In a micron- and millisecond-based world, you are constantly adjusting to the demands of your business. The facestock or adhesive formulation is often the focus of attention in the highly-staked arena of the self-adhesive labels manufacturing. You know that the unnoticed hero in determining the accuracy of die cuts or the speed of an entire labeling line is the release coat.
The chemical interface is the key to the performance of Pressure-Sensitive Adhesive. The surface chemistry of the release liner is critical to success, whether you’re running a matrix removal process at high speed or designing a transdermal medical patch. You can get broken webs if the release force of your product is too high. If it’s too low, then you may have problems with pre-dispensing.
The complexity of silicon and non-silicone system will be dismantled. The Rheology and cross-linking mechanism will be discussed in detail. Here is the blueprint to mastering interfaces.
Release Coating is a coating that prevents rust.
A Release Coating, a chemical treatment applied to the surface of a substrate, is designed to reduce or eliminate the force needed to release a pressure sensitive adhesive.
The system works by lowering the Surface Energy on the liner, preventing permanent adhesion while maintaining the adhesive’s tackiness upon transfer. The chemical integrity is preserved until application.
The Chemistry of Thermal and Radiation Curing Radiation Curing Systems
It is important to consider the cost of your chemistry, but also the compatibility of it with the substrate you are using and any specific requirements for the application. These are generally classified into thermal and radiation-cured system.
Thermal Solventless Silicones
The workhorse for the industry. Thermal solventless systems, which account for 70-80% global sales, are based on Polydimethylsiloxane. Mixing a vinyl functional polymer and a crosslinker hydride, which is catalyzed with platinum complex.
It is the mechanism of “Addition cure” that makes this system so special. There are no residual products, unlike the old condensation-cure systems. Bath Life must be managed strictly. The clock begins to tick once the platinum is added. To ensure complete cross-linking, you should operate your oven between 120degC – 160degC.
Solvent-Based Systems
In an environmentally conscious era, why use solvents? Solventless curing is not suitable for some films, such as ultra-thin PE or PET. Solvents reduce the viscosity allowing extremely thin coatings to be applied (as low as 0.3gsm), and create a network resistant to Silicone transfer.
UV Curing and EB Curing
You can use UV or EB to cure heat-sensitive thermal paper or film. We will distinguish here between Free Radicals and Cationic Systems.
Free Radical UV: Quick curing, but sensitive to oxygen inhibitors. In order to maintain oxygen levels under 50ppm, you need an inert Nitrogen chamber.
Cationic: Cure continues after light source is removed (dark cure). This product offers better adhesion on difficult substrates, but it is susceptible to moisture and alkaline additions to paper.
Substrate interaction and hold-out
The substrate is the foundation of any discussion about coatings. Your liner’s “Holdout” ability determines the amount of silicone that stays on top vs. diving into fibers.
Glassine SCK (Super Calendered Kraft).
The glassine surface is densified mechanically to make it non-porous. The density of the glassine is important. Glassine that is porous will allow silicone to penetrate, leaving behind a surface “starved”. The result is erratic “Lock-up” and release values. For consistent die-stripping, you need to have a smooth profile of the caliper.
Poly-Coated Kraft
For applications requiring dimensional stability, like wide-format graphics, you use PCK. This polyethylene layer serves as a barrier. You must make sure that the coating extrusion has the right surface tension in order to adhere with silicone. You will immediately fail the rub-off test if you do not.
Filmic Liners
The ultimate in smoothness is achieved by films, which gives the label a “Clear on Clear” look. A rough liner will create a dull surface because the adhesive reflects the topography of the paper. The PET liner produces a surface that is similar to glass, increasing clarity. To anchor silicone to PET, you need specific primers and inline corona treatments.
Release Physics: Critical Performance Metrics
Don’t rely solely on data sheets. To ensure performance on the converter’s presses, you need to control and measure specific physical parameters.
Release force profiles (Rheology).
The release is not static; instead, it’s a curve that depends on the peeling speed.
Slow Release (0,3 m/s): Important for hand dispensing and application.
Release at High Speed (100-300m/s): Important for Automatic Labeling Lines (matrix Removal).
This is managed by strong>High-Release Additives (HRAs)/strong> or Control Release Agents (CRAs). Control Release Agents or High-Release Agents (HRAs). The resins in these compounds disrupt the silicone networks to create a flat release profile at all speeds.
The Subsequent Adhesion Strength is the strength of adhesion that occurs after the initial adhesive force.
It is a good indicator of contamination. This test measures how much adhesive is retained in the product after it has been exposed to the release liner. A SAS of >85% is the target.
When your SAS falls below 60%, this means that unreacted siloxanes of low molecular weight are migrating to the adhesive from the liner. The tack is destroyed. The silicone migration phenomenon is the primary reason for label failures in the field.
