ADDITIVES IN COATINGS - A NECESSARY EVIL?

Roger F G Brown

Pigment Dispersion Group, Technology Centre Decorative Coatings Akzo Nobel Decorative Coatings PO Box 37, Crown House Hollins Road, Darwen, Lancashire, BB3 0BG, UK.

1 INTRODUCTION

It is generally accepted that surface coatings are applied to decorate and protect (paints and lacquers) or to inform (paints and inks). Coatings in their simplest form as a dried film on the substrate consist of binder (lacquers) or binder and pigment (lacquers, paints and inks). Additionally, it is necessary for the paint to be applied to the substrate. With the exception of some curable liquid formulations (e.g. radiation cured, liquid two pack paints, etc.) some form of diluent is required to enable the coating to be applied. In the majority of cases, this diluent is some kind of liquid volatile material (organic or aqueous) though in the case of powder coatings the diluent enabling application is air. Hence, to generalise, liquid paint, in its simplest form, will consist of pigment, binder and diluent. In this simplified formulation, the only variables are pigment volume concentration (PVC) and solids content (SC) and it is straightforward to record any given measurable property of the wet or dry paint as a function of the composition.

An optimised formulation with the best balance of properties can then be readily determined. Unfortunately, in the vast majority of, if not all cases, this optimised balance point is deficient in a number of key areas and, in some areas, the required level of performance simply cannot be achieved in such a simplified formulation. This is where additives play a part. By selective use of additives, it is possible to achieve better properties in specific performance attributes that cannot be achieved with the simplified tertiary formulation. Additionally, it becomes possible to achieve an enhanced balance of properties, where some properties can be improved without the same degree of detrimental effect on other properties. Additives are used in coatings for a great number of purposes to improve specific properties throughout the lifecycle of a paint such as processing properties in manufacture, in-can stability, application properties, drying properties and final film performance. Unfortunately, each extra additive adds a degree of complexity to the formulation and achieving the optimum balance point between varying (often opposed) properties. Hence, from the paint developer's point of view, a minimalist approach to formulation is to be preferred. Nevertheless, because it is generally impossible to achieve the desired properties without additives, this increase in formulation complexity is justifiable and wholly necessary. With VOC legislation stimulating the development of alternative technologies such as waterborne, high solids, radcure and powder, the need for additives will not go away. If anything, with waterborne technologies supplanting solventborne in a number of sectors, it can be argued that the importance of additives is increasing due to the added complexity of formulating in water.

2 LEGISLATION

It is not intended to provide a comprehensive list of legislation relevant to the coatings industry. However, within the European market, a number of legislative acts have been passed with many subsequent amendments and adaptations to technical process which have great significance for the Coatings industry. A number of important Directives are presented in Table 1. These have profound implications for the formulator in terms of labelling, inclusion of specific products and the levels at which specific products may be used. Furthermore, there are a number of voluntary labelling initiatives within the Deco sector such as Ecolabel, Blue Angel and the Globe. Additional to this, there are also sector specific directives currently under consultation, such as the Deco Directive on VOC's, which will further add to the framework within which the formulator must work. In many cases, it is this framework which drives product development but also the conversion from one type of technology to another. Environmental issues, both legislative and voluntary, will continue to drive additive development and paint development simultaneously for a long time to come.

3 THE NEED FOR ADDITIVES

As already discussed in the introduction, additives are frequently essential to obtain properties that cannot be obtained from the simple combination of pigment, binder and diluent. Additives are multifarious in nature and are used to control a number of different properties. A list of properties that may be controlled by selective use of additives, by no means exhaustive, is presented in Table 2, along with examples of the chemical nature of such materials. Paint formulation is a balancing act. This is because paint is expected to fulfil a number of key requirements, which will vary, dependent on the application. To maximise profitability, paints should be cheap to manufacture both in terms of raw material costs but also ease of production with high throughput, low energy processes preferred. Technically, paints must have good in-can stability so that there is no separation of ingredients or colour. Paints are required to have good application properties, specifically associated with rheology, whether for brush application, spray application, dip coating, coil coating, etc. dependent on the sector. The paint should dry well with respect to coalescence or cure. Finally, the final paint should adhere well to the substrate, have the correct colour and level of gloss, show good retention of colour and gloss and have the desired protective properties, ideally for as long possible. Hence, there are a great number of properties that a paint is expected to have, some of which are specifically opposed (e.g. sag resistance vs. levelling) and there is no such thing as the perfect paint. It is obvious that a balance must be struck between opposed properties. One of the key requirements when formulating a paint is to establish what balance of properties should be achieved. Within this, tacit acceptance must be made that the perfect paint does not exist and that the paint may not be able to match all desired properties. In establishing the desired balance, the most important properties for the specific application must be defined so that, where a trade between properties is required, the relative weighting given to gains and losses reflects their relative importance. Less obvious to non formulator's, is that non opposed properties also need to be balanced because each additive, as well as affecting desired properties, coincidentally will usually have knock-on effects, often unexpected, on other properties....