Hydrophobic Recovery

Hydrophobic Recovery

Key Takeaways

• Treated surfaces do not stay treated. A treated polymer drifts back toward its untreated surface energy from the moment treatment stops.
• Four mechanisms drive it: polar-group reorientation into the bulk, migration of oxidised fragments, additive bloom and airborne contamination.
• Decay is fast, then slow. Most of the loss comes in the first hours to days; the plateau after that is what the ink or adhesive meets.
• The window is a specification item. Polyolefins hold an ink-grade level for hours to days, PET, PC and ABS for weeks to months, silicone rubber for minutes to hours.

Hydrophobic Recovery

Hydrophobic recovery is the progressive loss of surface energy on a plasma-, corona- or flame-treated polymer surface after treatment, as the polar groups created by oxidation are buried, covered or diluted and the surface drifts back toward its original low-energy, water-repellent state. Also called treatment decay or aging of treatment, it is why a dyne level measured at the treater is not the level the ink or adhesive will see.

Treatment may lift a polyolefin from 29–31 to 44–56 dyne/cm. Recovery rarely takes it all the way back; it settles at a plateau between the two, and that plateau against the level the process needs sets the usable window.

Mechanisms

Four processes run in parallel; which dominates depends on polymer, additive package and storage.

Kinetics and Typical Windows

Plotted as surface energy against log time, recovery is a steep fall followed by a plateau. On corona-treated polypropylene roughly half the gain above the untreated level can disappear within 24–72 hours at room temperature before the curve flattens and may hold for weeks. The rate follows chain mobility: polypropylene (Tg below 0 °C), polyethylene and silicones are rubbery at room temperature and reorient freely; PET (Tg about 70–80 °C), ABS (about 105 °C) and polycarbonate (about 147 °C) are glassy and reorientation is nearly frozen.

Measuring the Decay

Characterise recovery by measuring wettability on production material under production storage at 0, 1, 4, 24 and 72 hours, then 7, 14 and 28 days.

• Wetting tension solutions to ASTM D2578 give a bracketed value in 2 dyne/cm steps, fast enough for the full schedule.
• Sessile drop contact angle (ASTM D5946 for corona-treated film, or a goniometer with water and diiodomethane) gives a continuous number and, with two liquids, isolates the polar component, which is the part that decays. Dyne pens detect decay but cannot map it.

Measure three positions per sample, since treatment varies edge to centre, and log treater settings, resin lot, storage temperature and packaging with every point. The surface energy calculator checks a dyne level or contact angle pair against the margin the ink or adhesive needs.

Factors That Shorten or Extend the Window

Specifying and Verifying a Treatment Window

Recovery cannot be switched off, so the process is designed around it: inline treatment (corona discharge on film, atmospheric plasma or flame plasma on moulded parts, seconds before the ink or adhesive), a treat-to-bond window for batch treatment with a maximum interval taken from the decay curve, or re-treatment of aged stock, which reverses reorientation but not additive bloom.

The specification should then state:

• Required surface energy at the point of use, for example 42 dyne/cm minimum by ASTM D2578 or water contact angle below 60°, not at the treater.
• Maximum treat-to-bond interval, no more than half the time at which the measured curve crosses that requirement.
• Storage envelope: temperature range, packaging, no contact with untreated polyolefin, corrugated board or silicone.
• Verification: one measurement per shift and before using stock older than 24 hours.

How long plasma surface treatment lasts applies this to a moulded-part line; surface pretreatment covers method selection.

Troubleshooting

Related Terms and Reading

• Surface energy
• Contact angle
• Pyrosil flame plasma
• Cold gas plasma
• Accelerated aging
• Plasma surface pretreatments of polymers for improved adhesion bonding

Applying this in production

The Sabreen Group provides independent engineering support for pretreatment window qualification, decay curve measurement 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

How fast does hydrophobic recovery happen after plasma or corona treatment?

Fastest in the first hours. On corona-treated polypropylene about half the gain above the untreated 29 dyne/cm can be lost within 24–72 hours, after which the curve flattens into a plateau that may hold for weeks. Silicone rubber recovers in minutes to hours; glassy PET, polycarbonate and ABS take weeks to months.

Does the surface return completely to its untreated state?

Usually not. Reorientation buries polar groups without removing them, and cross-linking from the treatment restrains the surface, so most polymers plateau above the untreated value. Whether that plateau is usable depends on the liquid: 38 dyne/cm on polypropylene suits some solvent inks and fails a 42 dyne/cm water-based ink.

Can aged stock simply be treated again?

If reorientation is the dominant mechanism, yes: re-treatment rotates the polar groups back out and the surface behaves like fresh stock. If additive bloom is the cause, re-treatment activates the bloom rather than the polymer and the level collapses again within hours. Solvent-wipe one sample before re-treating; if only that sample holds, bloom is the problem.

How should a treat-to-bond window be written into a specification?

Measure the decay curve on production material under production storage, find where it crosses the level the ink or adhesive needs, and set the maximum interval at half that time. State the level at the point of use, the test method (ASTM D2578 or water contact angle), storage temperature and packaging, and a verification check per shift.

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