Cross-links biuret

Isocyanates have several more reactions that are important in some more specialized applications (Fig. 3.2). Cyclotrimerization produces the isocyanurate ring, which is extremely stable, and can be used to build more heat resistance into polyurethanes. Excess isocyanate can react with the N-H group in polyurethanes to produce allophanate crosslinks, which add to the cure of the polyurethane. And excess isocyanate can similarly react with the N-H groups in polyureas to produce biuret cross-links, which add to the cure of the polyurea. 

The reactions involved when a diamine is used in the preparation of a cast elastomer are more apparent. The amine and pre-polymer react to give an extended polymer containing urea groups, which then form biuret cross-links by reaction with terminal isocyanate groups ... 

Uncatalyzed, these reactions are quite slow below 80-100 C and may be reversed in the range 150 -200 C. Urea groups are normally much more reactive towards isocyanates than are urethane groups so that biuret cross-linking will be more favoured than allophanate cross-linking. 

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). 

With an excess isocyanate in the above systems, allophanate and biuret reactions take place (Eqs (2.25) and (2.26)), resulting in further cross-linking. When increased rigidity and high-temperature performance are desired, further crosslinking may be accomplished via isocyanurate formation (Eq. (2.29)). Base catalysts such as alkoxides, quaternary ammonium or phosphonium, etc., promote this reaction. Aromatic isocyanates give iso-cyanurates far more easily than aliphatic ones. 

Amines produce polyurethanes with better mechanical properties than when diols are used for curing. Amines produce polyurethanes with a lower temperature resistance than when diols are used. The use of catalysts has been found to direct the cross-linking reactions away from the biuret to the allophanate reactions. 

In the preparation of a prepolymer, every effort is made to prevent the formation of any unplanned branching such as biuret groups. The prepolymer is essentially linear except when some cross-link sites have been introduced by using a multifunctional isocyanate or triol. 

Some cross-linking takes place in the form of side reactions such as biuret and allophanate bonds. Deliberate introduction of covalent cross-linking can be carried out by the introduction of multifunctional agents (mainly triols) into either the prepolymer or chain extension system. 

A variety of polymeric subunits is used to make polyurethanes. These include polyesters and polyethers. The major interchain linkages are molecular forces such as hydrogen bonding and the London force. Depending on the type of chain extender and processing temperature, there also may be biuret or allophanate cross-links. 

Subsidiary chemical reactions can take place. The major of these is the formation of an allophanate cross-link, as illustrated in Figure 2.6. This reaction normally needs a temperature of between 120 and 140°C to take place. The presence of a urea group at 100°C can react with the isocyanate group to form a biuret linkage. This is shown in Figure 2.7. 

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 ... 

Isocyanates also can react with each other to produce carbodiimides with the loss of carbon dioxide. This reaction requires high temperatures unless catalyzed by specific phosphorus compounds. Formation of carbodiimides normally is not an important cross-linking mechanism in polyurethane adhesives. However, carbodiimides are sold by Dow Chemicals (Ucarlnk ), Nisshinbo Industries (Carbodilite ), and Stahl USA (XR-2569). They have been recommended as water scavengers, crosslinkers, and stabilizers for carboxyl functional polyurethanes. The carbodiimide can react with water to give a urea, which still can react with additional isocyanate to produce a biuret. 

At least 15 processes have been proposed for the synthesis of polyureas with the repeat unit —R—NH—CO—NH—. Most of these are unsuitable for commercial production because of side reactions. For example, in the conversion of diisocyanates with diamines, biuret groups readily occur, and therefore, cross-linked polymers (see also Section 28.4). Polymers from the reaction of diamines with COS cannot be obtained free of sulfur, etc. 

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