Chapter 7: Polymers Part 2 DMT125 Materials Science Chapter 7: Polymers Part 2 25 January 2013 Copyright © MAMA

Thermoplastics General properties Relatively low densities Relatively low tensile strength Good electrical insulative materials Dielectric strength voltage gradient that produces electrical breakdown through the material Unit: volts/mm Relatively low maximum use temperature Range from 54 deg C to 149 deg C 25 January 2013 Copyright © MAMA

Thermoplastics Polyethylene (PE) Clear to whitish, translucent thermoplastic material Often fabricated into clear thin films 2 types Low density (LDPE) – branched chain structure High density (HDPE) – straight chain structure 25 January 2013 Copyright © MAMA

Thermoplastics 25 January 2013 Copyright © MAMA

Thermoplastics LDPE lowers its degree of crystallinity & its density due to branched chain structure Also lowers strength because it reduces intermolecular bonding forces HDPE can pack more closely together to increase crystallinity & strength due to having very little branching on the main chains 25 January 2013 Copyright © MAMA

Thermoplastics Most extensively used plastic material Low cost Has many industrially important properties Toughness at room temperature Low temperature with sufficient strength Good flexibility over a wide range of temperatures Excellent corrosion resistance Excellent insulating properties Odorlessness & tastelessness Low water-vapor transmission 25 January 2013 Copyright © MAMA

Thermoplastics Applications Containers Electrical insulations Chemical tubing House wares Packaging & materials handling 25 January 2013 Copyright © MAMA

Thermoplastics Polyvinyl chloride (PVC) Presence of large chlorine atom that is essentially amorphous & does not crystallize Strong cohesive forces between polymer chains due mainly to strong dipole moments caused by chlorine atoms Large negative chlorine atoms cause steric hindrance & electrostatic repulsion, reducing flexibility of polymer chains 25 January 2013 Copyright © MAMA

Thermoplastics PVC homopolymer Relatively high strength (51 to 63 Mpa) plus brittleness Medium heat-deflection temperature ( 57 deg C to 82 deg C at 0.5 Mpa) Good electrical properties (16,745 to 51,220 V/mm) High solvent resistance Flame & chemical resistance produced by the presence of high chlorine content in PVC 25 January 2013 Copyright © MAMA

Thermoplastics PVC can only be used for a few applications without addition of a number of compounds to the basic material so that it can be processed & converted into a finished product Compounds added are Plasticizers: impart flexibility to polymeric materials Heat stabilizers: prevent thermal degradation during processing & extending life of finished product Lubricants: aid the melt flow of PVC compounds during processing & prevent adhesion to metal surfaces Fillers: lower the cost of PVC Pigments: used to give color, opacity & weatherability 25 January 2013 Copyright © MAMA

Thermoplastics Rigid PVC Addition of rubbery resins can improve melt flow during processing Absorb & disperse impact energy so that the impact resistance of the material is increased Used for pipe, siding, window frame & etc Plasticized PVC Produce softness, flexibility & extensibility Adjusting these properties via plasticizer-polymer ratio Outperforms rubber, textiles & paper Used for furniture, shoes, wire insulation & etc 25 January 2013 Copyright © MAMA

PVC pipe & valve 25 January 2013 Copyright © MAMA

Thermoplastics Polypropylene (PP) Presence of methyl group Stronger but less flexible material Increase glass transition temperature Higher melting & heat deflection temperatures Temperature max: 120 deg C without deformation 25 January 2013 Copyright © MAMA

Thermoplastics Good chemical, moisture & heat resistance Low density Good surface hardness Dimensional stability Applications House wares Packaging Appliance parts 25 January 2013 Copyright © MAMA

Thermoplastics Polytetrafluorothylene (PTFE) Fluorinated polymer formed by the free radical chain polymerization of tetrafluroethylene gas to produce linear chain polymers of ---CF2--- units 25 January 2013 Copyright © MAMA

Thermoplastics Crystalline polymer with crystalline melting point of 327 deg C Highly dense crystalline polymeric material Density is high (2.13 to 2.19 g/cm3) Exceptional resistance to chemicals Insoluble in all organics with exception of a few fluorinated solvents Impact strength is high Tensile strength, wear & creep resistance are low 25 January 2013 Copyright © MAMA

Thermoplastics Adding fillers such as glass fibres can increase strength Slippery & waxy & has low coefficient of friction Applications Chemically resistant pipe & pump parts, high temperature cable insulation, O rings & etc 25 January 2013 Copyright © MAMA

