PS
Styrene polymer used in solid packaging and lightweight protective foam.
- Also known as
- polystyrene, poly(phenylethene), PS, EPS, expanded polystyrene
In everyday products
- Protective packaging foam — EPS insert
- Yogurt pot — PS component of the pot
- CD jewel case — Clear case panels
Structure and behaviour
[–CH₂–CH(C₆H₅)–]ₙ
The styrene vinyl group makes the backbone while the phenyl ring remains pendant. Typical general-purpose PS is an amorphous glass at room temperature; its clear rigid form and its expanded foam have the same main polymer chemistry.
- Restricted chain motion gives a rigid clear case, but ordinary general-purpose PS can fracture at a sharp hinge or notch.
- Rubber-modified grades improve impact performance, usually with a change in transparency; they are not identical to clear general-purpose resin.
- A softened sheet can be thermoformed into a thin pot, while injection moulding makes detailed case panels.
- Expanded beads trap gas in many cells, producing a light cushion whose behaviour depends on density and shape.
- Foaming creates structure at a larger scale than the repeat unit; adding a blowing agent is separate from making styrene chains.
Preparation routes
Styrene route
Addition polymerisation. The vinyl groups join into a carbon backbone carrying phenyl side groups. Making foam is a later processing choice, not a second polymerisation reaction. The benzene ring is retained as a side group rather than opened into the growing chain. Solid PS and expanded PS therefore share this molecular route.
Initiation and temperature control support chain growth. Expandable grades also contain a blowing agent, which is not a monomer; rubber-modified grades have additional polymer content. Rubber addition or bead expansion changes material structure beyond this simple backbone trace.
- Polystyrene: from benzene and ethene to solid and expanded productsChemical Industry Education Centre, University of York
From benzene and ethene to styrene
- Benzene + Ethene → Ethylbenzene
Alkylation joins an ethyl group to the benzene ring. Both feedstocks belong to this step.
- Ethylbenzene → Styrene
Dehydrogenation removes hydrogen and creates the vinyl double bond used in the next polymerisation step.
- Polystyrene: from benzene and ethene to solid and expanded productsChemical Industry Education Centre, University of York
Material limits
A clear PS case, an impact-modified yogurt pot and an EPS block are different forms, despite their common origin. Severe crushing can permanently damage an EPS cushion. Low-density foam also occupies much space during collection; local acceptance and separation from other foams determine its actual recycling route.
Sources
- Polystyrene: from benzene and ethene to solid and expanded productsChemical Industry Education Centre, University of York
- Polystyrene for rigid food packagingINEOS Styrolution
- Pack and protect: expandable polystyreneBASF
- Polymer structure, crystallinity and physical propertiesOpenStax, Rice University
- Sustainable plastics: the role of chemistryRoyal Society of Chemistry