Biurets isocyanates

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). 

An excess of phosgene is used during the initial reaction of amine and phosgene to retard the formation of substituted ureas. Ureas are undesirable because they serve as a source for secondary product formation which adversely affects isocyanate stabiUty and performance. By-products, such as biurets (23) and triurets (24), are formed via the reaction of the labile hydrogens of the urea with excess isocyanate. Isocyanurates (25, R = phenyl, toluyl) may subsequendy be formed from the urea oligomers via ring closure. 

Some of these isocyanates are commercially available in derivatized form, such as biurets and carbodiimides, to provide materials having improved handling or processing characteristics. 

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). 

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]. 

If an isocyanate group reacts with a hydrogen within the polyurea structure, a branching point is formed, a biuret group ... 

During the hardening of PMDI urethanes, polyureas, biurets and triurets/poly-urets have been found [16,17,141,142]. The proportions of the various compounds depend on the working and hardening conditions. The forming of the network is especially influenced by the ratio between isocyanate and water. 

From these, prepolymers are prepared where the diisocyanates may be completely reacted as in the case of the urethane oils which resemble the oil-modified alkyds but have urethane (—NHCOO—) links in place of the ester (—COO—) links of the alkyds, or where one only of the isocyanate groups is combined, leaving the other to participate in crosslinking reactions. Such a reactive prepolymer is the biuret that may be prepared from hexamethylene diisocyanate, has the following structure ... 

Amines, too, possess active hydrogens in the sense required for reaction with an isocyanate group. Thus the products of Reaction 4.10 react further to yield substituted ureas by the process shown in Reaction 4.11. Reaction can proceed still further, since there are still active hydrogens in the urea produced in Reaction 4.11. The substance that results from the reaction between an isocyanate and a urea is called a biuret (see Reaction 4.12). 

Both pigmented and unpigmented polyurethane paints have been prepared using a polyester resin containing hydroxyl functional groups and the biuret trlmer of hexamethylenedllsocyanate as a crosslinker. The molar ratio of hydroxyl/isocyanate has been chosen 1.0 and the pig-ment/binder ratio 0.6. 

Moreover, triuret, ammeline, ammelide, melamine and other products may be formed from isocyanic acid, biuret and combinations of them. If urea is heated up very fast, these reactions are suppressed and the decomposition into ammonia and isocyanic acid is the preferred reaction. Due to the high reactivity of isocyanic acid, its primary formation may subsequently lead to the formation of the aforementioned compounds of higher molecular weight. In order to avoid the formation of by-products, the heating-up must be carried out fast. Only then ammonia and isocyanic acid are obtained as sole products. In any case, local undercooling of the gas duct should be avoided and rapid dilution of the thermolysis products in the exhaust gas has to be ensured in order to avoid locally high concentrations of reactive compounds. 

Examples of non-urethane linkages derived from isocyanates a) urea, b) urea, c) biuret, d) amide, and e) allophanate... 

In reactions (3) and (4) the isocyanate is capable of reacting with the active hydrogen in a urethane, or urea group, to give branching, or crosslinking by the formation of an allophonate or a biuret group. 

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. 

One can observe positive deviations in the region of rH < 1 (excess of isocyanate groups) which are due to side reactions (allophanate, urea and biuret groups). In the region of rH > 1 the agreement of wg values is good. In the case of i e, the predicted curves depend not only on the results of the branching theory but also... 

Loblolly pine modified by 1,6-diisocyanatohexane (HDI) was found to be resistant to attack by G. trabeum at a WPG of 26 % (Chen, 1992c). At 26 % WPG, 6 % of bonded chemical was lost during a 12-week soil decay test. When moist wood was used for reaction, the HDI reacted mainly to form ureas and biurets. It was stated that the decay resistance of HDI modified wood was probably due to the inability of the modified cell wall to absorb sufficient amounts of water to support decay. Although wood reacted with chloro-sulphonyl isocyanate lost only 1.3 % mass when exposed to G. trabeum in a decay test, it was reported that 50 % of the bonded chemical was lost in this test. 

Isocyanate Crosslinkers. A wide variety of both aromatic and aliphatic Isocyanate crosslinkers are used in coatings (4). Aliphatic isocyanates are used when external durability is required. The isocyanate crosslinker studied in this work is the biuret of hexamethylene diisocyanate (Figure 1). Although resins based on triisocyanurates have been claimed to be superior in durability (,22.) > biuret based triisocyanates are more commonly used. Urethane coatings are generally formulated with a ratio of isocyanate to hydroxy of around 1 1. 

The situation is even more complex since the N—H bonds of both urethane and urea linkages add to isocyanate groups to form allophanate and biuret linkages, respectively. These... 

The subsequent crosslinking probably occurs by reaction of the hydrogen atoms of the resulting urea groups with isocyanate groups still present in the starting polymer or the chain-extended polymer, with the formation of biuret groups ... 

Isocyanates are produeed almost exclusively by the reaction of amines with phosgene (COCy, with the speeifie reaetion eonditions varying particularly for aromatic and aliphatic isocyanates (Chadwick and Cleveland 1981 Codd et al. 1972 Ulrich 1989). Aliphatic diisocyanates are produced by reaction of phosgene with either a slurry of the carbamate salts obtained in the reaction of the aliphatic diamines with earbon dioxide, or with a slimy of the amine hydrochloride (Ulrich 1989). Hexamethylene diisocyanate (HDI) is produeed by the reaction of phosgene with the amine salt (Chadwick and Cleveland 1981). The trimerie HDI biuret (HDI-BT), which has a low monomer content and is widely used in the formulation of exeeptionally high quality polymer coatings, is produced by controlled reaction of HDI with water, a water generator, or an amine (Chadwick and Cleveland 1981). 

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. 

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. 

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