copyright joseph greene 2001 1 classes of polymeric materials elastomers professor joe greene csu,...

Copyright Joseph Greene 2001

1

Classes of Polymeric MaterialsElastomers

Professor Joe Greene

CSU, CHICO


2

Elastomers• Elastomers are rubber like polymers that are thermoset

or thermoplastic– butyl rubber: natural rubber

– thermoset: polyurethane, silicone

– thermoplastic: thermoplastic urethanes (TPU), thermoplastic elastomers (TPE), thermoplastic olefins (TPO), thermoplastic rubbers (TPR)

• Elastomers exhibit more elastic properties versus plastics which plastically deform and have a lower elastic limit.

• Rubbers have the distinction of being stretched 200% and returned to original shape. Elastic limit is 200%


3

Rubbers• Rubbers have the distinction of being stretched 200% and

returned to original shape. Elastic limit is 200%

• Natural rubber (isoprene) is produced from gum resin of certain trees and plants that grow in southeast Asia, Ceylon, Liberia, and the Congo.– The sap is an emulsion containing 40% water & 60% rubber particles

• Vulcanization occurs with the addition of sulfur (4%).– Sulfur produces cross-links to make the rubber stiffer and harder.– The cross-linkages reduce the slippage between chains and results in higher

elasticity.– Some of the double covalent bonds between molecules are broken, allowing

the sulfur atoms to form cross-links.– Soft rubber has 4% sulfur and is 10% cross-linked.– Hard rubber (ebonite) has 45% sulfur and is highly cross-linked.


4

Rubber Additives and Modifiers• Fillers can comprise half of the volume of the rubber

– Silica and carbon black.

– Reduce cost of material.

– Increase tensile strength and modulus.

– Improve abrasion resistance.

– Improve tear resistance.

– Improve resistance to light and weathering.

– Example,• Tires produced from Latex contains 30% carbon black which improves the body

and abrasion resistance in tires.

• Additives– Antioxidants, antiozonants, oil extenders to reduce cost and

soften rubber, fillers, reinforcement


5

Vulcanizable Elastomeric Compounds• Rubbers are compounded into practical elastomers

– The rubber (elastomer) is the major component and other components are given as weight per hundred weight rubber (phr)

• Sulfur is added in less than 10 phr• Accelerators and activators with the sulfur

– hexamethylene tetramine (HMTA) – zinc oxide as activators

• Protective agents are used to suppress the effects of oxygen and ozone– phenyl betabaphthylamine and alkyl paraphenylene diamine (APPD)

• Reinforcing filler – carbon black– silica when light colors are required– calcium carbonate, clay, kaoilin

• Processing aids which reduce stiffness and cost – Plasticizers, lubricants, mineral oils, paraffin waxes,


6

Vulcanizable Rubber• Typical tire tread

– Natural rubber smoked sheet (100),

– sulfur (2.5) sulfenamide (0.5), MBTS (0.1), strearic acid (3), zinc oxide (3), PNBA (2), HAF carbon black (45), and mineral oil (3)

• Typical shoe sole compound– SBR (styrene-butadiene-rubber) (100) and clay (90)

• Typical electrical cable cover– polychloroprene (100), kaolin (120), FEF carbon black (15) and

mineral oil (12), vulcanization agent


7

Synthetic Rubber• Reactive system elastomers– Low molecular weight monomers are reacted in a polymerization step with very little cross-

linking.– Reaction is triggered by heat, catalyst, and mixing

• Urethanes processed with Reaction Injection Molding (RIM) • Silicones processed with injection molding or extrusion

• Thermoplastic Elastomers– Processing involves melting of polymers, not thermoset reaction– Processed by injection molding, extrusion, blow molding, film blowing, or rotational molding.

• Injection molded soles for footwear

– Advantages of thermoplastic elastomers• Less expensive due to fast cycle times• More complex designs are possible• Wider range of properties due to copolymerization

– Disadvantage of thermoplastic elastomers• Higher creep


8

Thermoplastic Elastomers• Four types of elastomers

– Olefinics and Styrenics– Polyurethanes and Polyesters

• Olefinics (TPOs are used for bumper covers on cars)– Produced by

• Blending copolymers of ethylene and propylene (EPR) or ter polymer of ethylene-propylene diene (EPDM) with

• PP in ratios that determine the stiffness of the elastomer– A 80/20 EPDM/PP ratio gives a soft elastomer (TPO)

• Styrenic thermoplastic elastomers (STPE)– Long triblock copolymer molecules with

• an elastomeric central block (butadiene, isoprene, ethylene-butene, etc.) and • end blocks (styrene, etc.) which form hard segments

– Other elastomers have varying amounts of soft and hard blocks


9

Thermoplastic Elastomers• Polyurethanes

– Have a hard block segment and soft block segment• Soft block corresponds to polyol involved in polymerization in ether based

• Hard blocks involve the isocyanates and chain extenders

• Polyesters are etheresters or copolyester thermoplastic elastomer– Soft blocks contain ether groups are amorpous and flexible

