NONWOVEN FABRIC CONTAINING POLYOLEFIN FILAMENTS

Abstract: A nonwoven fabric containing not less than about 25%, based on total web weight of the nonwoven fabric, of poly­ olefin filaments having a delta cross-sectional configur­ ation, and initial spun denier not exceeding about 4 denier per filament (dpf), and a final drawn denier of not less than about 1 dpf.

Description: This invention relates to nonwoven fabric materials containing polyolefin filaments. Conventionally, fibers and filaments of polyolefins such as polypropylene are used to make nonwoven fabrics used as cover sheets for diapers, sanitary napkins, and other absorbent articles intended for use in contact with the human body. The nonwoven material used in cover sheets for such articles must have substantial cross directional (CD) strength and toughness (also referred to as energy) as well as desirable surface softness, absorbancy (short liquid strike-through time), and limited rewetting or wicking properties, and of course must be cost-competitive. It is also desirable for such materials to mask discoloration and staining of the inner absorbant material. Unfortunately, softness and absorbency in non-woven materials are not compatible with CD strength and toughness. Modifications that increase softness tend to lower (CD) strength and increase cost. On the other hand, if the CD strength is increased by adding more bonding, this decreases softness and absorbancy. It is difficult enough to achieve a satisfactory combination of these properties without having to solve staining and opacity problems by further modifying the non-woven structure to increase its opacity. Therefore, staining problems in nonwoven cover sheets have been dealt with in the past by introducing colorants and brighteners as spun melt components. This causes additional problems, involving leaching, allergenic effects, and increased cost. It would be desirable to provide a structure for polyolefin-containing nonwoven material that would increase its opacity, without sacrificing softness, absorbency, CD strength or toughness, or requiring high concentrations of colorants. According to the invention, a nonwoven fabric of polyolefin filaments is characterized in that it contains not less than about 25%, based on total web weight of the nonwoven fabric, of polyolefin filaments having a triangular cross-sectional configuration, an initial spun denier not exceeding about 4 denier per filament (dpf), and a final drawn denier of not less than about 1 dpf. Filaments having a triangular cross-sectional configuration are often referred to as having a delta (▵) cross-section or as delta filaments. The remaining 75% of the fabric may be the same as the 25%, or may comprise polyolefin or other filaments such as rayon having other conventional cross-sectional configurations, such as "y", "x", "o" (round filaments), or the like, including blends of such filaments in combination with fibrillated film such as polyolefin film. Polypropylene filaments are preferred. Preferably, nonwoven fabrics according to the invention consists of a uniform blend of 25% to 75% of delta filaments and 75% to 25% of round filaments. The most preferable blend, for a desirable combination of softness and CD strength, contains 50% of each of these components. In any case, the nonwoven fabric may consist of one or more individual webs; if the fabric contains more than one web, the specified proportions of any type of filament according to the invention apply to the combined weight of the webs. Preferably, the delta filaments have an initial spun denier within a range of about 2.0-4.0 dpf and a final drawn denier within the range of about 1.0-3.0 dpf, most preferably 1.9-2.5 dpf, in order to retain both strength and softness. Nonwoven fabrics according to the invention can achieve an opacity within the range of 32% to 45% or more, as measured by a Milton-Roy Spectrophotometer, (model Match Mate 3000), manufactured by the Milton-Roy Corporation. Also preferably, the filaments or fibers for use in the fabrics are from about 2.54 to 7.62 cm (1-3.0 inches) in length. The longer filaments tend to produce greater CD tensile strength, and mixtures of long and short staple fibers tend to increase the CD tensile strength and toughness of the fabric. The most preferred mixture for maximum toughness combined with optimum softness is a 50:50 mixture of about 2.54 cm delta filaments of polypropylene and 3.81 cm to 5.04 cm round filaments. Nonwoven fabrics according to the invention are made by well-known, conventional techniques for making nonwoven fabrics, which involve blending the fibers in the proportions required before feeding them into the opening, sheet-forming, and bonding equipment conventionally used. It is possible to obtain nonwoven fabrics according to the invention, having substantially improved opacity and stain-hiding properties by using any conventional bonding methods, such as spun bonding and needle punching, and particularly thermal or sonic bonding techniques using multiple webs in machine or cross-machine directions, to obtain materials as light as 17.94 to 35.89 gm/m² (15-30 gm/yd²). Thermal bonding is the preferred fabrication technique. The following examples and table further illustrate the invention. Example 1 A. Delta