20090330_flexible pvc foils based in photonics for automotive applications

FLEXIBLE PVC FOILS BASED IN PHOTONICS

FOR AUTOMOTIVE APPLICATIONS

by

A. Filipe F. Gonçalves

Dissertation

Submitted to the Committee of

Engineering Design and Advanced Manufacturing

for the degree of

Advanced Post-Graduate

in

Technology Management Enterprise

Conducted

In

TMG-Automotive & Universidade do Minho

Under supervision of

Prof. Dr. J. Higino Correia

Guimarães, 09th March, 2009

AUTOMOTIVE

Flexible PVC Foils Based In Photonics For Automotiv

e Applications

Name: Anselmo Filipe de Freitas Gonçalves E-mail: [email protected] Telephone: 93 8619330 National identity card Nº.: 10206063 Thesis Title: Flexible PVC Foils Based In Photonics For Automotive Applications Supervisor: Prof. Dr. José Higino Gomes Correia

2 Flexible PVC Foils Based In Photonics For Automotive Applications

Acknowledgments

My first acknowledgments are addressed to the EDAM faculty organization for making this thesis

possible, in particular to Dr. José Higino Gomes Correia for suggesting the topic, for acting as my

faculty advisor, and for the guidance through the related research.

Very special acknowledgements are addressed to Mrs. Isabel Furtado from TMG-Automotive

board for the overall support, for making the financial and industrial resources available and for

believing in my capacities, offering me the needed time and resources.

Furthermore, special thanks go to Mr. Carlos Vieira, Technical Manager at TMG-Automotive, for

his continuous assistance and support through the whole project. I would like to extend my thanks to

the technical staff of TMG-Automotive that kindly support the practical work in industry environment.

I am also sincerely grateful to Alexandre Manuel Teixeira de Barros Ferreira da Silva for the

jointly work performed within the scope of this thesis and to Fibersensing for gently facilitate the use

of their technology.

Finally, I would like to thank my family and specially my wife, Filomena for supporting and

challenging me during this chapter of my life.

Flexible PVC Foils Based In Photonics For Automotive Applications 3

FLEXIBLE PVC FOILS BASED IN PHOTONICS

FOR AUTOMOTIVE APPLICATIONS

Abstract

This thesis is conducted within the frame of the EDAM Project “Automotive smart flooring based

in photonics”, for development of advanced optoelectronic sensing devices for the automotive industry

with the goal of gathering sensitive information and sending them remotely (possibly over a wireless

channel) for further processing. For this purpose, optical fibers and sensors are embedded into flexible,

laminated, polyvinyl chloride (PVC) structures. The used intrinsic sensors are formed by Fiber Bragg

Grating (FBG).

The research is focused in the insertion of optical sensors and optical fibers into polymeric PVC

matrix. The suitable materials and method, enabling an automated manufacturing process for such

integrated optical sensitive systems are defined. The PVC structure is designed in order to guarantee

the optimal function of optical sensors. Materials and structure of the PVC laminated structure are

selected in order to assure a correct insertion of the optical elements, enabling an automated optical

fiber insertion method.

Smart polymeric foil prototypes, able to sense and gather sensitive information, and send it for

remote analysis were fabricated. The obtained flexible sensing foils are characterized with respect to

their most important mechanical and optical characteristics. The fabricated foil is capable of

transferring the full deformation to the optical sensor. Tests indicate that the polymeric foil influence

on the sensor performance may exist. However, the presented optical sensor incorporated in the

polymeric foil is fully functional with high sensitivity.


Table of Contents

Abstract .........................................................................................................................................3

1 Introduction ..............................................................................................................................8

1.1 Overview of fiber optics ................................................................................................... 8

1.2 FOS in the automotive industry ...................................................................................... 10

1.3 Considerations about FOS integration............................................................................ 12

2 Materials and technology .......................................................................................................13

2.1 Flexible integration approach ......................................................................................... 13

2.2 Flexible matrix ................................................................................................................ 13

2.2.1 Substrate............................................................................................................... 13

2.2.2 Substrate fabrication ............................................................................................ 14

2.2.3 FOS principle ....................................................................................................... 14

2.2.4 FOS integration .................................................................................................... 16

2.3 Flexible PVC technology................................................................................................ 17

2.3.1 Plasticized PVC.................................................................................................... 17

2.3.2 Spread-coating process ........................................................................................ 18

2.3.3 Formulation effect on the application method..................................................... 20

2.4 Prototype development approach.................................................................................... 21

3 First prototype of flexible PVC carrier foil ............................................................................23

3.1 Flexible PVC formulation concept ................................................................................. 23

3.2 Concept design................................................................................................................ 24

3.2.1 Influence of the plasticizer................................................................................... 24

3.2.2 Influence of the PVC resin type........................................................................... 25

3.2.3 PVC plastisol composition................................................................................... 27

3.3 Foil construction ............................................................................................................. 28

3.4 PVC foil prototype with hand inserted optical fibers ..................................................... 29

3.4.1 Laboratory scaled spread-coating process ........................................................... 29

3.4.2 Experimental results............................................................................................. 29

3.5 Evaluation of mechanical properties of PVC matrix ..................................................... 34

3.6 Evaluation of the optical properties ................................................................................ 38


3.6.1 Measurement system............................................................................................ 38

3.6.2 Signal transmission trough in flexible PVC embedded optical fiber................... 38

3.6.3 Multiple signal measurements.............................................................................. 40

3.7 Conclusions ..................................................................................................................... 43

4 Design for an automated manufacturing process .................................................................. 44

4.1 Integration of FOS in continuous spread-coating ........................................................... 44

4.1.1 The laminated structure interior trimmings ......................................................... 44

4.1.2 Two ways for integrate fiber optic insertion........................................................ 45

4.2 Direct deposition of optical fibers during spread-coating............................................... 45

4.2.1 Preliminary considerations................................................................................... 45

4.2.2 Adjustment of the viscoelastic properties of the PVC matrix.............................. 46

4.2.3 Development of high-viscosity intermediate PVC adhesion layer ...................... 52

4.2.4 Deposition of optical fibers with intermediate adhesive layer PLAST 07 .......... 54

4.2.5 Conclusion of the direct fiber deposition method................................................ 56

5 Prototype characterization ..................................................................................................... 57

5.1 Integrated construction of the flexible sensing foil......................................................... 57

5.2 Mechanical properties of the flexible sensing foil .......................................................... 58

5.2.1 PVC matrix........................................................................................................... 58

5.2.2 PVC matrix with embedded optical fibers ........................................................... 61

5.3 Optical properties of the sensing foils during elongation tests .......................................65

6 Conclusions............................................................................................................................ 68

References .................................................................................................................................. 70

Appendix A ................................................................................................................................ 72


Table of Figures

Fig. 1 : Principle of internal reflection at the core-cladding-interface.......................................9

Fig. 2 : Optical fiber structure..................................................................................................15

Fig. 3 : Photo-impression of Bragg Grating in optical fiber ....................................................15

Fig. 4 : FBG Sensor principle ..................................................................................................16

Fig. 5 : Flexible (plasticized) PVC formation stages...............................................................17

Fig. 6 : Attenuation of dipole-dipole attraction between PVC-chains.....................................18

Fig. 7 : Layout of industrial spread-coating process................................................................19

Fig. 8 : Industrial plastisol application unit .............................................................................19

Fig. 10 : Development loops for PVC foils with FOS...............................................................22

Fig. 11 : Influence of plasticizer in the hardness flexible PVC foils .........................................24

Fig. 13 : Viscosimeter Rheomat 115..........................................................................................25

Fig. 14 : Viscosity / shear rate relationship for tested PVC resins ............................................26

Fig. 15 : Sandwich structure for embedded optical elements in flexible PVC foil ...................28

Fig. 16 : Laboratory plastisol application equipment ................................................................29

Fig. 17 : Defects due to the deposition of optical fiber over uncured plastisol .........................31

Fig. 18 : Plastisol displacements due to fiber initial shape recovering......................................31

Fig. 19 : Defects due to the deposition of optical fiber over uncured plastisol .........................31

Fig. 20 : Manual release from paper support .............................................................................32

Fig. 21 : Undulation along the optical fiber path .......................................................................33

Fig. 22 : Universal testing machine Instron 4302 for elongation-to-break measurements........34

Fig. 23 : Load-Elongation relationship for PVC foils cured at two different temperatures ......36

Fig. 24 : Tensile properties of PVC foils cured at 180ºC and 200ºC.........................................37

Fig. 25 : FS 4100 - portable BraggMETER unit........................................................................38

Fig. 26 : Reflected spectrum in PVC prototypes P03 and P04 ..................................................38

Fig. 27 : Reflected spectra from manual induced deformation..................................................39

Fig. 28 : Flexible PVC sensing prototype P04...........................................................................40

Fig. 29 : Spectrum of in series connected horizontal and vertical oriented FBGs ....................41

Fig. 30 : Time dependent log-function of deformed reflected spectra ......................................42

Fig. 31 : Laminated fiber optics in flexible PVC.......................................................................44


Fig. 32 : Viscosity / shear rate relationship for different plastisol formulations ...................... 47

Fig. 33 : Visible irregularities in the surface of prototype P05................................................. 50

Fig. 34 : Elongation-at-break performance of PLAST 06.........................................................51

Fig. 35 :Tensile properties of PLAST 06 .................................................................................. 51

Fig. 36 : Viscosity / shear rate relationship of PLAST 07 ........................................................ 53

Fig. 37 : Laminated fiber optics with intermediate PLAST 07 layer........................................ 54

Fig. 38 : Top view of prototype P06 ......................................................................................... 55

Fig. 39 : Side view of prototype P06......................................................................................... 55

Fig. 40 : PVC foil without optical fibers................................................................................... 57

Fig. 41 : PVC foil with optical fibers ........................................................................................ 57

Fig. 42 : Tensile properties of PVC foils cured at 180ºC and 200ºC........................................ 58

Fig. 43 : Elongation-at-break test of multi-layer structures ...................................................... 59

Fig. 44 : Amplified cross section – foamed material structure of PLAST 08 layer.................. 60

Fig. 45 : Flexible PVC foils with different optical fiber path ................................................... 61

Fig. 46 : Sample gripping in elongation-at-break test............................................................... 62

Fig. 47 : Elongation-at-break of PVC foils with different “S-shape” optical fiber paths ......... 63

Fig. 48 : Elongation-at-break of PVC foils with linear oriented optical fiber path .................. 64

Fig. 49 : Deformation of curved fiber segment......................................................................... 65

Fig. 50 : Bragg response to applied displacements ................................................................... 66

Fig. 51 : Bragg response to applied displacement steps............................................................ 67


1 Introduction

1.1 Overview of fiber optics

The science of photonics is related to the study of the emission, transmission, amplification,

detection, and modulation of light. A mono-mode optical fiber is a monofilament made of glass or

polymeric material with the ability to carry light along its length. Optical fibers are widely used in

fiber-optic communications, which permits transmission over longer distances and at higher data rates

(bandwidth), than other forms of communications. Fibers are used instead of metal wires because

signals travel along them with higher speed, with less loss, and they are immune to electromagnetic

interference. Fibers are also used for illumination, as well for transporting images, allowing viewing in

tight spaces. Specially designed fibers are used for a variety of other applications, including as sensors,

the so called fiber optic sensors (FOSs) [1].

Optical fibers can be used to measure a variety of characteristics like strain, temperature, pressure

among many others. This is possible because the characteristic to be measured – the measurand -

modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. In some

applications, the sensor is itself an optical fiber. In those cases we are talking about intrinsic FOS. In

other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system and we are

talking about extrinsic FOS [1].

Fiber optic technology experienced a phenomenal rate of progress since the discovery of

separated glass coating to avoid signal losses through the optical fiber [2]. The internal region of the

fiber, or core, is used to transmit the light, while the glass coating, or cladding, prevent the light from

escaping out of the fiber, reflecting it at the core-cladding-interface. This concept is explained by the

principle of Snell-Descartes, which states that the angle at which light is reflected at the interface

between two different isotropic medias, depends on the refractive indices of the two materials - in this

case, the core and the cladding [3][4][5].


The lower refractive index of the cladding (with respect to the core) causes the light to be angled

back into the core as illustrated in Fig. 1.

Fig. 1 : Principle of internal reflection at the core-cladding-interface

In the beginning of the new millennium, photonics technology was largely focused on

telecommunications. However, photonics began to cover a huge range of applications, including laser

manufacturing, biological and chemical sensing, medical diagnostics and therapy, display technology,

optical computing, etc. The optoelectronics industry has leaded to revolutionary products like compact

disc players, laser printers, bar code scanners, laser pointers, among others [6][7].

In parallel with these developments fiber optic sensor (FOS) technology has been a major user of

the optoelectronic and fiber optic communication industry. FOS technology has often been driven by

the development and subsequent mass production of components to support these industries. The

inherent advantages of FOSs include reduced weight, very small size, low power consumption,

immunity to electromagnetic interference, high sensitivity, wide bandwidth functioning and high

resistance to harsh environment, etc. All these properties were heavily used to counterbalance their

major disadvantages; high cost and unfamiliarity to the end user [6][7][8].

In the early days of FOS technology most commercially successful FOSs were straight targeted at

markets where conventional - electronic - sensor technology was marginal or in many cases

nonexistent. The cost of FOS technology remains higher than electronic sensors, thus FOSs have

penetrated only where their unique advantages justify the higher expense, or in which they are the only

option for the existing conditions. As component prices have fallen and quality improvements have

been made, the ability of FOSs to substitute traditional sensors for measuring rotation, acceleration,

electric and magnetic field, temperature, pressure, acoustics, vibration, positioning, strain, humidity,

viscosity, chemical concentrations and a host of other special measurands, has been enhanced.

Cladding

Core

Light No light loss – reflection at core-cladding interface

Without cladding – light (signal)


Integrated optical devices that were not available in usable form at the beginning of the 21st

century are now more commonly used. This had lead naturally to a higher familiarity with fiber sensor

technology and today’s FOSs applications are much more diversified:

- Telecommunications - Fiber Optic Gyroscopes - Military - Aerospace

- Medical And Biomedical (Surgical Miniaturization) - Petrochemical - Construction (Infrastructural Monitoring) - Lighting - Automotive

The applications have become so varied that fibers are available in a variety of core sizes,

compositions, and constructions, each with specific advantages and requirements [6][7][8][9].

Temperature, strain and load are the most widely studied measurands and the fiber grating sensor

represents the most widely studied technology for fiber optical sensors. Fiber-optic gyroscopes and

fiber-optic current sensors are good examples of rather mature and commercialized FOS technologies.

Today, some success has been found in the commercialization of optical fiber sensors. Some other

types of FOS, based on very simple concepts have been commercialized. Examples of such concepts

include the displacement or pressure sensor based on the light coupling of two fibers, the liquid-level

sensor based on frustrated total internal reflection, the pressure sensor using a wiggled (periodically

bent) fiber, and the temperature sensor based on the detection of radiation from a heated sensor head

(blackbody cavity) [10][11].

Although FOS have not experienced the dramatic commercial success of optical fiber

communications, they have been continuously and enthusiastically studied. Fiber grating sensors have

been the most widely studied topic among various optical fiber sensor technologies. A few fiber bragg

grating sensors have been commercialized for health monitoring of civil structures and oil industries.

New ideas are continuously being proposed and tested for not only various traditional measurands such

as strain, temperature, and pressure but also for new applications such as chemical sensors. Papers on

FOS already begun to appear in many field application journals such as civil engineering journals,

sensor journals, power engineering journals, chemical engineering journals and bioengineering

journals.

1.2 FOS in the automotive industry

The efficiency of optoelectronic devices has prompted automobile manufacturers to develop more

applications based on this technology. Advancements in electronics and photonics have lead to


innovative illumination, sensing, and visualization solutions, in particular in safety features. Some

features are available today while others are on the horizon.

Perhaps one of the most interesting developments has been the use of polymer optical fibers

(POF) in automotive applications, taking advantage of their low weight and showing simplicity of

connection and interconnection. Increasing data volumes add more and more complexity to control

systems in the automotive field and make these technologies particularly useful for instrumentation

and control. However, POFs still have to meet a number of demanding constraints for use in

automotive applications, including resistance to chemical, thermal and mechanical stress, as well as

being available with a high optical quality at low cost. Issues as temperature, transmission losses, and

bandwidth are tremendous important in POF; and the automotive industry is now moving towards an

optical fiber with a silica glass core/plastic clad (PCS).

The rising demand for safety, comfort, multimedia, infotainment, and telematic applications in

automobiles are catalyzing the integration of photonic-based technologies in the automotive industry.

Automobiles are increasingly employing photonics for lighting, displays, sensors, and communications

systems. For a majority of these applications, photonics technologies have worked well in conjunction

with the associated electronic counterparts. Incremental improvements in passive safety have reached

the saturation point and are not likely to further improve vehicle safety. The active safety industry,

however, is still in its infancy and new automotive applications for photonics emerge every year

[9][10][11][13].

The properties of FOSs make them more useful than their electronic counterparts for automotive

applications. Some of these applications include gyroscopes for automotive navigation systems [12],

strain sensors for smart structures and for the measurement of various physical and electrical

parameters like temperature, pressure, liquid level, acceleration, voltage and current in process control

applications [13].

Although photonic technologies are superior to electronic solutions in terms of data rates,

bandwidth, reliability, and robustness, it remains not easy to achieve integration and miniaturization as

easily as in microelectronics. In the automotive industry, the performance to weight ratio of structural,

functional and decorative components is a key design driver, and much development activity is carried

out using new materials and alternative design methodologies. The application of optical sensors is an

advantage regarding weight saving as it allows to reduce dramatically the use of heavy electrical

cables and wires. But on the other hand, the use of optical fibers almost always implies a critical

integration problem, which is related to a manufacturing cost problem.


