Cellular Plastics Processes – reducing fire risks
Some processes involving cellular plastics (CP) may increase fire risk. Below are precautions that can be used to reduce the risks for some of those processes.
When a naked flame is passed over PU foam for bonding it can be overheated and become a fire risk. To reduce the risks of fire from the flame bonding process it should (where practicable) be separated from other operations and be contained within a fire-resisting enclosure.
The gas burner should:
- have a gas supply incorporating a flame failure device
- have a gas supply that’s interlocked with the machine drive motor so if the machine stops after bonding has started the burner will automatically be extinguished
- have good fresh air ventilation for good combustion.
Escape of fume into the workroom should be prevented by enclosing processes and providing local exhaust ventilation (LEV). LEV systems should:
- remove fumes from the laminating sections
- remove fumes generated near the burner and in the laminating nip
- have exhaust ducts that terminate outside the building
- have exhaust ducts that are located to prevent fume re-entering buildings and to ensure it disperses
Newly-bonded materials should be transferred to a well-ventilated area and kept there until they stop fuming.
When applying adhesive on at the coating head of the machine to bond laminates there is an increased risk of fire because of the flammable solvent that is used.
There are non-flammable adhesives available for most applications that should wherever possible be used. If it is not possible to use non-solvent adhesives then additional precautions are needed. Fire separation between adhesive lamination and other processes is recommended.
The bonding machine should:
- be enclosed as far as possible to prevent solvent vapour entering the workroom
- have Local Exhaust Ventilation (LEV) at the coating head and any other points where solvent could evaporate
- have an inward airflow at each opening of at least 0.7 m/sec (approximately 140 ft/min) to control vapour release (Note: toxic solvents this may need to be increased significantly)
- have adequate air inlets
- have exhaust ducts that terminate outside the building
- have exhaust ducts that are located to prevent fume re-entering the premises and to ensure it disperses
If drying ovens are used:
- heating units should be located away from any source of flammable vapours
- a device (e.g. a flow switch) positioned in the fresh-air inlet or exhaust duct should be used to monitor air flow which is interlocked with the heat supply and the feed mechanism
- explosion relief should be provided
- a recirculatory system can be used to conserve heat but a minimum volume of 60m3 of fresh air per litre of adhesive used should be introduced to ensure the concentration of vapour within the oven does not exceed 25% of the lower flammable limit.
More advice on storing highly flammable liquids can be found in the fire and explosion pages.
Fabrication is the process of manually bonding together sections of PU foam and/or other materials to make an article or component. This is normally done by an adhesive. There can be a fire hazard from large amounts of vapour given off when large areas of freshly-applied adhesive are exposed. The vapour can be worse if the adhesive is spray applied.
To reduce the risks:
- use low toxicity, non-flammable solvents and adhesives wherever possible
- do fabrication processes in a well-ventilated workroom
- ensure vapour concentrations in the atmosphere are below the work exposure limit (WEL) for the mixture of solvents contained in the adhesive
- apply adhesives in a non-flammable booth with local exhaust ventilation (LEV)
- plan the flow of work through fabrication workrooms e.g. apply adhesives in sections rather than large areas
- if using highly flammable adhesives for large scale fabrication processes then separate them and carry them out in a fire-resisting enclosure
If spraying solvent-based adhesives then controls are needed to ensure a flammable atmosphere does not build up. Spraying should be carried out, as far as is reasonably practicable, inside a spray booth or enclosure constructed of sheet steel and provided with mechanical LEV.
Where it is not reasonably practicable and spraying is done at a fixed location e.g. a work bench then mechanical exhaust ventilation should be used as near as possible to the source of vapour emission. There should be a minimum air flow rate of 0.7 m/s (approximately 140 ft/min) at collecting points, which should be positioned to draw vapours away from the operator.
Shaping sections of flexible PU foam can be carried out mechanically by grinding or routing. This generates lots of dust and granular material. Dust lying on surfaces can be a fire hazard and fine airborne dust can be an explosion hazard.
