Question 6 of 8: Environmental Degradation of Polymers

Question 6: Environmental Degradation of Polymers (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

6(a) — Three major degradation mechanisms

Polymeric materials degrade in service by three principal environmental mechanisms. Swelling and dissolution is a physical (not chemical-bond-breaking) process in which a compatible solvent diffuses into the polymer, separates the chains, and either swells the network (crosslinked polymers, which cannot dissolve) or fully dissolves the material (uncrosslinked thermoplastics) — governed by how closely the solvent's and polymer's solubility parameters match. Bond rupture (scission) covers chain-breaking reactions driven by radiation (particularly ultraviolet photons, which have enough energy to break C–C and C–H backbone bonds directly — photodegradation) and by thermal energy at elevated temperature (thermal degradation/depolymerization); scission shortens chains, lowering molecular weight and with it strength and ductility, and can also generate reactive free radicals that propagate further damage. Weathering (oxidation) is the combined, synergistic attack of atmospheric oxygen, ozone, heat and UV light acting together outdoors — oxygen adds across radical sites created by UV or heat to form carbonyl and hydroperoxide groups, which embrittle the surface, discolour the polymer, and can further catalyse chain scission, making weathering more damaging than any one of its three contributing factors (heat, oxygen, UV) acting alone.

6(b) — Polymer classification for each application

6(c) — Justification: (iii) O-ring for preserving jam in a jar

A jar-lid o-ring must be an elastomer, not a thermoplastic or thermosetting polymer, because its function is entirely mechanical: it must compress elastically against the jar rim and lid to form a hermetic seal, then recover its shape each time the jar is opened and re-closed, over what may be dozens of cycles across the jar's service life. A thermoplastic gasket would creep and take a permanent compression set after the first sealing cycle (losing the seal on re-use), and a thermosetting material is typically too rigid and brittle in the required cross-section to conform to minor imperfections in the glass rim and lid at all — only a lightly crosslinked, rubbery-plateau elastomer combines the very low modulus and near-full elastic recovery ("resilience") needed to seal repeatedly.

The specific elastomer choice must also resist the service environment: contact with an acidic, sugar-and-pectin-rich foodstuff, repeated exposure to boiling-water processing (canning/preserving) or at least warm fill temperatures, and long-term storage at typical pantry conditions. A silicone or EPDM (ethylene propylene diene monomer) rubber is the standard choice for exactly this reason — both have fully saturated backbones (EPDM) or an inorganic Si–O backbone (silicone) with no easily oxidised double bonds, so they resist the combined heat/oxygen exposure of repeated hot-fill or boiling-water processing far better than a general-purpose diene rubber (e.g. natural rubber or SBR), which would harden and crack under the same repeated thermal-oxidative cycling. Food-contact-grade formulations of either elastomer are also compounded without the leachable plasticisers or accelerators that would be a food-safety concern in a seal that touches the product directly.