Polymers - structure, families & processing

Composition. Oxygen in the chain

polyesters such as PET & acetal

composition. nitrogen in the chain

polyamides (nylon) - strong & tough, absorbs water

Composition. Chlorine in the chain

gives PVC - resists burning & releases hydrochloric acid gas when it does burn

(m) ethylene (PE)

(p) polyethylene - milk bottle

(m) propylene (PP)

(p) polypropylene - garden chair

(m) styrene (PS)

(p) polystyrene - clear salad cutlery

(m) vynil chloride

(p) polyvinyl chloride (PVC) - window frames, garden hose

(m) methyl methacrylate

(p) polymethyl methacrylate (PMMA) - acrylic

lifetime lens. Long, well-entangled chains

give tough, crack-resistant parts

why do plastics fail/age?

breaks in monomer/polymer chains.

what do plastics age because of

UV lights, heat, oxygen - cut chains

thermoplastics

can be reheated and remoulded, welding & heat forming

thermosets (thermosetting plastic)

once formed cannot be reheated, shaped before or during curing, stiff but often brittle

elastomers

when stretched the tangled chains straighten out.

amorphous polymers (PS, PC, PMMA, PVC)

chains that are tangled at randomsem

semi-crystalline (PE, PP, PET, PA, POM)

have regions where chain segments fold & line up in heat, tightly packed bundles

copolymers

built from two or more different monomers in the same chain

random copolymers

monomers mixed at random

block copolymer

long blocks of each monomer - rubbery behaviour

bio-based plastics

the carbon comes from plants instead of fossil sources

biodegradable plastics

microorganisms can break the material down into water, CO2, and biomass under certain conditions

compostable plastics

breaks down within a defined time under defined composting conditions

UV degradation effect

fading, chalky surface, then brittleness and cracks

heat & oxidization effects

yellowing, embrittlement, loss of gloss

creep (slow deformation under constant load)

sagging, loosening

fatigue (repeated loading)

cracks at corners and thin sections after many cycles

environmental stress cracking (ESC)

sudden cracks where where stress & a chemical meet

plasticiser migration

soft PVC becomes stiff, sticky or cracked

water & abrasion

worn-through surfaces, lost texture, particle shredding

code 1 - water and soft-drink bottles

PET

code 2 - milk bottles, shampoo bottles

HDPE

code 3 - pipes, blister packs, cable insulation

PVC

code 4 - bags, squeeze bottles

LDPE

code 5 - take-away boxes, caps

PP

code 6 - salad cutlery, CD cases

PS

code 7

everything else

obrabotka - pellets

injection moulding, extrusion & blow moulding

obrabotka - powder

rotational moulding

obrabotka - sheet & film

thermoforming

obrabotka - beads with a blowing agent

foam and bead moulding

obrabotka - liquid resins or two-component systems

casting, reaction injection moulding (RIM), composite lay-up (thermosets & PU)