Aging
Aging is a critical post-lamination process conducted at 45-55°C for 24-72 hours to ensure complete cross linking of two-component polyurethane adhesives, resulting in enhanced bond strength, thermal resistance, and chemical stability, while effectively reducing residual solvent content.
The cross linking rate of polyurethane adhesives increases with temperature. While curing occurs at room temperature, it is slow (7-10 days) and results in lower peel strength compared to heat-assisted aging. However, aging temperatures must be carefully controlled, as excessive heat accelerates the loss of slip agents in PE and PP inner layers. This raises the coefficient of friction (COF), potentially causing poor machinability, sealing issues, and pouch-making problems like inconsistent lengths and reduced opening performance.
The ageing time is determined by the cross linking characteristics of the adhesive and performance requirements. The higher the demands for heat and chemical resistance in downstream processing, the longer the ageing time required.
The reaction between the main agent and aging agent doesn't need to reach 100%, but for emergency retort pouch production, aging for the specified time is still crucial. While peel strength may be similar, heat and corrosion resistance will vary. A aging temperature of 40°C requires at least one more day than 50°C. Therefore, we typically require a aging temperature of 50-55°C for retort pouches.
