Spray painting and surface coating

Aim of this document

This document examines how spray painting and surface coating can threaten the environment. It gives some general principles and practical recommendations that users of these processes should follow to control air pollution.

Definitions

Afterburner

Fuel burning equipment (usually a gas burner arrangement) designed for destroying, by combustion, organic vapours before they are released to the atmosphere.

Airless gun

A spray painting device that atomises the paint as it enters the spraying system, using a high pressure-drop nozzle.

Electrostatic painting

A painting process that uses the particle-attracting property of electrostatic charges to reduce overspray.

Fugitive emissions

Air pollutants that enter the atmosphere without first passing through a stack or duct designed to direct or control their flow.

Overspray

Paint, powder and solvent that misses the item being coated.

Paint

A mixture of a pigment and a vehicle that together form a liquid that can be applied to a surface, providing an adherent coating that imparts colour and often protects the surface.

Paint vehicle

The liquid constituent of paint, consisting of solvent or thinner and resin (film-forming component).

Powder coating

A coating that contains no organic solvents. These coatings are either thermoplastic or thermosetting powders.

Solvent recovery

Recovery of vapours from solvents, for example by use of a solid adsorbent material that holds the vapour molecules on its surface, from which they can be regenerated for recycling. This is desirable if cost is a major consideration or if incineration is impractical, as with chlorinated solvents.

Surface coating

Coating of surfaces for decoration and/or protection. A number of basic coating operations are used, including spraying, dip coating, flow coating, roller coating and electrocoating. Variations and combinations of these operations may be used, each designed for a special task. For example, articles may be coated by spraying using air-atomised, airless-electrostatic or hot-spray methods. The composition and physical properties of these coatings vary widely. Organic solvents and thinners are required for many of these operations.

Wet scrubber

A piece of pollution-control equipment designed for removing particles from gas by capturing the particles on or in liquid (usually water) droplets and separating the droplets from the gas stream.

Why have this document?

In Australia, unlike in other parts of the world such as California, there are as yet no air pollution regulations affecting the sale and use of paints. The strategy behind any laws that may be passed in the future will be to reduce the emission of hydrocarbons to the atmosphere from spray painting and surface coating.

The future will also see the introduction of alternative technologies for using waterborne coatings, high solid paints and powder coatings.

In the meantime the DEC has formulated the following recommendations to minimise the pollution hazards of spray painting and surface coating. If authorised officers familiarise themselves with this document they will be able to help spray painters move in the right direction to minimise pollution.

Sources and effects of pollution

The air emissions from surface coating operations come from the evaporation of organic solvents in the coatings. They consist primarily of volatile organic compounds (VOCs). The VOC emissions can occur in a number of places along the production line, such as during atomisation and application of the coating, during initial air drying of the parts after they leave the spray booth, and in the bake oven. In the case of car refinishing, where neither spray booths nor bake ovens are used, VOC emissions occur during coating application and during drying.

Fugitive emissions occur when coatings are mixed and loaded into the application device, during transport of coated parts from the spray booth to the oven, and during postcuring.

Many of the solvents in common use are a health hazard and have the potential to contribute to atmospheric pollution.

Sources of air pollution from solvent and powder-based systems are:

solvent emissions from storage areas, spills, evaporation from painted articles and dust from pre-painting steps (sanding and abrasive blasting)
paint components (solvents, hardeners and plasticisers) volatilised during application or curing
toxic smoke should fires and dust explosions from solvent, paint and powder occur
inadequate ventilation, treatment and dispersion of fumes, vapours and paint overspray
drift of overspray from the premises causing damage to cars.

The environmental effects on workers and neighbours can be serious.