– Hard blocks can consist of polybutylene terephthalate (PBT)

• Polyertheramide or polyetherblockamide elastomer– Hard blocks consits of a crystallizing polyamide

Soft HardHard Hard

Soft Soft


10

Commercial Elastomers• Diene C=C double bonds and Related Elastomers

– Polyisoprene- (C5H8)20,000 • Basic structure of natural rubber• Can be produced as a synthetic polymer• Capable of very slow crystallization• Tm = 28°C, Tg = -70°C for cis polyisoprene• Tm = 68°C, Tg = -70°C for trans polyisoprene

– Trans is major component of gutta percha, the first plastic– Natural rubber was first crosslinked into highly elastic network by Charles Goodyear (vulcanization

with sulfur in 1837)• Sulfur crosslinked with the unsaturations C=C

– Natural rubber in unfilled form is widely used for products with• very large elastic deformations or very high resilience, • resistance to cold flow (low compression set) and • resistance to abrasion, wear, and fatigue.

– Natural rubber does not have good intrinsic resistance to sunlight, oxygen, ozone, heat aging, oils, or fuels.

CH

CC

H

H3HH

HC C ][

CH

CH

HH

HC C ][

CH3

Cis

Trans


11

Commercial Elastomers• Polybutadiene

– Basis for synthetic rubber as a major component in copolymers Styrene-Butadiene Rubber (SBR, NBR) or in

– Blends with other rubbers (NR, SBR)– Can improve low-temperature properties, resilence, and abrasion or wear resistance

• Tg = -50°C

• Polychloroprene– Polychloroprene or neoprene was the very first synthetic rubber– Due to polar nature of molecule from Cl atom it has very good resistance to oils and

is flame resistant (Cl gas coats surface)– Used for fuel lines, hoses, gaskets, cable covers, protective boots, bridge pads,

roofing materials, fabric coatings, and adhesives– Tg = -65°C.

H HCH

CH HHC C ][

H HCH

CCl HHC C ][


12

Commercial Elastomers• Butyl rubber- addition polymer of isobutylene.

– Copolymer with a few isoprene units, Tg =-65°C– Contains only a few percent double bonds from isoprene– Small extent of saturation are used for vulcanization– Good regularity of the polymer chain makes it possible for the elastomer to crystallize on

stretching– Soft polymer is usually compounded with carbon black to increase modulus

• Nitrile rubber– Copolymer of butadiene and acrylonitrile– Solvent resistant rubber due to nitrile C:::N– Irregular chain structure will not crystallize on stretching, like SBR– vulcanization is achieved with sulfur like SBR and natural rubber

• Thiokol- ethylene dichloride polymerized with sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing.

H H3

CCH3HCC

][


13

Thermoplastic Elastomers• Thermoplastic Elastomers result from copolymerization of

two or more monomers.– One monomer is used to provide the hard, crystalline features, whereas

the other monomer produces the soft, amorphous features.– Combined these form a thermoplastic material that exhibits properties

similar to the hard, vulcanized elastomers.

• Thermoplastic Urethanes (TPU)– The first Thermoplastic Elastomer (TPE) used for seals gaskets,

etc.

• Other TPEs– Copolyester for hydraulic hoses, couplings, and cable insulation.– Styrene copolymers are less expensive than TPU with lower strength– Styrene-butadiene (SBR) for medical products, tubing, packaging, etc.– Olefins (TPO) for tubing, seals, gaskets, electrical, and automotive.


14

Thermoplastic Elastomers• Styrene-butadiene rubber (SBR)

– Developed during WWII • Germany under the name of BUNA-S.• North America as GR-S,Government rubber-styrene.

– Random copolymer of butadiene (67-85%) and styrene (15-33%)– Tg of typical 75/25 blend is –60°C– Not capable of crystallizing under strain and thus requires reinforcing

filler, carbon black, to get good properties.– One of the least expensive rubbers and generally processes easily.– Inferior to natural rubber in mechanical properties – Superior to natural rubber in wear, heat aging, ozone resistance, and

resistance to oils.– Applications include tires, footwear, wire, cable insulation, industrial

rubber products, adhesives, paints (latex or emulsion)• More than half of the world’s synthetic rubber is SBR• World usage of SBR equals natural rubber

C C

H

H H

n

H HCH

CH HHC C ][


15

Acrylonitrile-butadiene rubber (NBR)

• Also called Nitrile rubber– Developed as an oil resistant rubber due to

• the polar C:::N polar bond. Resistant to oils, fuels, and solvents.

– Copolymer of acrylonitrile (20-50%) and butadiene(80-50%)– Moderate cost and a general purpose rubber.– Excellent properties for heat aging and abrasion resistance– Poor properties for ozone and weathering resistance.– Has high dielectric losses and limited low temperature flexibility– Applications include fuel and oil tubing; hose, gaskets, and seals; conveyer

belts, print rolls, and pads.– Carboxylated nitrile rubbers (COX-NBR) has carboxyl side groups

(COOH)which improve• Abrasion and wear resistance; ozone resistance; and low temperature flexibility

– NBR and PVC for miscible, but distinct polymer blend or polyalloy• 30% addition of PVC improves ozone and fire resistance

H HCH

CH HHC C ][C C

H C:::N

H H

n

m


16

Ethylene-propylene rubber (EPR)• EPR and EPDM

– Form a noncrystallizing copolymer • with a low Tg.

