Elsevier

Polymer

Volume 42, Issue 13, June 2001, Pages 5531-5541
Polymer

UV-radiation curing of acrylate/epoxide systems

https://doi.org/10.1016/S0032-3861(01)00065-9Get rights and content

Abstract

Interpenetrating polymer networks (IPNs) have been synthetized by light-induced cross-linking polymerization of a mixture of acrylate and epoxide monomers. The consumption of each monomer upon UV-irradiation in the presence of radical and cationic-type photoinitiators was monitored in situ by real-time infrared spectroscopy. The acrylate monomer was shown to polymerize faster and more extensively than the epoxy monomer, which was further consumed upon storage of the sample in the dark, due to the living character of cationic polymerization. Curing experiments carried out in the presence of air and under air diffusion-free conditions indicate that the radical polymerization of the acrylate monomer is hardly affected by the oxygen inhibition effect, while the cationic polymerization of the epoxy monomer is enhanced by the atmosphere humidity. The addition of a photosensitizer, like isopropylthioxanthone, was shown to speed up substantially the polymerization of the epoxide, with formation within seconds of two fully cured IPNs.

Introduction

Interpenetrating polymer networks (IPNs) form a special class of polymer blends in which both polymers generally are in network form [1], [2]. They can be synthesized by cross-linking polymerization of two multifunctional monomers or telechelic oligomers that polymerize by different mechanisms, e.g. radical and cationic types. The main advantage of IPNs is that they combine the properties of the two kinds of polymer networks. An easy way to produce IPNs is by photoinitiated polymerization of a monomer mixture, in particular by combining the two types of monomers, which are the most widely used in UV-radiation curing: acrylates, which polymerize by a radical mechanism, and epoxides, which polymerize by a cationic mechanism.

UV-radiation has already been used to produce semi-IPNs or IPNs by photopolymerization of acrylate monomers in various polymer matrixes [3], [4], [5], [6], [7], [8], [9] or in epoxidized polyisoprene [10], respectively. The main interest of using UV-light to induce the polymerization reaction lies in the high polymerization rates, which can be reached under intense illumination, together with the advantage of a solvent-free formulation curable at ambient temperature. As a result, the UV-curing technology has found a large variety of applications, in particular to achieve a fast drying of varnishes and printing inks, and a quick setting of adhesives and composites materials [11], [12].

The monomers selected for this study are compounds, which are typically used in UV-curable resins, namely a bisphenol A based diacrylate and a biscycloaliphatic epoxide. Phase separation does not occur in this particular mixture, which remains perfectly transparent, before and after UV-curing. Acrylate monomers, which are known for their high reactivity, polymerize rapidly in the presence of photogenerated free radicals. Epoxides, which undergo polymerization by a cationic mechanism, are less reactive but they are insensitive to oxygen inhibition. A hydroxy-phenylketone and an aryliodonium salt were used as photoinitiators because these compounds are known to generate upon UV exposure free radicals and protonic species, respectively, with high quantum yields [13]. This hybrid formulation contains no volatile acrylate diluent and therefore it does not show the strong odour and irritating character typically found in acrylate-based UV-curable formulations.

The objective of this work was to study the kinetics of such ultrafast polymerization processes in order to assess both the reactivity of the two monomers and the efficiency of the photoinitiators. It was achieved by using real-time infrared (RTIR) spectroscopy as analytical tool [14]. Indeed, this technique is unique as it allows one to monitor in situ the polymerization of each monomer of the mixture and to record directly the conversion versus time profiles in a timescale as short as 1 s. The important kinetic parameters can thus be determined, in particular the actual polymerization rate and the final cure extent, a quantity which governs the physico-chemical characteristics of the IPN formed.

Section snippets

Materials

A hydroxyphenylketone (Darocur 1173 from Ciba Specialty Chemicals) was used to generate the free radicals that are to initiate the polymerization of the acrylate monomer. The photocleavage of this molecule produces a benzoyl radical and an α-hydroxyalkyl radical, both of which are capable to react with the acrylate double bond [13]:

A diaryliodonium hexafluorophosphate salt (DAI from Ciba Specialty Chemicals) was used to generate the protonic acid, which will initiate the cationic ring-opening

Results and discussion

The photoinitiated polymerization of each monomer alone has first been examined, before studying the photopolymerization of the monomer mixture, which generates the IPNs, and in particular the effect of atmospheric oxygen and of the type of photoinitiator used. We have also examined the influence of a photosensitizer and of light-stabilizers on the polymerization kinetics, and on some of the polymer properties as well.

Conclusion

Photoinitiated polymerization of a mixture of difunctional monomers reacting by different mechanisms is one of the most efficient methods to generate rapidly IPNs. Such ultrafast polymerization is best followed by real-time infrared spectroscopy, a technique that records directly conversion versus time curves for each one of the two monomers. In the case of the epoxide/acrylate combination, UV-irradiated in the presence of both cationic and radical-type photoinitiators, the polymerization of

Acknowledgements

This work has been supported by the Centre National de la Recherche Scientifique of France and by Ciba Specialty Chemicals (Additives Division — Basle — Switzerland).

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