US2757100A

US2757100A - Process for forming permeable sheet material

Info

Publication number: US2757100A
Application number: US318732A
Authority: US
Inventors: Verne L Simril
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1952-11-04
Filing date: 1952-11-04
Publication date: 1956-07-31
Anticipated expiration: 1973-07-31
Also published as: GB763604A; BE523941A; DE1010945B; FR1090575A

Description

United States Patent PROCESS FOR FORMING PERMEABLE SHEET MATERIAL Verne L. Simril, Williamsville, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application November 4, 1952, Serial No. 318,732

6 Claims. (Cl. 117-7) This invention relates to synthetic leather and, more specifically, to a process for forming a synthetic leather having the desirable inherent physical characteristics of leather.

Patents relating to methods and techniques of preparing various synthetic leather compositions have been granted to inventors as far back as 1850'; and even before that time, a great number of attempts have been made to devise a single and rapid technique of producing synthetic leather compositions. In most of the early art, pyroxylin was used to coat or impregnate various types of fibrous base materials to prepare leather substitutes. As time went on, pyroxylin/oil/pigment compositions were widely used as coating or impregnating compositions for various woven or non-woven fibrous base materials.

In the early stages of the synthetic leather industry, the main objective was to simulate the general appearance of leather. In todays markets,.coated fabrics, particularly the vinyl coated fabrics, are outstanding as leather substitutes in such applications as handbags, bookbindings, brief cases, card table covers, luggage, etc. In such applications, the coated fabrics are satisfactory because the general appearance of leather is simulated and the coated fabrics possess some of the desirable properties of leather. However, as compared to leather, the coated fabrics lack the properties of good tear strength, softness, and the ability to breathe or transpire water vapor and air, which are characteristic of leather; and, although the coated fabrics are used insuch applications as chair coverings, much is left to be desired, especially with respect to water vapor and air permeability. Up to the present time, synthetic leather compositions have made little or no inroads into the boots, shoes, and glove markets, mainly because of their inability to breathe, in addition to lack of good tear strength and softness. As used herein, the term breathe means transpire water vapor and air.

In general, the use of a synthetic leather composition in boots, shoes, gloves, etc., is mainly dependent upon its ability to breathe (usually expressed in terms of water vapor permeability). Physical tests on the water vapor permeability of leather indicate that leather transpires water vapor about /a as readily as free air. In general terms, shoe upper leather samples having a thickness of 0.0l6"0.104" have a water vapor permeability within the range of LOGO-18,000 grams/ 100 sq. meters/hr., when tested according to the method of Kanagy and Vickers, Journal of American Leather Chemists Association, 45, 211-242 (April 1950), in an atmosphere of 23 C. and 90% relative humidity. Hereinafter, the ability of synthetic leather compositions to transpire Water vapor will be expressed in terms of leather permeability (LPV), as measured by the above test, in grams/ 100 sq. meters/hr. Based upon comfort tests which have been carried out, under the above conditions, the minimum tolerable leather permeability for shoe upper leather is about ],6002,000 grams/ 100 sq. meters/hr. Preferably, for shoe upper leather, the leather permeability should be 4,00020,000. Actually, there is no upper limit, but the composition must be substantially resistant to penetration by liquid water and have acceptable strength properties.

An object of the present invention is to provide a synthetic leather having outstanding breathing qualities. A further object of the present invention is to provide a process of preparing a synthetic leather having the requisite properties for fabricating boots, shoes, gloves, chair coverings, and other articles wherein a composition capable of breathing is required. A still further object is to provide a process of preparing a synthetic leather having a tenacity, flex life, elongation, tear strength, modulus and leather permeability equal or superior to the various types of genuine leather. A still further object is to provide a process of preparing a synthetic leather in which the aforementioned physical properties may be tailored to the desired end use. Other objects will be apparent from the description given hereinafter.

These objects are realized by the present invention which, briefly stated, comprises forming a compact, essentially water vapor-impermeable, and continuous composite sheet by hot pressing a composition comprising essentially a relatively non-extensible material, either in the form of fibers and/or discrete particles, thoroughly impregnated with a relatively extensible, substantially non-adherent, binder material, and thereafter stretching said sheet within the limits hereinafter set forth, followed by relaxing, to produce a sheet having outstanding water vapor and air permeability.

The expression relatively extensible material, as used herein, means that the material must be stretchable under the stretching forces to be employed and must elongate during the stretching and substantially recover upon releasing the stretching forces. The expression relatively non-extensible material means that the material stretches to a substantially lesser extent, if at all, than the relatively extensible material under the stretching forces imposed.

The expression substantially water vapor-impermeable as applied to the initial composite sheet (i. e., before stretching) means that the sheet has a leather permeability value (LPV) of less than 1,000 grams/ 100 sq. meters/hr.,

. measured at 23 C. and 81% relative humidity. In general terms which may be applied to any combination of relatively extensible and relatively non-extensible materials useful in the present process, the water vapor permeability of the initial composite base sheet is substantially no greater than that of homogeneous sheet of the relatively extensible material or the relatively non-extensible material, whichever has the higher water vapor permeability. Usually, providing the initial composite base sheet is prepared in such a manner that there are substantially no voids present in the sheet, the water vapor permeability of such a base sheet is intermediate between the permeabilities of homogeneous sheets of the two major components of the base sheet. Actually, the leather permeability values (LPV) of the initial compacted substantially water vapor-impermeable sheets which are transformed into permeable sheets by following the present process are low not only because the sheet is compacted by hot pressing, but also because of the thickness of the sheets which are useful for converting to synthetic leather sheets. That is, the initial sheets are usually between 0.015"0.05 in thickness, and homogeneous sheets of either the binder polymer or the structural fiber having thicknesses within this range also have low LPVs. Normally the LPV is appreciably less than 1,000, and in no case greater.

