Why Recyclable Plastics Can Be Hard to Recycle in Practice

Take two plastic packages. The first is a rigid container made from a single polymer, high-density polyethylene (HDPE). The second is a flexible pouch made from bonded layers of polyethylene (PE) and polyethylene terephthalate (PET).

Both contain thermoplastic polymers. Both can be made into useful products. Yet the first may be straightforward to mechanically recycle, while the second can be difficult to process into a usable material.

Why?

A plastic can be recyclable in principle because its molecular structure allows a possible recovery pathway. It can be difficult to recycle in practice because the finished product may contain several incompatible materials, be difficult to sort or clean, require different processing conditions, or produce recycled material with unsuitable properties.

The key distinction is between a polymer being chemically reprocessable and a discarded product being practically recyclable. These are not the same thing.

01Four different questions hide inside the word recyclable

When someone says a plastic is recyclable, ask what they actually mean.

There are four separate questions to consider.

A plastic might pass the first test but fail the other three.

For example, a thermoplastic polymer might soften and flow when heated. That establishes that it can potentially be mechanically reprocessed. It does not establish that a recycling facility can separate it from other materials or that the resulting product will have sufficient strength.

Let’s work through why.

01Start with molecular structure

A polymer consists of long molecules made from repeating structural units. These long molecules are called polymer chains.

The arrangement of these chains determines how the material responds to heating.

Thermoplastics: chains that can move

Thermoplastics generally contain long, linear or branched polymer chains without permanent covalent cross-links connecting the chains into one continuous network.

The individual chains are held near one another by intermolecular attractions. When the polymer is heated, increased molecular motion allows the chains to move past one another more readily.

The material softens and can eventually flow.

When it cools, the polymer solidifies again.

This makes many thermoplastics suitable for mechanical recycling, where waste plastic is heated, reshaped, and cooled into a new product.

Think of thermoplastic chains as long shoelaces lying together in a tangled bundle. They can move relative to one another if enough energy is supplied.

A thermoset is more like those shoelaces tied together at many points. The permanent connections restrict movement.

The analogy has limits: real polymer chains are three-dimensional molecules, and their movement depends on temperature, intermolecular forces, chain entanglement, and other structural factors.

Thermosets: a permanent network

Thermosetting polymers contain covalent cross-links between their chains.

These cross-links create a rigid, interconnected network. Heating does not allow the chains to slide past one another in the way they can in a thermoplastic.

Instead of melting into a flowable liquid, a thermoset will eventually undergo chemical degradation if heated sufficiently.

This makes conventional melt-and-remould mechanical recycling unsuitable for thermosets.

However, this does not mean every thermoset is completely unrecyclable. Other approaches, such as grinding the material for use as a filler or using suitable chemical recovery processes, may be possible.

The important distinction is that a thermoset cannot simply be melted and reshaped like an ordinary thermoplastic.

For a closer explanation of this structural difference, see Thermoplastics vs Thermosets: Structure and Properties.

Why melting does not automatically mean degradation

There is another important distinction.

Melting changes the physical arrangement and movement of polymer chains. It does not necessarily break the covalent bonds along those chains.

Degradation involves chemical changes to the polymer, potentially including the breaking of covalent bonds.

Repeated heating, exposure to oxygen, moisture, and mechanical forces during processing can contribute to degradation.

This matters because a thermoplastic may be suitable for remoulding once but experience declining properties after repeated processing.

The difference between these two processes is explained further in How to Distinguish Polymer Degradation from Melting.

02The finished product may contain several different materials

Here’s where the original question becomes more interesting.

Imagine a flexible food package made from two bonded polymer layers:

• An outer PET layer.
• An inner PE layer.

The manufacturer has chosen these materials because different layers can provide different properties, such as stiffness, barrier performance, and heat-sealing capability.

The finished package works well for its intended purpose.

But what happens when it reaches the end of its useful life?

