Maya Ellison
Bonding hard plastic can be challenging, and the wrong adhesive can result in a weak bond or damage to the object. Different types of plastics require different adhesives, and some plastics are nearly impossible to glue. For instance, hard plastics like buckets or milk crates are likely made of polyethylene or polypropylene, which require a specific adhesive. Clear, brittle plastics like imitation glass or CD cases are usually polystyrene, which can be glued with poly cement. To bond hard plastic effectively, it is essential to identify the type of plastic and select the appropriate adhesive, such as epoxy-based adhesives or hot melt adhesives, ensuring a strong and lasting bond.
Characteristics and Values Table for Hard Plastic Bonding
| Characteristics | Values |
|---|---|
| Plastic Type | Polyethylene, Polypropylene, Polystyrene, Polycarbonate, Acrylic, ABS, PVC |
| Glue Type | Epoxy, Cyanoacrylate, Poly Cement, Solvent Cement, Super Glue, Hot Melt Adhesives, MMA Structural Adhesives |
| Surface Preparation | Clean with soap, specialised plastic cleaner, or isopropyl alcohol. Sand or abrade the surface to create a rough texture for better adhesion. |
| Joint Design | Ensure the joint design is optimised for adhesive bonding, especially if using welding or mechanical fasteners. |
| Surface Energy | Consider the surface energy of the plastic. If it is low, prepare the surface or use a specialist adhesive. |
| Drying Time | Consider the drying time of the glue. Some epoxies require drying overnight for best results. |
| Bond Strength | Choose an adhesive that will provide the required bond strength. Epoxy adhesives are known for their strong bonds. |
| Applicator | Use an applicator bottle or a dual-syringe dispenser to apply the adhesive. |
| Clamping | Use clamps, vise, masking tape, or rubber bands to hold the pieces together while the adhesive dries. |
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What You'll Learn
Choose the right glue for the type of plastic
When gluing plastic, it is important to determine what type of plastic you are dealing with. Plastic comes in many forms, each with distinct properties, and knowing what type of plastic you are working with enables you to choose the appropriate adhesive for the job.
The recycling symbol, consisting of three arrows that form a triangle, is the easiest way to identify a plastic type. The number in the centre of the symbol or a code underneath it indicates the plastic category. Items stamped with a 1 are made from Polyethylene Terephthalate (PET), while 2 indicates High-Density Polyethylene (HDPE). 3 is used for Polyvinyl Chloride (PVC), 4 means Low-Density Polyethylene (LDPE), 5 represents Polypropylene (PP), 6 is for Polystyrene (PS), 7 indicates other plastics, and 9 is used to represent Acrylonitrile Butadiene Styrene (ABS).
Plastics in categories 1, 2, 4, and 5 are very difficult to glue. For instance, plastics that are hard like a bucket or milk crate are likely to be polyethylene or polypropylene, and these plastics are nearly impossible to glue unless you choose an adhesive marked specifically for use with these materials. For gluing PVC, there is a two-part primer and PVC solvent cement commonly available in most hardware or home improvement stores. For bonding polystyrene, you can use poly cement, epoxy, or cyanoacrylate.
For bonding materials such as polycarbonate, acrylic, Nylon, Mylar, Delrin, phenolic, aluminum, and stainless steel, a strong adhesive to use is Loctite Plastic Bonder. Its specialised acrylic formula combines two components to produce a strong, impact-resistant bond that sets in less than half an hour. Once fully cured, it is resistant to water and common shop fluids and solvents. For gluing flexible plastics used in household items such as shoes or toys, Loctite Vinyl, Fabric & Plastic Flexible Adhesive is recommended. It is a clear liquid adhesive that is waterproof, dries clear, and creates a flexible bond that won't break down or yellow from UV/sunlight exposure.
When selecting an adhesive, it is also important to consider the specific application. Different types of glue have varying strengths, clarity, and drying times, so choose the one that will work best for your needs. For instance, if you are gluing plastic piping together, the material that bonds them is called solvent cement. ABS solvent cement will join two ABS pipes together, while PVC solvent cement joins two PVC pipes. Transition solvent cement is used to join an ABS pipe to a PVC pipe. If you are gluing plastic to another substance such as wood, metal, glass, or another type of plastic, further research is required to determine the appropriate adhesive.