– The % PP and PE units determines properties• Tg = -60°C for PE/PP of 67/33 to 50/50

– Unsaturated polymer since PP and PE are saturated• Resistant to ozone, weathering, and heat aging• Does not allow for conventional vulcanization

– Terpolymer with addition of small amount of third monomer (Diene D) has unsaturations referred to as EPDM

• 1,4, hexadiene (HD); 5-ethylidene-2-norbornene (ENB); diclopentadiene (DCPB) feature unsaturations in a side (pendant) group

• Feature excellent ozone and weathering resistance and good heat aging

– Limitations include poor resistance to oils and fuels, poor adhesion to many substrates and reinforcements

– Applications include exterior automotive parts (TPO is PP/EPDM), construction parts, weather strips, wire and cable insulation, hose and belt products, coated fabrics.

C C

H H

H H

n

C C

H CH3

H H

m

C C

H CH2

H H

mCH

CHCH3


17

Ethylene Related Elastomers• Chlorosulfonated Polyethylene (CSPE)

– Moderate random chlorination of PE (24-43%)– Infrequent chlorosulfonic groups (SO2Cl) – Sulfur content is 1-1.5%. – CSPE is noted for excellent weathering resistance

• Good resistance to ozones, heat, chemicals, solvents.• Good electrical properties, low gas permeability, good adhesion to substrates

– Applications include hose products, roll covers, tank linings, wire and cable covers, footwear, and building products

• Chlorinated Polyethylene (CPE)– Moderate random chlorination

• Suppresses crystallinity (rubber)• Can be crosslinked with peroxides• Cl range is 36-42% versus 56.8% for PVC

– Properties include good heat, oil, and ozone resistance– Used as plasticizer for PVC

C C

H H

H H

n

C

Cl

H

m

C

S

H

kClO O


18

Ethylene Related Elastomers• Ethylene-vinylacetate Copolymer (EVA)

– Random copolymer of E and VA• Amorphous and thus elastomeric• VA range is 40-60%• Can be crosslinked through organic peroxides

– Properties include• Good heat, ozone, and weather resistance

• Ethylene-acrylate copolymer (EAR)– Copolymer of Ethylene and methacrylate

• Contains carboxylic side groups (COOH)– Properties include

• Excellent resistance to ozone and• Excellent energy absorbers

– Better than butyl rubbers

C C

H H

H H

n

C C

H O

H H

O=CCH3

m

C C

H H

H H

n

C C

H C

H H

OOCH3

m


19

FluoroElastomers• Polyvinylidene fluoride (PVDF)

– Tg = -35°C

• Poly chloro tri fluoro ethylene (PCTFE)– Tg = 40°C

• Poly hexa fluoro propylene (PHFP)– Tg = 11°C

• Poly tetra fluoro ethylene (PTFE)– Tg = - 130°C

• Fluoroelastomers are produced by – random copolymerization that – suppresses the crystallinity and – provides a mechanism for cross linking by terpolymerization

• Monomers include VDF, CTFE, HFP, and TFE

C C

H F

H F

n C C

F Cl

F F

n

C C

F CF3

F F

n

C C

F F

F F

n


20

FluoroElastomers• Fluoroelastomers are expensive but have outstanding properties

– Exceptional resistance to chemicals, especially oils, solvents– High temperature resistance, weathering and ozone resistance.– Good barrier properties with low permeability to gases and vapors

• Applications– Mechanical seals, packaging, O-rings, gaskets, diaphrams, expansion joints, connectors, hose liners, roll covers, wire and cable insulation.

• Previous fluoroelastomners are referred to as – Fluorohydrocarbon elastomers since they contain F, H, and C atoms with O sometimes

• Two other classes of elastomers include fluorinated types– Fluorosilicone elastomers remain flexible at low temperatures– Fluorinated polyorganophosphazenes have good fuel resistance


21

Silicone Polymers• Silicone polymers or polysiloxanes (PDMS)

– Polymeric chains featuring • Tg = -125°C• Very stable alternating combination of • Silicone and oxygen, and a variety of organic side groups attached to Si

– Two methyl, CH3, are very common side group generates polydimethylsiloxane (PDMS)• Unmodified PDMS has very flexible chains corresponding to low Tg• Modified PDMS has substitution of bulky side groups (5-10%)

– Phenylmethlsiloxane or diphenylsiloxane suppress crystallization• Substituted side groups, e.g., vinyl groups (.5%) featuring double bonds (unsaturations ) enables

crosslinking to form vinylmethylsiloxane (VMS)• Degree of polymerization, DP, of polysiloxane = 200-1,000 for low consistency chains to 3,000-

10,000 for high consistency resins.• Mechanism of crosslinking can be from a vinyl unsaturation or reactive end groups (alkoxy, acetoxy)

Si

CH3

CH3

m

O


22

Silicone Polymers• Silicone polymers or polysiloxanes (PDMS)

– Properties• Mediocre tear properties• High temperature resistance from -90C to 250C.• Surface properties are characterized by very low surface energy (surface

tension) giving good slip, lubricity, and release properties (antistick) nand water repellency.

