Since the 1980s, China's plastic packaging industry has achieved sustained and rapid development. Especially plastic flexible packaging , with the popularization of supermarkets in major cities, the development is particularly rapid. In general, any single plastic film cannot meet the overall needs of flexible packaging. Multilayer composite films are widely used in food, agriculture, and Japan for their comprehensive advantages such as barrier properties, chemical resistance, high and low temperature resistance, and mechanical properties. With chemicals and other fields. At present, domestic multi-layer composite films are mainly processed by dry lamination using polyurethane adhesives.
First, the effect of glue viscosity on quality
Polyurethane dry composite adhesives with a solid content of more than 70% began to be applied in foreign markets as early as the 1980s. This glue can still maintain a very low viscosity when sizing at a solid content of more than 40%. Due to the reduced use of solvents, it has a low cost of use, is more environmentally friendly, and has excellent performance, which is welcomed by the market.
Today, we have noticed that 50% solids dry composite adhesives still occupy a large share in the domestic market, which indicates that our dry compound manufacturers and composite packaging manufacturers need to further improve their understanding of this aspect.
In dry composite, the control of the amount of adhesive applied is very important, it affects the quality of the composite material to a large extent.
If the amount of glue is too small, the adhesive cannot form a continuous film, and spots appear, which affects the appearance quality of the composite material, and the adhesion is poor, reducing the cooking resistance and heat seal strength of the composite material;
Excessive amount of glue will not only waste the adhesive, but also make the film wrinkle and harden, a tunnel phenomenon will occur, and the opening will be poor. At the same time, the excessive amount of glue will make the solvent difficult to volatilize. Peculiar smell.
The amount of adhesive applied should be determined according to the type of substrate, the use of the composite material, and the printing situation. An important process technology for the amount of glue is the viscosity control of the glue.
Generally, when we talk about glue, we always say what is the solid content or concentration of the glue, and the peel strength and various resistance after compounding, etc. In fact, the following indicators should be used to assess the state of the glue:
1. Glue transfer rate;
2. Transparency of glue;
3. The state after the glue is transferred. This includes: whether there are white points and the number and size of white points, whether there are crystal points and the number and size of crystal points, and whether there are "glue points" and "glues points".
The formulated adhesive must conform to the operating viscosity of the machine (generally, the viscosity transfer is the best when the viscosity is controlled at 15-20s). Under the conditions that the adhesive's working viscosity permits, choose high-concentration operations as much as possible. High concentration operation can reduce residual solvent and save solvent cost. But the premise is that there must be a suitable screen roller to ensure the appropriate amount of dry glue. General high temperature cooking film bag, dry glue volume is 4.5 ~ 5.0g / m2. Therefore, when the concentration of the adhesive is high, a shallower screen roll can be selected, provided that an appropriate amount of dry glue is ensured. Screen rolls for processing cooking and packaging are generally selected at 80 to 100 lines, and the startup speed is 70 to 100 m / min.
When selecting an adhesive, it is best to choose an adhesive with a high solid content, which can generally be used at a higher solid content (working concentration is 35% ~ 45%) to ensure the amount of glue, reduce the amount of ethyl ester, and reduce production. Cost and improve production efficiency. General purpose polyurethane adhesive CX-2022 / CA-875LT (the best use concentration is 40% ~ 45%) and 125 ℃ high temperature cooking glue CX-2522 / CA-775LT (the best use concentration is 30) % ~ 35%) can be purchased by customers.
In the flexible packaging industry , Zahn 3 # cup is generally used to measure the viscosity of ink or glue. At present, some companies are equipped with 4 cups. The diameter of Zahn 3 # cup is 3mm, and the diameter of 4 cups is 4mm. For the same glue sample, the measured data (seconds) with Zahn 3 # cup will be larger than the data measured with 4 cups.
For an enterprise, it doesn't matter which viscosity cup is used, as long as it can provide the necessary instructive data for the operator to achieve the goal.
Factors affecting the viscosity of glue
Ambient temperature has a significant effect on the viscosity of the glue. For glue of the same concentration, the higher the temperature, the lower the viscosity.
2. Concentration of rubber
The higher the concentration of the compound, the higher its viscosity.
3. The nature of glue
Glues of different brands of the same enterprise have different initial viscosity due to different synthetic process conditions, and their viscosities are also different when formulated at the same concentration.
4. Volatility of solvents
The amount of solvent volatilization is affected by factors such as temperature, humidity, the area where the glue is in contact with the air, and the air flow near the glue tank.
Generally speaking, the higher the viscosity of the glue, the worse its fluidity, the worse the flatness of the glue film after application, and the "glue spots" and "orange peel" tend to form. When the viscosity of the glue is too low, it is easy to form a "water streak". Of course, the appearance and degree of the above quality defects are also related to the number of lines, the shape of the cells, and the depth of the coating roller used.
In summary, the viscosity of glue is an important process indicator in the dry composite processing process, and it must be paid great attention.
Third, the way to control the viscosity of glue
1. Control the temperature and humidity of the environment;
2. Control the air circulation near the plastic bucket and tank;
3. Control the area where the glue in the plastic bucket and the plastic tank contacts the air;
4. Equipped with glue viscosity control system for the glue application unit;
5. Equip the glue tank with a special solvent bucket, and replenish the solvent to the glue tank regularly or continuously.
When the above measures are taken, the viscosity of the glue must be measured at any time to understand the effectiveness of the measures taken, and adjust and replenish the solvent in a timely manner.
Fourth, the conclusion
With the strengthening of people's awareness of environmental protection and the Group's vision, in addition to solvent-based adhesives, Golding Group has also successfully developed a series of solvent-free adhesive products. In the future, with 30 years of professional experience, outstanding development talents, technology and high-quality equipment, we will continue to develop high-quality products to meet customer expectations and needs, and continue to adhere to the concept of environmental protection and green, to provide customers with more More environmentally friendly products and services, hoping to create a better living environment for everyone on the planet!