FIELD OF THE INVENTION

This invention relates to water-based polyurethane dispersions (PUD).

BACKGROUND OF THE INVENTION

Water-based polyurethane dispersions (PUD) are well known, environment friendly resins for different coatings, inks and adhesives applications. There are two different approaches in commercial production of polyurethane dispersions, the acetone process and the prepolymer process. As acetone is a flammable solvent, the prepolymer process is the widely used one. In this process, PUDs are made from a di-isocyanate and a polyol. In this two-step process, a polymer is firstly made by reacting the di-isocyanate and the polyol in the presence of a tin catalyst. A polyol containing an acid group, e.g., 2,2-dimethylolpropionic acid (DMPA), is used to react with the di-isocyanate and incorporate acid functionality into the polyurethane (PU) prepolymer. In the second step, the acid and the residual di-isocyanate are neutralized with an amine, and the neutralized PU polymer is dispersed in water to obtain the PUD. In step one, a solvent like N-methyl-2-pyrrolidone (NMP) has been used for many years to dissolve DMPA during the prepolymer synthesis due to its good affinity for DMPA.

NMP is a particularly important, versatile solvent and the preferred reaction medium for the PUD chemical industry because of its low volatility, thermal stability, high polarity, aprotic, noncorrosive and good solubility properties. However, it has been demonstrated that NMP shows reproductive toxicity in animal testing. As a result, NMP has recently been classified as a potential reprotoxic substance under the Registration, Evaluation, Authorization and Restriction of Chemical Substances (REACH), which drives the increasing safety and regulatory concerns at global level.

Therefore, a solvent with a better environmental, health and safety (EHS) profile and similar solubility properties is expected to replace NMP. Dipropylene glycol dimethyl ether (DPGDME) and N-ethylpyrrolidone (NEP) are two commercially available solvents which manufacturers use to replace NMP. NEP is less harmful than NMP but still toxic. DPGDME has no risk to health but has limited solubility for DMPA so the PU polymer preparation usually proceeds at a slower rate than with NMP. Additionally, the final solvent concentration in the PUD is about 5 to 8% by weight. So, the water solubility of the solvent is important in producing a stable dispersion. NMP is completely water-soluble but DPGDME is only partly water soluble. PUD producers have also replaced NMP with ketones, e.g., methyl isobutyl ketone, or esters, but these solvents need to be removed from the prepolymer before dispersing in water since they are not water-soluble. The ideal solvent in PUD synthesis should be compatible with water-based PUDs with good affinity for the polyol containing an acid group, e.g., DMPA.

SUMMARY

In one embodiment the invention is a process for making a prepolymer comprising acid group, the process comprising the step of contacting(i) a di-isocyanate,(ii) a polyol containing an acid group,(iii) a polyol without an acid group, and(iv) a metal salt catalyst,
the contacting conducted under reaction conditions and in a solvent consisting essentially of:(A) a first component consisting of at least one dialkyl amide, and(B) optionally, a second component consisting of at least one an aprotic glycol ether.
In one embodiment the acid group of the polyol containing an acid group is a carboxylic acid group. In one embodiment the polyol containing an acid group is DMPA. In one embodiment the metal salt catalyst is an organic tin salt. In one embodiment the optional aprotic glycol ether is present.

In one embodiment the invention is a three-step process for making a water-based polyurethane dispersion (PUD), the process comprising the steps of:(1) forming a prepolymer with acid group by contacting:(i) a di-isocyanate,(ii) a polyol containing an acid group,(iii) a polyol without an acid group, and(iv) a metal salt catalyst,
the contacting conducted under reaction conditions and in a solvent consisting essentially of:(A) a first component consisting of at least one dialkyl amide, and(B) optionally, a second component consisting of at least one an aprotic glycol ether;(2) neutralizing the acid group of the prepolymer and any residual di-isocyanate with a base; and(3) dispersing the neutralized prepolymer in water.
In one embodiment the acid group of the polyol containing an acid group is a carboxylic acid group. In one embodiment the polyol containing an acid group is DMPA. In one embodiment the metal salt catalyst is an organic tin salt. In one embodiment the optional aprotic glycol ether is present. In one embodiment the base is an amine.

In one embodiment the invention is a polyurethane dispersion comprising (i) a prepolymer comprising an neutralized acid group, (ii) a dialkyl amide, and (iii) water. In one embodiment the dispersion further comprises an aprotic glycol ether. In one embodiment the neutralized acid group is an amine-neutralized carboxyl group.

