8 Elastomer Technology ALT - University of Akrongozips.uakron.edu/~wwschlo/POLYFiles/POLY108.pdf · Rubber Compounding 8-1 ELASTOMER TECHNOLOGY • Understand the functions of the

Rubber Compounding 8-1

ELASTOMER TECHNOLOGY

• Understand the functions of the various components of rubber compound recipes.

• Explain the significance of vulcanization parameters, including scorch time, cure time, and cure rate index.

• Understand how crosslink structure (crosslink density and crosslink distribution) affects the mechanical properties of rubber compositions.

• Explain how fillers, particularly reinforcing fillers, affect the mechanical properties of rubber compositions.

• Identify the chemical systems most suitable for vulcanizing common elastomers.

Rubber Vulcanization

• Vulcanization is the process of forming a molecular network of linked polymer chains.

• Networks are formed by chemical crosslinks between chains:– Carbon-carbon bonds– Sulfur atoms or chains of sulfur atoms– Polyfunctional organic molecules– Polyvalent metal cations

• Goal: a thermoset product with desirable physical properties.

• Vulcanizate properties depends on the type and number (density) of crosslinks.

X X

X

X

X


Rubber Compound Components

• Elastomer(s)• Cure system

– Crosslinking agent– Accelerator(s)– Cure activators (co-reactants with accelerator)

• Processing aids (improve post-cure processing properties)– Oils – Waxes – Tackifying resins

• Antidegradant(s)– Antioxidants– Antiozonants

• Pigments• Particulate fillers and extenders

– Reinforcing (carbon black, silica)– Non-reinforcing (clay, CaCO3, TiO2)

Crosslink Density

Vu

lcan

izat

e P

rop

erty

Crosslink Density:Effects on Polymer Properties

• Crosslink density (degree of crosslinking) = number (mols) of crosslinks/unit volume.

• Crosslink formation affects elastomer properties: – Hardness increases.

– Elastic behavior favored.

– Hysteresis losses decrease.

– Tensile and tear strength increase until crosslink density exceeds optimum levels.


Effects of Crosslink Type & Distribution

• Crosslink distribution: in sulfur vulcanization, the molar densities of monosulfide (C−S−C), disulfide (C−S−S−C), and polysulfide (C−Sx−C) crosslinks.

• Thermally stable crosslinks (C−C bonds, C−S−C bonds)contribute to– Low reversion (thermal stability)– Low compression set

• Thermally labile or flexible crosslinks (ionic bonds: −COO−

M2+ −OOC−; di- or polysulfide bonds: C−Sx−C) contribute to– Increased thermal reversion (crosslink cleavage)– Increased tear resistance– Increased fatigue resistance

• Crosslink structure affects the mechanical properties of NR vulcanizates more strongly than it affects those of synthetic elastomer vulcanizates.

Monitoring Vulcanization:Oscillating Disk Rheometer

0Time, min

Torq

ue,

dN

·m

Maximum TorqueMH

Minimum TorqueML

Scorch TimetS2

Cure Timet'90 (tC90)

M = 0.9(MH − ML) + ML

100CRI = —————

t'90 − tS2

ISO 3417, ASTM D2084

PlatenDie

Die

Platen


Vulcanization Parameters

• The time until crosslinking starts– Scorch time, scorch resistance, scorch delay (tS2)– Goal: adequate time for mixing, forming, or molding the rubber

compound

• The rate of crosslink formation– Cure rate index (CRI)

100CRI = ——————

t'90 − tS2– Goal: rapid, controllable rate

• The extent and type of crosslinking– Maximum torque (MH), crosslink density, crosslink distribution

(sulfur vulcanization: monosulfide, disulfide, polysulfide)– Goal: desired physical properties

Torq

ue,

dN

·m

Time, min

MarchingCure

Reversion

OvercureOnset UndercureOptimum

Cure

tS2

t'90

Vulcanization Profiles


Sulfur Vulcanization

• Suitable for diene-based elastomers:– NR, IR– BR– SBR– NBR– IIR– EPDM

• Low-cost• Rate easily controlled

– Accelerators– Retarders

• Crosslink distribution easily controlled (accelerators)– Monosulfides: heat resistance– Di-, polysulfides: tensile strength, elastic properties

S8S S2

Sx

S

Sx

Sulfur Vulcanization: Basic Rxn

CH C

CH3

H

CHS··S

CH C

CH3

· CH SH·S+

CH C

CH3

· CHS8

CH C

CH3

CHSx·

CH2 C

CH3

CH

CH C

CH3

CHSx

·CH2 C

CH3

CH−H·

CH C

CH3

CHSx

CH C

CH3

CH

Alternative ionic mechanism also possible.

