Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A Process for ~he Production of a Shoe Sole Having a Tread-Like Pattern from a
Closed-Cell Foamed Cross-Linked Ethylene-vinyl Acetate copolymer (EVA)
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It is already known that foamed ethylene-vlnyl acetate copolymers can be
used for the production of shoe soles. The vlnyl acetate content of the
polymer material that is used for this purpose a~ounts to approximately 14-28%
and materials of this kind are distinguished by particularly good flexibil~ty,
low weight, good resistence to wear, and good amenability to colouring.
O a special significance for most of the aforementioned properties is the
e~tremely finely formed~ homogeneous pore s~ructure. The shaping a~d
processing of materials o this kind must thus always be carried out in such a
manner that disadvantageous efEects or changes in the pore structure are
avoided as far as possible.
German patent publication DOS 16 ~5 383 describes a process for the
production of a non-skid shoe sole from rubber, having a pore structure such
that a controlled quantity of the rubber mixture that corresponds to the size
of the sole is placed in the sole mold, this being larger than the volume of
the finished sole during this process. The rubber mlxture is subsequently
distrlbuted withln the hollow mold by the raislng of the base plate and during
6ubsequent lowering of the base plate the mixture is after some delay foamed
from the completed mass. The finished sole can then be removed from the mold
whereupon it is characterized by a very fine regular pore structure. This
process could not be developed commercially however, since the sequence used
for the indlvidual steps of the process is extremely time consuming and
precludes any form of mass production at competitive prices. The use of this
process in the production of a shaped so]e from EVA is thus impossible.
British published application 2011243B descrlbes a process for the
production of a shaped portlon of a shoe having a surface that bears a
relief-type pat~ern consisting of a closed-cell foamed cross-linked
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polyolefin. The Eirst step in this process ls the productiorl of a foamed blank
from the material that is used, this belng of specific dimensions, and being
given the desired shape by the application of a combined thermo-pressure
treaement in a hollow mold. This leads to a very great concentration of the
foam blank throughout its whole cross-section and in turn to an increase in the
specific weight and some impairment of flexlbility. The pore size and pore
distribution vary very greatly depending on the particular degree of
concentration, this being particularly disadvantageous in relation to wear
resistance. For this reason, a shoe sole cannot Se produced in this manner.
The processing of EVA is not addressed in either of these references.
This invention undertakes the task of developing a rational procedure for
the mass production of a shoe sole shaped in such a manner as to have a
tread pattern, the sole being produced from a closed cell-foamed cross-linked
~VA, the process ensuring a homogeneous and regular fine pore structure, and
thus the production of a shoe sole that is characterized primarily by very good
flexibility at low weight and which is particularly resistant to wear.
The present invention provides a process for the production of a shoe sole
bearing a relief-like tread pattern, and being of a closed-cell foamed
cross-linked ethylene-vinyl acetate copolymer (EVA), characterized in that the
material that is used is shaped into a slab, cross-linked, and then expanded by
the use of a foaming agent contained therein and then tempered; the slab is
then separated into several partial slabs by at least one process step; the
surface of each partial slab is heated on one or both sides to a tempera~ure of
at least 120C up to a maximum value of 200C, provided that the temperature at
a depth that corresponds to the depth of the desired pattern does not exceed
75C; and each surface so heated is shaped by the impression or a stamping tool
heated to a temperature of 65 to 80C. Preferably the surface of the partial
slab can be ground or polished prior to heating or coated to a thickness of 0.1
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to 0.3 mm with an elastomeric ~aterial, for exal~ple elastomeric polyurethane.
The process proposed by this lnvention makes it possible, in contrast to
other processes, to structure the even surface of the foam blank which i8 used
without any noteworthy increase in the specific weight, and to provide this
structure in the manner of a relief pattern. The original homogeneous and
regular fine pore structure is retained, this being particularly advantageous
in relation to the characteristics required for a shoe sole.
The slabs that are required for this process can be shaped in conventional
multi-stage presses from closed-cell foam cross linked ~VA and then
vulcanized. The homogeneous addition of a foaming agent having a decomposition
point beneath the vulcanizatlon temperature results, after the multi-stage
press has been opened, in a spontaneous lncrease in volume that is
approximately th~ee-fold and in the desired fine pore structure. The surfaces
are smooth and even, and thus, it is not necessary to carry out any additional
mechanical processing prior to subsequent preparation by splitting. Because of
the closed pore structure the surfaces are completely closed. Th2 pore
diameter varies between 30 and 100 microns (at an average value of 60 microns).
As a result of mechanical splitting of the slabs into several partial
slabs, the pores that are located in these areas, and which are at first
closed, are opened. This results in a velvet-like configuration that differs
signiflcantly from that of the surface that is oriented towards the pressed
; plates during the vulcanization process. ~owever, this change is not connected
with any disadvantageous change ln the mechanical characteristics and it is
thus sufficient to arrange the split slabs according to ehe dictates of fashion
or taste. This, of course, does not preclude the fact that the surfaces can be
ground or coated with an elastomer material in order to standardize their
appearance.
