Crosslinking allophanate

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

The chemistry of the glycolysis of polyurethanes is complicated by the fact that there are additional groups in the polymer such as ureas, allophanates, and biurets, and die PURs may be crosslinked. In die presence of the appropriate glycols and at about 200°C, PURs undergo transesterification to form polyols. Under the same conditions, ureas undergo glycolysis to form urethanes and amines (Fig. 10.5). 

The extent of crosslinking in polyurethanes depends on a combination of the amount of polyfunctional monomers present and the extent of biuret, allophanate, and trimerization reactions [Dusek, 1987]. The latter reactions are controlled by the overall stoichiometry and the specific catalyst present. Stannous and other metal carboxylates as well as tertiary amines are catalysts for the various reactions. Proper choice of the specific catalyst result in differences in the relative amounts of each reaction. Temperature also affects the extents... 

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. 

The isocyanate (2270 cm"1) uretedinedione ring carbonyl (1780 cm"1) and urea carbonyl (1660 cm"1) groups can usually be identified. Carbonyls from ester, urethane, allophanate, isocyanuric acid ring and Biuret groups all absorb near 1730 cm 1 and are difficult to distinguish. Hydrogen bonds which can function as physical crosslinks in PU have been... 

This is an interesting comparison with regard to control of polymer crosslinking by biuret- or allophanate-forming side reactions. For example, 1,5-naphthalene diisocyanate showed the greatest tendency toward alloph-anate formation, and hexamethylene diisocyanate the least. 

Other hydrolyzable groups consist of biuret and allophanate crosslinks formed from the reaction of Isocyanate radicals with active hydrogen atoms of the urea and urethane linkages, respectively. 

However, another factor that could contribute to the Initial improvement In physical properties of the polyurethanes Is that the allophanate and biuret crosslinks, which are highly susceptible to hydrolysis, may be early victims of the degradation process. 

Formation of allophanates and biurets in polyurethane chemistry, especially when an excess of isocyanate is used, is in fact a supplementary source of crosslinking. 

The concentrations of the allophanate links varies with the time of cure. Also, if the crosslinking reactions are conducted in inert nitrogen atmospheres, veiy little scission of crosslinks takes place and a network structure forms during the cure. In open air, however, the scissions of crosslinks are extensive and the products have poorer physical properties. 

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. 

Yashitake and Furukawa investigated the thermal degradation mechanism of a.y-diphenyl alkyl allophanates and carbanilates as model compounds for crosslinking sites in polyurethane networks by pyrolysis-high-resolution GC/ FTIR (Py-HR GC/FTIR). Pyrolysis was performed at 250°C, 350°C, 450°C, and 500°C. 

When the difunctional precursors (diisocyanate, polyol and extender) are allowed to react in a stoichiometric amount, a thermoplastic PU is formed. Thermosetting PU are made by using excess diisocyanate (excess diisocyanate reacts with a urethane structure to form allophanate bonds) or by using a trifunctional extender like glycerin or trimethylol propane [92-94]. The unique feature of PU resin is that the change in UPE between crosslink offers a wide change in properties, especially the strain (which reflects flexibility). For example, a PU system with a molecular weight between two... 

The conventional mixture of urethane, biuret and allophanate crosslinks formed during the final chain-extension step by means of the standard 16h at 120 C hot-air cure. 

Acids influence the NCO/OH reaction by accelerating chain extension a little, and retarding crosslinking. If p-nitrobenzoylchloride is added to a urethane system in which active hydrogen compounds must be present, this additive has a mild catalytic effect on chain extension, no effect on allophanate formation, and a strong retarding effect on biuret formation. If water is present the reaction is strongly catalyzed. 

The network structure of linear and branched TPUs is obviously different. The branched thermoplastics are capable of forming allophanate and/or possibly biuret crosslinks under suitable conditions. These conditions are partly met by the processing temperatures but usually a subsequent hot-air cure is necessary to achieve the optimum set properties. Inevitably, therefore, the branched TPUs have considerably lower compression and tension set properties than the truly all-linear thermoplastic TPUs, and in this respect approximate to the castable polyurethanes. 

The basic polymer structure, which results from the formation of urethane groups and may be linear or crosslinked, largely determines the polymer s properties. However, other than these primary reactions, secondary reactions such as biuret, allophanate, dimer and trimer formation also have an effect on the properties of the polymer and their presence can also be detected by IR techniques. 

Isocyanate-urethane and isocyanate-urea reactions then yield biurets and allophanates, which produce crosslinking and branching ... 

These products can be broadly defined as coatings that contain the urethane or urea groups. To a lesser extent, groups such as allophanate or biuret can be present. They are available one or two component systems, able to cure at room or higher temperature. These coatings can be based on linear PU dispersions, or crosslinkable dispersions and they can also be produced by solvent free processes. 

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