Ethan Clarke
Plastic is a synthetic polymer, a type of matter that can be molded into various shapes and forms. The first man-made plastic was revealed in 1862 by Alexander Parkes, who showcased a material that could be molded when heated and retained its shape once cooled. Plastic is typically categorized as either a thermoplastic or a thermoset, with the former being easily recyclable and the latter not. In physics, the state of matter or phase of matter refers to the distinct forms in which matter can exist. Four common states of matter observable in everyday life are solid, liquid, gas, and plasma, each distinguished by the arrangement and behavior of their particles. While plastic may be perceived as a solid due to its ability to retain its shape, it can also undergo phase transitions and exhibit fluid-like behavior under certain conditions.
| Characteristics | Values |
|---|---|
| Phase of matter | Plastic crystal, a molecular solid |
| Constituent molecules | Retain rotational freedom |
| Behaviour | Behaves as a solid substance |
| Plastic type | Thermoplastics or thermosets |
| Examples | Polyethylene terephthalate, polyvinyl chloride, polystyrene, polyacetal, polyamide, polycarbonate |
| Recycling | Thermoplastics can be recycled, thermosets cannot |
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What You'll Learn
Plastic is a molecular solid
Plastic is a synthetic or semisynthetic material composed primarily of polymers. It is a type of molecular solid, which means it consists of discrete molecules held together by intermolecular forces. These intermolecular forces include van der Waals forces, dipole-dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, halogen bonding, and London dispersion forces.
Molecular solids can be ductile, brittle, or a combination of both, depending on the crystal face stressed. Ductile solids are characterized by isotropic intermolecular interactions, allowing for dislocation between layers of the crystal, similar to metals. Plastic crystals, for example, are soft, wax-like, and easily deformed. On the other hand, brittle solids fracture immediately after passing the yield stress.
The classification of plastics as molecular solids is based on their bonding characteristics. Plastics, such as PET, consist of long molecules or chains that can be considered single units bonded by intermolecular forces. These molecules are often interconnected, forming a giant molecule. However, it is important to distinguish plastics from thermosets, which are not considered molecular solids. Thermosets, such as epoxy, are formed when multiple molecules cross-link to create a continuous material.
Plastics can be further categorized into two types based on their chemical composition: those made up of polymers with only aliphatic (linear) carbon atoms in their backbone chains and those made of heterochain polymers containing atoms like oxygen, nitrogen, or sulfur in addition to carbon. The first category includes commodity plastics like polypropylene, while the second category includes engineering plastics like polycarbonate.
In terms of their physical state, plastics are considered amorphous solids, lacking a highly ordered molecular structure. However, some plastics exhibit a combination of amorphous and crystalline domains, giving them both a melting point and glass transitions. These semi-crystalline plastics include polyethylene, polypropylene, and polyvinyl chloride.
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Plastic crystals and their applications
Plastic is a solid material with a variable shape that can be moulded. It is typically composed of polymers.
Plastic crystals, on the other hand, are molecular solids with long-range positional order but with constituent molecules retaining rotational freedom. The term "orientationally disordered solid" is now preferred over "plastic crystal". This is because the latter is sometimes too limiting, and the former term accurately describes the physical picture. Plastic crystals are composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. They are mechanically soft and easily deformed, resembling waxes. They are usually made up of almost spherical objects.
Plastic crystals are closely related to condis crystals, conformationally disordered crystals that nevertheless possess translational and rotational order. Certain liquid crystals go through a plastic crystal phase before melting. In general, liquid crystals are closer to liquids, while plastic crystals are closer to true crystals. X-ray diffraction patterns of plastic crystals are characterized by strong diffuse intensity in addition to the sharp Bragg peaks.
Plastic crystals have various applications. For example, liquid crystal displays, thermometers, and the preparation of glasses with anisotropic properties. Additionally, there are emerging ionic plastic crystals, particularly organic ionic plastic crystals (OIPCs) and protic organic ionic plastic crystals (POIPCs). POIPCs are solid protic organic salts that have been found to be promising solid-state proton conductors for high-temperature proton-exchange membrane fuel cells.
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Plastic is a heterochain polymer
Plastics are synthetic materials that can be moulded into a variety of shapes and have a range of physical properties. They are typically defined by their engineering behaviour rather than their chemical composition. Plastics can be divided into two categories based on their chemical composition: those made of polymers with only aliphatic (linear) carbon atoms in their backbone chains, and those made of heterochain polymers.
