BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an air-laid composition useful in diapers, incontinence pads, sanitary napkins and other absorbent pads needed for body fluids; filters (air filters, liquid filters); and fabrics for window treatments, upholstery, pillows and bedding. In particular, the present invention comprises an air-laid web composition of absorbent, binder, and high modulus fibers that has an improved loft, compression resistance, and an increased capacity through the forming head of the air-laid apparatus. The modulus of the fibers is greater than about 2 grams per denier and has a percent crimp of less than about 30%.
2) Prior Art
Disposable absorbent articles such as disposable diapers, have found much success in the marketplace, however, there is always a need to improve these products in terms of their low density, high loft, and compression resistance. Prior to the present invention it was known to form existing air-laid composites from natural absorbents (and optionally up to 25% super absorbent polymers, SAP), bicomponent fibers as binder, and short cut synthetic fibers for loft and compression resistance. This existing composition contained approximately 10% bicomponent fibers, about 10% regular polyester fibers, and approximately 80% absorbent. This product had adequate loft, fluid intake rate and good wet strength. Generally this product was created by mixing a natural absorbent (wood pulp), and optionally the SAP, with the bicomponent fibers and the synthetic fibers and blending this mixture and ejecting the composition through forming heads of the air-laid equipment. The composite was then introduced into a heating zone, such that the lower melting material of the bicomponent fiber would melt and would run to the intersection where the fibers cross one another. Next, the composite was introduced into a cooling zone where the web was cooled, thus solidifying the molten lower melting material, thereby binding the mixture into a unitary web structure.
In this existing composition, the purpose of the natural absorbent such as the wood pulp and/or SAP is to absorb the body fluids, while the purpose of the bicomponent fiber is to bind the entire web together, and the purpose of the synthetic fibers, including the higher melting synthetic fiber component of the bicomponent fibers, is to provide loft and compression resistance so that the maximum surface of each individual natural absorbent fiber/particle may be exposed to the bodily fluids.
As the industry seeks to increase capacity with the existing equipment, a problem has arisen in that the forming heads of the air-laid apparatus often clog with the current composition as the throughput is increased.
It is an object of the present invention to improve the composition so as to increase the capacity without clogging of the air-laid equipment forming heads.
SUMMARY OF THE INVENTION
The present invention relates to the use of short cut fibers, which have a higher modulus than previously used synthetic polymer fibers. Employing a modification of the conventional formulation of about 3-15% by weight binder fiber, about 40-80% by weight absorbents, such as wood pulp fluff, and 10-50% by weight of the synthetic fiber having a higher modulus and lower percent crimp than conventionally employed fibers, permits an increase in through-put of the forming head of the air-laid apparatus.
The increase in modulus means that the fiber is stiffer. Prior to the present invention, it was generally thought that a flexible fiber was necessary to obtain good throughput in the forming head.
In the broadest sense, the present invention comprises an air-laid composition having absorbent, binder, and synthetic fibers having a high modulus and low percent crimp.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The webs of the present invention are useful in diapers, incontinence pads, sanitary napkins, and other absorbent pads needed for body fluids, filters (air filters, liquid filters); and fabrics for window treatments, upholstery, pillows and bedding. In particular, the present invention comprises a composition comprising absorbents, such as wood pulp or synthetic absorbent polymers, binder, and high modulus synthetic fibers.
The synthetic fibers in prior art processes were short cut in length between 3 and 18 millimeters and preferably about 6 millimeters in length. Typically such fibers have a modulus of approximately 1.5 grams per denier (gpd) or less. With the present invention, the fibers employed are heat set longer such that the modulus is greater than about 2 gpd and preferably greater than about 2.5 gpd. The increased modulus causes the overall fiber to be stiffer and contrary to popular belief, it actually exits the forming head of air-laid apparatus much cleaner without clogging and blocking the forming head compared with conventional compositions. The fibers of the present invention also have a low percent crimp, and particularly less than about 30%. These fibers will have a preferred range of 4 to 8 mm in length and a denier of about 1.5 to 4, and preferably a denier of about 3. The weight of the webs of the present composition range from about 50 to 500 grams per square meter (gsm).
