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2013-12-19

Studies of Environmental Stress Cracking of High

Density Polyethylene Liner in Alkaline Surfactant

Polymer Enhanced Oil Recovery Floods

Li, Zhong

Li, Z. (2013). Studies of Environmental Stress Cracking of High Density Polyethylene Liner in

Alkaline Surfactant Polymer Enhanced Oil Recovery Floods (Unpublished master's thesis).

University of Calgary, Calgary, AB. doi:10.11575/PRISM/27913

http://hdl.handle.net/11023/1215

master thesis

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UNIVERSITY OF CALGARY

Studies of Environmental Stress Cracking of High Density Polyethylene Liner in Alkaline

Surfactant Polymer Enhanced Oil Recovery Floods

by

Zhong Li

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

CALGARY, ALBERTA

DECEMBER 2013

© Zhong Li 2013

ii

Abstract

 

Alkali surfactant polymer (ASP) flooding is an enhanced oil recovery (EOR)

process that is used to increase the amount of crude oil that can be extracted from

conventional oil reservoirs. With the increasing use of high density polyethylene (HDPE)

pipes in EOR process, environmental stress cracking (ESC) poses a threat to the

integrity of the pipe in ASP floods.

In this research, the ESC behaviour of HDPE pipes was studied in ASP floods under

conditions that simulated the EOR reality. Particularly, tensile tests were conducted on

three types of materials, i.e., HDPE4710, HDPE3608 and polyethylene 100+ (PE100+),

which have been soaked in ASP under various pressures and temperatures. Results

demonstrate that HDPE can experience degradation upon ASP soaking, resulting in a

reduction of elongation of the material, especially at elevated temperatures. Moreover,

HDPE 4710 is more resistant to ASP soaking in terms of the stress-strain behavior,

while 3608 is least resistant to ASP soaking.

The resistance of HDPE to ESC was also investigated on pre-cracked specimen in

ASP flood with various concentrations. Various analysis techniques, including

differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron

microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), were used to

characterize their ESC behaviour mechanistically. It was found that ASP floods

concentration, applied stress level, ambient temperature and the mechanical

characteristics of various HDPEs affects the ESC behaviour on the HDPEs.

Furthermore, the water permeation property of HDPE 3608 was investigated by

iii

determining the water vapour transmission (WVT) rate at various temperatures and the

specimen thicknesses. A model was developed to illustrate the parametric effects on the

water permeating rate. It was found that WVT is quadratic function of ambient

temperature and power function of film thickness. As a result of combination of three

parameters, it can be found that ambient temperature is quadratic function to WVT and

thickness is quadratic coefficient of temperature term. The modeling results are

validated by the experimental data.

iv

Acknowledgements

 

I would like to express my sincere gratitude to my respected supervisor, Dr. Frank

Cheng, for his instruction, encouragement and support throughout my whole research

career here at the University of Calgary. His enthusiasm, diligence, perception and

professional attitude to science and research have been inspiring and guiding me to

move forwards in my future life and career.

Appreciations are absolutely worth extending to the members in my group, Dr.

Luyao Xu, Dr. Yang Yang, Xun Gong, Dr. Cheng Zhong, and Dr. Ruiling Jia and those

whose names cannot all be listed here, for their unforgettable support and generous help

in this work.

This work was supported by Canada Research Chairs Program, Natural Science and

Engineering Research Council of Canada (NSERC) and Husky Energy.

v

Dedication

This work is dedicated to my parent for their incessant and altruistic support.

vi

Table of Contents

Abstract ·············································································· ii 

Acknowledgements ································································ iv 

Dedication ··········································································· v 

Table of Contents ·································································· vi 

List of figures ······································································ ix 

List of Tables ······································································· xi 

List of symbols, abbreviations and nomenclature ···························· xii 

Chapter 1: Introduction ···························································· 1 

1.1 HDPE pipes in industry uses ··························································· 1 1.2 Research background ··································································· 2 1.3 Objectives ················································································ 3 1.4 Structure of thesis ······································································· 3 

Chapter 2: Literature review ······················································ 5 

2.1. Development of HDPE pipe technology ············································· 5 2.1.1. Fundamentals of PE materials ················································ 5 2.1.2. HDPE and its application as a pipe material ······························· 7 

2.2. Technical testing of HDPE materials ················································ 9 2.2.1. Mechanical testing ······························································ 9 2.2.2. Burst testing ····································································· 11 2.2.3. Environmental stress cracking testing ······································· 12 2.2.4. Permeability testing ···························································· 13 

2.3. Environmental stress cracking behaviour of HDPE materials ··················· 14 2.3.1. Principles ········································································ 14 2.3.2. Testing and assessing methodologies ········································ 15 2.3.3. Factors contributing to ESC ·················································· 18 2.3.4. State-of-the-art of research on ESC of HDPE ······························ 19 

2.4. Alkaline-surfactant-polymer flooding in enhanced oil recovery ················ 20 2.4.1. Principle ········································································· 20 2.4.2. Actual applications ····························································· 21 

Chapter 3: Experimental ························································· 23 

vii

3.1. Materials and solutions································································ 23 3.2. HDPE pipe soaking ···································································· 24 3.3. Tensile testing ·········································································· 26 3.4. Environmental stressing testing on pre-cracked specimens ······················ 26 3.5. Water permeability measurements ··················································· 27 3.6. Microstructure characterization ······················································ 28 

Chapter 4: Effect of ASP soaking on mechanical properties of HDPE

materials ··········································································· 30 

4.1. Introduction ············································································· 30 4.2. Tensile testing ·········································································· 31 

4.2.1. Testing on as-received materials ············································· 31 4.2.2. Testing on ASP-soaked specimens in air ···································· 32 4.2.3. Testing on specimens soaked at various temperatures ···················· 33 

4.3. Discussion ·············································································· 34 4.3.1. Effect of internal pressure of ASP fluid on mechanical properties of HDPE ··················································································· 34 4.3.2. Effect of surfactant on mechanical degradation of HDPE in ASP fluid · 36 4.3.3. Effect of temperature on mechanical degradation of HDPE in ASP fluid ··························································································· 37 

4.4. Summary ················································································ 37 

Chapter 5: Environmental stress cracking behavior of HDPE in ASP fluid

······················································································ 39 

5.1. Introduction ··········································································· 39 5.2. Results ·················································································· 39 

5.2.1. Effect of ASP concentration ··················································· 39 5.2.2. DSC measurements ····························································· 41 5.2.3. Measurements of Fourier transform infrared (FTIR) spectroscopy ····· 43 5.2.4. SEM characterization ·························································· 44 5.2.5. XRD characterization ·························································· 46 

5.3. Mechanistic aspects of environmental stress cracking of HDPE ················ 47 5.3.1 Effect of surfactant ······························································ 47 5.3.2 Effect of stress ··································································· 48 5.3.3 Effect of the type of HDPE material ·········································· 49 

5.4. Summary ················································································ 50 

Chapter 6: Permeability of HDPE to water ··································· 52 

6.1. Effect of HDPE specimen thickness ················································ 52 6.2. Effect of temperature ·································································· 53 6.3 Water permeability model ····························································· 54 

6.3.1 Mechanism of water permeation through a HDPE membrane ············ 54 6.3.2 The Experimental model the dependence of WVT on temperature and membrane thickness ··································································· 56 

viii

6.3.3 The experimental model of WVT-thickness at certain temperature ······· 60 6.4. Mechanistic aspects of water permeability of HDPE ····························· 62 

6.4.1 Temperature ······································································ 62 6.4.2 Thickness of HDPE3608 film ·················································· 63 

6.5. Summary ················································································ 64 

Chapter 7: Conclusions and recommendations ······························· 66 

7.1. Conclusions ············································································· 66 7.2. Recommendations ····································································· 68 

References ········································································· 69 

ix

List of figures

Fig. 2.1 High pressure process in making PE ··············································· 5 

Fig. 2.2 Low pressure process in making PE ··············································· 6 

Fig. 2.3. Polyethylene molecular chain structure ··········································· 6 

Fig. 2.4. Typical HDPE composites pipe [16] ·············································· 8 

Fig. 2.5 The micro scale of tensile ductile fracture [23] ·································· 10 

Fig. 2.6 Theoretical stress-strain curves of HDPE after immediate application of internal stress [25]. ············································································ 11 

Fig. 2.7 Internal pressure and hoop stress, axial stress ··································· 12 

Fig. 2.8 Brittle Fracture; (a) lamellae start pulled away, (b) the tie-molecules are stretched tight, (c) clean break [33]. ························································· 15 

Fig. 2.9. Schematic representation of bent strip method [34]. ··························· 16 

Fig. 2.10. Schematic representation of tensile creep test system [35]. ·················· 17 

Fig. 2.11. Schematic representation of ball or pin-impression method [36]. ··········· 17 

Fig. 3.1 (a) Pressurized HDPE pipe samples (b) dimension of specimen (c) the location of tensile specimen in the pipe ···················································· 24 

Fig. 3.2 Pre-cracked specimen stressing testing samples and dimension of specimen 27 

Fig. 3.3 Testing set up of the water permeability measurements. ······················· 28 

Fig. 4.1. The stress-strain curves measured on three as-received HDPE materials in air. ······························································································· 31 

Fig. 4.2 The stress-strain curves measured on ASP soaked specimen in air. ··········· 33 

Fig. 4.3 The stress-strain curves of HDPE3608 after 3 months of soaking at 25°C and 75°C. ······················································································· 34 

Fig. 5.1 The Failure Points of HDPE4710, HDPE3608 and PE100+ specimens soaked in ASP fluid with different concentrations at 50°C. (a) 7.5%; (b) 10%; (c) 12.5%; (d) 15%. ··············································································· 40 

Fig. 5.2 The F50 value of three types of HDPE materials in ASP fluids. ··············· 41 

Fig. 5.3 DSC curves measured at 1/2 notch, 3/4 notch and endpoint of three HDPE specimens at a heating rate of 10oC min−1. (a) HDPE4710; (b) HDPE3608; and (c) PE100+. ························································································· 42 

Fig. 5.4. Disentanglement energy of the feature points in each specimen. ············· 43 

Fig. 5.5 FTIR spectra measured at 1/2 notch, 3/4 notch, 7/8 notch and endpoint on the specimen (a) HDPE4710; (b) HDPE3608; and (c) PE100+. ························ 44 

Fig. 5.6 SEM views of specimens after 300 hours of soaking in 10% ASP fluid. (a) HDPE4710; (b) HDPE3608; and (c) PE100+. ············································· 45 

x

Fig. 5.7 Typical XRD spectrum of HDPE4710, HDPE3608, PE100+ range from 10

o to 90o at the scan rate of 2o min−1, unmilled HDPE4710 and crystallinity of HDPE4710, HDPE3608, PE100+ which calculated depends on XRD. ················ 47 

Fig. 6.1 The amount of water evaporated through a HDPE3608 membrane per unit area versus the time for three different thickness of the HDPE3608 membrane. The experiment was conducted under three different temperatures: (a)25°C, (b) 50°C, and (c) 75°C. Linear fits to the evaporation data have been performed and are shown in the figures as dashed lines. ··················································· 53 

Fig. 6.2 The amount of water evaporated through a HDPE3608 membrane per unit area versus the time for three different temperatures. The experiment for three different membrane thicknesses: (a) 1mm, (b) 1.5mm, and (c) 2mm. Linear fits to the evaporation data have been performed and are shown in the figures as dashed lines. ····························································································· 54 

Fig. 6.3 The physical process of water vapor transmission ······························ 55 

Fig. 6.4. The relationship between WVT and temperature. ······························ 56 

Fig. 6.5 The mathematical relationship between 2a and the membrane thickness. ··· 58 

Fig. 6.6 The relationship between |log (|WVT|)| and |log (t)| for experiments conducted at three different temperatures. ················································· 61 

xi

List of Tables

 

Table 3.1 Characteristics of HDPE Pipes Used in this Work ···························· 23 

Table 3.2 Characteristic of ASP floods Used in this Work ······························· 23 

Table 3.3 Parameters used in pressure rating calculation ································· 25 

Table 3.4 Test pressure based on HDPE pipe pressure rating ···························· 25 

Table 4.1 Summarize the yield points and elongation of each conditions. ············· 33 

Table 4.2 Strength-loss rate and the elongation-loss rate of each condition ··········· 35 

Table 5.1 Ic, Ia and Crystallinity of HDPE4710, HDPE3608, PE100+ ················· 46 

Table 6.1. The WVT of HDPE3608 measured under various membrane thickness and temperatures. ·············································································· 52 

Table 6.2 Saturated Water Vapor Pressures at Different Temperatures ················· 56 

Table 6.3 The values of the temperature sensitivity coefficient in WVT-Temperature formula. The subscript values on a denote the thickness (in mm) of the HDPE membrane studied in each case. ····························································· 57 

Table 6.4 The value of m, n, S, r, q for HDPE3608 WVT testing ······················· 60 

Table 6.5 The values of C and m of each temperature. ··································· 62 

Table 6.6 Average relative humility of the air and the saturated vapor pressure ······ 63 

xii

List of symbols, abbreviations and nomenclature

ASP Alkali Surfactant Polymer

EOR

HDPE

PE

Mw

Tm

Tg

Enhance Oil Recovery

High density polyethylene

polyethylene

Molecular weight

Melting temperature

Glass transition temperature

a(σ) time–stress superposition function

D(t) tensile creep compliance function

εc Creep strain

σ Stress

HDS hydrostatic design stress at 73°F, psi

fE environmental design factor

fT operating temperature multiplier

DR pipe dimension ratio

σair yield strength in air

σsol the yield strength in ASP flooding solution

Ibe

ESC

ESCR

breaking elongation-loss rate

environment stress cracking

environment stress cracking resistant

F50 the time when the fifth fails in a ten-specimen test, 50 % failure point

DSC

FTIR

SEM

Differential Scanning Calorimetry

Fourier Transform Infrared Spectroscopy

Scanning Electron Microscope

XRD X-Ray Diffraction

Ia

Ic

peak intensities for crystallinity

peak intensities amorphous

xiii

Xc crystallinity

∆g weight loss

A exposed area of the coating, m2

ED effective activation energy for diffusion

D water vapour diffusion constant

S water vapour solubility coefficient

ΔP the gas partial pressure

xiv

1

Chapter 1: Introduction

1.1 HDPE pipes in industry uses

High-density polyethylene (HDPE) is a plastic material which has a linear structure

with very little or no branching and possesses a high degree of crystallinity [1]. In

recent years, HDPE pipes have been widely used in various industrial applications, such

as energy transmission, water supply and drainage, and heating systems[1, 2], due to

their remarkable advantages, including an excellent corrosion resistance, low cost,

convenient installation in the field, etc. Moreover, HDPE does not contaminate the

medium which has been conveying. It can be recovered and recycled. At present, HDPE

is the third-largest commodity in plastic material [2, 3].

