5 Causes of Odor in Recycled Plastics and How to Reduce It

Recycled plastics are increasingly used in packaging, automotive components, construction products, consumer goods and industrial applications. However, one problem continues to limit the amount of recycled material that manufacturers can incorporate into new products:

Odor.

A recycled polypropylene (rPP) or recycled polyethylene (rPE) may have acceptable mechanical properties but still produce an unpleasant smell during extrusion, molding or in the finished product.

The problem is particularly common with post-consumer recycled (PCR) plastics, where the material has already been exposed to food, detergents, cosmetics, chemicals, labels, adhesives and other contaminants.

Research on recycled HDPE and other recycled polyolefins has identified numerous volatile organic compounds (VOCs) originating from previous product contents, polymer degradation and residual additives.

Understanding where these odors come from is the first step toward reducing them.

Here are five of the most common causes of odor in recycled plastics and practical ways manufacturers and recyclers can control them.

1. Contamination From the Plastic's Previous Use

One of the largest sources of odor in post-consumer recycled plastics is contamination from whatever the packaging contained during its first life.

Examples include:

• Detergents

• Shampoo and cosmetics

• Food residues

• Cooking oils

• Cleaning products

• Fragrances

• Household chemicals

Polyolefins such as HDPE, LDPE and PP can retain small amounts of these substances, even after the plastic has been collected and washed.

For example, compounds associated with fragrances and cleaning products have been detected in recycled HDPE after mechanical recycling. Some volatile compounds can remain present even after regranulation.

How can this odor be reduced?

Improving the quality of the incoming recycling stream is extremely important.

Possible strategies include:

• Better sorting of post-consumer plastics

• Separating food packaging from detergent or chemical containers

• More effective washing

• Hot washing where appropriate

• Improved rinsing

• Removing labels, adhesives and residual contents before extrusion

The cleaner and more consistent the incoming material is, the easier it becomes to manufacture low-odor recycled plastic compounds.

2. Polymer Degradation During Recycling

Odor does not always come from external contamination.

Sometimes, the polymer itself produces odor during processing.

Recycled plastics may experience several thermal histories:

Original manufacturing → product processing → recycling extrusion → compounding → final product manufacturing

Every heating cycle can contribute to polymer degradation, particularly when processing conditions are severe.

During thermal or oxidative degradation, polymers can generate low-molecular-weight compounds such as:

• Aldehydes

• Ketones

• Organic acids

• Hydrocarbons

• Other volatile organic compounds

Some of these compounds have noticeable odors even at relatively low concentrations.

Studies examining recycled PP and HDPE have found hydrocarbons and other volatile compounds associated with polymer degradation and processing.

How can degradation-related odor be reduced?

Recycling and compounding conditions should be carefully controlled.

Important parameters include:

• Processing temperature

• Residence time

• Shear

• Oxygen exposure

• Number of recycling cycles

• Stabilizer package

Using an appropriate antioxidant and processing stabilizer system can also help reduce further polymer degradation during extrusion.

This is especially important when the recycled material has already experienced significant thermal aging.

3. Residual Organic Matter and Microbial Contamination

Food packaging and household plastic waste may contain very small amounts of organic material.

If the waste is stored for long periods before washing or recycling, this contamination can degrade biologically and create strong odors.

Possible sources include:

• Food residues

• Oils and fats

• Organic liquids

• Moisture

• Microbial activity

Research on odor-active substances in post-consumer plastics has associated some undesirable odors with contamination and microbiological spoilage processes.

Even when most visible contamination is removed, some odor-active molecules may remain on the surface or inside the plastic.

How can this odor be reduced?

Good waste handling is critical.

Recyclers can consider:

• Reducing storage time before washing

• Keeping contaminated feedstock dry where possible

• Improving washing efficiency

• Controlling washing temperature

• Improving drying after washing

• Preventing cross-contamination during storage

A good recycling process therefore starts before the material enters the extruder.

4. Labels, Inks, Adhesives and Additives

A plastic package is rarely made from polymer alone.

It may also contain:

• Printing inks

• Labels

• Adhesives

• Coatings

• Pigments

• Stabilizers

• Processing aids

• Fragrances

• Other additives

During recycling, these components can enter the recycled polymer stream.

Some may produce volatile compounds during extrusion, while others may degrade during repeated processing.

Research on mechanically recycled HDPE has identified odor-related contamination associated with inks, adhesives, additives and residues from previous applications.

This explains why two recycled PP or PE materials with similar melt flow rates can have completely different odors.

Their contamination history may be very different.

How can this odor be reduced?

Better separation is again important.

Possible solutions include:

• Improved label removal

• Better washing

• More selective feedstock sourcing

• Avoiding highly contaminated recycling streams

• Melt filtration

• Degassing

• Deodorization

• Odor-control additives during compounding

For high-quality recycled compounds, controlling the source of the waste is often just as important as optimizing the extrusion process.

