Abstract:
In the production of a nonwoven fabric of thermally bonded fibers, a heavy web of fibers is continuously fed to a toothed cylinder at a slow speed to form a layer of fibers, and a portion of this layer is removed and formed into a lightweight uniform web at a faster speed. The second web is conveyed without draw to a calender having a bonding nip, and the fibers of the web are rearranged by compression and heating and are supported on a hot surface of one of the calender rolls prior to entering the nip to additionally improve uniformity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a method and apparatus for rapid formation of a highly uniform nonwoven web of staple fibers and is particularly suitable for the formation of the low bases weight webs of thermoplastic fibers at a high rate of speed.  
           [0002]    Nonwoven fabrics are produced by a variety of methods, and in general, such methods involve the continuous laydown of fibers or filaments in the form of an unconsolidated flat web on a conveyor, followed by consolidation of the web, such as by bonding or locking the fibers together to form the web into a cohesive fabric.  
           [0003]    The carding of staple fibers into an unconsolidated web followed by point bonding with a hot calender is one well known method of producing a nonwoven fabric. In such a process the fibers, which are received in bales, are first opened with standard textile opening equipment. The opened fibers are then fed to single or multiple cards which are installed in line, each forming a thin web. The webs are then layered together, then usually spread to increase web width, and fed to a hot calender for thermal bonding. The customary calender consists of two heated rolls, one being a smooth steel anvil roll, the other being a roll with an embossed pattern. The high points of the pattern are the area where the fibers are bonded together through partial melting. Such systems can produce webs which are reasonably uniform at a given speed and basis weight. Typically, a reduction in unit weight or an increase in speed results in a noticeable degradation in the uniformity of the fiber distribution. More precisely, at lower basis weights the web develops a more blotchy appearance due to areas of higher and lower concentrations of fibers. In the worst case, holes will form where the concentration of fiber is low. The degradation in web uniformity for the traditional system is also linked to the need of additional draw on the unbonded web to eliminate the bulging of the web which would otherwise occur at various points in the process. The amount of draw used to control the web during transport to the calender is inversely proportional to the cohesion of the unbonded web. A low cohesion web will require a higher draw. The spreading section and the calender nip point are prime areas where the bulging occurs. This bulging, if not eliminated, causes extremely poor web uniformity. A lighter web, when submitted to such increase in draw, develops even greater defects because the extremely light areas are now deformed into holes in the web.  
           [0004]    The prior art has tried to minimize the requirement for draw by using equipment transfer geometry and higher cohesion fiber to produce nonwoven material at higher production speeds. Both modifications have produced only moderate improvements in speed or uniformity.  
           [0005]    Other prior art has been the development of a machine which reorganizes the carded unbonded web (with minimal or no increase in output speed) by reforming it on a vacuum collector such as described in U.S. Pat. No. 4,475,271. This process can produce a web with a more uniform balance in tensile strength between the MD and CD direction but, it does not deliver the desired level of uniformity in fiber distribution as judged by visual appearance.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, a slow moving thick or high basis weight web of fibers having a high degree of cohesion, is formed using conventional cards, or other mechanisms. This web may be first spread in the cross machine direction.  
           [0007]    The thick web is fed into a relatively fast moving toothed reforming roll, which carries a layer of excess recirculating fibers needed to form the final web. A uniform portion of the layer of fibers is continuously removed from the reforming roll by a toothed web forming roll, and this web layer is transferred as a web to a conveyor by a transfer roll. The web is subsequently bonded.  
           [0008]    In the preferred embodiment, the reformed web is fed from the conveyor around an air control transfer roll, which allows the web to change direction without lifting or disruption, with the exit of the air control roll being located closely adjacent the upper heated roll of the rotating calender rolls.  
           [0009]    The web is not fed directly into the nip between the calender rolls. Rather, the web is transferred to the upper hot calender roll into a secondary nip between the transfer roll and hot calender roll, in an area upstream of the nip. The unconsolidated web is then heated and compressed in the secondary nip and is supported on the hot roll prior to entry into the calender nip to become thermally bonded.  
           [0010]    As the web passes through the secondary nip, the web is compressed, causing fibers to move relative to each other in a more uniform arrangement. This effect is aided by contact of the web with the heated roll in which individual heated fibers may shrink, curl or relax as they are being physically rearranged by compression. The rearranged web is partially wrapped and supported on the heated roll, which tends to eliminate any bulging of the web due to passage through the calender.  
           [0011]    Downstream of the reformer roll, all rolls and conveyor operate at substantially the same surface speed, and no substantial machine direction draw is imparted to the reformed web due to transport or thermal bonding. Thus, very light weight or low cohesive webs may be processed at high speeds without any loss in uniformity, and, in fact, uniformity is increased in the final stages of processing.  
