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In the process of flexible packaging lamination, whether it is dry lamination or solvent-free lamination, the adhesive does not dry is one of the common problems. Many companies pay less attention to the production process and subsequent testing, and only discover defects when making bags, or when downstream customers file quality complaints about packaging materials, they discover that the adhesive is not dry.
In order to improve product quality and avoid losses, we should tailor our production process to local conditions and eliminate factors that affect the dryness of the lamination film adhesive layer based on actual situations. At the same time, appropriate process conditions should be developed and monitored during production, which is the best way to avoid the problem of adhesive not drying.
The non drying of adhesive directly affects its temperature resistance, thermal peeling, and reduced lateral shear force.
After the substrate is heated, the thermal shrinkage rate between different materials is different, which will generate certain internal stress. When the shear force of the adhesive after heating is less than the internal stress, relative displacement delamination and other phenomena will occur between the composite substrates. Common problem phenomena include: heat sealed edge rolling, heat sealed tunnel, water retorting delamination, steaming delamination, exposed aluminum, poor heat sealing strength, white folded edges, zipper delamination, and decreased tear resistance.
According to the degree of non drying of the adhesive, different degrees of adverse phenomena may occur in subsequent processing. The higher the temperature resistance requirement and the unchanged performance of the glue itself, the higher the requirement for the dryness of the adhesive, such as zipper bags, octagonal seals, and packaging that require boiling or retort. Therefore, during the lamination curing process, it is necessary to try to make the adhesive layer as dry as possible.
Usually refers to the curing process of laminated film that does not reach the recommended curing temperature and curing time by the adhesive supplier, the adhesive reaction rate does not reach the ideal state, there is surplus of – OH and – NCO, and the adhesive starts subsequent processing before it dries completely.
The influencing factors are as follows:
① Insufficient aging temperature
When formulating the process for flexible packaging enterprises, for a certain purpose (such as reducing the friction coefficient), they intentionally set the curing temperature lower than the recommended value by the adhesive supplier, resulting in a slower reaction rate and inability to achieve the desired cross-linking degree and reaction rate of the glue within the specified time.
② The actual value is lower than the set value
The placement of thermocouples in the curing chamber is unreasonable, with hanging positions too high or too close to the heat source, resulting in the end face and surface temperature of the rolled film being lower than the ideal curing temperature.
③ Uneven aging temperature
The design or aging of the maturation room results in uneven internal temperature cycling, with significant temperature differences between the upper and lower layers, as well as in the middle of the periphery, leading to poor curing in some areas.
④ Insufficient maturation time
Some flexible packaging companies aim to improve efficiency, shorten construction periods, or rush orders in order to reduce aging time.
Insufficient curing agent is usually due to excessive consumption of curing agent by external factors under the recommended ratio by the glue supplier, resulting in excess – OH and causing the adhesive to not dry.
From the above principle of adhesive reaction, it can be seen that – NCO easily reacts with – OH, – NH2, and other substances in H2O or alcohols. The influencing factors are as follows:
① Water vapor consumption curing agent
When the environmental humidity is high, especially during the hot and humid summer season, a large amount of water vapor is easily carried into the glue, diluent, substrate, and composite process. Solvent free lamination is mainly prone to the introduction of water vapor during the lamination process and material storage. Solvent free materials are sensitive to moisture, and without adjusting the ratio of A and B adhesives when the environmental humidity changes, it is easy for the adhesive to not dry.
During dry lamination, solvent evaporation takes away heat, causing the temperature of the carrier (adhesive itself, mesh roller, scraper, adhesive film) to be lower than the ambient temperature. When the water vapor dew point is reached, the water vapor will condense into small water droplets and merge into the adhesive, consuming too much – NCO, especially in high temperature and high humidity seasons, the phenomenon is particularly obvious.
② Alcohol consumption curing agent
There are usually small amounts of alcohol substances in the dilution solvent of ink or glue, such as isopropanol used in printing, methanol and ethanol contained in ethyl acetate.
③ Ink consumption curing agent
The – NH2 and ternary chloroacetic acid in the ink react with – NCO during curing. The degree of NCO consumption varies among different manufacturers and colors of ink. The order of the amount of curing agent consumed from high to low is green, bright red, ultramarine, peach red, gold, silver, and transparent colors. Some soft packaging companies may add an appropriate amount of curing agent or adjust the adhesive ratio to the main color ink when encountering the above-mentioned colors in the process of developing the technology.
④ Additive consumption curing agent
In my work, I have found that some special additives in PE (whose specific components have not been determined) also consume – NCO. When the matting OPP additive precipitates and sticks back, and the matte oil sticks back to the composite layer, it also consumes – NCO.
Generally, adhesive manufacturers will allow an excess of – NCO when recommending the ratio of A and B adhesive, which is a margin left after considering the environment and normal ink consumption. However, when external factors decrease or the summer ratio is not adjusted to the winter ratio in a timely manner, it can cause an excess of – NCO. But as the aging time prolongs, the final – NCO will gradually be consumed by the water molecules that penetrate into the adhesive layer. Therefore, excessive NCO cannot dry completely within the normal curing time, and further curing is necessary to dry completely, but the strength attenuation is more significant.
Excessive solvent residue during ink printing or drying can also cause the glue to not dry. Excessive solvent molecules form a “barrier” between the A and B adhesive groups, hindering the reaction between some A and B molecules. After curing, the adhesive layer shows a sticky state.
To determine whether the dryness of the adhesive meets the requirements for subsequent applications, the method of digitizing the dryness of the adhesive is to detect the “secondary adhesion” of the peeling layer, commonly referred to as the “rebound strength”. Cut the sample to the specified size, peel off the lamination layer, and then manually reattach the peeled part. Test the “secondary peeling force” on the tensile machine, and the data obtained is the “adhesive strength”.
According to the regulations of some large flexible packaging companies, the “adhesive strength” should not exceed 0.5N/15mm, which will not affect the subsequent bag making performance. However, in practice, users also need to have different requirements for the dryness of the adhesive based on the different substrate structures, subsequent bag making temperature and pressure, and different applications (boiling, steaming).
When the adhesive layer is found to be sticking back, the usual method is to continue curing the composite film and observe whether the sticking situation is weakened. If there is improvement, it is due to insufficient curing time causing insufficient adhesive reaction rate or excessive – NCO. On the contrary, it is an excess of – OH, and no matter how long it is left for, the sticking situation will not change.
Alternatively, the peeled surface can be “deflated” after peeling and placed in an open environment to ensure that the peeled surface is not contaminated by grease on the hands or dust in the environment. After leaving for several hours, it can be reattached and observed for any reduction in adhesion. If there is improvement, it indicates that the excess – NCO will dry out after being consumed by water vapor.
In summary, there are multiple reasons and factors that can affect or cause the adhesive of lamination films to not dry. If users discover the phenomenon of “adhesive not drying” in the actual production process of lamination films, they should investigate and analyze it from multiple dimensions, confirm and solve it one by one, and form effective corrective and preventive mechanisms. The above content is a summary and analysis of the relevant problem-solving methods that the author has developed through practical communication and on-site problem investigation with users.
Mostly importantly, when it comes to mono-material PE or PP lamination, this will become more important for the adhesive to become dry. Cause the mono-material itself is more difficult to do lamination, and easier for problems occur.
Longdapac engineers are doing sufficient Research & Development on the lamination of mono-material film. So the quality is very good now.