PNEUMATIC TIRE COMPOUNDING
by Walter H. Waddell, Roop S. Bhakuni,
William W. Barbin, and Paul H. Sandstrom
The Goodyear Tire 8 Rubber Company
Akron. OH
The pneumatic tire is a toroidal, high performance polymeric composbte
which exhibits characteristics of aflexible membrane pressure container pro
viding load carrying capabllity,cushioning, and road handling abilities. Itencir
cies the rim of a wheel to absorb end control forces between the axle and the
road in a transient environment and thus transmits driving and braking torque,
transmits corneringforces, and performs all functions essential to locomotion
These features can be attributed to its deformabiiity and damping characleristics (Ref. 1). The first tires were solid rubber carriage tires exhibited in London,
United Kinudomin 1851 and were used for transoorlation until theearlvvears
..
of this century.~herequirementsof speed and cdmfort led to the development
of pneumatic tires. The pneumatic tire is a complex system of interacting rub
bery components, each with specific properties for maximum effectiveness
The performance of a tire depends on the component properties, the interac
tions of these components, and the service conditions of the tire (Ref. 2).
Tire Components
Thetireis acompositeofseverairubberycomponentseachofwhich servas
a specific and unique function, These components are developed or fw
mulated, designed, and fabricated to meet the mobility requirements of
vehicles which include carrying of the load, control of vehicle, handling, rida,
tractton, durability, etc. Tire components are made from materials such as
natural rubber andlor synthetic rubbers (Ref. 3),chemicals which function as
antidegradants, curatives, processing aids (Ref. 4), reinforcing fillers such as
carbon black and silica, and cordscamposed of textile, f~berglass,and steel
wire (usually brass coated). These materials are selected based on thdr
physico-chemical properties and their interactions with other constituenl
materials to provide a broad range of mechanical properties. Figure 1 show
the componentsof a radial passengertire. A tire can be divided into two majoc
segments:(i)thetread area, which includes the tread compound,base,cushion
and overlay; and (ii) the casing, which includes the carcass plies, beits.
sidewall, bead, inneriiner, apex, chafer, etc.
A p e x . The apex Is formuiated for good dynamic stiffness, flex fatigue, tea
strength, adhesion, and durability.
Base. The base is formuiated to have low hysteresis, good adhesion, fatigue,
tear, and durability.
Beadlnsulation. The bead usually consistsof multiplestrandsofhightensile,
brass-plated steel, coated with rubber and formed into Inextensible hoops to
seal the tire against the rim and provide hoop tension to prevent air leakage. it
acts as a load transfer mechanism between the tire and the rim.
Be11 Coat. The belts are layers of rubber-coatedcord, eithertextile, fiberglass
or steel, which wrap circumferentiallyaround the plies. The coat compound is
maliyformuiatedto providegoodadhesiontothecord, tear,fatigue,end age
resistance.
Chafer. Thechafer protects the plies from wearing and cutting against the rim,
distributes the hoop stress of the bead and prevents moisture and dirt from
penetrating into the tire.
Innerliner. The inneriiner is formulated to provide good air and moisture
'mpermeability, flex fatigue resistance, and aged durability.
Ply Coat. The ply is a rubber-coatedcord that serves as the reinforcing eiemeotofatirecarcassby locking the bead intothecarcass.Pliesprovidetensiie
resistance to Inflationpressure along the cord direction.
Sidewall. The sidewall compound is formulated for resistance to weathering,
ozone, abrasion, tear, radial and circumferential cracking, and for good
Mgue life.
Tread. The tread is des~gnedand compounded for abrasion resistance, traC-
.
Mn
lo* roll~noresistance, durablllty and protection of the caslng It provides
~
hiclional contact for transtkssion of driving, braking, and cornering forces.
Wedge. The shoulder wedge is placed under the belt edge to reduce lnterpiy
shear strain and is formulated for high dynamic stiffness, resilience, good
laligue, adhesion and tear resistance.
