Packaging bag solvent residue control, Shunxingyuan has a trick!

To achieve the relevant indicators for solvent residue control in printing, stricter measures must be taken in terms of ink selection and process control. This also poses a challenge to the drying efficiency of printing and laminating machines. Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will explain how to control the solvent residue in packaging bags to better meet customer packaging needs.

I. Hazards and Adverse Effects of Solvent Residue

1. Affects Packaging Safety

Residual solvents can contaminate the contents of the packaging, even causing the contents to deteriorate or change flavor, leading to packaging safety issues. In particular, benzene substances have been identified by the World Health Organization as carcinogens. How to reduce solvent residue, especially reducing or eliminating benzene solvent residue, has become a problem that the flexible packaging industry must solve.

2. Causes Print Adhesion, Reduced Lamination Strength, and Other Quality Issues

After printing, when the residual solvent reaches a certain amount, it is easy to cause problems such as sticking during winding and placement. It will also affect the quality of the lamination process. When the residual amount of alcohol solvents is high, it will affect the peel strength of dry lamination (usually using polyurethane adhesives) or cause "non-drying"; for solvent-free lamination processes, the residue of alcohol solvents will directly affect the stability of the peel strength of solvent-free lamination.

3. The Impact of Solvent Residue in the Printing Process on Solvent Residue and Total Residue in Subsequent Processes

If benzene-containing solvent inks are used, toluene residue will inevitably remain after printing. The residual toluene will not only be an important component of the residual solvent in the printed product, but will also further affect the volatilization of ethyl acetate in the lamination process. Therefore, in the control of the total solvent residue, the key is to control the toluene residue in the printing process.

II. Main Factors Affecting Solvent Residue in Gravure Printing and Their Control

1. Solvent Release of the Ink Itself

Different inks have different solvent release properties. In particular, some low-grade inks, due to the use of lower-grade binders, resins, and pigments, have poor solvent release properties, resulting in a larger amount of residual solvent in the final printed product. Although alcohol-soluble inks do not contain benzene solvents, the solvent residue of the printed product is also affected by the solvent release properties of the ink itself. Therefore, before using the ink, it is best to understand its residual solvent tendency from the ink manufacturer in order to make appropriate adjustments.

Commonly used resins in inks generally include polyamide, polyvinyl butyral, chlorinated polypropylene, polyester, etc. Different resins have different solvent release properties. Some resins have poor solvent release properties, but good printability and relatively low prices, often becoming mainstream products. However, when flexible packaging companies use this ink to print flexible packaging products, it is also easy to cause the problem of exceeding the solvent residue standard.

Inks with good solvent release properties have relatively less solvent residue. The systems of inks from different manufacturers are different, and the difference in solvent release properties mainly lies in the different binders, resins, and pigments of the ink.

In the same ink system, the solvent release properties of inks of various colors are also different. Among them, magenta and black inks have relatively poor solvent release properties, mainly because the pigment particles of magenta and black inks are very small and have strong adsorption capacity.

Due to stricter solvent residue requirements, many flexible packaging companies are now using benzene-free inks, benzene-free and ketone-free inks, and alcohol-water white inks, effectively reducing the residue of solvents, especially benzene solvents.

2. Volatilization Rate and Viscosity of Working Ink

In the printing process, the solvents used in inks are basically mixed solvents. Their volatilization rate has the greatest impact on solvent residue. If the mixed solvent contains a large amount of slow-drying solvents, it will lead to serious solvent residue. Therefore, the reasonable design of the mixed solvent formula is a very important technical issue. In addition, the purity of the solvent will directly affect the volatilization rate, and the water content in the solvent has the greatest impact on the volatilization rate. Ink viscosity also has a significant impact on solvent residue.

3. Drying Conditions

Drying conditions include the structure, length, air volume, and temperature of the drying box, which determine the drying efficiency of the printed product. Poor drying will increase the amount of solvent residue. Increasing the air volume and temperature will improve drying efficiency, but when the ink layer is thick, it may also cause the surface of the ink layer to dry quickly, thereby hindering the escape of the solvent inside the ink layer, leading to an increase in solvent residue. In addition, the drying equipment should be regularly maintained and cleaned to avoid blockage.

With the increase in printing speed and the increasingly stringent requirements of the market for the environmental protection performance of flexible packaging processing equipment, the drying systems of printing and laminating machines have been continuously improved. For example, the last color unit of the gravure printing machine uses two drying ovens or an extended drying oven; a guide roller is added to the last color unit of the gravure printing machine to transfer the printed product back to the front of the first color unit, thus increasing the drying distance and allowing the solvent to fully volatilize; effective zoning is carried out in the printing drying oven (divided into upper and lower parts, all the gas with high solvent content in the lower part is discharged, while the gas with low solvent content in the upper part is subjected to hot air secondary circulation), so that the high-concentration solvent gas is directly discharged without participating in the secondary return air, greatly reducing the solvent residue of the printed product. For example, the drying system fan uses frequency conversion control to achieve reliable control and adjustment of air volume and speed.

4. Printing Speed

The speed of printing determines the length of the drying time of the printed product. The printing speed should be gradually increased while ensuring that the printing ink layer is fully dried.

5. Humidity and Organic Solvent Concentration of the Drying Medium

The humidity of the drying medium (air) has a significant impact on the amount of solvent residue. Firstly, moisture in the air entering the small ink printing machine will inevitably cause the comprehensive volatility of the solvent to deteriorate; secondly, the large amount of moisture in the drying medium will also inhibit the volatilization of the solvent. If the humidity doubles, the drying speed of the ink will slow down by almost twice. Therefore, under high humidity conditions, the printing speed should be slowed down as much as possible and the contact between the ink and air should be reduced to ensure that the ink can be fully dried. At the same time, the solvent formula should be adjusted immediately to formulate a special formula suitable for this climate. The air humidity in the printing workshop should generally not exceed 70%, and of course it cannot be too dry, otherwise it will cause electrostatic problems.

6. Selective Absorption of Solvents by Substrate Films

Different substrate films have different adsorption capacities and tendencies for the solvents used in printing. PP materials easily adsorb hydrocarbon solvents, and have a large adsorption capacity for substances such as toluene; PVDC has an even stronger adsorption capacity for toluene; PA, PT and other water-absorbing materials easily adsorb alcohol solvents.

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