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8/21/2019 A Review of Two Component Water Borne Polyurethane
1/14
ndustrial maintenance coatings systems
have evolved through a number of
stages over the last several decades. For
many years, many standard systems utilized
solvent-borne vinyl wash primers and coat-
ings based on chlorinated rubbers.1 Until
the 1970s, alkyd resin systems, filled with
red lead and iron oxide, had been perhaps
the most popular choice for conventional,
high solvent content, heavy-duty mainte-
nance applications. As volatile organic
compound (VOC) content has been restrict-
ed and the use of lead-containing pigments
has been eliminated, epoxy-based primers
and urethane topcoats have become widely used for heavy-duty applications. The
epoxy primers, filled with zinc or utilizing
various barrier pigments, have inherently
good chemical resistance. The exceptional
durability of the urethane topcoat provides
long-term gloss and color retention while
protecting the integrity of the corrosion-re-
sistant primer.
Pushed by environmental concerns,
development has continued today toward
high performance, water-borne coatings
for heavy-duty maintenance applications.
This review article addresses the develop-
ment of two-component, water-borne
polyurethane coating systems. These coat-
ings consist of an isocyanate, often modi-
fied to improve its water dispersibility,
mixed with a dispersion of a hydroxyl
functional polymer (Fig. 1).
This article first describes the basics
of urethane chemistry and the types of ure-
thane systems used in industrial mainte-
nance applications. After a discussion of
the solvent-borne, two-component ure-
thane benchmark, this article describes for-mulation, application, and performance of
two-component water-borne systems devel-
oped thus far. Once these systems have
been described, formulation and applica-
tion difficulties associated with two-compo-
nent water-borne polyurethanes are ad-
dressed. The article ends with comments
on the state of the art in reactive water-
borne urethanes and areas of current and
future development.
A Review ofTwo-Component
Water-Borne Polyurethane
Coatings forIndustrial Applications
by S.L. Bassner and C.R. HegedusAir Products and Chemicals, Inc.
52 / Journal of Protective Coatings & Linings
I
Copyright ©1996, Technology Publishing Company
8/21/2019 A Review of Two Component Water Borne Polyurethane
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Urethane Chemistry
The isocyanate functionality, -N=C=O, can
react with a number of different functional
groups at ambient temperature, as shown
in Fig. 2. The reaction of isocyanate with
hydroxyl functionality forms a urethane
group, while reaction with amine function-
ality forms a urea group. One of the most
useful—and troublesome—reactions of iso-
cyanates is with water. The reaction of iso-
cyanate with water first forms an unstable
carbamic acid. This intermediate slowly de-
composes to amine, releasing carbon diox-
ide. The amine that forms reacts rapidly with additional isocyanate to form a urea
group. At higher temperatures or with ap-
propriate catalysis, isocyanate groups also
will react with urethane and urea groups to
form allophanate and biuret structures, re-
spectively. Isocyanates also undergo trimer-
ization, forming an isocyanurate ring struc-
ture (Fig. 3).
One of the most useful aspects of ure-
thane chemistry, which encompasses all of
these reactions, is the breadth of the struc-
tural variations that can be used. A variety
of different polymeric backbones can be
functionalized with the isocyanate group,
while a large number of NCO-reactive ma-
terials (mainly hydroxyl and amine groups)
are available for use.
For example, isocyanate groups can
be attached to aromatic rings, cycloaliphatic
rings, or linear aliphatic structures. Hydrox-
yl groups can be used to functionalize
acrylic, polyester, or poly-ether polymers
(Fig. 4). Because of this wide variety, ure-
thane coatings can be formulated to be
elastomeric or rigid, ultraviolet light stable,highly chemical resistant, hard but tough,
all within a wide range of formulated
cost/performance specifications.
The Solvent-borne, Two-Component Benchmark
The sections above have presented a
general introduction to polyurethane chem-
Developments in Water-Borne Urethanes
SEPTEMBER 1996 / 53
Fig. 1 - Isocyanatedispersed in a polyoldispersion yields atwo-component water-borne polyurethane.
Figures courtesy of the authors
Copyright ©1996, Technology Publishing Company
8/21/2019 A Review of Two Component Water Borne Polyurethane
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Copyright ©1996, Technology Publishing Company
tions, urethanes are most often used as sol-
vent-borne, single-component, moisture-curing primers or two-component (2K) top-
coats. The moisture-curing primers utilize
polyurethane prepolymers that are formed
by reacting an excess of a diisocyanate
such as toluene diisocyanate (TDI) or
methylene diphenylisocyanate (MDI) with a
polyether polyol.
These prepolymers, which have reac-
tive isocyanate groups, are then formulated
with sacrificial pigments such as zinc dust,
or barrier pigments such as aluminum or
micaceous iron oxide. Upon application,the isocyanate groups react with ambient
moisture, as shown in Fig. 2, ultimately
forming urea groups. These single-compo-
nent primers are known for their fast curing
characteristics and flexibility.2,3
Two-component, solvent-borne top-
coats are based on either a polyisocyanate,
such as an isocyanate trimer4 (Fig. 3), or an
isocyanate-terminated prepolymer.5 Both of
these materials are derived from aliphatic
diisocyanates, such as isophorone diiso-
cyanate (IPDI) or hexamethylene diiso-
cyanate (HDI).
The co-reactant with these iso-
cyanates, when used in weatherable top-
coat applications, is usually an acrylic poly-
ol or a polyester polyol.
These high-performance, two-compo-
nent systems are also known for rapid cure
and flexibility, as well as exceptional gloss
and long-term gloss and color retention.