*S

S

SS S

S

S

SS··S

*


Sulfur Vulcanization: Accelerators

• Sulfur is a sluggish cross-linking agent, especially for synthetic rubber.

• Sulfur/amine-based accelera-tors allow faster vulcanization (ionic mechanism) at relatively low temperatures.

• Accelerated vulcanizations use lower levels of sulfur.– Aging properties improved– Overcuring reduced

• Cure onset (scorch) can be controlled.

• Crosslink distributions can be controlled (mono-, di-, or poly-sulfide).

Thiazoles:

SHN

SS

N

S2

MBT MBTS

Thiurams/Dithiocarbamates:

TMTD

N C

2

H3C

H3C

S

S

ZDBC

Zn2+N C

2

C4H9S

S−

C4H9

Sulfenamides:

SNN

S HCBS MBS

SNN

SO

Guanidines:

DOTG

CH3 H3C

NH C

NH

NH

Sulfur Vulcanization: Accelerated Rxn

CH2 C

CH3

CHSNR2

N

S

S8SSx

N

SNR2

−HNR2

CH2 C

CH3

CHCH C

CH3

CHSx

SN

S

+

CH C

CH3

CH

CH C

CH3

CHSx

SHN

S• Slower cure onset

• Faster cure rate

• Slower cure onset

• Faster cure rate

Adapted from M.H.S. Gradwell & N.R. Stephenson, 2004, Rubber Chem. Technol. 77 931-946.


Accelerator Characteristics

Adapted from H.W. Engels et al., 2011 “Rubber, 9. Chemicals and Additives”, in Ullmann'sEncyclopedia of Industrial Chemistry, 6th ed., DOI: 10.1002/14356007.a23_365.pub3, 66 pp.

100

50

150

200

250

50 100 150Relative Scorch Time

Rel

ativ

e C

ure

Tim

e

Sulfenamides

Thiazoles

Thiurams

Dithiocarbamates

CBS

MBT

MBTS

TMTD

ZDBC

MBS

Cure Temperature EffectsNR Gum Stock*

5 10 15 20

5

10

15

20

25 170°C 160°C 150°C 140°C

Time, min

Torq

ue,

dN

·m

* 3.0 phr CBS, 2.5 phr sulfur

Adapted from L.F. Ramos-de Valle, 1981, Rubber Chem. Technol. 54, 24-33.

With increasing cure temperature:• Decreased scorch delay• Increased cure rate• Increased reversion

With increasing cure temperature:• Decreased scorch delay• Increased cure rate• Increased reversion


Cure Efficiency (Sulfur:Accelerator)Effect on NR Crosslink Distribution

Adapted from K. Suchiva et al., 2000, J. Appl. Polym. Sci. 78 1495-1504.

* 0.8-5.0 phr CBS,3.0-1.0 phr sulfur

80

60

40

20

1 2 3Sulfur/CBS Ratio*

Distribution,%

“Efficient” cure systems increase the formation of monosulfide crosslinks, decrease the formation of polysulfide crosslinks.

“Efficient” cure systems increase the formation of monosulfide crosslinks, decrease the formation of polysulfide crosslinks.

R−S−R

R−S X−R

R−S2−R

Cure Efficiency

Cure Efficiency (Sulfur:Accelerator)Effects of NR Crosslink Distribution

21

20

19

18

17

16

50

45

40

1 2 3 4 5 6Sulfur/DCBS Ratio*

Tensile Strength,MPa

Crosslink Density,mol/m3

Cure Efficiency

Adapted from G.R. Hamed & K. Boonkerd, 2011, Rubber Chem. Technol. 84 229-242.

