Isocyanate reactivity

Reactivities of isocyanates depend on their structure. Table 2.6 gives the main isocyanates used for polymer network synthesis. Conjugation with aromatic nuclei makes ArNCO particularly reactive. The reactivity of diisocyanates is well documented in the literature. For symmetric diisocyanates such as diphenylmethane 4,4 -diisocyanate (MDI) or para-phenylene 4,4 -diisocyanate (PPDI), both NCO groups have initially the same reactivity. But as the NCO group itself exhibits an activating effect on isocyanate reactivity, the fact that one NCO group has reacted introduces a substitution effect that usually decreases the reactivity of the second NCO group. 

This pheonomena, if present in the molded part, will usually be self-evident. Adhesive qualities to materials such as scotch tape and paint, for example, will be very poor. For the most part, however, this undesirable feature can be controlled or avoided. Raw material manufactures will usually select only those IMR compounds which are either (1) isocyanate reactive, or (2) have polymer solubility characteristics which are free of this problem. 

Apart from the development of polyurethane adhesives and sealants with isocyanate reactive groups, a new type of silane-terminated polymer adhesive has been developed called Polyurethane (PUR)-Hybrids. They and modified silicones (MS) and provide the benefits of ... 

Reactivity is calculated by a kinetic procedure in which the percentage of the original phenyl isocyanate addition, which reacts with the sample in a given time, is taken as an index of its reactivity. A calibration graph can be prepared in which the determined phenyl isocyanate reactivity is plotted against the ethylene oxide content, enabling a determination to be made of the ethylene oxide content of unknown samples from the calibration graph by interpolation. 

In this way, the Sivak group also synthesized a PU network using doxorubicin (Figure 12.20), another anticancer drug which contains several isocyanate reactive groups [153]. 

Isocyanate-reactive compositions containing internal mold release agent ... 

Cure characteristics of one-component systems categorise adhesives as moisture curable, UV curable, powder blocked isocyanates, etc., and for two-components systems as either polyol-isocyanate reactive curable or polyol-amine reactive curable. 

Blends of PVC and TPU are typically plasticized to some extent with monomeric esters. Hydrocarbons such as paraffin wax should be limited to very low levels, because of incompatibility with TPU ester or amide lubricants should be used instead. Active hydrogen (stearic acid, alcohols) should not be present, because of isocyanate reactivity. Low levels of Stabaxol P are advisable with PVC/VA to scavenge acetic acid. With PVC homopolymer, efficient heat stabilizers are adequate. Active hydrolysis catalysts such as antimony oxide and organotin carboxylates will adversely affect the water resistance of polyester TPU blends. 

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