Abstract:
A method for production of disposable pant diapers, the method including providing a length of a substantially homogeneous elastic laminate web having a maximum elastic extensibility in a first direction of at least 40% under a peak load F p ; advancing the length of the substantially homogeneous elastic laminate web in the first direction; attaching individual absorbent cores to the length of the elastic laminate web during the advancing to create a strip, while maintaining the length of the substantially homogeneous elastic laminate web under a tensioning load F t  in the first direction; folding the strip onto itself; maintaining the length of the substantially homogeneous elastic laminate web under the tensioning load F t  in the first direction during the folding; the tensioning load F t  satisfying the condition: 0.03 F p ≦F t ≦0.25 F p ; and welding the substantially homogeneous elastic laminate web and severing the individual pant diapers along the weld lines.

Description:
[0001]    The present application is a continuation of U.S. Ser. No. 11/658,539, filed on Jan. 25, 2007, which is a national stage application of PCT/SE2004/001384, filed on Sep. 29, 2004. The entire contents of U.S. Ser. No. 11/658,539 and PCT/SE2004/001384 are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method for production of disposable hygienic absorbent articles such as pants-type absorbent articles. 
       BACKGROUND OF THE INVENTION 
       [0003]    In order to improve comfort and fit, disposable hygienic articles, and particularly those which are intended to be worn around the waist of a user, are commonly provided with elastic components in selected regions of the article. In a pants-type absorbent article, i.e. an article which is pulled on in the same manner as a traditional pair of pants, the selected regions comprise the waist opening and the leg openings. Conventionally, the elastic components used in these regions are in the form of elastic threads or bands which are secured to a substrate, e.g. the outer cover of the article, in a stretched state under a tensioning force. When the tensioning force is released, the elastic components contract and thereby gather the substrate to which they are secured. In this manner, an otherwise substantially inelastic material web can be provided with elastic properties. 
         [0004]    The above-described manner of providing elasticised regions on an absorbent article suffers from several drawbacks. For example, securing the elastic components to a running material web in a continuous process is relatively complicated. The elastic components are generally secured to the material web with adhesive, for example hot-melt adhesive or thermosetting adhesive. To ensure that the elastic components do not release from the material web during the production process, it is necessary to ensure that the elastic components bear against the material web until the adhesive has set, dried or hardened. Problems may also arise in synchronizing the tensioning of the different elastic components and in achieving uniform tensioning of the different elastic components independently of the speed of the running web material. Since the extent of elasticity of the thus elasticised material web is directly proportional to the degree to which the elastic components are tensioned, typically 100%, when attached to the material web, during production the articles occupy an area which is substantially greater than that occupied by the finished articles in a relaxed state. This implies that the production facility takes up an undesirably large area. In addition, the production equipment has to be dimensioned to accommodate the tensioning force. A further difficulty is that of handling the finished articles once they have been severed from the running material web, since the articles assume an irregular three-dimensional shape as soon as the tensioning of the elastic components ceases. Folding and packaging of the finished articles have additionally proven to be very difficult steps because of their creased and three-dimensional shape. 
         [0005]    Although such elasticised disposable absorbent articles may provide satisfactory comfort and fit, the thus gathered material web can impart a bulky, creased appearance to the article. This implies that it may be difficult to conceal the article under normal clothing. This is particularly problematic for adult users of disposable absorbent pants. 
         [0006]    To overcome at least some of these drawbacks, rather than using an elasticised material in disposable absorbent articles, it has been proposed to make at least some regions of disposable absorbent articles of elastic material per se, for example an elastic nonwoven or elastic laminate. By using such material, there is no need to secure a tensioned elastic component to a gatherable substrate. As such, theoretically at least, it is possible to manufacture disposable absorbent articles in a process in which the constituent elastic components are substantially unstretched. Indeed, in WO-A-03/070140 it is proposed to stretch the elastic material web during production to a maximum of 5%. 
         [0007]    The present inventors have found that, in order to ensure accurate control of the elastic material web during production, it is necessary to subject the web to a certain minimum tensioning load. On the other hand, to enjoy as many as possible of the advantages that, at least theoretically, are available using an elastic material web vis-à-vis an elasticised web, it is important that the tensioning load be significantly less than the load to attain maximum elastic extensibility of the elastic material web. 
       OBJECTS AND SUMMARY 
       [0008]    It is therefore an object of the invention to provide a method of production of disposable hygienic absorbent articles which permits efficient production of the articles. 
