Environmental Stress Cracking Resistance Research Articles

Relationships between impact performance and structures of rotationally molded crosslinked high‐density polyethylene

AbstractCrosslinked high‐density polyethylene (XL‐HDPE) is a preferred material for chemical and fuel tanks due to its superior environmental stress crack resistance and impact strength. The impact performance of rotationally molded specimen is important for final products. In the research the drop weight impact strength (defined as ARM impact strength) of rotationally molded XL‐HDPE is tested between −40°C and 25°C. The crosslinking content, crystallization characteristics, and dynamic mechanical properties (DMA) of different thickness gradients are examined to illustrate the relationships between the impact strength, brittle‐ductile transition (BDT) and microstructures. The innermost surface layer (about 0.3 mm) has lower gel content, higher crystallinity, and average lamellar thickness compared with the body part. The ARM impact strength is about 1 J/mm at −40°C and −30°C, and about 29 J/mm at −20°C ~ 25°C. There is a BDT between −30°C and − 20°C. After removing the innermost surface layer, the sample breaks in ductile manner in the entire tested temperature range, and the ARM impact strength is about 24 ~ 26 J/mm. The DMA results show that the BDT is consistent with the structure transition of the innermost surface layer. The microstructures of rotationally molded XL‐HDPE in the innermost surface layer dominate the low temperature impact performance.

Application of short-term methods to estimate the environmental stress cracking resistance of recycled HDPE

Environmental stress cracking is a serious problem for polyethylene because it can cause failure without any visible warning due to the slow crack growth accelerated by aggressive agents. Tie molecules and entanglements are the main macromolecular characteristic increasing environmental stress cracking resistance, thus in this work mechanical and thermal properties governed by those macromolecular characteristics are determined by performing simple tests executable in the industrial laboratories for quality control on recycled high-density polyethylene. The mutual relation between the determined properties confirms their dependence on the investigated macromolecular characteristics and allows to predict in a comparative way the expected environmental stress cracking. The mechanical properties related to the environmental stress cracking resistance are the strain hardening modulus and the natural draw ratio. The strain hardening modulus is an intrinsic property that measure the disentanglement capability of the inter-lamellar links and the natural draw ratio is a highly sensitive parameter to the macromolecular network strength via the intercrystalline tie molecules. Since the measurement of these properties according to the standard ISO 18,488 requires a temperature chamber not often available in the industrial laboratories, the tensile test was performed also at room temperature and displacement rate 0.5 mm/min; a proportionality between the data obtained at different test condition emerged. The thermal property related to the environmental stress cracking resistance is the stepwise isothermal segregation ratio that state the chain fraction that generates a high rate of tie molecules responsible of environmental stress cracking resistance.

On the Environmental Stress Cracking Resistance of High Density Polyethylene (HDPE)

This research reports a visualization investigation of environmental stress cracking (ESC) of a high-density polyethylene (HDPE) prepared in house and a commercial high-density polyethylene. The in house made HDPE showed an excellent resistance against ESC as none of the tested samples cracked under chemical solution and that was due to higher value of molecular weight and less percentage of chain branching. On the other hand, the commercial HDPE had poor resistance against ESC under chemical solution, as all tested samples was cracked and completely splitted up into two pieces. An interesting finding of this research was that the commercial HDPE of relatively lower molecular weight cracked when stressed within the environment of ordinary water. Finally, this research recommended a thorough investigation of the role of additives and fillers on the ESC resistant of Polyethylene.

Influence of talc fillers on bimodal polyethylene composites for ground heat exchangers

AbstractIn this study, a commercial grade of talc is used as filler in a bimodal high‐density polyethylene (HDPE) used for the pressure pipe application. The composites are characterized by thermogravimetric analysis (TGA), differential scanning calorimetry, dynamic mechanical thermal analysis, and tensile testing. The results illustrate that the presence of talc has a considerable effect on the material properties and the pipe life‐length. It is presented that the thermal stability measured by TGA is enhanced, while the oxidation induction time decreases in cooperation of the talc. The nucleation behavior of talc particles during crystallization has no obvious effect on melting temperature; however, an increase in crystallization temperature is evidenced. Storage modulus as recorded from the dynamic mechanical analysis is also increased in all composites, furthermore, the temperature of the α relaxation is shifted toward higher temperature and finally the strain hardening modulus for the HDPE/talc composites is assessed and compared to the neat HDPE as a measure of environmental stress crack resistance.

