Bad Odor (VOCs) from Recycled Plastics
Plastics have become an indispensable part of our daily lives, finding applications in our kitchens, packaging, medical devices, clothing, and much more. These versatile synthetic materials, derived from organic polymers, have given rise to a multitude of polymeric resins like polystyrene, polyethylene terephthalate, polyurethane, polyvinyl chloride, polyethylene, and polypropylene. The world's insatiable appetite for plastics, especially for catering to the needs of the packaging industry, has posed a growing environmental challenge globally in the form of improper disposal systems and large plastics production, totalling over 8.3 million tonnes since World War II.
The accumulation of plastics in our environment, particularly when not collected and recycled, gives rise to several problems, key among which is the release of noxious odors, volatile organic compounds (VOCs), and contaminants. These VOCs can have detrimental effects on human health and can originate from various sources, including microbiological spoilage, detergent residues, ink, adhesives, and additives which come with the complete package of polymeric material. Addressing these odor issues is crucial for promoting the recycling of post-consumer plastic waste. This blog explores the pivotal role of VOCs in the recycling industry, examining the material-based techniques and technologies employed to mitigate emissions from recycled plastics. As the world aims to transition towards a circular plastic economy, understanding and addressing the issue of VOCs in recycled plastics is important to achieving sustainable and odor-free solutions.
Image Credits: https://datatopics.worldbank.org/what-a-waste/tackling_increasing_plastic_waste.html
From such a perspective, decontamination technologies in mechanical recycling are essential for converting plastic residues into reusable raw materials. Sorting, the initial phase, separates impurities like paper, wood, metal, and rubber. Different sorting methods, both dry and wet, are employed, including air density separation and magnetic density sorting, offering promising solutions for comprehensive material separation. The washing phase, particularly crucial for PET resins, involves high-friction washing and caustic ingredient addition to remove surface contaminants. However, technologies are not as advanced for polyolefins like polyethylene and polypropylene, with only a few patented methods focusing on deodorizing these materials. Additionally, addressing ink content in recycled plastics is essential for odor reduction. Some technologies aim to remove ink for prevention of the generation of VOCs in recyclates, augmenting the range of applications. While considerable progress has been made in sorting and cleaning processes, further advancements are needed, especially in the context of polyolefins and ink removal, to enhance the quality and odor control of recycled plastics. Continued evolution of decontamination technologies is vital for the success and sustainability of mechanical recycling in mitigating VOCs and unwanted odors.
Let’s focus on the main operation which is carried out in the extrusion machinery now. It is a critical component in the recycling process of polymers, especially concerning the formation of VOC emissions. In this unit, the plastic undergoes melting using screw extruders, which operate at high temperatures and rotating screws, potentially leading to material degradation and the release of breakdown products. Operating conditions, such as temperature, the presence of oxygen, and heating rate, significantly impact extrusion performance. Temperature plays a pivotal role, with higher temperatures leading to greater VOC emissions. Additionally, the presence or absence of oxygen and the heating rate can influence the release of VOCs during extrusion. To address the issue of VOCs during extrusion, a devolatilization step is often incorporated using vent sections on screw extruders. This process efficiently removes volatile impurities from the polymer melt. The effectiveness of this method is attributed to the mass transfer rate, which is enhanced when polymers are extruded under vacuum conditions, reducing the concentration of VOCs in the vapor phase. Some design changes have also been explored to improve the decontamination of recycled polymers. These changes include processes like supercritical CO2 extraction to remove VOCs, which can be effective but come with the drawback of high operating pressures.
Image Credits: https://www.coperion.com/en/industries/plastics-recycling/plastics-recycling-odor-reduction
Coperion offers a range of technology solutions aimed at effectively eliminating unpleasant odors. The ZSK high-performance extruder primarily focuses on removing highly volatile, low-molecular-weight compounds. To ensure complete odor removal from plastic recyclates, specialized deodorization systems, sometimes in conjunction with the Coperion Bulk-X-Change (BXC) heat exchanger, are employed. The initial step in processing plastic recyclates involves cleaning, separating, sorting, and shredding plastic waste. The resulting regrinds can be introduced into the compounding process through conveying lines and feeders. Inside Coperion's twin screw extruder ZSK, raw materials are melted and homogenized. Moreover, the melt undergoes thorough devolatilization before reaching the extruder discharge, effectively removing volatile substances in varying quantities depending on the nature and severity of the odor. Several factors contribute to this efficient degassing process within the Coperion extruder, including the high torque, well-matched process parameters, and constant surface renewal of the melt facilitated by the co-rotating twin screws. Volatile substances and unwanted odors are efficiently expelled through degassing domes and atmospheric vents on the process section. For specific recycling processes, a ZS-EG twin screw side degassing unit can be installed, ensuring odor removal while maintaining the melt within the process section, even under maximum specific torque, thus significantly enhancing product quality.