Anchorage and Rub off
If yes, can you remove the silicone with your thumb. You have anchorage failure if you answer yes. It can happen when the catalyst absorbs into the substrate (poisoning), or the moisture in the paper disrupts cure. A Non-Nutrient Index test can be used to verify whether the substrate you are using is compatible.
Process Control: Coating Techniques
Precision cannot be compromised. The standard coating weight for silicone thermal is between 0.8 and 1.3 gsm. A difference of 0.2 grams can affect the release profile.
Multi-Roll Coaters (5-Roll / 6-Roll)
Standard for silicones without solvent. Different roll speeds are used to get a uniform, thin film of silicone using these systems. It is important to control both the roll temperature and the pressure of nip. If you do not manage the heat generated by friction, (shear-heat), it can cause your bath to gel prematurely.
Gravure Coating
Preferable for emulsion and solvent systems. Gravure cylinder cell volume determines coat weight. This cylinder provides good transverse uniformity, but it is not flexible enough to adjust coat weights quickly without having to switch cylinders.
Problem-solving in the Field
Things go wrong fast when they do. How to diagnose production failures.
The “Zipper Effect”
Symptoms: Loud, jerky sounds during stripping. The web vibrates.
Cause of the Slip-stick Phenomenon: An aggressive release profile when driving at high speeds.
The solution: Reformulate the product with a flatter profile of rheology. To stabilize the high-speed release, increase the cross-linking density or change the ratio of CRA/HRA.
Silicone Lock Up
Symptom Label will not come off without tears.
Cause of the problem: Most often humidity-induced reaction after cure or cross-linker that has not reacted bonding with adhesive.
Solution Verify the silicone curing completeness by using an extraction test. Check that your liner wasn’t stored at high humidity.
Ghosting
Symptom The facestock displays a pattern that matches the texture of the release liner.
Cause: Differences in coating or curing weights transferring an imprint on the adhesive.
Improvement of “lay-flat characteristics” in the coating. Inconsistent silicone application may be caused by worn out metering rollers.
Comparison of Curing Systems
When selecting the best system, you must balance energy costs, substrate sensitivities, and performance needs.
| Feature | Thermal Solventless | Thermal Solvent-Based | UV Free Radical | UV Cationic |
|---|---|---|---|---|
| Cure Mechanism | Heat (Pt Catalyst) | Heat (Pt Catalyst) | UV Light (Photoinitiator) | UV Light (Acid Catalyst) |
| Substrate Stress | High (140°C+) | Medium (Evaporation) | Low (Cool Cure) | Low (Cool Cure) |
| Inerting Required | No | No | Yes (Nitrogen <50ppm) | No |
| Anchorage Difficulty | Low | Low | Medium | Low (Post-cure helps) |
| Capital Cost | Moderate | High (Solvent Recovery) | Moderate | Moderate |
| Typical Application | Volume Label Stock | Specialty Films, Medical | Thermal Liner, Film | Variable Info Printing |
Non-Silicone Future Trends and Sustainability
Industry is looking for alternative solutions. Coatings that do not release silicone and are based on fluorochemicals or carbamates have gained popularity for electronics applications in which the outgassing of silicone is forbidden.
Liner recycling represents the next frontier. To meet the modern sustainability requirements, you need to reduce the thickness of your coating (downgauging) as well as use release coatings that are compatible with the repulping process.
FAQs
How do addition cure silicone and condensation cure silicon differ?
The industry standard is addition cure (Platinum), which produces no harmful byproducts, and allows for a fast low temperature curing. The condensation (Tin), which releases alcohol or moisture during the reaction and causes shrinkage, is no longer used for high-speed coatings.
What is the effect of coating thickness on release force?
In general, higher weight coatings (above 1,2 gsm), provide better coverage on the surface and smoother release. This is especially true for porous paper. A heavy coating can increase costs and cause migration. It is important to find the weight of the coating that will provide continuous coverage, often called “critical coat weight.”
What is the importance of controlling Platinum (Pt), catalyst ratio?
The platinum is responsible for the reaction. The silicone will not cure properly if the ratio (50ppm) is too low. This can lead to migration or rub-off. It’s not good to have it too high. You waste money, and the pot life is reduced. The silicone will gel up in the machine trough.
How can I print on the release coating directly?
The surface energy of standard silicone is very low, which makes it hard for inks and other materials to adhere. Specific “printable releases” have higher surface energies or texturizing components to enable backside printing.
What can cause “blocking or confusion” of the release liner rolls?
When the silicone is rolled in a wound form, the backside of a liner can stick to it. It is caused either by a lack of silicone curing or by combining high tension with heat to cause the layers physically fuse.
