Melt blown webs

Abstract: Low viscosity ethylene acrylic copolymers, especially ethylene/alkyl (meth) acrylates, containing from 35-99 weight percent ethylene comonomer, and their blends with other fiber-forming polymers, are useful in producing fibers suitable for non-woven materials.

Description: This invention relates to melt blown webs of fibers, especially hydrocarbon fibers, as well as sheets and laminates made therefrom. Many thermoplastic resins may be extruded to form fibers of the monofilament type (relatively large) and very fine denier fibers, especially in nonwoven products. The most commonly used thermoplastic resin for formation of the very fine fibers are polypropylene and polyester, although many other resins have been suggested. It has not been possible to prepare acceptable nonwoven fabrics, webs or mats from ethylene acrylic copolymers because the extruded copolymers, e.g., ethylene acrylates, due to their high melt strength do not attenuate well to fibers by conventional methods. Thermoplastic resins such as ethylene vinyl acetate copolymers have been used; however, the EVA type copolymers are stable only to about 450 °F (232°C) and are not useable to blend with polypropylene which has an optimum processing temperature in the range of 500-550°F (260-288°C). EP-A-239 080, with a priority date earlier than the priority date of the present application, but published after the filing date of the present application, describes microfibres having a diameter of 100 nm (millimicron) or less by melt-blowing and spinning an ethylene-methyl acrylate copolymer. Also described are blends with ethylene or propylene homo or copolymers. This document does not describe however melt blown webs of an ethylene alkyl (meth) acrylate having a high elongation at break of at least 90 % at a basis weight of one ounce per square yard (34 g/m²). It is the combination of the finess of the fibres, and the high elongation which provides a particularly advantageous melt-blown web. GB-A-2 045 300 discloses melt blow spinning of blends of ethylene methacrylic acid ester copolymers with other polymers where a non-woven fabric is formed by receiving the fibres on a porous member directly after spinning. Again the combination of finely attenuated fibres and high elongation at break is not disclosed. The ethylene acrylic copolymers of the invention are stable up to about 610°F (321°C) and are therefore suitable for blends with polypropylene for optimum temperature processing. Small fiber diameters are important for producing many nonwoven applications due to the bacterial efficiency that small fibers produce. The linear low density polyethylene/ethylene acrylic copolymer blends of the invention may be formed into fibers having such small diameters of around 4-12 microns in size. The melt blown webs of the invention are especially useful in nonwoven structures. Examples of applications of nonwoven materials are diaper interfacings, wound dressings, clothing, sanitary products, medical products, sheeting, drapes, disposable clothing, protective clothing, outdoor fabrics, industrial fabrics, netting, bagging, membranes, filters, rope, cordage, wiping cloths, synthetic papers and tissue papers, and other products. The melt blown webs of the invention exhibit improved properties such as softness and low bonding temperatures in comparison to other materials. They have good tenacity and exceptional elongation. Stretch of fabrics and other nonwoven products made from the blends and copolymers of the invention are especially advantageous in certain applications such as clothing where it is important for the clothing to stretch rather than tear. Another likely application for the nonwoven products of these materials is form-fitting germents, drapes, and the like wherein it is necessary to stretch the fabric somewhat after it is positioned for its intended use. Summary of the invention The invention provides a melt blown web of fibers of from 1-50 µm (microns) diameter comprised of a blend of (a) from 30 to 70 wt% of an ethylene alkyl (meth) acrylate copolymer having a melt flow rate of from 25 to 200 and an ethylene comonomer content of from 60 to 90 wt% and (b) from 70 to 30 wt% of a second olefin fiber-forming polymer said web having an elongation at break in the cross direction of at least 90 percent at a basis weight of one ounce per square yard (34 g/m²). Processing in a melt blowing line is possible avoiding production of a mass of material which often fell short of the collection drum or self bonded so extensively that a nonwoven product was not formed. Description of the Preferred Embodiments A preferred embodiment of the present invention is a nonwoven web of fibers having a diameter of 1-40 microns. The melt blown nonwoven product may be produced by a process including extruding a fiber-forming thermoplastic polymer resin or resin blend in molten form from orifices of a heated nozzle into a stream of gas which attenuates said molten resin or blend into fibers and collecting said fibers on a receiver to form said nonwoven web. A preferred embodiment of the present invention is the use of the copolymers and blends of the invention in a melt blowing process to form a nonwoven product, such as in the manner described in U.S. Patent No. 4,078,124. Nonwoven products have not been readily formed from ethylene acrylic copolymers because the viscosity of the copolymers was found to be so high as not to permit formation of a nonwoven product. However, the present invention exploits the discovery that blends of certain ethylene acrylic copolymers with other fiber-forming materials can in fact be used