Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to stress cracking. Polycarbonate is an incredibly useful plastic for applications requiring transparency and high impact resistance. How to prevent polycarbonate rimless lenses from cracking. Even a brief exposure to naoh during the test resulted in a remarkable effect on the stressstrain behavior. Lenscrafters calls their poly lenses featherweights. 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. 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. Polycarbonates pc are a group of thermoplastic polymers containing carbonate groups in.
At creative mechanisms, we have used polycarbonate in a number of applications across a range of industries. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Polycarbonate lenses are thinner, lighter, and much more durable than standard. I dont think cheap polycarbonate is out of the question here. Improvement of environmental stress cracking resistance of. In freezing temperatures it can happen as well as when the drill holes have been drilled with a dull drill or too fast with a new one, that heated up and microscopically damaged the drilled hole. Pc is only fairly resistant to solvents and harsh chemicals, and can be subject to stress cracking depending on structural design and environmental conditions. Behaviour of polycarbonate in different chemical by determination of the timedependence of stress at constant strains. Doing some research into this problem we came up with a solution to cure to this. Polymers studied using this technique include polycarbonate pc. Furthermore, its considerable heat resistance can be enhanced with a variety of flame retardants without significantly lowering any of its other properties. Test method the strain in the test environmental stress. 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 widely regarded as the leading cause of plastic. The type of stress involved with surface crazing is most usually a tensile stress. 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. Therefore, a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. When the screw is tightened, it puts stress on the edge, therefore causing cracks.
As telephone construction and maintenance operations eliminate ozonedepleting solvents, questions on the compatibility of replacements with various telephone components arise. Environmental stress cracking esc is not the result of the continual battle that the plastics industry has in convincing environmentalists and politicians that plastics can be the solution to environmental problems and not the cause esc is the consequence of a combination of mechanical stress and exposure to an aggressive chemical. The potential for environmental stress cracking is of paramount concern when plastics are used in medical device components such as luers and stopcocks. Environmental stress cracking the the chemical agent. Also, at the optical i work at, ive seen quite a few lnses cracking due to the polycarbonate material itself.
Plasticizer chemical type, level of use, polarity, and severity of environmental conditions all play a role in the development of cracking in stressed polycarbonate. However, it has poor scratch resistance, somewhat poor chemical resistance and is prone to environmental stress cracking. Polycarbonate is the first commercially useful thermoplastic material which incorporates the carbonate radical as an integral part of the main polymer chain. Crazes surface cracks produced by esc in pmma drinking beaker. Phenol and its chemical derivatives are essential for production of polycarbonates, epoxies, bakelite, nylon, detergents. Since this is responsible for 15% of all failures of plastics components, environmental stress cracking has long been the subject of intensive research. The main cause of lenses cracking at the edges is the lenses were edged a hair too large. However, due to its susceptibility to environmental stress cracking, its use is limited to low stress applications. The strain in the test environmental stress cracking of.
Environmental stress cracking is the cracking of certain plastic products that. Environmental stress crazing 799 ntetradecane hexane isooctane 1400 1600 600 1200 nhexane 7. The waterbased degreaser resulted in environmental stress cracking esc of the polycarbonate test bars at all strain levels tested above zero strain. Environmental stress cracking of glassy polymers request pdf. Under the conditions of the test method, high local multiaxial stresses are developed through the introduction of a controlled imperfection 2, 3. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons.
Many sunglasses manufacturers choose polycarbonate because it can be easily shaped without problems like cracking or splitting, resulting in extremely lightweight, distortionfree, fashionable glasses that feature all of the health benefits doctors recommend. Environmental stress cracking of poly3hydroxibutyrate. Typically a bad drill job, crap quality polycarbonate or bad fitment into frame, where the frame is applying way too much pressure is the culprit of this kind of cracking. It is a lighter alternative to glass and a natural uv filter, so it is often used in eyewear. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. 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 esc is a common phenomenon that affects commercially available polymeric materials exposed to liquid agents under external loading, and it is one of the most common. The base lens material was tested via dsc and a glass transition. In general this problem falls into two subcategories. Deny kyriacos, in brydsons plastics materials eighth edition, 2017. 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. It can be laminated to make bulletproof glass, although bulletresistant is more accurate for the thinner windows, such as are used in bullet. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature.
