Surface Energy
Key Takeaways
- Surface energy decides whether a liquid wets or beads — it is the single most useful predictor of printing and bonding success.
- The liquid must be lower in surface tension than the solid, by a working margin of roughly 7–10 dyne/cm.
- Polyolefins sit around 29–31 dyne/cm, below almost every ink and adhesive, which is why they need pretreatment.
- Treatment decays. Surface energy is a temporary state, not a permanent property of the part.
Surface Energy
Surface energy is the excess energy held by molecules at a solid surface compared with those in the bulk, and on plastics it determines whether an ink, adhesive or coating spreads across the surface or draws up into beads. It is conventionally reported in dynes per centimetre (dyne/cm), numerically equal to millinewtons per metre.
Molecules inside a solid are surrounded on all sides and their attractive forces are satisfied in every direction. Molecules at the surface are not, so they carry unsatisfied bonding capacity. That unsatisfied capacity is what a liquid can interact with, and a surface with little of it — a low energy surface — gives a liquid nothing to hold onto.
The Rule That Governs Wetting
For a liquid to wet a solid, the surface tension of the liquid must be lower than the surface energy of the solid. Where it is higher, the liquid minimises its contact area and beads up. In practice a margin is needed rather than mere equality:
Target surface energy ≈ liquid surface tension + 7 to 10 dyne/cm
This single relationship explains most adhesion failures on plastics. A water-based ink at
around 35 dyne/cm applied to untreated polypropylene at 29 dyne/cm cannot wet it, and no
amount of drying time, pressure or ink reformulation changes that arithmetic. The surface
has to be raised first.
Typical Values by Polymer
| Material | Untreated surface energy | Implication |
|---|---|---|
| PTFE and fluoropolymers | About 18–20 dyne/cm | The lowest in common use; deliberately non-stick and very difficult to bond |
| Silicone | About 24 dyne/cm | Also a contaminant — airborne silicone lowers the energy of whatever it lands on |
| Polypropylene | About 29 dyne/cm | Below every common ink and adhesive; pretreatment effectively mandatory |
| Polyethylene | About 31 dyne/cm | As polypropylene; slip additives lower it further in service |
| ABS | About 35–42 dyne/cm | Moderate; pretreatment often about consistency and cleaning rather than raising energy |
| Polycarbonate | About 42–46 dyne/cm | Generally printable and bondable as moulded |
| PET | About 41–44 dyne/cm | Film usually still treated, to 46–52, for reliable ink anchorage |
| Nylon | About 40–46 dyne/cm | Hygroscopic, so readings vary with ambient humidity |
| Clean glass and metals | Several hundred dyne/cm | Wet readily — which is why contamination, not energy, is their adhesion problem |
Raising Surface Energy
Every industrial pretreatment works by the same basic route: oxidising the top few
molecular layers so that polar oxygen-containing groups are grafted onto a non-polar
hydrocarbon backbone. The methods differ in geometry, cost and durability rather than in
principle:
- Corona discharge — the standard for film and web at high speed and low cost per square metre.
- Flame plasma — suited to moulded and contoured parts with enough thermal mass to tolerate it.
- Atmospheric plasma — selective, robot-mountable and low in thermal load, for three-dimensional parts.
- Cold gas plasma — batch vacuum process reaching shadowed geometry, with the most durable result and the ability to tailor surface chemistry.
- Pyrosil — deposits a reactive silicate layer rather than activating the existing surface, giving a far longer usable window.
Measuring It
| Method | What it gives | Use it for |
|---|---|---|
| Dyne pens | A pass/fail against one value | Fast shop-floor checks. Convenient, but the ink dries out and pens age, so results drift. |
| Wetting tension solutions (ASTM D2578) | A bracketed dyne value | The standard production check. More reliable than pens; still a subjective read of whether the film holds for two seconds. |
| Contact angle goniometry | A quantitative angle, and with multiple liquids the polar and dispersive components | Process validation, qualification and troubleshooting. The method to use when a number has to be defended. |
| Water break test | Presence or absence of continuous wetting | A quick qualitative screen for gross contamination. It does not give a value. |
Measure across the part or the web width rather than at a single point. Edge-to-centre
variation is common, easily missed, and a frequent cause of a failure that appears random.
Why Treatment Does Not Last
Raised surface energy is a temporary state. Two mechanisms pull it back down, and both
run from the moment treatment stops:
- Chain reorientation. The grafted polar groups are attached to mobile
polymer chains, which rotate to bury them below the surface where they are more
energetically comfortable. Warmth accelerates this. - Additive migration. Slip agents, antiblock additives, plasticisers and
residual mould release migrate to the surface and cover the treated layer. On polyolefin
film this is usually the larger effect, and it is why a formulation change can shorten the
usable window without anything on the treater changing.
The practical consequence is that the interval between treating and bonding belongs in
the process specification with a stated maximum. Treated stock that has aged should be
re-measured before use, not assumed good.
Related Terms and Reading
- Contact angle
- All polymer surface pretreatment methods
- The science of solving plastics adhesion problems
- Adhesion failures
- Inkjet printing and adhesion of low surface energy polymers
Applying this in production
The Sabreen Group provides independent engineering support for surface energy measurement, pretreatment selection and adhesion troubleshooting. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What surface energy does a plastic need before printing or bonding?
Enough to exceed the surface tension of the liquid by roughly 7 to 10 dyne/cm. For most inks and adhesives that means 38–44 dyne/cm on polyolefins and 46–52 on PET film, but the figure should come from a trial with the actual chemistry rather than a generic target. A specification quoting an industry-standard number that nobody has tested against the real ink is a specification waiting to fail.
Why do polyolefins need pretreatment when other plastics do not?
Because polypropylene and polyethylene sit at roughly 29–31 dyne/cm, below essentially every common ink and adhesive, so no wetting occurs at all. Polycarbonate, ABS and PET start in the 35–46 range, above or close to many liquids, so they can often be printed or bonded as moulded. The chemistry that makes polyolefins inert and cheap is the same chemistry that makes them unprintable.
Is a dyne pen reading reliable enough for production control?
For a quick check, yes; for anything that has to be defended, no. Pen inks dry out and age, and the read is subjective. Wetting tension solutions to ASTM D2578 are the standard production method, and contact angle goniometry gives the quantitative value needed for validation or for settling a dispute with a customer. Whichever is used, measure across the width rather than at one point.
Why does adhesion fail when the dyne reading is correct?
Almost always contamination. Pretreatment activates whatever is on the surface, so a layer of mould release, slip additive bloom or compressor oil gets activated in place of the polymer, and a dyne pen will read that activated contaminant quite happily. Raising the treatment level does not help. Trace the contaminant to its source, which is frequently an upstream process rather than the treatment station.
How long does surface treatment last?
Hours to weeks, depending on the polymer, its additive package and storage conditions. Polymer chains reorient to bury the grafted polar groups, and slip agents, plasticisers and mould release migrate to the surface and cover them. Elastomers and plasticised grades decay fastest. Establish a maximum treat-to-bond interval empirically, put it in the process specification, and re-measure aged stock rather than assuming it is still good.
