Why Material Properties Matter More Than You Think in Plastic Recycling

By Lou Farrell

Material properties play a major role in plastic recycling, sometimes more heavily than the plastic itself. Numerous characteristics of the material and everything accompanying the product can influence its ability to be processed, broken down, and reused. Sometimes what looks recyclable is impossible to reuse. 

Understanding the factors that influence recyclability can make organizations and individuals more eco-conscious, especially when they manufacture and dispose of products, respectively.

Polymer Purity and Degradation

Every environmentalist knows that plastics are classified by composition, including the polymers they contain. Some could have polyethylene terephthalate (PET), polypropylene, or high-density polyethylene. Every ingredient’s density contributes to the plastic’s responses to processing. For example, density can alter the melting points or the separability of different materials, making it easier or harder to isolate polymers from one another.

Typically, mixed plastics make a less stable recycled material. Technicians do not want PET to contaminate a batch of polyvinyl chloride because it alters the degradation rate. If multiple polymers melt together, the recycled product will perform worse, having less strength or malleability.

This contamination can also alter the recycled material’s appearance and increase its fragility, making the plastic unsuitable for certain products. If this occurs, plastics can be downcycled into similar products that require the metrics the reused plastics can handle.

The Trouble With Multiple Layers

Many plastic products, especially in the food industry, use multiple layers of material to protect the item. Layers can include pigments that block light from entering the bottle, potentially leading to spoilage. 

The same goes for oxygen deterrents. Yogurt pouches, chip bags, and similar products require several layers of plastic to stay fresh and safe for consumption, but this makes them difficult to separate for recyclers. Plastics may be melted or broken down, but this can cause a contaminated mixture.

Some products may even say they are made from 100% recycled materials, but this does not mean the entire product can be recycled again. They can have plastic and more, including:

• Paperboard
• Aluminum
• Plastic films
• Plant-based plastics

Many recycling facilities lack the specialized mills needed to even attempt to separate these from plastics, making layered waste one of the most cumbersome pain points in the sector.

Additives and Contaminants

Similarly to how layers keep food fresh, some additives protect consumer products from degradation or the elements. Plastic polymers can include additives that enhance their properties, such as their fire resistance or stability against intense ultraviolet light. 

These ingredients will pollute plastic mixtures in recycling plants, and their inclusion could potentially harm the environment if they are chemical-based or volatile. Some older plastics even contain lead for pigmentation, which could make the product noncompliant with modern regulations.

The Invisibility of Carbon Black

Carbon black is a common ingredient to add pigmentation to plastics. While black plastics look sleek, they are almost incompatible with modern facility technologies. Most organizations use near-infrared equipment to pick, place, and sort recyclable plastics on rapidly moving conveyors. Carbon black absorbs this light and can go undetected, ending up in a landfill or incinerator.

When the infrastructure cannot determine whether plastic is present, it hinders recyclers’ overall effectiveness.

However, the industry must adapt and find ways to address these realities, as carbon black should remain a component of packaging manufacturing. It is a highly sustainable product, made from waste materials from many industries, offering a versatile alternative material.

The complex relationship between carbon black and recyclers presents opportunities for further research and development of advanced processing and sorting machinery, especially as more eco-friendly materials enter the plastics sector.

The Impact of Color

Carbon black is one example of how color can be a recurring problem for recyclers. It is easier to process clear plastics. The by-product can maintain a similar clarity, but if opaque or mixed colors are melted down, it can create a muddy mixture.

The appearance limits its use cases, making it difficult to resell. Some soda makers have even discontinued making bottles in colors, despite the branding advantages, because they had sustainability targets to meet.

Density and Shape

The weight, size, and shape of a mold can also directly influence its recyclability. Many sorters send plastics into a vat of water to see which ones float. This process helps employees identify the material properties, including distinct polymers. In bottle processing, it helps them separate cap plastics from the rest of the container. 

The strategy is ideal for a product, like a bottle, but less helpful for flat, thin plastics that would float regardless. Facilities may experience quality control issues due to these sorting concerns, which is why many recycling guidelines prohibit thin materials.

Labels and Adhesives

Labels can be made of paper or plastic, leading to another sorting concern. However, the adhesive that sticks it to the container is even more problematic for facilities. These sticky substances have always been a challenge for recyclers to wash off plastic, and they may not even be fully removed. This can lead to contamination, depending on the adhesive’s ingredients. 

The Scope of Material Properties in Plastic Recycling

Corporations are responsible for designing plastic products that are easier to recycle, making it easier for citizens to follow proper waste management practices. While many ignore the minutiae of plastic recycling, every color, label, and molding decision dictates how sustainable the end of its life cycle can be, if recycled.

Spreading awareness of how these factors determine a plastic product’s future is crucial for standardizing recycling behaviors and expectations.

Lou Farrell

Lou is the Senior Editor of sustainability and technology for Revolutionized Magazine. He has

over 4 years of experience crafting compelling articles on a variety of topics, from energy

efficiency trends to eco-friendly construction. He loves to write, and is passionate about sharing

his knowledge with others.