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. Environmental stress cracking the the chemical agent. George wypych, in pvc formulary second edition, 2015. Teknor apex has been an innovator in the pvc compound industry for more than six decades. It has been estimated that 15% of all failures of plastic materials in.
How to prevent stress cracking what is stress cracking. Plasticizers within pvc, such as phthalates can trigger stress cracking and result in damage to the polycarbonate. When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. Analysis of polycarbonate stress cracking and cleaning. Jansen, stork technimet abstract four case studies are presented to illustrate environmental stress cracking esc within automotive components. The use of polycarbonate in automotive applications is limited to low stress applications. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to. Words like ethylene dichloride and vcm are not typically used in daily conversation. The polymer chains eventually creep toward the pull and deform.
Environmental stress cracking examples from the automotive indsutry jeffrey a. Polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. You may be surprised to find out what two naturally occurring ingredients are the base for all vinyl compounds. Certain highviscosity polymeric plasticizers over 100,000 centipoise, and several in the i 0,000 to 25,000 range, did not stresscrack polycarbonate. Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials. Injection molded part defects cracks in molded parts. As telephone construction and maintenance operations eliminate ozonedepleting solvents, questions on the compatibility of replacements with various telephone components arise. This stress can be caused by a cavity pressure that is either too high or too low. Evaluating environmental stress cracking of medical plastics.
Polycarbonate is an incredibly useful plastic for applications requiring transparency and high impact resistance. Curbell plastics supplier of high quality thermoplastic materials, technical assistance, custom plastic fabrication. Either to itself or to other materials, there are glues available for most applications. Environmental stress cracking an overview sciencedirect. Flexible polyvinyl chloride pvc is the most common material used to manufacture clear flexible tubing for medical applications. Although polycarbonate rates high for impact strength when compared to abs, polyvinyl chloride pvc or acrylic, it is subject to stress cracking. This failure for glassy polymers bisphenol a polycarbonate pc. Stress cracks or surface crazing near stress concentrators like threads can be a. 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. The sunscreen analyzed contains various chemical species t hat may be stress cracking agents for polycarbonate. Plasticizerinduced stress cracking of rigid pvc and polycarbonate. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking. Polycarbonate cracking polymer engineering engtips.
The process for making pvc polyvinyl chloride is a mystery to most. Plasticizerinduced stress cracking of rigid pvc vs. Molecular degradation many of tuffak properties are determined by the size of its molecules. 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. Environmental stress cracking esc is a common cause of plastic product failure. A guide to chemical resistance of polycarbonate sheet. With force and time, crazing or stress cracking occurs. High stress can result in chemical stress cracking of polycarbonate film. If the molded part is completely stress free or is subject solely to compressive stress, then no stress cracking will occur. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms.
There are a variety of adhesive choices for bonding polycarbonate. 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. Stress cracking is one of the most common problems associated with field failures in plastic products. So me of the chemicals included are aromatic esters, ketones and amines. Its the reason you cant put stickers on your motorbike helmet. Oct 25, 2011 the stress required to produce failure in 50,000 hr produces crazing in less than 1 hr.
If a plasticized pvc tube is mounted on a stress free polycarbonate pipe, for instance, the. High stress levels in plastic are a common cause of component field failures. This damage will look like environmental stress cracking, or crazing. At 50,000 hr, crazing will occur at a stress level of 4000 psi 27. Certain highviscosity polymeric plasticizers over 100,000 centipoise, and several in the i 0,000 to 25,000 range, did not stress crack polycarbonate. Pvc pipe can be used for most any drain, water, or ventilation application. Stress cracking resistance of polycarbonate in contact with plasticized pvc compounds polymeric polyester plasticizers cause less cracking in polycarbonate than monomeric esters. Pdf pushbuttons, knobs and plastic faceplates in automotive interiors are constantly exposed to chemicals such as sunscreen. Environmental stress cracking esc is widely regarded as the leading cause of plastic. Polycarbonate pc bonding tips and adhesive selection. 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.
