characterization of epoxy and polyurethane … · characterization of epoxy and polyurethane resin...

CHARACTERIZATION OF EPOXY AND POLYURETHANE RESIN SYSTEMS FOR MANUFACTURING OF HIGH-PERFORMANCE

COMPOSITES IN HIGH-PRESSURE RTM PROCESS

Philipp Rosenberg1, Dr. Bernd Thoma1, Prof. Frank Henning1,2 1Fraunhofer-Institut fuer Chemische Technologie, Pfinztal, Germany

2Institut fuer Fahrzeugsystemtechnik, Karlsruher Institut fuer Technologie, Germany

Abstract The High Pressure RTM (HP-RTM) process is a manufacturing technology capable for high-

volume production of fiber reinforced plastics (FRPs) in the automotive industry. In the last years, robust equipment and process routes have been developed capable to produce FRPs in an industrial scale environment mainly by use of highly-reactive Epoxy resin systems (EP). However, alternative matrix systems like Polyurethanes (PU) are highly attractive but there is still a lack concerning know-how in process specific behavior and resulting mechanical properties compared to EP resin systems.

This study compares two matrix systems (EP and PU) for HP-RTM feasible to realize short process cycle time of less than five minutes. To ensure the comparability of the manufactured laminates, the study was carried out on same press and mold setup with constant process parameters with two HP-RTM injection machines for precise high-pressure dosing, mixing and injection of resin and hardener components of Epoxy and Polyurethane up with high throughput rates during infiltration step. The analysis of relevant process steps and characterization of mechanical properties of the manufactured carbon fiber based laminates using the different resin systems demonstrates the potentials of Polyurethane compared to Epoxy resin.

Introduction and state of the art The HP-RTM process has been intensely investigated in various studies in the last years [1-4].

Compared to classical RTM process, one of the infusion processes where low pressures of 1-20 bar are used for the infiltration of the fabrics, the HP-RTM uses high pressures of up to 150 bar for mixing and dosing of resin and hardener in the mixing head and cavity pressures of 30 to 120 bar during injection/compression step depending on selected process variant [3, 5]. The combination of high-pressure pumps and self-cleaning mixing head of such equipment guarantees high flow rates (20 - 200 g/s) resulting in short infiltration time of the preform. This enables the use of highly reactive resin/hardener systems with gel time of less than one min leading to short cycle times below five minutes. Two different variants, namely high-pressure injection RTM (HP-IRTM) and high-pressure compression RTM (HP-CRTM) can be implemented for the manufacturing of continuous-fiber reinforced composites [5, 6, 7]. Various studies have been conducted by different researchers to investigate the effects of certain process parameters on the quality of manufactured laminates by the CRTM, HP-CRTM and HP-IRTM processes which are summarized below [1-5, 8].

The HP-RTM process variants have been investigated to understand the influence of different process parameters (injection rate, vacuum, mold gap size and influence of binder) on resulting laminate properties. The selected reinforcements were based on glass fiber and fast curing Epoxy resin from Dow. The results of this study concluded that the variation of resin injection rate as well as mold gap led to no significant influence on the laminate properties. The use of vacuum during the process resulted in higher flexural properties and lower void content compared to the trials conducted without use of vacuum. The binder concentration was concluded as an important parameter affecting mechanical properties of the laminates. The highest mechanical properties were measured for the laminates with lowest binder content [8].

Another study conducted by the co-authors of this paper investigated the HP-IRTM and HP-CRTM process variants intensively using different mold gaps and different laminate layups with carbon fibers to demonstrate the high-volume production capability of the HP-RTM process variants. A main subject of the investigations was the variation of the injection gate geometry which was varied from point gate to film gate at different resin flow rates to understand the influences of injection gate design. Fiber volume fractions in a range of 50 to 56%, depending on number of layers, were realized at injection times between 7.5 s and 30 s. Based on the results of these studies a new state of the art HP-RTM mold having several pressure sensors was conceptualized and constructed to enable more detailed investigations of the cavity pressures in HP-RTM process variants [1, 2].

The co-authors of this study have published another paper where investigations were conducted to understand the influence of the process variants (HP-IRTM and HP-CRTM) on the cavity pressure built-up in HP-RTM process steps based on carbon fiber reinforcements and Epoxy resin. The results showed continuous increase of cavity pressures during injection step in HP-IRTM whereas the pressures in HP-CRTM were significantly lower during injection step. After end of injection, the pressures in HP-IRTM reached maximum values and in HP-RTM the maximum cavity pressure was observed after end of subsequent compression step. A linear dependency was found between the applied press force and the resulting maximum cavity pressure for both process variants [19].

The latest study of the co-authors in field of HP-RTM investigated the cavity pressures in HP-IRTM process with variable press force during injection and curing step and different end mold gaps in HP-CRTM process. An important result was the variable end mold gap in HP-IRTM process which was only controlled by the applied press force during injection step. An additional press force increase after injection was necessary to achieve final part thickness. The press force applied during injection was the main influencing parameter for the cavity pressure level. In comparison to the variable mold gap in HP-IRTM the trials with HP-CRTM process variant showed a constant end mold gap regulated by the distance control system of the press leading to variable press force during injection step. For the reference parameters used in this study (press force controlled HP-IRTM process variant), a linear dependency between applied press force and resulting cavity pressure was experimentally verified. A difference between theoretical cavity pressure and resulting cavity pressure of 20% was found. This difference was caused by the press force needed for compression of the resin gasket and fiber clamping [18].

State of the art HP-RTM process variants - HP-IRTM

The process steps of the high-pressure injection resin transfer molding (HP-IRTM) process variant are shown in figure 1 which has been created based on the process know-how and research activities of the co-authors in the last years [4, 5, 18, 19]. Three key parameters of the process are considered - press force, end mold gap (end mold gap of 0 mm = final part thickness) and cavity pressure - in correlation to the different process steps.