Since the 1950s, HDPE has become an excellent pipe material in industry because

of its superior performance, which has greatly changed the pipe systems in

manufacturing and distributing. HDPE pipes are relatively easy to lay, maintain, and

renew, making it easy for use in large-scale applications. Nowadays, HDPE pipes have

been widely used in energy transportation systems. HDPEs are preferred over steel

pipes in corrosive environments because they are highly resistant to corrosion. This

means that using HDPE pipes can save significant costs incurred for corrosion

prevention [4].

The disadvantages of HDPE as a pipe material include potential of stress cracking,

high degree of thermal expansion, and anisotropic mechanical properties [5]. It has been

reported that polyethylene (PE) is sensitive to environmental stress cracking (ESC) [6].

The combination of long-term loads, even small in magnitude, with certain

environmental conditions often results in premature failure of PE, including HDPE,

2

leading to cracking of the stressed PE materials. These cracks are generally thought to

initiate at microscopic imperfections and propagate through the crystalline regions of

the polymer structure. Furthermore, the presence of surface active wetting agents such

as alcohols, soaps and various surfactants can be responsible for the ESC of HDPE [7].

It has been found in industry that the fracture would not occur in any reasonable period

of time in the absence of surfactant in the environment [8]. Moreover, the ability of a

polymer to resist slow crack growth is known as ESC resistance, which is strongly

dependent on their structures and environmental conditions such as solution chemistry,

temperature and stress [9].

1.2 Research background

Petroleum production from the Enhance Oil Recovery (EOR) method has continued

to increase in the past decades [10]. The EOR method uses alkaline–surfactant–polymer

(ASP) flooding to reduce the interfacial tension and mobility ratio between oil and

water phases, thus increasing the oil production [10]. In the ASP process, low

concentrations of an alkali chemical (e.g. NaOH, KOH) and surfactant are injected into

the reservoir. The surfactant is used to achieve ultra-low interfacial tension between the

trapped oil and the formation water. The ultra-low interfacial tension allows the alkali in

the injection fluid to deeply penetrate the formation and react with the acidic

components in the crude oil to form additional surfactants. Polymer is sometimes added

to increase the viscosity of the injection fluid to minimize channeling and provide

mobility control. Depending upon the reservoir characteristics and applicable EOR

process, an additional 10% - 30% of the oil can be recovered compared to primary or

secondary recovery methods.

3

Large pipeline networks are required to transport the ASP fluids to injection wells

and the produced fluids (combination of crude oil, water and residual ASP fluids) to a

central processing facility. Due to the high corrosion resistance, HDPE pipes and/or

HDPE lined steel pipes are commonly used in oil production systems. Industrial

experience confirmed the occurrence of ESC of HDPE pipes in ASP flooding during

petroleum production [11]. However, there has been no relevant study committed to

understand the mechanistic aspects of the ESC behaviour of HDPE in this environment.

1.3 Objectives

The overall objective of this research is to develop a mechanistic understanding of

the ESC behaviour of HDPE materials under environmental conditions that are relevant

to ASP EOR in petroleum production. Progress has been made in the following areas:

-- The effects of ASP floods soaking on the mechanical properties of three industry

used HDPE material have been determined under various temperature and pressure.

-- The effects of ASP floods soaking on the ESCR of three industry used HDPE

material have been determined under ASP fluid concentration, various time

recording and analysis techniques.

-- The effects of water permeation behaviour of HDPE3608 materials have been

determined under specimen thickness and temperature, and develop a model to

quantify the water permeation through HDPE membrane.

1.4 Structure of thesis

The thesis contains seven chapters, with Chapter 1 giving an introduction of

research background and objectives.

Chapter 2 contains a comprehensive review of literature relevant to this research.

4

This includes a discussion of the basic properties of HDPE, specifically its high-strength

and non-conducting properties, and the state-of-the-art of HDPE in applications as a

pipeline materials, as well as a pipeline coating for steel pipes. Furthermore, the current

theories regarding ESC behavior of HDPE materials are presented.

Chapter 3 describes the experimental set up, including specimens and solution

preparation, testing methods, measurement techniques and analysis methods.

Chapter 4 discusses the effect of ASP soaking on mechanical properties of three

types of HDPE materials under various pressures and temperatures. The degradation of

mechanical properties of HDPE in ASP floods is discussed.

Chapter 5 describes the ESC resistance of pre-cracked HDPE materials in ASP fluid

with various concentrations. A number of analysis techniques are used to characterize

the crystallinity, structure and properties of HDPE upon ASP soaking. The resistance to

ESC of three types of HDPE material is ranked finally.

Chapter 6 describes the water permeability of HDPE 3608 material under various

temperatures and specimen thicknesses. A model is developed to quantify the water

permeation through the HDPE membrane.

Finally, the main conclusions of this research are summarized in Chapter 7, together

with the recommendations for the future work.

5

Chapter 2: Literature review

2.1. Development of HDPE pipe technology

2.1.1. Fundamentals of PE materials

The PE is a semicrystalline polyolefin thermoplastic material with a favourable

property combination which has been widely used in various industrial applications

including the petroleum production. PE consists of the basic repeating unit (-CH2–CH2-),

as shown in Fig. 2.1, with both crystalline and amorphous phases. The crystalline

lamellae provide structural integrity of PE, while the amorphous parts provide its elastic

properties. Currently, polyethylene manufacturing processes generally classified in

"high pressure" and "low pressure" operations. The former is generally applied in

producing conventional low density polyethylene (LDPE) and the latter makes high

density (HDPE). The “high pressure” operation was first produced in England in the

1930s. In high pressure, ionized ethylene gas could be converted into solid phase by

heating on the presence of oxygen trace amounts (a little confuse what you write in

there):

Fig. 2.1 High pressure process in making PE

The initial discovery of the low pressure process was an accident in 1952. In

Germany, this process was discovered when a new aluminium based catalyst was found,

Ethylene + (gas)

108 - 3×108 bar 80 – 300oC

< 10 ppm oxygen Polyethylene (solid)

6

successfully allowing for the ethylene polymerization process at much lower pressure

than the former operation [12]:

Fig. 2.2 Low pressure process in making PE

 

Fig. 2.3 Polyethylene molecular chain structure

The density of PE which generally range from 0.880 to 0.960 g/cm3 determines the

mechanical properties of PE such us strength, elongation, thermal stability [13]. From

the point of molecular chain, there are two parameters influence the properties, chain

length and degree of side branching. Chain length is directly related to polymer strength,

chemical inertness and high corrosion-resistance. With increasing chain length, strength,

elongation, thermal properties (list the “former”) improved correspondingly. Side

branching in PE is the random bonding of short polymer chains to the main polymer

chain. Entanglement means the topological restriction of molecular motion by other

chains. Since branched chains are unable to entangle very tightly, the resulting PE had a

relatively low density relatively poor yielding strength and elongation at break. On the

contrary conditions, the resulting PE had a relatively high density and it had advanced

performance. The thermal stability is also determined by the density of the resultant PE.

The density is primary controlled by the formation of crystalline regions and the

Ethylene + (gas)

106 – 8×106 Pa

70 – 300°C Al-based catalyst

Polyethylene (solid)

7

quantity of crystalline materail in relation to amorphous regions of the material. In

particular, the crystalline regions have chains that interact move strongly than those in

the amorphous regions, yielding enhanced mechanical and thermal properties. Therefore,

as crystallinity increases so does. [14].

In pipeline application, HDPE is preferred compared to middle density PE (MDPE)

or low density PE (LDPE).

2.1.2. HDPE and its application as a pipe material

HDPE, with fewer branches than MDPE or LDPE, has a greater proportion of

crystals, which results in greater density and greater strength. The density of HDPE is

range from 0.93 to 0.97 g/cm3 while the yield strength typically lies between 22MPa to

31MPa and maximum elongation ranges from 10% to 1200% [1]. Moreover, HDPE has

the distinction of being the strongest, toughest, most chemical resistant and least

flexible of the three types of PE. The strength of HDPE comes from its tightest cell

structure than MDPE and LDPE. When applications ask for very large liners such as

pond liners, HDPE is the most easily seamed together. HDPE is used as secondary

containment liners for oil system, and most widely used in industrial ponds and canal

liners, where chemical resistance is necessary [15].

HDPE has been used either solely or as a key component to make pipes and

composite pipes in petroleum production. The dominant advantage of HDPE pipes is

their corrosion resistance in environments relevant to petroleum production and

transportation oil. In composite pipes, HDPE can be used as an inner liner or outer

“coating” to steel pipe, as shown in Fig. 2.2 [16]. As a linear to steel pipes, the smooth

surface of HDPE provides excellent flow characteristics. The HDPE liner also has

8

ability to support large loads. As outer coatings to steel pipes, the HDPE’s high

chemical resistance helps the steel pipe resist corrosion from the surrounding. Moreover,

HDPE can be combined with other materials, such as fusion bonded epoxy (FBE), to

create high performance composite coating (HPCC) to protect steel pipe from corrosion

attack. FBE and composites pipes have two-layer or three-layer while normal HDPE

pipe has one-layer, which means that FBE and composites have better performance on

protection [16].

Fig. 2.4 Typical HDPE composites pipe [16].

The HDPE pipes were produced and used in market as early as 1955 [17].

Nowadays, the HDPE pipes have been increasingly used in petroleum industry. In the

US, HDPE offered an important role as a pipe or pipeline in oil field industry. In the

1950s and 1960s, HDPE was discovery by the gas company as a cost effective choice to

coat and wrap steel. In western Canada, the original HDPE pipe installations were

completed over 10,000 km of lining system is in variety of service conditions from 1986

until now [17].

In adverse environments, HDPE can achieve the required reliability goals, and also

can save cost. For example, HDPE composite pipes do not need cathodic protection and

the associated maintenance. HDPE is a non-conducting material and it was determined

that no electrochemical reaction occurred on HDPE [18]. There is almost no change in

9

chemical resistance of HDPE for exposure periods of greater than 3 months even though

the chlorine ion content in steel pipe is relatively high for non-conducting. This suggests

that chemical resistance of HDPE is excellent. In water distribution and transmission

applications, HDPE pipe is widely accepted for its corrosion resistance while the steel

pipe is highly corrosive in a tap-water filled pipe. Additionally, the excellent corrosion

resistance and beneficial maintenance of HDPE pipe provide for a distinct and

significant long-term cost advantage over steel pipe [7, 8].

2.2. Technical testing of HDPE materials

2.2.1. Mechanical testing

Tensile testing has been a standard method to determine the mechanical properties

of HDPE pipe materials. There are two methods for mechanical testing, i.e., ASTM

D638 [20] and ISO527 [21], where specimens are installed in the grips of the instrument

and pulled under a certain strain-rate until failure. In the ASTM D638, the test speed is

determined by the material specification and full-range value of instrument. In the

ISO527, the typical test speed is 5 or 50 mm/min for measuring the strength and

elongation. The main difference between the two standards is the tensile modulus

measurement. For tensile modulus, according to ISO 527-1 is required an extensometer

with a gauge length of 50mm and an accuracy of 1%. In the ASTM Standard there are

no fixed strain limits for the determination of modulus.

The point at which a stress causes the specimen to deform beyond the elastic limit,

or the point where the specimen is permanently deformed, is the yield point. The

deformation resulted from tensile testing can be observed on the specimen under

sufficient stress levels [22].

