Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

The interaction between poly(ether ether ketone) (PEEK) and three solvents (i.e., methylene chloride, tetrahydrofuran, and acetone) was studied by means of several complementary techniques. A series of eleven 0.3 mm thick PEEK films was produced. Each film had a certain crystallinity index and crystal morphology, as revealed by optical microscopy, differential scanning calorimetry (DSC), and wide angle X-ray scattering (WAXS). Dynamic solvent uptake measurements were performed on each film with the three solvents. Methylene chloride swelled both amorphous and semi-crystalline PEEK to high degrees, while tetrahydrofuran and acetone swelled amorphous PEEK only. After desorption, the samples were carefully analyzed to characterize solvent-induced crystallization (SINC), which occurred in amorphous PEEK exposed to all three solvents. Diffusion of methylene chloride and tetrahydrofuran in amorphous PEEK was observed in fractured specimens by scanning electron microscopy, while SINC was followed by DSC. The SINC process was found to be diffusion controlled. Diffusion of both solvents through the polymeric film took place in the first third of the equilibrium time, while swelling occurred in the remaining time. The mechanical properties of all solvent-exposed PEEK films were tested by three types of experiment (i.e., glass cone technique, microtensile tests, and tensile test in the environmental scanning electron microscope (ESEM)), which were first verified with another semi-crystalline thermoplastic polymer, isotactic polypropylene. PEEK specimens were stressed in the inside of glass cones and immersed in a series of solvents. Differences among solvent uptakes of stressed and unstressed specimens were explained in terms of crazing and SINC. Microtensile tests were performed on completely swollen PEEK specimens. Plasticization and delocalized crazing were found in the case of amorphous PEEK exposed to the three solvents and semi-crystalline PEEK exposed to methylene chloride, while classical crazing occurred in the other specimens. Finally, an amorphous PEEK specimen was swollen in acetone and stretched in the ESEM in acetone vapor. A very ductile deformation was observed, which occurred at the necked region between two notches. The results are discussed in terms of T_g depression and plasticization. / Ph. D.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / This study concerns the phenomenon of environmental stress cracking resistance (ESCR) in three impact polypropylene copolymers (IPPCs). The main purpose was to correlate the ESCR with their properties such as microstructure, molecular weight (MW), molecular weight distribution (MWD), crystallinity and morphology. Initially the selection of a suitable test method and an active stress cracking agent (SCA) were the preliminary concerns. The Bell telephone test was used to evaluate SCAs, while a published procedure for determining ESCR of ethylene based plastics was adapted for the purpose of this study. Isopropanol was selected as SCA. Polymers were fully characterized by FTIR, 13C NMR, DSC and high temperature GPC. Optical microscopy was used to investigate craze formation and crack growth, and scanning electron microscopy (SEM) was used to study the morphology of the polymers. Since IPPCs are known to have multi-fraction copolymeric structures and each of these fractions has significantly different average properties, fractions were selectively removed from the materials, either by solvent extraction at room temperature, or by TREF fractionation. The effect of removing these fractions on the ESCR was determined. The effect of the molecular composition of the three IPPCs on the ESCR of these materials, as well as the effect of the removal of the selected molecular fractions on the ESCR, morphology and molecular characteristics are discussed and compared. Conclusions are drawn as to the factors controlling ESCR in these materials.

