US3619339A

US3619339A - Porous nonwoven film-fibril sheet and process for producing said sheet

Info

Publication number: US3619339A
Application number: US840049A
Authority: US
Inventors: William Lee Garrett
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1969-07-08
Filing date: 1969-07-08
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09
Also published as: CA934916A; DE7025718U; GB1286345A

Abstract

A nonwoven film-fibril sheet of substantially continuous plexifilamentary strands is point-bonded and perforated, within specified limits, to provide a porous, strong, nonpapery fabriclike sheet for, e.g., garment end uses.

Description

United States Patent Inventor William Lee Garrett Wilmington, Del.

Appl. No. 840,049

Filed July 8, 1969 Patented Nov. 9, 197 l Assignee E. l. du Pont deNemours and Company Wilmington, Del.

POROUS NONWOVEN FILM-FIBRIL SHEET AND PROCESS FOR PRODUCING SAID SHEET 10 Claims, No Drawings US. Cl 161/109,

Int. Cl B32b 7/00, D04h 3/ 14 Field ofSearch l61/73,80,

Primary Examiner- Robert F. Burnett Assistant Examiner-Raymond O. Linker AttorneyEugene Berman ABSTRACT: A nonwoven film-fibril sheet of substantially continuous plexifilamentary strands is point-bonded and perforated, within specified limits, to provide a porous, strong, nonpapery fabriclike sheet for, e.g., garment end uses.

POROUS NONWOVEN FlLM-FIBRIL SHEET AND PROESS FOR PRODUCTNG SAID SHEET BACKGROUND OF THE INVENTION This invention concerns a porous, strong, nonpapery nonwoven film-fibril sheet of substantially continuous plexifilamentary strands. In particular, it concerns an improved sheet of this type having a specific combination of point-bonded and open hole area as well as processes for making these sheets.

Nonwoven sheets prepared from continuous networks of film-fibril elements are known. These film-fibril elements are tiny molecularly oriented elements often less than four microns thick. They may be prepared by a number of methods such as by fibrillating an oriented film, or by the flash spinning methods of Blades and White U.S. Pat. No. 3,081,519. ln the latter patent, the film-fibrils are described as continuous three-dimensional networks along and across a substantially continuous strand. These continuous networks regardless of their origin are generally, as well as herein, termed plexifilaments. The plexifilaments may be formed into nonwoven sheets by a variety of methods. The network material for example may be deposited on a moving belt in intersecting and overlapping layers. After cold rolling, a coherent film-fibril sheet is obtained. This highly suitable method for preparing a film-fibril sheet by flash spinning is described in Steuber U.S. Pat. No. 3,169,899.

The processing of film-fibril sheets to improve their textile quality has been the subject of much research. Particularly desirable qualities include the strength, porosity and softness of the sheet. However, a given desirable quality in film-fibril sheets is often obtained only with a sacrifice of one or more other desirable qualities. For example, although it is possible to improve the strength of such sheets by hot embossing, the products often tend to be stiff, papery, and relatively nonporous. These products are not suitable for end-uses wherein a fabric like nonwoven sheet is required, e.g., for apparel, for wrapping the inner springs of mattresses and the like.

The main object of the present invention therefore is to provide a nonwoven film-fibril sheet that is porous, strong and nonpapery and suitable for end uses wherein a fabric like nonwoven sheet is required. Another object of this invention is to provide suitable processes for making these nonwoven sheets.

SUMMARY OF THE INVENTION The product of this invention comprises a porous, nonwoven, film-fibril sheet of substantially continuous plexifilamentary strands, the sheet having a. about 200 to 800 point-bonds per square inch of the planar area of the sheet, the point-bonds extending through the thickness of the sheet and comprising about 5 to 10 percent of the planar area of the sheet, and

b. a multiplicity of perforations extending through the thickness of the sheet, the total cross-sectional area of the perforations being at least about 3 percent of the planar area of the sheet with the average cross-sectional area per perforation being less than about l square inches.

The process of this invention comprises:

a. providing a film-fibril sheet of substantially continuous plexifilamentary strands,

b. contacting about 200 to 800 spaced portions per square inch of at least one surface of the sheet with sufficient heat and pressure to point-bond about 7 and 13 percent of the planar area of the sheet, and

c. perforating the sheet to at least about 500 perforations per square inch, each of the perforations being provided by separating film-fibril elements of the sheet less than about 0.02l in. with the points of perforation being such that at least about percent, but not more than about percent of the sheet remains point-bonded, the perforating providing the total and average cross-sectional areas of perforations stated above.

DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENTS THEREOF The basic material for the product and process of this invention is a nonwoven plexifilamentary film-fibril sheet structure. The plexifilaments are preferably of the type disclosed in Blades and White U.S. Pat. No. 3,081,519 and may be collected into sheet form as disclosed in Steuber U.S. Pat. No. 3,169,899. The plexifilaments are prepared from synthetic filament-forming polymers or polymer mixtures. A preferred class of polymers is the crystalline, nonpolar group consisting mainly of crystalline polyhydrocarbons, most preferably linear polyethylene. Common textile additives such as dyes, pigments, antioxidants, delusterants, antistatic agents, reinforcing particles, adhesion promoters, ion exchange materials, and U.V. stabilizers may be mixed with the polymer solution prior to extrusion.

The film-fibril sheet is generally consolidated (e.g., compacted in a direction normal to the plane of sheet) to a density of between about 0.1 and 0.4 g./cm. (about 6.24 and 25.0 lb./ft.") or greater. Generally the sheet thickness is in the range of about 0.07 to 0.40 mm. (about 0.003 to 0,0l6) and has a surface area of at least 2 meter /gram. This sheet is then modified by point-bonding and perforating, as described more fully hereinafter to provide a unique combination of structural characteristics which is essential to the utility of the nonwoven sheet as a porous, strong, nonpapery material suitable for fabrication into comfortable garments or satisfactory springwrap' pocketting. The characteristics that must be properly combined in the product of this invention are the amount of point-bonding, the amount of the perforation area and the average size of the perforation in the sheet.

The first criterion for satisfactory nonwoven sheets of this invention concerns the percent of the sheet area that is covered with point bonds. As described below, in the test procedure for measuring point-bonded area, a point-bond is essentially a translucent window which extends through the thickness of the plexifilamentary sheet. The amount of pointbonding is the chief contributor to the tensile, delamination and Mullen-burst strengths of the sheet. Although point bonding" techniques (more fully described hereinafter) are known and described for nonperforated sheets in commonly assigned, copending U.S. application Ser. No. 575,843 filed Aug. 29, 1966 now U.S. Pat. No. 3,478,l4l and Dutch application No. 6,707,792, published Aug. 25, l967, it has been found in'perforated sheets of this invention that the pointbonded area as defined in the test descriptions below must be maintained between 5 and 10 percent to maintain strength properties at satisfactorily high level. In addition, satisfactory sheet surface stability can be maintained. If the point-bond area exceeds about 10 percent in perforated sheets (that would otherwise be within this invention), the sheet becomes excessively stifi" and papery, as indicated by noise level measurements described below. lt is essential that the percent point-bonded area be provided as a multiplicity of smaller bonds rather than fewer large ones, i.e., between about 200 and 800 points per square inch of the planar area of the sheet. The preferred range is between about 300 and 500 pointbonds per square inch of sheet.

The second criterion for satisfactory performance of the nonwoven sheet concerns the percent open area. It has been found that as long as an open, or perforated, area of at least 3 percent is provided, the fabric has a Frazier permeability or porosity of at least ftP/ftF/min. A porosity of at least 60 ftF/ftF/min. is desirable in fabrics intended for garments that will give an active wearer moderate body comfort in humid conditions. Although 3 percent open area is a satisfactory minimum, open areas up to about 6 percent or even 10 percent, providing the sheet with a Frazier porosity of up to I50 ft."*/ft. /min. are preferred. A large number of small perfora tions is preferred. For example, as few as 600 pertoralions per square inch amounting to at least 3 percent open area is quite satisfactory, but about l,500 to 2,000 perforations/in. are

preferred. The large number of small perforations permit retention of high strength while minimizing or eliminating the papery rattle and stiffness of the material.