         [0009]    According to one method, a disposable hygienic absorbent article is produced in a process in which an elastic material web is a substantially homogenous elastic laminate web having a maximum elastic extensibility in a first direction of at least 40%, preferably at least 60%, and most preferably at least 80%, under a peak load F p . The elastic laminate web is advanced in a direction of travel corresponding to the first direction while being maintained under a tensioning load F t  satisfying the condition: 0.03 F p ≦F t ≦0.25 F p . 
         [0010]    For the purposes of this disclosure, an elastic laminate web is to be regarded as substantially homogenous if the properties of the web at any two sections of the web are essentially the same in the same direction. 
         [0011]    Elastic extensibility here refers to the lengthening of the elastic laminate web in the direction of applied load which the web permits without plastically deforming or rupturing. For a material to be deemed to be elastically extensible it must also strive to recover its original length once the tensioning load is removed. For present purposes, a laminate web is deemed to be elastically extensible if it can be extended in at least one direction to at least 130% of its initial length, and will revert to at most 120%, preferably no more than 110%, of its original length upon removal of the tensioning load, while still meeting the requirement of maximum elastic extensibility outlined above. 
         [0012]    Due to the non-linear load/elongation property of elastic laminate webs caused by the inter-engagement of fibres of the less elastic layer or layers of the laminate, the %-elongation per unit load decreases at higher values of elongation. By selecting the tensioning load F t  so as to satisfy the condition 0.03 F p ≦F t ≦0.25 F p , adequate extension of the web is assured at the same time that the production equipment is not subjected to unnecessarily large forces. 
         [0013]    Under certain circumstances, it may be preferable to increase the elongation of the laminate during production. As such, in a preferred embodiment, the tensioning load F t  may be selected so as to satisfy the condition 0.05 F p ≦F t ≦0.25 F p . Similarly, where it is desirable to keep the load on the laminate as low as possible, F t  may be selected so as to satisfy the condition 0.03 F p ≦F t ≦0.20 F p . In a further preferred embodiment, F t  may be selected so as to satisfy the condition 0.05 F p ≦F t ≦0.20 F p . 
         [0014]    Preferably, the elastic laminate web comprises at least one nonwoven layer affixed to a film layer. The film layer is preferably an apertured three-layer PE/styrene-based film/PE elastomeric film. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    Embodiments of the invention will be described in the following in greater detail by way of example only and with reference to the attached drawings, in which: 
           [0016]      FIG. 1  is a schematic perspective view of a length of a production line for the production of a disposable hygienic absorbent article in accordance with an embodiment of the present invention; 
           [0017]      FIG. 2  is a schematic graphic representation showing load vs. strain for an elastic laminate; 
           [0018]      FIG. 3  is a graph showing load vs. strain for an elastic laminate comprising 25 gsm nonwoven material; 
           [0019]      FIG. 4  is a graph showing load vs. strain for an elastic laminate comprising 20 gsm nonwoven material; 
           [0020]      FIG. 5  is a graph showing load vs. strain for an elastic laminate comprising 18 gsm nonwoven material, and 
           [0021]      FIG. 6  is a cross-sectional view through an elastic laminate web for use in an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    In  FIG. 1 , reference numeral  10  generally denotes length of a production line for the production of a disposable hygienic absorbent article  12 . In the present example, the absorbent article is a pull-up pants-type diaper. 
         [0023]    The various components of the absorbent article are united on a (not shown) continuous conveyor belt running in a direction of travel denoted by arrow A. In the broadest form of the invention, a continuous length of a substantially homogenous elastic laminate web  14  is placed on the conveyor belt and maintained under a tensioning load F t . The elastic laminate web has a maximum extensibility in a first direction corresponding to the direction of travel denoted by arrow A of at least 40%, preferably at least 60%, and most preferably at least 80%, under a peak load F p . The continuous length of the elastic laminate web  14  is advanced in the direction of travel A to a station at which individual absorbent cores  16  are attached to the continuous length of the elastic laminate web  14  at spaced intervals. Individual articles  12 , in this case pull-up pants-type diapers, are thereafter formed from the thus assembled individual absorbent cores  16  and the continuous length of the elastic laminate web. 
         [0024]    The continuous length of the elastic laminate web  14  is maintained under a tensioning load F t  in the direction of travel during the advancing, with the tensioning load F t  satisfying the condition: 0.03 F p ≦F t ≦0.25 F p . 
         [0025]    In order to carry out the above described method, it is necessary to determine the peak load F p , i.e. the load under which the elastic laminate web displays elastic extensibility. The peak load is determined using the ASTM D 882 tensile strength test method. The tensile strength and elongation of a well-defined test piece is tested by means of a tensile tester. 
       Apparatus: Instron 4301 
       [0026]    Tensile tester connected to a computer
 
