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Year : 2002  |  Volume : 4  |  Issue : 14  |  Page : 25-29
The challenge of solvent substitution in coatings

DuPont Performance Coatings Ltd., Dagenham, United Kingdom

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A brief outline of the role of solvents in surface coatings is given, to provide the background to the problems encountered in solvent substitution. The direct substitution of one solvent by another is relatively straightforward. Computer programs greatly simplify the process. However, the real challenge lies with the reduction in solvent use, through the development of low solvent coatings. While this is a process that has been going on for many years, there is no doubt that recent regulatory pressures regarding both occupational health and environmental standards have made it the single most important challenge for the industry. Examples are discussed of the development of high solids, low solvent industrial coatings as well as water borne industrial coatings. Although there has been rapid progress in recent years, there is no prospect in sight for the complete elimination of solvents. Good workplace hygiene must therefore remain a key element in the safe use of coatings.

Keywords: coatings, solvent substitution

How to cite this article:
Giordan B. The challenge of solvent substitution in coatings. Noise Health 2002;4:25-9

How to cite this URL:
Giordan B. The challenge of solvent substitution in coatings. Noise Health [serial online] 2002 [cited 2020 May 26];4:25-9. Available from: http://www.noiseandhealth.org/text.asp?2002/4/14/25/31812

  Why use solvents? Top

Coatings are typically mixtures of three main components - binders, pigments/extenders, and solvents. The binder forms the film, and may provide the surface gloss. Pigments provide colour. Extenders are not necessarily cheap fillers, but provide key properties in products ranging from wall paints to industrial primers. Together with any additive required to give special properties, these make up the dry paint film, and provide the protection and decoration sought by the user.

Solvents are unique, in that they are a temporary component of the paint. They are used purely as a means to transfer the coating film on to the substrate, and then they are lost. It follows that the coatings chemist will try to use solvents that are low in cost, but he also has to consider their health and environmental impacts.

Solvents function by dissolving the polymeric binder. To do this, they have to separate the individual binder molecules, and this means that different chemical species are effective with different types of binder. The solvent molecules (generally rather small) have to penetrate between and separate the long strands of binder. Unless they have a higher affinity for these strands than the strands have to each other, then they will not dissolve the binder. No useful binder dissolves fully in water - if it did, it would remain permanently water sensitive. So water is almost never a coatings solvent. Where water is used, it is typically as a carrier, which allows dispersed binder, pigments and solvent to be transferred to a surface.

Coating films are thin - they can be less than 10 microns, and they seldom exceed 1000 microns. Their function is both to decorate and to protect the surface that they are applied to, so they must be uniform in film thickness, free from defects, and provide a regular surface profile. By reducing the viscosity of the paint during application process, the solvent allows a regular, uniform film to be laid down. It then helps the film to integrate and flow out, and thus ensures a smooth surface profile.

Other means of achieving these ends exist, but they all have some limitations. Examples are powder coatings, and radiation cured coatings.

Powder coating became an important process in 1960s, and has grown very rapidly, replacing many industrial uses of solvent borne paints. Solvent emissions are eliminated, though small amounts of VOC may result from curing reactions or thermal degradation of the polymer. Further, application efficiency is typically very high, so that waste levels are reduced. However, temperatures of 150 C are typically required, and this limits their use on wood and plastic substrates, and where the painted article is bulky (e.g. structural steel, ships). Articles are also often painted after assembly, and cannot tolerate high temperatures. Powder coatings have been very successful, though, in some areas of metal finishing. An interesting, relatively new, application is the final clear coat applied to car bodies, showing that the process is capable of achieving high standards of appearance and weather resistance.

Radiation cured coatings use as binder relatively low molecular weight oligomers, which can be cross-linked using UV or electron beam radiation. This is typically an ambient temperature process, and so is particularly suited to some applications where powder coatings are not practicable. It is, however, sensitive to the shape of the article, and works best on large flat areas. The reactive oligomers that are used are relatively hazardous compared with the polymers and solvents used in conventional coatings, and this makes the process most suited to applications where effective containment is possible, like roller coating and printing.

Important as these alternative processes are, their limitations mean that many application processes still rely on coatings containing solvents.

  Direct substitution of one solvent by another Top

The direct substitution of one solvent by another has always been, and remains, a challenge for the paint chemist. The driving force may be cost or availability, but it can also be the publication of new information, indicating that the solvent is less safe - to health or to the environment - than had been thought. A topical example is the current focus on the contribution of secondary organic particulates to air pollution (Dusek, 2000). This could well lead to pressure to replace some of the higher boiling aromatic hydrocarbon solvents.

To achieve this substitution, the ability of the solvent to dissolve the different types of binder must be fully characterised. Hildebrand and Scott (1964) sets out the physico-chemical principles involved. In addition, two other solvent properties are important in practice - the rate of evaporation and the flash point. The former is controlled by the process. Sometimes this needs a complex solvent mixture: part must evaporate quickly, to set the film, and prevent running and sagging, part must evaporate at medium speed, to ensure smooth integration of the film, and part - the tail - must remain until last to produce a sealed, glossy surface. The flash point broadly follows the rate of evaporation, but the main consideration is to avoid low flash points, for safety and regulatory reasons.

Knowledge of all the parameters of the individual solvents is still not enough. For example, the evaporation rate of a solvent can change when it is mixed with another. So computer programs are essential to handle the resulting complex mathematics. Such programs have been described in the literature for many years. For example, Rocklin and Bonner (1980) review literature back to 1958. Programs developed on this basis by the major solvent suppliers are available to formulators. While the results of the computer predictions must always be tested in practice - no model is perfect - the industry can react quite rapidly to a demand for substitution of this sort.

