Novel Super High Solids Polyurethane Topcoat for

Protective Coatings with Increased Productivity
Novel Super High Solids Polyurethane Topcoat for Protective

Coatings with Increased Productivity

Dr. Malte Homann, Dr. Ralf Lemmerz, Thomas Bäker

(Bayer AG, D-51368 Leverkusen)

Introduction

Current developments for anti-corrosion coatings are driven both by the productivity

of the coating and by demands for reduced environmental impact of such coatings.

The productivity of an anti-corrosion coating is calculated as the cost of the corrosion

protection system per square meter and year. Service life has thus become a

determining factor in productivity calculations. In light of this, a three-coat coating

system comprising primer, intermediate coat and topcoat, each based on chemically

crosslinking binders, has established itself as the standard solution for heavy-duty

corrosion protection. Epoxy resins and – much less commonly – polyurethanes are

used in the primer containing active pigment and the intermediate coat on top of it.

The topcoat is almost always a polyurethane coating based on hydroxyfunctional

acrylates and aliphatic polyisocyanates as these provide good corrosion protection

and superior weather stability.

Demands for reduced environmental impact caused by organic solvents in the

coatings are another driving force for research activities. New developments in anticorrosion coatings are oriented primarily toward restrictions on solvents already in

place or restrictions that are expected in the medium term. A solvent content of 250g

VOC/L or less (corresponds to a solids content of 75 % by volume) is currently the
medium-term objective within the EU for the reduction of solvent emissions from anticorrosion coatings. Near-zero emission coatings are being pursued as a long-term

goal.

In principle, water-reducible coating systems and powder coatings make it possible to

significantly reduce or completely eliminate solvent emissions. However, both

approaches also bring with them some inherent properties which complicate the

application of the coatings on site.

High solid coatings are formulated with very little solvent content

One method for formulating low-solvent (high solid) coatings which in practice has

proved easier is the use of coating raw materials which require very little solvent.

One of the greatest difficulties associated with low-solvent coating raw materials is

the requirement that the pot life and drying speed of low-solvent coatings must at

least equal those of the coatings currently in use. The processes employed today in

corrosion protection, beginning with the pretreatment of the steel components and

coating application, followed in the case of new construction by intermediate storage,

transport and installation, have been tailored to the pot life and drying speed of the

coatings used. The application characteristics of the coatings often determine the

rate of the overall process, which has been optimized for productivity. Disruption of

these processes due to adverse changes to the pot life and drying time of the

coatings results in significant cost increases, thus every advancement in the coatings

sector is also intended to increase the productivity of the overall process.
Novel super high solid polyurethane topcoat meets low solvent requirements

and complies with established application processes

This paper describes a super high solid anti-corrosion topcoat based on newlydeveloped polyurethane raw materials and offering significant productivity

advantages over the conventional two-component polyurethane (2K PUR) topcoats

currently used for corrosion protection. The designation “super high solid” is intended

to convey the fact that the solvent content is so low as to equal the solvent content of

water-reducible coatings. The super high solid coating described satisfies the most

stringent requirements for corrosion resistance. Its application properties facilitate

productivity-optimized processes in corrosion protection applications. With its

excellent tolerance to high film thicknesses, it also offers economic advantages over

the coatings in use today without sacrificing quality.

Capabilities and limits of today’s conventional topcoat binders

The conventional two-component polyurethane anti-corrosion topcoats used today

are based on hydroxyfunctional polyacrylates that react with aliphatic

polyisocyanates – generally HDI biurets or isocyanates – to form polyurethanes.

The hydroxyfunctional polyacrylates have high molecular weights, e.g. between

10,000 and 20,000 g/mol. In their pure polymer form, these polyacrylates are solids.

The formulated coating can be adjusted to the required viscosity for application in

airless spray equipment through the use of significant amounts of organic solvent.

Typical 2K PUR anti-corrosion topcoats have a solids content between 50 – 65 % by

volume.
Figure 1: Application characteristics of 2K PUR topcoats based on hydroxyfunctional acrylates and

aliphatic polyisocyanates

Hours

Vol.-%

solids

Status quo:

50 - 65 % vol.- solids Higher vol .- solids

Pot life:

> 3 h

Sand

dry:

1-2 h

Slower

drying Shorter

pot life

An increase in the solids content of 2K PUR anti-corrosion topcoats based on

hydroxyfunctional polyacrylates and aliphatic polyisocyanates is theoretically

possible. 2K PUR topcoats with significantly higher solids contents can be achieved

by using viscosity-optimized hydroxyfunctional acrylates of low molecular weight.

