In the compound production of flexible packaging, there is a kind of hidden quality defect: the off-line detection strength of the finished product is up to standard completely, and there is no abnormality in curing, cutting and bag making, but after storage and standing, high-temperature transportation and long-term storage, there are some problems such as slow delamination, foaming, whitening and strength attenuation. This lagging aging failure is not an immediate process defect, but the lack of hydrolysis resistance, aging resistance and damp-heat stability of
Polyurethane Adhesive system, which is also the core reason for the high-end packaging and cooking packaging customer complaints.
The molecular structure defect of glue is the root cause of poor aging resistance. The molecular chain of general-purpose polyurethane adhesive contains a large number of easily hydrolyzable ester groups, which are easily broken and degraded in humid, hot and acid-base media for a long time. At the initial stage, the cross-linking was normal and the strength reached the standard. However, with the passage of time, the adhesive layer gradually aged and softened, and the cohesion decreased, and finally interlayer peeling and bubbling delamination appeared. Low-quality glue has many small molecular residues and loose cross-linking structure, and the aging decay rate will be greatly accelerated.
The hidden danger of production process will bury the hidden danger of later failure. Insufficient coating amount and thin adhesive layer can not form a complete and dense protective film, and water vapor, grease and solvent can easily penetrate and destroy the bonding interface; The mixture ratio is deviated, the curing is not complete, and there are a large number of unreacted active groups in the adhesive layer, which continue to hydrolyze and oxidize slowly in the later stage, gradually destroying the adhesive system. In addition, the water content in the solvent, the high humidity in the workshop and the residual water vapor on the film surface will weaken the aging resistance of the adhesive layer and accelerate the poor delamination in the later stage.
Accelerated aging failure of media and storage environment. Grease, salt, acid-base soup in food packaging, high temperature and humidity in storage and large temperature difference between day and night will continue to erode the composite interface. Ordinary PU adhesive is weak in medium resistance and hydrolysis resistance. Under long-term immersion and infiltration, the bonding interface gradually fails, resulting in batch lag delamination, resulting in serious after-sales losses.
Improving aging delamination requires glue selection and process closed-loop control. Cooking and long-term storage products must use modified
Polyurethane Glue with hydrolysis resistance and aging resistance, and the dense cross-linked structure can resist medium erosion and molecular degradation. The production strictly controls the coating dry amount, the accurate proportion and the full curing, and eliminates the half-baked curing. Strictly control the solvent purity and workshop humidity, ensure the high cross-linking density of the adhesive layer, improve the long-term stability of the composite film from the source, and completely solve the hidden danger of aging and delamination in the later stage.
