Solvent-Free Polyurethane Adhesives and Laminating Technology (II)

      When used, the two components are uniformly mixed together, and the macromolecules are formed by mutual reaction to achieve cross-link curing. When solvent-free two-component agents are used, the product is required to have a lower viscosity to ensure that the operating temperature is between 40-50°C. The viscosity of the solvent-free adhesive is inversely proportional to the content of isocyanate groups in the prepolymer, and decreasing the content of free diisocyanate groups in the prepolymer necessarily results in a decrease in the content of isocyanate groups in the product, thereby increasing the viscosity of the product. The operating temperature is increased and the operating conditions are not easily controlled. The new two-component solvent-free polyurethane adhesive developed by our institute with low free isocyanate monomer not only reduces the content of free diisocyanate groups in the product, but also enables the product to have an appropriate viscosity, and the operating temperature is moderate to meet the customer's requirements.

5. Solvent-free adhesive coating and use

The coating of solvent-free polyurethane adhesives is different from the coating of solvent-based adhesives. When the solvent type adhesive is applied, in order to make the coating uniform, the viscosity that is easy to apply is adjusted by adding an organic solvent. Since solvent-free adhesives do not contain any solvents at all, the viscosity at room temperature is quite large (especially when solvent-free one-component adhesives exceed tens or hundreds of thousands of mpas at room temperature). According to the particularity of the structure of the solvent-free polyurethane agent, as long as the temperature is slightly changed, the viscosity has a great change. In order to make the viscosity drop suitable for coating, the adhesive is usually heated during use. Therefore, a solvent-free adhesive coating composite device should be a dedicated device with a heated coating and gluing system. Solvent-free laminating machine is composed of mixing metering glue system; coating gluing system; compound cooling system; unwinding, winding system and other major components.

1). Solventless coating machine coating system

Now our country's introduction of solvent-free coating machine coating system is mostly dedicated solvent-free coating machine coating system, also known as four-roll coating system (see Figure 1). It consists of A. B. CD composed of four rollers, A roller is a coating roller; B roller is a floating roller; C roller is a metering roller; D roller is a plastic fixed roller, fixed by the fixed roller D and metering roller C between the storage glue.

2). Use and coating of solvent-free adhesives

1 coating viscosity and temperature:

Solvent-free adhesives have large differences in molecular weight compared to solvent-containing adhesives. Solvent polyurethane adhesives generally have a molecular weight of tens to hundreds of thousands. When there is no solvent, the viscosity is also large, and it reaches millions of pascals per second (mpas) at room temperature. It is even a gel and cannot be coated directly. So it is dissolved by solvent to dilute to a coatable viscosity. When the structure of solvent-free adhesive and solvent-based adhesive is very similar, in order to achieve the viscosity required for solvent-free coating operation and reduce the molecular weight of adhesive, the viscosity is much lower than that of solvent adhesive adhesive, but it cannot be directly coated. The structural characteristics of solvent-free polyurethane adhesives are: the attraction between molecules is large (hydrogen bonds can be formed), the viscosity is high at room temperature, and when the temperature rises, the hydrogen bonds break and the viscosity drops. (Figure 2 is a one-component solvent-free adhesive viscosity vs. temperature curve.) Thus, the temperature is lowered to a coatable viscosity. Temperature increase Viscosity Decrease Coating is easy; but temperature is too high (one-component glue is greater than 100°C, two-component glue is greater than 80°C), the internal reaction speed of the adhesive accelerates gelation, easy to form polymer resin, resulting in coating Difficulties and uneven coating. Therefore, it is necessary to determine the optimum temperature for coating and maintain the minimum temperature difference. General one-component adhesive coating temperature is controlled at 70-100 °C, the viscosity is controlled at 600-3000mpas; two-component adhesive coating temperature is controlled at 45-80 °C, viscosity 500-1500mpas.

2 One-component adhesive coating and curing.

In addition to the temperature and viscosity required for the control of one-component adhesives, since one-component polyurethane adhesives contain NCO groups (in the range of 3-6%), they cure by reacting with water. When the coating amount is lg/m2, the amount of water required to completely cure is 6.5-13 mg. Water adhering to the surface of the dried film alone is not sufficient (OPP measured at 38% RH at 22°C, and water adhering to both sides of the CPP film is approximately 2-4 mg/m 2 ). The supply of water must be replenished from the outside, so the environment must be humidified.

3 coating amount

In solventless compounding, the amount of adhesive applied is critical to the quality of the composite film. The amount of coating is insufficient, it is easy to cause the adhesive force of the composite film is not large, easy to peel or even delamination. The selection of the optimal coating amount of the adhesive should be based on the comprehensive consideration of the different quality requirements of the composite film products, the properties of the film, the printing effect, and the ink and adhesive properties. General use of one-component coating amount of 0.5-1.8g/m2. When the amount of coating is more than 2g/m2, the coating is thick, which is not conducive to the curing reaction of the adhesive and water, causing the center of the film layer to not cure for a long time, and affecting the adhesive strength of the composite product. The use of a two-component solvent-free polyurethane adhesive is No such limit. However, too much coating is not conducive to the composite quality, the general coating amount is controlled at 1-3.5g/m2, when the special needs of the composite film (such as: high temperature cooking) can be increased to no more than 5. Og/m2.

4 initial adhesion and bonding strength.

Since the molecular weight of the solvent-free adhesive used is lower than the molecular weight of the solvent-based adhesive, the cohesion force at room temperature is small, and the initial adhesion before the uncured adhesive is low. Therefore, the tension and the traction force should be strictly controlled in the composite production operation, otherwise the compounding may occur. Membrane product quality issues. Now adhesives manufacturers are developing products with strong adhesive strength in the early stage, and this problem is expected to be better resolved.

6. Conclusion

The solvent-free adhesive dry composite technology for manufacturing composite film packaging materials has become a mature industrial technology after more than 20 years of development. In particular, in the past five years, China's composite film processing industry has realized and understood this new technology, and has successfully introduced more than 20 non-solvent compounding devices. The technology of solventless dry compounding to manufacture composite film packaging materials will be obtained in China. Greater development. With the localization of solvent-free composite equipment and the diversification of solvent-free adhesives, this new processing technology will gradually be recognized and accepted in the country with its outstanding outstanding advantages. It is expected that China will develop at a faster rate in the next few years.

Author / Wang Yongjiang

(Beijing Institute of Chemical Industry