Maya Ellison
The surface tension of plastics can be increased to improve their adhesion properties. Adhesion is the process by which two materials are held together by interfacial tension, which is the inherent instability of a coating. The lower the interfacial tension, the lower the tendency of the layer to become detached. The surface tension of a liquid and the surface energy of a material must be compatible for the application. If the surface tension of a solid is higher than that of a liquid, there will be increased molecular attraction, resulting in a stronger bond. Conversely, if the solid's surface tension is lower, the attractive forces will weaken, resulting in repelling of the coat.
Characteristics and Values for Increasing Surface Tension of Plastics
| Characteristics | Values |
|---|---|
| Surface Tension | Should be higher than the surface energy of the solid to increase molecular attraction |
| Surface Energy | Should be at least 5 mN/m (dyn/cm) above the surface tension of the adhesive |
| Surface Treatment Methods | Plasma, corona, flame, heat, oxidation, chemicals, primer or adhesion promoter |
| Surface Conditions | Dust-free, wettable, clean, smooth, dry, non-porous, and roughened |
| Measurement Tools | Goniometer, tensiometers, contact angle measuring instruments, drop shape analysis measuring instruments |
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What You'll Learn
Plasma surface treatment
Plasma treatment is a versatile process that can be applied to various plastic materials and products. It can be used to treat 3D parts, reaching areas that traditional treatment methods cannot. This includes treating the inside of small parts like syringes or closed plastic bags. Plasma treatment is also effective for plastic packaging, ensuring that printing inks adhere securely and increasing the wettability of the film. This is particularly important for marking expiration dates and other essential information.
In the plastic packaging industry, plasma treatment is a critical step in manufacturing cost-effective and environmentally responsible materials. It increases the storage life of products, with plasma activation lasting up to 12 months. This treatment can also facilitate the use of new material composites, creating innovative packaging solutions.
Plasma treatment is often compared to corona treatment, another method of increasing the surface energy of plastics. However, plasma treatment offers several advantages. It achieves higher levels of surface tension, up to 64 mN/m and beyond, while corona treatment's surface tension declines over time. Plasma treatment also ensures single-sided pretreatment of plastic films, avoiding punctures on the reverse side of the material.
The shelf life of pre-treated materials with plasma treatment can vary from hours to years, depending on factors such as the type of plastic, its formulation, treatment method, and exposure to elevated temperatures. It is recommended to print or bond the material soon after treatment to ensure optimal results.
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Primer or adhesion promoter
Adhesion promoters are used to increase the surface energy of plastics, which is relatively low, to ensure that coatings flow well and that there are no fisheyes, pinholes, gaps, or air bubbles. Adhesion promoters are particularly useful when bonding polyethylene, which has low surface energy, reducing the effectiveness of adhesives.
Adhesion promoters are aggressive solvents that soften the surface of the plastic a few minutes after they are applied. They are often used in conjunction with primers or sealers to create a secure bond. The adhesion promoter is applied first, followed by the primer or sealer while the plastic surface is still softened. This process creates a chemical adhesion, which is superior to mechanical adhesion, which is created by sanding.
Adhesion promoters are particularly useful when bonding new, bare plastic. If the plastic is painted, sanding and repainting may be sufficient. If there is no paint left and the plastic is bare, adhesion promoters can be used, followed by priming or sealing.
Adhesion-promoted primers are also available. These are high-performance adhesion promoters that can be used on difficult surfaces such as silicone or contaminated surfaces. They can be applied by hand or with HVLP in a fine mist.
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Corona treatment
The key mechanism behind corona treatment is the oxidation of the plastic surface. During the treatment, the high-frequency discharge splits carbon molecules and breaks oxygen molecules into ions. Some of these ions penetrate the surface layer of the plastic, forming new carbonyl groups with higher surface energy. This process improves the chemical connectivity between the plastic and liquids, such as inks or adhesives, enhancing their adhesion.
The equipment used for corona treatment includes a high-frequency power generator, a high-voltage transformer, a stationary electrode, and a treater ground roll. The electrical power is converted into higher-frequency power and applied to the plastic surface through ceramic or metal electrodes, creating the corona discharge. This treatment is safe, economical, and versatile, capable of treating multiple surfaces in a single pass without altering their appearance or strength.
