Furniture Sector: Solvent Use Reduction

Last updated: (February 2005)

Introduction

Solvents are organic liquids that evaporate easily at normal temperature and pressure, giving rise to volatile organic compound (VOC) emissions. Within the furniture industry, the bulk of solvents are used in wood coating and related processes, such as the thinning of coatings and spray-line cleaning. Solvents and associated VOCs contribute to a number of environmental problems and hence they are controlled by legislation.

This Reference Note provides a summary of relevant legislation and best practice for the cost-effective reduction of solvent use in furniture manufacture.

Background

VOCs are controlled because:

• they contribute to photochemical smog
• some are toxic and cancer causing
• some contribute to high level (stratospheric) ozone depletion
• most contribute to global warming.

Solvent reduction makes environmental and economic sense. The following sections provide an overview of relevant legislation and they outline best practice for the furniture sector. More general information on reducing the use of solvents in all industries can be found in Reference Note 078: The Solvent Directive

PG6/33(97) Wood Coating Processes

Under Part I of the Environmental Protection Act 1990, sites require an authorisation to operate any wood coating process which is likely to use more than 5 tonnes of organic solvent in any 12 month period.

As with all processes falling under this Local Air Pollution Control (LAPC) regime, authorised sites are expected to upgrade their operations to comply with the requirements laid down in the relevant Process Guidance Note: PG6/33(97). These controls can be split into:

• final compliance options
• other clauses.

Final compliance options

• All authorised solvent users must choose one final compliance option. This should have been in place by April 1999 for sites using >15 tonnes of solvent, whilst 5 to 15 tonnes users have until 2007. The options are:
• Compliant coatings – all coatings must have a solvent content not more than that specified in Clause 20, eg 435 g/l VOC for clear coatings applied by spray, roller or dipping techniques. Many sites are finding it difficult to switch to compliant coatings throughout their operation.
• Abatement technology to reduce VOC emission levels below 50 mg/m3 (>15 tonnes) or to below 150 mg/m3 (5 to 15 tonnes). Certainly for larger users, the emission limit could only be reached via the use of unrealistically expensive ‘bolt on’ abatement plant, eg incinerators.

Mass balance approach (also known as the 'bubble’ approach) with which sites will be deemed to comply where they have achieved a level of solvent usage below their ‘target emission’. Such limits are related to the solids content of coatings, which allows flexibility in accordance with varying levels of production. They are calculated as follows:

• 5 to 15 tonnes users can apply up to 1.6 times as much solvent as solids, ie 1:1.6 (and they are allowed until 2007 to achieve such a ratio or better)
• 15 tonnes and upwards users can apply the same amount of solvent as solids, ie 1:1 (and should have achieved such a ratio or better by April 1999)

The mass balance is the approach that the majority of sites favour due to its relative flexibility and cost effectiveness. It allows the use of low solvent coatings where they are practical, with the retention of solvent based coats where essential.

Other clauses

Authorised processes must ensure that all other clauses of the Guidance Note were complied with by April 1999 - regardless of the size of site. Such clauses include:

• 24 Solvent inventory - records should be kept of the amount of solvent used.
• 45 Spray efficiency - conventional spray equipment must be replaced by high efficiency types such as high volume low pressure (<10 psi atomisation pressure) or airless air assisted.
• 47-52 Cleaning operations must be reviewed. Where possible, water or less volatile solvents should be used for cleaning spray lines etc.
• 58-67 Chimney heights and dispersion considerations.
• 68-72 General operational requirements relating to the need for maintenance, training and good housekeeping.

Other Legislation Applicable to Wood Coating Operations

It should be remembered that other environmental legislation would also apply to wood coating operations. Brief details are given below:

Environmental Protection Act 1990 – Part II: Duty of Care

Waste solvent must be stored in a safe and contained manner. Containers should be labelled and transfer must only occur only to an ‘authorised person’, eg a registered carrier. At least basic details should be requested about the site receiving the waste eg name, location and what they do to the waste solvent.

Special Waste Regulations 1996

Waste wood coatings and thinners should be segregated and stored in drums. They may be recovered on-site or sent away for recovery or incineration. Removal of solvent from site must take place in accordance with the Special Waste Regulations. Regulators must be notified prior to the disposal of a batch of waste. This is achieved via the generation of a consignment note, which must be approved by the regulator prior to shipment. Consignment notes describe the waste, how it is packed, the process generating it, its chemical components, as well as providing details of the parties involved. Each consignment note has five copies and the fifth copy (green) must be retained by the site for at least three years.

Water Resources Act 1999

Care must be taken to prevent the spillage of solvents. These will run into surface water drains leading to a contravention of the Water Resources Act. Waste solvent should never be disposed of to surface water drain or foul sewer.