1.3 Considerations about FOS integration

Optical sensors have evolved considerably from their beginning as experimental devices. Better

instruments and sensor packages have helped make installation easier, especially in monitoring the

health of civil structures – one of the industrial scale applications for FOS that has most evolve in the

last years [14][15]. Nevertheless still about 50% of the cost of such systems is for installation. This is

mainly due to the use of manual or semi-manual deposition or integration techniques of optical fibers

and sensors [15][16].

One major issue to overcome FOS integration difficulties, it is the reproducibility and constant

quality of the used packaging or integration processes. The lack of existing industrial scaled optical

fiber integration processes, applied to the production of parts or elements which are used in

automobiles, is responsible for the relative low-market acceptance of FOS, despite the superior quality

and performance in comparison to the conventional electronic sensors [16][17].

Therefore automated ways for FOS and optical fiber installation and integration in sensing

structures is the crucial point. Costs for manual fiber alignment issues, have to be reduced, in order to

find industrial friendly applications of optical sensor modules. Furthermore, the concept of the FOS

itself has to be developed in such a way that the sensor element or module can be manufactured in line

or in the same time as the automotive part or element itself. This would significantly simplify the

installation of sensor modules in automobiles.

Another important issue for a more simplified use of automotive sensor modules is the question of

their flexibility. Optical fibers and sensors have less flexibility than electronic cables and wires. This

problem has to be solved with a flexible integration approach.


2 Materials and technology

2.1 Flexible integration approach

The lack of automated optical fiber integration techniques and the reduced flexibility - optical

sensing is not yet implemented in a widespread practical and industrial way, for automotive

applications. The goal is the development of a generic technology that offers an integrated

breakthrough solution for the industrial manufacturing of flexible optical sensing foils for automotive

applications. Subsequently, the project aims to develop a flexible substrate or matrix in which signal

transporting and sensing elements are integrated in line during the manufacturing process of the

substrate itself. This artificial and flexible optical sensing structure can be applied to regular or even

irregular surfaces, enabling distributed sensing that can easily be installed in automobiles. The flexible

and stretchable matrix can be sensitive to touch, temperature, pressure, deformation, etc. Such optical

sensing structure combines two features of interest: integrated optical sensing and high flexibility.

2.2 Flexible matrix

2.2.1 Substrate

In automotive applications, the need for discrete (sometimes hidden) and integrated sensing

solutions is much more relevant for areas where free space can not be misused. This is particularly the

case in interior areas, where safety concerns and comfort may generate strong needs for sensing

devices. In automotive interiors, any kind of functional device should be integrated in the most discrete

and elegant manner, in harmony with what the eyes of occupants can see. The car interior is mostly

made of soft-touch surfaces, like leather or leather like materials, textiles, carpets, mats, etc. In the

context of a flexible integrating approach for FOS, a flexible skin-like foil for automotive interiors is

therefore the base material for the scope of this work.

Flexible skin-like foils for automotive interiors can be made of different polymers. Polyurethane

(PUR) may be one of them, feeling like leather, with very long durability and high-performance in

regard to abrasion resistance and flexibility. But PUR-based artificial leather is one of the most

expensive skin materials for automotive interior trimming. Polyolefin based artificial skins are a


suitable alternative for the required objective, but their flexibility and performance related to softness,

abrasion and flexibility is in general more difficult to adjust.

As the research is focused on the development of a generic manufacturing technology for a

flexible optical sensing foil, it was decided to choose a polymer matrix that can be applied for the most

interior trimmings, with an acceptable average quality and price. The choice was set on plasticized

PVC, for its general good cost/performance ratio and ease of use during manufacturing processes.

PVC certainly is one of the most versatile plastics, still playing a major role in the building, packaging

and automotive market. Furthermore, PVC exhibits many advantages like highly competitive

production cost, high versatility in interior trim applications, high resistance to ageing, ease of

maintenance [18][19].

2.2.2 Substrate fabrication

PVC can be processed by techniques compared to other polymers. Extrusion, calendering as well

as paste (plastisol) techniques like spread-coating, slush moulding and dip moulding are techniques

predominantly used for PVC.

Spread-coating technology allows the manufacturing of foils for a broad range of applications,

such as clothing, footwear, home decoration, waterproof tablecloths, tarpaulins, conveyor belts,

wallpapers, floor mats, artificial leather and automotive interior trimmings [19].

Spread-coating is a process that consists of depositing one or more layers of plastisols (viscous

paste obtained by suspension of polymer resins in plasticizers) on a support such as natural or synthetic

fiber mats, textiles or paper (release paper). Afterwards, the deposited layer is gelated in an oven.

Because of its versatility, this technique constitutes an optimal choice for the development of flexible

optical sensing PVC foils.

2.2.3 FOS principle

In automotive application, reliability and reduced handling concerns of the used optical fibers are

mandatory. Therefore, the chosen optical fibers should have excellent performance and good market

acceptance. Glass fibers are preferable due to reduced signal-loss. Also, photonic fibers should be

provided with a cladding glass. Silica is so widely used because of its outstanding properties, in

particular its potential for extremely low propagation losses and its remarkably high mechanical

strength against pulling. For protecting the fiber during its manipulation and insertion operations, a


coating - polymer jacket

cladding

core

protective polymeric coating - polymer jacket - is desirable. The selected fiber is a single-mode fiber,

composed by a fiber core made of glass, surrounded by a glass cladding and coated with a polymer.

Easy optical connection is also a very important factor. With respect to this characteristic, the chosen

optical fiber has a coating that is very easily strippable, facilitating the preparation of fiber ends for

connecting to light sources or measurement devices.

Fig. 2 : Optical fiber structure

Optical signal stability through highly stable geometry along the entire length of the fiber is very

important in order to maintain the nominal mode-field performance. This performance is related to the

so-called mode-field diameter (MFD) - size of the light-carrying part of the fiber, including the core

and a small part of the cladding glass - that can determine the effect of bend-induced loss as well as

splice loss.

A Fiber Bragg Grating (FBG) is a special type of optical sensor obtained or formed in the optical

waveguide itself, a so-called intrinsic FOS. A FGB is created in optical fibers by forming a particular

profile in the fiber core so that its refractive index is modified, causing an internal reflection of light.

The refractive index changing profile is formed by "inscribing" or "writing" a patterned microstructure

in the fiber core. A narrow band of the incident optical signal is reflected by the inscribed

microstructure – gratings – resulting in a wavelength specific resonance. The FBG sensors are

fabricated by photo impression of photosensitive optical fibers with side-exposure to patterned UV

laser radiation:

Fig. 3 : Photo-impression of Bragg Grating in optical fiber


The gratings produced typically have lengths in the order of 10 mm [20]. When a light beam is

send into the “grated” fiber, the wavelength spectrum corresponding to the grating pitch will be

reflected, while the remaining wavelengths will pass through the grating undisturbed, as exemplified in

Fig. 4 [22][23]. Since the grating period structure is sensitive to strain and temperature, these two

parameters can be measured by the analysis of the reflected light spectrum. This is typically done

using a tuneable laser containing a wavelength filter - such as a Fabry–Perot cavity - or a spectrometer

[20].

Fig. 4 : FBG Sensor principle

2.2.4 FOS integration

For the chosen flexible sensor integration approach, a good insertion of optical fibers and sensors

in the flexible foil is mandatory. If the sensor is designed to monitor strain or deformation, it is

necessary to have a good bonding of optical fiber with the flexible polymeric matrix in which it is

embedded, i.e. to achieve good transfer of strain from the host material to the sensor.

The flexible polymeric substrate gives to the FOS protection against accidental damaging during

handling, placing and live period.

In the advanced manufacturing process, the choice of the raw-materials, in order to find the

optimum way to produce flexible PVC foils with integrated sensor capacities in a “one shot” process is

considered. Therefore, an integrated manufacturing process for such automotive PVC flexible foils

with sensing skills must be designed and implemented.


2.3 Flexible PVC technology

2.3.1 Plasticized PVC

PVC is never used in form of pure resin. The most PVC applications are based on plasticized

PVC, with a very broad spectrum of products that have either very high flexural modulus such as pipe

or building profiles or high flexibility such as footwear, refrigerator liners or even artificial leather,

under many others [18][19]. A possible way for manufacturing plasticized PVC products, is the

dispersion of fine PVC particles in a liquid plasticizer. This premix is called a plastisol. A plastisol is a

solvent free paste-like composition, usually referred to as "vinyl paste". It consists of a physical

mixture of finely sized PVC polymer particles and liquid plasticizers, such as phthalates and epoxy

oils. Other additives commonly used in formulating PVC plasticizers are inert fillers (e.g., calcium

carbonate), heat and UV stabilizers, anti-oxidants, etc.

PVC is a semi-crystalline polymer, obtained in form of powder. Its particle size and morphology

play an important role in processing and performance of products. The absorption of plasticizer into

PVC resin particles is a kinetic process whose rate varies inversely with the viscosity of the

plasticizer. The PVC grades designed for plastisols need to have small particle size (less than 2

microns) with a uniformly spherical hollow shape, in order to achieve an acceptable porosity of the

particles and subsequently a good surface adsorption of the plasticizer molecules; so they can stay in

suspension without sedimentation over longer period of time. Another key requirement is that the resin

must resist to solvation at room or storage temperatures so that the final product will have a practical

shelf life.

Liquid plasticizers have relatively good viscosity stability at ambient temperatures. However,

when exposed to elevated temperatures in the 130°C to 400°C range, the plasticizer solvates the

suspended PVC polymer particles, resulting in a permanently fused product. This is a chemical

reaction that passes trough several stages:

Fig. 5 : Flexible (plasticized) PVC formation stages

[a] [b] [d] [e] [f]

[a] Suspension: adsorbtion of plasticizer to PVC particles [b] Intermediate phase: plasticizer begins to penetrate the

outer shell of PVC particles � swelling of a thin layer of the PVC particle surface

[c] Gelation: solvation of the PVC molecules by the plasticizer � plastisol is converted to a semi-solid but weak structure, which doesn’t flow anymore.

[e] Fusion [f] Solidification and curing


The first one is the solvation stage, where an intermolecular diffusion of the plasticizer particles

into the PVC resin occurs, attenuating the strong intermolecular dipole-dipole connections between

PVC-chains responsible for the brittle character of pure, unplasticized PVC.

Fig. 6 : Attenuation of dipole-dipole attraction between PVC-chains

The plastisol material can be formulated to be either a soft foam or a dense hard solid, which

retains toughness even at low temperatures with hardness ranging from 30 to 90 Shore A and tensile

strength ranging from 750 to 3000 psi. It can be designed to resist to chemical attack, it is self-

extinguishing due to the chlorine groups, and provides good weatherability.

Plastisols can be processed for coatings, films, sheets, and foams, made by roller or spread-

coating (knife-edge coating), dip coating, rotational casting, and slush molding, as well as extrusion

and injection molding.

2.3.2 Spread-coating process

This industrial process relies on a coating being applied to a substrate which passes through a

“gap” between a “knife” and a support roller or counter roller. This technology is also referred to as

“Knife-Spreading” or “Knife-edge Coating”. The coating substance is in this case a PVC plastisol. As

the coating and substrate pass through the gap, the excess is scraped off by the knife. This process can

be used for high-viscosity coatings and very high coat weights.

There are innumerable variants of this relatively simple process. The designed process is adapted

for the manufacturing of flexible laminates that can be composed of several layers, made of PVC or

other substrates. Fig. 7 illustrates schematically the process. The plastisol is pumped from a

transportable container to a coating reservoir (�) formed by the blade (�) that is obliquely disposed

over the support roller (�). On the support roller, the plastisol is applied over release paper (�) that

Strong dipole-dipole attraction in unplasticized PVC

Attenuated dipole-dipole attraction in plasticized PVC


plays the role of a carrier and can later on be removed. The amount of applied plastisol is controlled by

precision setting of the gap between the paper surface and the blade. The applied PVC layer is

transported by the release paper to an oven, in which the geletion and fusion of the PVC plastisol

occurs.

Fig. 7 : Layout of industrial spread-coating process

The above described equipment sequence composed of an application unit followed by an oven, is

repeated three times to form an industrial production unit that enables the manufacturing of complex

laminates for automotive applications. Such complex laminates can be composed of several polymeric

layers bonded to flexible substrates like foams, webs and other textile materials.

A more realistic view of an industrial plastisol application unit is shown in Fig. 8.

Fig. 8 : Industrial plastisol application unit

� Blade

PVC plastisol

� Release Paper (or textile Substrate)

Industrial Oven

�

�

Application Unit

Oven Unit

to the oven

coating reservoir adjustable blade

support roller

PVC paste (plastisol)

release paper


2.3.3 Formulation effect on the application method

Viscosity

The viscosity of a PVC paste influences directly its manipulation and the whole foil

manufacturing process. High viscosity often requires low-speed handling with complex high-pressure

systems. Mixing difficulties may arise when a significant gradient exists in the viscosities of the

components. The pressure decreases, the pumping is difficult and it will be a critical situation.

Viscosity also influences the final characteristics of the obtained flexible PVC foil. Since,

viscosity is in any cases related to the PVC resin particles – in regard to geometry and dimension – and

to the type and amount of plasticizer, it affects the flexibility of the end product.

Fillers

The most common types of fillers for PVC plastisols are silica and calcium carbonate based.

Fillers are added to PVC pastes in order to adjust process parameters and final product properties.

Influence of fillers in plastisol processing:

- Increase processing temperatures

- Increase of viscosity

- Increase abrasion, causing wear of piston cups, packing seals, and precision parts.

- Increase risk of sedimentation during pot life of plastisol

Influence of fillers in final PVC characteristics:

- Increase of density

- Increase in hardness and impact resistance

- Lower product-cost

- Increase of whitening effect when foil is bent (if filler content is too high)

Temperature characteristics

Processing temperatures defined for gelation and fusion of the PVC affects directly the final PVC

foil. Gelation and fusion temperatures are typically material related, but can be varied in a certain

range. For a given PVC resin, higher gelation temperatures may result in a higher elongation at break

capacity. This temperature influence in the flexibility, should therefore be taken into account and

estimated. Another important process temperature dependent property is the quality of the fused PVC

matrix itself. With lower gelation temperatures, the solvation of the PVC molecules may not occur in a

homogeneous way and the final result can be a weakened PVC molecular structure that tends to


liberate plasticizer and become brittle or to develop fissures over time. Generally, such weak PVC

polymer is much more vulnerable to chemicals and weathering.

But gelation and general processing temperatures have to be balanced with respect to the

temperatures that the optical elements can endure. This means that the PVC plastisol system has to be

adjusted in order to achieve an acceptable quality of the flexible carrier, without loosing or damaging

the general performances of the embedded optical elements.

Degasification

Even if the plastisols systems used in this project aren’t solvent based and even the gelation and

PVC fusion reaction aren’t gas reactions, air is introduced in the plastisol during the manufacturing

process, due to the stirring process. When PVC resins, together with other additives are mixed into the

liquid plasticizer or plasticizer mixture, air is trapped in the plastisol. This causes the formation of very

small air bubbles with diameters ranging from a fiew microns up to 300 microns. Such air inclusions

may result in a weakened PVC matrix, less resistant to chemical attack and to abrasion. Furthermore,

air bubbles, even very small, originate discontinuities in the PVC matrix. Such discontinuities are in

fact weak points, where a correct adhesion of the PVC to the inserted optical elements fails. This may

cause errors in transferring strain or flexure from the host material to the sensor.

Fig. 9 : Air bubbles in translucent PVC foil

To remove air insertion in plastisols, vacuum has to be applied after mixing of the plastisols

compounds.

2.4 Prototype development approach

For the development of automotive smart flexible flooring material based in photonics, a first

development loop was planned. In this loop a starting or initial plastisol formulation is defined, based

on real material testing. Advisory information as well as product and process requirements, provided

by the industrial automotive partner of the project, are of crucial importance for enabling an initial

500µm

250µm

Light Microscope : Mag.: X20


tuning of the flexible PVC carrier. With this initial formulation, first laboratory prototypes with

incorporated optical elements have to be produced and analysed in respect to their general quality and

efficiency as flexible sensing elements. The analytical information coming from the first loop,

appropriate modifications to the material combination and the process should lead to a 2nd

development loop. In this loop, the effort is focused on the insertion method for the optical elements.

A 3rd and last development loop should benefit from the conclusions of the explored

manufacturing method(s) and lead to the proposal of an industrial automated manufacturing process

for flexible optical sensing foils. The composition of the sensing foils should be based on an improved

foil structure and PVC recipe.

Fig. 10 : Development loops for PVC foils with FOS

Initial Construction of

flexible PVC carrier foil

Material Selection

Manufacturing Process Design

Automated Manufacturing

Process Proposal

Process Conditions

Structure Selection

Recipe Optimization Fiber Integration Solutions

Properties Enhancement


3 First prototype of flexible PVC carrier foil

3.1 Flexible PVC formulation concept

The PVC matrix has to be roughly defined without neglecting the important compromise between

its flexibility and dimensional stability. Dimensional stability is important to ensure enough protection

for the optical fibers and sensors embedded in the foil. The PVC carrier has to be conceived to be

flexible but with enough dimensional stability in order to be suitable for several automotive interior

applications, ranging from flooring to upper parts like panels, e.g. door panels, backseat panels,

instrument panels, etc.

Dimensional stability can be measured by determining the hardness of the foil, whether flexibility

can be estimated with its ultimate elongation or elasticity modulus – also called young’s modulus.

Elasticity or flexibility, as well as the dimensional stability are mainly dependent on the choice of the

PVC resin itself. But the choice of the plasticizers and the plasticizer concentration in the plastisol

formulation are also strong influents.

