In the gravure printing process and the solvent-based dry lamination process of plastic flexible packaging , a large number of ester alcohol organic solvents such as isopropyl alcohol and ethyl ester are required. According to the ideal goal, the more completely these solvents evaporate in the production process, the better, but in actual production, due to various reasons, there will always be more or less solvents that are not completely released, leaving in the packaging products, causing " solvent remains". When the solvent residue reaches a certain value, the human sense of smell can sense its existence and affect people's physical and mental health.
China's national standard requirements for solvent residues in flexible packaging products are: total solvent residues ≤5mg / m2, of which benzene solvents are ≤0.01mg / m2. And many well-known companies require total solvent residues ≤ 3mg / m2, such as Nestle and other large brand customers have such requirements.
So, what are the factors that affect the solvent residues of gravure plastic printing and packaging products, and how can we effectively reduce the solvent residues in plastic packaging products?
I. Factors affecting solvent residues
The residual solvent of printing and packaging products is not a single solvent, but a mixture of several solvents. It comes from each stage of the production process. The final residual amount is the comprehensive result of various influencing factors. It mainly has the following effects on it. :
1.Main sources of residual solvents
Solvents in printing inks, solvents in compounding glues, diluent solvents added during printing, etc. These solvents are the main sources of residual solvents.
2. Impact of production process
Printing equipment performance, printing speed, inking capacity, drying system, and choice of compounding glue will directly affect the solvent residue.
3, the impact of printing substrates
Different printing substrates have different adsorption capabilities and different adsorption tendencies for the solvents used in printing, which affects the solvent volatilization.
4. Impact of product structure
Packaging products have a single-layer and multi-layer structure. Due to the barrier properties of the materials of different structures, the degree of solvent release in subsequent processing will also vary, affecting the final solvent residue.
Control of influential factors of solvent residues
Ink is an organic mixture composed of pigments, resins, solvents, and many other structural substances. The structure of the resin and the quality of the ink have an important effect on the solvent residues in plastic packaging. This is mainly reflected in the ink resin In terms of the release performance and the proportion of various solvents in the ink, the appropriate ratio of fast-drying solvent to slow-drying solvent in the ink can adjust the ink to be in the best equilibrium state during the drying process, which can significantly reduce the solvent residue in plastic packaging .
So that the solvent can quickly volatilize in the process of printing transfer and film formation, this need must be fully considered when designing the ink formula.
In the ink formulation, the selected solvent has strong solubility to the resin, and the affinity between the resin and the solvent is large. The fluidity of the ink system will be better, and the ink printing transfer will be better. However, the resin and the solvent are too strong. The release of the solvent by the resin is not necessarily excellent, and the solvent may not be completely released during the film formation of the ink. If the selected solvent has a weak solubility to the resin, although the solvent release is better during the ink printing transfer process, because the resin is not completely dissolved, it will cause printing failures such as knife wire, haze, etc., so we must ensure the printing It is trouble-free and the solvent release is more thorough. When designing the ink formula, it is necessary to understand the factors such as the printing environment and printing equipment conditions, grasp the relationship between printing transfer and solvent release, and design a suitable volatilization suitable for the printing machine speed Ink.
2. Production process
With the type of ink identified, the impact of packaging printing production processes on solvent residues becomes even more important. Therefore, it is necessary to understand clearly the performance of printing equipment, printing speed, plate depth, inking capacity, drying system performance, compound glue, etc., and adopt corresponding process control to achieve the purpose of minimum solvent residue.
(1) Selection of printing dilution solvent
Printing solvent is generally a mixture of multiple solvents with different volatilization speeds. Therefore, we must pay attention to the balance of the solvent and formulate the dilution solvent according to the speed of the printing machine. Pay particular attention to the amount of slow-drying solvent in the dilution solvent. For example, the amount of n-propyl ester should be controlled in chlorinated polypropylene ink, and the amount of slow-drying solvents such as n-propyl ester, propylene glycol methyl ether, etc. should be controlled in polyurethane ink. If there are too many slow-drying solvents, it will directly cause incomplete solvent evaporation and high solvent residues. If there is less slow-drying solvent, it will also cause ink crusting, which is easy to block the consequences of poor plate transfer. Therefore, when the proportion of the diluted solvent has been determined, do not easily change the proportion of the diluted solvent.
