How does evolving compatibiliser technology enable the recycling of mixed plastic waste?
Additive suppliers and academics have a large number of off-the-shelf or under-development solutions for blending incompatible polymers together to form higher performing, more stable materials. Many are developed with the aim of upgrading recycled plastics, often but not exclusively post-consumer recycled material (PCR). Many, but not all, are using maleic anhydride grafting techniques to bridge the interface between polar and non-polar polymers, or between non-polar but still incompatible polymers (e.g. PE and PP).
US-based Intermix Performance Materials is a start-up company established last year to commercialize the development of an ethylene-propylene multi-block compatibilizer additive technology licensed from Cornell University. It was founded by Geoff Coates, Tisch School Professor of Chemistry and Chemical Biology at Cornell, Andrew Arriz, Business Analyst, and Ting-Wei Lin, Principal Investigator.
“HDPE and isotactic polypropylene (iPP) are the most produced commercial plastics in the world and they have similar optical properties, which makes the process of sorting the two plastics on a large scale challenging,” says Lin. Despite their similar chemical structures, HDPE and iPP are incompatible with each other and therefore produce brittle and worthless material when melt-blended. This hinders the recycling efficiency of these plastics. It has been reported that less than 7% and 1% by weight of HDPE and iPP respectively are recycled.
Through a partnership with an established plastics recycler, Intermix obtained a batch of pellets made from old rope and fishnets, mainly recycled from the sea. These marine plastics have an HDPE to iPP weight ratio of approximately 1:1. The company says the blends are brittle and fail to stretch without much elongation. By adding 2% by weight of the company’s compatibilizer, the toughness can be increased to over 800% elongation at break.
Lin says compatible HDPE/iPP blends “are now becoming promising materials that can be reprocessed into products regardless of the composition and non-separability of the recycled HDPE/iPP blend. This can significantly reduce the cost of plastics recycling and increase recycling efficiency.”
Intermix currently has five employees focused on product research and business development and is working with a number of foundry manufacturers to scale up production of compatibilizers from the gram to the kilogram scale. Lin says, “Products made from recycled HDPE/iPP blends may soon be possible.”
Nexam Chemical of Sweden has developed a concept known as “reactive recycling” to improve the performance of mixed recycled polymers that are difficult to recycle. One example is recycled PP containing HDPE. “Nexamite R201 reacts with the polymer to form a PP/PE blend to provide PP and PE compatibility,” says Lars Öhrn, the company’s chief marketing officer. 85% rPP and 15% rHDPE blends, tensile tests showed less variation.”
Another example of application is PE containing PA. When Nexamite R405 was added to a blend of 93% HDPE and 7% PA6, tensile tests showed improved mechanical properties and surface appearance. Yield stress was also improved and the fracture strength was said to be comparable to that of pure HDPE.
Öhrn did not disclose the specific chemical reaction used, but said it was not based on maleic anhydride grafting. He says: “It works on polyolefins, but can also bridge and react with polycondensates. As with all reactive extrusions, you need the right mix and energy input to get the best results.” Nexamite R201 and R405 are both available in masterbatch form.
The latest addition to Nordic Grafting Company’s (NGC) portfolio is Acti-Tech Compatibilizer 16MA11F, based on grafted maleic anhydride (MA) Vistamaxx, a semi-crystalline copolymer from ExxonMobil. The new additive complies with EU 10/2011 food contact regulations and provides recyclers and processors with an easy-to-add universal compatibilizer to enhance the value of blends containing a variety of packaging polymers, including polyolefins, EVOH, PA and PET, according to NGC, which is licensed to ExxonMobil for the technology.
Acti-Tech products can be added at all stages of the recycling process: during in-house or pre-consumer recycling by the moulder, during re-granulation with or without filtration by the recycler, or during re-granulation by the recycler or modifier. The company says: “It represents the ideal solution for upcycling blended polymer feedstocks from post-industrial recycling (PIR) or post-consumer recycling (PCR) as well as internal waste, allowing these recycled plastics to be used in new or existing products without compromising end-use performance.”
According to NGC, Acti-Tech 16MA11F exhibits improved dispersion and reduced dispersed phase size of PA and PET particles in polyolefin substrates (typical PA/PE ratio of 40/60). Quentin Le Piouff, the company’s business development manager, says: “In addition, the addition of 4% Acti-Tech Compatibilizer 16MA11F to recycled barrier films (LDPE 60% / PA 30% / EVOH 10%) resulted in higher film and optical property recovery compared to non-compatibilized blends and lower cost.”
Acti-Tech additives are currently being tested in a variety of pre-consumer, post-industrial and post-consumer wastes, including multi-layer barrier film packaging as well as shrink sleeves, blow-moulded cosmetic and plant protection chemical containers, rigid packaging, reinforced/multi-layer tubes and hoses, fishnets, mats and carpets.
According to Le Piouff, Acti-Tech 16MA11F can also be used as a coupling agent for bio-based (starch, cellulose), wood composite (WPC) and natural fibre (NFC) materials. Several applications using this universal compatibilizer will be commercialized in the coming months,” he says.
Extending the reach
Netherlands-based Polyscope Polymers, whose core business is based on styrene-maleic anhydride, claims to have grown its market share in this sector significantly over the past decade. Its acquisition by Vertellus earlier this year is expected to support its future growth ambitions. “With this acquisition, Polyscope expands its presence in the specialty additives market,” said Patrick Muezers, CEO of Polyscope, “and we will be able to leverage Vertellus’ global resources, capabilities and strong market leadership position to better serves our existing and new customers.”