P PTFE flexible exhaust 25 January 2013 Copyright © MAMA

PTFE flexible exhaust inside shot 25 January 2013 Copyright © MAMA

Thermosetting plastics (thermosets) Formed with a network molecular structure of primary covalent bonds Some are cross-linked by a chemical reaction that occurs at room temperature (cold setting thermosets) Although cured parts can be softened by heat, their covalent bonding cross links prevent from being restored to the flowable state that existed before plastic resin was cured Cannot be reheated & remelted as thermoplastics 25 January 2013 Copyright © MAMA

Advantages High thermal stability High rigidity High dimensional stability Resistance to creep & deformation under load Light weight High electrical & thermal insulating properties 25 January 2013 Copyright © MAMA

General properties Density slightly high Tensile strength relatively low (28 to 103 Mpa) With addition of glass filling, tensile strength can be increase up to 207 Mpa High impact strength Good dielectric strength (0.5 to 2.6 x 104 V/mm) Maximum use temperature ranges from 77 deg C to 288 deg C 25 January 2013 Copyright © MAMA

Phenolics Low cost Good electrical & heat insulating properties Good mechanical properties Limited in color (usually black or brown) High hardness High rigidity High strength Applications Wiring devices Electrical switchgear & etc 25 January 2013 Copyright © MAMA

Epoxy resins Low cure shrinkage Good adhesion to other materials Good chemical & environmental resistance Good mechanical properties Good electrical insulating properties 25 January 2013 Copyright © MAMA

Allows the liquid epoxy resin to quickly & thoroughly wet surfaces Low molecular weight gives the material exceptionally high molecular mobility during processing Allows the liquid epoxy resin to quickly & thoroughly wet surfaces High reactivity of epoxides group with curing agents like amines provides high degree of cross-linking & good hardness, strength & chemical resistance Applications High voltage insulators Encapsulation of transistors & etc 25 January 2013 Copyright © MAMA

Deformation & strengthening Can be primarily Elastic Plastic (permanent) Combination of both Below Tg, thermoplastics deform by elastic (-40 deg & 68 deg C) Above Tg, deform by plastic (122 deg C & 140 deg C) 25 January 2013 Copyright © MAMA

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As temperature increases thermoplastics gradually soften Secondary bonding forces between molecular chains become weaker Strength decreases When heated through its Tg Strength decreases due to a pronounced decrease in the secondary bonding forces 25 January 2013 Copyright © MAMA

Creep & fracture Creep – deformation under a constant applied load at a constant temperature continues to increase with time Temperature at which creep of a polymeric material takes place is also important factor to determine creep rate Creep rate is relatively low due to restricted molecular chain mobility Above Tg deforms easily by a combination of elastic & plastic deformation – viscoelastic behavior Molecular chains slide past each other more easily – viscous flow Reinforce with glass fibres greatly increase the creep moduli & reduce creep rate 25 January 2013 Copyright © MAMA

Creep & fracture Fracture Can be considered either Brittle Ductile Intermediate between two extremes Unreinforced thermosets fracture primarily in brittle mode Thermoplastics fracture in either both Above Tg, it will fracture in ductile mode Below Tg, it will fracture in brittle mode 25 January 2013 Copyright © MAMA

Creep & fracture Temperature greatly affect the fracture mode of thermoplastics Thermosets heated above room temperature become weaker & fracture at lower stress level but still fracture primarily in brittle mode Due to covalent bonding is retained at elevated temperature Strain rate is also an important factor Slower strain rate favoring ductile fracture due to molecular chain realignment 25 January 2013 Copyright © MAMA

Creep & fracture Brittle fracture Crazes – distorted localized regions Formed in a highly stressed region of the material & consists of an alignment of molecular chains combined with a high density of interdispersed voids Ductile fracture Exhibit plastic yielding before fracture Molecular linear chains uncoil & slip past each other & gradually align closer together in the direction of the applied stress Eventually, the stress on the chains becomes too high, the covalent bonds of main chains break & fracture 25 January 2013 Copyright © MAMA

25 January 2013 Copyright © MAMA

Copyright © Mr.Mohd. Azarulsani b. Md. Azidin END Copyright © Mr.Mohd. Azarulsani b. Md. Azidin 25 January 2013 Copyright © MAMA