Control the residual amount of soft packaging solvent, these details must be considered in place
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Solvent residues in flexible packaging are mainly derived from the inks used in printing and the adhesives used in combination. Solvents commonly used in printing inks are toluene, ethyl acetate, methyl ethyl ketone, isopropanol, xylene, and the like. The solvent commonly used in the ester-soluble adhesive used in the dry compounding is ethyl acetate, and the solvent used in the alcohol-soluble adhesive is ethanol, methanol, and the like.
Factors affecting solvent residue in the printing process
1. The effect of consumables on solvent residue
The solvent absorption of the film substrate. The adsorption capacity and adsorption tendency of different film substrates are different. For example, PP film has a large adsorption capacity for toluene, PA and PT films are easy to adsorb alcohol solvents, and PA and PET films have less adsorption capacity to PP film than PP film. The adsorption of the solvent by the film substrate also affects the residual amount of solvent in the soft package.
The release of the ink itself to the solvent. The better the solubility of the solvent in the resin in the ink, the worse the release of the resin to the solvent. Different inks have different release properties to solvents, especially low-end inks produced by some small ink manufacturers. Due to the lower-grade resin and pigment, the solvent release is poor, and the solvent of the resulting print is poor. The amount of residue will be larger. Therefore, before selecting inks, flexible packaging companies should better understand the release of solvents from their ink products to ink manufacturers in order to deal with them. In addition, the solvent release properties of various color inks in the same ink system are also different, and the solvent release properties of the red ink and the black ink are poor, which is mainly due to the small pigment particles of the red ink and the black ink, and the large specific surface area. Strong adsorption capacity. The volatilization rate of the mixed solvent in the ink. A variety of solvents are mixed in the ink, called mixed solvents. The volatilization rate of the mixed solvent has a great influence on the final solvent residue of the flexible package. If the slow drying solvent is used too much in the ink, the final solvent residue of the flexible package will be larger. Therefore, it is important to properly select and design the type and proportion of solvent in the ink.
2. The influence of graphic design and product structure on solvent residue
In general, it is not advisable to design too many overlapping colors when designing flexible packaging, because if the ink layer is thick, the surface ink may dry quickly and hard, which may hinder the solvent from escaping inside the ink layer, eventually leading to flexible packaging. The solvent residue increases. Spot color inks can be used to prevent the ink layer from being too thick, which effectively reduces the solvent residue of the flexible package.
If there is a large area on the same plate and a very shallow mesh pattern, it is extremely difficult to control the solvent residue. Because when the mixed solvent is dried too fast, the shallow mesh will block the plate, and the drying rate of the mixed solvent will be lowered, which will cause a large amount of solvent residue. Therefore, this design should be avoided as much as possible during plate making.
When designing the structure of the flexible packaging product, the film with poor barrier property should be used as the printing layer as much as possible, and then combined with the barrier layer and the heat sealing layer with higher barrier property to avoid contamination of the inner packaging.
3. The effect of gravure printing process on solvent residue
Drying conditions. The structure, ventilation and wind temperature of the gravure press oven determine the drying efficiency and drying effect of the printed matter.
Today, the drying system of the gravure press is constantly improving. For example, the last color set of the imported gravure press uses two ovens, and the domestic gravure press lengthens the length of the oven. For example, some gravure printer manufacturers have also implemented effective partitioning inside the oven, which is divided into upper and lower parts. This design can discharge all the gases with high solvent content in the lower part, and the gas with low solvent content in the upper part. The hot air secondary cycle greatly reduces the solvent residue of the printed matter.
Printing speed. The speed of printing determines the length of time the print is dried. In order to effectively control the solvent residual amount of the flexible package, the printing speed should be appropriately increased while ensuring that the printing ink layer is sufficiently dried.
The humidity in the printing shop. The humidity in the printing workshop also has a great influence on the residual amount of solvent. There are two reasons: First, the moisture in the air will enter the ink, reducing the volatility of the solvent in the ink. Second, the presence of a large amount of moisture in the air will also inhibit the ink. Volatilization of the solvent. In general, when the humidity is doubled, the drying speed of the ink is nearly doubled. Therefore, under high humidity conditions, the printing speed should be reduced as much as possible, and the ink should be kept out of contact with air to ensure that the ink layer is sufficiently dry. The humidity in the printing workshop is generally controlled at 50% to 65%.
The printed product should be placed in a ventilated place for a day or two before compounding.
Factors affecting solvent residue in the composite process
At present, more than 80% of the adhesives used in domestic food and pharmaceutical composite films and bags are alcohol-soluble two-component polyurethane adhesives using ethyl acetate as a solvent. The following focuses on the effect of dry compounding process using ester-soluble adhesives on the residual solvent content of flexible packaging.
