Precautions for the production process of high-temperature retort pouches

1. The thickness of the base metal is balanced, the corona treatment effect is good, and the heat shrinkage rate is high

The commonly used high-temperature retort pouch substrates are PET, NY, AL, RCPP, etc. The corona treatment effect of these substrates directly affects the composite strength of high-temperature retort bags; During the high-temperature cooking process, the substrate shrinks to varying degrees. The corona treatment of the material did not achieve the expected effect, the composite strength was low, and there was not enough force between the composite bag layers to hinder the shrinkage of the substrate, resulting in serious delamination of the bag layer after high temperature cooking. In general, the surface tension of PET and NY used for high-temperature retort pouches should reach 50mN/m, and RCPP should reach 40~42mN/m. In addition, certain additives in the substrate, such as anti-blocking agents, anti-slip agents, and antistatic agents, are released to the surface at an accelerated rate of high temperatures, which can lead to a decrease in adhesion. Since the high-temperature and high-temperature retort pouch is basically in water during the sterilization process, when measuring the matrix shrinkage, the matrix shrinkage should be measured at 120 °C for 30min. The shrinkage rate should be small and stable, especially for BOPA film, RCPP film, SCPP film, and the longitudinal and horizontal moisture-proof shrinkage rate is recommended to be no more than 1.5%, which can be called cooking grade film.

The balance thickness of each layer of the material should be uniform, the thickness deviation should not exceed 7%, and the physical and mechanical properties should meet the requirements.

With aluminum foil products, in addition to its mechanical property parameters, the surface of aluminum must be dry, A and B and clean must meet the standards, that is, when checking distilled water, they must be wet, and the number of pores must be as small as possible. Well, it can't be stored for more than three months before use. In order to improve the fastness to the compound, the BOPA of the intermediate compound should be double-sided corona, and the corona value should not be less than 50 dynes.

2. Compatibility of cooking inks, glues and substrates

The production of composite packaging bags is a process of superimposing various properties of various materials, so it requires that the substrate, ink, glue, etc. have good compatibility and high temperature, humidity and heat resistance. The degree of matching between them is very important. If the match is not good, the processed product may have a poor appearance or delamination during the cooking process. According to the bonding theory, when the adhesive and the adherent have the same surface tension and the same polarity, the interfacial tension is the lowest and the bond strength is the highest. As a result, the surface condition of inks and glues after curing into films and the surface condition of substrates may vary depending on the manufacturer's product. Whether groups on different surfaces can be attracted to form intermolecular forces or chemical reactions to form chemical bonds. It determines whether the various surfaces have good adhesion or not. Therefore, the resin type of ink and glue should be fully considered when making steaming bags, so as to avoid poor overall performance of steaming bags due to incompatibility of inks, glues and substrates.

The ink or glue used for high-temperature retort pouches is generally a two-component polyurethane resin type. The heat resistance of an ink or glue depends mainly on the heat and moisture resistance of the polyurethane resin. The high temperature improves the reactivity characteristics of the ink adhesive. If the temperature and moisture resistance of the ink and glue is insufficient, hydrolysis will occur at high temperatures, and the composite strength will be reduced. From the characteristics of the polyurethane resin system, the amino and ester groups in the resin have extremely strong polarity, and are susceptible to moisture erosion and hydrolysis under humid and hot conditions. The hydrogen bond between the polyurethane resin and the surface of the substrate is easily broken, reducing the bond strength and even peeling off the bond layer. At present, the inks and glues used in high-temperature retort pouches are modified with resins or additives to improve the adhesion and heat resistance of the inks and glues. Therefore, the quality of the modified resins used in inks and adhesives determines the quality of inks and adhesives. RCPP and SCPP of the heat sealing layer are selected according to the different cooking temperatures.

RCPP or SCPP is usually used for heat sealing materials above 121°C, and the thickness must be greater than 60um. It is generally recommended to use imported products. The inner layer of imported RCPP contains rubber components, which are block copolymers and impact-resistant polypropylene.

3. Control of curing time and amount of glue

Due to the special sterilization method and use conditions of the high-temperature retort bag, it is determined that the high-temperature retort pouch needs to have good composite strength to reduce the influence of damp heat conditions on the quality of the high-temperature retort pouch, and the composite strength is closely related to the amount of glue and the curing time. Generally, the gluing amount of high-temperature retort pouch needs to reach more than 4g/m2; Curing time is another important factor affecting the composite strength, the curing time of the high-temperature cooking bag is generally more than 72h, sometimes although this time can not be reached, the adhesive layer is also dried after peeling off the bag, it seems that the curing has been complete, but in the actual high-temperature treatment process, because there is still residual stress inside, it will still be far from the product with enough curing time in terms of heat resistance and humidity resistance, and in the subsequent bag-making process, it is easy to produce the phenomenon of thermal separation. Therefore, in the production process of high-temperature and high-temperature retort bags, we should not blindly seek speed, and should focus on the quality of high-temperature retort pouches and produce in strict accordance with the process requirements.

4. Gold, silver and ink should not be used as high-temperature retort bags

Gold and silver ink is very easy to penetrate the pigment color, such as red, light blue, purple ink when exposed to water, coupled with high temperature, it is certain that it will dissolve. The copper powder and aluminum powder used in gold and silver ink are relatively active metals, which are easy to chemically react with moisture, grease and oxygen at high temperatures. After the reaction oxidation, the appearance is blackened, greenish or even faded. Therefore, gold, silver ink and inks that are easy to penetrate cannot be used as high-temperature retort pouch products.