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ONE HUNDRED PERCENT SOLIDS ALIPHATIC POLYURETHANE COATINGS -- FROM DREAM TO REALITY

Shiwei Guan, Ph.D., Research and Development Manager, Madison Chemical Industries Inc., 490 McGeachie Drive, Milton, Ontario, Canada L9T 3Y5

ABSTRACT

Recent advances in high solids aliphatic polyurethane coatings technology for high performance/heavy duty industries are reviewed. A new technology has been developed which results in a 100% solids, VOC free, fast set, aliphatic polyurethane coating system. The performance properties of this 100% solids aliphatic polyurethane system are discussed, comparing them with those of commercial 100% solids aromatic polyurethanes and high solids aliphatic polyurethanes.

INTRODUCTION

100% solids polyurethane coatings have become one of the most important families in the coatings industry today. These polyurethane systems have been and are still effective because of their outstanding life expectancy and performance, resistance to aggressive corrosive environments, high abrasion and impact resistance, curing capability at temperatures as low as -40^oC, strong adhesion, high film build, fast application, and compliance with the most rigorous regulations on volatile organic compound (VOC) emissions.

Existing 100% solids polyurethane technologies, however, have been confined mainly to aromatic polyurethane systems. Aromatic polyurethanes are made by reacting aromatic isocyanates, such as diphenyl-methane diisocyanate (MDI) or toluene diisocyanate (TDI), normally with polyether polyols. They oxidize much more easily than do polyurethanes prepared from aliphatic isocyanates when exposed to UV light. Aliphatic isocyanates reacted with either polyester or acrylic polyols have thus become indispensable in the formulation of high performance weatherable coatings. They present the industry with its first durable ambient curing, thermosetting protective coating with long term gloss and color retention.

Traditionally, aliphatic polyurethane coatings incorporate high-molecular-weight compounds. These compounds, typically, require high levels of solvents to make them usable in spray applications. These levels of solvent were acceptable previously but will not be permitted in the near future due to increasingly stringent environmental and safety regulations coupled with consumer demands for ecologically friendly materials.

Over the past several years many efforts have been made to develop higher solids aliphatic polyurethane systems. These efforts include: 1) the use of low molecular weight compounds which have lower viscosity resulting in minimum solvent usage for application; 2) the use of reactive diluents and 3); the use of new polyurethane prepolymers. New technologies have also been developed by the author recently at Madison Chemical which leads to three novel approaches to high solids aliphatic polyurethane coatings: a 100% solids, VOC free, instant setting, aliphatic polyurethane coating system; a high solids mix-and-apply aliphatic polyurethane coating system; and a high solids single component aliphatic polyurethane coating system.

This paper is the first in a series about the above three novel approaches to high solids aliphatic polyurethane coatings. Beginning with a discussion of the basic aliphatic polyurethane chemistry, the paper reviews the existing and commonly-used approaches towards higher or 100% solids aliphatic polyurethane coatings. Performance properties of the newly developed 100% solids aliphatic polyurethane technology are also discussed, comparing them with those of commercial 100% solids aromatic polyurethanes and high solids aliphatic polyurethanes.

ALIPHATIC POLYURETHANE CHEMISTRY

Typically, very high solids (both aromatic and aliphatic) polyurethanes are in a two-component format. This consists of one component being a polyisocyanate rich liquid, and the other hydroxyl functional coreactants. The typical aliphatic polyurethane reaction is shown below:

O O O

| | | | | |

R-N=C=O + R’-C-O-CH₂CH₂OH ® R-NH-C-O- CH₂CH₂-O-C-R’ (1)

The reaction advances readily and has an exothermic nature, thus providing polyurethane systems with low temperature curing ability. This is a significant advantage over many of the other reactive resin systems used in the coatings industry, such as epoxies.

While there are virtually unlimited arrays of hydrogen donors for preparing polyurethane coatings, hydroxyl based systems are used almost exclusively. In an aliphatic system, polyurethane coatings produced from the reaction of aliphatic isocyanates with a hydroxyl functional coreactant were initially formulated with polyester polyols. These coatings possessed good weathering performance. With the increased demand for improved durability at low cost, the use of acrylic polyols was then adopted. Acrylic polyols have low isocyanate demands and also attain superior hydrolytic stability over polyesters. Both polyester polyols and acrylic polyols tend to be more expensive than polyether polyols, and they are usually more viscous and therefore more difficult to handle. The high viscosity of polyesters and acrylics obviously makes it more difficult to develop a 100% solids aliphatic polyurethane system.

