Abstract:
A method of securing an elastic strand to a sheet of material includes moving the elastic strand and the sheet in a converging manner from a position in which the elastic strand is spaced from the sheet to another position in which the elastic strand contacts one surface of the sheet. Discrete dots of adhesive are intermittently applied to the strand. The dots of adhesive secure the strand to the sheet. Various articles of manufacture may be formed using the method, including hygienic articles such as diapers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/364,811 filed on Mar. 15, 2002, and the disclosure of which is hereby incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to technology associated with securing elastic strands to flat substrates and, more particularly, to the securement of elastic strands to substrates such as those used in producing hygienic articles such as diapers.  
         BACKGROUND OF THE INVENTION  
         [0003]    In various types of manufacturing operations, it is necessary to bond thin elastic strands to one or more sheets of material, such as woven or nonwoven materials. This practice is especially prevalent in the area of hygienic article manufacture, such as during the manufacture of diapers. Diaper manufacturing involves the application of fiberized adhesives, including temperature and/or pressure sensitive adhesives, onto flat substrates and stretched elastic strands, for example, in the areas of the waistband, leg cuffs, and standing leg gathers of the diapers. In these situations, it has been common practice to dispense continuous adhesive fibers or filaments onto either single elastic strands or multiple elastic strands at the same time, either before or after the strand has been laid against the substrate, to bond the strand(s) to the substrate(s). In this manner, overlapping portions of the same material may be bonded together with the stretched elastic strand(s) secured therebetween or two distinctly different substrates may be bonded together with the stretched elastic strand secured therebetween. This is a popular manner to elasticize specific areas of an article comprised of at least one flat substrate.  
           [0004]    One type of adhesive dispenser which has been used extensively for bonding one or more elastic strands to one or more flat substrates is Controlled Fiberization™ (CF™) technology. This well known technique imparts a generally back and forth motion to a dispensed filament of adhesive in the preferred form of a swirl by impacting the filament with a plurality of jets of air. In this manner, a wider region of a substrate may be covered with adhesive dispensed from a single discharge orifice of a nozzle. The width of the adhesive pattern placed on the substrate may be widened to many times the width of the adhesive filament itself. Controlled Fiberization techniques have also been used to secure single or multiple strands of elastic to a flat substrate by dispensing the adhesive onto the strand(s) of elastic while the strand(s) are separated from the substrate. In this manner, the adhesive filament wraps itself around the strand(s) of elastic prior to joining the elastic strand(s) to the substrate thus providing a stronger bond between the elastic strands and the substrate.  
           [0005]    Other adhesive filament dispensing techniques and apparatus have been used for producing vacillating generally sinusoidal pattern of adhesive on a substrate or, for example, a stitching pattern in which the adhesive moves back and forth generally in a sinusoidal or a zigzag motion on the substrate. Still other elastic strand securing methods include extruding a continuous layer of adhesive onto the strand after the strand has contacted the substrate. Various meltblowing techniques have also been used which essentially use randomly dispersed filaments of adhesive discharged onto one or more elastic strands either before or after the elastic strands have contacted the substrate.  
           [0006]    Some of the main goals in this area of technology relate to achieving the necessary bond strength between the elastic strands and the substrates while at the same time transferring the desired elastic properties of the strands to the substrates. Another goal is to use as little adhesive as possible. In addition to undesirable cost increase, using too much adhesive tends to stiffen the substrate and reduce the elastic properties of the strand(s). This latter effect leads to reduced elasticity in critical areas of the diaper, such as the waistband, leg cuffs, and standing leg gathers. In addition, large fiber patterns may obstruct the communication of moisture between layers, such as between an inner layer and an absorbent outer layer.  
           [0007]    For these and other reasons, it would be desirable to provide a method of securing one or more elastic strands to a flat substrate or sheet in a manner suitable for a high speed manufacturing environment, while also achieving the necessary bond strength, creep resistance, efficient use of adhesive, and optimization of other desired characteristics of the resulting product.  
