Abstract:
There is disclosed a novel process to form a water-absorbing sheet by extruding an aqueous solution of superabsorbing polymer as a fibrous stream onto a high velocity, hot fibrous stream of melt-blown fibers of thermoplastic polymer, causing entanglement of the fiber and forming a superabsorbent non-woven mat free of dusting problems.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to a process for melt-blowing a composite web, and more particularly to a process for melt-blowing superabsorbent fibrous composite webs and the product produced thereby. 
     (2) Description of the Prior Art 
     To increase the sorbency of fibrous webs by addition of superabsorbent particles has been the object of several prior workers. U.S. Pat. No. 4,429,001 describes the prior art of this approach, where superabsorbent particles are entrapped in a web of fine fibers. The disadvantage of this method is that the particles are either too well entrapped and shielded from the liquid to be sorbed, and therefor the absorbency is limited, or bonding of the particles is incomplete and the particles, prior to use, are &#34;dusting out&#34;. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a process for forming a superabsorbent fibrous composite web using melt-blowing techniques. 
     Another object of the present invention is to provide a novel apparatus and process to intermingle melt-blown thermoplastic fibers with fibers made from superabsorbent polymers. 
     Still another object of this invention is to provide a composite web of improved absorbency and physical strength in the dry and wet state, with an absence of &#34;dusting out&#34; of superabsorbent particles. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are achieved by pumping an aqueous solution of uncatalyzed superabsorbent polymer at room temperature to a melt-blowing die. A cross-linking catalyst is mixed to the solution shortly before introduction into the die. Hot air of about 280° F. is introduced into an air manifold of the die at no more than 15 psi air pressure, and the solution is spun vertically downwardly as a viscous stream of fibers surrounded by laminar air flow. At approximately 36&#34; below the first die, the downward stream of the viscous aqueous solution of the superabsorbent fiber is impacted by a high velocity stream of melt-blown fibers at an angle of 60 to 90 degrees, coming from a melt-blowing system such as described in U.S. Pat. No. 4,380,570. Such thermoplastic fibers are at about 700° F. and are propelled by the hot air to about 500 meter per second. At the point of impact of the two fiber streams, the fibers intermingle intensely and the heat from the melt-blown fiber stream evaporates the water from the superabsorbent fibers and activates the cross-linking catalyst to make the superabsorbent fibers water-swellable, but insoluble. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic bottom view of the extrusion dies for both the superabsorbent polymer solution and the melt-blown polymer; 
     FIG. 2 is a cross-sectional side view of the extrusion dies of FIG. 1; 
     FIG. 3 is a schematic diagram of the entire process showing all its essential components; 
     FIG. 4 is a schematic diagram of the composite web produced by the process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 and 2, 1 is the resin cavity into which resin or solution is pumped, the cavity leads to the spin nozzles 2, which is held by the mounting plate 3. Hot air enters the air manifold and exits through the screen 5, held by the retainer plate 4. Air thus surrounds each nozzle, blowing fibers downwardly at a velocity controlled by the air pressure entering the air manifold. 
     Referring to FIG. 3, there is provided a storage tank 6 for superabsorbent polymer solution of aqueous or other suitable solvent, feeding metering pump 7 to the transfer line 8; 9 is a smaller tank feeding cross-linking catalyst through pump 10 to the transfer line 8 shortly before entering the melt-blowing die 11; hot compressed air is fed into the air manifold of die 11, and viscous aqueous fibers 13 leave the die surrounded by a laminar flow of hot air, starting the evaporation of water from the superabsorbent fiber, thus strengthening the fibers. The extrusion die design is similar to those disclosed in the U.S. Pat. No. 4,380,570 incorporated herein by reference. 
     14 is an extruder, melting and pumping fiber forming thermoplastic polymer to metering pump 15 into the heated melt-blowing die 16. High pressure air of about 700° F. is fed into the air manifold of die 16 and blows fibers 18 at approximately sonic velocity onto fiber stream 13; at 19 the fiber streams mix, and the heated air of die 16 assists in evaporating the water from the superabsorbent fibers 13 and propels the composite web onto the moving screen 20; 21 is a vacuum chamber removing water vapor and heated air from the web. The web is further heated by radiation heaters 22, mounted in chamber 23. The web exits chamber 23 and is wound on winder 24. 
     Preferably both the superabsorbent fibers and the thermoplastic fibers are essentially continuous in length. 
     FIG. 4 shows a schematic diagram of the resulting composite web. The superabsorbent fibers 25 are entangled in the thermoplastic polymer fiber matrix 26, and are well separated from each other. This results in a higher degree of absorbency and a lack of &#34;dusting out&#34; of the superabsorbent fibers. 
     EXAMPLES OF THE INVENTION 
     The following examples are included for the purpose of illustrating the invention and it is to be understood that the scope of the invention is not to be limited thereby. 
     EXAMPLE 1-8 
     For Examples 1 to 8, the apparatus of FIG. 3 is used. The extrusion dies 11 and 16 of FIG. 3 are shown in FIGS. 1 and 2 and have the following nozzle dimensions: Die 11 has 4 rows of nozzles, 2 cm long, spaced 0.42 cm apart from center to center, the inside diameter of the nozzles is 0.91 mm. Each row has 21 nozzles, a total of 84. Die 16 has 3 rows of nozzles 1.5 cm long, spaced 0.21 cm apart, the inside diameter of the nozzles is 0.33 mm, each row has 55 nozzles, a total of 165. Tank 6 holds a solution of high molecular weight polyacrylic acid supplied by Chemdal Corporation, 60% (percent) by weight solids in water, tank 9 is filled with a 3% (percent) emulsion of benzoyl peroxide in water. 14 is a 1&#34; diameter, 24&#34; long extruder equipped with 3 heating zones, feeding thermoplastic polymer through a &#34;Zenith&#34;  gear pump to die 16. The vacuum box 21 is connected to a suction fan driven by a 2 HP motor. 
     Eight types of highly entangled melt-blown webs were made under conditions listed below in Table I. 
     
