Solvent-free lamination process
In the solvent-free composite process, tension control is crucial and must be very precise. Tension control includes several aspects such as the tension of the main unwind, tension after coating, tension of the secondary unwind, winding tension, and winding taper. The initial tack of the solvent-free composite adhesive is zero, and tension matching must be strictly controlled. This characteristic of solvent-free composite imposes high demands on equipment, and a stable tension control system is the heart of solvent-free equipment. The setting of tension process parameters is an accumulation of experience for flexible packaging manufacturers. Different equipment and different film composites have significant differences, and only repeated trials can determine the optimal tension parameters. Nylon film becomes soft and wrinkled after absorbing moisture, and a slightly higher tension is suitable during the composite process.
Generally, the tension after coating the film should be slightly higher than the tension of the main unwind, and the winding tension should be slightly higher than the unwinding tension. It is good to control the winding taper within 20%. For different materials of films, the tension in each part during the composite process varies. Even films of the same material produced by different manufacturers may require slight adjustments to the tension. For example, in the composite film structure of PA/PE, the tension of the PE film is generally between 1.5 and 2.5N, and the tension of the PA film can be controlled between 7 and 15N, depending on the actual situation. A method to check if the tension is appropriate is to stop the machine during the composite process, use a blade to cut a cross at the winding position, observe the direction in which the composite film curls. If the film curls in a specific direction, it indicates that the tension of this layer of film is too high and needs to be reduced, or the tension of another layer of film should be increased. The ideal state is that the composite film remains flat after cutting the cross. Tension adjustment should be based on the adjustment of different material combinations.
The coating amount is a critical factor affecting product quality. To control the coating amount, it is necessary to adjust the distance between the two glue rolls. Pay attention to maintaining a consistent distance on both sides to ensure uniform coating from left to right. Also, maintain the good cleanliness of the coating roller, without any foreign particles. When adjusting the gap between the glue roll ends, it should be done after the steel rolls are fully preheated. The adhesive liquid level between the glue rolls should be kept as low as possible to add fresh glue to the rolls at any time. Since solvent-free composite adhesives have zero initial tack, it is challenging to predict the appearance or composite strength during the process. Unlike dry composite, where quick-curing small samples can be tested, the final performance can only be evaluated 24 hours after the composite product has cured. Due to low initial tack and low viscosity, specific requirements must be communicated when purchasing nylon film, and higher standards for surface smoothness should be specified. Different films require different coating tensions during the composite process.
Solvent-free composite cannot reduce the residual solvents caused by printing, so higher requirements are placed on controlling residual solvents from printing. Solvent-free adhesives do not contain organic solvents, so they do not increase residual solvents, but they also do not reduce the solvents left from printing. Only by controlling the residual solvents from printing to below 3mg/m2 can the odor of the final product be eliminated.
The commonly used intaglio ink for printing on nylon film is polyurethane resin ink. In the polyurethane system, the amine groups in the ink's binder can digest the curing agent of the adhesive, resulting in the ink not drying. Additionally, polyurethane ink systems often include trihydroxy modified chloroacetic resin, which is one of the factors causing the ink not to dry. Therefore, when using polyurethane ink, a hardening agent should be added, or when formulating the adhesive, ketone solvents should be used as true solvents to dissolve the resin in the ink. Commonly used solvents for polyurethane resin inks include butanone (methyl and ethyl ketone), isopropanol, toluene, and other solvents. Among these solvents, butanone has the fastest volatility. During the printing process, if the quantity of butanone is not replenished in time, the ink in the ink tank may expand due to poor solubility of the resin, causing an increase in viscosity and affecting the ink's adhesion to the film. Therefore, when using polyurethane ink, an adequate amount of butanone in the diluent solvent must be maintained to meet the resin's solubility requirements. In summer when printing with polyurethane ink, the ink viscosity rises quickly. During operation, solvents should be added timely to maintain a relatively low viscosity of the ink in the ink tank, ensuring stable printing quality and preventing problems such as missing printing, small dot loss, color difference, blocking plates, and back sticking. Additionally, 8% to 15% of curing agents can be added to prevent the adhesive from not drying.
In the mixed solvents added to printing ink, some solvents can dissolve the binder in the ink, called true solvents, while others, when used alone, cannot dissolve the binder, called false solvents. Due to the different volatilities of solvents, if the true solvent evaporates first, the proportion of solvents in the ink may be disrupted, leading to phenomena such as the precipitation of ink resin. Therefore, true solvents should be added in a timely manner. When the balance of solvents is poor, some slower-evaporating true solvents should be added to maintain the balance of solvent volatilities. Different types of inks use different true solvents depending on the ink resin. It is crucial to determine the type of resin ink used before selecting the appropriate true solvent.
Moisture entering into adhesive systems containing isocyanate (--NCO) groups can cause gelation and whitening. This is mainly because 1 mole of water (H2O) can react with 1 mole of curing agent containing isocyanate groups (-NCO) in the adhesive to produce amine and carbon dioxide: R-NCO + H2O → R-NH2 + CO2↑. Although this reaction is not very fast, it is much faster than the reaction with the main agent (some experiments have shown it to be more than 10 times faster). Since the curing agent reacts with water first, it changes the ratio of the main agent to the curing agent, preventing the adhesive from curing properly. The amine produced from the reaction with water continues to react with 1 mole of isocyanate group (-NCO), producing a biuret: R-NCO + R2--NH2--RNHCONHR↓. The product is a white crystal, insoluble in ethyl acetate, causing whitening of the adhesive, and the accumulated biuret makes the composite roller clog, resulting in insufficient glue and producing defective products.