Abstract: This study investigates the influence of adhesive viscosity on the quality of flexible packaging, with a focus on polyurethane dry lamination adhesives with a solid content exceeding 70%. Introduced in the 1980s in foreign markets, these adhesives exhibit low viscosity even when applied with a solid content of over 40%, resulting in reduced solvent usage, lower costs, environmental friendliness, and excellent performance, making them highly popular.
As of today, it is observed that domestically, 50% solid content dry lamination adhesives still hold a significant market share. This indicates that there is room for improvement in the understanding of dry adhesive producers and lamination packaging manufacturers in the domestic market.
Control over the adhesive application quantity is crucial in dry lamination processes, significantly impacting the quality of lamination materials. Insufficient adhesive application leads to discontinuous film formation, resulting in spots that affect the visual quality of lamination materials. Additionally, it reduces bonding strength, compromising the material's resistance to boiling and heat seal strength.
Conversely, excessive adhesive application not only wastes adhesive but also causes film wrinkling and hardening, leading to tunneling and reduced opening performance. Moreover, excessive adhesive application makes solvent evaporation difficult, resulting in an accumulation of solvent in the adhesive layer, producing unwanted odors.
The adhesive application quantity should be determined based on factors such as substrate type, intended use of lamination materials, and printing conditions. An essential technological aspect of determining the quantity is controlling the viscosity of the adhesive.
In discussions about adhesives, emphasis is typically placed on solid content or concentration, post-composite peel strength, and various resistances. However, evaluating adhesive status should also consider factors such as adhesive transfer rate, adhesive transparency, and post-transfer conditions, including the presence, quantity, and size of white spots, crystal points, and adhesive points.
Formulated adhesives must comply with operational viscosity (generally controlled between 15 to 20 seconds for optimal adhesive transfer when using a viscosity cup). Operating at permissible viscosity conditions, it is advisable to choose higher concentrations to reduce residual solvents, save solvent costs, and ensure appropriate dry adhesive application. This choice is contingent on having suitable mesh rollers to guarantee the right dry adhesive quantity. For processing steam-cooking packaging, mesh rollers with 80 to 100 lines are typically selected at an operating speed of 70 to 100 meters per minute.
When selecting adhesives, opting for adhesives with higher solid content is preferable. This choice allows for higher solid content usage (operating concentration between 35% to 45%), ensuring optimal adhesive application, reducing ethyl acetate usage, lowering production costs, and enhancing production efficiency. General-purpose polyurethane adhesives, such as CX-2022/CA-875LT (best used at concentrations of 40% to 45%) and CX-2522/CA-775LT for 125°C high-temperature steam cooking (best used at concentrations of 30% to 35%), produced by Gaoding Fine Chemicals, are recommended for customer selection.
In the flexible packaging industry, a Chahn #3 cup is commonly used to measure the viscosity of ink or adhesive. Some companies have equipped themselves with a #4 cup. For the same adhesive sample, viscosity data obtained using a Chahn #3 cup (with a 3mm aperture) is generally higher in seconds than data obtained using a #4 cup (with a 4mm aperture).
For any given company, the choice between viscosity cups is not critical, as long as the selected cup provides essential guiding data for operators.
II. Factors Influencing Adhesive Viscosity
Environmental Temperature
Environmental temperature significantly influences adhesive viscosity. Higher temperatures result in lower viscosity for adhesives at the same concentration.
Adhesive Concentration
Higher adhesive concentrations lead to increased viscosity.
Nature of the Adhesive
Different adhesive brands from the same company may have different initial viscosities due to variations in synthetic process conditions, leading to different viscosities at the same concentration.
Solvent Evaporation
Solvent evaporation is influenced by factors such as temperature, humidity, the surface area of adhesive exposed to air, and airflow conditions near the adhesive trough.
In general, higher adhesive viscosity results in poorer flowability, leading to reduced film smoothness after adhesive application and the potential formation of "adhesive points" and "orange peel" effects. Conversely, excessively low adhesive viscosity can lead to the formation of "water ripple" patterns. The occurrence and severity of the mentioned quality defects also depend on factors such as the line count of the coating roller, mesh hole shape, and depth.
In summary, adhesive viscosity is a crucial process indicator in the dry lamination process, requiring careful attention.
III. Approaches to Controlling Adhesive Viscosity
Control of Environmental Temperature and Humidity
Control of Airflow Conditions near the Adhesive Bucket and Trough
Control of the Surface Area of Adhesive Exposed to Air in the Bucket and Trough
Installation of Adhesive Viscosity Control Systems for Coating Units
Provision of Special Solvent Buckets for the Trough, Regularly or Continuously Supplying Solvent to the Trough
While implementing these measures, it is essential to measure adhesive viscosity regularly to assess the effectiveness of the adopted measures, enabling timely adjustments and appropriate solvent replenishment.