A comparison between $K$ and $G$ approaches for a viscoelastic material: the case of environmental stress cracking of HDPE

According to linear elastic fracture mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson’s ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the slow crack growth and the environmental stress cracking resistance of two high-density polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behavior of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behavior exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

Influencing the environmental stress cracking resistance of amorphous thermoplastic parts by the example of polycarbonate and water

Plastic components in use are often exposed to a large number of influences which strongly affect the functionality of the component and are often the main cause of their failure. Here, the environmental stress cracking (ESC) is considered as the main cause for the failure of plastics. Until now, there are only a few possibilities on how to predict the ESC resistance of a plastics component in use and the engineer is often confronted with this failure after the product development process is already completed. This investigations deal with that problem and it is shown how the stress cracking resistance of plastics can be influenced according to the already completed product development by the example of polycarbonate under the influence of water. The focus is on the targeted influencing of the internal part properties, the influence of additional fiber reinforcement and as a last option the selection of a new material under the elimination of elaborate experiments using molecular dynamics simulations. POLYM. ENG. SCI., 59:E361–E366, 2019. © 2018 Society of Plastics Engineers

Operational maps between molecular properties and environmental stress cracking resistance

ABSTRACTEnvironmental stress cracking (ESC) is one of the main failure mechanisms involved in polymer fractures. This work focuses on the ESC resistance (ESCR) of polyethylene (PE) in which ESC occurs through a slow crack growth mechanism. To predict the ESCR of PE, it is necessary to fully understand the molecular structure of the resin. This work demonstrates the relationships between molecular structure characteristics and material responses based on experimental characterization and published literature trends. Relationships between ESCR, molecular weight, percentage crystallinity, and density were used to create ESCR and molecular structure maps, which can be used to improve the development of PE resins with a desirable (better/higher) ESCR. These maps along with a logical flow chart offer practical prescriptions and describe pathways toward the development of PE with a desirable ESCR. In addition, this work presents case studies that demonstrate the effectiveness of this approach. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47006.

Time-temperature equivalence in environmental stress cracking of high-density polyethylene

In this work the environmental stress cracking resistance of two high-density polyethylene grades, employed for the manufacturing of bleach bottles, was investigated by using a fracture mechanics approach. Two aqueous solutions similar to common bleach products (with and without sodium hypochlorite) were considered as the active environment, whose effect on both the initiation and propagation phases of fracture was evaluated for the two materials. Tests at different temperatures were performed and a time-temperature superposition scheme was applied both in air and in the active environment. Temperature was shown to influence fracture behaviour only through the material inherent viscoelasticity.

A New Concept of using Transverse Loading to Characterize Environmental Stress Cracking Resistance (ESCR) of Polyethylene (PE)

Research & Development in Material Science A New Concept of using Transverse Loading to Characterize Environmental Stress Cracking Resistance (ESCR) of Polyethylene (PE) PY Ben Jar* Department of Mechanical Engineering, University of Alberta, Canada *Corresponding author: PY Ben Jar, Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada Submission: December 12, 2017; Published: February 23, 2018 DOI: 10.31031/RDMS.2018.03.000574 ISSN: 2576-8840Volume3 Issue5

Controlling crystal polymorphism of isotactic poly(1-butene) by incorporating long chain branches.

Isotactic poly(1-butene) (iPB-1) is a high performance plastic with outstanding properties, such as flexibility, superior creep, environmental stress cracking and abrasive resistance. However, it exhibits a complex crystal polymorphism and polymorphic transformation behavior, which has limited its commercial development. In this paper, the incorporation of long chain branches (LCBs) causes coil contraction in the melt, which favors the direct melt-crystallization of form III that was generally crystallized from solutions and made of unconventional highly twined lamellae. Consequently, low-to-moderately branched iPB-1 samples as-crystallize from the melt into mixtures of form II and form III by compression-molding and fast cooling of the melt to room temperature, and the fraction of crystals of form III (fIII) increases with increasing concentration of LCBs, whereas highly branched samples can as-crystallize into pure form III with uniform crystal size distribution. The corresponding thermomechanical properties can be modified by controlling fIII.