1. The effect of consumables on solvent residue
(1) Effect of toluene on solvent residue
If a solvent-based ink containing toluene is used, there will be some residual toluene after printing, and residual toluene will further affect the volatilization of ethyl acetate in the adhesive. Generally, the final residual amount of ethyl acetate is 1.5 of the residual amount of toluene. ~2.0 times or so, which is why we often emphasize in practice that "controlling the residual amount of toluene in printing is the key to controlling the total amount of solvent residues in soft packaging." The most direct and effective way to control the residual toluene is to use a new benzene-free ink.
(2) The influence of the substrate
Even though the inks and adhesives used in the gravure and dry compounding processes are the same, the production process is the same, but the solvent residue in the flexible packaging will be different. This is because the rate of adsorption and release of solvents by different coated film substrates is different. The release rate of the commonly used rubberized film substrate to ethyl acetate is: PET>NY>BOPP (the opposite adsorption rate). Generally, the drying temperature at the time of compounding is close to the melting point of the BOPP film, and the irregular activity of the molecules is more intense, and the adsorption rate of the solvent such as ethyl acetate and toluene is accelerated. Therefore, it is more difficult for the BOPP film to further reduce the solvent residue.
(3) Effect of water content of ethyl acetate
The moisture in the ethyl acetate affects the volatilization of the ethyl acetate, which in turn affects the solvent residual amount of the composite membrane. Therefore, ethyl acetate with higher purity should be used.
(4) The influence of adhesive
Under the same substrate and the same process, the adhesives of different manufacturers are selected, and the solvent residue of the final composite film may also be different. This is because the polyurethane adhesive contains a reactive hydroxyl group in the molecular bond and forms a hydrogen bond with ethyl acetate, thereby binding the volatilization of ethyl acetate. The change of the main component of the adhesive will affect the strength of the hydrogen bond. The stronger the hydrogen bond, the more difficult it is to release the ethyl acetate. Therefore, the adhesive of different manufacturers will cause different solvent residual results. Before selecting an adhesive supplier, the main component of the adhesive should be fully understood to avoid unnecessary difficulty in solvent residual control.
2. Effect of dry compounding process on solvent residue
(1) Gradient setting of oven temperature and influence of ventilation effect
The temperature gradient and ventilation effect of the oven are the most important factors affecting the solvent residue in the dry compound stage. It is the key to control the drying temperature and ensure good ventilation. The standard temperature of the three-stage oven is usually 60 ° C, 70 ° C, 80 ° C, which is for the normal compounding speed (80 ~ 120 m / min), and the composite process conditions with ethyl acetate as solvent . In general, good ventilation combined with such a temperature gradient, the solvent residue of the composite flexible packaging product should be lower.
To control the solvent residue to a lower level (eg less than 5 mg/m2), the oven temperature should be increased by 5 to 10 °C. However, it should be noted that the first stage temperature of the oven should be kept as far as possible to stabilize the evaporation rate of ethyl acetate and toluene in the first stage. If the first stage of ethyl acetate and toluene volatilize too quickly, a dense layer of hard rubber will form on the surface of the adhesive, which will seriously affect the adhesive and ink in the inner layer of ethyl acetate and toluene. Volatilization, resulting in an increase in solvent residue. The temperature of the second section of the oven can be increased by 5 to 10 ° C, which can accelerate the volatilization of ethyl acetate and toluene in the adhesive and ink in the inner layer. In the third stage of the oven, ethyl acetate and toluene are difficult to volatilize because the hard layer of the surface has formed and the remaining ethyl acetate and toluene are difficult to volatilize, which is why the oven temperature in the last stage is higher. . The temperature of the third section of the oven can be increased by 5 to 15 ° C, but this data is the experimental result of the composite speed of 80 ~ 120 m / min. If the speed of the machine is reduced or increased, the temperature should be lowered or adjusted accordingly.
(2) Influence of composite speed
Similar to the principle that the printing speed affects the solvent residual amount, the composite speed is also a factor affecting the residual amount of the solvent, so the compounding speed should be set reasonably according to the drying capacity of the oven during production.
(3) The influence of the amount of adhesive applied
The larger the amount of the adhesive applied, the greater the hindrance to the evaporation of the inner solvent due to the hardening of the surface layer of the adhesive when passing through the oven, and the greater the residual amount of solvent. Therefore, on the basis of meeting the performance requirements of flexible packaging products, the coating amount of the adhesive should be controlled as much as possible.