The effectiveness of physical pretreatment tends to be influenced by the type of polymer and the manufacturing process used to create the substrates intended for bonding. Additionally, it is advisable to assess whether the quality of pretreatment is affected by the duration of the treatment. Acid pretreatment can also be utilized for certain plastics; for example, chromic acid is employed to surface treat polyolefins.
Low-Pressure Plasma
Plasma is generated by applying a high-frequency voltage to a gas within a low-pressure chamber using electrodes.
When substrates, especially plastics, are exposed to this plasma, their surface energy is elevated. This heightened surface energy facilitates complete wetting of the substrate surface by an adhesive, resulting in a robust and long-lasting bond. Various plasmas can be created using gases such as argon, ammonia, oxygen, or nitrogen, making the process adaptable to a wide range of substrate types.
This method is often preferred when treating small parts with complex geometry or for parts where the surface to be bonded is challenging to access.
Atmospheric Plasma
In this method, plasma is generated in air at atmospheric pressure without additional gases, and the beam generated does not have any electrical potential. The advantages of this approach are comparable to those of the previous method. It is typically favored for treating larger flat surfaces. Factors that can affect the quality of treatment include exposure time and the distance between the surface and the plasma nozzle.
Flame Treatment
Flame treatment oxidizes the surface layer to create polar groups; the greater polarity of these groups results in greater surface energy and improved wetting by the adhesive. This surface pretreatment method has been applied to polyethylene and polypropylene with success. Flame treatment quality can be influenced by the type of gas-to-air (oxygen)-ratio, exposure time, flow rate, and the distance between the flame and substrate.
Corona Treatment
Under standard atmospheric pressure, a high-voltage discharge produces oxygen and ozone molecules, which can activate the plastic surface by oxidation. All of the above methods have limited stability, varying from hours to weeks, depending on the substrate.
Learning Lab: How to Prepare Plastics for Adhesive Bonding
Want to achieve a good preparation for plastics ?. Video Credit: Huntsman Advanced Materials
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