Beside the elasticity and hardness, some other important aspects like price and organic volatile

emissions are of crucial importance and have to be considered at the early development stage. Volatile

organic compound (VOC) emissions are in the basis of a lot of severe quality standards for automotive

interior applications. Such emissions in flexible PVC products are mainly due to the volatility or

migration ability of the used plasticizers. Weight loss tests play a major role in the quality control of

interior trimmings for vehicles, as they deliver the amount of volatilized plasticizer when the flexible

PVC foil is submitted to higher temperatures.

The fine tuning of the end product properties is a complex task based on experimental testing of

numerous different formulations, combining at least a dozen of different raw materials. Such a fine

tuning of the foil characteristics include special properties like resistance to UV, aging, abrasion and

others.


3.2 Concept design

3.2.1 Influence of the plasticizer

The effect of plasticizer depending on its chemical nature was tested with a basis formulation

without any kind of additives, composed of a widely used microsuspension polymerisation PVC resin.

The used PVC resin is of low-viscosity and perfectly suitable for light to heavy plasticized

applications. Various films were produced with the same basis formulation, but with different

plasticizer type and plasticizer concentration. For each film (formulation) the Shore A Hardness was

measured and plotted in function of the plasticizer concentration.

Fig. 11 : Influence of plasticizer in the hardness flexible PVC foils

The migration of the tested plasticizer types was evaluated by weight loss. Samples of a reference

PVC formulation with 35% (+/- 0.5%) of plasticizer in pure microsuspension polymerisation PVC

resin were exposed to higher temperatures in a ventilated oven. The weight loss was measured with

changing exposure time.

45

55

65

75

85

95

105

20 30 40 50 60 70 80 90 100Plasticizer concentration ** [%]

DIBP

DOP

DIDP

TOTM

Di-isobutyl phthalate

Di-octyl phthalate Di-isodecyl phthalate Tri-octyl trimellitate

* Shore A Hardness according DIN 53505; ** Concentration = plasticizer amount per 100 g plastisol

rigid range

remi-rigid range

flexible range

highly flexible range

Plasticizer Concentration ** [%]

Sh

ore

A H

ard

nes

s*


Fig. 12 : Plasticizer weight loss vs. exposure time to high temperature

3.2.2 Influence of the PVC resin type

The choice of the resin type is primarily focused on grades for artificial leather foils that are

generally recommended for automotive applications. Several resin types were tested in regard to their

rheological behaviour in plastisols. The plastisol samples were prepared with a concentration of 35%

(+/- 1%) of a high molecular, linear, C10+C12 phthalate plasticizer.

For process purposes and for the use of the PVC paste to obtain flexible PVC foils by spread-coating,

the viscosity/shear-rate characteristics of 8 different PVC suspensions were evaluated using a

rotational viscosimeter with coaxial measuring system: Rheomat 115 (Contraves).

Fig. 13 : Viscosimeter Rheomat 115

0

5

10

15

20

25

15 30 45 60 75 90 120 135 150 165 180 195 210 225 240

DIBP

DOP

DIDP

TOTM

Sample size: 100 x 100 mm Exposure Temperature: 165ºC

Heating Time [hrs]

Wei

gh

t Lo

ss [

wt %

]


* Viscosity according to DIN114 measured at 20ºC (+/- 2ºC)

(ambient temperature)

�� Shear rate range for ideal industrial spread coating conditions

1000

10000

5 55 105 155 205 255 305 355 405 455 505 555 605 655 705 755 805 855905 955 1005

Shear Rate [s-1]

Vis

cosi

ty *

[P

a·s]

For each plastisol formulation, the viscosity was measured at 8 different shear rates and constant

temperature. Viscosity readings were performed after 24 hours rest-time, after plastisol preparation.

Fig. 14 : Viscosity / shear rate relationship for tested PVC resins

Resin Description Rheological behaviour ■ PVCRES 1 emulsion polymer (MW: 104000) Dilatant

■ PVCRES 2 emulsion polymerisation high gloss Dilatant

■ PVCRES 3 emulsion polymerisation low penetration Mixed

■ PVCRES 4 emulsion polymerisation high molecular Mixed

■ PVCRES 5 micro-suspension homopolymer Dilatant

■ PVCRES 6 micro-suspension homopolymer low visc. Dilatant

■ PVCRES 7 micro-suspension homopolymer high load Complex

■ PVCRES 8 emulsion polymerisation foam Thixotropic


3.2.3 PVC plastisol composition

The plasticizer influence analysis shows for higher molecular weight of plasticizer a low

plasticizing effect. Also Fig. 11 shows that for a given plasticizer concentration, the smallest hardness

value is obtained with the plasticizer of low molecular weight, like DIBP. One can achieve a more

“solid” or harder foil by using less plasticizer quantity if the plasticizer is of lower molecular weight,

which is not of practical interest, as this may result in low flexibility. It means that for a given hardness

of the PVC foil, more plasticizer is needed if it is of high molecular weight and a high amount of

plasticizer is favourable to have more flexibility in the foil. The plasticizer of choice should be TOTM.

But this plasticizer is by far more expensive than other plasticizers. The second choice is DIDP.

Furthermore, its performance it is relatively close to performance of TOTM and for a low price. The

exposure to high temperatures over long time shows that the highest volatility is registered with the

plasticizer of less molecular weight. “Heavier” plasticizer molecules are less able to migrate. Fig. 12

shows that high molecular weight plasticizers DIDP and TOTM have very low volatility if compared

with DOP or DIBP. It’s also interesting to notice the performance of the DIDP is very close to the one

of the expensive TOTM.

Definitely the plasticizer type selected for manufacturing of flexible optical sensing foils is DIDP.

First experiments are done with a target shore A hardness between 85 and 95, which should be reached

with a concentration of 40 to 55% of DIDP.

The PVC resin type is very important for the properties of the final product, but it is also a major

player in the foil manufacturing process. At the early development stage of this product, the industrial

manufacturing process has to be taken into account. For industrial spread-coating it’s generally

established that the ideal production speed should be in the range of 15 to 20 meters/minute. This

production speed corresponds to a speed at which the release paper is pushed through the plastisol

application unit and it is equivalent to an average shear rate of approximately 25 s-1. In a first approach

for the construction of a flexible PVC carrier foil, low viscosities of the used plastisol should be

targeted, in order to avoid a bad spreading of the PVC paste over the release paper and by the way,

also over the optical elements that should be covered by PVC. Attending to this initial requirement, the

viscosity analysis performed with 8 different PVC resins, demonstrated the suitableness of the grade

PVCRES6. This resin exhibits the lowest viscosity in the shear rate range between 25 and 55 s-1, which

constitutes a realistic shear value interval for ideal industrial spread-coating conditions. PVCRES6 is

therefore the suggested resin for making flexible PVC foils as first laboratory prototypes of flexible

sensing foils for automotive applications.


According to the results obtained with the material selection tests, the following plastisol

formulation is defined:

3.3 Foil construction

When thinking about a flexible sensing foil for automotive applications, the need for an “easy to

apply” product becomes evident. In this context the sensing product should be handled without

difficulties or at least, without friction or damaging of the included optical elements. For the insertion

of optical fibers and sensors in a PVC carrier, there are two major possibilities. First possibility

consists in bonding the optical elements on the carrier surface, resulting in higher friction on the

sensing foil surface and consequently a higher risk for damage of optical elements. Second possibility

consists in inserting the optical elements in the carrier matrix itself, lowering friction and risk for

damaging optical elements. This second possibility is more advantageous in respect to the protection

given to the optical elements. Furthermore, the insertion into the carrier matrix, should guarantee a

better bonding of optical fiber with the PVC matrix and subsequently a better transfer of external

stimuli from the host material to the sensor. For this purpose and considering the used manufacturing

process, where PVC is spread coated as viscous plastisol over a rolling release paper, a suitable way to

bring the optical fiber into the PVC core has to be designed. The PVC plastisol is pumped from a

transportable container to the coating reservoir of the plastisol application unit (see section 2.3.2 p.

18). Obviously optical fibers and sensors cannot be pumped in this way into the reservoir to be spread

coated. As the main concept is based on the use of glass fibers as optical elements, no printing of any

optical structure is applicable. The remaining solution is the deposition of the optical elements. In this

case, a sandwich structure approach is the most suitable:

Fig. 15 : Sandwich structure for embedded optical elements in flexible PVC foil

PVC Base Layer: 1st layer over Release-Paper

PVC Cover Layer: Final layer covering the optical elements

Deposited Optical Fiber / Optical Sensor

Plast01 Raw Material Type Concentration Function PVC Resin PVCRES6 55 % base polymer Plasticizer DIDP 40 % lowering paste viscosity + higher flexibility UV Stabilizer STAB1 1.5 % anti-aging effect + light stabilization + colour fastness Heat stabilizer EZ1 1.0 % improving weatherability Filler CaCO3 2.5 % improving impact resistance and hardness


3.4 PVC foil prototype with hand inserted optical fibers

3.4.1 Laboratory scaled spread-coating process

The process consists on a manual spreading of the plastisol over a substrate or carrier (e.g. release

paper) fixed on a metal frame. The spreading is done by moving horizontally a metallic knife over the

substrate. The movement of the knife makes the plastisol pass through a “gap” between the knife and

the substrate. The excess of plastisol is scraped off by the knife. The amount of applied plastisol is

controlled by the adjustment of the gap between knife and substrate. After the coating applied, as

uniform plastisol layer over the substrate, the whole metal frame is inserted by a mechanical

mechanism into an oven.

Fig. 16 : Laboratory plastisol application equipment

This laboratory technique enables to simulate industrial process conditions and is perfectly

suitable for industrial scale ups. The above described process enables the production of prototypes

with A4 dimensions.

3.4.2 Experimental results

A practical construction of the described structure in Fig. 15 is based on the application of

multiple PVC layers. The first PVC layer is applied over release paper and cured (gelation + fusion).

This layer plays the role of a protective skin for the optical fiber. Optical fibers are flexible and can be

easily bent so far the bending radius is not too sharp, unless the fiber breaks. A bent optical fiber tends

to recover its initial shape. It is therefore mandatory to bond the fiber to the substrate over which it is

deposited. An intermediate layer can be applied for avoiding the use of adhesives or any other polymer

type of a different composition. This 2nd layer plays the role of an adhesive layer for optical fiber if it

is pre-cured. If it is not, this uncured PVC plastisol layer will simply not allow the fiber to stand

steady, because the fiber would cause a plastisol displacement, while trying to recover its initial shape.

In order to pre-cure this intermediate PVC layer, a short exposure to heat in the oven should be

Knife

Plastisol

Metal frame

Lab Oven

Counter Cylinder � moves horizontally with knife

Release paper fixed on metal frame


enough. Afterwards the optical fiber is deposited over this “sticky” PVC layer and finally a 3rd PVC

layer is applied as cover layer. A Werner Mathis coating equipment was used for the production of

laboratory scaled flexible PVC foils with embedded optical fibers. In a first approach for this

prototyping step, the goal was to deposit manually optical fiber, (with FBG) just to understand the

behaviour of fiber and plastisol during handling. For this purpose the plastisol formulation Plast01 was

used.

Process steps for laminated configuration:

1. Application of PVC-layer 1 over release-paper 2. Heat-curing of PVC-layer 1 (insertion of frame into oven) 3. Application of intermediate PVC-layer 2 over PVC-layer 1 4. Partial heat-curing of PVC-layer 2 (5 seconds heating) 5. Manual deposition of optical fibers in PVC-layer 2 6. Final heat-curing of PVC-layer 2 7. Application of PVC-layer 3 over PVC-layer 2 with deposited optical fibers 8. Heat-curing of PVC-layer 3 9. After cooling down to room temperature � manual release from paper.

Process Conditions: Four different laboratory prototypes where generated: P01 – P02 – P03 – P04

Condition Step Operation

Gap (thickness) [µm] Temperature [ºC] Heating time [ '' ]

1 Application of PVC-layer 1 200 2 Heat-curing of PVC-layer 1 200 60 3 Application of intermediate PVC-layer 250 4 Partial heat-curing of PVC-layer 2 200 5 5 Manual deposition of optical fibers 6 Heat-curing of PVC-layer 2 200 60 7 Application of PVC-layer 3 350 8 Heat-curing of PVC-layer 3 200 60

P01

9 cooling + manual release while still on release paper – during 5 min. Condition

Step Operation Gap (thickness) [µm] Temperature [ºC] Heating time [ '' ]


P02

9 cooling + manual release while still on release paper - during 5 min. Condition

Step Operation Gap (thickness) [µm] Temperature [ºC] Heating time [ '' ]


P03

9 cooling + manual release while still on release paper - during 5 min.





P04

9 cooling + manual release while still on release paper - during 5 min.

Unsteadiness of the deposited fibers:

Fig. 17 : Defects due to the deposition of optical fiber over uncured plastisol

Fig. 18 : Plastisol displacements due to fiber initial shape recovering

Impossibility to achieve straight corners:

Fig. 19 : Defects due to the deposition of optical fiber over uncured plastisol

� Optical fiber tends to recover its initial shape, causing displacements of the partially cured PVC plastisol in the intermediate layer 2.

� Furthermore, the movement of fiber into uncured plastisol, makes the fiber to be embedded in an completely irregular way. This irregular configuration in the PVC layer 2 and 3 makes the surface of the foil itself becomes irregular.

�

�

Side view (transversal cut)

Top view

� It is not possible to form sharp corners. Fiber tends naturally to undo sharp bendings, describing more oval corners.

straight corner � �

Plastisol displacement leads to an irregular film formation with:

(A) plastisol-poor areas and (B) plastisol accumulation zones

(A)

(B)

Photo taken during PVC foil fabrication: PVC plastisol spread over release paper with manual deposited optical fibers.


Temperature and heating time:

Specific temperatures and exposure time were used for the insertion of optical fibers in PVC

matrix. Apparently no damage was observed on the fibers by direct observation after sample

manufacturing. A more detailed evaluation concerning the effect of curing temperature on the

mechanical properties of the obtained prototypes is needed (see section 3.5, page 34).

Thickness of the layers:

PVC-layer 1 (skin layer): Visual examinations of the obtained foils showed that once the skin

layer is cured, the optical fibers deposited on its surface don’t penetrate in its core. Therefore, after the

first prototype P01, produced with a skin layer thickness of 200 microns, the next prototypes (P02, P03

and P04) were manufactured with skins of 150 microns. With a thicker skin layer the whole

construction becomes less flexible and heavier. Furthermore, a thicker skin layer may contribute for a

less effective stimuli transmission from the outside into the fiber optics.

PVC-layer 2 (intermediate “adhesive” layer): The thickness of this layer started for prototype P01

with a thickness of 250 microns, which proved to be satisfactory. The progressive reduction of the

thickness demonstrated that the bonding of the optical fibers can be achieved with the minimum

thickness of 150 microns. But with this thickness, the unsteadiness of the deposited fibers becomes

more visible. The experiments showed that the thickness of 200 microns is more appropriate.

PVC-layer 3 (cover layer): The thickness of 350 microns for the PVC-layer 3 was chosen due to

the thickness of the optical fibers. With less coating thickness, the blade of the coating device would

plough out or even break the fibers when these pass trough the coating gap.

Undulated prototype surface:

The surface of the obtained foils is undulated. This effect differs from the irregularities caused by

the unsteadiness of the fibers in the way that “waves” are formed in the foil along the paths where

optical fibers are embedded. This particular surface defect is not observable before the foil is separated

from the release paper.

Fig. 20 : Manual release from paper support

Pulling in longitudinal direction


Fig. 20 shows how the release from support paper provokes a stretching of the foil. In the picture

it is visible that the elongation in the longitudinal way of the sample is more significant, since it’s on

this direction that the foil is pulled out from the release paper.

Fig. 21 : Undulation along the optical fiber path

Immediately after releasing from paper support, the foil presents an undulation in its surface,

along the optical fiber path. This effect is particularly intensified in the longitudinal direction of the

foil (see Fig. 21).

Apparently the effect seems to be related with internal stress at the interface fiber/matrix. These

waves are the result of different contracture modes between optical fiber and PVC matrix. During the

curing in the oven, the PVC matrix expands by a much higher coefficient than the optical fiber. When

the foil is cooled down, the PVC matrix contracts more than the optical fiber. This difference in the

contraction mode is responsible for inducing internal stress in the interface of optical fiber with the

PVC matrix.

The produced prototypes P01 to P04 were separated from the release paper in relatively short time

after their formation (5 min.). This might have been insufficient. If the cooling of the foil happens

while it’s still fixed to the supporting release paper, the PVC molecules of the matrix may rearrange

themselves, to form a uniform and stable matrix. If the foil with embedded optical fibers is released

from the support paper when material contraction is still happening, the induced internal stress leads to

the observed defect.


3.5 Evaluation of mechanical properties of PVC matrix

The mechanical properties were evaluated with respect to the influence of the selected gelation

temperatures (heat curing) in the flexibility or elasticity of the prototypes. Changes in flexibility were

measured with tensile experiments. For this purpose the elongation-at-break or ultimate tensile

strength was measured in specimens cured at two different temperatures, 180ºC and 200ºC, according

to the selected conditions for the obtained prototypes P01 to P04. The tested samples were fabricated

without insertion of optical elements. The objective of the experiment was to determine which of the

selected curing temperatures is more suitable for the PVC matrix obtained with the formulation

Plast01.

The measurements were performed with a universal testing machine Instron 4302, enabling a

controlled load to be applied to the samples. The applied load is measured in function of the resulting

elongation. Load is applied until the maximum or ultimate tensile strength of the material is reached.

At this point the material breaks and the maximum elongation and load are registered. The maximum

elongation is representative for the elasticity of the material.

