【正文】 on that sets off a degradative process.In theory, many pure polymers should not absorb UV radiation, and thus, not be subject to photodegradation. However, in practice the most polymers contain impurities such as carbonyl or carboxy groups or hydroperoxides that readily absorb radiation in the 290–400 nm range causing them to break down. Thus, generating sites within the polymer structure where chemical reactions can be initiated and propagated by free radicals. The active groups may be unavoidably present as a result of reactions that occur during polymerization. Similarly, metallic ions are present in most polymers as residues from polymerization catalysts, or as constituents of pounding additives such as heat stabilizers, antioxidants, colorants, fillers and others. The metal ions are highly receptive to the absorption of UV radiation, and are efficient in transferring the absorbed energy to the polymer molecules around them, thus, they act as photosensitizers and can promote degradation at the same time that they perform their desired functions.Another contributor to photodegradation of polymers is oxygen, which helps any free radicals that may be liberated by the UV to initiate and propagate oxidation of the polymer, hence, the term photooxidation.Polyvinyl chloride suffers from poor heat stability. Its degradation occurs by autocatalytic dehydrochlorination initiated at the labile sites in the polymer chains. This leads to severe discoloration and loss of mechanical properties. The dehydrochlorination most probably proceeds by a chain mechanism involving radical intermediates. Various defect sites in PVC are branching.Inorganic and organic thermal stabilizers are monly added to protect the polymer from heat degradation. Among the most widely used ultraviolet stabilizers is titanium dioxide pigment. Filling a polyvinyl chloride position with this pigment substantially reduces the effective depth of penetration of ultraviolet light into the surface of an article formed from such a position.Mohamed et al. pointed out that barbituric acid and thiobarbituric acid are nontoxic organics, thermally stable materials of high melting point. Both contain active methylene groups, and can act as Hdonor through their enolic hydrogen groups, which can intervene with the radical species derived from the thermal degradation of PVC. They investigated the possibility of using barbituric acid and its thioanalogue as thermal stabilizers for rigid PVC.The effective stabilization often requires a bination of antioxidant system in which plementary overlap of different mechanistic pathways involved. This act often referred to as synergism, is the motivation for the use of admixing position of dibutyltin maleate and trinitro and its ester homologues. The stabilization agents of dibutyltin maleate and trinitro esters could retard somewhat the photodegradation of PVC. It is hoped that the total stabilizing effect of this admixed system should be greater than the sum of the individual effects when PVC is subjected to an environment where the effects of heat and UV are bined. Turoti et al. investigated the effect of the stabilizing action of admixed mixtures of dibutyltin maleate and trinitro and its ester homologues on polyvinyl chloride exposed to natural atmosphere. In their study, the degradation and stabilization reactions were monitored by color formation, tensile strength and elongation at break, reduced viscosity as well as determination of time to embrittlement. It is observed that the stabilized PVC sample has an effective reduction in degradation reactions.Titanium dioxide is by far the most important of white inorganic pigments and possesses allround suitability. While rutile titanium dioxide is highly reflective at visible wavelengths, it is also highly absorptive at ultraviolet wavelengths. However, although titanium dioxide is a highly effective ultraviolet light stabilizer for polyvinyl chloride positions, it does have several serious drawbacks. An important disadvantage is the cost of titanium dioxide which has historically tended to be high pared with filler