Understanding the Compressive Behavior of Linear and Cross-linked Poly(vinyl chloride) Foams

The work reported in this article was initiated by the IUPAC Subcommittee 'Structure and Properties of Commercial Polymers' to study the 'Structure and properties of linear and cross-linked structural poly(vinyl chloride) (PVC) foams', and intends in particular to give a good understanding of the compressive response of these PVC foams with various foam densities. From a careful review of chemical reactions of polymeric methylene diphenyl diisocyanate [a mixture of 4,4'-methylene-bis(phenyl-isocyanate) and oligomeric isocyanates] with water or phthalic acid, it is known from the material supplier that a semi-interpenetrating polymer network of polyureas and/or polyamides is formed by cross-linking. The amide groups in these polymers are hydrophilic and can affect the overall mechanical response of PVC foams, especially under water. Scanning electron microscopy is used to study the morphology of linear and cross-linked foams and their heterogeneous foam deformation. Compression testing of these PVC foams is conducted in air and water, and illustrates the influence of foam density and cross-linking. Correlation of compressive testing of foams with the corresponding cross-linked PVC bulk materials suggests that elastic collapse of cells during compression may be linked to strain-softening of the corresponding bulk material. Finally, new predictive models are adopted for the PVC foams to give reasonable estimates of their compressive modulus and yield strength.