Reactive Polymer
Two-component liquid resins that cure through a chemical reaction without the evaporation of organic solvents provide strong bonds for flexible packaging. This material, known as solventless polyurethane adhesive, consists of an isocyanate and a polyol that are mixed just before application. Because there is no carrier fluid to remove, the lamination process does not require large drying ovens or solvent recovery systems.
This significantly reduces the energy consumption and the environmental footprint of the converting plant. The adhesive provides excellent heat resistance and chemical stability, making it ideal for food packaging that must withstand sterilization or boiling. However, the lack of solvent means the adhesive has a high initial viscosity that must be managed with heat.
Mixing Control
Success of the lamination depends on the precise ratio of the two components and the temperature at which they are applied. During the use of solventless polyurethane adhesive, the mixing unit must maintain an exact balance to ensure that the chemical reaction goes to completion. If there is too much isocyanate, the adhesive can become brittle or produce unwanted chemical byproducts.
If there is too much polyol, the bond may never fully cure, leading to sticky edges and low peel strength. The application temperature is usually held between forty and sixty degrees Celsius to lower the viscosity and allow for a smooth, thin coating. Because the curing process begins as soon as the components meet, the pot life of the mixture is limited.
Operators must monitor the coating weight closely to ensure that enough adhesive is present to bridge the gap between the two films. Typical coat weights are much lower than those used for solvent-based systems, often falling between one and two grams per square meter. This requires high-precision rollers and constant monitoring during the production run.
The curing continues after the film is wound onto a roll, often taking several days to reach full strength.
Laminate Quality
Achieving a clear and bubble-free bond requires that the substrate has a sufficiently high surface energy to be wet by the reactive resin. If the film is not properly treated, the solventless polyurethane adhesive will bead up, creating optical defects known as orange peel. These defects not only look poor but also represent areas of weak adhesion where the layers could separate.
The absence of solvents means that there is no swelling of the film surface, which can lead to better dimensional stability in the final laminate. This is a major advantage for thin films that are easily distorted by heat or chemicals. Once fully cured, the adhesive forms a tough, cross-linked network that resists moisture and oils.
This makes it a preferred choice for snack food bags, coffee pouches and medical packaging. Proper handling of the raw materials is essential, as isocyanates are sensitive to moisture in the air. If water enters the system, it can react with the adhesive to form carbon dioxide gas, leading to tiny bubbles in the laminate.
Regular maintenance of the pumping and mixing equipment is the only way to ensure consistent quality over long production runs.
