If your siliconizing procedure fails, you can have the best adhesive formula and most advanced face stock in your warehouse but your product will be nothing more than scrap. The release liner in the self-adhesive sector is treated like waste, but it’s actually the main carrier for label adhesion.
If you examine the construction of a pressurized device, it is clear that the silicone layer plays a critical role. The silicone layer controls the difference between the holding of the label when converting it and the releasing during dispensing. It isn’t just about coatings; this is managing surface energy on a micron level.
Understanding the cross-linking and rheology of your silicone systems is essential, regardless of whether you run a high speed rotogravure or narrow web 5-roll coating stations. This guide delves into the chemistry and mechanics of silicone systems, as well as the necessary troubleshooting.
Siliconizing is used in the manufacture of labels.
Siliconizing involves the application of a polysiloxane coating to a surface (such as PET or glassine film), in order to produce a release lining. The surface energy is reduced, which allows pressure-sensitive glues to adhere to the material and ensures clean separation at the dispensing stage.
This process must be viewed as the construction of a barrier. It is important to have a surface that has a level of dyne low enough (usually below 25 dynes/cm), but high enough, to enable the adhesive to coat without reticulation.
Balance is key. You can contaminate the adhesive surface if you use too much silicone. If you use too little silicone, the adhesive surface can become contaminated.
Release Chemistry: UV vs. Thermal Systems
The chemistry you choose will determine the parameters of your machine. The industry is evolving, but the main battles are between UV-curable and thermal-solvent-free technologies. It is important to understand the mechanism of cross-linking in order to predict performance when under pressure.
Thermo-solventless Systems (Additional Cure).
Thermal addition cure is the workhorse in the industry. You are looking at a platinum-catalyzed reaction of a vinyl functional siloxane with an hydride functional crosslinker.
The Platinum catalyst in your drum is expensive, but it’s what drives the reaction. The “pot-life” clock begins to tick when you combine these ingredients. In essence, you are managing a race to see who can cure the fastest in the oven and how long your bath will last in the pan.
The SiH to Vi ratio is a critical parameter. A ratio of 1.5:1 or less can lead to unreacted vinyls, a soft cure and even rub-off. You can achieve a hard cure by pushing the ratio up to 2.5:1 but the post-cure force will change over time due to the reaction of the excess hydride.
UV Curing: Cationic and Radical
UV siliconizing is likely to be used if you have heat sensitive substrates such as thermal paper, thin films or thin sheets. The challenge is no longer oven dwell times, but inerting chambers or photon energy.
UV Free Radical: The system cures immediately but oxygen inhibits it. Maintaining an atmosphere of inert Nitrogen is essential. The oxygen residual levels should be kept below 50 ppm. The surface will remain tacky if your nitrogen blanket fluctuates.
Cationic Ultraviolet: The chemistry of this chemistry does not require oxygen to cure, so it can be used in open air. It is however sensitive to moisture, and alkaline substrates. In cationic systems, the “dark cure”, or cross-linking of the web continues even after it leaves the UV light. This can lead to a significant drop in the release value 24 hours after production.
Substrate interaction and surface topography
It is not possible to apply the same coat weight on all surfaces. Your coating strategy is determined by the substrate topography. This “hold-out” is determined by the interaction of the silicone fluid with the film or paper surface.
Glassine (super calendered Kraft)
To minimize porosity, glassine is densified mechanically. Nevertheless, differences in density may lead to differential absorbency. You are wasting chemistry if the silicone is absorbed into the fibers of the paper, rather than just sitting on the surface.
You should aim for a coat weight between 0.8 and 1.1gsm when using standard glassine. Any less will result in inconsistent or fiber-picking release peaks.
Clay Coated Kraft Boards (CCK), and Polycoated Boards
CCK seals the surface with a layer of kaolin. It provides excellent holding power, allowing for coats as light as 0.70 – 0.90 gsm. The clay coating is brittle. The moisture content will drop below 4% if the temperature of drying is high enough. This can cause the clay coating or liner to be brittle.
Filmic Liners
The smoothest film surface is ideal for applications that require a “no label look”, where the adhesive’s clarity is of paramount importance. You can get a stable release even with a coat weight as low as 0.4-0.6gm.
Anchorage is the biggest challenge when it comes to film. Silicone will immediately rub off without a corona or primer treatment. The anchorage must be monitored continuously, because plasticizers from the film may migrate and affect the bond.
Process Control: Metrics and Critical Parameter
You must rely on quantitative data to produce reliable liners. This is the process that defines a reliable siliconizing.