• Excellent adhesion is obtained for curing compounds for caulk.

Si

CH3

CH3

m

O


23

Silicones• Unmodified PDMS has very flexible chains with a low Tg.

– Regular structure allows for crystallization below Tm– Addition of small amount of bulky side groups are used to

suppress crystallization• Trifluoropropyl side groups enhance the resistance to solvent swelling and

are called fluorosilicones• Linear form (uncrosslinked) polysiloxane corresponds to DP of 200-1000

for low consistency to 3,000-10,000 for high consistency resins• Mechanism for crosslinking (vulcanization) can be based upon vinyl

unsaturations or reactive end groups (alkoxy)

– Silicone polymers are mostly elastomers with mediocre tear properties, but with addition of silica can have outstanding properties unaffected by a wide temp range from –90°C to 250°C

• Surface properties have low surface energy, giving good slip, lubricity, release properties, water repellency, excellent adhesion for caulks

• Good chemical inertness but sensitive to swelling by hydrocarbons• Good resistance to oils and solvents, UV radiation, temperature• Electrical properties are excellent and stable for insulation and dielectric


24

Silicones• Properties

– Low index of reflection gives silicone contains useful combination of high transmission and low reflectance

– Can be biologically inert and with low toxicity are well tolerated by body tissue

– Polymers are normally crosslinked in the vulcanization stage. Four groups

• Low consistency-room temperature curing resins (RTV)• Low consistency-high temperature curing resins (LIM,LSR)• High consistency-high temperature curing resins (HTV, HCE),• Rigid resins

– RTV elastomers involve low molecular weight polysiloxanes and rely on reactive end groups for crosslinking at room temperature.

• One component, or one part, packages rely on atmospheric moisture for curing and are used for thin parts or coatings

• Two component systems have a catalyst and require a mixing stage and result in a small exotherm where heat is given off.


25

Silicones• Properties

– LSR elastomers involve low molecular weight polysiloxanes but a different curing system

• Relatively high temperature (150°C) for a faster cure (10-30s)• Mixed system is largely unreactive at room temp (long pot life)• Suitable for high speed liquid injection molding of small parts.

– HTV elastomers contain unsaturations that are suitable for conventional rubber processing.

• Heat curable elastomers (HCE) are cross linked through high temperature vulcanization (HTV) with the use of peroxides.

– Rigid silicones are cross linked into tight networks.• Non-crosslinked systems are stable only in solutions that are limited to paints,

varnishes, coatings, and matrices for laminates• Cross-linking takes place when the solvent evaporates.• Post curing is recommended to complete reaction, e.g., silicone-epoxy systems for

electrical encapsulation.


26

Silicones Applications• Most applications involve elastomeric form.

• Flexibility and hardness can be adjusted over a wide range

– Electrical applications high voltage and high or low temperatures• Power cable insulation, high voltage leads and insulator boots, ignition cables,

spark plug boots, etc..• Semi-conductors are encapsulated in silicone resins for potting.

– Mechanical applications requiring low and high-temperature flexibility and chemical inertness

• ‘O-rings’, gaskets, seals for aircraft doors and windows, freezers, ovens, and appliances, diaphragms flapper valves, protective boots and bellows.

– Casting molds and patterns for polyurethane, polyester, or epoxy– Sealants and caulking agents– Shock absorbers and vibration damping characteristics

• “Silly-Putty”: Non-crosslinked, high molecular weight PDMS-based compound modified with fillers and plasticizers.

– Biomedical field for biological inertness include prosthetic devices


27

Miscellaneous Other Elastomers• Acrylic Rubber (AR)

– Polyethylacrylate (PEA) copolymerized with a small amount (5%) of 2-chloro-ethyl-vinyl-ether CEVE, which is a cure site.

– The Tg of PEA is about -27°C and acrylic rubber is not suitable for low temperature applications.

– Polybutylacrylate (PBR) has a Tg of -45°C.

– Applications

• Resistant to high temperatures, lubricating oils, including sulfur-bearing oils.

• Include seals, gaskets, and hoses.

• Epichlorohydrin Rubber (ECHR)– Polymerization of epichlorohydrin with a repeat unit of PECH.– Excellent resistant to oils, fuels and flame resistance. (Cl presence) – Copolymer with flexible ethyleneoxide (EO) provides Tg = -40C– Applications include seals, gaskets, diaphragms, wire covers


28

Miscellaneous Other Elastomers

• Polysulfide Rubbers (SR)– One of the first synthetic rubbers. Tg =-27°C, PES Thiokol A

– Consists of adjacent ethylene and sulfide units giving a stiff chain.