           [0012]    In summary, the invention can be considered as having several general aspects. First, a web of staple fibers having a first basis weight and moving at a first speed is converted into a second, more uniform web having a second, lower basis weight and moving at a second, higher, surface speed. This is accomplished by continuously metering a layer of fibers from the first web onto a rapidly rotating toothed cylinder and removing a uniform portion of said layer to form the second web moving at the second speed. The second web is subsequently bonded.  
           [0013]    In a broad second aspect, a web of individual fibers, including at least some thermally bondable fibers, is subjected to preconditioning immediately prior to passage through a nip of a bonding calender. The preconditioning involves subjecting the web to heat and compression which is sufficient to at least partially rearrange the fibers in a more uniform array, but insufficient to thermally bond the fibers.  
           [0014]    A third broad aspect comprises supporting a web of unbonded thermoplastic fibers on a heated surface immediately prior to entry into the nip of a calender. The second and third aspects are preferably accomplished using a heated roll of the calender to heat, compress and support the web upstream of the bonding nip.  
           [0015]    A fourth broad aspect is to support the web of individual fibers to be thermally bonded at a substantially constant surface speed between the zone of formation and into and through the bonding zone in order to minimize any draw on the web after final web formation and to prevent loss of uniformity due to draw.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a side schematic view of the overall apparatus for carrying out the method of the present invention.  
         [0017]    [0017]FIG. 2 is an enlarged portion of a first part of the apparatus shown in FIG. 1.  
         [0018]    [0018]FIG. 3 is an enlarged portion of a second part of the apparatus shown in FIG. 1.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 shows the overall apparatus representative of a production line capable of carrying out the various aspects of the present invention.  
         [0020]    A relatively thick or high basis weight of a web  10  of unconsolidated fibers is first prepared. The web  10  may be formed by use of one and preferably a series of a plurality of conventional cards  12  which serve to separate clumps of fibers from a bale into individual fibers and to deposit the fibers via a take-off roll  14  onto a moving conveyor  16 .  
         [0021]    The web  10  comprises individual staple fibers which are capable of being bonded by conventional techniques. The initial part of the present method may be used to form uniform webs of fibers which are subsequently consolidated by thermal or non-thermal means. Non-thermal methods include techniques in which the surfaces of the fibers are not melted or softened to achieve bonding, including techniques such as chemical or adhesive (liquid or solid) bonding and hydraulic entanglement. In such cases, polymer fibers having higher melting points can be employed, such as polyester and polyamide, as well as other fibers such as polyolefin.  
         [0022]    The web as initially formed may also be bonded by thermal methods such as the application of heat, pressure and heat, or by the use of sonic techniques. Thermal bonding methods include, for example, through-air bonding using hot air, and passage of the web through the nip of a pair of heated calender rolls, one having an embossed surface with raised areas to define the bond sites. In such a case, fibers which have a relatively low melting point are used alone or in admixture with other fibers. Suitable thermoplastic fibers of this nature include polyolefins, such as polyethyle and polypropylene, multi-component fibers having an outer polyolefin surface, uand mixtures thereof. In the preferred embodiment, the fibers or fiber mixtures are capable of being bonded by passage through a conventional bonding calender.  
         [0023]    The initially formed web  10 , typically having a basis weight of from about 30 to about 90 grams per square meter (gsm) may be conveyed from the conveyor  16  to a conventional spreader  18 , which functions to increase the width of the web  10  in the cross machine direction. Since the web is relatively thick and cohesive at this stage, the spreading operation does not cause excessive loss in gross uniformity. At this stage, the web will be moving at a speed in the order of from about 50 to about 80 meters per minute.  
         [0024]    The above apparatus is conventional in nature and provides an initial feed web for subsequent processing in accordance with the present invention. The use of any known process for opening and individualizing fibers to form the initial web  10  is expected to suffice for the purpose of the present invention.  
         [0025]    In accordance with the present invention, the initial web  10  is first processed through a web reformer station  20  which results in a highly uniform web  22  having a basis weight of from about 20 to about 70 percent of the basis weight of the initial web, typically 10 to 30 gsm, and moving at a line speed of from about 150 to 500 percent greater than the line speed of the initial web, typically in the order of 150 to 250 meters per minute. The web  22  is then conveyed to a final fiber rearrangement and bonding station  24 , wherein the fibers are subjected to additional mechanical and thermal rearrangement shortly prior to bonding.  
         [0026]    The reformer station  20  is shown in FIG. 2, with the feed web  10  and reformed web  22  being omitted between rolls for the sake of clarity. The initial web  10  exits a conveyor  26  and is deposited between a lower curved support  28  and a toothed feed roll  30 . The feed roll 30 meters fibers onto a toothed cylinder  32  operating at substantially a faster surface speed and in the same direction (see arrows) than the feed roll. Semi-cylindrical covers  34  are preferably provided around the moving periphery of cylinder  32  in a closely spaced relation to uniformly guide the flow of air created by the cylinder and to prevent disturbance of fibers residing thereon by outside influences. The fibers are not carded by the cylinder  32 , as this would reduce production speed.  
