A new method of manufacturing virgin PET/PBT from recycled PET bottle and textile products offers energy saving in addition to addressing environmental issues. The production of PBT from recycled PET is one example of green method that has emerged recently. Post-consumer PET is a very attractive feedstock, with USA alone generating about 2000 tpa of PET bottle waste. Only 20% of PET waste is recycled, while the rest is dumped into landfills across the country.
The construction of the PET bottle makes it amenable for recycling. It is a relatively clean polyester feedstock once the glue, labels and the caps have been removed. PET bottles are typically formed into bales at recycling-collection centers. To make clean PET flakes, the bales go through cleaning and other processes, including manual separation to remove contaminants, grinding, washing, and drying. Additional extrusion and pelletization may also be involved.
Recycled PET finds applications in carpet fibers, geo-textiles and fiber fill. The feedstock can be converted into PBT resin, which is widely used in injection or blowmolding applications. A better way to reuse PET might be to convert it into more-valuable virgin PBT. To make PBT, cleaned PET flakes are fed into a high-temperature reactor along with a diol such as ethylene glycol or butanediol (BDO) and a catalyst.
Chemical depolymerization breaks the large molecular chain of PET into small repeating sections called oligomers. The process strips the chain�s backbone of its functional groups, spur molecules that give each plastic its unique properties. The next step involves swapping of the PET functional groups for PBT functional groups through transesterification, another catalyst-assisted process. Finally, the altered building block oligomers are rejoined or polymerized into a modified PBT copolymer that is chemically identical to one from monomer first time. The process releases ethylene glycol (EG) that�s collected for other uses. The EG represents 28% of the PET�s original mass. The polymerized PBT contains 60% recycled mass and reduces solid waste by up to 900 Kg for each ton of PBT produced.
But the big benefit in making PBT from recycled PET is energy savings. Producing PBT from oil takes approximately 50 GJ/ton; the PET-to-PBT process uses about 20 GJ/ton of PBT. The 30 GJ/ton energy savings is the equivalent of keeping 2 tons of carbon dioxide out of the atmosphere.
A big problem of recycling polymers is that the mechanical properties suffer as every heat cycle lowers molecular weight as chains break apart. Consequently, many postconsumer feedstocks are used into less-demanding applications. The PET to PBT process described above, however, decomposes the PET to its oligomers, erasing the molecules� heat histories. The monomers are then used to build a PBT molecule that resembles a virgin PBT molecule.
Tests have shown that resins developed by Sabic Innovative Plastics using PBT derived from postconsumer PET have equivalent performance to virgin PBT resins. The company�s Valox iQ PBT resins and Xenoy iQ PBT-polycarbonate (PC) alloy have the same mechanical, physical, and chemical properties as their virgin-derived counterparts.
Several formulations showed much higher flow in the post-consumer version than in the virgin PBT version. The Valox iQ 8280SF system, a 40% glass-filled PBT-PET blend formulated for high flow, had a MVR over four times higher than the virgin version. The 30% glass-filled, flame-retardant Valox iQ 4860HR had more than three times the MVR that its virgin counterpart did. The Valox iQ and Xenoy iQ resins are cost competitive with standard PBT and PC-and-PBT grades.