Mechanical recycling is a physical process used to reclaim materials, primarily plastics, from waste streams. This method focuses on processing discarded materials into secondary raw resources without causing a significant change to their fundamental chemical structure. The process involves multiple steps to transform used items, such as bottles and containers, into a form that manufacturers can incorporate into new products.
Core Principles of Mechanical Recycling
Mechanical recycling relies on physical forces, such as heat, pressure, and friction, to reshape the material. Unlike other methods, this approach ensures the long polymer chains that define the plastic’s identity remain largely intact. The goal is to reuse the polymer itself, preserving its original chemical makeup.
However, the repeated application of heat and sheer force during processing can cause some shortening of the polymer chains, leading to a slight degradation of material quality over time. This often results in a phenomenon known as downcycling, where the recycled plastic is used to create a product of lower quality or functionality than the original item. For example, a clear plastic bottle might be recycled into a darker, less structurally demanding product like plastic lumber.
The ideal, though less common, outcome is closed-loop recycling, where the material is reprocessed back into an equivalent product with minimal quality loss, such as a plastic bottle becoming another plastic bottle. Achieving this perfect loop requires highly pure and uncontaminated waste streams. The material’s identity is maintained throughout the process, allowing for its re-entry into the manufacturing cycle as a raw material substitute.
The Standard Multi-Step Process
The recovery of plastic waste begins with a rigorous sorting phase, which is a decisive step in determining the final quality of the recycled material. Automated systems use technologies like near-infrared light to identify and separate different types of plastics based on their unique polymer composition. This sorting is often supplemented by manual checks to ensure accuracy and remove any non-plastic contaminants.
Following sorting, the material moves to the cleaning stage, which involves washing to remove physical contaminants like dirt, food residue, labels, and adhesives. The cleaned plastic is then fed into a shredder or grinder, reducing the items into small, uniform pieces called flakes.
The dried flakes are introduced into an extruder, where they are melted down using heat and pressure. The molten plastic is pushed through a die to form continuous strands, similar to thick spaghetti.
The strands are quickly cooled, typically in a water bath, and then cut into small, uniform pellets. These pellets represent the secondary raw material, also known as recyclate, which manufacturers can purchase and use in place of virgin plastic resins to mold new products.
Materials Suitable for Mechanical Recycling
Mechanical recycling is most effective for thermoplastic materials, which can be repeatedly melted and reshaped without significant chemical alteration. The most common examples of these are Polyethylene Terephthalate (PET), frequently used for beverage bottles, and High-Density Polyethylene (HDPE), found in milk jugs and detergent containers. These materials are favored because they typically exist in relatively pure, single-polymer streams.
Other common plastics that are effectively recycled through this method include Polypropylene (PP) and Low-Density Polyethylene (LDPE). In contrast, thermoset plastics, like epoxy resins, are generally unsuitable because they undergo an irreversible chemical reaction when first heated, meaning they cannot be melted down and re-molded.
Material suitability is also heavily dependent on the consistency of the waste stream and the ease of separation. Multi-layered plastics, which combine different polymer types, pose a significant challenge because they cannot be cleanly separated by physical means, making them difficult to process mechanically. Clean, single-polymer streams yield the highest quality recyclate, ensuring the material can be used for more demanding applications.
Distinguishing Mechanical from Chemical Recycling
Mechanical recycling is fundamentally different from chemical recycling, often referred to as advanced recycling, in its approach to material recovery. Mechanical methods focus on preserving the polymer structure, using physical manipulation to process the plastic into a new form. The resulting output is a recycled polymer pellet that retains the molecular chains of the original plastic.
Chemical recycling, conversely, uses thermal or chemical processes to intentionally break down the polymer chains into their basic molecular building blocks. Techniques like pyrolysis use high heat in the absence of oxygen to break down plastic into oils or fuels, while depolymerization uses chemical agents to revert the polymer back into its original monomers. This process effectively resets the material’s history.
Mechanical recycling produces recycled plastic pellets that are still polymers, which may have reduced properties due to degradation. Chemical recycling yields monomers, which are the same chemical components used to make virgin plastic, or an oil/feedstock that can be used to produce new, virgin-equivalent polymers.