Polypropylene copolymer ppc is a softer, more flexible plastic, which also tends to have more stresscrack resistance and has greater impact strength. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure. Nov, 2012 this type of cracking is caused by molecular orientation at the surface of the plastic part. Polypropylene has excellent dielectric strength, excellent chemical resistance and outstanding heat resistance. Environmental stress crack resistance escr testing of.
Although stress cannot be eliminated, excessive stress can be molded into the parts by using too high an injection pressure, too high a holding. 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. Although these plastics are less prone to stress cracking, care should still be taken to ensure. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. To avoid problems, select your material with care and employ the fundamental rules of good design and processing. Use this chart to help guide your component selection. We are having a problem with one of our parts cracking about 2 days after molding. It is light in weight, resistant to staining, and has a low moisture absorption rate.
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. Stress cracking is caused by a combination of tensile stress, a stress cracking chemical and the inherent susceptibility of a thermoplastic like polypropylene to stress cracking. Polypropylene pp is a thermoplastic addition polymer made from the combination of propylene monomers. Pp, like polyethylene see hdpe, llldpe and polybutene pb, is a polyolefin or saturated polymer. 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. Excess or exposed adhesive is cured with a quick filler setter such as permabond qfs16. Environmental stress cracking the the chemical agent. 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. Plastic failure through environmental stress cracking.
Polypropylene, while similar in many others aspects, is not similar to polyethlyene with respect to stress cracking. Environmental stress cracking resistance of plastics. This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces. Stresscracking is not an issue for pp as it also displays good levels of corrosion resistance.
Sometimes, these stresses are intentionally produced to impart desirable characteristics, such as in oriented pet films and bottles. Stress marks on the surface of plastic injection parts. Pp, like polyethylene see hdpe, llldpe and polybutene pb, polypropylene is a polyolefin or saturated polymer. Certain surface active materials, although they have no chemical effect on polyethylene, can accelerate its cracking when under stress, such as liquid detergents and ultra pure water. When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. Polypropylene testing polypropylene compressive strength. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. Environmental stress cracking may account for around 1530% of all plastic component failures in service. Cracks in injection molded parts how to correct this. Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials.
It is harder than polyethylene and has greater tensile strength. For polyethylene, this may range from several months for a milk or juice bottle to several decades. The first option was to change to a filled polypropylene. Polypropylene plastic sheets johnston industrial plastics. Polypropylene copolymer is a bit softer but has better impact strength. Stress corrosion cracking of polyethylene practical. Its moisture absorption is practically nil and unsurpassed in flexural strength. When an injection molded part cracks or breaks there are three possible reasons. Wire and cable polymer compounds thermal stress cracking. Environmental stress cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. It is much less likely to stress crack in the presence of the grease. 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.
Over time, chemical attack or sunlight will reduce the strength of the plastic until the. A plastic required enduring the entire life of a product before it fails. Polypropylene provides excellent resistance to organic solvents, degreasing agents and electrolytic attack. It tends to have better stress crack resistance and lower. The polymer chains eventually creep toward the pull and deform.
Vinegar, on the other hand, is 5 percent acetic acid and the rest is mostly water. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges, and textiles. A recent study indicated that approximately 25% of plastic component failures are associated with esc1. How to prevent stress cracking what is stress cracking.
Polypropylene is one of those most versatile polymers available with applications, both as a plastic and as a fibre, in virtually all of the plastics enduse markets. Of the 3 causes of injection molded part failure, increased internal stress, increased external stress, and reduced plastic strength, the molder has control over 2 of them. Standard test method for environmental stresscrack. There is always some stress on the surface of a plastic part. Here is a list of physical and chemical properties of polypropylene. It offers good chemical resistance, has a high resistance to stress crack, and. Above a temperature termed the gel temperature, solvent penetration leads to a polymeric gel and the initial cracks noted in fig. For the polyoelfins, they do sometimes indicate stress cracking potential, particularly with oxidizers. Environmental stress cracking esc is one of the most important and common reason of failure in plastics pipes and packaging. Astm d2561 17 standard test method for environmental stress. Environmental stress crack resistance of polyethylene. The use of fillers is primarily designed to match the shrinkage of the. Stress cracking sc is recognized as one of the major concerns for high density polyethylene hdpe pipes. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking.