Figure 1: Process steps in HP-IRTM process variant: 1) mold closed by press to vacuum gap; 2) vacuum step -

vacuum ports open/lock for preselected time; 3) mold closed with preselected force (change from distance/speed controlled mode to force controlled press profile; 4) injection of matrix system by HP-RTM injection machine at preselected press force; 5) application of press force for obtaining final part thickness if necessary; 6) final part

thickness obtained and curing of resin, opening of mold and demolding of part

After the preform was placed in the mold cavity the mold is closed by the hydraulic RTM-press to a preselected vacuum gap (step 1). The end mold gap for the vacuum step (step 2) and the time of vacuum application depend on various factors like part geometry, mold concept and fabric type. The vacuum ports which are connected to the hydraulic auxiliary functions of the press open to exhaust the air out of the mold cavity for a preselected time to obtain vacuum. A good vacuum reduces void content and increases the impregnation quality in the final part [8]. After the vacuum time expired the vacuum ports are locked and the mold is closed by the press to apply a preselected force (step 3). At end of step 3 the press profile changes from distance/ speed controlled to press force controlled. The preform compaction grade and thus the permeability depend on the applied press force. At high press force the fibers may be compacted too much and a resin flow at high injection rate causes too high pressure in the equipment feeding pumps which results in an injection abort. In this case the press force needs to be reduced for injection step to obtain an end mold gap with a height of more than 0 mm (dashed line in Figure 1). Due to the need of preform and resin gasket compaction a small end mold gap arises if low press force has been preselected. In step 4 the infiltration of the resin system starts, resin and hardener are mixed in the mixing head and the preform is impregnated with resin. The selected injection rate depends on resin viscosity, gel time, amount and part size which are infiltrated at injection rates of 20 to 120 g/s in most cases. The high throughput rate allows the cavity to fill quickly and during resin injection, high cavity pressure may be created depending on the applied press force. During step 4 the end mold gap is only controlled by applied press force and the high cavity pressure may cause a raise of the end mold gap during resin flow - in particular at low press force. This leads to an increase of the preform permeability during injection and a cavity pressure reduction during ongoing injection. To obtain final part

Cavi

typr

essu

re

0

max.

Pres

s fo

rce

0

max.

1 2 3 5 6

HP-IRTM variant

0

Distance / speedcont rolled

Force cont rolled

End

mol

d ga

p

4Process step

thickness an additional compressions step (step 5) by increase of the press force after injection is necessary which involves an additional raise of cavity pressures [18]. After the final press force was applied, final part thickness was achieved and curing of the resin completed the mold is opened by the press to demold the part [6, 7].

State of the art HP-RTM process variants - HP-CRTM

A second variant of the HP-RTM process which has been investigated in detail in recent years is the high-pressure compression resin transfer molding (HP-CRTM) process which represents a combination of resin transfer molding (RTM) and compression molding [5, 2]. The same high pressure dosing equipment is used for processing highly reactive resin systems. If conducted without the use of high pressure dosing and mixing equipment, the process is referred in the literature as the Compression RTM (CRTM) process [9, 10].

Figure 2: Scheme of the process steps in HP-CRTM process variant: 1) Mold closed by press to vacuum gap; 2) Vacuum step - vacuum ports open for preselected time; 3) Mold closed to preselected end mold gap > final part

thickness; 4) Injection of matrix system by HP-RTM injection machine at preselected end mold gap;5) speed controlled compression step to final part thickness. After final part thickness was obtained, press profile changes to

force controlled mode; 6) force controlled curing of matrix system

Cavi

typr

essu

re

0

max.

1 2 3 4 5 6

Pres

s fo

rce

0

max.

0

End

mol

d ga

p

HP-CRTM variant

Distance / speed cont rolled

Force cont rolled

Process step

In the HP-CRTM process as shown in figure 2 the first two process steps are equal to the HP-IRTM process variant. After locking the vacuum ports the mold is closed to the injection mold gap (step 3) which is higher than the end mold gap. Depending on component design and mold concept the injection gap varies in a range of higher than part thickness to smaller than vacuum gap (max. 2 mm). A very small gap can have a similar height compared to HP-IRTM with low press force at injection start but in HP-CRTM the gap is controlled by the distance control system of the press and not by the press force. In step 4 the resin injection is carried out at significantly lower cavity pressure compared to HP-IRTM process variant. A high injection gap may lead to incomplete wetting of the fibers during injection due to the additional volume in the cavity. During the injection step (4) the press distance control system keeps a constant injection gap height - if necessary (small gap) by raise of the press force [3]. Once the required amount of resin was introduced into the mold cavity and the resin mixing head is closed, the compression step 5 is carried out by the hydraulic press and the mold is closed completely to obtain final part thickness controlled by the distance/speed control system. The resin is squeezed into the preform in order to infiltrate all fibers of the reinforcement and high cavity pressure is build up. In this step, the preform is compacted to achieve the desired part thickness and fiber volume fraction. As the end mold gap is equal to final part thickness, the press profile changes to force controlled process and after curing of the resin the mold is opened by the press to demold the final part [1, 3, 4].

The scientific investigations in field of HP-RTM have created basic process know-how. Current development activities for industrialized production of parts within the HP-RTM process chain aim on two major topics: Precise process monitoring and control and reduction of overall cycle time. Considering a high-volume production (up to 50,000 parts per year per production line), reliable and precise process control is evident to guarantee high process stability at low scrap rates. Therefore it is necessary to install and approve interfaces between mold, injection equipment and press. Further, increasing complexities of the parts, e.g. integration of pressure sensitive sandwich foams, require the implementation of high-speed infiltration cycles at low cavity pressure profile by using the interfaces of the equipment [3, 5]. The second major topic aims on achieving of short cycle times in production. Production cycles far below 5 minutes require resin systems with cure time of less than 100 s after injection start. Resin system suppliers have been developing highly reactive resin systems in recent time [11]. Beside developing new generations of Epoxy resin systems the use of alternative resins is part of actual research activities. Polyurethane systems have gained high interest to realize short cycle times in HP-RTM process because of the quick curing and lower processing temperatures. Suppliers further promise high fatigue resistance in dynamic tests and fracture toughness (G1c) [12, 13]. Due to these benefits it is essential to evaluate such alternative resin systems for their process - material performance behavior to obtain a general statement about usability of Polyurethanes in HP-RTM. This paper presents a first basic investigation by comparison of an Epoxy resin system with a Polyurethane resin system under equivalent process conditions.

