Method For Manufacturing A Synthetic Leather Base

Abstract

A needled sheet of synthetic or artificial fibers is bonded by a polymerizable binder. The method comprises four successive continuous phases, to produce a thick, dense needled sheet. A first phase consists of preparing two non-woven primary sheets from fibers of fine denier, at least to a major extent unshrinkable, having a high level of curl and a good curl permanence. A second phase consists of needling separately the two primary sheets. A third phase consists of superposing the two thus needled primary sheets and a fourth phase consists of subjecting the sheet thus formed to intense needling. This sheet once bound is then subjected to splitting operations dividing it into layers of the desired thickness, which layers can then be buffed individually.

Description

The invention relates to methods for the manufacture of a synthetic leather base, which consists in preparing a thick and dense needled batt or sheet from synthetic or artificial fibers and in binding this sheet by a polymerisable binder and in then dividing it into several layers which can if necessary be buffed.

It is known that such a synthetic leather base can especially receive a finish or a coating to constitute a shoe upper or similar product. A similar synthetic leather base, produced with a mixture of suitable fibers, can result in a range of new articles usable for the inner lining of shoes.

In the methods and installations known hitherto, the preparation of the thick and dense needled sheet was done by discontinuous operations by superposing primary batts of limited lengths which have to be passed successively into the same needlers.

It is an object of the invention to provide a method for manufacturing a synthetic leather base which overcome the drawbacks mentioned above whilst being simple and economical.

Moreover, in the previously known methods, the fibers composing the needled sheet were generally constituted, to a relatively large extent, of polyester fibres shrinkable or retractable by heat so as to enable a shrinkage operation to be effected between the preparation of the needled sheet and the bonding. These methods lead to articles which had generally a good "hand" but of which the physical characteristics were inferior to those of natural leather. There was noted, especially, a mediocre breaking strength, too great a deformability as well as a low resistance to bursting and to repeated flexing. In addition, the desirable homogeneity was not realized, the properties of the product in the "machine" direction and in the "cross" direction showing often considerable differences.

It is a further object of the invention, in certain cases, to overcome the latter disadvantage.

The method according to the invention is characterized by the fact that it is continuous and comprises four successive phases, to produce a thick needled sheet of high density, namely a first phase consisting of preparing two non-wowen primary sheets from fibers of fine denier, at least a major proportion of which are substantially non-shrinkable and have a high curl or crimp ratio and a good curl permanence, a second phase consisting of needling the two primary sheets separately, a third phase consisting of superposing the two thus needled primary sheets, and a fourth phase consisting of subjecting the sheet thus formed to intense needling, after which a curable binder is applied to this sheet the binder is cured and the bonded sheet is subjected to splitting operations dividing it into layers of the desired thickness, which layers can then be individually buffed.

The needling phase of each primary sheet preferably comprises continuously passing each sheet through a pre-needler then discontinuously passing each sheet through a needler.

Preferably, the operation of binding is effected by coagulation of a synthetic latex in situ.

The method according to the invention lends itself particularly well to continuous manufacturing in large quantities, under particularly economical and uniform conditions.

In addition, the very extensive needling effected according to the invention orients vertically (that is to say in the direction of the thickness) a large number of fibres and there is thus obtained an article having a good "hand" and a good "fold." In the usual methods, on the contrary, needling is of low density, the retraction completing the arrangement of certain number of fibers in the vertical direction; too considerable a needling would give, in fact, after shrinkage, an article having a consistency similar to that of cardboard, which would be a defect for a synthetic leather base.

The production of a thick and dense needled sheet, rendered possible by needling in two successive principal phases and followed by splitting after bonding, has the advantage of giving a finished product of better quality since, with a large thickness, there is obtained a better entanglement of the fibers and a closer texture than when the manufacture is effected so as to provide directly a single finished layer or so as to needle together more than two primary sheets. The needling effected according to the invention is also more economical for the same density of needling since several layers are perforated at the same time.

As a specified below, the product obtained has properties similar to those of a split hide (or flesh-side hide) of natural leather.

