Polyethylene, polypropylene, PTFE, and related polyolefin and fluoropolymer materials present a fundamental bonding challenge: their surfaces have very low surface energy, which means most adhesives cannot wet them properly. A structural adhesive applied directly to untreated polyethylene will bead up, fail to spread uniformly, and achieve a fraction of its strength on metal or glass substrates. Understanding the surface energy relationship explains why this happens and what surface activation approaches make reliable bonding possible.

Surface Energy and Adhesive Wetting

Adhesive bonding requires the adhesive to spread uniformly across the substrate surface and form intimate molecular contact. Whether an adhesive spreads depends on the surface energy balance: the adhesive must have lower surface tension than the substrate’s surface energy. When the substrate surface energy is below the adhesive surface tension, the adhesive cannot spread — it beads up on the surface rather than wetting it.

Surface energy is expressed in units of milliNewtons per meter (mN/m) or dynes per centimeter. Common values:
– Steel: 46–72 mN/m (high, good wetting)
– Glass: 70–80 mN/m (high, excellent wetting)
– Nylon (PA): 40–46 mN/m (moderate)
– Polyethylene: 31–35 mN/m (low)
– Polypropylene: 29–32 mN/m (low)
– PTFE: 18–20 mN/m (very low)

Most structural adhesives have surface tensions of 30–50 mN/m. Adhesive applied to PTFE at 18 mN/m cannot wet — the surface tension exceeds the substrate surface energy. Adhesive on polyethylene is marginal. The result is poor contact area, weak adhesion, and failure at the interface.

The Chemical Reason for Low Surface Energy

Low surface energy in polyolefins and fluoropolymers results from the chemical nature of their surfaces. Polyethylene and polypropylene surfaces consist of –CH₂– and –CH₃ groups — saturated hydrocarbon segments with no polarity, no hydrogen bond donor or acceptor sites, and only weak van der Waals interactions with other materials.

Fluoropolymers (PTFE, FEP, PVDF) replace hydrogen with fluorine. C–F bonds are highly non-polar, and the fluorine atoms shield the carbon backbone from external interaction. PTFE has the lowest surface energy of any solid polymer and resists adhesion from virtually all conventional adhesives without surface treatment.

These same chemical features that make polyolefins and fluoropolymers useful — chemical inertness, low friction, moisture resistance — are precisely what makes them difficult to bond.

How Poor Wetting Leads to Bond Failure

When an adhesive is applied to a low surface energy substrate and appears to bond (the adhesive cures and sticks initially), the joint typically has low initial strength and poor durability. Several failure mechanisms are active:

Low contact area. Even if the adhesive does not visibly bead, it wets the surface incompletely at the microscopic level, leaving un-bonded spots throughout the apparent contact area. Under load, stress concentrates at the bonded spots and the average stress is higher than the nominal joint area would suggest.

Weak interfacial bonds. On high-energy surfaces like metals, adhesives can form polar bonds, hydrogen bonds, or even covalent bonds with the substrate. On low-energy polyolefin surfaces, only weak van der Waals forces are available. These weak forces are overcome by low loads, producing low adhesive strength and peel resistance.

Moisture sensitivity. Joints relying solely on van der Waals forces at low-energy surfaces are very sensitive to moisture. Water has higher affinity for the adhesive’s polar groups than the non-polar polyolefin surface, and it readily displaces the adhesive from the surface at the interface.

Stress concentration at edges. Even when the center of a bonded area has adequate initial adhesion, the edges — where peel stress concentrates in overlap joints — peel away from the low-energy substrate under service loads. Once the edge disbonds, the peel front propagates rapidly through the remainder of the joint.

Email Us to discuss surface activation strategies for bonding low surface energy plastics in your application.

Surface Activation Methods

Achieving strong, durable bonds to low surface energy plastics requires surface activation — modifying the substrate surface to introduce polar functional groups, increase surface energy, and provide sites for adhesive bonding.

Flame treatment — passing the plastic surface through an oxidizing flame for a fraction of a second creates polar oxidized groups (C=O, –OH, –COOH) on the surface through thermal oxidation. Surface energy increases to 40–50 mN/m within seconds of treatment. Flame treatment is widely used for polyethylene and polypropylene in automotive applications. The treatment is not permanent — surface energy decreases over time as polar groups are buried by chain reorientation — so bonding must occur promptly after treatment.

Plasma treatment — atmospheric or low-pressure plasma with air, oxygen, or nitrogen gas oxidizes and activates the surface similarly to flame treatment but with more uniform and controllable results. Plasma treatment achieves higher surface energies than flame treatment and is preferred for complex geometries and precision applications. Like flame treatment, it is not permanent.

Corona discharge — an electrical discharge in air at the substrate surface generates ozone and reactive oxygen species that oxidize the polymer surface. Corona treatment is the standard industrial process for activating polyolefin films before printing or coating, and it works well for thin film bonding applications. Less suitable for three-dimensional parts than plasma or flame.

Chemical etching — chromic acid etching of polyethylene and polypropylene, and sodium-naphthalene complex treatment of PTFE, create heavily oxidized, roughened surfaces with high surface energy and good adhesion. Chemical etching produces more durable activation than flame or plasma and is used in aerospace and high-performance bonding applications. Chromate solutions involve hazardous chemicals requiring controlled handling.

Primers — adhesion promoter primers, including chlorinated polyolefin primers and specialized silane-based primers for polyolefins, create chemically compatible layers between the adhesive and the low-energy substrate. Primers are widely used for polyolefin bonding in automotive and consumer products because they are simple to apply and do not require specialized equipment.

Mechanical abrasion — sanding or abrading low-energy plastics increases surface roughness, providing mechanical interlocking sites for the adhesive. Abrasion alone is less effective than chemical or plasma activation for improving adhesion quality but can be used in combination with activation methods.

Adhesive Selection for Activated Low Surface Energy Substrates

After surface activation, adhesive selection should prioritize:

• Low viscosity (for better wetting of the activated surface)
• Flexibility (to accommodate differential thermal expansion between plastic and metal substrates)
• Compatible cure chemistry (some cure systems are inhibited by polyolefin surfaces even after activation)

Modified acrylic adhesives (structural acrylics, methacrylate adhesives) are particularly effective on activated polyolefins because of their ability to wet and bond to the activated surface and their flexible cured properties.

Incure’s Solutions for Challenging Substrates

Incure offers adhesive systems and compatible primers for bonding low surface energy plastics. Technical guidance on surface activation selection and qualification supports reliable bonding of polyolefin and fluoropolymer materials.

Contact Our Team to discuss bonding challenges with low surface energy plastics and identify Incure products appropriate for your substrate and application.

Conclusion

Adhesives fail on low surface energy plastics because non-polar polyolefin and fluoropolymer surfaces cannot be wetted by conventional adhesives, resulting in poor contact area and only weak van der Waals bonding at the interface. Surface activation through flame, plasma, corona, chemical etching, or primers is required to introduce polar groups and increase surface energy to levels where adhesive wetting and bonding are effective. Promptly bonding after activation and selecting compatible adhesive chemistries ensures that the surface energy benefit translates into durable joint performance.

Visit www.incurelab.com for more information.