The present invention resides in the discovery that stretching a substantially water vapor-impermeable composite sheet comprising an extensible or stretchable material and a relatively non-extensible or non-stretchable material results in the formation of a sheet which is permeable to water vapor and air. Generally, the degree of permeability imparted to the stretched sheet essentially depends upon the particular relatively non-extensible material employed, e. g., particulate or elongated (fibers); the particular relatively extensible material employed; the ratio of extensible and relatively non-extensible materials in the initial sheet; the extent of stretch (either in one direction or two directions); and the degree of adhesion between the two major components of the sheet. Other specific factors which affect not only the resulting permeability but also other physical properties of the resulting sheet will be discussed further hereinafter. It is believed that the stretched sheets are vapor-permeable because the internal structure of the initial sheet is modified. Stretching the initial sheet appears to pull the extensible component away from the relatively non-extensible component, and this results in the formation of voids or capillaries around the relatively non-extensible component. Hence, the resulting stretched sheet has a crosssectional structure essentially consisting of a binder material reinforced With the relatively non-extensible particles or fibers, the areas immediately adjacent the particles or fibers being voids. These voided areas form a structural pattern substantially identical to the distribution of the relatively non-extensible material throughout the extensible binder material. Preferably, the relatively nonextensible material is in the form of a fibrous mat, the fibers being intertangled. The use of this type of fibrous mat as the relatively non-extensible material results in the formation of interconnecting voids or capillaries throughout the stretched sheet.

The total volume of voids formed in the stretched and relaxed sheet depends not only on the quantity of structural fibers present in the initial, i. e., before stretching, sheet but also upon the degree of stretch and the amount of recovery of the binder polymer upon relaxing. Normally, the binder polymer after stretching does not recover completely, i. e., 100%, especially after being stretched as much as 40-50% in one or both directions. Incomplete recovery is evidenced by an increase in the thickness of the sheet after stretching and relaxing. However, so long as the adhesive bonds between fiber and binder are broken in stretching, it may be desirable to obtain almost complete recovery or a high degree of recovery because the resulting sheet would be softer and yet highly permeable because of the internal surface area formed by breaking the adhesive bonds between binder polymer and structural fibers.

The relatively extensible material may be selected from the large class of soft, thermoplastic polymers and, more specifically, from the class of polymers which may be generally classified as elastomers or elastomeric materials, as set forth by H. L. Fisher (Industrial and Engineering Chemistry, Aug. 1939, page 942). Those polymers which are only partially elastic or elastomeric and are not strictly classified as elastomers include ethylene polymers such as polyethylene, chlorinated polyethylene; vinylidene chloride/acrylonitrile copolymers; various polyamides such as N-methoxymethyl polyhexamethylene adipamide; and copolyesters made from ethylene glycol, terephthalic acid and sebacic acid of the general types described and claimed in copending applications U. S. Serial Nos. 150,811 and 150,812, filed March 20, 1950, in the name of M. D. Snyder, and now Patents 2,623,033 and 2,623,031, respectively polyvinyl acet'als such as polyvinyl butyral and polyvinyl laural; ethylene/vinyl acetate copolymers in which the ratio of ethylene to vinyl acetate ranges from 1.411 to 11:1. Polymers generally classified as elastomers include plasticized polyvinyl chloride; natural rubbers; synthetic rubbers such as neoprene (poly- 2-chloro-1,3-butadiene polymer), chloro-sulfonated polyethylene butadiene/acrylonitrile copolymers, and other butadiene copolymers. All of the foregoing may be employed with or without plasticizers.

With respect to the binder polymer, it is to be understood that the term thermoplastic is meant' to include those polymeric materials which are at least initially thermoplastic under the conditions of hot pressing. in other words, the term initially thermoplastic includes binder materials which melt and flow under the conditions of hot pressing; and at higher temperatures, these materials, e. g., rubber and synthetic rubber compositions, may cure and be converted to a substantially t'hermoset condition.

The relatively non-extensible material may be selected from a wide variety of materials in the form of particles of various shapes or in the form of elongated shapes such as fibers. The use of relatively non-extensible materials in fiber form is preferred. This is because it is desired to obtain interconnecting voids or capillaries in the resulting stretched sheets. Since the structural fibers are serving to reinforce the binder material and provide for the formation of interconnecting capillaries or pores having substantially the same interconnecting network pat' tern as that of the structural fibers, selection of fibers of a particular length and denier must be considered with these two functions in mind. For the purpose of providing adequate tensile strength, tear strength and flex life, the structural fibers should be at least about 0.5" in length. As a general observation, the strength properties of the resulting compositions do not increase appreciably when using structural fibers greater than about 1.5" in length. On the other hand, from the standpoint of handling non-woven mats of fibers on standard textile machines, it may be more convenient to use longer fibers, for example, as long as 8 or longer. With respect to the network of interconnecting pores formed throughout the cross-section of a sheet treated in accordance with this invention, the use of very short fibers, e. g., 0.01" flock, does not produce a composition having optimum permeability. When the length of structural fibers is increased from 0.25 to 0.5", the permeability appears to increase; but no appreciable increase is obtained when fibers longer than 0.5" are employed.

