At 50,000 hr, crazing will occur at a stress level of 4000 psi 27. Another feature of polycarbonate is that it is very pliable. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present. Stress crack reduction in polycarbonate parts american.
Nov, 2012 this type of cracking is caused by molecular orientation at the surface of the plastic part. A variation of the iso 4599 test was performed by the authors on medicalgrade versions of pctg, polycarbonate, and an acrylic resin. Plasticizerinduced stress cracking of rigid pvc vs. Environmental stress cracking is involved with some 25% of plastic part failures. Pvc drains can become brittle if exposed to large amounts of chemicals, such as those. Have you noticed stress cracking in polycarbonate parts that contact flexible pvc for prolonged periods. 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. Other plastics such as polyethylene, polypropylene, and polytetrafluoroethylene teflon are semicrystalline thermoplastics and. A guide to chemical resistance of polycarbonate sheet.
One example is the contact between polycarbonate and plasticized pvc. Several laboratories have used the concepts of fracture mechanics to evaluate stress cracking in plastics. In fracturemechanics testing, a welldefined flaw or crack is machined into a. Cracks in injection molded parts how to correct this. For your convenience and information, acme plastics created this reference guide to compare popular plastics, including acrylic, petg, polystyrene hips, polycarbonate, and expanded pvc. Jul 17, 2017 pvc pipe can become brittle due to chemical exposure. Plasticizers within pvc, such as phthalates can trigger stress cracking and result in damage to the polycarbonate. 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. Environmental stress cracking the the chemical agent. 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. Plasticizerinduced stress cracking of rigid pvc and polycarbonate. By environmental stress cracking, we understand a failure in which materials in contact with a.
Bonding polycarbonate with industrial adhesives surface. Adhesive types uv curable adhesives, cyanoacrylates, structural acrylics and. Stress cracking is a frequent problem in plastic applications. Evaluating environmental stress cracking of medical. The process for making pvc polyvinyl chloride is a mystery to most. Stresscracking resistance of polycarbonate in contact. Difference between polypropylene and polycarbonate. Jun 28, 20 this article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane. Esc is a failure mechanism whereby a plastic resin is effected by a chemical agent while simultaneously under tensile loading.
And finally, as is the case with so many properties, molecular weight is critical. Chemical resistance can be dependent upon exposure time, temperature, stress molded in and any external stress, type of chemical and the chemical concentration. Polycarbonate cracking polymer engineering engtips. 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. Flexible pvc medical tubing contains plasticizer, the component added to pvc to impart flexibility to the inherently rigid pvc polymer. Environmental stress cracking an overview sciencedirect. So me of the chemicals included are aromatic esters, ketones and amines.
Chemical resistance and stress cracking in polyurethanes. 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. The use of solubility parameters to predict critical stress. To avoid problems, select your material with care and employ the fundamental rules of good design and processing. Pvc pipe can be used for most any drain, water, or ventilation application. 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. This results in cracking when the part is exposed to any stress, such as that of the ejection system. Mar 31, 2017 the process for making pvc polyvinyl chloride is a mystery to most. 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. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to stress cracking. Stress crack will only develop, however, if tensile production andor are applied from the outside. Other commonly used medical rigid polymers, such as abs, polystyrene, and acrylic, can exhibit the same phenomenon. Brittle failure is thought to arise from crazes which reach a critical size.
Injectionmolded tensile bars were placed under fixed strains of 0%, 0. Click on any product names below to bring up additional information. 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. Plastic failure through environmental stress cracking. Its the reason you cant put stickers on your motorbike helmet. Unlike most other plastics, polycarbonate is capable of undergoing large hot moulding deformations without cracking or breaking. 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. Pvc pipe is a popular construction material for its low cost and durability. These are amorphous plastics, and they bond readily. However, it is also extremely sensitive to liquids that come in contact with it and will become weakened and crack. Environmental stress cracking of polyvinyl chloride in. This research studies the stress cracking of polycarbonate plastic by a variety of solvents and solvent blends. Tek tip cold forming or bending polycarbonate films tekra.
This interaction between the plasticizer in the plasticized pvc compound and the surface of a second plastic part can lead to reduced physical. This type of cracking is caused by molecular orientation at the surface of the plastic part. High stress can result in chemical stress cracking of polycarbonate film. Words like ethylene dichloride and vcm are not typically used in daily conversation. High stress can result in chemical stress cracking of polycarbonate film high stress in polycarbonate is usually any level that is over 1500 psi.
