Tuesday, August 11, 2015


I've mentioned several times that plastic can be produced from agricultural waste and/or CO2.

 The plastic of interest is polyethylene, or rather ultra high molecular weight polyethylene (UHMWPE, UHDPE, polyethene or trade names Spectra or Dyneema). It is formed of very long single chains of carbon, so the unit formula is CH2. This material is a thermoplastic, melting around 130 °C (or less if it contains solvents and/or crystalline defects). In normal use it should be kept between -150 °C and about 80 °C, so in-space applications will typically require a protective coating such as thin-film aluminum.

 So, we will assume we have available a quantity of ethanol (C2OH6) from other processes. Fermentation of sugar and/or cellulose is one possible source, as is syngas fermentation. It does not have to be perfectly pure; ethanol-water eutectic produced by distillation is acceptable. See the link for chemical structure and other data. This is passed through a fluidized bed reactor with alumina or zeolite dehydration catalyst that removes one molecule of water from each molecule of ethanol, yielding ethylene (C2H4) at high purity. The catalyst has to be regenerated periodically to recover the water and remove carbon and trace contaminants. The ethylene can be stored in high-pressure tanks.

 An alternative is to use the reverse water gas shift reaction to convert H2 and CO2 to CO; with additional H2 added this is syngas. The H2 would come from electrolysis of water. If methane is available it can be used with some O2 to produce syngas directly. Apply the methanol process, then apply something like the Mobil methanol to gasoline process to yield ethylene. Proper choice of zeolite will yield pure gaseous ethylene.

 The ethylene can be polymerized using titanium tetrachloride as a catalyst. This process has been used commercially for 60 years and currently yields high-quality resin with masses of 5.5 to 6 x10^6 grams per mol and contaminants of titanium (<40 ppm), aluminum (<20 ppm) and chlorine (<30 ppm), a total of 110 grams of catalyst lost per ton of plastic {UHMWPE biomaterials handbook, Steven M. Kurtz; chapter 2, tables 2.1 and 2.2}. Of these, chlorine is the hardest to replace unless there are convenient chloride salt deposits available; even so, 10kg of chlorine would be sufficient for 333 tons of plastic. The catalyst also requires magnesium chloride and a structural scaffold (usually microporous silica beads, sometimes zeolite or activated carbon), both of which can be completely recovered. The catalyst must be activated with triethylaluminum (TEA); this material is very dangerous and also useful as a rocket igniter. It can be produced using metallic aluminum, hydrogen gas and ethylene gas; if no TEA is available to jump-start the reaction then a small amount must be made using another process involving lithium hydride or ethyl chloride.
 A related catalyst, metallocene titanium, zirconium or hafnium chloride is used in solution with methylaluminoxane (MAO). Recent work has developed related catalyst systems using MAO; it is also very dangerous and is related to TEA. Synthesizing metallocene looks straightforward, but the input materials are fairly complex.
 These reactions can be terminated by hydrogen, so care must be taken to avoid any excess hydrogen gas in the ethylene or catalyst bed (particularly avoid water gas shift). The reactions normally occur in a solvent which does not react with the catalysts; this allows the catalyst active sites to remain exposed as the polymer molecules are carried away. Details about separation of metallocene from solvent appear to be kept secret, but supported catalyst in solvent should be straightforward as a continuous process.

 Solvents are a tricky question. The ideal solvent will be an organic oil which can be extracted from biomass, can dissolve PE, boils above 150 °C, does not thermally polymerize below that temperature and is nontoxic. Orange oil (mixed terpenes) fit that description, but the yield of terpene per m² of growing area is so low that it is disqualified for use in space. Proven solvents are xylene and toluene. This is an input that requires additional research; plant oils, silicone oils and possibly alcohols should be examined. Prototype production of PE could use Earth-sourced solvents while demonstrating the process is feasible, as could projects with a specific mass of plastic as a goal (early tethers for example).

 The product of this polymerization process is purified to contain only long-chain polymer and solvent. If necessary this can be done by centrifuge but the proportion of shorter chains and branches should be very low with these catalysts. Solvent is removed until the concentration of polymer is around 20% by mass. This results in a gel which is loaded into a ram extruder or similar heat and pressure device with spinneret holes. As the gel is passed through these holes the molecules are drawn into alignment; the resulting gel fibers are cooled and passed through water or ethanol to gradually remove the rest of the solvent as the fiber is drawn further. This gel spinning causes a very high degree of alignment and crystallization and can even reduce lattice defects; this is the secret of the incredibly high strength of Spectra fibers. The fiber is eventually wound onto a bobbin and then heat treated to drive off any remaining solvent and to further improve strength. Individual fibers are woven into yarn which is then used as needed. Again, fibers that will be exposed to space or to monatomic oxygen (low-Earth orbit) should be coated with a protective layer such as vapor deposited aluminum or a UV and oxygen resistant polymer film.

 For bulk use the freshly made polymer will have all solvent removed, yielding a powder that can be pressed into pellets for later use or pressed directly into shape. The powder can be melted under inert gas or redissolved in solvent and cast or extruded into the necessary shapes; pellets can sometimes be difficult to redissolve without shredding or patience.

 The bulk plastic is suitable for cutting boards, artificial joints, low-friction or low-wear contact surfaces, storage containers for water or moderate acids / bases, lightweight mechanical parts like rollers, cams, gears, etc., etc. Extruded rods can be used in 3d prototyping machines for additive manufacturing.

 The fiber is suitable for composite overwrap pressure vessels, tensile reinforcing members (including in regolith blocks), habitat hulls, meteoroid hull patches, cut-resistant cloth, ballistic armor, sutures and space tether strands.

Actually doing any of this would require the services of a good process chemist / chemical engineer at the least (plus possibly other engineers) to set up the various reactions, required equipment and input streams. Many of these processes if operated in batch mode will have unreacted or partially reacted inputs, and all of the processes will have some byproducts; these materials will need to be destroyed, most probably in a wet gasifier (supercritical water + O2 reactor) to recover the C, H and O. Any trapped catalyst material will find itself in the ash/salts, so any metals in the waste stream will end up as oxides or chlorides. Because the inputs are generated slowly, the overall process does not need to be particularly time-efficient as long as it is reliable, low-maintenance and low-mass-loss. For edge cases where a large supply of water and methane or CO2 are available a different solution might be used that takes less time but more manual effort.

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