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Prepare the plastic surface
Preparing the plastic surface is one of the most important steps in the bonding process. The surface energy of the plastic plays a significant role in how receptive it will be to the adhesive. If the surface energy is low, you will need to prepare the surface to ensure a strong bond.
Firstly, identify the type of plastic you are working with. Different types of plastics require different types of adhesives. For instance, if the plastic is hard, like a bucket or milk crate, it is likely to be polyethylene or polypropylene, which require specific adhesives. Clear, brittle plastics like imitation glass or CD cases are usually polystyrene, which can be bonded with poly cement. If the plastic feels similar to a Lego block, epoxy is the best choice. For plumbing work, PVC pipes require PVC primer and glue.
Once you have identified the type of plastic, you can select the appropriate adhesive. If you are using solvent cement, clamp the pieces together first, then use an applicator bottle to apply a thin line of solvent cement along the edge between the pieces. Press the pieces lightly together, ensuring that air bubbles are removed, but be careful not to push too hard, as this may cause the adhesive to ooze out. Keep the pieces firmly in place with clamps, vise, masking tape, or rubber bands.
Before applying the adhesive, it is important to clean the plastic surface to remove any grease or contaminants. Wash the plastic with soap, use a specialized plastic cleaner, or soak it in isopropyl alcohol. Ensure that the plastic is thoroughly dried, and avoid touching it with bare hands afterward to minimize oil residue.
To further prepare the plastic surface, you can create a rough surface for the glue to bond to. Briefly sand the plastic with 120-200 grit sandpaper, or use steel wool or emery cloth. Mechanical pretreatment methods like sanding, grinding, or sandblasting increase the surface area of the plastic, creating peaks and troughs, which can lead to a stronger joint. However, these methods do not alter the chemical structure of the surface, and the surface will still have low surface energy, making it hydrophobic. Therefore, it is crucial to select the appropriate adhesive that can form a strong bond with the plastic surface.
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Use an activator
Bonding hard plastics can be challenging as many adhesives cannot properly bond with the plastic. Different types of plastics require different types of glue. For instance, hard plastics such as buckets, crates, and containers are made from polyethylene and polypropylene, which are nearly impossible to glue unless you use an adhesive specifically designed for these plastics.
Some special plastic bonding systems include an activator as a preliminary step. This prepares the surface of the plastic to accept the adhesive and creates a stronger bond. Firstly, pre-fit and clean the parts. Apply the activator to both surfaces and allow it to dry for about a minute. Then, apply the adhesive as you would with super glue and join the parts. Let it dry by waiting for at least 10 minutes before moving the combined parts. Leaving the glue to dry overnight will typically yield the best results.
The Loctite two-part plastic bonding system has an activator that helps roughen the surface. For precise, detailed plastic glue application, use a small paintbrush or the tip of a needle. Mix the components if required. Two-part epoxies or adhesives are durable but they need some preparation. Read the product's instructions carefully for blend ratios and mix as directed to activate the adhesive. Clamp the pieces for a firm seal. Sealing pieces tightly is crucial. Use a clamp, tape, or elastics to hold parts firmly together while the glue dries. Curing times can range from a few minutes up to 24 hours, so check your product directions.
For larger repairs or when you are unsure of the type of plastic involved, the best way to glue plastic together is Loctite Super Glue All Plastics. This two-step system will bond almost any type of plastic in just seconds. Apply the activator to both surfaces, wait 60 seconds, then apply the adhesive to one surface and join the parts. When you need the extra strength of epoxy, your best choice for gluing plastic is Loctite Universal Instant Epoxy Liquid 5 Minute. This special formula bonds to PVC, ABS, FRP, Mylar, Delrin, and many other plastics and reaches handling strength in about 20 minutes.
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Apply the adhesive
When applying the adhesive, it is important to follow the manufacturer's instructions. Here are some general guidelines to follow:
Prepare the Surface
Firstly, ensure the plastic surface is clean and free of any grease or contaminants. You can wash the plastic with soap, use a specialised plastic cleaner, or soak it in isopropyl alcohol. Dry the surface thoroughly after cleaning. To maximise the surface area available for bonding, you can sand the surface briefly with fine-grit sandpaper, steel wool, or emery cloth. This creates a rough surface for the glue to adhere to, resulting in a stronger bond.
