Manufacture of film fabrics

Abstract: An apertured thermoplastic fabric is formed by extruding a film of a molten thermoplastic polymer, embossing the film in a nip between a smooth surfaced roll and a patterned roll, wherein the patterned roll includes a plurality of discrete, raised projections disposed in a pattern and each of which includes a sharp distal end; applying sufficient pressure at the nip for causing a plurality of small apertures to be formed in the extruded film in substantially the same pattern as the pattern of discrete, raised projections; orienting the apertured film in both the cross-machine direction and in the machine-direction, and thereafter embossing the oriented film on at least one surface with a plurality of raised members spaced closer to each other than the spaces between the apertures in the oriented film, to thereby provide a matte-type finish on at least one surface. The formed fabric also forms a part of the invention.

Description: FIELD OF THE INVENTION This invention relates to a process of fabricating an apertured film fabric and to the fabric prepared thereby. More especially this invention relates to a process of forming an extruded, apertured film having a generally cloth-like appearance and "hand", rendering it highly suitable for a variety of applications wherein fibrous, woven and non-woven fabrics have been employed, e.g. facing sheets for absorbent sanitary products such as sanitary napkins, incontinence pads, and the like. BACKGROUND OF THE INVENTION It is well known to fabricate apertured thermo-plastic webs. Most of the prior art apertured thermoplastic webs lack the desired appearance and hand to make them suitable for use as a facing sheet for sanitary products as well as for a variety of other applications wherein substantially cloth-like properties are desired and/or required. In a substantial number of processes high density polyethylene, polypropylene, and other high modulus, crystal-line polymers have been formed into plastic sheets, and then embossed with a variety of patterns designed to form thin areas in such sheets. The embossed sheets are then oriented either uniaxially or biaxially to cause the crystalline web to open up or fracture into a network of apertures in the thinned regions, see US-A-3,914,365; US-A-3,441,638; US-A-3,488,415; US-A-4,075,379; US-A-3,922,327; US-A-4,207,375; US-A-4,186,781; US-A-4,274,251; and US-A-4,568,596. In US-A-3,386,876 there is disclosed a process for manufacturing an apertured, thermoplastic web in which the apertures are defined by a plurality of uniaxially drawn strands which are separated by undrawn portions. One approach to forming the apertures before stretching the film is to punch the apertures into the web in a repeating, hexagonal pattern. The material used is polypropylene. As is shown in Figure 10 of this patent, the biaxially oriented web, which initially is formed with a hexagonal pattern of apertures, has a plurality of spoke-like uniaxially drawn strands emanating from a central undrawn hub portion, and terminating at their opposite ends in similar undrawn portions. In US-A-4,842,794, there is disclosed a two-sided embossing arrangement which, unlike prior art single-sided embossing arrangements, can be used to form an apertured film from soft, non-crystalline polymers. The two-sided embossing pattern formed slits through the web during melt embossing, and the film is subsequently biaxially stretched to open up the slit regions into a pattern of defined apertures. It has also been realised that subjecting extruded low density polyethylene to a two-sided embossing will form a product having strand-like polymer sections projecting in multiple planes, providing an undesired tactile feel to the product. It has now been realised that if a thermoplastic film is extruded from a polymer having a relatively low secant modulus and embossed using a single-sided embossing technique, an apertured thermoplastic film having a textile like appearance, softness and hand will be formed. SUMMARY OF THE INVENTION According to the invention in one aspect there is provided a process for the manufacture of an apertured thermoplastic fabric characterised by the steps of: a) extruding, in the machine direction, a film from a molten thermoplastic polymer having a secant modulus of no greater than about 50,000 psi; b) passing the extruded molten film in the machine direction through the nip of two rollers, one of said rollers having a substantially smooth surface and the other including a pattern comprising a plurality of separate sharp projections; c) applying sufficient pressure at the nip to form a pattern of small apertures in the extruded film; d) orienting the apertured film in both the cross-machine direction and in the machine-direction thereby provide a matte-type finish on said at least one surface. Preferably the oriented film is provided with a micro-matte