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Characteristics and Applications of Common Plastics - Part 1

Characteristics and Applications of Common Plastics - Part 1

Definition of Plastic

Material consisting chiefly of carbon, oxygen, hydrogen, nitrogen, and other organic or inorganic elements in a solid, final form obtained after processing it in a molten-liquid state that permits it to be melted, molded, and solidified into the desired shape. Such a material is termed plastics.

Plastic

Thus, plastics are high molecular weight synthetic resins, generally with additives like plasticizers, stabilizers, flame retardants, lubricants, and colorants. They can be flexible or rigid, cross-linked, and cured. Plastics find a huge and diversified spectrum of applications, ranging from domestic appliances to electrical wiring and building materials, electronics, and even the automotive and aerospace industries, through common hardware.

Properties of Plastics

  • High molecular organic compounds
  • Can be in liquid, solid, gas, colloid or solution forms
  • Mouldable
  • Types: Various monomers that can be combined
  • Uses: From simplest to most specialized
  • Properties: Such a wide variety exists
  • Several ways of processing

Advantages of Plastics

  • Highly resistant to corrosion and non-conductive to acids and bases
  • Low production cost
  • Strong, waterproof, light
  • Can be molded into various shapes
  • Good electrical insulator
  • Some of them can be used in the manufacture of fuel oils and gases, hence reducing the amount of crude oil consumed

Disadvantages of Plastics

  • Not readily recyclable and are not economically feasible
  • Have a high inflammability; some upon burning produce toxic fumes, such as polystyrene toluene and PVC gives off hydrogen chloride gas
  • At high temperatures, they break down into benzene rings known to be lethal in nature
  • Petroleum-based; hence, an example of a non-renewable resource
  • Non-biodegradable

The Classification of Plastics

Centered on The Performance When Heated:

  • Thermoplastics: Those plastics that soften when heated and, hence, can be reshaped for re-use. Examples include polyethylene, polyvinyl chloride, polypropylene, and polystyrene.
  • Thermosetting Plastics: Those that cure irreversibly do not dissolve or melt and decompose at high temperatures. Examples of this include phenolic and epoxy plastics.

By Characteristics of Use:

  • General-purpose Plastics: Such plastic material is mass-produced and consumed with fine moldability obtained at a low price. Examples include polyethylene, polypropylene, polyvinyl chloride, and polystyrene.
  • Engineering Plastics: Resistant to external forces, resistant to changes in temperature and provides dimension stability for special use. Examples include fluoroplastics and organic silicone.
  • Specialty Plastics: Includes reinforced and foam plastics because they bear high strength and cushioning ability.

Methods of Plastic Molding

  • Compression Molding: Applied to thermoset plastics, the main ones of which are phenolic resins. Other names include mold-forming and press-molding.
  • Extrusion Molding: Resin, upon heating, is pushed through a mold to form continuous shapes. The technique is suitable for high productivity but less precise.
  • Injection Molding: Molten thermoplastics are applied in large pressure into a mold. It is associated with complex parts and mass production, which is also related to high equipment and mold costs.
  • Blow Molding: The resin, after heating, is extruded and inflated in a mold cavity to develop a shape, which takes the form of a hollow object. Several techniques to be used in making films and hollow containers are also introduced.
  • Calendering: The material in solid form is processed into thin films or sheets using calender rolls; primarily used for PVC resins.
  • Foam Moulding: Introduction of blowing agents to give foam structures to plastics like PVC, PE, and PS.
  • Vacuum Forming: Flat plastic sheets are heated, and a vacuum is created between the sheet and the mold to create the shape. Common uses include packaging and trays.

ABS – Acrylonitrile Butadiene Styrene

Properties of ABS include its impact resistance, heat resistance, low-temperature resistance, stability against chemical corrosion, and proper electrical properties. It is easy to process with good dimensional stability and a very glossy appearance on the surface. ABS can be painted, colored, metal-plated, electroplated, welded, and heat pressed. The application includes industries related to machinery, automobiles, electronics, textiles, and buildings.

Properties of ABS:

  • Mechanical Properties: Highly resistant to impact, corrosive to chemicals, and having good electrical properties.
  • Flowability: Slightly poorer than HIPS and better than PMMA and PC.
  • Molding Performance: It requires good drying, high material and mold temperature, and fast cooling. Careful control of melting is necessary to avoid stress concentration and cracking.
  • General Properties: Non-toxic, odorless, ivory-colored semi-transparent or transparent granules or powder.
  • Density: 1.05-1.18 g/cm³.
  • Shrinkage Rate: 0.4%-0.9%.
  • Melting Temperature: 217-237°C.
  • Thermal Decomposition Temperature: >250°C.
  • Mechanical Properties: High impact strength, good wear resistance, suitable for medium load, and low-speed applications.

Thermal Properties:

  • Heat Deflection Temperature: 93-118°C, about 10% higher post-annealing.
  • Temperature Range: Good toughness at -40°C; end-use temperature range -40°C to 100°C.

Electrical Properties:

  • Good electrical insulator, not significantly affected by temperature, humidity, and frequency.

Environmental Properties:

  • Good resistance to water, inorganic salts, alkalis, and many acids but dissolves in ketones, aldehydes, and chlorinated hydrocarbons; has poor weather resistance, and its impact strength decreases on outdoor exposure.

PP (Polypropylene)

Polypropylene—Thermoplastic resin, CH₂=CH-CH₃. It can be isotactic, atactic, and syndiotactic; however, isotactic PP has higher crystallinity and molecular weight than the rest and is excellent both in heat and in solvent resistance. Atactic PP is noncrystalline and soft, which is suitable for elastomers.

Properties of PP:

  • Thermal Properties: MP ∼ 160–170°C; Tg −18°C
  • Mechanical Properties: High tensile strength; low density; good fatigue and impact resistance
  • Electrical Properties: Good insulators with high resistivity and low dielectric constant
  • Chemical Resistance: Resists acids, alkalis, and organic solvents fairly well; oxidation and UV degradation are potential concerns

Applications of PP include:

  • Packaging: Containers, bottles, films
  • Textiles: Fibers, nonwoven fabrics, carpets
  • Automotive: Bumpers, interior parts, trim
  • Household Products: Kitchen utensils, appliances, storage containers
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