Nonwoven Sheets Made From Rectangular Cross Section Monofilaments

Abstract

Nonwoven sheets of continuous synthetic polymer, e.g., stereoregular polypropylene, monofilaments which have elongated, e.g., rectangular, cross sections with aspect ratios of at least about 3:1 and are disposed randomly and are substantially discrete from each other except at crossover points in the sheet. These sheets may be made by extruding the polymer through appropriately shaped orifices, partially cooling the resulting monofilaments, drawing them with a pneumatic jet and depositing them on a collecting device.

Description

FIELD OF THE INVENTION

This invention concerns nonwoven sheets made from continuous synthetic polymer monofilaments which have elongated cross sections. More particularly, it relates to such sheets in which the monofilaments have substantially rectangular cross sections and are discrete from each other except crossover points.

BACKGROUND OF THE INVENTION

Nonwoven articles of continuous oriented monofilaments have been made and used for a variety of textile and related uses. Primarily, they have been used as carpet backings, insulation and disposable clothing. In general, such articles have been made by spinning continuous monofilaments of spinnable synthetic polymers, drawings the freshly spun filaments and depositing then on a collecting surface. A large portion of the art relating to these articles concerns the way in which the freshly spun filaments are drawn to improve the tensile strengths of the filaments. The strengths of such freshly spun monofilaments have been improved by drawing then between rolls rotating at different speeds. More recently they have been drawn by "spin drawing" techniques. In "spin drawing" partially cooled, partially crystalline filaments are fed into a pneumatic jet. The jet works on the ejector principle. That is, the expanding gaseous medium passing through the jet carries and accelerates the filaments causing then to be drawn or oriented.

Developments concerning methods for depositing the drawn filaments have been directed towards improving the isotropic properties of the finished, nonwoven product made from the filaments. Such developments have usually concerned particular patterns for laying the drawn filaments down on the collecting surface for keeping the filaments discrete, i.e., not entangled or bundled.

For the most part filaments which have been used to make nonwoven sheets have had circular cross sections. Certain noncircular cross section filaments have been suggested as equivalent to circular cross section filaments for use in nonwoven materials. In the related yarn art noncircular cross section monofilaments have been used to simulate natural fiber shapes, provide a particular appearance or increase covering power. Heretofore, it has not been recognized that the cross-sectional shapes of the filaments may materially affect the tensile properties of finished nonwoven sheets made therefrom.

INVENTION DESCRIPTION

It has been found that nonwoven sheets of continuous oriented synthetic polymer monofilaments having an elongated cross section in which the aspect ratio is at least about 3:1 and in which the monofilaments are deposited randomly and are substantially discrete from each other except at crossover points have unexpectedly better tensile properties than corresponding sheets of circular cross section monofilaments. This finding is especially surprising in view of the fact that individual monofilaments of circular cross section and individual monofilaments of elongated cross section and comparable denier have substantially similar tensile properties.

Filaments having elongated cross sections are used to make the unexpectedly strong nonwoven sheets of this invention. These filaments are characterized as having an aspect ratio (the ratio of cross section length of cross section width) of at least about 3:1 and usually in the range 3:1 and about 8:1. The shape of the cross section of these filaments will usually be substantially rectangular. Substantially rectangular cross sections include those having two sets of essentially parallel flat surfaces which intersect each other essentially at 90.degree. angles (true rectangle) as well as those having two slightly rounded oppositely disposed planar surfaces, the respective ends of which are joined by rounded, smaller surfaces. These latter cross sections are characterized as elliptical. The surfaces of these filaments will be substantially regular. That is, they should be relatively smooth and free of large bumps, protrusions or lobes.

The particular shapes of the elongated cross section monofilaments used in this invention are dependent upon the shaped of the orifice or die from which they are spun and extent to which they are oriented or drawn. Oriented monofilaments of elliptical cross sections are formed by extruding the polymer melt from orifices having a substantially rectangular cross section. Such monofilaments emerge from the orifices with rectangular cross sections; but, as they are drawn their cross sections reshape to ellipses. Oriented monofilaments which are substantially rectangular are made by spinning the melt from a die of rectangular cross section in which the longitudinal sides are somewhat concave. When such monofilaments are drawn the concave sides of the rectangle flatten so that they are substantially parallel. Devices for making elongated cross section monofilaments are well known. See for instance U.S. Pat. No. 3,179,770.

The freshly spun monofilaments are drawn from an original cross-sectional area as the filament emerged from the die of about 0.004 to 4.0 mm..sup.2 down to a cross-sectional area of about 0.00005 to 0.008 mm..sup.2. In terms of a circular filament this is equivalent to reducing the diameter from about 0.10-1.0 mm. down to 10-100 microns (1 to 60 denier). This drawing is done after the filaments are partially cooled and while the polymer is in at least a partially crystalline state.