It is within the scope of the present invention to employ fibers having a denier within a relatively wide range. Normally, textile fibers having a denier Within the range from 1-3 denier/filament are employed. On the other hand, fibers having a denier as low as 5.5 l0 denicr/ filament have been used in conjunction with fibers of greater denier, e. g., 1-3 denier/filament. Usually, it is entirely practical to employ very thin denier fibers so long as the cohesive bonds within the fibers are greater than the adhesion between fiber and hinder, or, in other words, so long as the fibers are distinguishable as such in the fiber/binder composite. it is especially advantageous to position very thin denier fibers at the surface of the present compositions in order to produce sheeting which will retain its original surface appearance after repeated abrasions. An extreme condition with respect to employing fibers of higher denier is the use of fibers substantially greater than about 16 denier/filament, these fibers being relatively stiff and bristle-like.

It is to be understood that oriented or unoriented fibers or mixtures thereof may be employed as the structural fibers in the present compositions. The use of orientable, but unoriented, textile fibers is highly advantageous in preparing compositions of high tear strength and improved pliability and flex life. Furthermore, it is within the scope of the present invention to employ mixtures of fibers of various sizes, i. e., length and denier, for the purpose of producing compositions which retain their original surface after repeated abrasions. This is accomplished by using short fibers, i. e., less than /2", in combination with the longer fibers in the structure. The chemical composition of the short fibers may be the same or difierent from that of the longer fibers. Usually, the short fibers are strategically concentrated near the surface of the initial structure in order to reduce fuzzing of the surface resulting from abrasion. On the other hand, the short fibers may be uniformly dispersed throughout the binder with the longer fibers. Non-extensible materials in the form of particles include pigments such as zinc oxide, talc, clay, diatomaceous earths, and various polymeric materials in particulate form such as polyamides, polyethylene terephthalate, polytetrafiuoroethylene, etc.

It is to be understood that the non-woven fibrous mats employed in preparing the polymer-impregnated, substantially impermeable, initial sheets may be fabricated in accordance with any well known batch-wise or continuous technique such as by carding machines, air deposition apparatus, and water deposition or paper-making techniques. Furthermore, the resulting fibrous mats may have their component fibers oriented substantially in one direction or randomly arranged. Individual mats having the fibers oriented in one direction may be cross laminated. In any event, the fibers must be interconnecting so that the resulting capillaries or void spaces are interconnecting.

In selecting the components, i. e., the extensible and relatively non-extensible materials, to be combined in preparing the initial composite sheet, two main factors must be taken into consideration: (1) the extensibility of the two components must be different; the relatively non-extensible material must be relatively non-extensible as compared with the extensible material under the stretching forces (expressed in terms of per cent elongation) to which the sheet is to be subjected. For example, if a polyamide fiber is employed as the relatively non-extensible material, the selected binder or extensible material must be one which can be elongated under forces which stretch the fibers to a substantially lesser extent, if at all. Generally speaking, a polyamide fiber is not considered to be a non-extensible material; but in accordance with the definition employed herein, the relativelynon-extensible material is defined with respect to or in relation to the extensible material or binder employed in combination therewith. (2) Adhesion between the extensible and relatively non-extensible material should be such that the adhesive bonds can be readily broken by stretching. The preferred or ideal case is one in which there is substantially no adhesion between the relatively extensible and relatively non-extensible material. With composite sheets fabricated from such basic components, only moderate stretchingforces are required to break the adhesive bonds and thereby form voids within the stretched sheet. On the other hand, it follows that initial composite sheets composed of a binder polymer and a fiber which form strong adhesive bonds must be subjected to drastic or extensive stretching in order to rupture these bonds for the purpose of producing voids within the stretched sheet. This is not generally practical from the standpoint of the excessive forces required. Hence, it is preferred that the degree of adhesion between the extensible and relatively non-extensible materials be as low or insignificant as possible.

The ratio of relatively extensible material to relatively non-extensible material in the initial sheet may vary from 30:70 to 70:30. However, the optimum quantity of extensible material in the initial composite sheet ranges from 40-60%, based upon the total weight of the two major components. Normally, the densities of the extensible material and the relatively non-extensible material are relatively close; and in such cases, the ratio of the two components may be expressed on either a volume or weight basis. Generally, as the quantity of extensible or binder material in the compositions is reduced, the initial composite sheet becomes more and more permeable to vapor (the stretched sheet is highly permeable); but other properties and characteristics of the material (the stretched sheet) are not satisfactory for use as a leather replacement, for example, in boots, shoes, gloves, chair coverings, etc. Stretched compositions which contain too low a quantity of extensible material are extremely fuzzy at the surface, and the abrasion resistance is very poor. The general feel is more like that of felt instead of leather, and the tensile strength and tear strength are below the tensile and tear strength of materials having from 40-60% of the extensible material. This is because in most cases the amount of extensible or binder material is insutficient to hold the relatively non-extensible material, e. g., fibers, together. On the other hand, as the amount of extensible or binder material is increased, the initial composite sheet has properties approaching those of a homogeneous film or sheet of the extensible or binder material. For example, the elongation and modulus increase as the extensible or binder content is increased; and the initial vapor permeability is very low (the sheets are substantially impermeable) as is the permeability of homogeneous films of the extensible or binder polymers employed. Furthermore, the tear strength also falls off rapidly at high binder content because the film is tearing essentially as a homogeneous (non-reinforced) film. All these factors are also true for corresponding stretched sheets. The content of the binder or extensible material may also be varied depending upon the particular combination of extensible and relatively non-extensible materials in the initial composite sheet. As previously stated, those components which, when combined into the initial sheet, form strong adhesive bonds, offer greater resistance to stretching and usually require greater amounts of stretch to obtain the desired vapor permeability. In such cases, the content of extensible or binder material in the initial composition may be. reduced somewhat in order to lessen the forces required for stretching. It is to be understood, however, that certain compromises must be made with respect to those physical properties mainly affected by reducing the content of extensible or binder material. In addition to the major substituents, the initial sheet may contain minor proportions of various additives such as plasticizers, dyes, etc., plasticizers usually being incorporated with the extensible material.

