Plasma treatment: how to bond PP, PE and PTFE for good?
Non-polar plastics such as PP, PE and PTFE are almost impossible to bond without pre-treatment because their surface energy is too low. This guide explains how plasma and flame treatment lift surface energy past the critical 38 mN/m mark, which processes exist and how to verify the result.
View bonding productsWhy do adhesives fail to stick to PP, PE and PTFE?
Polyolefins such as polypropylene (PP) and polyethylene (PE) and fluoropolymers such as PTFE have a very low surface energy. An adhesive can only wet a surface if its own surface tension is lower than the substrate's surface energy. If it is not, the adhesive beads up and the joint fails.
Untreated PP sits around 29 mN/m, PE around 31 mN/m and PTFE at just about 18 mN/m. Common adhesives and coatings, however, need a surface energy of at least 38 mN/m to wet reliably and form a durable adhesive bond.
- PTFE: about 18 mN/m - especially difficult, often needs chemical etching.
- PP: about 29 mN/m - readily activated by plasma or flame.
- PE: about 31 mN/m - responds reliably to pre-treatment.
- Target after treatment: durably above 38 mN/m, ideally 42 to 46 mN/m.
Which pre-treatment processes are available?
The most common processes are atmospheric plasma, low-pressure plasma and flame treatment. All three oxidise the surface and introduce polar groups such as carbonyl and hydroxyl groups, which raise the surface energy and improve wetting.
Atmospheric plasma works without a vacuum directly on the line and treats a spot in seconds. Low-pressure plasma reaches shadowed areas and fine structures evenly inside the vacuum chamber. Flame treatment passes a sharply tuned, oxygen-rich flame over the surface in under a second and suits large parts such as bumpers.
How do you measure and secure the result?
The result is checked through surface energy. In practice you use test inks or dyne pens to ISO 8296 that just form a closed film at a defined surface tension. If the ink pulls into droplets within two seconds, the surface energy is below that value.
- Test inks from 30 to 56 mN/m in steps for a fast check.
- Contact-angle measurement as the exact lab reference for surface energy.
- Carry out treatment and bonding on the same day where possible.
- Document treatment parameters, distance and speed.
Frequently asked questions
Why is 38 mN/m the target value?
Because common adhesives have a surface tension around 33 to 36 mN/m. Only when the plastic's surface energy sits above that at 38 mN/m or more does the adhesive wet stably and form a durable bond.
How long does the treatment effect last?
It depends on the plastic. PP and PE age back within hours to a few days as polar groups migrate into the bulk. That is why bonding should follow as soon as possible.
Can PTFE be bonded with plasma alone?
PTFE is especially difficult. Atmospheric plasma helps, but for durable strength a chemical etch with sodium-naphthalene or a specialised low-pressure plasma is often required.
What is the difference between plasma and flame?
Both oxidise the surface and raise the surface energy. Plasma uses ionised gas and suits small, delicate parts, while flame treatment is faster for large flat areas.
Bonding plastics reliably?
We support pre-treatment, test inks to ISO 8296 and the right adhesive for PP, PE and PTFE.
Measurably verified
Surface energy checkable with test inks to ISO 8296.
Process-safe
Defined parameters for reproducible joints.
Durable strength
An adhesive bond instead of beading adhesive.
Expert advice
Specialists guide pre-treatment and adhesive choice.