Recyclability of plastics: Additives for a second life

Polymer additives are chemical substances incorporated into plastics during manufacturing to modify their properties. Specific additives can address some of the challenges in plastic recycling. Thus, making the process more efficient and cost-effective. Some of the additives that enhance recyclability are:

 

Compatibilizers

Compatibilizers stabilize immiscible polymer mixtures by reducing the interfacial tension between the two polymers. This enhances the processability and mechanical properties of the mixture. Therefore, it offers a pathway for successful mechanical recycling with mixed waste streams. This is crucial because mixing incompatible plastics can lead to weak or brittle recycled materials.

 

Stabilizers
 

Multiple processing cycles can degrade the plastic, making it weaker and less functional. Stabilizers help maintain the desired properties of the plastic during recycling.

 

Processing aids

Processing aids improve the flow characteristics of recycled plastics during processing. Recycled materials can be more difficult to handle than virgin plastic due to degradation or contamination. Processing aids help the plastic melt more easily. Thus, they require less energy and improve overall processing efficiency.

 

Color neutralizers

They address aesthetic and sensory issues that can sometimes arise in recycled plastics. Recycled materials may have residual colors from their previous use. Color neutralizers can be pigments or dyes that mask unwanted colors.

 

Odor neutralizers
 

Odor neutralizers can be encapsulating agents that trap odor molecules or fragrances that provide a pleasant scent. Odor can be trapped by the following mechanisms:
 

• Adsorption mechanism: By using adsorbents, for example, zeolites, activated carbon, etc.
• Absorption mechanism: By using absorption materials like cyclodextrins.

Polymer additives address various challenges in plastic recycling through specific mechanisms.

 

Improving polymer compatibility

Compatibilizers act as the key players here. They improve the interaction between different types of plastics during the recycling process. Imagine them as molecular bridges that allow incompatible plastics to blend more effectively (Figure 1). This creates a uniform mixture with improved mechanical properties compared to simply mixing non-compatible plastics. Common compatibilizers include:
 

• styrene-ethylene/butylene-styrene,
• styrene-isoprene-styrene,
• maleic anhydride grafted polyolefins,
• ethylene vinyl acetate graft polystyrene,
• maleic anhydride,
• bisphenol A diglycidyl ether,
• triphenyl phosphate
   

By enabling the blending of different polymers, compatibilizers allow the creation of recycled materials with a wider range of properties, suitable for various applications. For example, a recycled HDPE/PP sample with ENGAGE™ polyolefin compatibilizer and a recycled HDPE/PP sample without Dow's ENGAGE™ compatibilizer are compared. Elongation at break is increased by a factor of 3 with compatibilization, from 130% to 450%².

 

Figure 1: Schematic Representation of the Effect of the Addition of a Compatibilizer in a Biphasic Immiscible Polymer Blend³

Maintaining material properties
 

Stabilizers come to the rescue to combat degradation. Multiple processing cycles during recycling can weaken the plastic, making it less functional. Examples of stabilizers include:
 

• Antioxidants: They help maintain the desired properties. Antioxidants extend their useful life by protecting polymers from oxidation and preventing degradation or crosslinking during the processing as shown in Figure 2.
  • Primary antioxidants: Directly react with free radicals, terminating them before they can damage the polymer chains.
  • Secondary antioxidants: Decompose harmful hydroperoxides formed during processing, preventing further degradation. Mixtures of primary and secondary antioxidant additives will also provide a synergistic effect. In other words, both act more efficiently together than separately.

• UV stabilizers: They include UV absorbers & hindered amine light stabilizers (HALS). These protect the plastic from degradation caused by sunlight exposure. This is particularly important for outdoor applications where recycled plastic products may be exposed to sunlight for extended periods.

Figure 2: Anti-oxidation Mechanism of Antioxidants in the Aging Process of LDPE⁴

Facilitating processing

Processing aids improve the flow characteristics of the plastic during processing. They essentially help the plastic melt more easily, requiring less energy and improving overall processing efficiency. This translates to:
 

• lower production costs,
• smaller environmental footprint, and
• higher quality recycled materials with less waste generation

Examples include internal lubricants (e.g., fatty acid amides) or external lubricants (e.g., calcium stearate).
 

Challenges associated with recyclable plastics

Recyclable plastics offer a more sustainable solution compared to virgin plastics. However, there are still significant challenges and considerations to address for a truly circular economy. These include⁸:

 

Presence of hazardous substances
 

This is related to the content of hazardous substances in plastics and in some cases in the polymers themselves (e.g., polyvinyl chloride). Hazardous substances in plastics pose a significant obstacle to increased recycling. These include over 13,000 substances such as:
 

• vinyl chloride in PVC (found in piping, construction, and healthcare items),
• additives,
• colorants,
• plasticizers,
• stabilizers, etc.
   

These hazardous substances can pose health risks. Therefore, they are undesired in recycled plastics used for new products.

 

Product design and contamination

Plastic products are often designed without reuse, repair, or recycling in mind. Some products include laminates of different materials (e.g. plastic, metal, and paper), which makes sorting and recycling difficult. Not all recycling facilities have advanced sorting technologies. Manual sorting can be slow and imperfect, further contributing to contamination issues.