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 cracking escr is the formation of cracks in a part when exposed to chemicals, if the parts have residual stress in them after molding. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure. If a plasticized pvc tube is mounted on a stressfree polycarbonate pipe, for instance, the. Stresscracking resistance of polycarbonate in contact with plasticized pvc compounds polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. Polycarbonate pc labware thermo fisher scientific us. Molecular degradation many of tuffak properties are determined by the size of its molecules.
Environmental stress cracking may account for around 1530% of all plastic component failures in service. All of the aforementioned materials are valued for their strength, clarity, and long history of safe use in. Microhardness is used for general screening and generation of data on plastic fluid compatibility. 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. Polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. 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 crack resistance escr testing of. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking. It is one of the most common causes of unexpected brittle failure in thermoplastic. 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.
George wypych, in pvc formulary second edition, 2015. It has been estimated that 15% of all failures of plastic materials in commercial use are related to environmental stress cracking esc 3. Over time, chemical attack or sunlight will reduce the strength of the plastic until the internal stress causes these cracks. Polycarbonate material exhibits aromatic sensitivity or is prone to absorb odors, according to the plastics web.
Processing polycarbonate at the manufacturers recommendations is the key to stress free and crack resistant parts. Evaluating environmental stress cracking of medical plastics. The test provides analysis for the materials oxidation and alcohol absorption, as well as its resistance to environmental stress cracking. 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. 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. 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. As little as 10 percent glass fiber can reduce problems with stress cracking in materials like polycarbonate and polysulfone. Pdf stress cracking of polycarbonate exposed to sunscreen. Revised stresscracking resistance of polycarbonate in. 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. If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots. Pvc can become brittle and crack when placed in a few.
Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. The sunscreen analyzed contains various chemical species t hat may be stress cracking agents for polycarbonate. 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. 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. A recent study indicated that approximately 25% of plastic component failures are associated with esc1. It is the premature embrittlement and subsequent cracking of a plastic. You may be surprised to find out what two naturally occurring ingredients are the base for all vinyl compounds. 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. Microhardness is used for general screening and generation of data on plasticfluid compatibility. Screw threads are a feature in a plastic component that benefits from annealing. 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. Possibilities of why a polycarbonate pc part is cracking. Polycarbonate pc bonding tips and adhesive selection.
This article discusses polyurethane benefits and challenges, and also provides a test case for a proprietary aliphatic polycarbonate alc polyurethane. Various methods are discussed such as uv, cyanoacrylate, epoxy, and acrylic. Plasticizerinduced stress cracking of rigid pvc 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. Environmental stress cracking esc is considered a leading cause of plastic part failure. Teknor apex has been an innovator in the pvc compound industry for more than six decades. Certain highviscosity polymeric plasticizers over 100,000 centipoise and several in the i0,00025,000 range did not stress crack polycarbonate. If the molded part is completely stress free or is subject solely to compressive stress, then no stress cracking will occur. 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 of pvc and pvccpe springerlink. Among these is environmental stress cracking, which. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. Analysis of polycarbonate stress cracking and cleaning. Many homeowners believe pvc to be a maintenancefree product, however, which may or may not be accurate depending on its application.
Polycarbonate is a highly durable industrial plastic, with exceptional transparency to visible light. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. The stress required to produce failure in 50,000 hr produces crazing in less than 1 hr. There are a variety of adhesive choices for bonding polycarbonate. Although deformation may be simpler with the application of heat, even small angle bends are possible without it. There is always some stress on the surface of a plastic part.
This problem is well known and is called environmental stress cracking. This phenomena, although rare and preventable, is known to occur in many types of. Keywords polymethylmethacrylate, environmental stress cracking, chemiluminescence, radical 1. 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.
Tek tip cold forming or bending polycarbonate films. Although polycarbonate rates high for impact strength when compared to abs, polyvinyl chloride pvc or acrylic, it is subject to stress cracking. Two rigid pvc formulation technologies designed to minimize the impact of plasticizerinduced stress cracking were evaluated. High stress in polycarbonate is usually any level that is over 1500 psi. But there are methods for getting to the root of the problem. As it so happens, this doesnt only happen to polycarbonate. The ductile failure is ascribed to plasticization andor plastic deformation around the craze.
454 1166 144 1644 1547 173 957 377 1305 1061 1532 364 1621 1547 1080 1369 378 548 9 302 1421 94 218 1457 569 1204 1402 1264