Abstract: This paper analyzes the thorny production problems such as appearance bubbles, ink dissolving, slow opening speed and low strength that are prone to appear in the solvent-free composite of the aluminized structure of flexible packaging, and introduces in detail the solution of "overall supply of ink and solvent-free composite of flexible packaging", which can also solve the safety problems of composite products at the same time, and contribute to the environmental protection and sustainable development of the soft packaging industry.
 Solvent-free composite adhesives are widely used in flexible packaging fields such as food, cosmetics, daily necessities, agricultural products, health care and medicine, electronics, and decoration because of their environmental protection, pollution-free, low energy consumption, low cost, and non-flammable production safety.
 I.
The application status of solvent-free composite adhesives

As can be seen from the above table, solvent-free composite adhesives have been maturely applied to ordinary packaging structures, but there are still some practical application problems, which affect a wider range of applications, mainly the following problems:
In the production of aluminized structures, there are often problems such as white spots, ink dissolving, low speed, and insufficient strength;
Plastic 121 °C cooking products do not dare to use solvent-free compounding, for fear of problems;
The friction coefficient increases abnormally after curing, which affects the opening;
Risk control of peel strength attenuation of PE film;
odor problems;
Product hygiene and safety issues, PAA and other harmful substances are not well retained, such as the compliance of China's GBGB4806.1-2016, GB9685-2016, the latest revision of the EU plastics regulation EU10/2011 (EU) 2020/1245 (implemented on September 23, 2020), etc.
 II.
How to solve application problems in production
This paper focuses on the failure analysis of the production thorny problems such as white spots, ink dissolving, slow opening and low strength that are prone to occur in the solvent-free composite aluminized structure, and discusses the solutions!
Solvent-free lamination of flexible packaging is a systematic project that requires the cooperation of equipment, materials, processes and operators.

Even if the same equipment and materials are in different environments, regions, climates, temperatures, and humidity, the quality of the products will be different, and the problems will be different.
(I) Analysis of the causes of compound white spots and countermeasures

The real causes of the formation of white spots or bubbles are:
Due to various factors, a trace amount of air is sandwiched between the two layers of composite substrate.
1. The flatness of the microscopic surface of the ink and the amount of glue
Specifically, there are no white spots in the blank area, and there are white spots/bubbles in the ink layer, and it is irregular (not circular).
The thickness of the ink layer on the gravure plate can reach 3~5 μm or even higher, while the thickness of the glue coating layer is only 1~2 μm.

2. Hardness, temperature and pressure setting of composite rubber roller
The pressure of the composite roller determines whether the composite material can be compacted between the layers, and the gas between the two layers of materials is discharged, so that the adhesive can be fully leveled under pressure and maintain a good appearance.
When we are in production, we need to adjust the compound pressure according to the different material interfaces!
Effectively increase the laminating temperature of the composite roller, which is conducive to the spreading of glue and enhances the uniformity of coating.
The composite rubber roller is generally a medium and high roller, which is mainly to make up for the bending deformation caused by the force of the roller, so that the rubber roller and the steel roller contact pressure are uniform on the entire transverse pressing surface, so as to discharge air bubbles and make the composite uniform and flat.
Hardness control of composite rubber rollers:
For materials with different flexibility or rigidity, composite rubber rollers with different hardness should be selected to ensure a smooth and uniform composite effect.
Check the hardness regularly to prevent the waste of defective products!
The hardness of composite rubber rollers of different materials is different;
The service life of various materials is different, and the composite rubber roller generally uses polyurethane material, and its hardness will also change with time, use wear and tear and other changes and different degrees of aging, resulting in uneven surface of the composite rubber roller and poor parallelism at the left and right ends, the following phenomena will occur:

3. Tension control and winding processing
In solvent-free lamination processes, tension control is extremely important and must be very precise.
Solvent-free lamination has a low initial tack, so it is particularly important to match the tension of each section of the composite substrate and control the winding tension.

For the high barrier material compounding, the appropriate winding tension is very important!
 
To come, the following highlights can be locked in the January 2021 issue of Packaging Frontiers
4. The leveling and spreading of the glue on the surface of the ink
III. Practical Cases
1. Application of inner AL/PE in PET/AL/PE structure
2. Application in thin PE aluminized structure OPP18/VMPET/PE37
3. "Special" applications in OPP aluminum foil structure
4. Application of plastic/plastic 121°C cooking
IV．Phenomenon of Compound Ink Dissolution and the Countermeasure
V. Conclusion