Moisture-Curing One-Component Solventborne Polyurethane Coatings

Adducts of diisocyanates or polyisocyanate/polyol combinations with an excess of isocyanate groups (prepolymers) can cross-link with atmospheric moisture (Figure 1) to give insoluble higher molecular weight polyurethanes/polyureas. This reaction describes the curing principle for moisture-curing polyurethane coatings.

There are two ways of formulating one-component coatings of this kind:

• Preparation of prepolymers from suitable polyisocyanates and polyols
• Use of DESMODUR® E polyisocyanates

Prepolymers from Polyisocyanates and Polyols
DESMODUR® N (HDI), DESMODUR® CB (TDI), MONDUR® MR and MRS (MDI) polyisocyanates are suitable for preparing prepolymers used in formulating one-component moisture-curing coatings. The prepolymers are usually prepared from polyols of low functionality and a corresponding stoichiometric excess of polyisocyanate. A moisture-curing clear varnish can be made as follows: The polyisocyanate and solvent are mixed for a short time in a dissolver, being sure that moisture is excluded. If a moisture scavenger is required, paratoluene sulfonyl isocyanate (pTSI) can be added. After this, the polyol, other additives and accelerators are added and the mixture is packed in a dry, airtight container. The coating reaches its final viscosity in approximately seven days. The storage stability of the final product is dependent on the polyol used and should be closely examined.

DESMODUR® E Polyisocyanates
The name DESMODUR® E covers a range of ready-to-use polyisocyanates for one-component coatings which require no other compounding when used in clears except for the addition of solvent where necessary. Because they are far simpler to use than two-component polyurethane coatings, the DESMODUR® E polyisocyanates have become very popular.

Properties
The properties of moisture-curing one-component coatings are principally determined by the nature of the particular base isocyanates used. For example, one-component coatings based on aliphatic polyisocyanates (HDI, IPDI, DESMODUR W diisocyanate) generally need longer drying times than those based on aromatic isocyanates (TDI, MDI). The drying times depend not only on the temperature but also on the amount of atmospheric moisture present. With very low absolute moisture content the drying times may be increased.

The weathering properties of one-component coatings also largely depend on the type of isocyanate used. Coatings based on TDI or MDI have a tendency to yellow in the light and show a relatively rapid loss of gloss on weathering. Those based on HDI, IPDI, and DESMODUR® W diisocyanate are light stable. Depending on the composition, these one-component coatings may be equivalent in gloss retention and chalk resistance to two-component polyurethane coatings based on DESMODUR® N polyisocyanates. One-component polyurethane coatings have very good mechanical properties.

The films can range from hard to very flexible. Surfaces are particularly high in mar and abrasion resistance. The films also have good resistance to chemicals, including strong organic acids, alkalis, alcohols, solvents and water.

Figure 1