POLYPROPYLENE CHAIN SCISSION UNDER FLOW WITH A LARGE PRESSURE GRADIENT IN A NEAR CRITICAL SOLVENT

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

Iguazú, Argentina

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

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

The polypropylene (PP) chains have tertiary carbons that are very susceptible to undergo 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 agent additions (i.e. peroxides), applications of high shear strains, etc. In this work, the mechanical degradation of 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 kinds of experiments 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 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.