Urethane link, formation

The types of chemical reactions that are frequently used in reactive blending can be grouped into imidization, ring opening and amidation reactions, and interchange reactions between polycondensates. Other types of reactions that are less frequently used include esterification, urea and urethane link formation, ionic bonding, and concerted addition in which a comonomer such as maleic anhydride is copolymerized with a double bond of an unsaturated polymer. 

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). 

As indicated earlier in this chapter, although diisocyanates are the intermediates responsible for chain extension and the formation of urethane links or a variety of crosslinks by further reaction, much of the ultimate polymer structure is dependent upon the nature of the components carrying the groups with which the isocyanates react initially. Such components can be simple acu-diols, such as were employed in early work on linear polyurethanes, giving polymers linked only by —NHCOO—. Examples of such polymers are listed in Table 1.2. Linear polyurethanes of this type are crystalline, fibre-forming polymers but are lower melting than the corresponding polyamides, and none has become of real importance either as a synthetic fibre or as a thermoplastic material. 

Using triplet sensitizers it has been shown that the photodegradation of polyurethane involved triplet states. In the early stages the excited states formed excimers involving polyurethane but in later stages the products of the reaction participated in excimer formation. Another investigation proposed that in the initial stages urethane links were broken by an electrophilic mechanism but that later in the reaction free radicals became important intermediates. ... 

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... 

Flexible polyurethane foams have good dry heat stability, withstanding temperatures up to 150°C for long periods without serious loss of strength. Above this temperature the formation of biuret and allophanate cross-links is reversed and there is a reduction in load bearing properties. Urethane links are stable up to about 200°C. Exposure to ultraviolet light causes discoloration but no apparent deterioration in physical properties. 

The term polyurethane is used to describe polymers which have been prepared by uniting intermediates containing at least two hydroxyl groups with isocyanates via the formation of a urethane link. This is a rearrangement reaction and unlike condensation polymerizations does not lead to the splitting out of a small molecule such as water. In the case of a dihydroxyl compound and a diisocyanate the reaction leads to the formation of a linear polymer ... 

The second component in the two pack is a polymeric multi functional isocyanate. The isocyanate group is extremely hydrogen acquisitive and will therefore abstract hydrogen from the hydroxy functional acrylic resin, forming a urethane link in the process. The multi functionality of the isocyanate ensures the formation of a network structure. Reactions take place at ambient temperatures or can be forced along at slightly above ambient temperatures, e.g. 60°C for 20 minutes as a typical cure cycle. 

The addition polymerization of diisocyanates with macroglycols to produce urethane polymers was pioneered in 1937 (1). The rapid formation of high molecular weight urethane polymers from Hquid monomers, which occurs even at ambient temperature, is a unique feature of the polyaddition process, yielding products that range from cross-linked networks to linear fibers and elastomers. The enormous versatility of the polyaddition process allowed the manufacture of a myriad of products for a wide variety of appHcations. 

The water reaction evolves carbon dioxide and is to be avoided with solid elastomers but is important in the manufacture of foams. These reactions cause chain extension and by the formation of urea and urethane linkages they provide sites for cross-linking, since these groups can react with free isocyanate or terminal isocyanate groups to form biuret or allophanate linkages respectively (Figure 27.5). 

Network Formation and Degradation in Urethane and Melamine-Formaldehyde Cross-Linked Coatings... 

Other reactions which lead to branching and cross-linking are the formation of allophanate and biuret linkages. The allophanate linkages occurs when the hydrogen on the nitrogen atom of the urethane group reacts with an isocyanate ... 

The exceptional stability of the urethane bond results from thermodynamic determinants of bond strength, from the crystallisation that results from the structure of both the polyol and the strongly hydrogen bonding urethane and from cross-links that result from the formation of hard chain segments (Evans and Greene, 1982). 

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