However, these are associated with severe degradation of the application

characteristics of the coatings. Instead of a pot life of at least 3 hours and a sand-dry

time of 1 to 2 hours to suit application requirements, the pot life is decreased

significantly and drying retarded. The price for the ecological advantage of low

solvent content is disadvantages in terms of overall process efficiency as a result of

longer drying times. The lowest achievable VOC content in a 2K PUR topcoat that

suits application requirements is approximately 250 g VOC/L. It should be possible to

achieve the medium-term goal of reducing solvent emissions with hydroxyfunctional

polyacrylates and aliphatic polyisocyanates. Further significant reductions of the

solvent content require a fundamental change in the chemical approach to the

binders.
A novel chemical approach to binders with very low solvent requirements

We therefore based our approach to super high solid 2K PUR anti-corrosion coatings

on a fundamentally new orientation for the resin components that are reacted with

the low-viscosity aliphatic polyisocyanates. We did this by combining two different

chemical approaches:

 - a low-viscosity, sterically hindered aliphatic diamine that has only moderate

reactivity with isocyanates

 - an aldimine reactive thinner to further reduce viscosity.

Aliphatic polyamines have long been used as reaction partners for polyisocyanates in

the formulation of weather-stable PUR coatings. The reaction products of polyamines

and polyisocyanates are polyureas and yield highly resistant coatings which exhibit

pronounced elasticity with high mechanical strength. The reaction of a polyamine and

polyisocyanate is significantly faster than that of a polyol and polyisocyanate. It is

slowed less at low temperatures than is the polyol/polyisocyanate reaction and at the

same time is less sensitive to undesired blistering caused by the

water/polyisocyanate reaction.

However, until now no aliphatic polyamines were available whose reactivity with

polyisocyanates was low enough for the formulation of coatings for manual

application. The extremely high reactivity of commercially available aliphatic

polyamines with polyisocyanates required the use of multi-component spray

equipment with very effective mixing devices. This significantly reduces the versatility

of the coatings.

Aliphatic polyamines with reduced reactivity ensure a reasonable potlife
Only after aliphatic polyamines were modified by the addition of sterically demanding

side chains on the aminofunctional groups was it possible to reduce reactivity

sufficiently to formulate a coating with a pot life comparable to that of conventional 2K

PUR anti-corrosion coatings.

Figure 2: Aspartic acid modification

O

O

O

O

R'

R'

2NH

R

NH2 O

O

O

O

R'

R'

O

O

O

O

R'

R'

N

H

R

N

H

di-aspartic acid ester

2 +

R‘ = short chain alkyl R = HMDA derivative

The side chains are inserted via the Michael addition of amine nitrogen atoms to

maleic acid diester to create a structure similar to aspartic acid. Polyamine A is an

example of such a sterically modified aliphatic polyamine. Viscosity is 150 mPa s

(23 °C, 90 % in butyl acetate). The steric hindrance reduces the reactivity of

polyamine A such that the viscosity increases only slowly after mixing with aliphatic

polyisocyanates. After curing, the films are hard, weather-stable and resistant to

abrasion.

Figure 3: Reaction of aspartic acid ester and isocyanate

O

O

O

O

Et N

Et

R + R'

O C N R'

O

O

O

O

Et

Et

N

C

N

H

R

O

H

aspartic acid ester isocyanate compound urea structure

A peculiarity of aliphatic polyamines modified with aspartic acid ester is the fact that

water accelerates their reaction with aliphatic isocyanates. This means that the mixed
coating remains easy to apply for a long time but quickly hardens once applied to the

substrate and exposed to humidity.

Reactive thinners are low-viscosity coreactants. Like solvents, they serve to reduce

viscosity but unlike solvents are incorporated into the binder matrix during curing.

Polyaldimines are extremely well-suited as reactive thinners because of their low

viscosity and their reactivity with polyisocyanates.

Figure 4: Aldimine formation

2NH

R

NH2 N

R

N

R'

H

H

R'

+

 diamine

R = cyclohexane derivative

- 2 H2O

aldimine

O

R'

H

2

aldehyde

R' = alkyl

Polyaldimines are formed from the reaction of aldehydes and polyamines.

Polyaldimine B is the reaction product of an aliphatic diamine and short-chain

aldehyde. Its viscosity is only 25 mPa s (23 °C). In contrast to polyisocyanates, the

reaction of polyaldimine B primarily results directly in the formation of an unsaturated

urea (en-urea). This reaction can be considered a cycloaddition reaction between

aldimine and isocyanate.

