CA1167213A

CA1167213A - Process for the production of a shoe sole having a thread-like pattern from a closed-cell foamed cross linked ethylene-vinyl acetate copolymer (eva)

Info

Publication number: CA1167213A
Application number: CA000399362A
Authority: CA
Inventors: Rudiger Voll; Walter Hoche; Gerhard Graab
Original assignee: Carl Freudenberg KG
Current assignee: Carl Freudenberg KG
Priority date: 1981-04-08
Filing date: 1982-03-25
Publication date: 1984-05-15
Also published as: GB2101922A; FR2503624A1; ES8304479A1; GB2101922B; DE3114105C2; BE891480A; ATA138182A; IT1147623B; FR2503624B1; DE3114105A1; IT8247809A0; ES511225A0; AT378734B; JPS57177702A

Abstract

ABSTRACT OF THE DISCLOSURE

A process is disclosed for the production of a shoe sole bearing a relief-like tread pattern, made from a closed-cell foamed cross linked ethylene/vinylacetate copolymer in which the material that is used is shaped into a slab, cross-linked, and then expanded by activation of a foaming agent contained therein, subsequently tempered, and the slab split into several partial slabs in at least one process step. The surface of each partial slab is then heated on one or both sides to a temperature of at least 120°C, maximum 200°C, provided that the temperature at a depth that corresponds to the depth of the desired pattern does not exceed 75°C, whereupon the surface so heated is configured in a relief-like pattern by the use of a stamping tool that has been heated to a temperature of 65 to 80°C.

Description

~'7~3 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) _ _ _ . _ 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 ~ 2 --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 _ 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 %

, i7~a3 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 : `~
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.

5 _ ~: ., ' ' ' ` ~ . ': ' ' ' 7;~3 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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 temperature of at least 120°C, up to a maximum value of 200°C, provided that the temperature at a depth that corresponds to the depth of the desired pattern does not exceed 75°C; and each surface so heated is shaped by the impression of a stamping tool heated to a temperature of 65 to 80°C.

2. A process according to Claim 1, characterized in that the surface of the partial slab can be ground or polished prior to heating.

3. A process according to Claim 1 or Claim 2, characterized in that the mechanically treated surfaces can be coated prior to heating with a 0.1 to 0.3 mm thick continuous film consisting of an elastomer material.