Ethan Clarke
Plastic is notoriously difficult to glue because it is not porous and is made of long molecules, meaning there are fewer ends for the glue to connect to. This is why surface preparation is so important when it comes to gluing plastic. The right preparation will improve adhesion and strengthen the bond between the substrates. The first step is to identify the type of plastic you're working with. This is important because different plastics have different properties and require different types of glue and preparation. Once you know what type of plastic you're dealing with, you can choose the right adhesive and prepare the surface accordingly. This may involve cleaning the surface with a specialised cleaner, drying it thoroughly, and roughening the surface with sandpaper to improve adhesion.
| Characteristics | Values |
|---|---|
| Surface Preparation Techniques | Degreasing, cleaning, abrasion, plasma treatment, primers, ion beam etching, flame treatment, chemical products |
| Degreasing Techniques | Vapor degreasing, cleaning/rinsing with volatile solvents (toluene, acetone, methyl ethyl ketone, methyl alcohol, isopropyl alcohol, trichloroethylene), immersion in a solvent bath |
| Abrasion Techniques | Roughening the surface with sandpaper, steel wool, or other abrasives |
| Cleaning Techniques | Soap, isopropyl alcohol, specialised industrial cleaning sprays, lint-free cloths |
| Plastic Welding | Melting plastics back together |
| Surface Energy | Increasing surface energy improves wettability and adhesion |
| Contaminants | Dust, dirt, oil, grease, rust |
| Adhesive Selection | Solvent-based adhesives, plastic cements, industrial adhesives, epoxy, super glue |
| Testing | Conduct small tests on similar materials before full application |
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What You'll Learn
Clean the surface
Preparing the surface before gluing is a critical step in achieving a strong bond and glue performance. The first step is to ensure the plastic is clean, dry, and free of any oily or greasy residues. Depending on the type of plastic, soap, isopropyl alcohol, or a specialised cleaning spray may be ideal for this step. For example, if you are working with sensitive plastics, a specialised Plastic Cleaner Spray can be used to clean without damaging the underlying plastic surface.
After cleaning, it is important to dry the plastic thoroughly using a soft, lint-free cloth. Any loose fibres left behind may interfere with the adhesion of the glue. It is also recommended to avoid touching the plastic with bare hands after cleaning, as the natural oils on the skin can interfere with the glue.
Degreasing is another important step in surface preparation, which helps remove any loosely held dirt or contaminants. This can be done using volatile solvents such as toluene, acetone, methyl ethyl ketone, methyl alcohol, isopropyl alcohol, or trichloroethylene. It is important to ensure that all environmental, health, and safety regulations are met before selecting a solvent. Common methods include vapour degreasing, cleaning, or rinsing the parts with the appropriate solvents.
If the plastic surface already has a rough texture, you may skip this step. However, if the surface is smooth, it is recommended to roughen the bonding area to improve adhesion. This can be done by using sandpaper, steel wool, or another abrasive material. For most plastics, a light run-over with 120-200 grit sandpaper will suffice.
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Remove contaminants
It is important to remove contaminants from plastic before using adhesive. This is because adhesives will stick to anything, including dirt, dust, and oily residues. Therefore, it is critical to ensure that the plastic surface is clean, dry, and free of any oily or greasy residues.
There are various methods to clean plastic surfaces and remove contaminants. Firstly, if there is sticky residue on the plastic, this can be removed using common household items. For example, oils such as coconut oil, olive oil, vegetable oil, or cooking oil spray can be used by gently rubbing the sticky surface with a cloth or old toothbrush. The glue residue will start to slide off. Alternatively, a non-toxic solution of warm water, white vinegar, and mild dish soap can be applied to a clean cloth or paper towel, which is then rubbed over the adhesive. After letting it sit for five minutes, the residue can be scraped away.
Another method is to use volatile solvents such as toluene, acetone, methyl ethyl ketone, methyl alcohol, isopropyl alcohol, or trichloroethylene to degrease the surface and remove any loosely held dirt or contaminants. However, it is important to ensure that all environmental, health, and safety regulations are met before selecting a solvent. This can be done by vapor degreasing, cleaning, or rinsing the parts with the appropriate solvents, or by immersing the substrates in a fresh bath of solvent.
Additionally, for light surface roughening, 120-200 grit sandpaper can be used to create a better adhesion by modifying the surface chemistry of the plastic. However, caution must be exercised, and the exact type of plastic must be considered before employing this technique.
Finally, for more sensitive plastics, a specialized plastic cleaner spray can be used to clean the surface without damaging the underlying plastic. After cleaning, it is important to dry the plastic thoroughly using a soft, lint-free cloth to avoid any loose fibres that may interfere with the adhesion.
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Roughen the surface
Roughening the surface of plastic before applying adhesive is a crucial step in the preparation process. Increasing the surface roughness provides a greater contact area for the adhesive to adhere to, resulting in a stronger bond. This can be achieved by using sandpaper with a grit of 120-200, or steel wool, to abrade the plastic surface. However, it is important to note that this step may be skipped if the plastic already has a rough surface.
