Hot melt pressure sensitive adhesive has the characteristics and advantages of zero pollution, safety and health, saving storage and transportation space, automatic high-speed processing and high initial viscosity. It has been successfully applied in the field of labels for more than 30 years in European and American countries and regions Time, and China has only begun to try to apply hot melt pressure sensitive adhesive to the label market in the last 10 years. Because hot-melt pressure-sensitive adhesives have unparalleled advantages for other adhesives for labels, their application prospects are unquestionable. Correct understanding and use of hot-melt pressure-sensitive adhesive is an important task for current label practitioners.
The source of adhesive force of hot melt pressure sensitive adhesive
To provide proper adhesive force, the adhesive needs to pass the following five functions: ①physical adsorption; ② mechanical anchoring; ③ internal penetration of polymer; ④ chemical bridging; ⑤ electrostatic attraction. The adhesive method of hot melt pressure sensitive adhesive mainly depends on physical adsorption and mechanical anchoring.
1. Physical adsorption
The physical adsorption force can be divided into van der Waals force, even electric property, hydrogen bond and ionic bond from weak to strong. Practical application tests have confirmed that adhesives with high polarity can induce a large interface polarity difference between the attached objects and attract each other. The adhesion between the two is also improved, resulting in better adhesion. Appropriate selection of chemical substances with higher polarity to synthesize hot-melt pressure-sensitive adhesive can improve the adhesive strength and enhance the adsorption force to the object.
However, the chemical components (elements) with higher polarity usually have higher activity and easily react with oxygen to cause aging. In addition, the chemical components with higher polarity are usually darker in color, which will affect the appearance of the adhesive or patch and reduce the value of the product.
For materials with low surface energy or polarity, such as PE, PP, etc., in theory, very low polarity adhesives need to be selected to obtain the smallest interface contact angle or the largest contact area, so as to obtain the best Van der Waals force. However, actual experience shows that the contribution of Van der Waals force to the adhesive force of the overall hot-melt pressure-sensitive adhesive is not as significant as the galvanic property produced by the polarity difference between the two substance interfaces.
2. Mechanical anchoring
Regardless of the color and polarity of the adhesive, any adhesive has a unique viscoelasticity. The viscous part can make the adhesive permanently flow, deform and permanently extend; the elastic part can make the adhesive instantaneously deform, rebound, tensile strength and heat resistance. The change of the viscoelasticity of the adhesive has a great relationship with the type, molecular weight, composition ratio, compatibility, etc. of the polymer resin, tackifier, softening oil and various additives. Changes in temperature, speed, time and pressure will also change the viscoelasticity of the adhesive.
Factors affecting the adhesive force of hot melt pressure sensitive adhesive
It is impossible to obtain a versatile hot melt pressure sensitive adhesive formulation. In general, to design an ideal adhesive property combination, in addition to understanding the contribution of physical adsorption, it is also necessary to consider the roughness of the surface of the object and the fracture mode of the interface.
1. The surface roughness of the object
The surface is completely smooth, regardless of the fluidity and mechanical anchorage of the adhesive, the maximum contact surface area that can be obtained is 100%. However, when the surface roughness is increased, the contact surface area of ​​the adhesive with insufficient fluidity will be less than 100%, and the adhesive with better fluidity can get more than 100% of the contact surface area. In the unit projected area, the larger the contact surface between the adhesive and the adherend, the higher the energy required for peeling.
2. Interface break mode
The fracture mode of the adhesive layer interface is closely related to the strength of the adhesive and the attached object. If the object must be torn after pasting, the cohesive force and interface energy of the adhesive must be greater than the cohesive strength of the object itself.
Hot melt pressure sensitive adhesive viscosity test
There are many ways to apply hot-melt pressure-sensitive adhesive, but no matter which method is used, the hot-melt pressure-sensitive adhesive must be pre-heated into a molten flow state in the melt tank, and then the appropriate glue equipment Spray or transfer it directly to the surface material or the object to be attached. The most commonly used label gluing devices are Roller and Slot-Die. Generally speaking, the hot melt pressure sensitive adhesive with lower viscosity is easier to apply, and the processing temperature can also be lowered moderately. It is suitable for heat-resistant surface materials such as PE, PP film and heat sensitive paper.
Application examples of hot melt pressure sensitive adhesive
Over the past 30 years, many hot melt pressure sensitive adhesives with different characteristics have been successfully used in various self-adhesive tape and label markets.
In the practical application market of hot-melt pressure-sensitive adhesive labels, there are roughly the following five categories of products:
1. General purpose;
2.High initial viscosity;
3. High cold resistance;
4. High heat resistance;
5. Repeatable paste (removable).
Hot-melt pressure-sensitive adhesives with different uses will have different adhesive properties, and they are usually not interchangeable at will. However, no matter what kind of application hot melt pressure sensitive adhesive must have good processing performance. Generally, formulators of hot-melt pressure-sensitive adhesives are guided by the acceptable price, processability and terminal adhesive properties of the market and customers, choose the most appropriate raw materials, and formulate products with the best combination of cost and physical properties. In theory, as long as the formula that meets the market requirements is the ideal formula, there is no so-called all-round formula, and there is no so-called secret recipe.
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