It has a good "hand" and a good "drape" or "fold."

The values of tensile strength and bursting strength are high; the characteristics of deformation observed by a dynamometer and by a tensometer are advantageous since, approaching those of natural leather, they enable the avoidance of using an intercalary fabric between the non-woven base and the finish, in the production of the synthetic leather, especially for shoe uppers. This therefore avoids "blocking" the finished article and leaves it a certain "give" which is indispensable if the shoe is to be comfortable.

The loss of strength in a humid environment and in the presence of solvents employed in manufacturing shoes remains within reasonable limits.

The product, highly needled, has excellent delaminating resistance.

It has good resistance to abrasion and to repeated flexing.

Finally, the absorbing power is, as for all synthetic materials, less than that of natural leather, but moisture is eliminated much more rapidly.

It must also be mentioned that the product obtained is perfectly adapted to the new techniques used for the manufacture of shoes, which techniques consist in placing on a mold a plastisol, a preparation of suitable resins or a thermoplastic film, in then placing thereon a synthetic leather base and in finally subjecting the assembly to the action of pressure and heat treatment, by high frequency, ultrasonics or the like, to ensure gelling and shaping.

All in all, the product lends itself fully to the manufacture of quality synthetic leather for shoe uppers.

It is even possible to obtain by this method suitable products for lining the inside of the shoe, and more especially the rear part called "anti-slipper."

For such use a portion of the mixture of fibers is constituted of synthetic fibers having a relatively low melting point, comprised between 60.degree. and 120.degree.C, and undergoing a certain shrinkage with heat. These fibers will give the finished article advantageous properties of thermoweldability and of deformability.

A thick and dense needled sheet is made, as indicated previously, with however a lower number of perforations than for the product intended for the upper of a shoe, so as to preserve good deformability.

A shrinkage of the sheet by 10 to 25 percent is obtained on thermal treatment effected at the moment of bonding. It contributes to bestowing on the product a good "hand" comparable to that of natural leather.

The bonding is effected preferably by coagulation in situ of a synthetic latex. It is advisable to watch that a good attachment of the binder to the fibers is obtained in order for the abrasion resistance to be adequate. If necessary, the latter can be improved, after splitting, by a suitable surface treatment, based on polyurethane for example.

After needling, bonding, splitting and if necessary surface treatment, the product obtained has excellent characteristics for use as a shoe lining, and more particularly as an anti-slip material. It has good properties for the shoe manufacture: "handle" similar to that of natural leather breaking and tear strength, deformation and thermoformability, thermoweldability and more generally ease of employment. In the same way, for the user, it offers qualities of appearance and comfort due to good absorbing power and solidity.

For the application of the method concerned, an installation is provided comprising successively four continuously operating means, namely a first means in two parts to manufacture separately two non-woven primary batts or sheets from fibers of fine denier, a second means in two parts to needle separately the two primary sheets thus formed, a third means to superpose the two primary sheets thus needled and a fourth means to subject the resulting assembly to intense needling, and, following these means, a means for bonding fibers and splitting device to divide the bonded sheet into layers of the desired thickness and, if necessary, a buffing means.

The invention will, in any case, be well understood with the aid of the supplementary description which follows as well as of the accompanying drawings, which supplement and drawings relate to preferred embodiments of the invention, given purely by way of illustrative but non-limiting example.

In the drawings:

FIG. 1 illustrates diagrammatically the successive operations of the method according to the invention.

FIGS. 2 and 3 show, respectively in elevation and in cross-section along the lines III--III of FIG. 2, a needle used in one of the abovesaid operations.

FIG. 4 shows, on a larger scale, a detail of the needle of FIGS. 2 and 3; and

FIG. 5, lastly, illustrates a needling operation.

According to the invention and more particularly according to that of its methods of application, as well as according to those of its methods of production of its various parts, to which it would seem that preference should be accorded, in order to prepare a synthetic leather base for shoe uppers, procedure is as follows or in analogous manner.

