Making Plastic from Carbon Dioxide

Today’s post is a little different from my normal craft related posts, but as I use this blog to catalog the things I’ve made I reckon this counts. So today I’m going to be telling you about my PhD thesis. It took me three years and was an interesting if not always pleasant experience.

I was working on turning carbon dioxide into plastic, specifically ‘switchable catalysis to make block copolymers‘. But don’t worry I’ll explain that later.

To a chemist, carbon dioxide is just another chemical, with which we can make new things, and actually its a pretty safe chemical – it’s not toxic and it’s not flammable. Chemists have been looking into using carbon dioxide to make other things for a while because it is a waste product. We all know about how we are producing significant quantities of carbon dioxide from our industrial processes and why that’s a bad idea. I do have to point out, that even if all chemicals where made from carbon dioxide, it still would not affect the levels of carbon dioxide in our atmosphere. However, using waste products is attractive because it saves money and prevents other ‘new’ chemicals from being made (which would produce even more carbon dioxide). Also as more incentives are being placed on manufacturers to capture waste carbon dioxide, having an out put for this captured carbon dioxide is a useful goal (as opposed to just storing it in a cave somewhere).

Carbon dioxoide is turned into a plastic by coupling it with another chemical called an epoxide. The plastic is called polycarbonate and research into this plastic is quite far along, there are several companies who are bringing it to market. It is being sold in the form of a polyurethane. Polyurethanes are made by taking the the polycarbonate and joining it with another chemical – an isocyanate. They come in many forms – but the carbon dixoide based polyurethanes are mostly being sold as foams (the solid sort) for things like insulation, mattresses and the cushioned bits of car seats.

The reason why polycarbonate is being turned into polyurethanes in order to be sold is the same reason why I was still carrying out research on it. Polycarbonate on its own makes a very brittle plastic, so if you try and make something from it, the object will crumble into pieces. One of the most common ways to change how a plastic behaves is to join a section of a different plastic – making a block copolymer, where the overall plastic has sections or blocks of different types of plastic.

My project was focused on developing methods of attaching the polycarbonate from carbon dioxide to other plastics which also have green credentials. The relative newness of these green plastics means methods of attaching them hasn’t been researched as much as with traditional plastics. Also even with traditional plastics, attaching them isn’t always easy and often involves several steps – which is not very efficient, in terms of time, money and environmental impact.

I was looking to find a method where I could use a single ‘pot’ for all my materials and make the block copolymer in one go. This is tricky because you have the materials for each plastic all mixed up and want to control which plastic gets made and when it gets made. Kind of like putting all the ingredients for a lemon meringue pie in a bowl, mixing it up and expecting to form the pastry, the lemon curd and the meringue and also for it to assemble into the pie. So it pretty complicated but fortunately for me I had a magic ingredient – a catalyst. A catalyst is a separate chemical, which makes it possible for a reaction to happen. In my case if you mix carbon dioxide and an epoxide together, nothing happens – it is only when you add the catalyst that the polycarbonate gets made. The catalyst I use to make polycarbonate has a trick up its sleeve – It can also make another green polymer, polycaprolactone under certain conditions. Polycaprolactone is considered green because it has the potential to be made from biomass (material that can be grown in some manner) and the plastic is biodegradable (it breaks down when in landfill). The key thing about this is that the catalyst only makes polycaprolactone under certain conditions, and these conditions are orthogonal (opposite) to the conditions needed to make polycarbonate.

This means if you mix the catalyst, the epoxide needed to make polycarbonate and the material, caprolactone, needed to make polycaprolactone, you can control which plastic is being made by controlling whether you add carbon dioxide gas or nitrogen gas (nitrogen gas just fills the space when you remove the carbon dioxide but doesn’t react).

When you add carbon dioxide, the polycarbonate is made. The polycaprolactone is never formed when carbon dioxide is present. If you take away the carbon dioxide, the polycarbonate can’t be made (because it is made from carbon dioxide). This gives the polycaprolactone a chance to be made. By switching the gas supply from carbon dioxoide to nitrogen, you are switching between making the two types of plastic – this is switchable catalysis.

During my PhD I used this method of switching to make several different types of block copolymer. I made plastics which contained three blocks, which had an ABA structure (the blocks on either end are the same but the middle block is different).The central B block was always polycarbonate  but by changing the A block  I could change the properties of the block copolymer. I used polycaprolactone, polyvalerolactone and polydecalactone as the A block and this meant the overall plastic changed from a slightly flexible tough plastic to a weak and inflexible plastic to a very flexible tough plastic respectively.

I then made a plastic containing 7 blocks of polycarbonate (A) and polycaprolactone (B), which had an BABABAB structure . I also discovered that you can add other starting materials to the mixture and switch between this new reaction (A) and the formation of polycarbonate(B) and polycaprolactone (C) to make a plastic containing 5 different blocks of three types of plastic (CBABC structure).

I hope you followed me all the way through and if anyone is interested in reading about my work in full detail, its been published here: http://pubs.acs.org/doi/abs/10.1021/acs.macromol.5b01293

 

 

 

 

 

 

 

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