Plastics are a byproduct of fossil fuel refining; they're largely made from what is left over after fuel oils are refined out of crude oil.
My initial notes for this series of posts were written while I was sitting in campsite. I was looking around at a tent and a screen house and various other things that make modern recreational camping possible and realizing that almost all of them are made of petroleum byproducts, all the synthetic textiles, all the plastics that make up the various containers. The main thing that wasn't is the little stove – which uses fossil fuel, it's a propane stove. If we stop using fossil fuels what are we going to replace these materials with?
The answer seems to be, plant-based products. In principle, with enough energy and engineering, any chemical, including any plastic, can be made from any other chemical which contains the right elements, any usable “feedstock” in the language of chemical engineering. In practice, some feedstocks are more suitable than others for particular products. Since the energy for producing plastics from fossil fuels itself comes from the petroleum used to produce the plastics, there is going to a significant increase in the demand for energy to the process. To some extent, this perhaps can be mitigated by using embodied solar energy in appropriately-chosen plants, but this is going to take time to work out.
So there is going to be a generation where we rebuild our base of knowledge and products. Time to get started.