The Effect Of Polyester Polyol Structure On The Reactivity Of Photocurable Epoxide Systems

For the cationic polymerization of epoxides, hydroxyl functional materials such as glycols,
polyether polyols, and polyester polyols are known to function as chain transfer and
flexibilizing agents. Crivello et. 1 described quantitatively the effect of several polyethers
and polyesters on the mechanical properties of the copolymers obtained in the
copolymerization of aliphatic and aromatic epoxides. The flexibilizing effect achieved in
this study was relatively modest with most polymers exhibiting elongations below 10%.
Koleske studied the properties of polymers obtained from poly(propylene oxide) diols and
2 and polycaprolactonediols and 3 with cycloaliphatic epoxides. In this study
a more pronounced flexibilization effect was achieved with the polyethers and the
polycaprolactone diols. As expected, difunctional materials yielded more elastic polymers
than trifunctionèl materials. In addition, the epoxide to hydroxyl ratio and the molecular
weight of the hydroxyl functional polymer was found to have a pronounced effect. Carter
et. 4,5 provided more insight into the effect of key variables such as epoxide and
hydroxyl equivalent weight and epoxide to hydroxyl ratio on flexibility and retortability of
coatings based on such polymers. Common to these previous studies is their focus on
the properties of the cured polymers. Varying the reactivity of the hydroxyl functional
polymers was not investigated.

Our initial interest was to design polymers with varying flexibility as required for their use
as adhesives. For this purpose, copolyesters of varying structure and molecular weight
were studied as chain transfer and flexibilizing agents in photocurable epoxide systems.
Vast differences in reactivity and properties of the resulting polymers were observed. To
obtain better insight into the factors affecting the reactivity, a series of different
homopolyester diols was chosen. The same acid was used for each polyester diol, but
.the nature of the glycol was varied.

Photo Differential Scanning Calorimetry (photo-DSC) and Real Time Fourier Transform
Infrared (RT-FTIR) were used to determine the rate of polymerization. The contents of
the soluble fraction in the cured polymer films and RT-FTIR were used to investigatethe
degree of conversion.