Aliphatic isocyanates are much slower reacting with hydroxyl groups than the aromatic isocyanates.  IPDI is used to make many waterborne polyurethane dispersions where the prepolymer contains reactive isocyanate end-groups.  The prepolymer is inverted into water and then chain extended with a diamine such as ethylene diamine. If the prepolymer had aromatic isocyanate end-groups they would react with the water before the ethylene diamine.  This shows the difference in reactivity.  Now, to prepare a IPDI polyurethane one needs to used a catalyst, such as a tert. amine  (TEA) or more commonly a tin compound (dibutyl tin dilaurate; DBTDL) at 0.1 to 2.0%.  Typical temperatures are from 50 C to 100C.  NCO to OH mol ratio typically run from 10 : 7 or high.  Termination is by chain extention with a diol or diamine or one could use mono-hydroxyl or mono-amine.  The patent literature is a good resource for reaction conditions.

Regards,

Chuck Berge

Polyurethane acrylate oligomers were prepared from isophorone diisocyanate, mixture of polyethylene glycol 1000 and 2000 and hydroxyethyl acrylate using dibutyl tin dilaurate as a catalyst. The rheological properties and its utilization as a UV-curable binder for inks of ink jet printing and pigment dyeing of cotton, viscose, wool, polyester and nylon 66 fabrics using pigment dyes were thoroughly investigated. It was

found that the prepared binder is characterized by low viscosity (0.0042 Pa S ¼ 4.2 cP) at a rate of shear of 10.0007 S1. Furthermore, the results obtained indicate that the aqueous UV-curable binder of polyurethane acrylate oligomer based on the mixture of PEG1000 and PEG2000 can be used safely for inks of ink jet printing and also in pigment dyeing to give coloured goods characterized by soft handling and from good to excellent color fastness properties. Infra-red spectra of the mixture of PEG1000 and PEG2000 before and after the preparation of polyurethane acrylate oligomer indicate some physicochemical changes in the structure.

For solution PU's one can use almost any aprotic solvent.  Solids of the prepolymer solution can be high or low.  I have made prepolymers at 45% to 80% solids.  The molecular weights of the prepolymer will depend on the diol molecular weight and the NCO:OH ratio.  The closer to 1:1 the higher the mol.wt.  One can run either a single stage or a two stage process. The most common single stage prep  starts with the solvent, diol/diamine  monomer(s) and catalyst in the reactor.  Reaction is heat to 50-80C and the IPDI is fed into the reactor at a rate that will control the exotherm.  The NCO number is monitored by IR or (better) by titration. When all the hydroxy and/or amine is consumed there should be residual NCO. If the original NCO:OH is less than 1:1 then the prepolymer is NCO terminated.  I make the prepolymer around 3000-5000 Mn then invert into water or invert by adding water to the prepolymer.  Once dispersed (less than a minute after addition) a chain extender (a diamine) is added.  The amount of diamine is based on the measured NCO number just prior to inversion.  In most dispersion polyurethanes there is an 'internal' stabilizer that provided the dispersion stability and determines the particle size.  Almost universally this is dimethylol propionic acid (DMPA).  For dispersions the final acid number is between 15 and 30 mgKOH/gram poymer.  Neutralization with amines (tert amines) can be done to the prepolymer or the amine can be added to the water.  For solution PU's the DMPA is not needed. For the dispersion PU's (PUD) the mol wt can be very high (40,000 Mn). Non-aqueous PU's generally are less than 15,000 Mn.

Regards,

Chuck

Good comments all;

very interesting the inversion that I never tried before;

In my experience I did polyurethanes both with aromatic and aliphatic isocyanates. Aromatics are more reactive and aliphatics requires a catalyst; DBDTL works well, amines are a catalyst that helps more the decarbossilation of the isocyanates (foaming).

In polyurethanes foams are used usually two catalysts one for the isocyanate polyol reaction (DBDTL) and the other, that is an amine , usually DABCO for the reatction water - isocyanate (that make foaming the polymer). Their ratio is important for the well done polyurethane foam.

So the choice of the catalyst in my opinion has to be the DBDTL because water and polyol are competitors for the isocyanate.

It is well known that water is contained in polyols especially PEG that is hygroscopic so its removal is mandatory if one wants a well done polyurethane coating or foam. I worked with PPG that is less hygroscopic but to remove the last traces of water I used an acid dehydrating agent (that if you need I can describe). In this case I removed all water from polyol and so I could make the reaction I wanted and find the best parameters that are the same described by chuck; (I worked also at 90-100°C to speed up preparations sometime).

Solvent: here is always a problem the best choice, and it is bonded to the final application and the substrate where the polyurethane is applied. I found often a good solution the addition of flowing agents that acted like a solvent to lower the viscosity if polymers were  too viscous.

Regards

Luca

Solventless waterborne PUD's are made commercially.  The prepolymer is made in acetone or MEK or ethylacetate or maybe other low boiling aprotic solvents.  It needs to be miscible with water and form solutions with PU.  After the inversion and chain extension or termination the solvent is distilled out. Remaining solvent levels are generally below 1% for shipping reasons (flash point needed to be about 141 C). Commercial distillation is a slow process taking as much time as the reaction itself.  Process time = cost so that is why many PUD's have high boiling solvents such as NMP.  These high boiling solvents also aide in particle coalescence and film formation.

Regards,

Chuck

Thanks very much for your comments, which are very interesting and help me a lot in my research.

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Polymer 99, 2016, 404-416