Foamed closed-cell structures

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

Rigid polyurethane foams are mostly based on polyether alcohol and are highly cross-linked. Rigid foams are many times blown by halogenated alkanes like trichlorofluoromethane. These foams have closed cell structures and are used for thermal insulation. Semi-rigid foams are used in car crashpads and packaging. 

Details are given of the fabrication of foams with uniform closed-cell structures. LDPE was used as polymer feedstock. Thermal analysis was performed using DSC and morphologies were examined using SEM. 12 refs. 

A collection of polyolefin foams with closed cell structure and different chemical compositions and densities was studied by using SEM, DSC and dynamic mechanical analysis. Deformation mechanisms were also studied. 26 refs. 

Airofom PA 230 is a non-crosslinked PE foam which while having a slightly lower temperature resistance than crosslinked types, is claimed to be substantially cheaper to process. It has a closed-cell structure and is available in rolls, sheets and fabricated products, or as a self-adhesive backed or laminated to other materials. A sample of the foam is included. 

Moreover, flexible foams are characterized by utilization of special emulsifiers in their synthesis yielding an open-cell architecture, whereas for rigid foams emulsifiers are chosen that create more closed-cell structures. As diisocyanate for both types, the commercially available mixture of 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate is especially suitable. If foam formation is to take place at room temperature, and especially when hydroxy compounds with secondary hydroxy groups are used [poly(propylene glycol)s], the presence of a catalyst is generally required (see Sect. 4.2.1). 

Cellular Glass. Composed of glass processed to form a rigid foam usually having a predominantly closed-cell structure... 

The carbonization of a syntactic foam opens up the previously closed cell structure. The size and proportion of micropores formed depend mainly on the size and shape homogeneity of the filler. Thus, there is quite a narrow size distribution of open pores in the carbonized material made from a novolac oligomer and Krecasphere carbon microspheres (Fig. 9)39). 

Use of unmodified starches in combination with mild acids and carbonates has been claimed.214 Starch moisture content should be no greater than 25%. Acids such as tartaric, citric or malic acids are added at levels between 0.2% and 7%, while the carbonate level is 0.1% to 2%, both based on total starch composition. The acid is stated to serve a two-fold function it depolymerizes the starch during extrusion via hydrolysis, improving expansion at the same time, it liberates carbon dioxide, which acts as a blowing agent, from the carbonate. A typical formula extruded in a twin-screw extruder at 170-195°C yielded a product with a density of approximately 1 lb/ft3 (16kg/m3) and resilience of 60-85%. The foam had a continuous skin with a closed cell structure. 

Foams Wave by Mater-Bi, foamed sheet packaging is a biodegradable alternative to conventional protective foam packaging such as polystyrene, polyurethane and polyethylene. Wave by Mater-Bi is starch-based, and is expanded using water, extruded into sheets and then assembled into blocks that can be cut into any shape. The foams have a robust and resilient closed-cell structure. 

The moisture resistance, low cost, and low-density closed-cell structure of many cellular polymers resulted in their acceptance for buoyancy in boats, floating docks, and buoys. Because each cell is a separate flotation unit, these materials cannot be destroyed by a single puncture. Foamed-in-place polyurethane between thin skins of high tensile strength is used in pleasure craft [98]. Other cellular polymers that have been used where buoyancy is needed are produced from polystyrene, polyethylene, poly(vinyl chloride), and certain types of rubber. Foams made from styrene-acrylonitrile copolymers are resistant to petroleum products [99,100]. 

According to Thomas elastic polymers often produce open-cell foamed plastics, whereas rigid polymers generally form closed-cell materials. However, there are many exceptions to this rule, owing to the variety of blowing techniques. Closed-cell structures are more likely to be produced from polyurethanes, epoxy resins, silicones, poly(vinyl chloride), polystyrene, etc., whereas open-cell materials mainly result from phenolic and carbamide foamed plastics. 

The structure of syntactic foams is composed of closed-cell microspheres and matrix resins, and the resultant foam has the advantages of (a) isotropic properties not available in other types of foams (b) very low water absorption due to highly closed-cell structures and (c) very high compressive strengthAveight ratio. Table 40 shows properties of syntactic foams composed of epoxy resin and glass microspheres. 