Environmental effects in biaxially orientated Polymethyl Methacrylate

Environmental stress cracking (ESC) is commonly found in polymers when submerged in an environment under applied stress. In such conditions, a crack may initiate and propagate from a material defect until it reaches a critical size and causes catastrophic failure. It is known that materials with a denser molecular structure are less prone to ESC, hence amorphous polymers often suffer greatly from this effect. A fracture mechanics approach was employed to investigate the fracture mechanism and crack growth in both air and environment. Two different grades of PMMA (amorphous and biaxially stretched PMMA) were tested in solvents with similar solubility parameters as this is known to hasten crazing. Time for crack initiation and crack speed were obtained and plotted against their corresponding fracture toughness. Thus, the ESC resistance of each material can be compared and component life expectance can also be predicted. To validate the experimental results, scanning electron microscopy (SEM) was used to examine the fracture mechanism of ESC.

Effect of Fluoride‐Modification on the Phillips Cr/SiO2 Catalyst for Ethylene Polymerization

AbstractIn this work, a fluoride‐modified Phillips catalyst was investigated by using combined experimental and computational methods. The addition of fluoride to the Phillips Cr/SiO2 catalyst can increase the activity of the catalyst calcined at a low temperature and the molecular weight of the polyethylene product. DFT calculations were performed to show that the difference of the Gibbs free energy barriers between chain transfer and chain propagation increased after the introduction of the fluoride onto the silica surface, which is in accordance with the increased molecular weight of the polyethylene produced by the fluoride‐modified Phillips Cr/F‐SiO2 catalyst. The computational results also reflect the increase of the activity of the ethylene/1‐hexene copolymerization after the introduction of fluoride, although the modification has little effect on the regioselectivity of the produced ethylene/1‐hexene copolymer. Moreover, copolymers produced by the fluoride‐modified Phillips catalyst with more short‐chain branches (SCBs) in the high‐molecular‐weight fraction and a significant enhancement of the environmental stress crack resistance can be explained from the results of the DFT calculations.

Environmental stress cracking (ESC) resistance of polycarbonate/SiO2 nanocomposites in different media

ABSTRACTPolycarbonate (PC) nanocomposites containing different nano‐SiO2 contents were prepared using a twin‐screw extruder followed by injection molding. The mechanical behavior and environmental stress cracking (ESC) resistance of the nanocomposites were investigated. Compared to neat PC, the incorporation of nanofiller leads to a slight improvement in Young's modulus, tensile strength, and notched impact strength properties. Interestingly, the nanofiller enhanced the environmental stress cracking resistance of PC in different fluids (isopropanol, methanol, aqueous urea solution, and deionized water) vastly. The correlation of the mode I critical stress intensity factor with the Hansen solubility parameter shows a very good agreement for different fluids and thus allows the prediction of the stress cracking behavior as a function of the filler content for different stress‐cracking agents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45451.

A fracture mechanics approach to characterising the environmental stress cracking behaviour of thermoplastics

Environmental stress cracking (ESC) is known to affect certain thermoplastics and occurs under simultaneous exposure to both applied stress and a hostile environment. The combination of these can cause a crack to form from a flaw in the material; upon reaching a critical size, the crack may accelerate thus causing catastrophic failure in the component. Various tests have been utilised to measure the resistance of different polymers to ESC, but these are often material- and application-specific and overlook the different stages of the failure process. In the present work, a fracture mechanics approach has been developed and applied, with a view to developing a test method that has wide applicability and provides both insight into the failure mechanisms as well as information for engineering design. Experimental results are presented for the following polymer-environment combinations: linear low-density PE in Igepal solution, HIPS in sunflower oil, and PMMA in methanol. It is shown that the representation of the results in the form of G versus crack velocity and G versus time can distinguish between materials of varying ESC resistance, identify the important regions of the failure process, and enable component life prediction.

