Ethan Clarke
Plastic is a polymer derived from crude oil, natural gas, or coal. It can also be made from renewable sources such as carbohydrates, fats, and oils. Plastic is composed of long chains of carbon and hydrogen atoms, which can be manipulated to form different types of plastic. The process of polymerization involves linking hydrocarbon monomers through chemical bonding to form polymers. These polymers are then used to create plastic products. The type of plastic determines the adhesive required for bonding, and various treatments can be applied to improve adhesion. Plastic bonding can be achieved through various methods, including the use of adhesives, solvents, heat, ultrasound, vibration, and radiation.
| Characteristics | Values |
|---|---|
| How plastic is made | Plastic is made by imitating nature's process of creating polymers. Synthetic plastics are derived from hydrocarbons in crude oil, natural gas, or coal. |
| Plastic bonding | Plastic can be bonded to other plastics, metals, or glass using adhesives, solvents, heat, ultrasound, vibration, radiofrequencies, or radiation. |
| Adhesive selection | The choice of adhesive depends on the type of plastic. For example, polystyrene can be bonded with poly cement, epoxy, or cyanoacrylate. |
| Surface preparation | The surface energy of the plastic plays a crucial role in its receptiveness to adhesives. Physical or chemical surface treatments can increase surface energy and improve bonding strength. |
| Chemical pretreatment | Primers can be applied to create a new 'bondable' layer on the plastic surface. However, they are polymer-specific and must be compatible with the chosen adhesive. |
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What You'll Learn
Plastic is a polymer
Plastic is a synthetic polymeric material derived from petroleum. It is made through a polymerisation or polycondensation process, where oil and natural gas are refined to form gases like ethane and propane. These gases are then heated to form monomers, which are bonded together to form polymers. The monomers and a catalyst are mixed to form a polymer, which is then extruded, cooled, and cut into pellets. These pellets are shipped to plastics fabrication companies worldwide.
Plastics are a specific type of polymer, composed of long chains of chemically bonded molecules or monomers. Polymers can be characterised as homopolymers, which are made up of one type of monomer, or copolymers, which are made up of two or more monomers. The chemical composition and size of the individual monomers determine how the polymer interacts with its environment.
The repeating units in polymers are often carbon and hydrogen, and sometimes oxygen, nitrogen, sulfur, chlorine, fluorine, phosphorus, and silicon. The most familiar plastic polymers, such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), are derived from petroleum hydrocarbons. These materials are used in a diverse range of applications but pose recycling and disposal issues due to their difficulty in breaking down.
Synthetic polymers present environmental challenges due to their production reliance on petrochemical products, which contribute to rising global temperatures, and their slow biodegradability, which can harm ecosystems. However, natural polymers, such as cellulose, latex, and rubber, are not derived from crude oil and can be biodegradable.
Plastics have long, linear polymer chains that are weakly chemically bonded. When heated, these bonds are easily broken, allowing the polymers to glide past each other and reform when cooled, enabling the plastic to retain its new shape. This property makes plastics easy to recycle and remould.
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Plastic bonding methods
Plastic is a polymer derived from crude oil, natural gas, coal, or renewable products such as carbohydrates, fats, and oils. There are about 50 groups of plastics, with hundreds of varieties, and different types of plastics require different types of adhesives. The recycling codes on plastics can help determine what type of plastic an item is made of and, consequently, what type of adhesive to use. For example, plastics in categories 1, 2, 4, and 5 (Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP), respectively) are very difficult to glue.
There are several methods for bonding plastics, including:
- Solvent bonding: This method involves coating plastics with a solvent and clamping them together. The solvent softens the plastics, and when it evaporates, the plastics are bonded together.
- Induction welding: Plastics that are difficult to weld, such as polypropylene, are usually bonded by induction welding.
- Vibration welding: This method is suitable for forming pressure-tight joints in irregularly shaped or larger plastic parts, or those composed of dissimilar materials with different melting points. Vibrational heating causes friction that, when paired with pressure, melts and bonds plastics.
