Dehydrochlorination stabilisers

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

As already indicated, the measurement of dehydrochlorination rates is not a practical way of assessing the effect of a stabiliser. Thus the congo red test sometimes specified in standards, in which a piece of congo red paper is held in a test tube above a quantity of heated PVC and the time taken for the paper to turn blue due to the evolution of a certain amount of hydrogen chloride, cannot be considered as being of much value. 

In addition to stabilisers, antioxidants and ultra-violent absorbers may also be added to PVC compounds. Amongst antioxidants, trisnonyl phenyl phosphite, mentioned previously, is interesting in that it appears to have additional functions such as a solubiliser or chelator for PVC insoluble metal chlorides formed by reaction of PVC degradation products with metal stabilisers. Since oxidation is both a degradation reaction in its own right and may also accelerate the rate of dehydrochlorination, the use of antioxidants can be beneficial. In addition to the phenyl phosphites, hindered phenols such as octadecyl 3-(3,5-di-tcrt-butyl-4-hydroxyphenyI)propionate and 2,4,6-tris (2,5-di-rcrt-butyl-4-hydroxybenzyl)-1,3,5-trimethylbenzene may be used. 

The hydrohalide is liable to dehydrochlorination, particularly when moist acid is used in its preparation, so that hydrochloric acid acceptors such as lead carbonate are useful stabilisers. Dibutyl phthalate and tritolyl phosphate are effective plasticisers. Rubber hydrochloride is used as a packaging film (Pliofilm) and as a rubber-to-metal bonding agent (e.g. Typly). 

The mechanisms of degradation and the mode of action of the various PVC stabilisers have both been widely studied. Often at least one aspect of their operation is some sort of reaction with the first trace of hydrogen chloride evolved. This removes what would otherwise act as the catalyst for further dehydrochlorination, and hence significantly retards the degradation process. In addition, many stabilisers are themselves capable of reacting across any double bonds formed, thereby reversing the process that causes discoloration and embrittlement. 

Polyvinyl Chloride. It is well-known how various attempts have been made to stabilise PVC against dehydrochlorination by salts, usually of divalent metal ions - as long chain alkylcarboxylates of Cd(II), Ba(U), Zn(II) [96]. Biswas and Moitra [102] recently established that the 3d metal ions incorporated in PVC-DMG-complex enhance the thermal stability of PVC in the order. 

Heat stabilisers retard dehydrochlorination and autoxidation and reduce fragmentation. In addition, they also cure existing damage. 

Poly(vinyl halides) - Photodehydrochlorination occurs in PVC with a foam backing as determined by Raman spectroscopy. For PVC stabilised with Zn/Ca stearates phooxidation indicates that dehydrochlorination dominates over oxidation at the inner layers beyond 100 microns " . The effect of polyene formation also appears to be a function of light intensity as well as oxygen diffusion rate. 

The more expensive organotin compound stabilisers replace tertiary chlorine atoms in PVC by more stable groups, as well as reacting with HCl. Therefore, they can prevent dehydrochlorination and the formation of polyenes. 

The method of production has been found to have a striking influence on the thermal stability of polyvinylchloride (PVC) over a temperature range up to 340 °C [1, 2]. Thus, PVC obtained as a result of y-irradiation and benzoyl peroxide (BP) initiation has approximately the same stability, while PCV obtained by initiation with azobisisobutyronitrile (AZBN) is noticeably less thermally stable over the temperature range of 220-270 °C. However, stabilisation towards further thermal degradation of all PVC samples tested is observed at about 60% weight loss, possibly due to the considerable dehydrochlorination of the polymer to form polyene and crosslinked structures. 

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