The lenses were injection molded from a polycarbonate resin and. Plastic offers better eyewear options advancing materials. The strain in the test environmental stress cracking of. Environmental stress cracking the the chemical agent. Doing some research into this problem we came up with a solution to cure to this.
Polycarbonate lenses are thinner, lighter, and much more durable than standard. Some copolymers of ldpe are available with improved stresscrack resistance. In general this problem falls into two subcategories. 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. Furthermore, its considerable heat resistance can be enhanced with a variety of flame retardants without significantly lowering any of its other properties. Improvement of environmental stress cracking resistance of. It also occurs in most fiberreinforced plastics, at the timedependent knee in the stressstrain curve. How to prevent polycarbonate rimless lenses from cracking. Combined with its superior chemical resistance, this lower residual stress level can significantly reduce the risk of product failure from environmental stress cracking esc. Phenol and its chemical derivatives are essential for production of polycarbonates, epoxies, bakelite, nylon, detergents. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present.
The base lens material was tested via dsc and a glass transition. There are factors other than viscosity that can also effect the stresscracking problem. Plastic failure through environmental stress cracking. 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. 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. The main cause of lenses cracking at the edges is the lenses were edged a hair too large. Analysis of polycarbonate stress cracking cleaning solvent compatibility with telephone components. However, due to its susceptibility to environmental stress cracking, its use is limited to low stress applications. Bpa is a precursor to important plastics, primarily certain polycarbonates and epoxy resins, as well as some polysulfones and certain niche materials. 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. Behaviour of polycarbonate in different chemical by determination of the timedependence of stress at constant strains.
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. Applications for polycarbonate include cds, dvds, bluray discs, drinks bottles, drinking glasses, lab equipment, animal enclosures, lighting lenses, to name but a few. Analysis of polycarbonate stress cracking and cleaning. Before trivex came along we were throwing way too many lenses read. Why do my daughters polycarbonate lenses keep on cracking. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure. Test method the strain in the test environmental stress. Environmental stresscracking has been found to occur most readily under such. Polycarbonate is an incredibly useful plastic for applications requiring transparency and high impact resistance. Environmental stress cracking last updated march 07, 2020. Environmental stress crazing 799 ntetradecane hexane isooctane 1400 1600 600 1200 nhexane 7. 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. 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.
The potential for environmental stress cracking is of paramount concern when plastics are used in medical device components such as luers and stopcocks. Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small number of chemical environments. I dont think cheap polycarbonate is out of the question here. 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. Even a brief exposure to naoh during the test resulted in a remarkable effect on the stressstrain behavior. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. Wetting agents, such as detergents, accelerate stress cracking. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused. One of the fastest growing plastics today is linear lowdensity polyethylene lldpe, used mainly in film applications but also suitable for. Environmental stress cracking esc is widely regarded as the leading cause of plastic. Polycarbonates pc are a group of thermoplastic polymers containing carbonate groups in. Pdf stress cracking of polycarbonate exposed to sunscreen. However, it has poor scratch resistance, somewhat poor chemical resistance and is prone to environmental stress cracking. Polymers studied using this technique include polycarbonate pc.
It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. Crazes surface cracks produced by esc in pmma drinking beaker. 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. Therefore, a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior. A very popular use for polycarbonate is as a base material for some cellphone housings and cases.
Figure 3 and figure 4 show a cracked mirror housing and headlight lens. In this effort we also conducted initial studies to evaluate a criterion for craze initiation proposed by gent. It is a lighter alternative to glass and a natural uv filter, so it is often used in eyewear. 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. Environmental stress cracking of glassy polymers request pdf. At creative mechanisms, we have used polycarbonate in a number of applications across a range of industries.
It can be uv stabilized with additives, and is frequently used in the production of. Deny kyriacos, in brydsons plastics materials eighth edition, 2017. Standard test method for environmental stresscracking of. 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. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. As telephone construction and maintenance operations eliminate ozonedepleting solvents, questions on the compatibility of replacements with various telephone components arise. A guide to chemical resistance of polycarbonate sheet. 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 of poly3hydroxibutyrate. Plasticizer chemical type, level of use, polarity, and severity of environmental conditions all play a role in the development of cracking in stressed polycarbonate. Environmental stress cracking is involved with some 25% of plastic part failures.
Polycarbonate is the first commercially useful thermoplastic material which incorporates the carbonate radical as an integral part of the main polymer chain. 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. Evaluating environmental stress cracking of medical plastics. When the screw is tightened, it puts stress on the edge, therefore causing cracks. Since this is responsible for 15% of all failures of plastics components, environmental stress cracking has long been the subject of intensive research.
Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in. Stress cracking is the main reason that high pressure overbraided hoses fail. 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. Some plastics such as abs, acrylic, polycarbonate, and polystyrene are more susceptible to stress cracking. Pc is only fairly resistant to solvents and harsh chemicals, and can be subject to stress cracking depending on structural design and environmental conditions. The type of stress involved with surface crazing is most usually a tensile stress.
Stress from fasteners, plastic welding and molding render polycarbonate susceptible to stress corrosion cracking when it comes in. 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. Lenscrafters calls their poly lenses featherweights. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Also, at the optical i work at, ive seen quite a few lnses cracking due to the polycarbonate material itself. 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.
It can be laminated to make bulletproof glass, although bulletresistant is more accurate for the thinner windows, such as are used in bullet. There is much printed information available regarding stress crazing in thermoplastic sheet products. 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. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. Under the conditions of the test method, high local multiaxial stresses are developed through the introduction of a controlled imperfection 2, 3.
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. 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 is the cracking of certain plastic products that becomes exposed to. Environmental stress cracking is the cracking of certain plastic products that. Environmental stress cracking is exhibited by many materials, including plastics. The antimicrobial solution, an aqueous solution of sodium hydroxymethylglycinate, resulted in cracks at 1. Phosgene is a valued industrial building block, especially for the production of urethanes and polycarbonate plastics.
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