Temperature and Humidity Settings for the Nylon Film Production Environment and Suitable Solvents for Printing

Due to the hygroscopicity of nylon film and the series of adverse consequences caused by moisture absorption, the environment in which nylon film is used and stored is very important for ensuring product quality.

1. Temperature and humidity control in the production environment

Nylon film should generally be stored at approximately 23℃ ±3℃, with relative humidity ideally in the range of 60%RH ±5%RH. Nylon film must not be placed directly on the floor or exposed to the open air, to prevent moisture absorption and dampening. The relative humidity in the production environment should generally not exceed 75%RH; if it reaches 80%RH, production (especially lamination production) should be stopped to avoid adverse consequences.

If production must proceed to meet a deadline, exhaust fans and dehumidifiers can be added to the production site to enhance indoor air circulation and reduce the production environment humidity to below 70%RH before resuming production. In short, use must be handled with caution when humidity exceeds 70%RH; when using at low temperatures or high humidity, the preheating equipment of the printing and lamination machines must be turned on to preheat the nylon film first and dry the moisture attached to its surface, so as to facilitate smooth printing and lamination production.

At the turn of winter and spring, because temperature changes are relatively large, when using mixed solvents, effective adjustments should be made according to temperature changes to prevent quality accidents caused by excessively rapid solvent evaporation or excessive residual solvent. Workshop environment control can maintain humidity at 50%–60%RH; in summer, 20–22℃ is optimal. This is because the printing workshop releases a large amount of heat energy during production, and controlling temperature and humidity well is the primary condition for ensuring printing quality.

In dry and solventless lamination processes, different models of two-component polyurethane adhesives have different sensitivities to water and alcohols. Exceeding the range may cause a non-drying phenomenon. The more the sensitivity exceeds the range, the poorer the flexibility; laminated products may sometimes become hard and brittle. Therefore, the lamination effect can be seriously affected. This is because solventless polyurethane adhesive contains NCO groups, which can react with active hydrogen atoms. These reactions consume isocyanate, reducing the amount that participates in the adhesive liquid reaction, while also releasing CO2, causing the adhesive layer to contain bubbles. This is extremely unfavorable for lamination and can also cause surface non-drying or reduced bond strength.

For materials with strong hygroscopicity such as nylon film and cellophane, residual solvents after printing and humidity in the air must be strictly managed and controlled. In the nylon film family, PA6 film has the highest water absorption rate, with a saturated water absorption rate of up to about 9%. After biaxially oriented nylon film absorbs moisture, in addition to wrinkling, it also stretches in the transverse direction and shrinks in the machine direction, and the elongation increases as humidity rises, with a maximum elongation of up to 1.5%. For example, at 75%RH, the transverse elongation is about 0.37%, while the machine direction is -0.28%; if humidity rises to 85%RH, the transverse elongation is 0.5% and the machine direction is -0.38%.

After nylon film absorbs moisture, its stress-strain curve changes significantly. Moisture absorption by nylon film does not greatly affect the corona effect, but it greatly affects the adhesion of inks and adhesives. Generally, when the moisture content of nylon film exceeds 3%, it not only affects the adhesion of inks and adhesives, but also makes the film feel stiff and the surface yellow, affecting appearance. Therefore, special attention must be paid to the storage and use of nylon film to prevent moisture absorption.

1. Use as little alcohol-based solvent as possible or no alcohol-based solvent in printing

Printing nylon film requires its dedicated polyurethane resin-based ink. When using polyurethane resin-based ink, pay attention to adding little or no alcohol-based diluent solvent. This is because the polyurethane resin itself is terminated with -OH, and -OH can react with the isocyanate group -NCO in the curing agent of the polyurethane adhesive, reducing the amount of curing agent participating in the reaction with the adhesive main agent and affecting bond strength. If an alcohol-based diluent solvent must be added, it should also be controlled to within 5% as much as possible (pay attention to checking the purity of the solvent used), and the residual solvent level should be reduced to a minimum during printing (generally total amount >3mg/m2).

When using the solventless lamination process, residual solvents should be thoroughly removed in the printing process. Because solventless lamination uses no solvent, there is no solvent residue problem. Solventless lamination cannot reduce residual solvents caused by printing, so higher requirements are placed on the control of residual solvents in printing. Solventless lamination adhesive contains no organic solvents and will not increase residual solvents, but at the same time it will not reduce the solvent residue caused by printing. Only by minimizing residual solvents in printing can the odor of the final product be truly eliminated.

For films used for water-based ink printing, corona treatment is best carried out at random before printing to ensure ink adhesion to the film. When printing jobs with a white ink base, it is best to add a small amount of general-purpose curing agent to the white ink to improve lamination bond strength. (Use according to the amount required by the ink manufacturer.) After printing, lamination production should be carried out as soon as possible. The remaining material can be wrapped well with packaging film containing aluminum foil to prevent water vapor in the air from intruding and causing moisture absorption. The remaining film should be placed in a dry and ventilated place; before reuse, it is best to place it in the curing room 2–3 hours in advance to dry the water vapor on the surface of the nylon film, so that the material is protected from moisture and product quality is not affected. (This article is excerpted from "The Impact of Temperature and Humidity Changes in the Production Environment on Printing and Lamination Quality", by Chen Mengjie. The full text has been published in "Packaging Frontier". Welcome to subscribe: Mr. Xie: 020~85626447/605, 13310896471. More technical content can be viewed at the Packaging Frontier Information Network at www.pack168.com)