How Silicone Release Coatings Help Pressure Sensitive Adhesive Functionality
Silicone release coatings are used in PSA applications to allow easy delamination or PSA label transfer from liners. Silicone is used for their application and curing properties, as well as lower release forces when compared to other materials.
A wide range of applications utilize silicone release liners with pressure-sensitive adhesives, ranging from release labels to diaper closures, medical applications (e.g., wound dressings), building insulation, and health and beauty products.
Release liners are part of a composite made of a label with its own adhesive on a facestock facing the release coating on a liner. The release coating allows easy delamination or easy label transfer from the liners onto the object to be labeled.
In its simplest form, a release coating is a liquid or solid that forms a weak boundary layer between two potentially interacting substrates. This boundary layer prevents intimate contact and the resulting development of adhesive strength. Any transfer of release agent should not hinder use of the separated surfaces.
There are several chemical types of materials that can be used as a release coating — such as polyacrylates, carbamates, polyolefins, fluorocarbons, chromium stearate complexes, and silicones. Silicones enjoy a unique position because they can be applied and cured into a polydimethylsiloxane (PDMS) network on various backing substrates,limiting migration; They also allow substantially lower release forces than other materials.
One of the key properties of silicone is its low surface tension. This is a consequence of low intermolecular forces and high chain flexibility. Unlike more rigid carbon-carbon backbones, PDMS polymers, because of their backbone flexibility and because at room temperature they are substantially above their Tg, can easily expose their low interacting/surface active methyl groups to provide low adhesion; or in other words, low release forces against adhesives they are exposed to.
Adhesives used on labels cannot easily wet such a low-energy silicone surface, as there are no groups to interact. This results in ease of delamination and ease of transfer of the label from the liner to its point of use.
But low surface energy is not the only aspect to consider. Even fluorocarbons, despite a lower surface energy than silicones, do not match silicone release performance. Another key component is the rheological behavior of the cured PDMS network applied onto the backing substrate. This rheological behavior helps in developing interfacial slippage in the systems, which plays a key role in the low release values observed on the release of pressure-sensitive adhesives from silicone-release-coated liners.
A high degree of slippage (low friction) is cited as the predominant reason PDMS has lower release force than the lower surface energy (but higher friction) fluorocarbon polymers.
Source: Adhesives Magazine | Texochem