Environmental stress cracking (ESC) is one of the main, and probably the most common, failure mechanisms involved in polymer fractures. This type of failure is critically important as it occurs suddenly, without any visible pre-fracture deformation. Such failure can be catastrophic and costly in cases where structural integrity is required. In polyethylene (PE), ESC occurs through a slow crack growth mechanism. Cracks initiate from stress-concentrated imperfections, propagate through the bulk of PE, and ultimately result in a brittle fracture. In order to predict the environmental stress cracking resistance (ESCR) of PE, it is necessary to fully understand the molecular structure of the resin. In this thesis, attempts were made to find relationships between molecular structure characteristics and material responses, mainly inter-lamellar entanglements and strain hardening behaviour of PE resins, through mechanical and rheological experiments. Inter-lamellar entanglements are believed to be the main factor controlling slow crack growth of PE. Extent of entanglements and entanglement efficiency were investigated by monitoring the strain hardening behaviour of PE resins in the solid state through a uniaxial tensile test, and in the melt state, through extensional rheometry. ESCR is usually assessed by unreliable and time consuming testing methods such as the notch constant load test (NCLT) on notched PE specimens in the presence of an aggressive fluid and elevated temperatures. In this thesis, a practical, yet reliable, tensile test was developed for the evaluation and prediction of ESCR. The developed test offers a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and presence of aggressive fluids. Through this test, a factor called ???corrected hardening stiffness (cHS)??? was developed, which can easily be used for a relative ranking of ESCR of different PE resins. Studies were next extended to the melt state via shear and extensional rheometry. Through studies in the shear mode, a molecular weight-normalized average characteristic relaxation time (??N) was found to be efficient in predicting the extent of chain entanglements in resins. This provided a potential melt indicator for a relative measure of ESCR, for linear low density polyethylene (LLDPE), with different short chain branching levels. Extensional studies were conducted to evaluate the strain hardening behaviour in the melt state. An inverse correlation was obtained between ESCR and the melt strain hardening coefficient (MSHC), found from Sentmanat Extensional Rheometry (SER). This indicated an inverse relationship between ESCR and chain extensibility in the melt. In addition, a new factor called ???melt hardening stiffness (mHS)??? was developed from the slope of a stress-strain line, obtained from SER. This factor, analogous to cHS, can be used for a practical and reliable ranking of ESCR of PEs. ESCR is usually associated with classical crystalline phase property indicators, such as crystallinity and lamella thickness. In this thesis, the effect of processing and post processing temperature on the extent of inter-lamellar entanglements were investigated, evaluated, and correlated to ESCR. Also, analysis of the lamella surface area (LSA) was pursued since LSA reflects changes in phase interconnectivity more precisely. The focus of this part of the study was on the effect of temperature on LSA to identify the optimum processing and post-processing conditions which yield a higher LSA. It was reasonable to presume that PE with larger lamella lateral surface areas will have more inter-lamellar entanglements, hence higher ESCR. Finally, a well-controlled ultraviolet (UV) photoinitiated reactive extrusion (REX) process was developed for selective formation of long chain branches in the PE structure. This was conducted to impose restrictions against stretching of the polymer chain, which consequently enhanced ESCR.

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The influence of two lubricating and cleaning agents and diesel fuel on environmental stress cracking of polyamide reinforced by glass fibers and polycarbonate was studied in this master thesis. Testing of environmental stress cracking was performed by the method of critical bending deformation. Bergen elliptical strain jig was used for testing. The test liquids caused varying levels of environmental stress cracking of amorphous polycarbonate, but they did not cause environmental stress cracking of glass fiber reinforced polyamide. The test method used allows testing the resistance of both materials against environmental stress cracking of any liquid. The results can be used to design products that are expected to be exposed to corrosive liquids. Quantification of the influence of stress free corrosion on the static and impact properties of polycarbonate and glass fiber reinforced polyamide was performed by accelerated test. Specimens were exposed to test liquid and an elevated temperature of 70 °C. The mechanical properties of the tested materials were significantly affected by elevated temperature exposure. The yield strength of the polycarbonate has increased and the toughness has decreased due to physical aging. The tensile strength of glass fiber reinforced polyamide has increased due to a decrease of the moisture content of the material.