Process study - comparison of Epoxy and Polyurethane resins in HP-RTM The presented study compares two resin systems, an Epoxy resin and a Polyurethane resin,

for their process specific behavior and their basic mechanical properties. HP-RTM laminates were manufactured with constant parameters for both resin systems and the process parameters were recorded with an online monitoring tool. The cavity pressures in process steps injection (4), increase of press force to achieve final part thickness (5) and curing (6) were analyzed based on a HP-IRTM process with reduced press force during injection step (see also figure 1). The infiltration trials were conducted using eight layers of carbon biaxial non crimp fabric and every trial was carried out three times to study the process reproducibility. The investigations of the manufactured laminates comprise evaluation of fiber volume content, tensile tests, three-point bending tests and evaluation of interlaminar shear strength and impact strength. The equipment, materials and process parameters used for the study are described below.

Equipment

A HP-RTM mold designed for research activities with possibility to install different inlay geometries was used for the study. To characterize the process and measure material properties the plate mold inlay was installed (Figure 3). The cavity has a size of 900 x 550 mm² (length x width) and adjustable cavity height of 1.8 to 2.9 mm to study different matrix systems, fabrics and layups with the HP-RTM process variants. A fiber clamping mechanism (3) based on a different polymer gasket geometries was used in the upper and lower mold halves to avoid race-tracking effects. The position of the resin injection gate, which designed as film injection runner, can be changed from middle to the side of the lower mold half. For this study the center position was selected. Nine pressure sensors (sensor type 6167A from Kistler Instrumente AG) were installed in the upper mold half with option to extend the number of sensors to 18; their respective position coinciding with the lower mold half is shown in figure 3 (a). The sensor positions over the mold length and mold width are also shown in figure 3 (a). For the study, six sensors were located (S_I and S1 to S5) along the mold length, one of them (S_I) was connected to the injection equipment to allow pressure controlled injection control, e.g. if the cavity pressure exceeds allowed maximum. Sideways, three pressure sensors were installed (S6 to S8).

(a)

(b)

(c)

Figure 3: Specially designed HP-RTM mold; (a) sensor positions and runner details, fiber clamping shown in red, (b) Sealing concept of the mold - vacuum gap, (c) sealing concept - injection gap

On either side of the mold (left side of S5 and right side of S1), vacuum ports are mounted to apply vacuum in the mold cavity before resin injection. They are moved by auxiliary hydraulic circuits of the press. Figure 3 (b & c) shows the sealing concept of the HP-RTM mold. To obtain the vacuum gap, a specially designed Lip gasket (1) was used which was mounted on the lower mold half. Once the mold was closed, a contact is established between the Lip gasket and the upper mold half to seal the mold cavity against air entry during the vacuum step (figure 3 (b)). Once the predefined time for vacuum application in the mold is reached, the vacuum gap is then closed by the press. In HP-CRTM process variant a predefined end mold gap is approached by the press distance control system (as shown in figure 3 (c)) or in case of HP-IRTM variant the press closes the mold and predefined force is applied. For the injection sequence it is essential that the resin gasket (2) is in contact with both mold halves to avoid resin leakage. For both process variants the fiber clamping mechanism (3) is individually adapted to be in contact with the fibers (4) placed in the mold cavity to avoid race-tracking and to ensure a good infiltration of the fibers.

The HP-RTM mold was designed for mounting it in a hydraulic compression press. For this study a compression press installed at the technical center of Fraunhofer ICT was used (press type Dieffenbacher Compress Plus DCP-G 3600/3200 AS). The press is capable of applying a maximum press force of 36.000 kN without the parallel holding system and 32.000 kN with use of the active parallel holding system. For Epoxy resin mixing, dosing and injection a three component HP-RTM equipment from KraussMaffei Technologies GmbH was used (equipment type RTM 8/3.2 K). For processing the Polyurethane resin system a two component machine from KraussMaffei Technologies GmbH, type RimStar Compact 4-8-PU-RTM was used. Both HP-RTM machines use the same processing principle with self-cleaning mixing heads directly connected to the film gate of the mold. In order to run a fully automated infiltration sequence after placing the preform in the mold cavity the HP-RTM equipment communicates with the hydraulic press which permits reproducible timing of all process steps. For recording and live-view of relevant process data from the mold and the press a Daisylab Software tool was connected to all measuring devices of the equipment. The data of the pressure sensors S1 to S8, temperature sensor T1 and press data like press force, positions of parallel holding system and press ram and the press force were recorded for all the conducted experiments.

S1S2S5 S4 S_I

S6 S7

900 mm

550

mm

Fiber clamping

S3

Film gate

Sensor connected to injection machine

S8

200 mm200 mm

225

mm

T1

Vacuum gap

3

2 14

Upper mold

Lower mold

Injection gap

3

2 14

Upper mold

Lower mold

Resin systems

Two resin systems were used for studying the process and resulting mechanical properties. The Epoxy resin used was a Sika AG type Biresin CR170 with hardener type Biresin CH150-3. The resin was preheated to 80°C and the hardener was held at room temperature in the HP-RTM equipment. An external release agent from Wuertz GmbH & Co., type PAT 657/BW, was dosed and mixed with the Epoxy resin directly in the mixing head during injection step. The mixing ratio of the resin: hardener: release agent was 100:24:2 by parts.

