Patent Publication Number: US-5296289-A

Title: Stretchable spun bonded nonwoven web and method

Description:
BACKGROUND OF THE INVENTION 
     Spun bonded nonwoven webs having autogenous spot bonds are described in U.S. Pat. No. 3,855,046, Hansen et al. U.S. Pat. No. 3,949,128 illustrates elastic spun bonded nonwovens having spot bonds arranged in a pattern which is elongated and distorted in response to forces exerted in the machine direction causing the filaments to buckle between the spot bonds with subsequent heat setting. Stretchability in the cross direction may be achieved by a later microcreping process. An increase in basis weight is caused by such bulking. U.S. Pat. No. 4,374,888 illustrates a non-woven laminate including exterior layers of spun bonded nonwoven webs each having autogenous bonds arranged in widely dispersed patterns of spot bonds in side by side relation and in rows both in the machine direction and in the cross direction and having an intermediate melt blown layer. These layers are pattern bonded utilizing a sonic horn. 
     It is an important object of this invention to provide a spun bonded nonwoven web which has been stretched to provide a soft &#34;hand&#34; and greater coverage with minimal sacrifices in strength as a result of stretching in either the machine direction or in the cross direction although there is a substantial reduction in basis weight of the web commensurate with the amount of dimensional increases. 
     Another important object of the invention is the provision of a spun bonded nonwoven web having a symmetrical pattern of autogenous spot bonds arranged in such a way as to provide marginal &#34;secondary bonding&#34; from which portions of continuous filament become disengaged when the web is subjected to stress in the machine direction or in the cross direction prior to release of the filaments in the spot bond. 
     Still another object of this invention is to increase the stretchability of a spun bonded nonwoven web having spot bonds arranged in a pattern conducive to symmetrical displacement upon the application of stress in the machine direction or in the cross direction. 
     SUMMARY OF THE INVENTION 
     A soft stretchable spun bonded nonwoven web has spot bonds distributed in a pattern providing spaced rows of spaced soft bonds in side by side relation in the machine direction and in the cross direction producing continuous strips or ribs of unbonded or partially bonded filaments in the machine direction as well as similar continuous strips or ribs in the cross direction throughout the web. 
     Such a &#34;cornrow&#34; bond pattern produces a larger primary bond area as a percentage of total area as well as a &#34;secondary&#34; bonding of the marginal filaments. The cornrow bond pattern produces a material which is stronger than would normally be expected because of the secondary bonding. This secondary bonding may be caused by the relative closeness of the bond points, such that the bond points merge at the edges producing secondary bonding. 
     Not only is there less primary bond area but also secondary bonding is not present in the &#34;staggered&#34; bond pattern of the prior art. Such prior art is illustrated in the aforementioned U.S. Pat. No. 3,949,128. In such a conventional staggered pattern, spot bonds are each isolated in a wider staggered pattern making discrete bonds with minimal secondary bonding. 
     When spun bonded nonwoven webs constructed in accordance with the invention are stretched, the secondary bonds are broken, and tensile strength and elongation are slightly reduced. However, stretching of prior art webs breaks primary spot bonds or breaks filaments from the bond points, producing similar effects which will greatly reduce the relatively lower strengths and elongation values possessed by such webs before stretching. 
     Here stretching will improve the hand and &#34;loft&#34; of the material with acceptable losses in tensile strengths and elongation characteristics. Stretching of prior art materials will improve the hand somewhat, but with unacceptable losses in strength and stretchability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The construction designed to carry out the invention will be hereinafter described, together with other features thereof. 