Both PET and PE are thermoplastics. A student might therefore predict that the whole package should be recyclable by simply heating it.

That prediction overlooks an important problem: different polymers do not necessarily have compatible processing conditions or form a useful homogeneous mixture.

Consider what happens in a PE recycling process.

The PE layer softens and flows under suitable processing conditions. The PET layer, however, may remain solid.

The PET does not magically disappear. It remains within the material as separate fragments or inclusions.

If the temperature is increased to process PET, the PE may be exposed to conditions that cause significant degradation.

Even if both materials could be softened, that would not guarantee that they would mix into a uniform material with suitable properties.

This is a problem of polymer compatibility and processing, not simply whether each polymer is a thermoplastic.

The Australian Packaging Covenant Organisation (APCO) specifically identifies PET as incompatible with PE and PP soft-plastic mechanical recycling pathways. Its guidance also highlights the problems created by incompatible secondary layers and other contaminants. This guidance applies to the specified soft-packaging recycling pathway, not every possible recovery technology. citeturn403260view0

Other components create similar problems

A plastic product might also contain:

• Adhesives that hold different layers together.
• Paper labels or other polymer labels.
• Pigments and inks that affect the colour of recycled material.
• Metal foil or other non-polymer components.
• Food residues and other contamination.
• Additives that change the material’s processing behaviour.

Not every additive makes a plastic unrecyclable. Some additives are compatible with a particular recycling process.

The issue is whether the complete mixture can be processed into a material with acceptable properties.

A clean, single-polymer product is generally easier to process than a contaminated mixture of several different materials.

03What actually happens during mechanical recycling?

To understand the practical problem, follow a discarded plastic product through a typical mechanical recycling pathway.

The process involves several stages.

Stage 1: Collection and sorting

Discarded products must reach an appropriate collection system.

The collected material is then sorted into suitable groups. Different polymers and product formats may need separate processing pathways.

If incompatible materials enter the same stream, they can contaminate the recovered material.

Stage 2: Washing and preparation

The plastic is cleaned to remove unwanted substances.

It is then shredded into smaller pieces, often called flakes.

The flakes may also need to be dried before further processing.

Moisture can be particularly problematic for some polymers because heating them in the presence of water can contribute to chemical degradation.

Stage 3: Melting and extrusion

The prepared material enters an extruder.

Inside the extruder, a rotating screw moves the polymer through a heated barrel. The material softens and flows through the machine.

Filters can remove some unwanted solid contaminants before the polymer exits through a die.

Stage 4: Pelletising and manufacturing

The extruded polymer is cooled and cut into small pellets.

These pellets can become feedstock for manufacturing new plastic products.

Notice how many things must go right before the polymer becomes useful recycled material.

The Australian Government’s national plastics flows report identifies clean, homogeneous or sortable feedstocks with low contamination as particularly suitable for mechanical recycling. Mixed-polymer materials can require separation or be directed towards a more limited range of products. citeturn845162view0

A polymer being thermoplastic is therefore only the beginning of the story.

04An experiment: does successful remoulding prove recyclability?

Consider an investigation designed to answer a specific question:

If a thermoplastic can be melted and reshaped in a laboratory, does that prove that a finished product made from it can be recycled?

The experiment needs to distinguish the behaviour of a single polymer from the behaviour of a more complicated product.

The following is an illustrative investigation, not a report of an actual industrial trial. Its numerical results are invented for teaching purposes.

The original experimental design

Two samples are prepared:

• Sample A: 50.0 g of clean, single-polymer HDPE.
• Sample B: 50.0 g of a bonded PE/PET laminate.

Both samples are dried and cut into small flakes.

Each sample is processed separately using a laboratory hot press under the same controlled conditions. The selected temperature and pressure are appropriate for processing the HDPE sample.

The material is placed inside a mould between heated plates. Pressure is applied, the material is allowed to soften, and the mould is cooled before the resulting specimen is removed.

