High performance coatings - where are we now?

Environmental legislation is having a profound effect globally on the coatings industry. A target has been set in the UK to reduce volatile organic compounds by 30% by the year 2000. Dr Nigel Whitehouse reviews the types of high performance coatings which are now available.

Over the

last 10 years, initially in the USA and more recently in Europe, paint manufacturers and their customers have come under increasing legislative pressure to reduce VOC contents and emissions. The response of the coatings industry has been to develop a range of compliant coatings containing less than a defined quantity of organic solvent.

Environmental protection acts

In the USA, the Environmental Protection Agency (EPA) has established air quality standards under a Clean Air Act. Industrial areas are required to comply with this legislation and reduce air pollution. Limits have been set for maximum VOC contents of generic coating types and paint application shops are subject to a daily or yearly quota of total VOC emissions.

In Europe, environmental legislation is developing rapidly. At present, existing legislation in member states has not been harmonised in a Council Directive, but such a document will come. In the UK, the Environmental Protection Act, 1990: Part 1, has established a framework for air pollution control. The Act claims to reduce pollution of air, water and land. Whilst it addresses a wide range of environmental issues, it is particularly concerned with VOC emissions and their reduction. The Act takes into consideration:

* Solvents and thinners added in use.

* Transfer efficiency of painting equipment (spray guns).

* Methods used for cleaning application equipment.

* Dust emissions from processes such as blast cleaning.

Her Majesty's Inspectorate of Pollution (HMIP) and Local Authorities are the regulatory bodies and operators of prescribed processes and are required to obtain operating permits. They are also required to eliminate, or minimise and render harmless, releases of prescribed substances from the process using Best Available Techniques Not Entailing Excessive Cost (BATNEEC). Permits contain emission limits and process operating conditions to ensure environmental protection.

The UK EPA applies to all companies involved in coatings, especially those companies carrying out coating operations in their own workshops in an enclosed environment. Companies using or emitting less than five tonnes of organic solvent (or 20 tonnes of paint) are excluded.

These regulations apply to the use of solvent-borne coatings in or as part of a manufacturing process. They apply to both factory and on-site processes. However, it would be rare for more than five tonnes of solvent to be used under the latter circumstances.

Process guidance notes

In the UK, the Secretary of State for the Environment has issued a series of Process Guidance Notes. These documents present the environmental performance standards, which are expected to be achieved by new or substantially modified processes and to which existing processes arc to be upgraded, in terms of emission limits and process operating conditions. The guidance notes also specify the period for upgrading of existing processes.

The relevant Process Guidance Note for the coating of metal and plastic is PG6/23. First issued in 1992, this Guidance Note is presently being revised. An amended document is expected to be issued later this year.

How are compliant coatings defined?

A `compliant' coating is one which contains less than a defined quantity of solvent usually expressed in grams per litre). For high performance coatings, the VOC contents defined in PG6/23 are set out in Table 1.

Table 1: Permitted VOC contents for compliant coatings.
                           1st April 1998   1st April 1996
                           (see 1 and 3)    (see 2 and 3)
Blast/weldable primer      780              780
General primer/undercoat   250              400
Topcoats                   420              520

The original target figure was 250 grams per litre by 1st April, 1998. As a result of consultation with the paint industry, a selective relaxation of this limit has been negotiated for topcoats and for blast/weldable primers. For topcoats and finishes, it was felt that the high solids products could not produce the desired standard of finish for some industries.

Blast/weldable primers need to have a low solids content by their very nature. A relaxation was agreed because it was recognised that water-based equivalent products are not generally available at present. However, VOC content levels are still under constant review and may be amended before the start of 1998, if appropriate new technology emerges. The preferred approach to meet EPA requirements is to use:

* Solvent free coatings.

* High solids coatings.

* Waterborne coatings.

Applicators will choose the type of coating best suited to their particular process. However, few fabricators are willing to apply water-based systems at present because of the environmental control required. Table 2 illustrates the potential advantages of compliant coatings.

Table 2: Potential advantages of compliant coatings.

Coating                 Advantage
Solvent free coatings   No solvent emission
                        Low fire risk
                        Reduced explosion hazard
                        Less wastage
                        Reduced number of coats
                        Safer working conditions
High solids coatings    Reduced solvent emission
                        Reduced number of coats
                        Less wastage
                        Safer working conditions
                        Existing application equipment may be
                        used
Waterborne coatings     Reduced solvent emission
                        Low odour
                        No solvent emissions from cleaning up
                        Lower fire risk
                        Reduced explosion risk
                        Safer working environments

In the UK, the general view is that present and future primer requirements will be met by high solids epoxy coatings. VOC levels of 340 grams per litre have been achieved and the target figure of 250 grams per litre should be attainable. Present technology can meet the relaxed VOC content for topcoats (420 grams per litre). Relaxation of the VOC content for blasl/weldable primers to 780 grams per Selitre means the present range of products can continue to be used. The use of alkyd primers for new construction is likely to be phased out, unless suitable high solids materials can be developed. At present, this possibility is remote.

High solids, surface tolerant epoxies and polyurethanes have been successfully used for several years. Typically, these coatings have volume solids in excess of 80% and so comply with both the 1996 and 1998 requirements (250 grams per litre equates, typically, with a volume solids of 70-75%).

Coal tar/pitch epoxies or polyurethanes have been used successfully to protect structural steelwork in immersed conditions. As a replacement, hydrocarbon modified pure epoxies are developing a good track record. They are high solids coatings with figures of typically 150 grams per litre.