When the filaments are made of substantially crystalline, stereoregular polypropylene, after the drawing they will have tenacities of about 2 to 5 g. per denier and elongations of about 50 to 400 percent, depending, of course, upon the particular drawing conditions.

The drawing orients the polymer structure and greatly increases the filament's tensile strength. While roll drawing or spin drawing may be used, it is preferable to spin draw the monofilaments used to make the unique, nonwoven sheets of this invention.

Accordingly, the freshly spun monofilaments are fed, usually in a bundle of about 5-500, into the main chamber of a pneumatic jet. Air, or other inert gases may be used as the gaseous drawing medium. In order to draw the monofilaments sufficiently the air velocity will usually be about 200-800 m. per sec. within the main chamber. Air travelling at these velocities will pick up the monofilament bundle and draw the filaments at speeds in excess of 1,500 m. per min. and up to the speed which causes the filaments to break. The draw rate will preferably range between 2,500-5,000 m. per minute. Particular jet designs may be used to keep the individual monofilaments separate while they pass through the jet. Other techniques, such as charging the filaments, may also be used to keep then from entangling.

The thus drawn monofilaments may be laid down onto a collecting surface as they emerge from the pneumatic jet. If the filaments have been drawn over rolls instead of spin drawn they may be passed into a pneumatic filament-handling device. Such devices are well known and operate on the same principle as the pneumatic spindrawing jet. However, the gas velocities are below those which stretch the filaments in the handling device. In order to keep the gas emerging from the jet with the filaments from entangling the filaments as they are deposited on the collecting surface or from blowing them off the surface, the surface may be a wire screen or other porous medium which allows the gas to pass through it.

Withdrawal and/or dissipation of the high-velocity gas emerging from the jet may be facilitated by applying suction to the side of the porous collecting surface opposite that on which the filaments are being laid.

The collecting surface will move away from the zone in which the bundles of nonentangled substantially parallel filaments are laid down. This motion away from the lay down zone may be conveniently achieved by using an endless moving screen belt as the collecting surface. In order to form webs having substantially isotropic tensile properties the rate at which the filaments are forwarded onto the collecting surface will be several times that at which the surface is moving away from the lay down zone. Usually, the filament forwarding speed will be about 10-1000 times the speed at which the surface is moved away from the lay down zone.

Filament patterns in the laid-down web will depend upon the relative motion of the filaments and the collecting surface. By moving the lay down device transverse to the direction of the take away or by keeping the lay down device stationary and moving the filaments either by oscillating the lay down device or baffling the filaments, various filament patterns will be achieved. The pattern should not be such as to affect substantially the isotropicity on the laid-down web. It is within the scope of this invention to incorporate within the nonwoven sheets a variable amount of monofilaments having a nonelongated cross section shape; provided such added filaments do not entirely eliminate the improved strength properties obtained through the use of elongated cross section fibers in accordance with this invention. Such fibers of other shapes affect other properties of the sheet as well as the strength of it. In particular, fibers having a round or circular cross section may be incorporated into the nonwoven sheet. The presence of such circular fibers improves the hand of the resulting sheet by reducing the stiffness thereof. By a proper choice in the ratio of round to elongated fibers, sheets having both improved hand and high strength may be obtained.

The thus laid web of continuous monofilaments of elongated cross section is dry--and even in this form has superior tensile properties relative to corresponding webs of circular cross section monofilaments.

Weight of the webs of this invention normally ranges from about 0.5 to 44 ounces per square yard. Their densities will usually be about 0.2 to 0.7 g. per cc. They will normally be 0.005 to 0.3 inch thick.

This web or batt is useful as such for insulation, paper reinforcement, nonwoven fabric reinforcement and filters. It may be further treated if desired by needle punching, calendering, heat sealing, sewing or knitting depending upon its intended end use. This web is also susceptible to other conventional treatments such as adhesive bonding. These bonded webs are useful as carpet backing, sacking, paper and fabric reinforcement, felt carpets, nonwoven fabrics, etc. As a general rule, the finished nonwoven product with the elongated cross section filaments have superior tensile properties to corresponding products made from conventional monofilaments.

The synthetic polymers which may be used in this invention are those which may be spun or otherwise formed into continuous monofilaments. Such polymers include crystalline polypropylene, crystalline polyethylene, poly-4-methyl-1-pentene, copolymers thereof, polyvinylchloride, polyesters such as polyethylene terephthalate, polyamides such as nylon and the like.

EXAMPLES

The following examples illustrate the nonwoven sheets of this invention. These examples are not intended to limit the invention in any manner. Unless otherwise indicated, percentages are by weight.