The specimens are then tested using a tensile testing machine.

The machine grips each specimen at opposite ends and pulls it apart. The maximum force before failure can be used to calculate tensile strength.

\[ \sigma = \frac{F_{\max}}{A} \]

Where:

• \(\sigma\) is tensile strength, measured in pascals (Pa).
• \(F_{\max}\) is the maximum force before failure, measured in newtons (N).
• \(A\) is the original cross-sectional area, measured in square metres (m²).

The prediction might be that both samples will soften and form a uniform new material because both contain thermoplastic polymers.

But that prediction is not necessarily correct.

The observation

The following illustrative results are obtained.

Sample A forms a coherent strip because the HDPE can flow under the selected processing conditions.

Sample B behaves differently. The PE softens, but the PET-rich material remains as solid fragments. The different materials do not form one uniform polymer melt.

A single meaningful tensile strength value cannot be assigned to the resulting heterogeneous material using the same specimen method.

Why the original experiment was incomplete

The investigation has demonstrated something useful: a clean, single-polymer HDPE sample can be remoulded under suitable conditions.

However, it has not tested the entire recycling process.

Both samples were already separated from other waste. They were also cleaned and prepared before entering the hot press.

A real discarded product may not arrive in this condition.

The experimental design needs to be revised.

The revised investigation

A better investigation would begin with complete discarded products rather than pre-prepared polymer samples.

The products would first pass through a sorting stage. The accepted material would then be washed, dried, shredded, and processed through the same type of extrusion equipment used in mechanical recycling.

The investigation would measure:

• The mass accepted during sorting.
• The mass remaining after washing and preparation.
• The amount of usable material produced during extrusion.
• The mechanical properties of the recovered material.
• Changes in those properties after repeated processing cycles.

This revised design tests both the polymer’s reprocessing potential and the practical difficulties of recovering it from a finished product.

It also highlights an important scientific principle: a laboratory result obtained from a purified sample cannot automatically be applied to a complex waste stream.

05The same polymer can produce different bulk properties

There is one more complication.

Even when a recycling process successfully recovers a single polymer, the resulting material may not have exactly the same properties as the original.

This is because molecular structure, processing, and bulk properties are related, but they are not interchangeable.

Consider three levels of description.

A polymer’s bulk properties depend on more than the identity of its repeating units.

For example, two samples made from the same polymer can have different tensile strengths because their chains are arranged differently.

The following factors can affect the final material:

Chain length

Longer polymer chains generally have more opportunities for entanglement and interaction. If degradation causes chain scission, the resulting shorter chains may produce different mechanical properties.

Chain alignment

During processing, polymer chains can become oriented in particular directions.

This can increase strength along the direction of alignment, producing a material with different properties in different directions.

Crystallinity

Some polymer chains form ordered regions, while others remain in less ordered arrangements.

The proportion and distribution of these regions can affect stiffness, density, and other physical properties.

Processing defects

Air pockets, cracks, contamination, and poor bonding between different polymer phases can weaken a finished material.

This explains why successfully producing plastic pellets does not automatically mean the material is suitable for its original application.

A recycled polymer might be suitable for a lower-strength product but unsuitable for an application requiring greater tensile strength or impact resistance.

This is sometimes described as downcycling: recovering material for an application with less demanding performance requirements than the original.

It is also important not to assume that every recycled polymer is weaker. The outcome depends on the polymer, its processing history, contamination, and the properties required of the final product.

06How to decide whether a plastic is recyclable in practice

When analysing an unfamiliar plastic product, use the following sequence.

A useful Australian example: plastic identification codes

You may have seen plastic products carrying a numbered triangle.

These numbers identify the type of plastic resin. They do not guarantee that the entire product can be recycled through your local collection system.

For example, a product might contain an identifiable thermoplastic polymer but also have a construction or format that makes it unsuitable for a particular recycling facility.

