A Guide to Plastics: Types, Properties, and Applications
Plastics: An Overview
Plastics are substances with diverse structures and properties. They typically lack a fixed boiling point and exhibit elasticity and flexibility, allowing them to be molded into various shapes. In a narrower sense, plastics refer to synthetic materials obtained through polymerization, involving the propagation of carbon atoms in long molecular chains derived from petroleum and other natural substances.
Properties of Plastics
Most plastics share these characteristics:
- Easy to work with and mold
- Low production cost
- Low density
- Impermeable
- Good electrical insulators
- Acceptable acoustic insulation
- Good thermal insulation (but most cannot withstand very high temperatures)
- Resistant to corrosion and many chemical factors
- Some are not biodegradable or easily recyclable, and if burned, produce pollutants.
Elastomers
Elastomers are polymers with elastic behavior. The term is sometimes used interchangeably with “rubber,” which more accurately refers to vulcanizates. The monomers forming the polymer typically consist of carbon, hydrogen, oxygen, and/or silicon. Elastomers are amorphous polymers above their glass transition temperature (Tg), giving them significant strain capacity. At room temperature, they are relatively soft and deformable. They are primarily used for seals, adhesives, and flexible parts. Their first use in the late 19th century revolutionized applications like car tires.
Types of Elastomers
Thermoset Elastomers
These elastomers remain strong upon heating until they degrade above a certain temperature. Most elastomers belong to this group.
Thermoplastic Elastomers
These elastomers soften and become malleable when heated. Their properties remain unchanged even after repeated melting and molding. This relatively recent material type was first synthesized in 1959.
Thermoplastics
A thermoplastic is a plastic that is pliable at room temperature, melts when heated, and hardens into a glassy state when cooled. Most thermoplastics are high-molecular-weight polymers with chains connected by weak Van der Waals forces (e.g., polyethylene), strong dipole-dipole interactions and hydrogen bonds, or aromatic ring stacks (e.g., polystyrene).
Unlike thermosetting polymers, thermoplastics can be repeatedly melted and reshaped. Their physical properties change gradually with repeated melting and molding (thermal history), generally decreasing.
Common thermoplastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyethylene terephthalate (PET), Teflon (PTFE), and nylon (a type of polyamide).
They differ from thermosets (e.g., Bakelite, vulcanized rubber), which do not melt at high temperatures but decompose, making them impossible to remold.
Many thermoplastics are blends of different polymers, such as vinyl, a mix of polyethylene and polypropylene.
Examples of Thermoplastics
- Acrylonitrile Butadiene Styrene (ABS)
- Polymethylmethacrylate
- Cellulose nitrate (celluloid)
- Cellulose acetate
- Styrene Acrylonitrile (SAN)
- Ethylene vinyl acetate (EVA)
- Ethylene vinyl alcohol (EVAL)
- Fluoropolymers (or fluoroplastics)
Thermosetting Plastics
Thermosetting plastics are infusible and insoluble polymers. Their chains form a three-dimensional network linked by strong covalent bonds, creating a giant molecule with a fixed shape. The chain mobility and rotational degrees of freedom are practically zero.
Thermosets offer advantages over thermoplastics, such as better impact resistance, solvent resistance, gas permeation resistance, and resistance to extreme temperatures. However, they are generally difficult to process, require curing, and are brittle. They can be reinforced with fibers like fiberglass, forming reinforced plastics. Thermoplastics can also be reinforced using this technique.
Examples of Thermosets
- Vulcanized natural rubber
- Bakelite (a phenol-formaldehyde resin used in electronics)
- Duroplast
- Urea-Formaldehyde Foam (used in wood imitation and boards)
- Melamine (used in work boards)
- Unsaturated polyester resins (often reinforced with fiberglass)
- Epoxy resin (used as adhesive and in reinforced plastics)
- Polyurethanes
- Silicones
Biodegradable Plastics
Interest in biodegradable plastics declined in the late 20th century with falling oil prices but has resurged due to rising oil prices and awareness of depleting oil reserves. There is increasing research into environmentally degradable polymers and plastics (EDPs). Manufacturing biodegradable plastics from natural materials is a significant challenge across various industrial, agricultural, and service sectors. Polyhydroxyalkanoates (PHAs) are a promising alternative.
Replacing conventional plastics with biodegradable ones can reduce environmental pollution. Biodegradable plastic waste can be treated as organic waste, degrading quickly under suitable conditions.
Types of Biodegradable Polymers
- Polymers extracted directly from biomass (e.g., starch, cellulose, casein, keratin, collagen)
- Polymers synthesized from biological monomers using renewable sources
- Polymers produced by microorganisms (e.g., bacteria)
PHAs, produced by bacteria, are considered “doubly green” due to their renewable origin and biodegradability. Polylactic acid (PLA), produced from sugar-rich sources, has similar properties to petroleum-based thermoplastics but biodegrades under favorable conditions.