The improvement in the water vapor permeability of the initial composite sheets effected by stretching is directly related to the amount of elongation. At very low elongation, i. e., 510%, the sheets remain substantially impermeable even though they are stretched in two directions. However, as the amount of elongation is increased, the water vapor permeability increases; and the improvement is outstanding when the sheets are stretched in two directions. Generally, the nature of the extensible and relatively non-extensible material employed in the initial composition determines the amount of stretch required to effect the desired improvement in water vapor permeability. However, in all cases, stretching in two directions, that is, biaxially, appears to produce the greatest improvement in water vapor permeability for a given extent of stretch or elongation. Furthermore, it has been generally observed that extremely high elongation, i. e., 50% and above, although imparting excellent porosity or breathing qualities, usually degrades the other physical properties necessary to produce a good synthetic leather. Specifically, the tensile strength decreases appreciably at high elongation. In general, as the extensible material or binder polymer is extended to the limit of its extensibility, the binder ruptures; and from this point, in the case of a fiber-reinforced composition, only fiber-fiber separation is measured. Preferably, the initial sheet should be stretched biaxially from 10-40%, the degree of stretching being substantially the same in each direction.

Since the tensile strength of the present synthetic leather compositions decreases as the extent of stretch increases, the tensile modulus also decreases. In general, a low modulus indicates a soft material; and this is usually desirable in a synthetic leather composition, es pecially for use in boots, shoes, gloves, etc. However, the degree of softness which may be imparted to these compositions by stretching is limited by the decrease in tensile strength which may be tolerated. From the foregoing, it is manifest that the present process is highly flexible; and the resulting synthetic leather compositions may be tailored to the desired end use by obtaining a balance between the desired vapor permeability and other physical properties, such as tensile strength, tear strength and tensile modulus.

Any desired expedient for stretching the initial sheet may be employed. Where the degree of adhesion between the extensible and relatively non-extensible material is high, drastic conditions of stretching may be required. So-called drastic conditions of stretching usually means soaking or conditioning the initial composite sheet in a hot liquid followed by stretching in one or both directions while the sheet is at an elevated temperature. On the other hand, mild stretching may be carried out by hand working, i. e., crumpling and/or flexing a sheet. Working the sheet by hand or mechanical means, however, does not always lead to producing uniform permeability throughout the entire sheet. In most cases, uniform permeability is only obtained by elongating the sheet in one direction and then in the opposite direction, or just in one direction, or by stretching the sheet simultaneously in both directions on a tentering frame.

As set forth hereinabove, in order to obtain synthetic leather compositions having outstanding breathing qualities and strength properties for use in fabricating boots, shoes, gloves, chair coverings, etc, by following the process of the present invention, the initial composite sheet, i. e., before stretching, must be substantially water vapor-impermeable. Initial composite sheets which are substantially permeable to water vapor, i. e., have an LPV substantially greater than 1,000 grams/ 100 sq. meters/hr. (measured at 23 C. and 8l% relative humidity), are not normally convertible into a synthetic leather composition of satisfactory strength properties by following the present process. Initial composite sheets having a content of relatively extensible or binder polymer substantially less than 30% are considered to be within this classification; that is, the water vapor permeability of the initial sheet is high because the content of extensible or binder material is too low.

In the preparation of the initial sheet, any technique which will provide a composite sheet having particles or fibers of the relatively non-extensible material distributed uniformly throughout the relatively extensible material may be employed. Usually, in order to prepare a uniformly composited sheet, it is necessary to compact the sheet by subjecting it to heat and pressure. Pressures in the neighborhood of 500 p. s. i. appear to be satisfactory, but any combination of pressure and elevated temperature which causes the relatively extensible or binder material to flow is satisfactory. The pressing temperature is usually above the flow temperature of the selected extensible or binder polymer. After the pressing or compositing step, the initial base sheet is substantially free of voids and substantially impermeable to air. Furthermore, the water vapor permeability is low and it is not substantially greater than the water vapor permeability of a homogeneous sheet of either the binder material or the reinforcing material. In preferred form, a mat of intertangled fibers (the relatively non-extensible material) is thoroughly impregnated with the extensible or hinder material so that each individual fiber is surrounded by the impregnant. Furthermore, it is important that the fibers protrude through the surfaces of the sheet to provide for entrances and exits for the passage of vapor through the stretched sheet, the internal structure of the stretched sheet comprising a network of interconnecting pores or capillaries. Various other methods of fabricating the initial composite sheets of the present invention are as follows:

(1) A fibrous mat may be pressed into a film or sheet of the extensible or binder polymer. The pressing temperature must be above the flow temperature of the polymer. In some applications, one sheet of the binder polymer-may be used, or the fibrous mat may be inserted between adjacent sheets.