The Polyurethane resin system was a two component system from Rühl Puromer GmbH, type purorim® 185 IT (polyol which contained an internal release agent) and puronate® 900 (isocyanate). The mixing ratio of polyol: isocyanate was 100:121 by parts. The components were preheated to 70°C (polyol) and 50°C (isocyanate) in the HP-RTM equipment. For the purpose of good demolding an external release agent was applied from ACMOS CHEMIE KG, type ACMOS 36-7224.

Fiber reinforcements

A biaxial fabric made of 50 K carbon fiber roving with trade name Panex35® from Zoltek Corporation was used. The biaxial construction stitched with Tricot pattern had 150 g/m² layers in 0° and 90° direction respectively and additional 4 g/m² of stitching yarn weight (polyester). The Panex35® fiber with carbon content of 95 % has a fiber diameter of 7.2 microns, a density of 1.81 g/cc and a modulus of 242 GPa at a tensile strength of 4,136 MPa. Before running the experiments the area weight on single fabric layer was characterized according to DIN 538541. The results of textile characterization are shown in table 1.

Table 1: Results of surface area weight measurement of the used Saertex fabric according to DIN 53854

Surface are weight of used fabrics

Material Number of samples

Area weight as per data sheet

Weight of stitching yarn

Measured area weight

Standard deviation

Zoltek biaxial non-crimp fabric

36 300 g/m² 4 g/m² 299.94 g/m² 6.91 g/m² / ±2.3%

Process parameters for the HP-RTM study

The main purpose, as mentioned above, was to investigate the infiltration behavior of the two resin systems (Epoxy resin and Polyurethane resin) during the injection step, increase of the press force after injection and in the curing step. The resin systems were characterized for their rheological behavior. For conducting the HP-RTM trials two different injection machines designed for the requirements of each resin system, were used for the study. A reference parameter set was defined based on the HP-IRTM process variant. After fabric placement in the mold cavity, the mold was closed with a predefined press closure profile. The vacuum ports were opened during the press closure sequence at a specific remaining mold gap. Subsequently, the mold was closed using 0 kN press force and vacuum was applied for 60 s after the mold halves contacted each other. After locking the vacuum ports the press force was raised to 500 kN and the resin injection started to impregnate the fabrics at a flow rate of 40 g/s with a total resin amount of 710 g. After closing the mixing head the press force was raised to 5.000 kN at a rate of 750 kN/s and the resin cured for 300 s. Every trial was repeated for three times to investigate the reproducibility during the injection step.

1 Resolution of scale: 0.01 g; area of sample: 100 cm²

Constant process parameters: All the studies were carried out using a layup consisting of eight layers of the biaxial carbon fiber fabric: [0/90 0/90 0/90 0/90]s. After stacking four layers together with Z-stitching pattern to the outside the next four layers were turned upside down to ensure the symmetry of the layup. The parameters such as vacuum application time, resin mixture injection rate, resin injection amount and curing time for all the experiments were constant and these parameters were not varied.

Resin specific parameters: Some parameters had to be adjusted to resin specific requirements such as mold temperature, mixing head pressure and temperatures of the raw components in the used injection machines. The process parameters used are shown in table 2.

Table 2: Process parameters used for the study

Process type, constant parameters and resin specific adjustments

Process type HP-IRTM process with 500 kN press force during injection step (during injection step the end mold gap was defined by applied press force of 500 kN)

• Raise of press force after injection step to 5000 kN at a rate of 750 kN/s reach final part thickness • Repetition of each trial for three times (verification of reproducibility)

Constant process parameters

Fiber Layup: [0/90 0/90 0/90 0/90]_s, Vacuum time: 60 s, Curing time: 300 s, Resin mixture injection rate: 40 g/s, Resin injection amount: 710 g

Resin specific parameters

Epoxy resin: Mold temperature on cavity surface: 120°C (± 3°C), Mixing head pressure: 120 bar (± 5 bar), Resin/Hardener temperature in the injection equipment: 80°C / 25°C Polyurethane resin: Mold temperature on cavity surface: 90°C (± 3°C), Mixing head pressure: 150 bar (± 5 bar), Polyol/Isocyanate temperature in the injection equipment: 70°C / 50°C

Results The results of the study are described and discussed hereinafter. The first step was the

characterization of the resin specific viscosities at process temperature in correlation to the cavity pressures during injection step, increase of press force and curing. The results of the infiltration trials during injection for both resins and the maximum cavity pressures during press force increase in combination with a pressure analysis are shown. The mechanical properties measured for both resins were analyzed and compared to each other.

Rheological investigations of the resin systems

The rheological characterization of highly reactive resin systems capable for HP-RTM process is a challenge in laboratory environment due to the time needed from mixing of resin and hardener to start of measurement. In this time the curing reaction has already started and the results do often not correlate to the real system viscosity in real process. Due to this issue an alternative approach was selected. The viscosities of the single components of both resins were measured using a plate-plate rheometer2 with plate diameter of 25 mm. The selected shear rate was 100 1/s. The resin and polyol were heated from room temperature to 150°C and the hardener and isocyanate components were heated from room temperature to 130/100°C respectively. The viscosity of the mixtures of both resins was calculated using mixture law by Arrhenius with ηs = mixture viscosity; Nx = component amount [%]; ηx = dyn. viscosity; [14, 15].

2 Anton Paar GmbH, type Physica MCR 501

ln 𝜂𝑠 = N1 lnη1 + N2 ln η2 (1)

The results are shown in figure 4. The temperature dependent viscosity of the Epoxy resin system shows high difference between resin and hardener component compared to the Polyurethane system. The calculated mixture viscosities at comparable temperature are even lower for the Polyurethane but it has to be noted, that the Epoxy is processed at higher temperature than the Polyurethane system.

Figure 4: Result of the rheological investigations of the single components of the resin systems used for the study.