10

Fig. 2.5 The micro scale of tensile ductile fracture [23]

The tensile test process includes three stages. The first stage is elastic deformation

which is temporary deformation occurring before the yield point, as shown in the first

two steps in Fig. 2.3. During this stage, when the applied stress is released, the

specimen can return to its original shape. There is no visible deformation observed. The

second stage occurs after yielding. The load is at a relatively stable level between the

yield point and the beginning of strain hardening, which is when the material is strained

beyond the yield point as the strain and stress increase. The deformation in second stage

is attributed to a combination of amorphous phase rearranging and crystal lamellae

slipping past each other while the individual crystal is still intact. The deformation

degree of amorphous phases and the crystalline orientation become larger with the

increasing stress and strain. Strain hardening will occur immediately. The amorphous

phase has reached its full extension during the strain hardening. Further deformation

occurs gradually, as shown in the third and fourth steps in Fig. 2.3. With a further

increase in the stress and strain, ultimate failure occurs and the sample is broken as

shown in the final step in Fig. 2.3. The characteristic of the breaking area is a rough

fibrous surface.

11

2.2.2. Burst testing

The burst testing is usually applied in analyzing the rapid or slow crack propagation

for pipelines while a constant internal pressure is applied by inert gas. The crack

propagation strongly depends on the internal pressure value. A full scale internal

pressure generally results in a rapid loading, which can break the aged pipes rapidly.

[24]. Relative small internal pressures, which generate slow rate of loading, can be used

to predict the slow crack growth. The relationship between stress and strain is shown in

Fig. 2.4 upon immediate application of internal pressure on HDPE pipes.

Fig. 2.6 Theoretical stress-strain curves of HDPE after immediate application of

internal stress [25].

12

Fig. 2.7 Internal pressure and hoop stress, axial stress

(a) Section (b) Isometric

Furthermore, the internal pressure generates hoop stress which usually leads to the

axial cracking. Axial cracking is generally due to the slow crack growth (SCG) through

the HDPE pipe. The crack propagates at a relative constant speed after initiation

because the internal pressure value is usually constant. The cracking will arrest after a

short distance. In most cases, the strength and thickness of HDPE pipes are sufficent to

prevent continuing crack propagation.

2.2.3. Environmental stress cracking testing

The ESC results of long-term small loads under specific environmental conditions,

which often results in the premature failure of HDPE structures. The ESC failure is

characterized by the presence of macroscopic ‘cracks’ on the material surface.

Microscopic fibrils are often observed around the fracture surface or crack tip. The ESC

testing always focuses on a certain fatigue life of materials under certain environmental

13

conditions, such us temperature, stress level, and immersion fluid [26, 27].

During ESC test, standard specimens are notched on the surface with specified

length and depth. The specimens are then bent into 180o and confined in a metal holder.

The entire holder, including the notched specimens, is when immersed in a corrosive

solution at constant temperature. Periodically, the specimens are observed by naked

eyes to observe visual cracks perpendicular to the cuts. At each inspection, the number

of cracked specimens is recorded. The test duration varies from 24 to 1000 hours,

depending on the various HDPE applications [28].

2.2.4. Permeability testing

The permeability testing of HDPE is important since HDPE pipes have been widely

used in transportation of various fluids including petroleum hydrocarbons. Water

permeability testing can be conducted by either wet cup method or dry cup method. For

the former, the test specimen is a HDPE sealed water-containing cup. The assembly is

placed in a chamber, such an oven, or in air. For the dry cup method, HDPE acts as

sealed cover of a containing desiccant cup. In both methods, the HDPE film thickness,

temperature and the types of the HDPE can be changed for the purpose of comparison.

The permeability of a HDPE film depends on the weight change of the cup assembly

and reflects the permeability of the film [29]. From the macroscopic point of view, the

permeability of a material mainly depends on its density. Moreover, the permeability of

the material also depends on temperature which can influence the relative humidity in

air and the evaporation rate of solvent/water. The thickness of the film is important

because it affects the amount of solution transmitted. Generally, high density materials

would have a low permeability to oxygen, water vapor, carbon dioxide, and aromatic

14

substances. The smaller molecular size of permeate usually results in a higher

permeability [30].

2.3. Environmental stress cracking behaviour of HDPE materials

2.3.1. Principles

The ESC in polymer materials means that the material failure is due to the

continuing external or internal stress in the presence of environmental substances at a

certain temperature. The surface active substances presents in the environment, such as

soaps, alcohols, chemicals containing moisture, are known as the stress cracking agents.

ESC of polymers is similar to the stress corrosion cracking process in metals.

Howard [31] described the relationship between the solvent activity and the

polymeric structure, and suggested that ESC is a purely physical process, i.e., there are

no chemical changes occurring in the polymer. In oxidative stress cracking, obvious

chemical changes are observed and the most apparent being manifested by the marked

reduction in the overall strength properties. Further, Howard suggested that the most

efficient cracking agents are liquids with high polarity, low viscosity and low surface

tension. Brown et al. pointed out that the speed of crack growth in polymer depends on

the disentanglement degree of molecules in the crystals. The number of tie molecules

and the crystalline degree of polymer are considered as the controlling factors [31].

Various molecular mechanisms of ESC had been proposed during recent years.

Lustiger et al. [32] proposed that the inter-lamellar failure is the controlling mechanism

of ESC and the tie molecules concentration is an influencing factor. The HDPE material

comprises an ordered crystalline region and a random amorphous region. The crystalline

region consists of packs of folded molecules named lamella, which are separated by the

15

amorphous region. The inter-crystalline polymer chains play an important role in the

deformation. There are three types of inter-crystalline chains. Tie molecules begin and

end in adjacent lamellae.

Fig. 2.8 Brittle Fracture; (a) lamellae start pulled away, (b) the tie-molecules are

stretched tight, (c) clean break [33].

When the stress is low, tie molecules disentangle slowly and relax with time. As

shown in Figs. 2.6(a) and 2.6(b), in the initial step of brittle fracture, the amorphous

materials start to stretch under stress. After a certain time, the inter-lamellar links start to

relax and untangle from each other until the number of remaining linkages becomes

very low. When the few remaining inter-lamellar links are stretched to their limit, they

are unable to pull apart lamellae, and consequently, the brittle fracture of the polymer

occurs, as shown in Fig. 2.6(c).

2.3.2. Testing and assessing methodologies

The testing and assessing methodologies of ESC include bent strip method, tensile

creep test and ball or pin-impression method.

16

The bent strip test is carried out according to the standard ISO22088-3 [38]. As

shown in Fig. 2.7, the specimen with a constant radius of curvature is fastened on a

plate. The stress value is controlled by adjusting the curvature radius of the specimen.

The specimen is immersed in the testing agent, and the specimen morphology is

observed by naked eyes.

Fig. 2.9 Schematic representation of bent strip method [34].

The tensile creep test system follows ISO 22088-2 [35]. As shown in Fig. 2. 8, the

specimen is loaded under a constant stress which is lower than its yield strength.

Meanwhile, it is immersed in a liquid at a specified temperature. The time and stress are

recorded. In this way, the tensile creep strength is defined as the tensile stress at break,

which is considered as a function of time to fracture.

17

Fig. 2.10 Schematic representation of tensile creep test system [35].

In the ball or pin-impression test, a fixed diameter hole is drilled in the specimen,

and an oversized ball or pin is put into it. The specimen is soaked in a test liquid. The

test method is repeated with gradational increasing diameters of balls or pins. As the

diameter increases, a time-constant multiaxial deformation is formed and grows

surrounding the hole. The crack morphology can also be observed by naked eyes [36].

Fig. 2.11 Schematic representation of ball or pin-impression method [36].

18

2.3.3. Factors contributing to ESC

The ESC behavior of polymers, including HDPE, is closely related with the

concentration of the surface-active agent, the level of internal stress or applied stress,

environmental temperature and time.

The presence of stress is critical to the ESC occurrence. There will be no stress

cracking occurring if the HDPE is free of stress. The swelling phenomenon is observed

when HDPE is immersed into liquids in the absence of stress. In practical applications,

the presence of stress is inevitable, which is generally caused by the flow of liquid in

HDPE pipes or residual stress from the manufacturing process. Thus, ESC of HDPE

occurs frequently in reality.

Recently, Howard has further studied the interfacial tension between the solution

and the polymer substrate as a critical variable in ESC [37]. From the perspective of the

surface-active agent, ESC occurs due to absorption of the surface-active agent. This

process locally reduces the yield strength of the material surrounding voids area, and

voids gradually expand and finally lead to fracture [38]. In general, surface active agent

will decrease the surface energy of the polymer. The surface energy can be further

decreased with the increasing concentration of surface-active agents. But surfactants do

not cause any swell or dissolve molecular chains itself. The swelling or dissolve

phenomenon of polymer is due to water in the solution. Surface active agent reduces the

energy required to create new surfaces in the polymer which caused the low yield point.

The effect of temperature on ESC of HDPE is complicated process, but the

commonly accepted perspective that increased temperature allows for increased

mobility of the molecular chains [39]. From the immersion perspective, higher

temperature generated that increasing rate of the chemical agent in circumstance to

19

diffuse into the plastic resin. The process allows for more rapid crack initiation and

accelerated crack propagation. From the HDPE molecular structure perspective, higher

temperature promotes the movement of inside molecular chain. Only local polymer

backbone and side chains are possible movement when temperature below glass

transition temperature (Tg). HDPE acts like a rigid solid in this temperature range. When

temperature is higher than Tg, larger scale chain movements in the amorphous phase

result in polyethylene becoming more soft. At even higher temperatures than Tg,

crystalline lamellae in the HDPE melt resulting in further chain slippage for

temperatures increasing to the melting temperature (Tm). When the temperature becomes

higher than the Tm, HDPE loses its structural integrity and becomes a viscous melt. The

ESC behaviour occurs more frequently under higher viscosity of the molecular

movements or agent penetration [33].

2.3.4. State-of-the-art of research on ESC of HDPE

Lustiger suggested that the cracking of HDPE pipes in the industry can be

classified into three types. The first one is third-party damage which is the result of

improper construction practices. The second one is joint failure which caused by

improper joining conditions or a material deficiency that inhibits proper fusion. The last

one is material failure. Material failure is influenced by inherent properties of HDPE,

quality of pipe design and manufacturing process in the field [40]. Until now, most

research of ESC has focused on microstructural properties, such as tie-molecules,

molecular weight and influence factor from environment conditions. Huang and Brown

clarified and developed further the theory of tie-molecules which is based on the

molecular weight (MW) of the material and proposed that the long polymer helping to

hold lamellae together and further reducing the rate of crazing. ESCR of polyethylene

20

increases with average molecular weight increases, since the tie-molecule concentration

increases [41]. Cheng proposed that physical chain entanglements also contribute to the

formation of inter-lamellar linkages and combining (micro) molecular science and

structural mechanics. Cheng shows that an increase in ESCR occurs at a much faster

rate than increases in tie-molecules density [42]. The latest research direction of ESCR

focused on entanglement degree of molecular chain. Tensile test and DSC are two usual

techniques for ESC behavior of HDPE. The former can directly measure the mechanical

parameters of HDPE and the latter investigate the structure characteristic of molecular

chains in the HDPE.

2.4. Alkaline-surfactant-polymer flooding in enhanced oil recovery

2.4.1. Principle

The EOR is a technique for increasing crude oil production by floods with low

concentration surfactant which is used to achieve interfacial tension between crude oil

and the injection fluid. With the progress of technology in petroleum production, it

experiences primary oil recovery, second oil recovery and EOR. The principle of the

primary oil recovery uses natural pressure of the reservoir to push crude oil to the

surface which allows about 5% to 10% of the oil in the reservoir to be extracted. The

secondary oil recovery  injects pressurised gas and water to motive the crude oil

remaining after the primary oil recovery phase to the surface which allows additional 25%

to 30% of the oil in the reservoir to be extracted. In the EOR, the principle is injecting

different materials, such as ASP flooding or CO2 flooding to improve the flow between

oil, gas and rock, and to recover crude oil remaining after the primary and secondary oil

recovery phases which allows additional 20% to 30% of the oil in the reservoir to be

21

extracted [43].

The objective of the application of ASP flooding is to reduce interfacial tension

between oil and water phases to achieve high oil extraction efficiency by adding

alkaline, surfactant and polymer flooding [44]. In the ASP flooding, the surfactant can

reduce the interfacial tension between the oil and water phases and promote the

separation of oil and water. The alkaline agent reacts with the acids to have partially

neutralization reaction in the crude oil to generate in situ surfactant (usually soap) that

shows synergistic effects between each other. The polymer improves the water viscosity

that can influence the surfactant transport so that the surfactant can affect the HDPE

within a short time.

Surfactants are the core part of the ASP flooding, and many types of surfactants

have been used for the purpose. The surfactant molecules are composed of a

water-soluble (hydrophilic) and a water insoluble (hydrophobic) functional group.

