Isocyanate-based

Applications for blocked urethane adhesives are small. However, they may be used as flocking adhesives or as crosslinkers for solvent-borne adhesives. Blocked urethane adhesives are also used as splicing adhesives for belts. Blocked isocyanates based on TDI, IPDI, and derivatives of hexamethylene diisocyanate are most commonly commercially available. 

The effective development of blocked isocyanate based coatings requires a complete understanding of the cure chemistry. Many materials have been identified which will improve cure response. Often these are reported as deblocking catalysts even though no direct evidence exists to support this claim. Most of these materials are well known catalysts of the cure reaction between hydroxyl and isocyanate and may be improving cure response solely by catalyzing this reaction. Effective development of catalyst systems requires a better understanding of the effect of catalysts on cure. 

Combinatorial approach to unsymmetrically tiered macromolecules [214] is a brand new area of research which would allow dendrimerization of materials (e. g., glass, classical polymers, fibers) and thus enable fine tuning of macromole-cular properties. For example, treatment of an amine terminated dendrimer with a mixture of complementary, isocyanate-based monomers [215, 216] affords a heterogeneous surfaced dendrimer. Selective transformation of the surface nitrile moieties via metal-catalyzed reduction to obtain a new polyamine dendrimer allows further combinatorial-based elaboration as illustrated in Fig. 42. 

Fig. 42. Combinatorial tier construction with isocyanate based dendritic monomers...

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

Isocyanate-based resins, in paper manufacture, 13 116 Isocyanate curing agents, 10 410 Isocyanate group, urethanes and, 22 36 Isocyanates... 

The results of dT/dt and T measurements made during curing of two systems - macro(di-isocyanate) based on poly(tetraethylene glycol) and 2,4-toluylene diisocyanate cured by diamine (MDA) and epoxy diene oligomer cured by triethanol(amine titanate) are shown in Figs. 3.9 and 3.10, respectively. The monotonic increase in the parameters, monitored in the experiment,... 

The synthesis of CP-96,345 (2) in optically active form, which relies on a chiral isocyanate-based resolution, is shown in Scheme 1. 

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. 

There are other proprietary systems such as polyacrylamate. It is Ashland Chemical s Airmax that is designed for use with preforms or glass mats. These reinforced plastics possess high flexural modulus, good impact resistance, and high temperature stability. Systems with similar performance from isocyanate-based polymers are also used. 

Chemistry MDl-based isocyanate prepolymer, polyether and/or polyester polyols, reacts with water to form polyurea-polyurethane backbone Often MDl-based isocyanate prepolymer (some are ahphatic isocyanate based), polyether and/or polyester polyols (some contain epoxy based materials), reaction forms polyurethane backbone MD1-, TDl- and some aliphatic isocyanate-based prepolymers, polyether and/or polyester polyols, reaction forms polyurethane backbone... 

Several other researchers have studied the effect of yellowing in aromatic isocyanate-based polyurethanes, and the effect of yellowing in aromatic isocyanate-based polyurethanes has also been studied. In general, products based on aromatic isocyanates are suitable only for indoor applications. 

Isocyanate-Based Adhesives. Generally diisocyanates or polyisocyanates are used in the preparation of polyurethane adhesives. The urethane link (-O-C(O)-NH-) is formed by the reaction of an alcohol with an isocyanate (-N = C = 0) group ... 

On the basis of these observatioqs and considerations, Baker and Holds-worth postulate that the catalyzed reaction depends on the reaction of the alcohol molecule with the complex formed from the isocyanate and the base. The formation of the isocyanate-base complex is a reversible reaction and its rate has to be taken into consideration in developing the over-all rate equation,... 

While the experimental confirmation of Equation (7) was considered by Baker and co-workers as supporting the proposed mechanism, the question was immediately raised as to why the fci s, the rate constants for the formation of isocyanate-base complex, should be dependent on the nature of the alcohol. 

According to the first hypothesis, the amine complexes with the alcohol and both the amine and this complex can function as catalysts for the reaction, the same isocyanate-base complex resulting from attack by either catalytic species. Therefore, the original mechanism is really a composite one of the following two steps ... 

In the presence of a tertiary base (B), there is a base-catalyzed reaction between the amines and isocyanate superimposed on the spontaneous and the product-catalyzed reactions. According to Baker and Bailey, the following steps are involved in the base-catalyzed reaction. First there is the reaction involving the isocyanate-base complex and the amine ... 

While Baker and Gaunt postulate that the mechanism of the base-catalyzed reaction of isocyanates and alcohols involves the attack of the isocyanate-base complex by the alcohol, according to Iwakura and Okada the higher acidity of the active hydrogen containing compounds changes... 

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

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. 

A number of isocyanate reactions for making isocyanate-based polymers are listed in Table 2 which is expressed by model reactions. 

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. 

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... 

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. 

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 ... 

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. 

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. 

---