(2) A fibrous mat may be impregnated by passing it through a solvent solution of the extensible or binder material. The solvent may be evolved by well known techniques.

(3) The binder or extensible material in the form of a powder may be distributed upon the surface of a moving fibrous mat, and thereafter the mat is subjected to a source of heat to melt the polymer and impregnate the mat. Various sources of heat may be employed, for example, infra-red, dielectric heat, heated rolls, hot plates, air oven, etc. This operation is followed by passing the sheet through pressing rolls to form a uniformly flat sheet.

(4) A moving fibrous web may be fed into the nip of a set of calendar rolls upon which the binder polymer is plasticated. The binder polymer and mat are fed between the nip, and the pressure of the rolls serves to impregnate the mat.

(5) A dispersion of particles or fibers (or a mixture thereof) of the binder or extensible material and particles or fibers of a relatively non-extensible material is prepared in aqueous medium or other inert liquid medium. This dispersion may be coagulated continuously to form a sheet consisting of a continuous matrix of the hinder or extensible material in which particles or fibers of the relatively non-extensible material are randomly dispersed throughout. The resulting sheet may be composited by subjecting it to light pressure by feeding between the nip of a pair of heated rolls.

(6) A fibrous mat may be passed through a liquid polymerizable organic compound to impregnate the mat with the polymerizable liquid. Thereafter, the impregnated mat is conducted through an oven or under a bank of infra-red lights to bring about polymerization of the liquid component. In such a process, the liquid polymerizable organic compound should be thickened to a suitable viscosity, probably by dissolving a small amount of the hinder or extensible polymeric material in the cor responding liquid monomer.

(7) A continuously moving fibrous mat may be passed under a flamespraying apparatus which sprays uniformly the molten extensible or binder material onto one or both surfaces of the fibrous mat. This type of apparatus would have to be set up in such a way as to provide for complete impregnation of the mat.

(8) A hinder or extensible polymeric material in the form of a granular powder may be distributed onto a hot moving belt in order to sinter the particles together to form a continuous sheet. This sheet of binder or extensible polymer along with a fibrous mat may be fed into the nip of a set of pressing rolls to impregnate the fibrous mat with the binder or extensible polymer.

(9) A solvent solution or dispersion of the binder or extensible polymer may be continuously sprayed upon one or both the surfaces of a moving fibrous mat. The resulting impregnated mat may be passed thereafter into the bite of heated pressure rolls to compact the sheet.

(10) A mixture of fibers of extensible and relatively non-extensible materials is carded, and the resulting mat is fed into heated pressure rolls to melt the extensible material to form a continuous matrix of the extensible material having fibers of the relatively non-extensible material distributed uniformly throughout.

In forming the initial composite sheets of the present invention, it is to be understood that the binder or extensible material must melt or flow at a substantially lower temperature, usually at least 20 C. lower than the relatively non-extensible material in order to provide for compositing or compacting the combination of materials to form the initial sheet.

The following examples will serve to further illustrate the preparation and nature of the synthetic leather compositions of the present invention.

The compositions tabulated and. specified in Table 1 were prepared in accordance with the following general procedure: Crimped staple fibers of polyhexamethylene adipamide, 2 /2" long and 3 denier/filament, were carded to form non-woven mats or webs. The web was cut into sections and the various sections were put between screens and immersed into an aqueous solution of wetting agents containing 2% octyl sodium sulfosuccinate and 2% of a pared in the manner described above except Example 17 which was prepared by impregnating the non-woven mat with a benzene solution of polyisobutylene. After impregnation, the solvent was evolved and the impregnated mat was subjected to heat and pressure as mentioned above.

The resulting substantially water vapor-impermeable composite sheets were then placed in a stretching apparatus and elongated a definite fraction of their original length or length and width, e. g., one-way or two-way stretching. The stretched sheets weer then subjected to various physical measurements, i. e., tenacity, elongation, modulus, tear strength and leather permeability (LPV). In all examples contained herein, the value of LPV was measured at 23 C. and 81% relative humidity.

TABLE 1 Percent Unit Tinius Stretch- Percent Weight of Thick- Elonga- Olsen L. P. V., Example Extensible (Binder) Material 1D=one Exten- Initial ness Tenacity tron Modulus Tongue gms./100 sq.

direction; sible Sheet (lnches) (p. s. 1.) (percent) (p. s. r.) Tear meters/hr. 2D=two Material (gms./sq. (gms) directions meter) 1 plasticized neoprene 40% 1D 53.5 713 MD 4,122 127 9, 756 16, 500 4, 375

