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Precautions for the Production Process of High-Temperature Retort Pouches

Release date:

2023-01-13 08:37

Author:

Bean bag

1. Balanced base material thickness, excellent corona treatment performance, and high heat‑shrink resistance.
 
Common base materials for high‑temperature retort pouches include PET, NY, aluminum, and RCPP. The effectiveness of corona treatment on these substrates directly affects the composite bond strength of the pouch; during high‑temperature retorting, the substrates exhibit varying degrees of shrinkage. If the corona treatment fails to meet expectations, the composite strength will be low, and insufficient interlayer adhesion will fail to counteract substrate shrinkage, resulting in severe delamination after retorting. In general, the surface tension of PET and NY used in high‑temperature retort pouches should reach at least 50 mN/m, while that of RCPP should be 40–42 mN/m. Moreover, certain additives—such as anti‑blocking agents, slip agents, and antistatic agents—can accelerate migration to the surface under high temperatures, potentially leading to reduced adhesion. Since high‑temperature retort pouches are typically immersed in water during sterilization, the dimensional shrinkage of the substrates should be measured at 120°C for 30 minutes. Shrinkage should be minimal and stable; in particular, for BOPA, RCPP, and SCPP films, the longitudinal and transverse moisture‑induced shrinkage is recommended to remain below 1.5% to qualify as a retort‑grade film.
 
The balanced thickness of each layer of the material shall be uniform, with a thickness deviation not exceeding 7%, and its physical and mechanical performance indicators shall meet the specified requirements.
 
When using aluminum foil products, in addition to their mechanical performance specifications, the aluminum surface must be dry, and both sides A and B must meet strict cleanliness standards—specifically, they must be wet when tested with distilled water, and the number of pinholes should be kept as low as possible. Furthermore, the storage period before use should not exceed three months. To enhance composite adhesion, the BOPA layer in the laminate should undergo corona treatment on both sides, with a corona treatment level of no less than 50 dynes.
 
2. Compatibility of cooking inks, adhesives, and substrates
 
The production of composite packaging bags involves superimposing the diverse properties of various materials, which necessitates that the base film, inks, adhesives, and other components exhibit excellent compatibility as well as resistance to high temperatures, humidity, and thermal stress. The degree of compatibility among these materials is critical; poor matching can lead to undesirable appearance or delamination during retort processing. According to adhesion theory, when the adhesive and the substrate possess identical surface tensions and similar polarities, interfacial tension is minimized and bond strength is maximized. Consequently, the surface characteristics of inks and adhesives after curing into a film, as well as those of the substrate, may vary depending on the manufacturer’s product. Whether functional groups on different surfaces can interact—either through intermolecular forces or by forming chemical bonds—determines the overall adhesion performance. Therefore, when manufacturing retort pouches, careful consideration should be given to the resin types of inks and adhesives to prevent poor overall performance resulting from incompatibility among inks, adhesives, and the substrate.
 
Inks and adhesives used for high‑temperature retort pouches are typically two‑component polyurethane resin systems. The heat resistance of these inks and adhesives depends primarily on the heat‑resistance and moisture‑barrier properties of the polyurethane resin. Elevated temperatures can alter the reactive characteristics of the ink or adhesive; if their temperature and humidity resistance is inadequate, hydrolysis may occur at high temperatures, leading to a reduction in bond strength. From the perspective of polyurethane resin systems, the amino and ester groups within the resin exhibit strong polarity, making them susceptible to moisture attack and hydrolysis under humid‑heat conditions. Hydrogen bonds between the polyurethane resin and the substrate surface can be easily disrupted, weakening adhesion and even causing the adhesive layer to delaminate. Currently, inks and adhesives for high‑temperature retort pouches are modified with resins or additives to enhance their adhesion and heat resistance. Consequently, the quality of the modified resins employed in inks and adhesives directly determines the overall performance of these materials. Depending on the specific retort‑cooking temperature, appropriate RCPP and SCPP films are selected for the heat‑seal layer.
 
RCPP or SCPP is typically used for heat-sealable materials at temperatures above 121°C, and its thickness must exceed 60 µm. It is generally recommended to use imported products. The inner layer of imported RCPP contains rubber components, consisting of block copolymers and impact‑resistant polypropylene.
 
3. Control of curing time and adhesive application amount
 
Due to the unique sterilization process and operating conditions of high‑temperature retort pouches, they must exhibit excellent lamination strength to mitigate the adverse effects of humid‑heat environments on product quality. Lamination strength is closely linked to both the amount of adhesive applied and the curing time. Typically, the adhesive application rate for such pouches should exceed 4 g/m². Curing time is another critical factor influencing lamination strength; for high‑temperature retort pouches, it generally needs to be at least 72 hours. Even when this duration is not fully met, the adhesive layer may appear dry upon opening, suggesting complete curing. However, during subsequent high‑temperature processing, residual internal stresses persist, resulting in significantly inferior heat‑ and moisture‑resistance compared with products that have undergone adequate curing. Moreover, during downstream pouch‑forming operations, these pouches are prone to delamination under heat. Therefore, in the production of high‑temperature retort pouches, haste should be avoided; product quality must take precedence, and manufacturing should strictly adhere to established process specifications.
 
4. Gold and silver inks, among others, are not suitable for use in high-temperature retort pouches.
 
Gold and silver inks are highly prone to color bleeding; for example, when cinnabar red, iridescent blue, and purple‑toned inks come into contact with water—and especially under elevated temperatures—dissolution is virtually inevitable. The copper and aluminum powders used in these inks are highly reactive metals that readily undergo chemical reactions with moisture, oils, oxygen, and other substances at high temperatures. Following oxidation, the resulting discoloration may manifest as darkening, greening, or even fading. Consequently, gold and silver inks, as well as other inks susceptible to penetration, should not be employed in high‑temperature retort pouch applications.

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