How Silicone PSA Works
Pressure-sensitive adhesives (PSAs) bond instantly under finger pressure without solvent activation or heat. Silicone PSAs achieve this through a polymer blend: a high-molecular-weight polydimethylsiloxane (PDMS) gum supplies the viscoelastic flow and peel strength, while an MQ or MDT silicone resin provides tackiness and cohesive strength. Neither component alone delivers a functional PSA—the synergy of gum plus resin is essential.
This silicone-based architecture outperforms acrylic PSA above 150 °C, in contact with medical skin, and in applications exposed to UV or aggressive solvents. The tradeoff is cost: silicone PSA is 3–6× more expensive than acrylic on a dry-film basis.
MQ Resin: The Tackifier
MQ resin contains only M units [(CH₃)₃SiO₀.₅] and Q units [SiO₂], with no D or T units. This highly branched, cage-like structure (Mw 3,000–8,000 g/mol) acts as a tackifier: pendant methyl groups on the M units create the low surface energy and wettability that generate tack. Key parameters:
- M:Q molar ratio: 0.6–1.0 (higher M = softer, tackier; lower M = harder, more cohesive)
- Solubility: soluble in aliphatic and aromatic solvents; typically supplied as 50–70 wt% solution in xylene or VM&P naphtha
- Functionality: most MQ resins carry residual SiOH groups that participate in crosslinking
MDT Resin: Balancing Tack and Cohesion
MDT resin adds D units [(CH₃)₂SiO] and T units [CH₃SiO₁.₅] to the M and Q units. The D units introduce linear PDMS-like segments that improve compatibility with the PDMS gum; the T units add crosslink density and cohesive strength. MDT resin is preferred when high shear resistance is required alongside moderate tack—typical in protective film and semiconductor wafer dicing tape.
Typical Formulation
| Component | Role | Typical Range |
|---|---|---|
| PDMS gum (HMW, Mw >500k) | Peel strength, viscoelastic flow | 40–60 wt% of dry film |
| MQ or MDT resin | Tack, cohesion | 40–60 wt% of dry film |
| Platinum catalyst | Crosslinking (addition cure) | 5–20 ppm Pt |
| Crosslinker (SiH-functional) | Network control | 0.5–2.0 wt% |
| Solvent (xylene / naphtha) | Coating viscosity | to 30–50% solids |
MQ:PDMS ratio of 40:60 produces a soft, high-tack adhesive (medical tape). A 60:40 ratio yields a stiffer, high-cohesion adhesive (protective film, electronics).
Cure and Processing
Peroxide cure (legacy): benzoyl peroxide or 2,4-dichlorobenzoyl peroxide at 150–180 °C. Simple but leaves residues that can discolor substrates. Still used in medical silicone tape.
Platinum-catalyzed addition cure (dominant): vinyl-terminated PDMS gum + SiH crosslinker + Pt catalyst. Cure at 100–150 °C in 1–3 min on a web coater. No by-products. Inhibited by sulfur, tin, and nitrogen compounds—substrates and equipment must be free of these.
Key Performance Targets
| Property | Typical Range | Test Method |
|---|---|---|
| Tack (loop tack) | 20–80 N/25 mm | PSTC-16 |
| Peel strength (180°) | 5–40 N/25 mm | PSTC-101 |
| Shear resistance (1 kg, 25×25 mm) | >10,000 min | PSTC-107 |
| Service temperature | −40 to 200 °C | — |
| Medical grade | USP Class VI / ISO 10993 | — |
Silicone PSA vs. Acrylic PSA
| Property | Acrylic PSA | Silicone PSA |
|---|---|---|
| Max service temp | ~120 °C | 200 °C+ |
| UV / ozone resistance | Moderate | Excellent |
| Biocompatibility | Moderate | Excellent (USP VI) |
| Low-surface-energy adhesion | Poor–Moderate | Excellent |
| Cost | Low | 3–6× acrylic |
Related Reading
MQ resin product page for grade specifications and M:Q ratios. MDT resin for high-cohesion PSA resin grades. Silicone PSA formulation guide for blending ratios and coating process parameters.