The Australasian Recycling Label provides more useful information about the disposal requirements of individual packaging components. The City of Sydney also explains the distinction between resin identification codes and actual recycling acceptance. citeturn744795view0

The important point is that identifying a polymer is not the same as establishing a complete recycling pathway.

08Questions and solutions

Question 1

A recycling laboratory receives 80.0 g of clean, single-polymer polypropylene (PP).

After mechanical processing, 68.0 g of usable recycled polymer is recovered.

Calculate the mass recovery percentage. Explain why this result does not establish that every polypropylene product is practically recyclable.

Solution 1

The mass recovery is 85.0%, but this only describes the recovery of the particular sample under the specified conditions.

Mass recovery is calculated using:

\[ \text{Mass recovery} = \frac{\text{usable output mass}} {\text{initial feed mass}}\times100 \]

Substituting:

\[ =\frac{68.0}{80.0}\times100 \]

\[ =85.0\% \]

The laboratory successfully recovered 85.0% of the starting mass as usable polymer.

However, the sample was already clean and consisted of one polymer.

A finished polypropylene product might contain other polymers, adhesives, pigments, or contaminants. These could interfere with sorting or processing.

Therefore, the experiment establishes the recovery potential of this prepared sample, not the practical recyclability of every product containing polypropylene.

Question 2

A flexible packaging product contains 100.0 g of bonded polymer layers:

• 70.0 g polyethylene (PE).
• 30.0 g polyethylene terephthalate (PET).

A recycling facility has a mechanical recycling line designed to process PE.

Its supplied processing data indicate:

• PE flows within a processing range of 130-160°C.
• PET requires 245-260°C to melt.
• This particular PE grade begins significant degradation above 210°C.

The layers are bonded together, and the facility has no equipment for separating them.

A student claims that the facility can recover 70.0 g of PE simply by increasing the processing temperature until both polymers melt.

Explain why this prediction is incorrect. Identify a suitable change to the recycling process that could improve material recovery.

Solution 2

The facility cannot assume that it will recover 70.0 g of clean PE by simply increasing the temperature.

The problem is that the two polymers have incompatible processing requirements under the supplied conditions.

At 130-160°C, the PE can flow, but the PET remains solid.

Increasing the temperature to 245-260°C would allow the PET to melt, but this exceeds the temperature at which the supplied PE grade begins significant degradation.

The PE may therefore undergo chemical changes before the PET can be processed under suitable conditions.

There is another issue: even if both polymers could be softened, this would not guarantee that they would form a homogeneous mixture with suitable properties.

The layers are bonded together, so melting one layer does not automatically separate it from the other.

A possible improvement would be to introduce a suitable delamination or separation process before mechanical recycling.

The separated PE and PET could then be directed towards appropriate processing pathways, provided sufficient purity and recovery can be achieved.

The key distinction is that the composition of the original package does not equal the mass of each polymer that can actually be recovered.

Question 3

A recycling facility receives 250.0 kg of dry mixed plastic waste.

The following masses are recorded:

A student argues that the recycling process has a 90.0% overall recovery because 90.0% of the flakes entering the extruder become usable pellets.

Calculate:

1. The mass recovery at each processing stage.
2. The overall mass recovery from the original collection.
3. Whether the student’s conclusion is correct.

Solution 3

The overall mass recovery is 64.8%, not 90.0%. The student’s calculation uses the mass entering the extrusion stage rather than the original collection as the denominator.

Step 1: Calculate sorting recovery.

\[ \frac{200.0}{250.0}\times100=80.0\% \]

The sorting stage retains 80.0% of the original feed mass.

Step 2: Calculate washing and drying recovery.

\[ \frac{180.0}{200.0}\times100=90.0\% \]

The washing and drying stage retains 90.0% of the sorted material.

Step 3: Calculate extrusion recovery.

\[ \frac{162.0}{180.0}\times100=90.0\% \]

The extrusion stage retains 90.0% of the dry flakes entering it.