Suitable absorbents are natural absorbents or synthetic absorbents primarily know as super absorbent polymers, or a mixture of these. The absorbents comprise 40-80% by weight of the web. Natural absorbents are hydrophilic materials such as cellulosic fibers, wood pulp fluff, cotton, cotton linters, and regenerated cellulose fibers such as rayon, or a mixture of these. Preferred is wood pulp fluff, which is both inexpensive and readily available.
While conventional wood pulp fibers are readily absorbent, compacted wood pulp fibers do not absorb as much bodily fluid as when a portion of the wood pulp fibers has been replaced with synthetic fibers, and preferably polyester fibers, which provide loft to the composite. Providing loft to the composite exposes more surface area of the wood pulp fibers to the bodily fluids and thus the wood pulp fibers are much more efficient in absorbing the bodily fluid.
Absorbent pads employing natural absorbents may not provide adequate fluid intake for all circumstances. Also natural absorbents are very bulky. Accordingly, many absorbent pads employ SAP in relatively low quantities. This is because the cost of SAP is much higher than the cost of natural absorbents. Replacing some of the natural absorbents with SAP can reduce the overall bulk of the pad and/or provide superior fluid intake.
As used herein, the term “super-absorbent polymer” or “SAP” refers to a water-swellable, generally water-insoluble material capable of absorbing at least about 10, desirably about 20, and preferably about 50 times or more its weight in water. The super-absorbent polymer may be formed from organic material, which may include natural materials such as agar, pectin, and guar gum, as well as synthetic materials such as synthetic hydrogel polymers. Synthetic hydrogel polymers include, for example, carboxymethyl cellulose, alkali metal salts of polyacrylic acid, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine, and the like. Other suitable polymers include hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers and mixtures thereof. The hydrogel polymers are preferably lightly crosslinked to render the materials substantially water insoluble. Crosslinking may, for example, be by irradiation or covalent, ionic, van der Waals, or hydrogen bonding. Suitable materials are available from various commercial vendors such as the Dow Chemical Company, Allied Colloid, Inc., and Stockhausen, Inc. The SAP may be in the form of particles, flakes, fibers, rods, films or any of a number of geometric forms.
The high modulus, low percent crimp synthetic fibers may be formed from any polymeric material capable of forming fibers that can be formed into a fibrous web. Suitable polymeric material, from which the synthetic fibers may be formed, include polyolefins, such as polyethylene, polypropylene, and the like; polyesters such as polyethylene terephthalate or polybutylene terephthalate, or copolyesters such as polyethylene terephthalate-isophthalate or polyethylene terephthalate-adipate and the like; polyamides such as nylon 6, nylon 6,6, poly(iminocarboxylpentamethylene) and the like; acrylics; as well as mixtures and copolymers thereof. Preferred is polyester fiber such as polyethylene terephthalate. The synthetic fibers must have a modulus of greater than about 2.0 gpd, and preferably greater than about 2.5 gpd, and most preferably greater than about 3.0 gpd. The synthetic fibers must also have a low percent crimp, preferably less than about 30%, and more preferably less than about 25%.
The binder of the present invention can be based on conventional latex systems, hot melt adhesives, or binder fibers, or a mixture of these. Conventional latex systems such as styrene-butadiene copolymer, acrylate, and polyvinyl acetate systems, as well as mixtures of these are well known. Hot melt adhesives are generally solid powder materials or non-latex paste and liquid compositions well known to those in the art. Binder fibers can be conventional low melt fibers or bicomponent fibers. Conventional low melt fibers can be polyolefins, for example, and in particular can be linear low density polyethylene. Bicomponent fibers having a denier of between 2 and 6 are the preferred binder fiber. Bicomponent fibers can be of the type in which the low melting point portion is adjacent to the high melting point portion such as a side-by-side configuration, or in a sheath-core configuration wherein the sheath is the lower melting component and the core is the higher melting component. The binders are thermally bonded by conventional means such as by using an oven (hot air, radiant or microwave), or calender roll(s), or by ultrasonic energy. It is contemplated that the web of the present invention will comprise between 3 and 15% by weight binder fiber, such as bicomponent fiber. This amount of binder fiber is deemed to be adequate to bind the web into a unitary structure. Preferably, about 10% by weight binder fiber (based on the weight of the web) gives most satisfactory results. When a conventional latex system is employed with the present invention, the amount of binder may range from 5-60% by weight of the web.