Poly-oxy-ethylene and poly-oxy-ethylene-alkylether are the most commonly used

surfactants. Common alkaline chemicals used in ASP flooding are sodium hydroxide

and sodium carbonate [45]. Polymers which are used in ASP flooding must remain

stable for a long time. The two most commonly used polymers are polysaccharide

biopolymers and synthetic polyacrylamides. Chemicals contained in the ASP flooding

are nontoxic in order to avoid pollution to the environment.

2.4.2. Actual applications

While the primary and secondary recovery techniques are able to recover about

35−50% of the oil in place, the ASP flooding which is used in the EOR process has been

one of the technologies that can further improve the EOR by 35% [46]. Since the 1990s,

22

the ASP flooding has been used in nearly all oil fields in North America and China. In

America, the first application of the ASP flooding technology was performed in a

nearby Minnelusa field. Other ASP flooding projects included a series of pilot tests in an

Oklahoma field. In Canada, the ASP flood technology is now being applied to certain

mature reservoirs Husky in the Taber area, Black Creek, Crowsnest field and Suffield.

Oil rate from the pattern increased from 8 to 28m3/day during early stage of

development. As of December 2010, nine ASP projects had been approved. In Alberta,

the polymer pipe or ASP floods projects were active or planned during 2011. In China,

there are two successful applications in DaQing oil field and ShengLi oil field in China

[47, 48, 49].

The ASP flooding technology has been used in petroleum industry. Generally, the

average concentrations of individual components contained in ASP fluid are 1.28%,

0.28%, and 0.15% for alkali, surfactant and polymer, respectively [50]. For example,

typically consists of ASP flooding is 0.5-1% alkali, 0.1% surfactant and 0.1% polymer

in Alberta oil industry [51]. The concentration of surfactant is usually lower than that of

the alkaline because the latter can generate a synergistic effect to replace the partial

function of surfactant has high cost. Alkaline and polymer plays a relative synergetic

role in the ASP floods to help the surfactant to achieve the best efficiency in EOR

process.  

23

 

Chapter 3: Experimental

3.1. Materials and solutions

The materials used in this research were HDPE4710, HDPE3608 and PE100+ pipes

supplied by Husky Energy. The characteristics of the HDPE materials are shown in

Table 3.1, and the properties of the HDPE pipes are summarized in Table 3.2. These

three types HDPE are widely used for well mechanical performance, corrosive- resistant

performance and impact-resistant performance. Melt index is essentially an indirect, and

inversely proportional, measure of the viscosity of the polymer when molten.

Table 3.1 Characteristics of HDPE Pipes Used in this Work

Specimens Density (g/cm3)

Melt index (gm/10 min)

Pipe radius-thickness

ratio (DR) HDPE 4710 0.947-0.955 <0.15 17 HDPE 3608 0.941-0.943 0.05-0.11 17

PE 100+ 1.156-1.168 0.08 11

Table 3.2 Characteristic of ASP floods Used in this Work

pH ESC system (mass fraction)

Tensile tests (mass fraction)

7.5% 14

10% 12.5%

0.2%

15%

Tests were conducted in distilled water with the addition of an alkali (1.5% NaOH)

and surfactant, 0.5% surfactant, which has been commonly used in Western Canadian

ASP floods.

24

3.2. HDPE pipe soaking

To investigate the effect of ASP soaking on the HDPE properties, the as-received

HDPE pipes were soaked in ASP fluid under various conditions. The HDPE pipe

samples were filled with the ASP solution, mounted in a pressure-containing aluminum

jig assembly and pressurized to the respective test pressure. Fig. 3.1 shows the

experimental setup.

Fig. 3.1 (a) Pressurized HDPE pipe samples (b) dimension of specimen (c) the location

of tensile specimen in the pipe

The ASP soaking of HDPE pipes was conducted at various temperatures and the

applicable pressure rating for the HDPE pipe sample. According to WL118 pressure

rating [52], the pressure rating (PR) can be calculated by:

25

                   (Eq. 3-1)

where HDS is hydrostatic design stress at 73°F, psi, fE is the environmental design

factor, fT is operating temperature multiplier, and DR is the pipe dimension ratio.

Particularly, the HDS is the fE is determined by media and environment conditions., and

the fT is chosen by environment condition when the pipes be applied to transport

internal liquids, gases and external liquids that are chemically benign to polyethylene

which can been found in the table in WL 118. The DR is defined by the ratio of

diameter to thickness of pipe. Table 3. 3 shows the relevant parameters used in pressure

rating calculation.

Table 3.3 Parameters used in pressure rating calculation

HDPE4710 HDPE3608 PE100+

HDS (psi) 1000 800 1000 fE 1 1 1

fT (25oC/75oC) 0.98 (25oC) 0.97/0.30(25oC/75oC) 0.98(25oC)

Table 3.4 shows the test pressures and temperature for the applicable pipe samples

in this work. The high-purity nitrogen gas (99.999%) is purged into the pipe specimens

to achieve the inner pressure rating.

Table 3.4 Test pressure based on HDPE pipe pressure rating HDPE type Pressure (psi)

25°C 75°C HDPE4710 122 HDPE3608 97 37

PE100+ 196/125(practice)

In practice, the operating conditions for HDPE4710 and HDPE3608 pipes follow

their theoretical temperature/pressure values [52]. However, the internal pressure rating

2

1T EHDSf f

PRDR

26

for PE100+, 196 psi, is too high for the laboratory safety. The operating PR for PE100+

pipe was thus lowered to 125 psi. Although this likely resulted in a lower stress in the

PE100+ pipe sample, the stress is still higher than would be allowed in ASP service [53].

The HDPE pipes were cut into segments with a length of 14 cm. Both openings of each

segment were sealed with steel plates by high quality sealant and four fastening screws,

as shown in Fig. 3.1. ASP solution was added into each segment and nitrogen is purged

to increase the internal pressure.

3.3. Tensile testing

After soaking of 30 days, the individual HDPE pipe was cut and machined into test

samples. As showed in Fig. 3.2, the length, width and thickness of the tensile test

specimens were 115 mm, 6 mm and 4 mm, respectively. The tensile testing was

conducted on the soaked specimen in air through the SmartTest Test Instruments and the

WinTest Digital Control System. The stress-strain curve was recorded and the

mechanical properties were analyzed.

3.4. Environmental stressing testing on pre-cracked specimens

The pre-cracked specimens used for environmental stress testing were machined

according to standard ASTM D1693 [58]. Fig. 3.3 shows the geometry and set up of the

specimen in the testing rig. The specimens were immersed in ASP solutions (pH 14)

with various concentrations. The bath temperature was controlled at 50 oC. The number

and percentage of the specimens experiencing cracking as a function of time were

recorded. The F50 value will be the time when the fifth specimen fails in a ten-specimen

test, for each material was obtained according to ASTM D1693 [59]. The specimen are

machined in machine shop.

27

 

Fig. 3.2 Pre-cracked specimen stressing testing samples and dimension of specimen

(a) Test assembly, (b) Test sample, and (c) the location of ESCR specimen in pipe

3.5. Water permeability measurements

Using to ASTM D1653 [56], the water permeability of HDPE3608 was

characterized by the water vapor transmission rate (WVT). The facility for the water

permeating testing is shown in Fig. 3.4. The cup was sealed by the HDPE3608 film with

different thicknesses. The cup was filled with water and then weighed by a high

precision balance (0.0001 g). The cup was placed in an oven or in air in order to

maintain the experimental temperatures of 25, 50 and 75°C, respectively. The

corresponding relative humidity of the testing environment was 21%, 7% and 0%,

respectively. The relative humidity is the average value in experiment days. Whole

facility was weighted every 24 h. Nine parallel tests were performed to ensure the

28

reproducibility of results.

To calculate the WVT, a slope of the weight loss in gram versus time was calculated.

The relationships among WVT, temperature and the HDPE film thickness were studied

and an experimental-dependent WVT formula for HDPE3608 was developed to

estimate the WVT value under variable temperatures and sample thicknesses.

Fig. 3.3 Testing set up of the water permeability measurements.

3.6. Microstructure characterization

The crystalline structure of HDPE specimens before and after ASP soaking was

characterized by X-ray diffraction (XRD) through a D/MAX2500 model facility, with a

detector operation at 40 kV and 100 mA. A 2θ range from 10 to 90° in reflection mode

was scanned at 2° min−1. A computer-controlled wide angle goniometry coupled to a

sealed-tube source of nickel filtered Cu Kα radiation (λ = 1.54056 Å) was used.

Thermal analysis of the HDPE specimens upon environmentally stress cracking

tests at 50°C was carried out by differential scanning calorimetry (DSC) (Perkin-Elmer

DSC-2C). The specimen with 2 mg weight was finely grinded and mixed with white oil

as swelling agent for data correction, and then put into a sealed sample cell for DSC

29

measurement. The disentanglement energy of the individual material at different

locations was calculated from the DSC spectrum by the associated software system.

A Fourier transform infrared spectrometer (FTIR) was used to obtain the FTIR

spectra at different locations on the pre-cracked HDPE specimens after environmental

stress cracking tests. The HDPE materials were finely grinded and mixed with KBr

solution. Spectra in the range of 650 to 4000 cm−1 were recorded for 32 scans at 2 cm−1

resolution.

Finally, the surface morphology of the HDPE specimen was characterized by a

scanning electron microscopy (SEM) (Zeiss DSM 960 model). A thin layer of gold was

coated on the specimen to increase its conductivity prior to SEM observation.

30

Chapter 4: Effect of ASP soaking on mechanical properties of HDPE

materials

4.1. Introduction

The mechanical properties of HDPE, including its strength and ductility, are critical

to the performance in petroleum production. It has been reported that the permeation of

environmental species can degrade the mechanical properties of the material. For

example, the crystalline zones in the HDPE act as impermeable barriers for diffusion

and sorption of organic contaminants in the petroleum and the non-crystalline zone are

permeable since the polymeric chains in the amorphous areas are relatively “active”.

“Active” is more easy to occur ESC [57]. Petroleum permeation can be degradation the

internal structure of molecular chains that direct influence the mechanical properties of

HDPE.

Furthermore, the environment the HDPE is used remarkably affects the mechanical

property and performance. The pipeline material is thus important to investigate the

potential effect of ASP fluid on the mechanical behavior of HDPE materials, and ensure

integrity of the pipe systems.

In this work, tensile tests of various HDPE samples were studied according to

ASTM D638 and WL118 pressure rating. This test method is chosen, primary because it

is most often used in industry, but also because this testing method is beneficial for

examining to comparing to other materials. This test method is used to determine the

effect of three influencing factors on the mechanical properties of various HDPE pipes,

which include internal pressure, ambient temperature and ASP floods on the mechanical

31

properties of various HDPE pipes.

4.2. Tensile testing

4.2.1. Testing on as-received materials

Fig. 4.1 shows the stress-strain curves measured on the as-received HDPE4710,

HDPE3608 and PE100+ specimens in air, respectively. The chosen curve is the best

example of a the stress-strain in the given HDPE sample from the 5 curves acquired. It

is seen that the yield points of HDPE4710 and PE100+ are similar at 27.2 MPa and 26.7

MPa, respectively. The yield strength of the HDPE3608 is at 24.6 MPa. The elongation

at break is defined as the elongation of the sample resulting from both elastic and plastic

deformation at the point just before the sample breaks. The elongation at break of the

three materials is quite different, with 0.65, 0.61 and 0.48 for HDPE3608, PE100+ and

HDPE4710, respectively.

Fig. 4.1 The stress-strain curves measured on three as-received HDPE materials in air.

32

4.2.2. Testing on ASP-soaked specimens in air

Fig. 4.2 shows the stress-strain curves on ASP soaked HDPE4710, HDPE3608,

PE100+ materials, respectively. The term “original” describes the specimen that has not

been soaked. The label “25°C” in Table 4.1 describes the specimens that have been

soaked in the ASP solution at 25°C. Similarly, the label “75°C” describes the samples

that have been soaked in the ASP solution at 75°C. The average yield points and

elongation calculated from 5 tensile tests of the three materials are shown in Table 4.1.

The reported errors are the standard deviation in the calculated means from the 5 tests.

Compared to Fig. 4.1, it is seen that, upon soaking in ASP floods 30 days, there is little

effect on the yield point of the material. However, the elongation of the material is

changed significantly. Particularly, for HDPE3608, the elongation is reduced from 0.65

to 0.52. For PE100+, the elongation is 0.49 while the as-received material has an

elongation value of 0.61. The effect of soaking on elongation of HDPE4710 is marginal.

A quick comparison of the elongation of HDPE materials before and after ASP soaking

shows that the HDPE4710 is resistant to degradation of elongation upon ASP soaking,

while the HDPE3608 is sensitive to the ASP soaking and the resulting mechanical

degradation.

33

Fig. 4.2 The stress-strain curves measured on ASP soaked specimen in air.

Table 4.1 Summarize the yield points and elongation of each conditions.