The reaction is much faster than the undesired hydrolytic reverse cleavage of

polyaldimine B, which produces polyamine and aldehyde. It also prevents the release

of aldehydes when the coating is cured in contact with atmospheric moisture, thus

avoiding unpleasant odors. Instead, the low solvent content means that the resulting

coating is atypically odor-free for a coating.
Figure 5: Reaction of aldimine and isocyanate to produce en-urea

H

C R''

R'''

H

N

R'

N

C

R

O N

H

O

N

R'

H

R

R''

R'''

unsaturated urea

electrocyclic

reaction

Formulation of the super high solid coating

Combining polyamine A modified with aspartic acid ester and polyaldimine reactive

thinner B with low-viscosity polyisocyanate C (HDI isocyanate) yields an anticorrosion coating with a solids content of 93 % by volume, which corresponds to a

VOC content of 90 g/L. The coating formulation is essentially as follows:

Table 1: Coating formulation and composition

A component

Polyaspartic amine 21.85 binder

Polyaldimine reactive thinner 6.21 binder

Zeolite paste, 50 % in castor oil 5.27 dewatering agent

Multiple 45.34 pigment, charge,

additives

B component

Low-viscosity HDI isocyanurate 21.33 binder

 100%

Coating composition

Crosslink ratio (NH/NCO) 1 : 1.1

Solvent content 7 %

(90 g/L)

Solids content

( calculated)

93 %

Pigment volume concentration 23 %

Characteristics of the super high solid coating

Novel super high solid coating is even superior to conventional 2K PUR

topcoat

The high gloss of the coatings is preserved if a hydroxyfunctional acrylate is replaced

with an amine/aldimine combination. The high gloss values of e.g. 90 units (60°)

typical for conventional 2K PUR topcoats can also be easily achieved in the novel

super high solid coating. The novel super high solid coating formulation is actually
superior to conventional 2K PUR topcoats with respect to gloss retention after

weathering. Another benchmark can be seen in Epoxisiloxane coatings which are

marketed as being superior to standard polyurethane coatings in their weathering

performance and in terms of solvent content of the formulations. However, with the

new super high solid polyurethane similar or even better weathering results are

obtained which is shown here.

Figure 6: Change in gloss after weathering of Epoxisiloxane topcoat and the super high solid

polyurethane coating (test according to QUV A)

Application characteristics: long pot life, fastdrying

The super high solid coating has a pot life of 3 hours (time until viscosity doubles). It

is therefore equally suited for application using airless spray equipment or manual

application using a roller or brush.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0h

500h1000h1500h

2000h2500h3000h

3500h4000h4500h

5000h5500h6000h

6500h7000h7500h

8000h

Time / h

Loss of gloss / %

Super High Solid

Epoxisiloxan
Table 2: Comparison of characteristics of the super high solid topcoat and a conventional 2K PUR

anti-corrosion topcoat (Binder = polyisocyanate D, 75 % in solvent; NCO content = 16.5 %;

and hydroxyfunctional polyacrylate E, 60 % in solvent, OH content = 1.6 %. Both are

commercial binders for 2K PUR topcoats)

*: Time until viscosity doubles under standard conditions (23 °C, 50 % rel. humidity)

**: After storage for 7 days under standard conditions

Super high solid

coating

Conventional 2K

PUR topcoat

Pot life* 3 h 5 h 15 min

Sand dry time T1

(120 μm, dry)

70 min 90 min

Through drying T6

(120 μm, dry)

 3 h approx. 12 h

Pendulum hardness

(König)**

 140 s. 115 s.

With respect to film drying, the super high solid coating is superior to the

conventional 2K PUR topcoats used in corrosion protection applications. The sanddry time (T1) is only 70 minutes under standard conditions. T6 drying, i.e. the time

until the applied topcoat is capable of withstanding moderate mechanical loads, is

only 3 hours and thus significantly shorter than the corresponding drying times of the

2K PUR topcoats used today. Because of the accelerated T1 and T6 drying, steel

components coated with the novel super high solid coating are sooner resistant to

contamination of the paint film by dirt, e.g. due to blasting operations being

performed nearby, and to transportation damage. This means a significant increase

in productivity for the coating applicator.

This advantage over today's 2K PUR topcoats becomes even more pronounced with

decreasing temperatures, as has been demonstrated and quantified on the basis of

measurements with an ultrasonic film rheometer. This method of measurement

determines the storage modulus of a paint film via the reflection of an ultrasonic wave

from the interface between the coating film and the measuring cell. It is thus possible

to track the increase in film hardness during curing under realistic conditions. The
transition from low-viscosity liquid after mixing of the components to cured coating

can be quantitatively documented using the change in storage modulus.