In comparison, the isocyanates, whether in their original or in a modified form, permit only few variations. The most common aliphatic isocyanates are hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI). The nature of the isocyanate-bearing species will greatly affect reactivity. Aliphatic isocyanates are considerably less reactive than aromatic isocyanates such as TDI and MDI. Differences also exist between individual aliphatics, for example, HDI is faster than IPDI. For the above reasons, commonly available aliphatic polyurethane coatings have a much longer curing time (e.g. 5-8 hours) compared with aromatic polyurethanes (e.g. 10 minutes to 1 hour).

A two component polyurethane system normally has a mixing ratio of 1:1 by volume. Higher mixing ratios are normally used to obtain a coating system whose two components are mixed together just before application. In this case, the pot life of the coating system must be of sufficient length (i.e. at least 45 minutes) for easy application. Coating systems with higher mixing ratios tend to be less sensitive to quality problems encountered during both the coating production and application, where possible mismetering of one or both components may take place. The 1:1 mixing ratio is an ideal choice for achieving not only 100% solids, but an instant setting polyurethane system. It is commonly used for most 100% solids aromatic polyurethane coatings. One of the major problems which formulators often meet, however, is to design a 1:1 coating system so that each component has the same viscosity, requiring the same pressure to cause them to flow at the same rate. The equal viscosity helps to simplify the set up and maintenance of the spray equipment, and also to avoid many application problems due to mismetering of the two components while using plural component spray equipment¹.

EXISTING APPROACHES TOWARDS HIGHER SOLIDS

The 1990 Clean Air Act has imposed very stringent controls upon air emissions and restricted the amount of organic solvents (the primary source of VOC’s) emitted from a coating system. Together with the technologies of powder coatings and water-borne coatings, high solids coatings technology evolved to meet the demand for reduced solvent levels. 100% solids technologies have been developed for epoxy and aromatic polyurethane systems. For aliphatic polyurethane systems, however, 100% solids technology has been rarely achieved by manufacturers in the industry. This is perhaps due to: 1) most commercially available 100% solids aliphatic polyisocyanates, polyester and acrylic polyols have a high molecular weight and a high viscosity. These polyols require high levels of solvent to make them usable in spray applications; 2) when their individual viscosity values are in the suitable range, different polyisocyanate demands of those 100% solids polyols makes it difficult to design a coating system with the right volume mixing ratio and/or balanced viscosities of the two components; and 3) greatly reducing the solvent content makes a major impact on formulation and performance properties of a coating system, and thus becomes a challenge to coatings chemists.

Over the past ten years, three main routes to compliance were pursued to achieve a high solids aliphatic polyurethane coating system:

the use of low molecular weight compounds which have lower viscosity resulting in minimum solvent usage for application^2,3

the use of reactive diluents⁴, and

the use of new polyurethane prepolymers⁵.

 

The use of low molecular weight compounds

Higher solids aliphatic polyurethane coatings have been achieved by reducing the molecular weight of the reactive components. This applies to both polyisocyanates and coreactants. In response to this need, polyisocyanates and polyol components have been optimized to give the lowest viscosity through molecular weight adjustment.

Compared with coreactants such as polyester and acrylic polyols, it is apparent that the aliphatic polyisocyanates are already the high solids component of the coatings. Commercially, HDI based isocyanate and biurets are being made to viscosities as low as 500 centipoise. All of the commercial and developmental polyisocyanate adducts used in high solids coatings are free of monomeric diisocyanate to meet safety and handling requirements.

Acrylic polyols have been favored for most aliphatic polyurethane systems because of their superior weathering, low cost, hydrolytic stability, and low isocyanate demands. However, these products have in the past been of very high molecular weight (10-20,000 g/mol). The major drawback of high molecular weight acrylic polyols is their high solvent demand, yielding coatings with less than 50% solids at application viscosities. Low molecular weight acrylics have therefore been developed with molecular weights typically below 2,500 g/mol. The functionality of these low molecular weight acrylics, however, has had to be increased to maintain the excellent resistance characteristics of these types of systems. Coatings compositions with solids contents of greater than 75% have been achieved. Altering the molecular weight/functionality to obtain compliant coatings has, however, resulted in shorter pot lives and longer drying times when compared to existing polyurethane coatings. To overcome the slower dry times, cure rates are increased by adding catalysts or increasing the amount of catalysts, which improves cure times, but also leads to the problem of short pot lives.

