Omar Khalid
Plastic is a versatile material used in a variety of applications, from everyday items like toys and shoes to more specialized uses such as in spacecraft. Its smooth, non-porous surface makes it challenging to work with when it comes to gluing or bonding. Different types of plastics require different adhesives, and choosing the right glue is essential for effective repairs. While super glue is a popular choice for quick fixes, it may not work on all plastics and can even damage certain types. This is where knowledge of specific plastic types and their suitable adhesives becomes crucial. Understanding the various glues available, such as cyanoacrylate adhesives, hot-melt adhesives, and plastic cements, can ensure successful bonding and prevent further damage.
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What You'll Learn
Identifying the type of plastic
For example, plastics marked with a 6 or "PS" are polystyrenes. These are used for simple items like disposable cutlery, plastic bowls, or fashion bags. For gluing polystyrene, you can use a poly cement, epoxy, or cyanoacrylate. Category 1 pertains to polyester and PET plastics, which covers the most common plastic types, including clothing, plastic bottles, and materials used for insulation. Category 3, PVC, is most commonly used in plumbing and can be glued using a two-part primer and PVC solvent cement.
Another way to identify plastics is through their appearance and feel. Polyethylene has a smooth texture and is flexible but can leave marks. Low-density polyethylene (LDPE) is thin enough to crumble in your hands and will make a loud crackling noise. High-density polyethylene (HDPE) is tougher to deform and usually has a translucent and milky white coating, but uncoloured HDPE is waxier in appearance. Polypropylene is bendable but will never break easily due to its resistance to fatigue.
The float test and burn test are also methods that can be used to identify plastics, but they are not as precise as the recycling symbol or appearance and feel. Burning plastic can be harmful to your health and the environment, so it is not recommended.
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Choosing the right adhesive
Plastic is a tricky material to bond with glue, as its smooth, non-porous surface makes it difficult for an adhesive to bond with. However, there are plastic glues designed to work with the properties of plastic, and choosing the right one will ensure your repair is effective and long-lasting.
The first step is to identify the type of plastic you are working with. Most plastics have a label or recycling symbol, usually a triangle of arrows with a number in the centre. This number will tell you what kind of plastic you're dealing with. For example, plastics marked with a 6 or "PS" are polystyrenes, and the best glue for these is a poly cement such as Loctite Plastic Bonder. Category 3 plastics are made of PVC, and can be glued with a two-part primer and PVC solvent cement. Flexible plastics are commonly used in household items such as shoes or toys, and Loctite Vinyl, Fabric & Plastic Flexible Adhesive is recommended for these.
Once you know your plastic type, you can choose the right adhesive. Different types of plastics require different types of glue. For instance, hard, high-density plastics, such as those used to make buckets, crates, or containers, are made from polyethylene and polypropylene. Ordinary white glues, wood glues, or common adhesives won't react well or form permanent bonds with these plastics. Instead, try a two-part cyanoacrylate adhesive such as Loctite Plastics Bonding System. For bonding materials such as polycarbonate, acrylic, Nylon, Mylar, Delrin, phenolic, aluminium, and stainless steel, Loctite Plastic Bonder is recommended.
There are several types of glue that can be used to bond plastic. Cyanoacrylate, or super glue, is a popular choice for household fixes, and works well for minor plastic repairs that need a quick set time. However, it can damage certain plastics. Plastic cement is another option, which slightly melts the surfaces it touches, which then fuse as the adhesive dries. Epoxy adhesives are durable, long-lasting, and versatile, and are often used where strength is a priority. They can also be used on a range of other materials. Acrylic adhesives are a reliable option for a strong, transparent bond, and are popular for hidden repairs. Hot-melt adhesives are applied with a glue gun and are great when speed and flexibility are important, but they don't work on all plastics.
Before gluing, always clean the plastic surface with soap, a specialised plastic cleaner, or isopropyl alcohol. Sand the surface to create a rough texture for the glue to bond to, and always follow the manufacturer's instructions for preparation and application.
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Surface preparation
Preparing the surface of plastic before gluing is critical to ensuring a strong bond. The right adhesive must be chosen for the specific plastic type, as different plastics require different types of glue. For instance, hard, high-density plastics, such as those used to make buckets, crates, or containers, are made from polyethylene and polypropylene, and ordinary white glues, wood glues, or common adhesives won't react well with these plastics.
The first step in preparing the surface of plastic for gluing is to ensure that the plastic is clean, dry, and free of any oily or greasy residues. Soap, isopropyl alcohol, or a specialised industrial cleaning spray may be used for this. The plastic should then be thoroughly dried.
The next step is to roughen the surface of the plastic to create a coarse texture that the glue can adhere to. This can be done by sanding the plastic with 120 to 200 grit sandpaper. If the plastic surface is already rough, this step can be skipped.
Additionally, degreasing can be done to remove any loosely held dirt or contaminants from the surface. This can be achieved using volatile solvents such as toluene, acetone, methyl ethyl ketone, methyl alcohol, isopropyl alcohol, or trichloroethylene. It is important to follow environmental, health, and safety regulations when selecting and using solvents.