– Flexibility is increased with addition of ethylene oxide for polyethylene-ether-sulfide (PEES), Thiokol B

– Mechanical properties are not very good, but are used for outstanding resistance to many oils, solvents and weathering.

– Applications include caulking, mastics, and putty.

• Propylene rubber (PROR)– Does not crystallize in its atactic form and has a low Tg = -72°C.

– Has excellent dynamic properties

HCH

CH

S S ][H S S

C C

H CH3

H H

n

O


29

Miscellaneous Other Elastomers• Polynorborene (PNB)

– Norborene polymerizes into highly molecular weight PNB.– Tg = 35°C but can be plasticized with oils and vulcanized into an

elastomer with lower Tg = -65°C.– Excellent damping properties that can be adjusted.

• Polyorgano-phosphazenes (PPZ)– Form an example of a new class of polymeric materials involving

inorganic chains.• Atoms of Nitrogen (azo) and Phosphorous form, the chain and a variety of

organic side groups, R1 and R2 can be attached to the phosphorous atom.• Side groups include halo (Cl or F), amino (NH2 or NHR), alkoxy (methoxy,

ethoxy, etc.) and fluoroalkoxy groups.• High molecular weight is flexible with a low Tg• Excellent inherent fire resistance, weatherability, and water & oil repellency• Applications

– coatings, fibers, and biomedical materials


30

Commercial Elastomers• Characteristics

Name Chemical Name Vucanization agentNatural rubber cis polyisoprene sulfurPolyisoprene cis polyisoprene sulfurPolybutadiene Polybutadiene sulfurSBR Polybutadiene-styrene sulfurNitrile Polybutadiene-acrylonitrile sulfurButyl Poly isobutylene-isoprene sulfurEPR (EPDM) Poly ethylene propylene- diene Peroxies or sulfurNeoprene Polychloroprene MgOSilicone Polydimethylsiloxane peroxidesThiokol Polyslkylenesulfide ZnOUrethanes Polyester or polyether urethanes Diisocycanates


31

Commercial Elastomers• Costs

Name Consumption 1983 (metric tons) $/lb TypeNatural rubber 676,267 $0.44 General PurposePolyisoprene $0.72 General PurposePolybutadiene 335,541 $0.74 General PurposeSBR 887,005 $0.66 General PurposeNitrile 57,239 $1.10 Solvent ResistantButyl $0.76 General PurposeEPR (EPDM) 141,490 $1.01 General PurposeNeoprene 85,096 $1.29 Solvent ResistantSilicone $4.40 Heat ResistantThiokol Psulfides $1.83 Solvent ResistantUrethanes $3.70 Solvent Resistant


32

Polymerization Mechanisms• Step-wise (Condensation) Polymerization

– Monomers combine to form blocks 2 units long

– 2 unit blocks form 4, which intern form 8 and son on until the process is terminated.

– Results in by-products (CO2, H2O, Acetic acid, HCl etc.)

• Chain Growth (Addition) Polymerization– Polymerization begins at one location on the monomer

by an initiator

– Instantaneously, the polymer chain forms with no by-products


33

Condensation Polymerization Example• Polyamides

– Condensation Polymerization• Nylon 6/6 because both the acid and amine contain

6 carbon atoms

NH2(CH2)6NH2 + COOH(CH2)4COOHHexamethylene diamene Adipic acid

n[NH2(CH2)6NH2 ·CO(CH2)4COOH] (heat)

Nylon salt

[NH(CH2)4NH · CO(CH2)4CO]n + nH2O Nylon 6,6 polymer chain


34

Condensation Polymerization Example• Polyurethane

– Reaction of isocyanate and polyether-alcohol (polyol)

• Polyester– Polymerization of acid and and alcohol

• Polycarbonate– Polycarbonates are linear, amorphous polyesters because they contain esters of carbonic acid and an aromatic bisphenol (C6H5OH)

Phenol + Acetone Bisphenol-A + water

2

OH

+H2O+

C CH2CH3

OC

CH2

CH2

OHOH


35

Other Condensation Polymers• Thermoplastic Polyesters

– Saturated polyesters (Dacron). • Linear polymers with high MW and no crosslinking.• Polyethylene Terephthalate (PET). Controlled crystallinity.• Polybutylene Terephthalate (PBT).

– Aromatic polyesters (Mylar)

O C

O

R O C

O

R

C

O

C

O

R R


36

Step-Growth PolymerizationCondensation Polymerizatio

• Main feature is that all molecular species in the system can react with each other to form higher molecular weight species.– Step-growth polymerization reactions fall into two

classes• A-R1-A + B-R2-B => A-R1-R2-B + AB

• A-R1-A + B-R2-B => A-R1-AB-R2-B– where A and B are repeat polymer groups which react with each other;

» Example, for polyurethanes A = Isocyanate and B = Polyol and the by-product is water.

