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  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
. 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
, 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
Flexural tests on transparent
rubber-toughened PMMA show a transition from yielding without
cavitation above 60·C to yielding with cavitation below 50·C
. 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.
- A. Lazzeri and C.B. Bucknall,
Polymer, J. Mater. Sci., 28 (1993) 6799.
- D.S. Ayre and C.B. Bucknall,
Polymer, in press, May 1998.
- C.B.Bucknall, D.S.Ayre and
D.J.Dijkstra, Submitted to Polymer, May 1998.
- C.S.Lin, D.S.Ayre and
C.B.Bucknall, J. Mater. Sci. Lett. 17 (1998) 669.