     The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein: 
     FIG. 1 is a perspective view illustrating conventional apparatus for manufacturing a spun bonded nonwoven web which is thereafter spot bonded in accordance with a roll bearing a special pattern producing a web constructed in accordance with the present invention; 
     FIG. 2 is a developed view illustrating the surface of a patterned roll constructed in accordance with the staggered pattern of the prior art; 
     FIG. 3 is a developed view illustrating the pattern of a roll constructed in accordance with the invention illustrating spaced continuous rows of spaced raised portions arranged side by side in both the machine direction and in the cross direction; 
     FIG. 4 is an enlarged transverse sectional elevation illustrating a patterned roll and a heated roll forming spot bonds in a spun bonded nonwoven web in accordance with the present invention; 
     FIG. 5 is a top plan view illustrating a web constructed in accordance with the present invention; 
     FIG. 6 is a further enlarged transverse sectional elevation illustrating a web being constructed in accordance with the invention with the cross section thereof taken in the longitudinal direction; 
     FIG. 7 is a sectional elevation similar to FIG. 6 further illustrating a nonwoven web of FIG. 6 after bonding; 
     FIG. 8 is a further cross sectional elevation of the web after having been stretched in both the machine direction and in the cross direction; 
     FIG. 9 is an enlarged top plan view further illustrating a nonwoven web having continuous strips or ribs in the machine direction and in the cross direction in accordance with the present invention; 
     FIG. 10 is a bottom plan view illustrating the web illustrated in FIG. 9; 
     FIG. 11 is a schematic top plan view illustrating apparatus for sequentially stretching a spun bonded nonwoven web which has been spot bonded in accordance with the present invention in both the machine direction and in the cross direction; 
     FIG. 12 is a graph comparing the cross direction strengths both before and after stretching a web constructed in accordance with the present invention with a web constructed in accordance with the prior art; and 
     FIG. 13 is a graph comparing the strengths of a web constructed in accordance with the present invention in the machine direction before and after stretching with such values of a web constructed in accordance with the prior art. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The drawings illustrate a stretchable spun bonded nonwoven web as well as apparatus and method for making same creating a soft high loft web. A plurality of continuous and randomly deposited, molecularly oriented filaments of a thermoplastic polymer form a web having a plurality of spaced spot bonds A of autogenous releasable filament bonds having been formed by passing said web between a roll with a raised pattern thereon and another opposed roll exerting pressure and applying heat to the web. The spot bonds are distributed in a pattern providing continuous strips or ribs B of filaments therebetween in the machine direction as well as continuous strips or ribs C in the cross direction throughout the web. 
     The pattern is such that the spot bonds are in rows and in side by side relation in both the machine direction and in the cross direction. The continuous strips are of such width that respective spot bonds are sufficiently close as to provide secondary bonding between the spot bonds resulting in enhanced stretchability in both the machine direction and in the cross direction without excessive loss of strength. The nonwoven web is subjected to stretching in either the machine direction or in the cross direction or both resulting in permanent elongation in either direction in excess of about thirty (30%) percent and preferably about one hundred (100%) percent as illustrated in the graphs herein. In any event the coverage is greatly increased with commensurate reduction in basis weight of the web. The releasable filaments of the spot bonds are held for stabilizing the web, but the release strength of the filaments is such that the bonded intensity approaches but is less than the pull required for fracturing the filaments when the web is subjected to stress. 
     The spun bonded nonwoven web of the present invention is manufactured by depositing continuous filaments 10 of thermoplastic material such as from spinnerettes 11. The filaments pass through an oscillating guide 12 and are randomly deposited upon an endless conveyer 13 in the customary manner for making a spun bonded web. The web thus formed is fed between the nip of a roll 14 having a raised pattern and a heated smooth roll 15 producing a web 16 which is thereafter stretched in accordance with the invention as described below. 
     The raised pattern of the roll 14 is illustrated in FIG. 3. Raised portions are in spaced rows 17 and 18 in the machine direction and in the cross direction respectively. The serially spaced spot bonds are closely spaced and arranged in side by side closely spaced relation. This pattern is in contrast to the asymmetrical or staggered portion of the prior art illustrated in FIG. 2 wherein the raised portions 19 of the pattern are scattered in a widely spaced relation in diagonal spaced rows. 
     The nip of the patterned roll 14 and the heated roll 15 is illustrated in FIG. 4 producing a spun bonded web 16 with autogenous spot bonds arranged in the pattern illustrated in FIG. 3. FIG. 5 illustrates a fabric 16 utilizing rolls illustrated in FIGS. 1, 3, 4 and 6. 
