Poly Vinyl Chloride comes in a variety of forms dependant on what fillers are added to the basic material. One of the components of flexible PVC is a plasticiser, and this evaporates over time turning what used to be a flexible material into a stiff material. This can be seen with things like electrical cable that when it is old is very difficult to cut with a knife. Beautifully soft fabric of a new motor car becomes hard and rigid and eventually starts to crack. My mother had a one-gallon PVC container holding rainwater in her garden for a couple of years. The container had been in the sun most of this time. I knocked the container over on the concrete and it shattered like this glass. I imagine UPVC has something in it that resists the ultra-violet radiation from the sun. My dictionary states that the 'U' stands for 'unplasticised'. But if it were only that, old UPVC doors doors would shatter when slammed.
Various building artefacts are made from PVC, and when they get old they tend to crack or break easily. But there is a useful bodge to add to the life of some items. I have done this to things like a watering can and a WC cistern that cracked. These items can be welded; but I strongly advise not to try welding anything like a PVC ladder, as even metal welding can be unreliable when it is not carried out by a skilled operator.
Welding is melting the two parts to be joined, and allowing the liquid material of each part to merge. Anything that can be melted can (in theory) be welded. PVC can be melted but it requires a very closely controlled temperature. It is very easy to char the material instead of simply melting it. I had an old car that had a black plastic bumper. It was probably not PVC, but one of the other plastics. I got some plastic of the same type from a scrap bumper and using a heat-controlled soldering iron, I repaired the fracture quite well. The fumes that melting plastic give off are rather noxious, so it is best to have a well-ventilated position.
I suggest using a heat-controlled iron as one can choose what bits to use to get a controlled temperature. But use a bit that you never want to use again for soldering, as the surface tinning is soon corroded away.
I am uncertain if Solvent Cement actually dissolves the surface of PVC, or whether it is just a good glue. If it does dissolve the surface, this may be an easier way to make a repair.
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