Compounding
Individualcompoundsare designed and formulated to meet the specificset
d performancerequirements of the tire component. This is primarily accomplished through selection of elastomer type(s), selection of chemicals for
vulcanization,selection of materials for facile processing and tire manufactur'kg, and selectionof materials for in-service performance.Table 1 lsa summary
d !he desirable properties of tire compounds and Table 2 a summary of the
tatMratorytests used to evaluate tire compound physical performance.Table 3
is a summary of the general composition of individual tire compounds: elasbmers, reinforcing fillers, cure system, and additional Important Ingredients.
Itis important tonote that each compound hasaspecific recipe formulated
kr its expected tire application, i.e. tread, sidewall, inneriiner, etc. However, in
addition lo the exact recipe, compoundphysical properliesandfinal tire performanceareverydependentupon exact processingconditions.They includethe
nixingsequence (sequence of adding the chemical ingredients), mlxing time,
nixing energy and stock temperature, and downstream processing methods
such as remiliing, calendering, and extruding, etc.
from the expandable biadder before vulcanization begins. Since each componentof the tire undergoes a different rate of cure (time-temperature cycle) and
can have very different proximities to the bladder and mold, it Is important to
produce the optimum state of cure (plateau) for each component during the tire
curing cycle. This is accomplished by selection of and adjustment of the cure
ingredients, namely sulfur, activators, acceierators, and retarders. The cured
Greshouidhave the final form and physical properties, including final cure State.
required for service.
Tables 4 through 11 provide formulas and physical properties for importanl
tire components. Table 12 provides examples of innertube and curins biadder
formulas. Finally it is important tonotethal theexampies provided heriare representative in that each compounderwiii adiust the recioe for soecilicorocessing constraints and performance requirernents of the barticuiar tire broduct
interactions
Interactions between the individual compounds and components of the tire
system occur during tire fabrication, curing, and service life. The first type of
interaction is physical, but also may have important chemical implications. it Is
based upon the migration of soluble irigredients. This occurs whenever
chemicals used as curatives (sulfur, accelerators, activators), processing aids
(oils, peptizers, tackifiers) and antidegradants (antioxidants, antiozonants,
waxes), for example, differ in concentration in two adjoining components
Migration of curatives will result in levels different from the original optimized
formulation and may lead todifferent statesof cure, particularly at theinterface.
Migration of antidegradants from the sidewall compound into the carcass will
resuit in lower concentrations and may lead to premature weathering.
The second type of interaction is again physical since compounds are
designed to have acertain modulus.The hardestcompositionsare thoseof lhe
bead area and the belts, which contain the highest amounts of the most rein.
i0rc:ng carbon blacks. The m~iticomponenl!;re system is designed lo have
C3mPoundsoecreaseste~v1isein
moouius 2s thevare sit~atedtanherirom
~. lne ~
high modulus componenis in order to distribute the stress and minimize con.
ditions for interfacial separation.
~
Curing
Both the cure rate of and the final cure state of a rubber compound are
important. Without exception, accelerators are used lo increase the rate of
sulfur vulcanizationoftirecompounds and to improve their physical properties
and age resistance.The most important characteristics of acceleratorsare their
effect on scorch (period after addition before vuicanization begins), cure
(period required for full vulcanization) and plateau times (period during which
Propertiesare constant). Scorch time is important because once vulcanization
has initiated, stocks cannot be processed as readily. The rate of cure or cure
time is important since production must achieve the desired final state of cure
within the shortest time after cure has initiated.
The tirecuring process involvesapplying specified temperatures and pressures over a period of time to effect simultaneous vulcanization of the various
rubbercompoundsin the tiretoformonec0m~osite.Thisisaccomplishedinan
automatic curing press which uses steam to maintain temperature in the sheU
around the metal mold. it also has an attached heat-resistant rubber bladder to
supply internal heat and pressure. When the press is completely closed, the
tread and sidewall components are forced into the mold pattern by pressure
~
The components of a tire composite have optimum physical properties
when removed from the mold and cooled to ambient temperature. However,
thereisno assurancethat the same condition will persist after the tire is subioctedtoserviceconditionssincechangescanoccurfrom (i) heat generated in the
tire by energy loss when high temperatures are achieved by friction due to
haking, and (ii) any increased crosslinking caused by thermo- and mechanodiemical oxidation, which can potentially lead to embrinlement, cracking or
decreased resistance to tear. Accelerated aging tests commonly used in the
hboratoryshowtrendsbut may notcorrelateclosely withallservice~onditions.