Table 4 shows the process relevant viscosities of the resins and their components which are continuously preheated in the equipment. During injection step the resins are mixed in the mixing head chamber at specific temperature and viscosity. The resin flow through the injection gate into the mold cavity (having usually higher temperature than resin mixture) loaded with fiber reinforcements causes a temperature increase of the resign mixtures. The results of the rheological investigations show that the Polyurethane system has a mixture viscosity/ temperature of 67.3 mPas at 59°C which is slightly lower compared to the Epoxy system having a mixture viscosity/temperature of 74.5 mPas at 69.3°C. As the resin flows through the fiber reinforcements during the injection step it is heated up to mold temperature which was 120°C for Epoxy resin and 90°C for Polyurethane resin. The viscosity results for mold temperature indicate a lower viscosity of the Epoxy resin having 10.3 mPas compared to the Polyurethane system having 19.1 mPas (+85.4%). It shall be noted that these measurements do not consider the viscosity increase during the curing reaction but they provide a realistic basis for the real viscosity in short timeframe after mixing of the components. As the injection was carried out in total time of 17.5 s, it is assumed that the flow front viscosity increased at end of the injection step. The characterization of the resin kinetics and the heating of the resin mixture from mixing temperature to mold temperature in HP-RTM is a challenge due to high reactivity in very short time after mixing which needs to be further investigated.

20 40 60 80 100 120 140100

101

102

103

104

105

Mix.-viscosity EP

Visc

osity

[mPa

s]

Temperature [°C]

EP Resin

Mix.-viscosity PU

EP Hardener PU POLY PU ISO

Table 3: Viscosity values of the rheological investigation correlated to the process specific temperatures

Rheological investigations of the resins

Resin Component Temperatures HP-RTM equipment Mold temperatures for the resins

Temperature [°C] Dynamic viscosity [mPas]

Temperature [°C] Dynamic viscosity [mPas]

EP Resin 80 82.8 120 16.9

Hardener 25 28.1 120 1.28 Mixture* 69.3 74.5 120 10.3

PU POLY 70 73.8 90 30.0 ISO 50 63.8 90 13.1

Mixture* 59 67.3 90 19.1 *Viscosity of the mixture calculated by mixture law of Arrhenius [14, 15]

Cavity pressure analysis for injection step and reproducibility

The HP-RTM infiltration study was carried out using constant parameter setup for the first four process steps - closing of the mold, vacuum step, application of preselected press force for injection and injection step. Figure 5 shows the recorded process data of three manufacturing cycles for both resin systems manufactured in HP-IRTM reference process: a) Epoxy resin; b) Polyurethane resin. The parameters during injection step were held constant at 500 kN press force, injection rate of 40 g/s and a total resin amount of 710 g. Three of the ten pressure sensors were used to study the cavity pressures, S3 was installed 25 mm from injection runner in resin flow direction, S4 was placed 200 mm from injection port in flow direction and S5 was located 400 mm from injection port near the vacuum ports of the platen mold (see also figure 3).

All laminates manufactured in this study were fully impregnated with resin and no dry spots or air entrapments were visually observed. This indicates that both resins are suitable for use in HP-RTM process. A drop of the pressures at S3 was observed during injection of the resin which was caused by slight raise of the end mold gap at press force of 500 kN during injection (process controlled only by applied press force).

a)

b)

Figure 5: Cavity pressures during resin injection step. The cavity pressure sensors S3, S4, S5 were distributed over the flow length of the mold. Every chart comprises the data of three infiltration trials. a) Epoxy; b) Polyurethane

Trial 2 - S3 Trial 2 - S4 Trial 2 - S5



83 84 86 88 90 92 94 96 98 100 1020

10

20

30

40

50

60

Injection start

Cavit

y pr

essu

re [b

ar]

Time [s]

Injection end

0

100

200

300

400

500

600

700

800 Press force

Pres

s fo

rce

[kN]

Epoxy resin




84 86 88 90 92 94 96 98 100 102 1040

10

20

30

40

50

60

Cavit

y pr

essu

re [b

ar]

Time [s]

0

100

200

300

400

500

600

700

800 Press force

Pres

s fo

rce

[kN]

Polyurethane resin

Injection start

Injection end

An important value for evaluation of cavity pressures is the pressure increase rate near the injection port during volumetric filling (resin flow through the fibers) of the cavity. The pressure increase rate was calculated in the timeframe of the first two seconds after injection start at constant end mold gap (linear increase of the injection pressure), after the flow front reached S3. The pressure of the Epoxy resin increased with a rate of 5.1 bar/s while the pressure increase rate of the Polyurethane resin was 59 % higher (8.1 bar/s). As the Polyurethane resin had 85.4 % higher viscosity than Epoxy resin in rheological tests, it assumed that the main factor for the differences in pressure gradient at beginning of the injection start was the different resin viscosity. The reduction from 85.4% difference in viscosity at mold temperature to 59% difference in pressure increase rate in real process may have been caused by different reasons which have to be further investigated (e.g. heating of the resin from mixing temperature to mold temperature, influences of the carbon fiber fabrics like capillary effects). The average maximum cavity pressures measured during injection at S3 for Epoxy resin were 32 ± 1.8 bar (12 to 13 s after resin flow front reached S3) and 36.1 ± 5 bar (7 to 8 s time to S3) for Polyurethane. The difference of 12.8% higher average maximum cavity pressure is another evident proof for the higher viscosity of the Polyurethane resin. The cause for the low difference of 12.8% is the variable end mold gap which changes the permeability of the reinforcements during injection step. At S4 maximum injection pressures of 11.3 ± 0.6 bar were measured for Epoxy resin and 13.1 ± 1.4 bar for Polyurethane resin (+16%) which strengthens the hypothesis of higher Polyurethane viscosity during injection step. The resin flow front of the Epoxy system reached S5 at different times, at trial 3 the pressures increased two seconds later compared to the first two trials. In contrast the flow front of the Polyurethane resin has not reached the sensor during injection only after increasing the press force to achieve final part thickness. This is another indication for higher viscosity of the Polyurethane leading to slower flow front progression at higher cavity pressure. Table 4 summarizes the most relevant values of the injection trials.

