PETpla.net Insider 06 / 2016

MATERIAL / RECYCLING 18 PET planet insider Vol. 17 No. 06/16 www.petpla.net UV light exposure negatively impacts next generation PET bottles Suitable storage solutions A research study conducted by Plastic Technologies, Inc. (PTI) has found that exposing post-consumer bottles to ultraviolet (UV) light prior to recycling has a severe impact on the physical properties and visual attributes of next gen- eration, recycled PET (rPET) bottles. However, the degree of yellowness can be decreased by using suitable storage conditions. Intrinsic viscosity, yellowing, haze are just some of the attributes that provide challenges when using rPET along with virgin material. rPET pro- ducers have engineered elaborate processes to minimise those char- acteristics in order to create material that provides the desired environmen- tal attributes along with key visual and processing properties. One of the problems that limits the amount of recycled PET (rPET) used in rigid packaging applications is the degree of yellowness that it can cause. The more rPET that is added to virgin PET during bottle manufac- ture, the more yellow the resulting bottle tends to become. The source of the rPET used has a great influence on yellowing. Deposit-grade materials are in the greatest demand as they result in less yellowing than curbside- grade material. Even virgin PET will discolour and yellow with each additional melting cycle. But there are many other causes that contrib- ute to this problem. For example, it is known that low levels of nylon coming from multilayer bottles can remain trapped in the PET flake after washing and cleaning. This residual nylon will cause considerable yellowing when the rPET is extruded. But one cause of yellowing that has not been widely addressed is the effect that UV radiation from the sun, as well as that emitted by artificial sources, such as fluorescent lighting, can have on PET. PET is sensitive to UV light especially at elevated tem- peratures, under high humidity, and in the presence of oxygen - all of which are present when PET bottles are exposed to the weather. Free radicals form within the plastic that cause sub- sequent degradation. The degradation effects seen are also compounded by the presence of oxygen in the air. The free radicals created by the UV radiation react with this oxygen to form hydroperoxides that can result in polymer chain breakage. Thus the end result in many plastics, including PET exposed to an outdoor sunny envi- ronment, is that they will discolour, embrittle and crack over time. While antioxidant and UV absorb- ers can be added to the plastic to help stabilise and prolong the mate- rial’s useful life, PET bottles normally do not contain significant amounts of these additives. The Association of Postconsumer Plastic Recyclers (APR) notes in their model bale specification for PET bot- tles, that bales should not be stored outdoors uncovered for a period exceeding two weeks to prevent UV degradation. There have been many papers published that describe the degradation effects that UV light has on PET. But these papers have primarily focused on the degradation seen on the actual PET article being studied and not what happens to the properties of PET when it is recycled. 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. But the most prob- able time that PET bottles might see significant exposure to the elements occurs when bales of PET bottles are stored outside while waiting to be brought into a reclamation facility to be ground into flakes, washed and re-pelletised for sale back into the industry. The study This study was undertaken to understand the effects weathering would have on PET bottles exposed to Toledo, Ohio weather for one year. 2l PET bottles produced using a com- mercial grade of PET were used for this study, not densely packed into large bales. The resin did not contain any ultraviolet absorbing additives. These virgin bottles were crushed, stacked five to six deep, and placed in uncovered open sided crates to afford maximum exposure to the ele- ments. Sunlight could easily penetrate through the several layers of bottle sidewalls, even reaching the bottles located at the bottom of the open crate. These crates of bottles were then placed on the roof of Plastic Technolo- gies, Inc. building in early January. Every three months, the bottles in the crates were agitated so that those on the bottom had a chance over time to move to the top or outside edges. Another set of bottles was stored indoors approximately 18-inches under a fluorescent light source. This light source was left on continuously, exposing the bottles for two months. A third set of bottles was stored and protected from light exposure for one year for use as a control. Periodically bottles were taken off the roof to study the effects of sunlight and weathering over the course of 12 months. Each aged set of bottles was ground into flake. The flake was then aggressively washed to remove accumulated sur- face dirt at 88°C for 15 minutes. Fol- lowing the hot caustic wash, the flake was rinsed twice with fresh water, dried to less than 50ppm moisture and injec- tion moulded into 3mm thick plaques. PET bottle bale RECYCLING S P E C I A L