Compounding of Plastics: Binders & Fillers

Compounding of Plastics

• Plastic is compounded with other ingredients for desirable properties in finished products.
• The added constituents either impart a useful function during molding or give a useful property to the finished product, forming a mixture.
• The constituents added are binders, fillers, plasticizers, dyes and pigments, lubricants, catalysts and stabilizers.

Binders

• Plastics are generally classified according to the type of binder used in their manufacture.
• A binder is responsible for holding the other constituents of the plastic together.
• Binders determine the type of treatment needed to mold articles from the plastic material.
• Binders can make up 30-100% of the plastic's composition.
• Binders can be natural or synthetic resins or cellulose derivatives.
• In the presence of a catalyst, binders are changed to an infusible cross-link form.
• Low molecular weight binder results in plastic molding more easily.

Fillers

• Fillers are added to plastics to:
  • Reduce the cost of the plastic.
  • Reduce shrinkage on setting and brittleness.
  • Enhance tensile strength, hardness, opacity, finish, and workability.
• Fillers are added to impart special characters to the finished products.
  • Barium salts make the plastic impervious to X-rays.
  • Asbestos provides corrosion and heat resistance.
  • Carborundum, quartz, and mica provide extra hardness.
  • Carbon black increases tensile strength.
  • Shredded textiles increase tensile impact strength.
• Other common fillers include cotton, corn husks, graphite, clay, paper pulp, wood flour, pumice, metallic oxides (ZnO, PbO), sawdust, and metal powders (Fe, Cu, Pb, Al).
• The proportion of fillers can reach as high as 50% of the plastic's composition.

Plasticizers

• Plasticizers improve plasticity and flexibility, reducing the temperature and pressure needed for molding.
• They play an important role in determining the properties of the finished product.
• Plasticizing properties are due to the partial neutralization of intermolecular forces of attraction in the resin molecules, increasing flexibility and plasticity.
• Plasticizers reduce the strength and chemical resistance of the material.
• The proportion of plasticizers can be up to 60% of the plastics, especially in thermosetting plastics.
• Cellulose derivatives tend to discolor when molded unless a plasticizer is added to reduce the molding temperature.
• Commonly used plasticizers are vegetable oils, camphor, esters of oleic, stearic, or phthalic acids, tributyl phosphate, triphenyl phosphate, and triacetin.
  • Camphor increases surface hardness when used with cellulose acetate.
  • Tributyl phosphate and triphenyl phosphate impart flame proofness.
  • Triacetin and tributyl phosphate improve toughness.

Dyes and Pigments

• Dyes and pigments provide decorative colors to plastics in high-polymer artifacts.
• Main coloring materials consist of organic dyestuffs and opaque inorganic pigments.

Lubricants

• Lubricants additives promote a good finish to the plastic materials.
• They improve flow characteristics and friction reduction in processing machines.
• Waxes, oils, stearates, oleates, and soaps are employed to make the molding of plastics easier and help in providing a glossy finish.

Catalysts

• Catalysts (accelerators) are added to fusible resins in thermosetting plastics.
• Catalysts accelerate the polymerization during the molding process, into crosslinked infusible form.
• Catalysts include hydrogen peroxide, benzoyl peroxide, acetyl sulphuric acid, ammonia and its salts, metals (Ag, Cu, and Pb), and metallic oxides like zinc oxide.

Stabilizers

• Stabilizers, such as alkaline earth oxides, organometallic salts, epoxy compounds, and amine-type compounds, prevent polymer degradation.
• Stabilizers improve the thermal stability of the polymer during processing.
• Heat stabilizers are used during the molding of vinyl chloride and vinylidine chloride polymers, as these polymers tend to undergo decomposition and discoloration at molding temperatures.
• Lead salts, such as white lead, lead chromate, litharge, and red lead, serve as opaque molding compounds.
• Stearates of lead, cadmium, and barium create transparent moulding compounds.

Molding or Fabrication of Plastics

• Plastic fabrication or molding shapes plastic using a mold.
• Molding involves plastic fabrication under severe heat and pressure and are applicable to both thermosetting and thermoplastic resins.
• Before molding, resins must be dried to ensure optimum performance of the finished product.
• Moisture lowers density, impairs mechanical, and optical properties.

