Urea cross-links, formation

THPC—Amide Process. The THPC—amide process is the first practical process based on THPC. It consists of a combination of THPC, TMM, and urea. In this process, there is the potential of polymer formation by THPC, melamine, and urea. There may also be some limited cross-linking between cellulose and the TMM system. The formulation also includes triethanolamine [102-71-6J, an acid scavenger, which slows polymerization at room temperature. Urea and triethanolamine react with the hydrochloric acid produced in the polymerization reaction, thus preventing acid damage to the fabric. This finish with suitable add-on passes the standard vertical flame test after repeated laundering (80). 

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). 

Figure 15.5. Reaction sequences leading to the formation of linear and cross-linked network urea resins.

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. 

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. 

Urea-formaldehyde resin, like phenol-, or furfuryl alcohol-formaldehyde resins, is typically thought of as resulting from simple condensation chemistry. The ultimate hardening of the resin is thought to be the result of the formation of a cross-linked network brought about by acid catalysis. Current reviews are available (1, 2) which discuss this traditional preception of UF resin chemistry. 

In Chapter 2 we indicated that the formation of a polymer requires that the functionality of the reacting monomer(s) must be at least 2. Where the functionality of one of the monomers is greater than 2, then a cross-linked polymer is formed. Thermosets like phenol-formaldehyde, urea-formaldehyde, and epoxy resins develop their characteristic properties through cross-linking. In this section our discussion is confined to those polymeric systems designed with latent cross-linkability that under appropriate conditions can be activated to produce a polymer with desirable properties. 

Curing with Atmospheric Moisture. Type 2 in Table 2.5. High molecular mass polyaddition products of polyols with excess diisocyanate contain reactive isocyanate groups. They are used to formulate one-pack polyurethane paints that cross-link with the formation of urea groups under the influence of atmospheric moisture to produce paint films with excellent resistance to chemical and mechanical attack. Solvent-free products and dissolved products with isocyanate contents of 5- 15% (based on the solid resin) are commercially available. 

The reaction between urea and formaldehyde is divided into two stages. The alkaline condensation to form mono-, di-, and trimethylolureas. (Tetramethylolurea has never been isolated.) The second stage is the acid condensation of the methylolureas, first to soluble and then to insoluble cross-linked resins. On the alkaline side, the reaction of urea and formaldehyde at room temperature leads to the formation of methylolureas. When condensed, they form methylene-ether links between the urea molecules. The alkaline products from urea and formaldehyde, and from mono- and dimethylolureas, are as follows (Formula 1) ... 

---