What are the features of label adhesives?
Label adhesives mainly have features of initial viscosity, final viscosity, shear strength (holding viscosity), UV resistance, solvent resistance, cold flow capacity, minimum labeling temperature, and application temperature range.
1. Initial viscosity
When the adhesive on the label and the substrate are in contact with a slight pressure, the adhesion of the adhesive to the substrate is called the initial adhesion of the adhesive, also known as the initial viscosity. A label with a high initial viscosity will immediately produce a large adhesive force after touching the substrate, and a certain amount of force is required to remove the label; while a label with a low initial viscosity will show little adhesion after contacting the surface of the substrate, and the label is easy to remove.
2. Final viscosity
After the adhesive has penetrated the surface of the substrate, the maximum adhesion that the label can achieve is called the final tack. The time to achieve a final viscosity of maximum viscosity depends on the strength of the adhesive, the roughness of the substrate surface and the ambient temperature. The maximum time after labeling is 2~24h. Generally speaking, there is no correlation between initial viscosity and final viscosity. Sometimes a label has a high initial viscosity, but does not end up with a high final viscosity.
3. Shear strength (holding viscosity)
It is a measure of adhesive cohesion and indicates adhesive softness. The low shear strength allows the adhesive to have a greater tendency to flow (resulting in high initial viscosity), requiring greater pulling force to separate the label from the substrate. When the adhesive has a high bond strength, due to the high cohesion (may have a low initial viscosity), it is easy to separate the label from the substrate without using a lot of force. How well the adhesive flows not only affects its initial viscosity, but also determines how well the adhesive will bleed around the label. Since the bleeding of the self-adhesive material will also affect a series of problems in the printing process, such as pollution adhesion, unfavorable paper feeding and double-sheet process problems, the fluidity of the adhesive must be controlled within a certain range.
4. UV resistance
It is the ability to measure the adhesive's ability to not lose its cohesive force and change color under the irradiation of ultraviolet light.
5. Solvent resistance
It means that the adhesive is applied in an environment with solvents and does not reduce the ability of bonding strength. These solvents include water, alcohol, solvents in petrochemical products, organic solvents, plasticizers, etc. These solvents exist in the application environment of the label or on the surface of some substrates.
6. Cold flow capacity
It is the ability of the adhesive to flow at lower than normal temperature. Adhesives with good cold flow are called low temperature or general purpose adhesives.
7. Minimum labeling temperature
The lowest temperature on the surface of the substrate before the adhesive loses its function. The minimum labeling temperature is determined by the "glass transition point" of the adhesive. Below the transition point, the binder gradually hardens, crystallizes and becomes a solid as the temperature decreases. The minimum labeling humidity for ordinary acrylic adhesives is 10°C, and the minimum labeling temperature for rubber-based adhesives is -25°C.
8. Application temperature range
The temperature range over which the adhesive achieves maximum cohesion without changing its properties after the label has been used. For acrylic adhesives, the application temperature range is between -20~120°C, and for rubber-based adhesives is -40~80°C. The application temperature range is related to the type of label fabric, the surface condition of the substrate and the environmental conditions. Different adhesives have different application temperature range indicators.