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. At the higher loads stresses breaks occurred in a ductile mode to generate the shallow slope segment of the curve. Jan, 2020 polyvinyl chloride pvc is a popular thermoplastic thats odorless, solid, brittle, and generally white in color. Polyethylene flooring vs polypropylene flooring and pvc floor tiles. One of the most common causes of cracking and splitting in pvc pipes is overpressurization. Comparison of different pond liner materials which is. Over time, chemical attack or sunlight will reduce the strength of the plastic until the internal stress causes these cracks. Environmental stress cracking an overview sciencedirect.
Cpvc pipe failure investigation community engineering. Microhardness is used for general screening and generation of data on plasticfluid compatibility. Jan 18, 2011 the premature failure of meltfused joints is also common failure where cracking initiates at stress concentrations created by poor installation practices. Evaluating environmental stress cracking of medical. Polypropylene tubing exhibits low water absorption less than.
Polypropylene copolymer ppc is a softer, more flexible plastic, which also tends to have more stresscrack resistance and has greater impact strength. 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. Side notches are made to facilitate the cracking in the main notch. Visually, brittle cracks are typically smooth, featureless and devoid of any yielding and deformation process, as shown in figure 2. Polypropylene testing polypropylene compressive strength. Chris oconnor is a senior consultant at gl noble denton, a leading independent technical advisor to the oil and gas industries. In polyethylene and polypropylene geomembranes ian d.
Environmental stress cracking the the chemical agent. The addition of a surfaceactive chemical, an environmental effect, accelerates this type of failure, thus this mechanism is termed environmental stress cracking esc. Environmental stress cracking is involved with some 25% of plastic part failures. See typical properties of polypropylene listed below. Excellent corrosion resistance to a wide range of item. Polypropylene is a lowcost thermoplastic elastomer, which consists of aliphatic hydrocarbon chains and is mainly used in packaging industries owing to its high fatigue resistance, good chemical and environmental stress cracking resistance, low density, and ease of processing. This unique material can be steamsterilized or autoclaved without damage and resists environmental stresscracking when subjected to most chemical. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in.
Polypropylene is available in several grades and under the following names. Machining guide for engineering plastics polycarbonate. 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. Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials. Nov, 2012 this type of cracking is caused by molecular orientation at the surface of the plastic part. On predicting environmental stress cracking in polymers. Environmental stress cracking esc is the premature cracking of a polymer due to the combined action of a stress and a fluid. It is produced via chaingrowth polymerization from the monomer propylene polypropylene belongs to the group of polyolefins and is partially crystalline and nonpolar. Plasticizerinduced stress cracking of rigid pvc and polycarbonate. 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. Polyethylene pe has revolutionized lowpressure pipe system design on a global basis and is testimony to pes unique combination of properties, which have driven the replacement of. Copolymer polypropylene tends to have greater lowtemperature endurance with only modest compromises over basic polypropylene properties. Use our interactive properties table below to explore by property group, sort, or compare two or more plastic materials.
The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. This type of cracking is caused by molecular orientation at the surface of the plastic part. Tensile tests conducted over a wide spectrum of temperatures and strain rates indicate that, for a given test temperature, there exists a critical strain rate above which crazing is the dominant deformation mode of pp. Polypropylene has excellent dielectric strength, excellent chemical resistance and outstanding heat resistance, its moisture absorption is practically nil and it is unsurpassed in flexural strength. Esc is a failure mechanism whereby a plastic resin is effected by a chemical agent while simultaneously under tensile loading. Its moisture absorption is practically nil and unsurpassed in flexural strength. 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. Exceeding the maximum pressure will cause the pipe walls to fail.
Polypropylene is one of those most versatile plastic materials. A number of myths have arisen as to the causes and remedies for stresscracking. Polypropylene, while similar in many others aspects, is not similar to polyethlyene with respect to stress cracking. This is precursor to a failure due to excess mechanical forces applied to threads and the propensity for cpvc to absorb contaminants at such stress zones. Pp is commonly used in packaging, containers, toys, automotive interiors and. Environmental stress crack resistance of polyethylene this test method is shown in figure 3. The most widely used member of the vinyl family, pvc is excellent when used for corrosionresistant tanks, ducts, fume hoods and pipe. A tube support should be used with this tubing for maximum holding power where end loading, vibration or pressure spikes may occur. Machining forces being lower for engineering plastics than for metals and the former deforming more easily if clamped too tightly, clamping pressures should be reduced. 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. Jul 17, 2017 pvc pipe can become brittle due to chemical exposure. Several laboratories have used the concepts of fracture mechanics to evaluate stress cracking in plastics. Pvc drains can become brittle if exposed to large amounts of chemicals, such as those. Polypropylene pp, also known as polypropene, is a thermoplastic polymer used in a wide variety of applications.