Other surface preparation techniques include flame treatment and plasma treatment. Flame treatment involves exposing the plastic surface to a gas flame for a few seconds, which oxidises the surface and increases surface energy. Plasma treatment, on the other hand, is carried out under a partial vacuum, where gas plasma is activated to produce excited species that react with the plastic substrate, providing better stability compared to other treatments.
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Application techniques
Firstly, identify the type of plastic you are working with. This is an important first step as different types of plastics require different types of glue. The quickest way to identify the type of plastic is to look for the recycling symbol, which usually consists of a triangle of arrows with a number, letter, or code in the centre.
Once you have identified the type of plastic, you can choose the most suitable adhesive. Some common types of glue for plastic include plastic cement, super glue, epoxy, cyanoacrylate, acrylic, and hot-melt adhesives. It is important to follow the specific instructions for the type of glue you are using, as different glues may have different preparation and application requirements. For example, two-part epoxies require the mixing of two ingredients in specific ratios to activate the adhesive.
Before applying the glue, it is important to prepare the plastic surface. Clean the plastic with soap or a specialized plastic cleaner, or degrease it with isopropyl alcohol. Dry the surface thoroughly. To create a rough surface for the glue to bond to, sand the plastic with 120-200 grit sandpaper. Steel wool or emery cloth can also be used, but only briefly.
When you are ready to apply the glue, use a small paintbrush or the tip of a needle for precise, detailed application. Apply an even layer of adhesive to both surfaces where they will be attached and press them together lightly. Use a clamp, tape, or rubber bands to hold the pieces together until the adhesive has dried.
It is important to note that many plastic adhesives continue to cure and develop a stronger bond for days or even weeks after application. Avoid applying stress or heat to the glued piece for at least 24 hours after application, even if the bond seems strong.
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Drying and curing processes
The process of drying and curing plastic glue involves several steps that ensure a strong and durable bond. Here is a detailed guide:
Surface Preparation
Before applying any adhesive, it is crucial to identify the type of plastic you are working with. Different types of plastics require different adhesives. Once you've identified the plastic type, choose an adhesive specifically designed for it. Clean the surfaces to be bonded with soap, a plastic cleaner, or isopropyl alcohol. This step ensures the removal of dust, grease, or other impurities that might compromise the adhesion. Dry the surfaces thoroughly, minimizing any oil residue. You can use a clean cloth or let the surfaces air-dry.
Roughening the Surface
As plastic surfaces tend to be smooth, it is beneficial to create a rough surface for better glue adhesion. You can use sandpaper, steel wool, or emery cloth to briefly rub and roughen the surface. Some two-part adhesives come with an activator that helps achieve this rough texture.
Applying the Adhesive
Follow the manufacturer's instructions for mixing and applying the glue. For precise application, use a small paintbrush or the tip of a needle. Generally, spread a thin, even layer of glue on both surfaces. Avoid applying excess glue to prevent leakage and maintain aesthetics.
Bonding the Surfaces
After applying the adhesive, carefully align the surfaces and join them together. Hold the pieces firmly in place during the drying process. You can use clamps, tape, or elastics, or tongs to maintain pressure and ensure the surfaces remain in contact.
Drying and Curing
The curing time can vary from a few minutes to up to 24 hours or more, depending on the adhesive used. Some adhesives cure rapidly when exposed to ultraviolet (UV) light, while others cure at room temperature without requiring additional heat. Always refer to the manufacturer's instructions for specific curing requirements and times.
Post-Curing Inspection
Once the curing process is complete, inspect the bond to verify its strength and durability. Check for any excess glue and remove it immediately using isopropyl alcohol, denatured alcohol, or mineral spirits. Clean the bonded item with soap and water to eliminate any remaining solvent residue, ensuring the removal of any harmful substances.
Remember, the specific drying and curing processes may vary slightly depending on the type of plastic and adhesive used, so always refer to the manufacturer's instructions for detailed information.
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Frequently asked questions
There is no one-size-fits-all solution when it comes to gluing plastic. Different types of plastics require different adhesives. It is important to determine what type of plastic you are dealing with before choosing an adhesive. For example, polystyrenes marked with a 6 or "PS" are best glued with a poly cement such as Loctite Plastic Bonder. For hard plastics made from polyethylene and polypropylene, try Loctite Plastics Bonding System, a two-part cyanoacrylate adhesive. For flexible plastics, Loctite Vinyl, Fabric & Plastic Flexible Adhesive is recommended.
Plastic is a very smooth material that is non-porous, which makes it difficult for adhesives to bond with anything on the surface. Low surface energy plastics are especially hard to bond with glue.
First, clean the surfaces to be glued. Then, apply a thin, even layer of glue on both surfaces, being careful not to apply too much to avoid leaks. Finally, join the two surfaces and hold them together for a few minutes so that the glue can adhere properly.
