Maya Ellison
Plastic is a material with low surface energy, which reduces the effectiveness of adhesives. This is because the surface tension of the liquid and the surface energy of the material must be compatible for adhesion. There are several methods to increase the surface energy of plastics, such as plasma, corona or flame treatment, and the application of oxidizing gases. Plasma treatment is a popular method as it is environmentally friendly and can treat complex surfaces. Another method is to roughen the surface of the substrate by sanding or chemical etching, creating peaks and valleys that give adhesives a greater ability to grip.
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What You'll Learn
Plasma surface treatment
Plasma treatment can effectively increase the polar fraction of the surface energy of plastics. This is achieved by attaching polar end groups to the surface of the material, which enhances the adhesion between the plastic and the coating or adhesive. The plasma activation of the plastic surface improves the wettability of the surface, allowing liquids such as ink, adhesive, or varnish to spread and bond securely.
One common technique used in plasma surface treatment is plasma polymerization, which involves depositing ultra-thin coatings that can lower the surface energy of the plastic. Fluorocarbon plasmas, for example, can be used to create a Teflon-like coat on the surface, resulting in superhydrophobic properties. Alternatively, depositing a thin film from a hexamethyldisiloxane plasma can create a glassy but hydrophobic surface on top of a hydrophilic plastic.
Another advantage of plasma surface treatment is its ability to treat complex geometries uniformly. Vacuum plasmas, for example, can penetrate small crevices or pores, ensuring consistent treatment across all exposed surfaces. This makes it ideal for batch processing, as multiple parts can be treated together with consistent results.
While plasma surface treatment offers significant benefits, it is important to consider potential drawbacks. Prolonged exposure to plasma can erode the plastic or degrade its mechanical properties. Additionally, polymers are delicate and can be susceptible to chain scission, crosslinking, or roughening during the plasma treatment process.
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Primer or adhesion promoter
Plastic adhesion promoters and primers are used to improve the bonding of plastics, composites, and metals during the production of car parts and components. They are particularly useful for improving the adhesion of coatings, paints, sealants, and adhesives to surfaces that may be difficult to bond with, including low-surface-energy plastics.
The key difference between adhesion promoters and primers is their function. Adhesion promoters act as additives to the paint to promote the adhesion of the paint film to the substrate. They are designed to make paint stick to surfaces. Adhesion primers, on the other hand, conserve the surface of a material that has already been bonded. Primers are often used as a barrier between paint and plastic to prevent damage from fast-drying "hot" solvents in the paint.
The application process for adhesion promoters and primers is quite similar. The surface should be cleaned, sanded, and treated with heat before applying a thin layer of primer or adhesion promoter. After the primer is applied, the surface should not be sanded or cleaned, and a filler can be applied. The filler primer fills any voids left by sanding and sands smoothly before continuing with the base paint.
Adhesion promoters are typically applied after the primer and before the base coat. It is important to note that primers and adhesion promoters are not always interchangeable, and specific projects may require one or the other, or both. For example, when working with Polyolefin-based substrates, an adhesion promoter can be used to improve the adhesion of acrylic foam and acrylic plus tapes.
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Roughen the surface
Roughening the surface of a plastic material is one way to increase its surface energy and make it more amenable to adhesion. This process can enhance the adhesion of paints, inks, coats, glues, and other coatings, improving their bonding strength.
There are various methods to roughen a plastic surface, including physical and chemical treatments. Physical treatments can involve using an abrasive material to manually create a rough texture on the plastic surface. This can be done through mechanical processes such as sanding, sandblasting, or abrading the surface with a rough tool. These methods can effectively increase the surface area and create micro-scratches, improving the adhesion properties of the plastic.
Another physical treatment is known as surface deformation, where the plastic is subjected to mechanical stress, strain, or tension, causing it to deform and create a roughened surface. This method has been observed in metals and polymers, where the deformation mechanisms at different length scales contribute to the overall roughness. The degree of roughening can be controlled by adjusting the applied strain, grain size, and heating rate during deformation.