cross-sectional isotactic polypropylene filament of 4.0 dpf spun denier is produced in a conventional manner by melt spinning at 290°C using PRO-FAX® 6501 polypropylene polymer (commercially available from Hercules Incorporated of Wilmington, Delaware), degraded in the usual way with .025% Lupersol to an MFR (Melt Flow Rate as measured according to ASTM D 1238-82) value of 16 and spun, using a 700 hole delta spinnerette to obtain a final drawn denier of 2.1 dpf. Crimped (10 crimps/cm or 25 crimps/inch) bundles are then cut into one inch 2.54 cm (1 inch) length, collected, and compressed into bales for later testing. B. Round cross-section polypropylene filament of 2.8 dpf spun denier is similarly produced in a conventional manner by melt spinning PRO-FAX® 6501 polypropylene polymer degraded to an MFR value of 13, spun at 290°C to obtain a final drawn denier of 2.1 dpf, crimped as above, cut into 5.08 cm (2 inch) lengths, collected, compressed and baled for later testing. C. Delta cross-section polypropylene of 2.6 dpf spun denier is produced by melt spinning at 285°C , using PRO-FAX 6301 (commercially available from Hercules Incorporated of Wilmington, Delaware), and finally drawn to 2.2 dpf, crimped as above, cut into 5.08 cm (two inch) bundles, collected, compressed, and baled for later testing. D. Delta cross-section fiber of Example 1A (2.1 dpf denier) is crimped as above and cut into 3.81 cm (1.5 inch) bundles collected and compressed into bales for later testing. E. Round-cross-section fiber of 2.8 dpf spun denier is drawn to 2.1 dpf as in Example 1B, crimped as above and cut into 3.81 cm bundles, collected, and compressed into bales for later testing. F. Staple cut fiber of delta and round cross-sectional configuration treated as described in C. and B. supra is combined in a homogeneous ratio of 50-to-50 parts by weight, collected, compressed and baled for later testing. G. Round cross-section polypropylene filament of 1.5 dpf is produced in the manner of Example 1B by melt spinning PRO-FAX 6501 polypropylene polymer degraded to an MFR value of 12 at 285°C and drawn to obtain a final drawn denier of 1 dpf, crimped as above, cut into 3.81 cm lengths, collected, compressed and baled for later testing. H. Delta cross-section polypropylene of 1.5 dpf spun denier is produced in the manner of Example IC by melt spinning PRO-FAX 6501 at 285°C and drawn to 1.0 dpf, crimped as above, cut into 3.81 cm bundles, compressed, and baled for later testing. I. Round cross-section polypropylene filament of 8.0 dpf is produced from the same melt and in the manner of Example IB, spun to obtain a 6 dpf final denier, crimped as above, cut into 3.81 cm lengths, collected, compressed, and baled for later testing. Example 2 A. Baled 2.54 cm crimped polypropylene staple of delta cross-sectional configuration as described in Example IA is broken, and formed into two identical homogeneous webs in a conventional manner, and the webs superimposed in machine direction as they are transferred onto a continuous fiber glass belt, and thermally bonded, using a hot diamond-patterned calendar at 165°C/40 psi roll pressure to obtain a nonwoven weighing 23.92 gm/m² (20gm/yd²). The resulting material, identified as NW-1, is then cut into convenient dimensions for conventional testing purposes and test results reported in Table I below. B. Baled 5.08 cm crimped polypropylene staple of round cross-sectional configuration as described in Example IB is broken, and formed into two identical homogeneous webs in a conventional manner, the webs being superimposed in machine direction as they are transferred onto a continuous fiber glass belt, and thermally bonded as in Example 2A, using a hot diamond-patterned calendar to obtain a semi-opaque nonwoven weighing 23.92 gm/m². The resulting material, identified as NW-2, is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported as control in Table I below. C. The 2.54 cm and 5.08 cm crimped staple of delta and round configuration of Examples IA and IB is added to separate openers and conveyed into separate cards to form two homogeneous webs with a 25/75 weight ratio of 2.54 cm delta and 5.08 cm round in a conventional manner, the webs being transferred onto a continuous fiber glass belt, and thermally bonded as before, using a hot diamond-patterned calendar to obtain a nonwoven material weighing 24.76 gm/m² (20.7gm/yd²). The resulting material, identified as NW-3, is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported in Table I below D. The 2.54 cm and 5.08 cm crimped staple of Examples IA and IB is added to separate openers, broken, conveyed into separate cards, and formed into two homogeneous webs having a 50/50 ratio of 2.54 cm delta/5.08 cm round in a conventional manner, the webs being superimposed in machine direction as they are transferred onto a continuous fiber glass belt, and thermally bonded as before, using a hot diamond-patterned calendar to obtain a nonwoven material weighing 24.76 gm/m². The resulting material, identified as NW-4, is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported in Table I below. E. The 2.54 cm and 5.08 cm crimped staple of Examples IA and IB is added to separate-openers, broken and conveyed into separate cards and formed into two identical homogeneous webs of 2.54 