Fig. 22 : Universal testing machine Instron 4302 for elongation-to-break measurements


Test conditions and foil construction: 2 x 2 different testing specimens were generated:

T01 and T02 at 180ºC Curing temperature

T03 and T04 at 200ºC Curing temperature

Foil construction applied to the 2 x 2 specimens: (foils without insertion of optical elements; each layer is composed of PLAST 01)

Geometry of tested specimens: rectangular

Dimensions of the tested specimens: 50mm x 200mm

Thickness of tested specimens: 700microns (± 20microns)

Sample conditioning according DIN 16 906 (24 hours at 23 ± 2ºC e 50 ± 6% rel. humidity)

Grip-geometry: rectangular

Grip distance between: 100mm

Extension speed: 50 mm/min

The obtained load-elongation curves are linear at the beginning, for elongations below 30%. This

initial linear behaviour defines the elastic or recoverable strain region of a tensile test. The

proportionality of stress and elastic strain in this region defines the elastic modulus (Young's modulus

E) by Hooke's law and the tested specimens demonstrated linear elastic properties. The slope of the

line in this region where stress is proportional to strain corresponds to E. The elastic modulus is a

measure for the ability of a material to deform under load in an elastic way – recovering its initial

shape after the load is removed. With the decrease of E, the material can be easily deformed [24][25].

Condition

Temperature [ºC]

Step

Operation Gap

(thickness)

[µm] T01 / T02 T03 / T04

Heating time [ '' ]

1 Application of PVC-layer 1 150 2 Heat-curing of PVC-layer 1 180 200 60 3 Application of intermediate PVC-layer 200 4 Partial heat-curing of PVC-layer 2 180 200 5 5 Heat-curing of PVC-layer 2 180 200 60 6 Application of PVC-layer 3 350 7 Heat-curing of PVC-layer 3 180 200 60 8 cooling + manual release


Fig. 23 : Load-Elongation relationship for PVC foils cured at two different temperatures

Lo

ad

[kN

]

Elongation * [mm]

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450

T01

T02

T03T04

* is equivalent to percentage of elongation [%]

�T01 curing temp. 180ºC �T02 curing temp. 180ºC �T03 curing temp. 200ºC �T04 curing temp. 200ºC

T01 T02

T03

T04


412.2% 401.9%

378.2%366.4%

32

5.6

N

33

4.5

N

33

2.4

N

32

4.8

N

0

50

100

150

200

250

300

350

400

450

T01 T02 T03 T04

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

From the obtained load-elongation curve, corresponding stress values can be obtained by dividing

the load by constant values calculated from the specimen geometry:

LA

LF

straintensile

stresstensileE

∆⋅⋅

==0

0

Maximum Elongation Ultimate Elongation

[%]

Maximum Load Ultimate Load

[N]

Young Modulus E [MPa]

366.4 324.8 4.102 378.2 332.4 3.996 412.2 334.5 3.184 401.9 325.6 3.064

Fig. 24 : Tensile properties of PVC foils cured at 180ºC and 200ºC

From the performed analysis it was possible to conclude that the specimens cured at 180ºC

temperature exhibit low ultimate elongation and their elasticity modulus is high, which reflects a high

load needed for deforming the foil. Specimens cured at 200ºC are more stretchable and this with less

load.

The curing temperature of 200ºC had proved to be more suitable in order to enhance the elastic

properties of the foils and to reduce their brittleness.

F = applied load A0 = original area through which the force is applied L0 = original length of the object ∆L = amount by which the length of the object changes (elongation)

�T01 �T02 �T03 �T04

E

180ºC 200ºC


3.6 Evaluation of the optical properties

3.6.1 Measurement system

The used measurement equipment to analyse the optical signal transporting and sensing properties

of the produced prototypes was a portable Bragg-meter unit from FiberSensing: FS 4100 Portable

BraggMETER™.

The intrinsic high dynamic range and high output power allows high resolution to be attained even for

long fiber leads and lossy connections. The FiberSensing BraggMETER measurement unit includes a

built-in NIST traceable wavelength reference that provides continuous calibration to ensure system

accuracy over long term operation. The broadband tuning range combined with the optical multiplexer

makes this measurement unit particularly suitable for large scale sensing networks.

Fig. 25 : FS 4100 - portable BraggMETER unit

3.6.2 Signal transmission trough in flexible PVC embedded optical fiber

By connecting the embedded fibers to the inbuilt laser source of the measurement system, it was

possible to detect a reflected spectrum. Tested prototypes: P03; P04.

Fig. 26 : Reflected spectrum in PVC prototypes P03 and P04


This type of optical fiber is capable to sense deformation and temperature. In order to verify this

behaviour, a deformation (stress) was applied to the polymeric foil and differences on the reflected

wavelength were registered.

Fig. 27 : Reflected spectra from manual induced deformation

In Fig. 27 it is possible to see the initial spectrum reflected from position #1, and how much it

slides when stretched to position #2. After that, the stress was released, and the spectrum came back to

the position #3. Another stretching deformation resulted in a reflected spectrum in position 4.

The experiment showed that an elastic linear deformation of the PVC foil with embedded optical

fibers (with FBG), results in a linear displacement of the initial optical spectrum. The displacement of

the spectrum is proportional to the linear deformation of the foil. When the deformation load is

removed, the foil recovers entirely its initial shape and the displaced optical spectrum recovers almost

completely its initial position. Comparing the green and blue lines in the spectra in Fig. 27, it is

possible to see a very small difference between initial and recovered spectrum position. It was also

possible to see that the spectrum shape does not change significantly during the application of

deformation force. This demonstrates that the optical signal loss is approximately zero, since there is

no reduction in the amplitude of the signal.

The obtained results suggested that the optical fiber was well embedded in the PVC matrix.

Position #1 Position #3 Position #2 Position #4


3.6.3 Multiple signal measurements

In a second experiment, a prototype containing two optical fibers with FBGs was used – prototype

P04. In this prototype, one FBG is horizontally oriented, while the second one is vertically oriented.

The combination of the two sensors with perpendicular orientation enables the measurement of

deformation forces in different axes. According to the force applied to the prototype, the deformation

read by the sensors can be different, since one sensor can be subject to compression while the other

may suffer elongation.

Fig. 28 : Flexible PVC sensing prototype P04

For the purpose of multiple signal measurements the flexible prototype was glued to a metallic

support, a stainless steel sheet with 0,8 mm of thickness. The adhesion to a stiff carrier should provide

a more realistic approach for the end-use of a flexible PVC sensor foil in automotive interior

applications – e.g. for flooring applications.


For this analysis, the sensors were connected in series, allowing in this case a simultaneous

reading of both by only one source.

Fig. 29 : Spectrum of in series connected horizontal and vertical oriented FBGs

In Fig. 29 is possible to see the reflection spectrum of both bragg sensors – two peaks, one for

each sensor. The proximity of both peaks, is approximately 0,7 nm, which later during deformation

analysis will have impact on the results.

Fig. 30 shows a function of the deformation sensed by the bragg structures in a time scaled axis.

Each curve corresponds to a bragg sensor and the positive and negative value of deformation

represents tensile and compression deformation force, respectively.


Fig. 30 : Time dependent log-function of deformed reflected spectra

Analysing the time dependency of the sample deformation (signal displacement in the spectrum),

it can be seen that in few situations, the reading of one of the sensors (blue line) goes to zero. This

behaviour happens by the fact that both spectrums were very close in terms of wavelengths. So, when

a deformation force displaces the spectrum of one of the sensors to the top of the other, the

measurement system rejects it and follows only one of the sensors.

Scan 1

Scan 0


3.7 Conclusions

The manual insertion of optical fibers into flexible PVC foils was successfully realized. The

realized prototypes demonstrated a surprisingly high resistance of the optical fibers to the relative

aggressive processing conditions used for the manufacturing of flexible PVC automotive foils.

The used PVC formulations were selected in function of the desired properties for a flexible optic

sensing system. Selected plasticizer DIDP may be replaced by another plasticizer type, according to

the specifications required for the end-use of the flexible sensing foil. Within the framework of this

project, DIDP proved to be perfectly suitable for creating the needed prototypes and developing an

industrial automated manufacturing process.

The process conditions for the PVC foil production with integrated fiber optics were selected in

order to achieve a good foil quality and to facilitate the definition of an automated fiber integration

method. A particular aspect of the tested process conditions was the number of heating cycles needed

for the construction of the multilayered foil structure. The needed partial heat-curing of PVC-layer 2

(see section 3.4.2 page 29) elevates the number of heating cycles up to four. This number of heating

cycles for the manufactured prototypes is too high. The pre-cure of intermediate layer makes it

necessary to use an industrial spread-coating unit with 4 plastisol application units. This is not

desirable; since, the manufacturing process becomes more expensive. Furthermore, it’s quite unusual

to find such industrial equipments. The intermediate layer over which the optical fiber has to be

deposited should be modified. A formulation has to be found that enables the fixing of the deposited

fibers without need of a partial heat curing.

The sensing capability of the developed flexible foils was demonstrated, opening the way to

intensify further characterization of the flexible PVC foils fabrication, in order to understanding their

limitations and features.


4 Design for an automated manufacturing process

4.1 Integration of FOS in continuous spread-coating

4.1.1 The laminated structure interior trimmings

Flexible skin-like foils for automotive interiors are in general made of several layers, composed of

different materials and each layer with a specific function. This multi-layer structure for interior

decoration and trimming is necessary in order to guarantee special functionalities like stretchability,

softness, dimensional stability, resistance to shear, impact resistance, according to the specific end-use

and area in the car interior where the foil has to be placed. The bonding of such multi-layer foils or

laminates is always related to lamination, this means a process that consists in merging two or more

layers of different materials together. For merging the different layers, adhesive can be used. But a so-

called “direct lamination” can be used, if the materials of the different layers are welded together or

sealed with heat and/or pressure, making use of the specific properties of the selected materials.

When spread-coating process is used, the laminates are obtained by direct lamination. Each spread

coated PVC layer constitutes a single layer of the final laminate. Each layer can be composed of a

different polymeric composition, adjusted to specific needs and desired product characteristics.

Furthermore, in addition to the spread-coating layers, other flexible substrates can be deposited over an

uncured PVC layer, like for instances foam-foils, textiles, webs or mats, etc.. Such a substrate is than,

once the spread coated layer is cured, automatically bonded to the layer over which it was deposited.

The used structure for the first flexible sensing prototypes P01 to P04, is in fact a typical laminate:

Fig. 31 : Laminated fiber optics in flexible PVC

1st layer: PVC “skin” layer � applied over Release-Paper

3rd layer : Cover Layer � final layer covering the fiber optics elements 2nd layer: intermediate “adhesive” layer containing fiber optics


4.1.2 Two ways for integrate fiber optic insertion

The goal for an integrated fiber optic insertion process during the manufacturing of a flexible

automotive laminate is to find a way to bring the optical elements in the core of the flexible laminate.

In fact there are only two possible approaches:

▪ Direct deposition of the optical elements

▪ Deposition of a substrate containing the optical elements (web, textile, flexible polymeric substrate, etc).

In both cases, the signal transporting and sensing elements have to be integrated in an automated

way, avoiding manual processes in order to guarantee high process reproducibility and lower

manufacturing prices.

The direct deposition approach implies the development of an automated fiber deposition

mechanism. Furthermore, the observed defects related to the unsteadiness of the fibers have to be

solved for such a direct deposition approach.

For the deposition of a substrate containing the optical elements, the manufacturing of the

substrate itself has to design as automated process. A possible approach would be the manufacturing of

a textile fabric with optical elements integrated in the textile structure. This fabric can then later be

laminated to the PVC foil.

4.2 Direct deposition of optical fibers during spread-coating

4.2.1 Preliminary considerations

Prior to the development of a specific automated mechanism which enables optical fibers to be

deposited over a rolling substrate in a predefined configuration, the defects observed during the

fabrication of the first flexible prototypes P01 to P04 have to be solved.

Unsteadiness of the deposited fibers: This problem is related to the adhesion of the optical fibers to

the substrate over which these are deposited. If deposited over a totally cured and tack-free surface,

bent optical fibers easily recover their original form. Obviously, the solution should be the

employment of an adhesion promoting material over the substrate. Uncured PVC had proven to be

ineffective, mainly due the relative low viscosity of the chosen plastisol formulation. Even if partially

cured, the used PVC formulation didn’t provide enough stiffness as adhesion promoting intermediate

layer; bent optical fibers were able to displace the partially cured plastisol, creating irregularities in


foil’s interior and surface. The initially selected PVC formulation was defined to be of low-viscosity,

considering the advantages of good flowing and spreading plastisols for industrial spread-coating. A

compromise for the viscosity has to be found, balancing good flowing properties of the fiber and

higher viscosity for a stronger fixing of the deposited fiber over uncured at least for the formulation of

the intermediate PVC adhesion layer.

Undulated prototype surface: This surface crinkle due to divergent contraction coefficients of PVC

matrix and optical fibers, may be solved with a slower cooling of the formed films. A longer cooling

time between foil formation and release from paper support may also be a positive factor for the

elimination of this defect.

Impossibility to achieve straight corners: The possibility to design straight corners during fiber

deposition may even not be a need. The needed pattern for the deposited optical fibers is dependent on

the final application and this is related to the form and the dimension of the final automotive part

where the sensing foil has to be applied. Nevertheless, it may be necessary to design deposition

patterns with sharper corners. Thus may be achieved by improving the adhesion of the deposited fibers

over the substrate during spread-coating. Another important aspect for the possibility to perform

sharper corners is the mechanism for a precise pattern drawing with the optical fiber.

4.2.2 Adjustment of the viscoelastic properties of the PVC matrix

Rheological behaviour as function of selected plastisol formulations

In order to understand the effect of the different plastisol compounds in the overall rheological

behaviour of the paste, several formulations were subjected to rotational shear rates. Reference

formulation was PLAST 01. Five other formulations were created with different PVC resin mixtures.

The variations in the resin mixtures were oriented based on detailed analysis of the obtained

rheological data from the material selection experiments (see section 3.2.2, page 25).

The mixing of the resins together with the other components was performed in a mechanical

mixer with rotating shear-blade agitator at 1750 rpm, during 5 minutes (same conditions for each

formulation). In order to remove the air in plastisols, medium vacuum was applied during 30 minutes

after mixing of the plastisols compounds.

The measurements were performed with a coaxial rotational viscosimeter, Rheomat 115. For each

plastisol formulation, the viscosity was measured at 15 different coaxial velocity steps or shear rates

and constant temperature. Viscosity readings were performed 24 hours after plastisol preparation. The

viscosity is given as a function of the shear rate. At each velocity, the viscosity was registered after 15

seconds.


Tested formulations and obtained results:

Formulation PLAST 01 PLAST 02 PLAST 03 PLAST 04 PLAST 05 PLAST 06

Raw Material Type Resin 1 PVCRES6 55% PVCRES6 25% PVCRES6 25% PVCRES6 25% PVCRES6 25% PVCRES3 40%

Resin 1 PVCRES4 25% PVCRES3 25% PVCRES8 25% PVCRES3 12.5% PVCRES8 13%

Resin 3 PVCRES8 12.5%

Plasticizer DIDP 40% DIDP 40% DIDP 40% DIDP 40% DIDP 40% DIDP 37.5%

UV Stabilizer STAB1 1.5% STAB1 1.5% STAB1 1.5% STAB1 1.5% STAB1 1.5% STAB1 1.5%

Heat stabilizer EZ1 EZ1 1% EZ1 1% EZ1 1% EZ1 1% EZ1 1% EZ1 1%

Filler CaCO3 2.5% CaCO3 2.5% CaCO3 2.5% CaCO3 2.5% CaCO3 2.5% CaCO3 7 %

Step Shear Rate Viscosity Viscosity Viscosity Viscosity Viscosity Viscosity

Modul [s-1] τ [Pa · s] [Pa · s] [Pa · s] [Pa · s] [Pa · s] [Pa · s]

1 6.65 977.0 1514 1560 1671 2259 2009 2931 2 9.51 683.0 1507 1556 1717 2259 1944 2732 3 13.61 478.0 1489 1578 1791 2231 1910 2458 4 19.48 334.0 1500 1614 1959 2148 1862 2338 5 27.90 233.0 1507 1702 2056 2105 1860 2270 6 39.90 162.9 1560 1875 2121 2070 1905 2281 7 57.20 113.6 1648 2009 2159 1967 1922 2272 8 81.80 79.5 1754 2089 2188 1817 1925 2226 9 117.10 55.5 1866 2125 2168 1701 1982 2220 10 167.60 38.8 1959 2193 2118 1652 1995 2212 11 240.00 27.1 2032 2143 2080 1626 2042 2222 12 343.00 19.0 2113 2089 2032 1596 1950 2217 13 492.00 13.2 2193 2160 1986 1550 1905 2193 14 704.00 9.2 2158 2175 1973 1482 1820 2132 15 1008.00 6.5 2143 2244 1875 1456 1778 2025

Fig. 32 : Viscosity / shear rate relationship for different plastisol formulations

2000

1000

5 55 105 155 205 255 305 355 405 455 505 555 605 655 705 755 805 855905 955 1005

PLAST 04 PLAST 02 PLAST 06 PLAST 03 PLAST 01 PLAST 05

Shear Rate σ [s-1]

3000

4000

Vis

cosi

ty υ

* [

Pa·

s]

�� Shear rate range for ideal industrial spread coating conditions

∆υ∆υ∆υ∆υ = −−−− 23%


Data interpretation

From the obtained shear rate dependent viscosity curves, it can be seen that the reference

formulation PLAST 01 ( ) has the lowest viscosity for shear rates under 85 s-1. The initial

formulation PLAST 01 was made with the resin PVCRES6, which exhibits a typical dilatant behaviour

when suspended in plastisol. Dilatant pastes become more viscous with increasing shear-rates.

Therefore the plastisol has it lowest viscosity when it is still. When shear force is applied, its viscosity

starts to increase.