Step 4: Calculate overall recovery.

Use the final usable mass and the original collection mass.

\[ \text{Overall recovery} = \frac{162.0}{250.0}\times100 \]

\[ =64.8\% \]

Step 5: Evaluate the student’s claim.

The student has correctly calculated the extrusion-stage recovery but incorrectly applied it to the entire process.

The original collection also experiences losses during sorting and washing.

This demonstrates why recycling performance depends on every stage, not just the ability of the polymer to flow through an extruder.

Question 4

A manufacturer investigates how repeated mechanical recycling affects the tensile strength of HDPE.

The same polymer is processed through different numbers of recycling cycles. Each specimen is moulded into the same geometry and tested in the same direction under identical tensile testing conditions.

The following illustrative results are obtained:

A student concludes:

“Every time the HDPE melts, covalent bonds in its polymer chains must break. This is why its tensile strength decreases.”

Calculate the percentage decrease in tensile strength after four recycling cycles. Then evaluate the student’s explanation, distinguishing melting from degradation.

Solution 4

The tensile strength decreases by 25.0%, but the results do not prove that melting itself breaks covalent bonds.

Step 1: Identify the original and final tensile strengths.

Original tensile strength:

\[ \sigma_0=24.0\text{ MPa} \]

Tensile strength after four cycles:

\[ \sigma_4=18.0\text{ MPa} \]

Step 2: Calculate the percentage decrease.

\[ \text{Percentage decrease} = \frac{\sigma_0-\sigma_4}{\sigma_0} \times100 \]

\[ =\frac{24.0-18.0}{24.0}\times100 \]

\[ =25.0\% \]

The tensile strength has decreased by 25.0%.

Step 3: Evaluate the chemical explanation.

The student’s conclusion confuses melting with degradation.

Melting is a physical change. It allows polymer chains to move and rearrange without necessarily breaking their covalent backbones.

However, repeated mechanical processing can expose the polymer to heat, oxygen, and mechanical forces that contribute to chemical degradation.

For example, chain scission can break covalent bonds along the polymer backbone, producing shorter chains.

Shorter chains may have different entanglement behaviour and mechanical properties.

Step 4: Consider the limitations of the evidence.

The decrease in tensile strength is evidence that the material’s bulk mechanical performance has changed.

However, tensile strength alone cannot identify the molecular mechanism responsible.

Changes in crystallinity, chain orientation, void formation, or contamination could also contribute to the result.

Further investigation would be required to establish whether chain scission occurred.

The correct conclusion is that repeated processing is associated with declining tensile strength in this experiment, but the data do not prove that melting itself causes covalent bond breaking.

Question 5

A manufacturer wants to produce new HDPE storage crates using recovered plastic.

Each crate must be made from material with a tensile strength of at least 20.0 MPa.

The factory has one HDPE mechanical recycling line. It does not have equipment for separating bonded polymer layers.

Two waste streams are available.

Stream A: Previously processed HDPE

The factory receives 100.0 kg of clean, single-polymer HDPE that has already undergone three recycling cycles.

Processing produces 82.0 kg of usable recycled HDPE pellets.

The tensile strength of these pellets is 18.0 MPa.

The factory can blend these pellets with virgin HDPE. The following experimental blend data are available:

Stream B: Multilayer packaging

The factory also receives 100.0 kg of bonded PE/PET packaging containing:

• 60.0 kg PE.
• 40.0 kg PET.

The PE and PET are bonded together. The available recycling line has the same processing limitations described below:

• The PE can flow within its suitable processing range.
• The PET requires a substantially higher processing temperature.
• The PE begins significant degradation before the PET can be processed under suitable conditions.
• No layer-separation equipment is available.

Assume that blending Stream A with virgin HDPE causes no mass loss, that all tensile measurements are comparable, and that the factory requires at least 50% recycled polymer by mass in its final material.

