POLYMERS - 國立中興大學

1 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e POLYMERS Fundamentals of polymer Technology thermoplastic POLYMERS Thermosetting POLYMERS elastomers Guide to the Processing of POLYMERS 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e PolymerA compound consisting of long-chain molecules, eachmolecule made up of repeating units connectedtogether There may be thousands, even millions of units in asingle polymer molecule The wordpolymeris derived from the Greek wordspoly, meaning many, andmeros(reduced tomer),meaning part Most POLYMERS are based on carbon and aretherefore considered organic chemicals 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Types of POLYMERS POLYMERS can be separated intoplasticsandrubbers As engineering materials, it is appropriate to dividethem into the following three (1) and (2) are plastics and (3) are rubbers 2002 John Wiley & Sons, Inc.

2 M. P. Groover, Fundamentals of Modern Manufacturing 2/e thermoplastic POLYMERS -Thermoplastics(TP) Solid materials at room temperature but viscousliquids when heated to temperatures of only a fewhundred degrees This characteristic allows them to be easily andeconomically shaped into products They can be subjected to heating and cooling cyclesrepeatedly without significant degradation 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Thermosetting POLYMERS -Thermosets(TS) Cannot tolerate repeated heating cycles asthermoplastics can When initially heated, they soften and flow formolding But elevated temperatures also produce achemical reaction that hardens the material into aninfusible solid If reheated, thermosets degrade and char ratherthan soften 2002 John Wiley & Sons, Inc.

3 M. P. Groover, Fundamentals of Modern Manufacturing 2/e ElastomersPolymers that exhibit extreme elastic extensibility whensubjected to relatively low mechanical stress Also known as rubber Some elastomers can be stretched by a factor of 10and yet completely recover to their original shape Although their properties are quite different fromthermosets, they share a similar molecular structurethat is different from the thermoplastics 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Market Shares Thermoplastics are commercially the most importantof the three types, constituting around 70% of thetonnage of all synthetic POLYMERS produced Thermosets and elastomers share the remaining30% about evenly, with a slight edge for the former On a volumetric basis, current annual usage ofpolymers exceeds that of metals 2002 John Wiley & Sons, Inc.

4 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Examples of POLYMERS Thermoplastics: Polyethylene, polyvinylchloride, polypropylene,polystyrene, and nylon Thermosets: Phenolics, epoxies, and certain polyesters elastomers : Natural rubber (vulcanized) Synthetic rubbers, which exceed the tonnage ofnatural rubber 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Reasons Why POLYMERS are Important: Plastics can be formed by molding into intricate partshapes, usually with no further processing required Very compatible withnet shapeprocessing On a volumetric basis, POLYMERS : Cost competitive with metals Generally require less energy to produce thanmetals Certain plastics are translucent and/or transparent,which makes them competitive with glass in someapplications 2002 John Wiley & Sons, Inc.

5 M. P. Groover, Fundamentals of Modern Manufacturing 2/e General Properties of POLYMERS Low density relative to metals and ceramics Good strength-to-weight ratios for certain (but not all) POLYMERS High corrosion resistance Low electrical and thermal conductivity 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Limitations of POLYMERS as EngineeringMaterials Low strength relative to metals and ceramics Low modulus of elasticity (stiffness) Service temperatures are limited to only a fewhundred degrees Viscoelastic properties, which can be a distinctlimitation in load bearing applications Some POLYMERS degrade when subjected to sunlightand other forms of radiation 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Synthesis of POLYMERS Nearly all POLYMERS used in engineering are synthetic They are made by chemical processing POLYMERS are synthesized by joining many smallmolecules together into very large molecules, calledmacromolecules, that possess a chain-like structure The small units, calledmonomers, are generallysimple unsaturated organic molecules such asethylene C2H4 2002 John Wiley & Sons, Inc.

6 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Figure of polyethylene from ethylene monomers:(1) n ethylene monomers yields (2a) polyethylene of chainlength n; (2b) concise notation for depicting the polymerstructure of chain length n 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Polymerization As a chemical process, the synthesis of polymerscan occur by either of two polymerization Production of a given polymer is generallyassociated with one method or the other 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Addition Polymerization In this process, exemplified by polyethylene, thedouble bonds between carbon atoms in the ethylenemonomers are induced to open up so that they joinwith other monomer molecules The connections occur on both ends of theexpanding macromolecule, developing long chains ofrepeating mers It is initiated using a chemical catalyst (called aninitiator) to open the carbon double bond in some ofthe monomers 2002 John Wiley & Sons, Inc.

7 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Figure of addition (chain) polymerization: (1) initiation,(2) rapid addition of monomers, and (3) resulting long chainpolymer molecule with n mers at termination of reaction 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Step Polymerization In this form of polymerization, two reacting monomersare brought together to form a new molecule of thedesired compound As reaction continues, more reactant moleculescombine with the molecules first synthesized to formpolymers of lengthn= 2, then POLYMERS of lengthn=3, and so on In addition, POLYMERS of lengthn1andn2alsocombine to form molecules of lengthn=n1+n2, sothat two types of reactions are proceedingsimultaneously 2002 John Wiley & Sons, Inc.

8 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Figure of step polymerization showing the two types ofreactions occurring: (a) n-mer attaching a single monomer toform a (n+1)-mer; and (b)n1-mer combining withn2-mer to forma (n1+n2)-merSequence is shown by (1) and (2) 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Some Examples POLYMERS produced byadditionpolymerization: Polyethylene, polypropylene, polyvinylchloride,polyisoprene POLYMERS produced bysteppolymerization: Nylon, polycarbonate, phenol formaldehyde 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Degree of Polymerization Since molecules in a given batch of polymerizedmaterial vary in length,nfor the batch is an average;its statistical distribution is normal The mean value ofnis called thedegree ofpolymerization(DP) for the batch DP affects properties of the polymer : higher DPincreases mechanical strength but also increasesviscosity in the fluid state, which makes processingmore difficult 2002 John Wiley & Sons, Inc.

9 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Molecular Weight Themolecular weight(MW) of a polymer is the sumof the molecular weights of the mers in the molecule; MW =ntimes the molecular weight of eachrepeating unit Sincenvaries for different molecules in a batch,the molecule weight must be interpreted as anaverage 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Typical Values of DP and MW forSelected PolymersMW300,000100,00015,00040,000DP(n )10,0001,500120200 PolymerPolyethylenePolyvinylchlorideNylo nPolycarbonate 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Linear, Branched, and Cross-linkedPolymers Linearstructure chain-like structure Characteristic of thermoplastic POLYMERS Branchedstructure chain-like but with sidebranches Also found in thermoplastic POLYMERS Cross-linkedstructure Loosely cross-linked, as in an elastomer Tightly cross-linked, as in a thermoset 2002 John Wiley & Sons, Inc.

10 M. P. Groover, Fundamentals of Modern Manufacturing 2/e Linearstructure of a thermoplastic polymerFigure structures of polymer molecules:(a) linear, characteristic of thermoplastics 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Branchedstructure that includes side branches alongthe chainFigure structures of polymer molecules: (b) branched 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Loosely cross-linked, in which primary bonding occursbetween branches and other molecules at certainconnection pointsFigure structures of polymer molecules:(c) loosely cross-linked as in an elastomer 2002 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 2/e Tightly cross-linkedornetwork structure-in effect, theentire mass is one gigantic macromoleculeFigure structures of polymer molecules:(d) tightly cross-linked or networked structure as in a thermoset 2002 John Wiley & Sons, Inc.