for the formation on nonwoven products, especially by the melt blowing process. The use of low viscosity ethylene acrylic copolymers for spunbond and melt blown nonwoven applications is disclosed herein. The ethylene acrylic copolymers of the invention may vary in ethylene content from 60 to 90 wt%, especially above 70 wt%. The acrylic comonomers of the invention are of the alkyl (meth) acrylate type. They may have the formula wherein R₁ is H or methyl (CH₃-) and R₂ is an alkyl group, preferably methyl, ethyl, propyl, or butyl, more preferably methyl. R₁ is preferably H rather than methyl but the (meth) acrylate or mixtures may be more available in some situations/locations. The most preferred acrylic comonomer of the invention is methyl acrylate CH₂CHCOOCH₃. Another preferred acrylic comonomer is ethyl acrylate CH₂CHCOOCH₂CH₃. Generally, the weight percent of acrylic comonomer content may be decreased somewhat where the comonomer is derived from ethyl acrylate rather than methyl acrylate. The amount of acrylic comonomer present in the ethylene acrylic copolymer of the invention may vary significantly depending upon the type of polymerisation used, choice of acrylic comonomer, type of process to be used for the copolymer, desired elongation characteristic for a nonwoven product of the copolymer, and process considerations. A useful range of acrylic comonomer content is from 10-40 weight percent, more preferably at least about 20 weight percent in the case of methyl acrylate or methyl (meth) acrylate and at least 10 weight percent in the case of ethyl acrylates or larger alkyl acrylates. According to the invention, fibers may be formed blends of the invention wherein the fiber diameter is from 1-50 µm (microns) (up to 16.7 dtex (15 denier)). A preferred range of fiber diameters for the fibers of the invention, especially in the case of spunbond or melt blown fibers is from 1-40 µm (microns), more preferably from 1-15 µm (microns) diameter. It has been found that fibers and nonwoven products made from the fibers of the invention have a softer "hand" or feel than polypropylene fibers of comparable size, polypropylene being the most commonly used melt blown thermoplastic material. The copolymers and blends of the invention comprise an ethylene acrylic copolymer having a melt flow rate of at least 10. As used herein, the melt flow rate is expressed in terms of grams per 10 minutes as determined by ASTM D1238 (condition E - 190°C). Accordingly, a copolymer having a melt flow rate or melt index of 10 has a flow rate of 10 grams per 10 minutes as determined by ASTMD1238 (condition E). Preferably, the ethylene acrylic copolymers of the invention have a melt flow rate of from 25 to 200. Preferably, melt blown web is from fibres prepared from a blend of from 40-60 weight percent blend of the ethylene acrylic copolymer and a second fiber-forming polymer, most preferably 50:50. In one highly preferred embodiment, materials other than the blends or copolymers of the invention are not present in any significant amount. Various fiber-forming polymers suitable as component (b) for the blend of the invention include polyolefins, polyamides, polyvinyls, and other polymers. Included are polypropylene, polyethylene, reactor copolymers of propylene with small amounts of ethylene, polyesters, poly(methyl meth acrylate), poly(ethylene terephthate), poly(hexamethylene adipamide), poly(omega-capropamide), poly(hexamethylene sebacamide), polystyrene, and polytrifluorochloroethylene. Favored among these are the polyolefins, especially polyethylene and polypropylene. Useful polyethylenes include low density polyethylene, high density polyethylene and linear low density polyethylene (copolymers of ethylene and lower alkyl comonomers). Highly preferred are linear low density polyethylene and polypropylene. A useful blend composition is 50 % of the acrylic copolymer of the invention with 50 % polypropylene or linear low density polyethylene. A highly preferred blend for forming fibers, especially by the melt blowing process, is a composition of 50% polypropylene or 50% linear low density polyethylene with an ethylene methyl acrylate copolymer having 10-30 weight percent methyl acrylate, preferably 20% methyl acrylate, and having a melt index of preferably 50-150. Typical operating temperatures for the melt blowing die when using the copolymers or blends of the invention are about 193 to 371 °C (380-700 °F), preferably 204 to 343 °C (400-650°F). The ethylene acrylic copolymers of the invention may contain additional components including fillers. However, a preferred embodiment of the invention is a fiber or a nonwoven web formed of an ethylene acrylic copolymer which consists essentially of the copolymer of ethylene and an acrylic comonomer. Similarly, blends of the preferred copolymer are also preferred. The blend of the invention may be formed by any of the various methods available for forming compounded polymers including various heating and high temperature blending processes. Such processes include Banbury mixing, dry blending, or melt extruding such components to form the polymer for producing the fiber. The fibres for the web of the invention may be made by melt blowing. Very fine fibers may be formed especially by melt blowing. These fibers may in turn be collected as mats, rovings, or other forms of nonwoven product. They can thereafter be processed further by known fiber handling equipment and processes to make garments and other objects of commercial use. The processes of forming the fibers benefit from the ability of the copolymers and blends of the invention to attenuate into fibers so as to