– and R1 and R2 are long chain polymers


37

Formation of Polymers• Condensation Polymerization

– Step-growth polymerization proceeds by several steps which result in by-products.

• Step-wise (Condensation) Polymerization– Monomers combine to form blocks 2 units long

– 2 unit blocks form 4, which intern form 8 and son on until the process is terminated.

– Results in by-products (CO2, H2O, Acetic acid, HCl etc.)


38

Chain Growth (Addition) Polymerization

• Chain Growth (Addition) Polymerization by Free Radical Mechanism– Involves three primary steps

• Initiation- formation of free radicals through homolytic dissociation of weak bonds (e.g., peroxides). Results in opening up unsaturated (C=C) bonds to saturated (C-C) bonds)

• Propagation- formation of long chain polymers of the now free C-C bonds

• Termination- reactions at the ends of the polymer cause C-C to terminate with a functional group that does not have any free electrons to bond with and results in unsaturated end group (C-C=CX)


39

Chain Growth (Addition) Polymerization

• Special case of Diene polymerization– Very important in elastomers- mostly addition– Polydienes are the backbone of the synthetic

rubber are produced by free radical polymerization

– Early attempts of polymerization was slow and produced low molecular weight polymers (oils)

– Emulsion polymerization (1930s) was introduced to speed up polymerization and higher Molecular weights


40

Polymerization Methods• 4 Methods to produce polymers

– Some polymers have been produced by all four methods• PE, PP and PVC are can be produced by several of these

methods

• The choice of method depends upon the final polymer form, the intrinsic polymer arrangement (isotactic, atactic, etc), and the yield and throughput of the polymer desired.

– Bulk Polymerization– Solution Polymerization– Suspension Polymerization– Emulsion Polymerization


41

Formation of Polymers• Polymers from Addition reaction

– LDPE HDPE PP

– PVC PS

C C

H H

H H

n

C C

H H

H H

n

C C

H CH3

H H

n

C C

H Cl

H H

n

C C

H

H H

n


42

Other Addition Polymers• Vinyl- Large group of addition

polymers with the formula:– Radicals (X,Y) may be attached to this

repeating vinyl group as side groups to form several related polymers.

• Polyvinyls– Polyvinyl chloride– Polyvinyl dichloride (polyvinylidene chloride)

– Polyvinyl Acetate (PVAc)

C C

H X

H Y

orC C

H X

H H

CC

H Cl

H H

C C

H Cl

H Cl

C C

H OCOCH3

H H


43

Manufacturing of Emulsion SBR• Free-radical emulsion process

– Developed in 1930s and still in use– Typical process (Figure)

• Soap stabilized water emulsion of two monomers is converted in a train of 10 continuous reactors (4000 gallons each)

• Water, butadiene, styrene, soaps, initiators, buffers, and modifier are fed continuously

• Temp is 5 to 10°C and conversion proceeds until 60% of the reactants have polymerized in the last reactor.

• Shortstop is added in the emulsion to stop the conversion at 60%• Unreacted butadiene is flashed off with steam and recycled• Unreacted styrene is stripped off in a distillation column that separates

liquid rubber emulsion from the gas styrene.• Rubber is recovered from the latex in a series of operations.

– Introduction of antioxidants, blending with oils, dilution with brine, coagulation, dewatering, drying, and packaging the rubber


44

Manufacturing of Emulsion SBR• Polymerization

– Cold SBR: at 5 to 10°C is called the cold process, • Better abrasion resistant, treadwear, and dynamic properties.

– Hot SBR: at about 50°C is called the hot process.• Conversion is allowed to proceed to 70%

• Higher branching occurs and incipient gelation.

– Typical SBR recipes, Table from Morton’s Rubber technology


45

Manufacturing of Emulsion SBR• Compounding and Processing

– Similar to natural rubber– Materials for large scale use, e.g., tires, based on

• Rubber, fillers (carbon black), extending oils, zinc oxide, sulfur, accelerators, antioxidants, antiozonants, and waxes.

– Materials are mixed in a mill or twin rollers or calendered– Processing into smooth compounds that can be quickly

pressed, sheeted, calendered, or extruded

• Recipes – Large parts, e.g., tires and hoses, are given in Tables 7.6,

7.7, 7.8, and 7.9


46

Polymerization of Elastomers• Butadiene-Acrylonitrile (Nitrile) Rubber

– Produced by emulsion polymerization – Nitrile rubbers have nitrile contents from 10 to 40%.

• Chloroprene rubber– Produced by emulsion polymerization– Produced as a homopolymer that has a high trans 1,4

chain structure and is susceptible to strain-induced crystallization, much like natural rubber.

• Leads to high tensile strength

– Does not lead itself to copolymerization


47

Polymerization of Elastomers• Butyl Rubber-

– Only important commercial rubber prepared by cationic polymerization

• Processes with AlCl at –98 to –90C

– Copolymer of isobutene and isoprene with isoprene used in 1.5 % quantities

• The isoprene is introduced to provide sufficient unsaturations for sulfur vulcanization.