Potential synergies between the Polyscope and Vertellus portfolios are currently being investigated. muezers says: “New synergies will continue to emerge, but the ethylene MA copolymer ZeMac offered by Vertellus has proven to be highly complementary to Polyscope’s current offering. ”
The combination of our additives will help us to answer our customers’ questions about the optimization of Xibond blends,” he said. We are already seeing an increasing demand for more sustainable solutions. For example, we have initiated a number of projects to support the processing of bio-based plastics in PLA/PBAT combinations with our additives. In this project, our Xibond compatibilizers are critical to achieving optimal performance in applications and developments, including the move from traditional PE bags to biomaterial shopping bags.”
“In addition, we are increasingly seeing opportunities to provide solutions for up cycling various polymers. Some of our customers are even finding that our compatibilizers are helping to blend ‘plastic hodgepodge’ with ABS to make blends that can make PET/ABS blends available for applications such as electrical housings,” explains Muezers.
“Finally, we have gained experience around the Xibox service model, which we use to help our customers speed up product development, cutting weeks, and in some cases months, from the development process. We use small bench top mini extruders to perform rapid screening of our customers’ formulations.”
The Xibox screening concept is said to have been adopted by several customers facing the challenge of recycling multiple materials.
Tisan Engineering Plastics of Turkey’s Olebond products are maleic anhydride grafted polymers produced by reactive extrusion. The company claims, “Depending on the level of grafting and the type of polymer, Olebond helps to improve material properties by acting as a compatibilizer, coupling agent, impact modifier and binder.”
Olebond 7401 grade is based on PP with a high content of maleic acid added to act as a coupling agent between the polymer and the filler. The company says that the 7401 grade also helps to improve the mechanical properties of blends of PP and incompatible polymers such as PA.
Olebond 7402 grade is used as a compatibilizer and coupling agent in applications such as halogen-free cable materials, multilayer tubes, laminates and blends of PE with other polymers. As a compatibilizer, Olebond 7402 is said to improve the interphase adhesion of PE to polymers such as PA and other polyolefins. says Tisan: “Olebond 7402 provides optimum dispersion between mineral fillers, fibres and polymer substrates and improves the mechanical properties of modified plastics. For example, when used in talc-filled modified materials, Olebond 7402 doubles the dispersion of talc.”
Other Olebond grades include ABS and EVA based products which can be used as compatibilizers in different products. 7404 is maleic anhydride grafted ABS and 7405 is maleic anhydride grafted ethylene vinyl acetate (EVA).
Korea-based SK Functional Polymer (SKFP) supplies a full range of compatibilizers and recycling accelerators that are said to enable users to convert mixed plastic scrap into high-value materials (the company was created through the acquisition of Arkema’s functional polyolefin business by SK Geo Centric in 2020).
Lolita Hauguel, the company’s business development manager in France, says: “SKFP has focused a lot of its research efforts on developing reactive polyolefins that make mechanically recycled plastic blends compatible. Lotader reactive terpolymers and Orevac graft resins are widely used as compatibilisers for various blends. SKFP not only has solutions for recycled solutions for blends such as rPET/rPE, rPET/rPP, rPA/rPE, rPA/rPP, rPA/rPET …… also for PE/EVOH blends.”
Lotader 4210 maleic acid grade is commercially available for upgrading recycled PE and EVOH multilayer scrap (final material is limited to non-food applications). SKFP says another successful example uses Lotader AX in recycled PET contaminated with polyolefins that were not completely eliminated during the sorting process.
SKFP says it has also demonstrated that its new range of highly heterogeneous Lotryl T acrylate copolymers is a highly efficient booster solution for recycled styrene resins, offering significantly improved impact properties. The company also offers solutions to improve the impact resistance and modulate the viscosity of rABS materials by combining non-reactive Lotryl highly polar copolymers with reactive Lotader AX. Hauguel says: “The combination of polarity and reactivity of these resins is the key to successful upgrading of recycled plastic waste.”
Orevac grafted polyolefins are said to be widely used as coupling agents for filled PP and PE modified materials. The product range includes grafted PP, grafted HDPE/LLDPE and grafted EVA. The content of reactive maleic anhydride in these products varies, but the higher the content, the more effective they are as coupling agents. They are suitable for use with ATH, MDH, other mineral fillers as well as glass and natural fibres.
Petrochemicals president Salvatore Monte says compatibility of different polymers can be achieved using reactive grafting polymers: such as maleic anhydride grafted PP, bipolar copolymers (e.g. SEBS), free radical graft initiators (e.g. diisopropyl benzene peroxide), and organometallic coupling agents (e.g. titanates). Most modifier manufacturers will only consider heteroatomic titanate coupling agents when there are problems with fillers or adhesion to various substrates,” he says. But in fact, they are thermally stable catalysts that can repolymerise different polymers in the extruder melt, just as titanocene and Z-N catalysts are used for monomer polymerisation.”
He said, “One benefit of the heteroatomic titanate catalyst is that the resulting polymer flows faster at lower temperatures, with a slight increase in tensile strength and a significant increase in elongation. Using a 0.2% by weight addition of catalyst to reduce processing temperatures is a new way to widen the processing window for different polymers, such as PC which typically requires high processing temperatures (above 300°C), and PVC which is typically heat sensitive.”
Monte cites that at this addition rate, the additive doubled the extrusion output of a clear uPVC computer chip housing profile at a reduced processing temperature of 47°C. It also improves melt flow at a reduced processing temperature of 100°C for the injection moulding of a 40% glass fibre reinforced PC modifier that customers have been using for many years to produce millstone bases by moulding the modifier onto steel sheets.
Modifiers and compatibilizers will play a key role in the shift from a linear to a circular economy, says Dow, which is developing additional products to support the recycling of incompatible material combinations. Its Retain 3000 polymer modifier, designed for recycling post-industrial films containing EVOH, is claimed to minimise gelation, while the Fusabond grade is used as an impact modifier and compatibilizer in post-consumer polyolefin recyclates.