Mixtures of diisocyanates

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

Diisocyanates have also generated interest as adhesives for plywood, but they have mainly failed in use because of economics and adhesive application problems (adhesive distribution, penetration into veneer, etc.). Further experiments have shown that mixtures of diisocyanates with the usual fillers and extenders have a very short potlife or give nonhomogeneous glues (5,6). 

Because a mixture of diisocyanate isomers is actually used, a random mixture of 2,4- and 2,6-substitution patterns results. 

Structural studies on polyurethane elastomers obtained with mixtures of diisocyanates... 

WAXS results for polyurethane elastomers with mixtures of diisocyanates. Crystallinity... 

The morphology of a series of polyester or polyether PUs based on mixtures of diisocyanates has been also examined by ourselves by using the AFM technique. Example PUs that were smdied, based on the macrodiol PEA and chain extended with EG are depicted in Table 2.4. The molar composition of the materials was diisocyanate macrodiol chain extender = 4 1 3 giving an isocynate index I = 100. The preparation procedures in order to obtain PUs with single or mixtures of the diisocyanates MDI and/or DBDI has been described elsewhere [127,207]. 

The DMA investigations have been also employed by us for PUs obtained with mixtures of diisocyanates. The materials were obtained in the Romanian laboratory. The two isocyanates were employed, MDI and DBDI, alone or as mixtures. The materials were synthesized with ethylene glycol (EG) as chain extender. The SS macrodiol was poly(ethylene adipate) PEA or polytetrahydrofuran (PTHF) of molar masses 2 000 g/mol. 

Shown in Fig. 3.3 are the schematic of the synthesis routes to obtain the materials with mixtures of diisocyanates listed in Tables 3.1 and 3.2 ... 

By using DSC measurements, we also have studied the thermal behaviour of a series of PUs based on mixtures of diisocyanates. The compositions of the family of PUs prepared and studied, is given in section 2.3 (Tables 2.2 and 2.3) where we also report SEM results obtained by us for this family of materials achieved with mixtures of diisocyanates. The molar proportions were hard segment macrodiol chain extender = 4 1 2.64, giving HS mass fractions in the region of 40%, and isocyanic index I = 110. 

As observed by us, all the PUs obtained by using mixtures of diisocyanates (PUci PEA and PUci pthf) did not display significant DSC maxima in the region between 20°-150°C characteristic mainly to the ordered soft segment structure as observed in the case of stretched PUd pthf model when the small maximum at 50°C reveals the PTHF moiety crystallization tendency which maintains also after removing the stress. Fig. 3.10 (e), curve 2. Such a tendency was observed also in many other PUs. 

Note that the compositions of the family of materials prepared and studied in this work, is similar to that of the PUs achieved with mixtures of diisocyanates which were characterized by WAXS and SEM in section 2.3 (Tables 2.2 and 2.3) and also by DSC in section 3.2.2. So, for abetter understanding of the PUs types investigated in this work, the readers should see Tables 2.2 and 2.3 in section 2.3. 

Note that polymers PUci, PUc2 and PUc3 are achieved with mixtures of diisocyanates (MDI-fDBDI) as shown in Tables 2.2 and 2.3 section 2.3. Polymers PUci were achieved from the two diisocyanates randomly distributed (see Table 2.3). 

Of great interest for use of the polymers as elastomers is the degree of strain-recovery on unload. Strain-recovery data for the three polymers are compared in Fig. 4.4. It may be seen that polymer C, obtained with mixtures of diisocyanates MDI+DBDI shows he highest degree of strain-recovery. 

Table 4.3 PU stress-strain data of the materials depicted in Table 2.3 of section 2.3.2. It includes two analogous families of PUs with single or mixtures of diisocyanates. The two PUs families vary in the type of macrodiol (PEA or PTHF)[60]...

The amount of NCO available in a pure or impure mixture of diisocyanates for reaction is the percentage free NCO. This is determined as follows for a diisocyanate such as MDI ... 

An interesting class of monomers which polymerize into different polymeric materials at different temperatures was discovered. At room temperature, the linear pol3rmer was obtained either from the polymerization of a dicyanourea monomer or directly from the polymerization of a mixture containing a diisocyanate and cyanamide. At elevated temperature (>100 C), the dicyanourea monomer or the mixture of diisocyanate and cyanamide crosslinked to a rigid foam or flexible film depending on the structure of the monomer. IR, DSC, and NMR were used for characterization of the polymers. 

Water-based polyurethane products can also be prepared by homogenizing a mixture of diisocyanates and diols in an aqueous surfactant solution, followed by heating the resultant miniemulsion to the prescribed step polymerization temperature [76]. Satisfactory miniemulsion polyurethanes are obtained only when the following requirements are fulfilled. 

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