Isocyanate-based foams

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates. TDI (toluene diisocyanate) is widely used for flexible foams. Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings. Modified TDI and modified MDI are used for high-resilience flexible foams. Polymeric isocyanates (polymeric MDI or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams. 

The first method of making isocyanate-based foams was based on the reaction of a carboxyl-terminated polyester with an organic diisocyanate, e.g., toluene diisocyanate. The simultaneous reactions resulting in carbon dioxide generation and polyamide formation produced cellular plastics. 

Both methods could be recognized as a new combination of a known gas-generation and a known polymer-formation reaction. Since then, a variety of isocyanate-based foams were developed as described below. New combinations of new gas-generation reactions and a known polymer formation lie outside of the prior art. This new area was studied by Ashida and his collaborators, and the work was reviewed (33). 

Isocyanate-based foams include polyurethane, polyisocyanurate, polyurea, polycarbodiimide, polyamide, polyimide, and polyoxazolidone foams. 

The major raw materials for making isocyanate-based foams include the following compounds polyisocyanates, polyols, catalysts, blowing agents, surfactants, epoxides, and flame retardants. 

Flame Retardants. Among the isocyanate-based foams, polyurethane foams, both flexible and rigid, are flammable. Due to serious fire hazards of polyurethane foams, strict fire regulations have come out on the use of foams in the areas of furniture and public transportation. In addition, the use of rigid urethane foams in building insulation have resulted in stricter fire regulations. 

Figure 26 shows a comparison of bum-through time between the two foams based on polymeric isocyanate (polymeric MDI) and a TDI-prepolymer (65). In the case of polymeric isocyanate-based foams, a remarkable increase in bum-through time resulted after increasing the NCO/OH ratio, which reached a maximum value at 3.0. 

Therefore, the preparation of foams with a high percentage of closed cells or high-density foams is difficult. These foams are quite different from other isocyanate-based foams, e.g., urethane foams, isocyanurate foams and oxazolidone foams. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

An ingeniously simple screening method was used by Britain and Gemeinhardt [146] to evaluate catalysts for the isocyanate/hydroxyl reaction. To approximate as closely as possible actual polymerization conditions, the 80 20 ratio of 2,4- and 2,6-tolylene diisocyanate (80 20 TDI) isomers and a polyether triol of 3000 molecular weight were mixed at NCO OH ratio of 1.0. A 10% solution of catalyst in dry dioxane was added, the final catalyst concentration being 1% of the weight of polyether. The time for the mixture to gel at 70°C was noted as an indication of catalytic strength. This technique used the same reactants employed in one-shot flexible polyether-based foam systems, almost completely eliminated solvent, and was used to screen quickly hundreds of possible catalysts. 

Many professional books on isocyanate-based plastic foams are available (1-24, 36, 115, 116, 227, 228, 229). 

Other types of isocyanate-based polymer foams, such as polyiso-cyanurate foams modified by oxazolidone, carbodiimide or imide linkages, have outstanding properties in flame retardance and fire endurance without the addition of any flame retardants... 

Manufacturing Process. Both prepolymer and one-shot processes are available, but the polymeric isocyanate-based one-shot process is used in preference because of the easy processing due to the low viscosity of the system, relatively low toxicity of polymeric isocyanates, and fewer environmental problems. However, a disadvantage of the one-shot process is a possible risk of shrunken-foam formation due to its higher closed-cell content. In contrast, the TDI-based prepolymer process has advantages including better in-mold flowability and higher open-cell content. 

Polymeric MDI—One-Shot Process. Polymeric isocyanate-based fluorocarbon-blown rigid urethane foams are the most widely used rigid urethane foams at the present time because the foams have a high thermal... 

In this section isocyanate-based polyurea foams ja-oduced by the reaction of the water-isocyanate or amine-isocyanate reactions will be described. Urea-formaldehyde foams will be excluded. The isocyanate-based urea-linkage formation is shown by the following model reactions ... 

In this section, isocyanate-based imide foams, according to model reaction (2) will be described, and other imide foams prepared by model reaction (1) will be described in a separate section. 

Methods for making isocyanate-based polyimide foams include a one-shot process by admixing carboxylic dianhydrides with an organic polyisocyanate at room temperature in the presence of a dipolar aprotic organic solvent (103, 106, 142, 251). The resulting foams from this method exhibited outstanding thermal resistance, as shown in Figure 47. 

It has been demonstrated that a variety of enzymes exhibited enhanced mechanical and chemical stability when immobilized on a solid support, producing a biocatalyst. Munnecke first immobilized a pesticide detoxification extract from bacteria by absorption on glass beads. The absorbed extract retained activity for what was then a remarkable full day. Wood and co-workers, using isocyanate-based polyurethane foams (Hypol ), found that a number of enzymes unrelated to OP hydrolysis could be covalently bound to this polymer. Later, Havens and Rase immobilized a parathion hydrolase. Furthermore, Turner observed that polyurethane foams are excellent adsorption materials for OP such as pesticide vapors. ... 

Isocyanates are formed by reacting phosgene with an appropriate hydrocarbon substrate. Many isocyanates are possible depending upon the hydrocarbon starting material. The commercially important polyurethanes are manufactured from toluene diisocyanate, based on toluene, and methylene diphenyl isocyanate, based on aniline. Both toluene diisocyanate (TDI) and methylene diphenylene isocyanate (MDI) can be used to manufacture foamed products, but only MDI is used as the primary feedstock for elastomeric polyurethanes. 

Major polyurethane products today include cellular materials such as water-blown flexible foams or fluorocarbon-blown rigid foams, elastomers, coatings, and elastic fibers, which are described subsequently. Closely related to polyurethanes is an isocyanate-based product called isocyanurate foam. 

An amide-based ohgomer containing pendant carboxylic acid and a polyfimctional aziridine crosshnking agent is used for production of non-isocyanate spray foam. Talc is used as nucleating agent. ... 

The basic reaction involved in the formulation of PU is between hydroxy compounds and isocyanates. The first record of this reaction was made by Wurtz and Hoffmann in 1848. Commercially significant advances did not occur until 1937, when 0. Bayer of Germany developed diisocyanate. Bayer and co-workers continued the development of polyester-based urethane polymers as nylon substitutes in the 1940-45 time period. Polyester toluene-diisocyanate (TDI) foams became commercially available in the United States in 1952-53. Two further developments occurred that allowed the use of PU foam to reach the present level. The first of these was the development of polyether polyol and the subsequent "one-shot" foaming process. Rigid polyether-based foams were available in 1957. The second development was the incorporation of halocarbons as blowing agents in 1958. 

The use of PMDI as a binder for foimdry cores, rubber waste products, and solid rocket fuel are also known. Isocyanate-terminated prepolymers, often prepared from TDI or MDI with polyether polyols are also used as binders for composite products that require elastomeric properties. Athletic surfaces are sometimes prepared from groimd rubber tire scrap bonded with isocyanate-based prepolymers. Similarly, flexible polyurethane foam scrap is bonded with isocyanate prepolymers to form rebonded foam usefiil as carpet imderlay. Solidification of incineration ashes with PMDI-based binder systems is another waste disposal application. In this manner hazardous waste materials imdergo chemical fixation and detoxification. 

The techniques have also been applied to isocyanate- and ester-based foams [52]. See also Section 9.4.6. 

The majority of hydroxy groups in the polyesters used for foam making are primary groups and are comparatively reactive towards isocyanates. It is therefore sufficient to include only simple tertiary amine catalysts in the formulation to obtain a satisfactory foam. The reactions which occur during the foaming process are the same as those described for polyether-based foams. 

---