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Coating of adhesives
The adhesive is generally cast-cast. The basic principle is that the adhesive is injected through the pressure of the coating head cavity. The tip of the coating head is an adjustable-size slit, which is coated with the base paper when coated. In operation, the adhesive is uniformly discharged from the slits of the coating head and coated on the surface of the base paper. The amount of coating can be adjusted by adjusting the slits. The amount of adhesive applied is the key to the performance of the self-adhesive material. The generally recommended coating weight is 24 g/m2 and the tolerance is ±3 grams. When the consistency of the adhesive changes, the overall coated grammage can be kept constant by adjusting the pressure. The advanced casting coating device feeds the coating thickness information back to the computer control device through the infrared scanning sensor, and after the analysis and processing, the relevant data is automatically adjusted to ensure the stability of the adhesive coating quality. The amount of adhesive applied should vary according to the season and the temperature and humidity of the area, and it can also be adjusted according to customer requirements.
The general rule is that the amount of coating in the winter should be greater than the amount of coating in the summer. In winter, the use in the north is greater than in the south. For different substrate surfaces, the rough surface has a greater amount of coating than the smooth surface. The amount of coating is related to the adhesive strength of the material. In a certain range, the adhesive force is proportional to the coating amount. Excessively large and small amounts of binder can have a direct effect on the processing and storage of materials.
Problems caused by excessive coating amount:
Overfilling occurs: In the summer, especially in the end of the roll, sticking of the end face of the roll or sticking of the sheets together may result in waste.
Cutting difficulties: The blade is easy to glue and stick to the paper and end faces, causing hidden troubles in the printing process.
Difficulties in transporting paper on the press: Adhesives often adhere to the paperboard, affecting normal feeding and registration.
When a label with an excessively large amount of glue is applied to the product, an overflow of glue may occur around the label, and a black border may be formed after the dust is adhered. When the temperature is high, the label appears to be misaligned on the product.
Problems caused by too small a coating amount:
Influencing the viscosity: The label is easily detached from the product, especially rough surfaces or large curvature surfaces.
Influencing the material structure: The separation force between the surface material and the backing paper is too small to stratify the material, or the label is peeled off together with the waste paper edge during die-cutting and waste discharge, which affects the normal production.
Sometimes the size of the coating should be adjusted according to the customer's needs. The label used in the supermarket requires a smaller amount of coating. But if you need to stick to a special surface such as rubber, you need to increase the amount of coating to strengthen the adhesion.
Classification of adhesives
According to different classification methods, adhesives can also be divided into different types. According to the coating technology can be divided into: hot sol type, solvent adhesives, emulsion adhesives; according to chemical composition can be divided into: rubber substrate, acrylic; according to the adhesive properties can be divided into: permanent, removable In addition to categories; according to the scope of application can be divided into: general-purpose, special sticky, medical, low temperature, high temperature and so on.
Adhesive properties
Initial viscosity: When the adhesive on the label is in contact with the substrate with a slight pressure, the adhesive effect of the adhesive on the substrate is referred to as the initial viscosity of the adhesive. When the label with a large initial viscosity touches the substrate, it immediately generates a large amount of adhesion, and a certain force is required to remove the label. After the label with a small initial viscosity touches the surface of the substrate, the adhesion is small and the label can be easily removed. For the determination of the initial viscosity, a tensile test method and a slant rolling method may be used.
Final Viscosity: When the adhesive penetrates the surface of the substrate, the maximum adhesion that the label can obtain is called the final viscosity. The maximum final viscosity obtained depends on the strength of the adhesive, the roughness of the substrate surface, and the ambient temperature. The maximum viscosity after labeling is 2-24 hours. In general, there is no direct relationship between the initial viscosity and the final viscosity. Labels with higher initial viscosity do not necessarily have higher final viscosities.
Shear strength: Shear strength is also called stickiness. It is an indicator of adhesive cohesion and indicates the softness of the adhesive. Adhesives with low shear strength have a greater tendency to flow, generally have higher initial viscosities, and greater tension is required to separate the label from the substrate. When the adhesive has a high adhesive strength, due to the higher cohesion, a lower initial viscosity is possible at this time, and the label is easier to separate from the substrate.
The degree of flow of the adhesive not only affects its initial viscosity, but also determines the situation of overflowing glue around the label, which in turn affects the problems of contamination, adhesion, paper feeding, and double sheets in the printing process. Therefore, the shear strength must be controlled at a certain level. Within range.
UV resistance: Determine the adhesive's ability to not lose adhesion and change color when exposed to UV light.
Solvent resistance: The adhesive solvent is applied in a solvent environment without the ability to reduce the adhesion. Solvents include water, alcohol, petrochemical solvent, organic solvents, and plasticizers. These solvents are present in the label's application environment or substrate surface.
Cold flow capacity: Indicates the ability of the adhesive to flow under room temperature conditions. Adhesives with good flowability are often low temperature or universal adhesives.
Lowest labeling temperature: The lowest temperature of the surface of the substrate without losing its functionality. The lowest labeling temperature is determined by the "glass transition point" of the adhesive. Below the conversion point, the binder gradually crystallizes as the temperature decreases and hardens to a solid. The minimum temperature for labeling a typical acrylic adhesive is 0°C, and the minimum labeling temperature for a rubber substrate adhesive is -25°C.
Application temperature range: refers to the temperature range where the adhesive reaches its maximum adhesion without changing its characteristics after the label is used. For acrylic adhesives, the application temperature range is -20 to 120°C, and the rubber substrate adhesive is -40 to 80°C. Apply the temperature range to the label's face material. The surface condition of the substrate is related to the ambient temperature, and different adhesives have different application temperature ranges.
Permanent and removable adhesives
Permanent adhesives are adhesives that make it difficult to peel off the entire label without damaging the surface of the label. Such as beer and most of the daily chemical products, stickers and anti-counterfeiting labels.
Removable adhesives are adhesives that can be completely peeled off without damaging the bonded surface, such as stickers on sunglasses.
Permanent and removable are actually just a relative concept. Permanent adhesives may become removable over time, and removable adhesives may become permanent over time. This has a lot to do with the properties of the adhesive, the condition of the adhesive surface, the length of time, and the environment in which it is used. Ultimately, it must be defined by experimentation.
Author / Liu Qing
Source: Packaging Printing and Processing 2005/12
The sticker adhesive is the medium between the label material and the bonding substrate and acts as a connection. According to the characteristics can be divided into two kinds of permanent and removable, adhesives have a variety of formulations for different surfaces and the use of the environment. Adhesives are the most important part of self-adhesive labeling technology and are key to the application of adhesive labels.