Ethan Clarke
Plastic deformation refers to irreversible deformation, during which chemical bonds between atoms and their nearest neighbours are broken and reformed. The bonds in the plastic region are influenced by factors such as the type of plastic, the presence of cross-linking, and the temperature. Different types of plastics have varying abilities to undergo plastic deformation due to differences in their bonding mechanisms. For example, ceramics are unable to easily break and reform their bonds, resulting in brittleness. On the other hand, metals exhibit metallic bonding, which allows for rapid and easy disruption and reformation of bonds, making them tough and resilient. Cross-linking in polymers, formed by extensive covalent bonds, increases the hardness and melting difficulty of the material. Additionally, temperature plays a crucial role, as some plastics exhibit a softening point when thermal kinetic energy enables internal rotation within the bonds, making them more flexible and deformable.
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What You'll Learn
Covalent bond formation in plastics
Plastics are synthetic materials derived from organic compounds containing carbon. The most common sources for these carbon compounds are oil and natural gas. Plastics are polymers, which are long molecule chains often mixed with other substances such as colouring agents and softeners.
The atoms of a molecule are held together by primary bonds. The three main types of primary bond are ionic, covalent, and coordinate. An ionic bond is formed by the donation of an electron by one atom to another, resulting in a stable number of electrons in the outermost shell. Ionic bonds are seldom found in polymers used in plastics. The most important interatomic bond in polymers is the covalent bond.
In a covalent bond, the outer electrons of some atoms are close enough to overlap and are shared between the nuclei, forming a strong primary bond. Each pair of electrons is called a covalent bond. Covalent bonds are found within the molecules of plastics and are very strong, rupturing only under extreme conditions.
The bonds between the molecules that allow sliding and rupture to occur are called van der Waals forces, or secondary bonds. Secondary bonds are weak forces of attraction between molecules. Thermoplastics are linear chain molecules with weak secondary bonds between the chains. Deformation of thermoplastics can occur in two ways: elastic, where the material returns to its original size and shape when the load is removed; and plastic, where the secondary bonds between the chains weaken and allow the molecular chains to slide over each other, resulting in the material not returning to its original size and shape when the load is removed.
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Ionic bonds in plastics
Plastics are synthetic materials derived from organic (carbon-containing) compounds. The most common sources for carbon compounds are oil (petroleum) and natural gas. Plastics are polymers, but not all polymers are plastics. Polymers are composed of many simple molecules that are repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure.
The atoms of a molecule are held together by primary bonds. The three main basic types of primary bonds are ionic, covalent, and coordinate. An ionic bond is formed by the donation of an electron by one atom to another so that each has a stable number of electrons in the outermost shell (usually eight). Ionic bonds are seldom found in polymers of current interest as plastic materials, although they are important in ion-exchange resins and "ionomers".
The most important interatomic bond in polymers and organic chemistry is the covalent bond. This is formed by the sharing of one or more pairs of electrons between two atoms. An example is the bonding of carbon and hydrogen to form methane. In the case of carbon, the stable number of electrons for the outer shell is eight, and for hydrogen, it is two. Thus, all the atoms possess or share the number of electrons required for stability. Where a pair of electrons is shared between two atoms, they are bound by a single bond.
Secondary bonds are weak forces of attraction between molecules that exist in substances such as water. They are also called secondary valence forces, intermolecular forces, or van der Waals forces. These forces hold groups of polymer chains together to form the polymeric material.
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Plastic deformation
In the case of metals, their ability to undergo plastic deformation makes them attractive structural materials, especially under tension. This is because plastic flow can limit the rate of spread of Mode I fracture, where tensile forces act to pull apart fracture surfaces. Metals exhibit toughness and resilience due to metallic bonding and their crystalline structure, which allows for the rapid disruption and reformation of bonds.
On the other hand, ceramics are less likely to undergo plastic deformation because they cannot easily break and reform their bonds. Ceramics typically fail by the extension of flaws under most loading conditions, and higher stresses would be required for dislocation motion to occur. This results in ceramics being harder but more brittle compared to metals.
At the atomic level, plastic deformation involves the displacement of atoms from their initial positions, leading to the breaking and reforming of chemical bonds with their nearest neighbours. This process transforms elastic energy into chemical potential energy or generates heat through disorganized vibrations throughout the material.