Compression Molding

• Applicable to both thermoplastics and thermosetting resins.
• Synthetic plastic material mixed with filler is placed in the mold using proper proportions.
• The mold is closed under low pressure and heated with simultaneous application of pressure (100 to 500 kg/cm² and 100 to 200°C).
• Cavities get filled with fluidized plastic.
• The material is withdrawn after cooling.
• Curing is done by heating (thermosetting) or cooling (thermoplastics).
• The molded article is removed by opening the mold parts.
• Fully automatic molding presses quicken the process.
• Door handles, electrical iron handles, bottle caps, and screw caps are obtained.

Injection Molding

• Applicable to thermoplastic resins.
• Plastic powder is fed into a hot cylinder through a hopper.
• Softened plastic is forced at a controlled rate into a tightly locked mold using a screw arrangement or piston.
• Temperature at the nozzle is increased (130-260°C).
• This makes the plastic fluid and injected into the mold.
• The mould is cooled to enable the hot plastic to be cured and become rigid.
• The molded object is mechanically ejected without any deformation.
• Telephones, buckets, and dustbins are made by this technique.
• Advantages:
  • Widely used for molding thermoplastics.
  • High speed production and low finished costs.
  • Low material loss.
• Limitation: A large number of cavities cannot be filled simultaneously, limiting the design of articles.

Transfer Molding

• Used for thermosetting materials.
• Operates on the injection molding principle.
• Molding powder is placed in a heated chamber, maintained at the minimum temperature to become plastic.
• Plastic material is injected through an orifice, into the mould by a plunger, working at high pressure.
• Friction at the orifice increases the material's temperature, making it liquid.
• Curing occurs under heat and pressure.
• The molded article is then ejected mechanically.
• Advantages:
  • Ejection of material from the orifice to the mold is at high speed.
  • Highly plasticized material handles delicate articles without distortion.
  • Intricate shapes unattainable by compression moulding get produced.
  • Blistering is almost eliminated.
  • Articles freed from flow marks are produced.
  • Thick pieces cure uniformly.
  • Fabricated pieces have greater mechanical strength and density.
  • Finished cost of fabrication is reduced.

Extrusion Molding

• Applicable for thermoplastic resins.
• Used for continuous molding of thermoplastic materials into uniform cross-section articles.
• Thermoplastic ingredients are heated to a plastic condition.
• Ingredients are pushed by a screw conveyor into a die, shaping the outer form of the article.
• The extruded product is cooled by atmospheric exposure, blowing air, or spraying water.
• A long conveyor carries away the cooled product.
• Articles like tubes, rods, strips, and insulated electric cables are manufactured.
• Processes can be either Vertical or Horizontal.

Melting and Glass Transition Temperatures

• Melting is a transition in crystalline polymers when polymer chains fall out of their crystal structures, becoming a disordered liquid.
• Glass transition is a transition in amorphous polymers, whose chains are not arranged in ordered crystals.
• Crystalline polymers have some amorphous portion (40-70% of the sample).
• A crystalline polymer sample can exhibit both glass transition and melting temperatures.
  • The glass transition is when an amorphous portion undergoes the glass transition
  • The crystalline portion undergoes melting.
• The glass transition temperature (Tg) is above which there is a reversible transition in amorphous materials to a molten, viscous, or rubber-like state (viscoelastic state).
• Viscoelastic materials exhibit both viscous and elastic characteristics.
• When a polymer cools below Tg, it becomes hard and brittle.
• Temperature above which polymer becomes soft, flexible, and viscoelastic is (Tg)
• Temperature above which polymer becomes liquid or viscofluid is (Tm).
• Hard plastics (polystyrene, polymethyl methacrylate) are used below their glass transition temperatures, in their glassy state.
• Their Tg values are above room temperature (around 100°C or 212°F).
• Rubber elastomers (polyisoprene, polyisobutylene) are used above their Tg, in their rubbery state.
• They are soft and flexible, and their Tg values are far below room temperature.
• Significance of melting and glass transition temperature:
  • Tg value indicates the temperature region where a polymeric material transforms from a rigid solid to a soft viscous state.
  • Tg and Tm are useful for selecting the right processing temperature.
  • Tg gives an indication of the flexibility of the polymer and its response to stress.

