Formation of highly crosslinked polymer films in the presence of bio-based epoxy by photoinitiated cationic polymerization

Abstract

Oleic acid was used as bio-based starting material for manufacturing of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (EEDE). In contrast to epoxidized triglycerides, which contain also saturated structures, EEDE comprises exactly two epoxy groups that is advantageous for comparison with commercial diepoxides such as 1,4-butanediol diglycidylether (BDDE), 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate (EEC), and diglycidylether of bisphenol-A (DGEBA). Although 9,10-epoxystearic acid methyl ester (EME) and EEDE using bis-(t-butyl)-iodonium-tetrakis(perfluoro-t-butoxy)aluminate and isopropylthioxanthone as photoinitiator system are less reactive compared to the commercial diglycidylethers BDDE and DGEBA, the crosslinked films made from EEDE alone or a stoichiometric EEDE-DGEBA mixture showed higher hydrophobicity compared to crosslinked films derived from the commercial monomers as shown by both contact angle measurements and surface free energy calculation. Furthermore, DMA analysis resulted in detection of a secondary relaxation (β relaxation) in addition to the glass transition in case of poly-DGEBA and networks made from EEDE DGEBA mixtures using DGEBA excess. Although the glass transition temperature determined by both DSC and DMA decreases for the networks containing EEDE segments in comparison with pure DGEBA networks, the crosslink density is nearly not affected by the EEDE segments. This may be attributed to the structure of EEDE containing exactly two epoxy groups that is the case for DGEBA as well. The more flexible aliphatic chain causes the lower glass transition temperature. In addition, crosslink density and Mc value of poly-EEDE-DGEBA made from a 1:5 M ratio of EEDE: DGEBA by photoinitiated cationic polymerization show a nearly ideal crosslinked polymer network structure. The crosslinked poly-EEDE-DGEBA films may be interesting for application in coatings because of their high crosslink density.