This is an excerpt from the final chapter of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes. 

Introduction

In recent decades, the global demand for thermoplastics has surged, primarily due to their versatility, durability, and cost-efficiency. Global plastic production has risen from approximately 270 million metric tons in 2010 to 367 million metric tons in 2020. However, this substantial increase in plastic production has led to a corresponding increase in plastic waste.

It is estimated that around 381 million tons of plastic waste were generated in 2015 alone, and this figure is projected to double by 2034. This means that the amount of plastic waste per year currently surpasses the amount of plastic produced. Landfilling and incineration are still the most common disposal methods for thermoplastics. ^[23] However, these practices lead to detrimental effects on soil ecosystems and the creation of toxic byproducts. In this context, it is crucial to develop new ways to deal with thermoplastic waste, not just at the end-of-life, but across the entire product life cycle.

In this chapter, you will learn about the most important recycling methods for thermoplastics and thermoplastic composites. You will discover the significance of Life Cycle Assessment (LCA) and sustainable design. Finally, the future challenges of thermoplastics sustainability will be addressed.

Can Thermoplastics Be Sustainable?

Despite the challenges in waste management, thermoplastics can still be a sustainable option. Compared to thermoset resins, in fact, thermoplastics are much easier to recycle, since they can be melted and remolded repeatedly. Due to their low density, when thermoplastics and thermoplastic composites are used to substitute metals and other heavier materials, they can lead to energy savings and reduced CO2 emissions. This is especially significant in the aerospace and automotive sectors.

In addition, thermoplastics tend to be very durable. In industrial applications, using advanced and engineering thermoplastics can significantly prolong the lifetime of components, thanks to the outstanding mechanical, chemical, and wear resistance of these materials.

In order to promote a more sustainable use of thermoplastics, however, circularity must always be a priority. This means focusing on sustainable design principles, evaluating the material’s carbon footprint, integrating recycled materials, prolonging the lifetime of virgin materials, and promoting recycling and waste management practices.

Recyclability of Thermoplastics

Recycling thermoplastics requires different methods depending on the type of plastic waste. Generally, plastic waste can be categorized into postindustrial (PI) waste, generated during manufacturing, and postconsumer (PC) waste, produced at the end-of-life of consumer products. PI waste is usually clean and has a known polymer composition, making it relatively straightforward to recycle. In contrast, PC waste often consists of mixed plastics with unknown compositions and potential contamination by organic and inorganic residues, making it more complex to recycle. ^[24]

The two primary recycling methods for thermoplastics are mechanical and chemical recycling. Mechanical recycling involves reprocessing plastic waste into new products. It includes several steps, such as collection, sorting, washing, and grinding of the material. It is the most straightforward and cost-effective option. However, for mechanical recycling, careful collection and sorting of waste are paramount. Challenges can arise from coatings, paints, and contaminants that impact the mechanical properties of the recycled thermoplastics.

On the other hand, chemical or feedstock recycling breaks down plastic waste into its basic chemical components, which can be reprocessed into new polymers. This method is suitable for heterogeneous and contaminated plastic waste, where separation is difficult or too expensive. However, most chemical recycling processes require solvents, with potentially increased environmental risk. Chemical recycling also requires substantial investments and it is only used for large volumes of waste.

Finally, quaternary or energy recycling focuses on recovering the energy stored in plastic waste through combustion. It is important to control volatile emissions to prevent environmental contamination. While it is a way to recover energy, this is widely considered the least sustainable approach compared to conventional recycling methods. ^[25]

With engineering thermoplastics, recycling is especially important. This is because these materials are often expensive and challenging to produce. However, when recycling thermoplastics for advanced applications, it is crucial to preserve the materials’ mechanical and functional properties. 

To address this challenge, Mitsubishi Chemical Group’s Statera™ sustainability platform offers an integrated approach to engineering thermoplastic recycling. This includes Sterra™, a portfolio of high-performance engineering plastics with recycled content, including advanced thermoplastics such as PEEK. These materials are guaranteed to maintain the reliability and mechanical properties of their virgin counterparts. The Statera™ program also provides verified lifecycle assessment data and simplifies responsible disposal by reclaiming production scrap and end-of-life parts.

Recyclability of Thermoplastic Composites

Thermoplastic composites offer a clear advantage over thermoset composites when it comes to recyclability. Unlike thermoset composites, which have a cross-linked matrix, thermoplastic composites can be efficiently and fully recycled using cost-effective methods, such as remelting and remolding.

In thermoplastic composites, mechanical recycling is the most widely used method for recycling both the fiber and the matrix. It involves shredding the composite into smaller parts and reprocessing it through melting and molding. With this process, there is no need to separate the fibers from the matrix. Mechanical recycling of thermoplastic composites is typically regarded as the most economical and environmentally friendly approach.