The final criterion in the combination of essential structural characteristics of the nonwoven products of this invention concerns the maximum size of the perforations placed in the material. It has been found that if all other product characteristics are within this invention, high tensile, tear, delamination and burst strengths can be maintained when the average perforation size is below 10" in As one increases perforation size above this level, the strength properties of the nonwoven sheets decrease markedly. Unexpected, sudden decreases occur in the delamination and burst strengths as average perforation size is increased beyond 10" in. per perforation. Smaller perforation size is preferred, e.g., about 10" in. or smaller; perforation sizes as low as the physical limitations of the perforating equipment used e. g., needle size) will permit, are most preferred For the nonwoven sheets of this invention, it is preferred that they have an average tensile strength of at least about 8 lb./inch, a tongue tear of at least about I lb./inch, a Mullen burst of at least about 40 psi, a delamination strength of at least about 0.2 lb./inch, a Frazier permeability at least about 70 ftP/ftF/min. and noise level below about 15 decibels.

In order to produce a sheet of this invention with to 10 percent point-bonded area, it is generally necessary to pointbond the starting material to between about 7 and 13 percent point-bonded area. The extra amount of point-bonding is necessary to compensate for the point-bonds, usually destroyed during the subsequent perforating steps. Although it is theoretically possible to pick the points of perforation such as to destroy either all of the bonds or none of the bonds, a random perforation of the sheet poinhbonded to about 7 to l3 percent will be such that between about 5 and 10 percent point-bonds remain.

The point-bonding steps may be carried out by the general method disclosed in commonly assigned, copending application Ser. No. 575,843 now Pat. No. 3,478,141 and in Dutch application No. 6,707,792, published Aug. 25, 1967. In this method, the starting nonwoven material is passed between the nip of two rolls. One roll has a heat-conductive surface with about 200 to 800 hard bosses/in. of roll surface. The bosses extend radially from the surface of the roll by about 0.020 inch, or at least about 2.5 times the thickness of the sheets being treated. The total area of the tips of the bosses is equivalent to about 5 to 15 percent of the planar area of the sheet. The opposing roll has a resilient surface with a Durometer hardness between 60 and 90. Sufficient heat is provided through the heat-conducting roll and sufficient pressure is provided between the rolls to form point-bonds (translucent windows). The remaining area of the sheet remains essentially unaffected or is lightly bonded. For the preferred linear polye hylene plexifilamentary sheets of this process, the temperature of the heated roll is usually maintained between about 150 and I85 C., the pressure in the nip between about and 125 pounds/inch depending on temperature and resilient roll hardness and speed between 25 and ISO yards per minute. Temperature is controlled so that excessive melting or perforation are avoided in this step. It is preferred that the point-bonds be uniformly distributed through the planar area of the sheet.

The perforating step may be carried out in a number of ways; in single or multiple passes, on one or several sheet thickness at a time. In order to retain the high tensile, tear, burst and delamination strengths that have been imparted to the sheet by the preceding steps in the process, while providing the degree of open area needed for satisfactory porosity, it is necessary that at least about 500 perforations be placed into each square inch of the material and that the perforations be provided by separating film-fibril elements of the sheet less than about 0.021 inch. A conventional needle loom operating with the usual vertical movement of the needles into and out of the sheet is a suitable apparatus for the perforating step.

Perforation may be carried out equally as well by means of a rotary pin perforator. The needles may be circular or triangular in cross section. In perforating the sheets, the needles should in the main, rearrange (separate) the fibrous elements of the sheet rather than break them. Some breakage and destruction of point bonds is acceptable, as previously men tioned. However, excessive disruption with its attendant loss of strength properties, must be avoided. It has been found that satisfactory results can be obtained when between about 500 and 2,000 perforations/in. are placed in the sheet by tapered needles of circular cross section which measure no more than about 0.018 inch in diameter at any point on the needle which enters the sheet. The corresponding maximum dimension for equilateral triangular cross section needles is about 0.018 inch in the height of the triangle and about 0.021 inch at the widest point. At perforation densities of 500 to 2,000 per in."', needles of dimensions greater than those just described cause excessive damage to the sheet and significant losses in the strength characteristics. It is preferred that the perforations be uniformly distributed throughout the planar area of the sheet.

TEST PROCEDURES In the following examples, sheet characteristics are measured by means of the following methods, in which TAPPI indicates Technical Association of Pulp and Paper Industry and in which ASTM indicates American Society of Testing Materials:

Basis weight is measured according to TAPPl-T4l0 08-61 or ASTM Method D646-S0.

