Abstract:
A composite sheet of a nanoweb bonded to a second web, such that fibers from the second web protrude through the nanoweb in a multiplicity of discontinuous regions.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is related to the field of nanoweb structures and in particular nanowebs bonded to substrates by needlepunching. 
       BACKGROUND 
       [0002]    “Nanowebs” are nonwoven webs comprising primarily, or even exclusively, fibers that have a number average diameter of less than one micrometer. Due to their extremely small pore dimensions and high surface area to volume ratio, nanowebs have been expected to be utilized as substrates for many applications such as, for example, hot gas filtration, high performance air filtration, waste water filtration, filtration membranes for biological contaminants, separators for batteries and other energy storage devices. However, one disadvantage of nanowebs for these applications is their poor mechanical integrity. 
         [0003]    The number average diameter of nanofibers are less than 1000 nm and sometimes as small as 20 nm. In this dimension, even if they are layered and formed as thick membranes, the mechanical strength of the resulting structures is not sufficient to withstand macroscopic impacts for filtration applications such as normal liquid or gas flows passing through them or higher strength required for winding and handling during end use manufacturing steps. Nanowebs made, for example, by electrospinning or electroblowing also tend to have low solids volume content (solidity), typically less than about 20%. 
         [0004]    Unsupported nanowebs also exhibit an excessive reduction in width (“necking”) when tension is applied in the machine direction (MD), such as when winding or post processing, for example, when applying surface treatments and laminating for some product applications. Where the material is unwound and wound again, varying tensions can result in different widths and potentially create variations in sheet properties. A material is desired which is more robust with regard to applied tension. Such a material can be obtained by bonding the nanoweb to a supporting web or scrim. 
         [0005]    Needlepunching is a form of mechanical bonding of fibers which have normally been produced by a card or other equipment. The process converts the web of loose fibers into a coherent nonwoven fabric using a needle loom. Needle looms of various types are well known in the art and function to bond a nonwoven web by mechanically orienting fibers through the web. The process is called needling, or needlepunching. Barbed needles, set into a board, punch fiber into the batt and withdraw, leaving the fibers entangled. The needles are spaced in a nonaligned arrangement. By varying the strokes per minute, the number of needles per loom, the advance rate of the batt, the degree of penetration of the needles, and the weight of the batt, a wide range of fabric densities can be made. The needle loom can be operated to produce patterned or unpatterned products. 
         [0006]    It is known in the art that needlepunching generally increases the air permeability of a nonwoven web. However, it is possible to use needlepunching to reduce the permeability of a nonwoven web under certain conditions. According to literature published by Foster Needle Co. on their website “Lower permeability is more difficult to achieve than higher permeability”. The key to lower permeability in a fabric, is to “close” the felt up as much as possible. The more the felt is “closed” (in other words, needled tightly and as densely as possible) the lower the permeability. Using this logic a person skilled in the art would expect a large amount of needle penetrations per square inch in order to just maintain the small pore structure of a web comprised of fibers with diameters of less than 1 micron. 
       SUMMARY OF THE INVENTION 
       [0007]    A first embodiment of the present invention is a composite sheet comprising a first web of polymer fibers having a fiber diameter less than or equal to one micron bonded to a second web of fibers having a fiber diameter greater than one micron, wherein some of the fibers of the second web protrude through the first web of polymer fibers at a multiplicity of discontinuous regions. 
         [0008]    Another embodiment of the present invention is a process for bonding a polymeric nanoweb to a felt to form a composite sheet, the process comprising providing the nanoweb and the felt in a face-to-face relationship and needlepunching the felt to the nanoweb, such that some fibers from the felt protrude through the nanoweb. 
         [0009]    The composite sheets of the current invention may be useful for many filtration applications, such as, but not limited to, bag house filters, vacuum cleaner filters, air purification filters and other gas or liquid filtration applications. 
     
    
     DETAILED DESCRIPTION 
       [0010]    The term “nonwoven” means a web including a multitude of randomly distributed fibers. The fibers generally can be bonded to each other or can be unbonded. The fibers can be staple fibers or continuous fibers. The fibers can comprise a single material or a multitude of materials, either as a combination of different fibers or as a combination of similar fibers each comprised of different materials. 
