What are monomers?

Monomers are small, reactive molecules that serve as the building blocks for plastics and other polymers. Through chemical reactions such as polymerisation, monomers combine to form long chains or networks, known as polymers. The properties of the resulting plastic depend largely on the chemical structure of the monomers used. They are used in numerous industries, including the packaging, construction, automotive and textile industries.

Formation

Monomers are produced industrially in a variety of ways, often through petrochemical processes such as cracking of crude oil fractions, oxidation, chlorination or biotechnological processes. Ethylene and propylene, for example, are produced by thermal steam cracking, while styrene is obtained from benzene and ethylene. Acrylic acid and its esters are produced in multi-stage oxidation and esterification processes. Many monomers are intermediate products in the chemical industry and are processed directly on site to prevent unwanted polymerisation during storage and transport.

Identification

Depending on the compound, monomers occur in liquid, gaseous or, more rarely, solid form. Many are colourless to slightly yellowish and have a characteristic, often pungent or sweetish odour. Some monomers are highly volatile and flammable, while others contain reactive functional groups that can polymerise spontaneously under heat, light or in the presence of catalysts. Inhibitors are often added for stabilisation, which reduce reactivity and improve shelf life.

Limit values

The permissible limit values for monomers are highly dependent on the substance. In the EU, they are specified in the Technical Instructions on Air Quality Control (TA-Luft) and in the Technical Rules for Hazardous Substances (TRGS). For example, the occupational exposure limit for styrene is 20 ppm (86 mg/m³) and for ethylene oxide 1 ppm (1.8 mg/m³). Compliance with these values is essential to avoid health risks.

Hazards

Monomers can pose different hazards depending on their chemical structure. Many have an irritating or sensitising effect on the skin, eyes and respiratory tract. Some, such as vinyl chloride or acrylamide, are carcinogenic. Highly reactive monomers also pose a risk of uncontrolled polymerisation, which can lead to heat generation and pressure build-up in closed containers. Environmental hazards exist in particular with water-soluble or persistent monomers that can enter ecosystems.