Homopolymer, random copolymer and block (impact) copolymer
A homopolymer is made from one monomer (e.g. propylene only); it is hard, stiff and cheap, used for raffia, BOPP film and general injection moulding. A random copolymer has a little ethylene randomly built into the chain, making it clearer and more flexible – preferred for transparent containers and hot-water pipe (PP-R). A block or impact copolymer contains a rubbery phase giving high impact strength at low temperatures, used for automotive parts, pails and crates. Because the wrong type causes cracking or clarity problems, the order must state the type clearly.
Related terms
Melt flow index (MFI / MFR)
The mass of molten polymer that flows through a die in 10 minutes at a set temperature and load – 230 °C/2.16 kg for PP, 190 °C/2.16 kg or 5 kg for PE – expressed in g/10 min (ISO 1133, ASTM D1238). A low MFI means high molecular weight and strength, a high MFI easy flow: pipe and blow-moulding grades are about 0.1–1, film 1–4, injection 10–60, and fibre and meltblown 25–1,500 g/10 min. When comparing offers, always check the test conditions.
Polyethylene types: HDPE, LDPE, LLDPE and mLLDPE
Polyethylene is classed by density and chain structure. HDPE (0.941–0.965 g/cm³) is rigid and chemical-resistant, used for pipe (PE100), jerrycans, bottles and crates. LDPE (0.910–0.930) has a branched structure that makes it flexible and clear, used for extrusion coating, shrink and food film. LLDPE is a linear low-density copolymer with butene (C4), hexene (C6) or octene (C8) comonomer; with high puncture and tear resistance it is the main raw material for stretch and packaging film. mLLDPE, made with metallocene catalysts, offers even higher toughness and clarity.