The selection and use of cooking resistant compound ink

At present, the homogenization of flexible packaging industry is becoming more and more obvious. In particular, the relatively low technical threshold for ordinary composite packaging, such as BOPP//PE, is highly competitive. Many flexible packaging manufacturers can only maintain low profitability through large-scale and mass production. High-temperature cooking bags used for packaging of meat and other products, especially three- or four-layer composite bags such as BOPA//A]//CPP and BOPET//A1//BOPA//CPP, due to high technical difficulty There is still a considerable amount of profit. Therefore, many flexible packaging manufacturers hope to increase their orders for cooking and packaging products. However, due to the high production technology of retort pouches, it is necessary to establish an optimal production process route to ensure that the final product is qualified. This article only starts from cooking-resistant compound inks for simple analysis for your reference.

The choice of cooking resistant compound ink

Before 1998, few manufacturers were able to manufacture cooking-resistant compound inks, mainly DIC and Toyo Ink. In the past five years, cooking-resistant compound inks have developed rapidly. Domestic well-known ink manufacturers have corresponding supply of ink products, such as Lamistar of Dongyang, Univure of DIC, 300LM of Zhejiang New Oriental, and GLP of Yip's ink, making domestic flexible packaging. The choice of manufacturing companies has greatly increased.

At present, common domestic cooking-resistant composite inks include one-component polyester inks and two-component polyurethane inks. For a two-layer structure with boiled and sterilized bags (such as BOPA//CPP) at about 100°C, one-component polyester composite inks can be selected, such as 321F from Zhejiang New Oriental. Yip's GLP, Yongcheng's Type 99 ink. If it is the production of three- or four-story high temperature cooking bag more than 1 21'c, generally choose two-component polyurethane ink, the current domestic market is more widely used Dongyang discretion Lamistar, D, C Univure, Zhejiang New Oriental 300LM.

In short, because steamed packaging bags are high value-added products, flexible packaging manufacturers should choose to buy brand products that are slightly more expensive but have more stability when they choose to use retort-retarded compound inks. They must not be cheap at the moment and buy poor quality inks, causing quality problems. The accident caused economic losses. There are many small-scale ink manufacturers can also provide cooking-resistant composite ink, but the poor product quality and stability, flexible packaging manufacturers need to be careful when buying.

The main features of polyurethane ink

The performance of polyurethane inks is slightly different from the chlorinated polypropylene inks used in BOPP printing on the market today, and the method of use is also different.

First, the composition of the diluting solvent is different. The diluent solvent for chlorinated polypropylene inks is benzene-based solvent, supplemented with esters and ketones. Polyurethane ink uses butanone as the main solvent and acts as a dissolving resin. Because the volatilization rate of butanone is faster than that of toluene, it is necessary to ensure the use of butanone in the mixed solvent when using a cooking-resistant polyurethane ink in order to prevent poor solvent solubility or poor ink transferability caused by excessive volatilization of the true solvent. The content reaches a certain proportion. Taking 300LM cooking-resistant composite ink of New Oriental Ink Company as an example, the content of butanone in the mixed solvent is required to be not less than 30%.

Second, polyurethane inks have a two-liquid reaction type. When production requires retort pouches that have been sterilized at temperatures above 121°C, a two-fluid reactive polyurethane ink must be used, and an appropriate amount of hardener must be added. Stir well before printing. The remaining two-fluid reaction type ink after use is preferably filtered first, and then partially added to the new ink, or used as a diluting solvent, but when the residual ink shows agglomerates or severe swelling, it cannot be used anymore.

Third, the polyurethane ink has good composite properties, and the peel strength after compounding is quite high. Generally, the peel strength of chlorinated polypropylene ink after printing is 1.5-4. ON/1 5mm, while the polyurethane ink can reach 3-8N/15mm, and the compound printed with the cooking-resistant polyurethane ink is sterilized at a temperature of 1 21°C, and its peel strength is not reduced.

Some points in use

Let's talk about the following points in the use of cooking-resistant composite ink.
First of all, the printing ink should pay attention to distinguish between ordinary resistance to cooking type, high temperature cooking type and resistance to high temperature cooking type, according to actual needs.

Secondly, it is advisable to perform confirmatory tests before using retort-resistant composite inks, especially the compatibility of retort-resistant composite inks with BOPA films, and the compatibility of retort-resistant inks and compounding adhesives so as not to affect the printing and compounding quality. . For example, the resin used in cooking-resistant composite ink is similar to the main component of two-component polyurethane adhesive. Due to the different reactivity of the curing agent in different types of adhesives, there is a big difference in the balance between use and large-format inks. When the printed pattern is compatible, wrinkling or delamination of the compound bag after steaming may occur.

Third, the pattern design of the steamed packaging bag should be simple and appropriate, and the color number should not be too much. Moreover, if the printing method is used for printing, taking into account the composite strength and heat sealing strength of the subsequent process, it is necessary to avoid full-page printing when designing the manuscript. In particular, the heat-seal position of the package should be designed to be transparent and not printed. .

