Safe use and storage of cellular plastics: Controlling the fire risk

Introduction

Cellular plastics (CP) include a wide range of polymers such as polyurethane (PU) and polyisocyanurate foams that are used in a wide range of applications. They are used mainly for upholstering furniture, packaging and insulation. Most are fire hazards and may be ignited easily with a small ignition source. Once established a fire can spread quickly often producing a large volume of thick, black, toxic smoke. The smoke can contain chemicals that are hazardous to people and the environment. Some types of plastic can continue to smoulder once the main fire has been extinguished giving off thick black smoke.

These pages give guidance to manufacturers, converters and users of CP on controlling the risk of fire.

General guidance on fire safety in the workplace can be found on the fire safety pages of the Gov.uk website.

HSE is responsible for enforcement of 'process' fire precautions eg fire hazards associated with particular processes. Local fire authorities are responsible for enforcing general fire precautions eg means of escape, layout of the building, fire alarm systems etc. Your local Fire Officer can also be a good source of advice on general fire precautions and design/layout of buildings. For further information on these areas of fire safety you should contact your local Fire Officer.

Please use the links below to find out more information about reducing the risks of using and storing CP.

Getting Started

When using and storing CP an assessment carried out under the Dangerous Substances and Explosive Atmosphere Regulations 2002 (DSEAR) should be completed to identify the package of controls that will be needed to reduce the risks. The DSEAR assessment should consider how to:

prevent fires starting;
restrict the rate of fire spread
ensure that in the event of fire everyone can escape safely
help fire brigades tackle any large fire; and
reduce the cost of a fire.

More information on completing a DSEAR assessment can be found here.

General safety

The following general safety points along with guidance on storage and processes can be used to reduce the risks of storing and working with CP.

Workrooms:

Keep just enough CP for processes in workrooms
Plan for processes that require large quantities of CP
Do not keep more that half a days supply in the workroom at any one time
Implement strict procedures for stacking, housekeeping and training of personnel

Housekeeping:

Keep work and storage areas clean and tidy
Do not allow dust from handling and cutting CP to accumulate on guards, machinery, electrical equipment and surfaces
Implement cleaning policies and procedures throughout the working day not just at the end of it.

Controlling Ignition Sources

One of the main ways to reduce the risks of fire when using CP is to control ignition sources. This includes ensuring other materials that can be easily ignited (eg loose paper) do not become an ignition source. Other ways to control ignition sources are below.

Naked flames should not be allowed in storage areas or workrooms unless in connection with a properly controlled manufacturing process such as flame-bonding;
Unavoidable hot work around CP, such as welding and cutting should only be carried out under strict controls. Permit-to-work systems may be appropriate.
Heating systems in CP storage areas should be chosen to reduce the risk of ignition. Portable direct gas or oil-fired heating appliances are not appropriate. Hot water systems or indirectly-fired hot-air systems may be used. If fixed solid fuel, gas or oil-fired heater units are used they should be located outside working area and stores.
Electrical equipment in CP areas should be considered in the DSEAR assessment to ensure the right type and standard is used. The assessment may consider but not be limited to:
- preventing accumulation of electrostatic charges eg earthing;
- prevention of CP dust or crumb contacting hot surfaces;
- protection required if there is likely to be an explosive atmosphere;
- preventing CP causing overheating by being pressed against electrical equipment;
- what British and/or European standards should apply to the required equipment.
Vehicles in workrooms and stores should be kept to a minimum. Hot parts of the engine and exhaust system can ignite CP. Electric vehicles are preferable and charging stations for batteries should be located away from CP areas.
Highly flammable liquids should be stored separately to CP.
Flammable gases such as pentane should be considered in the DSEAR assessment. The assessment should identify controls eg forced extraction at low levels, extraction during transport that will prevent a build-up of vapour igniting.
Machinery and plant should be properly maintained to eliminate ignition sources eg heat caused by friction or plant failure.
After work has finished check the workplace to ensure no sources of ignition have been left eg plant left on and that there are no signs of smouldering.

Storing Cellular Plastics to reduce fire risks

The safest storage area for cellular plastics (CP) is in a detached single-storey building, located away from fire escape routes and assembly points.

If the CP store is in an occupied building then:

it should be separated from occupied parts of the building
a partition giving at least half an hour fire resisting separation should be provided
doors through the partition should be self-closing and give a minimum of 30 minutes fire resistance
fire doors held open by fusible links are not recommended as they don't close quickly enough to prevent smoke spread.

