Analysis of solvent residues in the hottest flexib

The standard of solvent residue analysis in flexible packaging lags behind the current national standard on solvent residue of flexible packaging composite products, which was formulated more than 10 years ago in GB/t10005. The standard stipulates that the total amount of solvent residue of the composite product cannot exceed 10mg/m2, including benzene, alcohol, ester, ketone and other solvents left during printing, as well as ester solvents left during compounding. And according to the gas chromatograph record, it also includes the gas produced by all solvents in the process of chemical reaction. At the same time, GB/0005 also stipulates that the residue of benzene solvents shall not exceed 3mg/m2. If this recommended standard is upgraded to mandatory, it will play a great role in improving the safety of flexible packaging products

at present, there is still a big gap between China's standards and those of other countries. According to relevant parties, the limit of isopropanol, ethyl acetate and other solvents in Europe is 5mg/m2, and that in Japan is 3mg/m2; The limit of toluene in the United States is 2mg/m2, which is much more advanced than the domestic standard in China

since flexible packaging products generally adopt gravure printing first, and then dry compounding or tape casting compounding, the national standard only stipulates the upper limit of solvent residue in the final product, but does not involve the solvent residue in the printing stage. The author consulted the table printing ink standards and printing ink standards, which stipulated that the solvent residue should not exceed 30mg/m2. Compared with national standards and industry standards, these ink standards are indeed too backward

due to the discharge of organic solvents in the printing and compounding production of plastic films, this involves the issue of gas concentration licensing in the production environment. The author learned that the hygienic standard for industrial enterprise design of the Ministry of health, which is still being implemented, stipulates that the maximum allowable concentration of harmful substances in the air of workshops is: benzene 40mg/m3, toluene 100mg/m3, xylene 1oomg/m3, ethyl acetate 300mg/m3, butyl acetate 300rug/M3. It is understood that the standards of the former Soviet Union stipulated that toluene and xylene should not exceed 50mg/m3, ethyl acetate should not exceed 200mg/m3, and butyl acetate should not exceed 200mg/m3. The United States formulates standards according to the volume concentration value (PPM), which stipulates that toluene and xylene shall not exceed 100ppm, butanone shall not exceed 200ppm, ethyl acetate shall not exceed 4ooppm, and butyl acetate shall not exceed 150ppm

according to the author's long-term experience in the flexible packaging industry, environmental requirements are crucial to the control of solvent residues in the production process of flexible packaging. When the ambient temperature and humidity are high and the air pressure is low, it is dangerous even if it is close to the critical parameter. The gravure printing, dry compounding and bag making processes of some flexible packaging factories are all in a place without division. The high concentration of gas in the environment and the non emission of waste gas are also one of the important reasons for this consequence. In addition, we should also talk about the detection standard of gas chromatograph and the product sampling and inspection standard. There is a big gap between the test results of plastic bags that have been exposed to the environment for a long time and those that have just been unsealed. Similarly, the sampling position of coil products is also closely related to the final test data. The author once learned that Coca Cola company's practice is to cut 100mm along the diameter direction with a saw on the product film roll with a diameter of 600mm, and then peel off the outer layer and take samples for detection. Therefore, the adjustment of the new standard is bound to affect the formulation of a series of other relevant testing standards

several difficulties in gravure printing process under normal conditions, it should not be difficult for the traditional gravure printing process to meet the above index requirements. However, due to many influencing factors in the production process, it is difficult to control solvent residues

1. Intaglio electronic engraving intaglio holes are generally inverted pyramids, with a hole depth of 50 60um. Due to the shadow direction of the shape, it is difficult to transfer the ink at the bottom of the pyramidal hole during the printing process, and the actual hole depth is generally 30 40um. Over time, it is easy to block the plate, especially the small holes in the highlight part are more likely to block, resulting in the loss of dots on the print. Although such problems can be alleviated or reduced by adjusting the position of the scraper or the hot air in the drying oven, it is not always effective

therefore, many operators have to add slow drying solvents (such as xylene, butanone, butyl ester, etc.) to the ink. These slow drying solvents have high boiling points, so we must master the addition amount, otherwise there may be hidden dangers of solvent residue