The normal aging process can be retarded by addition of antioxidants, for
example, secondary arylamines added to inhibit mechano-chemical oxidation
such as flex-cracking. Thermo-oxidation stability Is promoted by using low
.
suifurlhigh
accelerator (semi-efficient) cure systems.
Weathering effects the outer surface of the tire, particularly the sidewalls.
Thus, chemical antiozonants are added to the black sidewall compound at
levels tested for effectiveness under both static and dynamic tire service conditions. However, the antiozonant is continually depleted from the sidewall
surfacebyreactionwithozone,and by physicalabrasioncaused bycurbscuffing
and washino. In addition.. ail .oractical chemical antiozonants for rubber are
s!aining materials and therefore can only be used in limited amounts. Suitably
Mended petroleum waxes serve as effective protectants against ozone under
static conditions, but must be used in conjunction with highly active chemical
antiozonants for dynamic protection of the tire.
-
References
1. F. J. Kovac. Tire Technology, The Goodyear Tire & Rubber Co., Akron,
OH (1978)
2 J. C. Ambeiang, Chpt. 11, Tke Vanderbilr Rubber Handbook, 12th Ed.
(1978)
3.M. Morton, Editor, Rubber Technology, 2nd Ed., Krleger Publishing (1981)
4. H. Long, Editor, Basic Compounding and Processing OJ Rubber, Rubber
Division, American Chemical Society, Inc., Lancaster Press (1985)
600
60 I
Table 2
Laboratory Testing of Tlre Compounds
ASTM Tests
Compound
Tread
Wire Coat
Sidewall, Wire Coat, Ply Coat,
lnnerliner
Sidewall
uium Tread, Wire Coat, PLY Coat
Sidewall
Ply coat, lnnerliner
Table 1
Desirable Properties of Tlre Components
Tread
Sidewall
Wire Coat Ply Coat
Polymers and Coated Fabric
lnneriiner
Wire Coat
Tread, Wire Coat, Ply Coat
Maximum
Traction
Adhesion
Wire
Oxidative
Stability
Cleanability
Afinimum
Rolling
Resistance
Wear
Cut
Growth
Groove
Cracking
Weather
Cracking
Flex
Cracking
Heat
Buildup
Curb
Scuffing
Heat
Buildup
Buildup
Permeability
602
Table 3
Composltlon of Tlre Compounds
Tread
Sidewall
NR
BR
SBR
NR
BR
SBR
EPDM
Table 4
Passenger Tlre Tread Reclpe
:
WOFLEX 1 5 0 2 SBR
Elastomers
Reinforcing Fillers
Blacks: N-1lo,
N-220,
N-299,
N-330
Non-blacF:
N-550,
N-660
Silica
Silica
Cure System
Low/Normal
Sulfur
Adapled lo
Polymer
Additional I n p d i e n ~ s
H l Q h l ~ Anlldegradants:
Adhesion
~
d
Aromatic VANWAX H Special Promoters:
promoters:
0
1
1
AGERITE RESIN D
Coball HsxamethyleneANTOZlTE 67P,
Stearate,
tetramine,
VANOX 2
Naphthenale. Rerorclnol
or .Born
Decanoate
h
~
~
~
~
~
100°C
Storage Modulus, MPa (psi)
004
Table 5
Truck Tread Reclpes
Radial
TSR 20 Natural Rubber
BUDENE 1207
PLIOFLEX 1712
SAF Black (N-110)
ANTOZITE 67P
AGERITE RESIN D
VANWAX H Special
Aromatic Oil
Stearic Acid
Zinc Oxide
Sulfur
DURAX
VANTARD PVi
MORFAX
METHYL TUADS
Total
100
..
-.
50
2
2
1.5
4
2
4
1.75
1.75
0.5
-.
-169.50
Rheonieter at ISOOC ( 3 0 0 " ~ )
Isl, minutes
t'c (SO), minutes
Table 6
Passenger Tire Sldewall Recipes
Bias
Black,
Radial
..
50
68.75
55
2
1.5
3
12.5
2
3.3
1.55
'
..
-.
