Thermodynamic Wetting and Surface Free Energy in Polyolefin Lamination
Thermodynamic wetting in polyolefin lamination demands substrate polar surface energy above 6.0 mN/m to achieve complete adhesive spreading and bond durability.
Gas Flame Activation
Thermal Treatment
Direct exposure of a substrate to a lean gas flame increases surface energy by depositing oxygen-containing functional groups onto the material. This method of surface modification, called gas flame activation, is widely used for thick materials or substrates with complex shapes that are difficult to treat with electrical methods. The process involves passing the material through a specialized burner that uses a mixture of air and natural gas or propane.
In the high-temperature environment of the flame, the polymer surface undergoes a rapid oxidation reaction. This creates reactive sites like hydroxyl and carboxyl groups that allow for better wetting. It is particularly effective for high-density polyethylene and other nonpolar plastics used in rigid packaging.
Chemical Oxidation
Success of the treatment depends on the precise chemistry of the flame and the speed at which the material passes through it. During gas flame activation, the burner must be set to a lean mixture, meaning there is more oxygen than is needed for complete combustion. This excess oxygen is what reacts with the polymer chains to change the surface energy.
The flame also cleans the surface by vaporizing small amounts of oils or contaminants that might interfere with adhesion. Unlike electrical treatments, this thermal process does not produce ozone, which makes it easier to manage in many production environments. However, the heat must be carefully controlled to prevent the material from warping or melting.
The distance between the burner and the substrate is a critical variable that determines the intensity of the activation. If the burner is too close, the surface may be damaged or scorched. If it is too far away, the oxidation will be insufficient to achieve the desired dyne level.
Modern systems use advanced sensors to monitor the flame color and temperature in real time. This ensures that every part of the substrate receives the same level of treatment across the entire width of the line.
Process Control
Integrating this activation method into a converting line requires a focus on safety and consistency. Because gas flame activation involves open flames and flammable gases, the equipment must include robust safety interlocks and ventilation systems. The treatment is often more durable than corona treatment, showing slower dyne decay over time.
This makes it a preferred choice for parts that may be stored for several days before being printed or coated. In the automotive and industrial packaging sectors, this longevity is a significant advantage for logistics planning. The depth of the oxidation is usually slightly greater than that achieved by electrical discharge, providing a very stable base for heavy coatings.
Testing the effectiveness of the flame treatment is done using dyne solutions or contact angle measurements. If the dyne level is too low, the operator can adjust the line speed or the gas-to-air ratio to increase the power. Consistent results depend on maintaining the burner heads and ensuring that the gas supply is pure and steady.
Proper setup leads to a surface that can bond with a wide variety of inks and adhesives.