Orientation in polypropylene sheets produced by die‐drawing and rolling

Two orienting techniques for stiffening semicrystalline polymers, rolling and die‐drawing, are compared with respect to the anisotropy they produce in isotactic polypropylene (PP). Billets of PP were either drawn at 145°C through a tapered slotted die in the Leeds large‐scale die‐drawing machine to reduction ratios R of 2.2, 5.1 and 7.6, or rolled between rolls of 65 mm diameter at 120°C to R = 2 to 5. Drawing increased the crystallinity, as estimated from differential scanning calorimetry, density and wide‐angle X‐ray diffraction (WAXD); it thus disrupts the original PP structure, developing an oriented crystalline structure. WAXD pole figures showed that both die‐drawing and rolling oriented the molecular chain axis nearly parallel to the machine direction and the b axis perpendicular to the drawing plane. This approximate uniaxial symmetry was confirmed by ultrasonic measurements of the stiffness matrix. In tensile and falling‐dart impact tests, samples failed by delaminating in the drawing plane. Although stresses are applied to the material in quite different ways in die‐drawing and in rolling, the geometry of deformation in both is similar, close to plane strain.