Plastic Compounding: Binders and Fillers

• Plastic is compounded with other ingredients to impart specific properties to finished products, with the added constituents either serving a function during molding or improving the finished product.
• The addition of these ingredients is called a mixture.
• Constituents added include binders, fillers, plasticizers, dyes and pigments, lubricants, catalysts, and stabilizers.

Binders

• Plastic classification often depends on the type of binder used in manufacturing.
• A binder's main purpose is to hold the other constituents together.
• Binders determine the treatment needed to mold plastic material articles.
• Binders can range from 30% to 100% of the plastic composition.
• Binders can be natural or synthetic resins or cellulose derivatives.
• In the presence of a catalyst, binders convert into an infusible cross-link form and influence plastic properties.
• Plastics mold very easily when binders are of low molecular weight.

Fillers

• Fillers are substances added to plastic to lower its cost and reduce shrinkage and brittleness during setting
• Fillers can improve tensile strength, hardness, opacity, finish, and workability.
• Fillers impact special characteristics to finished products.
• Barium salts make 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.
• Common fillers include cotton, corn husks, graphite, clay, paper pulp, wood flour, pumice, and metallic oxides like ZnO and PbO and sawdust, metal powders(Fe, Cu ,Pb, and Al); up to 50% of the plastic can be filler.

Plasticizers

• Plasticizers enhance plasticity and flexibility and lower the temperature and pressure needed for molding that plays a vital role in determining finished product properties
• Plasticizing properties result from partially neutralizing intermolecular forces of attraction in resin molecules, increasing the freedom of movement between polymeric macromolecules, increasing flexibility and plasticity, and decreasing strength and chemical resistance.
• Plasticizer proportion can be up to 60% of the plastic and are often in thermosetting plastics.
• Adding a suitable plasticizer can reduce the discoloration of cellulose derivatives when molded.
• Common plasticizers include 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 flameproof properties to cellulose acetate while triacetin and tributyl phosphate improve toughness.

Dyes and Pigments

• Dyes and pigments provide decorative colors to the plastic artifacts.
• Main coloring materials are organic dyestuffs and opaque inorganic pigments.

Lubricants

• Lubricant additives improve the finish of plastic materials, flow characteristics, and friction reduction in processing machines.
• Waxes, oils, stearates, oleates, and soaps make plastic molding easier and provide a glossy finish.

Catalysts

• Catalysts(Accelerators) are added into thermosetting plastics to speed up the polymerization of fusible resin into a crosslinked infusible form during molding.
• Catalysts include hydrogen peroxide, benzoyl peroxide, acetyl sulfuric acid, ammonia and its salts, and metals/metal oxides Ag, Cu, Pb, and zinc oxide.

Stabilizers

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

Fabrication of Plastics

• Plastic fabrication/molding shapes plastic using a mold and involves plastic fabrication under severe heat and pressure, applicable to both thermosetting and thermoplastic resins.
• Drying the resins before molding is essential for the optimum performance of the finished product as moisture lowers density and impairs mechanical and optical properties.

Compression Molding

• Compression molding applies to both thermoplastics and thermosetting resins.
• The synthetic plastic material is mixed with filler and other ingredients in proper proportions and placed in the mold, closing the mold under low pressure.
• It is heated with simultaneous pressure (100 to 500 kg/cm² and 100 to 200°C), cavities filled with fluidized plastic.
• After molding, the material is withdrawn after cooling and cured by heating (thermosetting) or cooling (thermoplastics).
• The molded article is removed after opening the mold parts.
• Fully automatic molding presses speed up this process; door handles, electrical iron handles, bottle caps, and screw caps are made this way.

Injection Molding

• Injection molding is applicable to thermoplastic resins.
• Plastic powder is fed into a hot cylinder through a hopper, with the softened plastic forced at a controlled rate into a tightly locked mold via a screw or piston.
• The temperature at the nozzle is increased (between 130°-260°C), fluidizing the plastic and injecting it into the mold that is then cooled to cure and rigidify the hot plastic.
• The molded object is ejected mechanically without deformation, used in making telephones, buckets, and dustbins.

Advantages of injection molding

• It is widely used for molding of thermoplastics
• High speed production is possible
• It has low finished cost
• It has low loss of material

Limitations of injection molding

• A large number of cavities cannot be filled simultaneously.
• There are designs of articles to be molded

Transfer Molding

• Transfer molding is used for thermosetting materials.
• The principle of injection molding is operative.
• Molding powder is placed in a heated chamber at the minimum temperature it plasticizes.
• The plastic is injected through an orifice into the mold via a high-pressure plunger.
• The friction at the orifice increases the material temperature, liquifying it, and flows into the mold.
• Curing occurs under heat and pressure, and the molded article is then ejected mechanically.

Advantages of transfer molding

• Delicate articles can be handled without distortion or displacement because the material is ejected at high speed and in a highly plasticized condition.
• Intricate shapes not possible with compression molding can be produced.
• Blistering is almost eliminated.
• Articles produced are free from flow marks.
• Thick pieces cure completely and uniformly.
• Mechanical strength and density of fabricated piece is higher.
• The finished cost of the fabrication article is eliminated

Extrusion Molding

• Extrusion molding is applicable to thermoplastic resins and primarily for continuous molding of thermoplastic materials into uniform cross-section articles.
• Thermoplastic ingredients are heated to a plastic condition and pushed by screw conveyor into a die with the required outer shape.
• The finished product extruding out is cooled via atmospheric exposure, blowing air, or spraying water on a long conveyor to continually carry away the cooled product.
• Uniform cross-section articles such as tubes, rods, strips, and insulated electric cables are manufactured using this method.
• Extrusion molding can be carried out in two ways: vertical and horizontal extrusion molding.

