The 100year stress crack resistance scr of corrugated hdpe pipes was evaluated using a 600 mm diameter pipe. 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. This instance leads us to a discussion of one of the methods that can be used to test for the presence of stresses in a part that might lead to stress cracking. High stress levels in plastic are a common cause of component field failures. Sometimes stress is not visible until the component is subject to loading or exposure to chemicals by the end user. Why the annealing process is crucial for plastics gmn. Aug 01, 2007 since the introduction of the first 2liter, onepiece pet bottle by continental can co. The number of specimens that crack over a given time is. Materials 2 medical plastics 3 chemical resistance and stress cracking in polyurethanes aliphatic polycarbonate polyurethanes demonstrate broad chemical resistance for in vivo. The stresscracking behavior of different polyethylenes is investigated in the temperature range from 23 to 90c and constant applied stress. Specific molded polycarbonate parts were found to stress crack during. One survey performed by rapra estimated that while polycarbonate makes up 1 percent by weight of the global consumption of plastic resin, it constitutes 10 to 15 percent of the case studies for field failures. The proposed test methods suggest the exposure to the gaseous product or the. After fixed period of time, use the crack point and thickness, for measure the critical strain of the specimen.
To assure product quality, manufacturers must opt the right test solution such as environmental stress cracking resistance tester. Medical device recalls on the rise the nature of the medical device manufacturing market requires companies to manage. Analysis of polycarbonate stress cracking and cleaning. Cosmetic surface defects, called water spots, often develop during humid months due to moisture condensation on the chilled molds. Standard test method for environmental stress crack resistance escr of threaded plastic closures. The specified chemical reagents were previously calibrated by use of environmental stress cracking esc techniques to cause crazing in sulfone plastics sp at specified stress levels. In this test, strips of a plastic are positioned in a fixed flexural strain and. Basic understanding of stress cracking in hdpe products. Pdf stresscracking of pet bottle as an unpredictable. Components pass final stress test 100% of the time. Stress cracking while the adhesive is curing, is also a possibility. A number of myths have arisen as to the causes and remedies for stresscracking. Environmental stress crack resistance of polyethylene.
The petalloid design of onepiece pet bottles creates more stress and results in less resistance to stress cracking. 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. Stress cracking of csd bottles blow molding advisors at. The low molecular weight polymer fractions and the chain ends within the bulk of the polymer become more ordered during the diffusion and swelling process by their partial solubility in the crazing agent, causing crystallization. Stress cracking of polycarbonate exposed to sunscreen.
It is not suitable for use in applications which require impact strength or applications where water exposure is inevitable. Chemical resistance and stress cracking in polyurethanes. The notched constant ligament stress ncls test astm 2163 was used for the pipe liner assessment. Standard test method for coronatreated polymer films using water. First, by reducing the stress, the plastic part will have better mechanical and thermal properties because there are fewer sites in the polymer that could propagate a crack or. This polycarbonate is not highly solvent resistent, and should show greater stress cracking tendency to develop conservative cleaning solvents. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present. Fiveyear inspection of plastic tanks der16 nys dept. The cadence of such chemical exposure tests does not always allow a priori. In response, molders raise the mold temperature to stop condensation, but without adding a corresponding increase in cycle time. Cracks in injection molded parts how to correct this. The test involved three stacked containers of chicken soup held at a specific temperature. The test specimens are bent with the notch pointing upwards in a metal ushaped specimen holder to generate excess residual stress.
In a set of experiments, certain cleaning solvents were applied to stressed 0. Standardize stress crack testing across your orginization. The natural oils in chicken soup could result in stress cracking under stress temperature. The surface transitions from a rubber to a glass state and the contraction of the surface due to loss of water yields stress sufficient to cause surface cracking. The location of the crack can tell us a lot about its specific cause. All these methods mimic possible degree of contact in the field. D5748 952019 standard test method for protrusion puncture resistance of stretch wrap film. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. Stress cracking of pet bottle as an unpredictable factor limiting the commercial life of carbonated soft drinks article pdf available in italian journal of food science 244. Environmental stress cracking is involved with some 25% of plastic part failures. Environmental stress cracking in plastics environmental stress cracking esc is a welldocumented phenomenon in plasticpolymer components and can occur when a polymer component is exposed to an aggressive chemical agent while under tensile stress, whether residual or applied. Often we tend to group all the problems into the environmental category, but in reality most of the field problems we see have to do with solvent stress cracking.
Environmental stress cracking an overview sciencedirect topics. Stress cracking resistance testsdurability and fatigue test. It can typically be formed at room temperature without cracking or breaking, similar to aluminum sheet metal. Such phenomena are called environmental stress cracking esc and are often observed among plastic products when a. The polycarbonate used was a colorless, transparent makrolon 2600 from mobay corporation. Pdf stress cracking of polycarbonate exposed to sunscreen. The fracture behaviour of the two materials was evaluated in terms of both crack initiation and propagation. Stress cracking is a frequent problem in plastic applications. To determine the crack initiation time and the crack growth rate a procedure based on.
Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure. Evaluation of residual stress in molded resin jfe techno. Another variable to be considered in polycarbonate stress crack testing is solvent. Investigations on environmental stress cracking resistance of. The bent strip method described in astm d1693 will be described below as the environmental stress cracking test method for the molded plate.
The bent strip test is a constant strain test, but polyethylene relaxes when strained. The intrinsic viscosity of the preform used to blow the bottle. 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. Environmental stress cracking esc or stress corrosion cracking scc seemed the most likely cause of the brittle cracks seen in the retained samples, but no records had been kept by the hospital of what fluids the connectors had made contact during service. In both methods, the bottle contains a known stress cracking agent. Chemical resistance and stress cracking in polyurethanes arlo n. Distortion of the cracking point is calculated to the following formula. A recent study indicated that approximately 25% of plastic component failures are associated with esc1. This month we want to conclude the discussion with a few more case studies and then discuss tests and material selection techniques that can keep us out of trouble with this challenging problem. Plastic failure through environmental stress cracking. Constant stress test can be another contributor to stress cracking.
To avoid problems, select your material with care and employ the fundamental rules of good design and processing. A 15minute soak with air drying no water rinse or mechanical drying. Stress cracks or surface crazing near stress concentrators like threads can be a sign of future problems. 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 department of chemical engineering. Although deformation may be simpler with the application of heat, even small angle bends are possible without it.
Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. Evaluating environmental stress cracking of medical. Stress crazing on acrylic surfaces and its causes stress crazing, which appears as an area of tiny microcracks, occasionally shows up in areas on the surface of most thermoplastic sheets.
An alternate test method that allows the evaluation of smaller samples or. Environmental stress cracking of polycarbonate and a. This article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane. Pet stress crack simulator automating stress crack. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. Specimens are machined directly from pipe or from a molded plaque. Part requirements dictate a final stress test for lot acceptability. Environmental stress cracking escr is the formation of cracks in a part when. So me of the chemicals included are aromatic esters, ketones and amines. Environmental stress cracking the the chemical agent. Carbonate stress corrosion cracking acscc is a common problem in the oil and gas production, petroleum refining, and petrochemicalchemical processing industries. Astm d7474 17 standard practice for determining residual.
Stress corrosion cracking scc is interpreted to take place due to specific combinations of susceptible materials in suitable corrosion environments under adequate stress, specifically under constant stress, figure 1. If the tank does not meet 6 nycrr subdivisions 599. This course explains the mechanics that underlie stress cracking of pet bottles with petaloid bases. The sunscreen analyzed contains various chemical species t hat may be stress cracking agents for polycarbonate. For example, the test method astm f48402 stress crazing of acrylic plastics in contact with liquid or semiliquid compounds uses flexural stress levels of 3,000, 3,500, and 4,500 psi 211, 246, and 316 kgcm 2. Plastic stress test protocols connecticut plastics. A guide to chemical resistance of engineering plastics. Strips of a plastic, each containing a controlled defect, are placed in a bending rig and exposed to a stresscracking agent. Environmental stress crack resistance of polyethylene today, this test is specified less frequently because it is not as aggressive towards modern resins. Here are some of the most common scenarios for cracks in injection molded parts.
Evaluating environmental stress cracking of medical plastics. Lens materials such as trivex are far less likely to fall prey to stress. This is the classical location of a stresscrack failure fig. Although quadrant engineering plastic products quadrant epp stock shapes are annealed using a proprietary stress relieving. Measurement of rupture lifetime evaluation under static stress. The plastic was received as injectionmolded plaques, 18inch thick. Astm d 1693 describes a test for evaluating the stress crack resistance of ethylene plastics in environments such as soaps, wetting agents, oils, or detergents. Acscc is a form of alkaline stress corrosion cracking that often occurs more aggressively at higher ph and high concentrations of carbonate solutions. A guide to chemical resistance of polycarbonate sheet.
In an effort to improve the design process, a methodology using short time stress relaxation tests can be. Escr is a precisely designed test unit constructed to inspect the physical modifications in plastic material under environmental stress. In transparent parts, the easiest way to detect stress is by viewing the part with polarized light. In general this problem falls into two subcategories. The problem lies, in part, in the definition of stress cracking. Learn more burst pressure burst pressure this procedure evaluates the performance. Jun 28, 20 this article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane.
Polycarbonate is sensitive not just only to solvent cracking, such as those seen on. The results of this research can be used to develop targeted replacements for 1,1,1trichloroethane and cfc1, replacements that have equivalent or better compatibility with polycarbonate plastics. Individually, the stretching molecules absorb energy. Side notches are made to facilitate the cracking in the main notch. Astm d 1693 describes a test for evaluating the stresscrack resistance of ethylene plastics in environments such as soaps, wetting agents, oils, or detergents. Esc is a major problem in the longterm service behavior of plastic products. Analysis of polycarbonate stress cracking and cleaning solvent. The number of specimens that crack over a given time is recorded. Meanwhile, the astm d2561 test method uses a constant internal pressurization of 5 psig to produce a sustained stress.
