Biodegradable polymers; epoxy resin and polystyrene

Biodegradable polymers

Biodegradable polymers are polymers that can be broken down by the action of microorganisms (bacteria, fungi) and enzymes in the environment into CO₂, water and biomass. They contain links such as ester or amide groups that enzymes can attack.

Examples: PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate; from 3-hydroxybutanoic and 3-hydroxypentanoic acids), polylactic acid (PLA), polycaprolactone, starch- and cellulose-based plastics, and nylon-2-nylon-6 (glycine + aminocaproic acid). Uses: packaging, carry bags, disposable cutlery, surgical sutures, implants, and controlled drug delivery capsules.

Epoxy resin

Epoxy resins are thermosetting polymers containing epoxide (oxirane) groups. They are made by condensing bisphenol-A with epichlorohydrin in presence of NaOH (about 50–60 °C):

HO–R–OH + 2 ClCH₂–CH–CH₂   --NaOH, –2HCl-->
(bisphenol-A)      \ /
                    O  (epichlorohydrin)

CH₂–CH–CH₂–O–R–O–CH₂–CH–CH₂    (n = 0)
  \ /                 \ /
   O                   O
where R = C₆H₄–C(CH₃)₂–C₆H₄
Higher polymers: more –CH₂–CH(OH)–CH₂–O–R–O– units inside.

The product is a liquid or low-melting prepolymer with epoxide end groups. It is cured (hardened) by adding a hardener such as a polyamine (ethylenediamine, triethylenetetramine) or an acid anhydride, which opens the epoxide rings and forms a cross-linked network.

Uses:

• Strong adhesives (Araldite) for metal, glass and ceramics.
• Surface coatings, paints and protective linings for tanks and pipes.
• Electrical insulation and potting of electronic components; glass-epoxy laminates for printed circuit boards.
• Matrix of fibre-reinforced composites (aircraft, boats, sports goods).
• Flooring, castings, moulds and tools.

Polystyrene

Prepared by free-radical addition polymerisation of styrene using benzoyl peroxide: n C₆H₅CH=CH₂ → [–CH(C₆H₅)–CH₂–]ₙ. Uses: packaging, disposable cups and plates, toys, CD cases, appliance housings, and expanded polystyrene (thermocol) for insulation and cushioning.