         SUMMARY OF INVENTION  
         [0008]    A method of securing at least one elastic strand to a sheet of material in accordance with this invention includes moving the elastic strand and the sheet in a converging manner from a first position in which the elastic strand is spaced from the sheet to a second position in which the elastic strand is adjacent one surface of the sheet. Preferably, the elastic strand is in a stretched condition during the securement method. In the second position, the strand contacts one surface of the sheet. Discrete dots of adhesive are applied to the strand preferably by intermittently actuating a suitable adhesive dispenser such as an electrically operated valve. The intermittently applied dots of adhesive contact the strand and the sheet of material in the second position to secure the strand to the sheet. In one form of the method, the dots of adhesive are applied to the elastic strand while the elastic strand and the sheet are in the first position, that is, while the strand and the sheet are spatially separated. Alternatively or in addition, the dots of adhesive are applied to the elastic strand while the elastic strand and the sheet are in the second position, that is, while the strand contacts one surface of the sheet. The discharge orifice of the dispenser used to apply the dots of adhesive may be spaced from the elastic strand or it may contact the elastic strand. Also, the discharge orifice may be of any desired shape including, for example, circular or square shapes. Preferably, when the adhesive dots are applied at the first position, the adhesive discharge orifice is spaced from the elastic strand and when the adhesive dots are applied at the second position, the discharge orifice is in contact with the elastic strand.  
           [0009]    In another method performed in accordance with the inventive principles, a slot nozzle is used having an adhesive discharge slot with a length and a width. The length is greater than the width and the adhesive discharge slot may be oriented adjacent the elastic strand with the strand moving parallel to the length of the discharge slot or transverse to the length of the discharge slot. Discrete dots of adhesive are then intermittently applied to the strand from the discharge slot. In this embodiment, the dots of adhesive will typically have a more elongate shape and the lengthwise dimension of the dot may be oriented parallel or transverse to the strand. In accordance with this aspect of the invention, the discrete dots of adhesive may be applied to the elastic strand in any of the manners described above. A strand guide slot may be used directly adjacent each discharge slot to ensure that the strand is stabilized against any significant movement transverse to the direction of its travel through the slot. The strand is preferably spaced slightly from the adhesive discharge opening of the slot.  
           [0010]    Any of the methods discussed generally above may be applied to situations involving the securement of more than one elastic strand to a sheet of material. That is, multiple spaced apart elastic strands are used in the manufacture of various articles, such as in diaper manufacture, to form elasticized sections of the article. In these situations, a plurality of discharge orifices, which may be round orifices or elongate slots, are positioned adjacent a corresponding plurality of elastic strands. Discrete dots of adhesive are then intermittently applied in essentially parallel lines along each of the respective elastic strands in any of the manners previously described.  
           [0011]    The invention further contemplates articles formed from flat substrates with at least one elastic strand secured on at least one substrate and, preferably, between two substrates. The elastic strand, and the first and second substrates are secured together by a plurality of discrete dots of adhesive extending along the elastic strand between the first and second flat substrates. It will be appreciated that the first and second flat substrates may either be completely separate materials secured together or may be portions of the same material which have been folded over to form the first and second substrates With the elastic strand(s) held therebetween. It will also be appreciated that various articles may be manufactured in accordance with the invention including hygienic articles, such as diapers, or other articles formed of flat substrates with elasticized portions.  
           [0012]    The present invention generally provides superior process control in elastic strand securing applications. The elastic strands will be effectively coated with dots of adhesive at a desired frequency. The dots of adhesive will form localized areas characterized by high bond strength between the elastic strand(s) and the substrate. At the same time, the elastic properties of the strands will not be significantly compromised and this should provide for better creep resistance or retained elasticity in the final product. Since the adhesive attachment method of this invention provides for increased creep resistance or retained elasticity in the final product, it is possible to use finer denier elastic strands. The invention also enables the use of less adhesive. Each of these aspects of the invention results in reducing the costs associated with manufacturing the product. A further increase in creep resistance should be obtainable if the adhesive dots are completely separated, i.e., there is no adhesive tailing between dots. Low adhesive add-on rates achieved by the invention also lead to enhanced softness of the manufactured article, and less bleedthrough of adhesive through the substrate(s). This latter advantage allows the use of lower gauge substrates leading to further cost reductions. Moreover, available technology in the form of bead dispensing nozzles or slot nozzles may be used and this will lead to lower costs and production downtime.  
           [0013]    Although preferred embodiments are discussed in detail herein, it will be appreciated that the adhesive dispensing methods of this invention may be applied with the adhesive nozzle spaced from the elastic strand or in contact with the elastic strand. Also, the elastic strand may be spaced from the sheet or substrate or it may be in contact with the sheet or substrate when receiving the adhesive. Any combination of the above may be employed, using orifices or slots of any desired shape and orientation with respect to the strand, depending on the needs of the application.  
