PROBLEM SOLVING FORUM
How Moisture and Humidity
Affect Thin Film Urethanes
Answer
to be cleaned as frequently as begloss finish to reduce project cost.
P. Kronborg,
fore. This job is usually left to naUnfortunately, the curing agents of
Hempel,
ture.
the chemically curing urethanes do
Lyngby, Denmark:
Therefore, there is no reason to
not only react with the base compoChemically curing polyurethane nents of the coating material; but
use a high gloss topcoat. The
coatings are used for a variety of they also have a tendency to react
topcoat can just as well be a semipurposes. The better known uses much more willingly with water
gloss, the condition a high gloss
are related to their excellent
would reach after contagloss and color retention, as
mination from dirt and
well as their good chemical
debris in the atmosuestion
resistance. Typically, they
phere. Color, however, is
What are the effects of moisture and humidity
are used on automobiles,
still important. A semion newly applied thin film, chemically
pleasure boats, and passengloss topcoat will—to
curing urethane coatings before full cure is
ger train coaches, where ulthe human eye—lose
achieved, such as those applied to the exterior
timate appearance as well
much less gloss than a
of storage tanks?
as ease of cleaning, even
high gloss coat with
with strong cleaners, is reearly exposure to conquired.
densation. Apparent
Less well known coating
damage to the semi-gloss is therethan those used with other coating
applications for polyurethanes take materials such as epoxies. This
fore less. Additionally, the underlyadvantage of a fast curing rate (for means that if a chemically cured
ing epoxy coats still provide the
example, coatings for splash and urethane is exposed to condensation
bulk of the anti-corrosion protection.
inter-tidal zones of marine struc- before it has cured sufficiently, the
At the same time, modern chemitures) and tolerance of low tempera- curing agent will react irreversibly
cally-cured urethane resins have
ture application conditions.
been formulated to give earlier resiswith the water instead of the base.
In industrial maintenance, cost has The result will be a finish with a retance to water exposure—and, debeen a major limitation to the use of duced gloss level. This has occurred
pending, on the temperature—rain
thin film urethanes since their intro- on a number of structures in the
showers after 4 hours have no meaduction. Even today, on storage open. Typically, the high gloss uresurable effect on the gloss at 68 F
tanks, cheaper alternatives such as thane topcoat has been applied in
(20 C).
water-borne or solvent-borne the late afternoon in temperatures
The applicator must still pay attenacrylics are often used. Chemically around 50 to 59 F (10 to 15 C) or
tion to the sensitivity to water of
curing urethanes had their first in- lower, and during the evening, conchemically curing urethanes, includdustrial use as topcoats in systems densation has occurred (dew). The
ing following procedures for proper
based on epoxies. The epoxy was next morning, the finish is flat and
storage, mixing, and application of
the part of the system responsible has to be sanded before a further
the coating material; assuring that
for corrosion protection, and the coat of urethane can be applied to
application and drying, occur during
chemically curing urethane was used restore the desired appearance. Unproper climate conditions; and
to add gloss and color retention that expected rainfall can lead to the avoiding condensation and other
epoxies lack. For this reason, the same situation.
means of water contamination in the
chemically curing urethane has been
total process.
Nowadays, it is common for large
applied mainly as a thin-film high structures such as storage tanks not
continued
Q
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Copyright ©1996, Technology Publishing Company
FEBRUARY 1996
/ 17
PROBLEM SOLVING FORUM
Answer
From Melvin Y. Zucker,
Yale Corporation,
Portland, OR:
Since the 1980s, aliphatic urethanes
have become the topcoat of choice
for the exterior of storage tanks because of their excellent color retention, hardness, and ultraviolet resistance and, perhaps, most important,
because graffiti can be removed
from them easily with negligible
damage to the coating.
The most popular urethanes for
the exterior of storage tanks are
two-part aliphatic polyurethanes
with either hydroxy-terminated
acrylic or hydroxy-terminated polyester polyols. As every applicator
has learned, the urethane coatings
can be tricky to apply over large
surfaces outdoors.
When exposed to moisture before
cross-linking, a urethane coating will
18 / Journal of Protective Coatings & Linings
Fig. 1 - A change in temperature during
cure of an aliphatic urethane application
resulted in flattening.
