Discussion on the effect of viscosity of glue on dry compound

Abstract In combination with its own production practices, the effects of dry compound adhesive viscosity on dry compound dry-drying and further on the quality of composite products were analyzed. The key is to introduce the effect of viscosity on the infiltration of glue on the base rubber from two aspects: thermodynamics and dynamics.

Key words dry compound adhesive viscosity

Since the 1980s, China's plastic packaging industry has achieved sustained high-speed development. In particular, plastic flexible packaging, with the popularity of supermarkets in major cities, especially rapid development. In general, any single plastic film can not meet the overall needs of flexible packaging, multilayer composite film is widely used in food, agriculture, and comprehensive advantages of its excellent barrier properties, chemical resistance, high temperature resistance and mechanical properties. Daily chemicals and other fields. At present, domestic multi-layer composite membranes are mainly made of polyurethane adhesives and processed by a dry composite method.

Polyurethane dry composite adhesives with a solid content of more than 70% have appeared in foreign markets as early as the 1980s. This glue can maintain a very low viscosity with sizing at a solid content of 40% or more. Due to the reduction in the amount of solvent used, it has a low cost of use, is also more environmentally friendly, and has excellent performance, and is therefore well received by the market. Today, we have noticed that the 50% solids content of dry-type composite adhesives in the domestic market still occupies a large share, indicating that our understanding of dry-compound manufacturers and composite packaging manufacturers in this regard is yet to be further improved. .

This article will briefly discuss the effect of glue viscosity on dry compounding in order to promote the market's recognition of the market's solid content and low viscosity dry compound products, and also to share our understanding and experience of dry compound production.

Effect of viscosity on transfer of glue

In production, the viscosity of the sizing solution is too high, and tape is often applied to a series of problems. On high-speed dryers, excessively high viscosity glue may fly at the glue roller, drawing, or cause the base film to separate from the anilox roller. The high speed operation of the anilox roller will also bring air into the glue solution and cause a large amount of white foam to appear in the glue tank. When the viscosity of the glue solution is high, these foams will not break too much and can easily be transferred to the substrate, resulting in the appearance of the substrate. The lack of localized areas of gluing affects the appearance and peel strength of the product. In addition, the viscosity of the glue is too high, which can lead to incomplete transfer of the glue from the anilox roller to the substrate. The time is long, and these residual non-transferred glues solidify in the mesh, blocking the mesh, affecting the glue. the amount. Complex operation is often overlooked in this experience, the use of high solids and low viscosity composite glue, anilox roller cleaning cycle is much longer, usually with ethyl acetate is also easier to clean.

2. Effect of Viscosity on the Wetting Process of Glue

The infiltration of the adhesive on the material to be bonded is critical for forming a good bond. To give an example: Water can well wet the glass. Although it cannot be used as an adhesive, it is necessary to combine the two glass sheets coated with water and separate them from each other. If the water between the glass is frozen at low temperatures, it is very difficult to separate the two glasses. This example illustrates two basic conditions required for good adhesion: (1) the surface to be bonded is well-infiltrated with a liquid adhesive to form a close molecular contact; (2) the liquid adhesive cures.

The adhesive is transferred from the anilox roller to the substrate and adhered to the substrate at a small point. These small spots of glue must be fully spread on the adhesive to form a continuous film. The use of a low-viscosity glue solution can greatly increase its infiltration on the base film, thereby facilitating the spreading. A liquid falls on the surface of the solid. The necessary condition for the liquid to spread automatically is that the surface tension of the solid with respect to the gas phase (γSV) is greater than the surface tension of the liquid with respect to the gas phase (γLV), ie γSV>γLV

And the more they differ, the more beneficial the infiltration is. At the same concentration, the lower the viscosity of the glue, the lower the molecular weight of the polymer under the same chemical structure. Studies have shown that the lower the molecular weight, the smaller the surface tension.

Obviously, the molecular weight of the adhesive is low, its viscosity is low, its surface tension is also low, and its wettability to the base film is better. However, this is only a thermodynamic basis. The actual infiltration process of the adhesive on the base film also depends on the kinetic process, ie how long the infiltration is completed. Only this time is short enough to have practical significance for production.

The rate of change of the contact angle over time of the liquid on the substrate is proportional to the surface tension of the liquid and inversely proportional to the viscosity of the liquid. That is, the lower the viscosity of the glue solution, the faster the infiltration rate on the base film.
A good adhesive not only has a lower viscosity when sizing, but also, during the drying process, as the solvent volatilizes, the glue solution continuously concentrates, and its viscosity can be maintained at a low level over a large range, thereby prolonging the flow. The state of the time, so that it has a sufficient time in the drying process of the actual infiltration balance, to achieve a better bonding effect.

Effect of Viscosity on Diffusion and Adsorption of Adhesives

From the microscopic dimensions, the surface of the substrate is uneven. One of the purposes of the adhesive action is to fill the irregular rough surface of the substrate, which needs to be completed first by the infiltration process. However, to form a strong bond, filling is not enough. The adhesive should also be able to diffuse and adsorb to the surface of the substrate through molecular motion, so that interactions between the adhesive molecules and the substrate molecules will occur at the interface. When the distance between them reaches a molecular radius of 05 nm, physical adsorption occurs between them, ie, secondary bonds or bonding bonds are generated to achieve a good viscosity strength. At the same time, diffusion also creates conditions for the occurrence of surface chemical adsorption. This is particularly interesting for polyurethane adhesives used in dry compounding. There is a certain excess of isocyanate groups (-NCO) in the adhesive system. These groups can react chemically with the active hydrogen atoms on the surface of the substrate, thereby greatly increasing the viscosity strength and making the adhesion more durable. Obviously, the low-viscosity adhesive has a lower molecular weight and a smaller molecule, so it has a stronger diffusion capacity and is more conducive to adsorption.

4 Conclusion

Liquid viscosity is due to the fact that its molecules are subjected to frictional resistance due to movement and is mainly influenced by two factors: concentration and molecular weight. Reducing the viscosity by reducing the molecular weight of the adhesive is a very widely used and very effective means in the molecular design of adhesives. The highly popular high-viscosity and low-viscosity adhesives have adopted this principle. However, there are limits to the application of this principle. If the molecular weight is too low, the cohesion of the macromolecules is limited, and the initial tacking force is often low. In the compounding process, the phenomenon of “tunneling” is easily generated. Moreover, a large amount of oligomer molecules in the adhesive will precipitate from the adhesive layer toward the interface during the curing process and after curing, forming a so-called “weak boundary layer” and affecting the adhesive strength. This is also one of the reasons why we sometimes find that the strength of a composite product decreases over time as it ages. In addition, we have some complex manufacturers to reduce the viscosity of the glue, not to choose high solid content and low viscosity of dry reticule dyeing, and because the concentration of glue is too low, when the solvent evaporates, the glue shrinks excessively, which will cause serious stress concentration. As a result, the adhesive strength of the product is reduced, which also affects the transparency. These are all we should pay attention to and avoid.

Author (Deng Dechun Wang Weiguo)