Sheet thickness is measured by means of a BS] gauge at a pressure of 0.07 p.s.i. according to the ASTM Method Dl 777-64.

Tensile strength and elongation are measured on an lnstron Tester according to ASTM Method D828-60 except that the specimens are elongated at a constant rate of 40%lminute instead of being broken in 10-5 seconds. Measurements reported hereinafter are the average of machine and crossmachine determinations.

Tongue tear is measured on an lnstron Tester essentially according to an adapation of Method No. 5134 Federal Specifications Textile Methods CCC-T-l9 l b.

Frazier permeability (or porosity) is measured in accordance with ASTM Method D737-46 in cubic ft. of air per minute passed through each ft. of sheet at a pressure differential of 10 inches of water.

Mullen burst strength is measured in accordance with ASTM Methods D1 I17 or D77463T or TAPPI T-403 M-53.

Delamination strength may be measured essentially in accordance with ASTM Method D-825-54, Method BStandard Method of Test for Ply Adhesion of Paper.

Noise level: The lack of papery rattle in the products of this invention is easily recognized subjectively by crumpling or shaking a sample and noting the quality of the sound produced. Quantitatively this characteristic is determined as follows. An I8-inch-l8-inch square sample is attached at one corner to the top of the exit of a Z-inch-diameter nozzle through which 601-10 ftP/min. of air is passed. The nozzle is positioned so that the air flows at an angle of 30 to the horizontal surface (e.g., a large flat table) which is located 1% inches below the bottom edge of the nozzle. The air flow causes the specimen to flutter and produce sound. A capacitance microphone having essentially flat response in the range of 40 to l5,000 hertz audiofrequencies is used in conjunction with (l) a Sound and Vibration Analyzer (General Radio Type 1564A) capable of tuning to any rs-band in the range of audible frequencies and (2) a recorder (e.g., General Radio Type l521-A) to plot the amplitude vs. frequency of the sound generated by the sample. The microphone is positioned face down about 3 inches above the fluttering specimen and about two-thirds of the distance along the diagonal starting at the corner ofthe specimen attaching to the nozzle.

The above equipment is used to determine the noise level" of the sample as follows: With air blowing through the nozzle, but with the specimen removed, a %-octave scan of the background noise is made covering the range from 250 to 25,000 Hz. Background noise must be controlled so that it is to db. in the range of 5,000 to 10,000 Hz. A second 1S- octave scan over the 250 to 25,000 Hz. frequency range is then made with the specimen replaced in position as described above. It is convenient to record both scans (i.e., background and total sound including that generated by the fluttering sample) on the same plot. The difference between background noise and total sound level is determined at the four :r-octave band center frequencies of 5,000 6,300, 8,000 and 10,000 Hz. and the arithmetic mean of these differences is recorded as the "sound level of the specimen. The lower the sound level, the softer, more comfortable, more drapeable and less papery is the nonwoven sheet.

Perforation density, or number of perforations per unit area of sample, is detennined by counting the number of individual perforations visible in a representative area of the specimen when viewed with 3X magnifier.

The method for determining the percent open area, the average and maximum perforation sizes is as follows. A sample is placed between two 3%-inch by 4-inch projector-slide cover glasses along with a mask having an opening exactly one inch in diameter. The assembly is then taped on the edges to provide a slide for a photographic enlarger which is used to project an image of all the perforations within the 1-inchdiameter mask at a magnification of eight diameters. A print or photogram of this image is then made. The photogram is marked with a grid of one inch squares, starting from two perpendicular diameters. Eighteen representative squares, or about 36 percent of the photogram area, are selected for further examination. The area of each perforation in the representative squares is measured with the aid of a microscope providing 10X magnification and fitted with a Whipple micrometer disc. A Whipple micrometer disc is a glass reticule which fits the microscope eyepiece at its focal plane and has engraved on it a 7 mm. by 7 mm. square, which is divided into four squares, each of which is further divided into 25 squares; one of the small squares, located near the center of the large square, is again subdivided into 25 still smaller squares. The reticle is calibrated against a known dimension so that the area of each small square of reticule superimposed on the focal plane is known. The area of the image of each hole in the 18 representative squares is then measured and corrected for microscope and enlargement magnification to give the actual perforation area. These areas are tallied to provide the measured distribution of perforation sizes from which the average perforation size and the percent of the sheet area taken up by the perforations (i.e., open area") are determined. From the tally of perforation sizes the maximum perforation size is defined as the perforation area which is at least as large as 95 percent of the perforations counted.