Dust particles are often electrostatically charged and can be strongly attracted to certain surfaces, If dust on surfaces is ignited the fire can travel quickly along it. Fine dust can also penetrate motor casings and bearings and can cause electrical equipment to become overheated increasing the risk of fire.
Dust and fine particles produced during grinding and routing operations should be controlled. An enclosure with provision of mechanical exhaust ventilation as near as possible to where dust is being generated should be provided. A dust collector such as a dry filter and/or a suitable cyclone should be provided in accordance with the recommendations contained in the booklet HS(G)103 Safe handling of combustible dusts.
Where dust penetration of electrical equipment is a problem dust-tight casings to the correct electrical rating should be provided. If there is the risk of a build up of electrostatic charges then steps should be taken to prevent this. More information on electrical safety in the workplace, including information on electrical safety in potentially explosive atmospheres can be found in the electricity pages.
Regular cleaning to reduce dust levels should also be a part of your controls for managing the fire risks.
When using heated resistance wires to process blocks of flexible PU foam normally the risk of ignition is low. However, fires may result if the temperature of the wires is set too high or if the wires short-circuit during use. Ignition may also occur if the PU feed to the machine stops so the foam stays in contact with the hot wire. In each of these situations foam may be heated to temperatures above 300-400oC causing toxic fumes and flammable vapours to be given off.
To reduce the fire risks from hot-wire cutting:
- the machine should be properly set up by a trained operator
- the machine should not be left unattended during operation
- off-cuts should not be allowed to build up in work areas
- appropriate fire extinguishers should be provided
- freshly-cut PU foam sections should be allowed to adequately cool in a ventilated enclosure.
Crumbing is the process of reducing down the size of plastics e.g. PU foam or polystyrene for use as a cushion filler or for recycling. Most commonly this is done by a granulator. Modified rag-pulling machines, which tear PU off-cuts to fine shreds and hammer mills, which grind the material can also be used.
Unless strict controls are in place for producing and using crumb there can be a significant fire hazard. If not contained, the crumb is likely to spread and lye on floors and other surfaces increasing the risk of spread of fire in a workroom. Dust and fine crumb produced in some plant can lead to a dust explosion.
To reduce the risks:
- crumbing should be separated from other processes
- where practicable crumbing should be contained within a fire-resisting enclosure
- the crumbing machine should be fully enclosed as far as practicable to control crumb and dust
- the air rate drawn through the working opening of the crumbing should remove all crumb/dust to prevent them entering the workroom
- when crumbing polystyrene avoid ignition sources in the crumbing unit
- use good cleaning regimes to prevent fine dust e.g. from PU foam crumb accumulating to prevent fire and dust explosion hazards
- protect equipment – both local exhaust ventilation (LEV) and crumbing plant - from explosion hazards e.g. prevent electrostatic discharges by bonding and earthing all metal parts
- if flammable vapours are released during crumbing e.g. from polystyrene then electrical equipment should be appropriate.
More information on electrical safety in the workplace, including information on electrical safety in potentially explosive atmospheres can be found in the electricity pages.
Cutting processes include horizontal, vertical or inclined cutting on band knives, peeling, press cutting and rotary cutting. PU foam is not likely to be ignited during cutting processes but sparks from grinding wheels used to sharpen blades can ignite dust accumulations inside guards. In addition, cutting lubricants may contain highly flammable liquids (HFLs) that give off vapour when used. The vapour could be ignited e.g. by the discharge of static electricity and so could be a fire risk.
To reduce the risks:
- frequently clean inside machine guards to prevent dust build up
- it is advisable to fit a water tray under the grinding
- if HFLs are used then machinery should be earthed and placed in well ventilated areas
- store aerosol lubricants in a steel cupboard or bin with a lid
- provide arrangements for the safe disposal of empty aerosol containers from the building
- use a rotating tube bit rather than a heated rod when preparing cylinders of PU foam for peeling
- provide appropriate portable fire extinguishers within easy reach of all cutting machines