Polyamides weather stability

The influence of UV light, usually together with moisture, results in color changes and embrittlement and their consequences. Figure 5.95. Black polyamide 66 stabilized with carbon (typically 2 %) is the most weather resistant, so that black-colored polyamides And applications outdoors. Products with high carbon content are also weather resistant for years in tropical regions. 

In contrast to polyamides in polystyrene phenolic antioxidants are not able to reduce the decrease in tensile strength during UV irradiation [141]. UVAs as well as HALS stabilizers are effective in polystyrene (PS). The best protection can be reached by using combinations of a UVA and a HALS. According to Gugumus [133,134] the synergism between UVA and HALS is distinct when the time until a AYI = 5 is taken as failure criterion (see Table 17.12). However, when as criterion the yellowness index after 1600 h in a Weather-Ometer is taken, the synergism between a UVA and a HALS is much smaller (see Fig. 17.14). 

Since the effect of temperature varies with type of polymer aud its formulations, temperatures different than those encountered in end-use environments can distort the stability rankings of materials in addition to causing unrealistic aging behavior. For example, it was shown [141] that change in air temperature from 30 to 60°C in an artilicial weathering test changed the rank order of the stabilities of polyamide, polypropylene, and polyester yam based on reduction... 

Polybrominated diphenyl oxide (PBIX)) llecabromodiphenyl Widely used in plastics such as ABS uncertain future because possible pollution during incineration. Especially useful in polyamides and PBTs polyesters good resistance to high processing temperatures and weathering, good colour stability used at low concentrations. 

The polyamides (PAs) are commonly known as nylons. They are linear molecules with a high degree of crystallinity and have the following formulas, as shown in Figure 8.1. As can be seen from the chemical formulas, there are a variety of polyamides available. Shown are the varieties that are used for corrosion resistance. The nylons are resistant to UV degradation, weathering, and ozone. However, all nylons will absorb some moisture from environmental humidity. This can have an effect on the dimensional stability of a part. 

The polyamides have a very low absorption of moisture, which means that parts produced from these materials will have a good dimensional stability regardless of the humidity of the environment. They are resistant to sunlight, weather, and ozone. Polyamide is used to coat many metals to protect them from harsh weather. 

AQ dyes are used for a broader spectrum of resins than that of the azo dyes. They are heavily used in styrene, ABS, SAN, polycarbonate, acrylics, cellulosics, polyesters etc. and are well known for excellent weatherability in the transparent mode such as red taillights, but in tint applications the light stability is greatly reduced. Heat, photo and chemical stability vary considerably as the pendants on the base AQ structure are substituted. Awareness of these differences is essential in color formulating. Most AQ dyes are not recommended for polyamide applications as these resins react with amine pendants and can remove the color. We are... 

Effective thermai-oxidative stabilization is a prerequisite for good light stability. Thus, no disadvantageous material changes were observed over the years in polyamides stabiiized against thermal-oxidative degradation as long as extreme irradiation, heat and weather influences were exciuded [86]. 

The mechanical properties are changed by water absorption and as a consequence of photo-oxidative degradation. In actual practice, the mechanical properties are often maintained for years, even if the parts are severely weathered optically. Table 5.9 shows the reduction in tensile strength of carbon-black stabilized polyamides after twelve years exposure to various climate zones [14]. 

The development of polyamide led to products with particularly good weathering resistance that meet extreme requirements thanks to special stabilizer systems and weathering resistant pigments (seat buckets for outdoors, unpainted exterior automobile parts). Figure 5.96 [690]. 

Non-light-stabilized polyamide 11 and polyamide 12 embrittle after approx. 900 h in the Weatherometer or after 15 months of outdoor weathering in Central European climate, Figure 5.97. Some weathering results are listed in Table 5.10 [502]. 

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