Crosshead speed: 500 mm/min
 
Clamp distance: 50 mm
 
         [0027]    Sample preparation: Test samples are cut from the entire width of the material. The width of the sample shall be 25.4 mm and the length at least 50 mm longer than the clamp distance if possible. It is of importance that the edges of the sample are even and without break notches. The samples are conditioned for at least 4 h in 50% RH±5% RH and 23° C.±2° C. before testing. 
         [0028]    Procedure: The tensile tester is calibrated according to the apparatus instructions and set to zero. The sample is mounted and it is ensured that it is not obliquely or unevenly fastened. The material is prevented from slipping by using clamps covered with galloon or similar material. The tensile tester is started, and stopped after the material has broken (if not automatically controlled). Measurements resulting from premature failures (i.e. the sample breaks at the clamp, or is damaged during preparation) are ignored if possible. 
         [0029]    The following results are expressed by the tensile tester/computer: 
         [0000]    Maximum force, N/25.4 mm
 
Elongation at maximum force, %
 
Break force, N/25.4 mm
 
Elongation at break force,
 
Knee point, N/%
 
         [0030]      FIG. 2  is a schematic representation of the behaviour of an elastic laminate web for use in the method according to the present invention under stretching at a constant strain. The laminate web comprises 25 gsm Sofspan NW from BBA on both sides of a 40 gsm apertured elastic film, where one face is glue-laminated with approximately 5 gsm glue. 
         [0031]    From zero strain, the elastic laminate web exhibits substantially elastic behaviour in region (A) up to around a “knee point” (B), after which, the load increases rapidly through region (C). The knee point (B) is defined as the first point on the load-strain curve at which the gradient becomes greater than 0.3N/%. The laminate web shown is elastic up to about 80% strain. The applied load eventually reaches a maximum (the “peak load”) at point (D), at which the gradient of the load-strain curve is zero. The load then drops through region (E) as the material fails. Complete failure of the laminate web occurs at point (F). 
         [0032]    The peak load F p  is the applied load at point D. In the embodiment of the present invention, the elastic laminate web is maintained under a tensioning load F t  which satisfies the condition: 0.03 F p ≦F t ≦0.25 F p . 
         [0033]      FIGS. 3 to 5  show the actual behaviour of elastic laminate webs having a 40 gsm apertured elastic film, though glue-laminated with approximately 3 gsm glue to Sofspan NW of different basis weights. Thus, in  FIG. 3  the basis weight of the nowwoven webs is 25 gsm, in  FIG. 4  it is 20 gsm and in  FIG. 5  it is 18 gsm. In all cases, the “knee point” (B) lies within the range 0.03 F p ≦F t ≦0.25 F p . 
         [0034]    Under certain circumstances, it may be preferable to increase the elongation of the laminate during production. As such, in a preferred embodiment, the tensioning load F t  may be selected so as to satisfy the condition 0.05 F p ≦F t ≦0.25 F p . Similarly, where it is desirable to keep the load on the laminate as low as possible, F t  may be selected so as to satisfy the condition 0.03 F p ≦F t ≦0.20 F p . In a further preferred embodiment, F t  may be selected so as to satisfy the condition 0.05 F p ≦F t ≦0.20 F p . 
         [0035]      FIG. 6  is a cross-section through an elastic laminate web  14  for use in an embodiment of the present invention. The elastic laminate web comprises a first nonwoven layer  26  affixed to a first surface  28  of a film layer  30 . Advantageously, the elastic laminate web further comprises a second nonwoven layer  32  affixed to a second surface  34  of the film layer  30 . 
         [0036]    Preferably, the film layer comprises an elastic film having a basis weight from about 20 g/m 2  to about 100 g/m 2 , preferably between 20 and 60 g/m 2 . The film layer may be selected from the group consisting of low crystallinity polyethylenes, metallocene-catalyzed low crystallinity polyethylene, ethylene vinyl acetate copolymers (EVA), polyurethane, polyisoprene, butadiene-styrene copolymers, styrene block polymers such as styrene/isoprene/styrene (SIS), styrene/butadiene/styrene (SBS), styrene/ethylene-butadiene/styrene (SEBS) block copolymer and blends thereof. 
         [0037]    To increase the breathability of the elastic laminate web, the film layer  30  may be provided with apertures  37 . In a preferred embodiment, the film is an apertured three-layer PE/styrene-based film/PE elastomeric film. 