* 0.75-5.00 phr DCBS,5.00-1.31 phr sulfur

Crosslink density, crosslink distribution affect physical properties.

Crosslink density, crosslink distribution affect physical properties.


Rubber Compounds: Sulfur Cures

CureActivators

Componenta NR SBR IIR NBREPDM CR

Elastomer

ElastomerZnOMgOStearic acidSulfurAcceleratorsProcessing aids

100 100 100 1001005 4 3 55

3 2 1 112.5 1.8 1.75 1.01.00.6 1.25 1.5 0.61.55 10 1025

10054

0.50.50.510

Antioxidant 2 2 1.5 1

a Parts per hundred (phr) of rubber. b Optimum cure at 153°C.

Cure time, minb 20 25 60 2525 80Performance

Tensile str., MPaElongation, %

19.4 1.3 2.2 1.41.6690 310 440 590310

8.9620

Sulfur Vulcanization:Influence of Rubber Type

100

50

150

200

20 40 60Time, min

Torq

ue

NBRNR

EPDM

BRSBR

IIR

Gum stockSulfur cureMBS accelerator

Adapted from J.R. Beatty & M.L. Studebaker, 1975, Rubber Age 107(8) 20-35.


1914

• White tires ZnO filler

• Starting in 1912, carbon black replaced ZnO and other filler materials in tires.

• Now: ~90% of all carbon black produced is used in the manufacture of tires and other rubber goods.

Reinforcing Fillers

• Synthetic elastomers have no inherent reinforcing properties

– Low resistance to abrasion, tear– Low compound viscosity– Low hardness, toughness

• Non-reinforcing fillers– Show little or no physical inter-

action with polymer phase– Serve as extenders or pigments

• Reinforcing fillers– Create physical and chemical inter-

actions with polymer phase– Affect performance characteristics of

vulcanizates

• Reinforcement determined by– Particle size (upper limit: <1000 nm)– Particle surface area– Surface chemistry

Image adapted from S.-L. Kim & D.H. Renecker, 1993, Rubber Chem. Technol. 66, 559-566.

Non-Reinforcing

Clay (kaolin)CaCO3BaSO4

Reinforcing

Carbon blackSilica (precipitated)

CARBON BLACK

Scale bar: 100 nm


Reinforcement Mechanism

• Bound rubber: Not separated by solvent extraction

• Bound rubber layer is inter-penetrated by bulk polymer.

• Origin:– Rubber absorption on particle

surface (dispersion, dipole-induced dipole, dipole-dipole)and/or

– Chemical bonding between rubber and carbon black or activated silica

• Reactive functional groups or structures:– Carbon black: phenol, carboxyl,

lactone, lactol, ketone, qui-none, pyrone, fullerene

– Silica: silanols, gem-disilanols

Adapted from S. Wolff, 1996, Rubber Chem. Technol. 69, 325-346;J.-B. Donnet, 1998, Rubber Chem. Technol. 71, 323-338.

FillerParticle

OH

COOH

C OO

OH

O

C OO

Si O Si O Si O Si O

O

OHOHHO

O O O O

100

50

150

200

20 40 60Time, min

Torq

ue

NRBR

SBR

NBR

IIR

EPDM

50 phr carbon blackSulfur cureMBS accelerator

Sulfur Vulcanization:Effect of Reinforcing Fillers

• Faster cure onset• Higher cure rate• Higher MH

• Faster cure onset• Higher cure rate• Higher MH


100

50

150

200

20 40 60Time, min

To

rqu

e

NBRNR

EPDMBR

SBR

IIR

Unfilled stocks:


Rubber Compounds: Carbon Black-filled

ReinforcingFiller:

a Parts per hundred (phr) of rubber. b Optimum cure at 153°C. c Assumed; noaccelerator shown in source table.

NBREPDM

Componenta gum black gum gumblack black

Elastomer

Elastomer

ZnOStearic acidSulfurAcceleratorsProcessing aids

100 100 100 100100

4 4 5 552 2 1 11

1.8 1.8 1.0 1.01.01.25 1.25 1.5 0.61.510 10 25 1025

100

51

1.00.6?c

10Antioxidant 2 2 1.5 1.5

SBR

Carbon black 50 50 50


Cure time, minb 25 20 25 2525 20Performance

Tensile str., MPaElongation, %

1.3 23.6 1.6 1.415.4310 520 310 590410

19.3610

Antidegradants

• Elastomers with many carbon-carbon double bonds (NR, IR, BR, SBR, NBR, CR) are attacked by oxygen and ozone.

