Exxonmobil hdpe product grades combines toughness and stiffness with resistance to environmental stress cracking at elevated temperatures and warping. Ultimate durability will be a function of the stress cracking resistance of the specific hdpe resin. It is the tendency for polyethylene parts to fail prematurely in the presence of detergents, oils, and other active environments. Environmental stress cracking esc in plastics means the failure at about room temperature due to continuously acting external andor internal stresses in the presence of surface active substances known as stress cracking agents such as alcohols, soaps, dyes, agents containing moisture. Environmental stress cracking resistance of polyethylene. It is a physical process driven by the applied stress but accelerated by the presence of.
Environmental stress crack resistance lyondellbasell. The definition of stress cracking according to astm d883 is an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical strength. Many plastics owe their resilience to ductility the ability of the plastic s long, chainlike molecules to stretch, sometimes to several times their original length, explains greg rutledge, professor in mits. How to prevent stress cracking what is stress cracking. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. The most popular application of low density polyethylene is plastic bags. Polyisobutylene was originally used as a plasticiser for polyethylene but was later found also to improve the environmental stress cracking resistance. Environmental stress cracking esc can occur when a polymer is placed under tensile stress in the presence of an active chemical agent. Environmental stress cracking of polyvinyl chloride in.
Notice that each of these has stress crack resistance 15% nctl higher than most, if not all the current aashto resins for pipe. Stresscracking resistance an overview sciencedirect topics. When considering a polyethylene product for use in secondary containment applications, it is important to note that most. Such polyethylene types offer extremely high stiffness, toughness, strength, stress crack resistance and an increased crack propagation resistance. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to stress cracking. The bent strip test is a constant strain test, but polyethylene relaxes when strained.
Bottles or bags designed to contain detergents, solvents or acids must therefore have a high resistance to environmental stress cracking. These reactions may reduce the physical properties of polyethylene. This type of cracking typically involves brittle cracking, with little or no ductile drawing of the. Evaluating environmental stress cracking of medical. Environmental stress cracking occurs in polyethylene in the presence of certain environmental conditions. Escr test for ethylene based plastics was first used on a lldpe film. Standard test method for environmental stresscrack. When the sample was stressed, cracks developed in the treated area. Chemical resistance and stress cracking in polyurethanes. Environmental stress crack resistance of polyethylene. Therefore, these resins could be useful for enhancing the properties of post consumer hdpe. Plastics will exhibit environmental stress cracking when exposed to an aggressive chemical environment if and only if a tensile stress is present.
Environmental stress cracking of polypropylene springerlink. Like lldpe, it is produced by low pressure polymerization techniques using transition metal catalysts. This stress relaxation allows testing to run without failure for very long periods 1,500 hours. The cause of this type of part cracking is internal stress caused by high plastic pressure during molding. Lowdensity polyethylene ldpe is a very flexible material with unique flow properties that makes it particularly suitable for shopping bags and other plastic film applications. In the case of failure involving fracture, the determination of the failure mode involves identifying how the crack ini. Grilon cf 6s is particularly suitable for boilinbag applications. Stress corrosion cracking of polyethylene practical. Since variance of any of these factors can change the environmental stresscrack resistance of the container, the test results are representative only of a given test performed under defined conditions in. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical strength.
Polyethylene bag 4 teacher notes for polyethylene bags. Thus, if conditions are unfavorable, various oils or even common household cleaning agents may cause a polyethylene bag, pipe, tank. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by. This type of low density polyethylene can be processed into thinner films with better environmental stress crack resistance than hdpe and ldpe. The polymer chains eventually creep toward the pull and deform. Environmental stress cracking esc is the premature cracking of a polymer due to the combined action of a stress and a fluid. Get a handle on stresscracking in hdpe bottles one of the first applications for blow molded hdpe bottles was the replacement of glass for bleach packaging. Plastic failure through environmental stress cracking esc, the plastic killer, is a frequent cause of field failures by jeffrey jansen the madison group, madison, wisconsin, usa environmental stress cracking esc is considered a lead ing cause of plastic part failure. Another form of stress corrosion cracking is attributable to incipient aging effects on the polyethylene coating.
Plastic failure through environmental stress cracking. Failure analysis of lldpe based materials stellenbosch university. Polyethylene with multimodal molecular weight distribution consists of several polymer fractions, which are homogeneously mixed. This problem only occurs when using rigid plastics. The stresses can arise from multiple sources, many of which are dif. They are also much higher in cracking resistance than post consumer hdpe. Diagnose it before it ruins your parts residual stress is an inherent result of plastics manufacturing processes and occurs when molten polymer is cooled and shaped. Polyethylene is susceptible to attack by some chemicals which may cause stress cracking, swelling, oxidation or may permeate the polyethylene. Mdpe exhibited excellent scg and stress crack resistance, and dominated pe pipe manufacture for 30 years until the development of bimodal hdpe pe100.
It is interesting to note that environmentalists frequently claim that the plastic bags. Available in the latin american and north american markets. The tensile stress may be applied externally or may simply be a consequence of residual, or moldedin, stresses. Determination of environmental stress cracking resistance.