Environmental stress cracking (ESC) is a significant problem that has plagued the plastics industry since its discovery nearly 70 years ago. The accelerated brittle failure brought about when a stressed polymer comes in contact with an aggressive environment can happen suddenly with destructive results. Many classes of polymers are susceptible to this type of slow crack growth; however special emphasis has typically been placed on polyolefins due to their wide range of working environments, market dominance and their seemingly chemical resistance. Much research has been focused on formulating environmentally resistant materials, while the evaluation techniques for gauging environmental stress cracking resistance (ESCR) seem to have been left behind. This research focuses on developing a reliable testing technique for evaluating the ESCR of polyethylene resins. Passive acoustic monitoring was adapted to an industrially accepted ESCR test in an attempt to hear polymer damage before it was visually apparent. It was discovered that the low energy released during the early stages of damage and excessive background noise masked passive signals, making this method of evaluation impractical. Alternatively, active ultrasonic monitoring through velocity and attenuation measurements was investigated to see if probing techniques could be used to detect structural damage. Active ultrasonic monitoring of static and tensile stressed samples were able to differentiate plasticization after ageing, however no indication of ESCR properties could be inferred. A novel forced based monitoring system was developed in response to the acoustic testing techniques. Force monitoring was able to provide useful information regarding the failure cycle of ESC and the acquired profiles could describe a failure onset time. Several ageing environments were also tested with force monitoring and a traditional ESCR test to reveal the stress cracking ability of biodiesel, an important finding. / Thesis / Master of Applied Science (MASc) / Accelerated failure of stressed plastics can occur upon exposure to fluids through a phenomenon known as environmental stress cracking (ESC). The following research outlines the development of a novel testing technique to gauge a material’s environmental stress cracking resistance (ESCR). Adaption of passive acoustics to an existing stress cracking test was unable to provide any indication of ESCR, however the use of active ultrasonics was able to show sample plasticization. A novel forced based measuring technique was found to uniquely map the failure progression of a sample undergoing ESC, providing valuable information for understanding the phenomenon. Additional testing was also completed on various environmental fluids to reveal biodiesel’s ability to provoke ESC, an important observation.

The first paper, “Dye Penetrant Induced Microcracking in High-Performance Thermoplastic Polyimide Composites”, studied the possibility of spurious microcracking in three high-performance thermoplastic polyimide composite materials due to zinc iodine dye penetrant. The material systems were IM7/LaRC™-IAX, IM7/LaRC™-IAX2, and IM7/LaRC™-8515. Specimens from each material system were subjected to one of three immersion tests. The first immersion test involved soaking composite specimens previously prepared with different polishing techniques in dye penetrant. In the second test, specimens were immersed in the individual components of the dye penetrant. The final test involved exposure of specimens to one of six solvents followed by exposure to dye penetrant. Results showed that the composite materials have sufficiently high thermal residual stresses to drive microcracking in the presence of dye penetrant without external mechanical loading. There was no evidence that the different polishing techniques had an effect on dye penetrant-induced stress cracking. The dye penetrant components did not produce microcracks in the composites. Some combination of the components must be present to induce microcracking. Observations also revealed that polishing had an effect on the microcracking process of the composites that were initially exposed to solvents then dye penetrant. The second paper, “The Effect of Environmental Stress Cracking on High-Performance Polymeric Composites”, studied solvent stress cracking and solvent-induced strength degradation on four polyimide matrix materials developed at NASA-Langley Research Center. These materials are LaRC™-IAX, LaRC™-IAX2, LaRC™-8515, and LaRC™-PETI-5. Cross-ply specimens were used to characterize solvent stress cracking in composites. Matrix cracking due to solvent exposure was observed in all of the materials. The solvent exposure time of the materials ranged from 1 minute to 96 hours. The results show that residual thermal stresses due to processing in the cross-ply composite specimens are sufficient to drive solvent stress cracking in the matrix. Solvent application lowers the microcracking toughness, G_mc ,values such that the available strain energy, G_m, within the transverse ply groups is sufficient to initiate microcracking. In the absence of a solvent, the same G_m value would not induce microcracking. Transverse flexure tests were performed on unidirectional specimens to determine the effects of the solvents on the material strengths. The presence of certain solvents severely degraded the materials. The manner in which the solvents were applied to the materials determined the degree of material degradation. The results revealed a synergistic effect between stress and solvent. The tests showed that diglyme, MEK, and acetone produced the most severe damage to the materials. The most solvent resistant material was LaRC™-PETI-5. This is followed by LaRC™-8515, LaRC™-IAX2, and LaRC™-IAX respectively. LaRC™- PETI-5 is a thermoset whereas the remaining materials are thermoplastics. / Master of Science