                                           TABLE I__________________________________________________________________________      1   2  3   4  5   6  7   8__________________________________________________________________________Example Rate      35  35 35  18 18  18 18  35of SolutionFlow from Tank 6(cm.sup.3 /min)Rate of Catalyst      1.75          1.75             1.75                 0.9                    0.9 0.9                           0.9 1.75Flow from Tank 9(cm.sup.3 /min)Air pressure at      6   6  6   6  5   5  5   612 (psi)Air temperature      140 140             140 140                    130 130                           130 130at 12 (°C.)Fiber size 13 in      10  10 10  8  8   10 8   10Web (micrometer)Polymer type in*      PP  PP PP  PP PP  PET                           PET N-66Extruder 14Polymer Feed Rate      62  83 104 52 31  40 30  56at Pump 16(cm.sup.3 /min)Air Pressure at      35  45 55  55 55  55 55  3517 (psi)Air Temperature      300 300             300 300                    300 330                           330 340at 17 (°C.)Die Temperature      280 280             280 280                    280 320                           315 31016 (°C.)Fiber Size 18      4   4  4   2  2   2  2   4(Micrometer)Weight Ratio Super-      1:3 1:4             1:5 1:5                    1:3 1:5                           1:3 1:3absorbent to Thermo-plastic Fiber__________________________________________________________________________ *PP is polypropylene of MFR 300, PET is polyethylene terephthalate of intrinsic viscosity 0.65, N66 is Nylon 66 of intrinsic viscosity 0.8. The speed of screen 20 was adjusted to produce a web of 200 gram/m.sup.2 basi weight. The drying chamber 23 was kept at 130° C. 
    
     Average fiber diameters were measured with a graded microscope. The superabsorbent and thermoplastic fibers are easily distinguishable since the superabsorbent fibers readily absorb and stain with water-soluble ink, while thermoplastic fibers do not. 
     EXAMPLE 9 
     Example 1 was repeated except that the pump feeding the benzoyl peroxide emulsion to the polyacrylic acid solution was shut off. Fibers formed in the same manner as in example 1, but the resultant web was not superabsorbent, upon wetting, the superabsorbent fiber dissolved and leaked out of the polypropylene melt-blown web; cross-linking of polyacrylic acid is achieved by a mechanism described in U.S. Pat. No. 3,379,564. 
     EXAMPLE 10 
     The fabrics produced in Examples 1-9 were tested, for absorbency, along with a control fabric (Example 1, without any superabsorbent fibers blended in), in the following manner: 
     Samples of fabrics were immersed in tap water of 20° C. for 5 and 20 minutes, respectively, then laid on a cellulose paper towel for 30 seconds. The amounts of water absorbed are listed in TABLE II. 
     
                       TABLE II______________________________________                  Weight ratio of water                  sorbed after immersionSample Basis           to weight of sheetWeight No. of      Weight-percent                  productsheet (gram/m.sup.2)      absorbent fiber                  After 5 min.                             After 20 min.______________________________________1     202      25          71       732     203      20          58       613     199      17          50       524     198      17          75       785     200      25          85       916     203      17          72       807     198      20          83       878     201      20          84       889     204      25          disintegrated10    150      --          7        8______________________________________ 
    
     It is evident from TABLE II that the fabrics absorbed water approximately proportional to the superabsorbent content, the samples having finer fibers absorbed more water (compare sample 3 with 4). There was not noticeable difference between the webs having polypropylene, polyester or nylon fibers as the thermoplastic component. The webs could be handled without superabsorbent material dusting out. 
     While the invention has been described in connection with as exemplary embodiment thereof, it will be understood than many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations of variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.