The Role of Release Liners
Release liners are substrate sheets—paper or plastic film—coated with a thin silicone layer that temporarily bonds to a pressure-sensitive adhesive but releases cleanly when peeled. They protect the adhesive surface during storage and handling, then separate from it during label dispensing, tape unwinding, or wound-dressing application. The silicone coating must provide a precisely controlled release force: too low causes premature release; too high causes tearing of the liner or adhesive delamination.
Two Silicone Coating Chemistries
Solventless addition-cure platinum silicone (dominant)
The modern industry standard. A two-part system: vinyl-terminated PDMS (base) + SiH-functional crosslinker + Pt catalyst. Applied at 1–3 g/m² by multi-roll gravure or direct gravure coating, then cured at 130–160 °C in a forced-air oven in 1–5 seconds (web speed 200–600 m/min). No VOC, no solvent recovery needed, lowest energy cost. Coat weight is tightly controlled—variation of ±0.2 g/m² changes release force significantly.
Solvent-based condensation methyl silicone resin (legacy)
Methyl silicone resin dissolved in xylene or toluene (10–20% solids), applied at 0.5–2.0 g/m² and cured at 150–200 °C. Higher cure temperature makes it better suited for glassine paper and polycoated kraft substrates. Still used in high-temperature release liner applications and where precise anchor-coat control is needed.
Release Force Engineering
Release force (peel force at 300 mm/min, 180° angle) is tunable from 5 cN/25 mm (ultra-low, easy-peel) to 100 cN/25 mm (high-release for heavy labels):
| Lever | Effect on Release Force |
|---|---|
| Increase MQ resin content in formulation | Increases release force (tighter adhesive contact) |
| Increase crosslink density (more SiH crosslinker) | Increases release force |
| Reduce coat weight | Decreases release force (thinner silicone film) |
| Add bath-life extender / inhibitor | No force effect; extends pot life at room temp |
Subsequent Adhesion Retention (SAR) — the adhesive's retained tack after liner removal — must be ≥80% for most label specifications and ≥90% for medical dressings. Low SAR indicates silicone migration into the PSA layer.
Substrates and Primer Systems
| Substrate | Primer Needed | Typical Liner Application |
|---|---|---|
| Glassine paper (60–80 g/m²) | Usually not | Label facestock liners, tape liners |
| Polycoated kraft paper | Anchor coat (SiOH-functional) | Foodservice release, bakery paper |
| PET film (12–36 µm) | Anchor coat (Ti or Al chelate) | Electronic component tape, medical |
| PE film (50–75 µm) | Corona treatment + anchor coat | Agricultural mulch film, stretch wrap |
Anchor coats prevent "rub-off" — the catastrophic failure where the silicone layer delaminates from the substrate instead of releasing from the PSA.
Quality Testing (FINAT Standards)
| Test | Method | Typical Specification |
|---|---|---|
| Release force | FINAT FTM 10 | 5–100 cN/25 mm (target-specific) |
| Subsequent adhesion retention | FINAT FTM 11 | ≥80–90% |
| Residual adhesion | FINAT FTM 3 | ≥80% |
| Silicone coat weight | Fluorescence / XRF | ±0.2 g/m² tolerance |
| Cure completeness (MEK rub) | FINAT FTM 5 | ≥50 double rubs |
Key Market Segments
- Label facestock liners: largest volume; glassine and SCK paper, both faces coated for matrix stripping
- Medical dressing and surgical tape liners: USP Class VI silicone required; release force ≤20 cN/25 mm for easy patient removal
- Silicone-free food release paper: butter wrappers, interleaving paper, bakery parchment (no silicone; fluorochemical or wax alternatives)
- Electronic carrier tape (EIA-481): PET film liner for SMT component tape-and-reel packaging
- Protective film liners: PE or PET film protecting screen glass, stainless steel, painted aluminum panels
Related Reading
Silicone release coating product page for addition-cure and condensation systems. Silicone resin category for MQ resin and binder grades used in release formulation. Silicone oil category for low-viscosity PDMS used in solventless release bath preparation.