The objective of the research related with the adjustment of the viscoelastic properties of the PVC

Matrix, was to increase the viscosity of the plastisol, at least in the range of shear-rates used during

industrial manufacturing process. Compounding PVCRES6 with resins like PVCRES3, PVCRES4 and

PVCRES8, has lead to plastisol formulations with high viscosity in this shear-rate range.

Formulations PLAST 02 and PLAST 03 demonstrated high viscosities with a general shear-rate

dependency quite similar to the behaviour of PLAST 01. With a viscosity that is relatively low in static

situation and begins then dramatically to increase when shear forces are applied. This effect is

particularly visible with formulation PLAST 03 ( ), that exhibits a viscosity of 2056 Pa·s in the

ideal processing range of 25 and 55 s-1. But the targeted objective was to develop a PVC plastisol with

stronger capacity to fix the optical fibers when those are deposited over it before curing happens. For

this purpose a plastisol formulation with higher viscosity at steady-state is needed. Therefore the

plastisol should be more viscous when it is in static state, because this is the situation when optical

fibers are deposited over it. PLAST 05 ( ) shows a much higher zero-shear viscosity (viscosity at

steady state), higher than 2009 Pa·s. When shear force is applied, its viscosity drops down to 1860 Pa·s

in the shear-rate range of 25 and 55 s-1. This is a very interesting viscoelastic behaviour, as it enables

the deposited fibers to contact with a highly viscous static PVC layer, that was previously spread

without much effort, since its viscosity was lower when passing through the spread coat knife.

A similar and even more interesting effect was observed with the resin compounds PLAST 04 and

PLAST 06. Both plastisols demonstrated thixotropicity, with falling viscosities as shear-rate is

increased. Such formulations exhibit high viscosity at steady state. But when shear forces are applied,

for instance when the paste is spread coated, the viscosity falls, making it easier to be applied.

PLAST 06 ( ) shows the highest viscosity at steady-state. With increasing shear-rate its viscosity

drops roughly 23%.

Based on the obtained results, the new formulation PLAST 06 was considered to have appropriate

viscoeleastic properties, enabling a stronger adhesion of the deposited optical fibers. The next step of


the research was to evaluate the behaviour of this new formulation during fiber deposition in

laboratory scaled prototyping.

Laboratory scaled fiber deposition over PLAST 06

For the evaluation of the PVC compound PLAST 06 as adhesion promoting intermediate layer for

a stronger fixing of the deposited fiber, a laboratory prototype P05 was fabricated with a Werner

Mathis coating equipment. A three layer construction was chosen, where each layer is composed of

PLAST 06. Optical fibers were deposited manually in the 2nd layer (intermediate layer), according to

the following process steps and conditions:


1. Application of PVC-layer 1 over release-paper 2. Heat-curing of PVC-layer 1 (insertion of frame into oven) 3. Application of intermediate PVC-layer 2 over PVC-layer 1 4. Manual deposition of optical fibers in PVC-layer 2 5. Final heat-curing of PVC-layer 2 6. Application of PVC-layer 3 over PVC-layer 2 with deposited optical fibers 7. Heat-curing of PVC-layer 3 8. After cooling down to room temperature � manual release from paper.

Process Conditions: For prototype P05:



1 Application of PVC-layer 1 150

2 Heat-curing of PVC-layer 1 200 60

3 Application of intermediate PVC-layer 200

4 Manual deposition of optical fibers




P05

8 Cooling to roomtemp. + manual release while still on release paper - during 10min.

Practical observations of fiber deposition over PLAST 06

Processability of the formulation PLAST 06

Mixing of the components of PLAST 06 was perfectly executable, without any problem. The

trapped air during mixing was totally removed by middle vacuum after 30 minutes, enabling the

formation of flexible PVC foils without any insertion of air bubbles.

PLAST 06 has proven to be perfectly suitable for spread-coating process. No difficulty was

observed during spreading the plastisol through the gap between knife and release paper, nor happened

any damage to the carrier paper.


Condition

Temperature [ºC]

Step

Operation Gap

(thickness)

[µm] T05 / T06

Heating time [ '' ]








Reduced defects related with the optical fiber deposition

Unsteadiness of the deposited fibers: This problem was significantly reduced, but not totally solved.

There were still plastisol displacements due to the tendency of bent optical fibers to recover their initial

shape. The uncured layer of PLAST 06 as intermediate adhesion layer, has a notable improved

capacity to fix the fiber, but is still not strong enough to avoid paste displacements. The surface of the

obtained prototype was therefore still provided with few irregularities.

Fig. 33 : Visible irregularities in the surface of prototype P05

Undulated prototype surface: The undulation of the foil surface along the fiber path is still visible,

even with a longer cooling time. A longer cooling period before removing the foil from the release

paper may be necessary and should be evaluated in further experiments.

Impossibility to achieve straight corners: The possibility to design straight corners during fiber

deposition is still not possible with the formulation PLAST 06.

Tensile properties of PLAST 06 foils

The flexibility of the PLAST 06 Matrix was evaluated with elongation-to-break measurements

performed on a Universal testing machine Instron 4302. For this purpose two foil specimens were

fabricated without embedded optical fibers: T05 and T06. The tensile performance of these specimens

was compared with the test specimens T03 and T04.

Test conditions and foil construction: Geometry of tested specimens: rectangular Dimensions of the tested specimens: 50mm x 200mm Thickness of tested specimens: 700microns (± 20microns) Sample conditioning according DIN 16 906 (24 hours at 23 ± 2ºC e 50 ± 6% rel. humidity) Grip-geometry: rectangular Grip distance between: 100mm Extension speed: 50 mm/min

PVC paste displacements

Undulated surface along fiber path


439.3% 430.8%

401.9%412.2%

41

1.7

N

41

5.4

N

32

5.6

N

33

4.5

N

0

50

100

150

200

250

300

350

400

450

500

T03 T04 T05 T06

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500E

▬ T03 ▬ T04 ▬ T05 ▬ T06

Results:

Fig. 34 : Elongation-at-break performance of PLAST 06

Fig. 35 :Tensile properties of PLAST 06


[%]


[N]


412.2 334.5 3.184 401.9 325.6 3.064 439.3 415.4 3.900 430.8 411.7 4.053

Lo

ad

[kN

]

Elongation * [mm]


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 25 50 75 100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

T05

T06

T03T04

T05 T06

T03

T04


Conclusions of the viscoelastic properties adjustment

The compounding research performed with selected PVC resins has lead to the development of a

significantly improved basis plastisol for the manufacturing of flexible PVC foil with embedded

optical fibers.

The formulation PLAST 06 demonstrated a higher performance in elongation-at-break. The

specimens produced with PLAST 06 are more stretchable and able to support higher loads, which is of

practical interest for the purposed application as flexible, easy-to-use sensing device. For further

development the basis formulation to use for the flexible PVC foil should be PLAST 06.

Despite its general good tensile properties, this formulation is still not offering the possibility to

fix properly deposited optical fibers, without plastisol displacements. Therefore an appropriate

formulation for the intermediate adhesive layer has to be developed.

4.2.3 Development of high-viscosity intermediate PVC adhesion layer

The high viscosity intermediate PVC adhesion layer was developed based on the cumulated

information collected during material selection tests. The starting formulation was the simple resin

suspension PVCRES8, which demonstrated thixotropic behaviour and exhibited the highest initial

viscosity. Formulation adjustments were carried out in order to raise the initial viscosity (zero shear

viscosity), in order to emphasis the fixing capability of the uncured plastisol for deposited optical

fibers. A strong increase in viscosity was achieved with thickeners. Several thickeners were tested and

finally the best result regarding homogeneity and viscosity of the obtained suspension was obtained

with a silica based thickener. The new formulation for the intermediate PVC adhesion layer was

defined as PLAST 07.

The following table shows the comparison of PLAST 07 with the reference formulation PLAST

01 and PLAST 06:

Formulation PVCRES8 PLAST 01 PLAST 06 PLAST 07

Raw Material Type Resin 1 PVCRES8 65% PVCRES6 55% PVCRES3 40% PVCRES8 42%

Resin 1 PVCRES8 13%

Thickener TH1 1%

Plasticizer DIDP 35% DIDP 40% DIDP 37.5% DIDP 45%

UV Stabilizer STAB1 1..5% STAB1 1..5% STAB1 1..0%

Heat stabilizer EZ1 EZ1 1% EZ1 1% EZ1 1%

Filler CaCO3 2..5% CaCO3 7% CaCO3 10%


1000

5 55 105 155 205 255 305 355 405 455 505 555 605 655 705 755 805 855905 955 1005

Vis

cosi

ty υ

* [

Pa·

s]

Shear Rate σ [s-1]

The fixing capability of PLAST 07 was first evaluated through its rheological behaviour. The

viscosity was measured at 15 different shear rates at constant temperature. Viscosity readings were

performed 24 hours after recipe preparation. The viscosity is given as a function of the shear rate. At

each velocity step, the viscosity was registered after 15 seconds.

Formulation PVCRES8 PLAST 01 PLAST 06 PLAST 07

Step Shear Rate Viscosity Viscosity Viscosity Viscosity

Modul [s-1] τ [Pa · s] [Pa · s] [Pa · s] [Pa · s]

1 6.65 977.0 4885 1514 2931 36149 2 9.51 683.0 4355 1507 2732 26637 3 13.61 478.0 3824 1489 2458 20554 4 19.48 334.0 3310 1500 2338 15698 5 27.90 233.0 2796 1507 2270 12116 6 39.90 162.9 2534 1560 2281 9448 7 57.20 113.6 2272 1648 2272 7384 8 81.80 79.5 2052 1754 2226 5963 9 117.10 55.5 1832 1866 2220 4940 10 167.60 38.8 1729 1959 2212 4113 11 240.00 27.1 1626 2032 2222 3404 12 343.00 19.0 1553 2113 2217 2729 13 492.00 13.2 1480 2193 2193 2193 14 704.00 9.2 1427 2158 2132 1846 15 1008.00 6.5 1374 2143 2025 1406

Fig. 36 : Viscosity / shear rate relationship of PLAST 07


4.2.4 Deposition of optical fibers with intermediate adhesive layer PLAST 07

To evaluate the effective capability to fix properly deposited optical fibers during spread-coating

process, a laboratory prototype (P06) was fabricated with the following structure:

Fig. 37 : Laminated fiber optics with intermediate PLAST 07 layer


1. Application of PVC-layer 1 over release-paper � PLAST 06 2. Heat-curing of PVC-layer 1 (insertion of frame into oven) 3. Application of intermediate PVC-layer 2 � PLAST 07 4. Manual deposition of optical fibers in PVC-layer 2 5. Heat-curing of PVC-layer 2 6. Application of PVC-layer 3 � PLAST 06 7. Heat-curing of PVC-layer 3 8. After cooling down to room temperature � manual release from paper.

Process Conditions: For prototype P06:






4 Manual deposition of optical fibers




P06


Practical Observations:

The deposited optical fibers were fixed by the uncured layer PLAST 07 in a relative stable way,

avoiding further plastisol displacements due to initial shape recovering from the fibers.

No plastisol displacement is visible on the surface of the prototype. Reflected light over the obtained

foil (Fig. 38), shows a totally flat surface, without irregularities due to plastisol displacement – only

discrete plastisol displacements are visible, but they are due to fingers touch when depositing the

optical fibers.

1st layer: PVC “skin” layer � applied over Release-Paper PLAST 06

3rd layer : Cover Layer � final layer covering the fiber optics PLAST 06 (2nd layer: intermediate “adhesive” layer PLAST 07

Optical fiber


Fig. 38 : Top view of prototype P06

Fig. 39 shows an oblique view of the folded (rolled) prototype P06. It is visible that there is no

irregularity on the surface. At the interface PVC / optical fiber the surface is totally flat which proves

that the optical fibers were perfectly fixed by the uncured plastisol layer PLAST 07.

Fig. 39 : Side view of prototype P06

Furthermore, the optical fiber is not felt when touching the sample or rubbering the surface over

the fiber path with the fingers. This is also an indicator for an optimum integration level.

In the fabrication of P06, the obtained laminated foil was allowed to cool down to room

temperature over longer time (15 min.) and then released from the support paper, like shown in Fig.

20. No surface undulation was observed in prototype 06, what proves that a longer cooling time allows

the molecules of the PVC matrix to rearrange themselves and to withdraw the stress caused by

different contraction modules of optical fiber and PVC.

Totally flat surface


4.2.5 Conclusion of the direct fiber deposition method

The direct deposition of optical fibers during spread-coating needs specifically adjusted PVC

formulations in order to enable a stable deposition process. For this purpose research was focused on

the viscolestastic properties of the solid PVC matrix and on the rheological characteristics of the

plastisol. The basis formulation for the outer layers – skin and cover or backside layer – was adjusted

in order to achieve a more stretchable PVC film, with good dimensional stability.

The research in this second development loop was focused on designing the PVC film

construction in such a way that the manufacturing process can be automated, free from unstable steps

like the tricky fiber deposition over plastisol layers unable to fix correctly the fibers. This difficulty

was overcome with the development of a specialty plastisol unit, enabling a relatively strong adhesion

of the deposited optical fibers.

The way to the development of an automated fiber deposition was now open. A deposition

process has to be designed based on an automated mechanism that ensures a precise fiber deposition

with high reproducibility of the needed optical fiber pattern in the foil.


5 Prototype characterization

5.1 Integrated construction of the flexible sensing foil

The flexible PVC foil was defined as to be composed of three different layers, to involve and

protect the optical elements to be integrated. For an easier visual analysis of the obtained structure, a

flexible sensing sample was manually prepared, with colour variation by layer.

Fig. 40 : PVC foil without optical fibers Fig. 41 : PVC foil with optical fibers

The 1st layer is the so-called skin layer, as it represents the first spread coat layer, applied over

release paper. It plays the role of a barrier skin, avoiding the fiber to be directly exposed at the surface

of the foil. The 2nd layer has the function of an adhesive layer during the manufacturing process. It has

to fix the deposited optical fibers in such a way that no further movement of the fibers can occur, due

to their natural tendency to recover initial shape.

The 3rd layer has to cover the optical fibers, avoiding the exposure of the fibers at the foil surface. Fig.

1 shows the thickness of the different layers. Fig. 41 shows the optical fiber embedded in the PVC foil.

The fiber is properly embedded in the 2nd layer. The microscopic picture shows that during the

manufacturing process, the deposited fiber penetrates into the adhesive PVC layer, being totally

covered by the uncured PLAST 07 layer. This demonstrates that the 3rd layer doesn’t have to be 350

microns thick.

The microscopic cross section views of the flexible sensing foil demonstrate an optimal

surrounding of the fiber by the adhesive layer PLAST 07. This is a first indicator for a good bonding of

the fiber to the PVC matrix. Moreover, the pictures show that there are no irregularities on the foil

surface, due to the fiber insertion. Therefore the spread-coating process combined with the selected

← Cross section view →

350µm ± 10µm

200µm ± 10µm

150µm ± 10µm

Light Microscope - Mag.: X30

Layer 1: PLAST 06

Layer 2: PLAST 07

Layer 3: PLAST 06

250µm



Condition

Temperature [ºC]

Step

Operation Gap

[µm] T07 / T08 / T09

Heating time [ '' ]

1 Application of PVC-layer 1 -> PLAST 06 150







PVC formulations constitutes a perfect method to cover entirely and homogeniously a strong body like

optical fibers.

5.2 Mechanical properties of the flexible sensing foil

5.2.1 PVC matrix

The evaluation of the ductility of the PVC matrix is bared in three prototypes under tensile tests:

T07, T08 and T09. The flexibility of the laminated matrix composed of different PVC layers was

evaluated with elongation-to-break measurements performed on a universal testing machine Instron

4302. For this purpose the test specimens were fabricated without embedded optical fibers.

Test conditions and foil construction: Geometry of tested specimens: rectangular Dimensions of the tested specimens: 50mm x 200mm Thickness of tested specimens: 700microns (± 20microns) Sample conditioning according DIN 16 906 (24 hours at 23 ± 2ºC e 50 ± 6% rel. humidity) Grip-geometry: rectangular Grip distance between: 100mm Extension speed: 50 mm/min

Results:

Fig. 42 : Tensile properties of PVC foils cured at 180ºC and 200ºC


[%]


[N]


�T04 401.9 325.6 3.064 �T06 430.8 411.7 4.053 �T07 367.9 324.6 4.102 �T08 362.0 324.5 3.961 �T09 379.3 340.7 4.070

379.3%367.9% 362.0%

430.8%

401.9%

34

0.7

N

32

4.5

N

32

4.6

N

41

1.7

N

32

5.6

N

0

50

100

150

200

250

300

350

400

450

500

T04 T06 T07 T08 T09

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500E

4.000


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 25 50 75 100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

T04T06T07T08T09

Fig. 43 : Elongation-at-break test of multi-layer structures Prototypes T07, T08 and T09 compared with specimens T04 (PLAST 01) and T06 (PLAST 06)

The prototypes T07 to T09 show lower maximum elongation values than the prototype T06,

which was formed with 3 layers made of PLAST 06. This means that the defined sandwich structure

with PLAST 07 as intermediate adhesive layer can not be stretched as much as the foils made with the

single plastisol formulation PLAST 06. But on the same time, the Young modulus or elastic modulus

is almost the same as for sample T06. The relative high young modulus reveals a high stability of the

obtained foils, since the modulus of elasticity may also be characterized as the “stiffness” or ability of

a material to resist deformation within the linear range.

