High speed tensile performance and fractography of acrylic latex films

The performance of acrylic (50:50 MMA/BuA) latexes was investigated as a function of fusion level using high speed tensile testing. The structure of the copolymer was varied via the addition of molecular weight modifiers to the emulsion polymerizations. Chain transfer agent (CBr₄) was used to reduce the copolymer modulus, and crosslinker (EGDM, ethylene glycol dimethacrylate) was added to increase the modulus. The resulting materials exhibited a wide range of viscoelastic behavior (G^* varied from ～ 10⁷ dyne cm^?2 to ～ 10⁹ dyne cm^?2). Fracture energy and peak force at break were measured as a function of the complex modulus. It was found that both of these parameters showed a maximum with respect to G^* that corresponded to intermediate levels of crosslinking. This observation was explained in terms of the degree of coalescence of the films. Fully fused films (excess CBr₄) were brittle and performed poorly, conceivably because of insufficient entanglement to support stress. Marginally fused films also exhibited inferior, brittle behavior. The films cast from latexes synthesized with low levels of molecular weight modifiers showed intermediate fusion levels and superior tensile performance. The quantitative results were rationalized using scanning electron microscopy (SEM) of both the virgin films and the fracture surfaces. For comparison, the same materials were tested in a fully fused state following hot pressing. The behavior paralleled that expected for vulcanized rubbers in which lightly crosslinked materials exhibit the highest tensile strength. © 1993 John Wiley & Sons, Inc.