Humans can develop respiratory irritation and metabolic toxicity from inhaling paint components and spray drift, particularly from paints incorporating isocyanates.
Fine powders can damage adjoining properties as they settle and harden on curing. The effect on neighbouring industries may not be evident until the powder becomes incorporated into their products.
Solvents such as toluene and xylene are considered to be air toxic, and could cause health effects in humans.
Solvents add to the general load of photochemically reactive hydrocarbons in the atmosphere, causing photochemical smog in the presence of NO_xand sunlight.
Dust fallout and odour can be a nuisance.
Fire may arise from solvent and dust explosions.
Old paints with red, orange and yellow pigments may contain chromium and lead salts. Removing such paint coats can produce toxic dusts.
In the two-pack painting systems isocyanates are present in minor amounts, usually as toluene di-isocyanate (TDI), hexamethylene di-isocyanate (HDI) or isophorone di-isocyanate (IDPI). Overexposure to isocyanates may result in health problems such as eye and lung irritation and asthma. The effects may be delayed for years. As the odour thresholds for isocyanates are above the recommended standards for exposure, odour is not a reliable way of identifying the presence of hazardous levels of concentrations.

General principles for minimising air pollution

Air pollution control measures that can be applied to surface coating operations generally fall into three distinct categories. These are:

reducing the use of VOC coatings
using higher transfer efficiency equipment
adding on air pollution control devices, including thermal and catalytic incinerators, carbon adsorbers and condensers.

Solvent emission on a large scale can be controlled by using afterburners or by recycling. For small scale operations, such as automotive painting repair shops, such in-house technology is not practicable, but adequate dilution must be used to prevent nuisance to people working or living near the premises. Waste solvents should be drummed and transported to other premises for recycling.

One way of reducing the discharge of wastes to the environment is to use water-based coatings, particularly as the application of surface coatings containing solvents such as toluene and xylene gives rise to strong solvent odours.

Spraying with solvent-based coating should be done at premises at least 100 m away from sensitive receptors (such as private homes, schools, kindergartens and hospitals) and should be located in a special trades or general zone.

Spray booths

Spray painting must be done in a spray booth that can control the emissions of particles and solvents. Spraying in the open can lead to soil contamination, vegetation damage and fallout on to other sensitive surfaces such as cars.

Spray booths are usually classified by their types of construction and exhaust filtering systems. Baffles placed in the ducts will remove some overspray but require regular cleaning. Baffles are acceptable only for small paint rates or if the paint is applied electrostatically. If the spray booth is used more or less continuously, the extracted air should be passed through an efficient filter or water scrubber to remove the bulk of the entrained paint particles. If spray painting is done only a few times each week, an air extraction system is adequate to dilute the solvent fumes.

Another option is to install an activated carbon adsorber in the exhaust duct. Such a cartridge will adsorb solvents until it is 'full' and then must be desorbed by steam or heat. The solvent can be collected and recycled and the cartridge used again.

If a wet scrubber is used to remove paint particles and solvent vapour, the spray nozzles should be checked regularly and the water maintained at the correct level at all times.

Filters

Spray booths should be fitted with filters to catch overspray. Suitable filters for different application rates are:

Application rate	Suitable filter
more than 4 L/hour	wet
from 2 to 4 L/hour	wet, fibre
less than 2 L/hour	wet, fibre, baffle

Filters should be maintained according to the supplier's instructions. These instructions should be prominently displayed for the benefit of operators.

Officers should encourage owners and managers of premises to follow these points with regard to filters:

Wet filter

(In commercial units this is called a waterwash.)

Make sure the sprays function correctly.
Make sure the make-up water float level is correct.
Fit a manometer to indicate negative pressure between the entrainment and distribution plates.
Follow the supplier's recommendations for adding water, alkali and oil.

Fibre filter

Make sure the filter fully covers the support frame spaces.
Fit a dial gauge or manometer to indicate static pressure drop across the filter.
Follow the filter manufacturer's recommendation for the operating pressure drop and replacing filters.
Keep spare filters on the premises.

Baffle filter

The baffle filter must cover the filter area.
Keep spare filters on the premises if a replaceable type is used.

Other points that operators should follow

Systems for intermittent, light and electrostatic spraying

If spraying is light or intermittent, or overspraying is to be reduced by using an electrostatic application method, a dry disposable filter is enough to filter the exhaust air, provided the following points are taken into consideration.

