Because the molecular structure of polyurethane adhesives contains highly active polar gene isocyanate, so adhesives of this type of adhesive can be a wide variety of substrates, such as: metal, plastic, wood, paper, fabrics, ceramics, glass, etc., in flexible packaging There are a large number of substrates commonly used in composites: plastics, aluminum foils, and plastics that are used most often. How can we obtain a flexible package with excellent performance? The following micro-analysis of the bonding phenomenon is carried out in several aspects.
Adhesive substrate selection
In order to firmly bond the two substrates together under the action of the adhesive, the first necessary condition is to make the adhesive completely wet on the solid surface, ie, to obtain a uniform and complete film. Only by completely wetting the surface of the substrate with the adhesive can the various mechanisms work, which is the prerequisite for obtaining the best bonding effect. If the infiltration is incomplete, many stress concentration points will be generated where the film breaks, and when it is small, the damage will easily occur from there, and the adhesive strength will also drop a lot. How can a liquid automatically spread on a solid surface? The Young's Wetting Equilibrium Equation describes the surface tension of a solid, a liquid, and an interface when a drop of liquid drips on a solid surface and the liquid infiltrates the solid surface to reach equilibrium. The relationship followed.
When the surface tension of the adhesive is the same as the surface tension of the substrate, the adhesive completely wets the surface of the substrate, and the adhesive strength is maximum at this time. It is known that the surface tension of the polyurethane adhesive is about 40 dyne. Therefore, if it is desired to fully wet the surface of the substrate, the surface tension of the composite substrate must be about 40 dyne in order to achieve complete wetting.
Among the two major substrates commonly used in flexible packaging, aluminum foil is now being used more and more because of its irreplaceable properties. Aluminum metal is a high surface material whose pure metal surface tension reaches 1909 dynes, but because it is easily oxidized, it always has a certain thickness of oxide layer on its surface. In addition, because of the heavy use of lubricating oil in its production process, residual grease can affect the surface tension of aluminum foil if it is not thoroughly cleaned later. Usually, the manufacturer classifies it into four levels, A, B, C, and D. Then for the composite substrate, to choose A, B grade products, C, D grade products can not guarantee the coating effect, it can not guarantee the bonding fastness. In addition, during the storage and transportation of aluminum foil, the packaging should be tight, and the storage period should not be too long, so as to avoid excessive oxide film formation on the surface and affect the composite effect. All kinds of plastic films are the mainstream of packaging composite substrates, and the surface tension of plastic films is different due to their different chemical structures. From the microscopic point of view, only the chemical structure of the surface can change its surface tension.
The original surface tension of some substrates is much lower than the 40 dyne of polyurethane adhesives. If these substrates are to be used for recombination, their surface tension must be increased to above 38 dyne. Otherwise, the adhesive cannot be applied to the surface. Wetting achieves high composite fastness. Usually the most common method is to use corona treatment. Under a high-voltage electric field, oxygen in the air turns into ozone, which is decomposed into oxygen and new ecological oxygen atoms. This new ecological oxygen atom is a very strong oxidant, which causes the surface of the low surface energy substrate to oxidize. The polarity of the surface molecule after oxidation increases, and the surface tension increases, so that the adhesive can be wetted. However, it should be made clear that this surface tension is not stable. If it is not compounded in time after the treatment, this tension will be attenuated with time as it is placed. Therefore, the placement time should not exceed 7 days. If it has been stored for a long time, it must be tested with a surface tension test solution before use. If the surface tension is less than 38 dyne, the corona treatment must be performed again. Otherwise, it cannot be compounded.
In addition, when the corona treatment is performed, the electron flow bombards the surface of the substrate under a high-voltage electric field, and can also cause surface fluffing, which serves as a roughened surface. This change in physical form increases the surface area and enhances the wetting, while the convex and concave surfaces are also beneficial for the "anchoring" effect of the adhesive.
The formation mechanism of adhesive force
Under the premise that the surface tension of the substrate meets the requirements, the adhesive forms a uniform and complete film on the surface of the substrate, and the two kinds of substrates have sufficiently close contact conditions under the combined pressure, and at this time, the bonding interface is A series of physical actions and chemical reactions take place, and the collection of these reactions reflects the formation of adhesion.
1. Intermolecular force
When the distance between the molecules reaches 1 nm, intermolecular forces are generated between the molecules. This intermolecular force is the main source of adhesion. Polyurethane adhesive contains isocyanate group, its reaction activity is extremely high, when it is in contact with the surface of the substrate, it can be adsorbed on the surface of the substrate to form a strong urea bond, the urethane bond, ester bond, ether bond, etc. in the adhesive. It can also form hydrogen bonds with the polar groups on the substrate surface. This further increases the peel strength.
2. Mechanical force
Under the microscope, any solid surface is not absolutely smooth. It is composed of countless peaks and valleys. When the adhesive fills and cures these peaks and valleys, it forms numerous miniature “anchors†that will form two types of anchors. The material is firmly locked together, which enhances the bonding effect from the physical and mechanical aspects, which is also an important part of the adhesive force. It should be noted that the mechanical effect of the adhesive layer on the plastic film is very limited, and it is not a factor that determines the ultimate strength of the peeling, but merely a method of increasing the fastness. Of course, the mechanical action of porous materials such as fabrics or papers is very important.
3. Diffusion force
When the adhesive is in close contact with the substrate, due to the thermal motion of the molecules, mutual diffusion occurs at the interface, and the closer the compatibility is, the easier the diffusion proceeds. At the same time, the lower the molecular weight of the adhesive, the more conducive to diffusion. The substrate is a highly crystalline molecular structure. Due to the close packing of its molecular bonds, intermolecular diffusion is less likely to occur. For example, polyester, although it is a high surface energy material, it is not easy to bond. The reason is that it is a highly crystalline molecular structure with very tight materials. Diffusion makes the interface blurry, you have me, and I have you. This force is part of the bond formation.
Regarding the bonding mechanism, there are many theoretical explanations. Due to the complexity of the bonding phenomenon and the limitations of modern science and detection methods, various theoretical explanations are also in various words. See you for your reference.
(to be continued)
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