The Role of Inhibitors in Monomer Storage, Transport, Production, and Processing

Monomers need to contain an inhibitor to prevent polymerization during processing or storage. Oxygen can play many roles in processing and storage of monomers, mostly depending on the temperature conditions, and this leads to the choice of inhibitor.  There are two main types of inhibitors: 1) Storage and transport inhibitors, and 2) Production and process inhibitors.

STORAGE AND TRANSPORT INHIBITORS

During storage, monomers are usually not exposed to high temperatures, but are exposed to oxygen due to transfers, handling, and general opening and closing of containers. Therefore, an inhibitor that works best in the presence of oxygen is needed. During storage at room temperature, monomers are slowly forming free radicals (R•) on their own that, if left unchecked, can form polymers (RRRRRRRRR....). When oxygen is present, the monomer free radical reacts with oxygen to form a peroxide: 

R• + O₂ --> ROO•

This reaction is several orders of magnitude faster than R• self-polymerization. Therefore an inhibitor is needed that terminates the ROO• radical because ROO• outnumbers R• radicals in stored monomer by several orders of magnitude. Phenolic type inhibitors react very quickly with peroxide radical (ROO•) in a termination step.  The phenolic inhibitors do not react with the monomer free radical (R•), thus oxygen is necessary for these inhibitors to function.

Common phenolic inhibitors include butylated hydroxytoluene (BHT), tert-butylcatechol (TBC), hydroquinone (HQ), hydroquinone methyl ether (MEHQ), and 2,4-dimethyl-6-tert-butyl phenol (2,4-DiMe-6-tBu phenol).

 

Trace amounts of oxygen are sufficient for the inhibitor to work. The mechanism for inhibition indicates that an amount equimolar to phenolic inhibitor would be required - 10 ppm dissolved O₂ is equimolar with 40 ppm phenolic inhibitor. An equimolar ratio of dissolved O₂ to phenolic inhibitor is the minimum suggested level.  Industrially, for storage and transportation of monomers, contact with air or a 50/50 nitrogen/air mixture is maintained.

The most commonly used industrial phenolic inhibitor is hydroquinone monomethyl ether (MEHQ) due to its colorlessness and its efficiency, usually used between 10 and 300 ppm.

PRODUCTION AND PROCESSING INHIBITORS

During monomer production and processing, high temperatures are needed during reaction and/or distillation. Because of the high temperatures, monomers are quickly forming free radicals (R•) on their own which can lead to fast polymerization. In this high temperature environment, oxygen behaves mostly as an oxidant that oxidizes the monomer causing severe yellowing and tar formation. Therefore, to reduce product oxidation (yellowing and tar), oxygen must be excluded from high temperature processes. Since oxygen cannot be completely excluded, it will react with small amounts of R• to form ROO•. Therefore, an inhibitor system is required that can quench both R• and ROO• radicals. We have already discussed the phenolic inhibitors that can terminate ROO• radicals.  

Process inhibitors that react with R• radicals (in the absence of oxygen) include quinones, nitroso compounds, metal salts, dibenzofulvene derivatives, aromatic azo compounds, phenylacetylene, pyridinic derivatives, aromatic amines (for example, phenothiazine). Industrially, for acrylates and methacrylates, examples of process inhibitors used are p-benzoquinone, cupferron, manganese or cerium salts, hydroxylamine, copper alkyldithiocarbamate, phenothiazine, etc. Process inhibitor concentration generally ranges from 2 ppm to 2000 ppm.