Refastenable diapers and sheets therefor

Abstract: A refastenable disposable diaper having a backsheet of low density polyethylene and polypropylene.

Description: This invention relates to disposable diapers (and backsheets and compositions for use therein) having normally tacky and pressure-sensitive adhesive tape closures and is particularly concerned with composite closures (i.e. closures made with more than one tape) that can be opened and refastened without destroying either the diaper or the tape. The invention especially relates to a thermoplastic monolayer film suitable for use as a refastenable diaper backsheet comprising a blend of polyolefins particularly polypropylene and polyethylene. At least as early as 1955 it had been suggested to use strips of normally tacky and pressure-sensitive adhesive tape to hold conventional cloth diapers on an infant; see, e.g., US-A-2 714 889 and US-A-3 221 738. A few years later, when disposable diapers became extremely popular, strips of pressure-sensitive adhesive tape were again employed as closures; see, e.g., US-A-3 620 217. A disposable diaper typically has a thin, flexible, stretchy low density polyethylene film cover, an absorbent filler on the inside of the cover, and a porous inner liner overlying the filler. Such a diaper is positioned at the crotch of an infant, the two ends of the diaper extending, respectively, toward the front and back. Adjacent edges of the diaper at each side are then either positioned next to each other or overlapped, a strip of pressure-sensitive adhesive tape being adhered to the cover at the border adjacent each of the two edges and holding the diaper closed. Because most pressure-sensitive adhesives bond firmly to the thin polyethylene diaper cover, it is almost impossible to open the tape closure without destroying the tape and/or the diaper cover in the process. After a tape closure has been opened, it is frequently discovered that the diaper has not been soiled and hence that there is no need to replace it. If the cover has not been torn, a second strip of tape can sometimes be applied as a replacement closure, but this is often inconvenient. As a result, considerable work has been undertaken to develop a tape diaper closure that is not only capable of bonding firmly to the diaper cover, but is also capable of non-destructive removal and replacement. Closures of this type have generally involved a combination of two or more tapes, one of which remains permanently adhered to one edge of the diaper and is removably adhered to a so-called "target tape" mounted on the other edge of the diaper. Examples of such products are shown in US-A-3 951 149, US-A-3 987 793, US-A-3 999 546, and US-A-4 020 842. The patents referred to in the preceding paragraph do not discuss the manner in which the closures are prepared. Typically in making such closures, the manufacturers of diapers mount rolls of the appropriate tape in their equipment, combining them to form a composite strip of tape, the width of which is substantially the same as the length of the diaper closure to be fabricated. The composite roll is then severed at right angles to the edges of the composite strip at intervals corresponding to the width of the desired tape closure and adhered at an appropriate location along the border adjacent the sides of the diaper. Although this manufacturing process is effective, many relatively small manufacturers are unable to provide the machinery necessary to accomplish the superimposition of several rolls of tape. As a result, it is important for a tape supplier to provide the manufacturers with a composite roll, made up of two or more specific tapes from which closures may readily be prepared. A variety of diapers have been made with resealable tapes or refastenable disposable diapers. Representative are US-A-4 049 001, US-A-4 055 181, US-A-4 158 363, US-A-4 227 530, US-A-4 296 750, US-A-4 345 597, and US-A-4 369 786; EP-A-148 587; and DE-3 419 623. US-A-4 330 888 discloses a disposable bib or napkin with an upper edge portion carrying a pressure-sensitive adhesive capable of releasably adhering to the clothing or body of a user. GB-A-1 320 628 and GB-A-1 342 115 disclose adhesive sheets or adhesive composite material which will releasably adhere to itself. Additional diapers with tape fastening systems are described in US-A-3 646 937, US-A-3 848 594, US-A-3 874 386, US-A-3 951 149, US-A-3 987 793, US-A-3 999 546, US-A-4 014 339, US-A-4 014 340, US-A-4 020 842, US-A-4 041 949, US-A-4 047 004, US-A-4 194 507, US-A-4 207 895, US-A-4 209 016, US-A-4 237 889, US-A-4 573 986, and US-A-4 578 071. EP-A-0 082 727 is concerned with embossed polyolefine films suitable for use as backsheets of a diaper of which the adhesiveness has been improved by arranging for the mean profile height of the surface to be less than 150 and the mean maximum peak valley height to be less than 230. GB-A-2 152 515 describes blends of low density polyethylene and a major portion of linear low density polyethylene to which a small amount of polypropylene has been added with a view to improving stiffness but maintaining good optical properties, including good gloss, in films made therefrom. EP-A-0 144 642 