EXAMPLE 1

Commercial, substantially crystalline, stereoregular polypropylene with a melt flow rate of 4 was melted in an extruder with the final extruder zone having a temperature of 320.degree. C. It was then melt spun through a two-hole spinnerette having round holes with a diameter of 1.0 mm. The spinnerette temperature was 300.degree. C. The polymer was extruded at a rate of 4.1 g./hole/minute. The resulting circular cross section filaments then fell 32 feet to a pneumatic jet which accelerated the filaments to a linear velocity of 2.400 yards per minute thereby drawing them. The filaments were then collected on a screen with a vacuum behind it to form a web or batt. This batt was slightly compressed between two rollers for easier handling.

The tenacity of these filaments, the breaking strength of this web or batt were determined by conventional ASTM tests. These properties are tabulated in tables I and II, respectively.

EXAMPLE 2

Filaments and a batt were prepared under conditions similar to those of example 1 except that the filaments were accelerated by the pneumatic jet to 5,200 yards per minute after being melt spun. The tenacity of these filaments and the break strength of this are also shown in tables I and II.

EXAMPLE 3

Filaments and a batt were prepared under conditions similar to those of example 2 except that the filaments were extruded at a rate of 3.3 g./hole/minute and were accelerated by the pneumatic jet to 5,300 yards per minute. The tenacity of these filaments are shown in table I and batt breaking strength in table II.

EXAMPLE 4

Filaments and a batt were prepared under conditions similar to those in example 1 except that the spinnerette had 3 rectangular orifices which were 3.0 mm. long and 0.3 mm. wide. The filaments were extruded at a rate of 2.8 g./hole/minute and were accelerated by the pneumatic jet to 2,200 yards per minute. The filament properties are shown in table I and batt properties in table II.

EXAMPLE 5

Filaments and a batt were prepared under conditions similar to those of example 4 except that the filaments were accelerated by the pneumatic jet to 4,000 yards per minute. Filament properties are shown in table I and batt properties in table II.

EXAMPLE 6

Filaments and a batt were prepared under conditions similar to those of example 4 except the filaments were extruded at a rate of 3.3 g./hole/minute and accelerated by the pneumatic jet to 4,600 yards per minute. Filament properties are shown in table I and batt properties in table II.

EXAMPLE 7

Filaments and a batt were prepared under conditions similar to those of example 4 except that the spinnerette had 3 rectangular orifices which were 2.0 mm. long and 0.5 mm. wide. The filaments were extruded at a rate of 3.3 g./hole/minute and were accelerated to 5,200 yards per minute by the pneumatic jet. Filament properties are shown in table I and batt properties in table II.

EXAMPLE 8

The nonwoven batt of example 1 was passed through a needle loom to produce a needled felt with about 200 needle penetrations per square inch. The results of a breaking strength test on the resulting fabric are shown in table III.

EXAMPLE 9

The nonwoven batt of example 4 was needle felted as in example 8. The results of a breaking strength test on the resulting fabric are shown in table III.

EXAMPLE 10

The nonwoven batt of example 3 was pressed in a hot calendering machine at about 145.degree. C. to heat seal the fibers together. The breaking strength of the heat-sealed nonwoven fabric is shown in table IV.

EXAMPLE 11

The nonwoven batt of example 7 was heat sealed in a manner similar to example 10. The breaking strength of the heat-sealed nonwoven fabric is shown in table IV.

EXAMPLE 12

The nonwoven batt of example 6 was heat sealed in a manner similar to example 10. The breaking strength of the heat-sealed nonwoven fabric is shown in table IV. ##SPC1## ##SPC2## ##SPC3## ##SPC4##

The tenacity data in table I indicate there is no significant difference in tenacity between round filaments and rectangular filaments of comparable denier and orientation. In contrast the strength data reported in tables II through Iv illustrate that various types of nonwoven sheets made from rectangular cross section monofilaments have substantially better breaking strength than similar sheets made from comparable circular cross section monofilaments.

Claims

I claim:

1. Nonwoven sheet of continuous synthetic polymer monofilaments, said monofilaments having elongated cross sections in which the aspect ratio is in the range of 3:1 to 8:1, and said monofilaments being disposed randomly and substantially discrete from each other except at crossover points.

2. The nonwoven sheet of claim 1 wherein the cross sections of the monofilaments are substantially rectangular.

3. The nonwoven sheet of claim 1 wherein the individual cross-sectional areas of the monofilaments are about 0.00005 to 0.008 mm..sup.2.

4. The nonwoven sheet of claim 1 wherein the polymer is substantially crystalline, stereoregular polypropylene.

5. The nonwoven sheet of claim 1 wherein the polymer is substantially crystalline, stereoregular polypropylene, the cross sections of the monofilaments are substantially rectangular and have individual areas of about 0.00005 to 0.008 mm..sup.2 and the aspect ratio is in the range of 3:1 and 8:1.