The potential for environmental stress cracking is of paramount concern when plastics. The thermal expansion of the contents produces about 5 psi in the bottle. Another feature of polycarbonate is that it is very pliable. Under the conditions of the test method, high local multiaxial stresses are developed through the introduction of a controlled imperfection 2, 3. The purpose of annealing can vary for different plastic materials, but elite plastics uses it to ensure part stability over time. This failure for glassy polymers bisphenol a polycarbonate pc. Preform and bottle testing acetaldehyde aa container head space this procedure measures the amount of acetaldehyde in pet bottles 24 hours after blow molding.
Some combination of certain chemical agents and plastic materials sometimes causes an. Another variable to be considered in polycarbonate stress crack testing is solvent application method. Environmental stress cracking the the chemical agent does. Environmental stress cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. For example, standard plastic is most prone to cracking, polycarbonate is one of the biggest culprits when it comes to stress fracturing but the crack spreads slowly. This stress relaxation allows testing to run without failure for very long periods 1,500 hours. Typically pet environmental stress cracking originates around the base of the bottle.
Polycarbonate is sensitive not just only to solvent cracking, such as those seen on battery cases, but to a range of other common liquids that are likely to. Aug 18, 2012 one test employed for thermoformed low rubber content impact polystyrene used for container lids involved warm chicken soup. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Carbonate stress corrosion cracking acscc inspectioneering. This phenomena, although rare and preventable, is known to occur in many types of plastic sheet materials and occasionally shows up on the surface of. 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 an overview sciencedirect. It is the premature embrittlement and subsequent cracking of a plastic. 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 moldedin stresses in. Such myths are either wholly wrong or correct just part of the time.
The amount of crystalline content in the container depends on the rate of cooling. 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. Environmental stress cracking esc is considered a leading cause of plastic part. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Aside from being difficult to predict or test for, stress cracking is. Strips of a plastic, each containing a controlled defect, are placed in a bending rig and exposed to a stress cracking agent. The scr tests were performed on the finished pipe at the liner and junction locations. A variation of this test uses sealed bottles warmed in a 140 f oven. Superglue is not suitable if the gap between the materials being glued is in excess of approximately 0.
Preform and bottle testing plastic technologies inc. A single notch cut into the test specimen acts as a crack initiation point. It outlines measures that plant personnel can take in both injection and blow molding to prevent this failure. This present study is confined to the environmental stress cracking resistance of polycarbonate. Other anecdotal data from experienced plastic part designers tends to echo. Investigations on environmental stress cracking resistance. This type of cracking typically involves brittle cracking, with little or no ductile drawing of the. While crystallinity provides strength, it also can lead to cracking. The test provides analysis for the materials oxidation and alcohol absorption, as well as its resistance to environmental stress cracking. 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. Hdpe is a semicrystalline material, with no crystals in the melted state. In last months article we defined the terms environmental stress cracking and solvent stress cracking, briefly explained the mechanisms involved, and provided a couple of examples of actual case studies. 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.
Esc is a failure mechanism whereby a plastic resin is effected by a chemical agent while simultaneously under tensile loading. Test stress cracking resistance of plastic water containers. 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. The surface transitions from a rubber to a glass state and the contraction of the surface due to loss of water yields stress suf. Stress cracking resistance testsdurability and fatigue testby. Environmental stress crack resistance of polyethylene this test method is shown in figure 3. Determination of environmental stress cracking resistance of. Escr problems get more pronounced in the summer, because of several factors related to increased heat and humidity. This research studies the stress cracking of polycarbonate plastic by a variety of solvents and solvent blends. Stress corrosion cracking an overview sciencedirect topics. Learn more aa content, preform resin this procedure measures the amount of acetaldehyde in pet resin or a pet preform. Not only the externally applied load is of importance but also the internal stress level plays a big role. The three point bending method and the variation of stress with the immersion time at constant strain were used to. 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.
Cracks in the direction of flow if the crack develops in the direction of the plastic s flow into the mold cavity, the cause is molecular orientation. Certain lens materials are more prone to stress fracturing than others. Polysulfone is very susceptible to stress cracking over time. It helps the user to inspect the actual working life of the material.
The mechanism of environmental stress cracking esc of amorphous polycarbonate in isopropyl alcohol, ethylene glycol monomethyl ether and methanol were studied. Annealing cycle after machining removes material and machining stress. The mechanisms of environmental stress cracking proposed in the. Stress relaxation tests with molding compounds used to encapsulate microcircuits have been performed to measure the time dependent nonlinear constitutive relation between stress and strain as a function of temperature and imposed strain. Standard test method for environmental stresscracking of. Solvent analysis of stresses in molded polycarbonate parts. A facet of that work involved demonstrating that the composition of the gasoline may change during testing through preferential evaporation of volatile components, which, in turn, can affect the results. Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small number of chemical environments.