Chemical treatments for roughening plastic surfaces also exist. These treatments may involve exposing the plastic to specific chemicals or environmental conditions that react with the surface, causing it to etch or corrode slightly, resulting in a rough texture. For example, treating a plastic surface with a strong acid or base can lead to controlled corrosion, creating a roughened surface.
It is important to note that the roughening process should be carefully controlled to achieve the desired level of surface roughness without compromising the integrity or functionality of the plastic material. Excessive roughening may weaken the material or make it more susceptible to environmental degradation. Therefore, the specific parameters, such as the degree of roughness, the tools or chemicals used, and the duration of treatment, must be carefully calibrated and optimized for the particular plastic and its intended application.
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Use surfactants
Surfactants are compounds that can effectively reduce the surface tension of plastics. They are often used in paints, varnishes, and adhesives to improve wetting and adhesion. Surfactants can act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
When selecting a surfactant, it is important to consider the critical micelle concentration (CMC). The CMC represents the concentration of surfactants above which their effectiveness decreases. Force tensiometers can be used to determine the CMC of a surfactant and its efficiency in reducing surface tension.
Surfactants can be used as a pre-treatment to increase the surface energy of plastics. This enhances the adhesion of subsequent coatings, paints, or adhesives. Common surfactant pre-treatment methods include plasma, corona, or flame treatment. Plasma treatment, also known as the fourth state of matter, is particularly effective as it contains positive ions, electrons, neutral gas atoms or molecules, UV light, and excited gas atoms and molecules. It is also environmentally friendly and suitable for treating complex surfaces.
Corona treatment is another option that can be applied using machines like the RotoTEC-X, which is cost-effective and suitable for treating various plastic materials. Flame treatment is a less expensive option but may not be as environmentally friendly as it involves high temperatures.
In addition to surfactant pre-treatments, there are other methods to increase the surface energy of plastics. These include roughening the surface of the substrate through sanding or chemical etching, creating peaks and valleys that enhance the adhesion of coatings or adhesives. However, it is important to ensure that the surface is properly cleaned and free of contaminants before applying any treatments or coatings.
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Increase surface energy
The surface energy of plastics can be increased to improve their adhesion properties. Low surface energy in plastics can cause issues with the adhesion of coatings, glues, sealants, and paints. To increase the surface energy of plastics, several methods can be employed:
Plasma Surface Treatment
Plasma surface treatment is an effective way to increase the surface energy of plastics, creating more bond sites for adhesives to attach to the surface. This method has been used successfully to treat plastics such as PEEK, which has a surface energy of around 34-38 dynes/cm2. After treatment, the surface energy can increase to approximately 60-70 dynes/cm2, making it possible to achieve successful bonding.
Corona Treatment
Corona treatment is another method to increase the surface energy of plastics. This process involves treating the plastic surface to a high-voltage corona discharge, which activates the surface and enhances its wettability. Corona treatment is often used on plastics with low surface energy, such as Polyethylene, to improve the adhesion of inks, paints, or coatings.
Primer or Adhesion Promoter
Using a primer or adhesion promoter can also help increase the surface energy of plastics. These promoters enhance the wettability of the surface, allowing adhesives to spread more effectively and create a stronger bond.
High Temperature and Wet Chemicals
While not as environmentally friendly, treatments involving high temperatures and wet chemicals can also increase the surface energy of plastics. These methods modify the surface energy to improve adhesion but may have a more negative environmental impact.
Specialist Adhesives
In some cases, rather than treating the plastic surface, specialist adhesives are formulated to bond with low surface energy plastics. These adhesives are designed to meet the unique challenges posed by plastics with low surface energies, such as Nylon 12 and PEBA.
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Frequently asked questions
Reducing the surface tension of plastics can improve the adhesion of coatings, glues, sealants, and paints.
The surface tension of a liquid and the surface energy of a material determine the strength of the bond between them. If the surface tension of the liquid is higher, the attractive forces are stronger, and vice versa.
There are several methods to reduce the surface tension of plastics, including plasma, corona, or flame treatment, and the use of oxidizing gases such as ozone or fluorine. Sanding or roughening the surface of the plastic can also increase the surface energy and improve adhesion.