relates to multilayer film or sheet material having high adhesive bonding strength between the layers and good heat sealability comprising a first layer formed of a resin composition composed of a propylene-based resin and a linear-low density ethylene α-olefin copolymer having specified density and a second layer formed of a polypropylene-based resin or layer formed of a resin composition of a polypropylene-based resin and a low crystallinity α-olefin copolymer. EP-A-0 253 792 was published after the priority date of the present Application and proposes film compositions having improved tape adhesiveness and suitable for use in baby diapers, comprising 0% to 95% of linear low density polyethylene, 0.1% to 50% of polypropylene and 0% to 95% by weight, low density polyethylene. A wide variety of refastenable diapers have been constructed with varying degrees of success. One type of product has a one-piece pressure-sensitive tape tab system which permits multiple fastening and refastening of the pressure-sensitive diaper tape tab to the embossed thermoplastic film used as the outside or backsheet of the diaper. The present invention provides a refastenable disposable diaper comprising a facing sheet, a backsheet, an absorbent panel positioned between said facing sheet and said backsheet and an adhesive tab fastener means attached to said back sheet, said backsheet being characterized by comprising a resin blend consisting essentially of polypropylene and a low density polyethylene which is LDPE, LLDPE, LMDPE or a mixture of any two of such low density polyethylenes in which from 10 to 20 weight percent of the resin blend is polypropylene, said backsheet having a transverse direction tensile strength value at 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils), and a 120° tape adhesion value of at least 380 grams. The present invention also provides use in refastenable disposable diapers of a backsheet consisting essentially of a blend of a low density polyethylene resin which is LDPE, LLDPE, LMDPE, or a mixture of any two of such low density polyethylene resins, a polypropylene resin, and a colour pigment, 10 to 20 weight percent of the resins in the blend being the polypropylene, said backsheet having a transverse direction tensile strength value at 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils) and a 120° tape adhesion value of at least 380 grams. An embossed monolayer thermoplastic film suitable for use as an outside sheet or backsheet of a refastenable disposable diaper in accord with the invention preferably has a transverse direction (T.D.) tensile strength (stress) value at low elongations (< 25%) between 0.454 Kg (one pound) and 1.362 Kg (three pounds); T.D. tensile strength @ break values above 0.908 Kg (two pounds); gloss @ 45 values (non-treated side) ≦ 8; slip C.O. F. (coefficient of friction) values (non-treated side) between 0.3 and 0.9 and tape peel force value between 0.454 Kg (one pound) and 0.908 Kg (two pounds) at nominal film thicknesses (≦ 38 µm (1.5 mils)). A refastenable diaper sheet can be made using either blown-film or cast-film extrusion. Blown-film extrusion is preferred because films so made will be more balanced in the machine and transverse directions and have higher toughness values. Non-refastenable embossed monolayer thermoplastic films used as the outside sheet or backsheet of a disposable diaper have been made largely of low density polyethylene (LDPE) resin blended with white pigments and slip additives as required to obtain the opacity (ash) and slip, coefficient of friction (C.O.F.) values needed. With a film thickness of about 38 µm (1.5 mils) such resin provides a suitable refastenable diaper backsheet. At film thicknesses of less than about 38 µm (1.5 mils), films of such resins have insufficient transverse direction (T.D.) tensile strength (stress) values at low elongations (≦25%) and consequently the film tends to tear or excessively distort as a pressure-sensitive diaper tab is peeled off. To boost the low T.D. tensile strength (stress) values, polyolefins such as high density polyethylene (HDPE) or medium density linear polyethylene (LMDPE) are added to the resin blend for making the film. Larger amounts of each (percent by weight) are required to obtain the desired stress value as nominal film thickness decreases. Although these polyolefin blends may produce thermoplastic films of the desired T.D. tensile strength (stress) at low elongations, the film may have lowered tape adhesion, lowered drop dart impact (increased splittiness) and less than desired slip C.O.F. At nominal film thicknesses of 25.4 - 38.1 µm (1.0-1.5 mils), conventional LDPE film extrusion equipment is detrimentally influenced by the high head pressure and torque requirements of the high loadings (percent by weight) of HDPE or LMDPE required to obtain the T.D. tensile strength (stress) values needed for refastenability. It has been discovered that blending from ten weight percent to 20 weight percent polypropylene with LDPE, LLDPE, LMDPE, or mixtures of any two of the three resins, monolayer films can be produced which have the desired refastenability characteristic, i.e. increased T.D. tensile strength (stress) at