Footwear

Abstract

Footwear, such as a boot or shoe, having an insole comprising an extruded foamed resin sheet wherein the resin is an ethylene-vinyl acetate copolymer or a blend thereof with polyethylene and has a polymerized vinyl acetate content of 2-17 percent by weight. The insole has a substantially uniform thickness, a substantially closed cell structure, and a density of 17-40 pounds per cubic foot. Preferably, the insole is treated with an electric discharge prior to being bonded to the other components of the article of footwear. The insole can be bonded to these other components by any suitable means, preferably by the use of an adhesive.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to footwear and more particularly relates to a novel insole comprising an extruded foamed resin sheet and to an article of outer footwear, such as a boot or shoe, having such an insole.

2. Description of the Prior Art

In the construction of outer footwear such as boots and shoes, the upper is normally fastened to an assembly comprising an insole and a heel board while the assembly is held on a last, a sole and heel are then fastened to the underside of the assembly, and the parts of the insole and heel board visible within the shoe are finally covered by a lining or sock. The heel board is usually made of a relatively rigid material such as fiberboard, but the insole needs to be more flexible. The nature of the insole is indeed of extreme importance in determining the comfort and life expectancy of the shoe as a whole, since it serves to hold the upper to the sole and also to cushion the foot of the wearer. As has been mentioned, the insole needs to be flexible in the interests of comfort, but another important requirement that generally militates against flexibility is the need for good "recovery," i.e., ability of the shoe to regain its original shape after being subjected to deformation in use. Repeated flexure and impact when the shoe is in use result in deformation of the shoe as a whole, and, unless a shoe has the ability to recover overnight from the effects of being worn for a day, it will give only a few months' wear before generally deteriorating, becoming ill fitting, uncomfortable, and unattractive in appearance.

Hitherto, it has been usual to employ for the production of insoles such materials as cardboard, paper pulp, leather pulp, leather, and similar materials, usually treated with a resin in order to afford some resistance to moisture. However, such materials suffer from the very real disadvantage that they are not as flexible as is desirable, and, even when treated with a resin, they absorb moisture either from outside the shoe or from a perspiring foot. When moist, they are readily deformed and have a tendency to disintegrate or delaminate, thus admitting water through the edge of the insole, which in turn leads to very rapid disintegration of the structure of the shoe as a whole. Perspiration can also cause some of these materials to rot. Some synthetic materials have been tested as insoles but have been found to be defective either because of insufficient flexibility or because of a tendency to "spread," i.e., stretch so that the necessary shape retention is not obtained. It can therefore be seen that the choice of an insole material is not simple matter and that the materials that have hitherto been proposed fall well short of the ideal.

SUMMARY OF THE INVENTION

An object of the invention is to provide a novel insole having good flexibility, resistance to moisture, and recovery.

Another object is to provide an article of footwear having such an insole.

These and other objects are attained by (1) forming an insole comprising an extruded foamed resin sheet having a substantially uniform thickness of 0.04-0.2 inch, a substantially closed cell structure, and a density of 17- 40 pound per cubic foot from a resin which is an ethylene-vinyl acetate copolymer or a blend thereof with polyethylene and which contains 2-17 percent by weight of polymerized vinyl acetate, (2) bonding the insole to a relatively rigid heel board to form an assembly, and (3) bonding the other components of a shoe or the like to said assembly to form an article of footwear.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are given to illustrate the invention and are not intended as a limitation thereof. Unless otherwise specified, quantities mentioned are quantities by weight.

EXAMPLE I

Dry blend 98 parts of an ethylene-vinyl acetate (88:12) copolymer resin in granular form with 2 parts of anhydrous calcium acetate, and feed the resulting mixture into the hopper of a screw extruder having a barrel with a nominal diameter of 1.5 inches. Inject nitrogen under pressure into the molten mixture in the extruder barrel, and extrude the resulting foamable composition at a temperature of 130.degree. C. into the atmosphere through a slit die having a length of 6 inches and a width of 0.025 inch. The product is a soft foam strip having a thickness of 0.08 inch, a density of 24 pounds per cubic foot, and a fine closed cell structure.

Subject both sides of the sheet to the action of an alternating electric discharge of 10,000 volts and 3 kilocycles per second by passing the sheet between the two electrodes spaced 0.15 inch apart, each part of the sheet being exposed to the discharged for 0.1 second. Cut the sheet into pieces in the shape of insoles suitable for ladies' casual shoes, and adhere each insole to a heel board made of a cellulose fiberboard known as "shank board" by means of a neoprene adhesive at 70.degree. C. for 5 seconds under pressure. Then adhere the resulting assembly to a leather upper by means of a neoprene adhesive applied at 70.degree. C. for 5 seconds, and finally attach a rubber composition sole and heel to the assembly, also by a neoprene adhesive.