2. The ink gravure printing ink is mainly solvent based ink. Although gravure printing ink has appeared in the domestic market in recent years, it has not become the mainstream ink. Solvent based inks are divided into benzene and non benzene, and benzene inks are being phased out. All kinds of solvents used in solvent based gravure inks can form toxic and harmful emissions

commonly used resins for gravure printing 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 they have good printability and relatively low price, and often become mainstream products. However, when printing flexible packaging products with this kind of ink, flexible packaging printing plants are also prone to solvent residue problems. In order to avoid dot loss or false drying of ink, the temperature of the drying oven should not be set too high during the printing process. In addition, the last printing unit of the gravure press is too close to the winding unit, so that the printed film is wound up when the ink is not completely dry, which is also a great hidden danger of solvent residue. For this reason, the position of the winding part should be adjusted. The gravure printing used by the author in the past better provides various e-commerce support machines for upstream and downstream enterprises in the supply chain, that is, it does not roll up immediately after printing the last color; It is transferred through the guide roller and then returned to the part in front of the first color group, which increases a long drying distance, so that the solvent can volatilize fully and reduce the residual solvent as much as possible

4. During dry compounding and dry compounding, the setting of glue amount, drying temperature, compounding speed, exhaust air volume of compounding machine, waste discharge system, etc. will affect the solvent residue of composite products. However, it is not easy to completely solve this problem. Without solving the residual solvent in the printing stage, no matter how well the dry compounding is controlled, it will not help

to put it another way, the plastic flexible packaging of food and drugs in European countries mostly adopts flexographic printing and solvent-free composite technology, while China adopts gravure printing and dry composite technology. Previously, the author has listed some difficulties in controlling solvent residue in traditional printing and dry compounding. In view of these difficulties, adjusting and improving the production process is also possible to completely solve the problem of high residue of solvent, which we have just further optimized the Chinese production line this year. However, if we change our thinking and learn from the successful experience of European and American countries, what will be the result

China began to promote plastic film flexographic printing as early as a few years ago, and they are all promoted and applied according to foreign processes. However, because they are not as good as gravure printing in terms of fine points and hierarchical reproduction, many users still can't accept it. However, we have also realized that flexographic printing has the following advantages

(1) the inking volume of flexographic printing is significantly less than that of gravure printing, and the average inking volume of flexographic printing is only 2G/m2

(2) the color concentration of flexographic printing ink is higher than that of gravure printing ink. When transferring the same volume of ink, the color density of flexographic printing is higher than that of gravure printing. Therefore, under the same color density, the ink layer of flexographic printing is thinner than that of gravure printing

(3) the energy release of the resin in flexographic printing ink to the solvent under sine wave, 3-angle wave, square wave, trapezoidal wave, oblique wave, user-defined waveform, stretching, tightening, zigzag, low cycle and high cycle fatigue, crack expansion, fracture mechanics experiment, and the release capacity is better than the resin in gravure printing common ink. For example, the nitrocellulose resin used in flexographic printing has a smaller odor than the polyamide resin used in gravure printing, And the effect of adsorption solvent is also small

(4) the printing speed of flexographic printing water-based ink on film is much faster than that of gravure printing water-based ink, and the application of gravure printing water-based ink also requires major adjustments to the equipment

(5) according to statistics, the solvent residue of gravure flexible packaging products is generally about 10mg/m2. If polyurethane ink is used, the solvent residue can be controlled to 5mg/m2. However, the solvent residue of flexible packaging products printed by flexographic printing is generally about 2mg/m2, and the ink with higher quality will be lower

in recent years, with the improvement of corrugated roller technology and laser direct plate making technology, it is completely feasible to print high-quality (150175 lines/inch) in mold labels with narrow width unit flexographic printing machine

similarly, the application in the field of flexible packaging should also be feasible

in addition, if flexographic printing is combined with solvent-free compounding; Not only the production cost is low, but also more conducive to environmental protection. Solvent free compounding is a mature process in Europe and America. After it was introduced into China a few years ago, it was found that the peel strength was not very ideal in practice. People who have been engaged in the research of solvent-free lamination process have found an interesting phenomenon: if the solvent-free lamination is carried out on two kinds of films that have not been printed, the peel strength is completely OK; If the film printed by flexography is used for solvent-free compounding, there is no problem with the peel strength: but if the film printed by gravure is used for solvent-free compounding, the peel strength is sometimes good or bad. Moreover, the compounding effect of benzene ink and alcohol ink used in gravure printing is completely different. The peel strength of the film printed by benzene ink is significantly better than that printed by alcohol ink