The less likely but not out of the question culprit could possibly be that the lenses are reacting to the fumes from the glue that secure the inner shell of her eyeglass case. Pc is commonly used for plastic lenses in eyewear, in medical devices. This research studies the stress cracking of polycarbonate plastic by a variety of solvents and solvent blends. Plastic offers better eyewear options advancing materials. All of the aforementioned materials are valued for their strength, clarity, and long history of safe use in. Modern test methods for pressurized pipes determine the time. Pdf a better look at polycarbonate lens researchgate. My lab just redid 4 of the same lens within the last 2 months.
Also, at the optical i work at, ive seen quite a few lnses cracking due to the polycarbonate material itself. In fact, once you have the right tools all which you will learn here, you can doityourself. Doing some research into this problem we came up with a solution to cure to this. The less likely but not out of the question culprit could possibly be that the lenses are reacting to the fumes from the. Why do my daughters polycarbonate lenses keep on cracking. It is important to have an understanding of the amount of stress used in any given test to better evaluate liquids or cleaners that may cause crazing. This test is an indication of the number of voids present in. This method is usually preferred over the socalled senarmont or analyzer rotation method, which can be difficult to learn and even more challenging to apply to highstress products. Analysis of polycarbonate stress cracking and cleaning. Crazing can be induced at moderate to high stress levels by low molecular weight hydrocarbons. It can typically be formed at room temperature without cracking or. Environmental stresscracking has been found to occur most readily under such. Plasticizers within pvc, such as phthalates can trigger stress cracking and result in damage to the polycarbonate. Environmental stress cracking esc is widely regarded as the leading cause of plastic product failure.
My guess is that you have ar coating on the lenses and that the coating is cracking, not the lens itself. Products such as acetone and xylene may cause stress cracking even at very low stress levels and should therefore be avoided. The tensile strength of trivex makes it highly resistant to cracking around drill holes, so much in fact, younger optics guarantees its trivex products trilogy for life, against stress fractures. Uv2 polycarbonate is similar to sl and trupoly in the sense that its mainly used for clear sheeting. Revised stresscracking resistance of polycarbonate in. Dec, 2012 stress crack test makrolon 1239 bayer materialscience. 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. The mathematical relationship between outer fiber strain, radius of curvature r and thickness t is.
Optical studies of crazed plastic surfaces nvlpubsnistgov. People choose plastic over glass because plastic lenses are thin, light, and less. Environmental stress cracking is involved with some 25% of plastic part failures. A guide to chemical resistance of polycarbonate sheet. Environmental stress cracking esc is widely regarded as. Lenscrafters calls their poly lenses featherweights. Common questions and answers about stress test for polycarbonate. The variation of stress with time was recorded, and the surface cracking was observed. Measuring with a compensator is a standard astm test procedure 2 and is a particularly effective laboratory tool for quality control of clear plastics. Stress cracking is the main reason that high pressure overbraided hoses fail. 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 applied stress is plotted versus the timetofailure on a double logarithmic scale. Cracks in polycarbonate lenses my lab just redid 4 of the same lens within the last 2 months.
Consider all the options before your cataract surgery. Glass versus acrylic versus makrolon impact test gat. Aluminum test fixtures used to induce stress cracking of the polycarbonate specimens had varying radii of curvature, providing a range of strain values from 0. 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. Pdf polycarbonate lenses are commonly used in many optical. A visual examination of returned eyeglasses showed cracks were developing in. 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 mixed solvent test for polycarbonate is well developed. Environmental stress cracking the the chemical agent. 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.
Standard test method for environmental stresscrack. Pc is commonly used for plastic lenses in eyewear, in medical devices, automotive components, protective gear, greenhouses, digital disks cds, dvds, and bluray, and exterior lighting fixtures. Polycarbonate and trivex lenses have similar impact resistance, but remember all bets are of any time a coating of any kind is involved. Polycarbonate lenses are also used in quality sunglasses that.