– MW is in the range of 300,000 to 500,000


48

Processing of Elastomers• Rubber Products

– 50% of all rubber produced goes into automobile tires; – 50% goes into mechanical parts such as

• mountings, gaskets, belts, and hoses, as well as

• consumer products such as shoes, clothing, furniture, and toys

• Elastomers and Rubbers– Thermoset rubbers

• Compounding the ingredients in recipe into the raw rubber with a mill, calender, or Banbury (internal) mixer

• Compression molding of tires

– Thermoplastic elastomers• Compression molding, extrusion, injection molding, casting.


49

Processing of Elastomers• Rubber Processors

– Mills and Banbury mixers


50

Compression Molding Process• Materials

•Elastomers: •Thermoplastic

•Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE), Thermoplastic Rubber (TPR)

•Thermoset rubbers•Styrene Butadiene Rubber, isoprene

Thermoplastic:Heat Plastic

prior to molding

Thermosets:Heat Mold

during molding


51

Polyurethane Processing• Polyurethane can be processed by

– Slow process: Casting or foaming, or– Fast process: Reaction Injection Molding (RIM)


52

Injection Molding Glass Elastomers• Plastic pellets with copolymer elastomers.

– Similar processing requirements as with injection molding of commodity and engineering plastics

• Injection pressures, tonnage, pack pressure, shrinkage


53

Transfer Molding of Rubbers• Transfer molding is a process by which uncured rubber

compound is transferred from a holding vessel (transfer pot) to the mold cavities using a hydraulically operated piston. Transfer molding is especially conducive to multicavity designs and can produce nearly flashless parts.


54

Calendering of Rubbers• Calendering is the process for producing long runs of uniform

thickness sheets of rubber either unsupported or on a fabric backing. A standard 3 or 4 roll calender with linear speed range of 2 to 10 feet/minute is typical for silicone rubber. Firm compound with good green strength and resistance to overmilling works the best for calendering.


55

Curing of Rubbers• Extruded profile may be cured by hot air vulcanization (HAV),

steam vulcanization (CV) or liquid-medium cure. HAV consists of a heated tunnel through which the profile is fed continuously on a moving conveyor. Air temperature reaches 600°F to 1200°F, and cure times are usually short, on the order of 3 to 12 seconds. The recommended curing agents are DCBP-50 or addition cure, both of which provide rapid cure with no porosity.

• Steam cure commonly refers to the steam curing systems used by the wire and cable industry and consists of chambers 4” – 6” in diameter and 100 – 150 feet in length. Steam pressure varies from 50 psig to 225 psig depending on wall thickness of the insulation.

• For liquid-medium cure, continuous lengths of extruded profile are fed into a bath of moltenmaterial (salt or lead) which cures the extrudate.


56

Polymer Length

• Polymer Length– Polymer notation represents the repeating group

• Example, -[A]-n where A is the repeating monomer and n represents the number of repeating units.

• Molecular Weight– Way to measure the average chain length of the polymer

– Defined as sum of the atomic weights of each of the atoms in the molecule.

• Example, – Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole

– Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole

– Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole


57

Molecular Weight

• Average Molecular Weight– Polymers are made up of many molecular weights or a

distribution of chain lengths.• The polymer is comprised of a bag of worms of the same

repeating unit, ethylene (C2H4) with different lengths; some longer than others.

• Example, – Polyethylene -(C2H4)-1000 has some chains (worms) with 1001 repeating

ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units.

– The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .


58

Molecular Weight• Average Molecular Weight

– Distribution of values is useful statistical way to characterize polymers.

• For example,– Value could be the heights of students in a room.

– Distribution is determined by counting the number of students in the class of each height.

– The distribution can be visualized by plotting the number of students on the x-axis and the various heights on the y-axis.

Histogram of Heights of Students

05

10152025

60 70 80

Height, inches

Fre

qu

en

cy

Series1


59

Molecular Weight• Molecular Weight Distribution

– Count the number of molecules of each molecular weight

– The molecular weights are counted in values or groups that have similar lengths, e.g., between 100,000 and 110,000

• For example,– Group the heights of students between 65 and 70 inches in one group, 70 to 75

inches in another group, 75 and 80 inches in another group.

• The groups are on the x-axis and the frequency on the y-axis.

• The counting cells are rectangles with the width the spread of the cells and the height is the frequency or number of molecules

• Figure 3.1

• A curve is drawn representing the overall shape of the plot by connecting the tops of each of the cells at their midpoints.

• The curve is called the Molecular Weight Distribution (MWD)


60

Molecular Weight• Average Molecular Weight

– Determined by summing the weights of all of the chains and then dividing by the total number of chains.

– Average molecular weight is an important method of characterizing polymers.

