Polymeric materials weather factors

What is meant by weather resistance What environmental factors can alter the properties of polymeric materials ... 

Other factors affecting the life of an adhesive bond are humidity, water immersion, and outdoor weathering. Moisture can affect adhesive strength in two ways. Some polymeric materials, notably ester-based pol)nirethanes, will revert, i.e., lose hardness, strength, and in the worst case, turn to fluid during exposure to warm humid air. Water can also permeate the adhesive and displace the adhesive at the bond interface. Structural adhesives not susceptible to the reversion phenomenon are also likely to lose adhesive strength when exposed to moisture. 

The accelerated weathering of polymeric materials in laboratory instruments is covered by ISO Standard 4892. In the US, ASTM standards G23, G26 and G53 cover such testing, and other ASTM standards are derived from these, as shown in Fig. 6.2. Many car manufacturers have their own acceptance specifications. Reviews of these standards and of factors affecting results have been published [4, 9-11]. 

Exposure to the outdoor environment affects not only the polymeric material itself but also other components within the matrix, such as dyes, pigments, processing additives, and stabilizers. Complex interactions of the combination of weather factors with the polymer and its components result in irreversible changes in the chemical structures and physical properties in a direction that generally changes both the appearance and mechanical properties and reduces the useful life of the material. Most organic polymeric materials require the use of one or more types of stabilizers for protection against the effects of the environment in order... 

WEATHER FACTORS AND THEIR EFFECTS ON POLYMERIC MATERIALS... 

Weather Factors and Their Effect on Polymeric Materials... 

Acceleration factors are material dependent and can be significantly different for each material and for different formulations of the same material. Therefore, it is erroneous to attempt to establish a single acceleration factor for a laboratory accelerated test to be used to predict lifetimes under natural weather conditions for a variety of materials and formulations. Because of the complex nature of the interaction of the combined weather stresses with a material, there is presently no simple way to estimate the acceleration factor for a material. Increase in irradiance cannot be equated with acceleration of degradation. For most polymeric materials, the rate of degradation is not simply a linear function of the level of irradiance. Also, it does not take into account the effect of temperature, moisture, and other weather factors. Thus, there is no substitute for determining the acceleration factor for a given material experimentally. 

The highest resistance to heat flow comes from the air entrapped in the cell. Still air has one of the lowest thermal insulations, 25mW/mK (Table 4.4). Therefore, polymeric materials in the form of foam have more entrapped air and show improved thermal insulation, the As factor, which depends on the type of polymer itself and contributes approximately 30% of the total resistance to heat flow. Therefore, the type of polymer used in thermal insulators is the next major factor to be considered while designing a thermal insulator. The other two contributions, i.e. Aj, due to the radiation across the cell of the foam and due to the convection of the gas, is kept to optimum levels by proper design of the cell structure of the foam coating. Requirements for extreme cold weather are ... 

UV chambers play an important role in comparing and predicting the performance of construction materials (elastomers, plastics, polymeric composites and coatings) and determining the effect of different weathering factors on the performance of a construction material. Recently, an innovative integrating sphere UV chamber design has been proposed for enhanced repeatability and reproducibility of the exposure results [34]. 

Severe exposure conditions significantly reduce service life. Thus, the service life of unprotected PE in the dark at 20°C is 8-10 yr, and of stabilized or crosslinked PE in a moderate climate is 15-20 years [7, 13]. In India, Pakistan, or Saudi Arabia, where the sun intensity is greater (250 kW cm ), the weathering life is only 2-5 yr, even if the products (pipes, tubes) contain much more antioxidants (2%). For other PE products (films), the lifetime in Saudi Arabia is only 3 months, while in Florida it is twice as long [14—16]. Temperature variations in the environment can also be a potential factor for the degradation of the polymeric material, diminishing the lifetimes of the products. These factors are especially important in climates with large differences between summer and winter temperatures (tests for these environmental... 

In order to provide a reliable early evaluation of the weatherability of a polymeric material, ideally the laboratory accelerated test should satisfy all correlation criteria when compared with results of natural weathering. However, any accelerated weathering test, either artificial or natural, only approximates the field exposure conditions. Nevertheless, the closer the simulation of the natural balance of all important weathering factors by the accelerated test, the better the... 

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