Polymer crosslinking controlled by release of catalyst encapsulated in polycarbonate micro-spheres

Abstract

EVA crosslinking by ester–silane exchange reaction, controlled by catalyst released from polycarbonate micro-particles, was studied from rheological experiments. Firstly, dibutyltin dilaurate in polycarbonate (PC) (DBTDL) catalyst was encapsulated in polycarbonate micro-particles by an emulsion process. These PC micro-particles have a trimodal size distribution with a mean diameter around 25 μm. Furthermore, scanning electronic microscopy analysis revealed that the DBTDL catalyst is homogeneously dispersed in PC micro-particles. This was verified by DSC experiments which showed a T_g equal to 72 °C for the DBTL micro-particles. Secondly, the deformation and break-up studies of PC micro-particles in molten EVA matrix showed that PC micro-particles can only break up in EVA matrix of high molecular weight (EVA28-03) and for temperatures higher than 200 °C. Thirdly, rheological experiments showed that the crosslink reaction is controlled by the diffusion rate of DBTDL from PC micro-particles. However, this diffusion rate was observed to be independent on either the temperature or the PC micro-particles size.

Introduction

A polymer, in its amorphous state, undergoing a chemical crosslinking process, is converted from a viscoelastic liquid into a viscoelastic solid. Properties changes are more dramatic near the liquid–gel transition [1]. Generally, chemical modifications in reactive polymer processing may cause important changes in the rheological behavior and as a consequence in the flowing of the polymers. For example mixing and shaping operations require sufficient molecular mobility, which vanishes when the chain motions slow down near the gel point. Consequently, the mixing (and shaping) and the reactive step must be separated by using formulations temporally dormant. This is mainly the case of the crosslinking reactions when they are used on a tailoring line of crosslinked materials.

On the other hand, a new interchange reaction ester-alkoxysilane in presence of dibutyltin oxide (DBTO) as catalyst and its application to ethylene-co-vinylacetate (EVA) crosslinking was previously reported [2]. This reaction takes place in the range of temperatures 100–250 °C only in presence of DBTO as efficient catalyst. However, this reaction must be well controlled (kinetics, side reactions) in order to separate the mixing and reactive processes. This is particularly true in a twin-screw extruder process. Indeed, the melt temperature may greatly overtake the check temperature due to the viscous dissipation phenomenon. As the ester exchange reaction does not take place without any catalyst, one way to control the reaction of crosslinking is to encapsulate the catalyst in polymer micro-spheres as controlled release system.

This paper is devoted to the study of EVA crosslinking by ester–silane exchange reaction controlled by catalyst release from PC micro-spheres. It is divided into the following parts: In the first part, we will describe the preparation and characterisation of PC micro-spheres containing dibutyltin dilaurate (DBTDL). DBTDL was chosen instead of DBTO as catalyst for the exchange reaction because DBTO is a powder and DBTDL is a liquid at room temperature. Consequently DBTDL is more suitable for its encapsulation by an emulsion process. Following this, we will study the deformation of molten PC micro-spheres embedded in molten EVA matrix in order to understand the mechanism of catalyst release at high temperature (T≥180 °C). In the last part, we will study from rheological experiments EVA crosslinking by the exchange ester–silane reaction catalysed by free or released DBTDL from PC micro-spheres.