Some time ago, I handled a query from an architect who required a very complex and unusual set of coating parameters to solve a particular problem. Proudly, I was able to detail a brand new coating just out of the development laboratory that fit the bill precisely. Instead of being awed by our technical brilliance, he asked whether it was available in hot purple. Being a development product, it wasn't, and the architect settled on a product which required substantial technical compromise but was available in the color of his choice.
This story outlines the importance of color in the buying or specifying decision, particularly in a society that is somewhat spoiled with choices. Collectively, the New Zealand paint industry offers formally approximately 12,000 different shades (and informally any shade that a customer requires). It was not always like this, however, and in the past, apart from a few trim colors, the antithesis of Henry Ford's policy existed-"Any color as long as it was white, or off white!" The advent of universal tinters was the key to the color revolution, and they changed the face of the New Zealand paint industry.
The Early Days
Universal tinters, meaning tinters used in both waterborne and solventborne decorative paint finishes, were developed in the U.S. in the 1950s. The concept was, and still is by using a small range of tint bases containing varying levels of factory dispersed pigment, a total range of colors could be produced by adding a known recipe of tinters. It is the sales person's and accountant's dream, and the technologist's nightmare.
The ability to instantly produce an infinite number of colors and never be out of stock has obvious attractions for the salesperson. The ability to do so without carrying a huge number of stock units appeals to the accountant. The technical truth is, however, that universal tinters contain large amounts of excipients (water, glycols, surfactants) that can be deleterious to the base paint.
The Resene Experience
In the late 1960s Resene introduced a point-of-sale tinting system based on an earlier American tint system which, in truth, contained excellent inorganic dispersions but extremely weak organic dispersions. We celebrated the system by also introducing to New Zealand the British Standard 2660 color chart. To complete the package, we made the decision to become lead free. Architects loved the new, bright colors offered by the B.S.S. color range, but the bright yellow�0-001 based on a 20% addition of a weak Hansa Yellow tinter to an almost neutral base, which covered little better than a yellow varnish stain�had poor flow characteristics, took days to dry, and in some instances, the humidity and temperature balance would dry into a perfectly flat finish. It went down like a lead balloon with the painting fraternity. It was clear that painters in New Zealand were not prepared to accept quality compromises for the convenience of having a tinting system. This dilemma of setting quality still exists today, and the Achilles Heel of most tinting systems is the inability to produce deep, bright colors with adequate covering power.
The Resene Approach
The valid approach of using two sets of tinters for solvent- and waterborne decorative coatings was not open to us, and consequently we adopted the strategy of trying to reduce tinter levels at all times. While it is theoretically possible to produce every known color from a neutral base and a range of tinters, the quality limitations of such an approach are as obvious as are the commercial benefits to the tinter supplier.
Our approach was, in fact, to increase the number of titanium dioxide-containing bases and to also introduce a range of colored bases. For example, in the area of bright yellows, we produced a base utilizing nickel titanate yellow and pigment yellow 1, (later to be replaced by pigment yellow 74). Nickel titanate yellow proved to be an ideal pigment for tinting as it had a very low tint strength but excellent hiding power. In 1970 this was a pretty novel approach. We also produced a range of fully tinted undercoats.
These moves went some way to remedying the hiding strength problems, but there were still shortcomings. In 1973 we became aware of a range of universal colorants which were designed only for in-plant tinting but had exceptionally high strength. Indeed, some of the organic pastes were up to four times the strength of traditional pastes being offered. This allowed a significant breakthrough and the hiding of deep colors to truly approach the standard, high-quality, dry-ground analogues.
The Influence of Shelf Space
Our company has the luxury of owning our own single-brand outlets, which do not demur from stocking the six white and five colored bases in our system. More typically, constraints on shelf space push paint manufacturers competing for this space to have fewer bases within their system, often as few as two. We have designed such systems for colleagues in the Asia-Pacific area, but the compromise on quality is significant. Although the inclusion of a white tinter in the system does allow an infinite number of white bases, costs and excipient-induced weaknesses are heavy prices to pay.
High-strength tinters can be used to solve many of the hiding and quality problems associated with tinting systems, but they do have two weaknesses. One, in the production of pale colors, is small sizes, particularly in markets that have low levels of titanium in their white. The inclusion of some deliberately weak shading tinters was one approach to overcoming this. However, the ever-increasing precision of delivery equipment also solves the problem.
The second and most frustrating and persistent problem, exacerbated by high-strength tinters, is poor tinter compatibility. I use the word "compatibility" with a certain amount of trepidation as it does not accurately describe all the nuances of the intermixing of concentrated tinter pastes.
A Model System
A simple and idealized model for universal tinters would be as follows. First, firmly anchor a monomolecular layer of material onto the pigment surface, which presents a hydrophobic orientation away from the pigment. Double layer this with a simple hydrophilic/ hydrophobic structure which would convert the pigment particles to a hydrophilic surface. Carry these particles in a water/glycol blend. In a waterborne system these would diffuse and disperse spontaneously throughout the system.
In a solventborne system, the water and glycol would nucleate micelles, which could peel off the outer layer of surfactant, revealing a compatible, hydrophobic layer on the pigment and an equally compatible hydrophilic-centered micelle surrounded by an outwardly looking hydrophobic layer.