Table 4: Cavity pressure analysis during the three infiltration trials with Epoxy and Polyurethane resin at constant process parameters

Cavity pressure analysis for injection step

Resin Average pressure gradient at S 3 in first 2

seconds after resin reached S3

Average maximum cavity pressure at Sensor 3

during injection


during injection


during injection

[bar/s] [bar] [bar] [bar]

EP Resin 5.1 ± 0.15 32.0 ± 1.8 11.3 ± 0.6 12.6 ± 2.0

PU resin 8.1 ± 0.63 36.1 ± 2.5 13.1 ± 1.4 -

Cavity pressures analysis for press force increase after injection step and curing step

The recorded process data for the injection step, increase of press force after injection, curing of the resin and opening of the mold for demolding of the part is shown in figure 6. Analogue to figure 5, three pressure sensors were considered for the process analysis (S3, S4, S5). For both resins one injection trial is shown in figure 6. After injecting the resin, the press force was increased to a maximum value of 5000 kN in 6 s (750 kN/s) to achieve final part thickness. The resin cure time was set to 300 s after maximum press force was achieved and after the cure time elapsed, the mold was opened by the hydraulic press for demolding of the cured part.

Figure 6: Cavity pressures for process steps injection, press force increase after injection, curing step and opening of the mold

Increase of press force after injection step: After injection end and the mixing head closed

and the press force increase led to simultaneous increase of all cavity pressures for both resins. The cavity pressure of Polyurethane resin at S5 increased during raise of press force as well, even if no pressure increase was observed during the injection step. The maximum pressure values measured for S3 were 67 bar for Epoxy and 88 bar for Polyurethane resin (see also table 5). At S4 a maximum cavity pressure of 60 bar (Epoxy) and 83 bar (Polyurethane) was measured. At S5 higher cavity pressures were observed compared to S3 (Epoxy) and S4 (both resins) having values of 72.6 bar (Epoxy) and 87.3 bar (Polyurethane) respectively. Different reasons may have caused this effect – possibly a viscosity increase of the resin in the flow front at end of the flow length (S5) occured – another cause could be additional mechanical pressure applied on the sensor surface, e.g. from the fabrics due to compaction mechanisms during press force increase. In general the cavity pressures were 31 % (S3) and 38 % (S4) higher for the Polyurethane system having a higher mixing viscosity at mold temperature (see also table 3). Further, it is assumed that the full impregnation of the fibers was not finished before raise of the press force in case of Polyurethane (figure 5) which may have led to resin flow during compaction to final component thickness and therefore to higher cavity pressure in beginning of curing cycle.

Table 5: Maximum pressures after press force increase to 5000 kN to achieve final part thickness

Cavity pressure analysis for press force increase after injection step

Resin Maximum pressure at Sensor 3 at maximum press force [bar]

Maximum pressure at Sensor 4 at maximum press force [bar]

Maximum pressure at Sensor 5 at maximum press force [bar]

EP 67 60 72.6 PU 88.0 83.0 87.3

84 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 4004150

10

20

30

40

50

60

70

80

90

100Curing step

Increase of press force

EP S3 EP S4 EP S5

Cavit

y pr

essu

re [b

ar]

Time [s]

PU S3 PU S4 PU S5

Injection step

0

1000

2000

3000

4000

5000

Press force

Pres

s fo

rce

[kN]

Curing step: In curing step the resin changes from low viscosity fluid to a solid state material by cross-linking the components molecules having a specific shrinkage due to the chemical reaction. The cavity pressure sensors are calibrated to detect pressures of low viscosity fluids. The raise of the viscosity may have led to adulterated pressure values during the curing step. This indicates that a reliable pressure measurement can be conducted only within the gel time up to a specific maximum viscosity value.

Another characteristic observed in the curing cycle was the pressure drop at S3 in the timeframe between 190 to 220 s (Epoxy) and 150 to 170 s (Polyurethane). It is assumed that the curing grade of the resin at end of the flow length is higher compared to the curing grade near injection point where the resin mixture was introduced last during injection step. This indicates a delayed increase of resin viscosity near injection port compared to end of the flow length. Parallel the shrinkage of the resin system occurs from end to begin of the flow length. This mechanism may have caused the pressure drop at S3 near injection gate being in contact with lower viscosity resin than S4 and S5. This mechanism can lead to the assumption that a rough estimation of the resin gel time using pressure sensors may be executable. Considering the time from injection start to pressure drop at S3 in curing cycle, the Epoxy resin pressure drop occurred after 106-136 seconds and the pressure drop of the Polyurethane occurred after 66 to 86 seconds. To prove this phenomenon of pressure drop near injection gate in curing step further investigations are needed.

Characterization and comparison of the mechanical properties The second main objective of this study was the comparison of basic mechanical properties

of the plates manufactured at consistent process conditions. The test methods, standards and preparation of the samples as well as the results of the mechanical tests are described and discussed below. All samples were cut by waterjet in length direction (resin flow direction) of the laminates. The specimen dimensions prepared from the laminates and used test norms are mentioned in table 6. For the tensile test samples, the width of the samples was reduced from 25 mm to 15 mm (deviating from the norm standard) to ensure that the testing equipment was able to test the samples within its force capacity. The samples for flexural tests were cut as sample type III (L/h = 20; l/h = 30) for the flexural tests. For Charpy impact strength sample type 2b without notch and impact on the broad side of the samples was chosen.

Table 6: Methods for characterizing the laminates; the samples were cut by waterjet in length direction

Methods for characterization of the manufactured test laminates Test method Test standard Thickness of the

samples (h) [mm] Length of the samples (l) [mm] Sample width (b)

[mm] Fiber volume content DIN EN ISO

7822:2000

2.3 - 2.4

Diameter: 25 mm

Tensile strength DIN EN ISO 527-4 250 15 Flexural properties DIN EN ISO

14125:1998 Sample type IV: 30 x h 15

Interlaminar shear strength (ILSS)

DIN EN ISO 14130:1998

10 x h 5 x h

Charpy impact strength DIN EN ISO 179-1:2010

Type 2b (without notch, impact on broad side of the samples): 25 x h

15

The fiber volume content characterization was done by incinerating the samples at 650°C under inert gas atmosphere to avoid loss in fiber weight. The results are shown in table 7. The difference in average fiber volume content of the Epoxy (52.9%) and Polyurethane resin (51.5) is below the standard deviation (1.2 % for both resins).

