A new low-cost approach : in the form of liquid silicon used as a modifier, restores the properties of recycled PET bottle flakes to produce new food-grade preforms and pellets. PTP's plant started up two years ago and produces pellets and bottle preforms from recycled PET. Equipment for the process was developed by BKG Bruckmann & Kreyenborg Granuliertechnik GmbH in Germany which built PTP's 22 mln lb/yr demonstration plant in Prague.
The continuous process starts with washing PET flakes. Flake is then treated with the liquid silicone modifier and fed into a stirred mixing reactor, where it's heated to 140-150 degree C for about 90 minutes. In the reactor, the surface of the flakes softens, the modifier adheres, and moisture is removed. Flakes then pass through metal detection and are fed into a twin-screw extruder with high-vacuum degassing. The heat in the mixing reactor, followed by higher heat (6 min at 260 degree C) in the twin-screw extruder with vacuum degassing removes any remaining volatiles along with unreacted silicone.
The melt is filtered through 58 micron screens and goes into BKG's CrystallCut crystallizing pelletizer which uses residual heat in the material to crystallize the pellets. The finished product contains 0.15% silicone. The tetra-functional silicone molecules create silane bridges to the short PET chains and oligomers, creating a new matrix of larger, branched PET molecules with higher molecular weight. PTP says the silicone modifier selectively connects shorter polymer chains, removing low-molecular-weight oligomers, but doesn't attach to longer PET chains. The I.V. actually drops but becomes an irrelevant measure of material performance capability. Normally I.V. of around 0.70 would be produced. This modified material has better mechanical properties than traditional bottle-grade PET material with 0.82 I.V.
The new material produced is called as modified PET. It has better elasticity and better oxygen barrier. Elongation at break is 250% vs. about 120% for standard RPET. PET-M bottles passed a six-week shelf-life test for CO2 migration based on a standard industry test protocol for carbonated soft-drink bottles. Acetaldehyde content is less than 0.5 ppm, suitable for water bottles. PET-M has received food-contact approval from the FDA. Clarity is slightly less than that of virgin PET. The modified PET can be used in green or blue preforms, and up to 20% of it can be used in clear preforms without hurting clarity. PTP plans to license the new technology to others. PTP will supply the silicone modifier to licensees. The process uses 25 kg of liquid silicone (hexamethyldisilazane and tetraethylorthosilicate) per metric ton of clean flake, or 2.5%.