Lo

ad

[kN

]

Elongation * [mm]


�T04 Layer 1 -> PLAST 01 / Layer 2 -> PLAST 01 / layer 1 -> PLAST 01 �T06 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 06 / layer 1 -> PLAST 06 �T07 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 �T08 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 �T09 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06

T01

T02 T03 T04

T04


The reason for a low maximum elongation value is related with the 2nd PVC layer made of

PLAST 07. The question is why the maximum elongation is lower when applying an intermediate

layer is made of a plastisol with very high viscosity at low shear rates. First, it’s important to stress that

the viscosity of a plastisol is not related to the tensile properties of the foils obtained with it. The

answer to this behaviour was found on the structure of the obtained sandwich foil, visible in Fig. 44.

Fig. 44 : Amplified cross section – foamed material structure of PLAST 08 layer

The amplified cross section shows that there are pores in the intermediate adhesive layer made of

PLAST 08 – white middle layer in Fig. 44. In fact, the used resin in this formulation is made of

emulsion polymerisation PVC especially designed for the producing PVC foam. As no expansion

agent is used in the formulation, a foam reaction is not expected. But like it can be confirmed by a

spectroscopic analysis, some pores are formed during the gelation and fusion of the PVC. This leads to

a PVC layer that is not monolithic or absolutely compact, like the layers formed by PLAST 06. This

foam-like structure is more “soft” and easier compressible, like a “sponge”. The pores in this

intermediate layer are in fact discontinuities in the PVC matrix and constitute weak points, responsible

for an earlier stretch-break in the elongation tests. Despite this loss in the elongation capacity of the

obtained foils, the high elasticity modulus in the range of 4 MPa (see Fig. 42) confirms more stiffness

against deformation, while elasticity remains very high.



As complementary ductility test, the hardness of the obtained foils was measured in Shore A

according DIN 53505:

Shore A results:

The obtained average value of the prototypes T07 to T09 is 60 Shore A.

5.2.2 PVC matrix with embedded optical fibers

The study of the mechanics of deformable PVC sensing foils was based on tensile tests. The

resistive behaviour to mechanical stretching of the flexible sensing foils is altered due to the presence

of optical fibers. This behaviour is dependent of the configuration of the optical fibers or on the optical

fiber path in the PVC matrix. To better evaluate these dependencies, several prototypes were produced,

with different paths of the inserted optical fibers.

Fig. 45 : Flexible PVC foils with different optical fiber path

Shore A DIN 53505 Values �T04 Layer 1 -> PLAST 01 / Layer 2 -> PLAST 01 / layer 1 -> PLAST 01 81 �T06 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 06 / layer 1 -> PLAST 06 73 �T07 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 61 �T08 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 59 �T09 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 60

“S”-form path

1X

2X

3X

4X

P10 linear

P07 7 curves

P09 8 curves

Distance between grips = 100mm

P08 5 curves

Sample Grip


The optical fiber was manually deposited over the intermediate adhesive layer PLAST 07

describing different geometries or paths. For an improved elongation capacity in stretching direction,

the fiber path was chosen to be in “S”-shape (1 “S” = 2 curves). For each prototype the fiber path was

varied by describing different “S”-shape patterns, like illustrated in Fig. 45. Four different prototypes

were created; P07, P08, P09 and P10. P07 was formed with 7 curves. The fiber path in P08 describes 5

curves, while 8 curves are described in P09. The last prototype P10 was created with a linear fiber

path.

All the above mentioned prototypes were constructed according to the process steps and

conditions already described in section 4.2.4, page 54. The obtained prototypes were submitted to

controlled elongation-at-break tests, performed on a universal testing machine Instron 4302.

Fig. 46 : Sample gripping in elongation-at-break test

The elongation-at-break tests were executed with a distance between grips of exactly 100mm,

according to the normative specification DIN 16 906. This altered the configuration of the free fiber

segment length available for the course of the elongation deformation. Fig. 46 shows the case of

prototype P08 and how the gripping of the foil changes the fiber segment length available for

deformation. One can see that the initially described 5-curves geometry (Fig. 45) of the deposited

fiber, is reduced after foil gripping to more or less 3 curves. The same proportional reduction was

applied to the other prototypes and the free fiber segment length available for elongation deformation

can be seen in Fig. 45.


0

0.05

0.1

0.15

0.2

0.25

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

T04T06T09P07P08P09

Lo

ad

[kN

]

Elongation [mm]

Geometry of tested specimens: rectangular Dimensions of specimens: 50mm x 200mm Thickness of specimens: 700microns (± 20microns) Sample conditioning according DIN 16 906 (24 hours at 23 ± 2ºC e 50 ± 6% rel. humidity) Grip-geometry: rectangular Grip distance between: 100mm

f 08

j 08

m 08

f 09

j 09

j 07

f 07

Ultimate elongation results:

The tensile tests showed very different results for maximum elongation dependent on the

geometry of the optical fiber path in the PVC foils. It is important to notice in this experiment that

different maximum elongation values were registered and assigned to different material limits: optical

fiber limit, polymeric fiber jacket limit and PVC foil limit.

Ultimate elongation charts:

Fig. 47 : Elongation-at-break of PVC foils with different “ S-shape” optical fiber paths

Maximum Elongation Ultimate Elongation [%] Ultimate Load [N] Sample

optical fiber limit f fiber jacket limit j PVC matrix limit m optical fiber fiber jacket PVC matrix �T04 NA NA 431 NA NA 326 �T06 NA NA 368 NA NA 412 �T09 NA NA 379 NA NA 340 �P07 91 131 NA 145 172 NA �P08 45 68 141 131 171 208 �P09 98 122 NA 192 220 NA �P10 1.62 NA NA 9.7 NA NA

▬ T04 Layer 1 -> PLAST 01 / Layer 2 -> PLAST 01 / layer 1 -> PLAST 01



▬ P07 Layer 1 -> PLAST 06 / Layer 2 -> PLAST 07 / layer 1 -> PLAST 06 with fiber-path = 3,5 X “S”



T04

T09

T06

P08

P09

P07

Elongation rate : 50 mm/min

Prototypes P07, P08 and P09 compared with specimens T04, T06 and T09.


Fig. 48 : Elongation-at-break of PVC foils with linear oriented optical fiber path

The experiment demonstrates that the designed flexible sensible foils with embedded optical

fibers are stretchable. The optical fibers can support a maximal elongation of 1.62% when in linear

orientation parallel to stretching direction. If the fibers are oriented in “S”-shape geometries, the

elongation values can become relatively high, with examples of deformation above 90% without

visible damage of the optical fibers. The influence of the fiber path geometry was proved in this

experiment. The amount of consecutive “S”-shaped curves in the deposited fiber geometry is

proportional to the maximum elongation achievable by the optical fiber.

Three different kinds of events were observed during the performed tensile tests. The first event is

related to the optical fiber rupture. This event is invisible to human eye, but it can be seen in the load-

elongation curves; points f07, f07 and f09 in Fig. 47. The second event is related to the rupture of the

fiber jacket (see Fig. 2). The last event observed in the elongation-at-break tests of the flexible sensing

foils is related to the rupture of the PVC foil itself.

To better understand the fiber behaviour when exposed to elongation stress in a single curved

segment of an “S”-shaped fiber profile, an additional prototype P11 was created and tested.

The performed tensile tests demonstrated a very strong adhesion of the optical fiber to the

surrounding PVC. The fiber integration has proved to be very efficient and highly responsive to

deformation without visible damages of the integrated sensing construction. Fig. 49 shows how strong

the fiber integration is. The flexible PVC matrix with the partial foamed intermediate PVC layer works

perfectly with the stretched rigid optical fiber, avoiding its rupture or any tearing of the PVC matrix. A

single curved fiber segment can withstand at least 60% deformation without breaking. With increasing

number of successive “S”-shape segments, the deposited optical fibers can reach values in the range of

100% elongation deformation.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0.00 1.00 2.00 3.00 4.00

Lo

ad

[N]

Elongation [mm]

1.62% Elongation

Prototype P10 with fiber path parallel to elongation direction

Elongation rate: 1 mm/min


Fig. 49 : Deformation of curved fiber segment

5.3 Optical properties of the sensing foils during elongation tests

The optical performance of the produced flexible sensing model was evaluated. The elongation-to-

break tests performed in the universal testing equipment Instron® 4302 were conducted at the same

time as optical signal was sent through the optical fibers embedded in the foils. The reflected signals

were measured by a BraggMETER™ unit from FiberSensing company.

The tested prototypes were cropped to the 50x100 mm size. Two tests were made; one in which the

continuous longitudinal deformation was measured while applying a constant load, another in which,

the elongation was applied in incremental steps of 0.2 % (200 µm). The deformation rate was set to 16

µm/s (1 mm/min). The wavelength deviations of the reflected signals were registered for prototype

P10.

Relaxation in PVC matrix after break of optical fiber

0% 25% 40% 50% 60% > 70%

Elongation Values

50% 60% > 70%


Fig. 50 : Bragg response to applied displacements

Fig. 50 shows that the wavelength deviation has a linear behaviour from 0.4 % ahead. Two reasons

for the non-linear relation below 0.4 % can be pointed:

1. The testing machine does not have enough resolution for elongations below 400 µm, not

providing precise elongation data. In such a case the test may be conducted with samples twice

longer, since the sample’s height or length influences the measurement sensitivity.

2. The polymeric foil does not transmit such little deformation to the fiber.

Notwithstanding this fact, the constructed sensing model with linear embedded optical fiber was

able to sustain a stretching deformation of 1,62 %, which is equivalent to 1,62 mm displacement. At

it’s maximal elongation value, the fiber registered an ultimate load of 9,691 N. At this deformation rate

of 1,62 %, a wavelength deviation of 9,207 nm was measured and the sensitivity of the present model

is 0,6 pm/µε (picometer per microstrain).

To have a notion of the system resolution, one should consider a 1 meter long steel beam that has

been stretched 1 mm; the wavelength deviation that would be measured is 0,6 nm. The determined

sensitivity value provides information about the integration quality.

In the second test, the linear deformation was induced in steps of 0,2 % (200 µm) elongation. After

each elongation increment, the applied deformation load was kept constant during time intervals of

approximately 60 seconds. During this segmented deformation, an optical signal was sent through the

optical fiber and its wavelength deviation registered. The elongation dependent wavelength deviation

was plotted as time function.


Fig. 51 : Bragg response to applied displacement steps

The used method enables to determine if the optical fiber glides in the polymeric PVC matrix. If

this happens, for a given deformation, the registered wavelength deviation should start decreasing and

demonstrates typical relaxation behaviour, described in Fig. 51 as curve [A ]. Fig. 51 shows for each

deformation level a small decrease of the wavelength deviation. But after a few seconds, the line

stabilizes, becoming almost totally flat.

This small variation may be due to vibrations of the testing machine claw, since the sensing model

has a high sensitivity as stated before. The preservation of an absolutely constant wavelength deviation

is representative for a very strong fixing of the optical fiber in the PVC matrix, without any movement

after an applied deformation. This ideal behaviour illustrated in Fig. 51 as curve [B] is in fact not

observable in the tested specimen. Another possible reason for the deviation from the ideal behaviour

may be the partially foamed structure of the intermediate PVC adhesive layer that enables some

resilience into the core of the foil.

The observed deviation from the ideal deformation behaviour confirms a strong fixing of the fiber

to the PVC matrix and optimum fiber integration.

Curve [A]: relaxation behaviour not ideal -> fiber gliding

Curve [B]: ideal behaviour strong fixing to matrix


6 Conclusions

The full integration of FBG sensors in polymeric flexible foils, using standard industrial

fabrication processes was described. The flexible sensing model presented an excellent sensibility with

0,6 pm/µε (picometer per microstrain). The integration of FBG sensors into the polymeric foil was

evaluated in terms of integration quality (adhesion of the fiber to the polymeric matrix), measurement

capabilities and sensor sensitivity. The developed laminated structure showed good performance with

respect to the sensing capability of applied deformation. Reversible deformations were able to be

measured by the flexible sensing system, which demonstrated good repeatability, since it returned to

its initial position and no reduction of the signal amplitude was observable.

The integration of fiber optic FBG sensors was successfully achieved with a direct fiber

deposition technique. The developed technique was modified in order to be run in line during the

normal manufacturing process for flexible automotive laminated foils by spread-coating. The selected

direct deposition of optical fibers during the spread-coating process for PVC foil manufacturing

demonstrated to a practical route for an industrial, automated production of flexible sensing foils.

The proposed direct deposition process requires new PVC plastisol formulations in order to

enable the deposition of fiber optics without visible defects and with high integration quality. The

selected laminated PVC structure is composed of different PVC layers, namely a skin layer, an

intermediate adhesive layer and a final cover layer. The optical fibers are integrated into the

intermediate adhesive layer. A particular aspect of the selected plastisol formulation for the

intermediate adhesive layer is its partially foamed structure, which contributes for a soft fiber

integration. The surface of the obtained sandwich foil is totally flat and in the regions along the fiber

path, the optical fiber can’t be felt by touch. The partially foamed PVC surrounds perfectly the optical

fiber and acts as a soft encapsulation. The adhesion of the fiber to this PVC matrix is very strong,

demonstrated by tensile tests.

The fine-tuning of the PVC matrix properties can be redirected to specific end-uses of the sensing

foil. The selected construction itself can be adapted to special needs. The thickness of 3rd layer (cover

layer) can be reduced. More layers can eventually be added if there is a need for stronger protection of

the fibers or a damping effect of the transmitted external stimuli to the optical fibers. Furthermore, one

or more layers made of different materials than PVC can be laminated to the structure. All those

changes can be applied to the developed flexible sensing concept according to the needs for the final

application.


Integration of FBG based-sensors in PVC foils is demonstrated, promising production of large

FBG based-sensors network in PVC foils and mass production in industrial environment.

Future Work:

The results obtained with the study and development of appropriate flexibe PVC matrix for

embedding FOS, has opened the way for the design of an automated FOS integration mechanism. The

proposed concept is based on a dispensing system that has to be coupled to the industrial spread-

coating equipment. This dispensing system acts as a kind of “dispensing head” or “integration head”,

supplying optical fiber during the manufacturing of flexible PVC foils.

The proposed fiber dispensing and depositing concept consists in the use of a plotter unit for

patterning the sensing configuration needed for the defined end-use. The suggested concept allows a

relatively high freedom in defining the needed optical sensor pattern.

The dispensing head is fixed on a supporting platform of the plotter table. The plotter controller

determines the position and the speed of the head. The plotter controller additionally controls the

dispenser by controlling the gears assuring synchronization between them.

With the proposed system, the image of the desired pattern can be plotted on the moving substrate

– the plastisol layer – to form the laminated structure with embedded optical fibers for sensing,

transporting and processing external stimuli.

The computer ensures that the appropriate x-y co-ordinates are fed to the plotter interface so that

the required surface feature is formed at the requested location over the plastisol surface.

Also, the effect of molecular rearrangement of the PVC molecules should be better understood

and evaluated. In particular effort should be paid to understand if the observed rearrangement is of

permanent nature or if there are still residual internal stresses that may cause undesirable foil shrinking

during its practical application.


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transverse holographic method”; Opt. Lett. 14 (1989) 823–825.

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Colloquium (1995).

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Appendix A

[1] A F. Silva, F. Gonçalves, L.A. Ferreira, F. M. Araujo, I. Dias, P. M. Mendes, J.H. Correia; “Fiber

Bragg Grating Sensors Embedded in Polymeric Foils”; Proceedings of MME 2008, Aachen,

Germany, pp. 327-330, September 28-30, (2008); Conference: MME 2008 - 19th MicroMechanics

Europe Workshop.

[2] A F. Silva, F. Gonçalves, L.A. Ferreira, F. M. Araujo, P. M. Mendes, J.H. Correia; “Optical Fiber

Sensors Integrated in Polymeric Foils”; Materiais 2009 - 5th International Materials Symposium

MATERIAIS 2009 (XIV meeting of SPM - Sociedade Portuguesa de Materiais, 2009).

[3] A F. Silva, F. Gonçalves, L.A. Ferreira; “Smart Sensing Polymeric Foil with Integrated Optic

Fiber Sensors”; Conference: The Third International Conference on Sensor Technologies and

Applications; (2009).

FIBER BRAGG GRATING SENSORS EMBEDDED IN POLYMERIC FOILS

A. F. Silva1

I. Dias4, P. M. Mendes1, J. H. Correia1 , F. Gonçalves2, L. A. Ferreira3,4, F. M. Araújo3,4,

1 University of Minho, DEI, Campus de Azurem, Guimaraes, Portugal 2 TMG Automotive, Campelos, Guimaraes, Portugal

3 FiberSensing, Maia, Portugal 4 INESC Porto, Faculty of Science, University of Porto, Porto, Portugal

e-mail: [email protected]

Abstract This paper presents the fabrication and characterization of a polymeric foil able to sense deformation, gather sensitive information and send it for further analysis. Fiber Bragg Grating sensors are embedded in laminated polymeric sheets commonly used in the car’s flooring. The fabrication of the Bragg sensors and the integration of the optic sensors in the foil is described, using industrial fabrication processes. The obtained foil is capable of transferring the full deformation to the optic sensor and returning to the initial position when no deformation exists, providing good sensibility. Application of this sensing foil includes automobile chassis monitoring. Key Words: optical sensor integration, fiber Bragg gratings

I INTRODUCTION Optical sensing technologies have associated advantages that make them very attractive in a broad range of applications. Optical fiber sensors, in particular, provide low-cost solutions, with immunity to electromagnetic interference, multiplexing capabilities and a high degree of miniaturization/integration. Presently, optical fiber sensors offer a high performance alternative, in comparison to standard technologies, in many different areas, either for measuring physical parameters like strain, temperature or pressure, or for performing highly sensitive biochemical analysis [1, 2]. Integrated optics devices, on the other hand, are now emerging as next generation sensing structures where virtually any parameter can be determined with high accuracy in a highly miniaturized optoelectronic device [3]. Linking textiles or textiles-polymer-laminates (artificial leather) with optical devices and electronics is more realistic than ever. An emerging new field of research that combines the strengths and capabilities of electronics, optics and polymers like Polyvinyl Chloride (PVC) is opening new opportunities.