Table 7: Results of the fiber volume content measurement

Fiber volume contents

Resin / Fabric Average thickness [mm]

Fiber volume content [%]

Standard deviation [%]

Fiber weight content [%]

Standard deviation [%]

EP / Carbon fiber 2.31 52.9 1.21 63.8 1.13

PU / Carbon fiber 2.38 51.5 1.18 62.6 1.10

Figure 7 shows the strength-modulus relation of all tested samples for tensile and flexural properties of Epoxy and Polyurethane laminates. Table 8 summarizes all results of the mechanical tests including interlaminar shear strength and Charpy impact strength.

Figure 7: Results of the mechanical tests; left: tensile tests; right: three-point bending tests

50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80500

550

600

650

700

750

800

850

900

950

1000

+ 5.1%

EP-CF 741 MPa / 66.9 GPa PU-CF 783 MPa / 63.6 GPa

Tens

ile s

treng

th [M

Pa]

Tensile modulus [GPa]

+ 5.6%

50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80700

750

800

850

900

950

1000

1050

1100

1150

1200

+ 2%

EP-CF 969 MPa / 66.9 GPa PU-CF 950 MPa / 63.9 GPa

+ 4.7%

Bend

ing

stre

ngth

[MPa

]

Bending modulus [GPa]

The tensile strength of the Epoxy laminates (average value 741 MPa / minimum value 693 MPa) is slightly lower compared to Polyurethane laminate strength (+5.6% average value 783 MPa / 738 MPa minimum value). Considering the standard deviation (50.7 MPa for Epoxy and 36.6 MPa for Polyurethane) in relation to the average strength, no significant difference in tensile strength was observed. The tensile modulus of Epoxy (66.9 GPa) and Polyurethane (63.6 GPa) samples was measured in elastic range and dominated by the fiber reinforcements, no significant differences could be observed. The average strength of the three-point bending samples was 969 MPa for Epoxy and 950 MPa for Polyurethane. A slight increase of 2% for the Epoxy strength was observed which leads to no significance considering the standard deviations. The measured modulus was comparable to the tensile modulus with 66.9 GPa for Epoxy and 63.9 GPa for Polyurethane samples. It is noteworthy to mention that the failure mechanisms were different for Epoxy and Polyurethane samples. All Epoxy samples broke by failure on tension side whereas all Polyurethane samples failed on pressure side of the sample. It has to be noted, that tensile and flexural tests are mainly dominated by fiber properties. Further investigations will aim on extended test methods like fiber-push out, in-plane shear strength, compression after impact or compression shear test.

In case of interlaminar shear strength tests (ILSS) almost small differences in the resulting strength values were measured for the manufactured laminates with Epoxy (67.5 MPa) and Polyurethane resin (64.3 MPa). However the ILSS specimen exhibited no failure mode as recommended per norm. The Epoxy samples showed combined bending and shearing failure and the Polyurethane samples showed failure on pressure side of the sample analogue to three-point bending test results. The Charpy impact strength was measured on a drop impact tester with possibility to record energy absorption and the forces during impact. The results showed differences in impact strength for Epoxy (47.1 ± 2.6 kJ/m²) and Polyurethane (53.9 ± 3 kJ/m²) having a higher average impact value of 14.4%. This indicates an increased dynamic strength which needs to be further investigated.

Table 8: Summarized results of the characterized laminates

Results for mechanical properties of the manufactured test laminates

Test method Resin type Strength [MPa]/ Standard deviation [MPa]

Modulus / Standard deviation [MPa]

Elongation / Standard deviation [%]

Tensile properties

EP 741.7 ± 50.7 66.9 ± 4.3 1.09 ± 0.04 ()

PU 783.5 ± 36.6 63.6 ± 3.3 1.14 ± 0.04

Flexural properties

EP 970 ± 37.6 66.9 ± 2.8 1.66 ± 0.09

PU 950 ± 13.9 63.9 ± 4.1 1.75 ± 0.21

Interlaminar shearing properties

EP 67.5 ± 2.19 - -

PU 64.3 ± 1.25 - -

Impact strength [kJ/m²] Energy absorbed at max. force [J]

Charpy impact strength

EP 47.1 ± 2.63 1.61 ± 0.10

PU 53.9 ± 2.99 1.90 ± 0.11

Summary and Next Steps The aim of the study was the comparison of an Epoxy and a Polyurethane system capable

for HP-RTM process. The resins were investigated for their rheological behavior and industrial scale HP-RTM equipment was used to study the infiltration behavior of the resins based on carbon fiber fabrics with constant process parameters. The injection step at low press force during injection, the increase of the press force to obtain final part thickness and the curing step were analyzed in detail. Basic mechanical and physical properties of the manufactured laminates were evaluated for both resins.