The point-bonded area of the sheets of this invention can be determined by a variety of microphotographic techniques. A suitable technique is as follows. The point-bonds, as discussed above, are small isolated zones extending substantially through the thickness of the plexifilamentary sheet wherein the plcxifilaments are compacted and cohered so as to exclude air and provide continuous paths for light transmission from top to bottom surface of the sheet. The plexifilaments in these areas may be partially fused but in general do not substantially lose their fibrous characteristics. Under low magnification the areas appear transluscent.

The number of point-bonds per unit area is determined by direct count on representative portions of the sheet. The area" referred to in point-bonds per unit area" is the planar area of one side of the sheet. Representative samples of the sheet are placed on microscope slides. Each slide is examined under the microscope with the surface on which the embossed pattern was impressed facing the microscope objective. Light is transmitted through the specimen. The microscope is equipped for making photographs at known magnification, preferably 50 or lOOX. At least 10 point-bonds per sample are photographed. The point-bonded area on each photograph appears as a white or light area of somewhat irregular shape against the dark background of the opaque portion of the sheet. The area of the point-bond does not appear continuously clear in the photograph. Such appearance would denote destruction of the fibrous character of the plexifilaments and formation of fused film. Rather, the typical bond photograph has bright spots scattered through the white or light area. The point-bond area on the photograph is measured conveniently with a planimeter and then corrected for the magnification. The point-bond areas are thus measured on several specimens to provide a statistical average point-bond area, which when multiplied by times the number of point-bonds per unit area gives the percent point-bonded area of the sheet.

EXAMPLES The invention is further illustrated by reference to the following examples.

EXAMPLE I A point-bonded film-fibril plexifilamentary sheet is prepared in this example essentially in accordance with the method of Steuber US. Pat. No. 3,169,899. This sheet provides a control and is the starting material for the nonwoven sheets of the subsequent examples.

Linear polyethylene having a density of 0.95 gram per cubic centimeter and a melt flow rate of 0.9 gram per 3 minutes as determined by ASTM Method D-l238-57T, Condition E, is flash-spun from a 12.5 percent solution in trichlorofluoromethane. The solution is continuously pumped to a spinneret assembly at a temperature of 185 C. and a pressure above 1,245 p.s.i.g. The solution is passed in the spinneret assembly through a first orifice to a pressure letdown zone and finally into the surrounding atmosphere. The resulting plexifilamentary strand is spread by means of a contoured rotating battle, electrostatically charged by passage through a corona charging zone between a multiple-point corona discharge electrode and a grounded target plate, and collected on a moving belt in overlapping intersecting layers. The sheet is then passed between a pair of rolls about ll inches in diameter. These rolls lightly consolidate the sheet at a pressure of about 10 lb./in. of roll length to a thickness of about 0.005 inch. The basis weight is about 1. l4 oz./yd.

The sheet is then point-bonded by passage of the sheet between a patterned metal roll and a resilient roll. The roll having the pattern is of metal, is 12% inches in diameter and the resilient roll covered with a rubber layer of about 56-inch thickness and having a diameter of 15 inches. The Durometer hardness of the resilient roll is about 75. Heat is provided to the metal roll by means of pressurized steam inside the roll. Temperature of the steam is 165 C. The sheet is wrapped through an arc of about 90 on the heated roll and is embossbonded at a speed of about ftlmin. In each case, the pressure exerted between the two rolls is sufficient to create the point-bonded areas of the sheet at the pressure points. The embossing roll of this example has lines of bosses arranged circumferentially around the embossing roll surface. ln each line there are 28 bosses per inch. The lines of bosses are one-sixteenth inch apart, center-to-center (i.e., 16 per inch of roll axis). The top of each boss is approximately square in cross section, measuring 0.015 inch on a side. Each boss extends about 0.020 inch from the roll surface. The sheet is thus provided with 448 generally rectangular point bonds per square inch. The resulting sheet has a point-bonded area of l2.2 percent.

The percent point-bonded area of film-fibril sheets created on this apparatus can be varied by adjusting roll pressure and temperature. The characteristics of the sheet materials prepared in this example are summarized in table I.