Yield point (MPa) Elongation

In air In ASP solution In air In ASP solution

HDPE3608 24.6±0.04MPa 23.7±0.02

(25°C)/

25.2±0.04MPa

(75°C)

0.65±0.018 0.52±0.014

(25°C)

/0.44±0.017

(75°C)

HDPE4710 27.2±0.03MPa 26.9±0.02 MPa 0.48±0.020 0.45±0.018

PE100+ 26.7±0.03MPa 25.6±0.03 MPa 0.61±0.016 0.49±0.019

4.2.3. Testing on specimens soaked at various temperatures

Fig. 4.3 shows the stress-strain curves measured on HDPE3608 after 1 month of

ASP soaking at 25°C and 75°C, respectively. The soaking temperature does not affect

the yield point of the material, but reduces the elongation with the rising temperature.

For HDPE3608, the measured elongations are 0.65 and 0.52 at 25°C and 0.44 at 75°C

34

ASP soaking temperatures, respectively. This reduction elongation with increasing

temperature shows that an elevated temperature accelerates the degradation of HDPE.

 Fig. 4.3 The stress-strain curves of HDPE3608 after 3 months of soaking at 25°C and

75°C.

4.3. Discussion

4.3.1. Effect of internal pressure of ASP fluid on mechanical properties of HDPE

The internal pressure rating of HDPE pipes has been defined as a true stress that

would result in changes of mechanical behavior of the pipe material [58]. The inner

pressure can be decomposed into hoop stress and axial stress [59]. In this work, the

hoop stress has a larger influence on mechanical behavior of the material because it is

caused by the transverse force acting on the pipe, as demonstrated by the presence of

many transverse swelling crazes. A previous study has proved that the inner pressure

can lead to failure of HDPE pipes and the swelling processes [60]. The mechanism is

related to the permanent damage to the molecular chains and subsequently causes the

interfacial degradation. The failure of HDPE is initiated by plastic deformation on the

35

pipe specimens. It usually occurs on locations with a loose entanglement of molecular

chains [61]. A large inner pressure is associated with a high hoop stress, which can

accelerate the yielding process of HDPE material to break the tangled molecular chains.

The loss rate of yield strength of HDPE materials is defined as Iys by:

                  (Eq. 4-1)

where σair is yield strength of HDPE measured in air, and σsol is the yield strength

measured in the ASP flooding solution. Similarly, the breaking elongation-loss rate is

defined as Ibe by:

                                                                      (Eq. 4-2)

Based on the two formulas, the calculated loss-rates of yield strength and elongation of

HDPE materials are shown in Table 4.2. It can be found that both the yield strength-loss

rate and the elongation-loss rate follow the order: PE100+ > HDPE3608 > HDPE4710.

The maximum reduction in both strength and elongation upon ASP soaking is PE100+.

The yield point loss-rate and elongation loss-rate of HDPE4710 are moderate. In

industry applications, large radius-thickness ratio pipe is good for increasing the

transport volume.

Table 4.2 Strength-loss rate and the elongation-loss rate of each condition

Iys (%) Ibe (%)

HDPE3608 3.65 20 HDPE4710 1.1 3.25

PE100+ 36.6 19.7

air solys

air

σ σI

σ

air airbe

air

δ δI

δ

36

4.3.2. Effect of surfactant on mechanical degradation of HDPE in ASP fluid

In industry application, 0.2% ASP flooding is most common used. Surfactant is the

central role in EOR process and ESC. The effects of surfactants containing in ASP

solution on the mechanical degradation of HDPE are associated with three aspects. First,

the surfactant can reduce the surface tension of HDPE and improve the degree of

swelling. Second, the surfactant can improve the wetting and adhesion between the ASP

solution and HDPE. Finally, due to the reduced interfacial tension, the polarity of the

solution and HDPE (two different phases) and the adhesion between them can become

stronger [62].

In tensile tests, it has been observed that the ASP solution soaking decreases the

elongation of HDPE materials. It is believed that the contained surfactant plays an

essential role in the mechanical degradation of HDPE pipes. Moreover, the actual ASP

flooding is a mixed solution composed of polymers and surfactants. Some surfactant

molecules are bound to polymers to change the adhesion of the polymer [63].

Elongation of HDPE is reduced by high content of hydroxyl in solution with the

increasing adhesion between the ASP flooding solution and HDPE.

It is also seen that the yield points of soaked HDPE specimens are reduced

compared with those measured in air. In ASP solution, the low surface energy of HDPE

induces surfactant enrichment at the film-substrate interface, which can enhance the

solubility of HDPE molecular chains. The enhanced solubility of molecular chains can

allow the water invade the HDPE more easily. The molecular chains become more soft,

relaxed and disentangled by the continual water invasion [64]. As a result, the yield

point decreases and the capability of the pipe to bear internal pressures also decline.

37

4.3.3. Effect of temperature on mechanical degradation of HDPE in ASP fluid

All molecules, ASP floods tend to permeate at greater relative rates under higher

temperature with internal pressure [65]. The degradation of the yield point and

elongation at break of HDPE3608 is caused by the higher rate permeation of ASP floods

molecules driven by the elevated temperatures and the internal pressure. In this case, the

internal pressure value of HDPE3608 is lower than HDPE3608 under other conditions,

so the elevated temperature is the most influenced factors of HDPE3608 degradation.

ASP floods molecules get more energy to permeate through the HDPE under elevated

temperature and the internal pressure differential creates a concentration gradient, hence

a greater driving force for the molecules to permeate through the HDPE. As a result,

higher concentration ASP floods penetrated into HDPE3608 in the same soaking period

and it ruptures the HDPE3608 molecular in stronger way. On the other hand,

temperature effects the performance of HDPE 3608 itself. The mechanical property of

HDPE3608 is stable under 40oC [66]. The molecular chains of HDPE3608 become

more flexible and liquid-like under elevated temperature. As a result, the strength of

HDPE3608 decreases when the molecular chain is more flexible. In 75°C, the stable of

mechanical properties and strength of HDPE3608 samples were decrease which

responded the failure of HDPE3608.

4.4. Summary

The stress-strain behavior of three types of HDPE, i.e., HDPE4710, HDPE3608 and

PE100+, which has been soaked in ASP fluid for 1 months, were investigated.

The ASP soaking changes remarkably the elongation of the HDPE materials. Prior

to ASP soaking, the elongations of HDPE4710, PE100+ and HDPE3608 are 0.48, 0.61

38

and 0.65, respectively. After soaking, the elongations of the three HDPE materials

change to 0.45, 0.49 and 0.52, respectively. While HDPE4710 is resistant to degradation

of elongation upon ASP soaking, the HDPE3608 is sensitive to the ASP soaking.

Moreover, the degradation of mechanical property of HDPE materials is enhanced by

the internal pressure and soaking temperature.

39

Chapter 5: Environmental stress cracking behavior of HDPE in ASP

fluid

5.1. Introduction

It has been reported that, under certain tensile stress, HDPE can experience

environmental stress cracking in environments containing species, such as surfactants,

that can permeate into the material [67]. It was also reported that the density of the

amorphous chain in the interlamellar amorphous region decreases with an increase in

the extracted petroleum resin and the yield decreased while the amorphous chain

density is decreasing in the interlamellar amorphous region [68].

In this chapter, the effect of ASP floods chemistry on the structure and life cycle of

HDPEs in ASP floods will be examined, with a particular focus on the enhancement

ESC of HDPE in the presence of ASP floods at 50oC. Last chapter was research

mechanical properties of HDPEs in ASP floods. To characterize the influence of ASP

floods on HDPE, various characterization techniques were employed such us DSC,

infrared spectroscopy (IR), XRD, SEM, etc.

5.2. Results

5.2.1. Effect of ASP concentration

Fig. 5.1 shows the percentage of the number of cracked HDPE specimens as a

function of immersion time in solutions containing different concentrations of ASP fluid.

The process follows the ASTM D1693. Percentage is the ratio of cracked specimens of

total specimens. 10% is the standard concentration of soaking solution in ASTM D1693.

The others are reference system to research the ASP floods contribution on ESC. It is

40

seen that the percentage of the cracked specimens increases with increasing immersion

time. Furthermore, the time resulting in cracking of the specimen reduces when the

concentration of ASP fluid is increased. The PE100+ is associated with longest failure

time.

Fig. 5.1 The Failure Points of HDPE4710, HDPE3608 and PE100+ specimens

soaked in ASP fluid with different concentrations at 50°C. (a) 7.5%; (b) 10%; (c) 12.5%; (d) 15%.

The F50 value is defined as environment stress cracking resistance time according to

ASTM D1693 [69]. Fig. 5.2 shows the F50 value of each specimen calculated from Fig.

5.1 as a function of the ASP fluid concentration. It shows that the F50 value for all of the

investigated HDPE specimens decreases with the increasing ASP concentration. For

example, the F50 value of HDPE4710 drops from 160 h to 30 h when the ASP

concentration increases from 7.5 to 15 %. This indicates that the ESC of the HDPE

specimens is highly sensitive to the concentration of ASP fluid. Furthermore, it is noted

that the F50 value depends on the type of materials. In 10 % ASP fluid, the F50 values of

41

HDPE4710, HDPE3608 and PE100+ are 154 h, 133 h, and 268 h, respectively. It is

clearly seen that PE100+ has the highest F50 value, indicating its best resistance to ESC,

while the HDPE3608 is associated with the smallest F50 value. Thus, HDPE3608 is

highly susceptible to ESC in ASP fluid.

Fig. 5.2 The F50 value of three types of HDPE materials in ASP fluids.

5.2.2. DSC measurements

Fig. 5.3 shows the DSC curves measured at the different feature points, i.e., 1/2

notch, 3/4 notch and endpoint, on HDPE4710, HDPE3608 and PE100+ specimens,

respectively. The curve can be divided into three regions depending on the processing

temperature. There is a small weight loss of specimen during the initial stage at a

relatively low temperature range from 40 to 110 °C. This is followed by a melting

process of the specimen and an obvious peak around 125 °C. Noticeably, the majority of

weight loss occurs at a higher temperature ranging from 130 to 220 °C (the third part of

the DSC curve), indicating the exothermic decomposition of PE materials. The third

42

part of the DSC curve is associated with bond disentanglement in PE [70, 71, 72].

Fig. 5.3 DSC curves measured at 1/2 notch, 3/4 notch and endpoint of three HDPE

specimens at a heating rate of 10oC min−1. (a) HDPE4710; (b) HDPE3608; and (c) PE100+.

In Fig. 5.4, the portions of the DSC curves that correspond to the disentanglement

are enlarged. The disentanglement energy of the entangled bonds in HDE material can

be calculated by integrating the third area of DSC curve. Fig. 5.4 summarizes the

calculated disentanglement energy at different feature points on the specimen. It is seen

that the disentanglement energy of the specimen is in the order of PE100+ >

HDPE4710 > HDPE3608. Moreover, for individual specimen, the disentanglement

energy of the materials at different locations is generally in the order of endpoint > 3/4

notch > 1/2 notch.

43

Fig. 5.4 Disentanglement energy of the feature points in each specimen.

5.2.3. Measurements of Fourier transform infrared (FTIR) spectroscopy

Fig. 5.5 shows the FTIR spectra corresponding to different feature points (i.e., 1/2

notch, 3/4 notch, 7/8 notch and endpoint) on the HDPE4710, HDPE3608 and PE100+

specimen after ESC testing. There are three functional groups in the specimen, i.e.,

C=C, CH2-CH2 and –OH. A peak around 1640 cm–1 is associated with the C=C

stretching vibration. The small peak at 2900 cm–1 corresponds to the -CH2- stretching

vibration, and the –OH stretching vibration appears around 3410 cm–1. Since all the

specimens have the same chemical formula and have been immersed in the same ASP

fluid, they have the same functional groups. Of the three functional groups, CH2-CH2

are from the carbon chains in PE, while the –OH functional groups and C=C double

44

bond comes from the immersion solution. The variation of transmittance of CH2-CH2

describe that degradation and rupture were occur under long time soaking and stress.

The capillary phenomenon occurs when the specimens soaked in the ASP floods for a

long period. The voids in the specimen fill with ASP floods and it reflects in the

transmittance of –OH and C=C. Notably, the–OH and C=C peak differs with different

feature points and the type of HDPE material. This is due to the different amount of

penetrated –OH and C=C double bond [73, 74, 75].

Fig. 5.5 FTIR spectra measured at 1/2 notch, 3/4 notch, 7/8 notch and endpoint on

the specimen (a) HDPE4710; (b) HDPE3608; and (c) PE100+.

5.2.4. SEM characterization

Fig. 5.6 shows the typical SEM images of the surface morphologies of the craze

area from HDPE4710, HDPE3608 and PE100+ after 300 hours of immersion in 10%

45

ASP fluid at 50°C. The views are made at two different magnifications, as shown in the

figure. As shown in low-magnification images, there are many crazes formed on the

surface of the specimen, perpendicular to the artificial notch on the specimen. It is also

seen that the crack length and depth are dependent on the specimens, which might be

due to the different structures of the PE specimens [76].

Fig. 5.6 SEM views of specimens after 300 hours of soaking in 10% ASP fluid. (a)

HDPE4710; (b) HDPE3608; and (c) PE100+.

