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Plastics are materials comprising of a wide variety of synthetic and semi-synthetic organic polymers, which are malleable and easily molded to different shapes. Due to their low cost, ease of manufacturing and hydrophobicity, plastics are widely used to make a wide range of different products. Plastics have replaced many traditional materials such as wood, leather, metal and glass because of their versatility.
Global production of plastics is increasing each year, and it is estimated that approximately 299 million tons of plastics were produced in 2013 alone1. Of these, about 78 million tons of plastics were used only for packing materials such as water bottles, food containers, etc. Of this 78 million tons, 14 % was used in incineration and energy recovery, about 33.3% was leaked into the environment, 40 % ended up in landfills and unfortunately, only 2% of it was recycled or reused2.
Approximately 10-20 million tons of plastic end up in ocean every year, and an estimated 5.25 trillion plastic particles weighing 268,940 tons are subsequently floating on oceans around the world, which is leading to the ingestion by and entanglement of marine animals3.
Although most plastics are non-biodegradable, studies have shown that certain plastics decompose rapidly in the ocean, releasing potentially toxic chemicals such as bisphenol A4. These chemicals could cause toxicity in ocean fauna, and even travel up the food chain and cause toxicity to us human beings4.
Due to the availability of different types of plastics present in the waste, sorting through them presents the biggest challenge in recycling. For example, polyethylene (PE) and isotactic polypropylene (iPP), both of which account for two thirds of world’s plastics, cannot be repurposed together due to their differences in structural and physicochemical properties2. Even though these materials have been available for 60 years, an efficient technology to blend PE and iPP has been lacking until now.
Most of the PE and iPP are synthesized using heterogeneous chromium and titanium catalysts. As a result of the availability of a multitude of active sites on these catalysts, the synthesized polymers are of different molecular weight (MW), MW distributions and microstructures, resulting in poor interfacial adhesion and erosion of mechanical properties in these blends.
Only 5% of the value of the PE and iPP, which accounts for more than $200 billion in annual sales worldwide, is retained in the recycled blends as a result of these sorting expenses and degraded physical properties. Therefore, combining these materials to create an upcycled product would have great sustainability and economic importance.
Researchers at the University of Cornell’s Department of Chemistry and Chemical Biology have recently developed a polymer that can combine PE and iPP. Coate’s team reported the use of the catalyst 1/B(C6F5)3 to produce high MW PE/iPP di-block polymers and PE/iPP tetra-block polymers with precise control of the block length.
They used a simple peel test, a facile method for comparing the interfacial strength between the molded films, to assess the adhesion between heterogeneous grade PE and iPP in the presence and absence of block polymers.
The peel test involves the compression molding of two rectangular plaques of PE/iPP, and three layers with block co-polymer film in a melt and the monitoring the peel strength while pulling apart. The results from the peel test showed that the laminates, without block copolymers, peel apart easily, while the incorporation of the block copolymers increased the peel strength.
The polymer addition utilized in this study allows PE and iPP to have the potential to not only be recycled but also be upscaled. This is a great step forward in the recycling of the most common plastics used in the world, as the future elimination of the municipal waste produced by these products could save an unprecedented amount of money, and even lives.
Sources and Further Reading
- Osgood, Melissa. "New Polymer Additive Could Revolutionize Plastics Recycling." Cornell University. 23 February 2017.
- Eriksen, Marcus, Laurent C. M. Lebreton, Henry S. Carson, Martin Thiel, Charles J. Moore, Jose C. Borerro, Francois Galgani, Peter G. Ryan, and Julia Reisser. "Plastic Pollution in the World's Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea." PLoS ONE 9.12 (2014). Web.
- James M. Eagan et al. Combining polyethylene and polypropylene: Enhanced performance with PE/PP multiblock polymers, Science (2017).