Figure 7: Change in normalized storage moduli at various temperatures (Film thickness in each case

approx. 200 μm; Normalization = current value/final value, both in [Pa])

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80 90 100

Time [ h]

Normalized 

storage mo dulus

1

2

3

4

1: Super high solid coating, 23 °C, 50 % rel. humidity

2: 2K PUR standard coating, 23 °C, 50 % rel. humidity

3: Super high solid coating, 5 °C, 50 % rel. humidity

4: 2K PUR standard coating, 5 °C, 50 % rel. humidity

Figure 7 shows the change in the normalized storage modulus for the super high

solid coating and a standard 2K PUR coating (hydroxyfunctional polyacrylate E and

polyisocyanate D), both at 23 °C, 50 % rel. humidity and 5 °C, approx. 50 % rel.

humidity. It is clear that curing of the super high solid coating (amine/isocyanate

reaction) is slowed less by reducing the temperature than is curing of the

conventional 2K PUR topcoat (alcohol/isocyanate reaction). At 5 °C (Curve 2), the

super high solid coating cures almost just as fast as the conventional coating at

23 ° C (Curve 3).

This means that the productivity gained through the use of the super high solid

coating either outdoors or in partially unheated shops is particularly apparent during
the winter. That work at the job site can proceed faster or that stationary coating

equipment can be used more effectively is a true cost advantage.

High chemical resistance, good overcoatability, excellent mechanical

properties

The high pendulum hardness of the super high solid coating (cf. Table 2) reflects a

higher density of crosslinker groups (urea structures) than in the standard 2K PUR

topcoat (polyisocyanate D and hydroxyfunctional polyacrylate E). These urea

structures are capable of forming hydrogen bridge bonds and thus contribute to a

greater resistance to organic solvents and aqueous acids and bases. Nevertheless,

the super high solid topcoat can be overcoated easily. In a peeling test, a panel

weathered outdoors in an industrial climate for 6 months before rinsing and

overcoating exhibited adhesion of more than 11 Mpa between the old and new

topcoat layers (xyz fracture). This suggests that it should be possible to refinish the

super high solid coating even years later with no significant pretreatment of the

surface to be recoated.

Measurements yielded by a mandrel bend test reveal that the super high solid

coating is no less flexible despite its greater pendulum hardness compared to the

standard coating. The super high solid coating is also at least on par with the

standard 2K PUR coating with respect to impact toughness (Impact Test), chipping

and abrasion resistance (Taber Abraser, CS 10 roller).

Similar film thickness achievable with two-coat systems

Another substantial advantage of amine-based PUR anti-corrosion coatings

containing reactive thinner is the high film thickness in which the coatings can be
applied without blistering. For the formulation described above, this is approx.

250 μm when applied with airless spray equipment and significantly higher if applied

by hand. Susceptibility to blistering is also significantly reduced compared to 2K PUR

topcoats crosslinked in a conventional isocyanate/polyol reaction. The desired

crosslinking reaction in the isocyanate amine/aldimine system is faster than the

isocyanate/polyol reaction and thus the undesired isocyanate/water reaction which

can lead to blistering is more strongly suppressed.

Total film thicknesses of 220 μm can be reliably obtained in two steps by combining a

zinc-rich 1K PUR primer (film thickness 100 μm) with a 120 μm thick film of the super

high solid topcoat. Such a two-coat system effectively protects steel against

corrosion. This was demonstrated in a salt spray test (ISO 7523) on low-alloy

structural steel (pretreated by blasting to Sa. 2 1/2), which the described coating

system withstood for over 2,000 hours.

It is thus possible to reduce the number of coating steps from three to two while

yielding the same film thickness as achieved with the conventional three-coat system

comprising primer, intermediate and topcoat.

If small amounts of the aldimine polyaldimine B are added to the zinc-bearing 1K

PUR primer prior to application, drying of the primer is so strongly accelerated that it

can be overcoated with the super high solid topcoat after only 1 hour. Because the

topcoat dries within only 3 hours to the point where it can withstand moderate

mechanical loads, steel components for new construction can be completely coated

and transported (dry to handle) only 5 hours after pretreatment of the substrate. In

contrast, a conventional coating system of epoxy resin primer and intermediate coat

with a 2K PUR topcoat requires approximately 36 hours to achieve this state.
In the case of refinishing operations, the fast overcoatability of the primer combined

with the short sand-dry times of the topcoat means that a surface can be completely

coated and resistant to soiling resulting from work on adjacent surfaces only 3 hours

after pretreatment of the substrate. The comparison with the conventional coating

system described above, which requires approximately 25 hours to achieve the same

state, is another impressive illustration of the productivity advantages gained by the

combination of an accelerated 1K PUR primer and a super high solid topcoat.