         [0038]    Each of the first and second nonwoven layers may have a basis weight of from about 10 g/m 2  to about 40 g/m 2 , preferably from about 12 g/m 2  to about 30 g/m 2 , most preferably from about 15 g/m 2  to about 25 g/m 2 , and may comprise a spunbond or carded material selected from the group consisting of: polypropylene, polyethylene, polyester and other polyolefin homopolymers and copolymers. 
         [0039]    In order to provide the individual articles  12  with desirable properties, the basic production method outlined above may be complemented by one or more of the following steps. 
         [0040]    In the production of a pants-type diaper, adhesive  36  may be applied, for example by spraying or coating, to the elastic laminate web  14 , after which elastic members  38  in the form of continuous elastic bands or threads are secured in a curved pattern across the laminate web. These elastic members  38  will form elasticised leg openings in the completed article  12 . Alternatively, the adhesive may be applied directly to the elastic members themselves. 
         [0041]    A second material web  40  of liquid barrier material, such as an elastic plastic film, may thereafter be applied over the elastic laminate web  14  and the elastic members  38 . The second material web may be provided with adhesive on its surface facing the first web  14  to ensure its adequate adhesion thereto. 
         [0042]    In the illustrated embodiment a third material web  42  is placed over the elastic laminate web  14  and the second material web  40  and secured over the absorbent cores  16 . The third elastic material web will constitute the topsheet of the completed absorbent article. As such, it can consist of a nonwoven material, e g spunbond, meltblown, carded, hydroentangled, wetlaid etc. Suitable nonwoven materials can be composed of natural fibers, such as woodpulp or cotton fibres, manmade fibres, such as polyester, polyethylene, polypropylene, viscose etc. or from a mixture of natural and manmade fibres. The topsheet material may further be composed of tow fibres, which may be bonded to each other in a bonding pattern, as e.g. disclosed in EP-A-1 035 818. Further examples of topsheet materials are porous foams, apertured plastic films etc. The materials suited as topsheet materials should be soft and non-irritating to the skin and intended to be readily penetrated by body fluid, e.g. urine or menstrual fluid. Advantageously, the third material web will have elastic properties. Before the third material web is applied to the second material web, the third elastic web may be coated with an adhesive on its surface directed towards the second material web. 
         [0043]    In an alternative production method, the second material web  40  of liquid barrier material may be omitted. Instead, nonwoven material may be placed over the elastic members  38 . Liquid barrier material may then be incorporated in an absorbent packet comprising a plastic film, an absorbent core and a nonwoven surface layer. Although in  FIG. 1  the elastic members  38  as laid out on the elastic laminate web comprise crotch elastic, it is to be understood that the crotch elastic may be incorporated in the thus-described absorbent packet instead. 
         [0044]    The above-described assembly of components forms a production web  44 . Leg openings  46  are cut out from the production web and the production web is then folded double in the production direction so that a fold edge  48  and an open edge  50  are formed. The folded production web  44  is then welded intermittently transverse to the production direction along weld lines  52  extending from the open edge  50  to the edge at each leg opening  46 . The welded production web  44  is then divided by being severed along each weld line  52  so that individual pants-type diapers  12  are separated from the production web  44 . The individual diapers can then be subjected to further processing steps such as folding and packing. 
         [0045]    The elastic laminate web  14  of the thus-produced pants-type diapers  12  constitutes the outer cover of the diapers. In a similar manner, the elastic laminate web may also constitute the outer cover of many types of disposable hygienic absorbent articles, such as conventional diapers, incontinence garments, sanitary napkins and panty liners. It is to be understood that the elastic laminate web  14  need not cover the entire outer surface of such articles. Instead, in certain circumstances it may be desirable to have a non-elastic region of the outer cover. Thus, for example, the crotch region of a diaper may have an outer cover region which is constituted by a strip of non-elastic nonwoven material bounded on either side by an elastic laminate web. In this manner, front and rear panels of the diaper will enjoy elastic properties.