Pollution Prevention and Control Act 1999

EPA Part I is currently being replaced by PPC, due to the requirements of a European Directive. In real terms, the initial impact on LAPC processes will be mainly administrative. See Reference Note 032 Integrated Pollution Prevention and Control (IPPC) for more general information on IPPC.

Solvent reduction best practice

This section provides an overview of solvent reduction best practice. More detailed information is available in a variety of ETBPP publications eg GG177 Reducing solvent use in the furniture industry. Relevant documents are listed at the back of this Reference Note.

Measure to Manage

An old adage suggests that if you cannot measure then you cannot manage an issue. Therefore, the first task in any solvent reduction programme is an analysis of current solvent usage. A Measure to Manage programme can act as a form of route optimiser. It involves a number of stages:

• Pinpoint company’s current solvent consumption: request information on solvent and solids consumption for the past year from all coating suppliers. Summarise this in a solvent inventory.
• Identify which solvent to manage and how to measure it: the inventory will highlight coatings with the greatest solvent reduction potential. Typical target areas are thinner usage and tin residue losses.
• Identify and evaluate solvent management controls: suggestions are provided below.
• Implement controls and measure their impact: it is vital to measure the impact of controls and feed this information back to those on the shop floor in order to maintain momentum.

Thinner usage

The solvent inventory indicates the total amount of thinner that is used by the site, but does not show where the usage occurs. Thinners will typically have three main uses; stripping damaged items, thinning of coatings prior to application and cleaning spray lines and equipment. It is also common for thinners to be used unofficially eg in maintenance operations.

Therefore, the first stage in a programme focusing on thinners would be the breakdown of usage by area/process. This can be achieved by keeping thinners in the solvent store and recording the purpose and amount of each batch that is requisitioned. Discrepancies between the amount requisitioned and the purchase inventory should be investigated. Once it is known where the bulk of the thinners is used, unnecessary consumption can be reduced.

Thinners for thinning

Thinners are used to reduce the viscosity of coatings and facilitate their atomisation and application. However, if properly managed, the majority of coatings should not need the addition of thinning liquid. Keeping lids on containers will minimise the loss of the original solvent content, whilst ensuring that coatings are up to temperature will help to maintain the correct viscosity.

However, operators can become accustomed to adding thinners even when they are not required. Controls on the issue of thinning solvents will make operators think about their necessity as well as encouraging measures to minimise the loss of the coating’s original solvent content.

Thinners for cleaning

Cleaning solvent is most undesirable from the mass balance perspective - as it raises the solvent total without adding anything to the solids total. Therefore, reduction of cleaning solvent to a bare minimum is essential from a management perspective, as well as being a requirement of PG6/33(97) - see Clauses 47-52.

The following steps should be considered:

i. Eliminate: some cleaning solvent can be avoided, eg roller coaters for applying ultra violet cured acrylic coatings might only need cleaning once a week, rather than daily.
ii. Minimise: clean spray lines with water if possible. One of the benefits of converting a spray line to a water-based coating is that the majority of cleaning can be achieved using water – though an occasional flush through with solvent is still likely to be needed.
iii. Can cleaning be achieved using less volatile solvents? Generally, the lower the boiling point of a solvent, the quicker it will evaporate at room temperature. Many furniture sites use solvents such as acetone and toluene, which have boiling points of 56oC and 110oC respectively. Replacement with a solvent such as butyl acetate (165oC) should lead to less evaporative loss.
iv. Can solvent used for cleaning furniture be replaced with other substances? A number of companies have switched from traditional solvent-based cleaning solution to one based on residues produced from the manufacture of orange juice. This change results in significant solvent savings as well as health and safety benefits.
v. Reuse: if cleaning spray lines/guns by spraying solvent through the system, ensure that the used solvent is collected and either directly reused or cleaned for reuse. Even relatively dirty cleaning fluids may be suitable for additional cleaning or for use as a pre-wash. Solvent can effectively clean when it contains up to 12% solids. Dirty solvent sent for recovery typically contains only 5 to 8% solids - meaning that half the cleaning power is wasted - and twice as much solvent is used.
vi. Recycle: waste thinners can be sent off-site for reuse or recovery for reuse. However, it may be more cost effective to clean dirty solvents on-site via a solvent recovery still. This allows solvent to be reused many times, thereby preventing the need for re-purchase and greatly reducing the total for ‘solvent in’ (as well as saving money). These units often have a swift pay back eg 8 to 12 months.

