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