• Halogenated elastomers (CR, BIIR, CIIR) are susceptible to thermal decomposition (−HX).

• Outcomes:– Hardening or softening– Cracking– Loss of elastic properties– Reduced service life

• Antioxidants are added to rubber compounds to react with oxygen.

• Antiozonants are added to rubber compounds to react with ozone and oxygen.

C CH

CH2

CH3

CH2

C CCH3H

CHCH2

H

C CH

·CH2

CH3

CH2

+ ·OH +C CCH3H

CCH2

O

H

C CH

CH2

CH3

CH2

C CCH3H

CHCH2

OOH

O2

O3

Adapted from G.-Y. Li & J.L. Koenig 2005 Rubber Chem. Technol. 78, 355-390;S. Commereuc et al. 1997 Polym. Degrad. Stabil. 57, 175-182.

+CH

CH2

O C CH

CH2

CH3

CH2

CCH3

CH

H

O


Antidegradants (cont.)

• Contain functional groups (OH, NH, SH) that act as chain terminators.

• Antioxidants– Peroxy scavengers react with

peroxy radicals (RO2·)– Hydroperoxide scavengers react

with ROOH.– Types:

• Hindered phenols• Alkyldiphenylamines• Dihydroquinolines• Organophosphites

• Antiozonants– Function depends on ability to

migrate to rubber surface– Types:

• Dialkyl- or diaryl-p-phenylenedi-amines

• Dihydroquinolines

C7H15NH

C7H15 NH

N C8H17

HC8H17

NCH3

CH3

CH3

H

S

CH3

OH

C

CH3

HO

CH3H3C

H3C CH3CCH3

CH3

O P

3

CH3C

H3CH3C

CH3CCH3

CH3

Peroxide Vulcanization

• Suitable for elastomers with low — or no — residual unsat-uration:– EPM, EPDM– HNBR

• Exception: IIR (chain cleavage reactions)• Useful for blends of rubbers with different sulfur cross-

linking reactivities:– NR-EPDM– NBR-EPDM

• Benefits:– Carbon-carbon crosslinks– Excellent heat resistance– Low compression set

• Disadvantage: limited options for stabilization with anti-oxidants


Peroxide Vulcanization: Basic Rxns

C

CH3

O

2CH3

∆2 C

CH3

O·

CH3

CH C

CH3

H

CHR· CH C

CH3

· CH−RH

×2 CH C

CH3

CH

CH C

CH3

CH

R·

−RHCH C

H

CH2

CH3

CH3

·CH C CH2

CH3

CH3

Chain Cleavage

+ ·CH2CH C

CH3

CH3

Exception: IIR

2 ·∆

−2(CH3)2C=O

“R·”

Rubber Compounds: Peroxide Cure

a Parts per hundred (phr) of rubber. b Sulfur: cures at 166°C;peroxide: cures at 160°C (NBR) or 171°C (EPDM).

EPDM

Componenta sulfur peroxide sulfur peroxide

Elastomer

Elastomer

ZnOStearic acidSulfurS AcceleratorsPeroxide

100 100 100 100

5 51 1

1.75 2.01.6 1.25 3.9

4 6

NBR

Carbon black 75 100 100 85

Processing aids 10 10 90 12Antioxidant 2 1.5 2

Adapted from The Vanderbilt Rubber Handbook, 13th Ed., 1990, R.H. Ohm, ed., R.T. Vanderbilt Co.