After opening the fibers in a conventional apparatus 1 (FIG. 1), the fibers are made to pass into two carding machines or groups of carding machines 2a, 2b and the continuous webs issuing from these carding machines are superposed by means of two horizontal or vertical fleecers 3a, 3b (especially of the "camel-back" type) which form continuous primary sheets. The carding machines can be two, four or six in number, according to their type and the production desired and their linings must, in any case, be adapted for working with fine deniers. There may also, instead of the assembly 2a, 2b, 3a, 3b, be used the usual pneumatic equipments enabling the production of non-woven continuous sheets of considerable thickness.

Then, the two continuous primary sheets are subjected separately to a needling operation. This needling operation comprises, preferably, a continuous preneedling of each primary sheet on needlers 4a and 4b respectively and a discontinuous needling in a needler 5a or 5b, that is to say in which the sheet is stopped before each needling, which does not prevent the overall operation from being considered as continuous. For each primary sheet, the pre-needler 4a or 4b and the needler 5a or 5b are each equipped with needles striking in the same direction, in general from above to below.

Then, by means of an apparatus 6, the two continuous primary sheets thus needled are superposed, this apparatus 6 comprising a turning mechanism which acts on one of the primary sheets so that the surfaces of these sheets, which have been first in contact with the needles of the pre-needlers 4a, 4b and the needlers 5a, 5b, occur outside the sheet formed by their superposition, which ensures a better adhesion of the primary sheets.

The resulting continuous sheet is then subjected to intense needling in several successive needlers, such as 7,8, 9 and 10, each equipped with needles working from above to below and with needles working from below to above.

If necessary after reception in a reeling device 11, the sheet passes into a tank of binder 12, then into an oven 13 where the coagulation of the binder is effected. The latter is, preferably, a synthetic latex enabling, in the oven 13, a coagulation in situ to produce a homogeneous article by avoiding migration of the binder towards the outside of the sheet. The sheet containing the coagulated binder is then wrung by mechanical means 14, then dried and vulcanised in a heating dryer 15. Then, the sheet obtained is divided in the direction of the thickness into several layers each having the desired thickness for the synthetic leather base, in a splitting machine 16 identical with those used in the leather industry, and finally the split pieces are treated on each surface in a buffing device 17 to give them a completely regular thickness and a good touch. There are thus obtained synthetic leather bases which can receive, in the usual manner, a finish or a coating to constitute a shoe upper or similar product.

There will now be given more specific information as regards the initial materials and the various phases of the method.

1. General indications 1.1 -- Fibers

Curled or artificial synthetic fibers, alone or in mixture with other fibers.

Denier less than 3 (namely 3.3 decitex) preferably 1.5.

Length 40 to 60 mm.

Curl measured according to BISFA method:

Frequency greater than 6, preferably of the order of 10;

Curl ratio greater than 10, preferably at least equal to 14;

Curl permanence greater than 50 percent -- preferably 70 percent and more.

The BISFA method is an international method employed for synthetic and artificial fibers, principally in Europe: the frequency expresses the number of half-waves counted on the curled fiber, related to 1 cm of the uncurled fiber; the curl ratio is the difference between the uncurled length and the curled length, expressed in percentage of the uncurled length; the permanence is expressed as a function of the loss of curl ratio, or as a function of the residual curl after a given placing under tension and rest.

For products intended to produce a thermoweldable lining, it is necessary to replace 30 to 70 percent of the fibers described above by fibers with low melting point having the following characteristics:

Denier equal or less than 3,

Length 40 to 60 mm,

Softening point 60.degree. to 120.degree. C.

There can be used, for example, fibers of polyvinyl chloride, copolymers of vinyl chloride and vinyl acetate or copolymers of ethylene and vinyl acetate.

1.2 Oiling

The fibers are oiled with a suitable oiling preparation to avoid the formation of static electricity during the fabrication of the sheet and to ensure sufficient lubrication for needling.

