Materials of Engineering ENGR 151 CHARACTERISTICS, APPLICATIONS AND PROCESSING OF POLYMERS

Transcription

1 Materials of Engineering ENGR 151 CHARACTERISTICS, APPLICATIONS AND PROCESSING OF POLYMERS

2 POLYMER FORMATION Synthesis of large (polymer) molecules is called polymerization. There are two types of polymerization Addition (or chain) polymerization Monomer units are attached one at a time to form a linear macromolecule Composition of resultant product molecule is exact multiple of original reactant monomer Condensation (step) polymerization Formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species Usually small molecular weight by-product, e.g. water 2

3 ADDITION (CHAIN) POLYMERIZATION Three distinct steps: Initiation, propagation, termination Initiation: An active center capable of propagation is formed by a reaction between an initiator (or catalyst) species and the monomer unit E.g. Polyethylene 3

4 ADDITION (CHAIN) POLYMERIZATION Three distinct steps: Initiation, propagation, termination Propagation: Linear growth of polymer chain by sequential addition of monomer units to active growing chain molecule 4

5 ADDITION (CHAIN) POLYMERIZATION Three distinct steps: Initiation, propagation, termination Termination: May occur in different ways Active ends of two propagating chains may link together to form one molecule (combination) 5

6 ADDITION (CHAIN) POLYMERIZATION Three distinct steps: Initiation, propagation, termination Termination: May occur in different ways Two growing molecules react to form dead chains (disproportionation) 6

7 CONDENSATION (STEP) POLYMERIZATION Condensation (step) polymerization Formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species Usually small molecular weight by-product, e.g. water No reactant species has the has the chemical formula of the repeat unit Reaction times are generally slower than those for addition polymerization 7

8 CONDENSATION (STEP) POLYMERIZATION E.g. Polyester 8

9 CONDENSATION (STEP) POLYMERIZATION 9

10 POLYMER ADDITIVES Improve mechanical properties, tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, processability, durability, etc. Fillers Added to improve tensile strength & abrasion resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. 10

11 POLYMER ADDITIVES Plasticizers Improve the flexibility, ductility and toughness of polymers Added to reduce the glass transition temperature T g below room temperature Presence of plasticizer transforms brittle polymer to a ductile one Commonly added to PVC - otherwise it is brittle Stabilizers Antioxidants UV protectants 11

12 POLYMER ADDITIVES (CONT.) Lubricants Added to allow easier processing polymer slides through dies easier ex: sodium stearate Colorants Dyes and pigments Flame Retardants (e.g. textiles and children s toys) Substances containing chlorine, fluorine, and boron 12

13 PROCESSING OF PLASTICS Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat until soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene. Thermoset when heated forms a molecular network (chemical reaction) degrades (doesn t melt) when heated a prepolymer molded into desired shape, then chemical reaction occurs ex: urethane, epoxy 13

14 PROCESSING PLASTICS COMPRESSION MOLDING Thermoplastics and thermosets polymer and additives placed in mold cavity mold heated and pressure applied fluid polymer assumes shape of mold Fig , Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. Copyright 1984 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) 14

15 PROCESSING PLASTICS INJECTION MOLDING Thermoplastics and some thermosets when ram retracts, plastic pellets drop from hopper into barrel ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold Fig , Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. Copyright 1984 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) Barrel 15

16 PROCESSING PLASTICS EXTRUSION Thermoplastics plastic pellets drop from hopper onto the turning screw plastic pellets melt as the turning screw pushes them forward by the heaters molten polymer is forced under pressure through the shaping die to form the final product (extrudate) Fig , Callister & Rethwisch 9e. (Reprinted with permission from Encyclopædia Britannica, 1997 by Encyclopædia Britannica, Inc.) 16

17 PROCESSING PLASTICS BLOWN-FILM EXTRUSION Fig , Callister & Rethwisch 9e. (Reprinted with permission from Encyclopædia Britannica, 1997 by Encyclopædia Britannica, Inc.) 17

18 POLYMER TYPES FIBERS Fibers - length/diameter >100 Primary use is in textiles. Fiber characteristics: high tensile strengths high degrees of crystallinity structures containing polar groups Formed by spinning extrude polymer through a spinneret (a die containing many small orifices) the spun fibers are drawn under tension leads to highly aligned chains - fibrillar structure 18

19 POLYMER TYPES MISCELLANEOUS Coatings thin polymer films applied to surfaces i.e., paints, varnishes protects from corrosion/degradation decorative improves appearance can provide electrical insulation Adhesives bonds two solid materials (adherands) bonding types: 1. Secondary van der Waals forces 2. Mechanical penetration into pores/crevices Films produced by blown film extrusion Foams gas bubbles incorporated into plastic 19

