Molecular degradation many of tuffak properties are determined by the size of its molecules. Either to itself or to other materials, there are glues available for most applications. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. The lenses were injection molded from a polycarbonate resin and. Stress cracks will always result in sheet failure, which will develop from areas of greatest stress screws, fixings, bends, etc. 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 material selection guide at curbell plastics author. Stress cracking of polycarbonate parts during prolonged storage while assembled with plasticized polyvinyl tubing is greatly reduced by using tubing in which tri2ethylhexyl trimellitate is the predominant or sole primary plasticizer. 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. The issue is that when the plasticizer comes into contact with a rigid material adjacent to itunless that material is made of rigid pvcit will tend to soften the connector or fastener, resulting in damage. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. Plasticizers within pvc, such as phthalates can trigger stress cracking and result in damage to the polycarbonate. Stresscracking resistance of polycarbonate in contact with plasticized pvc compounds polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. Polycarbonate material exhibits aromatic sensitivity or is prone to absorb odors, according to the plastics web. Revised stresscracking resistance of polycarbonate in. Polycarbonate has thousands of applications, from cd manufacture to aircraft cockpit canopies, but the largest user of polycarbonate is the construction sector, where is a replacement for glass. Although both amorphous and crystalline plastics are susceptible to environmental stress cracking, it is generally recognized that amorphous plastics tend to be more at risk. Also, if the ejection speed is too great, the plastic will not have time to conform to the normal stress being applied and cracks will form from distortion. Stress cracking resistance of polycarbonate in contact with plasticized pvc compounds polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters.
When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. Environmental stress cracking the the chemical agent. The sunscreen analyzed contains various chemical species t hat may be stress cracking agents for polycarbonate. Plasticizerinduced stress cracking of rigid pvc vs. 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. Pvc pipe can be used for most any drain, water, or ventilation application. Evaluating environmental stress cracking of medical. Environmental stress cracking an overview sciencedirect. Environmental stress cracking occurs in amorphous polymers such as abs, pc, pmma, ps, pvc, and san but also in some in semicrystalline thermoplastics, such as pe, pp, pa, pb.
You may be surprised to find out what two naturally occurring ingredients are the base for all vinyl compounds. Curbell plastics supplier of high quality thermoplastic materials, technical assistance, custom plastic fabrication. Esc is a failure mechanism whereby a plastic resin is effected by a chemical. High stress in polycarbonate is usually any level that is over 1500 psi. Stress crack reduction in polycarbonate parts american. If this plasticized pvc compound is in direct contact with another polymer surface, the plasticizer can interact with the surface of that polymer and can often result in marring and or cracking of that surface. How to prevent stress cracking what is stress cracking. Polycarbonate displayed outstanding criticalstrain values in alcohols and aliphatic hydrocarbons but very low criticalstrain values polyvinyl chloride is a mystery to most. Various methods are discussed such as uv, cyanoacrylate, epoxy, and acrylic. This month we want to conclude the discussion with a few more case studies and then discuss tests and material selection techniques that. Many homeowners believe pvc to be a maintenancefree product, however, which may or may not be accurate depending on its application. High stress can result in chemical stress cracking of polycarbonate film. Stress from fasteners, plastic welding and molding render polycarbonate susceptible to stress corrosion cracking when it comes in contact with certain accelerants such as salt water and plastisol.
When properly formulated, its rigidity and clarity make it a candidate for use in many of the applications where polycarbonates are presently used. The presented cases illustrate how the failure analysis process. This problem is well known and is called environmental stress cracking. Plasticizerinduced stress cracking of rigid pvc and.
Polycarbonate cracking polymer engineering engtips. The two fixing screws go into nuts which are embedded in hard clear plastic possibly acetate or thin perspex on the far side of the case. Stress crack will only develop, however, if tensile production andor are applied from the outside. Analysis of polycarbonate stress cracking cleaning solvent compatibility with telephone components. 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 polymer chains eventually creep toward the pull and deform. Environmental stress cracking examples from the automotive indsutry jeffrey a. Certain highviscosity polymeric plasticizers over 100,000 centipoise, and several in the i 0,000 to 25,000 range, did not stresscrack polycarbonate. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking. Stresscracking resistance of polycarbonate in contact. A few plastics, among them pet and polycarbonate, can be entirely. Cracks in corners when cracks appear in the corners of plastic parts, the cause is too much plastic shrinkage onto the mold core in that corner. Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials.
Polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. At creative mechanisms, we have used polycarbonate in a number of applications across a range of industries. The cause of this type of part cracking is internal stress caused by high plastic pressure during molding. At 50,000 hr, crazing will occur at a stress level of 4000 psi 27.
By environmental stress cracking, we understand a failure in which materials in contact with a. Flexible pvc medical tubing contains plasticizer, the component added to pvc to impart flexibility to the inherently rigid pvc polymer. 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. 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. Certain highviscosity polymeric plasticizers over 100,000 centipoise, and several in the i 0,000 to 25,000 range, did not stress crack polycarbonate. High stress levels in plastic are a common cause of component field failures. Environmental stress cracking escr is the formation of cracks in a part when exposed to. Analysis of polycarbonate stress cracking and cleaning. Plasticizerinduced stress cracking of rigid pvc and polycarbonate. Our experience indicated that rigid pvc would be more resistant to plasticizerinduced stress cracking than polycarbonate and we decided to validate this observation. The mechanisms of environmental stress cracking proposed in the literature will be. Esc is a very important mode of plastic component failure.