Another problem with the use of low molecular weight polyisocyanates, combined with the reduced molecular weight of the co-reactants like acrylics, is an increase in the crosslink density of the coating. Too high a level of crosslink density can lead to a decrease in flexibility, impact and mar resistance.

Rather than low molecular weight polyester and acrylic polyols, blocked amine functional reactants have also been introduced to make higher solids aliphatic polyurethane (or more precisely, polyurea) coatings. These include aspartic acid esters, ketimines, and aldimines. A typical urea reaction is shown below:

O

| |

R-N=C=O + R’NH₂ ® R-NH-C-NH-R’ (2)

Aspartic acid esters are secondary amines resulting from the modification of primary amines by reaction with dialkyl maleates. The reactivity of aspartic acid esters is reduced to a much less extent than primary amines due to steric effects and hydrogen bonding. Their reactivity towards polyisocyanates is very dependent on the structure of the parent amines. They, however, still do not represent on their own a perfect partner for isocyanates in two component systems.

The use of ketimine groups as a source of blocked primary amine crosslinking agents has found some acceptance in high solids coatings. With aliphatic isocyanate based systems, the ketimines can react in a direct manner. This reaction is extremely slow in the absence of weak acid catalysts but proceeds very quickly to a high yield when exposed to ambient moisture. In a curing film the reaction is competing with the hydrolysis of the ketimine. The major drawback of using ketimines is their instability towards hydrolysis and self condensation. The shelf lives of high solids polyurethane coatings with ketimines are very limited and extreme care has to be taken in coating manufacture, storage, handling, and application.

Aldmines are products resulting from the reaction between amines with aldehydes. Compared with ketimines, aldimines exhibit much better hydrolysis stability and thus can be adapted to actual applications. Experimental formulations with solvent levels at or below 2.0 pounds/gallon have been reported⁶.

The use of reactive diluents

Reactive diluents are low-viscosity coreactants designed to reduce VOC’s in coating systems. While they generally provide the ability to reduce the viscosity of the polymeric composition similar to a solvent, they also undergo reaction with the polymer. Reactive diluents based on low molecular weight polyester or polyether polyols, castor oil derivatives, oxazolidines and acetoacetate chemistry are commercially available. Differing from those low molecular weight coreactants discussed above, reactive diluents tend to have a low reactivity toward isocyanates, a low functionality, and a relatively low viscosity. These make them very attractive for use in aliphatic polyurethane systems with long potlives and reasonable dry times. Aliphatic polyurethane systems with VOC levels as low as 2.58 lbs/gal have been reported⁴.

There are several problems associated with the use of reactive diluents which the author has found during the past few years. The first problem is that reactive diluents are normally much more expensive than commercially available polyesters, acrylics, and other coreactants. This increased the raw materials costs of the coatings systems to a point that end users found the coating systems too expensive. The second problem with the use of reactive diluents is possible performance property changes if large amounts of reactive diluents are added into a coating system. This is due to their low functionality and low molecular weight.

The use of prepolymers

Prepolymers have long been used in high solids polyurethane coatings systems, especially in aromatic systems. Conventional prepolymer manufacture often results in the production of a significant amount of high-molecular-weight oligomers. It is these oligomers that cause high-prepolymer viscosities.

New prepolymer technology has therefore been developed to achieve high-molecular-weight aliphatic polyurethane prepolymer systems with a low oligomer content and less than 1% of residual isocyanates⁵. By using the low viscosity, higher-molecular-weight prepolymers as the isocyanate rich component (often referred as the "A" side), a significant amount of the high solvent demand polyols from the polyol rich component (the "B" side) can be avoided. The low oligomer content of these prepolymers also results in the additional benefit of longer potlives. By using the new prepolymer technology, IPDI based aliphatic polyurethane coatings systems with VOC contents at 2.8 lbs/gal. have been reported⁵.

Table 1 summarizes these above existing approaches towards higher solids aliphatic polyurethane systems. Obtained VOC’s levels for individual approaches in Table 1 are typical for aliphatic polyurethane basecoat/topcoats.

