Plastic is a versatile class of synthetic or semi-synthetic materials fundamental to modern manufacturing and daily life. It is primarily composed of polymers, which are large molecules constructed from repeating smaller units called monomers. This molecular structure allows plastics to be molded and shaped under heat and pressure, a property known as plasticity. Plastic’s affordability, durability, and adaptability have led to its ubiquitous presence in medical instruments, electronics, packaging, and construction materials.
Structural Classification: Thermoplastics vs. Thermosets
The behavior of plastic when exposed to heat is determined by the molecular arrangement of its polymers, leading to two main structural classes.
Thermoplastics have long, linear, or branched polymer chains that are not chemically bonded. When heated, the weak intermolecular forces between these chains break, allowing the material to soften and flow. This enables thermoplastics to be repeatedly melted, reshaped, and cooled without chemical change. This reversible process makes materials like polyethylene (PE) and polycarbonate (PC) suitable for recycling and re-molding.
In contrast, thermoset plastics undergo an irreversible chemical reaction, known as curing, during their initial formation. This process creates strong, permanent covalent bonds that cross-link the polymer chains into a rigid network structure. Once cured, thermosets solidify permanently and cannot be softened or melted again. When exposed to excessive heat, materials like epoxy resins and phenolics degrade or decompose rather than transition back to a liquid state. This cross-linked structure provides dimensional stability and resistance to high temperatures.
Mechanical and Physical Properties
The internal structure of plastics allows for a broad spectrum of mechanical performance, ranging from pliable to rigid. Strength is measured by resistance to external stress, while stiffness determines the degree of deformation under load. Some plastics, like high-density polyethylene (HDPE), are flexible, while others, such as glass-fiber-reinforced polymers, possess high strength and rigidity.
Plastics generally have a low density, much lower than that of metals. Common plastics, such as polypropylene (PP), have a density around 0.9 g/cm³, while some engineering plastics reach about 1.5 g/cm³. This low weight and high strength-to-weight ratio make them a preferred choice for applications in the automotive and aerospace industries where weight reduction is a priority.
Plastics also demonstrate impact resistance, the capacity for absorbing energy without shattering. Materials like polycarbonate are known for their toughness, allowing them to withstand sudden impacts. This durability leads to their use in protective gear and shatterproof applications.
Optical properties vary widely based on internal molecular organization. Amorphous plastics, where molecules are randomly oriented, are typically transparent, such as acrylic and polycarbonate. Conversely, semi-crystalline plastics, which have ordered regions, tend to be opaque or translucent because the crystalline regions scatter light.
Thermal and Electrical Behavior
Most plastics are poor conductors of heat and electricity, a property stemming from their molecular composition. This low thermal conductivity is why plastics are used for insulating layers in buildings and for the handles of cooking utensils, preventing heat transfer. Foamed plastics, like polystyrene, further enhance this insulation by trapping pockets of air.
In the electrical field, plastics are excellent insulators due to their high electrical resistivity. They prevent the flow of electric current, essential for safety and functionality in electronic components and wiring. Polyvinyl chloride (PVC), for instance, is commonly used as a protective sheathing in cables.
The material’s response to high temperatures depends on its structural classification. Thermoplastics soften and transition to a fluid state at a specific melting point, which varies significantly by type. Low-density polyethylene (LDPE) melts around 105°C, while polyethylene terephthalate (PET) melts between 250°C and 260°C. Amorphous thermoplastics do not have a sharp melting point but soften gradually over a temperature range, defined by their glass transition temperature.
Chemical Resistance and Environmental Stability
Most plastics exhibit chemical inertness, making them resistant to general corrosion from water, dilute acids, and bases. This resistance allows plastics to be used for long-term storage of various household and industrial chemicals. Specific fluoropolymers, such as PTFE, are known for exceptional resistance, allowing them to withstand aggressive chemical environments.
A common limitation is the interaction of plastics with certain organic solvents. Solvents like aromatic hydrocarbons (e.g., benzene) or ketones (e.g., acetone) can cause certain plastics to swell, stress crack, or dissolve. Amorphous plastics, including polycarbonate, are particularly susceptible to stress cracking when under mechanical tension and exposed to polar organic solvents.
Over extended periods, plastics can be affected by environmental factors, primarily ultraviolet (UV) radiation and oxygen. Exposure to UV light causes photodegradation, which breaks down the long polymer chains. This breakdown is accelerated by oxygen, leading to the material becoming brittle, losing mechanical strength, and experiencing discoloration. Manufacturers incorporate UV stabilizers and antioxidants into the formulation to improve long-term stability.