Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Invention
The present invention relates to a pneumatic
vehicle tire having belt plies, sach with axial edge
portions, and a tread portion, which is divided into
two superimposed layers, whereby the belt plies
contain at most 8% plasticizer, and the radially
outermost of the tread portion layers contains at
least 25% plasticizer and is softer or more
resilient than the radially innermost of the tread
portion layers.
Conventionally, such tires contain 30 to 35
plasticizer in the outer tread portion layer, while
the inner tread portion layer has approximately 5%
less plasticizer. The Shore A hardness of the inner
tread portion layer is about 2 to 4 greater than
that of the outer tread portion layer. The outer
belt ply, which adjoins the inner tread portion
layer, conventionally has a Shore A hardness that is
about 7 to 10 greater than that of the inner tread
portion layer. Thus, in the constant increase of
hardness from radially outwardly to radially
inwardly up to the belt plies, the greatest increase
in hardness is at the connecting surface between the
inner tread portion layer and the outer belt ply.
The Shore A hardness of the outer tread portion
layer of known tires of the aforementioned general
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type is between 64 and 69.
A drawback of the heretofore known tires is
that several years after manufacture they exhibit a
plasticizer diffusion from the tread portion into
the rubber mixture that surrounds the load-carrying
cords of the belt. After a long period of use, this
has an adverse effect upon the service life of the
belt structure.
It is therefore an object of the present
invention to protect the belt structure from such an
influence ovsr as a long a period of time as
possible in order to prevent negative impacts upon
the service life.
Brief Description of the Drawing
This ob~ect, and other objects and advantages
of the present invention, will appear more clearly
from the following specification in conjunction with
the accompanying schematic drawing, which is a
cross-sectional view through one exemplary
embodiment of the inventive tire for a passenger
car.
Summary of the Invention
The pneumatic vehicle tire of the present
invention is characterized primarily in that the
inner tread portion layer extends over the axial
edge portions of the belt plies and in these regions
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is thicker than in the middle portion of the tire
between the edge portions, and in that the inner
tread portion layer contains at most 10
plasticizer.
Pursuant to the prevailing understanding, the
inventive approach can be explained by the fact that
the gradual diffusion into the belt plies of -the
plasticizer that is used in the tread portion is
restrlcted. This retards the softening of the belt
plies; in particular, the gradual increase of the
strain paths in the highly stressed tire shoulders,
and the previously accompanying aging, are retarded.
This explanation is supported, among other things,
by the recognition obtained in extensive tests that
the complained-about separations start at the edges
of the belt plies and spread toward the middle.
Thus, the essence of the inventive solution is that
the lower or inner tread portion layar is embodied
as a diffusion restrictor and collection base for
plasticizer, for which reason the inner tread
portion layer itself contains only little
plasticizer, and possibly none at all; the blocking
and collection action over the edges of the belt
plies is particularly defined. As a consequence of
the reduction in thickness of the inner tread
portion layer in the central portion of the tire,
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the ability of the profiled blocks formed in the
outer tread portion layer to move in the central
region, which is particularly important with regard
to the formation of running noise and traction, is
maintained. The inner layer of the tread portion
over the edges of the belt plies is preferably at
least twice as thick as in the central portion of
the tire.
The correct hardness in the edge portions of
the belt plies can be maintained for a particularly
long period of time if the inner tread portion layer
has the same amount of plasticizer as does the
ad~oining outer belt ply.
A particularly great resistance to aging
results if the inner tread portion layer has a Shore
A hardness of at least 70. The hardness of the
inner tread portion layer is preferably between 71
and 74.
Since the inventive construction is
particularly effective in restricting diffusion of
plasticizer into the edge portions of the belt
plies, it is possible to provide the outer tread
portion layer with a greater quantity of
plasticizers. This amount of plasticizer is
preferably great enough that the outer tread portion
layer has a Shore A hardness of no more than 63. In
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this connection, it is advisable for passenger car
tires, taking wear into consideration, not to reduce
the hardness of the outer tread portion layer below
60. For trailers for private use, such as single-
axle trailers, the hardness can be reduced to 57.
Due to the small amount of annual operation that is
involved, these tires up to now have generally
failed as a result of age rather than from wear.
The advantage of the retarded aging in the highly
stressed tire shoulders becomes particularly clearly
evident in these applications while at the same time
reducing the tendency of the trailer to swing as a
result of a more abrupt increase of the lateral
force over the inclined angle.
