January 30, 2012
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Recycle PET bottles into a more valuable polymer. Poly(ethylene terephthalate) (PET) is the only plastic recycled in large volumes. Even so, recycled material is a small fraction of the PET market. The economics of recycling are marginal; in many cases, depending on market conditions, virgin PET (made from ethylene glycol and terephthalic acid) is less costly to make than PET from recycled bottles.
One solution is to use discarded PET bottles to make higher value products than PET. P. Agarwal and co-inventors disclose a method for converting recycled PET bottles into modified poly(butylene terephthalate) (PBT) random copolymers. PBT is a much more expensive resin than PET and is often used in conjunction with fillers as a thermoplastic molding compound in automotive, electrical, and electronic applications.
In the first stage of the process, purified flake from PET bottles is depolymerized. The flake is combined with ethylene glycol (EG) in a ratio of 1:0.8 to 1:2.0. The reaction is conducted in the presence of a typical transesterification catalyst composed of titanium, antimony, or tin compounds. The depolymerization is carried out under 1–6 bar pressure and in a temperature range of 200–260 °C. Depolymerization time is 20–120 min. The resulting molten mass is filtered to remove any black specks.
The second stage is transesterification. The mass from the first stage is treated with 1,4-butanediol (BDO) in a molar excess of 2–4 times the stoichiometric requirement. The solution is heated to 210–235 °C at atmospheric pressure for 10–40 min. Any vapors formed are passed through a distillation column; EG and THF (formed by BDO dehydration) are removed, and the BDO is condensed and returned to the reactor.
In the third stage, the product from stage 2 is heated at 190–235 °C under reduced pressure (60–95 kPa). Again, any evolved EG and THF vapor is removed, and most of the BDO is returned to the reactor.
The product from stage 3 is polycondensed in stage 4. It is heated to 230–265 °C while the pressure is gradually reduced to 0.01–1 kPa over 45–120 min to allow the copolymer’s molecular weight to increase. EG, THF, and BDO vapors are removed. The final PBT product has an intrinsic viscosity of 0.5–1.5 dL/g and an EG–DEG (diethylene glycol) content of <0.4 wt%. The melting point of the polymer is in the range 215–222 °C. (Sabic Innovative Plastics IP B.V. [The Netherlands]. US Patent 8,088,834, Jan. 3, 2012; Jeffrey S. Plotkin)