2 TD 1,079 148 705 2 do 2D 59 8 46 MD 6, 660 115.7 5,892 19,100 3,020

' TD 2, 076 170. 7 2,222 3 -do 0 57 791 MD 5,824 107 13,478 16, 000 167.5

' i TD 2, 176 183 10, 339 4-; butadiene/styrene copolymer 10% 2D 51.2 654 .032 4, 734 107.8 8,870 10,683 3, 020 5 neoprene 2D 40. 5 958 .084 3, 200 90 5, 300 28, 500 11,000 d 40% 2D 50. 5 532 033 4, 900 95 6, 650 10, 000 5, 500 0 629 023 3, 700 142 23, 000 16, 300 25 0 63 85,7 .035 MD 5, 900 82 16,486 ,000 30 TD 3,004 130 13,182 16,000 d0 40% 2D 1, 151 .059 3, 83 4, 415 12, 000 1, 800 do 30% 2D 1 47.6 85,0 .037 MD 5, 255 80 10,200 8, 590 1, 436 TD 1, 931 138 6, 037 12, 740

neoprene BAG 0 53.5 733 .030 MD 4, 605 97 14, 631 9, 700 30 TD 496 1,27 7. 559 10,260 d0 1--------T----,- 30%.21) 52.5 709 .046 MD 2, 073 56.5 4,117 7, 510 5,000 TD 360 63. 75 2, 464

50 neoprene 700/50 neoprene BAG 0 56. 2 741 026 1, 589 161 8, 595 12, 000 40 do 30% 2D 58 840 040 MD 5, 152 97. 5 7, 066 11, 000 1, 100

TD 1, 025 174 2, 489 16, 000 50 methyl Ccllosolve acrylate/ 50 vinyl acetate 30% 2D 54.6 708 .039 MD 4,185 123 3,112 11, 000 1, 205

TD 1, 292 191 1, 052 do 0 54. 4 777 .031 MD 5, 200 108 1, 598 11,000 375 TD 1,379 265 2,734 polyrsobutylena 30% 2D 827 060 MD 3, 597 112 2,095 12,500 4, 568

, TD 1 116 171 3,000 do 0 65 854 .040 MD 5,800 121 6,440 9, 500 45 TD 1 880 187 4,560 methyl acrylate., 40% 2D 55. 3 819 034 MD 7, 687 83 13, 100 6, 300 1, 633

TD 3, 924 114 12,000 10 30% 2D 53 625 .027 MD 6,176 79 11, 000 4,500 1, 433

TD 2, 801 89 12,000 d0 0 53 701 .027 MD 6, 900 66. 5 22, 965 5, 500 151 TD 4,000 96 ,500 50 neoprene/50 butyl 1111213612-- 40% 56 1, 390 089 2, 400 150 2, 600 6, 500 8, 600 50 methyl Cellosolve methacrylate/50 methy1acrylate.. 40%; 62 987 .046 4, 000 108 4, 700 11, 000 2, 650 .,do 0 64 851. 034 5, 100 112 14, 500 10, 500 485 neoprene with stifiening agent. 40% 62 867 035 5, 090 87 10, 700 8. 600 2, 200 d0 0 63. 5 1, 113 041 4, 800 98 21,400 12,000 629 1 MD =machine direction (longitudinal).

3 TD =transverse direction.

sodium salt of alkyl benzene sulfonate. The webs were then squeezed through a two-roll wringer and allowed to dry. A dispersion or latex of the various extensible or hinder polymers indicated in Table 1 was prepared by various known techniques. Many of the latices are commercially available. After the webs were dried, they were then immersed in the dispersion or latex for a standard time, allowed to drain, and again put through the tworoll wringer. The polymer of the latex was thereafter immediately gelled by immersing the impregnated web into a 50% solution of acetic acid in methanol. The impregnated webs were then washed free of soap and acid with running water, and pressed free of excess water. Thereafter, the webs were dried at a temperature below 90 C. to prevent the polymeric binder from curing until the webs were placed between sheets of cellophane and Bristol board and pressed at 500 lbs. per sq. in. pressure and at a temperature above the flow temperature of the particular extensible or hinder polymer employed.

All of the compositions tabulated in Table l were pre- In stretching the initial substantially vapor-impermeable sheets of the present invention to improve vapor permeability, it was discovered that other changes in the general physical properties of the resulting sheet were 9 produced. To investigate these changes, a series of initial composite sheets was prepared, one series employing a butadiene/acrylonitrile copolymer as the binder or extensible material, and another series employing a neoprene binder. All of these were prepared in a manner similar to the procedure described hereinbefore; and, in all cases, crimped staple fibers of polyhexamethylene adipamide, 2 /2 in length and 3 denier/filament, were employed as the relatively non-extensible material. These initial sheets were then stretched from 10% to 50% in one direction or in both directions and then relaxed. After this stretching step, the sheets recovered almost completely their original dimensions except for thickness which increased. For example, after a 50% stretch in two directions, there was almost a 30% increase in thickness.

TABLEZ Change of physical properties with stretchzng Unit Thick- Ten- Elonga- Tinius- .P. V., Example Binder Percent Percent Weight, uess, acity, tion, Modulus, Olsen gm. 100

Binder Stretch gJm inches p. s. i. percent p. s. i. Tear, rn. r.

grams 27 Butadiene/Acrylonitrile copolymer 52.2 10% 1D 762 .033 1,893 195 6,894 18,450 69 d 51.0 20% 1D '742 .030 1, 456 192 3, 169 19, 300 752 53. 0 30% 1D 687 .038 4, 221 95 2, 996 2, 250 3, 870

1D =one direction.

