What chemicals are used in release paper?

What chemicals are used in release paper?
Silicone-coated release liners include two- and three-component systems for pressure-sensitive tapes and labels. Silicone-coated release papers are a range of silicones designed to be coated on one or both sides of a paper or film substrate.

Properties and Chemistry of Silicone Release Coatings
Silicones are uniquely positioned for use as release coatings due to their application and curing properties, as well as their low release forces compared to other materials.

In today's modern environment, silicone release films with pressure-sensitive adhesives have a wide range of applications, from release labels to diaper closures, medical applications such as wound dressings, building insulation and health and beauty products.

The release film is part of a composite material consisting of a label and its own adhesive, with the label's face material facing the coating on the release film. The release film can be easily peeled off or labels can be easily transferred from the release film to the object to be labeled.

In its simplest form, a release coating is a liquid or solid that creates a weak boundary layer between two substrates that may interact. This boundary layer prevents close contact and thus the development of bond strength. Any transfer of release agent should not prevent the use of the release surface.

There are several chemical types of materials used as release coatings, such as polyacrylates, urethanes, polyolefins, fluorocarbons, chromium stearate compounds, and silicones. Silicones are unique in that they can be applied to a variety of substrates and cured into a polydimethylsiloxane (PDMS) network, thereby limiting migration; they can also significantly reduce release forces compared to other materials.

Properties of optimal release characteristics
One of the key properties of silicone is its low surface tension. This is a result of low intermolecular forces and high chain flexibility. Unlike more rigid carbon-carbon backbones, PDMS polymers can easily expose their low-interacting/surface-active methyl groups due to their backbone flexibility and being well above their Tg at room temperature, thus providing low adhesion; In other words, low peel force against the adhesive in contact.

The adhesives used on labels cannot easily wet this low-energy silicone surface because there are no interacting groups. This makes the labels easy to delaminate and easy to transfer from the liner to the point of use.

But low surface energy is not the only aspect to consider. Even fluorocarbons, despite having lower surface energy than silicones, do not have the same mold release properties as silicones. Another critical factor is the rheological behavior of the cured PDMS network coated on the backing. This rheological behavior contributes to the development of interfacial slip in the system, which plays a key role in the low release values observed when peeling pressure-sensitive adhesives from silicone release-coated liners.

Slip (low friction) is thought to be the main reason why PDMS has lower peel forces than fluorocarbon polymers with lower surface energy (but higher friction).

Chemical Properties of Silicone Release Coatings
Silicone release coatings are generally divided into four different categories:

a. Solvent type

b. Solvent-free

c. Water-based emulsion

d. UV curing

The above four categories, except for the ultraviolet curing system, all adopt the addition curing mechanism for curing, in which the base polymer reacts with the cross-linking agent in the presence of a catalyst.

• Curing of silicone polymers is an important process for the following reasons:
• Prevent silicone migration
• Improve the bonding strength of the release coating to resist the pulling force of the adhesive layer
• Improve adhesion to substrate layer
• Promotes rapid drying
• Cure Chemistry
• To achieve cross-linking of silicone release coatings, the most common chemical method is through hydrosilylation.