The invention relates to laminated structures that are useful in the construction of disposable articles, e.g. diapers, etc. In particular, the invention relates to a high strength, through-bonding technique that enhances the strength and durability of stretchable laminates.
Many commercially available disposable articles include laminated components which are breathable and stretchable, for example absorbent articles like disposable diapers for infants and undergarments for incontinent adults use stretchable side panels. Many, if not most, of these disposable articles use layers of nonwoven material. Other products which use nonwoven laminates include bandages, body wraps, and the like.
Several types of nonwoven fabrics are known in the art. In general, nonwoven fabrics contain fibers of polyethylene, polypropylene, polyethylene terephalate, nylon, or rayon and are joined in any number of ways for example adhesives, thermal bonding, felting, or other methods known in the art. Thermally bonded nonwoven polypropylene is particularly prevalent in disposable articles. Thermally bonded nonwoven polypropylene is breathable and has a soft, cloth-like feel which is particularly suitable for these applications.
The assignee of this application has been involved in the development of various adhesives for use in nonwoven laminated structures, e.g. see copending U.S. patent application Ser. No. 08/632,117 entitled xe2x80x9cOil Resistant Polybutylene Based Hot Melt Adhesivexe2x80x9d filed on Apr. 15, 1996 by Bonnie M. Harris and Monina Kanderski; and U.S. patent application Ser. No. 08/914,523 entitled xe2x80x9cHot Melt Adhesive Having a High Acid Number for Disposable Soft Goodsxe2x80x9d, filed on Aug. 9, 1997 by Mark D. Alper and Diane Strelow, both incorporated herein by reference. As discussed in the above incorporated patent applications, it is difficult to find adhesives that are effective for bonding nonwoven material to the elastomeric films or meshes which are desirable for use in nonwoven laminate structures. Some elastomeric films or mesh webs (e.g., oil resistant materials or materials embodying incompatible process aids) are simply difficult to laminate with nonwoven fabric. In addition, effective and durable bonding to elastomeric film or mesh is particularly difficult because of the mechanical stresses put on the bonds when the laminated structure is repeatedly stretched.
The following discussion regarding elastomeric films and meshes is added to provide a more complete explanation of some of the difficulties encountered when fabricating laminated structures with commercially available elastomeric films or meshes.
Commercially available elastomeric films and meshes are typically manufactured from A-B-A block copolymers such as styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-ethylene/butylene-styrene, as well as other polyolefins such as, but not limited to, ethylene vinyl acetate (EVA), and low density polyethylene (LDPE). These elastomeric materials typically include about 20% to about 30%, by weight, of the elastomeric block copolymers, and about 20% to about 30%, by weight, of other polyolefins, or ethylene vinyl acetate. An example of these films and the specifics of their composition can be found in U.S. Pat. No. 4,476,180. Once these elastomeric webs are formulated and extruded, the webs can be very tacky. The tackiness makes it difficult to process the web on the extrusion line, and in subsequent converting equipment. To address problems associated with the tackiness of the elastomeric web, manufacturers of the elastomeric webs usually incorporate a relatively high amount of slip agent into their formulations. For example, fatty acid amides having the formula CH3xe2x80x94Rxe2x80x94COxe2x80x94NH2 are commonly used as slip agents. The letter R is a chain of repeatingxe2x80x94CH2xe2x80x94units typically about 12 to 24 carbon atoms in length. The most commonly used fatty acid amide is Erucamide. It is commercially available under the trade name Kemamide B from the Humko-Sheffield Chemical Company. This particular slip agent has a carbon chain which is 22 units long, a molecular weight of about 337.6, and a capillary melting point of about 82xc2x0 C.
The slip agents incorporated into the elastomeric films and meshes have caused significant problems for prior art hot melt adhesives which are commonly used in the production of nonwoven laminates. In particular, it is believed that the slip agents become mobile, under specific environmental circumstances, or are otherwise incompatible with the polymers used in the elastomeric films or meshes; and, these two factors combine to cause the slip agents to migrate to, or otherwise to become deposited on, or xe2x80x9cbloomxe2x80x9d to, the exterior surface of the film or mesh. The slip agents have a waxy nature, and they act as a lubricant once they bloom to the surface of the film or mesh. For the manufacturers of elastomeric film and meshes, this provides two valuable benefits. The first benefit is that the presence of the waxy layer on the exterior surface of the film or mesh helps prevent adjacent layers of film from adhering thereto (known in the industry as xe2x80x9cblockingxe2x80x9d) when it is placed into a roll form. The second benefit is that the waxy layer acts as a lubricant, thereby facilitating the use of the film when it is unwound, and later processed over rollers, guides, etc. during subsequent converting and/or processing.
While the slip agents referred to above facilitate and even enhance the use of these synthetic films and meshes in manufacturing, the slip agents which migrate to the exterior surface of the film or mesh appear to destroy, or substantially weaken bonds of prior art adhesives. The amount of slip agent present in the films currently in commercial production ranges from about 0.1% to 1%, by weight, although some films currently in production have as much as 4.5%, by weight. A further disclosure of the use of slip agents in films or meshes can be found in U.S. Pat. Nos. 4,476,180, 4,977,014 and 4,714,735, all of which are incorporated by reference herein.
In view of the difficulty surrounding the application of hot melt adhesives to elastomeric films and meshes, the industry has long sought a hot melt adhesive composition which would effectively bind these elastomeric films and meshes to breathable materials, such as nonwoven webs.
