Note: Descriptions are shown in the official language in which they were submitted.
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Field of Invention
This invention relates to pneumatic tires and parti-
cularly relates to pneumatic tires having a tread of a
cap-base construction.
Back~round of the Invention
Pneumatic tires are ge~erally constructed with the
application of outer, extruded, homogeneous tread stock
over a supporting carcass of a plurality of rubberized
fabric plies. The tread extrusion is generally of a
compounder rubber, sometimes referred to as a rubber
compound, that, when cured, provides the tread of the
tire with a high degree of resistance to tread wear
during its use on a vehicle, preferably without promoting
an excessive heat build-up in the tire itself. The
excessive heat build-up would be detrimental to the
durability of the tire, particularly its supporting
carcass.
~lternately a rubber tread of a cap-base construction
has been proposed (U S Patent 3,157,218). There the
outer, ground contacting portion of the tread, or cap,
was composed of a rubber designed to provide conventional
resistance to tread wear, good traction ability. The
outer cap portion of the tread was adhered to an inner
tread portion, or base, disposed between the cap and the
supporting carcass. The base rubber composition was
compounded with the intention of diminishing heat build
up in the overall tread. The patent disclosure achieved
its cap-base construction purpose by utilizing a substan-
tial amount of polybutadiene rubber in both the cap and
the base. It should be pointed out that its disclosure
primarily related to the reduction of tire failures
such as rib tear and groove cracking, in truck tires
of the bias ply type construction, particularly since
radial construction tires were not widely used at that
time.
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However, methods of reducing rolling resistance of
a tire to effect an energy savings while maintaining
adequate traction yet keeping heat build-up to acceptable
limits are still sought after.
Disclosure and Practice of the Invention
In accordance with this invention, a pneumatic rubber
tire is provided with a radial ply carcass having a tread
across its crown composed of (A) an outer rubber cap
composition of, based on 100 parts by weight rubber,
(1) about 85 to about 100 weight percent butadiene/styrene
rubber and, correspondingly, about zero to 15 weight
percent polybutadiene, (2) about 50 to about 100 weight
percent butadiene/styrene rubber and, correspondingly,
about 0 to about 50 weight percent cis 1,4-polyisoprene
or (3) about 50 to 80 weight percent butadiene/styrene
rubber, about 10 to 40 weight percent polybutadiene and
about 10 to 40 weight percent cis 1,4-polyisoprene; said
rubber reinforced by containing about 50 to 100 phr of
a high reinforcing carbon black and characterized by
having a hot rebound value in the range of about 55 to
about 70, according to ASTM 1054-66 and (~) a base com-
position disposed between said outer cap and said carcass
of the tire as an adherent interlayer comprised o~ a
rubber composition of. based on 100 parts by weight
rubber. (1) about zero to 30 weight percent butadiene/
styrene rubber, (2) about 20 to about 50 weight percent
natural rubber, and (3) about 40 to about 70 weight
percent polybutadiene; said rubber mixture reinforced by
containing about 30 to 70 phr semi-reinforcing carbon
black characterized by having a hot rebound value in
the range of about 75 to about 90 according to ASTM 1054-66.
The cap-base design was used to decrease heat genera-
tion or heat build-up in a tread which had been compounded
for improved, or reduced, rolling resistance. It is
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generally understood that changing a tread compound to
reduce its rolling resistance will also generally result
in a loss of traction and a decrease in heat build-up.
The cap-base concept of this invention is designed to
allow a reduction in rolling resistance without an
appreciable or substantial loss in traction nor increase
in heat build-up.
Reference to the drawing shows a radial tire 1 with
a tread 2 adhered to a supporting carcass 3 composed of
a plurality of rubberized fabric plies 4. The tread 2
is composed of an outer cap 5 which contains the lugs
and groove matrix 6 adhered to the inner base 7.
The thickness of the cap 5 and the base 7 portions
of the tread 2 construction can vary over a reasonably
considerable range. However, in general, for the green
and uncured tire the base portion should be at least
about 40 mils in thickness, and preferably in the range
of about 40 to about 120 mils. Upon shaping and curing
the tire in the mold, the previously extruded rubber
tread flows into the face of the tread contour of the
mold and the interface between the base and the tread
assumes a somewhat undulating shape 8. In this manner,
the base tends to be thicker in the lug configuration of
the tread 9 and thinner in the groove configuration of
the tread 10. In this final condition the cap portion
of the tread should be sufficiently thick to provide at
least about 20 mils of rubber thickness below the bottom
of the groove portion of the tread and preferably a
thickness in the range of about 20 to about 100, more
preferably about 50 to about 100 mils in this area.