This instance leads us to a discussion of one of the methods that can be used to test for the presence of stresses in a part that might lead to stress cracking. It can also happen other places along the edge of the lens usually when people take their frames off their face and throw them on a table over a period of time. The test specimens are bent with the notch pointing upwards in a metal ushaped specimen holder to generate excess residual stress. The variation of stress with time was recorded, and the surface cracking was observed to evaluate the esc resistance of samples. Stress cracks or surface crazing near stress concentrators like threads can be a sign of future problems. Stress crack test makrolon 1239 bayer materialscience. Either to itself or to other materials, there are glues available for most applications. Its the reason you cant put stickers on your motorbike helmet. This is a fancy way of saying that the lenses expand and contract with temperature.
How to prevent polycarbonate rimless lenses from cracking. Pdf stress cracking of polycarbonate exposed to sunscreen. Polycarbonate also has very good heat resistance and can be combined with flame retardant materials without significant material degradation. Reference 2 uses the following to describe the chemical resistance of polycarbonate to a solvent. For example, the test method astm f48402 stress crazing of acrylic. Stress cracking of polycarbonate exposed to sunscreen.
One example is the contact between polycarbonate and plasticized pvc. Since variance of any of these factors can change the environmental stress crack resistance of the container, the test results are representative only of a given test performed under defined conditions in the laboratory. Evaluation of residual stress in molded resin jfe techno. Stressfree parts are more likely to maintain their strength, optical clarity, stability, and resistance to environmental stress factors. Sometimes stress is not visible until the component is subject to loading or exposure to chemicals by the end user. Usually happens in a wire frame where the screw is. Hagan, md, fellow american academy of ophthalmology, fellow american college of. If your ar coating can make it through the test with a 4 or 5. Plasticizerinduced stress cracking of rigid pvc vs. The low molecular weight polymer fractions and the chain ends within the bulk of the polymer become more ordered during the diffusion and swelling process by their partial solubility in the crazing agent, causing crystallization. Therefore, a hard coating is applied to polycarbonate eyewear lenses and. It involves varying concentrations of npropyl alcohol as the inert ingredient and toluene as the stress crack agent. Rapra in the united kingdom states that in nearly onethird of all failed plastic applications the problem is stress cracking. Head light lens cracking a headlight lens had cracked after a brief time in service.
Plastic stress test protocols connecticut plastics. Its usually a workmanship defect from the edging process, although it can occur in a couple of other steps. The waterbased degreaser resulted in environmental stress cracking esc of the polycarbonate test bars at all strain levels tested above zero strain. Since variance of any of these factors can change the environmental stresscrack resistance of the container, the test results are representative only of a given test performed under defined conditions in. In general this problem falls into two subcategories. Possibilities of why a polycarbonate pc part is cracking. Polycarbonates pc are a group of thermoplastic polymers containing carbonate groups in. If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots. Has anyone had problems with high minus poly lenses that crack. The interactions between the fluid, the stress, and the plastic polymer include. Dec 19, 2009 the main cause of lenses cracking at the edges is the lenses were edged a hair too large. Astm d2561 17 standard test method for environmental stress. One is a stress crack, usually caused by the lenses being a millimeter or two too large when they come out of the edger. Under the conditions of the test method, high local multiaxial stresses are developed through the introduction of a controlled imperfection 2, 3.
Investigations on environmental stress cracking resistance. Jun 09, 2011 the test involved using four identical samples of polycarbonate light lenses of a major light manufacturer. Residual stress testing for transparent polymers mddi online. 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. Certain lens materials are more prone to stress fracturing than others. We can compute the amount of stress both ways, so that the reader can use the type of stress with which he is more familiar. Relaxation of residual stresses in plastic cover lenses. Feb 05, 2018 the tensile strength of trivex makes it highly resistant to cracking around drill holes, so much in fact, younger optics guarantees its trivex products trilogy for life, against stress fractures. As it so happens, this doesnt only happen to polycarbonate. The lens is not too tight in the frame and the lab said it is either due to how the lens is cleaned or the polishing.