This type of coating is also used indoors in
concrete floor applications, often with a
chemical-resistant polyester polyol co-reac-
tant, to give durable, high gloss, wear-resis-
tant coatings.
With this wide range of available re-
actants, polyurethane topcoats can be for-
mulated within a full spectrum range of
performance characteristics. There are,
however, some general performance char-
acteristics common to these systems. These
are described below.
istry. Before discussing the development
of two-component water-borne technol-
ogy for weatherable topcoats, it is first
necessary to describe the related solvent-
borne benchmark.
For industrial maintenance applica-
Developments in Water-Borne Urethanes
54 / Journal of Protective Coatings & Linings
Fig. 2 - Typical reactions of isocyanates: with hydroxylto form urethane groups; with amine to form urea groups; with
water to form an unstable carbamic acid, which dissociatesto form amine and carbon dioxide (this amine then reacts
with more isocyanate to form urea groups).Figures courtesy of the authors
Fig. 3 - Generalized polyisocyanate structures
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Copyright ©1996, Technology Publishing Company
Formulation and Application of 2K
Solvent-borne Polyurethane CoatingsIn 2K, solvent-borne polyurethane coatings,
all pigments, fillers, additives, and solvents
are added to the co-reactant side, because
of the sensitivity of isocyanates to water.
These coatings are most often formulated
with a slight excess of isocyanate (NCO/
OH = 1.05-1.2) to ensure that all the polyol
is reacted. The remaining isocyanate groups
react with ambient moisture over time to ul-
timately form urea groups, which tend to
incrementally improve film properties such
as hardness and chemical resistance.In polyisocyanate-cured formulations
(Fig. 3), volume mix ratios typically vary
from around 3:1 (co-reactant to isocyanate)
to around 6:1. In prepolymer cured formu-
lations, the mix ratios are more often 1:1 or
2:1 because of the higher equivalent weight
of the prepolymer. Usable pot lives for
these formulations range from over 8 hours
for the older, high VOC formulations (>360
g/L [3.0 lbs/gal.) to around 2-4 hours for
low VOC formulations (≤ 360 g/L [3.0
lbs/gal.]). While applicators must contend with a defined pot life, urethane coatings
are known for their ease of application and
relatively rapid cure under a wide range of
conditions. Most solvent-borne industrial
maintenance topcoats applied today range
from 420 g/L (3.5 lbs/gal.) VOC down to a
low of 335 g/L (2.8 lbs/gal.) VOC. The
practical limit of solvent-borne urethane
coatings for these applications appears to
be around 240 g/L (2.0 lbs/gal.) VOC.
Coating Properties of 2K
Solvent-borne Polyurethanes
Table 1 lists a set of typical properties for a
weatherable topcoat. Note that these are
only typical ranges and do not represent
the limits obtainable of these properties. Of
importance are the rapid cure times, high
gloss, excellent chemical and solvent resis-
tance, good hardness with good flexibility,
and long-term gloss and color retention.
Current 2K Water-bornePolyurethane Systems
Water-borne polyurethanes have been
available for some time as single compo-
nent, fully reacted polymers, modified
with carboxylic acid functionality to make
them water dispersible. These single-com-ponent polyurethane dispersions, or PUDs,
have found use in applications where
toughness combined with flexibility is im-
portant, such as in synthetic leather coat-
ings and in coatings for plastics. While
PUDs offer many of the performance ad-
vantages of reactive urethane coatings, they
do not provide the high level of perfor-
mance required in most heavy-duty indus-
trial maintenance applications.
Developments in Water-Borne Urethanes
SEPTEMBER 1996/ 55
Fig. 4 - Structures of common isocyanates (top) andgeneric polyols (bottom) used in industrial maintenancecoatings
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Copyright ©1996, Technology Publishing Company
Two-component, water-borne
polyurethane systems are a relatively recent
development. The remainder of this review
will discuss the various types of systems
that have been developed as well as areas
for improvement in application and perfor-
mance. Note that a number of these sys-
tems have been developed for applications
other than industrial maintenance (e.g.,
wood coatings, aerospace), but their rele-
vance to the development of systems suit-
able for industrial maintenance applications
is apparent.
After an introduction, this section will
be broken down into materials and formu-
lation, application characteristics, and prop-
erties and property development.
Introduction—2K Water-borne
Polyurethanes
As with the solvent-borne versions, two-
component water-borne polyurethane coat-
ings consist of a polyol and a multi-func-
tional isocyanate. (Further detail is
provided below.) These materials have
been used in a number of high perfor-
mance coating applications, including in-
dustrial maintenance, automotive,
aerospace, plastic, and wood coatings.6-11
The popularity and importance of these
coatings are growing significantly becauseof their potential for providing excellent
performance properties, equivalent to those
of their solvent-borne counterparts, com-
bined with a low VOC content. Most cur-
rent water-borne formulations have a VOC
content of less than 250 g/L (2.1 lbs/gal.);
the leading technologies have been used to
develop zero-VOC coatings. (See State of
the Art section.)
Materials and Formulations—2K
Water-borne PolyurethanesThe polyol dispersion in these coatings can
be from the acrylic, polyester,
polyurethane, or alkyd families. In all of
these cases, hydroxyl functionality is re-
quired in the polymer for reaction with the
isocyanate. For example, the hydroxyl
functional polyurethane dispersion is made
by reacting a difunctional isocyanate with a
low molecular weight diol and a bishydrox-
yfunctional carboxylic acid. Hydroxy func-
tional acrylic dispersions are obtained by
including both acrylic acid and hydroxy
acrylates in the polymerization reaction.