Clearly it is desired that the base does not extend to
the outer wearing surface of the tread lug because it
would be considerably faster wearing than the cap composi-
tion. However, if the tire is of a very good wearing
quality or,in other words, exhibits good tread wear
characteristics, then in some cases the base could strike
through a portion of the wearing surface and still provide
a tire with overall good wearing characteristics.
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In the description of this invention, it has been
pointed out that different types of carbon black are to
be used for the cap as compared to the base portions of
the tread.
Representative of the high reinforcing blacks utilized
for the outer cap portion of the tread are those having
various ASTM designations such as NllO, otherwise sometimes
known as Super Abrasion Furnace Black (SAF), N220, some-
times otherwise known as Intermediate Super Abrasion
Furnace Black tISAF), and N330, sometimes otherwise known
as High Abrasion Furnace Black (HAF).
The semi-reinforcing carbon blacks utilized for the
base portion of the tread have ASTM designations such
as N660, sometimes otherwise known as General Purpose
Furnace Black (GPF), N762, sometimes otherwise known as
Semi-Reinforcing Furnace Blacks (SRF) and N550, sometimes
otherwise known as Fine Extrusion Furnace Black (FEF).
The practice of this invention is further illustrated
by reference to the following examples which are intended
to be representative rather than restrictive of the scope
of the invention. Unless otherwise indicated, all parts
and percentages are by weight.
Example 1
Rubber compositions were formulated to prepare the
cap and base portions of an extruded tread stock,
according to the following general recipe shown in Table 1.
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Table 1
-
Components Tread Cap Formulation Tread Base Formulation
Polybutadiene
(oil extended) 12.50 Natural Rubber 40.00
Butadiene/styrene
rubber (oil Polybutadiene
extended) 123.75 60.00
Carbon black
(ISAF) 80 (FEF) 50
Waxes 4.00 Waxes 1.00
P,rocessing oil 12.00 Processing oil 15.00
Antioxidant 2.00 Antioxidant 3.00
Accelerator 1.00 Accelerator 0.50
Zinc oxide 3.00 Zinc oxide 3.00
Sulfur 1.50 Sulfur 2.25
The formulations cf Table 1 were mixed separately,
extruded together, co-extruded in one piece, and applied to a
radial ply rubber tire carcass as a cap-base tread and
accompanying sidewalls having the same formulation as
the base. The tire was then molded through shaping and
curing under pressure to form the resulting pneumatic vehi-
cular tire (size GR 78-15).
Samples of the cured cap composition of the tread were
analyzed and found to have a hot rebound value of about
62 according to ASTM 105~-66.
Portions o~ the cured base compound were tested and
found to have a hot rebound value of about 78 according
to ASTM 1054-66.
The pneumatic tire was mounted on a rim, inflated
and driven by 67.2 inch (271 cm) diameter fly wheel for
the purpose of evaluating its rolling resistance. The
rolling resistance factor was tested by adjusting fly wheel
torque and measuring the torque by transducer in the drive
line or fly wheel. The test tire's rolling resistance was
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compared to that of a control tire u-tilizing a tread
compound increasing to the experimental tire's cap
formula-tion. In this test, the experimental tire had
about 5% less rolling resistance than the control tire.
In Table 2 the results of various test values are
shown, particularly the hot rebound values, rolling resis-
tance and wet and dry traction. It is important to
appreciate that Table 2 shows that the enhanced rolling
resistance of the cap/base tire was achieved without
sacrificing the wet and dry traction factors.
Table 2
Ph,ysical Test Value Cap Base Tire Control Tire
1. Hot Rebound Value Tread 62 Base 78 Tread 62
2. Rolling Resistance
(pounds)l~ 105(5% more efficient) 100
3. Wet tractionl 100 100
4. Dry tractionl 100 100
lUsing a value determined for the control tire to be 100
2As determined by testing the pneumatic tire in its
inflated state as mounted on a rigid, centered, rim under
about a 80% of rated load against a 67.23 inch diameter
fly wheel as measured in pounds.
While certain representative embodiments and details
have been shown for the purpose of illustrating the invention,
it will be apparent to those skilled in this art that various
changes and modifications may be made there~ without departing
from the spirit or scope of the invention.
