What is Polymer? its Properties and Additives
Polymers are large chemical compounds with unique properties that allow them to be moulded and shaped. Plastics are classified into two major categories: thermoplastic and thermosets.
Thermoplastics, such as polyethylene and polystyrene, can be moulded and remoulded repeatedly, while thermosets are polymers that cannot be reprocessed through reheating due to a chemical reaction that makes them infusible and insoluble.
These thermoplastic polymers, known as commodity plastics, have a structure that allows individual molecules to flow past one another, allowing them to have high or low molecular weight and separability.
Thermosetting resins undergo a chemical reaction during initial processing, making them infusible and insoluble, making them unsuitable for reworking or heating.
Polymers, a common form of commodity plastics, are diverse in types and classifications. These common forms can be processed to achieve varying properties, making them valuable resources in various industries.
Polymers’ physical state is influenced by their morphology, which can be amorphous or crystalline. Amorphous molecules are randomly intertwined, while crystalline molecules are closely arranged. Semicrystalline materials display crystalline regions within an amorphous matrix.
Thermoplastics retain their moulded shapes to a certain temperature, known as the glass transition temperature (Tg).
The glassy state, where molecules are frozen, makes the material stiff and brittle. Above Tg, the amorphous parts enter the rubbery state, displaying increased mobility and becoming plastic and elastic.
Non-crystalline polymers like polystyrene directly lead to the liquid state, while partly crystalline polymers like low-density polyethylene or polyethylene terephthalate do not reach the liquid state until the melting temperature (Tm).
Beyond this point, crystalline regions are no longer stable, and the rubbery or liquid polymers can be moulded or extruded.
Polymers’ physical properties are influenced by their physical state and morphology, which affect their mechanical properties. Glassy polymers like polystyrene exhibit stiffness, while highly crystalline polymers like polyethylene and polypropylene have a higher elongation ratio.
PET is another semicrystalline plastic that has stiffness and high dimensional stability under stress, making it suitable for beverage bottles and recording tape.
Mechanical properties of polymers include breaking stress, stiffness, and tensile strength, which are quantified as the flexural modulus. Toughness is another crucial property, as it represents the energy absorbed by the polymer before failure.
Repeated stress below the tensile strength may result in fatigue failure.
Plastics are poor heat conductors, with conductivity reduced when a gas is induced into the material. They are also electrical insulators if designed for conductivity, with dielectric strength and dielectric loss being important factors.
Creep, a condition where elongation is not recovered when stress is eliminated, is a common occurrence in plastics.
Polymer additives are components in the production of plastic products. They are used to enhance the properties of the polymer, such as PVC, by altering its glass transition temperature (Tg).
PVC is unique in its ability to accept and retain plasticisers of varying molecular sizes and chemical compositions, which can affect the polymer’s flammability, odour, biodegradability, and cost.
Colourants are also essential for the final appearance of plastic products. Popular pigments include titanium dioxide, zinc oxide, carbon, and other inorganic oxides. Other organic compounds can be used as pigments or dyes.
Reinforcements are used to improve the mechanical properties of plastics, using materials like silica, carbon black, talc, mica, and calcium carbonate as particulate fillers.
Fibre reinforcement, especially with thermosets, can increase stiffness.
Stabilisers, such as antioxidants, help to prolong the useful life of plastic products by countering the effects of aging. These stabilizers are typically used in small quantities to counter the effects of ageing on the polymer.
FAQs
Is polymer a plastic?
Plastics are synthetic materials composed of long-chain molecules formed by large monomers, a sub-division of polymers, which are uniform molecules formed by small monomers.
What are 4 types of polymers?
Synthetic polymers, including addition, condensation, and rearrangement, are among the various types of polymers available.
What is called polymer?
The Greek words polymer and monomer are synonymous, as polymers are large molecules formed by bonding a series of building blocks, with each monomer representing a part, resembling a chain with monomer links.
What does polymer mean in engineering?
Polymers, characterised by their organic parts and repeating units, are long chains composed of various units.
What does a polymer engineer do?
A polymer engineer works with a variety of polymers, including nylon, epoxy, and plastic, in various roles, focusing on developing new polymers and finding uses for existing ones.
Related: Malleability Explained!
Source: mechlesson.com/polymer
