Journal of Cellular Plastics

Biodegradable foams are a key area of growth for packaging applications. Starch-based materials have been a successful environmentally degradable polymer. However, making foams out of these materials has been challenging due to their chemical composition and consequent low miscibility in CO₂. We examine the potential for developing biodegradable foams with one component being a starch-based polymer by introducing it into polycaprolactone. Polycaprolactone has thus far shown very high-expansion ratios for foams with supercritical CO₂. Blends with increasing amounts of starch-based materials were processed and foamed using isothermal treatments with supercritical CO₂. Characterization of the samples was done using X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. The melting enthalpies and temperatures of the starch-based materials phase decreased with decreasing starch-based materials indicating some an influence of polycaprolactone on the starch-based materials crystallinity. Foaming, however, caused a reversal in this effect with the foamed melting points similar to the pure components. Micrographs of the samples from the scanning electron microscopy revealed that the cell size of the foams reduced with the increase in starch-based materials concentration. Mechanical tests—tensile, compression, shear, and impact—were performed on the foamed samples. The results indicate a valuable approach to foaming materials that are compostable but not CO₂ miscible through blending with a highly foamable polymer such as polycaprolactone.

A phenomenological constitutive model for Divinycell PVC H100 foam undergoing crushing and hysteresis under cyclic compression loading was developed. Cyclic compression tests were done with strain amplitudes from 0.02 to 0.1 and strain rates ranging from 0.0005 s⁻¹ to 5 s⁻¹. Within this test range, the PVC H100 foam exhibited strain rate-dependency, damage, and hysteresis. Damage that occurred in the foam after yielding followed the pattern of Mullins damage, i.e. the damage was essentially fixed at a given strain amplitude, and more damage occurred with increasing the strain amplitude. A constitutive model based on damage initiation and viscoelastic damage evolution of the foam was proposed. A simple damage initiation criterion based on critical compressive strain was proposed to separate undamaged and damaged foam response. A standard model, an elastic spring in parallel with Maxwell element, was used to describe viscoelastic behavior before and after damage. Before damage, spring and damper constants were evaluated from the test data. The rate-dependent undamaged stress–strain response and flow stress were found to be in good agreement with the test results. After damage, the spring and dashpot resistances were found to be the functions of strain amplitude and flow stress, which depended on strain rate. These viscoelastic damage functions were shown to give very good predictions of the hysteresis and strain rate-dependent behavior of the foam after damage.

This paper examines the feasibility of using polyols from vegetable oils as base polyols (i.e. with 50% or more in a blend with petrochemical polyols) for flexible molded polyurethane foams. A series of hyperbranched (HB) polyols were synthesized by transesterification of hydroxy fatty acid methyl esters and different modifiers to control viscosity, hydrophilicity, molecular weight, and functionality. All HB polyols had hydroxyl numbers around 85 mg KOH/g, with the exception of one which was 105 mg KOH/g. When mixed with petrochemical polyols with OH numbers 35 and 28 mg KOH/g, the HB polyols acted primarily as high molecular weight crosslinkers that increased the stiffness of the polymeric network and the load-bearing properties but decreased the tensile strength, elongation, and tear strength. However, most of the foams met the targeted tensile and tear strength values while some of the foam formulations provided satisfactory elongation. The best mechanical properties were obtained from foams with phthalic anhydride-modified HB polyols. It was demonstrated that flexible molded foams with satisfactory properties can be obtained with 50% and 65% of HB soy polyols in a blend with PPO polyols.

This article reports on the compressive, dynamic mechanical and dielectric properties of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions fabricated using the environment-friendly supercritical carbon dioxide. The effects of cell morphologies such as relative density, cell-size distribution and porosity on the compressive strength, Young’s modulus, storage modulus, loss modulus, dielectric constant and loss tangent of the microcellular polycarbonate foams are investigated quantitatively. Experimental values of the compressive strength and Young’s modulus fit very well with the theoretical values calculated from the Gibson–Ashby model at low relative densities. The higher relative density leads to higher storage modulus and loss modulus. The bimodal foams significantly improve the compressive and dynamic mechanical properties compared to the unimodal foams with the same relative density. The dielectric properties of microcellular foams depend only on the total porosity, but not on the cell-size distribution or microstructure of the foams. With increasing porosity, the dielectric constant of the microcellular foams gradually decreases, and agrees very well with the curve calculated from the Maxwell-Garnett-spheres model.