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PHOTOCURED SYSTEMS INVOLVING onium salt photoinitiators (Chapter 4), epoxides, and polyols have certain advantages over free radical cured systems involving acrylates of various types. One important advantage of the cationic systems is improved adhesion due to low shrinkage of less than 2% to less than 5% during the photopolymerization reaction. In addition, these systems are easy to handle because all products used are low viscosity liquids and the compounds are readily miscible. As a result, usually simple stirring is all that is needed to combine the formulating ingredients. It should be mentioned that some polyols that can be used as formulating ingredients are low melting solids that have low viscosity after melting. Some of the polycaprolactone and poly(tetramethylene oxide) polyols are in this category. The ingredients have low skin and eye irritation potential, though investigators should read manufacturers' Material Safety Data Sheets and other available information when working with these and with other compounds. The epoxide-based formulations do not require an inert atmosphere during cure, since they are not oxygen inhibited. These items and others are given in the comparison of cationic and free radical systems presented in Table 6.1. A number of these items as well as others will be dealt with in the following discussion.