Polymers 2 Final (1) PDF

Module-I (12) Polymers Introduction to polymers, Thermoplastic and Thermosetting plastic. Ingredients of the plastic (Compounding of plastic.) Fabrication of plastic by Compression, Injection, Transfer, Extrusion molding. Preparation, properties and uses of Phe...

Module-I (12) Polymers Introduction to polymers, Thermoplastic and Thermosetting plastic. Ingredients of the plastic (Compounding of plastic.) Fabrication of plastic by Compression, Injection, Transfer, Extrusion molding. Preparation, properties and uses of Phenol formaldehyde, PMMA, Kevlar. Effect of heat on the polymers (Glass transition temperatures), Polymers in medicine and surgery. Conducting polymers, Industrial polymers. Rubbers Natural rubber (latex), Drawbacks of natural rubber, Compounding of rubber (vulcanization of rubber), Preparation, properties and uses of Buna‐S, Silicone and Polyurethane rubber. Polymers Definition Polymers are large molecules made up of repeating units called Monomers. Examples: Polyethene, PVC, Buna-S etc. Properties of a polymer Lightness High strength-to weight ratio High strength composites Low thermal conductivity Low electrical conductivity Resistance to chemicals Variety of colours and transparencies Low cost Recyclability Flammability Classification of a polymer I) Based on monomer unit  Homoplymer A polymer prepared from a single monomer is a homopolymer Eg.: Polyethene, PVC Types of Homopolymer  Linear  Branched  Cross-linked  Network (3D)  Coplymer If two or more monomers are employed, the product is a copolymer Eg.: Buna-N, Nulon-66  Types of Copolymer  Random copolymer : -A-B-B-A-B-A-A-B-  Alternating copolymer : -A-B-A-B-A-B-A-B-  Block copolymer : -A-A-A-A-B-B-B-B-  Graft copolymer : -A-A-A-A-A-A-A-A- II) Based on polymerization process  Addition polymer or Chain growth polymer Formed by addition reaction. Eg.: Polyethene, PVC  Condensation polymer or STEP growth polymer Formed by linking condensation reaction Eg.: Nylon-66, Terylene III) Based on stereoisomerism  Isotactic All R groups are on the same side of the chain  Syndiotactic R group occupies alternate side of chain  Atactic R group occupies random side of chain Plastics Properties of plastics  Do not conduct electricity  Poor conductors of heat  Unreactive –not affected by water or air or chemicals  Fall between elastomers and fibers  Harder to stretch than elastomers  Can be shaped and molded easily Factors affecting properties of plastics Factor Effect 1. reaction Temperature, pressure and catalysts affect the length and conditions branching of the polymer chain. 2. Monomer The type of monomer used affects the type of forces between polymer chains. 3. Additives Additives can ‘lubricate’ polymer chains, join them together with cross-links, or preserve them from decay Types Thermosoftening plastics Also called ‘thermoplastics’ Do not contain cross-links. Are flexible, stretchy Have low melting point Can be moulded and shaped after they have been made, many times. Examples : Polyethene, Natural rubber Thermosetting plastics Do not contain cross-links. Are rigid Have high melting point Can be moulded into shape when they are being made  E.g., Aircraft components, Racing car components, Sports equipment Glass transition temperature (Tg) Different polymers have different segments on their backbones. The ease of movement of these segments (portions of the chain) depends on the structure, physical environment of the chain etc. of the segment. Any movement of these segments requires energy. Then each different polymer would have different energy requirement for the movement of these segments (different polymer = different structure, different physical environment of the chain etc). Below glass transition temperature, these segments do not have sufficient energy to move. So, if some stress will be applied to bend a polymer which is below its Tg then the segments won’t be able to move into new positions, which makes the polymer brittle. Above Tg they would be flexible. Tg IS A PROPERTY RELATED WITH THE AMORPHOUS REGIONS OF THE POLYMER, NOT CRYSTALLINE! So that elastomers are elastomers above their Tg. Below, they are not elastomers, they are glassy, because they are not flexible anymore Moulding. Compression moulding Diagram Principle: The amount of plastic needed is calculated and placed inside the mould before it is closed like rotational casting. The mould is heated and closed under pressure. The plastic flows around the mould and the strong bonds form which prevent the plastic being reshaped again. These plastics are heat resistant so is used for electrical fittings, kitchen ware & handles. Generally used for thermosetting plastics. Process: It involves two steps Preheating and pressurizing Image courtesy http://course1.winona.edu Reaction conditions Pressure Range: 2000-3000 psi(13.8-20.7 MPa) Temperature Range: 300ºF to 375ºF (149⁰C- 191⁰C) Cure time Range: variable Advantages Lowest cost More uniform density Uniform shrinkage due to uniform flow Improved impact strength due to no degradation of fibres during flow  Dimensional accuracy Internal stress is minimized Disadvantages Large Curing time Uneven parting lines are present Scrap cannot be reprocessed. Difference between compression moulding and injection moulding Compression Molding Injection Molding 1. Heated mold Cooled mold 2. Thermoset material Thermoplastic material 3. Long cycle time (minutes) Short cycle time (seconds) 4. Can’t recycle flash or waste parts Can reprocess flash and waste parts 5. Low part complexity High part complexity 6. No undercuts Undercuts are possible 7 Reinforcement material can be long Some reinforcement material possible (inches) must be short (whisker length) Extrusion moulding Diagram Principle This is very similar to injection moulding but instead of injecting into a mould the plastic is pushed through a DIE. This gives a continuous length of a product with the same cross section. It is ideal for producing products like pipes, guttering & uPVC window frame sections. Process Raw materials in the form of thermoplastic pallets, granules, or powder, placed into a hopper and fed into extruder barrel. The barrel is equipped with a screw that blends the pallets and conveys them down the barrel. Heaters around the extruder’s barrels heats the pellets and liquefies them. Screw has 3-sections Feed section Melt or transition section Pumping section. Applications Solid rods, channels, tubing, pipe, window frames, architectural components Plastic coated electrical wire, cable, and strips Injection moulding Diagram Principle Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. Characteristics High Quality Material Energy Saving Environmental Protection Process involves following steps Mixed resin is placed in the transfer chamber Resin is heated to liquid state Plunger presses liquid resin through sprue and runner system to mold cavities Mold is heated After resin is cured, parts are removed Pressure ranges from 70 Mpa – 200 Mpa Process capabilities High production rates Good dimensional control Cycle time range 5 to 60 sec’s Mold materials- tool steels, beryllium - Cu, Al Mold life- 2 million cycles (steel molds) 10000 cycles ( Al molds). Materials used in this process Thermosetting polymers Thermoplastics Applications Plastic products Toys Milk containers Movie cases Combs kitchen utensils Dust pans Rubber materials Valve steam seals Oil seals for the engine Earplugs Rubber hoses  Telecommunications items