レトルトパウチの層間剥離の原因分析（Ⅰ）
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.