It is e8sential that the slabs be heated in such a manner that the
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required temperature gradient is obtalned across the area of the pattern
depth. Thi9 can be realized most simply by the use of a high energy infra-red
emitter. The cycle time for the subsequent shaping of the surface in a
stamping machine i8 relatively short and amounts to only a few seconds. No
noteworthy re-shaping of the stamped pattern could be observed after removal of
the stamping tool, and the shaped partial slabs contained in this manner can be
stamped out so as to produce shoe soles of the desired shape and size by the
use of conventional processes.
It is also possible to shape both sides of the partial slabs by the use of
lO a suitable process, for example, in order to provide improved adhesion during
the cementing on of the inner sole. In general, however, a procedure of this
sort i9 not necessary and it is sufficient to provide a tread pattern on only
one side of the sole.
The present invention will be described in greater detail below, this
being done on the basis of an example:
Example l
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The components listed below were placed in an internal mixer and blended
into a homogenous mass for approximately 8 min at a temperature of 120C~ The
percentage details to the particular proportion of the individual components
are related to the total welght of the finished product.
Ethylene-vinyl acetate copolymer having a
vlnyl acetate content of 20% 60 %
Silicic acid, pracipitated 17 %
. Calcium carbonate, coated 13 %
Zinc soap of a fatty acld used as a processing
substance 2.5 %
Azodicarbonamide (used as foaming agent) 1.7 %
Iron oxlde pigment, used as a dye 5.4 %
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The above mlxture was removed from the internal mixer after the completion
of homogenization and treated further in a friction roll mlll. The composition
was then made up to 100~ by the addition of 0.4% by weight of an ~ bis-
(t-butyl-peroxy)-diisopropylbenzene used as a cross-llnking agent.
The pre-product that was obtained in thls way was shaped into a slab that
was 203 cm thick, 82 cm long and 58 cm ~idea Thls size corresponded exactly to
the size of the recess in a shaping mold, preheated to a temperature of 170
into which the slab was subsequently placed. The shaping mold was then
immediately closed at a pressing pressura of at least 70 kp/cm2 and the slab so
obtained was heated for 15 min at the above temperatureO This resulted ln
vulcanization and activation of the foaming agent contained in the slab.
After the explry of the abovP mentioned period of time, the shaping ~old
was opened and the foaming agent caused the slab to expand to a 120 x 85 by
3.4 cm format because of the effects of the above temperature. The slab ~as
then placed in a tempering oven, preheated to a temperature of 80C, and
vulcani~ation continued for a period of six hours. The specific weight of the
slab after cooling was 0.35 g/cm3.
The slabs obtained in this manner were subsequently split into 6 mm thick
partial slabs with the help of a band saw. A pattern measuring 30 x 20 cm was
stamped out of such a sub-plate and irradiated for 2.5 seconds by an
infra-red-dark radiator having a power output of 6.5 W/cm2. This
resulted in a temperature of 130C at the surface and a temperature of 70C at
a distance of 3 mm beneath the surface.
The slab so heated was immediately passed to a shaping tool which was used
to produce a tread-like pattern in the surfaceO The stamping tool used for
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this proces3 consisted of steel and was preheated to a temperature of 80C.
The stamping consisted of columnar depressions having a diameter of 5 mm and a
depth of 3 mm arranged at intervals of 12 mm from each other.
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A pressure of 6 kp/cm2 was used and the stamp remained in the press for
10 seconds. Thus the surface was reshaped to correspond to the configuration
of the stamping tool and was characterized by a particularly good sharpness.
The specific weight after cooling was 0.36 g/cm3 and was thus virtually
unchanged.
Example 2
The procedure described in Example 1 was repeated with the use of a
stamping tool having a relief-like stamping surface that was configured in the
form of truncated pyramidal depressions. These recesses abutted immediately
onto each other and with 3 ~m depth for an edge length of 4 mm. This pattern
could al80 be transferred to the pressed piece to provide a sharp impression~
The unit weight was to 0.36 g/cm3 and was thus virtually unchanged. The pore
structure was characterized by an almost unchanged and excellent homogeneity.
A partial plate measuring 30 x 20 cm according to Example 1, was, unlike
the case described therein, placed in a heating chamber so as to produce a
special temperature gradient and heated in this chamber to a temperature of
130C that was equally distributed throughout the whole cross-section. The
slab was subsequently placed in a ~haping tool and the surface was configured
by using the ætamping tool described in Example 1, under the same conditions as
described therein~ The configuratlon obtained thereby was characterized by a
very poor degree of sharpness and the unit weight was increased to an
undesirable value of 0.56 g/cm3. Neither flexibility nor wear performance
correæponded to the properties required of a good shoe 801e material.
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