         [0027]    A toothed forming roll  36  is provided, at a close distance from the cylinder  32  and rotates in an opposite rotary direction. The cylinder  32  deposits a uniform layer of fibers resident as the outer layer of fibers on the cylinder onto the forming roll  36 . Thus, the cylinder  32  carries an amount of fibers in excess of that required to establish the reformed or second web  22 . As the cylinder  32  rotates past the feed roll  30 , areas lacking a sufficient population of fibers to form a uniform layer will tend to pick up more fibers from the feed. Thus, the feed roll, cylinder and forming roll work in dynamic conjunction to provide a highly uniform web of unbonded fibers at a high rate of speed. The surface speed of the cylinder  32  is substantially greater than the surface speed of the forming roll  36 , preferably in the order of from about 3.5 to about 10 times faster.  
         [0028]    A toothed take-off roll  38 , located at a close distance from the forming roll  36  and rotating in an opposite direction, removes the entire reformed web  22  from the forming roll and deposits the same on a moving conveyor  40 , which is preferably upwardly inclined relative to horizontal machine direction travel.  
         [0029]    The reformed web of individual fibers  22 , which is now in a highly uniform and fast moving state, may be consolidated or bonded by any suitable thermal or non-thermal technique as described hereinabove. Preferably, however, the web  22  comprises heat bondable fibers and is subjected to additional conditioning, followed by bonding by passage through a conventional heated calender having one or two pattern rolls.  
         [0030]    In the preferred embodiment, the reformed web  22  is subjected to final processing and bonding at the station  24  as shown in FIG. 3. The conveyor belt  40  is preferably of mesh construction allowing air flow therethrough of at least 300 CFM per square foot. An air flow transfer roll  42  supports the exit return loop of the conveyor belt  40 . A pair of spaced fixed radial air seals  44  and  46  are provided across the width of the roll  42 . The first seal  44  intersects the belt  40  and the supported web  22  at approximately the 12 o&#39;clock position on roll  42 , as shown.  
         [0031]    A calender apparatus is provided closely adjacent the air transfer roll  42  and comprises an upper smooth heated roll  48  and a lower embossed or patterned roll  50 , rotating in opposite directions as indicated by the arrows as shown. In the alternative, the upper roll  48  may have an embossed or patterned surface, and the lower roll  50  may be patterned or smooth. The upper roll  48  is in tangential relation with the air transfer roll  42  and is slightly spaced therefrom, as will be explained in greater detail. A first nip  52  is defined between the calender rolls  48  and  50 , where thermal/pressure bonding occurs, and a second nip  54 , upstream of the first nip, is defined between the air transfer roll  42  and the upper calender roll  48 . The second seal  46  intersects the second nip  54 .  
         [0032]    Suitable means, such as an air pump  56 , are connected to a plenum chamber  58  to cause a uniform flow of air to be drawn through the porous conveyor belt  40  and into and across the web  22  in the zone between the fixed seals  44  and  46 . Since the web will typically be light in weight and highly porous, the purpose of this air flow is not to provide a positive pressure drop or seal for the transfer process. Rather, the purpose is to control the boundary layer air which would normally move away from the roll as speed is increased. The negative air flow allows the web to be transferred without disturbance and also prevents the possibility of turbulence and hence disruptive forces at the second nip  54 .  
         [0033]    It has been found that the nip  54  established between the rolls  42  and  48  should be in the order or 0.250 in. (0.635 cm)or less. As the reformed web  22  enters the nip  54 , the web is compressed between the two rolls, and the fibers in the web are heated by the hot calender roll. The simultaneous heating and compression causes at least a partial rearrangement of the fibers due to mechanical and thermal influences, allowing the fibers to shrink and relax as well as to move relative to one another and in three dimensions into the most efficiently packed or uniform arrangement while the fibers remain unbonded.  
         [0034]    The web  22  adheres to and is supported by the heated calender roll through a quadrant of rotation  60  until the web passes through the first nip  52  where permanent point bonding between the fibers occurs. In prior art arrangements the web passes through an unsupported area prior to the nip of the calender, and due to compressive forces at the nip, a bulge in the web can form prior to the nip, with the only available solution being to increase the machine direction draw on the web by increasing the speed of the calender rolls relative to the speed of the web feed. In the present arrangement, the final rearrangement of the fibers and the support of the web on the roll  48  serve to eliminate any tendency to bulge.  
         [0035]    Apparatus of the prior art requires a substantial amount of draw to enable processing. In the present apparatus, the draw between the forming roll  36  and the bonding nip  52 , if any, is less than 5% and most preferably less than 3%. Thus, the surface speed of all components downstream of the cylinder  34  is substantially the same. As a result, bonded webs of a low basis weight and uniformity can be formed at a speed up to 30-40% greater than available on a conventional line. As a result, it is possible to produce light weight nonwoven webs of very high uniformity and at high production rates and low cost, in comparison to prior art methods.