     FIG. 6 is an enlarged sectional elevation illustrating the web 16 with the ribs C disposed in the cross direction. The spot bonds A having a depression broadly designated at 20 have an upper surface 21 and a lower surface 22 recessed a lesser amount opposite the heated roll 15. FIG. 7 illustrates the rib C, while FIG. 8 illustrates the rib C and the bonds A having been stretched utilizing the apparatus illustrated in FIG. 11. 
     FIGS. 9 and 10 illustrate respective top and bottom elevations of the nonwoven fabric 25 constructed in accordance with the present invention. It will be noted that the continuous filaments 26 in the machine direction are oriented in the continuous strips or ribs B, while the filaments 27 in the ribs or strips C in the machine direction are predominately oriented continuously in that direction. This may occur as a result of the raised portions of the pattern parting some of the loose randomly distributed filaments becoming oriented in this fashion as the raised portions separate them upon compression of the web in the marginal areas of the web adjacent the spot bonds. 
     Referring now to FIG. 11, apparatus is illustrated for imparting stretch in the machine direction through the use of rolls 30 which attenuate the web 16. Each of the rolls 30 is driven faster a preceding roll as through the use of a suitable DC motor 31. The web 16 which is stretched in the machine direction is then introduced to a tenter between chains 32 and clips 33 for stretching the fabric in a cross direction resulting in the finished web 25 which is wound into a roll 26. Examples of the characteristics of webs which have been manufactured and stretched in accordance with the present invention are set forth below. If desired infrared heaters providing an adjustable heat source of about 260° F. may be provided as illustrated at 40 to provide increased stretchability and to vary the characteristics of the web. 
     Prior Art--Average Weight--42.64 gly 2   
     Subject Nonwoven--Avg. Wt.--42.18 gly 2   
     Tensile strengths are averages taken from an Instron machine Model 1011. Gage length is 5.00 inches and speed 5.00 in/min. Samples were taken of the materials being 9&#34;×11&#34; with the length in the machine direction. The samples were weighed and cut into 1 inch strips with the exception of the material to be stretched. The samples to be stretched were first folded into a 1-2 inch bundle with the length being machine direction. Samples were then placed in the Instron machine and clamped using approximately 1/2 inch of material on each end. The material was than stretched 5.55 inches, released, refolded and stretched cross directionally for a distance of 4.45 inches. One (1) inch samples were cut from the resulting material. 
     The resulting data follows: Note elongation values are at break-point. 
     
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                Tensile in. Kg                                            
                            Elongation % (% of 5 in.)                     
______________________________________                                    
Prior Art                                                                 
         CD     2.102        78.2                                         
         MD     3.09         88.4                                         
Stretched                                                                 
         CD     1.42         43.2                                         
         MD     2.07         51.6                                         
Subject  CD     4.612       151.8                                         
Fabric                                                                    
Normal   MD     2.689       143.4                                         
Stretched                                                                 
         CD     2.857       121.4                                         
         MD     2.64        130.7                                         
Elongation                                                                
         CD     --          210.4                                         
Values                                                                    
With Pre-                                                                 
         MD     --          241.7                                         
Stretching                                                                
Factored In                                                               
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%    Actual Stretch                                                       
                  Deformation Def. as % of Stretch                        
______________________________________                                    
 5    .25 in      .0625 in    25                                          
10    .50 in      .0810 in    16.2                                        
15    .75 in      .2190 in    29.2                                        
20   1.00 in      .2500 in    25                                          
25   1.25 in      .5625 in    45                                          
Average               28.1                                                
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     Deformation occurs even at low percentages of stretch, and for these low stretches the deformation tends to be in the neighborhood of 25% of stretch. Therefore, lower limits of stretch without deformation of the material seems to be unobtainable. 
     Thus, use of the process of the invention results in a web which possesses improved strength and elongation characteristics over unstretched webs of comparable spun bonded construction. The web retains these qualities despite stress which would meet or exceed the capabilities of comparable prior webs such as a pre-stretching which meets or exceeds the elasticity limits of the similar weight of such prior webs. 
     While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.