cm delta and 5.08 cm round of 75/25 weight ratio in a conventional manner, the two webs being superimposed in machine direction, transferred onto a continuous fiber glass belt, and thermally bonded as before, using a hot diamond-patterned calendar to obtain a nonwoven material weighing 23.1 gm/m² (19.3gm/yd²). The resulting material, identified as NW-5, is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported in Table I below. F. Baled combined 5.08 cm crimped staple of 50:50 delta:round cross-sectional configuration by weight, as described in Example IF (1B and 1C) is broken and formed into two identical mixed fiber webs in the same general manner as before, the webs being superimposed in machine direction, transferred onto a continuous fiber glass belt, and thermally bonded as before, using a hot diamond-patterned calendar to obtain a nonwoven material weighing 19.1gm/yd². The resulting material identified as NW-6 is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported in Table I below. G. Baled 3.81 cm (1.5 inch) crimped staple of drawn 2.1 dpf delta cross-section, as described Ex ID is broken and formed into a web in the same manner as before. A second web is then prepared using 1.5 (1.5") crimped staple of 2.1 dpf circular cross-section as described in Example IE is broken and formed into a web of equal weight in the same manner as before. The two webs, consisting of different fiber cross-section are superimposed in a machine direction, transferred onto a continuous fiber glass belt, and thermally bonded as before, using a hot diamond-patterned calendar to obtain a nonwoven material weighing 21.5 gm/m² (18gm/yd²). The resulting material identified as NW-7 is then cut into convenient dimensions for testing purposes, standard tests run, and test results reported in Table I below. H. Baled 3.81 cm polypropylene staple of round cross-sectional configuration (extruded 1.5 dpf drawn 1 dpf) as described in Example 1G is broken and formed into two identical homogeneous webs, the webs being superimposed in machine direction as they are transferred onto a continuous fiber glass belt then thermally bonded, using a hot diamond-patterned calendar at 165°C/276 kPa (165°C/40 psi) roll pressure to obtain a nonwoven weighing 23.92 gm/m² (20gm/yd²). The resulting nonwoven, identified as NW-8, is then cut into convenient dimensions for testing purposes, and test results reported in Table I below as a control. I. Baled 3.81 cm polypropylene staple of delta cross-sectional configuration and 1 dpf from Example 1 2G supra to obtain an opaque nonwoven weighing about 23.92 gm/m². The resulting material, identified as NW-9, is then cut into convenient dimensions for testing purposes and test results reported in Table I below as a control. J. Baled 3.81 cm polypropylene staple of round cross-sectional configuration and a drawn dpf of 6 from Example 1 I is broken and formed into two identical homogeneous webs in the manner of as in Example 2H, to obtain a nonwoven, identified as NW-10, is then cut into convenient dimensions for testing purposes, and conventional test results reported in Table I below as a control.

Claims: A nonwoven fabric containing polyolefin filaments is characterized in that it contains not less than about 25%, based on total web weight of the nonwoven fabric, of polyolefin filaments having a delta cross-sectional configuration, an initial spun denier not exceeding about 4 denier per filament (dpf) , and a final drawn denier of not less than about 1 dpf. A nonwoven fabric as claimed in claim 1, further characterized in that it contains a blend of delta and round filaments. A nonwoven fabric as claimed in claim 2, further characterized in that it contains a blend of a uniform blend of 25% to 75% of delta filaments and 75% to 25% of round filaments. A nonwoven fabric as claimed in claim 2, further characterized in that it contains a blend of 50% of each of the delta and round filaments. A nonwoven fabric as claimed in claim 3 or 4, further characterized in that the delta filaments have an initial spun denier within the range of about 2.0 to 4.0 dpf and a final drawn denier within the range of about 1.0 to 3.0 dpf. A nonwoven fabric as claimed in claim 5, further characterized in that the delta filaments have a final drawn denier within the range of about 1.9-2.5 dpf. A nonwoven fabric as claimed in any of the preceeding claims, further characterized in that the filaments in the nonwoven fabric are from about 2.54 to 7.62 cm in length. A nonwoven fabric as claimed in claim 7, further characterized in that it comprises a 50:50 mixture by weight of 2.54 cm delta filaments and 3.81 cm to 5.04 cm round filaments. A nonwoven fabric as claimed in any of the preceeding claims, further characterized in that it has an opacity within the range of 32% to 45%. A nonwoven fabric as claimed in any of the preceeding claims, further characterized in that all the delta filaments are polypropylene filaments. A nonwoven fabric as claimed in any of the preceeding claims, further characterized in that the filaments of the nonwoven fabric are bonded by thermal bonding.

Inventors: MODRAK, JAMES PATRICK, ROBERTS, OWEN PHILLIP
Applicants: HERCULES INCORPORATED
Application year: 1988
Main classification: D04H1
IPC codes: D04H1, A61F13, D04H3