Isocyanate cross-linkers for these
systems are typically based on conven-
tional aliphatic isocyanate hardeners, pri-
marily HDI with a biuret, isocyanurate,
or uretdione structure (Fig. 3). Cyclo-
aliphatic (i.e., IPDI) versions also have
been used but they tend to be less compat-
ible with water-borne polyols and have
higher glass transition temperatures (Tg)
than linear isocyanates, making film forma-tion more difficult. In the most common
cases, these isocyanates have been modi-
fied with ethylene oxide and propylene
oxide to render them hydrophilic and thus
water dispersible.
However, the more traditional iso-
cyanates, which generally are hydrophobic,
have been used more recently in specially
designed systems. The hydrophilically-mod-
ified versions generally have both lower
Developments in Water-Borne Urethanes
56 / Journal of Protective Coatings & Linings
Table 1Properties of a Typical Two-Component,Solvent-Borne Urethane
Property Typical Value (Test Method)
Tack Free Time 2-4 hours (ASTM D 1640)
Pencil Hardness HB-2H (ASTM D 3363)
Persoz Pendulum Hardness 150-300 seconds (ASTM D 4366)
Adhesion to Treated Steel 5B (ASTM D 3359)
Impact Resistance 80-160 inch-lb (ASTM D 2794)
MEK Double Rubs 200+ (ASTM D 4752)
Resistance to Common Solvents Excellent (Spot Test)and Dilute Acids and Bases
60 Degree Gloss 95+ (ASTM D 523)
Gloss Retention (2,000 hrs, UV-B) 70-90 percent (ASTM G 53)
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Copyright ©1996, Technology Publishing Company
functionality and glass transition tempera-
ture than their unmodified counterparts.This is because the hydrophilically-modi-
fied versions have increased molecular
weight without increased reactive groups.
Use of these modified polyisocyanates re-
sults in applied coatings with increased
free volume and molecular mobility, and
with lower Tg and cross-link density. Con-
sequently, these films can be more suscep-
tible to organic solvents and other chemi-
cals, decreasing their swell, stain, and
etch resistance.
Although these are water-borne mate-rials, both the polyol dispersion and the
isocyanate components often contain or-
ganic solvents to improve compatibility,
viscosity, manufacturing and application
properties, or film formation. Using the
latest technology, coatings that display
excellent properties have been form-
ulated and applied with no added solvent
for thinning.12
In the formulation of 2K reactive coat-
ings, stoichiometry of the admixed coating
(i.e., NCO/OH ratio) plays an important
role in the properties of the cured coating.
Two-component, solvent-borne polyure-
thanes are typically formulated at stoi-
chiometries ranging from 1.05 to 1.2. In this
case, a slight excess of NCO ensures com-
plete reaction of the polyol and the desired
high level of properties. In contrast, 2K
water-borne polyurethanes are formulated
at NCO/OH stoichiometries ranging from
1.5 to 3.0. Due to the substantial amount of
water in these systems, some of the iso-cyanate will inevitably react with water. A
large excess of isocyanate is needed to en-
sure that all of the polyol will react into the
cross-linked polyurethane system. If this re-
action did not occur, coating properties
would be diminished because of unreacted,
low molecular weight, thermoplastic polyol
in the applied coating. Poor mechanical
and chemical resistance properties would
be the result.
Two critical issues with these 2K
water-borne polyurethanes are the viscosity relationship between the polyol dispersion
and the isocyanate, and the compatibility
between these 2 components. Mixing liq-
uids with similar viscosities is much easier
and more efficient than mixing liquids with
substantially different viscosities. In most
cases, the polyol dispersions have viscosi-
ties in the range of 100-400 cps. As a result,
isocyanates with a viscosity within or close
to this range are preferred.
The second issue concerns compati-
bility between the isocyanate and polyolparticles. Upon mixing, both the isocyanate
and polyol are in a dispersed state. For
the required reaction to occur, isocyanate
and polyol particles must mutually coalesce
by the diffusion of 1 particle into another.
For this to occur, the 2 different compo-
nents must be compatible and have molec-
ular mobility. Coalescence, then, becomes
more difficult with higher Tg or less com-
patible materials.
Organic solvents have been used to
address both of the above issues. Solvents
can be mixed with the isocyanate to form a
solution that reduces viscosity and increas-
es molecular mobility and isocyanate-poly-
ol compatibility. For example, IPDI trimer
is generally less compatible with the typical
polyols used in these applications and
therefore generally requires the use of ad-
ditional co-solvent.
Of course, this has the undesired side
effect of increasing VOC. Commonly used
solvents are acetates (e.g., methoxy propylacetate, butyl acetate) and ethers (e.g.,
propylene glycol ethers). Taking this
approach, coatings can be formulated
with organic solvent content of 8 percent
to 12 percent by weight and VOC of less
than 300 g/L (2.5 lbs/gal.). Solvent levels
of 150 to 200 g/L (1.25 to 1.67 lbs/gal.)
are common.
In addition to improving reactant
compatibility, co-solvents help film forma-
Developments in Water-Borne Urethanes
SEPTEMBER 1996 / 57
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Copyright ©1996, Technology Publishing Company
tion, addressed in more detail later. They
also can have an effect on optical proper-
ties (gloss and haze) and property develop-
ment. For example, less compatible co-sol-
vents can decrease gloss and increase haze.
We have also found that compatible, low
vapor pressure co-solvents decrease hard-
ness. This is probably caused by the sol-
vents’ extremely low vapor pressure and
slow release from the film, causing them to
be a fugitive plasticizer. This plasticization
can also have a negative effect on chemical
and solvent resistance.
