What Styrene Monomer Is
Styrene, also called styrene monomer, vinylbenzene or ethenylbenzene, is an aromatic hydrocarbon with the molecular formula C8H8 and a molecular weight of 104.15 g/mol. Its structure is a benzene ring carrying a vinyl group, and the conjugation between the vinyl double bond and the ring is what gives the molecule its polymerisable character. At room temperature styrene is a clear, colourless to light yellow liquid with a sweet, pungent odour. It is practically insoluble in water but dissolves in ethanol, ether and most organic solvents. Commercially it is produced mainly by the dehydrogenation of ethylbenzene, which is itself made by alkylating benzene with ethylene.
Key Physical and Chemical Properties
| Property | Value |
|---|---|
| CAS number | 100-42-5 |
| Common names | styrene monomer, vinylbenzene, ethenylbenzene |
| Molecular formula | C8H8 |
| Molecular weight | 104.15 g/mol |
| Boiling point | 145 °C |
| Melting point | -30.6 °C |
| Flash point, closed cup | about 31 °C |
| Density at 20 °C | 0.906 g/cm3 |
| Water solubility | practically insoluble |
| Typical commercial purity | 99.5% minimum |
The combination of a reactive double bond with aromatic rigidity is the reason styrene sits at the centre of so many polymer families: it polymerises and copolymerises readily, and it also dissolves many resins, which lets it act as a reactive diluent rather than a passive solvent.
Main End Uses
Styrene is polymerised to general-purpose polystyrene (GPPS), high-impact polystyrene (HIPS) and expandable polystyrene (EPS). These materials appear in packaging, disposable food service items, insulation boards and household products, where their clarity, rigidity and low cost are the deciding factors.
As a comonomer, styrene is combined with acrylonitrile to make SAN, with acrylonitrile and butadiene to make ABS, and with butadiene to make styrene-butadiene rubber (SBR) for tyres, hoses and footwear. These copolymers combine the stiffness of the styrene units with the impact resistance of the rubber phase, which is why ABS dominates appliance housings, automotive interior parts and consumer electronics enclosures.
GPPS, HIPS and EPS for packaging, insulation and disposable articles
SAN and ABS for rigid, impact-resistant moulded parts
SBR for tyres, hoses, belting and footwear compounds
Unsaturated polyester resins for composites, boat hulls, pipes and automotive parts
Paints, varnishes, paper coatings and carpet backings
In unsaturated polyester resins styrene works as a reactive diluent: it lowers viscosity so the resin can wet glass fibre and fill a mould, and then it crosslinks the resin during curing. In coatings and carpet backings, styrene-based polymers and copolymers contribute gloss, hardness and structural stability.
Quality Parameters to Verify When Buying
Commercial styrene is specified primarily by purity, typically 99.5% or higher, determined by gas chromatography. Purity alone is not enough, however, because several trace parameters decide how the monomer will behave in storage and in the reactor.
| Parameter | Why it matters |
|---|---|
| Purity by GC | main specification, typically 99.5% minimum |
| Colour on the Pt-Co scale, ASTM D1209 | indicates oxidation and contamination |
| Water content by Karl Fischer, ASTM E203 | affects polymerisation and product quality |
| Aldehyde and peroxide content | promotes premature polymerisation and colour bodies |
| Polymer content | plugging, fouling and loss of monomer |
| Inhibitor level, such as TBC | must be sufficient to suppress self-polymerisation |
Styrene is normally shipped with an inhibitor such as para-tert-butylcatechol to prevent polymerisation during transit and storage, and the inhibitor level has to be checked on receipt because it is consumed over time. A buyer should request a certificate of analysis for each batch, confirm the permitted storage temperature range, and verify the batch history and the condition of the tank or container before loading. Clear specification, delivery terms and quality claims should be written into the purchase agreement, since disputes over polymer content and inhibitor level are the most common commercial friction points.
Safety, Storage and Bulk Supply
Styrene is a flammable liquid with a flash point near 31 °C in a closed cup, and its vapour can form explosive mixtures with air. Vapour inhalation irritates the eyes and respiratory tract and can cause dizziness, and prolonged exposure has been associated with adverse health effects, so the monomer is classified by the International Agency for Research on Cancer as a probable human carcinogen. Closed systems, local exhaust ventilation and suitable respiratory protection should be used, the liquid must be kept away from ignition sources and from polymerisation initiators, and all transfer equipment must be bonded and grounded to prevent static discharge.
Storage demands equal care. Styrene must be kept with its inhibitor intact, at a controlled temperature, typically below 25 °C, and for limited periods only, because the monomer polymerises exothermically once the inhibitor is depleted or the temperature rises. Tanks should be inerted and protected from direct sunlight and heat.
Bulk styrene is supplied by dedicated tank truck, railcar or ocean tanker and is handled as hazardous cargo under the applicable transport regulations. Before placing a large order, a buyer should confirm the destination market import requirements, including customs classification, and verify that the supplier can issue the safety data sheet, the certificate of analysis and any required declarations. Because styrene prices track feedstock costs and supply-demand balances closely, annual contracts and spot purchases alike should state the specification, the delivery terms and the quality claim procedure in writing.
Frequently Asked Questions
Q: What is styrene monomer used for?
It is used to make polystyrene, ABS and SAN copolymers, styrene-butadiene rubber and unsaturated polyester resins for construction, packaging, automotive and consumer goods.
Q: Is styrene the same as polystyrene?
No. Styrene is the liquid monomer, while polystyrene is the solid polymer made from it. The monomer is hazardous to handle, whereas the finished polymer is an inert plastic.
Q: Why does styrene need an inhibitor?
Styrene polymerises spontaneously over time and the reaction releases heat, so an inhibitor such as TBC is added to delay polymerisation during storage and transport, and its level must be monitored.
Q: How should styrene be stored?
Store it in sealed, inert containers at a controlled temperature, typically below 25 °C, with the inhibitor level verified and away from ignition sources, oxidisers and polymerisation catalysts.
Q: What is the flash point of styrene?
The flash point is about 31 °C in a closed cup, which classifies styrene as a flammable liquid, so fire protection and grounding are required wherever it is handled.
Q: What documents should come with a styrene shipment?
The shipment should include the safety data sheet, a certificate of analysis stating purity and inhibitor content, the batch number, and transport documents covering the hazardous goods classification.