Industries like the automotive, aeronautics and biomedical look for ways to gather information from their systems status. Lower production cost, wider exploitation of integrated circuit technology and wider applicability to sensor arrays ensure the integration of microsensors in almost any structure, providing the desired system data.

I.1 FIBER BRAGG GRATINGS A Fiber Bragg Grating (FBG) is a small size microstructure (less than 10 mm long) that can be photo imprinted in photosensitive optical fibers by side-exposure to patterned UV laser radiation. Such a microstructure consists on a periodic modulation of the refractive index of the core of the optical fiber that is characterized by a narrowband resonance spectral reflection. Since this is a deeply embedded device in the optical fiber structure, the resonance behavior strictly follows external actions in the exact proportion as the silica matrix surrounding the component. This results in a localized sensor offering high sensitivity to temperature and strain, particularly suitable for quasi-distributed measurements ranging from few centimeters to tens of kilometers.

Figure 1 – Schematic representation of Bragg sensor.

[1]

mailto:[email protected]�

The main feature that makes Bragg grating sensors so competitive for structural monitoring is its inherent multiplexing capability, which arises directly from their intrinsic properties. Since each Bragg sensor has its own wavelength signature, in an array of sensors is possible to identify each sensors individually. This multiplexing capability enables many grating sensors to be interrogated using common optoelectronic instrumentation. In addition, fiber Bragg grating sensors offer immunity to EMI/RFI, remote monitoring capacity, small size and weight, electrical isolation, intrinsically safe operation in hazardous environments, high sensitivity and long-term reliability.

I.2 POLYMERIC FOIL Vinyl mixtures or so-called Plastisols, are liquid dispersions of a finely divided thermoplastic, polyvinyl chloride resin, in a plasticizer. It is a 100% non-volatile paste-like composition and consists of a physical mixture of finely sized PVC polymer particles and liquid plasticizers, such as phthalates and epoxy oils. These resulting pastes are highly viscous mixtures that, after heated above the curing-temperature (130 to 400 ºC), become homogenized and a solid phase results. When cooled, the Plastisol provides a tough material with specific physical characteristics. The Plastisol material is formulated to be a dense solid and elastic polymer, which retains toughness even at low temperatures. Plastisols can be formulated with hardness ranging from 30 to 90 Shore A (relative hardness of elastic materials) and tensile strength ranging from 750 to 3000 psi. It can be formulated to resist chemical attack, is self-extinguishing due to the chlorine groups, provides good weatherability and can be used as support structure when looking for flexibility

II FABRICATION PROCEDURE II.1 FIBER BRAGG GRATINGS

The mechanism responsible for the non-linear photoinduction change of the refraction index is known as photosensitivity and is associated with the silica matrix doped with germanium that is the core of photosensitive optical fiber [5]. This mechanism enables the direct fabrication of the microstructures in the core of optical fibers by

exposure across the periodic pattern of UV radiation. Each individual component of the microstructure is formed by irradiation spot of the core of the fiber, which locally causes a change of the refraction index on adjacent positions not exposed. Figure 2 illustrates the experimental setup used in the fabrication of Bragg structures by the phase mask method. The optical fiber is maintained along the surface of the mask aligned transversely with the depressions, and the UV radiation focused along the fiber optic through a cylindrical lens. It should be noted that, an ideal mask ideal – i.e., with total abolition of the order zero - the period of the modulated interference pattern is always half of the period of the phase mask, and independent of wavelength of the laser emission (Figure 2).

Figure 2 – Schematic of the FBG fabrication setup.

The period of the modulated interference pattern depends only on the period of the phase mask. The manufacturing of the Bragg structure with different wavelengths requires the use of different phase masks. However, the extreme simplicity of alignment and stability of the inherent pattern of interference can produce networks of Bragg with high reproducibility. The used optic fiber was a single mode Corning 28-e, a standard optic fiber for communication applications, with acrylate coating. The Bragg structures printed in the core of the fiber were in the communication wavelength range (1520-1570 nm).

II.2 POLYMERIC FOILS The fiber optic sensor is packaged with the polymeric foil that works also as a support structure. By this, is important that the optic fiber becomes very well embedded on the foil in order to

eliminate any possibility of losing sense sensibility by the flexible foil. In order to accomplish such goal, a three layer foil approach was considered (Figure 3).

Figure 3 – Three layer foil approach.

The fabrication relies on an industrial process. A first layer in applied in a substrate (support for the fabrication) and layer-by-layer, the structure passes through a gap between the “blade” and counter cylinder to ensure the desired thickness. As the coating and substrate pass through, the excess is scraped off. At the end, the layer goes to the inside of the oven to cure and become a solid state structure.

Figure 4 – Procedure schematic.

The second layer suffers a partial cure in order to increase the viscosity and facilitate the insertion of the optic fiber. The main concern of this process was the high temperature of the oven that would be sufficient to destroy the optic fiber coating and change the refraction index of the fiber, damaging it.

Table 1 – Polymeric foil fabrication procedure.

Step Operation Condition

Gap [µm]

Temp. [ºC]

Heating time [s]

1 Application of PVC-layer 1 150 - -

2 Heat-curing of PVC-layer 1 - 200 60



5 Optical fibres insertion - - -




9 Cooling + manual release - - -

III RESULTS Figure 5 shows the result of the fabrication process previously presented. The polymeric foil, with 210x297 mm size, has optic fiber based sensors embedded in it. By visual inspection, we can conclude that the fabrication process has run successfully, appearing only to be some deformation waves on the surface as result of the heating process followed by a cooling period (Table 1). The temperature difference made the polymeric material to initially expand and then contract, pushing the fiber along, creating the waves.

Figure 5 – Polymeric foil with optic fiber sensors

embedded.

Figure 6 presents the reflected spectrum of the FBG sensor. The side lobes come from the grating fabrication process, resulting from the radiation transmission function. They can be smoothed by apodization function.

Figure 6 – Reflected spectrum from the FBG sensor for

two distinct tensile forces.

When deforming the polymeric foil, the embedded FBG sensor follows the deformation and the reflected spectrum suffers a wavelength deviation. This variation, without changing the spectrum shape, can be translated to a deformation value by a mathematical expression. The high sensitivity verified is representative of a successful implementation of the optic fibers in the PVC foil. Figure 7 presents the progression of the deformation sensed, by the FBG structure, versus time. The FBG sensor presents a linear behavior while undergoing a tensile force. When the force is released, the spectrum comes back to the initial wavelength, intrinsic to the FBG sensor.

Figure 7 - Deformation sensed by the FBG over time.

A final analysis was done, in order to evaluate the ability to sense in a real structure. A foil with optic sensors was glued to a metallic structure that was later subject to force. The polymeric foil, as well as the optic sensors, was able to follow the deformation of the metallic structure with a sensitivity of 1pm/µε (picometer per microstrain).

IV DISCUSSION The integration step has been presented but a few problems must be taken into account. First, the temperature restraints in the foil fabrication need to be analyzed, since the use temperature (200 ºC) was below the temperature used in the industrial process (230 ºC). The possible damages to the optic fiber due to high temperatures may be overcome by the use of fiber coated by polyamide, capable of supporting higher temperatures. Secondly, there is a need to reduce the deformation caused by the heat and cooling process. A possible

solution will be the creation of a fabric mesh with optic sensors integrated that are later inserted in the polymeric foil. Further work, also includes the design of optoelectronic components capable of being also inserted in the polymeric foil. The main goal is to have a flexible polymeric foil with a sensing network inside it, capable of detecting deformation.

V CONCLUSION The prototype model presented an excellent behavior, 1pm/µε, allowing further improvements. The fabrication process of the optic fibers with FBG sensors is standard, not presenting any major problems. The manufacturing of the polymeric foil integrating the FBG sensors, presents restraints, especially in terms of temperature, requiring more research, but is clear that this flexible material can be use as structure material. In terms of performance, the structure showed that, not only the spectrum shape did not change during the force application, but also that it returned to its initial position (without any reduction of the signal amplitude). Integration of FBG based-sensors in PVC foils is demonstrated, promising production of large FBG based-sensors in PVC foils and mass production in industrial environment.

VI ACKNOWLEDGEMENT Alexandre Ferreira da Silva is supported by Portuguese Foundation for Science and Technology (SFRH/BD/39459/2007).

REFERENCES [1] Grattan, K. T. V. and T. Sun, Fiber optic sensor

technology: an overview. Sensors and Actuators a-Physical, 2000. 82(1-3): p. 40-61.

[2] Wolfbeis, O. S., Fiber-optic chemical sensors and biosensors. Analytical Chemistry, 2006. 78(12): p. 3859-3873.

[3] Blue, R., et al., Platform for enhanced detection efficiency in luminescence-based sensors. Electronics Letters, 2005. 41(12): p. 682-684.

[4] Norm Schiller, Hamamatsu Corporation. Selecting Optical Detectors for Automotive Designs, in ECN, December,1, 2004.

[5] Hill, K.O., Photosensitivity in optical fiber waveguides: from discovery to commercialization. Selected Topics in Quantum Electronics, IEEE Journal of, 2000. 6(6): p. 1186-1189.

Optical Fiber Sensors Integrated in Polymeric Foils

A. F. Silva1,a, F. Gonçalves2, L. A. Ferreira3,4, F. M. Araújo3,4, P. M. Mendes1, J. H. Correia1

1 University of Minho, DEI, Campus de Azurem, Guimaraes, Portugal 2 TMG Automotive, Campelos, Guimaraes, Portugal

3 FiberSensing, Maia, Portugal 4 INESC Porto, Faculty of Science, University of Porto, Porto, Portugal

[email protected]

Keywords: optical sensor integration, fiber Bragg gratings

Abstract. Optical sensors have hit their maturity and a new kind of systems is being developed. This paper deals with the development of a new sensing structure based on polymeric foils and optic fiber sensors, namely the Fiber Bragg Grating sensors. Sensor integration in polymeric foils, using industrial process is the proposed goal. To achieve this goal, Finite Element Analysis was used for prototype modeling and simulation. The model was subjected to loads and restraints in order to retrieve information about stress distribution and displacement of specific points. From the simulation was possible to predict the sections where the sensor should be positioned. A prototype was then fabricated using industrial processes. Tests indicate that the polymeric foil influence on the sensor performance may exist. However, the prototype was able of transferring the full deformation to the optical sensor. Moreover, the optical sensor, which is incorporated in the polymeric foil, is fully functional with high sensitivity, 0,6 picometer by microstrain, allowing deformation measurements, up to 1,2 millimeter.

Introduction

Optical sensing technologies have associated advantages that make them very attractive in a broad range of applications. Optical fiber sensors, in particular, provide low cost solutions, with immunity to electromagnetic interference, multiplexing capabilities and a high degree of miniaturization/integration. Presently, optical fiber sensors offer an high performance alternative, in comparison to standard technologies, in many different areas, either for measuring physical parameters like strain, temperature or pressure, or for performing highly sensitive biochemical analysis [1, 2]. Integrated optics devices, on the other hand, are now emerging as next generation sensing chips where virtually any parameter can be determined with high accuracy in an highly miniaturized optoelectronic device [3].

Many industries as the civil, automotive and aeronautic, among others, are already benefiting from the potential of optical sensing technologies while looking for ways to gather information from their systems status. The number and sophistication of optoelectronic systems found in smart structures is increasing at an unprecedented level. In this context, the development of advanced optoelectronic systems, with integrated sensing and data transport capabilities, is a key step for the implementation of a new generation of intelligent structures. In addition, a variety of optical sensors has been proposed for other different applications. Optical fiber sensors offer competitive and sometimes unique solutions to many different problems.

Linking textiles or textiles-polymer-laminates (artificial leather) with optical devices and electronics is more realistic than ever. An emerging new field of research that combines the strengths and capabilities of electronics, optics and polymer composites is opening new

[2]

opportunities. Lower production cost, wider exploitation of integrated circuit technology and wider applicability to sensor arrays ensure the integration of microsensors in almost any structure, providing the desired system data.

Fiber Bragg Grating Fiber Bragg Gratings (FBG) are periodic changes in the refraction index of the fiber core made

by adequately exposing the fiber to intense UV light. The gratings produced typically have lengths of the order of 10 mm [4]. When an optical beam is injected into the fiber containing the grating, the wavelength spectrum corresponding to the grating pitch will be reflected, while the remaining wavelengths will pass through the grating undisturbed, as exemplified in Figure 1 [5, 6]. Since the grating period structure is sensitive to strain and temperature, these two parameters are measured by the analysis of the reflected light spectrum. This is typically done using a tunable laser containing a wavelength filter (such as a Fabry–Perot cavity) or a spectrometer [4].

Figure 1 - Illustration of a Bragg sensor principle.

A resolution in the range of 1 µε (micro-strain) and 0.1 ºC can be achieved with the best

demodulators [4]. Since we are dealing with optical sensors that are sensitive to temperature and, in this case, also to strain by the same manner, a few issues may appear when measuring both parameters simultaneously. In this case, it is necessary to use a strain free reference grating that measures the temperature alone, in order to compensate the temperature error from the sensor network and measure the correct strain values.

A main advantage to use Bragg gratings is their multiplexing potential [5]. Many gratings can be written in the same fiber at different locations and tuned to interfere at different wavelengths. This leads to the possibility for measuring strain at different locations along a single fiber. However, since the gratings have to share the spectrum of the light, there is a trade-off between the number of gratings and the dynamic range of the measurements on each of them.

Polymeric Foil

Vinyl mixtures, or so-called Plastisols, are liquid dispersions of a finely divided thermoplastic, polyvinyl chloride resin, in a plasticizer. It is a 100% non-volatile paste-like composition and consists of a physical mixture of finely sized PVC polymer particles and liquid plasticizers, such as phthalates and epoxy oils [7, 8]. These resulting pastes are highly viscous mixtures that, after heated above the curing-temperature (130 to 400 ºC), become homogenized and a solid phase results. When cooled, the Plastisol provides a tough material with specific physical characteristics. The Plastisol material is formulated to be a dense solid and elastic polymer, which retains toughness even at low temperatures. Plastisols can be formulated with hardness ranging from 30 to 90 Shore A (relative hardness of elastic materials) and tensile strength ranging from 750 to 3000 psi [7, 8]. It

can be formulated to resist chemical attack, is self-extinguishing due to the chlorine groups, provides good weatherability and can be used as support structure when looking for flexibility

Finite Element Sensing Device Model Finite element analysis has been carried out using specific software. A three-dimensional model

of the optical fiber integrated in a polymeric foil was created using solid elements. The analyzed model had a length of 100 mm, a width of 50 mm and a height of 550 µm. The optic fiber is 250 µm thick, with a 9 µm core and a 125 µm cladding, and was placed in the center of the foil.

The material properties used for simulations were based on the available commercial materials properties from CES EduPack software. PVC-elastomer (shore 75A) was chosen for the polymeric foils, silica for the fiber core and cladding, and acrylate for the fiber buffer.

As the full model has a small number of elements, it was possible to simulate and observe the detailed behavior of the model along the load application time. For this reason, a 3D mesh was chosen for the simulation. The mesh was composed by 6215 brick elements that fill the model volume. The elements are defined by 20 nodes, having three degrees of freedom at each node: translations in the nodal x-, y- and z-directions. In some sections of the model, like the fiber volume and model borders, the mesh was refined, presenting smaller elements, ensuring better results.

Figure 2 – Finite element model representation in the FEM software: a) model mesh with applied loads and constraints; b) nodal stress (von Mises yield criterion).

Analysis The model was subjected to force loads applied at the 168 nodes that composes the limits of the

polymer front face, as depicted by the red arrows in Figure 2a. Besides the loads, it was also restraint at the back face of the polymer, by forcing a null displacement in the three directions. The force was applied as ramp function of 5x10-3 N per second, at each node, over a 20 second period.

Figure 2b shows the stress distribution at the middle section plane. The plane is at the half height of the model. It is interesting to see that, despite neither restraints nor direct forces were applied to the fiber, the polymeric foil deformation is sensed by the fiber, where the stress concentration was mainly in its middle section. The tensile effects in the fiber are more evident at the middle section, since the polymer drives the strain to that specific area. The sensor, according to these results, should be positioned in this region, where the effects were more significant.

From the available data, it is known that the fiber is capable of sustain a known yield strength of 0,7 GPa. To test fiber limits, during the analysis, it was subjected to higher stress. From Figure 3a, the fiber passes above the yield strength when 8,5x10-2 N is applied to the nodes. At this moment, the fiber presents a stretch of 600 µm.

a)

b)

a) b) Figure 3 – a) Stress and displacement variation over pressure application; b) Displacement differential between a node at the polymer surface and a node at the interior of the fiber.

The polymeric foil does not present any significant stress due to its low Young’s Modulus. This allows the PVC foil to be easily stretched. It is important that the polymer does not represent any obstacle for deformation transference. From the obtained and presented results, the fiber is fully sensitive to the deformation since it deforms along the polymer foil elongation. Besides, and yet more important, the difference between the polymer displacement and the fiber displacement is very small, ensuring that the foil deformation is completely transferred to the fiber sensor (Figure 3b).

Sensing Device Prototype The optic fiber used in the prototype fabrication was a single mode Corning 28e, a standard fiber

for communication applications, with acrylate coating. The Bragg structures were first printed in the fiber core using a communication wavelength range (1520-1570 nm). The fiber optic sensor was then packaged within a sandwiched polymeric foil (Figure 4a). The fiber integration within the foil is a crucial step since it must be avoid any significant loss of sensor sensitivity due to the foil. The fabrication relies on an industrial process (Figure 4b), where a first layer is applied in a structure that will work as substrate for the fabrication process. After the thickness homogenization, the layer is cured in the oven, to become a solid structure. The second layer suffers only a partial cure, in order to increase the viscosity and facilitate the insertion of the optic fiber. The third, and final layer, is placed over the fibers and is fully cured in the oven.