This results in cracking when the part is exposed to any stress, such as that of the ejection system. It has a lower impact strength, but its working temperatures and tensile strength are superior to low or high density polyethylene. Environmental stress cracking an overview sciencedirect. Because stress cracking results in the breakdown or failure of a plastic material, a plastic part needs to endure its entire designed life before it fails. Esc is a failure mechanism whereby a plastic resin is effected by a chemical agent while simultaneously under tensile loading. Escr problems get more pronounced in the summer, because of several factors related to increased heat and. Part designers rarely attempt to estimate the amount of internal molded in stress because this stress can vary widely due conditions during the injection molding cycle. Amorphous structure of polymer facilitates solvent diffusion, which is required for the process of damage to occur. This type of cracking is caused by molecular orientation at the surface of the plastic part. 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 the laboratory. It is the tendency for polyethylene parts to fail prematurely in the presence of detergents, oils, and other active environments. Environmental stress crack resistance of polyethylene stress crack reagent concentration 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. It is sensitive to microbial attacks, such as bacteria and mold. Polypropylene chemical resistance guide hmc polymers.
The addition of a surfaceactive chemical, an environmental effect, accelerates this type of failure, thus this mechanism is termed environmental stress cracking esc. Although esc is often described as a chemical attack, it is a very complex combination of plasticizing, migration, low level stress and mechanical failure initiated by exposure to specific classes of liquids, primarily surfactants soaps. Identifying incompatibility between a plastic resin and a chemical agent at the design and material selection stage can be the difference in avoiding failure. Plastic polyethylene resin tanks sierra 2152585602. Polypropylene is a rigid plastic that has a higher operating temperature limit than polyethylene. Stress cracking results in failure or breakdown of plastic materials. Polyethylene and polypropylene resin selection guide for storage tanks. Polypropylene is semicrystalline and therefore has much better chemical resistance than polystyrene. Pp has good resistance to environmental stress cracking.
It is interesting to note that stress cracking in this case occurs without application of an applied stress although frozen. 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. Re your query on environmental stress cracking of plastics, the corrosion resistance data given out by the manufactures sometimes indicate that a given thermoplastic in a given environment at a given temperature may not be suitable. This stress can be caused by a cavity pressure that is either too high or too low. Polypropylene has a significantly higher stress crack resistance and is therefore not susceptible to cracking at the same magnitudes at which hdpe may crack. It is one of the most common causes of unexpected brittle failure in thermoplastic. Unlike polyethylene, polypropylene is not prone to environmental stress cracking. It is very aggressive and usually produces cracks in all but the most stress free abs parts. For polyethylene, this can range from a few months for a juice bottle to several decades for natural gas pipelines and an automobile fuel tank. Effect of molecular structure parameters on crystallinity. This unique material can be steamsterilized or autoclaved without damage and resists environmental stresscracking when subjected to most chemical tests. Experimental study of stress cracking in high density. The solution to an internal part cracking problem is to change the packing pressure.
Environmental stress cracking refers specifically to a phenomenon observed in polyethylene. Troubleshooting cracking in injection molded parts. Environmental stress cracking is involved with some 25% of plastic part failures. Polypropylene pp is a linear hydrocarbon polymer, expressed as cnh2n.
Environmental stress cracking occurs in amorphous polymers such as abs, pc, pmma, ps, pvc, and san but also in some in semicrystalline thermoplastics, such as pe, pp, pa, pb. Experimental study of stress cracking in high density polyethylene pipes a thesis submitted to the faculty of drexel university by jingyu zhang. Cracking under normal load conditions can be strong evidence of internal residual stress. Several laboratories have used the concepts of fracture mechanics to evaluate stress cracking in plastics. Haward department of chemistry, university of birmingham. Although esc is often described as a chemical attack, it is a very complex.
