Rigid foamed polymers

Fig. 13. Effect of height-to-width ratio h/D of cells on compressive strength of rigid foamed polymers based on (1) polyurethane, y = 48 kg/m, polystyrene, y = 32 kg/m (2) polyurethane, y = 32 kg/m (3) polyurethane, y = 24kg/m phenolic resin, y = 32kg/m and aj, are the compressive strengths perpendicular and parallel to the direction of foaming...

Fig. 30a and b. Rod model of a cellular structure (a) and tension-failure diagram (b) of this model for rigid foamed polymers... 

Javni, L W. Zhang Z.S. Petrovic. Soybean-oil-based polyisocyanurate rigid foams. /. Polym. Environ. 2004,12, 123-129. 

Polyurethane. SmaU quantities of polyurethane film are produced as a tough mbber-like film. Polyurethane is more commonly used to produce foamed sheet, both flexible and rigid. The flexible foam is used as cushioning in furniture and bedding the rigid foam is widely used for architectural insulation because of its outstanding thermal insulation efficiency (see Urethane POLYMERS). 

Syntactic Cellular Polymers. Syntactic cellular polymer is produced by dispersing rigid, foamed, microscopic particles in a fluid polymer and then stabilizing the system. The particles are generally spheres or microhalloons of phenoHc resin, urea—formaldehyde resin, glass, or siUca, ranging 30—120 lm dia. Commercial microhalloons have densities of approximately 144 kg/m (9 lbs/fT). The fluid polymers used are the usual coating resins, eg, epoxy resin, polyesters, and urea—formaldehyde resin. 

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. 

Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

The flexible foams discussed in the previous section have polymer stmctures with a low degree of cross-linking. If polyols of higher functionality, i.e. more hydroxyl groups per molecule, are used, tougher products may be obtained and in the case of material with a sufficiently high functionality rigid foams will result. 

Polyurethane foams are widely used. Rigid foams, for example, are used in cavity wall insulation in buildings, while flexible foams have, until recently, been used in soft furnishing for domestic use. They continue to be used in car seating. In addition to foams another major use of polyurethanes is in surface coatings. A variety of polyurethane-based polymers, some of considerable complexity, are used for this purpose, but all share the common desirable features of toughness, flexibility, and abrasion resistance. 

Some other groups such as ester, ether, amide, or urea are present in the Polymer chain of commercial polymers. In 1937, O. Bayer found that reaction of diisocyanates with glycols fields polyurethanes which are useful as plastics, fibres, adhesives, rigid foams and surface coatings. 

To get polyurethane foams the polymer is formed along with gas evolution. When these two processes take place simultaneously the gas bubbles are trapped in polymer matrix yielding a cellular product. Slightly cross-linked products are flexible while highly cross-linked products are rigid. Both flexible and rigid foams are of commercial importance. 

The difunctional N-cyanourea compounds were found to polymerize into different polymeric materials at different temperatures. At room temperature, a linear polymer was obtained either from the polymerization of a di-N-cyanourea monomer or directly from the mixture containing a diisocyanate and cyanamide. At elevated temperature (>100°C), the di-N-cyanourea monomer, or the mixture of a diisocyanate and cyanamide, cross-linked to a rigid foam or flexible material, depending on the structure of the monomer. 

Foamed polymers are low-density, cellular materials that contain bubbles of gas and are made in a variety of ways out of thermoplastics and thermosets. Their properties vary from rigid to flexible. The rigid foams are best known for their insulation properties (like in ice chests). The flexible foams are used extensively in cushioning (seats, mattresses). 

Most thermoplastics and thermosets can be foamed, many of them into either flexible or rigid foams. The choice is controlled by the blowing agent, additives, surfactants, and mechanical handling. Some polymers can be expanded as much as 40 times their original density and still retain a substantial part of their strength. Most commercial foams are expanded to derisities of two to five pounds per cubic foot. (Water is 62 pounds per cubic foot.)... 

Foamed polymers. Thermosets and thermoplastics formed into low density, cellular materials containing bubbles of gas. Rigid foams have their gas bubbles in closed cells, inhibiting flexibility flexible foams have the bubbles in open cells, permitting the gas to escape as the foam is flexed. 

Structural foams. Rigid foams made from thermosets. See polymer foams. 

Isosorbide polyurethanes, especially those based on aliphatic isocyanates, may be useful in the same applications as conventional polyurethanes i.e. thermoplastics, coatings, and foams. In fact, excellent rigid foams have been obtained from P(I-MDI)(5). Isosorbide has a low melting point of 61°C and it is suitable for use in reactive injection molding processes alone or in the form of a mixture with other conventional diols. In addition, its polymers may also find specific applications due to the anticipated high complexation ability of the two tetrahydrofuran rings in their isosorbide units. 

Rigid foams are used for structural and insulation uses while the flexible materials are used for a vast variety of applications as seen in Figure 2.20. The versatility of polyurethane positions the product as unique in fire polymer world because of the breadth of applications. As we will show, small changes in chemistry can achieve a broad range of physical properties. This statement emphasizes the physical properties and serves as a testament, however, to the lack of chemical interest. It is supported by a description of the independent variables of density and stiffness and the range of products based on the primary attributes of polyurethanes. See Figure 2.21. 

Syntactic Cellular Polymers. Syntactic cellular polymer is produced by dispersing rigid, foamed, microscopic panicles in a fluid polymer and then... 

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