Moisture Curing One
Component Coatings
Moisture curing polyurethane
coatings are an effective and economical coating solution in many
new construction and refurbishment applications in which steel
is the substrate material.
Moisture cure polyurethane
coatings can be applied independent of the weather (temperature,
humidity and dew point), dry quickly and are surface tolerant.
In addition, the coatings remain elastic and resistant to UV radiation
to provide longterm durability.
Adducts of diisocyanates
or polyisocyanate and polyol combinations with an excess of isocyanate
groups (prepolymers) can crosslink with atmospheric moisture to
give insoluble higher molecular weight polyurethane/polyureas.
This reaction describes the curing principle for moisture-curing
polyurethane coatings.
Formulating One-Component
Moisture-Cure Coatings
There are two ways of
formulating one-component coatings (1K coatings) of this kind:
- Preparation of prepolymers from suitable polyisocyanates and
polyols
- Use of Desmodur®
E polyisocyanates
You
can find this information and accompanying illustrations in the
Bayer publication, “The Chemistry of Polyurethane Coatings” by
clicking
here.
Additionally,
learn more about waterborne coatings by browsing and downloading
our collection of technical
papers.
Prepolymers
from Polyisocyanates and Polyols
Desmodur®
N (HDI), Desmodur® L (TDI), and 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 trademark Desmodur®
E covers a range of ready-to-use polyisocyanates for one-component
coatings which may require no other compounding when used in clear
coatings except for the addition of solvent where necessary.
Properties
of Moisture-Curing One-Component Coatings
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 isocyanates (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 aliphatic polyisocyanate based
moisture-cured coatings often require metal catalysts (such as
dibutyltin dilaurate) to provide reasonable cure times.
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 mar resistant
and abrasion resistance is exceptionally high. The films also
have good resistance to chemicals, including the stronger organic
acids, as well as alkalis, alcohols, solvents, and water.
Aliphatic
polyisocyanate based moisture curing resins are available which
can be formulated into coatings which meet the requirements of
the SSPC - Paint 38 Specification “Single-Component Moisture-Cure,
Weatherable Aliphatic Polyurethane Topcoat, Performance-Based.”
Pigmented
One-Component Coatings
Due to the sensitivity
of one-component coatings to moisture, a special technique has
to be followed when formulating pigmented coatings. This involves
the use of pTSI, Additive OF, and splitting the grind procedure
into several steps. This process may be accomplished in the same
amount of time as the milling of a conventional coating and can
be done with the same equipment.
The formulation is divided
into four operations:
- Weighing
- Predispersing
- Dispersing in the sand mill
- Filling
The individual components
are weighed in the given sequence and predispersed with a dissolver.
In so doing, two processes occur simultaneously:
- Homogenization of the weighted materials
- Dehydration of the pigments, extenders, and solvents by pTSI
Carbon dioxide is released
through the chemical reaction between water and pTSI, acting as
a buffer gas preventing contact with air and humidity. While the
dissolver charge is warm, it can be dispersed in a sand mill.
After cooling, the catalyst,
and if necessary Additive OF, can be added and the batch adjusted
to the desired viscosity with anhydrous solvents. The material
should then be packed in dry airtight containers.
Another method of formulating
pigmented one-component coatings is to add the pigment in the
form of a pre-dispersed paste colorant. In this case, the pigment
mill base can be prepared in a binder which reacts as little as
possible with the NCO group. The incorporation of pTSI ensures
that the pigment paste is free of water. The paste may then be
mixed with Desmodur E polyisocyanate binders such as Desmodur
E-28.
Curing
The drying rate of one-component
coatings is dependent on the relative atmospheric humidity and
the temperature. Low temperature and low atmospheric humidity
may slow down the drying considerably.
As with two-component
systems, one-component coatings based on aliphatic polyisocyanates
often require the incorporation of reaction accelerators. Metal
compounds such as dibutyl tin dilaurate are especially suitable
for this purpose.
Application
Moisture-curing coatings
based on the Desmodur E polyisocyanates are normally applied by
brushing or spraying. Dip coatings and curtain coating cannot
generally be used because of the extended contact between the
liquid coating and atmospheric moisture.
Unpigmented one-component
coatings are used primarily for wood substrates such as parquet
flooring and other indoor wood flooring applications. Another
area of application is for the sealing or coating of concrete
floors, and decorative seamless floors.
Pigmented
one-component coatings can be used for anticorrosion coatings
for metal, for the coating of concrete substrates, and for various
other decorative and protective coatings.
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