2D two directions.

Table 3 contains the results .of a test which may be employed for the purpose of selecting the most desirable combinations of extensible and relatively non-extensible materials from the adhesion standpoint. Assuming that the forces required to break the adhesive bonds are available, the ultimate limit would be the point at which the adhesion is greater than the breaking strength of the relatively non-extensible fiber. On the other hand, in the case of employing a relatively non-extensible material in the form of particles, the ultimate limit would be represented by a situation wherein the adhesion is greater than the breaking strength of the extensible material.

To obtain a measure of the relative adhesion between various extensible and relatively non-extensible materials, sheets of nylon, i. e., polyhexamethylene adipamide, film (the relatively non-extensible material) were fastened together by means of selected relatively extensible or hinder materials in the form of a glue seal. This was done by placing a film of various samples of relatively extensible materials between adjacent layers of nylon film and pressing the layers at substantially the same temperature and pressure conditions employed to prepare the initial composite sheets. Thereafter, the seals were pulled in a Tinius-Olsen machine to obtain the force necessary to separate the layers of nylon film. Table 3 presents the results of these tests.

It is to be understood that the purpose of the adhesion test is to provide a method of selecting or screening various combinations of extensible and relatively nonextensible materials which exhibit little or no adhesion under the conditions required to composite the components to form the initial sheet. For example, as shown in Table 3, polyethylene does not adhere to nylon film under the conditions employed to form a composite sheet of nylon fibers impregnated with polyethylene, e. g., inserting a mat of intertangled nylon fibers between adjacent films of polyethylene and pressing the composite at 125 C. and 500 p. s. i. The advantage of employing extensible and relatively non-extensible components which exhibit little or no adhesion is illustrated by the fact that the initial sheet composed of nylon fibers impregnated with polyethylene was rendered highly permeable to water vapor by very mild stretching, i. e., hand working. On the other hand, N-methoxymethyl polyhexamethylene adipamide forms a very strong adhesive bond with polyhexamethylene adipamide; and a composite composed of polyhexamethylene adipamide fibers impregnated with N-methoxymethyl polyhexamethylene adipamide must be subjected to relatively drastic treatment, e. g., swelling with hot water and stretching while hot, to eifect any improvement in water vapor permeability.

For the purpose of comparison, Table 3 also includes data on composite sheets fabricated from the materials tested. In all cases, nylon fibers, 2 /z in length and 3 denier/filament, were impregnated with the polymers indicated in Table 3. The method of preparing the initial composite sheet was the same as described hereinbefore except in the case of nylon fibers impregnated with polyethylene. The initial composite sheet in this case was formed by inserting a carded web of intertangled nylon fibers between adjacent films of polyethylene and then pressing the layers at 125 C. and 500 p. s. i. to form a composite sheet.

As shown in Table 3, those combinations of materials exhibiting intermediate adhesion between that of polyethylene to nylon and N-methoxymethyl polyhexamethylene adipamide to nylon (polyhexamethylene adipamide) show intermediate values of leather permeability when stretched to the substantially same extent. As a general observation, it should be mentioned that in situations where it is desired to employ a particular combination of an extensible and a relatively non-extensible material which form relatively strong adhesive bonds, the Work required to produce a certain vapor permeability by stretching may be decreased by reducing the amount of the extensible material in the initial composition.

TABLE 3 Efiect of adhesion of various extensible materzals to nylon 1 Heat Percent Seal Extensible LPV, gmsJ Extensible Material Value Material in Percent sq.

(Adhe- Composite Stretch meters/hr.

sion) Sheet s) Polyisobutylene 39 57 30 8, 514 Polyethylene 0 45 11, 000 Neoprene 735 197 40. 5 30 11,000 MOA/VA 526 53 40 1,700 MCA/AN 536 61 30 2, 000 F/VA 456 50 30 7, 000 DOD 669 40 50 7,000 nylon 1,416 50 1, 500

1 Nylon=polyhexamethylene adlpemide. 1 MCAIVA=methyl Cellosolve aerylate/vinyl acetate copolymer. 3 M OAlAN=methyl Oellosolve aorylate/acrylonitrile copolymer. 4 E IVA=ethy1enelvinyl acetate copolymer. 5 DCD=modlfied dichlorobutadlene. Nylon=N-methoxymethyl polyhexamethylene adipamide. 7 Mild Stretch (Hand Worked). 8 Swelled in hot water and stretched two ways 50% while hot.

TABLE 4 particles are uniformly dispersed throughout the neoprene binder, and it has been found that when such particles are present in the initial composite sheet, less stretching is required to obtain a given LPV than in the case of stretching a similar composition which does not contain particles. I

As many widely different embodiments may be made without departing from the spirit and scope of this invention, it is to be understood that said invention is in no Variation of physical properties with content of extensible material 1 HAND WORKED Unit Tenacity] Tinius- Percent Exten- Weight Thick- Unit Elonga- Olsen L. P. V., sible Material (gms./sq ness Weight, tion, Modulus Tear/Unit gins/100 (Neoprene) meter (inches) p. s. i./ percent Weight, mfl/hr.

gins/m. lbs/gm.

STRETCHED 30% (TWO DIRECTIONS) UNSTRETOHED 1 Neoprene used as the extensible material.