46

5.2.5. XRD characterization

Fig. 5.7 shows the XRD spectra of as received HDPE4710, HDPE3608, and

PE100+ materials, respectively. It is seen that there are two well-defined peaks around

21.70° and 23.9°, which correspond to the diffraction peaks of the L(110) and L(200)

lattice planes of the HDPE material, respectively [77]. The feature of all XRD spectra is

similar, although the materials were prepared with different curing degrees and molding

techniques, indicating that there is no obvious effect of the processing conditions on the

crystal structure of HDPE. Depends on the XRD curves, the crystalline degree which

can order the mechanical properties of initial HDPEs can be calculated by following

equation:

(Eq. 5-1)

Where Xc is crystallinity, Ic and Ia are peak intensities for crystallinity and

amorphous peaks respectively, K is constant which is 0.75 for PE [78]

Table 5.1 Ic, Ia and Crystallinity of HDPE4710, HDPE3608, PE100+

Ic Ia Crystallinity

HDPE3608 157891 70903 59.9% HDPE4710 178756 80011 74.9%

PE100+ 164239 49672 76.8%

Fig. 5.8 shows the crystalline degree of the three HDPE materials calculated from

the integration of the area under the crystalline peaks in Fig. 5.7. It is seen that there is

the highest crystalline degree of about 63.51% for PE100+, while HDPE3608 exhibits

the lowest crystalline degree of about 56.77%. In polymer science, higher crystallinity

means that yield point and strength is higher. The order of crystallinity and order of

yield point of initial pipes are same.

47

Fig. 5.7 Typical XRD spectrum of HDPE4710, HDPE3608, PE100+ range from 10 o to 90o at the scan rate of 2o min−1, unmilled HDPE4710 and crystallinity of HDPE4710,

HDPE3608, PE100+ which calculated depends on XRD.  

5.3. Mechanistic aspects of environmental stress cracking of HDPE

5.3.1 Effect of surfactant

It has been generally accepted that alkaline and polymer have little effect on

degradation of HDPE [79]. The presence of surfactant has a significant influence on the

ESC of the material [80]. The percentage of the cracked specimens increases with the

immersion time and the ASP concentration. Moreover, the F50 value decreases with

increasing ASP concentration. It has been proposed that, under the immersion condition,

the surfactant absorbed by a HDPE specimen serves as a severe stress cracking agent

responsible for the ESC of the material [80]. The surfactant is able to lower the surface

energy of polymers. The potential of a surfactant to promote stress cracking in a HDPE

48

specimen is governed by the driving force by which the surfactant containing liquid

transports through the craze fibril structure [82]. Diffusion of the surfactant in HDPE

leads to an increased chain mobility and a corresponding reduction of the activation

energy for the deformation process. Therefore, once the surfactant containing liquid

penetrates to the tip of the craze, it begins to plasticize the HDPE and facilitates the

growth of the craze. Kambour reported that the critical strain for solvent-induced craze

initiation and stress cracking is related to the solubility parameter of the solvent [83].

The solubility parameter of a stress cracking agent is a measure of the total cohesive

attraction between the fluid molecules. According to Hansen, the solubility parameter

comprises three types of cohesive forces: dispersive, polar and hydrogen bonding [84].

Stress cracking agents, i.e., surfactants in the ASP flooding in the present work, act to

lower the cohesive forces between tie molecules in the crystallites, thus facilitating their

“pull-out” and disentanglement from the lamellae [85].

5.3.2 Effect of stress

As previously mentioned, the stress also plays an important role in the ESC of

HDPE. It has been found that, if the HDPE specimen is completely free of stress, no

stress cracking will occur. Even the specimens exposed to the same

surfactant-containing solution will not undergo ESC. Diffusion of surfactant molecules

into the HDPE specimen as a result of the stress could lead to an increased chain

mobility and therefore a reduction of the activation energy for the deformation process

[86]. When the stress concentration reaches a critical value, void formation takes place

through cavitation inside plastically elongated macromolecular chain [87]. In the

present work, different characteristic points were selected on each specimen, which

49

corresponds to different applied stresses (Fig. 3.3). Depending on the bending degree of

the specimen, the order of the stress concentration at the characteristic points is 1/2

notch > 3/4 notch > endpoint. The craze is first seen in the middle of the notch on the

specimen (observed by naked eyes), suggesting that the craze initiates at the location

with the largest stress. The disentanglement energy for each point of the specimen was

calculated by the DSC measurements from the exothermic decomposition area (Fig. 5.5).

It has been found that the disentanglement energy of entangled bonds varies by the

locations on the specimen, suggesting that the stress could affect the disentanglement

energy. For HDPE4710, the disentanglement energy decreases from 62.63 J/g at the

endpoint to 47.84 J/g at the 1/2 notch. This clearly demonstrates that with increasing

stress on the specimen, macromolecular disentanglement occurs more easily, leading to

the decreasing disentanglement energy. Previous studies showed that the as the stress

increased, the voids were elongated gradually in the direction of the applied stress,

resulting in a decrease in the disentanglement energy [87]. This is consistent with our

work. Furthermore, it is seen that the amount of the penetrated –OH varies with

different feature points on the specimen. Fig. 5.5 describes that the peak height for the

penetrated –OH is in the order of 1/2 notch > 3/4 notch > 7/8 notch > endpoint, which is

the same for the magnitude of the applied stress. Thus, the amount of the penetrated –

OH to the specimen increases with increasing applied stress. As discussed above, an

increase in stress induces the formation of voids in the PE, which contribute to the

penetration of –OH [89, 90].

5.3.3 Effect of the type of HDPE material

From the above results, the ESCR, thermal and mechanical properties have shown

50

to strongly depend on the type of PE. For example, PE100+ has the highest ESC

resistance among three types of PE as indicated by a much higher F50 (233 h) compared

to that of HDPE4710 (135 h) and HDPE3608 (123.37 h) (Fig. 5.6). After immersion in

the ASP flooding, the failure of the PE is also accompanied by a volume increase,

suggesting the presence of swelling phenomenon. This phenomenon is very common for

plastic polymers when they are in contact with the solution for enough time [91, 92].

The combined effects of the swelling by H2O, the penetrating surfactant and the stress

lead to the failure of PE. It is found that swelling degree varies for different PE

specimens and PE100+ has the lowest swelling degree compared to HDPE4710 and

HDPE3608. Besides, the craze distribution and morphology are also dependent on the

PE type. There are more crazes on HDPE4710 and HDPE3608 than PE100+ and the

crazes on HDPE4710 and HDPE3608 are deeper and wider compared to those on

PE100+. This can be explained by the XRD, DSC and FTIR analysis. XRD results

showed that PE100+ had a higher crystallinity than the HDPE4710 and HDPE3608 (Fig.

5.7). It also exhibits the highest disentanglement energy of 68.67J/g (Fig. 5.4) among

three types of PE, indicating it has the strongest entangled bond. Finally, PE100+ has

the lowest amount of penetrated –OH (Fig. 5.5(c)) compared to HDPE4710 (Fig. 5.5(a))

and HDPE3608 (Fig. 5.5(b)), suggesting its high compactness. Such structural

characteristics contribute to the high ESCR of PE100+.

5.4. Summary

        ESC behaviors of three different types of PE, (i.e., HDPE4710, HDPE3608 and

PE100+) in different concentration of alkaline-surfactant-polymer flooding were

investigated. ESCR behavior was studied according to ASTM D1693.

51

ESCR of the PE depends on the surfactant concentration, stress and the nature of

the materials. ESCR of three types of PE showed that high concentration of ASP

flooding and relatively low strength caused the accelerated cracking. Especially when

the concentration of ASP flooding reached 15 %, the crack formation time reduced

significantly. It was therefore recognized that a 15 % concentration of ASP flooding is

close to the critical ASP flooding concentration for these three types of PE.

DSC measurements showed that the disentanglement energy of entangled bonds

was highest for PE100+, while for individual specimen, the disentanglement energy

decreased with increasing stress.

FTIR showed the functional groups of HDPE including CH2=CH2 and –CH2-CH2-

and the main functional groups in ASP flooding (e.g. –OH). FTIR results also revealed

that the amount of the penetrated –OH was the in order of PE100+ < HDPE4710 <

HDPE3608. For individual PE specimen, the amount of the penetrated –OH increased

with the increase in the applied stress.

By SEM, it was observed that the failure of specimen was featured with swelling

phenomenon and formation of crazes on the surface after immersed tests. The swelling

resistance of the PE was directly related to the length of immersion time and the

mechanical properties of the three types PE.

52

Chapter 6: Permeability of HDPE to water

6.1. Effect of HDPE specimen thickness

Fig. 6.1(a)-(c) shows the amount of water evaporated per unit area of the

HDPE3608 membrane with increasing time for different thicknesses of the membrane

(i.e., 1.0, 1.5 and 2.0 mm), also for at three testing temperatures. Both temperature and

membrane thickness influences the water vapor transmission rate. The water

permeability through the HDPE membrane is calculated by fitting a line to the collected

data and determining the slope from the fit. The definition of the WVT is:

(Eq. 6-1)

where Δg is the weight loss (g), t is time (day) and A is the exposed area of the

membrane (m2) [93]. WVT is water vapor transmission has units of g/m2·day. The

WVT values of HDPE3608 were measured under various conditions and are outlined in

Table 6.1. Generally, the WVT decreases with increasing film thickness resulting from a

greater amount of material that the water must diffuse through to leave the system.

Although water vapor transmission decreases continuously with testing time, it is seen

that the relationship between WVT and the membrane thickness is not a simple linear

function from Fig. 6.1.

Table 6.1 The WVT of HDPE3608 measured under various membrane thickness and temperatures.

25°C (g/m2 day) 50°C (g/m2 day) 75°C (g/m2 day)

1.0mm -0.01574 -0.10934 -0.33071 1.5mm -0.01500 -0.07788 -0.28770 2.0mm -0.01319 -0.04918 -0.13550

=g

tWVTA

53

 

 

Fig. 6.1 The amount of water evaporated through a HDPE3608 membrane per unit area versus the time for three different thickness of the HDPE3608 membrane. The

experiment was conducted under three different temperatures: (a)25°C, (b) 50°C, and (c) 75°C. Linear fits to the evaporation data have been performed and are shown in the

figures as dashed lines.

6.2. Effect of temperature

Fig. 6.2 shows the water permeability of the HDPE3608 membrane measured at

three temperatures with different thicknesses. Fig. 6.2 describes the effect of

temperature and Fig. 6.1 describes the effect of thickness, but both Figs describes the

same experimental data. Temperature has shown to be more influential on the WVT

than the membrane thickness. WVT is the slope of data line which exhibits a larger

fluctuation with increasing temperature even though the fluctuation is reduced with the

increase in the thickness. The relationship between the WVT, temperature and thickness

will be further discussed in Section 6.3.

54

 

 

Fig. 6.2 The amount of water evaporated through a HDPE3608 membrane per unit area versus the time for three different temperatures. The experiment for three different

membrane thicknesses: (a) 1mm, (b) 1.5mm, and (c) 2mm. Linear fits to the evaporation data have been performed and are shown in the figures as dashed lines.

6.3 Water permeability model

6.3.1 Mechanism of water permeation through a HDPE membrane

The process of water permeation through a polymer can be divided into four steps,

as illustrated in Fig. 6.3. The first step is the continuous movement of water as it

changes from liquid to vapor. The water molecular movement is significantly affected

by temperature. In this work, the test cell is sealed so as to reflect the water molecular

moving velocity under saturated water vapor pressure. The theoretical values of

saturated water vapor pressure at normal ambient temperature are summarized in Table

6.2. The second step is adsorption. Water vapor diffuses from the water surface, where it

has just evaporated, to the surface of the HDPE. The distance between the surface of the

55

water and the HDPE barrier is fixed at 6 mm, so the transmission rate through HDPE

also depends on the rate of transport of water vapor from the water surface to the HDPE

surface. However, the air between the water and the HDPE rapidly becomes the

saturated water vapor, making and the relative humidity in the petri dish is 100 %

during testing, and thus the transport of water to the HDPE is not a limiting step. The

third step is the diffusion of water molecules through the HDPE membrane and is

usually referred as a solubility process between the HDPE3608 film and water vapor.

For a given type of HDPE film, the transmission rate in this step is mainly determined

by the film thickness. For different types of HDPE, the transmission rate can also be

affected by their densities. More specifically, water transmission through polymeric

materials is thought to only occur through the amorphous regions of the polymer, rather

than through the crystalline regions [cite the thickness paper]. Therefore, the degree of

crystallinity, which is directly related to the density of polymer, has a strong influence

on the WVT rate. The last step is the evaporation of water molecules, which results in

the weight-loss of the petri dish. [98]

Fig. 6.3 The physical process of water vapor transmission

56

Table 6.2 Saturated Water Vapor Pressures at Different Temperatures

Temperature

/(°C)

Saturated Water Vapor Pressures 

/(kPa)

Temperature

/(°C)

Saturated Water Vapor Pressures

/(kPa)

25 3.1690 75 38.563 50 12.344 100 101.32

6.3.2 The Experimental model the dependence of WVT on temperature and membrane

thickness

In ASTM D1653 [95], it is specified that the relationship between the WVT and the

film thickness or temperature is not linear. In this section, an experimental mathematical

model describing the relationship between the WVT and temperature, which also

includes the influence of the membrane thickness, is derived. From the literature, it is

known that WVT depends on temperature, but also relative humidity, which is also

dependent on temperature [96]. As the relative humidity could not be controlled in these

experiments, we develop an empirical model to predict the influence of temperature for

the present experimental study.