20 ADVANCED POLYMERS Ultrahigh Molecular Weight Polyethylene (UHMWPE) Molecular weight ca. 4 x 10 6 g/mol Outstanding properties high impact strength resistance to wear/abrasion low coefficient of friction self-lubricating surface Important applications bullet-proof vests golf ball covers hip implants (acetabular cup) UHMWPE Adapted from chapteropening photograph, Chapter 22, Callister 7e. 20

21 ADVANCED POLYMERS Thermoplastic Elastomers Styrene-butadiene block copolymer styrene hard component domain butadiene Fig (a), Callister & Rethwisch 9e. soft component domain Fig , Callister & Rethwisch 9e. 21

22 CHAPTER 17: CORROSION AND DEGRADATION OF MATERIALS ISSUES TO ADDRESS... How does corrosion occur? Which metals are most likely to corrode? What environmental parameters affect corrosion rate? How do we prevent or control corrosion? 22

23 EHStock/iStockphoto THE COST OF CORROSION Corrosion: -- the destructive electrochemical attack of a material. -- Ex: Rusting of automobiles and other equipment Cost: -- 4 to 5% of the Gross National Product (GNP)* -- in the U.S. this amounts to just over $400 billion/yr** * H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., **Economic Report of the President (1998). 23

24 ELECTROCHEMICAL CORROSION For metallic materials, corrosion process is normally electrochemical Transfer of electrons from one species to the other Metals give up electrons Oxidation reaction Electrons may be taken up by hydrogen to form hydrogen gas Reduction reaction 24

25 ELECTROCHEMICAL CORROSION Ex: consider the corrosion of zinc in an acid solution Two reactions are necessary: -- oxidation reaction: -- reduction reaction: Zinc flow of e - 2e - in the metal H + Oxidation reaction Zn Zn 2+ H + H + H + H + H + H 2 (gas) H reduction reaction + Acid solution Adapted from Fig. 17.1, Callister & Rethwisch 9e. (From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright 1986 by McGraw-Hill Book Company. Reproduced with permission.) Other reduction reactions in solutions with dissolved oxygen: -- acidic solution -- neutral or basic solution 25

26 ELECTRODE POTENTIALS Not all metals oxidize to form ions with the same degree of ease, e.g. electrochemical cell Reduction (deposition) occurs for copper at the expense of oxidation (corrosion or dissolving) of iron 26

27 ELECTRODE POTENTIALS Electron motion results in electric current 27

28 ELECTRODE POTENTIALS If copper electrode is replaced by zinc electrode, reaction is reversed Zinc corrodes while iron deposits 28

29 ELECTRODE POTENTIALS Behavior of electrochemical cell and associated corrosion/deposition reactions are dependent on nature of metals Relative properties of metals used for electrodes determines direction of flow of electrons (electric current) Concept of standard electrode potential Measured w.r.t. an inert reference metal (Pt) electrode 29

30 metal, M Platinum metal, M Platinum STANDARD HYDROGEN ELECTRODE Two outcomes: -- Corrosion ne - -- Metal is the anode (-) V e - M n+ ions o metal < 0 H 2 (gas) H + e - H + 2e - 25 C 1M M n+ sol n 1M H + sol n (relative to Pt) -- Electrodeposition ne - e - e - M n+ ions H 2 (gas) 25 C 1M M n+ sol n 1M H + sol n -- Metal is the cathode (+) V o metal > 0 H + H + 2e - (relative to Pt) Standard Electrode Potential Adapted from Fig. 17.2, Callister & Rethwisch 9e. 30

31 STANDARD EMF SERIES EMF series metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K more anodic more cathodic o V metal V o DV = 0.153V Data based on Table 17.1, Callister 9e. Metal with smaller V corrodes. o metal Ex: Cd-Ni cell - Cd o Cd 1.0 M o Ni V < V Cd corrodes Cd 2 + solution Adapted from Fig. 17.2, Callister & Rethwisch 9e. 1.0 M + 25 C Ni Ni 2+ solution 31

32 STANDARD EMF SERIES 32

33 GENERALIZED REACTIONS -- Potential Difference: 33

34 CORROSION IN A GRAPEFRUIT Cu (cathode) + H + Zn (anode) - H + Zn 2+ reduction reactions 2e - H + H + Acid H + H + H + oxidation reaction 34

35 EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE Ex: Cd-Ni cell with standard 1 M solutions Ex: Cd-Ni cell with non-standard solutions Cd 25 C 1.0 M Ni 1.0 M Cd 2 + solution Ni 2+ solution Cd X M Cd 2 + solution T Y M Ni Ni 2+ solution Reduce V Ni - V Cd by -- increasing X -- decreasing Y -- increasing T n = #e - per unit oxid/red reaction (= 2 here) F = Faraday's constant = 96,500 C/mol. 35

36 EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE R: Gas constant T: absolute temperature n: number of electrons participating in either of the half cell reactions F: Faraday s constant 36

37 EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE R: Gas constant T: absolute temperature n: number of electrons participating in either of the half cell reactions F: Faraday s constant 37

38 EXAMPLE 38

39 EXAMPLE 39