Have you noticed stress cracking in polycarbonate parts that contact flexible pvc for prolonged periods. Polycarbonate, an amorphous glassy thermoplastic, is susceptible to cracking or crazing when under stress and in contact with a chemical environment. However, it is also extremely sensitive to liquids that come in contact with it and will become weakened and crack. Analysis of polycarbonate stress cracking cleaning solvent compatibility with telephone components. Polycarbonate, pvc or polystyrene stress cracking is more prevalent in low molecular weight polycarbonates. 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. One example is the contact between polycarbonate and plasticized pvc. Jul 17, 2017 pvc pipe is a popular construction material for its low cost and durability. Evaluating environmental stress cracking of medical. The potential for environmental stress cracking is of paramount concern when plastics are. It is therefore recommended to select polycarbonates with higher molecular weight which have better resistance to solventinduced cracking. Stresscracking resistance of polycarbonate in contact.
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. It involves varying concentrations of npropyl alcohol as the inert ingredient and toluene as the stress crack agent. 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. Looking inside the casing, i see that where the screws go into the nuts, there are lots of little stress cracks in the plastic. The cause of this type of part cracking is internal stress caused by high plastic pressure during molding.
Luckily, polycarbonate is a relatively easy plastic to stick. Stress cracks or surface crazing near stress concentrators like threads can be a sign of future problems. Certain highviscosity polymeric plasticizers over 100,000 centipoise and several in the i0,00025,000 range did not stress crack polycarbonate. Plasticizerinduced stress cracking of rigid pvc and.
Esc is a very important mode of plastic component failure. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. 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. 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. 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. Polycarbonate displayed outstanding criticalstrain values in alcohols and aliphatic hydrocarbons but very low criticalstrain values polyvinyl chloride is a mystery to most.
In many cases, you will also see that the part is hanging up on the core. Chemical resistance can be dependent upon exposure time, temperature, stress molded in and any external stress, type of chemical and the chemical concentration. 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. High stress in polycarbonate is usually any level that is over 1500 psi. This problem is well known and is called environmental stress cracking. 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.
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. 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 crack reduction in polycarbonate parts american. The presented cases illustrate how the failure analysis process. Esc is a failure mechanism whereby a plastic resin is effected by a chemical. Environmental stress cracking of polyvinyl chloride 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. This month we want to conclude the discussion with a few more case studies and then discuss tests and material selection techniques that. Revised stresscracking resistance of polycarbonate in. Flexible pvc medical tubing contains plasticizer, the component added to pvc to impart flexibility to the inherently rigid pvc polymer. The environmental stress cracking esc effects on pvc of various high ph sodium hydroxide environments have been studied. 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.
The solution to an internal part cracking problem is to change the packing pressure. Stress cracks will always result in sheet failure, which will develop from areas of greatest stress screws, fixings, bends, etc. 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. Various methods are discussed such as uv, cyanoacrylate, epoxy, and acrylic. Plastic failure through environmental stress cracking. The mixed solvent test for polycarbonate is well developed. Pdf stress cracking of polycarbonate exposed to sunscreen. Environmental stress cracking is involved with some 25% of plastic part failures. 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. By environmental stress cracking, we understand a failure in which materials in contact with a. At creative mechanisms, we have used polycarbonate in a number of applications across a range of industries.
Amorphous structure of polymer facilitates solvent diffusion, which is required for the process of damage to occur. Stress crack will only develop, however, if tensile production andor are applied from the outside. 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. The lenses were injection molded from a polycarbonate resin and. Bonding polycarbonate with industrial adhesives surface.
Plastic material selection guide at curbell plastics author. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. 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. 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.
Environmental stress cracking escr is the formation of cracks in a part when exposed to. It is a lighter alternative to glass and a natural uv filter, so it is often used in eyewear. Environmental stress cracking esc is considered a leading cause of plastic part. 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. Plastic stress test protocols connecticut plastics. Two rigid pvc formulation technologies designed to minimize the impact of plasticizerinduced stress cracking were evaluated. This means that polycarbonate, polyacrylonitrilebutadienestyrene, and. A few plastics, among them pet and polycarbonate, can be entirely. 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. 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.
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