EXAMPLES ll-Vlll The sheet of example 1 is subjected to perforation on a rotary perforator (Model 18H, Robert A. Main 8L Sons, Co., Wyckofi, N.J.). The machine is equipped with a fractional horsepower electric motor drive and an Va-inch thick felt covering of F-3 hardness on the backup roll. The perforating roll, which is 2% inches in outer diameter, has steel pins of 0.030-inch base diameter projecting radially 0.125 inch from its surface. The pins are tapered to a sharp point over the full projecting length. The pins are spaced one-eighth inch apart eenter-to-center in axial rows. The rows are placed at a circumferential spacing of one-eighth inch along the surface of the roll diameter. The pin centers are offset in each succeeding row by 0.030 inch in the following sequence: (a) reference position, (b) 0.030 inch to the left of reference, (c) 0.030 inch to the right of reference position and (d) repeat of the sequence starting again with the reference position. Since the efiective diameter of the imaginary cylindrical surface connecting the pin tips is greater by one-fourth inch than the outer diameter of the metal roll, the pin density in the sheet is less than the nominal 64 64pins/in. of roll surface. A sheet of ordinary paper is passed through the nip formed by the pinned cylinder and the felt covered backup roller set to engage the pins to maximum depth of 0.010 inch. The holes per square inch produced in the paper show the effective pin density in the sheet to be 57.2 points/in". Successive passes through this nip accumulate penetrations over the area of the sheet in that the pins generally penetrate unperforated areas in successive passes.

A series of samples is prepared by varying numbers of passes through the nip and depths of fixed penetration as follows:

The physical, structural and sonic characteristics of these products are summarized in table l.

Whereas, the sheet of example 1 is stiff and papery as indicated by the sound level measurement of over 50 decibels, the sheets of examples 11 through V, which are sheets of this invention, are porous, nonpapery and flexible while still retaining satisfactory strength and surface stability characteristics. The sheets of examples VI, VII and VIII do not have the required percent open area (i.e., at least 3 percent). These sheets with less than 3 percent open area are insufficiently porous and too noisy to be useful for comfortable garments or for inner spring pocketting.

EXAMPLE lX-X In these two examples, samples of point-bonded plexifilamentary sheets with excessively large perforations are prepared.

Plexifilamentary sheet materials of about 1.1 and 1.6 oz./yd. are prepared and point-bonded with the same pattern as in example I but to a point-bonded area of 9.0 and 9.8 percent, respectively. These materials are needle-punched on a 12% inch wide, downstroke, single-board needle loom Fiber/Locker" laboratory felting machine made by James Hunter Machine Company of North Adams, Mass.) as follows to produce the sheets of examples IX and X. The loom has 46 needles per inch of board length in the cross machine direction.

For perforating the 1.1 oz./yd. material of example 1X, the needle board is equipped with round ZS-gauge barbless needles. Each needle has an average diameter of 0.033 inch, a sharp point and a taper extending over about 0.160 inch of the 1-inch total blade length. The machine is adjusted to permit penetration of the needles 0.180 inch into the sheet. The material is advanced 0.200 inch incrementally after the needles are withdrawn and remains stationary while being penetrated. Each square inch of material is provided with 230 penetrations per pass. The material is passed through the machine four times providing a grand total of 920 penetrations per square inch with the material being inverted for each successive pass, thus providing half the penetrations from each side.

In example X, the 1.6 oz./yd. point-bonded plexifilamentary sheet material is subjected to 1,840 penetrations per square inch by 25-gauge barbless felting needles. The needles are equilateral triangular in cross section having average height of the triangular cross section of 0.034 inch. The taper of the sharp point of the needle extends over about 0. inch of the total blade length of 1 inch. The machine is adjusted to permit penetration of the needles 0.500 inch into the sheet. The material is advanced 0.200 inch intermittently after the needles are withdrawn and remains stationary while being penetrated. The material is passed through the machine eight times, being inverted for each additional pass so that it receives half the perforations from each side.

As shown in table I, the resultant sheets of example IX and X are excessively weak in tensile, burst, and delamination strengths. in addition, the sheets are fuzzy, limp, and do not have the covering power desired for garment uses.