1. The Coat Weight Calculator
The uniformity of the weight is important. The web can “zip” if there is a variation in width of +0.1gsm. Use XRF scanners to monitor in real time.
2. The Rub-Off Test for Curing and Anchorage
Anchorage can be either mechanical or chemical. Curing refers to the crosslinking of the silicone. It is possible to have perfectly cured silicone which falls from the sheet due to its lack of anchorage.
Test the machine immediately after it has been removed. For a conclusive analysis, however, you should use the extractable test. To determine how much extractable material is unreacted, immerse the sample in Methyl Isobutyl Ketone or Toluene. Extractables in a well-cured thermal product should be below 4 to 5%.
3. Moisture Management
Hygroscopic paper is made of cellulose. Siliconizing ovens (reaching 140degC – 160degC) drive moisture out. You can maintain the dimensional stability of your product by re-moisturizing efficiently. When you send a lining with only 2% moisture it will later absorb the water in the air, leading to “hygroexpansivity”, which can lead to wrinkles and tunneling on the label stock.
Comparison of Silicone Technologies
Weigh the constraints of the production line against performance demands when selecting the system.
| Feature | Thermal Solvent-less | Thermal Emulsion | UV Free Radical | UV Cationic |
|---|---|---|---|---|
| Cure Mechanism | Platinum Catalyzed Addition | Platinum Catalyzed Water-based | Photo-initiated Radical | Photo-initiated Acid |
| Substrate Compatibility | Paper, High-temp Films | Porous Papers (Kraft) | Heat-sensitive Films, Thermal Paper | Heat-sensitive Films |
| Typical Coat Weight | 0.8 – 1.2 gsm | 0.4 – 0.8 gsm (solids) | 0.6 – 1.0 gsm | 0.6 – 1.0 gsm |
| Main Disadvantage | High energy consumption (ovens) | Swelling of paper fibers | Requires Nitrogen Inerting (<50ppm O2) | “Dark cure” / Post-cure drift |
| Equipment CAPEX | Medium | Low (if coating stations exist) | High (Inerting chambers) | Medium |
Siliconizing: FMEA and Troubleshooting
Things tend to get worse exponentially when they go wrong. These are three of the most common failure types and what you can do to address them.
Silicone Transfer
It happens when the silicone moves from the lining to the adhesive. This results in a label falling off because the adhesive is contaminated.
How to fix it: Look for undercuring. Slow down the speed of the conveyor or increase the temperature in the oven. Check the level of platinum catalyst. Check the UV output of your lamp and its cleanliness if you are using it.
Stick-Slip (Zipping)
The release can be jerky, noisy and cause the web to break. It is often due to the lack of slip additives, or a release profile that’s too aggressive.
Fix: It may be necessary to add a Controlled Release Additive or High Release Additive to the stripping solution to level out its release profiles at different speeds.
Ghosting
It appears on the front stock as a pattern that matches the print on the back of the liner, or the other way around. This is usually a result of chemistry.
Fix: Verify that your backside liner coating is compatible. Make sure the tension on the reel is not excessive when winding. This will exacerbate pressure transfer.
In the Future, Release Liners will be used in a circular economy
Conversations about siliconizing are shifting to sustainability. In the industry, there is a shift away from landfilling used liners. Prepare your process for linerless systems, and thinned PET liners to save raw materials.
In addition, non-platinum catalyst systems are gaining in popularity to help reduce dependence on precious metals. Your competitiveness will be determined by your ability to stay ahead of the trends in coming decades.
FAQs (Frequently Asked Question)
What causes my release force to increase after ageing?
It is usually due to “locking up.” This can happen if adhesive flows in the micron-sized peaks and valleys on the paper. Make sure your silicone is completely covered and think about a more rigid substrate.
What’s the difference between a tight and an easy release?
To prevent breaks in the web, an easy release is needed for automatic high-speed dispensing. For hand-applied labelling or rigid labels that need help keeping the liner in place during conversion, tight release (usually 30-50 g/25mm) may be used.
What is the weight of silicone coating?
X ray fluorescence is the standard industry method for production. The silicon atoms are detected directly. Use standards to calibrate your XRF on a regular basis. The base paper’s density will affect the scatter.
What is the best way to use thermal paper with silicone?
Generally, no. Thermal silicone cures at 140degC+, which activates the thermal coating on the paper facestock and turns it black. UV siliconeizing is used for thermal paper because it’s “cold.”
Why does silicone get pinholes?
Poor wetting is often the cause of pinholes. The silicone will not spread if the surface tension is greater than the surface energies of the substrate. Dust or contamination in the web before the coating head can cause this.
