Which Plastics Are Suitable for Pyrolysis

With plastic pollution becoming an increasingly severe issue, pyrolysis—a technique within the realm of chemical recycling—is regarded as one of the key methods for combating “white pollution.” However, it is important to note that not all plastics are suitable for pyrolysis. Selecting the appropriate type of plastic not only impacts recycling efficiency but also directly determines whether the process is economically viable and environmentally friendly.

The following section briefly outlines which plastics are suitable for pyrolysis and which are not, along with the underlying principles behind these distinctions.

Principles of Pyrolysis Technology

Simply put, pyrolysis involves heating plastics in the absence of—or with very limited—oxygen, causing their large molecular chains to break down into smaller molecules, ultimately yielding fuel oil, combustible gases, and carbon black.

The key lies in breaking the carbon-carbon bonds. Whether this can be achieved efficiently and in a controlled manner depends primarily on the molecular structure of the plastic.

Most Suitable Plastics for Pyrolysis

Polyethylene

Common Items: Shampoo bottles, milk jugs (HDPE); plastic wrap, plastic bags (LDPE).

Why They Are Ideal for Pyrolysis: Their molecular structure consists of long carbon chains that are easily broken apart.

High Oil Yield: At temperatures around 400°C, the oil yield for LDPE can exceed 83%, while that for HDPE surpasses 70%.

Primary Products: Heavy oils similar to diesel fuel; these possess high calorific values ​​and can be utilized as industrial fuels or subjected to further refining.

Technical Maturity: The technology is highly mature and serves as the primary feedstock for pyrolysis projects both domestically and internationally.

Polypropylene

Common Items: Takeout containers, car bumpers, woven bags, etc.

Why It Cracks Easily: Its molecular chains feature methyl side chains, making them more susceptible to cracking into smaller molecular products.

Oil Yield: The oil yield is approximately 78%; the resulting oil is relatively light and contains a high proportion of gasoline-like components.

Main Products: A light oil resembling gasoline, which can be used directly—or after blending—in internal combustion engines.

Additional Advantages: The cracking temperature is relatively low, resulting in lower energy consumption and greater cost-efficiency.

Polystyrene

Common Items: Disposable foam food containers, yogurt cups, CD cases, etc.

Why It Pyrolyzes Easily: The phenyl side chains present on its molecular structure make it prone to breaking at specific points when subjected to heat.

Oil Yield: The oil yield can reach up to 69%, and the resulting product features a very high content of styrene monomer (approximately 60–70%).

Primary Product: Styrene monomer, which can be directly used to manufacture new PS plastics, thereby enabling a closed-loop recycling system.

Value: The product possesses high added value, is highly profitable, and offers outstanding economic viability.

Polymethyl Methacrylate (Acrylic)

Common Items: Scrap acrylic sheets, automotive taillight covers, etc.

Unique Feature: Under high temperatures, it readily undergoes “depolymerization”—that is, it reverts directly to its original monomer: methyl methacrylate (MMA).

Recycling Efficiency: The monomer recovery rate can exceed 90%, making it one of the highest among all chemically recycled plastics.

Primary Product: MMA monomer, which can be used directly to manufacture new PMMA.

Key Consideration: The process involved is more akin to “depolymerization,” requiring more precise temperature control than standard pyrolysis.

Types Suitable for Pyrolysis but Economically Unviable—Condensation Polymers

Some condensation plastics (such as PET, polycarbonate, and polyamide).