– 3 ways to represent Average molecular weight• Number average molecular weight

• Weight average molecular weight

• Z-average molecular weight


61

Gel Permeation Chromatography• GPC Used to measure Molecular Weights

– form of size-exclusion chromatography– smallest molecules pass through bead pores, resulting in

a relatively long flow path– largest molecules flow around beads, resulting in a

relatively short flow path– chromatogram obtained shows intensity vs. elution

volume– correct pore sizes and solvent critical


62

Gel Permeation Chromatography


63

Number Average Molecular Weight, Mn•

• where Mi is the molecular weight of that species (on the x-axis)• where Ni is the number of molecules of a particular molecular species I (on

the y-axis).– Number Average Molecular Weight gives the same weight to all polymer

lengths, long and short.• Example, What is the molecular weight of a polymer sample in which the polymers

molecules are divided into 5 categories.– Group Frequency– 50,000 1– 100,000 4– 200,000 5– 500,000 3– 700,000 1

...

...

321

332211

NNN

MNMNMN

N

MNM

i

iin

000,260

)13541(

)700(1)500(3)200(5)100(4)50(1

...

...

321

332211

n

n

i

iin

M

KKKKKM

NNN

MNMNMN

N

MNM


64

Molecular Weight• Number Average Molecular Weight. Figure 3.2

– The data yields a nonsymmetrical curve (common)– The curve is skewed with a tail towards the high MW– The Mn is determined experimentally by analyzing the number of

end groups (which permit the determination of the number of chains)

– The number of repeating units, n, can be found by the ratio of the Mn and the molecualr weight of the repeating unit, M0, for example for polyethylene, M0 = 28 g/mole

– The number of repeating units, n, is often called the degree of polymerization, DP.

– DP relates the amount of monomer that has been converted to polymer.

0M

Mn n


65

Weight Average Molecular Weight, Mw

• Weight Average Molecular Weight, Mw

– Favors large molecules versus small ones– Useful for understanding polymer properties that relate to

the weight of the polymer, e.g., penetration through a membrane or light scattering.

– Example,• Same data as before would give a higher value for the

Molecular Weight. Or, Mw = 420,000 g/mole

...

...

332211

233

222

211

2

MNMNMN

MNMNMN

MN

MNM

ii

iiw


66

Z- Average Molecular Weight

– Emphasizes large molecules even more than Mw– Useful for some calculations involving mechanical

properties.– Method uses a centrifuge to separate the polymer

...

...233

222

211

333

322

311

2

3

MNMNMN

MNMNMN

MN

MNM

ii

iiz


67

Molecular Weight Distribution

• Molecular Weight Distribution represents the frequency of the polymer lengths

• The frequency can be Narrow or Broad, Fig 3.3• Narrow distribution represents polymers of about

the same length.• Broad distribution represents polymers with varying

lengths• MW distribution is controlled by the conditions

during polymerization• MW distributions can be symmetrical or skewed.


68

Physical and Mechanical Property Implications of MW and MWD

• Higher MW increases• Tensile Strength, impact toughness, creep resistance, and

melting temperature.

– Due to entanglement, which is wrapping of polymer chains around each other.

– Higher MW implies higher entanglement which yields higher mechanical properties.

– Entanglement results in similar forces as secondary or hydrogen bonding, which require lower energy to break than crosslinks.


69

Physical and Mechanical Property Implications of MW and MWD

• Higher MW increases tensile strength• Resistance to an applied load pulling in opposite directions

• Tension forces cause the polymers to align and reduce the number of entanglements. If the polymer has many entanglements, the force would be greater.

• Broader MW Distribution decreases tensile strength• Broad MW distribution represents polymer with many shorter molecules

which are not as entangled and slide easily.

• Higher MW increases impact strength• Impact toughness or impact strength are increased with longer polymer

chains because the energy is transmitted down chain.

• Broader MW Distribution decreases impact strength• Shorter chains do not transmit as much energy during impact


70

Thermal Property Implications of MW & MWD• Higher MW increases Melting Point

• Melting point is a measure of the amount of energy necessary to have molecules slide freely past one another.

• If the polymer has many entanglements, the energy required would be greater.

• Low molecular weights reduce melting point and increase ease of processing.

• Broader MW Distribution decreases Melting Point • Broad MW distribution represents polymer with many shorter

molecules which are not as entangled and melt sooner.

• Broad MW distribution yields an easier processed polymer

Mec

hani

cal

Pro

pert

ies

MW

Mel

ting

Poi

nt

MW

* Decomposition


71

Example of High Molecular Weight• Ultra High Molecular Weight Polyethylene (UHWMPE)

• Modifying the MWD of Polyethylene yields a polymer with– Extremely long polymer chains with narrow distribution– Excellent strength– Excellent toughness and high melting point.

• Material works well in injection molding (though high melt T)• Does not work well in extrusion or blow molding, which

require high melt strength.• Melt temperature range is narrow and tough to process.• Properties improved if lower MW polyethylene

– Acts as a low-melting lubricant– Provides bimodal distributions, Figure 3.5– Provides a hybrid material with hybrid properties

copyright joseph greene 2001 1 classes of polymeric materials elastomers professor joe greene csu,...

Documents

rubber silica

rubber additives

rubber elastomer

rubber stiffer

soft rubber

thermoplastic butyl

copyright joseph greene

natural rubber thermoset