It was first noticed in polyethylene wire coatings that were often. Hmc pp resins have excellent resistance to environmental stresscracking. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present. But there are methods for getting to the root of the problem. Environmental stress cracking may account for around 15. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. In general, failure is a serious problem when using polymeric materials. Proteus copolymer high impact resistant thermoplastic with excellent resistance to cracking at low temperatures. In fracturemechanics testing, a welldefined flaw or crack is machined into a plastic. Polypropylene chemical compatibility chart from ism. Environmental stress cracking of polypropylene springerlink. Jansen stork technimet new berlin, wisconsin e nvironmental stress cracking esc is a phenomenon in which a plastic resin is degraded by a chemical agent while under stress, and it is the leading cause of plastic component failure. Residual stress is an inherent result of plastics manufacturing processes and occurs when molten polymer is cooled and shaped.
Over time, chemical attack or sunlight will reduce the strength of the plastic until the internal stress causes these cracks. On predicting environmental stress cracking in polymers. The gap does not exceed the recommended gap for that product check the. Environmental stress crack resistance escr testing of plastics. Cracking andor crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general. It usually occurs at high levels of applied stress.
As a general class of materials, polyethylene pe and polypropylene pp are recognized for their. 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. Environmental stress cracking esc is the premature cracking of a polymer due to the combined action of a stress and a fluid. Sc is a brittle failure that occurs at a stress level lower than the shortterm. Environmental stress cracking resistance of polyethylene. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. Polypropylene copolymer ppc is a softer, more flexible plastic, which also tends to have more stress crack resistance and has greater impact strength.
Some reduction in tensile strength and an increase in. On predicting environmental stress cracking in polymers article in polymer degradation and stability 771. Basic understanding of stress cracking in hdpe products. With force and time, crazing or stress cracking occurs. Under many circumstances, environmental stress cracking can occur simply under the moldedin residual stress associated with injection molding and other forming processes. Notice the high electrical resistivity and low coefficient of thermal expansion, which give pp the outstanding resistance and durability against heat and electricity. Stress cracking is one of the most common problems associated with field failures in plastic products. Stress cracking is a frequent problem in plastic applications.
Copolymer polypropylene tends to have greater lowtemperature endurance with only modest compromises over basic polypropylene properties. Polypropylene is an economical material that offers a combination of outstanding physical, chemical, mechanical, thermal and electrical properties not found in any other thermoplastic. The copolymer is more pliable than the homopolymer. 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 cracking the the chemical agent does. The cause of this type of part cracking is internal stress caused by high plastic pressure during molding. Stress crack resistance one of the best chemicals for checking stress crack resistance in abs is concentrated acetic acid, called glacial acetic acid by chemists. Troubleshooting cracking in injection molded parts paulson.
By environmental stress cracking, we understand a failure in which materials in contact with a fluid are subjected to stress. Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. Problems caused by residual stress in plastic part generally fall into some of the following categories. 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.
Another related mechanism is environmental stress cracking esc. Although both amorphous and crystalline plastics are susceptible to environmental stress cracking, it is generally recognized that amorphous plastics tend to be more at risk. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. You will observe lots crazing or fine micro cracks from the surface appearance of product. Evaluating environmental stress cracking of medical. Environmental stress cracking escr is the formation of cracks in a part when exposed to chemicals, if the parts have residual stress in them after molding. Todays approach to stress corrosion cracking in polyethylene coatings of steel pipes for standard applications in water and gas distribution systems merely considers the coating types featuring the low peeling resistance of the early editions of din 30670, i. Everything you need to know about polypropylene pp plastic. A materials resistance against this type of failure is called environmental stress cracking resistance escr. Elevated temperatures tend to accelerate the cracking. 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. Its tougher and more durable than propylene homopolymer. Evaluating environmental stress cracking of medical plastics. Polypropylene is available in several grades and under the following names.
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