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. Environmental stress crack resistance escr testing of. Another feature of polycarbonate is that it is very pliable. Although deformation may be simpler with the application of heat, even small angle bends are possible without it. Its the reason you cant put stickers on your motorbike helmet. Bonding polycarbonate with industrial adhesives surface.
Processing polycarbonate at the manufacturers recommendations is the key to stress free and crack resistant parts. As little as 10 percent glass fiber can reduce problems with stress cracking in materials like polycarbonate and polysulfone. Several laboratories have used the concepts of fracture mechanics to evaluate stress cracking in plastics. Stress cracking is one of the most common problems associated with field failures in plastic products. Stresscracking resistance of polycarbonate in contact with plasticized pvc compounds polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. And finally, as is the case with so many properties, molecular weight is critical. A recent study indicated that approximately 25% of plastic component failures are associated with esc1.
George wypych, in pvc formulary second edition, 2015. There is always some stress on the surface of a plastic part. Chemical resistance can be dependent upon exposure time, temperature, stress molded in and any external stress, type of chemical and the chemical concentration. This article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane. Evaluating environmental stress cracking of medical. These are amorphous plastics, and they bond readily. You may be surprised to find out what two naturally occurring ingredients are the base for all vinyl compounds. Tek tip cold forming or bending polycarbonate films tekra. As it so happens, this doesnt only happen to polycarbonate. To prevent such stress cracking, which has been discovered to be due to the migration of the relatively small dehp plasticizer molecules from the pvc tubing along the surface of the polycarbonate part, assemblies constructed in accordance with the present invention use a special pvc tubing in which totm has been substituted for dehp as the.
Plasticizers within pvc, such as phthalates can trigger stress cracking and result in damage to the polycarbonate. Certain highviscosity polymeric plasticizers over 100,000 centipoise and several in the i0,00025,000 range did not stress crack polycarbonate. Other commonly used medical rigid polymers, such as abs, polystyrene, and acrylic, can exhibit the same phenomenon. For your convenience and information, acme plastics created this reference guide to compare popular plastics, including acrylic, petg, polystyrene hips, polycarbonate, and expanded pvc. Stress crack reduction in polycarbonate parts american. Polycarbonate pc bonding tips and adhesive selection. Injectionmolded tensile bars were placed under fixed strains of 0%, 0. Other plastics such as polyethylene, polypropylene, and polytetrafluoroethylene teflon are semicrystalline thermoplastics and. Although stress cannot be eliminated, excessive stress can be molded into the parts by using too high an injection pressure, too high a holding pressure, or. Teknor apex has been an innovator in the pvc compound industry for more than six decades. Environmental stress cracking of pvc and pvccpe springerlink. The ductile failure is ascribed to plasticization andor plastic deformation around the craze.
Chemical exposure may result in either physical degradation stress cracking, crazing, swelling and discoloration or chemical attack reaction of chemical with polymer and loss of. But there are methods for getting to the root of the problem. A guide to chemical resistance of polycarbonate sheet. Microhardness is used for general screening and generation of data on plastic fluid compatibility. Cracks in injection molded parts how to correct this.
The process for making pvc polyvinyl chloride is a mystery to most. This interaction between the plasticizer in the plasticized pvc compound and the surface of a second plastic part can lead to reduced physical. At 50,000 hr, crazing will occur at a stress level of 4000 psi 27. Polycarbonate is a highly durable industrial plastic, with exceptional transparency to visible light. Environmental stress cracking is involved with some 25% of plastic part failures. Pdf stress cracking of polycarbonate exposed to sunscreen. This results in cracking when the part is exposed to any stress, such as that of the ejection system. Pvc pipe is a popular construction material for its low cost and durability.
Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. Environmental stress cracking may account for around 1530% of all plastic component failures in service. Nov, 2012 this type of cracking is caused by molecular orientation at the surface of the plastic part. Tek tip cold forming or bending polycarbonate films. It is the premature embrittlement and subsequent cracking of a plastic. 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. Difference between polypropylene and polycarbonate. So me of the chemicals included are aromatic esters, ketones and amines. 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 widely regarded as the leading cause of plastic product failure. Environmental stress cracking an overview sciencedirect. One example is the contact between polycarbonate and plasticized pvc. Two rigid pvc formulation technologies designed to minimize the impact of plasticizerinduced stress cracking were evaluated. Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking. Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. Plastic failure through environmental stress cracking. Unlike most other plastics, polycarbonate is capable of undergoing large hot moulding deformations without cracking or breaking. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. This whitepaper explains the cause and impact of plasticizerinduced stress cracking, as well as the relative stress crackresistance of specialtyformulated rigid pvc versus polycarbonate et al when exposed to pvc plasticizers commonly used in pvc medical tubing applications. Stress crack will only develop, however, if tensile production andor are applied from the outside. To avoid problems, select your material with care and employ the fundamental rules of good design and processing. Plasticizerinduced stress cracking of rigid pvc and polycarbonate flexible polyvinyl chloride pvc is the most common material used to manufacture clear flexible tubing for medical applications.
