Plastic ages whether or not you use it

This looks at polymer ageing in products from the practical end — what holds up once conditions stop being ideal.

What ageing does chemically

Many flexible plastics contain plasticisers, small molecules that separate polymer chains and allow them to slide past each other. Those molecules gradually migrate out of the material, which is the sticky film that develops on old cables and grips. As they leave, the material stiffens and loses its ability to absorb strain, so it cracks where it previously flexed.

Ultraviolet light breaks polymer chains at the surface, producing the chalky, finely cracked appearance of weathered plastic. Oxidation continues throughout, accelerated by heat, which is why a part near a motor ages faster than an identical part elsewhere.

Where cracks appear and why

Injection moulding leaves internal stress frozen into the part, concentrated at sharp corners, thickness changes and around holes. Those locations become the crack initiation points years later, which is why plastic parts fail in the same place across many examples.

Weld lines, where two flow fronts met during moulding, are weaker than surrounding material and are a common failure path. Threaded plastic bosses fail through a combination of moulded stress and the hoop stress applied by a screw. Understanding this explains why identical failures are reported by many owners of the same model, and why it is design rather than misuse.

Chemical attack from ordinary substances

Environmental stress cracking occurs when a stressed plastic contacts a substance that would not harm it unstressed. Cleaning products, sunscreen, insect repellent, hand sanitiser and cooking oils are common causes on consumer goods.

After the warranty ends, the failure looks like a crack appearing spontaneously, and it is frequently blamed on material quality rather than on contact. Manufacturers usually specify compatible cleaning agents, and those instructions are among the more useful lines in a manual. Where a product will be handled with sunscreen or cleaned with solvents, the polymer chosen genuinely matters.

Which plastics behave how

Polypropylene and polyethylene resist chemicals well and are hard to bond, which is why they are used for containers and rarely glued. Polycarbonate is tough and impact resistant while being vulnerable to certain solvents and to stress cracking. Acrylonitrile butadiene styrene moulds well and finishes nicely, which is why it is used for housings, and it yellows under ultraviolet light.

In the small print, glass fibre filling increases stiffness and strength substantially at the cost of toughness, so filled parts crack rather than bend.

Product literature rarely names the polymer, though moulded recycling codes on the part itself often do.

Storage is its own hazard

Items stored unused still age, and the classic examples are perished rubber seals, brittle cable insulation and cracked battery compartments. Heat in lofts and garages accelerates all of these processes, and temperature cycling adds mechanical stress on top. Sunlight through a window delivers ultraviolet exposure indoors, which is why items left on windowsills fade and embrittle.

Judged against the category, this is why buying old new stock is a real gamble, since the warranty period may have elapsed while the item sat in a warehouse. It also explains why spare parts bought and stored for years are sometimes unusable when finally needed.

Judging a product for its plastics

Look at where load-bearing plastic parts sit, and whether any of them carry weight, torque or repeated impact. Prefer metal inserts at screw points on anything that will be opened, since plastic threads have a very limited number of cycles. Check whether the product is likely to sit in sunlight or heat, and whether the manufacturer specifies ultraviolet-stabilised material.

Read the cleaning instructions before purchase, because they hint at chemical vulnerabilities you will otherwise discover accidentally. This site does not perform materials analysis, so owner reports of specific cracking failures are the practical evidence to seek.

The takeaway

Plastic parts have a calendar as well as a duty cycle, and heat and sunlight run both clocks faster.

The question is rarely which is best. It is which is enough.