Fracture toughness and toughening mechanisms of poly(butylene terephthalate)/polycarbonate (PBT/PC) blends

Abstract

Poly(buty1ene terephthalate)/polycarbonate (PBT/PC) system was selected in the present study on the fracture toughness and toughening mechanisms of rigid-rigid polymer blends. A comprehensive review on the toughening mechanisms for traditional rubber toughened and rigid polymer toughened polymer blends was given. Techniques for fracture toughness characterisation and toughening mechanism analysis were introduced and compared. The equivalence of the critical J—integral and the specific essential work of fracture was proven experimentally. There is no geometry dependence of the specific essential work of fracture as verified with three different specimens, namely, Single-Edge-Notched Three-Point-Bend (SEN—3PB), Compact Tension (CT) and Double-Edge-Notched-Tension (DENT). The specific essential work of fracture concept has been successfully extended to impact testing and the kinetic energy effect is discussed.

A relatively thorough investigation on the morphology, mechanical properties, fracture toughness and toughening mechanisms was first conducted on a commercial grade PBT/PC/impact modifier (IM) blend under different testing conditions. Toughening mechanisms and sequence of toughening events were studied by SEM and TEM. It is suggested that the expansion of IM particles and matrix crazing under triaxial stress takes place first, which is followed by the cavitation inside of the IM particles and/or at the boundary between IM particle and matrix. Further expansion of the cavities reduces the thickness of the ligaments between them and eventually relieves the plane—strain constraint allowing massive shear deformation in the matrix to occur. It is believed that this matrix shear deformation provides the major contribution in toughening this commercial blend.

To be able to modify the microstructures and hence to study the structure-property relationship a series of PBT/PC blends with and without IM particles were subsequently fabricated using a twin screw extruder with different processing control successfully. It was found that the morphology of the blends changed gradually with the percentage of PET from PC-matrix/PBT-particle to PC-particle/PBT-matrix. Bi-continuous structure was observed with the PBT/PC (40/60) and (50/50) blends. A relatively strong boundary between PET and PC domains was found in the PC—rich blends which could be tentatively attributed to the PBT/PC copolymer generated by transesterification during processing.

In the toughened PBT/PC blends without IM, new toughening mechanisms are first observed. These include: (1) crazes formation in matrix under triaxial stress state, (2) crazes stabilisation by PC domains which prevent the crazes from developing into harmful cracks, (3) debonding—cavitation at the interface between PET and PC when the triaxial stress reaches its debonding strength, (4) relief of plane—strain constraint via debonding-cavitation which promotes massive shear yielding in matrix and (5) crack bridging by PC domains which is suggested as a possible major toughening mechanism. It is significant that in these blends the PC domains not only stabilise the growing crazes but also bridge the crack surfaces, enlarge the plastic zone size and result in a high fracture toughness. To enable these toughening mechanisms an appropriate bond strength between PET and PC is critical. It should be strong enough to provide the crack-bridging effect but not too strong to prevent debonding—cavitation to occur.

The effects of processing condition, testing temperature, strain rate and IM addition on the fracture behaviour of the laboratory prepared PBT/PC blends were also studied. Mechanisms responsible in toughening PBT/PC blends at impact loading rate were proposed; and possible correlations between fracture toughness and molecular relaxation processes were discussed. The optimum volume fraction of IM particle in this PBT/PC/IM system was found and the role of the IM in toughening was investigated using OM and SEM.

Based on the results of the present study, some suggestions on future work in this area and on the design of tough rigid—rigid polymer blends are given.