Quality issues caused by improper ripening control of packaging bags: a case study

I. Low strength of composite PE film

A Hunan factory produced BOPA/PE packaging bags that were returned due to low strength.

After peeling apart the two layers of film and wiping the PE film with fingers, a slippery feeling was felt between the two fingers. Excessive release agent was present in the film.

The adhesive used in this product is PU-1850, which has a certain resistance to release agents.

In this factory's curing room, the thermocouple for temperature measurement was parallel to the heat source, only 30-40 centimeters away. Under such control, the heat source would be in a state of stopped heating for a long time, and the actual curing temperature could not reach 50℃. Furthermore, the curing time with the power supply turned on was less than 16 hours.

Adjusting the thermocouple position and extending the curing time achieved the required peel strength.

When compounding PE films with high release agent content, an adhesive with certain resistance to release agents should be selected. The film rolls should be placed in the curing room as soon as possible after leaving the machine, and the curing temperature should be appropriately increased. Generally, ester-soluble two-component polyurethane adhesives have poor natural curing resistance to release agents.

II. Increased coefficient of friction

A Guangdong factory produced BOPP/VM-PET/PE structured film rolls, which had no problems during winter production. In June, the coefficient of friction increased, and the packaging machine could not operate normally. Even after natural curing, the film rolls were still unusable.

This product uses a 30μ thick PE film, which had good lubricity before lamination. During lamination, the cooling roller was not cooled with water, and the surface of the cooling roller was warm. The film still retained heat from the oven conveyor belt before winding, and the film was also warm during winding. Although it was naturally cured in the workshop, the summer temperature was high (the workshop temperature was 30℃ at 10 am), and the heat from the film roll was difficult to dissipate, so it was actually in a hot state.

The laminated film was taken from the site and placed in environments of 27℃ and 45℃ for about 20 hours. The film placed at 27℃ had good lubricity, while the film placed at 45℃ felt rough, and the lubricity was significantly reduced, indicating that temperature has a significant impact on the product's lubricity.

Similar to some thin transparent CPP films that are sensitive to temperature, the coefficient of friction of CPP is below 0.1 before lamination, but after lamination, only natural curing can be used.

III. Delamination during steaming

A Wenzhou factory produced salted egg packaging bags. After sterilization at 115℃ for 30 minutes in a food factory, delamination of the BOPA occurred in the top layers of bags in the sterilizer, and the ink transferred to the CPP.

Shunxingyuan Packaging believes that the quality of the adhesive and the amount of adhesive applied meet the product requirements, and the curing at 50℃～55℃ for 48 hours also meets the requirements. The problem is most likely caused by the ink.

After curing the sample bag again at 55℃ for 24 hours, there was no delamination after steaming at 121℃ for 30 minutes; however, all the un-cured samples steamed at the same time showed delamination.

The direct manifestation of this problem is ink transfer, and the cause of this phenomenon is insufficient curing.

Later, when other technical personnel from the company visited the factory, they found that the actual curing time was around 36 hours, indicating inconsistency between the factory's process requirements and actual execution.