Urethane-group

Further reaction of the active hydrogens on nitrogen in the urethane groups (3) can occur with additional isocyanate (1) at higher temperatures to cause formation of aHophanate stmctures. The active hydrogens in urea groups can also react with additional isocyanate to form disubstituted ureas which can stiU further react with isocyanate to form biurets (13). 

The formation of isocyanurates in the presence of polyols occurs via intermediate aHophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibiUty is improved, and the friabiUty of the derived... 

Although the first polyurethanes were similar to that shown above, several polymers currently used contain many linkages in addition to the urethane group. Because of this the term polyurethane is now generally extended to cover all the complex reaction products of isocyanates and polyhydroxy compounds (the latter frequently known in this context as polyols). 

Pot life is several hours versus several days for conventional non-reactive hot melts. A good reactive urethane is one which exhibits a viscosity rise of less than 10%/h. The slow increase in viscosity with urethane adhesives is due to chain extension via the slow reaction of the active hydrogen of the urethane groups with... 

Polyurethane adhesives are formed by the reaction of various types of isoeyanates with polyols. The polar urethane group enables adhesion to various surfaees. Depending on the raw materials, glue lines with rubber-like elastic to brittle-hard behavior ean be aehieved. The presence of reactive terminal groups provides a ehemieally hardened adhesive. When polymerized to a high enough molecular weight, the adhesive ean be physically rather than chemically hardened, i.e. a hot melt. 

Although the name polyurethane might be taken as implying that these materials contain urethane groups (—NHCOO—) in the backbone of the macromolecule, for those polyurethanes in major commercial use this is not tme. For such materials the initial macromolecule tends to be a polyester or polyether it is the crosslinks that involve the formation of a polyurethane stmcture. 

As explained in Chapter 1, the urethane group is the product of the reaction of a hydroxy compound with an isocyanate group (Reaction 4.8). This reaction occurs by step kinetics, yet is usually an addition process since no small molecule is lost as the reaction proceeds. 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

Apart from the Important reaction leading to the formation of urethane groups, carbon dioxide can be released during curing by hydrolysis of the Isocyanate group, leading to the formation of urea groups (10). 

Crosslinking and branching can be promoted either by the use of a triol as a chain extender, or by using less chain extender than is theoretically required the unreacted isocyanate end groups then react with urethane groups in the main chain to form allophonate or biuret crosslinks. 

The stabilization of the planar conformation in these materials is due primarily to intramolecular hydrogen bonding between the N-H and C=0 (urethane) groups on adjacent R groups. The resulting polymer conformation is shown schematically in Figure 3. This explanation of the color transition is clearly evident from infrared spectra (12), which will not be discussed here. 

Figure 3. Hydrolysis of pendant urethane groups as a function of medium. Pendant —NHCOO— substrate cellulose (A) basic medium (pH — 11.3) m acidic medium (pH = 3.1) (9) deionized HsO (pH = 7.0)...

If the equilibrium constant K has a value between 1 and 10, less than a thousandth of the total amount of water formed in the reaction mixture is sufficient to prevent the formation of really high-molecular-weight condensation polymers. Hence it follows that it is extremely important to remove as completely as possible the low-molecular-weight reaction products, for example, water, eliminated during a polycondensation. In principle, these equilibriums are also known in stepwise addition polymerizations (polyaddition) like the back-reactions of urethane groups. Since they mostly occur at higher temperatures only, they can be neglected. 

The addition polymerization reaction of dihydroxy compounds with diisocyanates sets in on mixing the two components and gentle warming. Under proper conditions, linear polyurethanes with molecular weights up to about 15,000 can be obtained. As in the case of polyamides and polyesters, the softening point of the aliphatic polyurethanes depends on the number of carbon atoms between the urethane groups. 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

Hydrolytic Cleavage. As a derivative of carbonic acid the urethane group can be cleaved hydrolytically but only under rigorous conditions (40). It is more stable than an ester group but can react similarly with alcohols and amines (20, 35). However, such conditions are unlikely to prevail in a propellant. 

Thermodynamic Calculations (Specific Impulse). Prepolymers in propellant binders keep the concentration of urethane groups small. Their effect on ballistic properties is therefore negligible and need not be considered in thermodynamic calculations. The contributions of the binder to specific impulse, density, etc., are determined by the chemical backbone structure of the prepolymer. [A more comprehensive treatment of interior ballistics can be found in References 37, 42.]... 

Figure 9.7. Comparison between IR spectra of Z1031-I (upper) and Z1030/1073 (lower) coatings (a) carbonyl of urethane groups (b) carbonyl of isocyanurate ring and urethane by-products (c) NH urethane and urethane by-products (d) CH2, CH3 (e) CFo, COC, backbone (f) isocyanurate ring (g) carbonyl of ureas and urethane by-products).

N nonamide(NH)unit 1 the Contribution from the NH units in those polymers where there is no carbonyl group adjacent to NH. NH units encountered in urethane groups are also included in this category. 

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