1
0.15
zm%
,
7.4
12.0
Physical Properties
Cured 22 minutes at ISO'C (300m/,.~
stress at 3009b, MPa (psi)
Tensile strength, MPa (psi)
Elongation at Break, 96
%
Rebound: at 2 2 " (72"~).
~
12.2
36.8
*/1STMDI054 cured32 minutes at 150°C ( 3 0 0 ~ 0
50
50
-.
..
50
2
10
*.
..
..
2
2
2
3
..
-.
3
1.75
1
AMAX
-,
12.3 (1790)
27.1 (3920)
550
81.9
SMR.5 Natural Rubber
BR1220
Chiorobutyl 1066
Mstalon 6505
HAF Black (N-330)
VANPLAST R
Naphthenic Oil
VANTALC 6H
Titanox 1000 Titanium Dioxide
Nucap 200 Treated Clay
Stearic Acid
AGERITE RESIN D
ANTOZITE 67P
VANWAX H Special
SP 1077 Resin
Ullramarine Blue
Zinc Oxide
Sulfur
Vultac 5
WAX
11.8(1710)
17.6 (2550)
41 5
70
Told
White. Exxon
Formula BA5152
..
..
-
176.75
21 5.3
Rheorneter at 160°C (320'0
:
(
tsl, minutes
tc'90, minutes
3.0
12.6
3.6
16.8
Properties Cured at
160" (320°@
13 min.
1 7 min.
7.1 (1030)
20.2 (2930)
580
70
5.3 (760)
11.3 (1630)
630
Stress at 300%, MPa (psi)
Tensile slrength, MPa (psi)
Elongation, %
Rebound*, % 22°C
..
;1S?'hI D l 054, cured 30 minutes
i. Ozone Craclcing
Stalic, cracks in mm
Dynamic, cracks in mm
0
0.5-1 O
.
0
0
BR 1220
HAF Black (N-330)
GPF Black (N-660)
Aromatic Oil
ANTOZITE 67P
AGERITE SUPERLITE
Arofene 1055B
Zinc Oxide
Stearlc Acid
AMAX
Sulfur
Total
Rheometer at:
..
55
..
5
1
..
..
10
2
0.8
4
177.8
150°C (300°@
tsl, minutes
t'c (SO), minutes
4.6
14.0
Physical Properties
Cured 18 minutes at 150'C (30O0@
Stress at 300%, MPa (psi)
Tensile strength, MPa (psi)
Elongation at Break, %
Rebound, % 22°C
100°C
SWAT Adhesion, N
18.5 (2680)
24.5 (3550)
405
56
80
577
610
61 1
Table 11
Tfre Apex and Bead Insulation Reclples
TSR 20 Natural Rubber
PLIOFLEX'?500 SBR
HAF Black (N-330)
GPF Black i~-moj
Aromatic Oil
Rosin Oil
VANPLAST R
Stearic Acid
Resorcinol
Hexamethylenetetramine
Zinc Oxide
AGERITE RESIN D
Diisopropylbenzothiazoie sulfenamide
VANTARD PVt
Sulfur
DURAX
Total
Apex
Innertube
100
100
..
70
8
25
-.
..
1
3
3
3
..
1.5
0.2
3
.202.7
Rheometer at 150°C (300°F)
4.2
17.2
Properties cured at
150°C (300°F)
18 min.
Stress at 3W%, MPa (psi)
Tensile strength, MPa (psi)
Elongation, %
Hardness, Shore A
11.8(1710)
13.9 (2010)
120
87
Dynamic Mechanical Properties
I'ibrotester, 100°C (212°F) ASTM 2231
- -
Storage Modulus, MPa (psi)
Resilience, %
Tan delta
..
80
..
tsl, minutes
tc'90, minutes
Table 12
Innertube and Curing Bladder Reclples
20.3 (2940)
63
0.076
..
..
1
5
2
0.5
1
..
Curing
Bladder
100
5
..
55
.5
-.
5
...
-.
10
-
-
204.5
180
oil is shown in U.S. Patent No. 3,031,423
oodyear Tire &Rubber Company
170°C (338°F) 190°C (374°F)
3.0
11.0
8
170" (338-4
4.9 (710)
10.4 (1510)
600
50
3.5
20.0
20
190°C (374°F)
4.6 (700)
12.8 (1850)
720
68