           [0014]    These and other features, objects and advantages of the invention will be more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic elevational view illustrating a method for intermittently applying adhesive dots to one or more elastic strands prior to joining the elastic strands with a flat substrate.  
         [0016]    [0016]FIG. 2 is a schematic view similar to FIG. 1, but illustrating a method of intermittently applying dots of adhesive after the elastic strand has contacted one surface of the flat substrate.  
         [0017]    [0017]FIG. 3 is a top view illustrating the preferred adhesive pattern of intermittent, discrete and separated dots of adhesive on multiple elastic strands.  
         [0018]    [0018]FIG. 4 is a fragmented front elevational view showing the nozzle portion of a slot nozzle dispenser for intermittently applying dots of adhesive onto three elastic strands.  
         [0019]    [0019]FIG. 5 is a bottom view of the slot nozzle shown in FIG. 4, but with the strand guide removed for clarity.  
         [0020]    [0020]FIG. 6 is a cross sectional view taken along lines  6 - 6  of FIG. 4 illustrating the method of applying discrete dots of adhesive onto the moving elastic strand.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    For purposes of this description, words of direction such as “upward”, “vertical”, “horizontal”, “right”, “left” and the like are applied in conjunction with the drawings for purposes of clarity in the present description only. As is well known, liquid dispensing devices may be oriented in substantially any orientation, so these directional words should not be used to imply any particular absolute directions for an apparatus consistent with the invention.  
         [0022]    [0022]FIG. 1 illustrates one embodiment of the method of this invention which uses an adhesive dispenser  10  including a nozzle  12 . Nozzle  12  may include a circular discharge orifice, a more elongate slot-shaped orifice, or other types of orifices suitable for dispensing discrete dots of adhesive. In this embodiment, one or more stretched Lycra strands  14  are moving in the direction of arrow  16  and a flat sheet  18  of substrate material, such as a woven or nonwoven material, is moving in the direction of arrow  20  around a conventional guide cylinder  22  rotating in the direction of arrow  24 . Strand  14  is stretched so that, upon attachment to sheet  18 , the sheet  18  will be elasticized generally along a line defined by strand  14 . Dispenser  10  is operated by a suitable controller  26  for intermittently actuating a valve (not shown) within the dispenser  10 . The valve actuates on and off to selectively allow and prevent the flow of adhesive through the nozzle  12 . The apex or tip  12   a  of the nozzle  12  is spaced a short distance from the Lycra strand  14  and accurately dispenses adhesive dots  30  directly onto the strand  14  immediately prior to or upstream from the point  32  where the strand  14  meets the substrate  18 . Thus, during the time that it takes for the strand  14  to reach point  32 , each adhesive dot  30  will flow around all sides of the strand  14  including the lower side (as viewed in FIG. 1) to ensure full bonding between the strand  14  and the upper surface of the substrate  18 .  
         [0023]    The frequency of the adhesive dots  30  will depend on the application conditions and needs, however, in one exemplary embodiment, there may be 4-7 dots 30 per inch in a diaper manufacturing application. The adhesive discharge orifice of the nozzle  12  may be circular and generally about 0.010 inch to about 0.020 inch in diameter when a pressure-sensitive hot melt-adhesive having a viscosity of 1000-4000 centipoise is used at a temperature of 275° F. to 350° F. and a pressure of 400 psi to 800 psi. Of course, many other specific parameters for either the dispenser  10  or nozzle  12 , or both, as well as for the adhesive, may be used depending on the application. The distance from the discharge orifice at the apex  12   a  of the nozzle  12  to the elastic strand  14  may be from about 0.010 inch to about 0.5 inch. Again, however, this distance may also change depending on the needs of the application. One preferred dispenser is the ES400 dispenser available from Nordson Corporation of Westlake, Ohio which uses an electromagnetic coil capable of cycling at a high frequency. As will be appreciated, higher speed production lines will require a higher frequency actuation to achieve the desired spacing of adhesive dots  30  on the elastic strands  14 .  
         [0024]    [0024]FIG. 2 illustrates another method performed in accordance with the inventive principles. In this method, the dispenser  10  and, more particularly, the discharge nozzle  12  is positioned upstream of the point  32  of joinder between strand  14  and substrate  18  such that the adhesive dots are dispensed after the Lycra strand(s)  14  have contacted the flat substrate  18 . Also, this embodiment illustrates the option of contacting the strand  14  with the apex  12   a  of the nozzle  12  while intermittently dispensing the dots  30  of adhesive. In this case, the dots  30  of adhesive are dispensed onto the Lycra strand(s) and the upper surface of the flat substrate  18  in a simultaneous fashion. This, as in the first embodiment, forms discrete bond points between the elastic strand(s)  14  and the mating surface of the substrate  18 .  