The effects of other additive formulat-
ing aids (wetting agents, flow and levelingaids, defoamers, rheology modifiers, light
stabilizers, and catalysts) depend on the
specific systems, desired properties, and
application scenario. Selection of additives
is much more critical than in solvent-borne
systems, and it seems that additives play a
much larger role in attaining acceptable
coatings. Catalysts can be used but are gen-
erally not required. Traditional tin catalysts
are common.
As with solvent-borne polyurethanes,
pigmentation is included in the polyolcomponent. Most available publications
describing 2K water-borne polyurethane
coatings provide high gloss white formula-
tions with titanium dioxide pigment. Jacobs
and McClurg7 provided a low gloss gray
formulation designed for aircraft applica-
tions. The literature has proclaimed (and
we have seen in our laboratory) that
with the use of appropriate dispersants
and dispersion procedures, pigment com-
patibility and stability in these systems
are excellent.
Application Characteristics—
2K Water-borne Polyurethanes
Since these are 2K reactive coatings with a
defined pot life (up to several hours), the 2
components must be mixed just before ap-
plication. The mixing process is made more
difficult by the fact that the applicator is
not simply mixing 2 solutions, but dispers-
ing 1 component (the isocyanate) in the
continuous phase of a second component,
the polyol dispersion.
Most of the available literature de-
scribes manual mixing; however, two-
component (in-line) mixing also has been
reported. At this stage of coating prepara-
tion, similarity in the viscosities of the reac-
tive components is critical. As stated previ-
ously, liquids of dissimilar viscosity are
difficult to mix, while those of similar vis-
cosity will mix much more readily. There-
fore, the formulator must adjust the compo-
sition of the 2 components to increasemixing efficiency.
Kahl and Bock6 reported that the
compatibility of the polyol and isocyanate
also has a dramatic effect on mixing of
these co-dispersed systems. They found the
best results in mixing, application, and per-
formance with hydrophilically-modified,
linear isocyanates. Hydrophobic iso-
cyanates could also be used but required
additional amounts of co-solvents to im-
Developments in Water-Borne Urethanes
58 / Journal of Protective Coatings & Linings
Table 2Comparison of Water-Borne vsSolvent-Borne Polyurethane Coatingsfor Industrial Applications8
Water-Borne Solvent-Borne
Solvent content (percent by weight) 5-12 40-60
VOC (g/L)* 300 500
Gloss 60 degrees/20 degrees 92/80 95/85
Appearance very good very good
Cross cut test on steel or electro 0-1 0deposition primer (GT)
Erichsen (proprietary) indentation 8-10 8-10tester (mm)**
Tack free dry time (hrs) 3-5 2-4
ASTM D 4366, Method A, König 130-160 170-200pendulum hardness (sec)
Solvent resistance good good
* grams/liter=pounds/gallon of VOC x 119.8** 1 mm=40 milsEditor’s Note: Not all test methods are reported
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Copyright ©1996, Technology Publishing Company
prove compatibility. Conse-
quently, a decreasing amountof solvent was required to ob-
tain acceptable polyol-iso-
cyanate compatibility.
The preferred method
with manual mixing is to slow-
ly add the isocyanate compo-
nent into the polyol compo-
nent. This causes a noticeable
viscosity rise due to emulsifica-
tion of the isocyanate. Water is
then added slowly to reduce
the viscosity to suit the desiredapplication scenario. Bock
and Petzoldt11 reported that
improvements in gloss, hard-
ness, and chemical resistance
were obtained by increasing
the shear used in the disper-
sion process.
Theoretically, this should
decrease the particle size of
the dispersed isocyanate. It is
suspected that as with latex
dispersions, smaller particle
size results in improved film
formation because of en-
hanced diffusion of polymers
across particle boundaries. In
the case of the 2K system, this
will enhance coalescence and
further promote the iso-
cyanate-polyol reaction.
Note that in newer, high
solids systems described later
in this article, the polyol itself emulsifies the isocyanate.
This eliminates the need for
modified isocyanates as well as the
need for co-solvents. (See State of the Art
section below.)
Properties and Property
Development—2K Water-bornes
As previously mentioned, 2K water-borne
polyurethanes have been developed and
evaluated for a number of high perfor-
mance applications. Each of these applica-
tions has unique performance require-
ments, and formulation of 2K water-borne
urethanes can be tailored to meet specific
needs. Certain properties of the water-
borne polyurethanes are consistent across
these application scenarios. The pot life of
Developments in Water-Borne Urethanes
SEPTEMBER 1996/ 59
Table 3Evaluation Test Results of the Two-Component
Water-Borne vs Solvent-Borne Topcoats13
Property Water-Borne Solvent-Borne
Wet Paint
VOC at application viscosity (g/L)* 148 501
Co-solvent at application viscosity 7 43(percentage by weight)
NCO to OH ratio (percent) 150 150
Solid content (percentage by weight) 60 56.7
Application viscosity (sec DIN Cup r) 30 20
Dust-dry time (hrs:min) 4:15 4:00
Tack-free time (hrs:min) 7:30 7:30
TNO drying phase 1 (hrs:min) 0:15 —phase 2 (hrs:min) 1:30 —phase 3 (hrs:min) 2:00 —
phase 4 (hrs:min) 6:30 —
Dry Paint
Film thickness (µm) 42 35
Hardness Persoz/König, ASTM D 4366, 281/142 322/196after 7 days at room temperature
Haze (Haze Units), ASTM D 4039 19 6
Gloss 20 degrees (Gloss Units) 81 91
Gloss 60 degrees (Gloss Units) 85 93
Appearance OK OK
Adhesion Gt 2 Gt 0
Erichsen indentation (mm)** 9 >9
Conical mandrel (diameter in mm), 0 0ASTM D 522
Impact face/reverse (kg•cm) >105 >105
Xylene resistance 5 min after OK OK7 days at room temperature
Yi (yellowing index) -0.1 -1.2
Wi (whiteness) 93 91
* grams/liter=pounds/gallon of VOC x 119.8** 1 mm=40 milsEditor’s Note: Not all test methods are reported
8/21/2019 A Review of Two Component Water Borne Polyurethane
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Copyright ©1996, Technology Publishing Company
these admixed coatings typically ranges
from 1 to 4 hours. Pot life is affected by the
surfactant/emulsifier, compatibility between
the isocyanate-polyol, and catalyst. Gener-
ally, pot life is longer with improved emul-
sification and compatibility. As expected,
formulations without catalyst tend to have
longer pot life and shorter dry time.
