Basics Plastics made
Basics Plastics made from CO 2 First plastics from CO 2 coming onto the market - and they can be biodegradable Carbon dioxide is one of the most discussed molecules in the popular press, due to its role as greenhouse gas (GHG) and the increase in temperature on our planet, a phenomenon known as global warming. Carbon dioxide is generally regarded as an inert molecule, as it is the final product of any combustion process, either chemical or biological in cellular metabolism (an average human body emits daily about 0.9 kg of CO 2 ). The abundance of CO 2 prompted scientists to think of it as a useful raw material for the synthesis of chemicals and plastics rather than as a mere emission waste. Traditionally CO 2 has been used in numerous applications, such as in the preparation of carbonated soft drinks, as an acidity regulator in the food industry, in the industrial preparation of synthetic urea, in fire extinguishers and many others. Today, as CO 2 originating from energy production, transport and industrial production continues to accumulate in the atmosphere, scientists and technologists are looking more closely at different alternatives to reduce flue-gas emissions and are exploring the possibility of using CO 2 as a direct feedstock for chemicals production, and first successful examples have already been achieved. The carbon cycle on our planet is able to recycle the CO 2 from the atmosphere back in the biosphere and it has maintained an almost constant level of CO 2 concentration over the last hundred thousand years. The carbon cycle fixes approx. 200 gigatonnes of CO 2 yearly while the anthropogenic CO 2 accounts for about 7 gigatonnes per year (3-4% of the CO 2 fixed in the carbon cycle). Even if this quantity looks small, we must bear in mind that this excess of CO 2 has been accumulating year after year in the atmosphere, and in fact we know that CO 2 concentration rose to almost 400 ppm from 280 ppm in the preindustrial era. In recent years different processes have been patented and are currently used to recover CO 2 from the flue-gases of coal, oil or natural gas, or from biomass power plants. The recovered CO 2 can be either stored in natural caves, used for Enhanced Oil Recovery (EOR), or can be used as feedstock for the chemical industry. The availability of a high quantity of CO 2 triggered different research projects worldwide that are aimed at finding a high added value use for what otherwise is a pollutant. Plastics from CO 2 When it comes to the question of CO 2 and plastics there are many different strategies aiming at either obtaining plastics from molecules derived directly from CO 2 or using CO 2 in combination with monomers that could either be traditional fossil-based or bio-based chemicals. Moreover, the final plastics can be biodegradable or not, depending to their structures. Noteworthy among already existing CO 2 derived plastics are polypropylene carbonate, polyethylene carbonate, polyurethanes (see also p. 38) and many promising others that are still in the laboratories. Polypropylene carbonate Polypropylene carbonate (PPC) is the first remarkable example of a plastic that uses CO 2 in its preparation. PPC is obtained through alternated polymerization of CO 2 with PO (propylene oxide, C 3 H 6 O) (Fig. 1). The production of PPC worldwide is rising and this trend is not expected to change. Polypropylene carbonate (PPC) was first developed 40 years ago by Inoue, but is only now coming into its own. PPC is 43% CO 2 by mass, is biodegradable, shows high temperature stability, high elasticity and transparency, and a memory effect. These characteristics open up a wide range of applications for PPC, including countless uses as packaging film and foams, dispersions and softeners for brittle plastics. The North American companies Novomer and Empower Materials, the Norwegian firm Norner and SK Innovation from South Korea are some of those working to develop and produce PPC. Today PPC is a high quality plastic able to combine several advantages at the same time. 44 bioplastics MAGAZINE [05/12] Vol. 7
Basics By Fabrizio Sibilla Achim Raschka Michael Carus nova-Institute, Hürth, Germany Thinking further ahead, in a future when propylene oxide will be produced from methanol reformed from CO 2 , PPC will be available derived 100% from recycled CO 2 , therefore making it very attractive for the final consumer. PPC is also a biodegradable polymer that shows good compostability properties. These properties, when combined with the 43% or 100% ‘Recycled CO 2 ’ can contribute to the development of a plastic industry that can aim at being sustainable in its three pillars (social, environmental, economy). Other big advantages of PPC are its thermoplastic behaviour similar to many existing plastics, its possibility to be combined with other polymers, and its use with fillers. Moreover, PPC does not require special tailor-made machines for its forming or extruding, hence this aspect contributes to make PPC a ‘ready to use’ alternative to many existing plastics. PPC is also a good softener for bioplastics: many biobased plastics, e.g. PLA and PHA, are originally too brittle and can therefore only be used in conjunction with additives in many applications. Now a new option is available which can cover an extended range of material characteristics through combinations of PPC with PLA or PHA. This keeps the material biodegradable and translucent, and it can be processed without any trouble using normal machinery (see also p. 48). It must be pointed out that it is not easy to give an unambiguous classification to PPC, but it falls more into a grey area of definitions. As discussed above, it can be prepared either from CO 2 recovered from flue gases and conventional propylene oxide, and in this case although not definable as ‘bio-based’ it may still be attractive for its 43% by wt. of recycled CO 2 and its full biodegradability. It can in theory also be produced using CO 2 recovered from biomass combustion, thus being classified as 43% biomass-based (25% biobased according to the bio-based definition ASTM D6866). As already mentioned above, if propylene oxide could be produced from the oxidation of bio-based propylene, then it can be declared 57% biomass-based or 100% bio-based if CO 2 and propylene oxide are both bio-based. As more and more different plastics and chemicals in the future will be derived from recycled CO 2 they will need a new classification and definition such as ‘recycled CO 2 ’ in order not to bewilder the consumer. Polyethylene carbonate and polyols Polypropylene carbonate is not the only plastic that recently came onto the market. Other remarkable examples are the production of polyethylene carbonate (PEC) and polyurethanes from CO 2 . The company Novomer has a proprietary technology to obtain PEC from ethylene oxide and CO 2 , in a process similar to the production of PPC. PEC is 50% CO 2 by mass and can be used in a number of applications to replace and improve traditional petroleum based plastics currently on the market. PEC plastics exhibit excellent oxygen barrier properties that make it useful as a barrier layer for food packaging applications. PEC has a significantly improved environmental footprint compared to barrier resins ethylenevinyl alcohol (EVOH) and polyvinylidene chloride (PVDC) which are used as barrier layers. H 3 C O CO 2 catalyst CH 3 O O C O n propylene oxide polypropylene carbonate Fig. 1: Route to PPC from CO 2 and propylene oxide bioplastics MAGAZINE [05/12] Vol. 7 45