PerformanceCure time, minb 5 20 20 10Tensile str., MPaElongation, %

18.6 13.6 16.2 15.9360 340 390 300


Metal Oxide Vulcanization

• ZnO, MgO

• Suitable for elastomers containing halogen or carboxyl groups:– CR– BIIR, CIIR– XSBR, XNBR (carboxylated SBR, nitrile rubbers)

• Benefits:– Low- or no-sulfur cure system– Improved strength properties– Low compression set

• Disadvantages:– ZnO-catalyzed formation of HCl (MgO alternative)– Rapid cure onset with carboxylated elastomers– Ionic crosslinks (XSBR, XNBR) thermally labile

Metal Oxide Vulcanization: Typical RxnsHalogenated Elastomers

CH2

CHH2CC

Cl

+ ZnClOHCHCHH2CC

ZnO

CH2

CHCH2

C

ClZnCl2

CH2

CHH2C+

C

ZnCl3−

+ ZnCl2CHCHH2CC

ZnClOH

−H2O

−ZnCl2, −HCl

CHCHH2CC

CH2

CHH2C

C

CH

CHClH2C

C

Dehydrohalogenation(loss of HCl)

Isomerization

Covalent CrosslinkFormation

Adapted from H. Desai et al., 2011 J. Appl. Polym. Sci., 105, 865-876

CH2

CHCH2

C

Cl~Cl



Rubber Compounds: Metal Oxide CureHalogenated Elastomers

a Parts per hundred (phr) of rubber. b Cure: 20 min at153°C.

Componenta sulfur peroxide metaloxide

Cure System

CR (Neoprene W)

MgO

Stearic acidSulfurAcceleratorsPeroxide

100 100 100

4 4 4

0.5 0.5 0.512 1.5

1


Plasticizer 5 5 5Antioxidant 2 4 2

ZnO 5 5 5

Performanceb

Tensile str., MPa 13.0 12.6 13.4Elongation, %Tear str., kN/m

590 470 53056.3 37.0 42.2

Metal Oxide Vulcanization: Typical RxnsCarboxylated Elastomers

ZnO

−H2O

CH2

C−O

CH

O

CH2

C

−O

CH

O

Zn2+

IonicCrosslink

CH2

CHO

CH

n

O

CH2CH

m

CHCH2

ℓ

CH2 CH

CN



Rubber Compounds: Metal Oxide CureCarboxylated Elastomer

a Parts per hundred (phr) of rubber.

Componenta 100:0 50:50 0:100

Recipe

NBR

Carbon black

Stearic acidSulfurAccelerators

100 50

40 40 40

2 2 20.5 0.5 0.53 3 3

XNBR 50 100

Plasticizer 5 5 5Antioxidant 1 1 1

ZnO 5 5 5

Performance

Tensile str., MPa 18.2 21.0 25.5

Elongation, %Tear str., kN/m(23°C)

500 415 430

48.5 50.9 53.4

200% Mod., MPa 4.8 10.0 11.0

Reactive Phenolic Vulcanization

• Suitable for elastomers with even low levels of unsatura-tion:– EPM, EPDM, IIR– FKM

• Cures activated by Lewis acids:– SnCl2– ZnCl2– FeCl3

• Disadvantages:– Rapid cure onset (+ activators)– Slow cure rates (− activators)

• Benefits:– C-C, C-O crosslinks reversion

resistance– Route to elastomer + polyolefin

thermoplastic vulcanizates and similar compatibility-enhanced blends.

XCH2

OH

C8H17

CH2

~2−3

OCH2

OH

C8H17

CH2 OCH2

OH

C8H17

CH2X

HO OHCF3

CCF3

X = OH, BrSchenectady Intl. SP-series (IIR, EPDM vulcanization)

DuPont VC-50 (FKM vulcanization)


Reactive Phenolic Vulcanization: Rxns

CH2 C

H3C

H3C

m

CH2 CH CH2

CH3

C

n R’OCH2

OH

R

CH2X

Lewis acidactivator

CH2 CH CH2

CH3

C+

R’OCH2

HO

R

CH2

+ X−

CH2 CH CH2

CH3

C

R’OCH2

O

R

CH2

−H+

H2C

C

H2C

H3CC

CH2

HO

R

CH2 C CH2

CH3

C

HO

R

CH2

OCH2

CH2

…

CHCH2

CH3

CH2C

−HX

CrosslinkedPolymer

−H+

CH2 C CH2

CH3

C

XCH2

HO

R

HO

R

CH2

OCH2

CH2

…

12.5

15.1

26

9.2

Reactive Phenolic Vulcanization:IIR Cure Systems

a Parts per hundred (phr) of rubber.

