Rigid foam

Replacement of CFCs in rigid polyurethane foams has been a much more serious problem, since the main use of these foams is as heat insulators and the main property required is therefore low thermal conductivity. Blowing gases of high 

The trimerization of isocyanates to form isocyanurate rings is particularly important in producing iire-resistant rigid foams, and a large volume of work has appeared on this topic. Much of it has been done by Kresta and co-workers, who have discussed the general mechanism, the co-catalytic effect of urethane groups, and the use of cyclic sulphonium zwitterions and aminimides as catalysts. Model systems have been used for these studies, but attention has also been paid to the kinetics and mechanism of commercial polyisocyanurate systems.  

Other rigid foam topics have been less extensively discussed in the literature. Improvements in processing of foam systems and in the friability of the final product have been claimed, by adjusting individual formulations. Plasticizers such 

FIGURE 14.34 Production of low-density polystyrene foam sheet from tubular film die. (Data from Collins, F. H., SPE J., 16, 705, July 1960.) 

Preheat Filling Fusion Dwell Cool Part ejection  

Drains closed Drains closed Drains open 

FIGURE 14.35 Molding polystyrene beads. (Reproduced by permission from PELASPAN Expandable Polystyrene, Form 171-414, Dow Chemical Co., 1966. Copyright 1966 Dow Chemical Co.) 

Foams of phenol formaldehyde resins can be made from a dispersion of a volatile diluent (isopropyl ether dispersed with the aid of a surfactant) in an aqueous solution of an incomplete phenol-formaldehyde reaction product [46]. Addition of an acid catalyst such as hydrochloric or sulfuric acid causes further condensation of phenol and formaldehyde to give a dimensionally stable, network structure. At the same time the heat of reaction volatilizes the diluent, yielding a foam. The foaming can be done in place. Phenolic foams are used as heat-stable, flame-retardant, thermal insulation. 

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Rifinah Rifle barrels Right of first refusal Rigid foam... 

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). 

The product contains 12.6% phosphoms and has an OH number in the 450 mg KOH/g range. Fyrol 6 is used to impart a permanent Class 11 E-84 flame spread rating to rigid foam for insulating walls and roofs. Particular advantages are low viscosity, stabiHty in polyol—catalyst mixtures, and outstanding humid aging resistance. Fyrol 6 is used in both spray foam, froth, pour-in-place, and slab stock. 

A number of commercial phosphoms-containing polyols have been made by the reaction of propylene oxide and phosphoric or polyphosphoric acid. Some have seen commercial use but tend to have hydrolytic stabiHty limitations and are relatively low in phosphoms content. BASF s Pluracol 684 is a high functionahty polyol containing 4.5% P, sold for Class 11 rigid foam use. 

One ASTM test procedure has suggested (24) that foamed plastics be classified as either rigid or flexible, a flexible foam being one that does not mpture when a 20 x 2.5 x 2.5 cm piece is wrapped around a 2.5 cm mandrel at a uniform rate of 1 lap/5 s at 15—25°C. Rigid foams are those that do mpture under this test. This classification is used in this article. 

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. 

The mechanical properties of rigid foams vary considerably from those of flexible foams. The tests used to characterize these two classes of foams are, therefore, quite different, and the properties of interest from an application standpoint are also quite different. In this discussion the ASTM definition of rigid and flexible foams given earlier is used. 

The properties of commercial rigid foamed plastics are presented in Table 2. The properties of commercial flexible foamed plastics are presented in Table 4. The definition of a flexible foamed plastic is that recommended by the ASTM Committee D 11. The data shown demonstrate the broad ranges of properties of commercial products rather than an accurate set of properties on a specific few materials. Specific producers of foamed plastics should be consulted for properties on a particular product (137,138,142). 

Table 2. Physical Properties of Commercial Rigid Foamed Plastics ...

Tensile strength and modulus of rigid foams have been shown to vary with density in much the same manner as the compressive strength and modulus. General reviews of the tensile properties of rigid foams are available (22,59,60,131,156). 

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). 

The isocyanates used with rigid foam systems are either polymeric MDI or specialty types of TDI. Both contain various levels of polymerized isocyanate groups which contribute to molecular weight per cross-link and also may affect reactivity due to steric hindrance of some isocyanate positions. 

Packaging constitutes another significant use and is often a foam-ia-place operation to protect iadustrial equipmeat such as pumps or motors. Furniture articles molded from rigid foam are used in the form of decorative drawer fronts, clock cases, and simulated wooden beams. Flotation for barge repair and sport boats as well as insulation for portable coolers are a few other uses. 

Economics. Rigid foam systems are typically in the range of 32 kg/m (2 Ibs/fT) and, in 1992, had a foam price of about 3.63/kg ( 1.65 per lb) with hquid foam systems at about 2.75/kg. Unit prices for pour-ia-place polyurethane packaging systems fall between the competitive expandable polystyrene bead foam at 3.30/kg and low density polyethylene foams at 5.80/kg. 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

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