PETpla.net Insider 03 / 2015

MATERIAL / RECYCLING 20 PET planet insider Vol. 16 No. 03/15 www.petpla.net Shedding more light on rPET Yellowing by Dr. Francis M. Schloss and Ms Julie Brown, Plastic Technologies, Inc. The amount of recycled PET (rPET) used in rigid packaging applications is limited because of the yellowness it causes. The source of the rPET has an influence on yellowing; deposit- grade materials typically suffer less than kerbside-grades. Residual levels of nylon trapped in PET flake after washing and cleaning cause considerable yellowing when the rPET is extruded and additives such as ultraviolet (UV) light blockers, slip agents and acetaldehyde and oxygen scavengers can all contribute to yellowing as the rPET is subjected to addi- tional melt processes. A study by Phoenix Technologies LLC into the effects of label adhe- sives found that they also could be a significant contributor, if not effectively removed from the flake during the cleaning process 1 . The effect of UV radiation, how- ever, has not been widely addressed. PET is sensitive to UV light, espe- cially at elevated temperatures, under high humidity, and in the presence of oxygen-all of which are present when PET bottles are exposed to the weather. UV radiation is found in the electromagnetic spectrum at wave- lengths from below 100 up to 400 nanometres (nm); the UVA segment of the spectrum represents the greatest amount of UV radiation that pen- etrates the atmosphere and reaches the Earth’s surface. This long-wave- length UV radiation is strong enough to cause chemical reactions and is responsible for the damage plastics experience when exposed to the ele- ments. Free radicals created by UV within the plastic cause degradation that is compounded by the presence of oxygen in the air leading to polymer chain breakage. The result is discol- ouration, embrittlement and cracking. The addition of antioxidant and UV absorbers can help to stabilise and prolong the material’s useful life but PET bottles do not normally contain these additives. Published papers that describe the degradation effects of UV light on PET 3-5 have focused on the degrada- tion in the article being studied, not what happens to PET’s properties when recycled. UV radiation above 320 nm will pass through unprotected PET. Prod- ucts that need protection will either have UV-blocking full-sleeve labels or will have UV absorbers added during manufacture. Only a small number of PET bottles in the market contain UV absorbers. While PET bottles and their con- tents do not experience significant exposure to sunlight, bottles under storage after manufacture as well as product-filled bottles sitting on a store shelf can be exposed to artificial light sources that can also emit low levels of ultraviolet light. Exposure of PET bottles to sunlight is most likely when bales of PET bottles are stored out- side, awaiting recycling. This study, which used two-litre commercial-grade PET bottles, was undertaken to understand the effects weathering would have on PET bot- tles exposed to Toledo, Ohio weather for one year. This resin used to make the bottles did not contain any UV- absorbing additives. Virgin bottles were crushed, stacked five to six deep and placed in uncovered open-sided crates, to afford maximum exposure to the elements. The crates were placed on the roof of the Plastic Technologies, Inc. building in early January. Every three months, the bottles were agitated so that those on the bottom were able to move to the top or outside edges. Another set of bottles was stored indoors approximately 18-inches under a fluorescent light source for two months. A control set was stored and protected from light exposure. Throughout the year, bottles were periodically taken off the roof to study the effects of sunlight and weathering. Each set of bottles was ground through a 3/8” screen into flake, which was then aggressively washed at 88°C for 15 minutes in a solution of 0.3% Triton X-100 and 1.0% caustic (NaOH). The flake was