Factors Affecting Melting and Glass Transition Temperatures

• Tg is directly proportional to the polymer's molecular weight.
  • Tg increases with increasing molecular weight up to 20,000
  • Little effect beyond that point.
• Greater cross-linking results in higher Tg values.
  • High branching/cross-linking brings polymer chains closer.
  • It reduces mobility, increasing Tg.
• Polymers with strong intermolecular forces have greater Tg values.
  • Polar groups in the polymer chain lead to strong intermolecular cohesive forces.
  • Reduces mobility and increases Tg.
• Side groups (benzene and aromatic) attached to the main chain increase Tg.
  • The side groups hinder C-C bond rotation in the polymer backbone.
  • It restricts mobility, increasing Tg.
• Tg of an isotactic polymer is greater than syndiotactic, which is greater than atactic.

Conducting Polymers

• Conducting or conductive polymers are organic polymers that conduct electricity; they can be semiconductors.
• Conducting polymers are finding increased use due to light weight, ease in processing, and mechanical properties.
• Materials are used in electronics, sensors, and microelectronics devices.
• Polyacetylene, polypyrrole, polyindole, and polyaniline are major classes of conductive polymers.
  • Polypyrrole and polyaniline protect metals as anti-corrosive coatings.
  • Conductive polymers create artificial muscles, biosensors, and controlled-release drugs in medicine.
• Polymers are poor conductors of electricity due to lack of free electrons, but polyconjugated polymers can be converted into electrical conductors.
• Valence electrons in non-conjugated polymers like polyethylene are present in sp³ hybridized covalent bonds.
  • Electrons involved in a σ-bond have low mobility and do not contribute to the electrical conductivity.
• Valence electrons exist in sp² hybridized covalent bonds in conjugated polymers like polyacetylene.
  • Each carbon atom has a valence electron in a p₂ orbital.
  • All p₂ orbitals combine to form a wide set of delocalized electrons.
  • The mobility of delocalized electrons is utilized to impart conductivity.
• Conjugated polymers can be semiconductors or insulators.
  • Energy gap between conducting and valence bands can be > 2 eV.
  • Undoped conjugated polymers such as polythiophenes and polyacetylenes have low electrical conductivity (around 10⁻¹⁰ to 10⁻⁸ S/cm).
• Higher electrical conductivity can be induced in conjugated polymers via:
  1. Excitation of π electrons in an electric field:
     • Electrons get excited and transported through the solid polymeric matrix.
     • Overlapping orbitals of conjugated π electrons form valence and conduction bands, making intrinsically conducting polymers.
  2. Disturbance in conjugated polymer matrix:
     • Removing electrons (oxidation) use electron acceptors like AsF₅ or iodine
     • Inserting electrons (reduction) use electron donors like alkali-metal ions (Li, Na, Ca).
     • This is referred to as doping.
  • p-doping involves adding electron acceptors to remove delocalized electrons, which creates a mobile positive charge on the polymer backbone.
  • n-doping involves adding electron donors to add electrons to an unfilled orbital.
• Conductivity due to the addition of external ingredients result in extrinsically conducting polymers.
  1. Conducting element-filled polymer:
     • The polymer binds conducting elements such as carbon black, metal oxides, and metallic fibers.
  2. Blended conducting polymer:
     • When a conventional polymer is blended with a conducting polymer.
• A polymer that has repetitive units as coordination complexes makes a coordination conducting polymer
  • Made when a metal atom with a polydentate ligand (charge transfer complex) is bound to the polymer.

Applications of Conducting Polymers

• Rechargeable lightweight batteries use the perchlorate-doped polyacetylene-lithium system.
  • About 10 times lighter than standard lead storage batteries.
• Used in wiring in aircraft and aerospace components.
• Electronic devices: transistors, diodes, telecommunication systems.
• As an antistatic coating for clothing.
• Electromagnetic screening materials and photovoltaic devices.
• Molecular wires and molecular switches.