Лако-красочные материалы — производство

The general autooxidation scheme for hydrocarbons shown in Fig. 1 represents the core reaction for hydrocarbon degradation [5]. As adhesives are mixtures of polymers and oligomers, this scheme can be used to understand the oxidative degradation of adhesives. Several aspects of the scheme are worth emphasizing before we examine the effect of UV light on polymer degradation. First, the formation of a peroxyradical (ROO*) from a polymer free radical (R*) is an extremely fast (diffusion-controlled) reaction in the presence of oxygen. Second, the hydroperoxides (ROOH) formed from the peroxyradicals can further decompose to generate two new free radicals (RO*, HO*), which can initiate

further reactions. This autocatalytic mechanism results not only in the cyclic nature of autooxidation but also in an exponential increase in the number of free radicals available to initiate new degradation reactions. Although the same scheme is the basis for photo­oxidation of polymers, the presence of UV light exacerbates the situation, especially with regard to the initiation reactions.

Thermolysis of hydroperoxides (ROOH) becomes significant only above 100° C. Although this high-temperature reaction is a major cause of degradation in hot-melt adhesives, it is less important at ambient temperatures. However, photolysis of ROOH does occur to a significant extent at ambient temperatures and is a major cause of photo­induced polymer degradation [6]. Other photoinduced reactions can occur due to light absorption by trace levels of carbonyl impurities resulting from thermal oxidation of the polymer during manufacture, storage, and processing.

Although we focus here on these photooxidative pathways, it is important to men­tion the purely photolytic degradation of adhesives. Polymers that are chromophores in the UV range can directly absorb UV light without first undergoing oxidation. These polymers may undergo photolytic rearrangements, providing another nonoxidative path­way for loss of physical or chemical properties. Examples of these polymers are aromatic polyurethanes or polyesters.