The air movement through the filter must correspond with the filter manufacturer's recommendations for maximum filtration efficiency.
The dry filter must be suitable for collecting the sprayed material. Certain spray materials, for example, lacquers and fast-drying enamels, may be hard to entrap. Get advice on the effect of materials on the filter from the suppliers of both the spray material and the disposable filters.
The efficiency of a dry filter spray booth is related to the state of cleanliness of the filter. Fix a manometer to the spray booth permanently and change the filter when the manometer reading reaches that specified by the manufacturer.

Systems for regular spraying

Ensure that spray booths are designed according to the Factories (Health and Safety, Spray Painting) Regulation 1977.
If paint spraying must occasionally be done in the open, ensure that:
- employees have been trained in overspray minimisation
- there is no painting on windy days
- the area chosen for painting operations is as protected as possible.
These techniques may also give a better product and reduce costs. Open air spray painting should not be done routinely.
Any reclaimed solvent may be recycled, or disposed of by an experienced contractor. Alternatively, it may be burned in an afterburner before being discharged into the atmosphere, provided that the final exhaust meets the requirements of the Clean Air Act. To fully combust the solvent liquid, the afterburner should be designed to achieve a minimum temperature of 750^oC and a residence time of 0.5 seconds.
If a process requires large quantities of paint to be sprayed on a regular basis, a spray booth incorporating an efficient wet collection device will be needed. An efficient wet collector would incorporate a spray chamber containing enough spray nozzles and with enough water circulation to remove entrained paint particles.
An alternative to the spray chamber is the 'No-Pump' Collection Device, in which water is entrained by air movements. To obtain maximum filtration efficiency with this type of collector, it is important to maintain the correct water level as recommended by the manufacturer. An automatic water level control device must be fitted to all spray booths of the No-Pump type.

If the level controller incorporates an electrically operated switch or valve, the device should be electrically interlocked with the ventilation fan. This makes sure it operates whenever the booth is used.

A water curtain is useful only to maintain booth cleanliness; it is not acceptable as a collection device. A separate wet collector should be incorporated in the unit along with the curtain.
The efficiency of any spray booth is related to its cleanliness.

Powder coating systems

Ensure that spray booths are designed in accordance with the Factories (Health and Safety, Spray Painting) Regulation 1977.
Design the ventilation system in such a way that the powder concentration cannot exceed 10 g/m³ in any operating circumstance, and the system is readily accessible and easy to clean. Ideally it should be cleaned regularly to ensure product quality and reduce reject rates and production costs.
Make sure the quality of the filter media and the design of the filter system are such that the concentration of solid particulate emission will not exceed 100 mg in each cubic metre of residual gases emitted to the atmosphere.
Take all practical steps to recover as much overspray as possible. As much as 50 per cent of the powder sprayed may be overspray. Most of the overspray can be removed by a cyclone separator and recycled. The balance, a very fine powder, can be removed by bag filters or air scrubbers (in large operations) and is usually dumped as it is not economical to re-use it. Smaller operations may substitute the cyclone and bag filter with cartridge filters.
Keep the working area clean to minimise the probability of dusty conditions or a dust explosion. Clean all equipment regularly. Operators should be trained in good manufacturing techniques to improve housekeeping, minimise wastage and product rejects and maximise powder recycling.
The pressure differential across the fabric filter should be continuously measured and readily observed.
Fabric filter installations must be fitted with inspection panels to provide adequate access for inspecting filter media and for cleaning. Panels should be fitted with fasteners that can be removed without special tools.
Regularly maintain the total powder coating system and the fabric filter in particular.

Stacks for spray painting and surface coating

Emissions from proposed and existing spray painting and surface coating applications should be designed to meet the following criteria.

The stack height for intermittent, light, or electrostatic spray painting should be at least 3 m higher than the roof ridge of the highest structure within 30 m of the stack.
If toxic contaminants are released from a stack, the stack height should be determined by using appropriate air pollution dispersion modelling.
The final discharge velocity should be at least 15 m/s.
Rain caps that restrict the upwards flow of the exit air should not be used; instead, a suitable vertical discharge cap should be used.
The final discharge should be directed vertically upwards.
A risk assessment may be required.

Summary

Emissions at spray painting and surface coating premises present a threat to workers, neighbours and the environment. If authorised officers encourage users of these processes to follow the general principles and specific recommendations given in this document, the threat of pollution will be minimised.