Isocyanate, urethane bonds

Although this reaction is reversible it could be driven to the right if the dissociated fragments undergo subsequent side reactions or in some way become less available to one another for recombination. Morphology could play a role since the polymer prepared from PPG 1000 is stable at 204°C. even though it has twice the number of polyol to isocyanate urethane bonds available for dissociation. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

The reaction of an isocyanate group with an hydroxylgroup leads to the urethane bond ... 

Phenolic-isocyanates (phenolic-urethanes). The binder is supplied in three parts a phenolic resin in an organic solvent (0.8%), methylene diphenyl diisocyanate (MDI) (0.5%), and a liquid amine catalyst. When mixed with sand, the amine causes a reaction between the resin and the MDI, forming urethane bonds, which rapidly set the mixture. The speed of setting is controlled by the type of catalyst. The optimum cure temperature is 25 to 30°C. Compression strength is typically over 4000kPa (600psi). 

There has also been a preliminary study of the thermal decomposition of polyurethanes using a carbon-14 labelled isocyanate (76). Changes in intrinsic viscosity were correlated with loss of carbon-14 from the polymer. The results tended to confirm earlier conclusions that scissions occurred at the urethane bonds. 

Urethane linkages tend to dissociate above about 200°C. Fabris48 indicated that urethanes from many isocyanates and primary and secondary alcohols begin to decompose at 150°C-200°C proceeding at a measurable rate above this range. Urethane bonds decompose by the following three mechanisms ... 

Acetyl- and isocyanate-bonded chemicals did not stabilize the components degrading at 325 °C, but showed the same thermogra-vimetric and evolved gas analysis profiles as did the controls. Because ester and urethane bonds are not as stable toward pyrolysis as ether linkages at high temperatures, there was a partial release of bonded chemical at low temperatures (117). 

In bonding wood, the reaction of isocyanate groups with the numerous hydroxyl groups that are present in the various components of wood—cellulose, hemicellulose, and lignin—is possible. The product of this chemical reaction with wood is the urethane bond as shown in Reaction I. Methyl, ethyl, propyl, and butyl isocyanates... 

IR spectroscopic evidence for covalent urethane bond formation in the reaction between isocyanates and wood has been obtained. Isolation of holocellulose by the sodium chlorite method, isolation of lignin by the H2SO4 procedure, and subjecting both to IR spectroscopy indicated that isocyanates reacted with both cellulose and lignin (194),... 

Polyurethanes are produced by the reaction of diols with diisocyanates. Nucleophilic addition of an alcohol to an isocyanate produces the urethane functional group. Although no (small molecule) by-products are produced, the urethane bonds are formed in discrete steps. 

The formation of a single urethane bond by the addition reaction of hydroxyl and isocyanate functional groups is represented simply by ... 

Choice of Isocyanate-Reaction Partners that Form Bonds Other than Urethane Bonds. 

Isocyanates swell wood and react at 100° to 120° C without a catalyst or with a mild alkaline catalyst. The resulting urethane bond is very stable to acid and base hydrolysis. There are no by-products generated from the chemical reaction of isocyanate with dry wood. 

Some possible answers to the foregoing question occur. Perhaps, in spite of our efforts to dry Polymer 20 granules some tightly absorbed water remained in the polymer. This moisture would then be on site to react with the isocyanate-terminated polymer chain ends as they are generated by the thermal/shear-induced cleavage of polymer urethane bonds. Such a process would reduce polymer average molecular weight, and thus melt viscosity, until the water is used up or has escaped. 

Urea or urethane bonds by reaction of isocyanates and amines or alcohols... 

There is several research stodies carried out that made use of palm oil for PU preparation (Chian Gan 1998 Tuan Noor Maznee et al. 2001). Palm oil consists of mainly triglycerides (Tri-Gs), thus, it is necessary to introduce hydroxyl groups to a molecule of Tri-G for urethane reaction (Badri 2012). PU which sometimes is also known as urethane or isocyanate polymer has once been of the most important segments in the polymer industry for decades (Healy 1963). The onset of urethane bond dissociation is somewhere between 150°C and 220°C, depending on the type of substituent the isocyanate and polyol. Saturated hydrocarbons are... 

Besides urethane bonds, other chemical functionalities are present due to reaction of isocyanate groups with water and other compounds. In fact, urea and isocyanurate bonds are more abundant than the urethane linkages in some commercial products that are designated as polyurethanes. Allophanate, biuret, urea, carbodiimide, and isocyanurate linkages may be found. These bonds certainly affect the properties of the final polymer, but unfortunately it is almost impossible to determine them quantitatively (Figure 6). 

Other information about bonding of wood can be found in Wood adhesives - amino-plastics, Wood adhesives - isocyanates/urethanes. Wood adhesives - phenolics, and Wood composites - adhesives. 

Wood adhesives - isocyanates/urethanes A PIZZI Bonding to damp wood... 

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