Bittner and Ziegler8 investigated
water-borne versus solvent-borne 2K
polyurethane coatings for industrial mainte-
nance applications. They formulated an
acrylic polyol dispersion with an HDI-
based polyisocyanate. VOC was 300 g/L
(2.67 lbs/gal.) for their water-borne materi-
als versus more than 500 g/L (4.2 lbs/gal.)
for the solvent-borne analog. They reported
excellent overall properties for both sys-
tems (Table 2) and concluded that bothcoatings displayed acceptable gloss, ap-
pearance, hardness, adhesion, and flexibili-
ty for these applications. Using hardness
testing, solvent resistance testing, and dif-
ferential scanning calorimetry, they found
that the water-borne coating reached ulti-
mate properties within 1 day (at ambient
conditions) while the solvent-borne coating
took up to 14 days.
However, the water-borne coating
was observed to be softer
with a lower Tg (30 C versus49 C) than the solvent-borne
analog. Of course, the hard-
ness and Tg of the water-
borne could be increased
by altering the polyol, iso-
cyanate, or the NCO/ OH ratio
of the formulation.
Increasing isocyanate con-
centrations from 1.5 up to 3
does affect coating properties.
Wingerde and Brinkman13
found that increasingNCO/OH from 1.0 to 2.0 in-
creases hardness and water
resistance. (They recommend
NCO/OH of 1.5 or higher.)
Our work indicated similar relationships:
increasing the NCO/OH ratio led to in-
creased hardness and chemical resistance
with decreased flexibility and toughness.
An NCO/OH of 1.5 was optimal for indus-
trial maintenance applications.
In developing coatings for aircraft
topcoats, Jacobs and McClurg7 found once
again chemical resistance increased and
flexibility decreased with increasing
NCO/OH ratios ranging from 1.5 to 3.5.
They found that an NCO/OH of 3.0 was
optimal for this application. Additionally, as
the NCO/OH ratio increases, formulation
cost typically increases due to higher cost
of the isocyanate component relative to the
polyol component.
Wingerde and Brinkman13 also uti-
lized an acrylic polyol dispersion with anHDI-based polyisocyanate to formulate
clear and pigmented industrial coatings.
Once again, the coatings were comparable
(Table 3) except that the water-borne coat-
ings (clear and pigmented) were slightly
softer than their solvent-borne counterparts.
The pigmented water-borne coating had
slightly lower gloss than the pigmented sol-
vent-borne coatings. Lower gloss with pig-
mented water-borne coatings is relatively
Developments in Water-Borne Urethanes
60 / Journal of Protective Coatings & Linings
Table 4Property Comparison of Solvent-Borne vs
Water-Borne Clearcoats and Topcoat9
Formulation A B C D
MEK 2X Rubs 200+ 200+ 200+ 100+
Pendulum Hardness (sec) 170 23 134 120
Reverse Impact (in. lbs) 160 160 160 160
Tensile Strength (psi)* 4900 6200 5755 —
Elongation (percent) 90 >90 >90 85
A = Solvent-borne HDI polyisocyanate and highly functional polyester (clear)
B = Solvent-borne HDI polyisocyanate and tri-functional polyester (clear)
C = Reactive 2K water-borne system (clear)
D = Reactive 2K water-borne topcoat (pigmented)
* 1000 psi=6.895 MPa
Editor’s Note: Not all test methods are reported
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Copyright ©1996, Technology Publishing Company
common due to film formation issues (dis-
cussed below) and pigment dispersion ef-fects, though again, the newer higher solids
systems are an exception.
Jacobs and Yu9 formulated industrial
coatings using a hydroxy-functional
polyurethane dispersion with a hydrophilic
(water-dispersible) HDI trimer. Once again,
properties of both clear and pigmented for-
mulations were comparable with control
solvent-borne formulations (Table 4) when
applied and cured at typical laboratory con-
ditions (e.g., 23 C [73F], 55 percent RH).
They also found property development of the water-borne version to be relatively
fast. Solvent resistance, impact resistance,
and hardness reached ultimate levels in 2
days. Tg reached its ultimate level in 3
days, and tensile strength was reached in
4-5 days.
Jacobs and Yu did report that ex-
tremes in temperature and humidity during
curing had substantial effects on properties.
At higher humidity (e.g., 90 percent RH),
properties were reduced. This reduction is
caused by water remaining in the film
longer, favoring the isocyanate-water reac-
tion. Increasing temperature up to 31-38 C
(88-100 F) can help overcome this effect
by helping to drive the water from the sys-
tem and promote coalescence of the polyol
and isocyanate.