Table 3: Productivity of super high solid coating on accelerated 1K PUR primer compared with

conventional coating system. All times from completion of substrate pretreatment (assumes

shop working one shift only)

5 h

36 h 25 h

3 h

Time until the coated parts

can be transported

Time until the topcoat is 

resistant to soiling

Front: Super high solid topcoat, two-coat system;

Rear: Conventional three-coat system

Economic coating process

Although the costs of raw materials for such a super high solid coating are higher

than those for the conventional PUR topcoat crosslinked in an isocyanate/polyol

reaction, the elimination of one coating step more than offsets the added cost of the

coating raw materials. If one also considers the increased productivity of the coating

process due to faster drying, the cost balance is improved even further. Taking into

consideration the entire corrosion protection process, the novel super high solid

coating achieves the high performance of a conventional 2K PUR topcoat with more

reliable application and at a lower overall cost. Thus, in terms of productivity, the
novel super high solid coating is in no way inferior to the conventional PUR topcoats

used in corrosion protection applications.

Table 4: Economic comparison of the technologies

Economic Comparison

2-coat system

1K- PUR / 2K- PUR super high solid

Conventional 3-coat system

Cost of raw materials higher lower

Application cost lower higher

Productivity higher lower

Overall cost lower higher

Summary

The use of low-viscosity, sterically-hindered aliphatic polyamines in combination with

aldimine reactive thinners and low-viscosity aliphatic polyisocyanates permits the

formulation of a super high solid anti-corrosion topcoat with low VOC values of the

same order as water-reducible coatings. The coating is characterized by its high

quality in terms of its very good weather stability similar to epoxisiloxane and low

soiling.

The elimination of one coat for anti-corrosion coatings per ISO 12944 C5 and the

further increase in productivity due to rapid drying even at low temperatures more

than offset the increased cost of the binder for the super high solid coating compared

to the coating systems used today. The newly developed PUR raw materials for anticorrosion coatings thus unite the advantages of reduced solvent emissions and

greater productivity.
The manner in which you use and the purpose to which you put and utilize our products, technical assistance and information (whether verbal, written

or by way of production evaluations), including any suggested formulations and recommendations are beyond our control. Therefore, it is imperative

that you test our products, technical assistance and information to determine to your own satisfaction whether they are suitable for your intended uses

and applications. This application-specific analysis must at least include testing to determine suitability from a technical as well as health, safety, and

environmental standpoint. Such testing has not necessarily been done by us. Unless we otherwise agree in writing, all products are sold strictly

pursuant to the terms of our standard conditions of sale. All information and technical assistance is given without warranty or guarantee and is subject

to change without notice. It is expressly understood and agreed that you assume and hereby expressly release us from all liability, in tort, contract or

otherwise, incurred in connection with the use of our products, technical assistance, and information. Any statement or recommendation not contained

herein is unauthorized and shall not bind us. Nothing herein shall be construed as a recommendation to use any product in conflict with patents

covering any material or its use. No license is implied or in fact granted under the claims of any patent.

Outlook

In addition to polyamine A, a range of aspartic acid ester-modified aliphatic amines

with even lower viscosity and greater reactivity is already available. These can be

used to formulate lightfast, solvent-free and extremely fast-drying coatings.

Recent tests show that although replacing the HDI isocyanurate with an IPDI

isocyanurate results in a slightly reduced solids content due to the higher viscosity, it

also results in even longer pot life and significantly accelerated drying. This can

further increase productivity, even at low temperatures. A more detailed study must

still be conducted.

The anti-corrosion effect of single-coat super high solid coating system is also the

subject of intensive study.

Bibliography

1. 'Two-Component Aliphatic Polyurea Coatings for High Productivity Applications',

C. Angeloff, E. Squiller, Kurt Best; Journal of Protective Coatings & Linings;

August 2002, pp. 42ff

2. 'New Very High Solids Protective Topcoats Reduce Costs and VOC Levels',

Dr. R. Lemmerz; Protective Coatings Europe, July 2002, pp. 17ff

3. 'Aliphatic Polyurea Coatings Based on Polyaspartic Esters', E. Squiller, Polyurea

Development Association Conference, December 2000

4. 'Imine-Isocyanate Chemistry: New Technology for High Solid Coatings',

L. Venham, D. Wicks, P. Yeske, presented at the Division of Environmental

Chemistry - American Chemical Society, August 1994