So much for fairy tales. Studies introducing drops of colorant into various media shows that there is almost no spontaneous diffusion in anything other than water and a few, simple, water surfactant systems. Polymers, thickeners and surfactants rarely ever set up simple solutions, but rather a complex of domains. Indeed, associative thickeners are designed to promote such domains.
In my experience, tinters find their way to the places in the paint system where they are most comfortable, and this is the cause of much of the problems with compatibility, color development, tint acceptance and rub up, which continues to frustrate us. The most dramatic example I ever saw was in a highly structured urethane-acrylic system designed for high-build, airless spray application. It was tinted to a light grey and produced a uniform color on application. When the applicator's airless spray gun broke down, he reverted to air assisted spray, thinning the product with the correct thinner, which brought about the appearance of black pigmented droplets in a very pale grey continuum. The pigment-friendly domains had obviously been finely dispersed through the coating and were held in place by high structure. Release of this structure by thinning allowed these domains to coalesce into the visible black droplets.
Factors Affecting Colorant Compatibility
Colorant compatibility is affected by all elements of the system. The continuous medium has probably the most significant impact, but these can be subtly affected by the dispersed phase. Identical paints varying only in the type of titanium used can show dramatic differences in tinter acceptance. One can only assume that the orientation of the first layers of material on the titanium surface provides some form of template for subsequent associations. We have even used the tactic of blending hydrophilic and hydrophobic grades of titanium in the same product to solve persistent acceptance problems.
Time can be a wonderful ally in solving these problems. Evaluating one particular alkyd, I have seen the worst tinter acceptance performance in my experience turn into the best simply by aging the untinted base paint three months. In this case I felt sure that a substantial high molecular weight fraction in the rather poorly made alkyd was problematic initially, but that this fraction eventually ended up on the surface of the titanium and completely changed the nature of the continuous phase. Indeed, the presence of a large amount of a low-molecula-weight fraction could have been beneficial in this configuration.
Although time is normally an ally, we have also experienced the reverse. At its most embarrassing, a tinted batch of paint was supplied and half of the 400-liter order was applied satisfactorily and produced the correct color. The application of the second coat was inadvertently delayed by about four weeks, during which time, due to selective flocculation of the titanium, the color was totally different.
Selective flocculation is an unusual phenomena. We have even seen it in a minor pigment component during a typical rub up test. I started this section referring to the fact that, to me, the process of incorporation of the tinter into the paint system is shrouded in mystery. I have challenged many people with this question but have found that few have given it consideration. I therefore commend Johan Bieleman, formerly of Condeo Servo BV, in a recent paper for at least attempting to describe the phenomenon.
Color as a Marketing Tool
It is interesting to note the marketing potential of color that tinting systems bring, and yet the architectural paint market has been very slow to adopt color as a point of difference. Even though the U.S. market has led with the "walls of color" concept, color offerings have generally been the same from supplier to supplier. It is only recently that differentiating color offerings such as those endorsed by Ralph Lauren and Martha Stewart have been used to differentiate paint companies.
The presence of a successful standard, such as British Standard 5252, can mitigate against individual paint company's exploitation of color.
South Korea has a single fandeck, which is produced by a paint manufacturers association, and all companies use it. To the best of my knowledge there are no individual color cards at all in that country.
We believe color can be a successful point of difference between paint companies, and we attempt to bring out a new range every year. We also include within the range anything which may be fashionable, be it paint effects, metallics or pearlescents. The color offering gets backed up with test pots, self adhesive color chips, drawdowns and electronic representations. Color renderings are offered, as are virtual painting schemes and the ability for users to generate their own colors using the RGB guns on a screen.
There is virtually no product that we make which is not designed to bolt on to our tinter system to instantly provide our full color library in that product. New specialized tinters get added for new finishes, such as transparent iron oxides for our wood stain range, and more inorganic colors for silicate and silicone emulsion finishes.
The full color range is offered in a high-performance range of coatings, and the requirements of the market are such that any difference in color standard expectations between a purely decorative and a high-performance coating have long since disappeared.
As for the future, the demise of universal colorants will parallel the swing to all waterborne technologies. However, as this swing is happening at vastly different rates throughout the world, universal colorants are likely to be around a long time. Some of the major pigment houses, however, are signalling their readiness for this change by expanding their traditional ranges of pastes offered for waterborne paints and by offering them calibrated volumetrically. We have designed tinting systems on such products for some colleagues in the Asia Pacific area who only have a waterborne product range.
I believe that the major thrust for the future will be tinting pastes with excipients, which have a secondary useful role in the coating. Already there is a suggestion from one surfactant manufacturer that its surfactant can also double as a coalescent.
Major advances in compatibility will only ensue when the pigment surfaces are fully stabilized with a tenaciously adhering surfactant system. Pigment manufacturers have been noticeably reticent in providing clear characterizations of their pigment surfaces. Without such knowledge, dispersing/stabilizing systems will invariably be haphazard and lacking precision, and the excessive use of surfactants will continue to be both an economic and technical weakness.
Colin Gooch is technical director at Resene Paints, Ltd., in New Zealand. This paper was initially presented at The Colour Delivery Challenge II, in April 2002, organized by the Paint Research Association (PRA). For more information on the conference, contact PRA, at 44�(0) 20�8614�4811, E-mail: email@example.com; Web: www.pra.org.uk.