Other Applications of 2K Water-borne
Polyurethane Coatings
Two-component, water-borne poly-
urethanes have been investigated for use inother coatings applications, such as auto-
motive6,11, aircraft7, and wood finishing.10
Bock, et al.11, have evaluated these sys-
tems for automotive coatings—exterior
clearcoats, primer surfacers, base coatings,
single layer topcoats, and soft feel coatings.
For exterior clear coats, they evaluated
acrylic and polyurethane polyols and mix-
tures of both, cross-linked with hydrophili-
cally-modified HDI trimer. They found that
a balance of properties can be obtained
with a mixture of the 2, resulting in excel-
lent overall properties.
Bock and Petzoldt11 found that 2K
water-borne primer surfacers have greater
flexibility and impact properties than theirsolvent-borne counterparts, especially at
low temperatures. This improves perfor-
mance in applications over plastic compo-
nents by providing a barrier to the notching
effect of a brittle topcoat, which can catas-
trophically crack the plastic substrate.
These mechanical characteristics also favor
application as pigmented topcoat systems
by providing a resistance to cracking and
chipping, while providing the chemical re-
Developments in Water-Borne Urethanes
SEPTEMBER 1996 / 61
Table 5Properties of High Solids PigmentedTwo-Component Water-BornePolyurethane Topcoat12
Dry Times at 72F (22 C)/50 percent RH Set to Touch 2.5 hrsTack-Free 4.0 hrs
Appearance 20 Degree Gloss 8960 Degree Gloss 94
AdhesionDry Tape (ASTM D 3359) 5ADry Scrape (ASTM D 2197) 6 KGWet Tape (24 hr/70 F [21 C]) 5AWet Tape (4 days/70 F [21 C]) 5A
Chemical Resistance Acid. 10 percent HCl Spot. (7 days) No EffectBase. 10 percent NaOH Spot. (7 days) No EffectAcid. 5 percent Nitric Spot (7 days) Very Slight
Solvent Resistance (ASTM D 4752) MEK >200 Double RubsToluene >200 Double RubsIsopropanol >200 Double Rubs
Impact Resistance (ASTM D 2794) Direct >160 in. lbsIndirect >160 in. lbs
Flexibility (ASTM D 4145) 0-T BEND PASS
Hardness Pencil (ASTM D 3363) 2HPendulum (ASTM D 4366) 190
All coatings were applied over zinc-phosphated cold- rolled steel to a dry film of 2.5 mils (63 micrometers) by conventional air spray application.Results obtained after 7 days’ room temperature cure.
Fifty percent RH although most properties reached full developmentwithin 24 hrs.
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Copyright ©1996, Technology Publishing Company
sistance of 2K solvent-borne polyurethanes.
The authors conclude that further develop-
ment can overcome existing limitations in
terms of application conditions such as at-
mospheric moisture, temperature, and
flash-off times.
Jacobs and McClurg7 used a
polyurethane polyol and a water-reducible
alkyd cross-linked with a water-dispersible
(hydrophilic) isocyanate to formulate clear
and pigmented coatings for aircraft top-
coats. They concluded that the perfor-
mance properties of these 2K water-borne
systems were comparable to the traditional
aircraft topcoats while reducing VOC by
greater than 50 percent. Coatings formulat-
ed with the hydroxy functional alkyd dis-
played high gloss and excellent chemical
resistance as required for commercial air-
craft applications. The polyurethane polyol-
based coating had excellent flexibility andtoughness, as well as the chemical resis-
tance needed for military aircraft.
Development Challenges with 2K Water-borne Polyurethanes
Now that the characteristics of the 2K sol-
vent-borne and water-borne polyurethane
coatings have been identified, it is impor-
tant to point out the difficulties that the
resin chemist, formulator, and paint appli-cator face in converting to a water-borne
analog. These difficulties are related mostly
to film formation issues. Related to film for-
mation, but especially troublesome in its
own right, is carbon dioxide generation
and entrainment. Finally, limitations on ve-
hicle solids affect a number of processing,
handling, and application issues.
Film Formation
Film formation is not a significant issue in
solvent-borne coatings, since all reactingcomponents are fully dissolved in the
solvent carrier and intimately mixed. Upon
application of the coating, a continuous
thin film forms, solvent evaporates, and
the well-mixed components react to
form the final film. In single component
dispersions, such as PUDs and acrylic latex-
es, the polymer is not dissolved in the
water, but dispersed as very small particles
or drops.
Upon application of the coating
and evaporation of the water, good film
formation is contingent on effective coales-
cence of these particles—or their ability
to flow into one another to form a continu-
ous polymer film. This process is often
assisted by a small amount of solvent in
the coating formulation that softens the
particles and allows them to coalesce.
The solvent then evaporates to allow the
coating to achieve its ultimate film proper-
ties (Fig. 5).
A reactive, 2K water-borne coatingformulation has to overcome not only the
coalescence issue described above, but also
the difficulties of getting good mixing of
the reactants so that once the film has been
applied and water evaporates, complete re-
action can occur. Two-component water-
borne polyurethane formulations also must
contend with the competing reactions of
isocyanate with polyol (desired) and with
water (not desired).
Developments in Water-Borne Urethanes
62 / Journal of Protective Coatings & Linings
Fig. 5 - Latex film formation mechanism
8/21/2019 A Review of Two Component Water Borne Polyurethane
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Copyright ©1996, Technology Publishing Company
There has been a full treatment of is-
sues surrounding film formation of 2K
water-borne polyurethane coatings in
which the polyol and a hydrophilically
modified isocyanate are co-dispersed in an
aqueous matrix.14 The specific events and
their timeframes are shown schematically in
Fig. 6.