Anti-sticking phenomenon analysis

When printing BOPA film or BOPET film with retort-resistant composite ink in summer, anti-sticking phenomenon may occur, especially in high humidity south. Since the BOPA film has strong hygroscopicity, when the relative humidity in the air reaches 80% or more, it is prone to quality defects. Therefore, it is necessary to control the ambient temperature and humidity of the workshop, otherwise it is impossible to produce a high-quality retort pouch.

There are two main reasons for the anti-sticking phenomenon.

One is that the retort-resistant composite ink itself has poor adhesion to the substrate. The adhesion fastness of polyurethane ink on BOPA film is not as good as that on BOPET film, and it is not as good as that of chlorinated polypropylene ink on BOPP film. If the winding is too tight, it may easily cause anti-adhesion. . Especially when the humidity in the printing room is high, the BOPA film absorbs moisture and shrinks after rewinding, causing anti-adhesion. So, when printing BOPA film in summer. The winding diameter should not be too large. The printing length of the film roll can be about 3000 meters. In addition, if printed BOPA film does not immediately recombine, it must be wrapped with aluminum foil, placed upright in a dry place, to prevent moisture absorption BOPA film.

The second reason is that the higher solvent residues in the prints cause anti-sticking, and this phenomenon often occurs on medium and low speed printers. Because the drying capability of the low- and medium-speed printing presses is not strong, and the printing viscosity of the cooking-resistant composite inks is high, the amount of ink applied is large, and the “dry-up” phenomenon of the printing patterns is more serious, and the residual solvent gradually volatilizes after winding up. Can cause anti-sticking phenomenon. Part of the pattern, especially the more overlapping parts of the edge lines. The ink layer is thick, and the tightness of the film roll after rolling is uneven, and it is also likely to cause anti-sticking phenomenon.

Polyurethane ink can adjust the viscosity of ink when it causes anti-sticking due to the phenomenon of ink surface “dry”. Make it print at low viscosity. At this point, less ink is printed and the ink layer is thin. With a large amount of dilution solvent, the binder molecules of polyurethane ink are relatively loose. The shielding ability against solvent evaporation is relatively weak. When the ink is transferred to the film, the solvent can quickly and completely separate from the ink layer in the drying tunnel, and it is not easy to produce a “dry” phenomenon.

Permeation Phenomenon Analysis

In the case of printing a high-temperature-resistance retort pouch with a BOPA film as the outer material when using a red-based cooking-resistant compound ink (such as pink, magenta, or pink hue ink), color penetration often occurs, resulting in serious quality accidents. . Moreover, because problems often occur in the later stages of processing, even after the contents have been packaged, the resulting economic losses are also large.

The author encountered a lot of this type of color penetration accident, through a lot of comparative tests found that the performance of the BOPA film and the characteristics of the cooking-resistant composite ink have a significant impact on the color penetration.

(1) Color infiltration often occurs in retort pouches with BOPA film as the outer layer material. Under the condition that other conditions remain unchanged, if the outer printing film is replaced with BOPET film or BOPP film, the color penetrant phenomenon will occur. disappear.

(2) Ink hues of color percolation appear mostly in red and purple. For this reason, we have selected different grades of red and purple pigments for comparison. When the moisture permeability of BOPA film is too high, the color penetration phenomenon cannot be completely eliminated. Only the degree of penetration is different.

(3) Temperature and water are the two basic factors that cause color penetration. In the same temperature conditions, the cooking packaging bag is placed in the oven heating, there will be no color penetration phenomenon; when the temperature is lower than 70 °C, after the usual time of sterilization treatment, cooking packaging bags will not appear color penetration phenomenon.

The results of the above three points were analyzed: Nylon resin is a semi-crystalline to amorphous thermoplastic polymer, the lower the degree of crystallization, the greater the change in alignment between molecules when the temperature rises, making the possibility of pigment migration more Big. The water vapor transmission rate of BOPA film is 20-40 times that of BOPET film. In the process of boiling water, water molecules continuously come in and out of BOPA film, washing and taking away the pigment components in some printing inks (relatively have water-soluble Sex pigments, such as red pigments, cause color penetration. At the same time, the inner layer material CPP film of the steamed packaging bag and the moisture permeability of the polyurethane glue layer are only 1/20 to 1/40 of the BOPA film, under the combined effect of the two. Prevents the penetration of water molecules into the inner layer of the package, and when the infiltration of water molecules is limited to the two layers of BOPA outer membranes in close contact with each other, a small amount of dissolved pigment particles (the amount of dissolved pigments varies depending on the grade of the pigment). As the water molecules infiltrate each other, we always see the color permeation or color diffusion between the bag and the bag, but rarely permeate the inner layer, unless it is printed with common non-retortable composite ink. Production of bags.

Zhejiang New Oriental Ink Group Co., Ltd.

Source: Printing Technology