The process of creating Bakelite involves reacting phenol with formaldehyde, which produces a complex, thermosetting network polymer. This reaction usually releases water and other by-products. By controlling the reaction, Baekeland was able to stop it while the resin was still in a fusible and soluble state, allowing it to be dissolved in solvents and mixed with fillers to create a usable plastic. This thermosetting plastic could also tolerate a significant amount of inert ingredients, making it highly modifiable. Due to its excellent insulating properties, Bakelite was widely used in radio electrical systems and automobile electrical systems.
Another important example of a heterochain polymer is polycarbonate, which contains two aromatic (benzene) rings in its molecules. Polycarbonate is an engineering plastic that can compete with die-cast metals in applications such as plumbing, hardware, and automotive components. Other examples of engineering plastics made from heterochain polymers include polyacetal, polyamide (nylon), polytetrafluoroethylene (Teflon), polyphenylene sulfide, epoxy, and polyetheretherketone. These plastics offer unique properties that make them valuable in various specialized applications.
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Phases of matter and their definitions
The term "phase" is often used interchangeably with the "state of matter". A single compound can form different phases, despite being in the same state of matter. For example, water can exist in different phases as ice or water, with the former being a solid state and the latter being a liquid state.
There are four common states of matter observable in everyday life: solid, liquid, gas, and plasma. Each phase is distinguished by the arrangement and collective behaviour of its constituent particles (atoms, molecules, ions, and electrons).
In a solid, particles are tightly packed and held in fixed positions, giving the material a definite shape and volume. This arrangement prevents solids from being compressed. Crystalline solids, such as ice, exhibit regular and repeating patterns in their particle structures. However, solids can also be amorphous, lacking long-range order in the positions of their atoms, like glass.
In a liquid state, particles remain close together but can move past one another, allowing the substance to adapt to the shape of its container while maintaining a nearly constant volume. This adaptability is due to the ability of liquids to be compressed.
Gases differ from solids and liquids as they take the shape and volume of their containers. Gas particles are far apart and move freely, allowing the substance to expand and fill its container.
Plasma is similar to a gas but contains charged particles (ions and free electrons) that move independently and respond to electric and magnetic fields. At extremely low temperatures, substances exist as solids, and as the temperature increases, they transition to liquids, then gases, and finally enter the plasma state at very high temperatures.
In addition to these common states of matter, there are other exotic states that have been observed, such as superfluid, supersolid, superglass, and various magnetic states like ferromagnetism and antiferromagnetism. These states occur under specific conditions, such as extreme temperatures or pressures, and exhibit unique properties that differentiate them from the more commonly encountered phases of matter.
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Plastic is not a single compound
Plastic is a polymeric material that can be moulded or shaped. It is made through the polymerization of starting materials (monomers) that are almost always derived from petrochemicals. These petrochemicals are usually fossil fuels such as crude oil, natural gas, and coal.
Plastics can be divided into two main categories based on their chemical composition. The first category comprises plastics made of polymers with only aliphatic (linear) carbon atoms in their backbone chains, such as polypropylene. The second category is made of heterochain polymers, which contain atoms like oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. Examples of this type include polycarbonate.
The engineering behaviour of plastics is a more critical factor than their chemical composition in defining them. This is reflected in the distinction between "commodity" resins and "specialty" resins. Commodity plastics, like polyethylene and polypropylene, are widely used and produced at high volumes, while specialty resins, like engineering plastics, are tailored for specific applications and produced at lower volumes and higher costs.
The variety of plastic types and subtypes, as well as the ability to mix different plastics together to form polymer blends, highlights the diverse and complex nature of plastics, which cannot be attributed to a single compound.
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Frequently asked questions
Plastic is a solid, specifically a plastic crystal, which is a molecular solid with long-range positional order but with constituent molecules retaining rotational freedom.
The four states of matter observable in everyday life are solid, liquid, gas, and plasma. However, there are other phases that occur under extreme conditions, such as Bose-Einstein condensates, quark-gluon plasma, and neutron-degenerate matter.
A phase of matter is a region of material that is chemically uniform, physically distinct, and often mechanically separable. Distinct phases can exist within a given state of matter, and they can be differentiated based on solubility as well as phase transitions.