Looking inside the casing, i see that where the screws go into the nuts, there are lots of little stress cracks in the plastic. It is therefore recommended to select polycarbonates with higher molecular weight which have better resistance to solventinduced cracking. Polycarbonate, pvc or polystyrene stress cracking is more prevalent in low molecular weight polycarbonates. As telephone construction and maintenance operations eliminate ozonedepleting solvents, questions on the compatibility of replacements with various telephone components arise. Chemical resistance can be dependent upon exposure time, temperature, stress molded in and any external stress, type of chemical and the chemical concentration. Evaluating environmental stress cracking of medical plastics.
It is a lighter alternative to glass and a natural uv filter, so it is often used in eyewear. If a plasticized pvc tube is mounted on a stressfree polycarbonate pipe, for instance, the. There are a variety of adhesive choices for bonding polycarbonate. Injection molded part defects cracks in molded parts. The mixed solvent test for polycarbonate is well developed. However, it is also extremely sensitive to liquids that come in contact with it and will become weakened and crack. The potential for environmental stress cracking is of paramount concern when plastics are. Two rigid pvc formulation technologies designed to minimize the impact of plasticizerinduced stress cracking were evaluated. Environmental stress cracking esc is considered a leading cause of plastic part. Environmental stress cracking of polyvinyl chloride in. This means that polycarbonate, polyacrylonitrilebutadienestyrene, and. So me of the chemicals included are aromatic esters, ketones and amines.
Environmental stress cracking esc is a common cause of plastic product failure. Polycarbonate is an incredibly useful plastic for applications requiring transparency and high impact resistance. Have you noticed stress cracking in polycarbonate parts that contact flexible pvc for prolonged periods. Although polycarbonate rates high for impact strength when compared to abs, polyvinyl chloride pvc or acrylic, it is subject to stress cracking. 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. Any evidence of cracking in ejector pin areas indicate the pins are too small, or the ejection speed was too great, or there was too much injection pressure used to fill the mold. With force and time, crazing or stress cracking occurs. Stress cracking is one of the most common problems associated with field failures in plastic products.
Pdf pushbuttons, knobs and plastic faceplates in automotive interiors are constantly exposed to chemicals such as sunscreen. The process for making pvc polyvinyl chloride is a mystery to most. Stress cracks or surface crazing near stress concentrators like threads can be a. Luckily, polycarbonate is a relatively easy plastic to stick. Polycarbonate, an amorphous glassy thermoplastic, is susceptible to cracking or crazing when under stress and in contact with a chemical environment. Its the reason you cant put stickers on your motorbike helmet.
Jul 17, 2017 pvc pipe is a popular construction material for its low cost and durability. 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. This stress can be caused by a cavity pressure that is either too high or too low. The use of polycarbonate in automotive applications is limited to low stress applications. This damage will look like environmental stress cracking, or crazing. Certain highviscosity polymeric plasticizers over 100,000 centipoise and several in the i0,00025,000 range did not stress crack polycarbonate. In many cases, you will also see that the part is hanging up on the core. If the molded part is completely stress free or is subject solely to compressive stress, then no stress cracking will occur. Plastic stress test protocols connecticut plastics.
Bonding polycarbonate with industrial adhesives surface. Environmental stress cracking esc is widely regarded as the leading cause of plastic. Words like ethylene dichloride and vcm are not typically used in daily conversation. The solution to an internal part cracking problem is to change the packing pressure. It involves varying concentrations of npropyl alcohol as the inert ingredient and toluene as the stress crack agent. Plastic failure through environmental stress cracking. This failure for glassy polymers bisphenol a polycarbonate pc. Stress cracks or surface crazing near stress concentrators like threads can be a sign of future problems. Pdf stress cracking of polycarbonate exposed to sunscreen. No matter how much experience you have working with plastics, sometimes a direct comparison of physical properties is the best way to determine which material is best for your applicationproject. Teknor apex has been an innovator in the pvc compound industry for more than six decades. The environmental stress cracking esc effects on pvc of various high ph sodium hydroxide environments have been studied.
Flexible polyvinyl chloride pvc is the most common material used to manufacture clear flexible tubing for medical applications. 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. If a plasticized pvc tube is mounted on a stress free polycarbonate pipe, for instance, the. A guide to chemical resistance of polycarbonate sheet. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. 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. 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. The most critical effect of chemical attack is stress cracking or crazing, which may range in size from being visible to the naked eye to being only observable under a microscope. One example is the contact between polycarbonate and plasticized pvc. Oct 25, 2011 the stress required to produce failure in 50,000 hr produces crazing in less than 1 hr. Environmental stress cracking is involved with some 25% of plastic part failures. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to. Polycarbonate pc bonding tips and adhesive selection.
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