A large part of plastic is used for packaging and these products are often contaminated by food and other substances, which hinder recycling. Consumers often struggle to differentiate between various plastic types. This leads to mixed recycling bins. This contamination makes it difficult to separate and reprocess plastics efficiently.

 

Downcycling and degradation
 

The recycling process can degrade the plastic's quality, especially with multiple recycling cycles. Most recycling processes result in downcycling. This shortens the lifespan of the material within a circular economy. Downcycling occurs when:
 

• recycled content is of lower quality than the original product
• recyclates are used in products of lower value than the original. For example, when polyethylene terephthalate (PET) from plastic bottles is used in textiles

This reduces the possible number of recycling loops. Thus, limiting its potential applications and reducing its overall value.

 

Economic hurdles
 

Recycled plastics are perceived to be of lower quality than virgin plastics. This results in relatively low (although increasing) demand. The low price of primary materials and the costs associated with recycling plastics impact the demand for recycled plastics. If the demand is low, the recycling process may not be economically sustainable. In addition, collecting and processing recycled plastics can sometimes be more expensive than using virgin plastics. This is due to logistical challenges and limited infrastructure.

 

Key strategies and benefits for advancing plastics recycling

By addressing the challenges and strategizing recycling through a collaborative approach involving industry, government, and consumers, we can unlock the full potential of recyclable plastics.
 

• Standardization and labeling: Sorting accuracy by consumers can be improved by clear and consistent labeling systems for different plastics. Additionally, standardized recycling symbols and regional guidelines are crucial.

• Innovation in recycling technologies: Advanced recycling technologies like chemical recycling offer promising solutions for processing complex plastic waste streams. They potentially improve the quality of recycled materials.

• Product design for recyclability: Designing products with recyclability in mind from the outset is crucial. This includes using single-material structures and avoiding complex multi-layer plastics that are difficult to separate and recycle.

• Consumer education and awareness: Raising public awareness about proper sorting practices and the importance of using recyclable products is essential for success.

• Policy and incentives: Government policies that incentivize the use of recycled plastics and penalize virgin plastics use can create a more favorable economic landscape for a circular plastic economy.

Figure 3: Process of Plastics Recycling

Recycling plastics offers a solution to the environmental and economic challenges posed by our reliance on virgin plastic.
 

• In terms of the environment: It reduces plastic pollution in oceans, landfills, and ecosystems. This protects wildlife and habitats, while also mitigating the growing problem of microplastics. Researchers estimate that plastic pollution results in a 1-5% loss in marine ecosystem services.

• In monetary terms: This equates to an annual loss of $500 billion to $2.5 trillion, reflecting the substantial economic implications of plastic pollution on industries such as fishing and tourism¹. Recycling also creates jobs in the collection, sorting, processing, and remanufacturing sectors. It can also be a cost-effective alternative for manufacturers and fosters a market for recycled materials.

Polymer additives are revolutionizing plastic recycling across various industries. Some of the real-world success stories of recycled plastics are summarised here.

 

Packaging industry

Traditionally, multi-material packaging posed recycling challenges. For example, combining PET for rigidity with PE for flexibility. Additives like compatibilizers bridge the gap, allowing these materials to blend effectively. This creates fully recyclable packaging while maintaining desired properties like strength and barrier protection.
 

• Dow REVOLOOP™ recycled resin is formulated with up to 70% post-consumer recycled (PCR) plastics. It is designed for use in secondary and tertiary packaging and has potential use in some non-food contact primary packaging applications. This high-performance PCR-rich resin is designed for film applications that require improved homogeneity, consistency, and toughness over other film-based PCR resin offerings.

• Mars and Berry Global are now putting post-consumer recycled plastics into plastic containers used for m&m's®, SKITTLES®, and STARBURST®. In 2022, Mars launched 15% recycled plastic in large-format jars and has lightened the packaging. The bars are produced from a single-pellet, food-grade rPET derived from mechanical recycling technologies. Available in three sizes, the jars will eliminate about 300 tons of virgin plastics per year.

• Amcor Rigid Packaging partnered with a winery in the US to commercialize a 100% recycled PET wine bottle. It combines the product protection and shelf-life benefits of glass packaging with PET's lightweight and durability. The bottle meets product protection requirements through the use of SiO_x. It is an ultra-thin, glass-like oxygen barrier coating applied inside the bottle. It protects against oxidation and preserves quality and flavor by keeping the wine from coming in contact with the rPET.

• Mars Petcare has launched a new pouch for its SHEBA® wet cat food. It incorporates, on average, 30% recycled polypropylene from advanced recycling technology. SHEBA® pouch is not only food safe but can also withstand the retort process where the product is sterilized at a high temperature. The pouch is made through an advanced recycling process where mixed plastic waste is recycled through pyrolysis into an oil. This can be used as the feedstock to produce a range of plastics with the same characteristics and functionality as virgin plastic.

