Flexible solvent residual adhesive ink mainly from the use of composite printed and used. 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. 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. The release of solvents by different inks is different, especially the 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 obtained printing 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. The adsorption capacity is strong. The volatilization speed 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 partial ink layer of the print from being too thick, thereby effectively reducing 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 air temperature of the gravure press oven determine the drying efficiency and drying effect of the printed matter.
Today, the drying system of gravure presses 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 also implement effective partitioning of the inside of 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 of 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 molecular irregular activity 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.
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