Isocyanate polyol reaction

Chem. Descrip. Organotin Uses Catalyst for PU coatings, adhesives, sealants Features Provides delayed action catalysis of isocyanate/polyol reaction Properties Lt. yel. liq. sol. in alcohols, org. soivs., insol. in water sp.gr. 1.11 vise. 33 cps f.p. -23 C decomp. pt. 255 C flash pt. (PMCC) 130 C 17.5% total tin... 

Catalysts (Figure 3.5). Isocyanate + polyol reactions go quite rapidly at room temperature. Isocyanate + amine reactions go rapidly at room temperature. However, most processors add catalysts to make the polymeiization/cure reactions even faster and to control the foaming process. 

Isocyanate/Isocyanurate. Isocyanates react with polyols to form rigid polyurethane foams, a major type of thermoset plastics. While these are very useful in thermal insulation, they are limited by failure at high temperature and by flammability. One way to solve these problems is to convert part of the isocyanate to isocyanurate by cyclotrimeriza-tion (Fig. 3.64). Whereas the isocyanate-polyol reaction forms polyurethane rapidly at room temperature, the cyclotrimeiization of isocyanate to isocyanurate requires strong alkaline catalysis and heat to compete successfully. The resulting isocyanurate rings build considerable heat resistance (150 to 250°C, short-term <800°C) and flame-retardance into the polyurethane foam. They are useful for insulating pipelines and boilers. 

Amine catalysts are primarily used to catalyze the isocyanate-water reaction ( blowing catalyst ), while tin or other metal catalysts are used to regulate the rate of the isocyanate polyol reaction ( gelling catalyst ). Surfactants are used up to 2 pph (parts per hundred) to regulate the cell size. Higher amounts of the surfactant produce thinner cell walls and smaller cells. An excessive amount would cause collapse of the foam as the walls and ribs of the foam cells could not support the pressure of the gas. 

It will be noted that in the production of rigid foam it is not necessary to use a complex catalyst system. Commonly tertiary amines are used alone to catalyze the isocyanate-polyol reaction metal catalysts are not widely used. 

The rapidity with which isocyanate-polyol reactions take place has been used to advantage in reaction injection molding (RIM), In the typical process (Figure 14.28), two components are mixed by high-pressure injection into a special chamber and injected almost immediately into a closed, low-pressure mold. The finished parts can be cellular or solid depending on the amount of material injected... 

Differences In Crosslink Chemistry. The main crosslinking reaction In Isocyanate-polyol coatings Is the reaction of the Isocyanate group with hydroxy groups to form a urethane crosslink. 

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

First-generation solventless polyurethane adhesives are one-component isocyanate terminated prepolymers formed by the reaction of MDI (4,4 methylene bis (phenyl isocyanate)), or other isocyanates with polyether and/ or polyester polyols. One-component 100% solids adhesives rely on moisture from the air or substrates or from induced moisture misting during the converting process, to cure the adhesive via an isocyanate/water reaction and subsequent polyurea-polyurethane polymer formation. Typically the high viscosity of the adhesive is such as to require adhesive delivery equipment and application rollers heated from 65-80 °C for use. They have a high level... 

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... 

Polyurethane chemolysis can be performed by processes similar to those applied to PET. Thus, polyurethane glycolysis yields a mixture of polyols, which can be reused in the formulation of new polyurethanes. Likewise, polyurethane hydrolysis leads to the formation of polyols, diamines and carbon dioxide. The diamine can be subsequently transformed into the corresponding isocyanate by reaction with phosgene, whereas the polymer-... 

Catalysts. To make the final polyurethane, common catalysts are tertiary amines and organotin compounds, often used in mixtures. Amine catalysts favor the reaction of isocyanate with water, producing urea linkages and CO2, which acts as a blowing agent. On the other hand, organotin catalysts favor the isocyanate/hydroxyl reaction. Additional catalysts and catalyst decomposition products may be present if graft polyols are part of the mixture. The most common of these catalysts are free radical initiators. Phosphorus compounds may be present in the case of carbodiimide-modi-fied MDI. 

In addition, it was observed that the values of k were reduced in the case of lower molecular weight polyols. In this case, phase separation occurred rapidly, which retarded the progress of isocyanate-hydroxyl reactions by entrapping unreacted isocyanate groups and thereby reducing the rate of conversion (Figure 3.5). 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

The increase in viscosity from 0% PFA to 30% PFA is of the order of 400-500% for the polyol while it is nearer 1000% for the isocyanate. This will create problems of mixing in the PFA given that the chemicals (polyol and isocyanate) commence reaction 20 seconds after mixing. 

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