Select commercial grades of various types of recycled polymer additives in our extensive database:
 

Consumer industry

The consumer goods industry is facing increasing pressure to improve product recyclability. Regulatory requirements and consumer preferences for sustainable products are driving innovation in this sector. Manufacturers are actively seeking solutions that enhance recyclability. For example, Green Toys makes 100% recycled kids' toys. The process includes the following steps:
 

• Step 1: Recycled milk jugs (HDPE, LDPE, and PP) and yogurt containers are collected by waste management.
• Step 2: They are shredded into flakes.
• Step 3: The flakes are reprocessed into raw materials.
• Step 4: The raw materials are finally mixed with food-safe, mineral-based products.

Figure 4: Green Toys' 100% Recycled Toys Made (with no BPA, PVC, or Phthalates) from Plastic at Risk of Polluting Oceans⁷

Similarly, Samsung announced that they will repurpose discarded fishing nets and use them to build their Galaxy S series. The Galaxy S23 uses recycled ocean waste and according to Samsung⁹, it is their most sustainable phone to date, emitting a total of 70.557 kg of CO₂-e. Samsung aims to incorporate recycled resin into 100% of the plastics in its products by 2050¹⁰.

Indesit has developed a greener washing machine, using recycled plastics to construct its Hotpoint washing machines. The washing machines are built using a high-grade plastic polymer made from shredded plastic waste. It was recovered from refrigerators. Indesit developed the project with the help of the Waste & Recycling Action Plan (WRAP) and Axion Recycling, as part of an overall effort to recycle waste materials. The washing machines are available in the U.K. and have good efficiency ratings, making them an all-around green choice.

 

Automotive industry

From the environmental point of view, the thermoplastic alternative proved as the material of choice for this particular vehicle application. This is due to its:
 

• lightweight design potential,
• material-efficient manufacturing process combined with the highest degree of material utilization, and
• excellent recycling properties

Car manufacturers are increasingly incorporating recyclable plastics with specific additives into components like dashboards, door panels, and interior parts. These additives help the plastic withstand the high temperatures encountered during car manufacturing and maintain its structural integrity throughout the vehicle's lifespan. Utilizing recyclable components with additives contributes to sustainability goals within the automotive industry. Additionally, they can offer cost savings compared to virgin plastics.

Ford has been repurposing recycled plastic to manufacture vehicle parts. One of how Ford has been doing this is by utilizing recycled plastic bottles for the manufacture of underbody shields on all their SUVs and other vehicles. The Ford Motor Company makes use of about 1.2 billion recycled plastic bottles annually. This equates to approximately 300 bottles per vehicle⁵.

BMW is also a leader in this area, incorporating recycled plastics with the help of additives in various interior components. This reduces their environmental footprint and potentially lowers production costs. BMW already uses recycled fishing nets for the fabrics used to make headliners and floor mats for the BMW i3, BMW iX, and other models⁶.

 

Construction industry

Companies like Dow are partnering with construction firms to develop large-scale recycling projects for building materials containing specific additives. Dow identified hundreds of potential uses of hard-to-recycle plastic waste for building materials. These can be used to develop infrastructure such as park benches, bus stops, and other outdoor structures. Through collaboration, it addressed the issue of hard-to-recycle plastics by diverting materials from landfills. Additionally, creative and circular uses for these materials were found.

 

Figure 5: Applications of Recycled Plastics in Different Industries

The future of plastic recyclability is brimming with exciting possibilities. Some emerging trends and innovations are:

 

Next-generation additives

Research is ongoing to develop new types of additives that are even more effective in enhancing recyclability. These might include additives that improve compatibility across a wider range of plastics or those that target specific challenges like decontamination during recycling.

 

Focus on sustainability
 

The focus is shifting towards developing sustainable and eco-friendly additives. This could involve using bio-based materials for additive production or creating additives that are themselves biodegradable, further minimizing their environmental footprint.

 

Improved sorting technologies

Advancements in sorting technologies like automated sorting using AI and near-infrared (NIR) spectroscopy can improve the efficiency of separating different plastic types for recycling. This allows for more effective utilization of additives specific to each plastic type.

 

Chemical recycling

Chemical recycling offers a promising avenue for processing complex plastic waste streams that are difficult to recycle mechanically. This technique breaks down plastic at a molecular level, allowing it to be remanufactured into virgin-quality plastic.

 

Role of additives

Additives can play a crucial role in facilitating these advanced recycling methods. For example, specific additives might be used to prepare the plastic waste for chemical breakdown or to improve the quality of the recycled plastics produced through these techniques.

 

Closed-loop recycling systems

The concept of closed-loop recycling systems is gaining traction. This involves designing products with specific additives that make them easily recyclable back into the same product or similar high-quality applications. This minimizes waste generation and maximizes resource use.

 

The future of plastic recyclability is promising, driven by innovative polymer additives and emerging technologies. These additives play a vital role in:
 

• enhancing recyclability,
• conserving resources, and
• supporting a circular economy

As industry demands grow, advancements in additive technologies and recycling methods grow. Technologies like chemical recycling and closed-loop systems are the future of sustainable plastics. By embracing these innovations, industries can significantly reduce plastic waste and create a more sustainable future for generations to come.