Epoxy foams are rigid. Some are predominantly of closed-cell structure and some of open-cell structure. Current efforts may lead to... 

Cellular solids are a class of materials with low densities and novel physical, mechanical, thermal, electrical, and acoustic properties. Low-density cellular metals can feature a wide variety of topologies to include open-cell foam, closed-cell foam, hollow-sphere foam, periodic/optimized truss structures, and honeycomb. Metallic foams consist of air dispersed in a solid matrix, similar to polymer foams such as polystyrene or food foams such as whipped cream. Closed-cell foams feature solid faces such that each cell is independently sealed from its neighboring cells, whereas open-cell foams (also known as porous metals, metal sponges and truss-type materials) do not contain cell walls they only have cell edges. Hollow-sphere foams consist of an assembly of individual hollow spheres. 

Foamed adhesive tapes Adhesive tapes without a substrate, with the total system consisting of a pressure-sensitive adhesive polymer with two-sided adhesive properties available in foamed and closed-cell structure. They are not to be confused with one or two-side adhesive tapes with foam-structure substrates. In... 

Generally, Eq. (17.21) is well obeyed, but for most solid foams the exponent is smaller than three, sometimes even as small as two, because even for a closed-cell structure most of the matrix material may be in the beams rather than in the walls. The theory can be extended for other rheological parameters, such as the yield stress, but these relations are mostly not well obeyed. As a general rule, however, one may state that for any rheological parameter Z... 

Onifine. [Otsuka] Azodkarbonamide blends blowing agent for prod, of cross-linked PVC foam of closed-cell structures cushioning and absmbing materials insulation, water flotation items sealant in motor and construction industries. 

Inhibited foam plastics are predominantly of a closed-cell structure, which means that the vapor phase is insulated in each cell from the neighboring cells by polymer walls. Diffusion of Cl through the cell walls inside polymer containers made of this inhibited foam plastic diminishes the possibility of corrosive damage of the packed hardware. 

A number of methods have been devised for producing cellular products from PVC, either by a mechanical blowing process or by one of several chemical blowing techniques. PVC foams are produced in rigid or flexible forms. The greatest interest in rigid PVC foam is in applications where low-flammability requirements prevail. It has an almost completely closed cell structure and therefore low water absorption. The rigid PVC foam is used as the cellular layer of some sandwich and multi-layer panels. 

Cellular rubber may be described as an assembly of a multitude of cells distributed in a rubber matrix more or less uniformly. The cells may be interconnected (open cells) as in a sponge or separate (closed cells). Foam rubber made from a Uquid starting material such as latex, described earKer, is of open-cell type. Cellular products made from solid rubber are commonly called sponge (open cell structure) and expanded rubber (closed cell structure). 

Freon 11 blows to give closed cell structures whereas water produces open cells through carbon dioxide. This formulation is typical for a rigid foam for thermal insulation purposes. The heat build-up due to the reaction exotherm (about 80 kj/mole for formation of urethane groups) is sufiiciently dissipated through the open cell to avoid thermal degradation. 

Poly (styrene-co-acrylonitrile). See Styrene/acrylonitrile copolymer Poly (styrene-co-allyl alcohol). See Styrene/allyl alcohol copolymer Poly (styrene-co-butadiene). See Styrene/butadiene polymer Poly (styrene-co-divinylbenzene). See Styrene/DVB copolymer Poly (styrene-co-maleic anhydride). See Styrene/MA copolymer Poly (styrene-co-methyl methacrylate). See Styrene/methyl methacrylate copolymer Poly (styrene-co-a-methylstyrene). See Styrene/a-methyl styrene resin Poly (styrene-divinylbenzene). See Styrene/DVB copolymer Polystyrene, expandable Synonyms EPS Expandable polystyrene Expanded polystyrene XPS Definition Amorphous PS beads contg. pentane as a blowing agent and coated with a lubricant the polymer is converted to foamed articles with a closed cell structure by applic. of steam Properties Beads (0.4-1.5 mm diam.)... 

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