Polyurea foams

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  

Reaction (a) was applied in preparing low-density packaging foams. Reaction (b) was recently used for preparing low-density flexible urea foams (81, 141) 

Some examples of urea foams prepared by the water-isocyanate reaction are as follows. ICI disclosed the foam prepared by the water-isocyanate reaction in the jn-esence of imidazole compounds (137). PRB NV disclosed a foam j epared in the presence of water-soluble saccharide and polyol (139). Bayer AG disclosed a foam prepared by using 1.5 to 50 parts of alkanolamine with water and 100 parts of polyisocyanate (138). Schaum Chemie disclosed foams prepared by using lower alkanols and alkylene diols (140). 

The urea linkage is thermally more stable than the urethane linkage due to its higher content of hydrogen bonding and urea foams are suitable for higher-temperature-service applications. 

Priester et al (141) have found a method of controlling the amine-isocyanate reaction by using substituted-amine-terminated polyethers, and they obtained high-resiliency polyurea foams by this method. Ashida et al (81) prepared low-density flexible polyurea foams by using a primary-amine-terminated polyether (Jefhunine D-2000, Texaco Chemical). 

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Polyisocyanurate foams, polyurea foams and phenolic foams are growing rapidly in recent years. Urea-formaldehyde foams disappeared recently from the U.S. market. Rubber foams and pyranyl foams are no longer available in the worldwide market. 

Physical blowing agents may be classified as CFCs (chlorofluoro-carbons), HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbon ethers) and non-fluorine-containing organic liquids. These fluorinated blowing agents can also be used in foaming polyisocyanurate foams, polyoxazolidone foams, and polyurea foams. 

Polyurea foams discussed in Chapter 2 above can be used for retrofitting of cavity walls. 

Uses Curative for cast elastomers and PU crosslinking agent for polyurea foam liq. replacement for methylene bis-2-chloroaniline... 

Borsus JM, Merckaert P, Jerome R, Teyssier Ph (1982) Catalysis of the reaction between isocyanates and protonic substrates. II Kinetic study of the polyurea foaming process catalysed by a series of amino compounds. Journal of Applied Polymer Science 27 4029-4042. 

The newer open-ceU foams, based on polyimides (qv), polyben2imida2oles, polypyrones, polyureas, polyphenylquinoxalines, and phenoHc resins (qv), produce less smoke, are more fire resistant and can be used at higher temperatures. These materials are more expensive and used only for special appHcations including aircraft and marine vessels. Rigid poly(vinyl chloride) (PVC) foams are available in small quantities mainly for use in composite panels and piping appHcations (see Elame retardants Heat-RESISTANTPOLYA rs). 

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

In a further vtu iation developed by Bayer, hydrazine (NH2NH2) is dissolved in the polyol and then allowed to react during the foaming stage with some of the 80/20 TDI present. This is of the form of reaction (2) shown in Section 27.2 and this leads to a polyurea of general form ... 

There is also growing interest in multi-phase systems in which hard phase materials are dispersed in softer polyether diols. Such hard phase materials include polyureas, rigid polyurethanes and urea melamine formaldehyde condensates. Some of these materials yield high-resilience foams with load deflection characteristics claimed to be more satisfactory for cushioning as well as in some cases improving heat resistance and flame retardancy. 

Another family of polyols is the filled polyols.llb There are several types, but die polymer polyols are die most common. These are standard polyether polyols in which have been polymerized styrene, acrylonitrile, or a copolymer thereof. The resultant colloidal dispersions of micrometer-size particles are phase stable and usually contain 20-50% solids by weight. The primary application for these polyols is in dexible foams where the polymer filler serves to increase foam hardness and load-bearing capacity. Other filled polyol types diat have been developed and used commercially (mainly to compete with die preeminent polymer polyols) include the polyurea-based PEID (polyhamstoff dispersion) polyols and the urethane-based PIPA (poly isocyanate polyaddition) polyols. 

Example 5. Glycolysis of Polyurethanes with Propylene Oxide after Pretreatment with Ethanolamine.55 A rigid polyurethane foam (ca. 100 g) was dissolved in 30 g ethanolamine by heating. Excess ethanolamine was stripped, leaving a clear solution. Infrared and GPC analysis indicated that the clear solution obtained contained some residual polyurethane, aromatic polyurea, aliphatic polyols, aromatic amines, and N,N -bis(f -hydroxyethyljurea. Next the mixture was dissolved in 45 g propylene oxide and heated at 120°C in an autoclave for 2 h. The pressure increased to 40 psi and then fell to 30 psi at the end of the 2-h heating period. The product was a brown oil with a hydroxyl number of485. 

The nature of a hydrophilic prepolymer permits the addition of large amounts of water. The isocyanate reacts with the water to abstract COt. The amine that also results from the reaction then reacts with an isocyanate group to produce a urea linkage. The reaction continues until the water or the isocyanates are consumed. If provisions are made to trap the CO2 in the mass, a foam is produced. If such provisions are not made, the CO, will bubble away, leaving behind a low gel-strength hydrogel. Careful examination of the resultant molecule might cause one to rename it a polyurethane/polyurea. 

Flexible PU foams are prepared from basically the same raw materials as PU elastomers. Instead of a short-chain extender, water is used. Water reacts with isocyanates to form an amine and C02 (Eq. (2.22)), which results in foaming. The diamine thus created can then react with isocyanate groups giving polyurea short segments. A surfactant is also introduced as a cell control agent. 

This reaction is often employed in the production of flexible urethane foams, which are frequently block copolymers of polyether or polyester segments joined to polyurea segments. (The polyester or polyether segments terminate in urethane segments resulting from reaction of polyether or polyester hydroxyl end groups with the isocyanate.)... 

Careful studies of polyurea formations are generally quite difficult. Low-molecular-weight disubstituted ureas have such low solubility in most solvents that precipitation causes experimental difficulties the problem is even worse with many polyureas. In addition the rather limited thermal stability of polyureas at temperatures of about 200°C has detracted from what otherwise might have been a significant interest in these polymers for fibres. The major commercial interest has not been in pure polyureas, but rather in polymers containing polyurea blocks, as in water-blown polyurethane foams and in polyurea—urethane elastomers. The complexity of these commercial systems, which are nearly always crosslinked, has made kinetic studies difficult. 

An unusually valuable study of the kinetics of polyurea—urethane formation from systems normally used to prepare water-blown polyurethane foams was described by Hartley et al. [195]. Some typical systems were found to consist of a single phase initially, while certain others were complicated by the existence of two phases, with phase inversion occurring. Studies included measurements of the rate of heat evolution, gas evolution and viscosity increase, as well as analysis for the presence of reaction products after intervals of time. 

They found the heat of reaction for 80 20 TDI and water to be 38.3 1.74 kcal mole", and that for 80 20 TDI and a poly(oxy-propylene)triol to be 42.5 0.76 kcalmole" (with two equivalents of NCO per mole in each case). Both separate reactions gave data which fit second-order kinetics up to 50% reaction. Preliminary attempts at calculating kinetic coefficients for both reactions during polyurea—urethane foam formation were given. It is believed that further efforts along this line will be needed to clarify the kinetics satisfactorily, however. 

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

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