Allophanate groups

Polyisocyanate types with free, excess isocyanate groups include adducts of diisocyanates with polyols, isocyanurates formed by trimerization of diisocyanates, and high molecular mass products containing biuret or allophanate groups. Monomeric MDI and its mixtures with oligomers have a low vapor pressure and can be safely handled at room temperature. 

A typical polyurethane adhesive may contain, in addition to the urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, and allophanate groups. Polyurethanes are formed by the addition reaction of diisocyanates or polyisocyanates with polyols (Figure 3.12) through a step-growth polymerization... 

Although it is expected that difunctional monomers will give a linear polyurethane, the polymerization reaction is subject to possible side reactions. The formation of allophanate groups can occur, particularly if reaction temperatnres exceed 400 K. Here, an isocyanate group adds onto the secondary amine in the methane link, and a branched or cross-linked structure is formed. 

Thus, the reactions of branching and formation of the network polymer take place on the siuface of MgCl2. The orienting effect of the boundary surface with a sohd body in the pol50irethane formation reaction may not only accelerate the principal process but also facilitate the course of side-reactions (in the case considered, these are the reactions of formation of branching through the allophanic groups). 

Since 2-9% excess isocyanate was used to prepare the polymers in this study, the polymers undoubtedly contained some allophanate groups. The effect of excess -NCO content compared to hydroxyl number of the polybutadiene on tensile properties of the polyurethane are shown in Figure 1. As the -NCO content increased, the ultimate tensile strength, a f passed through a maximum at about 6% excess -NCO and then stayed constant. As shown in Table 3, Shore A Hardness increased continuously with % free-NCO content. 

As in other similar cases [337], the main postcure process should be, the appearance of the urea groups according to equations 4.22(a)-(c), together with the slow reaction leading to the appearance of the allophanate group equation 4.22(d). 

As observed, the values of tensile stress and tensile strength were somewhat higher in thinner films probably because in this case the more polar urea group formation was favored to the prejudice of the appearance of allophanate groups. The 100% tensile stress value was similar in aU the cases due to the fact that, as... 

It is seen that exact frequencies shift, depending upon sample state (melt spectrum or KBr pellet) as well as on specific group influence. It is useful to note that the amide I band is specific for constitution since it contributes significantly to the allophanate group absorption and was utilized for evaluating the melt spectrum. This technique is useful, soundly based on literature correlations, but is highly empirical. Trial and error must be used... 

Thus, a urethane adhesive can be based on a wide range of polymers as long as they can be reacted to form a urethane linkage. In practice, polymers can contain aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea and allophanate groups. This leads to a very wide range of raw materials being available, and formulations tend to differ in the type of polymer used rather than on the variations in additives, as seen with many other adhesives. 

About 30 years later Kricheldorf and Schwarz [45] studied the addition of tetra(ethylene glycol), TEG, onto HMDI in benzene and in bulk at 60 °C. At that time MALDI-TOF mass spectrometry was available and the author observed that with 1/1 feed ratios a considerable fraction of linear chains having two TEG end groups existed in the reaction mixture indicating a consumption of HMDI by side reactions, such as the formation of allophanate groups. An excess of 4—7 mol % of HMDI was needed to transform all linear species into cycles. These results deviate from the comments of Stepto and Waywell, who concluded that a 1/1 feed ratio is... 

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