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Chapter II

I INTRODUCTION

The dynamic growth of radiation curing as an industrial process is dependent on continued innovation to support this technology. In the field of photoinduced curing, this is reflected by rapid development of new materials needed for the formulation of these systems and in particular in the.rapid improvements made with one of the key components of light curable formulations, namely the photoinitiator.

The role of the photoinitiator is to convert the physical energy of the incident light into chemical energy in the form of reactive intermediates. These intermediates are either radicals capable of adding to vinylic or acrylic double bonds, thereby initiating radical polymerisation, or reactive cationic, anionic or basic species which can initiate their respective polymerisation reactions.

Light induced radical polymerisation, particularly curing by ultraviolet light, is by far the most important application of radiation curing and the development and introduction of new radical photoinitiators, both as experimental and as commercial products, have proceeded at breath taking pace during the last decade. The developments in the field of photoinitiators are stimulated by various factors.

Firstly, there is the continuous improvement of photoinitiators for existing applications such as coatings, inks, photoresists, printing plates or adhesives. Since there is no single photoinitiator which can meet the particular requirements of all of these applications, there is a tendency to develop tailor-made products for each of these technologies. A trend towards higher production line speeds makes the development of more reactive photoinitiators imperative. The interest in using thick, opaque pigmented coatings containing high levels of pigments requires photoinitiators with strong absorptions at suitable wavelengths for complete cure. The continued improvement of the technical equipment, like the introduction of new radiation sources, also necessitates the development of suitable new photoinitiators. Furthermore, the optimisation of other properties of the initiator not directly linked to the curing step, like the discolouration of the cured formulation by the photoinitiator or its reaction products, or the extractability of both the photoinitiator and its reaction products, are just as important as further improvements to the efficiency of the initiation. These trends are exemplified by the continuing introduction of improved products during the last few years displaying outstanding performance in applications such as pigmented systems, photoresists or printing plates.

Secondly, the adoption of radiation curing by new technologies requires new types of photoinitiators to fulfil specific demands. As an example, the introduction of lasers as light (curing) sources in industrial processes stimulated the development of new photoinitiators tuned to the wavelengths of the main laser emission lines. Thus, the application of radiation curing for processes so far unfeasible for this technology became possible.

It is the objective of this chapter to summarise the major developments in the field of photoinitiators during the past years and give an overview of the existing classes of photoinitiators.

II PRINCIPLES OF PHOTOINDUCED) RADICAL POLYMERISATION

Photocurable systems are formulations which undergo crosslinking upon irradiation. Two types of photocurable systems are known, namely photosensitive high molecular weight polymers which undergo photocrosslinking reactions and photopolymerisable systems consisting of low molecular weight materials which are cured by a light induced polymerisation reaction. Since photopolymerisation may be defined as the process whereby light is used to induce an increase in molecular weight, both systems are commonly referred to as photopolymers.

1. SYSTEMS UNDERGOING PHOTOCROSSLINKING

These systems consist of macromolecules possessing functionalities which can undergo crosslinking reactions upon irradiation. Every crosslinking reaction is a photoinduced process which necessitates the absorption of a photon and, in contrast to photo- polymerisation (see Section II.2), no thermal processes which would amplify the photochemical step are involved. However, since the starting material is already a polymer, only a few chemical reactions are needed to ohtain a three-dimensional polymer network. Typical examples are the photoinduced [2+2]-cycloadditions of polymer-bound cinnamic acid esters or maleimide derivatives.

The crosslinking of the polymers by these processes creates a polymer network with a different solubility from the starting linear polymer. Since the unexposed starting material is a polymer with low crosslink density, but relatively high viscosity, these materials are unsuitable for coating applications and are mostly used in imaging applications. Image amplification in crosslinking is brought about through the immobilisation of large molecules by a small number of crosslinks.

Since the functionalities which undergo the crosslinking reactions are themselves photoreactive, photoinitiators are not needed for photocrosslinking. However, sensitisers can be added to extend the range of spectral sensitivity. These types of photopolymers will not be considered in this chapter. They are, however, considered in Volumes V and VI.

2. PHOTOPOLYMERISABLE FORMULATIONS

An insoluble polymer network can also be obtained by the copolymerisation of relatively low molecular weight species with multifunctional components. However, in contrast to high molecular weight polymers which undergo crosslinking, a large number of chemical reactions are necessary to achieve this goal. In photopolymerisable formulations only the first reaction step, which is the production of an initiating species, is a photochemical reaction. The polymerisation itself is exclusively a thermal chain reaction. This chain process amplifies the first photopolymerisation, and, in contrast to many other photochemical reactions, is very efficient and therefore an industrially

acceptable process.

Since the components of the formulations undergoing polymerisation are not photoreactive, a compound has to be added which can absorb light and subsequently undergo a reaction from its excited state. This reaction finally generates radicals which can initiate the polymerisation process. Molecules or molecular systems capable of forming radicals upon irradiation are termed radical photoinitiators. The initiating radicals can be generated directly by the photochemical reaction or they can be produced by rapid thermal reactions following the primary photochemical reaction.

The initiating radical subsequently adds to a reactive vinyl or acrylate double bond of a monomer, producing a new radical centre on the monomer. This process is the initiation of the polymerisation reaction, which when combined with the previously discussed generation of radical species constitutes the process of photoinitiation.

After the initiation step, the classical picture of radical polymerisation is valid for the photoinitiated polymerisation process. The following steps are usually distinguished.

i) Propagation: Repeated addition of monomeric units in a chain reaction to produce the polymer backbone.

ii) Chain transfer: Hydrogen abstraction by the radical on the growing polymer chain to terminate the growing chain with concomitant production of a new radical. If the newly formed radical can start another chain reaction, this is termed a chain transfer process.

iii) Termination: The chain reaction can be terminated by different processes. Examples are disproportionation or recombination reactions between two growing polymer chains. Termination can also occur upon recombination with any other radical including primary radicals produced by the photoreaction.

These steps are summarised in Figure 1. Also given are the different rate constants. For the purposes here, the low molecular weight species which are used to form the

bulk of a light curable formulation will be termed monomer and designated M. It does not mean that the reactions are restricted to monomers or monofunctional reactants. Indeed, multifunctional carbon-carbon double bond containing molecules are essential to form a crosslinked network. All stages in forming a crosslinked network are explained in detail in Volume II, Chapter I and Volume V, Chapter III.