Table 1 Existing approaches towards higher solids aliphatic polyurethanes

100% SOLIDS ALIPHATIC POLYURETHANE TECHNOLOGY

As shown in Table 1, the existing approaches towards higher solids aliphatic polyurethanes have provided various ways to reduce the solvent levels in the coatings systems, and each of them has their own advantages and disadvantages. These technologies are, however, still far from achieving a 100% solids, zero VOC, aliphatic polyurethane system. They represent two major problems in the development of higher solids aliphatic polyurethane systems.

First, the existing approaches have placed too much emphasis on the capability of a higher solids aliphatic polyurethane system being applied using conventional field pressure pot spray equipment. This has immediately forced the coatings chemist’s thinking towards developing an "ideal chemistry" which meets the following requirements: 1) the rate of reaction between aliphatic isocyanates and coreactants has to be suppressed before application; 2) the method of suppression must not generate excessive VOC’s, and then 3) the rate of the reaction should be greatly accelerated after application. The results of this "ideal chemistry" have therefore had to include all those problems discussed above (hydrolysis stability, short potlives, etc.).

The use of low molecular weight coreactants, reactive diluents, and prepolymers are really not new to the coatings industry. These ideas have been successfully used in the development of 1:1 mixing, plural component, 100% solids epoxy and aromatic polyurethane coatings. These 100% solids coatings systems normally require plural component spray equipment. The two components are delivered through individual fluid lines to a mixing device, which is located within the spray gun or directly before the spray tip. Compared with conventional spray equipment for low solids coatings, the plural component equipment is more complex. Successful application of these 100% solids coatings, however, has been and is being achieved by numerous end users on a regular basis. Therefore, the use of conventional spray equipment should not become an initial restriction in designing a higher solids coating system.

Secondly, too much emphasis has also been placed on finding a "super-ideal resin/coreactant solution" on an individual basis to reduce the solvent levels. However, formulating a good higher solids polyurethane coating system involves consideration of many parameters other than solvent demand and molecular weight. Research efforts towards higher solids or 100% solids aliphatic polyurethanes should, therefore, be made in a systematic way, combining all individual possible solutions together.

This systematic and "open-minded" concept has resulted in a 100% solids, VOC free, instant setting, aliphatic polyurethane technology. This new 100% solids technology produced a resilient polyurethane coating film with excellent tear strength, impact and abrasion resistance. The viscosity values of both isocyanate and coreactant components were perfectly balanced at 1,000 centipoise as per ASTM D2196. Handling characteristics were: end users’ choice of initial setting time from 5 to 30 minutes, cold weather cure ability (-10^oC to +65^oC), low and high film build, a one coat system for most general maintenance applications, non-flammable. Although application of this 100% solids aliphatic system is currently with plural component (1:1) spray equipment, it is interesting to note that this new 100% solids technology can lead to unique coating formulations so that a conventional single component spray equipment could be used.

PERFORMANCE RESULTS OF THE 100% SOLIDS ALIPHATIC POLYURETHANE

Physical and chemical properties of the 100% solids aliphatic polyurethane system, as well as its gloss and color retention, were all high. The following typical results are given together with those of a commercial 100% solids aromatic polyurethane and a commercial high solid (70% solids) aliphatic polyurethane systems.

Weathering was investigated by placing panels in a QUV cabinet fitted with UVB-313 bulbs for 5,000 hours as per ASTM G53. Panels were exposed to a cycle of eight hours UV at 50^oC followed by four hours condensation at 40^oC. Gloss values of the panels before and after exposure were measured as per ASTM D523. After 5,000 hours exposure, both the 100% solids and the 70% solids commercial aliphatic polyurethane systems had gloss retention of greater than 90%, compared with 65% with the 100% solids aromatic polyurethane system (Table 2).

Table 2 5,000 hours QUV 313B test results (ASTM G53)

Long-term outdoor weathering tests with these systems is currently underway but initial results for 18-months has already been obtained. The 100% solids aromatic polyurethane system showed chalking and darkened in color as was expected. Both the 100% solids aliphatic and the 70% solids commercial aliphatic polyurethane systems had gloss retention of greater than 95% and showed no color changes by a visual examination.

Chemical resistance was evaluated for 96 hours as per ASTM D714 in 10% H₂SO₄, 5% HNO₃, and 25% NaOH solutions. The 96 hours of continuous immersion exposure is far more than the average duration of a chemical spill. Yet, this has become the standard used by some coatings manufacturers as part of the "extra care" that they take for end users. Table 3 shows typical results obtained for these systems.

