Coating and use of solvent-free adhesives

The coating of solventless polyurethane adhesives is different from the coating of solvent based adhesives. When the solvent-based adhesive is applied, in order to make the coating uniform, the viscosity which is easy to apply is adjusted and controlled by increasing the organic solvent. Since solvent-free adhesives do not contain any solvent at all, the viscosity at room temperature is quite large (especially when solvent-free one-component adhesives exceed tens of thousands or hundreds of thousands of mpas at room temperature). According to the particularity of the structure of the solventless polyurethane agent, as long as the temperature is slightly changed, the viscosity thereof is greatly changed. In order to make the viscosity drop suitable for coating, the adhesive is usually heated during use. Therefore, the solvent-free adhesive coating compounding equipment should be a dedicated device with a heated coating and gluing system. The solventless compound machine consists of a mixed metering and dispensing system; a coating and gluing system; a composite cooling system; a unwinding and winding system.

1). Solventless compound coating system

At present, the coating system of the solventless laminating machine introduced in China is mostly a coating system of a special solventless laminating machine, which is also called a four-roll coating system (see Fig. 1). It consists of A. B. CD four-roller composition, A-roller is a coating roller; B-roller is a floating roller; C-roller is a metering roller; D-roller is a glue-fixing roller, and the glue between the fixed roller D and the metering roller C is stored.

2). Use and coating of solvent-free adhesives

1 coating viscosity and temperature:

Solvent-free adhesives vary greatly in molecular weight compared to solvent-containing adhesives. The molecular weight of the solvent Polyurethane Adhesive is generally from tens of thousands to hundreds of thousands. When there is no solvent, the viscosity is also large, and the normal temperature reaches millions of milliamperes per second (mpas), even a gel, which cannot be directly coated. Therefore, it is diluted to a coatable viscosity by means of solvent dissolution. When the solvent-free adhesive is similar in structure to the solvent-based adhesive, in order to achieve the viscosity required for the solvent-free coating operation, the molecular weight of the adhesive is lowered, and the viscosity is much smaller than that of the solvent-based adhesive, but it cannot be directly coated. The structural characteristics of the solvent-free polyurethane adhesive are: large intermolecular attraction (hydrogen bond formation), high viscosity at normal temperature, hydrogen bond cleavage and viscosity decrease when temperature rises. It is a one-component solvent-free adhesive viscosity versus temperature curve. Therefore, heating increases the viscosity to a coatable viscosity. Temperature increase viscosity drop coating is easy; but the temperature is too high (one-component adhesive is greater than 100 ° C, two-component adhesive is greater than 80 ° C), the internal reaction speed of the adhesive is accelerated to gel, easy to form polymer resin, resulting in coating Difficulty and uneven coating. It is therefore necessary to determine the optimum temperature for the coating and to maintain a minimum temperature difference. Generally, the coating temperature of the one-component adhesive is controlled at 70-100 ° C, the viscosity is controlled at 600-3000 mpas; the coating temperature of the two-component adhesive is controlled at 45-80 ° C, and the viscosity is 500-1500 mpas.

2 coating and curing of one-component adhesive.

The coating of the one-component adhesive is in addition to the temperature and viscosity controlled above, since the one-component polyurethane adhesive contains NCO groups (contents in the range of 3-6%), which is cured by reaction with water. When the coating amount is lg/m2, the amount of water that needs to be completely cured is 6.5 to 13 mg. The water adhering to the surface of the dried film alone is insufficient (OPP measured at 32 ° C at 38 ° RH, and the surface of the CPP film adhered to water of about 2-4 mg/m 2 ). Water must be replenished from the outside, so the environment must be humidified.

3 coating amount

In solventless composite processing, the amount of adhesive applied is critical to the quality of the composite film. Insufficient coating amount, easy to cause adhesion of the composite film is not large, easy to peel off or even delamination. The choice of the optimum coating amount of the adhesive should be based on the different quality requirements of the composite film product, the film properties, the printing effect, and the performance of the ink and the adhesive. Generally, the single component coating amount is 0.5-1.8g/m2. When the coating amount is >2g/m2, because the coating is thick, it 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, affecting the bonding strength of the composite product, and using the two-component solventless polyurethane adhesive. There is no such restriction. However, if the coating amount is too large, it is not conducive to the composite quality. Generally, the coating amount is controlled at 1 to 3.5 g/m2, and when the composite film of special needs is required (for example, for high-temperature cooking), it can be increased to not more than 5.0 g/m2.

4 initial adhesion and bond strength.

Since the molecular weight of the solvent-free adhesive used is lower than the molecular weight of the solvent-based adhesive, the cohesive force is small at normal temperature, and the initial adhesive strength before uncured is low. Therefore, the tension and the traction force should be strictly controlled in the composite production operation, otherwise the composite will be produced. Quality problems of film products. Now adhesive manufacturers are developing products with strong initial adhesion, and this problem is expected to be better solved.

 Conclusion

The technology of solvent-free adhesive dry composite manufacturing of composite film packaging materials has become a mature industrial technology after more than 20 years of development. Especially in the past five years, China's composite film processing industry's understanding and understanding of this new technology, and the successful introduction of more than 20 solvent-free composite equipment experience, solvent-free dry composite manufacturing composite film packaging materials technology will be obtained in China Great development. And with the localization of solvent-free composite equipment and the variety of solvent-free adhesives, this new processing technology will gradually be recognized and accepted domestically with its obvious outstanding superiority. It is expected that in the next few years, China will develop at a faster rate.