The dispersion of isocyanate in the
aqueous system appears to occur immedi-
ately upon addition and mixing. Particle co-
alescence during the admixed state was
found to be minimal by particle size experi-
ments. Isocyanate reaction with hydroxyl
groups occurs within 2 to 5 hours, as evi-denced by the maximum exotherm from
calorimetry data.
Reaction with water occurs at a much
slower rate, as shown in calorimetry work
on a water/isocyanate system. These results
correlate well with pot life studies and car-
bon dioxide generation from the iso-
cyanate/water reaction.
After application, evaporation of most
volatiles (including water) occurs within 30
minutes. During this time, a critical
solids content of the coating is reached,
so that particle-to-particle contact is com-
pleted throughout the film. When this oc-
curs, diffusion of polymer molecules
across particle boundaries leads to par-
ticle coalescence.
This also favors the isocyanate/hy-
droxyl reaction. Isocyanate reactions after
application are 80 percent complete
within 3 days, suggesting substantial cross-
linking by this time. To account for the
isocyanate/water reaction and ensure
complete hydroxyl reaction, NCO/OH
ratios are typically 2.0.Studies on film property develop-
ment illustrate that barrier properties
began to be established within 3 hours,
and chemical resistance develops within
3 days.
As noted earlier, solvent-borne
polyurethanes normally reach ultimate
properties at a slower rate. Chemical
resistance is usually reached within
7 days.
Developments in Water-Borne Urethanes
SEPTEMBER 1996 / 63
Fig. 6 - Time line of events in film formation mechanism of two-component polyurethane coatings
8/21/2019 A Review of Two Component Water Borne Polyurethane
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Copyright ©1996, Technology Publishing Company
Carbon Dioxide Generation
and Entrainment With 2K solvent-borne polyurethanes, the
end of pot life is defined by a rise in vis-
cosity and gelation of the admixed coating.
In contrast, the viscosity of most water-
borne versions is relatively constant
throughout the pot life. As the induction
time of these coatings increases, the poten-
tial for the isocyanate-water reaction greatly
increases. (A number of researchers have
reported that in these systems, the iso-
cyanate-water reaction is delayed by up to
2 hours after mixing.) As the isocyanate- water reaction occurs, carbon dioxide is
produced within the coating.
This has several consequences. First,
it increases the potential for defects from
foam and bubbling within the cured coat-
ing. Second, as the reaction of isocyanate
with water increases, the amount of iso-
cyanate available for reaction with polyol
decreases. This can result in unreacted
polyol in the applied coating, which will
plasticize the film and reduce hardness,
toughness, and chemical resistance. Finally,
as isocyanate begins to react with water,
urea particles form that inhibit good coales-
cence. Therefore, the pot life of these 2K
water-borne coatings is often controlled by
this isocyanate-water reaction, which can
be difficult to assess in an industrial appli-
cation scenario. Newer, high solids water-
borne systems, however, tend to exhibit a
viscosity increase similar to their solvent-
borne counterparts.
The isocyanate-water reaction alsohas consequences on limiting coating thick-
ness. This reaction will inevitably produce
carbon dioxide in the applied coating, es-
pecially if formulated at higher NCO/OH
ratios. This carbon dioxide must then dif-
fuse out of the film in such a manner as to
avoid film defects. As film thickness in-
creases, release of this carbon dioxide be-
comes more difficult, and bubbles will form
within the cured coating.
This effect is even more prominent
later in pot life. Most reports limit the po-tential dry film thickness obtained in 1 ap-
plication of these coatings to 2 to 3 mils (50
to 75 micrometers). Thicker films may be
obtained by multiple applications, but this
is usually not preferred from a logistics and
scheduling standpoint. Ideally, the formula-
tor would like to be able to produce water-
borne systems that mimic their high solids
counterparts in application characteristics as
well as properties. Recent developments in
2K water-borne polyurethane coatings12
have resulted in systems applied at highersolids (65 percent to 75 percent) that handle
very similarly to the solvent-borne analog.
State of the Art
and Future Work
Research and development in both resin
and formulation work for 2K water-borne
polyurethanes is continuing at a rapid pace.
This article captures works published
through April 1996. At the time of this writ-
ing, a very interesting development in this
area has been that of an extremely high
solids, water-dispersed polyol that allows
formulation of coatings quite similar in
handling and performance to high solids
solvent-borne coatings.12 This polyol, sup-
plied at 70 percent non-volatiles, is capable
of dispersing standard isocyanates, alleviat-
ing the need to use modified isocyanates
and the problems associated with them, as
noted above.Possibly because of the high solids
level of the formulated coating, these mate-
rials exhibit a definitive end of pot life
through viscosity build and gelation, with-
out loss of properties before that point. Ad-
ditionally, since the polyol itself disperses
the isocyanate, film formation issues
surrounding particle coalescence are
dramatically lessened, leading to exception-
al film properties (Table 5). As a result,
Developments in Water-Borne Urethanes
Dr. Sherri L. Bassner
received a BA inchemistry from Goucher
College in 1984 anda PhD in inorganic
chemistry fromPenn State
Univeristy in 1988.In the fall of 1988, she
began working atAir Products and
Chemicals, Inc. inthe Professional
Development Program.After spending 1 year
in the ElectronicsDivision developing
new compounds for metal vapor
deposition, she joined the Polyurethane
Chemicals Division,where her charge
was developing new polyurethane
prepolymers forhigh solids coatingsapplications. In the
following 6 years, she
was involved inthe development,applications,
scale-up, andcommercialization
of many new products for the industrial
maintenance market.She is now a Senior
Principal Applications Chemist in the
Polymer ChemicalDivision.