finish by directing the film at a temperature below its melt temperature through an embossing nip between a patterned roll and a smooth surface roll, the patterned roll having projections spaced closer together than the spaces between apertures in the oriented film, to thereby provide a matte-like finish on at least one surface of the film. In the preferred embodiment of this invention the thermoplastic film is formed from a polymer having a 2% secant modulus no greater than 50,000 psi. (This is in distinction to the prior art, high density polyethylene compositions, which have a 2% secant modulus in excess of 100,000 psi). In the preferred embodiments of this invention the thermoplastic film is made from a polymer selected from the group consisting of low density polyethylene, low density polyethylene copolymers (such as ethylene vinyl-acetate, ethylene butyl acrylate, ethylene methyl acrylate, ethylene ethyl acrylate, ethylene methyl methacrylate, ethylene acrylic acid, ethylene methacrylic acid, or ethylenic ionomers), linear low density polyethylene, very low density polyethylene, ultra low density polyethylene, or blends of polymers in which the major component is low density polyethylene; low density polyethylene copolymers, linear low density polyethylene, very low density polyethylene, ultra low density polyethylene, or the above-mentioned polymers and polymer blends containing pigments or processing aids. In one preferred embodiment of this invention the discrete raised projections providing apertures in the film are in the form of sharp cones which are disposed in a repeated regular geometric pattern. Most preferably the repeating pattern is hexagonal. In one preferred embodiment of the process the extruded film is extruded from a polymer having a secant modulus in the range of 1%-2% strain, of no more than 20,000 psi, as determined either by ASTM D790-86 or ASTM D882-83. All references to "secant modulus" refer to the secant modulus anywhere in the range of 1%-2% strain, as determined in both the machine direction and cross-machine direction on a one (1) mil blown film under ambient conditions (i.e. nominally 70°F, and 50% humidity) employing either ASTM D8820-83 or D790-86. In another aspect the invention provides a fabric comprising a film formed from a molten thermoplastic polymer having a secant modulus of no greater than about 50,000 psi, the film having a pattern of small apertures therein, being substantially planar and being free of continuous strand-like sections disposed in different planes. Film fabrics of this invention are well suited for use as facing sheets for absorbent sanitary products of the type that generally include an absorbent central core made from fluff or other well known absorbent material, and a moisture impervious plastic back sheet. DESCRIPTION OF THE DRAWINGS The above and other attendant advantages of this invention will become readily apparent from the detailed description which follows-when taken in conjunction with the following drawings wherein: Figure 1 is a fragmentary plan view of one the apertured web made in this invention; Figure 2 is an enlarged view of the circled portion on Figure 1; Figure 3A to 3D apparatus show schematically for carrying out the process of the invention continuously; Figure 4 is a fragmentary plan view showing the web of the invention being stretched in the cross-machine direction in the portion of the apparatus shown in Figure 3B; Figure 5 is a partial plan view of the embossing roll in the film, in accordance with the process of this invention; Figure 6 is a sectional view taken along line 6-6 on Figure 5; Figure 7 is a partial plan view of the embossing roll employed to form the matte finish in the apertured web; and Figure 8 is a photograph at 25X magnification of a portion of the upper surface of the formed web. DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS A molten, thermoplastic web 10 is extruded through a die 12 of a conventional extruder 14 (Figure 3A). The thermoplastic polymer, details of which will be described later, has a melting point of approximately 115°C, and the die 12 is maintained at a temperature in the range of about 240oC to about 250°C, at its exit end. As can be seen in Figure 3A, the web 10 exits the die in a substantially vertical orientation and immediately enters the nip 16 between a patterned, metal embossing roll 18 and a smooth surfaced, metal backup roll 20. Both the embossing roll and the backup roll preferably are maintained at a temperature in the range of between 20°C and 70oC, and most preferably at approximately 45°C. The smooth surface of the backup roll 20 should be formed with a crown, i.e. a convex curvature in the axial direction. In an exemplary embodiment the roll 20 is provided with a curvature such that the radius of the roll in the centre is approximately 4 mils greater than the radius of the roll at each axial end. A