Polypropylene Chaín scission under flor with a large pressure gradient in a near critical solvent

MARTINI, RAQUEL EVANGELINA; BARBOSA, SILVIA ELENA; BRIGNOLE, ESTEBAN ALBERTO

Foz de Iguazú. Brasil.

Conferencia; I Conferencia Iberoamericana de Fluidos Supercríticos (PROCIBA 2007); 2007

PLAPIQUI (Argentina), UNICAMP (Brasil), Universidad de Valladolid (España), Universidad Nova de Lisboa (Portugal)

The PP chains have tertiary carbons very susceptible to oxidation and degradation. There are several causes of PP degradation and they could act by themselves or combined. Some of these are thermal actions, chemical agents additions (i.e. peroxides), applications of high shear strains, etc. When the shear forces applied are sufficient to store elastic energy equal to the strength of the main chain, bonds are distorted, bond angles and distances are extended, and the rupture of bonds takes place, producing mechanical degradation on the polymer chain. In this work, the mechanical degradation of Polypropylene (PP), dissolved in near critical n-pentane, was studied. The PP solubilized during high pressure-high temperature polymer blend separation process was collected by rapid expansion throughout a narrow valve. The pressure gradient in this valve was about 30 MPa. Two kind of experiment were carried out. Initially, pure PP solubility was studied varying the solution temperature. In a second stage, a polymer blend separation was performed on PP/polystyrene (PS) with different relative content of both polymers. The molecular weight distributions curves (MWD) of all soluble samples, obtained by gel permeation chromatography (GPC), shift to the low molecular weight side and the polydispersity is reduced, indicating PP chain scission. Due to the high shear strains generated during discharge, the mechanical degradation was found to be the main chain scission mechanism. In all cases, thermal and oxidative degradation were previously analyzed. The effect of the temperature and the polymer concentration was investigated. A negative linear functionality of the chain scission was found with both variables. To analyze the relationship between polymer degradation and molecular weight, the chain scission distribution function was calculated. On this basis, a critical molecular weight for the beginning of chain scission was obtained. This value is a function of temperature but remains constant with concentration. Keywords: poly(propylene) (PP), chain scission, mechanical degradation, near critical solution