elongations and increased tape adhesion without significant degradation of other film physical properties. For monolayer films, the gauge of the film should be 25.4 to 38.1 µm (1.0 to 1.5 mils). Some examples of monolayer thermoplastic films suitable for refastenable backsheets of disposable diapers are set forth in Table I. MONOLAYER FILMS RESIN BLEND (Percent by Weight) Gauge (Mils) µm P.P. LDPE LLDPE LMDPE White Concentrate 7 86 7 (1.2) 30.5 5 88.5 6.5 (1.2) 30.5 10 83 7 (1.1) 27.9 10 83 7 (1.2) 30.5 11 83 6 (1.3) 33.0 12 83 5 (1.4) 35.6 20 50 23 7 (1.1) 27.9 10 60 25 5 (1.4) 35.6 15 55 25 5 (1.4) 35.6 20 55 20 5 (1.4) 35.6 10 65 20 5 (1.4) 35.6 10 45 40 5 (1.4) 35.6 15 50 30 5 (1.4) 35.6 P.P. = Polypropylene; 2 melt flow; .900 density; homopolymer LDPE = Low density polyethylene; 1.3 melt index; .925 density; liner grade LLDPE = Linear low density polyethylene; 1.0 melt index .917 density LMDPE = Linear medium density polyethylene; 1.0 melt index; .935 density Coextruded or multilayer films are also suitable for use as a refastenable diaper backsheet. Such films should have a core layer of 50-100 percent polypropylene. The values for the monolayer films are applicable to the coating layers of coextruded films. The core of a 25.4 µm (1.0 mil) coextruded film having 5.08 µm (0.2 mil) outer coatings is illustrated in Table II. COEXTRUDED OR MULTILAYER FILM RESIN BLEND (Percent by Weight) Core 15.24 µm (0.6 Mil) P.P. 60 LLDPE 33 White Concentrate 7 In embossed monolayer films containing polypropylene in combination with linear low density polyethylene and/or linear medium density polyethylene, the advantages of each of the resins is obtained. High stress and high tape adhesion qualities of polypropylene are obtained and the high impact values and high tensile strength values of the linear polyethylenes are obtained, thereby providing a film with significantly better overall physical properties than those customarily used in diaper backsheets. Stress at low elongations (≦25 percent) and tape adhesion are also similarly improved in coextruded films containing polypropylene. In Table III, a comparison is shown between LDPE film and films of various blends. It is readily seen that films containing polypropylene have increased adhesion values and increased T.D. stress @ 10%. T.D. STRESS AND 120° TAPE ADHESION VALUE FOR MIXED RESIN BLENDS Blend (Percent by Weight) T.D. Stress @ 10% (Grams) 120° Tape Adhesion Value Grams LDPE LMDPE HDPE P.P. Wh. Con. 93 7 400 295 83 10 7 464 286 88 5 7 470 329 83 10 7 525 351 73 20 7 650 348 88 5 7 515 380 83 10 7 560 >425 73 20 7 804 >425 LDPE - Low Density Polyethylene LMDPE - Medium Density Linear Polyethylene HDPE - High Density Polyethylene P.P. - Polypropylene Wh. Con. - White Concentrate In Table IV, comparisons between similar films with and without polypropylene are illustrated. It is readily seen that films containing polypropylene have increased T.D. tensile strength (stress) at low elongations (≦25 percent) and increased tape adhesion. T.D. TENSILE STRENGTH (STRESS) AND TAPE ADHESION OF COMPARABLE FILMS WITH AND WITHOUT POLYPROPYLENE Blend (Weight Percent) Embossing Pattern Gauge (Mils) µm T.D. Stress (Grams) Tape Adhesion (Grams) LDPE P.P. Wh. Con. 93 7 FSII-B (1.2) 30.5 651 417 86 7 7 FSII-B (1.2) 30.5 735 545 93 7 M-C (1.2) 30.5 494 299 84 8 8 M-C (1.2) 30.5 617 425 93 7 FSII-B (1.0) 25.4 586 294 83.5 9.5 7 FSII-B (1.0) 25.4 640 403 93 7 FSI-B (1.35) 34.3 660 467 86.5 7 6.5 FSI-B (1.35) 34.3 800 571 93 7 M-C (1.2) 30.5 606 296 86 7 7 M-C (1.2) 30.5 703 366 FSII - Fine Square II FSI - Fine Square I B - Blown M - Matte C - Cast LDPE - Low Density Polyethylene P.P. - Polypropylene Wh. Con. - White Concentrate A disposable diaper comprises in general, a facing sheet defining a diaper inside surface for direction toward an infant, a moisture-impervious backsheet substantially coextensive with the facing sheet and defining a diaper outside surface, an absorbent panel positioned between the facing sheet and the backsheet, and an adhesive tab fastener means. On a refastenable or releasable tape closure, the tape or tab has a free end and a fixed end attached to the diaper. The free end of the tab has an adhesive which is adapted to be releasably affixed to the backsheet. Refastenable diapers come in a variety of detailed constructions and releasable tapes. An example of such a diaper is illustrated in US-A-4 330 888. The present invention provides a unique backsheet for such a refastenable disposable diaper. A most preferred backsheet is constructed of a resin blend comprising by weight percent, about 12 percent polypropylene, about 83 percent low density polyethylene and about 5 percent white concentrate. It can readily be appreciated that the instant invention can be incorporated in a variety of diaper constructions and releasable tape constructions. It is essential though that the diaper backsheet comprise or be made of sufficient polypropylene as to provide the desired strength and refastenability feature. The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes in the illustrated film or diaper construction may be made without departing from the scope of the invention as defined in appended claims.

