What causes white spots and bubbles in solvent-free compounding?

In the production process, there are many common reasons for white spots and bubbles in solvent-free lamination, mainly including: insufficient glue application, lamination temperature and pressure, environmental humidity, workshop cleanliness, substrate reasons, lamination process and mechanical reasons.

Let us analyze them separately below

(1) Insufficient glue application

Insufficient glue application is a major factor causing white spots and bubbles, especially in light-colored inks. When the glue application is small, there are still gaps after the adhesive is leveled under pressure, causing gas retention and bubbles. Properly increasing the glue application will solve this problem.

(2) Mechanical reasons

The metering roller, glue transfer roller, glue coating roller and laminating roller of the solvent-free laminating machine are not smooth enough, damaged or aged, causing bubbles. In particular, the glue transfer roller contacts the metering roller and glue coating roller at the same time, and controls the glue transfer and glue application amount through the speed ratio of the three. If the surface hardness and roughness of the glue transfer roller are too large or the surface is not cleaned properly, it will cause changes in the glue application amount and coating uniformity, resulting in gaps causing white spots and bubbles. If the problem is caused by the quality of the composite roller itself, the equipment manufacturer should replace the composite roller. If the problem is caused by the improper cleaning of the composite roller, the composite roller should be thoroughly cleaned.

(3) Influence of composite machine speed

When the composite machine speed is high, the lamination time of the two layers of composite film is short, and white spots will appear. Appropriately reducing the machine speed can reduce the occurrence of white spots.

(4) Influence of composite pressure and temperature

Appropriately increasing the pressure of the composite roller and raising the composite temperature can increase the creep of the solvent-free adhesive, which is conducive to the secondary flow of the adhesive and better bonding between the two layers of film, reducing the occurrence of white spots.

If the winding tension is too small, some composite substrates with slightly larger rigidity will warp, and a small gap between the two substrates will form white spots or bubbles.

(5) Causes of the composite substrate itself

There are crystal points and impurities on the surface of the composite substrate. When it passes through the glue coating and laminating rollers, it will affect the extrusion and discharge of glue and gas. After laminating, the crystal points rebound and prop up the film to cause white spots or bubbles. When the surface tension of the composite substrate is low, it will affect the leveling of the adhesive and cause appearance problems. When the two composite substrates are both high-barrier materials, the carbon dioxide gas generated by the reaction of the film's own water vapor and the adhesive cannot be discharged and bubbles are generated. Composite substrates, especially materials with good water absorption (such as nylon and aluminum-plated materials), are opened too early before laminating, causing the film to absorb a large amount of water and react with the adhesive to produce a large amount of carbon dioxide that cannot be discharged and bubbles are generated.

(6) Influence of environmental humidity and workshop cleanliness

When the environmental humidity is too high, the adhesive reacts with water vapor to produce carbon dioxide that cannot be discharged and bubbles are generated.

In addition, poor workshop cleanliness causes dust or small mosquitoes to adhere to the surface of the substrate. The dust particles push the two layers of film open and create a gap around them, resulting in bubbles. If you look closely, you will find a small black dot in the middle of the bubble. Therefore, it is recommended to build an independent, constant temperature and constant humidity environment for the solvent-free laminating machine to ensure product quality.

(7) Others

Other factors that cause white spots and bubbles in product quality include: poor surface flatness of the winding paper core, residual solvent mixed into the adhesive after the solvent cleaning gluing system is used immediately, and there is too much residual solvent in the printing film printing ink. If white spots, bubbles and other quality problems occur in the product during the actual production process, corresponding measures can be taken according to the actual situation to solve them.