Mechanisms of antioxidant action: The effect of spin traps during the processing and photooxidation of PVC

Abstract

SynopsisI t is demonstrated by the use of spin traps that during the early stages of thermal processing of PVC considerable concentrations (> 3 x mol g-') of free radicals are produced which are primarily responsible for the initial products (hydroperoxides, unsaturation, and hydrogen chloride) previously shown to be formed in the polymer. From a semiquantitative analysis of these products, it is estimated that more than 50% of the radicals are formed from hydroperoxides and the rest by mechanmkion of the polymer chain. The spin traps are effective processing stabilizers in combination with a tin maleate HC1 scavenger. One of them (2-methyl-2-nitroso propane, MNP) has also been shown to be a photoantioxidant.

Photostabilization of Polymers

“…14. Transmittance spectra of CeCIJpoly(vinyl alcohol)-coated Lexan sheet sealed with a poly(vinyl acetate) overcoat after 20 h of UV irradiation: (a) 0·19 poly(vinyl alcohol); (b) 0.0059 CeCI 3 in 0·1 9 poly(vinyl alcohol); (c) 0·01 9 CeCI 3 in 0·1 9 poly(vinyl alcohol); (d) 0-059 CeCI 3 in 0·1 9 poly(vinyl alcohol).647…”

Analytical Methods

“…• several mechanisms of degradation were developed with support of ESR data: polaron mechanism of thermal degradation, 123 mechanism of PVC stabilization by tin stabilizers, 124 mechanism of chain branching explaining role of HCl, 125 unimolecular mechanism of HCl splitting of, 126 post-irradiation aging, 127 peroxy-radical formation during thermal degradation, 128 • identification of polyenyl radicals, 129 • understanding of UV degradation and stabilization, 122,130 • measurement of nitroxyl radical concentration, 131,132 • plasticizer diffusion and distribution, 133 The methods of measurement are standard techniques which permit determination of paramagnetic species. In some studies, HCl evolution was measured directly in ESR tube in addition to ESR studies.…”

Mechanodegradation

“…• extrusion, 1,2 • injection molding, 2,3 • milling and grinding, [4][5][6][7][8] • PVC mixing with additives, 9 • Brabender plastograph mixing, 10 • capillary and torque rheometry, [11][12][13] • processed materials. [14][15][16] Repeated extrusion is frequently used to estimate the effect of processing on a particular formulation.…”

“…Rapid increase in torque marks extensive degradative processes. [12][13][14] Radical processes and the effect of nitroxyl radicals on PVC stabilization were investigated by torque rheometry. 10 Capillary rheometry can also be used to estimate the effect of shear rate (Figure 8.8), although again, temperature together with mechanical shear are essential factors controlling the degradation rate of PVC and thus stabilizer reaction rate.…”

“…Similar to previously described studies on extrusion and injection molding, these data also do not allow us to separate the effect of temperature and shear energy on degradation which give a cumulative response. 12,13 Oxidation of the hindered amines occurs very rapidly and completely during mechanooxidation. 11 Torque rheometry is frequently used for various purposes, including estimation of the influence of mechanodegradative forces.…”