Base accelerated degradation of polyvinyl chloride

Abstract

The degradation at 150°C. of polyvinyl chloride in the presence of base is studied by means of ultraviolet, visible, and infrared spectra. The reaction of polyvinyl chloridebase systems at 150°C. is viewed as a base accelerated dehydrochlorination of polyvinyl chloride proceeding through initiation, propagation, and termination phases. Polyene units with sixteen double bonds in conjugation are considered to be the most probable component of polyene structures contributing to the longest wave length absorption maxima recorded at 550 mμu. Dichroism to polarized light is observed in a plasticized, oriented, base degraded polyvinyl chloride film. This dichroism is considered a confirmation of the assigned polyene structure. The infrared spectra obtained reveal the development, as the reaction progresses, of conjugated carbon‐carbon double bonds, conjugated carbonyl groups, bonded hydroxyl groups such as from hydroperoxides and/or alcohols, and the eventual formation of carboxyl groups. The mechanism of the oxidative degradation at 150°C. is attributed to the attack, after dehydrohalogenation, by molecular oxygen at the olefinic double bonds to form peroxide radicals which by radical chain transfer yield hydroperoxides that degrade to carbonyl containing products.

Infrared spectra of thermally degraded poly(vinyl-chloride)

“…A broad carbonyl band at 1730 cm-I is probably th e r esult of the oxidation of double bond struct ures followin g the pyrolysis. The CH vibration [3] in CH2 is shifted from 1430 cm-I in the original material to 1470 cm-I . A band at 1380 cm-I , assigned to the C-H deforma tion for the aliphatic C-CH 3 bond [1 3], is presen t in this sp ectrum as well as in the spectrum of the r esidue of the material p yrolyzed a t 400 0 C .…”

“…The color of the residu e has b een attributed to a polyen e stru cture exi ting after the deh~-dro chloriua tion [2] . The role of oxygen in the decomposition m echanism has not ~ret b een r esolved although h y droperoxide m echanism s have b een proposed [3].…”

“…The shifting of th e carbon-hydrogen vibration of CH2 [3] from 1430 cm-1 in the sp ectrum of untr eated polym er t o 1455 cm-I for the more highly decomposed samples may be a r esult of a loss of th e chlorine atoms or an in crease in the CH3 concent ration [1 3]. The band a t 1380 cm-I in the most bighly degraded material may b e assigned to the carbon-hydrogen deforma tion frequency for the aliphatic C-CH3 bond [1 3].…”

“…The r emoval of th e band at 690 cm-I, assigned to t b e C-Cl stretching vibration [3,4], from the spectrum of the sample pyrolyzed at 300 0 C correla tes with mass spectrometer and weight loss m eas urements [1] which showed that almost all t he chlorine atoms wer e r emoved from the polymer at t his temperature.…”

“…The application of infrared spectrophotometry to the explanation of the degradation process occurring in poly (vinyl chloride) has, however, not b een extensively r eported in the literature. Campbell and Rau scher [3] u tilized changes in the infrared spectra of poly(vinyl chloride) in a study of the base-accelerated degradat ion of this polymer. Others [10,11] limi ted their studies to changes in only one portion of the sp ectrum.…”