crown on the roll tends to compensate for flexing of the rolls, which occurs during embossing. (It may be possible to eliminate the crowning of the roll 20 by increasing the size of both rolls so that they do not flex). The embossed web 10 passes about the periphery of the smooth surfaced backup roll 20 and is directed through a predraw unit 30 including four smooth surfaced metal rolls 32, 34, 36 and 38. The number of rolls employed does not constitute a limitation on the broadest aspects of the present invention. In a much preferred embodiment of the invention the most upstream roll 32 in the predraw unit 30 is driven at a rotational speed of approximately 70 feet/minute (21.33 m/min), and each of the following rolls 34, 36 and 38 is driven at a rotational speed approximately one foot per minute (30 cm/min) faster than the immediately preceding roll; resulting in the web 10 being stretched approximately 6% in the machine direction. The primary purpose of the predraw unit is not to stretch the web, but to maintain tension on the web. Referring to Figures 3B and 4, the web 10 is directed from the predraw unit 30 into a tenter unit 40, in which the web is stretched in the cross-machine direction. As is illustrated in Figure 3B, the tenter unit 40 comprises three heated zones 42, 44 and 46, located between the entrance 48 and exit 49 thereof. Preferably the overall length of the tenter unit 40 is approximately 45 feet (13.71 m). The web enters this unit with a cross-machine direction width of 23 inches (58.4 cm) and exits the unit with a cross-machine direction width of 60 inches (1.524m). Thus, the web 10 is stretched approximately 2.6 times of its entrance width (approximately 260%) between the entrance 48 and the exit 49 of the tenter unit 40. The tenter unit 40 is a conventional unit sold by Marshall and Williams in Rhode Island, USA, and includes opposed, transversely spaced-apart chain-driven rows of clamping jaws (not shown) to automatically clamp the longitudinally extending edges of the web 10, as the web enters the tenter section at entrance 48. These jaws, as they are being moved in the machine-direction, are directed apart in the cross-machine direction to transversely stretch the web 10, as is illustrated in Figure 4. Referring to Figure 3C, after the web 10 has been stretched in the cross-machine direction in the tenter unit 40 it is directed into a machine direction stretcher unit 50. This latter stretcher unit includes an entrance section 52, an idler section 54 and an exit section 56. The entrance section 52 includes two large driven rolls 58 and 60 (e.g., 10 inches (25.4 cm) in diameter), which preferably are driven at a speed in the range of approximately 75 to 78 feet per minute (22.86 to 23.77 m/min) which is the speed at which the web 10 exits the tenter section 40 at 49. A small diameter roll 62 engages driven roller 58 to define a drive nip 64 at the entrance section 52, to direct the web into the stretcher unit 50 at the desired entrance speed. In a preferred embodiment of this invention the rollers 58 and 60 are driven at approximately 76 feet per minute (23.16 m/min). Still referring to Figure 3C, the exit section 56 includes a plurality of large diameter, driven rolls, four of which are illustrated at 70, 72, 74 and 76. The most downstream driven roll 76 engages roll 78 to define a nip therebetween, for controlling the exist speed of the web 10. Preferably the speed of the driven rolls 70, 72, 74 and 76 is in the range of approximately 130 to 140 feet per minute (39.6 to 42.6 m/min). Located between the entrance section 52 and exit section 56 is the idler section 54 including a plurality of separate idler (non-driven) rolls which are but spaced close together. The number of idler rolls can be varied, preferably eleven are present. In the illustrated embodiment the stretcher unit 50 includes four distinct and separated heated zones. Most preferably each of the heated zones is at a temperature slightly less than the melt point of the polymer. Referring to Figure 3, the rolls in the entrance section 52 and the most upstream idler roll of the idler section 54 are maintained at a temperature of approximately 106°C. The idler section 54 is provided with a set of 4 (or 5) idler rolls downstream of the first idler roll, which are heated to a temperature of approximately 106°C, and an immediately subsequent, or further downstream set of 5 (or 4) idler rolls, which are maintained at a temperature of approximately 103°C. This latter set of idler rolls is followed by a single idler roll and the rolls in exit section 56, which preferably are maintained at a temperature of approximately 100°C. As a result the web 10 is gradually reduced in temperature as it is directed through stretcher unit 50. In another embodiment of a machine-direction stretcher unit (not illustrated) only three distinct and separate heated zones are provided. In this