However, it is important to note that certain physical and chemical changes can occur during recycling. The initial size reduction step, such as shredding, leads to a reduction in fiber length. The subsequent melting and molding stages can further break down the fibers and even cause degradation in the mechanical properties of the matrix.

Thermal recycling is a method that involves the thermal removal of the matrix in the composite to recover the fibers. This process typically includes subjecting the material to high temperatures, ranging from 300 to 1000 °C. Although it is mainly used for recycling thermoset composites, it can also be applied to thermoplastic composites. Pyrolysis is a thermal recycling process in which the matrix is eliminated without any oxygen.

The recycling of fibers, particularly carbon fiber, is gaining increasing attention due to the demand for low-cost alternatives to virgin carbon fiber. On average, the energy consumption for producing 1 kg of recycled carbon fiber is approximately one-fourth of that required for producing the same amount of virgin material. This translates to recycled carbon fibers being 20–40% less expensive than their virgin counterparts. As a result, recycling offers an  appealing cost-saving solution. ^[20]

In this context, Mitsubishi Chemical Group’s carboNXT focuses on the recycling and reintegration of carbon fiber into the market. CarboNXT’s process involves the sorting of composite waste, the recovery of recycled carbon fiber through pyrolysis, the refinement and resizing of carbon fiber, and its use in high-quality customized products.

Waste Management and Sustainable Design Principles

Recycling alone isn’t sufficient to ensure a sustainable approach to manufacturing with thermoplastic materials. In fact, the sustainability of a product isn’t limited to the end-of-life but should be integrated into the design process from the very beginning. Among the tools currently available to address this necessity, Life Cycle Assessment (LCA) is the most important.

LCA is a crucial quantitative tool for evaluating sustainability. It offers a comprehensive analysis of environmental impacts throughout a product's entire life cycle, from material extraction to end-of-life management. This includes raw material acquisition, production, packaging, transportation, use, and waste disposal. LCA quantifies energy consumption, materials used, and emissions produced at every step. It offers valuable insights into opportunities for minimizing environmental impact.

LCA can also defy your expectations. For example, LCA has shown that replacing fossil-based polymers with bio-based materials doesn't always enhance sustainability. Waste management and recycling often provide more eco-friendly solutions than biodegradation, and switching from plastics to other materials isn't always environmentally sustainable. ^[26]

To foster sustainable product design in a circular economy, there is an increased need for collaboration along the entire value chain. Here are four key points you should keep in mind.

• Materials selection. Choose sustainable, recyclable materials. Assess their life cycle impact, including production, use, and disposal.

• Durability and longevity. Create long-lasting products with robust materials and design, considering their expected lifespan.

• Modularity and adaptability. Employ modular design for easy disassembly and reconfiguration.

• Repair, reuse, and recycling. Include features for easy repair, encourage product reuse, and design for recyclability.

 Mitsubishi Chemical Group is committed to supporting LCA and sustainable design principles. MCG’s CORACAL™ software, for example, is a precious tool for calculating the CO2 footprints of your applications. This software provides transparent information and consulting regarding carbon emissions and environmental impact.

CORACAL™ enables users to estimate carbon emissions for materials in the MCG portfolio, aiding in the selection of the most sustainable options. The software offers insights into waste reduction through takeback programs and helps identify more environmentally friendly solutions, fostering data-driven decisions and transparency.

Looking Ahead: Current Challenges and Future Solutions

Facing the growing concern towards the environmental challenges posed by thermoplastics, innovative solutions are being explored. Bioplastics, for example, have emerged as a promising alternative. They are plastics derived from biomass, such as plants, instead of fossil fuels.

It is essential to note that not all bioplastics are biodegradable, and not all biodegradable plastics are bio-derived. Biodegradability is defined as the ability of a material to naturally degrade at least 90% within six months, without any toxic residues. While biodegradability is valuable for certain applications, such as packaging, engineering applications need to prioritize durability to extend product lifetimes and reduce material waste. 

The development of bio-derived engineering plastics, combining sustainability and durability, is still an open challenge. To address this issue, MCG has developed DURABIO™, a bio-based transparent polycarbonate material with exceptional mechanical and wear resistance properties.

In the thermoplastics industry, an additional concern that has emerged in recent years is the environmental impact of per- and polyfluoroalkyl substances (PFAS). PFAS are a large group of polymers, widely employed in advanced applications for their unique heat and chemical resistant properties. 

However, PFAS raise concerns due to their adverse health effects and environmental accumulation. Because of this, the European Chemicals Agency is currently discussing a new restriction on PFAS. In response to these concerns, MCG is researching advanced solutions to offer suitable PFAS alternatives.

These examples show how the proactive adoption of circular design principles, along with responsible risk management practices and continued innovation, can play a crucial role in mitigating future pollution and promoting sustainable solutions in the thermoplastics industry.

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