Plastic Compounding: Binders and Fillers

• In prIn order to give finished products definite properties, plastic needs to be compounded with additives
• Additives discharge a use during moulding or impart some useful property to the finished product
• The various constituents added are binders, fillers, plasticizers, dyes and pigments, lubricants, catalysts, and stabilizers

Binders

• Plastic is classified based on binder type
• The main purpose of a binder is to hold the other constituents of the plastic together
• It determines the type of treatment needed to mould articles
• A binder is 30 to 100% of the plastics
• Binders may be natural or synthetic resins or cellulose derivatives
• esence of a catalyst, they convert into infusible cross link form
• Binders also influence plastic properties
• A low molecular weight binder will allow the plastic to be moulded easily

Fillers

• Added to plastics to reduce cost, shrinkage, and brittleness
• They impart better tensile strength, hardness, opacity, finish, and workability
• Fillers are added to impart special characters to finished products
• Barium salts make plastic impervious to X-rays
• Asbestos provides corrosion and heat resistances
• Carborundum, quartz, and mica provide extra hardness
• Adding carbon black increases tensile strength
• Shredded textiles increase tensile impact strength
• Other filler materials include cotton, corn husks, graphite, clay, paper pulp, wood flour, pumice, and metallic oxides (e.g. ZnO, PbO)
• Saw dust and metal powders (Fe, Cu, Pb, Al) can be used as fillers
• The proportion of filler added can reach as high as 50%

Plasticizers

• The important function is to improve plasticity and flexibility to reduce the temperature and pressure molding requires
• They determine finished product properties and are therefore chosen accordingly
• Plasticizing properties are mainly due to partial neutralization of intermolecular forces of attraction in resin molecules
• Impart greater freedom of movement between the polymeric macromolecules of the resin, increasing flexibility and plasticity
• It reduces the strength and chemical resistance
• The plasticizer proportion is up to 60%
• They are often used with thermosetting plastics
• Cellulose derivatives tend to discolour when moulded unless the moulding temperature is reduced via a plasticizer
• Commonly used ones 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

• Colour appeal is often of prime importance in high polymer artifacts
• They provide decorative colours to plastics, mainly organic dyestuffs and opaque inorganic pigments

Lubricants

• Additives that promote a good finish, improving flow and reducing friction in processing machines
• Waxes, oils, stearates, oleates and soaps make molding easier and provide a glossy finish

Catalysts

• Accelerators added to thermosetting plastics to polymerize fusible resin during moulding into a crosslinked infusible form
• Catalysts include hydrogen peroxide, benzoyl peroxide, acetyl sulphuric acid, ammonia and its salts, and metals like Ag, Cu, and Pb; metallic oxides such as zinc oxide

Stabilizers

• Alkaline earth oxides, organometallic salts, epoxy compounds and amine type compounds are stabilizers added to polymers to prevent degradation
• Their purpose is to improve thermal stability during processing
• During the moulding of vinyl chloride and vinylidine chloride polymers, heat stabilizers are used because these polymers show a tendency to undergo decomposition and discolouration at the moulding temperature
• Lead salts like white lead, lead chromate, and litharge serve as opaque moulding compounds
• Stearates of lead, cadmium, and barium are transparent moulding compounds

Moulding (Fabrication)

• Fabrication of plastic (moulding) is the technique of giving desired shape to plastic using a mould
• Fabrication involves the plastic under severe heat and pressure and is applicable to thermosetting and thermoplastic resins
• It is essential to dry the resins to achieve optimum performance of the finished product
• Moisture lowers the density and impairs the mechanical and optical properties

Compression

• Applicable to both thermoplastics and thermosetting resins
• Synthetic plastic mixed with filler and ingredients is placed in the mould in proper proportions
• The mould is closed under low pressure
• Heated with simultaneous pressure application based on specifications (100 to 500 Kg/cm² and 100 to 200°C)
• Cavities are filled with fluidized plastic
• Once moulding is over, the material is withdrawn after cooling
• Curing is done by heating (thermosetting) or cooling (thermoplastics)
• After curing, opening the mould releases the moulded article
• Door handles, handles of electrical iron, bottle caps, and screw caps are obtained via this method

Injection

• Applicable to thermoplastic resins
• Plastic powder is fed into a hot cylinder through a hopper
• Plastic is forced into a tightly locked mold at a controlled rate by screw/piston
• The temperature at the nozzle increases (between 130°-260°C) to make the plastic fluidized and inject into the mould
• The mould is cooled to cure the hot plastic so it becomes rigid
• The moulded object is ejected mechanically without any deformation
• Telephones, buckets, and dustbins are made by this technique

Advantages of injection Moulding

• Widely used for moulding thermoplastics
• Has high-speed production and low finished cost and material loss
• Limitation to not being able to fill a large number of cavities simultaneously which limits the design of articles to be moulded

Transfer

• Used for thermosetting materials
• Injection moulding is operative
• Molding powder is placed in a heated chamber maintained above the minimum temperature to turn plastic
• The plastic is injected into the mold through an orifice
• A plunger is used with high pressure
• Friction at the orifice raises temperature to liquify the material and flow into the mould
• Curing occurs due to heat and pressure and the final moulded object is ejected mechanically

Advantages of Transfer Moulding

• Ejecting material from orifice to the mould is fast and highly plasticized, delicate articles can be handled without distortion or displacement
• Cannot be attained by compression moulding
• Eliminate blistering and produce articles free from flow marks
• Thick pieces are cured completely and uniformly
• It produces higher mechanical strength and density with eliminated finished cost