It has been estimated that 15% of all failures of plastic materials in commercial use are related to environmental stress cracking esc 3. Cpvc pipe failure investigation community engineering services. Deterioration will manifest as brittleness and stress cracking. Thermal stress cracking can be minimized or eliminated by carefully choosing the polymer compound formulation and extrusion conditions. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present. For chemically resistant plastic, view our chemical resistance of plastics chart. The nature of polyethylene pipe failure plasticstoday. This interaction between the plasticizer in the plasticized pvc compound and the surface of a second plastic part can lead to reduced physical. Its properties are similar to polyethylene, but it is slightly harder and more heat resistant. Both cpvc and pvc are subject to environmental stress cracking from a variety of sources, including the chemicals used by the water treatment plant. Specimens are machined directly from pipe or from a molded plaque. Environmental stress crack resistance of polyethylene.
Basic understanding of stress cracking in hdpe products. 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. 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. Polypropylene is semicrystalline and therefore has much better chemical resistance than polystyrene. When bonding pvc to polystyrene, the plasticizers in the pvc can leach straight through the adhesive and attack the polystyrene. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure.
There is always some stress on the surface of a plastic part. Since the introduction of the first 2liter, onepiece pet bottle by continental can co. However, avoid any unsafe condition where the plastics part could come out of the clamping tools. Adhesive types uv curable adhesives, cyanoacrylates, structural acrylics and. Each size and grade or schedule of pvc pipe is rated to a maximum fluid pressure. In fracturemechanics testing, a welldefined flaw or crack is machined into a. Standard test method for environmental stresscrack. Stress cracking of pet bottles is caused by a combination of the reduced tensile strength of the base from chemical attack from the fillingline lubricant, internal stress from the carbonation pressure, and molded in stresses in. These can include weight gain if the plastic absorbs the chemical, weight loss if the plastic is degraded by the chemical or if the chemical extracts lowmolecularweight components of the plastic, dissolution if the chemical is a good solvent, or other changes such as variations in opacity or color. Astm d2561 17 standard test method for environmental stress. Microhardness is used for general screening and generation of data on plastic fluid compatibility. Difference between polypropylene and polycarbonate compare. Its currently ranked as the third most widely used plastic in the world behind polyethylene and polypropylene. Pp polypropylene pp as specified by astm d 4101, is a member of the polyolefin family of pure hydrocarbon plastics.
Plasticizerinduced stress cracking of rigid pvc vs. Resistant to fatigue used for plastic hinge applications. As a polymer, chemicals can have a profound negative effect on the makeup of pvc, causing loosening of bonds between molecules in the plastic and hastening the rate of plasticizer migration out of the pipe. Environmental stress cracking of polypropylene springerlink. Causes of contamination can be trace amounts of metals from pumps, plumbing, and fittings or poor quality sodium hypochlorite. Evaluating environmental stress cracking of medical plastics. Proteus copolymer high impact resistant thermoplastic with excellent resistance to cracking at low temperatures. Use this chart to help guide your component selection. These homopolymer sheets can be heat formed, shaped, and welded to fabricate ducts, hoods, and much more.
A single notch cut into the test specimen acts as a crack initiation point. Plastic failure through environmental stress cracking. On predicting environmental stress cracking in polymers article in polymer degradation and stability 771. The copolymer is more pliable than the homopolymer. 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. It is harder than polyethylene and has greater tensile strength. It does not present stresscracking problems and offers excellent electrical and chemical resistance at higher temperatures. Such myths are either wholly wrong or correct just part of the time.
Pp, like polyethylene see hdpe, llldpe and polybutene pb, polypropylene is a polyolefin or saturated polymer. It is much less likely to stress crack in the presence of the grease. A recent study indicated that approximately 25% of plastic component failures are associated with esc1. Escr problems get more pronounced in the summer, because of several factors related to increased heat and humidity. Stress rupture curves on five commercially available geomembranes.