- Spin welding: One of the two parts to be joined is spun at high speed while force is applied to hold the parts together. The resulting friction heats and melts the plastics, and more pressure is applied post-spinning to bond the materials together.
- Hot plate welding: This method joins two plastic parts by melting them against a precisely heated platen for a prescribed period, after which the platen is removed and the two parts are forced together.
- Laser welding: Laser welding uses a laser beam's concentrated heat and directionality to join plastics. It can be used on applications with plastics of varying thicknesses and is ideal for high-volume production of parts requiring narrow, deep welds.
- Infrared welding: Infrared welding is a fast, non-contact method that uses infrared radiation to generate heat and melt the surfaces of plastic parts. The parts are then joined by using pressure to hold them together until they are bonded.
- Mechanical fastening: This is the simplest way of joining plastics and is more suitable for joining stronger plastics and when precision bonding is not required. Mechanical fastening involves joining plastics together with simple fasteners like latches and nails.
Before bonding, the polymer may need to be prepared by increasing its surface energy and removing contaminants from its surface. Physical surface treatment involves chemically changing the surface by exposing it to flame, corona discharge, or plasma. Mechanical pretreatment involves abrading the surface through sanding, grinding, or sandblasting, which increases the surface area available for bonding and results in a stronger joint. Another method of preparing the polymer is chemical pretreatment, which involves applying a special chemical that will bond to the hydrophobic surface of the plastic and create a new layer that can be bonded.
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Plastic adhesives
The surface energy of the plastic also plays a role in how receptive it will be to the adhesive. Plastics with low surface energy are hydrophobic, and the adhesive will not be able to form a strong bond. To increase the surface energy, physical surface treatment can be used, which involves exposing the surface to flame, corona discharge, or plasma. Another method is mechanical pretreatment, which involves abrading the surface through sanding, grinding, or sandblasting to increase the surface area available for bonding. However, this method does not alter the chemical structure of the surface and can damage the surface. Chemical pretreatment, also known as priming, involves applying a special chemical that will bond to the hydrophobic surface of the plastic and create a new layer that the adhesive can bond to. However, primers are polymer-specific and may not be compatible with certain adhesives.
When choosing a plastic adhesive, it is also important to consider the chemicals that the bond will be exposed to. Some adhesives have poor chemical resistance, so it is important to choose an adhesive that can withstand the specific chemicals the plastic bond will experience. Water, for example, can permeate an adhesive and cause plasticization, making the adhesive more flexible and less strong.
Some common plastic adhesives include cyanoacrylate glue (CA glue), epoxy-based adhesives, solvent-based adhesives, and poly cements.
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Pretreating plastics
Plastic is a polymer derived from crude oil, natural gas, coal, or renewable products such as carbohydrates, fats, and oils. It is composed of long chains of carbon atoms that are weakly or strongly bonded together. The type of bonding depends on whether the plastic is a thermoplastic or a thermoset. Thermoplastics have long polymer chains that are entangled but only weakly chemically bonded, allowing them to be melted and reformed. On the other hand, thermosets have strong primary bonds between adjacent polymer chains, giving them a rigid 3D structure that cannot be melted and reformed.
When it comes to pretreating plastics for bonding or adhesion, there are several methods available. The main objectives of pretreating plastics are to increase the surface energy, remove contaminants, and modify the properties of the polymer to enhance biodegradation. Here are some common pretreatment techniques:
Mechanical Pretreatment
Mechanical pretreatment involves physically abrading the surface of the plastic to increase the available bonding area. This can be achieved through sanding, grinding, or sandblasting, resulting in a rougher surface with peaks and troughs. While this method increases the surface area for bonding, it does not alter the chemical structure of the plastic, and the surface remains hydrophobic with low surface energy. Mechanical pretreatment is also prone to surface damage and inconsistency.