Extrusion Moulding

• Extrusion is applicable to thermoplastic resins
• Used for continuous moulding of thermoplastic materials to articles of uniform cross section
• Thermoplastic ingredients are heated to plastic condition and pushed by a screw conveyor into a die with the shape to be manufactured
• The finished product is cooled via atmospheric exposure, blowing air, or spraying water
• A long conveyor carries the cooled product continuously
• Articles of uniform cross section like tubes, rods, strips, insulated electric cables are manufactured by this
• It can be carried out in two ways, vertical or horizontal

Melting and Glass Transition Temperatures

• Melting is a transition in crystalline polymers when polymer chains pull out of their crystal structures and become disordered liquid
• Glass Transition is a transition which happens to amorphous polymers, polymers whose chains that are not arranged in ordered crystals, even when in the solid state
• Crystalline polymers generally have amorphous portions that make up 40-70% of the polymer sample
• Crystalline polymers have both a glass transition and melting temperature (Amorphous undergoes glass, and crystalline portion undergoes melting)
• The glass transition temperature (Tg) is above, there is a reversible transition in amorphous materials or regions in semi-crystalline materials into a molten, viscous, or rubber-like state
• Viscoelastic materials exhibit viscous and elastic characteristics
• When a polymer is cooled below Tg, it hardens and becomes brittle like glass
• Tg is the temperature above at which a polymer becomes soft, flexible and viscoelastic
• Tm is the temperature at which a polymer becomes liquid or viscofluid

Melting temperature Usage

• Hard plastics like polystyrene and polymethyl methacrylate are used below their Tg in their glassy state
• Tg values above room temperature are around 100°C (212°F)
• Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, remaining soft and flexible in their rubbery state
• Tg values for rubber elastomers are below room temperature

Significance of Melting and Glass Transition Temperature

• Tg value indicates the temperature region at which a polymeric material transforms from a rigid solid to soft viscous state
• Tg and Tm are useful in choosing the right processing temperature in which materials are fabricated into finished products
• Tg indicates the flexibility and stress response of the polymer

Factors Affecting Melting and Glass Transition Temperature

• Tg is directly proportional to the molecular weight of the polymer, but increases beyond a molecular weight of 20,000 become negligible
• A greater degree of cross-linking or branching gives a higher Tg
• Stronger intermolecular forces give greater Tg, as numerous polar groups cause strong intermolecular cohesive forces, reducing the chain mobility and increasing Tg
• Side groups, like benzene and aromatic groups, increase Tg by hindering free rotation about the C-C bond
• Tg of an isotactic polymer is greater than syndiotactic, which is greater than atactic

Conducting Polymers

• Conducting, or conductive, polymers are organic polymers that conducts electricity
• These compounds can exhibit conductivity comparable to metals, and can be semiconductors
• Conductive polymers have increased in use due to their light weight, ease in processing, and good mechanical properties
• Applications include electronics, such as batteries, sensors, and microelectronics devices
• Polyacetylene, polypyrrole, polyindole, polyaniline, and their copolymers are the main class of conductive polymers
• Polypyrrole and polyaniline protect metals as an anti-corrosive coating
• They can be used in the production of artificial muscles, biosensors, and drugs with controlled-release agents

Polymers Conductivity

• Polymers are poor conductors of electricity due to non-availability of free electrons
• Pure polyconjugated polymers can be converted into polymers with electrical conductivities comparable to metals
• Valence electrons of 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

Polymers Electrical Conductivity

• In conjugated polymers like polyacetylene, valence electrons are present in sp² hybridized covalent bonds
• One valence electron exists in a pz orbital that is orthogonal to bonds
• All pz orbitals combine with each other to form a molecule wide set of delocalized electrons that can be utilized to impart electrical conductivity

Semiconductors Conductivity

• Conjugated polymers are semiconductors or insulators
• They exhibit an energy gap above 2 eV which is too high for thermally activated conduction
• Undoping conjugated polymers like polythiophenes or polyacetylenes have low electrical conductivity around 10⁻¹⁰ to 10⁻⁸ S/cm
• Higher electrical conductivity can be induced in conjugated polymers by excitation of electrons in electric field and disturbance in conjugated polymer matrix

Excitation of electrons in electric field

• Electrons get excited and transported through the solid polymeric matrix when an electric field is applied
• Overlapping orbitals of conjugated electrons form valence and conduction bands
• These are intrinsically conducting polymers

Disturbances in conjugated polymer

• Disturbances are matrix by removing electrons (oxidation) with electron acceptors like AsF₅ or iodine, or inserting electrons (reduction) with electron donors like alkali-metal ions
• This process is doping which give polymers doped with conductivity

Doping

• Doping by oxidation (p-doping) - Added electron acceptors remove some delocalized electrons, which creates a mobile positive charge on the polymer backbone
• Doping by reduction - Added electron donors donate electrons to unfilled orbital

Extrinsically conducting polymers

• The conductivity in polymers due to addition of external ingredients
• Conducting element-filled contains polymer acts as a binder that binds conducting elements like carbon black, metal oxides, or metallic
• Blended conducting polymer contain conventional polymer and a conducting polymer
• Polymers whose repetitive units are coordination complexes are called coordination conducting polymers

Applications

• Rechargeable light weight batteries based on perchlorate doped polyacetylene-lithium system, 10 times lighter than conventional lead storage batteries
• Wiring in aircrafts and aerospace components
• Electronic devices such as transistors, diodes, and in telecommunication systems
• Antistatic coating for clothing
• Electromagnetic screening materials
• Photovoltaic devices
• Molecular wires and molecular switches