Environmental Stress Cracking Resistance of Halloysite Nanoclay-Polyester Nanocomposites

The environmental stress cracking resistance of halloysite nanoclay-polyester nanocomposites was investigated using fracture mechanics approach. The incorporation of halloysite nanoclay was found to improve the environmental stress cracking resistance of the nano-composites. The storage modulus of nano-composites measured by dynamic mechanical analysis increased remarkably as a function of halloysite nanoclay content. At 0.7 wt% nanoclay, the Tg improved from 72°C to 76°C. The fracture toughness increased up to 33% and time to failure improved 155% with the addition of 0.7 wt% of halloysite nanoclay. The maximum microhardness was found 119% higher for the same nano-filler concentration compared to monolithic polyester. The reinforcement with 1 wt% showed lower fracture toughness due to agglomerations of nanoclay which act as flaws. The presence of agglomerates weakened the bond between nano-particles and matrix hence reduces the environmental stress cracking resistance by halloysite nanoclay reinforcement.

Improvement of environmental stress cracking resistance of polycarbonate by silicone coating

To explore the possibility of using surface coating to reduce environmental stress cracking (ESC) of transparent polycarbonate (PC) parts, silicone coated and SiO2 coated PC were tested in a self-made three-point bending apparatus in the presence of ethanol. The variation of stress with time was recorded, and the surface cracking was observed to evaluate the ESC resistance of samples. Slower stress relaxation rates and fewer surface cracks indicated that silicone coating improved the ESC resistance of PC, but SiO2 coated PC was found to be no better than that of uncoated PC. Silicone coating reduced the absorption of ethanol in PC, weakening the surface plasticization, thus hindering the formation and development of cracks in PC. Nanoindentation test results showed that the mechanical properties such as hardness and elastic modulus of silicone coating are a better match for PC than SiO2 coating. This allows the silicone coating to have a favorable effect in providing continuous protection for PC under the combined action of ethanol and stress.

Investigation of the resistance to environmental stress crack of high density polyethylene parts

The research methodology for determining of the environmental stress cracking resistance (ESCR) of polymeric injection molding parts was proposed. The investigations of ESCR were carried out on the original laboratory stand. For the tests samples prepared in accordance with the standards (ISO 22088 and ISO 16770) and real objects (threaded closures) were used. The values of destruction time of samples were determined using both stable and variable loads. Studies have shown that ESCR of prepared samples depends on both their design and material properties.

Effect of Surface Silicone Coating on Environmental Stress Cracking Resistance of Transparent Polycarbonate Parts

Effect of Surface Silicone Coating on Environmental Stress Cracking Resistance of Transparent Polycarbonate Parts

The influence of sub-Tg annealing on environmental stress cracking resistance of polycarbonate

To explore the effect of physical aging on environmental stress cracking (ESC) behavior of polycarbonate (PC), sub-Tg annealing was utilized as a method for accelerated aging. Injection molded samples were annealed at 130 °C for different time varying up to 96 h. A three point bending apparatus was used to evaluate critical stress for crazing and to record the variation of stress with immersion time at constant strain. The ESC results indicated that the critical stress for crazing initiation of PC in ethanol is increased by sub-Tg annealing. However, the resistance of annealed PC to ESC with immersion time during the stress relaxation test depends on the level of initial stress. When a relatively low initial stress was used, a short time (24 h) of sub-Tg annealing reduced the stress relaxation rate and decreased the number of cracks on the surface of PC. However, under higher initial stress, the stress relaxation rate of PC had a slight change only when the annealing time was prolonged about threefold (72 h). This can be explained by the formation of cohesional entanglement sites during the sub-Tg annealing process, which was demonstrated by the thermal and dynamic mechanical tests.

Effect of processing on the environmental stress cracking resistance of high-impact polystyrene

Processing conditions have a strong effect on the final mechanical properties of products made of polymeric materials. Relevant phenomena most commonly include thermal stresses, physical ageing, frozen-in strains and molecular orientation. In this work, two different high-impact polystyrenes, processed by thermoforming, were considered: a “standard” one and a grade specifically resistant to Environmental Stress Cracking (ESC). The main effect induced by thermoforming was molecular orientation. The local degree of orientation was measured on a thermoformed product and its effect on the material ESC behavior in sunflower oil was studied. A Fracture Mechanics approach was applied to evaluate the fracture resistance of the two materials. Results show that a higher degree of orientation increases the fracture resistance in air but has no effect on the (expectedly lower) resistance in the active oil environment.