Foamed polymers cell size

Microcellular foaming, bimodal cell size distributions, and high open-celled contents of molecular composites of HT-polymers were reported by Sun et al. [33], investigating blends of a rod-like polymer polybenzimidazole with an aminated PSU and poly(phenyl sulfone) by using carbon dioxide as a blowing agent. The complex foaming behavior was related to phase separation within the otherwise... 

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. 

Polymer foams are usually produced with the conventional foam extrusion process, either as rigid foams or flexible foams. A key performance property is thermal insulation, which critically depends on the foam cell size and the thickness of the polymer walls between the pores. Control of the foam cell size can be difficult due to the low solubility of blowing agents and the gas created in the polymer and inhomogeneous nucleation. In particular, the production of polymer foams with cell sizes in the micron and submicron range is a current challenge. 

Cell Structure. A complete knowledge of the cell stmcture of a cellular polymer requires a definition of its cell sizes, cell shapes, and location of each cell in the foam. 

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. 

It should be realized that flammability of foams is a complex subject area and the "mechanism by which cellular polymers with different physical forms (cell sizes, etc.) lose heat at high temperatures have received surprisingly little attention" [19]. The... 

An alternative means of generating a polyimide foam with pore sizes in the nanometer regime has been developed [80-90]. This approach involves the use of block copolymers composed of a high temperature, high Tg polymer and a second component which can undergo clean thermal decomposition with the evolution of gaseous by-products to foam a closed-cell, porous structure (Fig. 7). 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

Crosslinked low-density polyethylene foams with a closedcell structure were investigated using differential scanning calorimetry, scanning electron microscopy, density, and thermal expansion measurements. At room temperature, the coefficient of thermal expansion decreased as the density increased. This was attributed to the influence of gas expansion within the cells. At a given material density, the expansion increased as the cell size became smaller. At higher temperatures, the relationship between thermal expansion and density was more complex, due to physical transitions in the matrix polymer. Materials with high density and thick cell walls were concluded to be the best for low expansion applications. 16 refs. 

Gas compression in closed-cell polymer foams was analysed, and the effect on the uniaxial compression stress-strain curve predicted. Results were compared with experimental data for a foams with a range of cell sizes, and the heat transfer conditions inferred from the best fit with the simulations. The lateral expansion of the foam must be considered in the simulation, so in subsidiary experiments Poisson s ratio was measured at high compressive strains. 13 refs. 

Figure 5.75 Effect of gas cell size on the specific flexural modulus of 6-mm-thick polypropylene structural foam. Samples were tested at 23°C with and without their outer skins. Reprinted, by permission, from P. R. Hornsby, in Two-Phase Polymer Systems, L. A. Utracki, ed., p. 115. Copyright 1991 by Carl Hanser Verlag.

The porous structure of the polymer can clearly be seen from its scanning electron micrograph (SEM) (Fig. 12). The cell sizes are usually smaller than in conventional gas-blown polystyrene foams and of higher spherical symmetry (Fig. 13). 

The properties of a foamed plastic can be related to several variables of composition and geometry often referred to as structural variables. These variables include polymer composition, density, cell structure (i.e., cell size, cell geometry, and the fraction of open cells), and gas composition. 

Structural Foams. Structural foams are usually produced as fabricated articles in injection molding or extrusion processes. The optimum product and process match differs for each fabricated article, so there are no standard commercial products for one to characterize. Rather there are a number of foams with varying properties. The properties of typical structural foams of different compositions are reported in Table 4. The most important structural variables are again polymer composition, density, and cell size and shape. 

Foamed plastics can be classified in different ways, for instance by their nature (flexible vs. rigid), chemical composition of the matrix, density, cell size, cell structure (open-celled vs. closed-celled), processing method, and dimensions. It is the aimed combination of these properties that determines the final application of the cellular polymer. As an example, open-celled ultra-low density foams are highly desirable for acoustical insulation, while rigid foams with closed-cells and elevated densities are preferred as load-carrying core materials in composite materials. 

A complete knowledge of the cell structure of a particular polymer would require the size, shape, and location of each cell. Because this is impractical, approximations are employed. Cell size has been characterized by measurements of cell diameter [25] and of average cell volume [26,27]. Mechanical, optical, and thermal foam properties depend on cell size. 

Another study of foamed polymers uses modern physicochemical methods of structure elucidation of gas-filled polymers and more accurate mathematical descriptioa These studies have considerably contributed to the understanding of the general character of the spatial structure of plastic foams. Thus, quantitative estimations of the effect of each morphological parameter (specific gravity, size and shape of ceUs, type of communication between ceUs, cell distributbn in the bulk, etc.), on the properties of a given material could be made. 

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