Physical Surface Treatment
Physical surface treatment, also known as physical pretreatment, involves chemically altering the surface of the plastic by exposing it to a flame, corona discharge, or plasma. Plasma surface pretreatments are particularly useful for promoting adhesion between difficult-to-bond plastic substrates.
Chemical Pretreatment
Chemical pretreatment, often referred to as "priming," involves applying a special chemical that bonds to the hydrophobic surface of the plastic and creates a new layer that adhesives can adhere to. Primers diffuse into the plastic, intermingling with the polymer chains. However, primers are polymer-specific and must be compatible with both the plastic and the adhesive being used.
Advanced Pretreatment Techniques for Plastic Waste Recycling
In the context of recycling plastic waste, advanced pretreatment techniques are essential to address the challenges posed by contaminants and mixed polymer streams. These techniques include chemical washing to remove inks (deinking), extraction methods to remove unwanted additives, and dissolution-based pretreatments such as delamination and dissolution-precipitation. Deodorization techniques are also employed to remove odor constituents, while solvent-based extraction methods can effectively remove various additives from the polymer matrix.
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Plastic composition
Plastic is a polymer, a substance composed of long chains of molecules that are bonded together. These molecules are usually derived from hydrocarbons, which are typically sourced from fossil fuels such as crude oil, natural gas, and coal.
The first step in the process of creating plastic is to refine crude oil into different petroleum products, such as naphtha, which is a crucial compound for making plastic. These petroleum products are then converted into useful chemicals called "monomers," which are the basic building blocks of polymers. Examples of monomers include ethylene, propylene, and butene, which all contain double bonds that allow carbon atoms to react and form polymers.
The next step is polymerisation, where monomers are linked together through chemical reactions to produce polymers. This process generates thick, viscous substances known as resins, which are used to make plastic products. For example, when ethylene is subjected to heat, pressure, and a catalyst, it forms long, repeating chains of carbon atoms, creating a plastic resin called polyethylene (PE).
The PE-based plastic is then processed in a factory to create plastic pellets, which can be melted and cast into moulds to form various plastic products.
Plastics can also be categorised based on their chemical composition and recycling codes. There are eight recycling codes for plastic, indicating the specific material used:
- Polyethylene Terephthalate (PET)
- High-Density Polyethylene (HDPE)
- Polyvinyl Chloride (PVC)
- Low-Density Polyethylene (LDPE)
- Polypropylene (PP)
- Polystyrene (PS)
- Other plastics, including polycarbonate and acrylic
- Acrylonitrile Butadiene Styrene (ABS)
These different types of plastics have varying bonding properties and require specific adhesives for effective bonding. Some common adhesives include epoxy, cyanoacrylates, anaerobics, acrylics, and polyurethanes. Additionally, plastic additives and fillers can be used to enhance specific properties, such as flexibility, hardness, and chemical resistance.
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Frequently asked questions
Plastic is made from polymers, which are bonded chains of carbon atoms. Most plastic in use today comes from hydrocarbons derived from crude oil, natural gas, and coal.
Plastic can be bonded to other plastics, metals, and glass using adhesives, solvents, heat, ultrasound, vibration, radiofrequencies, and radiation. The type of adhesive used depends on the type of plastic. For example, polystyrene can be bonded using poly cement, epoxy, or cyanoacrylate.
The surface energy of the plastic plays a significant role in how receptive it will be to adhesives. If the surface energy is low, you can increase it through physical surface treatment, such as exposing the surface to flame, corona discharge, or plasma. Alternatively, you can use chemical pretreatment, which involves applying a special chemical that bonds to the plastic's surface and creates a new layer for the adhesive to bond to.
Water can permeate an adhesive to a certain degree, making it more flexible and less strong. Continual immersion in water may require a sealant bead to protect joint edges. Additionally, different adhesives have varying levels of chemical resistance, so it's important to consider what chemicals the bond will be exposed to and choose an adhesive that can withstand them.
