Crosslinking of a thermoset into a thermoplastic matrix: morphologies obtained under quiescent and dynamic conditions.
Françoise Fenouillot, Laure Meynié, Jean-Pierre Pascault
Laboratoire des Matériaux Macromoléculaires - UMR CNRS 5627
France
Keywords: thermoset, blend, morphology
The study concerns a thermoplastic / thermoset blend where the precursors of a network are initially solvent of the thermoplastic at the curing temperature. Then upon polymerisation, phase separation occurs. The blend becomes heterogeneous, composed of a thermoplastic rich phase and a dispersed thermoset rich phase.
Generally, such blends are prepared by melt mixing of the components into an internal mixer or a twin-screw extruder. Then, most of the time, the phase separation and curing of the blend is done afterwards, into an oven [1, 2, 3]. This work present what happens if the thermoset minor phase is polymerised and crosslinked directly into the processing machine, that is in the presence of flow/shear.
We have worked with a polystyrene /epoxy-amine model blend. The epoxy is a diglycidyl ether of bisphenol A (DGEBA) associated with a low reactivity amine, 4,4’-methylenebis(2,6-diethylaniline), (MDEA). With this model blend, the reaction induced phase separation process has been realized first under quiescent conditions, and then under shear into an internal mixer.
We have shown that despite the fact that the kinetic of the epoxy-amine reaction and the thermodynamic of the phase separation process were identical, the morphologies obtained under static or dynamic conditions were drastically different. Whereas spherical particles (diameter ~3mm) are obtained in the absence of flow, irregular particles and larger dimensions characterize the morphology under shear. It appeared that the morphology is strongly dependent, not only on the evolution of the viscosity ratio between the matrix and the dispersed phase, but also on the gelation of the dispersed phase. The classical deformation/coalescence behavior is observed only before the gelation region. Then, around the gel point, the particles tend to agglomerate in an irreversible way. The relaxation of shape normally leading to spherical particles becomes impossible due to the increasing elasticity of the dispersed phase. Coupled dynamic/static experiments allowed to identify the period of time during which the morphology evolves strongly, it happened to be very short compared to the total process time and centered around the gel point of the epoxy-amine.
1.Pascault J-P., Williams R.J.J., Polymer blends: formulation, Ed. By D.R. Paul and C.B. Bucknall, 1, 379-413, 2000
2.Saalbrink A., Lorteije A. Et Peijis T., Composite Part A 29A, 1998, pp. 1243-1250
3.Jansen B. J. P., Meijer H. E. H., Lemstra P.J., Polymer, 1999, vol 40, pp. 2917-2927