1.3. Needling

1.3.1. Pre-needling

The pre-needler 4a or 4b comprises a small number of rows of needles which strike from above to below. Their arrangement is such that, over a centimeter of width of board, there are two to five needles, and preferably three.

The striking speed is 600 punches/min.

The advance of the batt or sheet is continuous.

Thickness of batt before pre-needling: 150 mm.

Thickness of batt after pre-needling: 20 to 25 mm.

1.3.2. --First needling

The first needler 5a or 5b comprises at the minimum 20 rows of needles which strike from above to below. Their arrangement is such that, over 1 cm of width of the needle board, there is a minimum of 30 needles and preferably 45.

The striking speed is of the order of 300 punches/min. and can be higher according to the production effected.

The feed of the batt is discontinuous.

Thickness of the batt before needling: 20 to 25 mm.

Thickness of the batt after needling: 7 mm.

The weight of the batt after needling can reach 700 g/m.sup.2 and the number of perforations per cm.sup.2 is from 50 to 150, and preferably 100.

1.3.3. Needling of the assembled sheets

This operation is effected by passage of the assembled sheets, through needlers 7 to 10 arranged successively and comprising alternately needle boards striking from above to below and from below to above.

Each board comprises, per centimeter of width, at the minimum 30 needles and preferably 45, distributed over a minimum of 20 rows of needles.

The striking speed is from 250 to 600 punches/min. according to the production realized.

The feed of the sheet is discontinuous.

Thickness of the sheet after superposition: 14 mm.

Thickness of the sheet after the first needler 7 with multiple punches (two boards operating one from above to below and the other from below to above): 10 mm.

Thickness of the sheet after the second needler 8: 8.5 mm.

Thickness of the sheet after the third needler 9: 7 mm.

Thickness of the sheet after the fourth needler 10 : 6 mm.

At the outlet from these machines, the needled sheet weighs 1,000 to 1,400 g/m.sup.2. The total number of perforations is between 300 and 1,000/cm.sup.2.

The density of the needled sheet, that is to say the weight in kg per m.sup.2 for a millimeter thickness, must be between 0.150 and 0.250.

1.3.4. Choice of needles

This choice is important since it must achieve without needle breakage the maximum efficiency and the best appearance of the finished product.

As shown in FIG. 2, it is necessary to distinguish in a felting needle 18:

The heel 19 which positions the needle when it is placed in the needle board, 20 (FIG. 5);

The shank 21, which is implanted in the board, or in a plug of plastics placed in the board, and holds the needle in position;

The intermediate narrowed portion 22, which is of smaller diameter than that of the shank, and connects the shank and the triangulated portion 23;

The triangulated portion 23, which comprises the barbs 24, and constitutes the active portion of the needle.

The following type of needle appears to be the most suitable:

The length L of the needle is 3 inches, longer needles being more fragile;

The barbs are placed on three edges (the three edges being arranged with respect to the heel 19 according to FIG. 3);

There are two barbs per edge, placed according to the designation "regular barbs" used by the suppliers, since closer barbs involve too many fibers at once and cause breakages.

The Table below gives the different dimensions (in inches) with reference to FIG. 2:

A = 1.156 E = 0.250 B = 0.156 F = 0.083 C = 0.156 G = 3 D = 0.250

The barbs are "standard" barbs which form a projection with respect to the edge 25 (see FIG. 4).

The gauge is a number, given by the supplier, which corresponds to a diameter of steel wire before triangulation, for example:

Gauge 42 : 0.016 inch

Gauge 46 : 0.012 inch

It is these two gauges which are preferably used, the first being used for pre-needling and a portion of the needling, and the second for the rest of the needling.

1.3.5. Feed per punch and depth of penetration

The needlers are adjusted with one advance per punch of 3 to 7 mm, with a preference for 5 mm.

The passage of the sheet 26 is effected in each needler between two plates 27, 28 in which holes enable the passage of the needles 18. When the point of a needle, after having traversed the sheet, arrives flush with the opposite plate, it is said that the penetration is nil. The penetration p indicates therefore the distance traversed by the point of the needle from this level (see FIG. 5).