(2) Printing drying temperature and ventilation
Dry ventilation is the key to good solvent release during printing. First of all, it is necessary to develop the habit of maintaining and cleaning the drying and ventilating devices, and maintain a certain amount of air volume and make it enter and exhaust air balance. Secondly, when the last color is dried, the temperature should be increased by 5 ~ 10 ° C. If the color number of the equipment and the product allows, the roll film can be allowed to pass through the two-color unit after printing. Residual amount.
(3) Dry composite process
Because the glue and diluent used for the compound are ethyl esters, the volatility is high. Therefore, the problem of solvent residue caused by compounding is not as serious as that of printing, but if it is not controlled properly, it can also cause high solvent residue. This is because the composite drying tunnel is long and the temperature is high, so the most important thing is to control the air volume. When the pre-exhaust and normal exhaust of the composite equipment are normal, the normal release of ethyl ester can be guaranteed under normal processing speed. At the same time, compounding can further eliminate residual solvents in printing.
If the ethyl ester is found to exceed the standard after compounding, the composite film can be dried in the curing room for an additional 24 hours. The time should not be too long, otherwise the internal material additives will precipitate and increase the odor. But if it is a product with a double-layer barrier structure, more baking is not helpful. So be sure to do a good job of control in the early printing and compounding stages.
(4) Printing environment
The temperature and humidity in the printing workshop will also have a certain effect on the release of the solvent. The excessive humidity in the workshop will reduce the evaporation speed and balance of the solvent, which will cause the evaporation of the solvent to slow down during the printing process and cause residues, but the humidity will be too low. Also, it is easy to cause static electricity in the high-speed printing process. Therefore, pay attention to the temperature and humidity in the workshop and maintain a certain humidity to effectively eliminate static electricity in ink printing, so that excessive humidity does not increase the solvent residue.
Printing substrates are selectively adsorbed on solvents. Different printing substrates have different adsorption capabilities and different adsorption tendencies for solvents used in printing. PP and PVDC materials easily adsorb ester organic solvents, PA, PET, etc. The water-absorbing material is easy to adsorb alcohol solvents. In general, PA and PET have less solvent adsorption than PP, but they will increase the solvent residue of packaged products. For a certain packaging product, the printing base material and the composite base material are selected according to the packaging requirements of the product, and it is basically difficult to change. Therefore, under the condition that the base material cannot be changed, according to different base materials, the solvent species with different solvent ratios should be selected to reduce the solvent adsorption amount of the printing base material accordingly.
4. Product structure
Different packaging products have different packaging structures. Solvent residues in single-layer structures are relatively easy to control, and products with multilayer structures are relatively difficult to control. At the same time, the barrier properties of materials have different effects on solvent residues.
For product structures with poor barrier properties, such as BOPP // PE, the residual solvent can pass through a process such as drying in the oven, slitting and rolling, which can overflow a large part of the residual solvent, especially volatile ethyl esters. However, for product structures with good barrier properties on the outer layer, such as VMPET // PE, it is more difficult for residual solvents to overflow through this method.
For products with a double-layer barrier structure, solvent residues are the hardest to control and the hardest to remedy afterwards, such as PET // VMPET // PE, cellophane // AL // PE, PET // VMCPP and other structures are enclosed in the composite layer Residual solvents in it are difficult to escape. Therefore, when we produce products, we must consider the influencing factors of the product structure. For products with double-layer barrier structure, we must design the balance of the solvent at the beginning of printing to control the residual amount so that the product is in the early production process. Release the solvent as much as possible to avoid causing high solvent residues, which cannot be improved in the later processing.
5.Purity of solvent
A large amount of organic solvents are used in the ink production process and the packaging printing process. The purity of the solvent is not high. First, there are many components with high boiling points, which will slow down the volatilization rate of the overall solvent, especially the late rate of solvent volatilization, resulting in solvent residue. The other is that some impurities of coking components remaining in the solvent will produce a burnt smell in the printing of flexible packaging. Even if the residual amount of the solvent is within the controlled standard, it cannot be accepted by consumers.
Therefore, all solvents used in the production process of packaging products must be strictly controlled by the enterprise to prevent quality loss caused by impure solvents.
Third, the conclusion
In summary, every influencing factor of packaging solvent residues is important. Therefore, a qualified packaging product is the result of strict control of many process steps. When we produce any product, no matter how complicated the process, we must first analyze the product production process, the process elements involved, and then analyze each of them. Each process of the link will affect the solvent residue of the product, and then control them one by one, so as to reduce the solvent residue and produce a qualified product.