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How to Improve Bond Strength on Difficult Substrates (PP, PE and More)
Achieving a strong, reliable bond isn’t always straightforward. While many materials are relatively easy to bond, others present a significant challenge due to their surface properties.
Polypropylene (PP) and polyethylene (PE) are two of the most common examples, but they’re far from the only difficult substrates manufacturers encounter.
If you’re struggling to achieve consistent adhesion, understanding why these materials are difficult to bond is the first step towards finding a solution.
Why Are Some Materials Difficult to Bond?
For an adhesive to create a strong bond, it must first be able to wet the surface of the material.
Materials with a high surface energy, such as metals, glass, and wood, generally allow adhesives to spread and bond effectively.
Materials with a low surface energy do the opposite. Adhesives tend to bead up rather than spread evenly, resulting in weaker adhesion and an increased risk of bond failure.
Common low surface energy materials include:
- Polypropylene (PP)
- Polyethylene (PE)
- Thermoplastic olefins (TPO)
- Certain fluoropolymers
- Silicone-based materials
These substrates are widely used because they are durable, lightweight, and cost-effective, but they can be notoriously difficult to bond.
Understanding Polypropylene (PP) and Polyethylene (PE)
PP and PE are among the most widely used plastics in manufacturing.
They can be found in:
- Packaging products
- Automotive components
- Consumer goods
- Medical devices
- Storage containers
- Industrial equipment
Their resistance to chemicals and moisture makes them attractive materials, but these same properties make it difficult for many adhesives to achieve a reliable bond.
As a result, manufacturers often assume these materials cannot be bonded effectively. In reality, they can… but success depends on selecting the right process.
Improve Surface Preparation
When bonding difficult substrates, surface preparation becomes even more important.
Contaminants such as dust, grease, oils, mould-release agents, and fingerprints can further reduce adhesion performance.
Before bonding:
- Clean surfaces thoroughly
- Remove oils and contaminants
- Ensure substrates are completely dry
- Avoid touching prepared surfaces before application
Even minor contamination can significantly impact bond strength.
Consider Surface Treatments
One of the most effective ways to improve adhesion on low surface energy plastics is to modify the surface itself.
Surface treatments increase surface energy, allowing adhesives to spread more effectively.
Common methods include:
Corona Treatment
A high-voltage electrical discharge alters the surface chemistry of the material, improving wettability and adhesion.
This technique is commonly used in packaging and film production.
Plasma Treatment
Plasma treatment achieves a similar result but offers greater precision and control.
It is widely used in automotive, electronics, and medical manufacturing.
Flame Treatment
A carefully controlled flame briefly oxidises the surface, increasing surface energy and improving adhesive performance.
This method is often used for larger plastic components.
Use Primers Where Appropriate
Primers can act as a bridge between the substrate and the adhesive.
For difficult plastics, a suitable primer can dramatically improve bond strength and consistency.
Benefits include:
- Improved wetting
- Enhanced durability
- Greater long-term reliability
- Increased resistance to environmental factors
However, primer selection is critical and should always be matched to both the substrate and adhesive system.
Select an Adhesive Designed for Low Surface Energy Plastics
Not all adhesives are capable of bonding PP and PE effectively. Many general-purpose products simply aren’t formulated for these materials.
When selecting an adhesive, consider:
- Substrate compatibility
- Environmental conditions
- Required bond strength
- Production speed
- Flexibility requirements
Modern adhesive technologies have significantly improved the ability to bond traditionally difficult plastics, often eliminating the need for mechanical fastening in certain applications.
Don’t Forget Joint Design
Even the best adhesive can struggle if the joint itself isn’t designed appropriately.
When bonding low surface energy materials:
- Maximise bonding area where possible
- Reduce peel stresses
- Design for shear loading where appropriate
- Ensure consistent adhesive coverage
Good joint design can significantly improve overall performance and durability.
Test Before Full Production
One of the biggest mistakes manufacturers make is assuming a solution that works in one environment will perform identically in another.
Variables such as temperature, humidity, loading conditions, and substrate variations can all affect bond performance.
Before implementing a new bonding process:
- Conduct application-specific testing
- Evaluate long-term performance
- Assess environmental exposure
- Validate production consistency
Testing helps identify potential issues before they become costly production problems.
Final Thought: Difficult Doesn’t Mean Impossible
Materials such as polypropylene and polyethylene have a reputation for being difficult to bond, and for good reason.
However, with the right combination of surface preparation, treatment methods, adhesive selection, and process control, strong and reliable bonds are entirely achievable.
The key is understanding the material and choosing a bonding solution designed specifically for the challenge.
Need Help Bonding Difficult Materials?
At Anglo Adhesives, we work with manufacturers across a wide range of industries to solve complex bonding challenges.
Whether you’re working with polypropylene, polyethylene, composites, or other difficult substrates, our team can help identify the right adhesive and application process for your requirements.
Contact our team to discuss your application.