The resulting shoe has a very flexible and comfortable sole. The insole survives extended wearer trials without failure, whereas a pulped leather insole in an otherwise identically constructed shoe disintegrates after only a few months. The resistance to moisture of the insole of the invention is excellent. The recovery ability of the shoe of the invention is infinitely superior to that of the control shoe, the latter being misshapen and ill-fitting after only 3 months whereas the shoe of the invention retains its good shape and fit after 12 months of use.

Trials of other women's and children's shoes according to the invention show no case of failure due to insole cracking, the trials always being ended by failure of a different component of the shoe.

EXAMPLE II

Repeat Example I except for substituting an ethylene-vinyl acetate (96:4) copolymer for the ethylene-vinyl acetate (88:12) copolymer. The resulting shoe is similarly flexible and comfortable and is just as successful in passing wearer trials. The insole has the additional advantage of reduced sensitivity to the effects of heat during application of the neoprene adhesive, thus permitting the use of slightly more severe bonding conditions if required.

The resin used in the practice of the invention preferably contains from 3 or 7 to 15 percent by weight of polymerized vinyl acetate, e.g., 3-5 percent by weight. As demonstrated in the Examples, excellent results are obtained employing resins containing, respectively, about 4 percent and about 12 percent by weight of vinyl acetate. Preferably, the resin is in ethylene-vinyl acetate copolymer, but it can be a blend of polyethylene with such a copolymer. When it is a blend, the proportion of vinyl acetate in the copolymer can, of course, be greater than 17 percent by weight, e.g., up to 30 percent or even 45 percent weight, sufficient polyethylene being blended with the copolymer to reduce the vinyl acetate content of the resin as a whole to a value within the required range. The polyethylene can be of the "low-pressure" type made using a chromium oxide or organometallic catalyst system, but it is preferably a "high-pressure" polyethylene, e.g., a polyethylene having a density of 0.90-0.94 grams per cc. Ethylene and vinyl acetate are usually the only monomers polymerized into the resin, but a small proportion not exceeding 3 or 5 percent by weight of the total, of a third monomer such as propylene or methyl methacrylate can be polymerized into the resin if desired.

The sheet has a substantially uniform thickness, i.e., the thickness does not vary by more than 15 percent. Preferably, any thickness variation is not greater than 5 percent. The type of footwear being made determines the choice of insole thickness ordinarily, thinner insoles usually being needed for children's and ladies' shoes than for men's boots or shoes. Preferably, the thickness is from 0.05 to 0.1 or 0.15 inch; very often 0.06-0.085 inch is particularly suitable.

The foamed sheet has a substantially closed cell structure, i.e., the majority, normally at least 90percent, of the cells are closed. Preferably the majority, and more preferably at least 75 percent, of the cells have a diameter of 0.004-0.04 inch e.g., 0.01-0.03 inch.

The density of the sheet is very often 20-40 pounds per cubic foot, preferably 22-30 pounds per cubic foot; and a density at the lower end of this range; e.g., about 24 pounds per cubic foot, is usually most suitable. The sheet may have a nonfoamed skin on each side, but this is not usually thicker than about 0.002 inch.

The foamed sheet is an extruded one and is normally made by extruding a foamable mixture of the resin and a suitable blowing agent under pressure through a die into a zone of lower pressure (usually the atmosphere) so that the expansion of the blowing agent and foaming of the resin take place. Usually a slit die is used, but it is also possible to employ an annular die to extrude a tube of foamed resin that is later slit longitudinally and opened out into a flat sheet. When a slit die is used, it can have a flared outlet if desired, and a pair of rollers can be placed in front of it so that the extruding sheet passes between them; such rollers assist in the production of a smooth flat sheet free of thickness variations and corrugations.

As indicated above, any suitable blowing agent can be used, but the preferred blowing agents are those which are gases or vapors under normally atmospheric conditions. Volatile liquids can also be used. In may cases, the blowing agent is one that is normally gaseous but which, while under pressure before extrusion, will be present in solution in the molten or semimolten resin. Exemplary of volatile blowing agents that can be used are lower aliphatic hydrocarbons such as ethane, propane, butane, pentane, etc., lower alkyl halides such as methyl chloride, trichloromethane,1,2-dichlorotetrafluorethane, etc.; and inorganic gases such as carbon dioxide or nitrogen. Nitrogen and the lower aliphatic hydrocarbons, especially butane or isobutylene, are preferred; and a mixture of nitrogen and a lower aliphatic hydrocarbon is often particularly useful. The blowing agent can also be a chemical blowing agent, e.g., a bicarbonate such as sodium bicarbonate or ammonium bicarbonate, or an organic nitrogen compound that yields nitrogen on heating, such as dinitrosopentamethylenediamine or barium azodicarboxylate. In the case of a liquid or readily liquifiable blowing agent, 3-30 percent, especially 7-20 percent, based on the weight of resin, is often a suitable proportion. When the blowing agent is permanent gas, it is more convenient to consider relative volumes at standard temperature and pressure. For example, the use of 0.5-5 parts, preferably 1-2 parts, by volume of nitrogen at S.T.P. in conjunction with 1 part by volume of polyethylene gives excellent results. The use of amounts of blowing agent at a lower end of the above ranges often results in the production of a thicker sheet.