However, it is important to note that the effects of corona treatment may diminish over time, and re-treatment may be necessary to ensure continued adhesion. Additionally, the effectiveness of the treatment depends on the specific material's chemical composition, and testing is recommended to determine the optimal treatment parameters.
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Flame treatment
The flame treatment process involves exposing the plastic surface to a suitable oxidizing flame for a very short period, typically between 0.2 and 3 seconds. This process oxidizes the upper layer of the plastic, causing it to become more hydrophilic and increasing its surface energy. The result is improved wettability, allowing fluids to effectively wet out the surface and creating a strong adhesive bond.
One of the benefits of flame treatment is that it is a relatively clean process that produces less waste and does not use harmful chemicals when compared to chemical deburring processes. It is also effective at removing organic contaminants, such as oils or greases, resulting in a clean and polar surface.
However, one drawback of flame treatment is the use of fossil fuels, which raises sustainability concerns due to the increased carbon footprint. As a result, some companies are exploring alternatives to flame treatment to increase the sustainability of their products.
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Oxidation
The surface tension of plastics can be increased by altering their surface chemistry and energy. This is often done to improve the adhesion of coatings, inks, adhesives, paints, and varnishes.
Plastics are typically materials with low surface energy, which causes them to repel certain liquids, such as water-based coatings. This can be problematic when trying to bond plastics with adhesives, as the adhesive will bead up and fail to make intimate contact with the surface.
One method to increase the surface energy of plastics is through oxidation. This can be achieved using various surface treatment methods, including:
Plasma Treatment
Plasma treatment is a commonly used method to increase the surface energy of plastics. It involves exposing the plastic surface to plasma, which is the fourth state of matter, containing positive ions, electrons, and excited gas molecules. This process modifies the chemical structure of the plastic surface, increasing its surface energy and improving its adhesion properties. Plasma treatment is particularly effective for plastics like polyethylene, which has low surface energy and is challenging to bond with conventional adhesives.
Corona Treatment
Corona treatment, also known as electrical corona discharge, is another technique used to increase the surface energy of plastics. It involves treating the plastic surface with a high-voltage corona, which modifies the surface chemistry and enhances its energy. This method improves the wettability of the plastic surface, allowing adhesives, inks, and coatings to spread evenly and bond securely.
Flame Treatment
Flame treatment is a traditional method of increasing the surface energy of plastics. It involves exposing the plastic to a flame, which modifies the surface chemistry through a process known as flame pyrolysis. A thin layer of hydrophobic silicone oxide is formed on the plastic surface, resulting in a hydrophilic surface due to the presence of Si-OH groups. This treatment can be effective even for temperature-sensitive plastics, as the brief flame-substrate interaction does not affect the temperature of the plastic significantly.
Pyrosil® Flame Treatment
Pyrosil® is a newer flame treatment method that utilizes combustion chemical vapor deposition (CCVD) to deposit an amorphous silicon dioxide (SiO2) layer on the plastic surface. This treatment is effective for improving the adhesion of tough-to-bond polymers. Similar to traditional flame treatment, Pyrosil® creates a hydrophilic surface due to the high density of Si-OH groups on the deposited SiO2 particles. However, it differs in that it produces SiO2, while traditional flame treatment does not.
Oxidizing Gases
The use of oxidizing gases, such as ozone or fluorine, is another approach to increasing the surface tension of plastics. These gases interact with the plastic surface, causing oxidation and increasing the surface free energy. This, in turn, improves the wettability and adhesion of the plastic surface.
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Frequently asked questions
Surface tension is the inherent instability of a coating. The lower the surface tension, the less likely a coating will be to detach due to penetration by other substances through tiny cracks.
Plastics have comparatively low surface energy and low interfacial adhesion when bonding, coating, or printing. This means that adhesives, paints, and varnishes tend to repel plastics.
The surface tension of plastics can be increased by raising the surface energy of the plastic. This can be done by applying heat, oxidation, chemicals, corona treatments, or plasma treatments to the plastic substrates, altering the chemical structure of the surface.
Increasing the surface tension of plastics makes them more receptive to coatings, inks, or adhesives.
