DELAMINATION FAILURE MECHANISMS IN MICROLAYERS OF POLYCARBONATE AND POLY(STYRENE-CO-ACRYLONITRILE)

The peel strength and delamination failure mode of coextruded microlay er sheets consisting of alternating layers of polycarbonate (PC) and p oly(styrene-co-acrylonitrile) (SAN) were studied with the T-peel test. Four delamination modes were observed: two modes where the crack prop agated along the PC-SAN interface and two other modes where the crack propagated through crazes in the SAN. The SAN layer thickness determin ed whether crack propagation was interfacial or through crazes. Crazin g and crack propagation through crazes were observed only if the SAN l ayer was thicker than 1.5 mu m. As the thickness of the SAN layer incr eased, the amount of crazing in front of the crack tip and the amount of craze fracture gradually increased; the peel strength increased acc ordingly. If the SAN layers were thinner than 1.5 mu m and the PC laye rs were relatively thick, the crack propagated along a single interfac e. The peel strength for this delamination mode was the lowest and equ al to about 90 J/m(2), independent of layer thicknesses. This delamina tion mode came closest to providing a ''real'' measure of the adhesive toughness of PC to SAN. With both interfacial and craze delamination, the crack could move from layer to layer if the PC was thin enough. T earing of the relatively thin PC layers increased the peel strength of the multiple-layer delamination modes. (C) 1998 John Wiley & Sons, In c.