 

Fig. 6.4 The relationship between WVT and temperature.

57

In Fig. 6.4, a quadratic relationship can be found between WVT and temperature.

The choice of a quadratic function is justified because of the shape of the resulting

curves, in addition to a past study showing similar trends between the temperature and

the WVT rate [97]. At a constant temperature, a decrease of the thickness of HDPE

3608 membrane leads to an increase in water vapor weight-loss. The parabolic curves

can be written as:

WVT=a(T+b)2 +c (Eq. 6-2)

where a, b and c are constants. T is temperature. The parameter a, or the WVT

temperature sensitivity coefficient, is the main influencing factor in determining WVT

variation with temperature. An increase in the temperature sensitivity factory therefore

shows that the pipeline material is more sensitive to fluctuations in temperature (i.e.

higher temperatures result in much diffusion of water through the pipeline material).

Table 6.3 shows the variable a in the WVT-Temperature formula. Subscript indicates the

thickness of the membrane.

Table 6.3 The values of the temperature sensitivity coefficient in WVT-Temperature formula. The subscript values on a denote the thickness (in mm) of

the HDPE membrane studied in each case.

a1.0 (g/(m•oC)2•day) a1.5 (g/(m•oC)2•day) a2.0 (g/(m•oC)2•day)

Values -1.17552×10-4 -1.02216×10-4 -0.40344×10-4

In Table 6.3, a1.0 describes that the rate of change of the WVT with temperature as a

function of film thickness, for three specific thicknesses of 1mm, 1.5mm and 2.0mm.

a1.5 and a2.0 are similar. The value of the WVT temperature sensitivity coefficient

increases with increasing film thickness, showing that WVT decreases rapidly with

increasing film thickness. The parameter b is the critical WVT temperature, which

58

describes the temperature at which the WVT value is a minimum, or when water has the

most difficulty diffusing through the pipeline material. The parameter c gives the

minimum WVT value at the critical WVT temperature.

As it has been shown above in Figure 6.4, the WVT value not only depends on the

temperature, but also on the thickness of the pipeline material. In order to isolate the

influence of thickness on the value of WVT, the partial derivative of Eq. 6-2 with

respect to temperature is calculated:     

                  (Eq. 6-3)

Twice the temperature sensitivity factor, a, versus the thickness of the pipeline

material is plotted in Fig. 6.5. Again, the dependence of the temperature sensitivity

factor on the thickness is quadratic. Thus an engineering model is used to investigate the

thickness dependence on WVT that expresses the temperature sensitivity coefficient in

terms of thickness, as shown in Equation 6-4.

Fig. 6.5 The mathematical relationship between 2a and the membrane thickness.

2

2thkWVT a

T

59

The quadratic relationship between a and the membrane thickness can be expressed

as follows:

                  (Eq. 6-4)

where m, S, n are fit parameters. The thickness sensitivity factor, m, describes the

sensitivity of the value of WVT with thickness of the pipeline material. An increased

value of the thickness sensitivity factor therefore shows that the pipeline material and/or

WVT value is more sensitive to the thickness of the pipeline (i.e. a decrease in the

pipeline thickness would substantially increase the diffusion of water through the

pipeline material). The critical thickness, n, describes the thickness at which the

temperature sensitivity factor value is a minimum, or when water has the most difficulty

diffusing through the pipeline material. The parameter s gives the minimum temperature

sensitivity factor value at the critical WVT temperature.

WVT can now be express in terms of both temperature and thickness by integrating

equation 6-4. The result of this integral is written as follows:

                  (Eq. 6-5)

Based on the nine sets of WVT experimental data, the calculated values of m, n, s, r, q

are shown in Table 6.4.

a 1

2

WVTth

T 2

m Thickness n 2 S

2

2

2

2

m Thickness nWVT T T q

sr

60

Table 6.4 The value of m, n, S, r, q for HDPE3608 WVT testing

m

10-6g•(oC)2/day

n

109/m3•(oC)2

s

g/(m•oC)2•day

r (g/m2•oC•day )

q

(g/m2•day)

1.86×10-4

-1.09

-2.058×10-4

-2.295×10-4

-2.363×10-4

3.922×10-3

3.952×10-3

1.580×10-3

-0.050

-0.029

-0.028

Average 1.86×10-4 -1.09 -2.20×10-4 3×10-3 -0.04

Standard deviation

0.20×104 1×10-3 -0.01

For HDPE3608 we can determine an exact formula for calculating the dependence

of WVT on temperature and thickness by substituting the measured fitting parameters

outlines in the previous paragraphs into Equation 6-4. This final formula can be written

as:

                                 

(Eq. 6-6)

6.3.3 The experimental model of WVT-thickness at certain temperature

The film thickness primarily affects the physical process of the water vapor

transmission in step 3 in Fig 6.3. The previously developed model of WVT as a function

of temperature shown in 6.3.2 follows a quadratic function as Eq. 6-2, which also

dependent on thickness. We now explore the dependence of WVT on membrane

thickness, rather than temperature, to isolate its effect on WVT. By ASTM D1653, WVT

is not a linear function of the film thickness. Gilbert S. Banker proposed that in ultrathin

membrane around 10-3 to 10-2mm, the function between WVT and thickness follows

24 42 31.86 1.09 2.20

3.15 0.042

ThicknessWVT T T

61

power function [98]. However, there has been no detailed examination to date of the

variation of WVT with thickness for much thicker films. Based on the iterative fitting

process, it can be found that the relationship between |log | | | and|log | | |,

where t is the thickness of the membrane, is linear at a given temperature (Fig. 6.6).

Using this data, an empirical mathematical model describing the variation of WVT with

thickness for HDPE having a thickness comparable to that used in pipeline materials is

developed.

Fig. 6.6 The relationship between |log (|WVT|)| and |log (t)| for experiments

conducted at three different temperatures.

As shown in Fig. 6.6, the log-log plot of WVT against film thickness is linear. The

relationship can be therefore expressed by the following equation

          (Eq. 6-7)

It can be reduced to

WVT (Eq. 6-8)

where t is the film thickness (mm). All of the parameters are absolute values. C is the

log(| |) log logWVT C m t

62

antilog of ordinate intercept in |log (|WVT|)|-log (t) and m is the slope of line. The

physical meaning of C is weight loss of water vapor per cubic meter per day. The unit of

C is g/m2•day•mm which can be reduce to 103g/m3•day. C and m are

temperature-dependent constants. WVT is increasing with increasing C. This equation is

specific to this research system and it can be used to calculate the approximate values of

WVT at different film thickness. Table 6.5 shows the values of C and m at each

environment temperature and the corresponding final equations of WVT and thickness.

It is shown that C decreases with rising temperature, m shows normal distribution.

Table 6.5 The values of C and m of each temperature.

C

(103g/m3•day)

m

(constant)

Final formula

25oC

63.53

m1

-0.12

m2

-0.25

Average

-0.18±0.09

WVT163.53

.

WVT19.15

.

WVT13.02

.

50oC 9.15 -0.84 -1.15 -1.0±0.2

75oC 3.02 -0.34 -1.29 -0.8±0.3

6.4. Mechanistic aspects of water permeability of HDPE

6.4.1 Temperature

From the established experimental mathematical model in 6.3, temperature is

identified as one of most influential factors for WVT [99]. As shown in Fig. 6.3, in the

first physical process step of WVT, the velocity of water molecule movement is

determined by the temperature. At higher temperature, the molecular kinetic energy

increases and more water molecules can escape from the liquid surface which produces

63

a higher saturated vapor pressure (SVP). Higher temperatures accelerate the

vaporization of water in the sealed petri dish and water vapor can quickly reach to the

HDPE3608 film. The second and third steps can be considered as diffusion processes.

The effective activation energy for the diffusion, ED, is defined by the equation [100]

(Eq. 6-9)

ED decreases markedly with increasing temperature, suggesting that the diffusion

process can occur more easily. And consequently the WVT rate becomes faster [101].

In addition, relative humidity is another important parameter of the WVT affected

by the increasing temperature. The relative humidity in the sealed petri dish would

become nearly 100 % due to the continual water evaporation, while the relative

humidity in air outside the petri dish sharply decreases as the temperature rises (Table

6.6). The difference between the humidity values increases with the rising of

environmental temperature which is the driving force for the loss of water by

evaporation [100].

Table 6.6 Average relative humility of the air and the saturated vapor pressure

Temperature

Average relative humility in air

Average relative humility

in sealed petri dish

Average relative humility

difference

25oC

100%

50oC 100%

75oC 100%

6.4.2 Thickness of HDPE3608 film

Table 6.5 shows that the WVT rate which changes inversely with the thickness of

2D

InDE RT

T

Decreasing Increasing

64

the film. In Fig. 6.6，the absolute value of the log |WVT| decreases almost linearly with

the log|t| of film. Further fitting of the mathematical model suggests that the relationship

between WVT and film thickness probably follows a power function. Theoretically,

solubility and diffusion coefficient do not fluctuate with increasing thickness, but it does

not mean that the thickness has no effects on the WVT rate. The increasing thickness

increases the difficulty of water vapor to pass through the HDPE3608 film.

To discuss the effects of thickness, the temperature can be assumed as the same

value. There is another WVT formula associated with the film thickness:

                  (Eq. 6-10)

where D is the water vapor diffusion coefficient in the specimen(mm2/ day). WVT is the

water vapor transmission rate (g/mm2•day). Thickness is the total thickness of specimen

(mm) and ΔP is the density of the saturated water vapor at the testing temperature (g/

mm3) [102].

D and ΔP are subjected to influences from the environment temperature and the

properties of tested material. At the same temperature, the values of D and ΔP are

constant [102, 103]. Therefore, WVT is inversely associated with HDPE3608 film

thickness. As shown in Eq. 6-10, the increasing thickness leads to the decrease in WVT.

Eq. 6-10 is basically consistent with experimental model in Table 6.5.

6.5. Summary

The WVT rate increased with the rising temperature and decreased with the

increase in film thickness. Although not experimentally examined, a further important

factor which determined the WVT rate is the relative humidity, which in the case of the

experiments conducted in this chapter depended on the temperature. The WVT rate was

ΔPWVT D

thickness

65

fit using two mathematical models predicting the WVT rate variation with temperature

and with thickness. Examining the influence of temperature, a mathematical model

was developed showeing that WVT varied parabolically with temperature, in contrast to

the power relation that was observed for the variation of WVT with membrane thickness.

66

Chapter 7: Conclusions and recommendations

7.1. Conclusions

In this research, the tensile mechanical properties, resistance to environment stress

cracking, and WVT rate of HDPE were studied according to ASTM D638, ASTM

D1693 and ASTM D1653 under different experimental conditions including internal

pressure rate, ASP fluid concentration, temperature and the HDPE membrane thickness.

Surface analysis techniques, such us DSC, FTIR, SEM and XRD, were used to study

the degree of destruction of internal structure in the HDPE films through

characterizations of the disentanglement energy of molecular chain, the grade of voids,

and the degree of cracking and swelling. For WVT, a mathematical model was

established for the relationship between WVT, environmental temperature and film

thickness. Primary conclusions of this research are summarized as follows:

(1) The tensile test of the three different types of HDPE showed that stress

cracking susceptibility of the PE depended on the surfactant concentration, stress and

the type of materials. The ASP solution soaking under the internal stress rating would

decrease the elongation of the material. The effect was more significant at an elevated

temperature. In comparison, the strength of material was less affected by internal

pressure rating, ASP floods, heating process, and type of HDPE.

PE 100+ exhibited a higher crystallinity than HDPE4710 and HDPE3608. The

order of stress cracking resistance of the three types of PE is PE100+ > HDPE4710 >

HDPE3608 which was entirely consistent with the order of crystallinity. The SEM

images showed well-organize crazes on the surface of all HDPE specimens after

soaking.

67

(2) The percentage of the cracked specimens compared to the total number of

specimens in ESC tests increased, while the F50 value decreased with the ASP

concentration for all three types of HDPE. The PE 100+ had the highest F50 value,

suggesting the high resistance to ESC compared to the other two HDPEs.

Therefore, ASP flooding will increase the susceptibility of HDPE pipes to

environmental stress cracking. The potential of a surfactant to promote stress cracking

of PE materials is driven by the transport of the surfactant-containing liquid through the

craze fibril structure of PE. The diffusion of the surfactant in PE leads to the increase in

chain mobility and the reduction of the activation energy for deformation. Once the

surfactant-containing liquid penetrates to the tip of crazes, plasticization of PE starts and

facilitates the growth of crazes. This effect is further enhanced by the increasing applied

stress.

The susceptibility of HDPE to ESC depends strongly on the applied stress. Actually,

in the absence of stress, the crack would not propagate. In this work, the

disentanglement energy was determined at different locations on the pre-notched

specimen. The order of stress at these points was ranked as 1/2 notch > 3/4 notch >

endpoint due to the geometrically induced stress concentration. The voids of various

specimens can be described by the permeability of hydroxide radical to the specimens.