The best steering precision is achieved if the
inner tread portion layer is at least 7 Shore A
hardness degrees harder than the ad~oining outer
tread portion layer. Pursuant to tests, the dynamic
reliability at the phase boundary between the inner
and outer tread portion layers is adequately
maintained if the dlfference in hardness is not
greater than 20 Shore A degrees.
A particularly advantageous combination of high
resistance to aging, favorable driving conditions,
and low manufacturing cost is achieved for the tires
if the inner layer of the tread portion over the
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edges of the belt plies is at least twice as thick
as in the central portion of the tire and has a
Shore A hardness of at least 70, and if the outer
tread portion layer has a Shore A hardness between
57 and 63, with the inner tread portion layer being
harder than the outer tread portion layer by at
least 7 Shore A degrees.
In order to be able to manufacture the
inventive tires in a particularly economical manner,
it is advisable for the rubber constituents of the
rubber mixtures of the inner and outer tread portion
layers to be the same, i.e. for the polymers and the
proportions thereof to be the same. These two
rubber layers preferably begin to differ only after
the addition of the non-rubber constituents,
especially the plasticizer.
Further specific features of the present
invention will be descrlbed in detail subsequently.
Description of Preferred Embodiments
Referring now to the drawing in detail, in the
illustrated embodiment of a pneumatic vehicle tire
1, a single ply carcass 4 is looped about the bead
cores 2 in the bead portions 3. Disposed over the
carcass 4 in the region of the support surface of
the tire ars the belt ply 5, with its edges 6, and
the belt ply 7, with its edges 8. The thickness of
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the belt ply 7 is indicated in black. The white
points represent the cross sections of the load-
carrying cords 11 of the belt ply 7. In contrast, ~
the belt ply 5, as is the case with the carcass 4, ~ ~-
is merely diagrammatically indicated to facilitate ~ -
illustration. It is precisely with the illustrated
limitation of the number of belt plies to a minimum
(the carcass ply is used to form the triangular
structure that is necessary in order for the belt to
function properly), that it is particularly
important that the ~uality of the rubber of the belt
plies 5, 7 at their highly stressed edge portions 6,
8, in particular with regard to the highly stressed
modulus of elasticity thereof, be effectively
maintained over time to the greatest extent
possible.
Disposed immediately above, i.e. radially
outwardly of, the belt ply 7 are the inner layer 9
and the outer layer 10 of the tread portion. The
inner tread portion layer 9 has a thickness above
the belt ply edges 6, 8, in the shoulder regions, of
3.8 mm, and the central portion of the tire has a
thickness of 1.8 mm. The outer layer 10 of the
tread portion has a thickness in the shoulder
regions of 10.5 mm. The depth of the profiled
grooves is 7.5 mm.
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The Shore A hardness of the outer layer 10 of
the tread portion is set at 62 by adding softeners
or plasticizers. This great degree of softness or
resilience not only increases the riding comfort,
but also results in particularly high coefficients
of friction in the circumferential and axial
directions on wet roads. The following is one
example of a suitable rubber mixture having such a
low hardness for the outer layer 10 of the tread
portion:
Parts per hundred parts
rubber
Natural rubber 70
Polybutadiene 30
HAF black 80
Aromatic plasticizer 50
Ash 3.5
Stearic ~cid 2.5
Zinc oxide 3
Sulfur 1.5
Accelerator 1.5
In the above table, the weight proportions of
the components of the mixture are indicated relative
to 100 parts by weight rubber.
The rubber mixture for the inner tread portion
layer 9 differs from that for the outer tread
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portion layer 10 only in that the black content is
reduced to 70 phr, and the content of the
plasticizer is reduced to 12 phr.
A diffusion of the plasticizer out of the outer
tread portion layer lO into the belt rubber is
obstructed by the inner tread portion layer 9, and
in particular especially in the region of the highly
stressed belt ply edges 6, 8 via the particularly
thick construction of the inner layer 9 at that
location.
In addition to the advantage that the strength
of the belt is maintained over a greater time
interval, the present invention also makes it
possible to use particularly coft or resilient tread
mlxtures, and hence leads to further increa~ed
safety on wet roads.
The present invention ls, of course, in no way
restrlct~d to ths speciflc disclosure of the
specification and drawing, but also encompasses any
modifications within the scope of the appended
claims.
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