Лако-красочные материалы — производство

by PETER JONES Grad PRI
Edited by Peter Oldring PhD BA

Chapter One

I INTRODUCTION

The vinyl and acrylic group of resins is one of the most widely used in surface coating applications.

Vinyl and acrylic resins may be used on their own or in blends with other resins. Coating resins can be divided into two distinct categories:

• Thermoplastic polymer coatings

• Thermoset polymer coatings.

The thermoplastic types are long chain polymers with high molecular weights and they film form without external chemical reaction. The thermosetting types are shorter chain polymers containing reactive groups which can be ‘cured’ by the application of heat or usually by reaction with another chemical type to form a crosslinked film.

Thermoplastic films harden by solvent evaporation, whereas thermoset films harden by chemical reaction.

Vinyl resins are produced by the addition polymerisation of vinyl monomers. The term ‘vinyl monomer’ is applied to the species of molecules which contain a reactive C=C double bond. Individual molecules (termed ‘monomers’) of a vinyl compound are capable of undergoing a reaction to form long chains of monomer units linked via C-C bonds (termed ‘polymer’). The monomer units in the molecular chain may be exclusively of one species (a ‘homopolymer’) or the polymer may comprise two or more species of monomer (‘co-polymer’, ‘terpolymer’, etc.).

This mechanism for resin formation is totally different from that for any other class of resins.

Alkyds and polyesters are formed by condensation reactions which generate water of reaction. Polyurethanes are formed by chemical reactions which do not involve the generation of any by-product. Vinyl and acrylic resins alone are formed by addition polymerisation of the carbon carbon double bond.

In this chapter, the theory, mechanisms, kinetics and types of addition polymerisation will be considered. The acrylic monomers which can be used and the properties that they impart, will be discussed, as will their effects on polymerisation. Crosslinking reactions and functional monomers, which can participate in them, are the basis of industrial thermoset systems and are, therefore, also taken into account

The four methods of polymerising acrylic monomers, namely bulk, suspension, solution and emulsion are considered. Chapter I concerns itself with the chemistry of acrylic polymerisation and solvent based acrylics. Chapter II is devoted to waterborne resins, especially emulsion polymerisation, because of its significant differences from the other types of polymerisation.

Much of the discussion about solvent based solution polymerisation of acrylic monomers is also relevant to waterborne systems, where the monomers are polymerised in solution in a water soluble solvent, neutralised and then dispersed or inverted into water.

However, the basis of acrylic addition polymerisation needs to be considered first.