These maleffects are believed due to the excessive damage done to the sheet by the high penetration density with the large-diameter needles. Note that the average perforation size is greater than the required maximum of 10 in. for sheets of this invention. Another indication of the excessive damage wrought by perforating in this fashion to obtain excessively large perforations is the reduction in point-bonded area from about 9 to about 5 percent.

EXAMPLES Xl-Xlll In these examples, a sheet that is point-bonded, softened and creped is compared to products of this invention which are additionally perforated.

The point-bonded sheet of example 1 is softened by a conventional stroking and creping techniques. The properties and characteristics of this material are listed as example X1 in table 1. Note the high noise level and lack of porosity in the sample.

The plexifilamentary, point-bonded and softened material of example X1 is further processed to provide in examples X11 and X111 additional sheets of this invention. The further processing comprises perforating the material on a conventional needle loom of the double-acting type (i.e., having downstroke and upstroke needle bears in tandem). The loom is operated at a punching speed of 230 strokes per minute. Both needle beams are equipped with 43-gauge (Torrington System felting needles. Each needle is equilateral-triangular in cross section. The average height of the triangular cross section is 0.015 inch. The total blade length is 0.625 inch. The needle tapers to a sharp point over a distance of 0.060 inch from the tip. Each needle has one barb located 0.250 inch from the tip. There are 250 needles per inch of cross-machine direction length of each needle board, or a total of 500 nee dles per inch of machine width. In operation, the material is advanced incrementally through the machine. The sheet material is stationary while being penetrated by the needles and is advanced only while the needles are removed from the sheet. The machine is adjusted to permit 0.220 inch penetration of the needles into the sheet material. The barbs do not enter the material. In example Xll, the incremental advance of the sheet is 0.294 inch. Each square inch of material is then subjected to a total of 1,700 penetrations. 1n example Xlll, the

incremental advance is 0.588 inch and the material is subjected to 850 penetrations per square inch.

As shown in summary table I, the sheets produced in examples XI] and Xlll are of the present invention and a significant 3. Sheet of claim 1 wherein said total cross-sectional area of said perforations is at least about 6 percent.

4. Sheet of claim I wherein said total cross-sectional area of said perforations is less than about 10 percent.

improvement over the sheet of example XI. 5 5. Sheet of claim 1 having at least about 600 perforations per square inch. UTILITY 6. Sheet of claim 1 having between about 1,500 and 2,000 The sheets of this invention are suitable for use as disposaf gr r f' q i I I I ble multiple-use garments in a wide range of environments, as I a f if I gz f Iona dust covers on the underside of furniture, as fabric covers or 8 a 3: aerin $25232; fi fibril wrappings for the coil springs of mattresses, as table cloths, as p p p g p dra e linin and as man other such items The broad utili sheet compnsmg:

P W y a. providing a film-fibril sheet of substantially continuous ty of the fabrics derive from their superior combination of tr n th (tensile te r burst and del t' rf t plexlfilamemary strands 2.5 g t a amma f i b. contacting about 200 to 800 Spaced portions Per Square a l E f; f z fi pq Z :3 inch of at least one surface of said sheet with sufficient VLOUS a F 66 o ls on P heat and pressure to pointbond about 7 to 13 percent of ecte to urt er processing operations suc as pnntmg, yethe planar area of Said sheet and "if; coaung 'mpregnatmg heat Sealmg lammatmg and many c. perforating said sheet to at least about 500 perforations o ers.