Photo courtesy of Melvin Y. Zucker
flatten (lose its gloss) or turn milky,
depending on the amount of moisture to which it is exposed.
In an ideal situation, each mole of
polyol hydroxyl (polyester or acrylic
resin) reacts with each mole of isocyanate in the hardener. When
water is introduced, the balance is
disrupted, and unreacted polyol hydroxyls remain.
Some manufacturers add isocyanate to compensate for a certain
amount of moisture in a limited
range.
Resin to hardener ratios of 1 to 1
are less susceptible to moisture than
ratios of 4 to 1. However, even ratios of 1 to 1 will tolerate only a limited additional amount of moisture.
Fig. 1 shows an aliphatic polyester
polyurethane, 1 part resin to 1 part
hardener, that was applied to an elevated water storage tank that contained about 3 ft (1 m) of water in
the riser. When the urethane was
sprayed, conditions were such that
the applicator did not notice any
condensation on the epoxy-coated
steel. As temperature conditions
changed after application but before
polymerization was complete, more
condensation occurred, and, as
shown, the urethane turned milky,
and lost its gloss.
When a tank is completely filled
with water while being coated, the
continued
Copyright ©1996, Technology Publishing Company
PROBLEM SOLVING FORUM
effect may not be quite as noticeable, but it occurs to some degree. I
like my projects to have maximum
gloss and luster, so I never permit
painting with content in the tank.
Owners are willing to spend more
for a system of epoxy primer and
aliphatic urethane topcoat than they
would spend for an alkyd system to
enjoy the superior long-term attrac-
tiveness that the system offers.
Hence, I caution all applicators in
the bid documents that flat spots,
lap marks, overspray, or discoloration will not be acceptable, and
they should take precautions to
apply the topcoat in appropriate
conditions.
This means that they must take
into consideration conditions that
may not be addressed in manufacturer’s data sheets, such as wind,
temperature, and humidity, even
after the work is finished for the
day. I have seen many urethane
coatings go flat even after application under ideal conditions during
the day. They lost gloss because marine air conditions changed in the
evening before cross-linking was
sufficiently completed.
Some aliphatic urethanes are easier to use than others. It behooves
contractors to find products within a
generic class that are reliable in a
wide range of weather conditions.
✍
Answer
From Mike Mitchell,
International Paints,
Felling, UK:
Early water resistance is an issue
with almost all coatings. Before molecular weight build-up has occurred,
there is water sensitivity, which may
result in discoloration and other deficiencies.
Water sensitivity is obviously dependent on the rate of cure reaction
and thus is more likely to occur in
cool, northern climates. Other factors influencing water sensitivity are
the hydrophobicity of material and
solvent type.
The traditional long oil alkyd
finish is probably the least susceptible of all convertible coatings
(those that will cross-link, as distinct
from the low solids lacquer-dry thermoplastics, such as chlorinated
rubbers or vinyls). This characteristic
of the alkyd is because the oil
content of the alkyd is water-repellent .
There is a somewhat different situation in high humidity situations.
The phenomenon of amine bloom
was well recognized in the early
days of epoxies, when low molecular weight aliphatic amines were
widely used and “greasy” surface
20 / Journal of Protective Coatings & Linings
Copyright ©1996, Technology Publishing Company
PROBLEM SOLVING FORUM
layers were common on curing in
high humidity and low temperatures. The amine bloom and the
greasy oily surfaces were the reaction of free amine with atmospheric
moisture and carbon dioxide.
In chemically curing polyurethane
coatings, premature loss of gloss because of high humidity is not, in my
experience, a widely occurring phenomenon, especially in Europe.
Most comments on this in my
company have emanated from
the regions of South East Asia
and areas such as Papua, New
Guinea, to the north of Australia.
Surprising, there have been few
comments from the Gulf of Mexico,
which, at first sight, would have similar climatic conditions.
My belief is that in Europe much
polyurethane used at new construction is applied inside fabricating
shops in controlled conditions.
With polyurethanes, the reason for
the problem lies in the cross-linking
chemistry and in the move to higher
and higher solids, where lower molecular weight polyols, typically
acrylic polyols, are used.