Suitable bicomponent fibers are polyethylene/polypropylene; polyethylene/polyester (especially polyethylene terephthalate); polypropylene/polyester; and copolyester/polyethylene terephthalate, such as polyethylene terephthalate-isophthalate/polyethylene terephthalate; nylon 6/nylon 6,6; and nylon 6/polyethylene terephthalate; as well as mixtures of these. Preferably polyethylene/polyester are used, especially grafted polyethylene/ polyethylene terephthalate, such as linear low density polyethylene/polyethylene terephthalate.
The web of the present invention comprises between 3 and 15% by weight binder fiber such as bicomponent fiber. This amount of binder fiber is deemed adequate to bind the web into a unitary structure. Preferably, about 10% by weight gives most satisfactory results. Moreover, the web also has 40 to 80% by weight absorbent, such as wood pulp fibers, some being optionally being substituted by SAP. Lastly the web has synthetic fibers (of length between 3 and 18 mm), having a denier between 1.5 and 4, in an amount from 10% to about 50% by weight, based on the weight of the web. The weight of the webs of the present composition range from about 50 to 500 grams per square meter (gsm).
The web may be formed by an air-laid process by merely mixing or blending the various fibers together and subjecting them to a flowing air stream which carries the fibers to a surface, preferably a screen surface, upon which the fibers are deposited in a very random order. If binder fibers are employed, the web is then conveyed to a heated zone of sufficient temperature and having a sufficient residence time for the web such that the low melting material of the binder fiber melts, flows to the intersection of a group of overlaid, contacting and intersecting fibers. Next, the web is transported on the conveyor belt to a cooling zone where all molten material solidifies thus making the web structurally rigid. If the binder is a latex system, the web is coated with the latex (by spraying, dipping, etc.) and the latex is allowed to dry and cure, thereby solidifying. Thereafter, the web may be cut into various lengths and widths for the end use applications, namely, fenestration drapes, dental bibs, eye pads, diapers, incontinent pads, sanitary napkins, wound dressing pads, air filters, liquid filters, and fabrics such as drapes, bedding or pillows.
The web can also be used in conjunction with other components, such as part of a laminate with a woven or nonwoven material or fabric. For example, spun-bond/melt-blown /spun-bond fabric (“SMS”) are known and have many uses. Stitch-bonded fabric is another known example of a multicomponent structure comprising fabric and a fiber web sewn or stitched together.
The physical properties of the synthetic fibers were measured according to the following procedures.
Linear Density (Denier)
The denier of the synthetic fibers was measured according to ASTM 1577-96, option A, using uncut fibers (tow).
The modulus was measured according to ASTM 3822-96. Uncut fibers (tow) were used with a 5 inch gauge length and a strain rate of 60%/minute. The modulus is reported as the load at 10% elongation in gram/denier (gpd).
Crimp Frequency (Crimps Per Inch, CPI)
The crimp frequency was measured according to ASTM 3937-94, using option one for preparing the sample.
A length of tow, approximately 1.25 m, is clamped at one end and tensioned to remove the crimp. The tow is cut at a distance of 1 m from the clamp. The tow is allowed to relax, and the length of the relaxed, crimped tow band is measured (L, cm). The percent (%) crimp is 100−L.
Forming Head Capacity
Trials were conducted on a Dan-Web airlay former. The basis weight of the web was kept constant at 263 gsm by increasing the belt speed as the throughput of fibers through the forming head was increased. The maximum throughput was defined as the maximum rate without choking the forming head, which resulted in a decrease in web basis weight.