Solvent residue composition analysis in flexible packaging

The standard lags behind the current implementation of the national standard for solvent miscellaneous products in flexible packaging, is formulated in GB/T 10005 more than 10 years ago. The standard stipulates that the total residual solvent content of the compounded product should not exceed 10 mg/m 2 , including both residual solvents such as benzenes, alcohols, esters, and ketones during printing, as well as residual ester solvents. And according to the gas chromatograph, it also includes all the gases produced by the solvent during the chemical reaction. At the same time, GB/E 0005 also limits the residual amount of benzene solvents to no more than 3mg/m2. If the mandatory standard upgrade is mandatory, it will greatly promote the safety of flexible packaging products.

At present, China’s standards still have a small gap compared with other countries. According to relevant sources, the European limit for various solvents such as isopropyl alcohol and ethyl acetate is 5 mg/m2, Japan is 3 mg/m2, and the U.S. limit for toluene is 2 mg/m2, which is compared with the domestic standard in China. Many advanced.

Since flexible packaging products are generally printed in gravure and then compounded by dry compounding or casting, the national standard only stipulates the upper limit of the residual solvent content of the final product, but it does not involve the residual amount of solvent in the printing stage. The author reviewed the surface ink standards and ink standards, which stipulate that the residual solvent content should not exceed 30mg/m2. Compared with national standards and industry standards, these ink standards are indeed too late.

There is bound to be an emission of organic solvents during the printing and compounding process of plastic films, which involves the licensing of gas concentration in the production environment. The author understands that the hygienic standards for the design of industrial enterprises of the Ministry of Health are still in force. The maximum allowable concentration of harmful substances in the workshop air is: benzene 40 mg/m3, toluene 100 mg/m3, xylene 100 mg/m3, ethyl acetate 300 mg/ M3, butyl acetate 300 rug/m3. It is understood that the former Soviet Union's standard for that year would stipulate that toluene and xylene should not exceed 50 mg/m3, ethyl acetate should not exceed 200 mg/m3, and butyl acetate should not exceed 200 mg/m3. In the United States, standards are established in terms of volume concentration values ​​(ppm), which stipulate that toluene and xylene do not exceed 100 ppm, butanone does not exceed 200 ppm, ethyl acetate exceeds 4OO ppm, and butyl acetate does not exceed 150 ppm.

According to the author's previous experience of working in the flexible packaging industry for a long time, the environmental requirements are crucial for the control of the residual amount of solvent in the production process of flexible packaging. When the ambient temperature and humidity are high and the air pressure is low, even close to the critical parameters are more dangerous.

Some of the gravure packaging factories' gravure printing, dry compounding, and bag making processes are all located in an undivided place. The gas concentration in the environment is relatively high, and exhaust emissions do not go out, which is one of the important reasons for this consequence. Said gas chromatograph inspection standards and product sampling inspection standards. Plastic bags that have been exposed to the environment for a long time and plastic bags that have just been unsealed have a big gap between the results of the tests. Similarly, the sample location of the coiled product and the final inspection data also have a great relationship.

The author has learned that Coca-Cola's practice is to saw the diameter of the 600mm diameter product roll with a saw at a diameter of 100mm and the sample is peeled off after the outer layer is peeled off. Therefore, the adjustment of this new standard will inevitably affect the formulation of other series of related testing standards.

Several difficulties in gravure printing process under normal conditions, the traditional gravure printing process to achieve the above requirements should not be difficult. However, due to many factors influencing the production process, it is difficult to control the residual solvent.

1. The intaglio electronic engraving gravure has generally a chamfered pyramidal cell whose depth is 50 Um-60 Um and is influenced by the shape of the aperture. The ink at the bottom of the pyramidal cell hole is difficult to transfer out during the printing process. The actual depth of the cell site is generally 30Um-40um. Over time, it is prone to blocking phenomenon, especially in the small area of ​​the high-light site is more likely to clog, resulting in the loss of small dots on the print. Although it is possible to mitigate or reduce such problems by adjusting the position of the doctor blade or hot air in the oven, it does not always work.

As a result, many operators have had to add slow-drying solvents (such as xylene, butanone, butyl ester, etc.) to the ink. These slow-drying solvents have a high boiling point and must be well-stuffed, otherwise they may bury the solvent residue.

2. Ink gravure printing ink is mainly based on solvent type ink. Although gravure ink has appeared in the domestic market in recent years, it has not become mainstream ink. Solvent-based inks are classified as benzene and non-benzene, and benzene inks are being phased out. Solvent-based gravure inks use various solvents to form toxic and harmful exhaust gases.

Gravure ink commonly used resin generally have polyamide, polyvinyl butyral, chlorinated polypropylene, polyester and so on. Different resins have different releasability to solvents. Some resins have poor solvent release properties, but printability is good and prices are relatively cheap, often becoming mainstream products. However, the flexible packaging printing plant is also prone to solvent residue problems when using this type of ink to print flexible packaging products. In the printing process, in order to avoid the loss of small dots or the phenomenon of “false drying” of the ink, the temperature of the drying oven cannot be set too high.

In addition, the last group of printing units of the gravure printing press is too close to the winding unit, so that the printed film winds up when the ink is not dry, which is also a great hidden danger of solvent residue. For this reason, the position of the winding position should be adjusted.

The gravure printing machine previously used by the author did not immediately wind up after the last color was printed; it was passed through the guide roller and returned to the position before the first color group, thus increasing the drying distance for a long period of time, enabling the solvent to Fully volatile, minimize solvent residue.

4. When dry compound dry compounding, the amount of glue, drying temperature, compounding speed, exhausting capacity of the compounding machine, and waste disposal system will affect the solvent residue of the compound product. However, it is not easy to completely solve this problem, and the residual solvents in the printing stage cannot be solved. The dry compound control is no better, and it does not help.

Another way of thinking is that most of the plastic flexible packaging for foods and pharmaceuticals in the EU customs countries uses flexographic printing and solvent-free composite processes, while our country uses gravure and dry compound processes. In the past, the author has listed some difficulties in the control of solvent residues in traditional garden printing and dry compounding. It is also possible to completely solve the problem of excessive solvent residues in response to these difficulties in adjusting and improving the production process. However, if we change our minds and learn from the successful experiences of European and American countries, what will be the result?

China also began to promote plastic film flexographic printing methods several years ago, and they are all promoted and applied according to foreign technologies. However, due to the lack of gravure printing in the fine mesh and hierarchical reproduction, many users cannot accept it.