64 / Journal of Protective Coatings & Linings
8/21/2019 A Review of Two Component Water Borne Polyurethane
14/14
coatings based on this polyol can be for-
mulated with no co-solvent added—essen-tially zero VOC.
While the performance of 2K water-
borne polyurethane coatings has risen to
approach that of their solvent-borne coun-
terparts, work continues on the develop-
ment of improved resins and formulations.
Areas of focus include enhancing the ability
to mimic the handling characteristics of sol-
vent-borne coatings, increasing the range
of properties available by extending the
family of products, and reducing raw mate-
rial costs. JPCL
Notes
1. C. H. Hare, Protective Coatings: Funda-
mentals of Chemistry and Composition ,
SSPC 94-17 (Pittsburgh, PA: Technology
Publishing Co., 1994).
2. G. Gardner, “Moisture Curing Polyure-
thanes,” JPCL (February 1996), 81-100.
3. J. Kramer and S. L Bassner, “Polyure-
thane Prepolymers for Moisture Cure
Primers,” Modern Paint and Coatings (June
1994), 20-23.
4. R. R. Roesler and P.R. Hergenrother,
“Two Component Polyurethane Coatings,”
JPCL (January 1996), 83-94.
5. J. Kramer and S.L. Bassner, “Using Novel
Polyurethane Prepolymers in VOC-Compli-
ant, Two Component Weatherable Top-
coats,” Paint and Coatings Industry (Au-
gust 1994), 42-44.
6. L. Kahl and M. Bock, “Water-borne 2-Component PU Clear Coats for Automotive
Coatings: Development of Raw Materials
and Mixing Technology,” in Proceedings of
the 3rd Nurnburg Congress , Nurnburg, Ger-
many, March 13-15, 1995 (Middlesex, Eng-
land: Paint Research Association, 1995).
7. P.B. Jacobs and D.C. McClurg, “Water-Re-
ducible Polyurethane Coatings for
Aerospace Applications,” in Proceedings of
the Low and No VOC Coatings EPA Confer-
ence , San Diego, CA, May 25-27, 1993
Washington, DC: Environmental Protection Agency, 1993).
8. A. Bittner and P. Ziegler, “Water-borne
Two-Pack Polyurethane Coatings for Indus-
trial Applications,” in Proceedings of the 3rd
Nurnburg Congress , Nurnburg, Germany,
March 13-15, 1995 (Middlesex, England:
Paint Research Association, 1995).
9. P.B. Jacobs and P.C. Yu, “Two Component
Water-borne Polyurethane Coatings,” Journal
of Coatings Technology (July 1993), 45.
10. C.A. Renk and A.J. Swartz, “Fast Drying,
Ultra-Low VOC, Two Component Water-borne Polyurethane Coatings for the Wood
Industry,” in Proceedings of the Water-
borne, Higher Solids, and Powder Coatings
Symposium , New Orleans, LA, February 22-
24, 1995 (Hattiesburg, MS: Univ. of So.
Miss., 1995), 266-276.
11. M. Bock and J. Petzoldt, “Aqueous
Polyurethane Coatings Systems for Plastics,”
in Proceedings of the Water-borne, Higher
Solids, and Powder Coatings Symposium,
New Orleans, LA, February 14-16, 1996
(Hattiesburg, MS: Univ. of So. Miss., 1996),
502-513.
12. W.O. Buckley, E.H. Klingenberg, T.L.
Richards, and J.M. Snyder, “High Perfor-
mance Two Component Water-borne
Polyurethane Coating Systems,” in Proceed-
ings of the Water-borne, Higher Solids, and
Powder Coatings Symposium, New Orleans,
LA, February 14-16, 1996 (Hattiesburg, MS:
Univ. of So. Miss., 1996), 127-139.
13. M. Wingerde and E. Brinkman, “Two
Component Polyurethane Paints: A Com-parison Between Solvent and Water-borne,”
in Proceedings of the 3rd Nurnburg
Congress , Nurnburg, Germany, March 13-
15, 1995 (Middlesex, England: Paint Re-
search Association, 1995).
14. C.R. Hegedus, A.G. Gilicinski, and R.J.
Haney, “Film Formation Mechanism of Two
Component Water-borne Polyurethane
Coatings,” Journal of Coatings Technology
(January 1996), 51-61.
Developments in Water-Borne Urethanes
SEPTEMBER 1996 / 65
Dr. Charles R. Hegedus has worked as a Lead Applications Chemist in industrial coatings resins atAir Products and Chemicals inAllentown, PA, since 1993. He is responsible for research anddevelopment of high performance coatings for industrialapplications. Before joing Air Products,he was employed for17 years at the Naval Air DevelopmentCenter, where he was Technical Leader ofthe Protective Coatings Group.
Dr. Hegedusreceived his BS in chemical engineering and PhD in materials engineering fromDrexel University.He has published more
than 60 technical papers and reports.He recently receivedthe FSCT Roon and Corrosion Committee Publication Awards.He has 18 patents and 6 patents pending.He is a member of the
Journal of CoatingsTechnology editorial review board, and he chairs the FSCTCorrosion Committee.He is also a memberof SSPC and ACS.
The authors can
be reached at AirProducts andChemicals, Inc.,7201 HamiltonBoulevard, Allentown,PA 18195-1501,610/481-2561; 610/ 481-2225; fax: 610/ 481-7923.
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