Table 3 96 hours chemical resistance test results (ASTM D714)

No color changes were found with the 100% solids aliphatic and the 70% solids commercial aliphatic polyurethane systems. They were however slightly softened, which is typical for most high solids aliphatic polyurethane systems after immersion. As is to be expected from most of the advanced 100% aromatic polyurethane systems, the tested 100% solids aromatic polyurethane coating was discolored without any other physical property changes.

Taber abrasion resistance tests were conducted as per ASTM D4060 for the three systems. The Taber abrasion test rotated a sample of the coating, under a load pressure of the certain weight (1 kg), against a grinding wheel using a specified size (CS17) and a defined number of revolutions (1000 cycles). The samples were evaluated by measuring the change in mass before and after the abrasion. All three of the tested polyurethane systems showed good abrasion resistance (Table 4). However, it should be noted that the abrasion resistance of a polyurethane system is somewhat related to its rigidity. A polyurethane coating could be designed to be either very rigid or very elastomeric, and better abrasion resistance was normally found with an elastomeric system. One cannot therefore conclude from Table 4 that the aliphatic polyurethanes would have better abrasion resistance than the 100% solids aromatic polyurethane.

Table 4 Results of abrasion test (ASTM D4060, 1 kg load, CS17, 1000 cycles)

The adhesion of a coating is generally considered to be a good indicator of its longevity. Table 5 shows average adhesion testing values (ASTM D4541) when adhered directly to a near white blast (SSPC-SP10) steel. The tested 100% solids aromatic polyurethane coating showed excellent adhesion up to 2000 p.s.i. Both the 100% solids aliphatic and the 70% solids commercial aliphatic polyurethane systems had an adhesion value greater than 750 p.s.i., which was above normal for the exterior application. It should be mentioned that the excellent adhesion of the tested 70% aliphatic polyurethane, also developed by the author, is not commonly seen with most of the high solids aliphatic polyurethanes available in the market today. When applying the aliphatic polyurethanes to very rough substrate surfaces such as ductile iron pipes or concrete structures, an aromatic basecoat under the aliphatic polyurethanes should be recommended.

Table 5 Results of adhesion test (ASTM D4541)

A nationally recognized steel tank manufacturer had been using a commercial high solids coating to coat the exterior of their above ground tanks. Although the high solids system gave a satisfactory finish when cured, it created problems in production. One of the serious problems was the length of time required between coats as well as the time it took the coating to dry, especially at cold winter temperatures. This cut capacity and slowed production considerably. When the tank was coated with the new 100% solids aliphatic polyurethane technology, however, a number of advantages became immediately apparent: only one coat was required to completely cover the tank without a primer; the new coating cured in about 20 minutes at ambient temperature and the tank could be moved out of the painting room very quickly - this allowed them to bring the next tank in for coating quickly; and the new coating dried to a bright and shiny finish which has excellent resistance to UV and weathering (Figure 1).

Figure 1 Above-ground steel tank exterior application of the 100% solids aliphatic polyurethane coating technology

CONCLUSIONS

The use of a systematic, "open-minded" approach has resulted in a new 100% solids, VOC free, instant setting, aliphatic polyurethane technology. This technology provides a unique tool in developing aliphatic polyurethane coatings which are not only environmentally friendly but exhibit remarkable performance properties.

REFERENCE

1. Shiwei Guan, "Assuring Quality When Applying 100 Percent Solids Polyurethanes", Journal of Protective Coatings and Linings, 74, December, 1995.

2. J.L. Williams, "High-Solids Polyurethane Coatings,: Past, Present, and Future", Proc. Twentieth Waterborne, Higher-Solids and Powder Coatings Symp., New Orleans, LA, 1, 1993.

3. S.M. Lee, etc., "Aldimine-Isocyanate Chemistry: A Foundation for High Solids Coatings", Proc. Twenty-Second Waterborne, Higher-Solids and Powder Coatings Symp., New Orleans, LA, 69, 1995.

4. G.N. Robinson, etc., "High Performance Polyurethane Coating Systems Utilizing Oxazolidine-Based Reactive Diluents", Journal of Coatings Technology, 69, Vol. 66, December, 1994

5. Jeffrey Kramer and Sherri Bassner, " Using Novel Polyurethane Prepolymers in VOC-Compliant, Two-Component Weatherable Topcoats", Paint & Coatings Industry, 42, August, 1994.