I read this post about peak plastic earlier today and I feel compelled to throw my totally ignorant 2¢ in.
While that article makes the point that fossil fuels are also the source of plastic — and in fact many of the materials and chemicals of our everyday lives — and with peak oil comes peak, well, everything. This is not that obvious of a phenomenon right now. This unobviousness is probably because natural gas is a hugely important feedstock in the production of many important plastics (e.g. polyethylene). With any application there are many plastics that could work, and so while the one that does work the best gets too expensive the alternatives will be mustered to fill in for them.
While the concept of peak oil is more visceral every time we visit the gas station, natural gas is cheap and becoming cheaper — largely due to fracking and other methods of accessing previously inaccessible natural gas. In fact there has been such a glut of natural gas hitting the market that many northern gas fields are no longer looking that profitable as the market price is below the cost of bringing said gas to market. The surprisingly low prices have created a lot of movement in both vertically integrating gas fields and building new ethylene cracking units by the big chemical companies.
Natural gas is a hugely important feedstock for plastic, I would wager probably more so than oil, if only because it is a large source of ethylene — used to make polyethylene. This is likely to become even more acute as time goes on, but then again as the gap between the cost of oil and the cost of natural gas grows we will likely see more traction with CNG as the fuel for everything, and so this might change.
Anyways, in the long run natural gas will be exhausted eventually as well. This is just common sense: there is a finite amount of it. However I think the more obvious worry is not that there will no longer be enough ethane to make plastic water bottles — or whatever — but that the natural gas needed to run the plant will be too expensive. That is the other thing often neglected in this analysis: while fossil fuels are an incredible gift, a chemical legacy, for synthesis they are also the fuel we use to actually operate the vessels which turn that chemical legacy into the final products we all use.
Of more immediate concern to Chemical Engineering is the cost of energy. Utility costs are very significant, and are a big part of that driving force towards phenomenally large facilities in a few places as opposed to small facilities geographically dispersed. Natural gas is not only used directly as a feed stock — turned into ethylene, hydrogen, etc. — it is also often the fuel that is the ultimate source of process heat (directly or by steam).
This is not so doom-and-gloom as you might think. At least not if you are a Chemical Engineer. Finding new ways to produce intermediates and final products in ways that are primarily more energy efficient and secondarily do not use fossil fuels is a huge growth industry. I can think of no greater source of jobs for people in Chem Eng. or biotechnology than the opportunity to basically tear down and rebuild an entire industry.
Of course this has already started. Plastics made from corn starch or using bio-derived ethylene and propylene are already on the shelves. I read a while ago that the big tire companies are looking at bio-derived isoprenes as well. Which I find slightly funny since the isoprenes are used to make synthetic rubber, so they are looking for naturally sourced synthetic rubber to mix with natural rubber.