Some of the lenses lasted only 23 days before large cracks appeared one from below and one from above. Poly is also sensitive to a lot of different things. Fatigue crack propagation in polycarbonate materials technology. The dil ells ion s of the craze cracks produced by shorttime stress crazing and by stress sovent. Luckily, polycarbonate is a relatively easy plastic to stick. One of the most important things to understand related to this topic is that lenses have what is scientifically referred to as a coefficient of thermal expansion. For example, standard plastic is most prone to cracking, polycarbonate is one of the biggest culprits when it comes to stress fracturing but the crack spreads slowly. Effect of coatings and processing on failure mechanisms. Stress from fasteners, plastic welding and molding render polycarbonate susceptible to stress corrosion cracking when it. 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. Molecular degradation many of tuffak properties are determined by the size of its molecules.
Polycarbonate and trivex lenses both block 100 percent of the suns uv rays without the need for special uvblocking lens coatings. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease. The problem lies, in part, in the definition of stress cracking. The stress cracking characteristic of polycarbonate was evaluated by measuring the minimum strain required to produce serious damage to pcls, causing cracks, and those samples were exposed to propylene carbonate, ethyl acetate, and npropanol, where a nonhomogenous region of an isochromatic fringe pattern was identified on the pcls by. At low stresses a nearly stress independent failure mechanism. An effective way to test this type of sample is to scrape material from the visually. For transparent parts, the test methods described here can be performed using inexpensive, nondestructive procedures that can be used either on. 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. In transparent parts, the easiest way to detect stress is by viewing the part with polarized light. The part was considered to be typical of many other parts that had also cracked in service. Stress cracking is one of the most common problems associated with field failures in plastic products. Lens materials such as trivex are far less likely to fall prey to stress fractures. Environmental stress cracking the the chemical agent does. Stress free parts are more likely to maintain their strength, optical clarity, stability, and resistance to environmental stress factors.
Measuring residual stress in transparent plastics mddi. Standard test method for environmental stresscracking of. It can be laminated to make bulletproof glass, although bulletresistant is more accurate for the thinner windows, such as are used in bullet. Its unlike those two, however, in the sense that its tensile strength is measured by mpa instead of psi. Relaxation of residual stresses in plastic cover lenses with. In such ways, methods of taking mea sures against environmental stress cracking by using solubility parameters, and inferences regarding the causes of fractures by observing surface cracks 6, are being devised. The stress cracks on the lens now can almost be felt with your finger nail on the outside of the lens it doesnt catch, but you can feel them, but they all originate from inside of the lens.
I was hoping to see if there are any chemicals that can fill remelt the plastic, perhaps under a vacuum or with working pressure high heat buffing. For transparent parts, the test methods described here can be performed using inexpensive, nondestructive procedures that can be used either on or offline without slowing down production. For newbies, it can be a difficult process but trust me, i will make it easier for you. Polycarbonate lenses, as other high index materials, cr39, and glass have certain. The lenses were injection molded from a polycarbonate. 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. Stress from the frame on the lenses will help transform these microcracks into longer cracks across the lens. The ar coating can be cracked because it is less flexible than the polycarbonate lens. Improvement of environmental stress cracking resistance of. Processing polycarbonate at the manufacturers recommendations is the key to stress free and crack resistant parts. Polycarbonate cracking polymer engineering engtips.
The main cause of lenses cracking at the edges is the lenses were edged a hair too large. Parameters used for scratch hardness testing on polycarbonate lens. The test involved using four identical samples of polycarbonate light lenses of a major light manufacturer. This problem is well known and is called environmental stress cracking. This is known as the longterm hydrostatic strength lths. However, it is also extremely sensitive to liquids that come in contact with it and will become weakened and crack. The antimicrobial solution, an aqueous solution of sodium hydroxymethylglycinate, resulted in cracks at 1. 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.
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. High stress levels in plastic are a common cause of component field failures. When the screw is tightened, it puts stress on the edge, therefore causing cracks. Environmental stress cracking on light lenses by shield. The complete guide today, i want to show you how to bend polycarbonate sheet.
Have you noticed stress cracking in polycarbonate parts that contact flexible pvc for prolonged periods. The failure rate had remained low, but steady, over the course of several years. The content on this site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. Plastic failure through environmental stress cracking. 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. Other commonly used medical rigid polymers, such as abs, polystyrene, and acrylic, can exhibit the same phenomenon. The lenses were exposed soaked to chemicals for a period of 24 hours.
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