Since variance of any of these factors can change the environmental stress crack resistance of the container, the test results are representative only of a given test performed under defined conditions in the laboratory. Chemical resistance can be dependent upon exposure time, temperature, stress molded in and any external stress, type of chemical and the chemical concentration. 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. A materials resistance against this type of failure is called environmental stress cracking resistance escr. At a high enough stress a polymer will crack even with a poorlycompatible solvent.
Environmental stress crack resistance of polyethylene stress crack reagent concentration. It became apparent that environmental stress failure was an achilles heel for these otherwise high performance materials and exposure to application environments needed to be strongly considered. Hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Highdensity polyethylene hdpe resins are well suited for a range of extrusion film applications such as grocery, garbage and deepfreezer bags 10 to 25 microns and injection, blown and rotational molding processes. Chemical resistance chart for plastics an indepth look at. Environmental stress crack resistance lyondellbasell.
Both cause the plastic to crystalize which increases the risk for environmental stress cracking. Basic understanding of stress cracking in hdpe products. Esc is the occurrence of cracking in a polymer by contact with a liquid or vapour environment and stress. Cn103087267b formula for improving escr environmental.
Throughout a semicrystalline polymer, there may be small packets of crystals contained within the amorphous matrix, with some polymer chains being so long that they can be in two different crystals at the same time. 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. Jan 18, 2011 hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Modeling of the fracture mechanism of hdpe subjected to. Mdpe exhibited excellent scg and stress crack resistance, and dominated pe pipe manufacture for 30 years until the development of bimodal hdpe pe100. 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. Stress cracking agents, such as the liquids mentioned above, migrate into minute cracks in the crystalline areas of the polyethylene molecules forming the surface. Prediction of environmental stress cracking in plastics. Some performance evaluation tests that once took only days to run now take months with todays. A products thermal stress cracking resistance tscr depends on the polymer compound formulation and the processing conditions under which the wire or cable was extruded, in addition to the temperature of the environment. Environmental stress cracking resistance improves with increasing hbpe content, and, for a constant hbpe concentration, it increases with increasing octene content of the hbpe.
Environment stress cracking of hdpe products is quite common due to its semi crystalline nature which gets attacked due to chemicals present in the environment. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. Geomembranes made from high density polyethylene hope have a high percent crystallinity and are therefore of concern with regard to stress cracking. Apr 16, 2015 fracture mechanics testing for environmental stress cracking in thermoplastics duration.
Chemical resistance chart for plastics an indepth look. 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. Different polymers exhibit varying degrees of escr. Adding hbpe with a low octene content, however, results in reduced failure times of the blend compared with hdpe blends that exhibit relatively good environmental stress cracking behavior. The nature of polyethylene pipe failure plasticstoday. Semicrystalline polymers such as polyethylene show brittle fracture under stress if exposed to stress cracking. Packaging testing equipment by recommended for you. It will also describe some of the tests used to measure escr in hdpe products and how resin properties influence escr. Studies on the environmental stress cracking resistance of polyethylene blends have, so far, only been described for blends of lldpe and ldpe for polyethylene water tubes, and for hdpeldpe blends in the context of the testing of a new method to determine stress cracking resistance and to optimize polymer properties 15, 16. In this research, the esc behaviour of hdpe pipes was studied in asp floods under conditions that simulated the eor reality. Smithers provides an overview of environmental stress cracking for polymer materials, and how test programs can help reduce or even prevent failure. It has been estimated that 15% of all failures of plastic materials in commercial use are related to environmental stress cracking esc 3. During the summer months, stresscracking of hdpe containers reaches its peak. There are any number of reasons for stress cracking in hdpe including detergents and fatty acids.
