Foamed cell size, physical properties

Density and polymer composition have a large effect on compressive strength and modulus (Fig. 3). The dependence of compressive properties on cell size has been discussed (22). The cell shape or geometry has also been shown important in determining the compressive properties (22,59,60,153,154). In fact, the foam cell stmcture is controlled in some cases to optimize certain physical properties of rigid cellular polymers. 

Silicone foam thus formed has an open ceU stmcture and is a relatively poor insulating material. Cell size can be controlled by the selection of fillers, which serve as bubble nucleating sites. The addition of quartz as a filler gready improves the flame retardancy of the foam char yields of >65% can be achieved. Because of its excellent dammabiUty characteristics, siUcone foam is used in building and constmction fire-stop systems and as pipe insulation in power plants. Typical physical properties of siUcone foam are Hsted in Table 10. 

EFFECT OF CELL SIZE ON THE PHYSICAL PROPERTIES OF CROSSLINKED CLOSED CELL POLYETHYLENE FOAMS PRODUCED BY A HIGH PRESSURE NITROGEN SOLUTION PROCESS... 

Blends of poly (ethylene terephthalate) (PETP) and polypropylene (PP) with different rheological properties were dry blended or compounded, and extrusion foamed using both physical blowing and chemical agents, and the foam properties compared with those of foam produced from the individual components in the absence of compatibilisers and rheology modifiers. The foams were characterised by measurement of density, cell size and thermal properties. Low density foam with a fine cell size was obtained by addition of a compatibiliser and a co-agent, and foamed using carbon dioxide. The presence of PP or a polyolefin-based compatibiliser did not effect... 

While the primary reason for reticulation is to improve flow-through characteristics, it provides a further benefit by making the surface available to fluids passing through. The technology also produces a remarkable degree of uniformity in cell size. This contributes to the predictability of both flow and surface characteristics. If the surface is activated in some way, it is easy to see why this aspect of reticulation could be beneficial in designing functional devices. Table 2.4 and Table 2.5 show typical physical properties of commercially available reticulated foams. 

The resistance to fluid flow is a measure of the physical structure of the foam. In order to control the flow through a foam, ceU size, degree of reticulation, density, and other physical factors must be controlled. The control of these physical factors, however, is achieved through the chemistry and the process by which the foam is made. The strength of the bulk polymer is measured by the tensile test described above, but it is clear that the tensile strengths of the individual bars and struts that form the boundaries of an individual cell determine, in part, the qualities of the cells that develop. A highly branched or cross-linked polymer molecule will possess certain tensile and elongation properties that define the cells. The process is also a critical part of the fluid flow formula, mostly due to kinetic factors. As discussed above, the addition of a polyol and/or water to a prepolymer initiates reactions that produce CO2 and cause a mass to polymerize. The juxtaposition of these two reactions defines the quality of the foam produced. Temperature is the primary factor that controls these reactions. Another factor is the emulsification of the prepolymer or isocyanate phase with the polyol or water. 

Expandable polystyrene with improved properties has been obtained by copolymerization, postreaction, additives, or by physical means. Improvements have been achieved particularly in resistance to premature fusion of particles during expansion, in reducing cell size, in self-extin-guishability, in attractive coloration, in resistance of foams to attack by gasoline and in dimensional stability of foams at elevated temperatures. 

This survey deals with the fundamental morphological parameters of foamed polymers including size, shape and number of cells, closeness of cells, cellular structure anisotropy, cell size distribution, surface area etc. The methods of measurement and calculation of these parameters are discussed. Attempts are made to evaluate the effect and the contribution of each of these parameters to the main physical properties of foamed polymers namely apparent density, strength and thermoconductivity. The cellular structure of foamed polymers is considered as a particular case of porous statistical systems. Future trends and tasks in the study of the morphology and cellular structure-properties relations are discussed. 

In open cell structures the gas is air and for this reason they have a higher absorptive capacity of moisture, a higher gas and vapour permeability, less effective insulation capabilities for either heat or electricity, and a better ability to absorb and dampen sound. Eor any polymer composite, the proportion of open gas structural elements increases as the density of the foamed plastic decreases, because an increase in cell size means a decrease in the thickness of cell walls and ribs [60]. The combination of other advantageous physical properties with fair acoustic characteristics has led to the use of plastic foams in sound proofing [31]. 

Some MCPs also exhibit better physical properties. Many MCPs are tougher and have a longer fatigue life. Also the specific mechanical strength is much better than conventional large-cell foamed plastics. MCPs can have densities as low as 0.03 g/cm. At such low densities, the thermal insulation properties of MCPs are excellent because of the small cell size. 

In addition to the chemical properties of the material, physical properties such as surface area for cell attachment are essential. Various methods of creating pores in these materials to increase sirrface area are used. Scaffolds formed using the different techniques, which include fiber bonding, solvent casting/particulate leaching, gas foaming and phase separation are known, which result in different porosity, pore size, and the promotion of tissue growth [127]. 

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