Now, let us look at the changes that can be made from the material perspective for avoiding bad odors. The addition of external agents, such as adsorbents and stripping agents, can play an important role in mitigating VOC emissions during extrusion. Adsorbing agents aim to control emissions by being added to the melt polymer, while stripping agents increase the free volume within the molten polymer to enhance the release of VOCs. Commonly used stripping agents include water, nitrogen, air, and supercritical fluids like CO2, which have demonstrated efficacy in devolatilizing polyolefins.
Image Credits: https://www.recyclean.in/plastic-masterbatches-product-detail-3140.html
The VOCs available in post-consumer waste can be classified based on their molecular structure, comprising hydrocarbons, aromatic compounds, flavor and fragrance compounds, alcohols, aldehydes, carboxylic acids, ketones, esters, ethers, amines, amides, sulfur-containing compounds, and halogenated compounds. Around 437 VOCs have been documented from plastics, with 80 of these identified as substances responsible for odors in materials like post-consumer HDPE, post-consumer LDPE bags, mixed film fractions of post-consumer polyolefin, and PSW (including LDPE, HDPE, and PP resins). Some notable examples of odoriferous compounds include the fatty odorants 2-octenal and 2-nonenal, floral and fruity-scented compounds like α-isomethyl ionone, verdyl propionate, β-ionone, and tetrahydrolinalool, as well as substances with a cheese-like odor, such as butanoic acid and isovaleric acid, among others.
Ecosorb products align with OSHA regulations, ensuring accurate information presentation on Material Safety Data Sheets (MSDSs). These technologies effectively remove odors from recycled plastics, especially HDPE, which can be contaminated by household or industrial liquids, rendering them unsuitable for consumer goods. By incorporating trace amounts of polyethylene imines, they eliminate aldehyde odors associated with rancid fats and oils, making the recycled plastic odor-free and suitable for various applications. Evonik's TEGO® SORB PY 88 TQ is a concentrated deodorant suitable for controlling odors in masterbatches and compounds. It efficiently handles odors like hydrogen sulfide, mercaptane, thioether, isovaleric acid, amines, and ammonia. This additive, suitable for recycling of polyamides, polyolefins and rubber exhibits good heat stability in the compounding process, making it a valuable tool for managing odors in recycling of plastics. Ampacet's masterbatch additive, Ampacet 101787, is designed to remove taste and odor molecules as they pass through plastic packaging elements. This product is effective in LDPE and ethylene vinyl acetate films used in various applications, including trash bags and flexible food packaging. It has been claimed that the active component possesses a significant level of porosity, showing a preference for selectively adsorbing smaller molecules responsible for generating odors. Additionally, owing to its moderate hydrophobic nature, it effectively eliminates both organic and inorganic compounds even in the presence of water. BYK-Chemie's polymeric stripping agent, BYK-P 4200 is another notable development, serving as an adsorbent to remove odor-causing contaminants and VOCs during vacuum degassing in compounding operations. This granular material, added during extrusion, effectively surpasses conventional solutions, making it a valuable tool for addressing odor concerns in plastics such as PE and PP with limited suitability in ABS.
In the world of plastics, combating bad odors, often stemming from volatile organic compounds (VOCs) in recycled materials, is a critical challenge. Mechanical recycling processes have made strides in sorting, cleaning, and extrusion to mitigate these odors. Material-based strategies, like the use of adsorbents and stripping agents, show promise in reducing VOC emissions. Understanding the molecular structure of VOCs enables targeted odor reduction. Innovative solutions from companies/products like Ecosorb, Evonik, Ampacet, and BYK-Chemie are making significant progress. These efforts are vital for a sustainable, odor-free future in the circular plastic economy, ensuring the success of mechanical recycling and a cleaner environment.
If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at prashant.gupta@polymerupdateacademy.com
Author
Dr. Prashant Gupta
Faculty, Polymerupdate Academy