Sodium hypochlorite and its effects on polyethylene. Environmental stress cracking resistance of polyethylene. Polypropylene pp is a linear hydrocarbon polymer, expressed as cnh2n. The first option was to change to a filled polypropylene. Polypropylene has a significantly higher stress crack resistance and is therefore not susceptible to cracking at the same magnitudes at which hdpe may crack. Identifying incompatibility between a plastic resin and a chemical agent at the design and material selection stage can be the difference in avoiding failure. The environmental stress cracking esc effects on pvc of various high ph sodium hydroxide environments have been studied. Amorphous structure of polymer facilitates solvent diffusion, which is required for the process of damage to occur. Cpvc and pvc pipes are designed to last for decades, there are certain environmental factors that can cause them to fail prematurely. Polypropylene has excellent dielectric strength, excellent chemical resistance and outstanding heat resistance.
This contamination will cause accelerated deterioration of the tank over a period of time. As you can see from the chart to the right, tensile strength of pvc pipe increases as the size of the pipe increases. Polyethylene pe, polypropylene pp and polyvinyl chloride pvc are three most commonlyused thermoplastics in the flooring industry. Improving thermal stress cracking resistance in wire and cable polymer compounds share. 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. Unlike polyethylene, polypropylene is not prone to environmental stress cracking. Pp is commonly used in packaging, containers, toys, automotive interiors and exteriors, consumer products, and medical components. Lldpes, pvc, and pp do not have the distinct yield point that hdpe has, so they can be considered far more stable materials. George wypych, in pvc formulary second edition, 2015. Jul 07, 2016 cpvc and pvc pipes are designed to last for decades, there are certain environmental factors that can cause them to fail prematurely. Have you noticed stress cracking in polycarbonate parts that contact.
If the plastic is under stress, it may also experience environmental stress cracking, which can be defined as the crazing and cracking that may occur when a. Polycarbonate is among the most widely used plastics that consists. Cracks in injection molded parts how to correct this. Polypropylene cannot be cemented, but it is easily. It does not present stress cracking problems and offers excellent electrical and chemical resistance at higher temperatures. Polypropylene plastic sheets johnston industrial plastics. A materials resistance against this type of failure is called environmental stress cracking resistance escr. By environmental stress cracking, we understand a failure in which materials in contact with a. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Mdpe has good shock absorption and is more resistant to notching and stress cracking than hdpe.
Environmental stress cracking of polyvinyl chloride in. Pvc pipe tensile strength can be measured by hanging weight from the pipe until it bends or cracks. Difference between polypropylene and polycarbonate. Aug 18, 2017 polypropylene is a lowcost thermoplastic elastomer, which consists of aliphatic hydrocarbon chains and is mainly used in packaging industries owing to its high fatigue resistance, good chemical and environmental stress cracking resistance, low density, and ease of processing. Polypropylene chemical compatibility chart from ism. Under many circumstances, environmental stress cracking can occur simply under the moldedin residual stress associated with injection molding and other forming processes. 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. Environmental stress cracking the the chemical agent does. It is associated with the phenomenon of crazing and solvent plasticization of. This unique material can be steamsterilized or autoclaved without damage and resists environmental stress cracking when subjected to most chemical tests. 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.
Similarly, for both bonding methods, reduce stress, either by creative joint design or distributing the stress over a larger bond area. Polyethylene flooring vs polypropylene flooring and pvc. 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. Also, you may want to use our plastic material selection guide or interactive thermoplastics triangle to assist with the material selection process based on your application requirements. Wire and cable polymer compounds thermal stress cracking. Although pp has half the strength of pvc and cpvc, with a design stress of 1,000 psi at 73 f, it may have the most versatile chemical resistance of the thermoplastic materials identified as the sentinels of industrial piping. Aug 01, 2007 the petalloid design of onepiece pet bottles creates more stress and results in less resistance to stress cracking. The use of fillers is primarily designed to match the shrinkage of the.
759 1244 1048 1137 325 956 263 862 1467 1382 1138 137 185 1244 805 817 1437 51 632 503 767 123 1474 669 1082 1138 907 1167 171 1328 225 1428 367 1235 912