Precautions for high-temperature steaming bag production process

1. Balanced substrate thickness, good corona treatment effect, and high heat shrinkage resistance
 
Commonly used base materials for high-temperature retort pouches include PET, NY, Al, and RCPP. The corona treatment effect of these base materials directly affects the composite strength of the high-temperature retort pouch; during the high-temperature retort process, the base materials will shrink to varying degrees. If the corona treatment of the material does not achieve the expected effect, the composite strength is low, and there is not enough force between the layers of the composite pouch to prevent the base material from shrinking, resulting in severe delamination of the pouch layers after high-temperature retorting. Generally, the surface tension of PET and NY used for high-temperature retort pouches should reach 50 mN/m, and RCPP should reach 40-42 mN/m. In addition, some additives in the base materials, such as anti-blocking agents, anti-slip agents, and antistatic agents, accelerate their release to the surface at high temperatures, which may lead to a decrease in adhesion. Since high-temperature retort pouches are basically in water during the sterilization process, when measuring the shrinkage rate of the substrate, the substrate shrinkage rate should be measured at 120℃ for 30 min. The shrinkage rate should be small and stable, especially for BOPA film, RCPP film, and SCPP film. The longitudinal and transverse moisture-proof shrinkage rate is recommended not to exceed 1.5% to be considered a retort-grade film.
 
The balanced thickness of each layer of the material should be uniform, and the thickness deviation should not exceed 7%; the physical and mechanical performance indicators should meet the requirements.
 
When using aluminum foil products, in addition to its mechanical performance parameters, the surface of the aluminum must be dry, and the cleanliness of A and B must meet the standards, that is, when checked with distilled water, they must be wet, and the number of pores must be as small as possible. The storage time before use should not exceed three months. To improve the composite strength, the BOPA in the intermediate composite should use double-sided corona treatment, and the corona value should not be less than 50 dynes.
 
2. Compatibility of cooking inks, adhesives, and substrates
 
The production of composite packaging bags is a process of superimposing the various properties of various materials, so it requires that the base materials, inks, adhesives, etc., have good compatibility and resistance to high temperatures, humidity, and heat. The degree of matching between them is very important. If the matching is not good, the processed products may have poor appearance or delamination during the retorting process. According to the adhesion theory, when the adhesive and the adherend have the same surface tension and the same polarity, the interfacial tension is minimized, and the adhesion strength is the highest. Therefore, the surface condition of the ink and adhesive after curing into a film and the surface condition of the substrate may vary depending on the manufacturer's products. Whether the groups on different surfaces can be attracted to form intermolecular forces or undergo chemical reactions to form chemical bonds determines whether various surfaces have good adhesion. Therefore, when making retort pouches, the resin type of ink and adhesive should be fully considered to avoid poor overall performance of the retort pouch due to incompatibility between ink, adhesive, and substrate.
 
The inks or adhesives used for high-temperature retort pouches are generally two-component polyurethane resin types. The heat resistance of inks or adhesives mainly depends on the heat resistance and moisture resistance of the polyurethane resin. High temperatures improve the reaction characteristics of ink and adhesive. If the heat and moisture resistance of the ink and adhesive are insufficient, hydrolysis will occur at high temperatures, and the composite strength will decrease. From the characteristics of the polyurethane resin system, the amino and ester groups in the resin have strong polarity and are easily eroded and hydrolyzed by moisture under wet heat conditions. The hydrogen bonds between the polyurethane resin and the substrate surface are easily broken, reducing the bonding strength, and even causing the bonding layer to peel off. Currently, the inks and adhesives used for high-temperature retort pouches are modified with resins or additives to improve the adhesion and heat resistance of the inks and adhesives. Therefore, the quality of the modified resin used in the ink and adhesive determines the quality of the ink and adhesive. Select RCPP and SCPP for the heat-sealing layer according to different retort temperatures.
 
RCPP or SCPP is usually used as a heat-sealing material above 121℃, and the thickness must be greater than 60 μm. 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 adhesive application
 
Due to the special sterilization method and usage conditions of high-temperature retort pouches, high-temperature retort pouches need to have good composite strength to reduce the impact of wet heat conditions on the quality of high-temperature retort pouches, and the composite strength is closely related to the adhesive application and curing time. Generally, the adhesive application of high-temperature retort pouches needs to reach more than 4 g/m2; curing time is another important factor affecting composite strength. The curing time of high-temperature retort pouches generally needs to reach more than 72 h. Sometimes, even if this time is not reached, the adhesive layer is dry after the bag is peeled open, and it seems that the curing is complete, but in the actual high-temperature processing process, due to the residual stress inside, it will still be far different from the product with sufficient curing time in terms of heat resistance and moisture resistance, and in the subsequent bag-making process, it is easy to produce heat-induced delamination. Therefore, in the production process of high-temperature retort pouches, one should not blindly pursue speed, but should prioritize the quality of high-temperature retort pouches and strictly follow the process requirements for production.
 
4. Gold and silver inks are not suitable for high-temperature retort pouches
 
Gold and silver inks are very prone to penetration of pigments, such as crimson, bright blue, and purple inks. When they encounter water, coupled with high temperatures, dissolution is inevitable. The copper powder and aluminum powder used in gold and silver inks are metals with relatively high activity and are easily chemically reacted with water, oil, and oxygen at high temperatures. After oxidation, the appearance shows blackening, greening, or even fading. Therefore, gold and silver inks and inks that are easily penetrated cannot be used for high-temperature retort pouch products.