Causes Analysis of Delamination in Retort Pouches

Delamination in retort pouches manifests in interface damage at three key layers: the inner film/adhesive, adhesive/ink, and ink/outer film. It can also occur as cohesive failure within the adhesive layer or the ink layer. When delamination is minor, it appears as stripe-like separation in the stressed areas of the packaging, or the pouch can be easily peeled or torn apart by hand. In more severe cases, delamination can result in large-scale separation after the retort process. The main causes of the above phenomena are as follows:

1. The ink or adhesive doesn't have enough resistance to moisture and heat, which weakens the bond.

The ink or adhesive used in retort pouches, whether a two-component or single-component system, is generally made from polyurethane resin, which mainly determine the moisture and heat resistance of the ink or adhesives. Specifically, due to the presence of ester and urethane groups in polyurethane resin, which have strong polarity, the material is prone to hydrolysis under humid and high-temperature conditions. Additionally, the hydrogen bonds formed between the polyurethane resin and the substrate surface are easily broken under such conditions, leading to a decrease in bonding strength, and in severe cases, adhesive layer separation. Ink or adhesive manufacturers can technically improve the adhesion strength of retort inks after high-temperature processing by altering the chemical structure of the polyurethane resin and using additives.

2. There are issues with the application of the ink or adhesive.

① The amount of hardener does not meet the requirements. The amount should neither be too little nor too much. If the amount is too low, the crosslinking between the hardener and the resin will be insufficient, leading to a decrease in the adhesive strength, heat resistance, and resistance to hydrolysis of the adhesive layer. If the amount is too high, excessive crosslinking may occur, which can affect the crystallization and microphase separation between polymers, potentially damaging the cohesive strength of the adhesive layer. This can increase internal stress, cause excessive shrinkage of the ink layer, and lead to delamination.

② The hardener ratio is only one factor affecting the actual adhesive ratio. In practice, the hardener may be consumed by moisture, alcohols, and other substances in the ethyl ester, causing the actual reaction ratio to differ. Additionally, the storage time of the adhesive and insufficient drying capacity of the equipment can also affect the actual crosslinking density of the adhesive.

③ If the aging time is insufficient, it will result in incomplete crosslinking, which reduces the heat resistance and hydrolysis resistance of the ink layer.

④ If the ink or adhesive lacks sufficient activity, or if excessive expired or ineffective residues are mixed in, the number of groups available to bond with the substrate will be reduced before printing or laminating. This results in weak adhesion.

1. Incompatibility between the ink, adhesive, and film.

According to adhesion theory, when the surface tension of the adhesive matches that of the substrate and their polarities are similar, the interfacial tension is minimized, resulting in maximum adhesion strength. Therefore, the surface conditions of the ink or adhesive after curing and the film surface may vary depending on the products from different manufacturers. When the surface groups of various materials cannot effectively attract each other to form intermolecular forces or chemically bond to create chemical bonds, adhesion might occur at low temperatures due to weak interactions. However, under high temperatures, these weak adhesive forces can be disrupted, leading to delamination.

2. Issues with the film

This issue mainly manifests in two aspects: First, the film treatment does not meet the requirements, or the film has not been treated at all, resulting in inadequate peeling strength and heat seal strength after sterilization. Second, certain additives in the film may migrate to the surface at high temperatures, potentially weakening adhesion. These additives can include opening agents, anti-static agents, lubricants, and other substances added to plastic films.

If delamination occurs in retort pouches after sterilization, the first step is to identify the interface where the delamination occurs and determine which two materials are involved. It is also crucial to verify whether the production process adhered to the required standards. Additionally, swapping materials for comparison experiments can help pinpoint the true cause of the delamination.

3. Issues with the retort process

①When using retort pouches, food companies should strictly control the temperature within the range specified by the manufacturer. Even a few degrees over this range can be critical, as the materials used to make the retort pouch are often already at their maximum tolerable limits. Whether the retort pouch's performance can still be guaranteed with these extra degrees becomes a key factor.

②Affected by the packed contents, the physical and chemical properties of the contents inside the packaging, along with any chemical changes or new substances that may form under high temperatures, can penetrate the layers of the retort pouch, leading to a decline in its performance, such as causing delamination.

③The impact of vacuum level. Even the vacuum level inside the retort pouch can create differences in stress between the layers of the pouch, leading to delamination between film layers with weaker adhesion.

④Back-pressure during sterilization. Under high-temperature conditions, the heat seal strength of the pouch is much lower than at room temperature. At this point, the pressure difference between the inside and outside of the retort pouch can cause excessive expansion, similar to inflating a balloon, potentially leading to rupture or leakage.