embodiment the entrance section and idler section include the identical roll arrangement as entrance section 52 and idler section 54 of the machine direction stretcher unit 50. However, the rolls in the entrance section and the most upstream idler roll of the idler section in this embodiment are maintained at a temperature of approximately 103°C - 104°C, as opposed to 106°C. The first set of four (4) or five (5) idler rolls downstream of the first idler roll are heated to a temperature of approximately 105°C to 106°C (substantially the same as in the stretcher unit 50); however, the further downstream set of five (or four) idler rolls are maintained at a temperature of approximately 109°C (as opposed to 103°C in the stretcher unit 50). Unlike the stretcher unit 50, this stretcher unit does not include a single idler roll following the second set of five (or four) idler rolls, and does not include a separate exit section 56. When using stretcher unit 50, with the driven rolls 58 and 60 at the entrance section 52 maintained at a speed of approximately 76 feet per minute (23.16 m/min) and the driven rolls 70 and 72 at the exit section 56 are maintained at a speed of approximately 120 to 130 feet per minute (36.57 to 39.6 m/min) the stretcher unit 50 stretches the web 10 approximately 158%-171% in the machine direction. Referring to Figure 3D, the apertured web 10 leaving the stretcher unit 50 is directed into a texturizer and winder section 90. The stretcher unit 50 is a continuous extension of the web forming line, and includes a driven metal pattern roll 92 forming a nip 94 with a smooth surfaced, resilient backup roll 96. The backup roll 92 preferably is a metal roll coated with a rubber, plastic or paper, selected to reduce the gloss of the web. A preferred roll is made from a synthetic polymer, identified as Supertex by Beloit Corporation of Kalamazoo, Michigan, USA. Most preferably the roll 92 is temperature controlled to a temperature that is dependent upon a number of factors, including speed of rotation, diameter and the pressure at nip 94. As shown, the texturizer and winder section 90 is an off-line unit, adapted to receive the film after it has been wound into a roll form from the machine direction stretcher unit 50. In this off-line unit the pattern roll 92 is 12 inches (30.4 cm) in diameter, is run at 200 to 400 feet per minute (60.9 m to 121.8 m/min), is loaded to apply a pressure of approximately 1,600 pounds per lineal inch at the nip 94, and is not heated. The backup roll 96 is approximately 16 inches (40.6 cm) in diameter and preferably is temperature controlled to a temperature of approximately 100°F (37°C). However, when the backup roll is made of Supertex it may maintain a desired temperature without the use of any special temperature control equipment. The texturizer and winder section 90 can be included in line with the remainder of the film forming system, as is illustrated in the drawings. Where the web is not heated during the texturizing operation it is not necessary to provide any subsequent cooling operation. Accordingly, the completed web is wound into a continuous roll form, immediately after leaving the texturizer and winder section 90. Where it is necessary to heat the pattern roll and/or backup roll to effect texturizing, the patterned roll can be followed by a cooling roll 98, which will be maintained at a suitable temperature to set the web prior to the web being directed over guide rolls 100 and 102, and then on to a driven winder roll 104. A very important feature of the process of this invention is the combination or selection of an embossing pattern and the thermoplastic polymer composition. The same embossing roll pattern does not necessarily form the same pattern of apertures in the completed web of different polymers. The embossing roll pattern 18 shown in Figures 5 and 6 includes a plurality of discrete cones 200 arranged in a repeating, hexagonal pattern, as is illustrated by phantom lines in Figure 5. In the most preferred form of this invention the height of the cones is approximately equal to or slightly greater than 0.0105 inches (0.02667 mm) and the diameter of each cone, at the base thereof, is approximately 0.023 inches (0.5842mm) with the bases of adjacent cones being in engagement with each other. As can be seen best in Figure 5 phantom lines 202a and 202b, which interconnect adjacent cones forming part of each hexagon, are parallel to each other and extend substantially perpendicular to the circumferential direction (i.e., machine direction) of the roller. Most preferably the circumferential spacing between adjacent phantom lines 202a and 202b is approximately 0.040 inches (about 1 mm), resulting in a repeating pattern of approximately 25 hexagons per linear inch (25.4 mm) in the circumferential direction around the roll. The projections are disposed in a regular geometric pattern (most preferably