Channeled Resources Group analyzes release liner design

The goal of release liner design is to ensure that the liner peels smoothly away from the adhesive surface with the right amount of force.  A zippy release, however, occurs when the liner inconsistently sticks and releases during the peel, causing a sound that is similar to a zipper (also known as chatter). This jerking movement can cause problems during application, and in some cases, indicates the beginning of processes that lead to lock up and complete release failure.

There are two main contributors to zippy release: mechanical and chemical issues. Mechanical problems are caused by poor silicone coverage, which could be due to the silicone application method or equipment. Chemical issues can arise from under-cured silicone reacting with the adhesive or complications from the silicone formulation itself.

When sourcing pressure sensitive labels, it’s helpful to understand how each piece of the structure interacts with others. The relationship between adhesives and silicone release coatings is complex. Working with an experienced release liner supplier will help converters avoid a zippy release and achieve consistent liner performance.

Adhesives Come First

The type of adhesive used in a laminate structure is typically determined by its end use, and each application has its own unique requirements. Adhesives can be permanent, repositionable, or removable. There are a variety of adhesive technologies available, from skin-safe adhesives designed for wound care to bonding tapes used in automotive assemblies. Some of the most common adhesive types include acrylic-, rubber-, and epoxy-based formulas.

Types of Silicone Coatings

There are two types of silicones divided by curing mechanism. UV-cured coatings, which can be cationic or free radical, use UV light to initiate the curing reaction. These were initially developed for application on film substrates that were more susceptible to heat-curing methods. UV-cured silicones offer advantages for some applications, although they tend to offer less versatility and control in terms of release levels.

Thermal-cured silicones are cured in ovens. Thermal silicones also have several categories, including solvent-based, which are less commonly used today due to environmental regulations, emulsion or water-based, and 100% solids. Coatings with 100% solids do not have a diluent, and these types of silicones are the most versatile across coating equipment and end-use applications. With a wide variety of available additives and a significant amount of R&D investment, much of the release liner market is moving toward 100% silicone formulations.

There are many considerations that go into selecting the right silicone for a release liner.

Chemistry Considerations

A little chemistry lesson is necessary to understand how silicone behaves. Silicone is a polymer, meaning it is made up of long repeating chains of molecules. Its backbone consists of silicon and oxygen atoms.

Functional groups, which are specific groups of atoms that are reactive, are attached either at the ends (end blocked) or at various places throughout the chain (multifunctional). This molecular architecture is selected based on the desired properties of the silicone. The functional groups have important implications for how silicones cure and how they can potentially react with different types of adhesives.   

Crosslinkers are chemical agents that are designed to bond with functional groups. In the case of silicones used for release liners, hydrides on the crosslinker bond with the vinyl groups on the silicone polymer. Problems can start to occur if there is an excess of un-crosslinked hydrides after curing.

If there are too many hydrides left in the silicone, which could be caused by using too much crosslinker, the functional groups found in some types of adhesives can react with the silicone. For example, an acrylic crosslinking adhesive could begin to react with the silicone’s extra hydrides, resulting in an increased release level. This reaction between residual hydrides in a silicone and functional groups within an adhesive is a major contributor to age-up, or the increased difficulty to peel a release liner that can occur over time.

For this reason, time is another critical factor in release liner design. The longer the structure sits after application of the adhesive, the greater the risk of age-up and chemical reactions between the silicone and adhesive.

Silicone Curing

Excess hydrides can occur in fully cured silicone, as outlined above, but under-cured silicone can also result in un-crosslinked functional groups. If the silicone is not fully cured, some groups can still remain free. A reactive adhesive with functional groups of its own could bond to those sites on the uncured silicone, causing a tighter release. The main symptom of under-cured silicone is often a zippy release.

Silicone coaters can employ a variety of tactics to help ensure proper cure. One option is to add more crosslinker, but that can lead to an abundance of extra hydrides, increasing the risk for zippy release and age-up. Another avenue is to add more platinum catalyst to help the silicone cure faster, but that route is very costly.

When formulating the silicone for a specific release liner, there is a balancing act between the type and amount of crosslinker and the right amount of platinum that will produce a full cure reaction.

Silicone Coverage

While silicone formulation and curing can produce zippiness, inconsistent coverage is another major factor. Mechanical problems could be caused by a coat weight that is too low, which can result in coating defects and streaks.  

The equipment used to apply the silicone coating to the substrate can also have an impact. Direct gravure coating equipment utilizes rollers with laser-etched cells to transfer the silicone to the substrate, which can leave gravure lines or gaps in the coating surface. Knife-over-roll coaters use a knife blade to scrape off excess silicone after it passes over a roll. This method can be difficult to calibrate with the very low coating weights used for release liners, potentially resulting in less coat weight control and more variability from side to side.

Smooth roll coating is recognized as an efficient application for silicone coatings. As its name implies, it uses smooth rollers to evenly deposit silicone across the entire web, and roll coaters are typically constructed with four, five or six rolls. The ratio of the roll speeds in this type of coater are used to precisely control coat weight as the web progresses through the machine.

Application Method

The way the labels are removed from the release liner affects the severity of zippy release. Slower peel speeds tend to make zippiness more noticeable, while higher peel speeds can reduce its effects.

Release force must also match the requirements of the application. When the release is too tight, zippiness becomes more pronounced. Controlled Release Additives (CRAs) can be added to silicone formulations to increase release levels, but the excessive use of CRAs can cause inconsistent release, particularly in applications that would benefit from an easier release level.  

To formulate an effective release liner, silicone coaters need a wide breadth of expertise that ranges from silicone and adhesive chemistry to label and tape product uses. A release liner that performs consistently throughout storage and end-use applications is the result of managing all of these variables.

About CRG

Channeled Resources Group combines deep expertise in silicone technology, adhesive interactions, and application processes to formulate custom release liner solutions tailored to each application. By evaluating the complete laminate structure as well as its intended use, the company helps customers minimize zippy release and deliver high-performance products. 

CRG helps its customers create tapes and labels that perform as expected every time, backed by technical knowledge and a commitment to quality customer service.

This story originally appeared here on the Channeled Resources Group website.