This is because stress from different sources increases during the summertime. Studies on the environmental stress cracking resistance of polyethylene blends have, so far, only been described for blends of lldpe and ldpe for polyethylene water tubes, and for hdpe ldpe blends in the context of the testing of a new method to determine stress cracking resistance and to optimize polymer properties 15, 16. Some polymers, such as polypropylene, are not affected by slow crack growth. Environmental stress crack resistance service temperatures up to 60c 140f is highly resistant to a wide variety of rather reactive cracking of escr is carried out on molded specimens 18 in. 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 poissons ratio of the considered material. A recent report on a study of the expected lifetime of highdensity polyethylene hdpe pipes containing chlorinated potable waters adds a new, and perhaps primary, parameter to the conventional parameters of stress cracking resistance scr, oxidative induction time oit, and carbonyl index ci for the determination of remaining lifetime of hdpe piping. Standard test method for environmental stresscrack. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. This research taken an industrial focus, as a result, test methods were selected. New thoughts on hdpe pipe durability in chlorinecontaining water. Environmental stress cracking may account for around 1530% of all plastic component failures in service.
Polyethylene resins with higher short chain branch content have higher esc crack activation energy. Flexible geomembrane liner xr5 is 300% stronger than 60 mil hdpe, has no issues with environ. Experimental study of stress cracking in high density polyethylene pipes a thesis submitted to the faculty of drexel university by jingyu zhang. The commonly used test for determining escr of hdpe can be time consuming and rather imprecise. Environmental stress cracking resistance escr is an important indicator of performance for high density polyethylene hdpe in structural and polymer pipe applications. Environmental stress crack resistance is also important when selecting appropriate resins.
Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present. Hdpepolyethylenecovinylacetate eva and lowdensity polyethylene ldpeeva blends were tested and compared with respect to their environmental stress cracking resistance escr using the. Environmental stress crack resistance escr is a commonly used test to characterize cracking failure of highdensity polyethylene in applications such as wires, cables, blow molded containers, and other rigid packaging applications. Environmental stress cracking an overview sciencedirect. 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. The mechanisms of environmental stress cracking proposed in the literature will be discussed in detail along with testing methods commonly employed. Each phase of the application development process shown in figure 2 can influence environmental stress cracking. Escr problems get more pronounced in the summer, because of several factors related to increased heat and humidity. Environmental stress crack resistance escr testing of plastics. Various scientific approaches for estimating geomembrane servicelife in buried applications have been published see paragraph 2 below. Some grades of hdpe have very good resistance against esc, while some have marginal resilience.
The escr of polyethylene, one of the most widely used plastic materials, is of particular concern in the plastics industry and is the subject of this investigation. Wire and cable polymer compounds thermal stress cracking. It is the combination of stress and fluid contact that causes failure to occur much earlier than either component could achieve in isolation, if at all. Premature cracking can be caused by topload stress, internal pressure, moldedin stress, and tail swing. Due to its high density, hdpe is a crystalline material and when a high tensile stress is applied, the material will stretch and yield before ductile failure but at low tensile stress, the material will fail suddenly with a smooth break typical of brittle failure. Environmental stress crack resistance of polyethylene ineos. Enhancing the environmental stress crack resistance of hdpe. The ability of a polymer to resist slow crack growth or environmental stress cracking is known as escr. Environmental stress cracking resistance of polyethylene. Experimental study of stress cracking in high density polyethylene. Environmental stress crack resistance escr testing of. Environmental stress cracking can be accelerated in the presence.