Transfer efficiency

‘Transfer efficiency’ is a measure of how much coating actually ends up where it is intended ie on the wood. In large part, transfer efficiency is dictated by the type of application technology used. In simple terms, the transfer efficiency is calculated as:

• Amount of coating on work piece
• Amount of coating sprayed

PG6/33(97) requires that operations switch from conventional spray guns to more efficient varieties. In reality, the actual transfer efficiency of these new guns may be as low as half of that stated by manufacturers due to incorrect spraying pressures, poor spraying technique and awkward items, such as chairs, which only have a small surface area on to which solids can be transferred. Transfer efficiency may be improved by attention to the following:

i. Air and fluid pressures: many operators assume that raising air and fluid pressures will result in an increased rate of spraying – hence allowing a greater throughput of items. However, raising pressures above those recommended by the manufacturer will lead to excessive wear of equipment, unnecessary energy usage, over-atomisation, increased bounce-back and the formation of an excessive air-cage in front of the item being. coated. This results in lower transfer efficiency and higher operating costs.

Problems are particularly common with high volume low pressure systems (HVLP) designed to work at <10psi atomisation pressure. In reality, many operators use 12 to 14psi, effectively reducing their efficiency to that of a conventional spray gun (30 to 40%)

ii. Cleaning and maintenance: proper maintenance is essential to ensure the performance and life span of spray equipment. In order to ensure optimum atomisation, regular maintenance should include the checking and replacement of pressure pot valves and worn parts such as nozzles and needle valves. Electrical connections of electrostatic systems should be regularly checked. Any spray system working with two-pack materials should be flushed out and left filled with thinners at the end of the working day. Systems using single pack air drying coatings should be cleaned out on a weekly basis and filled with thinners whilst not in use for any prolonged period of time. Care must be taken with the choice of solvent, as one which is unsuited to the coatings in use may not clean thoroughly, leading to a build up of deposits over time.

Air-assisted airless (AAA) systems also require regular cleaning of the in-line filters to prevent blocking of the spray tips. Most systems have two filters - one large area filter on the outlet of the pump and one fine filter in the gun handle. Both of these should be matched in mesh size to the tip size used in the gun - thus preventing tip blockages. The larger pump outlet filter should be cleaned in solvent with a soft brush once a week, whilst the smaller gun handle filter should be cleaned every two working days.

iii. Operator technique and training: proper operator training is essential to maximise transfer efficiency. Most spray gun manufacturers will provide training to enable operators to understand how the equipment works and how to optimise its use. This is particularly important when switching from conventional to HVLP or AAA guns. In addition, there are a few simple measures to improve efficiency:

• Triggering at the end of a pass – ie releasing the trigger when the gun is not directly over the item being sprayed.
• Gun positioning - keeping the gun at right angles to the surface to ensure an even rate of application. Also, ensure that the gun is the correct distance from the work piece - eg around 15 cm to 30 cm for HVLP guns. If the gun is held too far away, there will be losses due to gravity. In addition, the coating may be too dry by the time it is applied to the wooden surface, leading to peeling. If the gun is too close, the applied coat may be too wet, leading to runs.
• Fan pattern adjustment - altering the fan pattern to suit the item being coated, rather than always using the largest spray pattern to increase application rates.

In-line heater systems

In-line heaters reduce the viscosity of coatings, without the addition of thinners. The systems are particularly useful for high-solids coatings which can cause flow and application problems. Heaters can lead to an increase in transfer efficiency of around 5% and can be retrofitted to existing equipment.

The in-line heater is a fully flameproof enclosed unit which is fitted on the outlet of any pressure system before the gun. The heater has an adjustable temperature gauge and is thermostatically controlled. To ensure a constant temperature during spraying, re-circulation systems are fitted to ensure that coating in the pipes does not cool down when spraying stops. Such systems produce a much more consistent temperature of coating and can maintain coating temperatures to within +/- 1oC. A temperature of between 40 and 50oC is generally optimum.

Viscosity is a measure of the extent to which a liquid resists a tendency to flow - the greater the viscosity, the less flowing the liquid. Viscosity can be measured in the number of seconds that it takes for a coating to run through a calibrated viscosity cup. Application and finish quality may be hindered if coating viscosity is too high. Viscosity may be reduced in a number of ways, eg by adding thinners or increasing temperature.

Variations of temperature can lead to quality problems as illustrated in the table below, which shows daily temperature fluctuations in a hypothetical workshop. During the course of the day, the viscosity of the paint has changed from 23 to 17 seconds – leading to a 22% increase in the output of the spray guns. Unless operators are properly trained, such a change may lead to excessive coating application and over-colouring.

Time	Temperature (oC)	Viscosity (seconds)	Spray gun output (oz/min)
Early morning - cool workshop	15	23	460
Mid morning – workshop heats up	20	20	520
Midday - workshop at full temperature	25	17	560

Surplus coatings

Two-pack materials can give rise to a large amount of wastage if handled incorrectly. It is essential to avoid catalysing more material than is required for a given period of work. Ideally, sufficient material should be mixed for no more than two hours of production. Any catalysed material remaining at the end of the day should be diluted with uncatalysed lacquer, with the balance of acid catalyst added at the start of the next day.

