Epoxies Modified by Palmitic Acid: from Hot Melt Adhesives to Plasticized Networks

C.E. HOPPE; M. J. GALANTE; P. A. OYANGUREN; R. J. J. WILLIAMS

MACROMOLECULAR MATERIALS AND ENGINEERING (PRINT)

Wiley V-CH Verlag

Año: 2005 vol. 290 p. 456 - 462

1438-7492

Reactions taking place in a homogeneous solution of an epoxy monomer based on the diglycidyl ether of bisphenol A (DGEBA) and palmitic acid (PA), in the presence of benzyldimethylamine (BDMA), were investigated using Fourier-transformed infrared spectroscopy (FTIR) and size exclusion chromatography (SEC). In the stoichiometric formulations prepared with equal molar ratios of epoxy (E) to carboxyl groups, E/PA¼1, the main reaction was the carboxyl addition to the epoxy giving a b-hydroxy ester. This reaction was followed by transesterification that occurred to a very small extent. In the formulations prepared with an epoxy excess, E/PA>1, the transesterification reactions were very significant as well as the homopolymerization of the epoxy excess that took place to an almost complete conversion. Reactions products synthesized in the range of 1<E/PA<2 were solids at room temperature due to the crystallization of a fraction of fatty acid chains. Above the melting temperature, reaction products recovered the liquid state. The formulation synthesized with E/PA¼2 exhibited a good behavior as a hot-melt adhesive of steel sheets with a single lap-shear strength of 2.5 MPa and an interfacial-cohesive failure. For E/PA>3, the gelation took place leading to the networks exhibiting a single glass transition temperature (Tg) without any evidence of crystallization or phase separation. Tg varied from 90 8C for the neat epoxy to 0 8C for the formulation with E/PA¼3.¼1, the main reaction was the carboxyl addition to the epoxy giving a b-hydroxy ester. This reaction was followed by transesterification that occurred to a very small extent. In the formulations prepared with an epoxy excess, E/PA>1, the transesterification reactions were very significant as well as the homopolymerization of the epoxy excess that took place to an almost complete conversion. Reactions products synthesized in the range of 1<E/PA<2 were solids at room temperature due to the crystallization of a fraction of fatty acid chains. Above the melting temperature, reaction products recovered the liquid state. The formulation synthesized with E/PA¼2 exhibited a good behavior as a hot-melt adhesive of steel sheets with a single lap-shear strength of 2.5 MPa and an interfacial-cohesive failure. For E/PA>3, the gelation took place leading to the networks exhibiting a single glass transition temperature (Tg) without any evidence of crystallization or phase separation. Tg varied from 90 8C for the neat epoxy to 0 8C for the formulation with E/PA¼3.