The reason solventless lamination can use lower adhesive quantities without significantly affecting bonding strength can be attributed to various process and technical factors. These reasons primarily include the following:
Differences in Coating Methods Resulting in Distinct Coating Cross-Section Shapes:Dry lamination typically employs gravure coating, where the coated surface on the substrate looks like a series of small "hills" or "peaks," with certain areas having adhesive and others remaining adhesive-free. In contrast, solventless lamination utilizes multi-roll transfer coating, resulting in a coating cross-section with a "wavy" appearance, but the entire surface is coated, resembling a highly similar wavy pattern. In other words, the entire surface is covered with adhesive in solventless lamination, with differences in localized coating thickness, representing the uniformity of the coating.
Differences in Process Modes and Adhesive Viscosity:Two critical viscosity concepts need to be introduced here: coating viscosity and lamination viscosity. Coating viscosity refers to the working viscosity of the adhesive during application to the substrate, while lamination viscosity refers to the viscosity of the adhesive on the substrate during the bonding process.In dry lamination, the adhesive has a low viscosity during coating, but its viscosity significantly increases after passing through the drying oven before entering the lamination process. Because of the high viscosity, dry lamination requires substantial pressure during lamination to flatten the high-viscosity "hills" and achieve proper substrate adhesion.Typically, in solventless lamination, there is no heating or cooling between coating and lamination, and the short time interval between coating and lamination, combined with high production speeds, results in coating and lamination viscosities that are almost the same. Therefore, only minimal pressure is required for effective bonding in solventless lamination.The differences in adhesive viscosity also determine their flow and flattening capabilities, leading to significant differences in the required pressure for bonding between dry and solventless lamination processes.
Differences in Initial Tack Sensitivity:Dry lamination and solventless adhesives have notably different initial tack characteristics, resulting in varying requirements for adhesive application quantities and different considerations during the roll-up or offloading process. For example, in solventless lamination, an excessively high adhesive quantity can lead to issues such as "blocking" during roll-up. This phenomenon occurs because, when the viscosity is low, the adhesive acts like a lubricant. When the adhesive quantity surpasses a certain threshold, the friction between the substrates may be insufficient to overcome various disturbances, causing lateral movement and resulting in blocking issues. Experienced operators know that, within the specified requirements, lower adhesive quantities in solventless lamination lead to smoother and more regular roll-ups.In contrast, dry lamination adhesives have higher initial tack properties, reducing the likelihood of blocking. Therefore, they allow for larger increases in adhesive quantities.