The depths of penetration are from 8 to 12 mm for the pre-needling (machine 4a or 4b), from 7 to 11 mm for the first needling (machine 5a or 5b), 10 to 20 mm and 8 to 12 mm for finishing (machine 10).

1.4. Conditioning

For preparation of the sheet and the operation of needling, it is necessary to work at a temperature above 20.degree. C and preferably at 22.degree. C. The relative humidity must be at least 65 percent.

1.5. Binding

The binder is a dispersion of synthetic latex, preferably a butadiene copolymer, containing vulcanizing agents and sensitized to heat. Its concentration is from 6 to 30 percent. The latex is destabilized when heated between 40 and 60.degree.C.

The product obtained after vulcanisation at 15 (FIG. 1), has the following characteristics: Weight per m.sup.2 15 to 3 kg Thickness 4 to 7 mm Density 0.300 to 0.500 Ratio of binder 25 to 50 %.

2. There are now given more specific indications regarding one embodiment of a synthetic leather base intended for shoe uppers.

______________________________________ 2.1. Fibers Polyamide - 1.6 decitex (1.4 denier) - 50 mm Curl measured according to the BISFA method: Frequency 10 to 11 Ratio 14 to 15 Curl permanence 70 to 75% ______________________________________

Proportion of grease : extraction with petroleum ether < 0.25 percent

2.2. Oiling

1 percent with respect to the weight of the fibers.

Condensed oxide of ethylene and propylene --non-ionic -- strongly antistatic.

2.3 Needling

Pre-needler 4a, 4b :

four rows of needles

3.15 needles per cm of board 20

600 punches/min.

Needles gauge 42

Penetration 10.5 mm

First needler 5a, 5b :

34 rows of needles

43 needles/cm of board

250 to 300 punches/min.

Needles gauge 42

Penetration 9 mm.

Superposition of Primary sheets or batts : two batts of 700 g/m.sup.2 having 100 perforations/cm.sup.2 are superposed

Four needlers 7 to 10 with multiple punch each comprising:

43 needles per cm of board striking from

above to below,

43 needles per cm of board striking from below

to above,

Rate : 300 punches/min.

Needles gauge 42 for the three needlers 7, 8 and 9.

Needles gauge 46 for the last needler 10.

Respective penetrations : 13, 17, 17 and 11 mm.

Needled sheet : 1,400 g/m.sup.2

800 perforations/cm.sup.2

Thickness : 6 mm

Density : 0.220.

2.5 Binding

The binder used is a butadiene Copolymer latex of which the concentration is 20 percent of polymer.

The product obtained has the following characteristics:

Weight per m.sup.2 2.500 g Thickness 6 mm Density 0.400 to 0.420 Ratio of binder 40 to 45%.

3. A second embodiment relates to the production of a product for shoe lining.

3.1 Fibers

Mixture of:

50 percent of fibers having the same characteristics as in 2.1, the length of the fibers being however of the order of 40 mm;

40 percent of fibers constituted of a copolymer of vinyl chloride (85 percent) and by vinyl acetate (15 percent);

3.3 dtex; length: 40 mm; softening point 70.degree.C;

10 percent of polyvinyl chloride fibers; 2 dtex; length : 60 mm.

3.2 Oiling

As in 2.2.

3.3. Needling

As in 2.3, the needled sheet having the following characteristics: 1,250 g/m.sup.2 ; 600 perforations/cm.sup.2 ; thickness: 6 to 7 mm; density : 0.190 to 0.200.

3.5 Binding

The binder is a butadiene acrylonitrile copolyper latex of which the concentration is 11 percent of the polymer. A shrinkage of 15 to 20 percent occurs in the course of thermal treatment, at the same time as the coagulation of the latex (at 13 FIG. 1), and of drying and vulcanisation (at 15) by reason of the presence of polyvinyl chloride fibers.

