8 Elastomer Technology ALT - UAkron
Transcript of 8 Elastomer Technology ALT - UAkron
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 Compounding 8-2
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.
Rubber Compounding 8-3
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
Rubber Compounding 8-4
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
Rubber Compounding 8-5
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
*
Rubber Compounding 8-6
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.
Rubber Compounding 8-7
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
Rubber Compounding 8-8
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 Compounding 8-9
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.
Rubber Compounding 8-10
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
Rubber Compounding 8-11
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
Adapted from J.R. Beatty & M.L. Studebaker, 1975, Rubber Age 107(8) 20-35.
100
50
150
200
20 40 60Time, min
To
rqu
e
NBRNR
EPDMBR
SBR
IIR
Unfilled stocks:
Rubber Compounding 8-12
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
Adapted from J.R. Beatty & M.L. Studebaker, 1975, Rubber Age 107(8) 20-35.
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
Rubber Compounding 8-13
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
Rubber Compounding 8-14
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
Rubber Compounding 8-15
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 Compounding 8-16
Adapted from The Vanderbilt Rubber Handbook, 13th Ed., 1990, R.H. Ohm, ed., R.T. Vanderbilt Co.
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
Carbon black 75 75 75
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 Compounding 8-17
Adapted from The Vanderbilt Rubber Handbook, 13th Ed., 1990, R.H. Ohm, ed., R.T. Vanderbilt Co.
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)
Rubber Compounding 8-18
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
Carbon black 50 50 50
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
Rubber Compounding 8-19
Reactive Phenolic Vulcanization:IIR Cure Systems (cont.)
* DTDM = 4,4’-Dithiodimorpholine; TMTD = tetramethylthiuram disulfide; MBT = 2-mercaptobenzothiazole.Cure system contains ZnO unless stated otherwise.
Adapted from The Vanderbilt Rubber Handbook, 13th Ed., 1990, R.H. Ohm, ed., R.T. Vanderbilt Co.
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
Rubber Compounding 8-20
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