Effect of sintering on processability vis-a-vis microcellular foaming of ultrahigh molecular weight polyethylene

Processing of ultrahigh molecular weight polyethylene (UHMWPE) involves sintering due to its high melt strength and no flowability above melting temperature. Variations in compression molding pressure during sintering lead to chain rearrangement at the sintered interphase and the boundary, affecting foamability. UHMWPE particles are sintered using compression molding; samples are prepared at two different pressures: UHPE-HP (80 bar) and UHPE-LP (40 bar) at 180°C. The sintering phenomenon of UHMWPE particles is observed through an optical microscope, and their effect on foaming was observed. UHPE-HP foams are systematically studied to obtain the foaming window. Increasing foaming pressure (80–120 bar) made UHPE-HP foams softer (0.350–0.219 g/cm³) with varying average cell size (26.37–46.1 μm) and foam cell density (3.98 × 10⁷–1.06 × 10⁸ cells/cm³), and compression modulus decreased from 9 to 5.4 MPa. DMA results showed a strong dependence of stiffness on crystallinity, and foamed samples exhibit higher stiffness than their unfoamed counterpart. The storage modulus for foamed samples decreases with increase in the gas content. The UHPE-LP foam is relatively softer, with a lower foam density (0.233 g/cm³), a higher expansion ratio, bigger average foam cells (35.13 μm), and lower foam cell density (9.33 × 10⁷ cells/cm³). This is due to constrained crystallinity at the interphase and pre-existing cavities, favoring the foaming.