Plasticizerinduced stress cracking of rigid pvc and. High stress can result in chemical stress cracking of polycarbonate film. Revised stresscracking resistance of polycarbonate in. Plasticizerinduced stress cracking of rigid pvc and polycarbonate. It is called environmental stress cracking and basically it means that solvents cause the material to crack due to built in stresses in the polycarbonate. Environmental stress cracking of polyvinyl chloride in. Pvc pipe can be used for most any drain, water, or ventilation application. If a plasticized pvc tube is mounted on a stressfree polycarbonate pipe, for instance, the. Polycarbonate pc labware thermo fisher scientific us. This research studies the stress cracking of polycarbonate plastic by a variety of solvents and solvent blends. At low stress intensity factors an increased temperature, increases the fatigue crack propagation rate, while at high stress intensity factors an increased temperature results in high fracture resistance.
Keywords polymethylmethacrylate, environmental stress cracking, chemiluminescence, radical 1. The test provides analysis for the materials oxidation and alcohol absorption, as well as its resistance to environmental stress cracking. It is one of the most common causes of unexpected brittle failure in thermoplastic. Polycarbonate cracking polymer engineering engtips. Molecular degradation many of tuffak properties are determined by the size of its molecules. Mar 31, 2017 the process for making pvc polyvinyl chloride is a mystery to most.
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. Over time, chemical attack or sunlight will reduce the strength of the plastic until the internal stress causes these cracks. High stress in polycarbonate is usually any level that is over 1500 psi. In fracturemechanics testing, a welldefined flaw or crack is machined into a. High stress can result in chemical stress cracking of polycarbonate film high stress in polycarbonate is usually any level that is over 1500 psi. 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.
It can typically be formed at room temperature without cracking or breaking, similar to aluminum sheet metal. 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. 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. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present.
Replace polycarbonate pieces when you see signs of aging occur to prevent failure during use. Pvc drains can become brittle if exposed to large amounts of chemicals, such as those. Brittle failure is thought to arise from crazes which reach a critical size. By environmental stress cracking, we understand a failure in which materials in contact with a. 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. There are a variety of adhesive choices for bonding polycarbonate.
Stress cracking is a frequent problem in plastic applications. This problem is well known and is called environmental stress cracking. Jun 28, 20 this article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane. The use of solubility parameters to predict critical stress. Jul 17, 2017 pvc pipe can become brittle due to chemical exposure. Environmental stress cracking the the chemical agent does.
A variation of the iso 4599 test was performed by the authors on medicalgrade versions of pctg, polycarbonate, and an acrylic resin. The stress required to produce failure in 50,000 hr produces crazing in less than 1 hr. Words like ethylene dichloride and vcm are not typically used in daily conversation. Polycarbonate and polysulfone are particularly sensitive to stress cracks. Have you noticed stress cracking in polycarbonate parts that contact flexible pvc for prolonged periods. Click on any product names below to bring up additional information. 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. 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. Stresscracking resistance of polycarbonate in contact. Possibilities of why a polycarbonate pc part is cracking.
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. Adhesive types uv curable adhesives, cyanoacrylates, structural acrylics and. 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. Esc is a failure mechanism whereby a plastic resin is effected by a chemical agent while simultaneously under tensile loading. Environmental stress cracking esc is considered a leading cause of plastic part failure. 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. 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. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to stress cracking. Polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. Pvc can become brittle and crack when placed in a few. Screw threads are a feature in a plastic component that benefits from annealing. Chemical resistance and stress cracking in polyurethanes. Plasticizerinduced stress cracking of rigid pvc vs. Various methods are discussed such as uv, cyanoacrylate, epoxy, and acrylic.
Polycarbonate material exhibits aromatic sensitivity or is prone to absorb odors, according to the plastics web. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Flexible pvc medical tubing contains plasticizer, the component added to pvc to impart flexibility to the inherently rigid pvc polymer. Environmental stress cracking the the chemical agent. The mechanism for plasticizerinduced stress cracking originally proposed by lacatus and summers is supported by our observations and appears to be applicable for other rigid amorphous polymers such as polycarbonate. The sunscreen analyzed contains various chemical species t hat may be stress cracking agents for polycarbonate. Introduction plastic components are used in various kinds of equipment and machinery, but depending on the environmental factors involved, various kinds of degradations may arise. Analysis of polycarbonate stress cracking and cleaning. Among these is environmental stress cracking, which. All of the aforementioned materials are valued for their strength, clarity, and long history of safe use in. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. It has been estimated that 15% of all failures of plastic materials in commercial use are related to environmental stress cracking esc 3.
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. This phenomena, although rare and preventable, is known to occur in many types of. Many homeowners believe pvc to be a maintenancefree product, however, which may or may not be accurate depending on its application. Microhardness is used for general screening and generation of data on plasticfluid compatibility. If the molded part is completely stress free or is subject solely to compressive stress, then no stress cracking will occur. Evaluating environmental stress cracking of medical plastics.
If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots. Polycarbonate is among the most widely used plastics that consists. Inspect polycarbonate regularly and before each use for signs of aging including permanent discoloration yellow, brown, orange, or pink, crazing spider web cracks, stress cracking vertical cracks, or permanent clouding or pitting. Part failure can occur since the screw places a steady longterm stress on the component in combination with the residual stress from the machining process.
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