The Kinetic Evaluation for Photo-Oxidation of PVA (Polyvinyl Alcohol)

Abstract

We previously reported the correlation of physical properties of plastics (ultimate strength and relative elongation at break) with changes of their internal structures during the ultraviolet ray deterioration from kinetic point of view.Hereunder presented is the report of kinetic studies about the oxidation of water soluble plastic, PVA (polyvinyl alcohol), under ultraviolet rays in the atmosphere of nitrogen and oxygen. The differences in the mechanisms of deterioration under the different conditions of surrounding gas were investigated by the infrared absorption method. At the same time, the crystallinity and stereospecific (syndiotactic diad) were determined from the ratio of absorbances of crystallization (1141cm-1) and stereospecific bands (916cm-1, 849cm-1).The results of these experiments are as follows:(1) The rate of increase or decrease in the absorption band for each functional group is a first order reaction.(2) The remarkable increase in the absorptions of carbonyl groups, such as esteric type (1738cm-1), aldehydic type (1720cm-1), ketonic type (1709cm-1) and acidic type (1700cm-1), are recognized and their quantities are in the order of ketonic, acidic, aldehydic and esteric in nitrogen atmosphere and of aldehydic, ketonic, acidic and esteric in oxygen.(3) The mechanism of the photo-oxidation is closely related with the mechanisms of formation of these carbonyl groups. Especially, it seems that the deterioration is remarkably accelerated by the chain breaking caused by the formation of aldehydic and acidic carbonyl groups and by the crosslinking caused by the formation of esteric carbonyl group.(4) The values of crystallinity and syndiotactic diad, calculated from the ratio of absorbances (D1141/D1420, D916/D849), tend to increase.(5) The formation of ozonides is recognized and the ozone-initiated reaction is expected to take place from the consideration of the reaction process or the mechanism of deterioration.(6) The coloration is attributed to the chromophores (>C=O) and the bathochromic effect of auxochromes (-OH), and the degree of coloration is larger in nitrogen atmosphere than in oxygen.