The order of ESCR of the three PE materials was same as order of their crystallinity and

tensile properties.

(3) The WVT of HDPE depends on the environmental temperature and film

thickness. For the same film thickness, as the environmental temperature increased, the

relative humidity in air decreased, resulting in an increase of the WVT in the

HDPE3608 film. At the same environmental temperature, WVT decreased as the

68

thickness of the HDPE3608 film increased.

It has been previously reported that the relationship between WVT and the

influencing factors was not a linear function. There are many theories describing the

mathematical relationship between WVT and the influencing factors. In this study, the

mathematical models showed that the WVT followed the quadratic function with

temperature and WVT followed the power function with thickness. The relationship

between WVT with temperature and thickness which is temperature following quadratic

function and thickness is in the second order item coefficients. The mathematical

models can be used to predict the WVT of HDPE3608 at various temperatures and film

thicknesses.

7.2. Recommendations

(1) To investigate the environmental effect (e.g., internal stress, ASP flooding and

room temperature or high temperature.) on the yield point and elongation of the

widely used HDPEs in industry.

(2) To study the ESCR property of the three HDPEs, under different ASP flooding,

temperature and stress, and to study the different factors affecting the interior

structures by advanced surface characterization techniques.

(3) To study the WVT of various HDPE films at different temperatures and film

thicknesses, and to develop mathematical models to predict the WVT of various

HDPE films at other temperatures and film thicknesses.

69

References

 

[1] Fink, Johannes Karl. High performance polymers. United States of America:

Published by: William Andrew Inc. 2008.

[2] Sachin Kumar, Achyut K. Panda, R.K. Singh, Resour.Conserv.Recy, 55 (2011) 893.

[3] X. M. Zhang, S. Elkoun, A. Ajji, M.A. Huneault, Polymer 45 (2004) 217.

[4] Johannes Karl Fink, High Performance Polymers, William Andrew Inc. United

States, 2008.

[5] Andreas Frank, Werner Freimann, Gerald Pinter, Eng Fract Mech, 76 (2009) 2780

[6] R.A. Bubeck. Polym. 22 (1981) 682.

[7] L. Vollrath. Polym. Test. 11 (1992) 83.

[8] A. Kianinejad, Energy Sources, Part A, 35:1929–1938, 2013.

[9] B.Ho. Choi, J. Weinhold, D. Reuschle, M. Kapur. Polym. Eng. Sci. 49 (2009) 2085.

[10] N. Ibrahim, D.M. Shinozaki. Mater. Sci Eng. 30 (1977) 175.

[11] Yoshihito Ohde, Hiroshi Okamoto. J. Mat. Sci. 15 (1980) 1539.

[12] Bernhard Wunderlich, Malcolm Dole. Polym. Sci. 24 (1957) 201.

[13] Preloshni Naidoo, A comparative analysis of the chemical composition of linear

low density polyethylene polymers synthesised with 1- hexene comonomer under

different catalytic conditions, University of Stellenbosch.

[14] Runzhou Huang, Xinwu Xu. Materials, 6 (2013) 4122

[15] J.Dale Ortego, Tejraj M. Aminabhavi. J. Hazardous Mater, 42 (1995) 115.

[16] Patrick Laney, Use of Composite Pipe Materials in the Transportation of Natural

Gas, INEEL/EXT-02-00992, United States, 2002.

70

[17] Website http://www.plasticpipe.org

[18] Warters, W. J., Cant, D. J., Tzeng, H. P. and Lee, P. J., 1997. Mannville gas

resources of the Western Canada Sedimentary Basin: Geological Survey of Canada

Bulletin 517, 101 p.

[19] G. Grundmeier, W. Schmidt, Electrochimica Acta, 45 (2000) 2515.

[20] ASTM. Standard Test Method for Tensile Properties of Plastics, D638-10.

[21] Tensile Properties ISO 527, International Organization for Standardization.

[22] Zhuo Zhuang, Yongjin Guo, Eng. Frac. Mech. 64 (1999) 271.

[23] Anantawaraskul, S., Soares, J. B. P., & Jirachaithorn, Macromolecular Symposia,

257 (2007) 94.

[24] H. R. Brown. Polym. 19 (1978) 1186.

[25] History and physical chemistry of HDPE, Lester h. Gabriel.

[26] David i.Bower, An Introduction to Polymer Physics, Cambridge, New York, 2002.

[27] Jingyu Zhang, Experimental Study of Stress Cracking in High Density

Polyethylene Pipes, Philadelphia, USA, 2005.

[28] Harold Schonhorn, H. L. Frisch, R. V. Albarino, J. Polym. Sci. Polym. Phys. 11

(1973) 1013.

[29] Shinji Kanehashi, Akira Kusakabe, Shuichi Sato, J. Membrane. Sci. 365 (2010) 40

[30] G. K. van der Wel, O.C.G. Adan, Organic Coatings. 37 (1999) 1

[31] Y. G. Hsuan, R. M. Koerner, Honorary Member. J. Geotech. Engrg. 119(1993)

1840.

[32] R. P. Kambour, Polym. Eng. Sci. 8(1968)281

[33] Borrajo, J., Cordon, C., Carella, J. M., Toso, S., & Goizueta. J of Polym Sci Part

B-Polym Physi, 33 (1995)1627

71

[34] Plastics -- Determination of resistance to environmental stress cracking (ESC) --

Part 3: Bent strip method, International Organization for Standardization, 22088-3:2006.

[35] Plastics -- Determination of resistance to environmental stress cracking (ESC) --

Part 2: Constant tensile load method, International Organization for Standardization,

22088-2:2006.

[36] Plastics -- Determination of resistance to environmental stress cracking (ESC) --

Part 4: Ball or pin impression method, International Organization for Standardization,

22088-4:2006.

[37] L. Vollrath. Polym. Test. 11 (1992) 83.

[38] Sid Ahmed Reffas, Mohamed Elmeguenni, Eng. Tech & Applied Sci Research 3

(2013) 452.

[39] D. J. Plazek, K. L. Ngai. Macromolecules. 24 (1991) 1222.

[40] Zouheir Sekkat. J. Opt. Soc. Am. B. 15 (1998) 401.

[41] J. Schellenberg, G. Fienhold. Polym. Eng & Sci. 38 (1998) 1413.

[42] Joy J. Cheng, Maria A. Polak. Tunnelling and Underground Space Technology.

26 (2011) 582.

[43] S. Thomas. 63 (2008) 9. IFP International Conference.

[44] N. Arihara, and T. Yoneyama, Asia Pacific Oil and Gas Conference and Exhibition

held in Jakarta, Indonesia, 20–22 April 1999. SPE54330.

[45] M. Y. Khan, A. Samanta, K. Ojha, Petrol. Sci. Technol, 27(2009)1926.

[46] Duy Nguyen and Nicholas Sadeghi, Nalco Company. SPE EOR Conference at Oil

and Gas West Asia, 16-18 April 2012, Muscat, Oman.

[47] Jay Vargo, Jim Turner, Barrett Resources. SPE Reservoir Evaluation & Engineering.

3 (2000) 552.

72

[48] Marc Charest. Canadian discovery. 1 (2013).

[49] Shutang Gao, SPE EOR Conference at Oil & Gas West Asia, 11-13 April 2010,

Muscat, Oman.

[50] James Sheng. Enhanced oil recovery field case studies. 2013.

[51] Website http://www.tiorco.com/tio/products/asp-sp.htm

[52] WL118 Pressure Rating

[53] Hou Jirui, Liu Zhongchun, Xia Huifen. SPE Rocky Mountain Petroleum

Technology Conference, 21-23 May 2001, Keystone, Colorado

[54] Charles M. Hansen. Ind. Eng. Chem. Res., 40 (1), 2001, 21–25.

[55] ASTM. Standard test method for environmental stress-cracking of ethylene plastics,

D1653-13.

[56] ASTM. Standard test method for water vapor transmission of organic coating films,

D 1653-93.

[57] Naylor, Polym. Sci. 2 (1989) 643.

[58] S. H. Teoh, E. H. Ong. Polym. 36(1995)101.

[59] N. Ibrahim, D. M. Shinozaki. Mat Sci Eng. 30(1977)175.

[60] J. Kubát, J. Petermann, M. Rigdahl. Mat Sci Eng. 19(1975)185.

[61] Scott M. Merry, Jonathan D. Bray. J. Geotech. Geoenviron. Eng. 1997.123:57-65.

[62] Kevin W. Evanson, Marek W. Urban J Appl Polym Sci. 42（1991）2309.

[63] Shuji Saito. J. Colloid. Inter. Sci. 22 (1967) 227.

[64] Maged A. Osman, Jörg E.P. Rupp, Ulrich W. Suter. Polym. 46 (2005) 8202.

[65] Jim Schmitz, 13th Middle East corrosion conference. Paper No. 10073.

[66] H. A. Alawaji, Temperature effects on Polyethylene (HDPE) and Fiberglass (GRP)

Pipes.

73

[67] Jingyu Zhang. Experimental study of stress cracking in High Density Polyethylene

pipes. November 2005. Drexel University.

[68] Mari Fukuoka, Polym. J, 38 (2006) 542.

[69] Byoung-Ho Choi, Jeffrey Weinhold. Polym. Eng. Sci 49 (2009) 2085.

[70] A. Galeski, M. Psarski. Macromolecular. 104 (1996) 183.

[71] J. Qin, P.P. Hu, J. M. Zhao, Z. Q Wu, B. J. Qian. J. Appl. Polym. Sci. 51 (1994)

1433.

[72] J. A. Reyes-Labarta, M. M. Olaya, A. Marcilla. Polym. Sci. 47 (2006) 8194.

[73] B. L. Karger, L. R. Snyder, C. Eon. J. Chromatogr. A. 125 (1976) 71.

[74] C. A. Fonseca, I. R. Harrison. Thermochim. Acta. 313 (1998) 37.

[76] R. Anbarasan, V. Dhanalakshmi. Appl. Spectrosc. 77 (2012) 619.

[77] G. Han, Y. Lei, Q. Wu, Y. Kojima, S. Suzuki. Polym. Environ. 16 (2008) 123.

[78] Hua huang, Chinese J of Polym. Sci. 18(2000)363.

[79] J. G. J. Beijer, J. L. Spoormaker, Polym. 41 (2000) 5443.

[80] Seung Won Jeong, Chem. Eng. 18 (2001) 848.

[81] Kawaguchi, Takafumi, Polym. Eng. Sci. 43 (2003) 419.

[82] J. P. Gibert, J.M.L. Cuesta, A. Bergeret, A. Crespy. Polym. Degrad. Stabil. 67 (2000)

437.

[83] P. Hittmair, R.Ullman. Appl. Polym. Sci. 6 (1962) 1.

[84] G. A. Bernier, R. P. Kambour. Macromolecules. 1 (1968) 393.

[85] Beth A. Miller-Chou, Jack L. Koenig. Prog. Polym. Sci. 28 (2003) 1223.

[86] J. Scheirs, Compositional and Failure Analysis of Polymers: A Practical Approach

[M], Wiley, Chichester. 558 (2000).

[87] E. Carrero, N.V. Queipo, S. Pintos, L.E. Zerpa. J. Petrol. Sci. Eng. 58 (2007) 30.

74

[88] B. J Qian, P. P. Hu, J. M. He, J.X. Zhao, C.X.Wu. Polym. Eng. Sci. 32 (1992) 1290.

[89] Yang Chen, J. APPL. POLYM. Sci. 8 (2013) 39880.

[90] F. Xia, F. Chen, Y. X. Hu, X. M. Wang, S.K. Lin. Scanning. 22 (2000) 366.

[91] R. M. Patel. Appl. Polym. Sci. 124 (2012) 1542.

[92] Seung Won Jeong, Chem. Eng. 18 (2001) 848.

[93] G. R. Howell, Y. F. Cheng, Organic Coatings. 60 (2007) 148.

[94] K. Nazan Turhan, Ferhunde S ahbaz, J. Food. Eng. 61 (2004) 459.

[95] ASTM. Standard test method for water vapor transmission of organic coating films,

D 1653-93.

[96] Jianhua Huang, Yubo Chen. Textile Research J. 80(5): 422.

[97] Zhongbin Zhang, Ian J. Britt. J of Appli. Polym. Sci, 82(2001)1866.

[98] Gilbert S. Banker, Ashok Y. Gore. J. Pharm. Pharmac., 1966, 18, 457-466

[99] W. C. Outhwaite, M. J. Livingston, D. H. Pashley. Arch. Oral. Biol. 21 (1976) 599.

[100] Y. Hu, V. Topolkaraev, A. Hiltner, Measurement of Water Vapor Transmission

Rate in Highly Permeable Films, 2000.

[101] George Q. Chen, Colin A. Scholes, Cara M. Doherty, Chem. Eng. J. 209 (2012)

301.

[102] Yisheng Chen, Yanxia Li, Int. J. Pharmaceut. 358 (2008) 137.

[103] Juan I. Mate´, John M. Krochta, J. Agric. Food Chem. 44 (1996) 3001.