The blowing agent is sometimes employed in conjunction with a nucleating agent, which assists in the formation of a large number of fine cells. A wide range of nucleating agents can be employed, including finely divided inert solids such as silica or alumina (optionally in conjunction with zinc stearate), or small quantities of a substance that decomposes at the extrusion temperature to give a gas can be used. An example of the latter class of nucleating agents is sodium bicarbonate, optionally in conjunction with a weak acid such as tartaric or citric acid. Boric acid, calcium acetate, calcium propionate, and calcium benzoate are also excellent nucleating agents. A small proportion of the nucleating agent, e.g., up to 5 percent by weight of the resin, is usually effective.

The resin can also contain such additives as coloring agents, antioxidants, stabilizers, lubricants, etc., if desired. For example, it is often desirable to color the resin when it is to form part of a sandal, the construction of which renders the edge of the insole visible as the welt. A clay or other brown pigment is very often suitable for this purpose.

According to a preferred feature of the invention, the sheet is subjected to the action of an electric discharge. This has been found not only to improve the adhesion of hot melt and other adhesives to the insole, but also surprisingly to reduce undesirable adhesion of the insole to any metal platen or press foot employed in assembling the shoe. A voltage, preferably alternating, above 3,000 volts, preferably 5,000-30,000 volts, is usually suitable, especially one having a frequency of 1,000-100,000 cycles per second. It is preferably a corona discharge but can be a spark or spray discharge. Preferably, both sides of the sheet are treated, with the electrodes situated, e.g., 0.005-0.2 inch away from the surfaces; and an application time of 0.003-1 second, especially 0.01-0.5 second, is generally satisfactory.

The production of the articles of footwear can follow normal practice, although it is necessary to ensure that the degree of exposure to elevated temperatures, e.g., when using a hot melt adhesive, is not sufficient to cause collapse of the foam structure. This can generally be arranged where the necessary bonding between insole and other components of the shoe is achieved by means of a cold or hot melt adhesive or with a polyurethane or neoprene adhesive, or even by injection molding of a plasticized polyvinyl chloride sole and heel, provided that contact with elevated temperatures is maintained for as short a time as possible. It is, for instance, surprising that a polyamide or polyester hot melt adhesive can be used at 240.degree. C. with an insole material having a softening temperature of only 120.degree. C. or even less, if the period of time for which the insole is exposed to this temperature is not more than 5 seconds. Even better results in this respect can be obtained if the resin of the insole material contains a proportion of vinyl acetate at the lower end of the range, e.g., 3-7 percent or 3-5 percent by weight. Any metal platen or press foot maintained in contact with the insole during the process should normally be at a temperature lower than the softening point of the resin. Rubber vulcanizing is not generally a suitable method because of the excessive temperature/time cycles involved. Traditional methods can such as stitching or nailing can, of course, be employed.

The excellent flexibility of the insoles of the invention renders them particularly useful as components of ladies' casual shoes and children's footwear, but they can also be very well employed in, e.g., men's shoes or boots, ladies' court shoes, ladies' fashion boots, or sports footwear for either sex.

It is obvious that many variations can be made in the products and processes set forth above without departing from the spirit and scope of this invention.

Claims

What is claimed is:

1. A process which comprises extruding a foamable resin composition wherein the resin is an ethylene-vinyl acetate copolymer or a blend thereof with polyethylene and has a polymerized vinyl acetate content of 2-17 percent by weight through a die into a zone of lower pressure to form a foamed sheet having a substantially uniform thickness of 0.04-0.2 inch, a closed cell structure wherein at least 90 percent of the cells are closed, and a density of 20-40 pounds per cubic foot, subjecting at least one side of the sheet to the action of an electric discharge for 0.01-0.5 second with electrodes situated 0.005-0.2 inch away from the surfaces, the discharge being an alternating voltage of 5,000-30,000 volts with a frequency of 1,000-100,000 cycles per second, and cutting the sheet into at least one insole-shaped piece.

2. The process of claim 1 wherein the insole-shaped piece is bonded to a heel board to form an assembly.

3. The process of claim 2 wherein an upper is bonded to the assembly and a sole and heel are then bonded to the underside of the assembly to form an article of footwear.