Figure 4 – a) Chosen approach and illustration; b) Industrial process schematic; c) Fabricated prototype.

a)

b) c)

Measurements and Analysis Figure 4b shows the functional sensing device prototype fabricated as previously described. The

polymeric foil, 210x150 mm2 size, has a Bragg sensor embedded in it. By visual inspection no damage is detected, being a good indicator of the fabrication process success. By touch, the fiber is not felt, sustaining the thought of a good integration level. Figure 5a presents the reflected spectrum of the FBG sensor. The side lobes come from the grating fabrication process, resulting from radiation transmission function, and not from the integration process. The lobes can be later smoothed by apodization if necessary. When stretching the polymeric foil, the embedded FBG sensor follows the deformation and the reflected spectrum suffers a wavelength deviation. When the sample is released, the spectrum returns to the initial position.

Since one of the goals is to produce this type of foils with integrated sensors on industrial environments, the restraints of the industrial process had to be evaluated. Considering that optical sensors are, in general, sensitive to temperature, one of the analyzed restraints was the fabrication process temperature ranges. Several foils, with integrated optic fibers, were fabricated at different temperatures, from 200 to 240 ºC with 20 ºC steps, and at different cures durations, from 60 to 180 seconds with 30 seconds steps. It was found that the PVC from the polymeric foil did not stand temperatures above 240 ºC during 150 seconds. At the optic fiber level, all the Bragg sensors did support the temperature and the duration of the cure without presenting any sensitivity loss or damage. For the industrial process, these results does not present any restraints since, in general, the polymeric foils are fabricated at a temperature of 220 ºC for 60 seconds.

To better evaluate the performance of the produced model, the prototype was tested with an Instron® 4302 testing machine at the same time that the optical signal was being measured by a BraggMETER™ unit from FiberSensing company [8].

a) b) Figure 5 – a) Reflected spectrum from the FBG sensor for two distinct tensile forces; b) Bragg response to applied displacements.

The prototype was cropped to the 50x100 mm size. During the test, the model was subject to a

displacement at the rate of 16 µm/s. As it is demonstrated over the graph (Figure 6), the wavelength deviation has a linear behavior from 0,5 % onwards. Two reasons for the non-linear behavior below 0,4 % can be pointed out. First, the testing machine does not have enough resolution for elongations lower than 400 µm, not providing precise elongation data, and in this case a test with a sample with double height may be used, since it duplicates the machine sensitivity. Second, the polymeric foil does not transmit such little deformation to the fiber.

It was also verified that the prototype was able to sustain a stretching of 1,62 % (strain), corresponding to 1,62 mm of displacement. In this situation, the fiber was subjected to a load of 9,691 N.

A displacement of 1,62 %, with a wavelength deviation of 9,207 nm, was measured and the sensitivity of the present prototype is 0,6 pm/µε (picometer per microstrain). In a different way, if we consider a 1 meter long steel beam that has been stretched 1 mm, the wavelength deviation that would be measured is 0,6 nm. The determined sensitivity value provides information about the integration quality.

Conclusions In this paper, a finite element model and a fabricated prototype model were investigated and,

although the results of each one may seem different, the overall model behavior was the same than the fabricated prototype. The finite element model provided relevant data about the stress distribution. This is crucial when dealing with the positioning of the Bragg sensor. It was showed that the difference between the deformation of the polymer and the fiber is very small. It is a good indicator for the loss of sensitivity that may exist. The fabricated prototype stretches a little more than the predicted by finite element model. This may be a result from the fact that the available material parameters do not match exactly the material properties of the fabricated device. For example, the PVC polymeric foil is a custom formulation from TMG Automotive®. It was especially made for a better fiber integration level, which may result on different properties when compared to the commercial polymer. The fabricated prototype presented a sensitivity of 0,6 pm/µstrain, but the sensor network resolution will be always related to the monitoring system resolution. The structure showed good performance, not only the spectrum shape continued the same during the force application, but also showed good repeatability, since it returned to its initial position and there was not any reduction of the signal amplitude. At the integration level analysis, the results demonstrated the successful integration of fiber sensor within the polymeric foil.

Integration of sensors, as the Fiber Bragg Grating ones, in new substrates as polymeric foils or textiles laminated are proving to be a very fertile research area and new developments can be expected.

Acknowledgements Alexandre Ferreira da Silva is supported by Portuguese Foundation for Science and Technology (SFRH/BD/39459/2007).

References 1. Grattan, K.T.V. and T. Sun, Fiber optic sensor technology: an overview. Sensors and Actuators a-Physical, 2000.

82(1-3): p. 40-61. 2. Wolfbeis, O.S., Fiber-optic chemical sensors and biosensors. Analytical Chemistry, 2006. 78(12): p. 3859-3873. 3. Blue, R., et al., Platform for enhanced detection efficiency in luminescence-based sensors. Electronics Letters,

2005. 41(12): p. 682-684. 4. Glisic, B. and D. Inaudi, Fibre Optic Methods for Structural Health Monitoring, ed. J.W. Sons. 2007. 5. Kersey, A.D., et al., Fiber grating sensors. Lightwave Technology, Journal of, 1997. 15(8): p. 1442-1463. 6. Rizvi, N.H. and M.C. Gower. Production of Bragg gratings in optical fibres by holographic and mask projection

methods. in Optical Fibre Gratings and Their Applications, IEE Colloquium on. 1995. 7. Petrie, E.M. (2005) PVC Plastisol Adhesives. Adhesives & Sealants. 8. Rodolfo Jr., A., L.R. Nunes, and W. Ormanji, Tecnologia do PVC. 2 ed, ed. P.A. Ltda. 2006: Braskem S.A. 448.

Smart Sensing Polymeric Foil with Integrated Optic Fiber Sensors Fabrication and characterization of a polymeric foil sensitive to strain.

A. F. Silva, P. M. Mendes, J. H. Correia Department of Industrial Electronics

University of Minho Guimaraes, Portugal

[email protected]

F. Goncalves R&D Department TMG Automotive

Campelos, Guimaraes, Portugal [email protected]

L. A. Ferreira, F. M Araujo INESC Porto

Faculty of Science University of Porto

Porto, Portugal

L. A. Ferreira, F. M Araujo FiberSensing

Maia, Portugal [email protected]

[email protected]

Abstract— A structural integrity monitoring system based on optic fiber sensors is an important development at the smart structures level. However, direct sensors incorporation, without a substrate structure, creates few difficulties in eventual sensor maintenance or replacement. This paper presents an approach to overcome this issue. The fabrication, using industrial fabrication processes, and characterization of a polymeric foil able to sense and gather sensitive information, and send it for remote analysis is explored. The described example uses Fiber Bragg Grating sensors embedded in laminated polymeric sheets commonly used in different industries, as automotive, aeronautic, civil, among others. The fabricated foil is capable of transferring the full deformation to the optical sensor. Tests indicate that the polymeric foil influence on the sensor performance may exist. However, the presented optical sensor incorporated in the polymeric foil is fully functional with high sensitivity, 0.6 picometer by microstrain, measuring deformation, up to 1.2 millimeter.

Keywords- optical sensors; smart structures; fiber Bragg gratings, sensor integration

I. INTRODUCTION Optical sensing technologies have several advantages

that make them very attractive in a broad range of applications. Optical fiber sensors, in particular, provide low-cost solutions in the overall system, with immunity to electromagnetic interference, multiplexing capabilities and a high degree of miniaturization and integration. Nowadays, optical fiber sensors offer a high performance alternative, in comparison to standard technologies, in many different areas, either to measure physical parameters like strain, temperature or pressure, or to perform highly sensitive biochemical analysis [1, 2].

However, the incorporation of such sensors creates some difficulties in eventual sensor maintenance or replacement. Alternatively, it is proposed in this paper to incorporate optoelectronic instrumentation in standard polymeric foils

that can be already found in different products, e.g., automotive and air craft floors and wall coverings. The advantages come from the easy applicability of foils in the monitored structure, allowing also the easy access for replacement. Moreover, integrated optical devices are now emerging as the next generation of sensing structures, where virtually any parameter can be determined with high accuracy in a highly miniaturized optoelectronic device [3]. Linking textiles or textiles-polymer-laminates with optical devices and electronics is becoming more realistic than ever. An emerging new field of research that combines the strengths and capabilities of electronics, optics and polymers like Polyvinyl Chloride (PVC) is opening new opportunities.

Industries, like the automotive, aeronautics, civil and biomedical are looking for solutions to gather information from their systems status. Lower production costs, wider exploitation of integrated circuit technology and wider applicability of sensor networks allows the integration of microsensors in almost any structure, providing the desired system data.

During this paper, the choice of Fiber Bragg Grating sensors and Polyvinyl Chloride is justified. The prototype is described in detail at the fabrication level and evaluate at the performance level.

A. Fiber Bragg Grating Sensors Fiber Bragg Gratings (FBG) are periodic changes in the

refraction index of the fiber core made by adequately exposing the fiber to intense UV light. The gratings produced typically have lengths of the order of 10 mm [4]. When an optical beam is injected into the fiber containing the grating, the wavelength spectrum corresponding to the grating pitch will be reflected, while the remaining wavelengths will pass through the grating undisturbed, as exemplified in Figure 1 [5, 6]. Since the grating period structure is sensitive to strain and temperature, these two

[3]

parameters are measured by the analysis of the reflected light spectrum. This is typically done using a tunable laser containing a wavelength filter (such as a Fabry–Perot cavity) or a spectrometer [4].

Figure 1 - Illustration of a Bragg sensor principle.

A resolution in the range of 1 µε (micro-strain) and 0.1 ºC can be achieved with the best demodulators [4]. Since we are dealing with optical sensors that are sensitive to temperature and, in this case, also to strain by the same manner, a few issues may appear when measuring both parameters simultaneously. In this case, it is necessary to use a strain free reference grating that measures the temperature alone, in order to compensate the temperature error from the sensor network and measure the correct strain values.

The choice of Bragg sensors may not be the clear one sometimes due to their cost. However, the maturation of the Bragg technology is bringing the prices down and improving the performance. Considering the capability to sustain high number of cycle loads, harsh ambient conditions, multiplexing capabilities and thus low weight of connection leads, the Bragg sensors are becoming more and more interesting to be applied [5].

B. Polymeric Foil Flexible skin-like foils can be made from many different

polymers. Polyurethane (PUR) may be one of the “noblest” materials, feeling like leather, with very long durability and high performance in regard to abrasion resistance and flexibility. However, PUR-based foils are one of the most expensive materials in the field of flexible polymeric foils. Polyolefin based artificial skins are a suitable alternative for the required objective, but their flexibility and performance related to softness, abrasion and flexibility is in general more difficult to adjust. As the research is focused on the development of a generic manufacturing technology for a flexible optical sensing foil, a polymer matrix with an acceptable average quality and price is desired. The choice

was set on plasticized Polyvinyl Chloride (PVC), for its general good cost/performance ratio and ease of use during manufacturing processes. PVC certainly is one of the most versatile plastics, still playing a major role in the building, packaging and automotive market. Furthermore PVC exhibits many advantages like highly competitive production cost, high versatility in interior trim applications, high resistance to ageing and ease of maintenance

At the process level, the PVC is a highly viscous mixture that becomes solid on heating. When cooled, the plastisol provides a tough material with good physical characteristics and with a service temperature range of 0°C to 125°C [7]. The plastisols commercial success is mainly due to the low cost and easy applicability. Long-term flexibility is the main advantage, which means that the plastisol supports relative motion between host substrates. The plastisol material can be formulated to comprise specific characteristics. It can be a soft foam or a dense hard solid material, resistant to chemical attack, self-extinguishing or to provide good weatherability [7]. The major drawback is the cure temperatures that may be too high, limiting their use on specific substrates or applications.

II. FABRICATION The optic fiber used in the model fabrication was a

single mode Corning 28e, a standard fiber for communication applications with acrylate coating. The Bragg structures printed in the core of the fiber, had a length of 10 mm and were in the communication wavelength range (1520-1570 nm). Many gratings can be written in the same fiber at different locations and tuned to interfere at different wavelengths. This leads to the possibility for measuring strain at different locations along a single fiber. However, since the gratings have to share the spectrum of the light, there is a trade-off between the number of gratings and the dynamic range of the measurements on each of them.

The fiber optic sensor is packaged within a sandwiched polymeric foil (Figure 2). The integration of the fiber in the foil is a crucial step since there may be some loss of sensor sensitivity by the foil.

Figure 2 – Sandwich approach.

The fabrication relies on spread-coating, a common industrial process (Figure 3). A first layer is applied in a structure that will work as substrate for the fabrication process. After the thickness homogenization, the layer is cured in the oven and become a solid structure. The second layer suffers only a partial cure in order to increase the viscosity and facilitate the insertion of the optic fiber. The third and final layer is placed over the fibers and is fully cured in the oven.

Figure 3 – Industrial process schematic.

III. RESULTS AND DISCUSSION Figure 4 shows the functional prototype produced as

previously described. The polymeric foil, 210x150 mm2 size has a Bragg sensor embedded in it. By visual inspection no damage is detected, being a good indicator of the fabrication process success. By touch, the fiber is not felt, sustaining the thought of a good integration level.

Figure 4 – Fabricated prototype.

Figure 5 presents the reflected spectrum of the FBG sensor. The side lobes come from the grating fabrication process, resulting from radiation transmission function, and not from the integration process. The lobes can be later smoothed by apodization if necessary.

When stretching the polymeric foil, the embedded FBG sensor follows the deformation and the reflected spectrum suffers a wavelength deviation. When the sample is released, the spectrum returns to the initial position.

Since one of the goals is to produce this type of foils with integrated sensors in an industrial environment, the restraints of the industrial process had to be evaluated. Considering that optical sensors are, in general, sensitive to temperature, one of the analyzed restraints was the fabrication process temperature. Several foils with integrated optic fibers were fabricated at different temperatures, from 200 to 240 ºC with 20 ºC steps, and at different cures durations, from 60 to 180 seconds with 30 seconds steps. It was found that the PVC from the polymeric foil did not stand temperatures above 240 ºC during 150 seconds. At the optic fiber level, all the Bragg sensors did support the temperature and the duration of the

cure without presenting any sensitivity loss or damage. For the industrial process, these results do not present any restraints since, in general, the polymeric foils are fabricated at a temperature of 220 ºC for 60 seconds.

Figure 5 - Reflected spectrum from the FBG sensor for two distinct tensile forces.

To better evaluate the performance of the produced model, the prototype was tested over a Instron® 4302 testing machine at the same time that the optical signal was being measured by a BraggMETER™ unit from FiberSensing company [8].

The prototype was cropped to the 50x100 mm size. Two tests were made over the model. One in which a displacement was applied at a constant increment until the model did not stand any more strain and the fiber snapped.

During this test, the model was subject to a displacement at the rate of 16 µm/s. As it is demonstrated over the graph (Figure 6), the wavelength deviation has a linear behavior from 0,4 % ahead. Two reasons for the non-linear part until de 0,4 % can be pointed. The testing machine does not have enough resolution for elongations lower than 400 µm, not providing precise elongation data, and in this case a test with a sample with double height may be used, since it duplicates the machine sensitivity. The polymeric foil does not transmit such little deformation to the fiber.

Besides this fact, the model was able to sustain the stretching of 1.62 % (strain), which is 1.62 mm of displacement. At this time, the fiber was subject to a load of 9.691 N.

A displacement of 1.62 % with a wavelength deviation of 9.207 nm was measured and the sensitivity of the present model is 0.6 pm/µε (picometer per microstrain). To have a notion of the system resolution, if we consider a 1 meter long steel beam that has been stretched 1 mm, the wavelength deviation that would be measured is 0.6 nm. The determined sensitivity value provides information about the integration quality.

Figure 6 – Bragg response to applied displacements.

In another test, the displacement was applied in steps of 0.2 % (200 µm) and left at that state during a period of time, in order to evaluate the behavior of the fiber when subject to loads (Figure 7).

Figure 7 – Bragg response to applied displacement steps.

The applied displacement can incremented to see if the fiber slips over the polymeric foil. If this happens, the optical signal should start decreasing. In Figure 7, it can be seen a little bump after each step, but in seconds the optical signal stays constant. The small variation may be due to some vibration of the testing machine claw, since the model has a high sensitivity as determined before. The preservation of a constant value ensures that the fiber does not slip and that it is well embedded in the foil.

IV. CONCLUSION The full integration of an optic fiber FBG sensor in a

polymeric foil, using standard industrial fabrication processes was described in this paper. The prototype model presented an excellent behavior, 0.6 pm/µstrain. The integration of FBG sensors into the polymeric foil was evaluated in terms of the fabrication process temperature, integration level (adhesion of the fiber to the polymeric foil), measurement capabilities and sensor sensitivity. The structure showed good performance, namely, it had a linear behavior and, not only the spectrum shape continued the same during the force application, but also showed good repeatability, since it returned to its initial position and there was not any reduction of the signal amplitude. At the integration level analysis, the results demonstrated the successful integration of fiber sensor within the polymeric foil. Considering the Bragg sensor 10 mm length, the fiber layout can be fully customized according to the desired smart structure and the density of the sensors is mainly defined by the needed sensor dynamic range and the interrogator system resolution.

Integration of FBG based-sensors in PVC foils is demonstrated, promising production of large FBG based-sensors network in PVC foils and mass production in industrial environment.

V. ACKNOWLEDGMENTS The author, Alexandre Ferreira da Silva, is supported by

Portuguese Foundation for Science and Technology (SFRH/BD/39459/2007).

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http://www.fibersensing.com/�

20090330_flexible pvc foils based in photonics for automotive applications

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