In contrast to prior attempts to develop improved adhesives, the invention involves the use of a high strength through-bonding fabrication technique for elastomeric laminates. In its preferred embodiment, the laminated structure includes two outer layers of nonwoven material and an inner elastomeric layer laminated between the outer nonwoven layers. The inner elastomeric layer is preferably a mesh, and in accordance with the invention, must provide openings therethrough so that a bonding side of the first nonwoven outer layer is exposed through the openings to a bonding side of the second nonwoven outer layer. An A important aspect of the invention involves the use of through-bond adhesive to directly bond the first and second outer layers of nonwoven material to each other through at least some of the openings in the inner layer. The direct bonds between the nonwoven layers that occur through the openings in the elastomeric mesh mechanically secure the elastomeric mesh within the trilaminate structure. In many applications, it may also be desirable to provide adhesive for bonding the outer nonwoven layers directly to the elastomeric mesh. It is the direct bond, however, between the nonwoven layers that occurs through the openings in the elastomeric mesh that is primarily responsible for enhancing the durability of the nonwoven trilaminate structure.
In some nonwoven/elastomeric mesh trilaminates or like structures, the nonwoven layers are not stretchable in the machine direction, but are spreadable in the cross direction. The elastomeric mesh allows the trilaminate to stretch in the cross direction when the trilaminate is stretched, and returns the trilaminate to the relaxed position when the stretching ceases. In nonwoven trilaminate structures where the outer nonwoven layers are bonded directly to the elastomeric mesh, but not bonded to each other through openings in the mesh, repeated stretching tends to promote delamination. This is due to the inherent difficulties in bonding elastic materials. Using a through-bonding technique in accordance with the invention substantially eliminates this problem, even when there is repeated stretching.
The invention is not limited to nonwoven elastomeric mesh trilaminates. For example, the through-bonding technique improves durability of laminated structures using types of breathable material other than nonwoven. Further, the invention should not be limited to trilaminate (i.e., three ply) structures, inasmuch as the structure could contain additional layers in accordance with the invention.
In another aspect, the invention is a method of fabricating a breathable, stretchable laminated web on a laminating machine which receives the two webs of breathable material (e.g. nonwoven) and one web of the elastomeric material having a plurality of openings therethrough. The method facilitates effective through-bonding of the breathable layers through the openings in the elastomeric web. It involves feeding a first web of breathable material into the laminating machine. Separately from the first web of breathable material, a second web of breathable material is also fed into the laminating machine. In addition, the elastomeric web having a plurality of openings is also fed into the laminating machine. The webs are aligned with the elastomeric web placed between the webs of breathable material. Before the webs are completely aligned, through-bonding adhesive is applied on the bonding side of at least one of the webs of breathable material. The through-bonding adhesive is preferably applied in beads via a bead applicator, but could also be applied using other known methods. The adhesive must be applied, however, under conditions (e.g. temperature, viscosity, machine speed, compression force, etc.) sufficient to allow the through-bonding adhesive to flow through the openings in the elastomeric web and wet the bonding side of the other web of breathable material when the webs are aligned and subsequently compressed together. The preferred through-bonding adhesive is an oil resistant, polybutylene-based hot melt adhesive, although the invention can be implemented using other types of through-bonding adhesive. After the adhesive has been applied and the webs have been aligned, the webs pass through a compression nip to achieve intimate contact between the wetted bonding sides of the first and second webs of breathable material. It is preferred that the compression force be at least 50 pounds per lineal inch. The preferred compression nip is formed by a hard roller (e.g. steel) and a resilient roller (e.g. rubber). Pneumatic cylinders can be used to control the compression force applied at the compression nip. It is important that there be a sufficient amount of through-bonding adhesive to wet the bonding side of both webs of breathable material through the openings in the elastomeric web, and that there be sufficient compression to insure intimate contact between the wetted bonding sides of the webs of breathable material to facilitate proper bonding. The laminated web can then be longitudinally sliced and perforated in the machine direction as is known in the art to create tear lines and form a plurality of laminated webs.
It is preferred that the beads of through-bonding adhesive be applied to the bonding side of one of the webs of breathable material, but not directly on the second web of breathable material. Instead, a sprayed-on adhesive is preferably applied on the second web of breathable material. Thereafter, the elastomeric web is aligned and brought into contact with the second web of breathable material and adhered thereto via the sprayed-on adhesive. Thereafter, it is preferred to spray-on additional adhesive. The resulting web (i.e. the second web of breathable material with the elastomeric web adhered thereto) is then aligned with the first web of breathable material to which the beads of through-bonding adhesive have been applied. The aligned webs then pass through the compression nip as described above.
The preferred bead applicator has an adhesive plenum and a plurality of adhesive application nozzles that may or may not be in contact with the bonding side of the first web of breathable material. In order to facilitate application of a sufficient amount of through-bonding adhesive, the nozzles can be arranged in a series of diagonal rows. Inasmuch as a relatively significant amount of through-bonding adhesive should to be applied to reliably secure the inner elastomeric layer within the trilaminate structure, the amount of through-bonding adhesive applied during fabrication should be selected carefully, otherwise the finished product may become overly stiff. In many applications, it may be desirable to apply the through-bonding adhesive only along selected portions of the web (e.g. along the longitudinal edges of the spliced laminated webs), thereby mechanically securing the inner elastomeric web within the trilaminate structure without creating an overly stiff finished product.
As should be apparent to those skilled in the art, the high strength through-bonding technique improves the durability of finished products using the laminated structure, without significantly compromising line speed of the lamination machine. Importantly, the through-bonding process and adhesive chemistry overcomes adhesion difficulties caused by process aids (e.g. slip agents, oils, etc.) on elastomeric film or mesh when fabricating laminated structures. The invention is also likely to reduce costs associated with the development of new laminates. The invention eliminates the need, at least in some circumstances, to find a suitable adhesive to bond a nonwoven layer directly to an inner elastomeric layer as is typically necessary during the development of conventional laminated structures.
Other features and advantages of the invention should be apparent to those skilled in the art upon inspecting the following drawings and descriptions thereof.
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