provide a nonwoven product of extremely soft "hand" having good strength and elongation characteristics. The invention shows improvement over specific properties of polypropylene and ethylene vinyl acetate copolymers or blends because of strength and elongation capability. Furthermore, the copolymers are advantageous over EVA's in blends with polypropylene and may be processed at favorable polypropylene temperatures (above 500°F). The fabrics are classified by base weight, usually in ounces per square yard. Thus thicker fabrics have a heavier base weight than thinner materials/fabrics. A better understanding of the invention may be gained by a review of the following examples and accompanying Table. Examples Nonwoven products in the form of mats were formed from a ten inch die head on a melt blowing process line fed by an extruder. The product collection drum was located about ten inches (254 mm) from the die head and the die head was operated at about 550°F (288°C). The mats were cut into appropriately sized portions and tested by standard methods to determine tenacity, break strength, and Young's Modulus as well as the percent elongation at break in the direction of takeup of the nonwoven product (machine direction) as well in the direction perpendicular to takeup of product on the product collector (cross direction). The die head/nozzle may be operated so as to extrude copolymer or blend at varying rates. An operable range is 0.1 to 1.0 gram per minute per orifice in the die, preferably 0.1 to 0.5, more preferably 0.2 gram per minute per orifice. The air "knife" may be operated at any rate suitable for forming fabrics. A useable range is 100-300 standard cubic feet (2832 to 8495 liters) per minute (SCFM). Below 200 SCFM (this corresponds to 5663 liters per min) is preferred and 150 SCFM (4247 liters per min) is highly preferred. The collector/drum may be positioned at various distances from the orifices where resin is expelled so long as the fibers are attenuated and collectable as a fabric. A useable range of separating the nozzle and collector roll is 6-24 inches (152.4 to 609.6 mm), preferably 6-20 inches (152.4 to 508 mm), more preferably 8-15 inches (203.2 to 381 mm). Young's Modulus reflects the stiffness of a fabric, lower values being a softer, more drapeable fabric. High elongation is desirable in many fabrics to provide stretchable, puncture resistant, form-fitting shapes. Tenacity is a measure of strength, higher values reflecting more strength per unit weight and the possibility of corresponding lower cost. Using a twenty inch die head having 401 orifices and the equipment described above an ethylene methyl acrylate copolymer having 20 percent by weight methyl acrylate and a melt index of 6 for comparison was processed. However, the extruded ethylene methyl acrylate copolymer did not attenuate to fibers in the melt blown process and a nonwoven fabric could not be formed. The following examples demonstrate formation of nonwoven fabrics from polypropylene, linear low density polyethylene, ethylene methyl acrylate copolymers of the invention, ethylene methyl acrylate copolymer/polypropylene blend of the invention, and ethylene methyl acrylate/linear low density polyethylene blend of the invention. The materials were processed in the twenty inch melt blowing die to form a nonwoven product at temperature and pressure settings which were consistent with their formation. The materials of each example and the characteristics of the examples are listed in the table below. Examination of the above table reveals that webs made from ethylene acrylic copolymers have excellent elongation while maintaining good fabric strength. Furthermore, the blends of the invention are noted to have exceptional elongation over that of either the polyolefin component of the blend or the acrylic copolymer component of the blend. Accordingly, the copolymers in blends of the invention are not only capable of producing valuable nonwoven products having soft 'hand' and good strength characteristics but provide materials which have an elongation characteristic especially suited for certain applications where stretching of the material (rather than tearing or puncturing) is important.

Claims: A melt blown web of fibers of from 1-50 µm (microns) diameter comprised of a blend of (a) from 30 to 70 wt% of an ethylene alkyl (meth) acrylate copolymer having a melt flow rate of from 25 to 200 and an ethylene comonomer content of from 60 to 90 wt% and (b) from 70 to 30 wt% of a second fiber-forming polymer said web having an elongation at break in the cross direction of at least 90 percent at a basis weight of one ounce per square yard (34 g/m²). Web according to claim 1 wherein said fibres are from 1-40 µm (microns) diameter, preferably from 1-15 µm (microns) diameter. Web according to claim 1 or claim 2 wherein said alkyl (meth) acrylate is methyl acrylate. Web according to any one of the preceding claims wherein said ethylene acrylic copolymer has an acrylic comonomer content of from 10-40 wt%, preferably from 20-40 wt%. Web according to any of the preceding claims wherein said second fiber-forming polymer is a polyethylene homopolymer or copolymer or a polypropylene homopolymer or copolymer. Web according to any of the preceding claims wherein said second fiber-forming polymer is 50 wt% of the blend. Web according to any of the preceding claims wherein said web has a base weight of one ounce per square yard (34 g/m²).

Inventors: GRAHAM BLAIR ALFRED, GRAHAM, BLAIR ALFRED
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1987
Main classification: D01F6
IPC codes: C08F220, D04H3, D01F6, D04H1, C08F210