Determine which waste stream can produce material meeting the crate specification using the current factory equipment. Your explanation must address:

• Whether Stream A can meet the tensile strength requirement.
• Whether Stream A can meet the minimum recycled-content requirement.
• Why Stream B’s 60.0 kg of PE is not automatically recoverable as 60.0 kg of clean PE.
• Why selecting the blend with the highest recycled content is not necessarily the correct decision.

Solution 5

Stream A can produce compliant crate material using a 50:50 recycled-to-virgin HDPE blend. Stream B cannot produce a suitable homogeneous HDPE feedstock using the current equipment.

The important point is that recyclability must satisfy both the processing requirements and the final product specifications.

Step 1: Assess Stream A without blending.

The recovered HDPE pellets have a tensile strength of 18.0 MPa.

The crate requires at least 20.0 MPa.

Therefore:

\[ 18.0<20.0 \]

The recycled pellets alone do not meet the required tensile strength.

A student who stops here might conclude that Stream A cannot be used.

However, the manufacturer has another option: blending the recovered HDPE with virgin HDPE.

Step 2: Evaluate the 50:50 blend.

The experimental data give:

\[ \sigma=21.0\text{ MPa} \]

This exceeds the required tensile strength:

\[ 21.0>20.0 \]

The blend also contains exactly 50% recycled HDPE by mass.

It therefore satisfies both requirements.

Step 3: Calculate the mass of compliant material that can be produced.

The factory has 82.0 kg of recycled HDPE pellets.

For a 50:50 blend, an equal mass of virgin HDPE is required.

\[ m_{\text{virgin}}=82.0\text{ kg} \]

The total blend mass is:

\[ m_{\text{total}}=82.0+82.0 \]

\[ =164.0\text{ kg} \]

The recycled content is:

\[ \frac{82.0}{164.0}\times100=50.0\% \]

The factory can therefore produce 164.0 kg of compliant material, containing 82.0 kg of recycled HDPE.

Step 4: Evaluate the 60:40 blend.

The second blend contains 60% recycled HDPE and 40% virgin HDPE.

Its measured tensile strength is only 19.5 MPa.

\[ 19.5<20.0 \]

It fails the crate specification despite containing a greater proportion of recycled material.

This exposes an important misconception: a higher recycled content does not automatically produce a better material for a particular application.

The mechanical properties of polymer blends are not necessarily proportional to the mass fractions of their components.

That is why the experimentally measured tensile strengths must be used rather than assuming a linear relationship.

Step 5: Evaluate Stream B.

Stream B contains 60.0 kg of PE, but this does not mean that 60.0 kg of clean PE can be recovered.

The PE and PET layers are bonded together.

At the PE processing conditions, the PE flows while the PET remains solid. The two materials therefore do not produce a homogeneous melt.

Increasing the temperature to process PET would expose the PE to conditions that cause significant degradation.

The factory also has no equipment for separating the two layers before processing.

Consequently, Stream B cannot provide a suitable homogeneous HDPE feedstock through the current mechanical recycling line.

A suitable delamination process, separate recovery pathways, or a redesign using compatible materials could change this outcome. However, any proposed method would need to be tested for polymer purity, recovery yield, and final mechanical properties.

Final decision: Stream A is the suitable choice under the current constraints. Although its recycled pellets do not meet the tensile strength specification independently, the measured 50:50 blend does. Stream B contains individually useful thermoplastic polymers, but its bonded construction prevents the available equipment from recovering a suitable HDPE feedstock.

This is the central lesson: being able to reprocess a polymer, recovering it from a finished product, and producing a material that meets a new product specification are three different achievements.

09Where this connects to next

The next useful step is to understand how processing changes the arrangement of polymer chains and therefore changes the properties of the finished material.

In particular, chain alignment can increase tensile strength in one direction without changing the chemical identity of the polymer.

That relationship is important when comparing virgin and recycled polymer products, especially when the recovered material is being considered for a new structural application.