The process and synthetic leather compositions of the present invention have been specifically illustrated in the foregoing examples in which nylon (polyhexamethylene adipamide) fibers have been employed as the relatively non-extensible or reinforcing material. It is to be understood, however, that the relatively non-extensible material, as disclosed hereinbefore, may be in the form of particles. Furthermore, other types of natural and synthetic fibers may be employed such as other types of polyamides and interpolyamides, such as polyhexamethylene sebacamide, polycaproamide, and various interpolyamides as described and claimed in U. S. Patent No. 2,285,009, polyethylene terephthalate, polyacrylonitrile, rayon and various natural fibers such as cotton and wool, and mixtures of two or more of the aforesaid fibers. Various polymeric materials may be employed as the relatively non-extensible materials in the form of particles of various sizes, such as particles of synthetic polyamides, and other synthetic linear thermoplastic materials. Furthermore, particles of thermosetting resins such as urea-formaldehyde, phenol-formaldehyde, etc., may be employed in particulate form. In addition, various inorganic materials may be employed in particle form such as titanium dioxide, zinc oxide, talc, clay and various diatomaceous earths.

It is within the scope of the present invention to employ mixtures of fibers and particles of a relatively nonextensible material. For example, a mat of intertangled nylon fibers may be thoroughly impregnated with a latex of neoprene having an amount of zinc oxide particles dispersed therein (as much as 40% of the particles, based on total solids, may be used). The zinc oxide wise restricted except as set forth in the appended claims.

I claim:

1. A process for preparing permeable sheet material which comprises hot pressing fibrous material with a dry, thermoplastic, extensible polymeric binder material, said binder material being substantially more extensible than said fibrous material and having a flow temperature below the deformation temperature of the fibrous material and constituting from about 30% to about 70% by weight of the total weight of fibrous and binder materials, at a temperature above the flow temperature of the binder and below the deformation temperature of the fibrous material to form a substantially water vaporimpermeable compacted sheet and stretching said compacted sheet from about 10% to about 50% of its original dimension to form a water vapor-permeable, substantially liquid-resistant sheet.

2. The process of claim 1 wherein the said compacted sheet is stretched in two directions from about 20% to about 50% of its original dimensions.

3. A process for preparing permeable sheet material which comprises hot pressing a non-woven mat of fibers with a dry, thermoplastic, extensible, polymeric binder material, said binder material being substantially more extensible than the fibers and having a fiow temperature below the deformation temperature of the fibers and constituting from about 30% to about 70% by weight of the total weight of fibers and binder, at a temperature above the fiow temperature of the binder and below the deformation temperature of the fibers to form a substantially water vapor-impermeable compacted sheet and stretching said compacted sheet from about 10% to about 50% of its original dimension to form a water vaporpermeable, substantially liquid-resistant sheet.

4. The process of claim 3 wherein the said compacted sheet is stretched in two directions from about 20% to about 50% of its original dimensions.

5. The process of claim 4 wherein the fibers comprise nylon.

6. The process which comprises impregnating a nonwoven mat of nylon staple fibers with elastomeric binder material having a flow temperature below the deformation temperature of nylon, said binder material constituting from about 40% to about 60% by weight of the total weight of fibers and binder material, drying said impregnated mat, hot pressing said dried, impregnated mat at a temperature above the flow temperature of the binder and below the deformation temperature of nylon, whereby to form a compacted sheet, and thereafter stretching said compacted sheet equally in two directions from about 20% to about 50% of its original dimensions to form a water vapor-permeable, substantially liquidresistant sheet.

References Cited in the file of this patent UNITED STATES PATENTS Nae rt Feb. 3, Meers July 31, 'Kenworthy July 22, Respess Dec. 23, Respess Apr. 14, Clifford June 14, Schur Jan. 1, Read Mar. 24, Austin Nov. 17, Francis Dec. 29, Kopplin Apr. 3, Hawley Jan. 7, Raymond et a1. June 21, Slack et a1. Oct. 25, Shearer Sept. 9, Rand Mar. 10, Biefeld et a1 Mar. 30,

FOREIGN PATENTS Great Britain Sept. 14,

Claims

1. A PROCESS FOR PREPARING PERMEABLE SHEET MATERIAL WHICH COMPRISES HOT PRESSING FIBROUS MATERIAL WITH A DRY, THERMOPLASTIC, EXTENSIBLE POLYMERIC BINDER MATERIAL, SAID BINDER MATERIAL BEING SUBSTANTIALLY MORE EXTENSIBLE THAN SAID FIBROUS MATERIAL AND HAVING A FLOW TEMPERATURE BELOW THE DEFORMATION TEMPERATURE OF THE FIBROUS MATERIAL AND CONSTITUTING FROM ABOUT 30% TO ABOUT 70% BY WEIGHT OF THE TOTAL WEIGHT OF FIBROUS AND BINDER MATERIALS, AT A TEMPERATURE ABOVE THE TEMPERATURE OF THE BINDER AND BELOW THE DEFORMATION TEMPERATURE OF THE FIBROUS MATERIAL TO FORM A SUBSTANTIALLY WATER VAPORIMPERMEABLE COMPACTED SHEET AND STRETCHING SAID COMPACTED SHEET FROM ABOUT 10% TO ABOUT 50% OF ITS ORIGINAL DIMENSION TO FORM A WATER VAPOR-PERMEABLE, SUBSTANTIALLY LIQUID-RESISTANT SHEET.