The antimicrobial solution, an aqueous solution of sodium hydroxymethylglycinate, resulted in cracks at 1. Bpa is a precursor to important plastics, primarily certain polycarbonates and epoxy resins, as well as some polysulfones and certain niche materials. Stress cracking is the main reason that high pressure overbraided hoses fail. Standard test method for environmental stresscracking of. Pdf stress cracking of polycarbonate exposed to sunscreen. Environmental stress cracking is the cracking of certain plastic products that becomes exposed to. Environmental stress cracking is exhibited by many materials, including plastics. Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in. Plastic failure through environmental stress cracking. Some copolymers of ldpe are available with improved stresscrack resistance. Why do my daughters polycarbonate lenses keep on cracking. There is much printed information available regarding stress crazing in thermoplastic sheet products. The lenses were injection molded from a polycarbonate resin and.
Combined with its superior chemical resistance, this lower residual stress level can significantly reduce the risk of product failure from environmental stress cracking esc. The mechanism of environmental stress cracking esc of amorphous polycarbonate in isopropyl alcohol, ethylene glycol monomethyl ether and methanol were studied. It also occurs in most fiberreinforced plastics, at the timedependent knee in the stressstrain curve. Before trivex came along we were throwing way too many lenses read. One of the fastest growing plastics today is linear lowdensity polyethylene lldpe, used mainly in film applications but also suitable for. Tableware, drinking tumblers, cutlery, jars and beakers, storage containers for foods. Environmental stresscracking has been found to occur most readily under such. A guide to chemical resistance of polycarbonate sheet. Analysis of polycarbonate stress cracking and cleaning. Applications for polycarbonate include cds, dvds, bluray discs, drinks bottles, drinking glasses, lab equipment, animal enclosures, lighting lenses, to name but a few. Along comes trivex and over the past 3 years we have a trackable record of a reduction in scratch warranties, chipped lenses, nonadapts, and other misc. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure. Phosgene is a valued industrial building block, especially for the production of urethanes and polycarbonate plastics. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present.
Often we tend to group all the problems into the environmental category, but in reality most of the field problems we see have to do with solvent stress cracking. Environmental stress cracking last updated march 07, 2020. Plastic offers better eyewear options advancing materials. Polycarbonate is also considerably less toxic than many other plastics, and the ease with which it can be recycled only adds to its environmental friendliness. The variation of stress with time was recorded, and the surface cracking was observed to evaluate the esc resistance of samples. The problem lies, in part, in the definition of stress cracking. A very popular use for polycarbonate is as a base material for some cellphone housings and cases.
It can be uv stabilized with additives, and is frequently used in the production of. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused. Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small number of chemical environments. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. Stress from fasteners, plastic welding and molding render polycarbonate susceptible to stress corrosion cracking when it comes in. Environmental stress cracking is involved with some 25% of plastic part failures. Figure 3 and figure 4 show a cracked mirror housing and headlight lens. There are factors other than viscosity that can also effect the stresscracking problem. To explore the possibility of using surface coating to reduce environmental stress cracking esc of transparent polycarbonate pc parts, silicone coated and sio 2 coated pc were tested in a selfmade threepoint bending apparatus in the presence of ethanol. This caused in a great reduction in both tensile strength and maximum elongation and, therefore, showed that naoh is a strong stress cracking agent to phb.
Wetting agents, such as detergents, accelerate stress cracking. Analysis of polycarbonate stress cracking cleaning solvent compatibility with telephone components. In this effort we also conducted initial studies to evaluate a criterion for craze initiation proposed by gent. Investigations on environmental stress cracking resistance. Simultaneous action of stress and strain on plastics in contact with specific fluids causes the onset of cracking and embrittlement of plastic known as environmental stress cracking. Products molded from tritan have less residual stress than ones molded in other types of thermoplastics, such as polycarbonate. Evaluating environmental stress cracking of medical plastics.
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