There is obtained before splitting a sheet having the following characteristics:

Weight per m.sup.2 2.1 to 2.3 kg Thickness 5.5 mm Density 0.420 Proportion of binder 33%

3.6 Splitting

Splitting is to 0.80 mm thickness. Buffing is not necessary.

3.7 Surface treatment

One of the surfaces is treated with a solution of polyurethane to deposit a layer of 5 g/m.sup.2 of colorless polyurethane, to give good resistance to abrasion. To differentiate this surface from the reverse, the latter can be given a inscription.

The 40 percent of copolymer fibers with low softening point ensure in the product, qualities of deformability and of self-welding by heat.

4. This example relates to a variation of Example 3, with the same possibility of use.

4.1 Fibers

Mixture of

50 percent of fibers having the same characteristics as in 2.1, with a length of the order of 40 mm;

50 percent of fibers constituted of a copolymer of ethylene and vinyl acetate; softening point: 105.degree.-110.degree..

4.2 Oiling

As in 2.2.

4.3 Needling

As in 2.3, the needled sheet having the following characteristics: 1,250 g/m.sup.2 : 400 perforations/cm.sup.2 ; thickness: 8 mm; density: 0.150.

4.5 Binding

The binder is a butadiene acrylonitrile copolymer latex of which the concentration is 9.50 percent of polymer. Shrinkage as in 3.5.

There is obtained before splitting, a sheet having the following characteristics:

Weight per m.sup.2 : 2.1-2.3 kg

Thickness : 6-6.5 mm

Density : 0.350

Proportion of binder : 33%.

There may be added to the binder resorcinol and hexamethylene tetramine to improve the attachment of the fibers and of the binder and, for this reason, the abrasion resistance.

4.6 Splitting

As in 3.6.

4.7 Surface treatment

As in 3.7.

Claims

I claim:

1. Method of manufacturing a synthetic leather base exhibiting a homogeneous cross-section, comprising preparing a thick, homogeneous sheet of high density from synthetic fibers, said fibers having a denier less than 3, a curl frequency over 6, a curl ratio over 10 and a curl permanence over 50 percent, said method being a continuous method and including four successive phases, the first phase consisting of preparing two continuous non-woven primary sheets from said fibers, the second phase consisting of needling the two primary sheets separately from one side only with needles striking in the same direction, the third phase consisting of turning over one of said primary sheets and superposing the two thus needled primary sheets such that the surfaces which have been initially contacted with the needles are on the outside of the sheet formed by their superposition, and the fourth phase consisting of subjecting the sheet thus formed to intense needling from both sides, applying a curable binder through the resulting sheet, curing said binder so as to bind together the fibers to form a bound sheet and subjecting the bound sheet to splitting operations dividing it into layers of the desired thickness.

2. Method according to claim 1, wherein the needling of the superposed primary sheets is effected in several successive needlers each equipped with a needle board working from above to below and with a needle board working from below to above.

3. Method according to claim 1, wherein the fibers are oiled before forming the primary sheets, with 0.5 to 2 per cent (with respect to the weight of the fibers) by suitable oiling agent to avoid the formation of static electricity on forming the sheet and to ensure sufficient lubrication for the needling.

4. Method according to claim 1, wherein said binding is effected by coagulation in situ of a synthetic latex.

5. Method according to claim 1, wherein the thickness of each primary sheet is of the order of 150 mm before preneedling, of 20 to 25 mm after pre-needling and of 7 mm after the last needling, and wherein the sheet emerging from the last needler has a thickness of the order of 6 mm, weighs 1,250 to 1,400 g/m.sup.2 and has a perforation density comprised between 600 and 1,000/cm.sup.2.

6. Method according to claim 1, wherein needles are used having a triangulated portion possessing two "regular" barbs per edge.

7. Method according to claim 1, wherein the needles used for the needling operations have gauges of the order of 42 to 46.

8. Method according to claim 1, including buffing the resulting layers individually.

9. Method according to claim 1 wherein the needling phase of each primary sheet comprises effecting pre-needling with continuous advance of the sheet in a pre-needler, then needling in a needler with discontinuous advance of the sheet.