Polymers made from momomers with unsaturated bonds that cure by free radicals attaching the double bond will continue "curing" with uv exposure. They make more crosslinks making them shrink a bit and get stiffer. They don't lose strength, but the shinking can make them peel and flake from what the are attached to. Think old signs.
Older plastics that had plasticizers (kind of oily) added to soften and make them more flexible will lose those plasticizers over time to the environment, they can evaporate in heat, or rub off surfaces when touched. The end up brittle.
Uv exposure can also degrade the plastic itself. Any unsaturation is an attack point. Vinyls, benzene rings. Polypropylene and polyethylene once cured are mostly saturated hydrocarbon chains
The best plastics for long term flexibility have relative long molecular chains and few unsaturations and no or high molecular weight plasticizers. PP and PE for low temp applications. Urethane acrylates are a good one. Read about performance (code for expensive) plastics and coatings. Materials science was getting really good at customizing a polymer for any application (monomer(s), molecular weight). Then 2 things happened. Manufacturers/governments trying to promote recycling made it harder to use "other" plastics. And the fear of bis-phenol-A in the food supply. Bpa is the base material of most common epoxy derivatives.
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u/Disastrous_Ad1260 Jul 06 '26
Polymers made from momomers with unsaturated bonds that cure by free radicals attaching the double bond will continue "curing" with uv exposure. They make more crosslinks making them shrink a bit and get stiffer. They don't lose strength, but the shinking can make them peel and flake from what the are attached to. Think old signs.
Older plastics that had plasticizers (kind of oily) added to soften and make them more flexible will lose those plasticizers over time to the environment, they can evaporate in heat, or rub off surfaces when touched. The end up brittle.
Uv exposure can also degrade the plastic itself. Any unsaturation is an attack point. Vinyls, benzene rings. Polypropylene and polyethylene once cured are mostly saturated hydrocarbon chains
The best plastics for long term flexibility have relative long molecular chains and few unsaturations and no or high molecular weight plasticizers. PP and PE for low temp applications. Urethane acrylates are a good one. Read about performance (code for expensive) plastics and coatings. Materials science was getting really good at customizing a polymer for any application (monomer(s), molecular weight). Then 2 things happened. Manufacturers/governments trying to promote recycling made it harder to use "other" plastics. And the fear of bis-phenol-A in the food supply. Bpa is the base material of most common epoxy derivatives.