Melting and Glass Transition Temperatures

• Melting is a transition occurring in crystalline polymers when the polymer chains fall out of their crystal structures and become a disordered liquid.
• Glass transition is a transition in amorphous polymers, where chains aren't in ordered crystals.
• Crystalline polymers have some amorphous portion (40-70%), allowing them both a glass transition (amorphous portion) and melting temperature (crystalline portion).
• The glass transition temperature (Tg) is above the reversible transition in amorphous materials (or within semicrystalline materials) into a molten, viscous, or rubber-like (viscoelastic) state.
• Viscoelastic materials show both viscous and elastic characteristics.
• As a polymer cools below the glass transition temperature, it hardens and becomes brittle and glass-like.
• At temperatures above Tg, polymers become soft, flexible, and viscoelastic, and at temperatures above Tm, they become liquid or viscofluid.
• Hard plastics like polystyrene and polymethyl methacrylate are used below their glass transition temperatures in their glassy state, around 100 °C (212 °F).

Uses of Rubber elastomers

• Uses of Rubber elastomers like polyisoprene and polyisobutylene occur above their glass transition temperatures, when they are soft and flexible.
• Their glass transition temperatures are well below room temperature.
• The Tg value indicates the temperature region where a polymeric material changes from a rigid solid to a soft viscous state.
• Tg and Tm are helpful in choosing the right processing temperatures for making finished products.
• Tg indicates the flexibility of the polymer and its response to stress.
• Tg is directly proportional to the molecular weight of the polymer, increasing up to a molecular weight of 20,000.
• The higher the degree of cross-linking, the higher the Tg, bringing polymer chains closer together, reducing mobility and increasing Tg.
• Polymers with strong intermolecular forces of attraction have greater Tg. Polar groups in the polymer chain lead to strong intermolecular cohesive forces, reducing chain mobility and increasing Tg.
• Side groups, especially benzene and aromatic groups, raise the Tg, hindering free rotation about the C-C bond on the polymer backbone.
• An isotactic polymer's Tg is greater than that of a syndiotactic polymer, which, in turn, is greater than an atactic polymer.

Conducting Polymers.

• Conducting polymers are organic polymers that conduct electricity, behaving as metals or semiconductors.
• Conducting polymers are becoming more common due to their light weight, ease of processing, and good mechanical properties, specifically in electronics, such as batteries and sensors.
• Main classes of polymers include polyacetylene, polypyrrole, polyindole, polyaniline, and their copolymers.
• Polypyrrole and polyaniline are used to shield metals, while in medicine, conductive polymers are used in artificial muscles, biosensors, and drug delivery systems.
• While lacking free electrons, polyconjugated polymers can be converted from insulators to ones with electrical conductivity similar to metals.
• In non-conjugated polymers like polyethylene, valence electrons are in sp³ hybridized covalent bonds involved in the σ-bond, having low mobility, and not contributing to the electrical conductivity.
• In conjugated polymers like polyacetylene, valence electrons exist in sp² hybridized covalent bonds, with one electron per carbon atom in a p₂ orbital, orthogonal to the other σ-bonds.

Conductivity of Polymers

• Combining p₂ orbitals creates delocalized electrons whose mobility can conduct electricity through the conjugated polymers.
• Conjugated polymers are either semiconductors or insulators compounds
• The energy gap between the conducting and valence bands is substantial (> 2 eV), preventing thermally activated conduction.
• Undoped conjugated polymers, like polythiophenes and polyacetylenes, have low electrical conductivity (around 10-10 to 10-8 S/cm), that can be raised with the methods below.

Methods of increasing conductivity of polymers

• An electric field excites electrons that are transported through the solid polymeric matrix. Overlapping orbitals form valence and conduction bands for intrinsically conducting polymers.
• Polymer matrix is disturbed by removing electrons (oxidation by electron acceptors like AsF5 and iodine) or inserting electrons (reduction by electron donors like alkali-metal ions Li, Na, and Ca). The process is referred to as doping.
• Doping through oxidation includes adding electron acceptors like AsF5 and iodine to remove delocalized electrons that creates a mobile positive charge on the polymer backbone
• Doping by reduction (n-doping) includes adding electron donors like alkali-metal ions Li, Na, and Ca to the matrix, which adds electrons to an unfilled orbital.
• Conductivity in polymers due to the addition of external ingredients results in extrinsically conducting polymers.

Types of Extrinsically conducting polymers

• Conducting element-filled polymer: Polymer binds conducting elements (carbon black, metal oxides, and fibers) that conduct electricity.
• Blended Conducting Polymer: A conventional polymer is blended with a conducting polymer to charge transfer a metal atom with a polydentate ligand binds to the polymer.

Applications of Conducting Polymers

• Rechargeable light-weight batteries based on perchlorate doped polyacetylene-lithium system are 10 times lighter than typical lead storage batteries.
• In wiring in aircrafts and aerospace components.
• In electronic devices such as transistors and diodes and in telecommunication systems.
• In antistatic coating for clothing.
• In electromagnetic screening materials.
• In photovoltaic devices.
• In molecular wires and molecular switches.