If the CP store is in a multi-occupancy building then whenever possible it should not be below any occupied floor. Stores below occupied floors may not give enough time to evacuate the building before the fire blocks escape routes. Also smoke and fire can spread rapidly from the stores to the upper floors.

If storage below occupied floors is unavoidable then an automatic fire-detection system linked to the fire alarm should be installed in the store to give early warning of fire. The installation of an automatic sprinkler system should also be considered.

When storing CP the following should be considered:

Storage area segregation – processing and storage areas should be separate. There are more likely to be ignition sources in processing areas so keeping bulk supplies away the risk is minimised.
Positioning storage areas – they should be positioned to minimise heat, smoke and the spread of fire to occupied areas.
Access through storage areas – access should be minimised and escape through a foam storage area should not be the only means of escape.
Temporary storage – if CP is being stored awaiting disposal or use (eg insulation waiting to be installed in buildings) then precautions should still be taken. This may include storing CP outside, in freight containers or a designated room away from fire exits etc.
Finished product storage – finished products may have a lower fire risk as they are covered. However, to reduce risks easily ignitable materials should be kept away.
New buildings/significant refurbishments – it is easier to reduce CP fire risks through building design eg fire resistant structures. Building regulations would apply so contact your local authority (who enforce the building regulations) or Fire Officer for more advice.
Highly flammable (HF) products – storage areas can be used for other HF solids eg polyester wadding but HF liquids eg cleaning solvents should not be stored here.

Positioning of stock in storerooms

The following arrangements should be followed so far as practicable:

stacks of loose blocks should be stable (racking can be used for cut pieces)
access routes round stacks should be more than 800 mm wide
stacks should be arranged to avoid creating dead-end gangways with only one direction of escape
there should be a minimum of 1 m between stack tops and ceilings, smoke detectors, or light fittings
stacks should not be positioned directly below incandescent lights unless the bulbs are protected
stacks should not be placed close up against a wall or pillar that supports steam pipes or electrical services

it is recommended that scrap material brought into the store in bins or bales is kept in a special area marked out and set aside for the purpose.

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.

Flame bonding

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.

Adhesive lamination

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 (eg 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

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 eg 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 eg 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

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.

Hot-wire cutting

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

Crumbing is the process of reducing down the size of plastics eg 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 eg 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 eg prevent electrostatic discharges by bonding and earthing all metal parts
if flammable vapours are released during crumbing eg 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

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 eg 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

Flexible polyurethane (PU) foam – reducing fire risks

Manufacturing PU foam is an exothermic reaction so the foaming process needs to be carefully controlled to prevent foam reaching excessive temperatures. The highest temperatures will normally be in the centre of a block.

When manufacturing PU foam you should:

know the degradation or charring temperature of the foam
periodically measure the temperatures within large blocks
have written procedures of what to do if a block overheats eg transfer to a safe place outside the building
plan workflow so minimal amounts of PU is kept in process areas
remove finished goods as soon as possible to the store or despatch area
where practicable keep bulk stores above workrooms in multi-storey factories with a downward workflow
carryout processes involving large-scale handling of easily ignitable substances (eg highly-flammable liquids or crumb) in a fire-resisting enclosure on the top floor
monitor and manage workroom stock levels (including raw materials and part/processed materials)
manage stock levels so they are generally not more than 10% the total area
where necessary put additional safeguards in place when stock levels exceed 10% the total area eg in conversion processes where products are almost entirely PU foam
minimise times when stock levels are over 10% the total area
may need to check the quality of chemicals used in foaming

Stacking arrangements

Flexible PU foam waiting processing and shaped materials eg finished cushions should be arranged in stable stacks using pallets or racks where necessary. Rolls of laminated material should be kept in racks. Upholstered furniture may be kept in single units, racks or stable stacks as appropriate.

All materials should be kept in well-defined areas separated by marked gangways and following the guidance on storage. PU foam and work-in-hand incorporating PU foam should be stacked in workrooms following the guidance on positioning stock in storerooms. Materials in workrooms should not be stacked within 1m of any part of a fire exit.

It Is not recommended to keep PU foam or finished products against internal glazing or directly under any elevated working area. Elevated working platforms and galleries are not recommended in PU foam workrooms. Where these exist, PU foam should be kept as far as possible away from them and adequate fire protection and means of escape must be provided.