Uncontrolled Polymerization Hazards - Acrylates

There is a brief article at NJ.com about a recent incident at a warehouse where six firefighters were hospitalized for chemical exposure related symptoms after responding to a reported fire at the warehouse. It appears that some chemicals in drums began to self-react causing a chemical vapor release that affected the firemen approaching the warehouse doors. The chemical involved was an unspecified acrylate.

Acrylates

Acrylates are a family of vinyl monomers that are based upon chemical modifications of acrylic acid. The modifications are made by adding carbon chains of varying lengths to either (or both) of the two carbons separated by the characteristic double bond that makes these chemicals important components of a wide variety of commercial polymers. While acrylate monomers are frequently used to make homopolymers (polymers containing just a single type of monomer) they are more frequently used to make copolymers where relatively small amounts of acrylates are added to other monomers to modify or tailor the characteristics of the resulting polymer.

All acrylate monomers have some level of toxicity associated with them. Generally speaking, the longer the side chains added to the monomer the lower the level of toxicity. See the manufacturers Safety Data Sheet (SDS) for toxicity information. They also have a characteristic odor that becomes less offensive and harder to detect as the side chains get larger.

For ease of handling, higher molecular weight (longer side chains) acrylates have organic solvents added; this increases the volatility and may increase the flammability of the products. The lower molecular weight acrylates are very volatile and flammable in their own right.

Autopolymerization

All vinyl monomers have the capability to auto-polymerize, that is to begin to react to form polymers without the addition of reaction initiating chemicals. A variety of inhibitors are added to these monomers to prevent that auto-polymerization process, but none of them are completely successful and a very-low rate of autopolymerization is almost always in progress in stored monomers.

Polymerization of vinyl monomers always results in the production of heat. In the polymer production process a great deal of energy is consumed in controlling the heat of reaction because the higher the temperature of the reactants the faster the polymerization process proceeds. It is important to control the rate of the reaction for both quality and safety purposes.

Each acrylate monomer has its own characteristic autopolymerization temperature. Generally speaking, the smaller the length of the side chains, the lower that temperature is. The autopolymerization temperature is important because when the monomer reaches that temperature it is not possible to control the autopolymerization process by applying external cooling.

Safety Concerns

If the heat of reaction is not removed by external cooling the amount of heat produced by the autopolymerization process raises the temperature of the acrylate/solvent mixture which increase the rate of reaction. At lower product temperatures air cooling of the containers is sufficient to remove the heat of reaction of properly inhibited acrylate mixtures.

As the inhibitor is consumed over time, the rate of the autopolymerization reaction increases and at some point air cooling is no longer adequate to control the temperature of the container. If additional inhibitor is not added, or more efficient cooling added you can reach the point where you have a significantly self-accelerating polymerization reaction taking place with an accelerating increase in the acrylate temperature.

At some point, if not controlled, the autopolymerization reaction will increase the temperature of the acrylate mixture to the point that the vapor pressure of the acrylate or solvent will exceed the pressure rating of the sealed container causing a failure of the container (potentially a catastrophic failure) and a release of the acrylate and/or solvent vapors to the atmosphere. This could result in an explosive flammable-air mixture.

Response

The autopolymerization reaction becomes a safety concern when there is a noticeable rise in the temperature of the storage container. Material stored in drums or other portable containers should have the temperature checked periodically (see manufacturers safe handling guidance for monitoring frequency) and the drum temperature recorded so that trends can be tracked.

When a significant temperature rise is noted (see the manufacturers safe storage guidance for what is considered ‘significant’ for the particular acrylate). After checking the drum for any signs of bulging or other indicators of pressure, carefully open the container {wearing ALL appropriate personal protective equipment (PPE) – see the SDS} and add the re-stabilization chemical (also known as a ‘backstop’) recommended by the manufacturer (typically a phenothiazine solution).

If the temperature continues to rise after the backstop solution is added, or there are signs of container pressurization (bulging top or leaking), you have a hazardous material response situation which requires specially trained personnel. Typically, this is going to include isolation of the material and the application of cooling water to the outside of the container to reduce the possibility of catastrophic loss of containment. Atmospheric and run-off water monitoring should also be conducted.

If you have loss of containment during the incident the liquid will be some combination of the base acrylate and any solvent used in the solution. In either case they are hazardous materials and should be treated appropriately. Both toxicity and flammability will be issues of concern. Because of the heat of the liquids escaping the containers, there will be a significant vapor cloud (again flammable and toxic) associated with any release.

Summary

Acrylates are valuable industrial chemicals used in the making of many specialty polymers and plastics. They are, however, hazardous materials and must be treated with care and respect. Facilities that store these materials need to understand the autopolymerization hazards and have a plan in place to deal with them. An essential part of that plan must be a close working relationship with the local first responder community, ensuring that the first responders are aware of the potential hazards and how to deal with them.