         [0025]    In the embodiments of FIGS. 1 and 2, it will be appreciated that nozzles  12  may be used which dispense only through one discharge orifice  15  or, alternatively, through multiple discharge orifices. In the latter case, for example, each orifice would be positioned adjacent to a corresponding elastic strand  14  such that discrete, separated dots of adhesive  30  would be dispensed onto a corresponding number of elastic strands  14  as, for example, illustrated in the top view of FIG. 3. Alternatively, in the various embodiments of the invention there may be thinner areas of adhesive between adjacent dots  30 . For example, this may occur if dispenser  10  is cycled “on” and “off” in such a manner that an adhesive tail is formed between each dot. That is, dispenser  10  might not fully shut off between adhesive dot dispensing cycles. The shape of the adhesive dots  30  themselves may also take different forms. The dots  30  may be essentially circular in shape as shown in FIG. 3, or the dots  30  may be more elongated in directions parallel or transverse to the length of the elastic strand(s)  14 . This is the case, for example, if a slot nozzle is used with the length of the slot extending parallel or transverse to the length of the elastic strand(s)  14  as discussed below.  
         [0026]    FIGS.  4 - 6  illustrate another embodiment of the invention utilizing an illustrative slot nozzle  40  including an elastic strand guide  42  for ensuring that the strands  14  are held steady as they move through the respective slots  44  of the nozzle  40 . Each strand  14  moves through a slot  46  in the guide  42  and then through a slot  40  in the adhesive discharge nozzle. An intermittently actuated valve (not shown) may be used as discussed in connection with the first embodiment to discharge elongate dots  48  of adhesive onto each strand from openings  50  associated with the respective discharge slots  40 . In this embodiment, the strands  14  are again preferably spaced from the discharge openings  50  as in the first embodiment by a spacing that may be determined according to the application needs. The length of the opening  50  may be also determined by application needs. The chart below illustrates one preferred set of dimensions for the strand size when, for example, Lycra is used, and the corresponding recommended minimum slot width and discharge orifice width.  
         [0027]    Comparison Between Elastic Strand Size and Nozzle Dimensions  
                                       Elastic               Strand       Discharge Orifice       (Denier)   Slot Width   Width                   420 mm   0.23 mm   0.20 mm       490 mm   0.25 mm   0.23 mm       560 mm   0.28 mm   0.25 mm       700 mm   0.32 mm   0.30 mm       840 mm   0.35 mm   0.30 mm       1120 mm    0.47 mm   0.42 mm                  
 
         [0028]    The dispenser in this embodiment may be the same as dispenser  10  of the first embodiment. As shown in the figures, slot opening  50  may be oriented with its length extending parallel to the corresponding strand  14 . As another alternative, slot opening  50  may be oriented with its lengthwise dimension extending transverse to or across the direction that strand  14  moves. This may better allow the adhesive to flow around the sides of the strand  14 .  
         [0029]    In accordance with the invention, one or more elastic strands are coated with discrete, separated dots of adhesive either before and/or after the strand contacts one surface of the sheet. As the elastic is stretched when applied and bonded to the sheet, the sheet is elasticized along the region of the strand or strands. In addition to contacting the sheet, it will be appreciated that the adhesive should contact enough of the strand to form a sufficient bond between the sheet and the strand. This may involve fully coating all sides of the strand or only partially coating the strand. In the embodiments described above, an additional linear formation of adhesive, such as one applied by a meltblown nozzle, may be applied to the sheet, prior to contact with the strand, to assist with the bond. Dots of adhesive may be applied from only one side of the strand, such as the upper side, or from opposite sides of the strand, such as upper and lower sides. If applied from opposite sides of the strand, the dots may be applied from directly opposite locations such that they contact the same point along the strand, or they may be applied in a staggered format with upper and lower dots of adhesive alternating along the strand. In this manner, it may be possible to apply more dots of adhesive per inch. The number of adhesive dispensers on either or both sides of the strand may be varied according to the needs of the application.  
         [0030]    While the present invention has been illustrated by a description of preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the appended claims to such detail.  
         [0031]    Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in numerous combinations depending on the needs and preferences of the user. This has been a description of the present invention, along with the preferred methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims, wherein we claim: