Paper Coating PLA for
Paper Coating PLA for Paper Coating Improved PLA-Based Compounds Provide a Competitive and Carbon- Neutral Alternative to Polyethylene for Coated Paper Applications Extrusion coating is an established processing technology resulting in production of multilayer flexible structures. The operation involves melting of a thermoplastic to be applied onto a substrate such as paper (as shown in Figure 1). Polyethylene (PE) has been traditionally used for imparting properties such as water & grease resistance to the paper and in some cases barrier properties. This article discusses the problems and issues arising from a polyethylene coated paper when biodegraded under composting conditions and the benefits of selecting a fully compostable coating. Substrate Nip Roll Die Chill Roll (Not drawn to scale) Air grap Figure 1: Schematic of an extrusion coating process (adapted from reference by B. A. Morris [1]) Line Speed Compostability Commonly used plastic-coated paper products include milk and juice cartons, hot and cold paper drinking cups, frozen food containers, plasticlined paper bags, take-out containers and some paper plates. The issue with plastic coatings such as PE coated products is its end of life i.e. what happens to the product after use when it enters the waste/disposal environment Although most of the manufacturers of these plastic-coated paper products make no claims that their products are compostable, many composters and waste diversion programs accept them either inadvertently assuming there is no polymer in a paper product or in hopes of composting the paper fibers along with the coating. As summarized by Narayan - Chairman of the ASTM Committee for Environmentally Degradable Plastics and Biobased Products (D20.96) [2], “to meet the requirements of compostability, a material must satisfy the primary requirements of complete biodegradability under composting conditions. In addition, it has to meet the disintegration and safety criteria to make a claim of compostability. ASTM D6400, D6868, ISO 17055, and EN 13432 are specification standards for compostable plastics and require: • 90%+ of the test material’s carbon conversion to CO 2 via microbial assimilation of the test polymer material in powder, film, or granule form in 180 days or less—a laboratory scale test method, as described in the previous section. • Disintegration of the test material in both shape and thickness of the products intended for use, such that 90% of the test material must pass through a 2 mm sieve using ISO 16929 or ISO 20200 standard test methods. • Safety considerations where the resultant compost should have no impact on plants, using OECD Guide 208, Terrestrial Plants Growth Test. Furthermore, regulated (heavy) metals content in the polymer material should be 50% or lower than prescribed thresholds in the country of use (e.g., 50% of the U.S, and Canadian Environmental Protection Agency’s prescribed threshold).” For a 230 gsm (grams per square meter) cupstock paper having a PE coating of 15 gsm on one side, the polyethylene content in the product is only 6.5%. However, the specification standard requires that any 34 bioplastics MAGAZINE [05/11] Vol. 6
From Science and Research By Shilpa Manjure Natur-Tec A Division of Northern Technologies International Corp. (NTIC) Circle Pines, MN, USA component added in excess of 1 % to other biodegradable material has to be itself completely biodegradable. PE is not biodegradable and is not completely assimilated by microorganisms in the compost system [2]. In fact, PE simply fragments as the paper is consumed by the microorganisms in the pile. Narayan further [2] reports that the fragments, some of which could be microscopic, can be transported through the ecosystem and could have serious environmental and health consequences. Figure 2: Examples of modified-PLA coated-paper applications that are certified by the BPI and are fully compostable A recent research from Eco-Cycle and Woods End Laboratories [3] demonstrated and verified that micro-plastics were shed from all plastic-coated paper products during composting. These micro-plastics may pose a significant risk to our soils, freshwater and marine environments, wildlife, and ultimately, human populations. The U.S.-based Algalita Marine Research Foundation reported [4] that degraded plastic residues can attract and hold hydrophobic elements such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) up to one million times the background levels. As such, it is increasingly important that with such a potential threat to our environment we ban non-biodegradable plastic-coated paper products in compost and practice use of fully compostable coated-paper products. Polylactide (PLA) is the most studied and easily available biobased and biodegradable polymer. As such PLA would be a good candidate to be coated on to paper for complete compostability at the end of life. PLA, however, has some property drawbacks as discussed later and is not the most suitable candidate for extrusion coating. This is mostly true when coating convertors like to utilize their current equipment that has been optimized considering properties and features of polyethylene for the past several decades. PLA properties can be tailored to be coatable and still meet the ASTM D6868, “Standard Specification for Labeling of End Items that Incorporate Plastics and Polymers as Coatings or Additives with Paper and Other Substrates Designed to be Aerobically Composted in Municipal or Industrial Facilities”. This implies that the coated paper is consumed as food by the microorganisms in the compost pile and there are no fragments of polymer left behind. Both the paper and coating are converted completely to carbon dioxide, water and hummus when disposed off in an industrial composting facility [5-7]. Natur-Tec in collaboration with ITC India’s Paperboard and Specialty Paper Division [8] has been able to successfully commercialize such a modified-PLA coated paper that is also certified by the Biodegradable Products Institute (BPI) and convert it to commercial end products as shown in Figure 2. bioplastics MAGAZINE [05/11] Vol. 6 35
