How to Control Solvent Residual Volume in Gravure Printing Soft Packaging (1)

At present, the solvent-based gravure printing inks used in the domestic flexible packaging industry contain a large amount of organic solvents. In addition, a certain amount of thinner is required to adjust the ink viscosity and volatilization rate during the gravure printing and processing of flexible packaging to adapt to the printing and compounding processes. Claim. Gravure ink commonly used diluents are toluene, butanone, isopropanol, xylene and so on. The solvent used for the ester-soluble adhesive in dry compounding is ethyl acetate, and the solvents used for alcohol-soluble adhesives include ethanol and methanol. During printing and compounding, although the substrate is dried by the oven system, some solvents are still not completely volatilized, thus forming solvent residues.
With the continuous progress of society, the effect of solvent residues in flexible printed packages on the environment and food safety has received more and more attention. In the physical and chemical index test items related to the national standards for flexible packaging composite film (bag) products, the state has made clear regulations for toluenediamine, evaporation residue, heavy metal content, and solvent residue in flexible packaging. Among them, the total amount of solvent residues in food composite films (bags) must be less than 10 mg/m2, and the residual amount of benzene-based solvents should be ≤ 2 mg/m2.

To achieve the printing of relevant indicators for solvent residue control, flexible packaging companies must adopt more stringent measures in terms of ink selection and process control. At the same time, this poses a challenge to the drying efficiency of printers and laminators.

Solvent residue hazards and adverse effects

1 affect the safety of packaging

Residual solvents can contaminate the contents of the package, and even cause the contents to degenerate and taste taint, leading to safety problems in food packaging. In particular, benzene-based substances have been identified as carcinogenic substances by the World Health Organization. How to reduce the amount of residual solvents, especially to reduce or even eliminate benzene-based solvents, has become a problem that flexible packaging companies must solve.

2. Quality problems such as adhesion of printed products and decreased peel strength in composite processes

After printing, when the residual solvent reaches a certain amount, problems such as sticking and the like may occur during the winding and placement of the printed product. It also affects the quality of the compounding process. When the alcohol solvent residue is large, it will affect the peel strength of the dry composite (usually polyurethane adhesive) or cause "not dry"; and for the solvent-free composite process, the residual alcohol solvent will directly affect the Solvent composite peel strength stability.

3. Effect of Solvent Residue on Residual Solvent Residue and Total Residue in Printing Process

If benzene-based solvent-based inks are used, there will inevitably be toluene residue after printing, and residual toluene will not only be an important component of the residual solvent of the prints, but will also further affect the volatilization of ethyl acetate in the compounding process. Therefore, in the entire solvent residue control, the key is to control the amount of toluene residue in the printing process.

Main Factors Affecting Solvent Residue in Gravure Printing Process and Its Control

1. The ink's own solvent release

Different inks have different solvent release properties, especially some low-grade inks. Due to the use of relatively low-grade binders, resins, and pigments, their solvent release is poor, and the final print product has a large residual solvent. Although the alcohol-soluble ink contains no benzene solvent, the solvent residue of the print is also affected by the ink's own solvent release properties. Therefore, before using the ink, it is best to understand the tendency of the residual solvent to the ink manufacturer in order to make corresponding treatment.

Commonly used resin inks are polyamide, polyvinyl butyral, chlorinated polypropylene, polyester and so on. Different resins have different releasability from solvents. Some resins have poor release properties from solvents, but printability is good and prices are relatively cheap, often becoming mainstream products. However, when flexible packaging companies use this type of ink to print flexible packaging products, they may easily cause excessive solvent residue problems.

The ink has good solvent release and the residual solvent is relatively small. The inks of different manufacturers differ in their systems, and the difference in solvent release is mainly due to the difference between ink binders, resins and pigments.

In the same ink system, the solvent releasability of each color ink is also different. Among them, the releasability of magenta ink and black ink solvent is relatively poor. The main reason is that the pigment particles of magenta ink and black ink are very small, and the adsorption capacity is small. Very strong.

Due to stricter solvent residue requirements, many flexible packaging companies have adopted benzene-free inks, benzene-free and ketone-free inks, and alcohol-water white inks to effectively reduce the residual of solvents, especially benzene-based solvents.

2. Volatilization speed and viscosity of working ink

In the printing process, the solvent used in the ink is basically a mixed solvent, and the volatilization rate has the greatest influence on the solvent residue. If the mixed solvent contains more slow-drying solvents, serious solvent residues will result. Therefore, the rational design of mixed solvent formulations is a very important technical problem. In addition, the purity of the solvent will directly affect the volatilization rate. The content of water in the solvent has the greatest effect on the volatilization rate of the solvent. The ink viscosity also has a great influence on the solvent residue.