TABLE I Example Number I II III IV V VI VII VIII IX X XI XII XIII Basis weight, oz./yd. 1.14 1.14 1.1:; 1.20 1.11 1.11 1.10 1.15 1.10 1.58 1. 30 1.28 1. 2? Thickness, Ill- 111 5.0 5.7 5.8 6.1 5.2 5.0 4.6 5.8 5.7 8.6 4.9 7.3 6. 5 Average tensile, lbs./in 12.9 7.7 8.6 9. 3 9.7 12.4 10.2 11.8 6.8 3.3 12.6 9.3 10. 8 Elongation, percent.. 28 24 25 27 NM 28 24 25 25 2s 2s 2 2 Tongue tear, lbs 2. 2 1. 2 1.4 1. 5 1. 6 1.8 2.0 2. 1 NM NM 2. 9 2. 3 2. Frazier permeability, c.1.rn./1t. 0.2 126 112 00 79 32 69 26 306 197 0. 4 100 7 g Mullen burst, p.s.i 63 48 46 44 47 53 51 53 32 32 71 51 6 Delamination, 0.27 0.21 0. 2s 0. 24 0.21 MN 0.28 NM 0.18 0.01 0. 34 0.25 0.36 Sound level, db +50 +7.0 +7.3 +10.d +12.5 +25.2 +18.1 +2l.1 +3 +3 +2.8 +8. 2 Point-bonded area, percent 12. 2 8. 4 8. 9 7.8 9. 7 11. 1 9. 8 11. 2 4. 5 3. 6 8. 1 5.6 6. 2 Perforation density, number/1m 0 1,444 1,129 1,5s2 1,817 991 916 417 753' 590 0 1,112 640 Average perforation size, 10-in. 0 70.1 42.1 25.8 18.0 38.6 26.3 37.5 246 114 0 38.7 45.5 Maximum perforation size, 10 111 0 99.2 82.6 50.0 37.5 62.9 51.8 48.3 474 295 0 60.6 67. 3 Open area, percent 0 10.1 4.8 3.9 3.4 1.5 2.4 1.6 18.6 6.9 0 5.0 3. 0

NOTE: NM=n0t measured.

What is claimed is: per square inch, each of said perforations being provided 1. Aporous, nonwoven film-fibril sheet of substantially conby separating film-fibril elements of said sheet less than tinuous plexifilamentary strands having about 0.02linch, with the points of perforation upon said a. about 200 to 800 point-bonds per square inch of the sheet being such that at least about 5 percent, but not planar area of said sheet, said point-bonds extending 40 more than about [0 percent of said sheet remains pointthrough the thickness of said sheet and comprising about bonded, said perforating providing a total cross-sectional 5 to 10 percent of the planar area of said sheet, and area of said perforations of at least about 3 percent of the b. a multiplicity of perforations extending through the planar area of said sheet with the average cross-sectional thickness of said sheet, the total cross-sectional area of area per perforation being less than about l0 square said perforations being at least about 3 percent of of the inches. planar area of said sheet, with the average cross-sectional 9. Process of claim 8 wherein said perforating is done in area per perforation being less than about 10" square multiple passes. inches. 10. Process of claim 8 wherein said perforating is to less 2. Sheet of claim 1 having about 300 to 500 pomt-bonds per than about L000 perforations per square inch square inch.

Patent No Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION William Lee Garrett Dated November 9,

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column l0, Claim 8, line i l,

line

Claim 1, line #7,

should read should read should read should read 10- "64 6 should read 64 3T, 10 should read 10 Claim 7,

line 10,

10 should read 1C? 10 should read 10 and d thiS 251th day of April 1912.

ROBERT GOT'ZSCHALK Commissioner of Patents should read l

Claims

2. Sheet of claim 1 having about 300 to 500 point-bonds per square inch.
3. Sheet of claim 1 wherein said total cross-sectional area of said perforations is at least about 6 percent.
4. Sheet of claim 1 wherein said total cross-sectional area of said perforations is less than about 10 percent.
5. Sheet of claim 1 having at least about 600 perforations per square inch.
6. Sheet of claim 1 having between about 1,500 and 2,000 perforations per square inch.
7. Sheet of claim 1 wherein said average cross-sectional area per perforation is at least about 10 5 square inch.
8. A process for preparing a porous nonwoven film-fibril sheet comprising: a. providing a film-fibril sheet of substantially continuous plexifilamentary strands, b. contacting about 200 to 800 spaced portions per square inch of at least one surface of said sheet with sufficient heat and pressure to point-bond about 7 to 13 percent of the planar area of said sheet, and c. perforating said sheet to at least about 500 perforations per square inch, each of said perforations being provided by separating film-fibril elements of said sheet less than about 0.021 inch, with the points of perforation upon said sheet being such that at least about 5 percent, but not more than about 10 percent of said sheet remains point-bonded, said perforating providing a total cross-sectional area of said perforations of at least about 3 percent of the planar area of said sheet with the average cross-sectional area per perforation being less than about 10 4 square inches.
9. Process of claim 8 wherein said perforating is done in multiple passes.
10. Process of claim 8 wherein said perforating is to less than about 2,000 perforations per square inch.