5. Residual VOCs That Survive the Recycling Process

One of the most important causes of recycled plastic odor is the presence of volatile organic compounds, or VOCs.

Mechanical recycling does not necessarily remove all volatile and semi-volatile contaminants.

A scientific review of recycled plastics identified hundreds of volatile compounds across different recycled polymer streams, with post-consumer materials generally containing a wider range of compounds associated with fragrances, previous use and degradation.

More recent research on recycled HDPE similarly shows that VOCs can remain after conventional recycling and contribute to persistent odor.

How can VOCs be reduced?

Depending on the polymer and application, several technologies may be used:

Vacuum degassing

Removing volatile compounds from the polymer melt during extrusion can significantly improve recycled material quality.

Thermal deodorization

Controlled heating can help volatile contaminants migrate out of the polymer.

Steam stripping

Steam-based processes have been investigated as a method for removing VOCs from post-consumer recycled HDPE.

Improved washing and extraction

More aggressive purification can remove contaminants before extrusion.

Odor-control additives

Odor scavengers or adsorbent-based masterbatches can help reduce residual odor when process improvements alone are insufficient.

In many applications, the best solution is a combination of feedstock control, optimized recycling conditions and an appropriate additive system.

Why Odor Control Matters for Recycled PP and PE

Odor can significantly restrict where recycled polymers can be used.

A material may perform well mechanically but still be rejected for applications such as:

• Automotive interiors

• Household products

• Consumer packaging

• Storage containers

• Furniture

• Appliances

• Closed indoor environments

This creates an important challenge for companies trying to increase recycled content.

Instead of simply asking:

“How much recycled plastic can we add?”

a more useful question is:

“How can we improve the quality of the recycled material so that we can use more of it?”

Odor control can be an important part of that strategy.

How to Choose the Right Odor-Reduction Strategy

There is no universal solution for every recycled polymer.

Before selecting an odor-control additive or changing processing conditions, manufacturers should first identify the likely source of the odor.

For example:

Odor Problem	Possible Approach
Food or detergent residues	Better sorting and washing
High VOC content	Degassing or deodorization
Polymer degradation	Improve processing conditions and stabilization
Ink or adhesive contamination	Better separation and washing
Residual odor after processing	Odor-control or scavenger masterbatch
Odor becoming worse after extrusion	Check temperature, residence time and antioxidant package

In some cases, simply increasing the amount of an odor scavenger is not the best solution.

The formulation and the recycling process should be considered together.

Reducing Odor in Recycled Plastics With Polymatica

At Polymatica Chemicals, we work with plastic recyclers, compounders and manufacturers to improve the performance and usability of recycled polymer materials.

For recycled PP and PE, odor-control solutions such as Polymatica ReFresh™ O and ReFresh™ OC can be incorporated into the formulation as part of an odor-reduction strategy.

When degradation is also contributing to material quality problems, stabilization can be considered alongside odor control.

Polymatica can also support companies with:

• Recycled polymer formulation development

• Additive selection

• Odor-reduction strategies

• Antioxidant and stabilization systems

• Recycled-content optimization

• Processing troubleshooting

• Development of customized polymer compounds

The objective is not simply to mask an unpleasant smell. It is to understand where the odor is coming from and develop a practical solution for the specific recycled material and process.

Have an Odor Problem in Your Recycled Plastic?

If your recycled PP, PE or PCR compound has persistent odor, the source may be contamination, degradation, residual VOCs — or a combination of several factors.

Polymatica can help evaluate the problem and develop an odor-control strategy for your formulation and processing conditions.

Contact Polymatica Chemicals to discuss your recycled plastic application.

Website: www.poly-matica.com

Frequently Asked Questions About Odor in Recycled Plastics

Why does recycled plastic smell?

Recycled plastic can smell because of residual product contamination, food or detergent residues, VOCs, polymer degradation, inks, adhesives, additives and other substances that remain in the material after recycling.

How do you remove odor from recycled plastic?

Odor reduction may involve better sorting, washing, drying, melt degassing, thermal deodorization, processing optimization and odor-control additives. The correct method depends on the source of the odor.

Why does recycled polypropylene smell?

Recycled polypropylene can contain contaminants from its previous application and can also generate volatile degradation products during repeated thermal processing.

Why does recycled polyethylene have an odor?

Recycled polyethylene, particularly post-consumer HDPE and LDPE, can retain VOCs originating from food, detergents, cosmetics, fragrances and other products that were previously in contact with the plastic.

Can additives reduce odor in recycled plastics?

Yes. Odor-control and scavenger masterbatches can be used to reduce residual odors in recycled polymers. For the best results, additives should be combined with proper feedstock selection and optimized recycling and processing conditions.

Can recycled plastic odor be completely eliminated?

It depends on the contamination level, polymer, recycling history and end-use requirements. Some materials can achieve very low odor levels, while highly contaminated PCR streams may require several purification and formulation strategies.