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Department of Chemical Engineering
Dilational Yielding In Rubber Toughening of Plastics

A. Lazzeri, University of Pisa


Dilatational yielding is known to make an important contribution to energy absorption in rubber-toughened plastics under critical conditions. The ability to yield and cold-draw while increasing in volume is a key requirement in developing crack propagation resistance in thick sections, and one that is best met by adding rubber particles. As the stress increases, small holes form in the rubber particles, and yielding takes place in the surrounding matrix. The aim of this project was to understand the factors affecting dilatational yielding in toughened plastics, and in particular the exact role of rubber particle cavitation. Before the study began, there was some uncertainty about whether the matrix yielded (or crazed) first, thereby initiating rubber particle cavitation, or cavitation occurred first, triggering shear yielding and crazing. A combination of modelling and experimental testing was used to address these questions.

The Lazzeri-Bucknall energy-balance model for rubber particle cavitation [1] was extended to include: the global energy input from the specimen, effects due to rigid polymeric cores or sub-inclusions in complex rubber particles, the formation of more than one void per particle, and differential thermal contraction [2]. Predictions of the model were compared with experiment.

Two new techniques were introduced to detect rubber particle cavitation, one based on thermal contraction testing, and the other on dynamic mechanical thermal spectrometry (DMTS). Rubbers have very high expansion coefficients, and therefore cause increased contraction of toughened plastics during cooling: triaxial tensile stresses are set up in the rubber phase, which therefore becomes less dense. Hole formation releases internal stresses, whereupon the toughened plastic increases in volume, and the rubber returns to its equilibrium density. The volume increase during cavitation gives rise to an anomaly in the thermal contraction curve [3], while the increase in density affects the DMTS curve. Under compression, the holes close, and the tan peak shifts to higher temperature because of the increase in density. Under tension, the holes open, and the peak temperature becomes independent of applied stress [4].

Flexural tests on transparent rubber-toughened PMMA show a transition from yielding without cavitation above 60·C to yielding with cavitation below 50·C [1]. This is accompanied by a shift in the neutral plane of the flexure specimen, indicating a decrease in the ratio of tensile to compressive yield stress, in accordance with the cavitation model. Separate tests in tension and compression confirm this transition, which in flexural impact shifts to a temperature above 80·C. Several of the test programmes supported the view that cavitation of the rubber particles is a necessary first step in the initiation of crazes internally in toughened plastics. Pre-cooling ABS to induce cavitation in the rubber causes an increase in the 23·C creep rate. Pre-straining HIPS at 23·C, followed by annealing above the Tg of PS and cooling back to 23·C, causes a fall in the yield stress of HIPS. Tensile dilatometry on toughened PA6 shows a change in deformation kinetics on reaching a critical stress, corresponding to the cavitation stress.

References

  1. A. Lazzeri and C.B. Bucknall, Polymer, J. Mater. Sci., 28 (1993) 6799.
  2. D.S. Ayre and C.B. Bucknall, Polymer, in press, May 1998.
  3. C.B.Bucknall, D.S.Ayre and D.J.Dijkstra, Submitted to Polymer, May 1998.
  4. C.S.Lin, D.S.Ayre and C.B.Bucknall, J. Mater. Sci. Lett. 17 (1998) 669.