Long-term stability, plasticizers degradation

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

The long-term stability of plasticized materials is a major concern for their successful application and it can be depreciated by chemical degradation of the polymer and/or plasticizer or by physical changes in the bulk properties of materials. 

The oxidative reaction forms new oxidative moieties due to the utilisation of a substantial amount of stabilisers, leading to a consequent decrease of long-term stability and deterioration of mechanical properties. For an effective recycling process, it is necessary to know the extent and effect that degradation has on the structure, mechanical properties and long-term stability of recyclates. Plastics face prolonged use before being discarded as waste [23]. 

Fillers may have a negative effect on the long-term properties of plastics. For one thing, fillers can absorb stabilizers at their surface, reducing their protective effect. For instance, filled polyolefins exhibit lower long-term stability than unfilled materials with comparable stabilizer concentration. Moreover, contaminations, such as metals and metal ions, may be introduced by fillers that can accelerate degradation [533]. 

The particular absorber to be used in a given application depends on several factors. One important criterion is whether the absorber will strongly absorb that portion of the ultraviolet spectrum responsible for degradation of the plastic under consideration. Compatibility, volatility, thermal stability, and interactions with other additives and fillers are other items that must be considered. When used in food wrappings, Food and Drug Administration approval must be obtained. While one or more of these considerations may rule out a given stabilizer or influence llie choice of one class over another, the final selection must await the results of extensive accelerated and long-term tests. 

This stability is important to plastics long-term performance. However, for some applications only short-term performance is desired before the product is discarded, as in the fast-food and packaging markets. In such cases it is considered advantageous for discarded plastic to degrade when exposed to microbes. There thus exists a requirement to develop or modify plastics possessing the properties required for their service life, but with the capability of degrading in a timely and safe manner, particularly to handle the worldwide waste situation. 

Their effects on long-term performance, particularly photostability is much more variable and depends on the photosensitization mechanism [20]. However, they can all be improved to the level of non-degradable plastics by the use of inexpensive stabilizing systems [14] should this be required in the future. 

After the introduction of the first HA(L)S in the 1970s, this class of Ught stabilizers rapidly became the most important UV stabilizer for the majority of plastics. Without the discovery of HA(L)S, the outdoor applicability of many polymers would be limited. So, the use of PP in automotive application without the use of HA(L)S would be impossible. Although HA(L)S stabilizers were developed as UV stabilizer they are more and more used as long-term heat stabilizers too [74,109]. Especially where phenohc antioxidants cannot be used due to their discoloration, HA(L)S is used to protect the polymer against long-term heat degradation [73]. 

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