hexagonal) that repeats less than 50 times per inch (25.4 mm) in the circumferential direction around the roll; more preferably less than 35 per inch (25.4 mm), and even more preferably no more than 25 per inch (25.4 mm). In the preferred form of this invention the concentration of projections is such that the geometric pattern repeats 25-35 times per inch (25.4 mm) in the circumferential direction around the roll. The hexagonal pattern of cones can be arranged such that the cones interconnected by phantom lines 202a and 202b are aligned in the circumferential direction around the roll 18, i.e. perpendicular to the axis of said roll. In other words, the hexagonal pattern can be reoriented 90 degrees from the position illustrated in the drawings. When the hexagons are reoriented such that the phantom lines 202a and 202b each are oriented in the circumferential direction around the roll, then the concentration of repeating hexagons referred to earlier will be in the axial direction along the roll, as opposed to the circumferential direction around the roll. It should be understood that when other geometric patterns of cones are provided they likewise may be reoriented 90 degrees, whereby the concentration of the repeating pattern will be in the axial, as opposed to the circumferential direction around the roll. In a much preferred form of this invention the thermoplastic film is made from an ultra low modulus thermoplastic polymer having a 2% secant modulus of no more than 20,000 psi, and most preferably is an ultra low density polyethylene/octene copolymer sold under the trade name Attane 4701 by the Dow Chemical Company of Midland, Michigan, USA. This polymer is an extremely tough polymer having a melt index of 1 gram/10 min, as determined by ASTM test method D-1238E. The density of this polymer is approximately 0.913, as determined by ASTM test method D-792. The above preferred polyethylene/octene copolymer includes slip and anti-block additives and a phosphonite secondary antioxidant. A fluoropolymer copolymer lubricant is present to enhance processibility. A preferred lubricant employed in this invention is Ampacet 10562, manufactured by Ampacet Corporation of Tarrytown, New York, USA. Also present is a whitening agent or concentrate such as one employing titanium dioxide (Tio₂). In a preferred embodiment of the invention the whitening agent employed is Ampacet 41914. This agent, which is utilised in the form of a master batch, includes 50% low density polyethylene, 50% titanium dioxide, less than 1% aluminum stearate (a lubricating agent) and 200 parts/million of Irganox 1010 (an antioxidant). Another ingredient is calcium carbonate filler. In a preferred embodiment of this invention the calcium carbonate is part of a master batch, sold under the designation Ampacet 11147. This product includes 50% calcium carbonate, and 50% low density polyethylene as a carrier for the calcium carbonate. Referring to Figures 1, 2 and 8 the unique web construction formed in accordance with this invention is illustrated. Specifically, the web 10 is a substantially planar, film-like member having a thickness preferably-in the range of from about 1.5 mils to 4.5 mils, more preferably in the range of 1.5 mils and 3.0 mils, and most preferably approximately 2.5 mils. The web includes a plurality of film-like areas 206 (as opposed to thin strand-like sections disposed in different planes) which are interrupted by a plurality of apertures 208 disposed in a hexagonal array. Each of the apertures 208 has a substantial elliptical configuration formed by a substantially smooth continuous curved surface free of sharp edges. Located in at least the upper surface of the film-like areas is a micro-texturizing pattern which is provided by the earlier identified pattern roll 92. As shown in Figures 2 and 8, this pattern includes a plurality of compressed, linear lines that are spaced closer to each other than the spaces between the apertures in the oriented film, to thereby provide the micro matte-type finish. Specifically, this matte-type finish includes two separate sets of compressed parallel lines 210 and 212, which are formed by a series of raised, linear ribs 210a and 212a, respectively, formed on the surface of pattern roll 92 (see Figure 7). The spacing between the lines 210 and between the lines 212 measured from centre to centre, is in the range of 3-10 mils, and most preferably is 6.7 mils. As can easily be seen in Figures 2 and 8, the sets of compressed lines 210 and 212 formed in the web 10 intersect each other, with each set of lines 210 and 212 being disposed at an acute angle to the machine-direction of web formation to form diamond-shape bosses 214 in the web. By employing the texturizing, embossing operation the film-like areas between apertures are provided with a more textile-like "hand" and appearance (e.g. lower gloss) than would be obtained without such an operation.

