For years, you have probably been dealing with the weights of coatings and their release values. You know that a label is not just an ordinary label. This is an engineering challenge. You must manage a delicate balance of chemistry and mechanics when you work with applications that are pressure sensitive. The fundamental behavior dictates whether you’re running a fast-paced flexo or troubleshooting failures in a packaging machine. Let’s cut through the hype and examine the mechanics and rheology that make the materials function.
Definition of Pressure-Sensitive Mechanisms
The pressure-sensitive system uses a viscoelastic adhesion mechanism to form a bond only by applying a light force externally. No water, heat, or solvents are required. The system uses the specific balance between flow (wetting), and resistance (cohesion), to produce instantaneous Van der Waals forces on the substrate surface.
Adhesion is more than just glue: The Physics of Adhesion
You are buying viscoelastic fluid when you choose a construction that is pressure sensitive. It must have liquid properties for it to adhere to the microscopically textured surface of the substrate. The Glass Transition Temperature becomes the most important metric.
The polymer chains will freeze if your application environment falls below the Tg. Material loses its tact and turns into a solid. If the temperature rises, then the modulus of loss (G’’) will overtake the modulus of storage (G’), which can result in cold flow. Standard acrylics have a temperature range from -20degC up to +80degC. However, specialized hot melts based on rubber might be pushed down even further for applications that require deep freezing.
Surface Energy Wet Out
The thermodynamics is important. Your pressure-sensitive labels will either anchor or lift based on the Dyne (surface energy). Surfaces with high energy, such as stainless steel and glass, are more forgiving. Labeling plastics with low surface energy (LSE), such as Polyethylene (PE) and Polypropylene(PP), can be a difficult task.
Poor wetting occurs when the surface tension and surface energy of an adhesive are not equal. Adhesive simply forms beads instead of flowing. You need to use a rubber high-tack adhesive, or an acrylic modified specifically for surfaces with a Dyne level below 38 Dynes/cm.
The Laminate Construction: Release, Face and Liner
Your final product’s integrity depends on how the facestock and release liner interact. Often, conversion issues are caused by this.
It is a dynamic release liner
Release liners are not just carriers; they’re precision tools. The silicone release curve and transfer must be monitored. You risk having labels predispensed on webs if the release force is too low (less than 10g/25mm). It is not tight enough (over 50g/25mm) and you may experience breaks in the web during dispensing at high speed.
Consider the pressure of the die during cutting. You can compromise the silicone coating if you die-strike too deeply into glassine or PET liners. The adhesive will migrate to the fibers of the paper, which can cause the label to break during the dispensing process. It is essential to have a perfect kiss cut.
Face Stock Conformity
The choice between Vellum, MC Prime or BOPP for rigid containers is largely aesthetic. Modulus matters most for squeezable tube or ampoules with tight radius. To prevent edge lift (flagging), you need a low-stiffness film. It is important to use a PE or conformable PP film because it has a lower restoring force than the adhesive’s shear strength.
Adhesive Systems: A Comparative Analysis
When choosing the best chemistry, you have to make a choice between cohesion and resistance to environmental factors. The major systems are compared.
| Feature | Emulsion Acrylic | Hot Melt Rubber | Solvent Acrylic |
|---|---|---|---|
| Initial Tack | Moderate | Very High | High |
| Shear Resistance | Good | Low to Moderate | Excellent |
| UV & Aging | Excellent | Poor (Yellows) | Excellent |
| Typical Tg | -20°C to -40°C | -10°C to +10°C | -30°C to -50°C |
| Application | General Purpose/Retail | Rough Surfaces/Chilled | Automotive/Chemical |
Identifying common failure modes
When a failure occurs in a pressure sensitive application, there are usually clues. It is important to perform a failure mode and effect analysis.
Failure of adhesive: Label lifts off product cleanly. The adhesive may not have been properly hydrated. Look for contamination on the surface (dust or oil), as well as low Dyne. This could be due to the application temperature being too low.
Adhesive failure: When the label is lifted, it leaves residue of adhesive on the front stock as well as the product. The adhesive’s internal strength was over-extended. It is common for hot melts to degrade in environments with high temperatures.
Failure of Substrate: Surface tear away from the product (common in cardboard). The substrate was weak, not the bond.
FAQs
What causes my labels to lift after 24 hours at the edge?
It is usually due to “memory”, or plasticizer migration, in the facestock. When you put a rigid label on a curve, it will try to go back to the flat position. The edges will lift if the shear force of the adhesive cannot overcome the energy. Make sure your facestock is flexible enough to conform with the radius of the container.
How do shelf life and service lives differ?
The shelf life of a laminate is the time it takes for that laminate to remain stable when stored (usually 1-2 years when 22degC/50%RH). The service life of a label is the longevity it will have after being applied to a product. This can vary from 1-2 years depending on the UV resistance and the chemical resistance.
What is the effect of temperature on initial tacking?
The adhesive becomes harder as the temperature decreases. The wet out capability is reduced, resulting in a significant reduction of initial tacky. Apply pressure-sensitive material above the specified minimum temperature for application (normally around 5degC – 10degC in standard grades).
Can I use standard paper labels for cryogenic applications?
Generally, no. Standard adhesives lose their adhesion and crystallize below -20degC. Cryogenic storage requires specialized adhesives, and synthetic faces like polypropylene or nylon that are able to withstand thermal shock.
Why does adhesive bleed during conversion?
The most common causes of bleeding or oozing are using adhesives with excessive cold flow or blunt blades in die-cutting machines that force the adhesive from the roll, as well as winding rolls at an excessive tension. To prevent this effect, it is important to control your rewind profile.
