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
There are two ways of
formulating one-component coatings of this kind:
- Preparation of prepolymers from suitable polyisocyanates and
- Use of DESMODUR® E polyisocyanates
Prepolymers from Polyisocyanates
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.
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.