Claims: Claims for the following Contracting States : BE, DE, FR, GB, IT, LU, NL, SE A refastenable disposable diaper comprising a facing sheet, a backsheet, an absorbent panel positioned between said facing sheet and said backsheet and an adhesive tab fastener means attached to said backsheet, said backsheet being a coextruded multilayer film comprising (a) a polypropylene inner core of 50 to 100 percent polypropylene, and (b) coating layers each composed of a resin blend consisting essentially of polypropylene and a low density polyethylene which is LDPE, LLDPE, LMDPE or a mixture of any two of such low density polyethylenes in which from 10 to 20 weight percent of the resin blend is polypropylene, said backsheet having a transverse direction tensile strength value at 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils), and a 120° tape adhesion value of at least 380 grams. Use in refastenable disposable diapers of a backsheet composed of from 10 to 20 weight percent polypropylene, from 20 to 40 weight percent LMDPE, from 45 to 65 weight percent LDPE, and up to 5 percent by weight of opacifying pigment, said backsheet having a transverse direction tensile strength value to 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils) and a 120° tape adhesion value of at least 380 grams. Claims for the following Contracting States : AT, CH, LI A refastenable disposable diaper comprising a facing sheet, a backsheet, an absorbent panel positioned between said facing sheet and said backsheet and an adhesive tab fastener means attached to said back sheet, said backsheet being characterized by comprising a resin blend consisting essentially of polypropylene and a low density polyethylene which is LDPE, LLDPE, LMDPE or a mixture of any two of such low density polyethylenes in which from 10 to 20 weight percent of the resin blend is polypropylene, said backsheet having a transverse direction tensile strength value at 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils), and a 120° tape adhesion value of at least 380 grams. A diaper as claimed in claim 1 in which the backsheet is a coextruded multilayer film comprising (a) a polypropylene inner core of 50 to 100 percent polypropylene, and (b) coating layers each composed of a resin blend of polypropylene and a low density polyethylene in which from 10 to 20 weight percent of the resin blend is polypropylene. A diaper as claimed in claim 1 in which the backsheet is an embossed monolayer blown film made from a resin blend of low density polyethylene and polypropylene, the polypropylene constituting 10 to 20 weight percent of the blend. Use in refastenable diapers of a backsheet consisting essentially of a blend of a low density polyethylene resin which is LDPE, LLDPE, LMDPE, or a mixture of any two of such low density polyethylene resins, a polypropylene resin, and a colour pigment, 10 to 20 weight percent of the resins in the blend being the polypropylene, said backsheet having a transverse direction tensile strength value at 10 percent elongation of at least 515 grams, a gauge of 25.4 to 38.1 µm (1.0 to 1.5 mils) and a 120° tape adhesion value of at least 380 grams. Use as claimed in claim 4 wherein the backsheet is composed of from 10 to 20 weight percent polypropylene, from 20 to 40 weight percent LMDPE, from 45 to 65 weight percent LDPE, and up to 5 percent by weight of opacifying pigment.

Inventors: MCBRIDE ROBERT KIMBALL, MCBRIDE, ROBERT KIMBALL
Applicants: TREDEGAR IND INC, TREDEGAR INDUSTRIES, INC.
Application year: 1987
Main classification: A61L15
IPC codes: B32B27, A61L15, A61F13