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Plastic Weathering Fundamentals


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Learn more about the effects of sunlight on plastics, test methods for assessing light stability, and the correlation between natural and accelerated weathering.

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Plastic Weathering Fundamentals

  1. 1. + 1 (732) 309-3476 Jim Botkin BotkinChemie Weathering Fundamentals
  2. 2. Introduction All plastics are susceptible to degradation upon long-term outdoor exposure. This phenomenon is caused by chemical and physical changes that occur in the polymer as a result of exposure to ultraviolet (UV) radiation in sunlight. Compared to visible light, UV radiation is characterized by shorter wavelengths and higher energy, and thus is much more damaging to plastics and other organic materials. Fluorescent lamps used for indoor lighting also emit UV light, but at a much lower intensity than normal sunlight. Characteristics of Sunlight Sunlight received on earth comprises radiation of wavelengths ranging from about 290 nm to 2800 nm and beyond). The visible and infrared components of sunlight (wavelengths > 400 nm) are relatively low in energy and are non- damaging to most polymers. UV radiation (wavelengths less than 400 nm) is higher in energy and produces degradation in plastics. UV radiation is subdivided into three diferent regions of wavelengths. UV-A and UV-B radiation are responsible for the degradation of plastics exposed outdoors, while the short wavelength (< 280 nm) UV-C radiation is screened by the ozone layer in the atmosphere. Solar radiation spectrum at the earth's surface
  3. 3. Efects of UV Radiation on Plastics Exposure of plastic materials to UV radiation in the presence of atmospheric oxygen initiates degradation through a photo-oxidative process. This chemical process produces changes in the polymer structure (e.g. breaking or cross- linking of the polymer chains, formation of new chemical functional groups) that result in a loss of the material’s useful properties over time. For example, photo-oxidation can produce undesired changes in appearance (e.g. discoloration, changes in gloss, or chalking), deterioration of mechanical properties (e.g. tensile, fexural, or impact properties), and/or the formation of visible defects such as cracks. Water tank showing chalking associated with severe UV degradation Stadium seats showing fading resulting from UV degradation
  4. 4. Relative Weathering Resistance of Plastic Materials Due to diferences in their chemical composition, various polymers respond diferently to the efects of UV radiation, resulting in very diferent weatherability characteristics. As shown below, some materials such as acrylics and fuoropolymers are inherently highly weatherable and require little in the way of UV stabilization. Other materials such as aromatic polyurethanes are inherently unstable to UV light and must be painted if they are to be used in long-term outdoor applications. Most other polymers (including polyolefns and some engineering plastics) fall into the middle range of this spectrum, and can be used successfully outdoors when properly stabilized. The inherent weathering characteristics of a polymer are an important consideration in material selection as part of the product design process. For example, the properties and processing characteristics of ABS and ASA are similar, but for outdoor applications ASA is preferred due to its superior weathering characteristics. Relative weatherability of selected plastics Less weatherable More weatherable PPO PUR - Aromatic ABS PC/ABS POM PS Nylon PBT, PET PC PC/ASA ASA PE PP, TPO PUR – Aliphatic PVC Acrylics Fluoropolymers Not weatherable Suitable for interior and short-term outdoor use “ Super weatherable” Suitable for outdoor use when properly stabilized
  5. 5. Light Stability Testing Natural outdoor weathering can be a slow process, even when conducted in tropical or desert climates with relatively high solar irradiance and temperature. Accelerated weathering devices (often referred to as weather-o-meters or WOM) have been developed to provide results more quickly. These devices provide faster results than natural weathering because they can be operated continuously (i.e. samples can be exposed for 24 hours per day rather than only during daylight hours) and also because the lamps used as radiation sources provide more intense radiation than natural sunlight. The most widely used weather-o-meters today utilize xenon-arc lamps as the radiation source, which when properly fltered produce a spectrum of radiation that is quite comparable to that of natural sunlight. Accelerated weathering methods utilizing xenon-arc weather-o-meters are described and specifed in industry standards such as ISO 4892-2 and ASTM G155. The resistance of a polymer to the efects of weathering is typically determined by measuring changes in the appearance or mechanical properties of a test specimen resulting from natural or accelerated weathering. Changes in appearance are most often assessed using instrumentation such as a spectrophotometer or a gloss-meter. In some cases changes can be assessed visually (e.g. onset of chalking) or microscopically (e.g. the formation of micro-cracks). Alternatively, the weatherability of a material can be assessed by measuring changes in a mechanical property such as elongation at break, tensile strength, or impact strength as a function of exposure time. Example of a xenon-arc weather-o-meter
  6. 6. Special Considerations for Automotive Applications The automotive industry has developed its own specialized test methods for accelerated weathering. The SAE J2412 protocol is used for automotive interior trim components and the SAE J2527 protocol for automotive exterior materials. These protocols are also described in ASTM G155 (cycles 8 and 7, respectively). Compared to the standard ASTM or ISO test methods, the automotive protocols feature higher UV irradiance. These methods also utilize extended UV flters that transmit more short wavelength radiation than the daylight flter employed in the ASTM and ISO methods. These features provide further acceleration of the test but may also produce aging processes that do not occur during natural weathering. The criterion for meeting OEM weatherability requirements for plastic materials typically involves a specifcation for color change (for example DE) after a given amount of exposure (for example DE < 3 after 2500 kJ/m2 exposure measured at 340 nm). The weatherability of automotive interior components is tested using the SAE J2412 protocol (ASTM G155, cycle 8)
  7. 7. Correlation between Natural and Accelerated Weathering Clients often ask how to determine the lifetime of a material outdoors based on accelerated weathering data, for example by defning an “acceleration factor” equating “x” hours of accelerated weathering in a weather-o-meter to “y” months or years of actual outdoor exposure based on UV irradiance levels. Unfortunately, this practice is discouraged given that the weatherability of a material in end use is afected by additional factors beyond irradiance, including precipitation, relative humidity, temperature, air pollutants, length of day/night cycles, and many others. A detailed discussion of these factors can be found in Annex A of ISO 4892-1:1999. The bottom line is that no laboratory exposure test can be specifed as a total simulation of end use conditions outdoors. A more appropriate use of accelerated weathering testing is to compare the relative weatherability characteristics of diferent samples of a material, for example to rank the performance of diferent light stabilization systems in the same polymer substrate. In this case, exposure conditions should be selected which either conform to end user specifcations or correlate with expected end use conditions as closely as possible. Exposure of multiple replicates of test samples side by side in the same weather-o-meter at the same time is also recommended in order to maximize the reliability of the results. However, even if the outdoor lifetime of one of the samples being tested is known, caution is advised in using the results to infer the lifetime of the other samples under test. Caution is advised in using accelerated weathering results to estimate the lifetime of plastics exposed outdoors
  8. 8. Jim Botkin provides training seminars and consultation to help clients formulate, test, and specify plastic additives in order to meet industry and customer performance requirements. Jim has over 20 years of formulation experience and is backed by a proprietary additive information database. He has helped his clients to reduce formulation costs, accelerate new product development, and solve or avoid problems relating to additive performance, regulatory compliance, freedom to operate, and product availability. Please contact me to put my expertise to work for you. Jim Botkin BotkinChemie +1 (732) 309-3476 Contact Us
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