Fire and Explosion Risk - Reactive Monomers

  • Issue : Dec 2004
  • Review : Dec 2005
  • OG Status : Fully Open
  • Author : CI 4

(Safety Alert)


This alert has been produced to draw attention to a number of recent incidents in the chemical industry where self-reactive monomers have been present, and were the most probable cause of the incidents.


In one incident, a drum of inhibited methyl methacrylate was returned to a chemical transport company with the intention of return to the original supplier. The transport company stored it in a shed to await other material for return to that supplier. After 6 months the drum exploded, causing considerable damage and contamination of the shed and adjacent warehouse, and the closure of a local motorway for 6 hours.

In another incident, in a batch reaction, insufficient inhibitor and inadequate process cooling control allowed a reactor to become over-pressurised, relieving the semi-reacted boiling mass into the adjacent roadway, with a massive release of solvent and acrylate vapour into adjacent housing and shops. The company have spent many hundreds of hours assessing the cause and effects of the incident. Below the line costs are estimated at £1m.

In yet another incident, inhibited acrylate monomer was stored in laboratory sized bottles on shelving with other flammable materials. The stocks had not been rotated, and some of the bottles were 5 years old. It is suspected that one of the bottles exploded, causing an adjacent container to break, releasing its' contents which then mixed with the acrylate. These two materials were chemically incompatible, and spontaneously combusted due to the heat of the reaction. The subsequent fire went out of control within minutes, and the store (a large chemical warehouse) burnt to the ground during the next 6 hours. Stock losses were estimated at >£10m, and property loss at £4m. Approximately 50 persons had to be made redundant.

Fortunately, in each of these cases, no one was injured, but the risk to injury is clear.

The root cause in each of these incidents appears to be related to the absence of sufficient inhibitor to prevent runaway polymerisation. The inhibitor incorporated with many monomers is adequate to provide protection for typically 6 months under ambient conditions. Many inhibitors degrade or become inert within that period as they react with free radicals that eventually chemically change the inhibitor such that it is no longer effective. Some inhibitors rely on the presence of dissolved oxygen in the monomer to keep them active. High summer temperatures can cause poorly inhibited monomer to runaway or self-react.

Monomers should be stored in accordance with the guidance from the supplier. The level of inhibitor should be checked at regular intervals and topped up if necessary. Failure to do so can result in the monomer becoming unstable with potentially serious results as indicated above. It appears that some sections of industry are unaware of the necessary precautions. It is important that all users recognise that inhibitor levels can become depleted, and should be maintained at manufacturer's recommended levels.

Action Required

HSE would suggest that if you have reason to use or store any reactive monomers, they should be stored in accordance with the advice from the supplier, ensuring that

  • the material is stored away from other incompatible materials;
  • inhibitor levels are monitored;
  • storage conditions allow adequate ventilation and cooling;
  • monomers are used in rotation order - oldest first.

If you suspect that you have partially reacted or poorly inhibited or uninhibited material, contact the original supplier who should be able to offer advice. If monomers are used in reactions, consideration should be given to minimising the amount of unreacted monomer within the batch.

When considering where these substances may be present, remember the less obvious laboratory store cupboards and benches. Contact the supplier to obtain methodology for testing inhibitor levels. Read the manufacturer's safety data sheet (MSDS) for each substance.

Relevant Legal Requirements

Health and Safety at Work etc Act 1974

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Updated 2022-01-21