A tensile strain hardening test indicator of environmental. This thesis studies the resistance of hdpe pipeline materials to environmental stress corrosion cracking due to the exposure to asp chemicals. The constant strain tests favor lower crystallinity lower density polyethylene as the resultant surface stress is much lower than hdpe expected. Investigations on environmental stress cracking resistance of. Chemical attacks are sometimes considered esc environmental stress cracking when related to vulnerabilities in the plastics structure. It can lead to a catastrophic breakdown or failure of a plastic material or part. Because parts made with vistamaxx enhanced hdpe are tougher than those made with pure hdpe, they are less likely to fail through environmental stress cracking. 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. A recent report on a study of the expected lifetime of highdensity polyethylene hdpe pipes containing chlorinated potable waters adds a new, and perhaps primary, parameter to the conventional parameters of stresscracking resistance scr, oxidative induction time oit, and carbonyl index ci for the determination of remaining lifetime of hdpe piping. Xr5 is thinner and stronger than hdpestandard 30 mil thick xr5 strength significantly exceeds 100 mil hdpe. Definition of environmental stress cracking esc 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, often referred to as esc, causes a stressed plastics part to become brittle and crack when in contact with a wide range of fluids that act as stress cracking agents. Grace hsuan, jy zhang and wk wong department of civil, architectural and environmental engineering, drexel university, philadelphia, usa abstract the 100year stress crack resistance scr of corrugated hdpe pipes was evaluated using a 600 mm diameter pipe. This technical publication defines environmental stress cracking esc and environmental stress cracking resistance escr and why escr is an important resin property. The invention provides a formula for improving escr environmental stress cracking resistance performance of highdensity polyethylene and a preparation method of the formula, and belongs to the technical field of high polymer materials. Investigations of environmental stress cracking resistance of hdpeuhmwpe and ldpeuhmwpe blends article in journal of polymer research 205 may 20 with 141 reads how we measure reads. Hdpe uhmwpe and ldpeuhmwpe blends were prepared and the environmental stress cracking resistance escr was compared using the belltelephone test. Esc is the consequence of a combination of mechanical stress and exposure to an aggressive chemical. Cn103087267a formula for improving escr environmental.
Studies of environmental stress cracking of high density. The results reported here confirm that environmental stress cracking phenomena in plastics depend on the size and shape of the test liquids molecules as well as their hansen solubility parameters hsp relative to those of the polymer. Environmental stress crack resistance of polyethylene introduction to environmental stress cracking and escr over the past decade, highdensity polyethylene hdpe materials have improved significantly and now meet more stringent performance standards. Jun 19, 2008 environmental stress cracking resistance escr is an important indicator of performance for high density polyethylene hdpe in structural and polymer pipe applications. It is one of the most common causes of unexpected brittle failure in thermoplastic. Environmental stress astm d 1693 test method for 2 24 hr duration crack resistance for environmental stresscracking of 50% failure max escr ethylene plastics. Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in. Environmental stress cracking is a physical phenomenon that is initiated as a slow surface crack. Another issue can be caused by too much heat and dwell time during the forming of the bottle. Investigations of environmental stress cracking resistance. Investigations on environmental stress cracking resistance. Ageing resistance of hdpegeomembranes evaluation of. Environmental stress cracking resistance of blends of high.
Esc occurs in conjunction with internal and external stresses on a plastic. Chemical exposure may result in either physical degradation stress cracking, crazing, swelling and discoloration or chemical attack reaction of chemical with polymer and loss of. The behavior of other, untested liquids can be estimated from hsp correlations based on a limited number of test liquids. Environmental stress cracking is a physical phenomenon that is initiated as a slow. Plastic failure through environmental stress cracking. The most common victims are hdpe containers for liquid detergents, motor oil, and industrial chemicals. Adding hbpe with a low octene content, however, results in reduced failure times of the blend compared with hdpe blends that exhibit relatively good environmental stress. 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. Wyzgoski, prediction of environmental stress cracking of polycarbonate from solubility considerations, journal of applied polymer science, 23, 11531166, 1979.
This concept has been extended to the analysis of linear viscoelastic materials, by introducing timedependent quantities. In the case of the most commonly used stress crack reagent, igepal co630, as the concentration of water in the stress cracking media increases, the faster the esc onset. Investigations of environmental stress cracking resistance of. Experimental study of stress cracking in high density. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. Environmental stress crack resistance of polyethylene. Astm d2561 17 standard test method for environmental stress. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. By understanding the basic structure of polyethylene, it is easy to see how this can affect environmental stress cracking. Approaches to resolve environmental stress failure have limited discussion in the open literature although a significant number of references can be found in the patent literature. All joking aside, the potential for environmental stress cracking escr of hdpe bottles has always existed because of the materials inherent. Environmental stress cracking esc occurs when pe is subjected to stresses in the presence of various environmental agents. Hdpe polyethylenecovinylacetate eva and lowdensity polyethylene ldpeeva blends were tested and compared with respect to their environmental stress cracking resistance escr using the. Environmental stress corrosion cracking resistance of high.
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