  Solvent reduction Top

The approach described solves problems of the exchange of one solvent for another, but it does not help with the wider target of reducing the overall quantity of solvent used. Solvent reduction requires a more radical approach, involving either high solids or waterborne coatings. In both cases, success depends on the redesign of the binder. Two examples from fairly recent experience follow.

Example 1: high solids vehicle refinish clear coat

The clear coat is typically the last finishing coat applied during the repair of a motor vehicle. It has to achieve the high gloss and weather resistance of the original finish. Typical conventional products are two pack, isocyanate cured acrylics, with a solids content by volume of up to 35%.

One way of reducing the solvent content is to increase the proportion of binder. Unfortunately, this brings in its wake some serious problems. The proportion of solvent can be reduced only by reducing the molecular weight of the binder, and hence its viscosity. The reduction in the viscosity of the binder means that the clear coat is now prone to run and sag during application, and the reduction in molecular weight means that it takes much longer to cure to a fully cross linked film.

These problems are soluble, but they require significant research and development efforts in order to develop satisfactory solutions. The polymer redesign must achieve a much more rapid viscosity increase as the solvent evaporates, and this typically involves the introduction of a measure of viscoelasticity. At the same time, reactivity must be improved.

The result is shown in [Figure - 1]. The comparison is between quantities of clear coat containing equal volumes of solids. Thus the effect of the quantities - the surface area painted - will be equal. For this effect, the amount of solvent used is reduced from 659 ml to 330 ml, or about 50%. (In parenthesis, it should be noted that the VOC expressed in the terminology of the UK Process Guidance Notes falls only from 560 to 420 g./l. The expression of VOC content in this way is convenient from the point of view of environmental regulation, but can give a misleading picture from an occupational health point of view.)

This emissions reduction has been achieved with no loss of coatings performance, and no overall process cost to the user (Rentz, Blumel and Lonjaret 1999). It illustrates the progress that can be made in some sectors towards reducing solvent use.

Example 2: automotive waterborne basecoat

A proportion of motorcar bodies are finished with a glossy coloured topcoat; but the majority undergo a two coat finishing process. A thin coating of highly pigmented basecoat is followed by a glossy clear coat. This process allows a wide range of metallic and pearlescent colours to be used. These colours only give the required effect at low solids content. The resulting low film thickness constrains the flake-like aluminium or mica particles to lie flat, which is a precondition for them to show their desired colour effects.

[Figure - 2] shows a typical solvent borne basecoat with a volume solids content of 10%. A "high solids" approach can increase this to perhaps 15 percent, but with some loss of brilliance in metallic colours. This approach only leads to a modest reduction in solvent used. An alternative is to achieve the required solvent reduction by replacing solvent by water, leading to the elimination of almost 90% of the solvent.

This is an even trickier problem, because, as already pointed out, use of a water soluble polymer leads to a film with unsatisfactory water resistance. In this case, the answer has been to develop water dispersible binders. While these are carried in water, solvent has a crucial role to play, both in stabilising the dispersion, and in integrating the drying film.

Aluminium is a highly reactive metal, particularly in the finely divided form in which it is included in the paint. Reaction with the water not only degrades the colour, but gives rise to hydrogen gas, with a consequent risk of explosion. In parallel to the development of new binders, therefore, it was necessary for the pigment manufacturers to develop aluminium flake pigments that are stable in water.

The development of these products twofold commercial status involved a very substantial research and development efforts. The result is a generation of products fully equal to the solvent borne products that they replace. This type of basecoat has now been almost universally adopted by the European car manufacturing industry.

  Conclusions Top

Although direct substitution of one solvent by another is relatively straightforward, the broader question of solvent substitution remains a significant challenge for the coatings industry.

Reduction of solvent use through "high solids" technology is well established in many sectors, and can often give reduction in line with current regulations. Water borne coatings are establishing themselves in selected areas, particularly where the volume of the end used justifies the very considerable research and development that can be involved.

The coatings industry has reduced solvent use, and will reduce it further - but it must be stressed that elimination of solvent use is not in prospect. Good workplace hygiene therefore remains a key element in the safe use of coatings for the foreseeable future.[4]

  References Top

1.Dusek, U. (2000), Interim report IR-00-066 Secondary Organic Aerosol - Formation Mechanisms and Source Contributions in Europe, Laxenburg, International Institute for Applied Systems Analysis  Back to cited text no. 1    
2.Hildebrand, J.H. and Scott, R.L. (1964) The solubility of non-electrolytes, 3rd ed.. New York, Dover Publications  Back to cited text no. 2    
3.Rentz, O., Blumel, F. Lonjaret, J-P. (1999), Stoffstrommanagement fu r kleine und mittlere Unternehmen aus dem Bereich der Autoreparaturlackierung, Karlsruhe, French-German Institute for Environmental Research (DFIU/IFARE)  Back to cited text no. 3    
4.Rocklin, A.L. and Bonner, D.C. (1980), A computer method for predicting evaporation of multicomponent aqueous solvent blends at any humidity. J. Coatings Technology 52: 27-36  Back to cited text no. 4    

Correspondence Address:
Bruno Giordan
DuPont Performance Coatings Ltd., Freshwater Road, Dagenham GB-RM8 1RU
United Kingdom
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Source of Support: None, Conflict of Interest: None

PMID: 12678925

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