In the context of polymers, such as thermoplastics, plastic deformation can occur when the thermal kinetic energy allows for internal rotation within the bonds and independent sliding of individual molecules. This results in increased flexibility and deformability, defining the glass transition temperature (Tg). At higher temperatures, the melting point (Tm) is reached, where crystalline regions disintegrate, and the material becomes a viscous liquid.
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Plastic bonding glues
Plastic is a non-porous and very smooth material, which makes it difficult for an adhesive to bond with its surface. However, there are plastic glues designed specifically to work with the properties of plastic, allowing for easy repairs or creations. The type of glue depends on the type of plastic being used.
There are eight recycling codes for plastic, which indicate the exact material the product is made of. For example, items stamped with a "1" are made from Polyethylene Terephthalate (PET), while "2" means High-Density Polyethylene (HDPE). Knowing the kind of plastic being used helps determine the adhesive best suited for the material. Plastics in categories 1, 2, 4, and 5 are very difficult to glue.
For bonding polystyrene (category 6), poly cement, epoxy, or cyanoacrylate will form a sufficient bond. Cyanoacrylate, or super glue, can be used for various types of repairs and projects, including use with plastics. It works best when surfaces are less smooth, so it may be helpful to sand the plastic surface before applying the adhesive.
For polycarbonate (category 7), an epoxy will work best, while acrylic (also category 7) requires an acrylic solvent adhesive. Cyanoacrylate may work, depending on the project. ABS plastics (category 9) can be glued using ABS solvent adhesives, but these tend to warp the plastic. For optimal results, an epoxy should be used, although cyanoacrylate may also work well.
Other plastic bonding glues include the Infinity SuperTAC 500 Plastic Bonding Glue Sticks, which offer heat resistance and bonding abilities to a wide range of plastics, and the Loctite Super Glue Plastics Bonding System, a two-part cyanoacrylate adhesive that dries transparent and successfully bonds plastics with no mixing or clamping required. MMA adhesives are another option, creating strong and effective structural bonds that cure quickly at room temperature.
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Adhesive suitability
The recycling codes on plastic products can be a helpful guide for determining the right adhesive. These codes indicate the specific type of plastic, with categories 1, 2, 4, and 5 being particularly challenging to glue. For instance, polystyrene (category 6) can be bonded with poly cement, epoxy, or cyanoacrylate, while polycarbonate (category 7) works best with epoxy. Acrylic, also in category 7, requires an acrylic solvent adhesive for optimal results. ABS plastics (category 9) can be bonded with ABS solvent adhesives, but these may warp the plastic; therefore, an epoxy or cyanoacrylate glue is recommended for better performance.
The joint design is another essential factor to consider when bonding plastics. A well-designed joint ensures the adhesive's success, as poor design can lead to bond failure. Maximizing the contact area between the surface and the adhesive is crucial for achieving a higher-strength bond. Additionally, minimizing peel force is important, as most adhesives cannot withstand significant peel forces.
The suitability of an adhesive also depends on substrate compatibility. Some grades of plastics, due to their low surface energy, can cause adhesion problems. These hydrophobic surfaces repel adhesives, resulting in weak bonds. Therefore, understanding the specific plastic type and its surface characteristics is vital for selecting the most compatible adhesive.
In conclusion, adhesive suitability for plastics involves a combination of factors, including plastic type, joint design, and substrate compatibility. By considering these aspects, manufacturers can achieve strong and lasting bonds when working with various plastic materials.
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Frequently asked questions
The three main types of primary bonds are ionic, covalent, and coordinate. An ionic bond is formed by the donation of an electron by one atom to another, resulting in a stable number of electrons in the outermost shell. Covalent bonds are formed by the sharing of one or more pairs of electrons between two atoms. An example of this is the bonding of carbon and hydrogen to form methane.
Firstly, different types of plastics require different types of plastic glue. The recycling symbol on plastics, which consists of three arrows forming a triangle, can help determine what type of plastic it is. For instance, items stamped with a "1" are made from Polyethylene Terephthalate (PET), while a "2" indicates High-Density Polyethylene (HDPE). Once you know the type of plastic, you can choose the most suitable adhesive. Additionally, it is important to maximise the contact area between the surface and adhesive to increase bond strength and minimise peel force.
Plastic deformation refers to irreversible deformation, during which atoms are displaced from their initial positions and achieve a new equilibrium location. This displacement results in the breaking and reforming of chemical bonds. The elastic energy associated with the initial strain may be transformed into chemical potential energy or lost as heat through disorganized vibrations in the material.