Claims: A process for the manufacture of an apertured thermoplastic fabric characterised by the steps of: a) extruding, in the machine direction, a film from a molten thermoplastic polymer having a secant modulus of no greater than about 50,000 psi; b) passing the extruded molten film in the machine direction through the nip of two rollers, one of said rollers having a substantially smooth surface and the other including a pattern comprising a plurality of separate sharp projections; c) applying sufficient pressure at the nip to form a pattern of small apertures in the extruded film; d) orienting the apertured film in both the cross-machine direction and in the machine- direction thereby provide a matte-type finish on at least one surface. A process according to Claim 1, characterised in that the secant modulus of the polymer from which the film is extruded is no more than about 20,000 psi. A process according to Claim 1 or 2 characterised in that the thermoplastic polymer comprises one or more of low density polyethylene, low density polyethylene copolymers, linear low density polyethylene, very low density polyethylene, ultra low density polyethylene, and blends thereof. A process according to Claim 3 characterised in that the thermoplastic film comprises an ultra low density polyethylene/octene copolymer having a secant modulus of no more than 20, 000 psi. A process according to any preceding Claim characterised in that the projections on the patterned roller are disposed in a regular pattern, and the concentration of projections in one of machine direction and cross machine direction is such that the pattern repeats less than 50 times per inch (25.4 mm). A process according to Claim 5 characterised in that the pattern is a hexagon. A process according to any preceding Claim characterised in that the projections are substantially cone-shaped, and adapted to form a plurality of small apertures, each of which is defined by a substantially smooth, continuous curved surface free of sharp edges in the film when the film is oriented in both the machine direction and the cross machine direction. A process according to any preceding Claim characterised in that the apertures film is provided with a micro-matte finish by passage through the nip between a patterned roll and a smooth roll, the patterned roll having raised regions spaced closer together than the apertures in the film. A process for the preparation of an apertured thermoplastic fabric comprising the steps of: a) extruding, in the machine direction,a film from a molten thermoplastic polymer selected from the group consisting of low density polyethylene, low density polyethylene copolymers, linear low density polyethylene, very low density polyethylene, ultra low density polyethylene, and blends thereof; b) passing the extruded molten film in the machine direction through the nip of two metal rollers, one of said rollers having a substantially smooth surface and the other of said rollers including a plurality of discrete, raised projections disposed in a pattern, each projection terminating in a sharp distal end, said distal ends of said projections being separated from each other and being completely surrounded by continuous, surface regions located proximally of said distal ends; c) applying sufficient pressure at the nip for causing a plurality of small apertures to be formed in the extruded film in substantially the same pattern as the pattern of discrete, raised projections of said other of said rollers; d) orienting said apertured film in both the cross-machine direction and in the machine-direction to increase the open area of said apertures, said oriented film, with the apertures formed therein, being susbtantially planar and being free of continuous strand-like sections disposed in different planes; and e) embossing the oriented film on at least one surface with a plurality of raised members spaced closer to each other than the spaces between the apertures in the oriented film to thereby provide a matte-type finish on said at least one surface. A process according to Claim 9, wherein the film is extruded from a molten thermoplastic polymer including blends of polymers in which the major component is selected from the group consisting of low density polyethylene, low density polyethylene copolymers, linear low density polyethylene, very low density polyethylene and ultra low density polyethylene. A fabric comprising a film formed from a molten thermoplastic polymer having a secant modulus of no greater than about 50,000 psi, the film having a pattern of small apertures therein, being substantially planar and being free of continuous strand-like sections disposed in different planes. A fabric according to Claim 11, including a micro-matte finish on at least one side.

Inventors: HOVIS EDWARD E, JOHNSON ERIC D, HOVIS, EDWARD E., JOHNSON, ERIC D.
Applicants: APPLIED EXTRUSION TECHNOLOGIES, APPLIED EXTRUSION TECHNOLOGIES, INC.
Application year: 1992
Main classification: B29D28
IPC codes: B29C59, B29C47, B29L7, B29D28