Foamed plastics continued

A cellular plastic has been defined as a plastic the apparent density of which is decreased substantially by the presence of numerous cells disposed throughout its mass (21). In this article the terms cellular plastic, foamed plastic, expanded plastic, and plastic foam are used interchangeably to denote all two-phase gas—soHd systems in which the soHd is continuous and composed of a synthetic polymer or mbber. 

Flexible foams are resilient open-cell structures with densities varying from 25—650 kg/m3, depending on the choice of the raw materials. Most flexible foams are produced in the form of a slab or bun in a continuous process in widths up to 2.4 m and thicknesses up to 1.2 m. A liquid foamable mixture is pumped onto a conveyor, which moves through a tunnel where reaction and foaming occur (101). Similar mixtures can be placed in a mold and allowed to foam. This process is used in the manufacture of automobile seats (see Foamed plastics). 

What are the trends in the polymeric foam industry The production of foamed plastics on the basis of high polymers will continue to expand mainly utilizing common raw materials (polystyrene, poly(vinyl chloride), polyolefins and synthetic resins). Apart from that, one should expect a strong increase in the commercial pro-... 

Frequently, differences in substrate absorbency form the reason for variations in adhesive strength in bonds between PVC films and wood, plastic foams, or textiles. To speed up evaporation of the water from nonabsorbent plastics, continuous feed ovens can be used or water binding substances such as ettringite can be added just before application. Coarsely dispersed adhesives should be used on highly absorbent substrates, since very small particles tend to migrate into the substrate. 

Unique Properties of Foamed Plastics. Foamed plastics have certain unique properties that distinguish them from solid polymers and are particularly useful in practical applications. Basically, these properties result from their composite structure—a continuous phase of polymer that is of relatively high modulus, and a gas phase of negligible modulus, which may be either dispersed as single cells in a closed-cell foam, or continuous and interpenetrating in an opencell foam. 

UNION CARBIDE CHEMICALS AND PLASTICS CO., INC. TRITON Low-Foam Surfactants (Continued) ... 

Air classification also uses density as the characteristic by which different plastics may be separated. A blast of air is blown through a steady stream of falling ground, mixed plastics. The dense plastics continue to fall while expanded foams and other lighter materials are blown into a separate collector. This technique is typically used to separate plastics with significantly different densities whereas liquid based density separations may be tuned more tightly. 

An important newer use of fluorine is in the preparation of a polymer surface for adhesives (qv) or coatings (qv). In this apphcation the surfaces of a variety of polymers, eg, EPDM mbber, polyethylene—vinyl acetate foams, and mbber tine scrap, that are difficult or impossible to prepare by other methods are easily and quickly treated. Fluorine surface preparation, unlike wet-chemical surface treatment, does not generate large amounts of hazardous wastes and has been demonstrated to be much more effective than plasma or corona surface treatments. Figure 5 details the commercially available equipment for surface treating plastic components. Equipment to continuously treat fabrics, films, sheet foams, and other web materials is also available. 

Flammability. The results of small-scale laboratory tests of plastic foams have been recognized as not predictive of their tme behavior in other fire situations (205). Work aimed at developing tests to evaluate the performance of plastic foams in actual fire situations continues. All plastic foams are combustible, some burning more readily than others when exposed to fire. Some additives (131,135), when added in small quantities to the polymer, markedly improve the behavior of the foam in the presence of small fire sources. Plastic foams must be used properly following the manufacturers recommendations and any appHcable regulations. 

One simple rheological model that is often used to describe the behavior of foams is that of a Bingham plastic. This appHes for flows over length scales sufficiently large that the foam can be reasonably considered as a continuous medium. The Bingham plastic model combines the properties of a yield stress like that of a soHd with the viscous flow of a Hquid. In simple Newtonian fluids, the shear stress T is proportional to the strain rate y, with the constant of proportionaHty being the fluid viscosity. In Bingham plastics, by contrast, the relation between stress and strain rate is r = where is... 

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