Choose the Right Adhesive
Select an adhesive that is specifically designed for bonding hard plastics, such as polyethylene or polypropylene. Look for products that advertise the ability to bond these specific materials. If bonding polystyrene, you can use poly cement, epoxy, or cyanoacrylate. For ABS plastics, epoxy is recommended, although cyanoacrylate can also be used. If using PVC pipes, a two-part primer and PVC solvent cement are typically used.
Different adhesives will have varying application methods, so refer to the manufacturer's instructions. Some adhesives may come in an applicator bottle, allowing you to apply a thin line of cement along the edge between the pieces. Gently press the pieces together, ensuring they are aligned correctly and removing any air bubbles. Avoid pushing too hard to prevent adhesive from oozing out. If adhesive does ooze out, wipe away the excess unless you are using an acrylic cement, which should be left to evaporate.
Secure the Joint
Use clamps, vises, masking tape, or rubber bands to hold the pieces firmly together while the adhesive cures. The curing time can vary depending on the type and brand of adhesive, so always read the instructions on the container. Some adhesives may take a few minutes to set, while others can take up to 24 hours or more. For the best results, leave the bonded pieces undisturbed overnight.
Post-Bonding Care
Once the adhesive has cured, you may need to sand, drill, or machine the joint for a smooth finish. Some adhesives may require additional curing time to reach full strength. Always refer to the manufacturer's instructions for specific care and handling recommendations after bonding.
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Clamp the pieces together
Clamping is an essential step in the plastic bonding process. Clamps are used to hold the pieces of plastic firmly together while the adhesive dries or sets. This ensures that the pieces do not shift or move during the curing process, which could result in a weak or uneven bond.
When clamping hard plastic pieces together, it is important to apply even pressure to both pieces. This can be achieved by using clamps with padded jaws or by placing a piece of scrap wood between the clamp and the plastic to distribute the pressure more evenly. It is also important to avoid over-tightening the clamps, as this could deform or damage the plastic.
The type of clamp used will depend on the size and shape of the plastic pieces being bonded. For larger pieces, C-clamps or bar clamps may be necessary to provide sufficient clamping pressure. For smaller pieces, spring clamps or hand screw clamps may be more suitable. It is also possible to use alternative methods such as masking tape, rubber bands, or vices to hold the pieces together if clamps are not available.
The clamping time will vary depending on the type of adhesive used. Some adhesives may take only a few minutes to set, while others can take up to 24 hours to fully cure. It is important to refer to the instructions on the adhesive packaging to determine the appropriate clamping time.
In addition to clamping, there are other factors that are crucial for successfully bonding hard plastic. Selecting the appropriate adhesive for the specific type of plastic is essential, as different plastics require different types of glue. It is also important to prepare the plastic surfaces before gluing by cleaning them with soap and water and sanding them lightly to create a rough surface for the glue to adhere to.
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Frequently asked questions
Different types of plastics require different types of glue. Hard plastics like buckets and milk crates are usually made of polyethylene or polypropylene, which are nearly impossible to glue unless you use an adhesive specifically designed for these plastics. Infinity SuperTAC 500 Plastic Bonding Glue Sticks are one such adhesive.
There is no single adhesive that is best for every plastic application. The best glue depends on the type of plastic and the materials you want to join together. For example, if you are gluing plastic pipes, you should use a solvent cement designed for the specific type of pipe. If you are gluing plastic to another substance, such as wood or metal, you will need to do some research to find a suitable adhesive.
Surface preparation is critical to achieving a strong bond. First, clean the plastic surface with soap, a specialized plastic cleaner, or isopropyl alcohol to remove grease and other contaminants. Then, briefly sand the surface with fine-grit sandpaper to create a rough surface for the glue to bond to. Finally, apply an activator to the surface to help the plastic accept the adhesive and create a stronger bond.
While there is no one-size-fits-all solution for bonding hard plastic, some adhesives are specifically designed for hard-to-bond plastics. For example, PUR hot melt adhesive creates an incredibly strong bond with hard plastics like polyethylene and polypropylene. Another option is to use a two-part epoxy adhesive, which creates a very durable bond that can be sanded, drilled, and machined if needed.
