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 without visible beading, the adhesive wets the surface incompletely at the microscopic level, leaving un-bonded spots across the apparent contact area — so under load, stress concentrates at the bonded spots and the effective stress runs higher than the nominal joint area suggests.

Weak interfacial bonds. High-energy surfaces like metals allow polar, hydrogen, or even covalent bonds to form; low-energy polyolefin surfaces offer only weak van der Waals forces, which low loads readily overcome, producing poor strength and peel resistance — a mechanical shortfall that shows up directly in ASTM D1002 lap shear testing.

Moisture sensitivity. Joints relying solely on van der Waals forces are highly moisture-sensitive — water has greater affinity for the adhesive’s polar groups than the non-polar substrate does, and it readily displaces the adhesive at the interface.

Stress concentration at edges. Even when the joint center shows adequate initial adhesion, the edges — where peel stress concentrates in overlap joints — disbond first under service loads, and the peel front then 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 through an oxidizing flame for a fraction of a second creates polar oxidized groups (C=O, –OH, –COOH) via thermal oxidation, raising surface energy to 40–50 mN/m within seconds. It is widely used for polyethylene and polypropylene in automotive applications, but the effect is not permanent — surface energy decays as polar groups are buried by chain reorientation — so bonding must occur promptly after treatment, following the same surface-cleanliness verification principles that apply to any prepared substrate.

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

Corona discharge — an electrical discharge in air generates ozone and reactive oxygen species that oxidize the polymer surface. Corona is the standard industrial process for activating polyolefin films before printing or coating, though it suits thin films better than three-dimensional parts.

Chemical etching — chromic acid etching of polyethylene and polypropylene, and sodium-naphthalene treatment of PTFE, create heavily oxidized, roughened, high-energy surfaces. Etching produces more durable activation than flame or plasma but involves hazardous chemicals requiring controlled handling.

Primers and mechanical abrasion — chlorinated polyolefin and silane-based primers create chemically compatible layers between adhesive and substrate without specialized equipment, while sanding or abrading adds mechanical interlocking sites. Abrasion alone is less effective than chemical activation but works well combined with it.

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. Where the assembly also runs hot in service, the primer compatibility of the chosen activation layer needs separate qualification, since a primer suited to room-temperature bonding may soften well below the adhesive’s own service temperature.

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 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, etching, or primers introduces polar groups and raises surface energy to levels where wetting is effective. Bonding promptly after activation, and pairing it with the cleaning discipline used for any other substrate, ensures the surface energy benefit translates into durable joint performance.

Visit www.incurelab.com for more information.