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