Two Component Polyurethane
Coating Systems
The
most frequently recommended products for two-component polyurethane
coatings, also know as 2K coatings, applications are Desmodur®
N polyisocyanates and Desmophen®
polyols. In addition, for some applications the Desmodur and Mondur®
aromatic polyisocyanates may also be appropriate.
You
can find this information and accompanying illustrations in the
Bayer publication, “The Chemistry of Polyurethane Coatings” by
clicking
here.
Calculations
for Two Component Coatings
When preparing two component
polyurethane coating formulations, the rules for chemical equations
are followed. In theory, a maximum molecular weight is reached
and those properties associated with molecular weight are optimized
when one equivalent of isocyanate reacts with one equivalent of
hydroxyl, that is, when the ratio of NCO to OH is 1.0 / 1.0. In
practice, a small excess of isocyanate, about 5-10%, is often
used to allow for the likely destruction of isocyanate by water
contained in solvents and pigments, so that the NCO to OH ration
is maintained at 1.0 / 1.0. It is sometimes desirable to vary
the NCO to OH ratio from about 0.9 / 1.0 to about 1.2 / 1.0 in
order to modify the properties of the coating. In any event, it
is important that the NCO to OH ratio be controlled.
In
order to achieve an NCO to OH ratio of 1.0 / 1.0, the weight of
one equivalent of the supplied form of the isocyanate is reacted
with one equivalent of the supplied form of the polyol. Usually
the product literature will list the equivalent weights, but if
not, then the polyisocyanate literature will give the %NCO and
the polyol literature will give the %OH or the OH number. The
NCO equivalent weight can be calculated from the %NCO, and the
OH equivalent weight from either the %OH or the OH number. These
equations can be combined to calculate the weight ratios of ingredients
for any NCO to OH ratio. Basic algebra is all that is necessary
for these calculations.
Properties
of Two Component Coatings
The properties of a
two-component polyurethane coating depend on certain factors,
such as the degree of branching of the reaction partners, the
content of the reactive groups themselves, and the physical state
of the raw materials.
It is possible to change
the film properties by using the same polyisocyanate with different
polyols while maintaining a constant NCO to OH ratio. Generally
speaking, the higher the hydroxyl content of the particular reaction
partner, the harder and more chemical resistant the films will
be. A low hydroxyl content normally gives softer and more flexible
films.
A
further possibility for modifying the film properties is to use
the same polyol and vary the quantity of the polyisocyanate. By
“undercrosslinking,” which means using an NCO to OH ratio less
than 1.0 / 1.0, the polyurethane film generally becomes softer
and perhaps more flexible, less weather resistant, and less resistant
to solvent and chemicals. On the other hand, by “overcrosslinking,”
using an NCO to OH ratio greater than 1.0 / 1.0, the resultant
films tend to be harder and more chemical resistant. Another possibility
for changing the properties of a coating is to use a blend of
polyols or polyisocyanates. For instance, a rigid, brittle acrylic
urethane coating could be made less brittle and more flexible
by blending in a softer, less-branched polyester. Also, a slower
drying polyurethane coating could be made to dry more quickly
by blending some faster reacting or faster drying polyisocyanate
with the original. For example, blends of Desmodur
Z and
Desmodur N polyisocyanates are used to take advantage of the faster
physical dry of Desmodur Z and the higher chemical crosslinking
of the Desmodur N polyisocyanates.
Curing
of Two Component Coating Systems
The curing of two-component
polyurethane coating systems can be carried out at room temperature.
The drying times can vary considerably depending on the types
of polyols and polyisocyanates used. In general, systems based
on the following polyisocyanates dry at room temperature in these
approximate relative times:
| Desmodur IL |
1 |
| Desmodur HL |
2 |
| Desmodur L |
6 |
| Desmodur N + catalyst |
6 |
| Desmodur N |
30 |
Elevated temperatures
are often used for forced drying in production line coatings.
The highly reactive Desmodur IL and Desmodur HL polyisocyanates
have particular importance for baked finishes as they allow extremely
short drying times. Coatings based on Desmodur L or Desmodur N
polyisocyanates can also be cured at elevated temperatures to
reduce the cure time.
Desmodur N aliphatic
polyisocyanates have low reactivity by nature, and longer drying
times can be expected. The incorporation of 0.005% dibutyltin
dilaurate, 0.2% zinc octoate, or up to 0.5% Desmorapid® PP
catalyst based on solid binder reduces the dry time to practical
levels.
While coatings based
on Desmodur
L polyisocyanate normally do not require a catalyst, the addition
of an accelerator may be advisable for certain applications. Metal-based
catalysts have less of an effect on Desmodur L polyisocyanates
than do tertiary amines such as Desmorapid PP catalyst, which
is normally used in the range of 0.05-0.2% based on solid binder.
Coatings
based on Desmodur IL or Desmodur HL polyisocyanates are normally
used without a reaction accelerator.
Two
Component Coating System Application
The chemical curing mechanism
of two-component polyurethane coatings makes it necessary to consider
the following points:
- Both components must be mixed together thoroughly.
- The reaction which begins immediately upon mixing results
in a gradual to rapid viscosity increase.
- The increase in viscosity eventually leads to a gelation of
the paint.
The usable potlife depends
on the following factors:
- Binder concentration and coating solids level
- NCO/OH ratio
- Polyol and polyisocyanate used
- Catalyst type and concentration
- Temperature of the coating mixture
- Choice and quality of the solvents
- Possible effects of additives, pigments, or extenders
If these factors are
taken into account, the potlife of conventional two-component
polyurethanes can be set to be one work shift. In principle, this
means any application method can be employed. Faster curing coatings
require an adjustment in the application method.
Two component polyurethane
coatings can be applied by a variety of methods. Application by
air, airless, or air-assisted airless spray, electrostatic spray,
brush, roller coating, and curtain coating is possible. Careful
formulation is required to maximize the application characteristics
of each formulation for its end use.
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