Journal of Cellular Plastics

An approach to improve the efficiency of the thermal insulation behavior of expandable polystyrene (EPS) particle foams by diminishing the thermal conductivity is the reduction of the radiation term of the thermal conductivity by an adjusted enlargement of the cell size of the particle foam. This correlation was investigated in detail by the determination of the dependences of cell size and thermal conductivity on the densities of the particle foam over a wider range using samples of expandable polystyrene particle foams showing conventional fine cell size as well as enlarged cell sizes. Based on the dependence of cell size on foam density of fine cell EPS foams, an equation is given also covering foams of larger cells. At the same mean diameter of the foam cells, the thermal conductivity of the EPS foam is increasing with a decrease in foam density in the whole range of diameters investigated from about 50—350 µm. At the same foam density, the thermal conductivity is in general independent of the mean cell diameter of the EPS foam at high foam densities, whereas at lower foam densities, the thermal conductivity is decreasing with increasing mean cell diameter of the foam, in a range of foam densities from about 10—35 g/L. Subsequently, a practical model to describe the dependence of thermal conductivity of expandable polystyrene particle foam on cell size and foam density is proposed and discussed.

Polyglycerol (PGL) is a polyhydroxyl compound obtained by selfcondensation of glycerol in the presence of alkaline catalysts. It is a very attractive polyol as a starter for the synthesis of polyether polyols for rigid polyurethane foams. It is liquid, easy to handle and has a very high average functionality of 4—20 (or more) hydroxyl groups/mol. By propoxylation of PGL or PGL—sucrose mixtures, we obtained new polyether polyols with very high functionalities, which are very difficult or impossible to obtain by other methods. A new technology for PGL-based polyether polyols preparation was investigated. In the first step the self-polycondensation of glycerol to PGL in the presence of potassium hydroxide or potassium methoxide as a catalyst was carried out. In the second step, the crude alkaline PGL was alkoxylated with PO without removing the catalyst, followed by purification of the resulting polyether polyols. Rigid polyurethane foams prepared from the synthesized PGL-based polyether polyols and crude MDI displayed good physical and mechanical properties, excellent dimensional stability, and low friability.

Foaming of blend systems is a promising approach to develop cellular materials with a set of desired properties. However, foaming of blend systems is not only a chance but also a challenge, as different polymers have to be foamed at the same foaming conditions. The best conditions during foaming are individual to each polymer and influence the obtained cellular structure. In immiscible polymer blends like poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) (SAN) the differences in glass-transition temperature (T_g) and viscosity are vast and can inhibit the foaming of the blend system. In order to minimize the above-mentioned difference in T_g and viscosity, one approach is to choose a blowing agent, which shows a selective solubility in one polymer phase, working as a selective plasticizer. The aim of the present study is to evaluate systematically the foaming behavior of immiscible PPE/SAN blend systems using ethanol and n-pentane as blowing agents. Mass uptake of ethanol and n-pentane in the PPE/SAN-blends as function of the blend composition is measured, offering a selective solubility of ethanol in SAN, while n-pentane shows a high affinity to PPE. The plasticizing effect on the blend phases PPE and SAN is theoretically estimated using the Chow-equation and the WLF-equation. After foaming the blowing agent saturated blend samples in an oil-bath at different temperatures, the density reduction and the cellular structures are analyzed and correlated to the blend morphology, the blowing agent solubility in the blend phase, the rheological properties and the glass transition temperature of the nonsaturated and saturated blend phases.

Composites of etylene vinyl acetate (EVA)/aluminum trihydroxide (ATH) (up to 70 wt%) were foamed to create new materials with good fire retardancy properties and low weight, proving the feasibility of developing cellular structures when high levels of halogen-free flame retardants (HFFR) are included. An experimental study was carried out to explore the effects of chemical composition on cellular structure as well as the effect of structure on thermal stability, mechanical and combustion properties. Sample fabrication was carried out using an improved compression molding route consisting of polymer compounding, precursor preparation and foaming under pressure. The polymer matrix consisted of EVA as well as certain amount of linear low-density polyethylene-maleic anhydride as coupling agent. The inorganic filler used was ATH ranging from 0 wt% to 70 wt%. Furthermore, azodicarbonamide was used as chemical blowing agent. Foamed samples with cell sizes below 100 µm were produced. These samples showed similar fire retardancy than their solid precursors. The compatibilization was proved indispensable to achieve a good adhesion between mineral filler and polymer and to improve the cellular structure. The increase of the amount of filler has an interesting effect on the cellular structure, going from a closed-cell (up to 50 wt% of ATH) to an open-cell cellular structure (60 wt% of ATH or more). The feasibility of producing HFFR cellular materials has been demonstrated as a result of this investigation, leading to a notable reduction of material compared to the solid one and to new properties which can result in new applications.