Silicone release coatings are essential in a wide range of modern industrial and consumer applications, especially where pressure-sensitive adhesives (PSAs) are involved. These include release labels, diaper closures, wound dressings, building insulation materials, and health and beauty products.

Understanding the Role of Release Liners

A release liner is part of a composite system comprising:

• A facestock (e.g., label material)

• A pressure-sensitive adhesive

• A liner coated with a release agent

The release coating facilitates clean and easy delamination, allowing the adhesive label or film to be transferred smoothly from the liner to the desired surface.

What Is a Release Coating?

At its core, a release coating is a liquid or solid layer that forms a weak interfacial boundary between two surfaces. This weak interaction prevents bonding and ensures that adhesive materials can be easily removed when needed. Ideally, release coatings should not transfer to the substrate or compromise the adhesive's performance after separation.

Why Silicone?

Release coatings can be made from various chemical classes, such as:

• Polyacrylates

• Carbamates

• Polyolefins

• Fluorocarbons

• Chromium stearate complexes

• Silicones

Among these, silicones are uniquely advantageous due to their ability to:

• Form Polydimethylsiloxane (PDMS) networks upon curing

• Exhibit exceptionally low release forces

• Offer minimal migration

• Adhere to diverse substrates

Properties for Optimal Release Performance

Low Surface Tension

Silicones have inherently low surface energy due to:

• Low intermolecular forces

• High chain flexibility

• Surface presentation of methyl groups

These properties hinder adhesive wetting and bonding, facilitating easy label removal.

Superior Interfacial Slippage

Even though fluorocarbons have lower surface energy, silicones outperform them due to their superior rheological behavior and slippage characteristics, which result in lower friction and easier release.

Types of Silicone Release Coatings

Silicone release coatings are available in four primary types:

1. Solvent-Based

2. Solvent-Free (100% solids)

3. Water-Based Emulsions

4. UV-Curable Systems

All but the UV-curable variants rely on addition-cure chemistry, involving the reaction of a silicone base polymer with a crosslinker in the presence of a catalyst.

Curing Mechanism: Hydrosilylation

The most widely used cure method is hydrosilylation, an addition reaction between:

• A Si-H functional polymer

• A vinyl or hexenyl-functional polymer

Reaction Catalysts

• Typically catalyzed by platinum

• Two key systems:

  • Vinyl-functional PDMS

  • Hexenyl-functional PDMS (faster curing due to less steric hindrance)

Inhibitors for Bath Life

To prevent premature curing, inhibitors such as:

• Acetylenic alcohols (e.g., 1-ethynyl-1-cyclohexanol)

• Maleate/fumarate esters
  are used to extend pot life while balancing cure speed.

Selection Considerations for Silicone Coatings

Several parameters must be considered when selecting a silicone release system:

Application Process Factors

• Coating equipment type (e.g., gravure, smooth roll)

• Substrate compatibility

• Cure temperature and humidity

• Environmental regulations (preference for solventless or emulsion systems)

Adhesive and Substrate Compatibility

• Plastic liners (e.g., PE, PP, PET) pose challenges due to:

  • Low surface energy

  • Additives that inhibit curing

• Proper formulation must consider surface treatments or additives to improve anchorage.

Formulation Guidelines

Crosslinker

• Typical loading: 0.25–1.2% by weight

• Higher levels accelerate cure but may reduce bath life or increase adhesive interaction

Catalyst (Platinum)

• Loading: up to 0.5%, or 1.0% for low-temperature applications

• Excess platinum may cause bath instability and acrylic lock-up

• Highly sensitive to contamination from elements like nitrogen, sulfur, tin, etc.

Anchorage Additives

• For paper substrates: 0.8–2.0% by weight to improve adhesion

Process Solvents

• Compatible solvents include ethyl acetate, toluene, hexane, heptane, and white spirits

• Solvents must not poison the catalyst or inhibit cure

Preventing Cure Inhibition

To avoid cure inhibition:

• Ensure all equipment, substrates, and solvents are free of contaminants (e.g., sulfur, amines, tin)

• Store and handle catalysts and inhibitors carefully to maintain formulation integrity

Conclusion

Silicone release coatings remain the industry benchmark for high-performance release liners in pressure-sensitive adhesive systems. Their low surface energy, excellent slippage, and customizable cure chemistries make them indispensable across medical, packaging, hygiene, and industrial applications.

Proper formulation, substrate compatibility, and cure process optimization are crucial to realizing the full benefits of silicone release coatings in high-performance converting environments.