Journal of Cellular Plastics

Polyurethane closed cell rigid foams have been widely used as insulants in the building and refrigeration industry and also for district heating pipes. It is the special combination of its excellent insulation performance and the mechanical stability due to the foaming process which makes this material very attractive for these applications. The replacement of CFCs, which have been banned as blowing agents since 1993, by hydrocarbons in Europe caused a decrease in the insulation capability of polyurethane (PU) rigid foams. The additional global requirement to reduce energy losses also adds an incentive to search for potentials which would improve the thermal performance of rigid foams. To overcome this problem, the heat transfer within PU rigid foams via

All LDPE resins are not created equal. It is well known that autoclave LDPEs are considered to be the products of choice for extrusion coating applications. On the other hand, LDPE resins produced using tubular reactors have a different structure that makes them more suitable for extruded foam applications. More specifically, within the choices of tubular LDPE resins for extruded foam applications, some are easier to process and produce better quality foam products than others. What makes one resin better than another?

This paper discusses the general chemical and rheological differences between autoclave and tubular LDPE resins then focuses in more detail on those differences for a variety of tubular LDPE products used in the extruded foam market.

Open-celled foams are three-dimensional networks of polymeric cells. The mechanical properties of a foam depend on the size and geometry of its cells. Since foams have a three-dimensional polyhedral structure, the two-dimensional characterization techniques currently used provide limited accuracy.

Nuclear magnetic resonance and x-ray tomography methods offeropportunities for three-dimensional imaging of these polyhedral structures. Software, which can use digital three-dimensional images to determine structural parameters such as strut length distribution, connectivity, and cell size, is being developed.

The image processing approach uses conformal curvature flow (CCF)segmentation to find the surfaces of foam struts in the 3-D images. Once these surfaces have been found, volume thinning is used to find the structural skeleton of the foam. The resulting data set can then be used to determine many statistical characteristics of the foam, including strut length distributions, window size and shape distributions, and cell size information.

Analysis of a reticulated polyurethane foam sample using these methods yielded a reasonable approximation of the structural skeleton of the sample.

A numerical simulation for polymeric foaming extrusion processes was conducted. Combining classical nucleation rate and bubble growth models with a non-Newtonian fluid model of a flow, a simultaneous bubble nucleation and growth behavior in a flow field was simulated. Simulation results were compared with the experimental data obtained by visual observations at a foaming extruder, where a polypropylene resin was physically foamed. The effects of physical parameters in foaming model on bubble size and number density calculation were intensively examined by sensitivity analysis.

Dissolution and solubility characteristics of carbon dioxide in polystyrene melt were studied in both single screw and twin-screw foaming extruders. The effects of the main processing conditions on the gas dissolution behavior were also studied. Solubility data obtained from the twin-screw extrusion experiments showed better consistency compared with those obtained from the single screw experiments, indicating the importance of enhanced mixing in affecting the gas dissolution and solubility behavior during an extrusion foaming process. Gas dissolution inside a single screw foaming extruder was studied for three atmospheric gases (carbon dioxide, nitrogen and argon) in polystyrene melts. The results were compared with those obtained in a twin-screw extruder for the carbon dioxide-polystyrene system in an effort to elucidate mechanisms for the complex process of gas dissolution inside a foaming extruder.