Componenta Recipe

IIR

Stearic acid

SnCl2·2H2OZnO

100 100 100

2 2 2

1.5 1.5 1.55


Processing oil 7 7 7

100

2

1.50.5

50

7

100

2

1.51

50

7

100

2

1.55

50

Methylol phenol-formaldehyde resin 5 5Bromomethyl phenol-formaldehyde resin 5 5 5 5

7

Cure characteristics

t'90, MPa 26

MH, dN·M 7.5

27 13 19

8.2 5.0 5.8

Adapted from L.T. Lukich, 1977 US Pat. “Bladder composition containing low unsaturation butyl rubber,” 4,022,848.

ZnO competes with any Lewis acid activator

ZnO competes with any Lewis acid activator

Bromomethyl (BrCH2−) resinsare more active XL agents thanhydroxymethyl (HOCH2−) resins

Bromomethyl (BrCH2−) resinsare more active XL agents thanhydroxymethyl (HOCH2−) resins


Reactive Phenolic Vulcanization:IIR Cure Systems (cont.)

* DTDM = 4,4’-Dithiodimorpholine; TMTD = tetramethylthiuram disulfide; MBT = 2-mercaptobenzothiazole.Cure system contains ZnO unless stated otherwise.


Cure System, phr*

ProcessingSafety

(Scorch) Cure Rate

IntermittentService T,Tmax, °C Application

DTDM 2.0TMTD 2.0

VerySafe Slow 100-121 Molded goods

Bromomethyl phenol-formaldehyde 12.0 Safe Slow 177-204 Curing bladder

Hydroxymethyl phenol-formaldehyde 12.0Halogenated polymer 5.0 Safe Slow 177-191 Curing bladder

Hydroxymethyl phenol-formaldehyde 12.0SnCl2 2.0 Scorchy Very

Fast 191-232 No ZnOCuring bladder

Sulfur 2.0TMTD 1.0MBT 0.5

VerySafe Moderate 100-121 Inner tubes

References and On-Line Resources

• General– The Vanderbilt Rubber Handbook, 14th Ed., M.F. Sheridan, ed., R.T.

Vanderbilt Co., 2010.

• Compounding– A.Y. Coran “Vulcanization”, in Science and Technology of Rubber,

3rd Ed., J.E. Mark, B. Erman & F.R. Eirich, eds., Elsevier/Academic Press, 2005, pp 321-366. ISBN: 0-1246-4786-3

– J.-B. Donnet & E. Custodero “Reinforcement of Elastomers by Particulate Fillers”, in Science and Technology of Rubber, 3rd Ed., J.E. Mark, B. Erman & F.R. Eirich, eds., Elsevier/Academic Press, 2005, pp 367-400. ISBN: 0-1246-4786-3

– H.W. Engels et al., 2011 “Rubber, 9. Chemicals and Additives”, in Ullmann's Encyclopedia of Industrial Chemistry, 6th ed., DOI: 10.1002/14356007. a23_365.pub3, 66 pp.

– B. Rodgers & W. Waddell, “The science of rubber compounding”, in Science and Technology of Rubber, 3rd Ed., J.E. Mark, B. Erman & F.R. Eirich, eds., Elsevier/Academic Press, 2005, pp 401-454. ISBN: 0-1246-4786-3


References and On-Line Resources

• Antidegradants– P.P. Klemchuk, 2005 “Antioxidants”, in Ullmann's Encyclopedia of

Industrial Chemistry, 6th ed., DOI: 10.1002/14356007.a03091, 22 pp.

– J.A. Kuczkowski, 2011 Rubber Chem. Technol. 84, 273–295.

• Testing– Handbook of Polymer Testing: Physical Testing. R. Brown, ed., CRC

Press, 1999. ISBN: 0-8247-0171-2

Documents

8 Elastomer Technology ALT - University of Akrongozips.uakron.edu/~wwschlo/POLYFiles/POLY108.pdf · Rubber Compounding 8-1 ELASTOMER TECHNOLOGY • Understand the functions of the