Tin residue losses

A surprising amount of unused coating is disposed of as tin residue. This is, of course, wasteful and if a significant quantity remains in any one container, the site runs the risk of being out of compliance with the Special Waste Regulations 1996. Operational changes will centre on operator training and avoiding mixing large batches of two-pack coatings.

Surface preparation

In order to obtain the best results from coating material, it is necessary to have a properly prepared surface on which to apply the coating. A work piece should be finish sanded to comprise short, cut fibres which appear as a smooth, matt surface which is free of dust. In subsequent finishing there should only be minimal rise in the fibres.

A sanded work piece with a shiny, glossy finish has been compressed and exposed to high frictional heat. It will have an unacceptable fibre rise during subsequent coating, resulting in a greater likelihood of uneven stain application as well as an increased need for denibbing. Such problems can occur as a result of incorrect grit sequence selection, too high sanding pressure and the use of abrasive belts past their useful life – ie worn or clogged. Properly prepared substrate will result in a reduction in the amount of coating required to obtain a satisfactory finish - leading to lower cost and reduced solvent consumption.

Delivery sizes

Most companies use 25 litre drums for wood coatings. These are easy to stir (to ensure complete mixing) and are suited to the relatively small quantities that most pressure pots are designed to hold. In addition, certain coating pump systems are designed to fit 25 litre tins. However, for the largest of users, it may be possible to switch to 205 litre or even 1,000 litre IBC containers for key coatings. Changing from 25 to 205 litre drums can reduce residue by 5 times, due to lower total residual coating in the containers, though the benefit is of course lost if decanting must then occur.

Lids

If lids are left off tins containing solvent, the contents will evaporate - leading to expensive and unnecessary wastage. If the solvent content of a coating decreases to a large enough extent, thinners will have to be added to achieve the required viscosity - which is a double waste. Containers without lids are also vulnerable to spillage.

However, the process of getting lids on tins is often an ongoing battle for management and requires some form of training for the operators. One of the main target areas will be raising motivation and overcoming apathy along the lines of "well it’s only cleaning/solvent/a small amount etc" - which is used to justify a lack of attention and care.

Storage

Unless trained otherwise, some sprayers have a tendency to store several days worth of coating around spray booths - to minimise the inconvenience of travelling to the coating store. It is therefore tempting to finish one job and switch to the next coating drum, leaving the original tin in the work place without a lid. By the end of the day, the spraying area is full of half used, un-lidded coating containers.

For reasons of legal requirements, health & safety, spillage prevention and evaporative loss reduction, it is preferable to minimise the amount of solvent stored in the workplace. It is a good policy to insist that the only tins allowed in the work area are those that are being used. The rest of the day’s requirement can be stored in local, lidded, metal solvent storage cabinets - with the bulk of coatings being kept in a central solvent store.

Consideration should be given to the temperature of storage areas. Cold liquids have a higher viscosity and may need thinning prior to application - unlike their warmer counterparts. Therefore, it may be advisable to bring the following day’s coatings inside and store them in the local solvent cabinets over night to warm up.

Dispensing units

Consideration should be given to the use of ‘plunger pots’ for the storage of solvent applied by rag/hand - eg application of pullover or general cleaning solvents. Plunger pots come in a variety of forms - eg a sprung lid which is pushed down to access solvent, or a plunger which is depressed to dispense a set amount of liquid. Any such system is an improvement on having a 10 or 25 litre tin with no lid for the majority of the working day.

Spillage

If liquids are spilled on site, the temptation may be to wash them down the drain – ‘out of sight and out of mind’. However, this will generally be the worst method of clearing a solvent spillage.

Spillage kits should be provided at key locations - eg solvent store, waste solvent collection point and near sensitive locations such as surface water drains located on solvent transfer routes. Procedures should be in place for dealing with such occurrences, with operators given some form of training. A reporting system should also be set up to record significant spillages eg more than 1 litre of solvent or more than 2 litres of solvent-based coating. This will discourage careless solvent handling and encourage effective spill management. The resulting information will also be required for Measure to Manage programmes.

Changing coating formulations

Once the above measures have been taken, companies will have minimised their unnecessary solvent usage and moved a significant distance towards achieving compliance with PG6/33(97). If further solvent reduction is still needed, it will have to be achieved via the change to low solvent coatings.

Legislation

Environmental Protection Act 1990 – Part I

PG6/33(97) Wood Coating Processes.

Environmental Protection Act 1990 – Part II: Duty of Care

Special Waste Regulations 1996

Water Resources Act 1991

Pollution Prevention and Control Act 1999

The future

The EU Solvent Directive is not expected to have an impact upon UK wood coating operations as its requirements are much more relaxed than those imposed by PG6/33(97). For example, the EU threshold for control is 15 tonnes of solvent usage and the main compliance deadline is 2007.