The process of roughening the surface removes any pre-existing contaminants and changes the chemical composition of the plastic. This is essential because contaminants like dirt, dust, oil, or grease can interfere with the adhesion process. Additionally, the natural oils present on human skin can also affect the glue's ability to bond, so it is recommended to avoid touching the plastic surface with bare hands.
When roughening the surface, it is crucial to choose the appropriate abrasion technique for the specific type of plastic being used. For example, plastics such as PEEK, nylon, and polycarbonate can be successfully roughened using sandpaper. However, other plastics may require different abrasion techniques or additional steps, such as degreasing with solvents or chemical treatments, to ensure a clean and properly prepared surface for adhesion.
While increasing surface roughness can enhance adhesion, it is not the sole factor in achieving a strong bond. The choice of adhesive, the chemical composition of the surface, and the joint geometry all play a significant role in the overall success of the adhesive bonding. Therefore, it is important to consider all aspects of surface preparation, including cleaning and modifying the chemical composition, in conjunction with roughening the surface to achieve optimal adhesion.
Lastly, it is worth noting that excessive roughness may not always be beneficial. The adhesive must be able to flow into the cracks and crevices created by abrasion. If the adhesive cannot flow properly, there may be reduced contact between the adhesive and the surface, leading to weaker bonds. Therefore, it is crucial to balance the level of surface roughness with the adhesive's ability to make optimal contact and form a strong bond.
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Treat with plasma
Plasma treatment is an effective method to prepare plastic for adhesive bonding. It is a two-fold process that involves surface activation and cleaning. The treatment parameters and efficacy depend on the material, and atmospheric plasma is administered using a defined recipe of power, exposure time, and source-to-sample distance.
Firstly, plasma treatment cleans the plastic surface by removing volatile organic compounds, dirt, and contaminants that can impede bonding. This step is crucial as any residue can negatively impact the adhesion strength. The cleaning process can be done using specialised cleaning sprays, or with volatile solvents such as acetone or isopropyl alcohol.
Secondly, plasma surface activation changes the surface chemistry of the plastic, making it more receptive to forming durable bonds with adhesives. This process increases the surface energy of the plastic, creating a hydrophilic surface that is wettable to adhesives, resulting in stronger and more durable bonds.
Plasma treatment also provides better stability compared to other treatments like corona discharge or flame treatments. It can be applied under low pressure or atmospheric pressure, and the risk of over-activation can be avoided with highly accurate setting options.
Overall, plasma treatment is an effective way to prepare plastic for adhesive bonding by improving the surface energy and wettability, resulting in stronger and more durable bonds.
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Choose the right adhesive
Choosing the right adhesive for plastic is crucial, as the wrong one might not hold up or cause irreversible damage. Different types of plastics require different types of adhesives. The first step is to identify the type of plastic you are dealing with.
Plastics are often labelled with recycling symbols, which can help determine the type of plastic and the suitable adhesive. For instance, plastics stamped with a '1' are made from Polyethylene Terephthalate (PET), and those with '2' are High-Density Polyethylene (HDPE). Knowing the plastic type, you can choose an adhesive suited to the material. Plastics in categories 1, 2, 4, and 5 are challenging to glue.
For instance, Polyethylene and Polypropylene (PP) plastics, often used in hard, high-density items like buckets and crates, are nearly impossible to glue unless you use an adhesive specifically for these plastics. Loctite Plastics Bonding System is a two-part cyanoacrylate adhesive suitable for such plastics.
For flexible plastics, Loctite Vinyl, Fabric & Plastic Flexible Adhesive is recommended. It is a clear, waterproof adhesive that dries clear and creates a flexible bond, making it ideal for items like shoes or toys.
If you are dealing with ABS plastic, commonly used in the transportation and construction industries, epoxies are the highest-strength adhesive option. They offer excellent chemical and water resistance but are prone to cracking with significant movement. For smaller DIY projects involving ABS plastic, cyanoacrylate glue (super glue) is a good option.
Other types of adhesives include solvent-based adhesives, which dissolve the top layer of plastic, creating a strong bond. Plastic Cement (Plastic Glue) is another option, formulated specifically for plastics, slightly melting the surfaces before fusing as it dries. Acrylic adhesives are ideal for a strong, transparent bond, making them popular for hidden repairs.
When in doubt, consult the glue manufacturer or your local hardware store for advice on choosing the right adhesive for your specific plastic type.
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Frequently asked questions
Clean the plastic surface to remove any dust, dirt, oil, grease, or other contaminants.
Use a detergent or solvent to remove contaminants. You can also use specialised industrial cleaning sprays, or for general-purpose plastics, a surface cleaner spray.
Dry the plastic thoroughly using a soft, lint-free cloth. Avoid touching the plastic with your bare hands as the natural oils on the skin can interfere with the glue.
Roughen the bonding area, especially if the plastic surface is smooth. You can use sandpaper, steel wool, or another abrasive.
Yes, you can modify the chemical composition of the plastic surface through plasma treatment, chemical treatment, or flame treatment. You can also use plastic welding or primers.