The rheological investigations showed low viscosity of both resin systems suitable for the HP-RTM process. The Polyurethane exhibited lower mixing viscosity at equipment temperature whereas the viscosity at mold temperature was 85.4 % higher compared to the Epoxy resin. During the trials the feasibility of used Polyurethane and Epoxy resins to manufacture high quality laminates at short cycle time was demonstrated. The cavity pressure study showed acceptable reproducibility, however it shall be noted that a more precise process control may lead to smaller deviations in cavity pressures. The analysis of the injection step showed higher pressure increase rate at begin of the injection and higher maximum injection pressure for the Polyurethane due to the higher viscosity at mold temperature compared to the Epoxy resin. A cavity pressure increase at end of the flow length was not seen for the Polyurethane during injection step until the additional press force increase after injection to achieve final part thickness. The analysis of the process steps for press force increase to achieve final part thickness and curing of the resin showed that the press force increase led to simultaneous cavity pressure raise in the mold. The Polyurethane resin achieved higher maximum cavity pressure compared to the Epoxy resin. This may have been caused by higher Polyurethane viscosity and incomplete fiber impregnation leading to resin flow during compaction to final part thickness. A drop of the cavity pressures near injection gate was observed for both resins at specific time after injection start in curing step. It is assumed that the raise of viscosity started at end of the flow length where the gel point was reached earlier than near the injection port. The pressure drop near injection port was triggered by resin shrinkage from end the resin flow length towards injection port. This effect needs to be further investigated. The characterization and comparison of the laminate mechanics showed almost comparable properties for both resin systems having no significant differences in tensile, three-point bending and interlaminar shearing properties. The results of Charpy impact strength showed an increase of 14.4% for the Polyurethane impact strength. Concluded, the mechanical properties of the laminates were on equal level for both resin systems but extended tests have to be considered for further studies.

Polyurethanes have high potential for use in HP-RTM due to lower process temperatures and good infiltration behavior resulting in comparable mechanical properties and even improved impact strength compared to Epoxy resin. The results of this study will be validated in future by extended studies considering both resin chemistries from different suppliers to underpin the presented results. The research in field of HP-RTM process will further address cycle time reduction which demands highly precise process control. Advanced HP-RTM programs have been defined based on the investigations in recent years and the experiences gained with high-speed curing Epoxy and Polyurethane resins. One key for significant shorter cycle times (<3 min) is a cavity pressure controlled high-speed injection sequence with active end mold gap variation by a mold/press/injection device interface to control the cavity pressures during injection cycle. The combination of the extended process control with ultra-high speed curing resins with demolding time of less than 2 min after injection start will be the second key. This will enable the HP-RTM process for obtaining high-quality impregnation of functions-integrated complex automotive structures (e.g. with integrated sandwich structures) in competitive cycle time at acceptable cavity pressure level.

Bibliography 1. Chaudhari, Raman: “Characterization of high pressure resin transfer molding process variants for

manufacturing high performance composites”, phD Thesis, Fraunhofer Verlag, ISBN 978-3-8396-0669-8, 2013.

2. Henning, F.; Chaudhari; R., Karcher; M., Elsner, P.: High pressure RTM - A manufacturing process for large scale part production, in Proceedings of the Polymer Processing Society 28th Annual Meeting, PPS-28, 11-15 December 2012, Pattaya, Thailand, (2012).

3. Rosenberg, P., Chaudhari, R., Albrecht, P., Karcher, M., Henning, F., “Effects of process parameters on cavity pressure and component performance in High-Pressure RTM process variants”, SPE ACCE, Sept. 2014, Novi (Detroit), MI, http://www.speautomotive.com/SPEA_CD/ SPEA2014/pdf/ET/ET11.pdf.

4. Rosenberg, P., Chaudhari, R., Karcher, M., Henning, F., Elsner, P.: “Investigating cavity pressure behavior in high-pressure RTM process variants”, In: PROCEEDINGS OF PPS-29: The 29th International Conference of the Polymer Processing Society - Conference Papers. Nuremberg, Germany, 15–19 July 2013, American Institute of Physics (AIP Conference Proceedings), pp. 463–466, 2014.

5. Gardiner G.: “HP-RTM one the rise”, Article on www.compositesworld.com, 14th April 2015 http://www.compositesworld.com/articles/hp-rtm-on-the-rise.

6. Graf M., Fries E., Renkl J., Henning F., Chaudhari R., Thoma B.: “High-Pressure Resin Transfer Molding - Process Advancements”, in SPE Automotive Composites Conference & Exhibition 2010, 15-16th September (2010), Troy (MI), USA.

7. Chaudhari, R., Rosenberg, P., Karcher, M., Schmidhuber, S., Elsner, P., Henning, F.: “High pressure RTM process variants for manufacturing of carbon fiber reinforced composites”, 19th International Conference on Composite Materials, 28 July-02 August 2013, Montreal, Canada, 2013.

8. Khoun, L.; Maillard, D.; Bureau, M.N.: “Effect of Process Variables on the Performance of Glass Fibre Reinforced Composites Made by High Pressure Resin Transfer Moulding”, In: Proceedings of the 12th Annual Automotive Composites Conference and Exhibition (ACCE 2012), (2012).

9. Naoto Ikegawa, Hiroyuki Hamada, Zenichiro Maekawa: “Effect of Compression Process on Void Behavior in Structural Resin Transfer Molding”, Polymer Engineering and Science (April 1996), Vol. 36, No. 7, 1996.

10. Prabhas Bhat, Justin Merotte, Pavel Simacek, Suresh G. Advani: “Process analysis of compression resin transfer molding”, Composites: Part A 40, p. 431–441, 2009.

11. Cate P., Bland S., “VORAFORCE at Dow”, Reinforce Plastics, June 2015.

12. Henningsen M., Desbois P., Neuhaus B.; „Innovative Matrixsysteme für schnelle RTM-Prozesse“, CCeV Automotive Forum, 29-30 June 2011, Ingoldstadt. http://www.carbon-composites.eu/sites/carbon-Composites.eu/files/anhaenge/gruppen/12/08/29/08_praesentation_ basf_dr_hennigsen.pdf.

13. Kreiling S., Fetscher F.; “Progress with Polyurethane matrix resin technology: High-speed Resin Transfer molding processes and application examples”, SPE ACCE, September 11-13, 2013, Novi (Detroit), http://www.speautomotive.com/SPEA_CD/SPEA2013/pdf/TS/TS1.pdf.

14. Grunberg L., Nissan A.; “Mixture Law for Viscosity”, Nature (November 5), Vol.164, p.799-800, 1949.

15. Arrhenius Z.; physic. Chem., 1, 282, 1887.

characterization of epoxy and polyurethane … · characterization of epoxy and polyurethane resin...

Documents