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