With the higher molecular weight
polyols used in the 1960s and 1970s,
considerable lacquer drying occurred. With the greater number of
hydroxyls on each chain, a further
rapid increase in the glass transition
temperature (Tg) occurred because
of the increased molecular weight.
The rapid increase in Tg improved
both early water resistance and resistance to high humidity.
With the onset of higher solids,
overcoatable acrylic polyurethanes
with a lower number of hydroxyls
per chain, lower molecular weight
resulted, and the above features
were lost. The opportunity for alternative reactions to occur then became much greater.
The principal (and the desired) reaction with two-pack acrylic urethanes is the following: an acrylic
polyol and a diisocyanate combine
Copyright ©1996, Technology Publishing Company
to form a polyurethane.
However, in all circumstances,
there is a potential competing
reaction, that between the diisocyanate and atmospheric moisture.
In this reaction, a polyurea is
formed, which in itself is not necessarily a major problem, but the byproduct of the reaction, carbon
dioxide, is a major problem. When
carbon dioxide forms at the surface,
small defects occur that detract from
the overall gloss.
The reaction with polyol is preferred and obviously can be accelerated by use of traditional accelerators such as dibutyl tin laurate, but
such additives severely shorten the
pot life of the product.
The most practical solution
in areas with high evening humidities is to paint in the morning
and allow curing to take place in the
afternoon before the onset of high
humidity.
✍
Reader Response
On Inspecting and Repairing
Concrete before Lining
From A.W.M. van den Hondel,
Nebest,
Groot-Ammers,
The Netherlands:
The November 1995 JPCL posed the
following Problem Solving Forum
question: “What types of inspection
and repair procedures are needed to
ensure that a previously uncoated
concrete structure is capable of accepting a lining?”
Concrete is a relatively inexpensive building material with structural
properties that make it capable of
supporting loads. However, concrete
can corrode when it is in contact
with aggressive chemicals such as
fertilizers, manure, acids, sulfates,
and salt water. Protecting it with an
impermeable lining is a straightforcontinued on page 137
FEBRUARY 1996
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PROBLEM SOLVING FORUM
continued from page 21
ward way of preventing deterioration.
A thorough investigation of previously uncoated concrete is needed
to determine whether it is capable of
accepting a liner. In the inspection,
one must assure that the following
conditions are met.
• The concrete has attained at least
80 percent of its design strength.
• The concrete quality at the surface
is such that it can withstand the occurring mechanical loads. (If not, a
liner will not do any good.)
• The structure must not be open to
water; water penetration into a
structure with an impermeable lining
can cause problems.
• The surface must be cleaned of alkali, efflorescence, sand (from construction), dust, dirt, oil, grease, fat,
chemical films in general, curing
compounds, form release agents
(containing waxes, silicones, or silicates), laitance, loose deposits, rust,
and (for most linings) water. These
contaminants may be removed by
abrasive blasting, mechanical scraping, or chipping.
For example, for new floors, light
sweep blasting is usually the most
effective way to remove laitance. All
dust must then be removed. Oily
contamination may be removed
using an emulsion cleaner, but heavily contaminated floors may require
specialized techniques such as flame
cleaning.
• Spalling and delamination of
concrete must be repaired. Precautions should be taken to assess the
nature and origin of the deterioration process. Is it, for example, rebar
corrosion (chloride vs. carbonation),
chemical corrosion of the concrete,
or an alkali aggregate reaction?
• Cracks larger than 100-200 micrometers (4 to 8 mils) should be
filled by resin injection. Whether or
not the injected polyurethane (or
epoxy) should be flexible will depend on the origin of the crack
Copyright ©1996, Technology Publishing Company
(e.g., structural or settlement causes,
temperature cycle deformation, accidental overload, or hardening
shrinkage).
• Small holes or voids in the concrete surface should be filled with a
suitable filler compound.
Once the necessary cleaning and
repair procedures have been performed, the concrete is ready for lin-
ing in accordance with the manufacturer’s instructions.
On the Advantages and
Disadvantages of Partnering
From Andrew Greig,
Kvaerner Installasjon a.s.,
Norway:
The December 1995 JPCL raised the
continued
FEBRUARY 1996
/ 137