Under the conditions of the test method, high local multiaxial stresses are developed through the introduction of a controlled imperfection 2, 3. When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. The inversely proportional relationship between m e and escr indicates that. One of the fastest growing plastics today is linear lowdensity polyethylene lldpe, used mainly in film applications but also suitable for. Advancing surface oxidation of plastic makes it less suitable for lowtemperature applications. It is a solventinduced failure mode, in which the synergistic effects of the chem ical agent and mechanical stresses result in. Failure caused by environmental stress cracking may be attributed to stored stresses acquired. Hdpe containers are opaque or translucent but never transparent due to the.
It is soft and pliable and has applications ranging from plastic bags. One of the most common causes of failure for polyethylene components is environmental stress cracking esc, a form of lowstress failure that is accelerated by environmental interaction. Timetemperature equivalence in environmental stress cracking of. Some copolymers of ldpe are available with improved stress crack resistance.
Investigations on environmental stress cracking resistance. In this context, possible damage mechanisms of polyethylene have to be considered, such as light or heat aging, oxygeninduced embrittlement, and stabilizer degradation. Standard test method for environmental stresscracking of. Environmental stress crack resistance of polyethylene figure 2 astm f1473 the polyethylene notch tensile pent test this method is used in the united states to test pe pipe grade materials that exhibit high escr values, as this test generally provides more timely results. Environmental stress cracking refers specifically to a phenomenon observed in polyethylene. Experimental study of stress cracking in high density polyethylene pipes a thesis submitted to the faculty of drexel university by jingyu zhang. Environmental stress cracking may account for around 1530% of all plastic component failures in service.
Environmental stress cracking esc is not the result of the continual battle that the plastics industry has in convincing environmentalists and politicians that plastics can be the solution to environmental problems and not the cause esc is the consequence of a combination of mechanical stress and exposure to an aggressive chemical. Environmental stress crack resistance escr testing of. A study, using the compressedring environmental stress cracking resistance test, 1 shows that polyethylene pipe can be degraded after only a short exposure time figure. In this work the environmental stress cracking resistance of two highdensity polyethylene grades, employed for the manufacturing of bleach bottles, was investigated by using a fracture mechanics. The higher relative amount of polyethylene created a part that was not as stiff as intended, making it more prone to deformation and cracking under longterm stress. Ten per cent pib in polyethylene gives a compound resistant to stress cracking as assessed by the severe btl test. It usually occurs at high levels of applied stress. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. These are amorphous plastics, and they bond readily.
This type of cracking typically involves brittle cracking, with little or no ductile drawing of. Other plastics such as polyethylene, polypropylene, and polytetrafluoroethylene teflon are semicrystalline thermoplastics and. Plastic components can fail via many different modes, including catastrophic mechanisms, such as brittle fracture, ductile overload, creep rupture, environmental stress cracking, molecular degradation, and fatigue. Wetting agents, such as detergents, accelerate stress cracking. Most studies on the environmental stress cracking of polyethylene focus on the effect of tiemolecules, but physical chain entanglements are another source of interlamellar linkages that contribute to esc of polyethylene. Environmental stress cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. Ldpe applications packaging thanks to its low cost and good flexibility, ldpe is used in packaging industry for pharmaceutical and squeeze bottles, caps and closures, tamper evident, liners, trash bags, films for food packaging frozen, dry goods, etc. It is the premature embrittlement and subsequent cracking of a plastic. And in the process, they can remove some stress from a visit to the supermarket or subsequent drive through the parking lot. Environmental stress cracking is a failure mechanism very similar to slow crack growth in terms of shape. Bottles or bags designed to contain detergents, solvents or acids must therefor have a high resistance to. Environmental stress cracking resistance escr of polyethylene has.
Ldpe has high ductility but low tensile strength, which is evident in the real world by its propensity to stretch when strained. Experimental study of stress cracking in high density. Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in yield point among various polyethylenes. Environmental stress cracking the the chemical agent. Hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Environmental stresscracking resistance of ldpeeva blends. This sample was treated with a detergent in the darker area shown. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. Environmental stress crack resistance of polyethylene today, this test is specified less frequently because it is not as aggressive towards modern resins. The total number of equations is equal to the total number of nodal parameters and the equations are usually of a banded nature. Polyethylene with multimodal molecular weight distribution.
Environmental stress cracking can take place through continuous crack growth or via a stepwise progressive mechanism of cracking and arrest, depending on the molecular structure of the plastic, the composition of the chemical agent, the level of stress, and other environmental conditions. Injection molded part defects cracks in molded parts. The detergent stresscracking of polyethylene 509 loaddisplacement relation for the complete structure may be obtained, resulting in a set of equations of the form f kgs. Characterising the environmental stress cracking behaviour. Environmental stress cracking esc is a slow crack growth process which. The nature of polyethylene pipe failure plasticstoday. When the environment was to be placed into a sealed ldpe bag with the. Paxon hdpe product grades provides outstanding quality in blow molding and rotational molding applications. With force and time, crazing or stress cracking occurs. This stress can be caused by a cavity pressure that is either too high or too low. Polymer network mobility and environmental stress cracking. In transparent parts, you will usually see these internal cracks before the molded part fails. It is one of the most common causes of unexpected brittle failure in thermoplastic.