a review of two component water borne polyurethane

8/21/2019 A Review of Two Component Water Borne Polyurethane

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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


2/14

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



3/14


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-



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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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.


SEPTEMBER 1996/ 55

Fig. 4 - Structures of common isocyanates (top) andgeneric polyols (bottom) used in industrial maintenancecoatings


5/14


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



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)


6/14


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-


SEPTEMBER 1996 / 57


7/14


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-



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


8/14


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


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


9/14


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



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


10/14


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-


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.


11/14


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).



Fig. 5 - Latex film formation mechanism


12/14


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.


SEPTEMBER 1996 / 63

Fig. 6 - Time line of events in film formation mechanism of two-component polyurethane coatings


13/14


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,


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.



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.


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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