|HDPE absorbs more volatiles as compared to PET and is processed at lower temperatures than PET. Hence it is more difficult to remove volatiles from HDPE as compared to PET to meet US FDA and EU standards for reuse in food bottles. Recycling HDPE homopolymer from milk and water bottles back into food-grade bottles is a new achievement. So far, two technologies have met the FDA protocol out of which one is proprietary, the other is commercially available.
The proprietary process relies on a fluidized bed aerated by hot gas. Volatiles are removed by blowing hot air through HDPE flake at 1-4 ft/sec at temperatures of up to 120 C for 1-10 hrs. The prototype decontamination vessel is used to produce sample quantities of EcoPrime FDA-grade post-consumer HDPE. The process can remove volatiles to a level substantially below the FDA’s protocol for letter of no objection (<320 ppb of volatiles). A production-scale unit has been designed, but the location and start up is not disclosed.
The first commercially available process to recycle food-grade HDPE bottles was developed in the UK over the last 4 years with funding from WRAP (Waste & Resources Action Programme), a government initiative to reduce waste. The process was developed by a consortium of Nampak Plastics, a South African blow molder of milk bottles, and several large dairies and supermarket chains. The Fraunhofer Institute in Germany tested and qualified the process. The critical decontamination unit in the WRAP process is a modified Vacurema system – It is akin to the Vacurema machine used in PET recycling to maintain high IV, but is modified for devolatilizing HDPE at a lower temperature but longer time in the vacuum chamber. HDPE flake is heated to 120 degree C under vacuum in a succession of two chambers to get longer dwell time. PET flake passes through one Vacurema decontamination chamber at 230 degree C. Compared with the PET version, the HDPE extruder also has a screw with a longer L/D, vacuum vent and die-face pelletizer, whereas the PET version is unvented and uses a strand pelletizer. Closed Loop Recycling has a 13 mln lb pa Vacurema system for HDPE, partly funded by WRAP, which started up last summer. The firm has been pelletizing PCR HDPE for three months, but the initial output was “technical quality,” not food grade, while it tuned the process to meet volatiles specifications. Greenstar started up last spring with the first Vacurema unit built for PCR HDPE, rated at 13 mln lb pa. Greenstar passes finished pellets past an “electronic nose” from Alpha MOS in France to “sniff” for any remaining volatiles. If volatiles are found, the material becomes “technical polymer,” not food grade.
(Source : PlasticsTechnology)