         [0011]    “Calendering” is the process of passing a web through a nip between two rolls. The rolls may be in contact with each other, or there may be a fixed or variable gap between the roll surfaces. An “unpatterned” roll is one which has a smooth surface within the capability of the process used to manufacture them. There are no points or patterns to deliberately produce a pattern on the web as it passed through the nip, unlike a point bonding roll. 
         [0012]    A “scrim” is a support layer and can be any planar structure with which the nanoweb can be bonded, adhered or laminated. Advantageously, the scrim layers useful in the present invention are spunbond nonwoven layers, but can be made from carded webs of nonwoven fibers and the like. Scrim layers useful for some filter applications require sufficient stiffness to hold pleats and dead folds. 
         [0013]    The term “nanofiber” as used herein refers to fibers having a number average diameter or cross-section less than about 1000 nm, even less than about 800 nm, even between about 50 nm and 500 nm, and even between about 100 and 400 nm. The term diameter as used herein includes the greatest cross-section of non-round shapes. A nanoweb is defined as a web of fibers wherein the number average fiber diameter is less than 1 micron. 
         [0014]    An as-spun nanoweb typically comprises primarily or exclusively nanofibers that are produced by electrospinning, such as classical electrospinning or electroblowing, and in certain circumstances, by meltblowing processes. Classical electrospinning is a technique illustrated in U.S. Pat. No. 4,127,706, incorporated herein in its entirety, wherein a high voltage is applied to a polymer in solution to create nanofibers and nonwoven mats. However, total throughput in electrospinning processes is too low to be commercially viable in forming heavier basis weight webs. 
         [0015]    The “electroblowing” process is disclosed in World Patent Publication No. WO 03/080905, incorporated herein by reference in its entirety. A stream of polymeric solution comprising a polymer and a solvent is fed from a storage tank to a series of spinning nozzles within a spinneret, to which a high voltage is applied and through which the polymeric solution is discharged. Meanwhile, compressed air that is optionally heated is issued from air nozzles disposed in the sides of, or at the periphery of the spinning nozzle. The air is directed generally downward as a blowing gas stream which envelopes and forwards the newly issued polymeric solution and aids in the formation of the fibrous web, which is collected on a grounded porous collection belt above a vacuum chamber. The electroblowing process permits formation of commercial sizes and quantities of nanowebs at basis weights in excess of about 1 gsm, even as high as about 40 gsm or greater, in a relatively short time period. 
         [0016]    Handling of nanowebs is extremely difficult due to their fragility. For this reason, it is sometimes advantageous to deposit the nanoweb directly onto a scrim to ease the handling of the nanoweb. Accordingly, the composite sheet of the present invention can further include a scrim upon which the nanoweb is supported prior to needlepunching with a felt or support scrim. The scrim can be arranged on the collector to collect and combine the nanofiber web spun on the scrim. 
         [0017]    Examples of the scrim may include various nonwoven cloths, such as meltblown nonwoven cloth, needle-punched or spunlaced nonwoven cloth, woven cloth, knitted cloth, paper and the like, and can be used without limitations so long as a nanofiber layer can be added on the scrim. 
         [0018]    Polymer materials that can be used in forming the nanowebs of the invention are not particularly limited and include both addition polymer and condensation polymer materials such as, polyacetal, polyamide, polyester, cellulose ether and ester, polyalkylene sulfide, polyarylene oxide, polysulfone, modified polysulfone polymers and mixtures thereof. Preferred materials that fall within these generic classes include, poly (vinylchloride), polymethylmethacrylate (and other acrylic resins), polystyrene, and copolymers thereof (including ABA type block copolymers), poly (vinylidene fluoride), poly (vinylidene chloride), polyvinylalcohol in various degrees of hydrolysis (87% to 99.5%) in crosslinked and non-crosslinked forms. Preferred addition polymers tend to be glassy (a T g  greater than room temperature). This is the case for polyvinylchloride and polymethylmethacrylate, polystyrene polymer compositions or alloys or low in crystallinity for polyvinylidene fluoride and polyvinylalcohol materials. One preferred class of polyamide condensation polymers are nylon materials, such as nylon-6, nylon-6,6, nylon 6,6-6,10 and the like. When the polymer nanowebs of the invention are formed by meltblowing, any thermoplastic polymer capable of being meltblown into nanofibers can be used, including polyolefins, such as polyethylene, polypropylene and polybutylene, polyesters such as poly (ethylene terephthalate) and polyamides, such as the nylon polymers listed above. 
         [0019]    It can be advantageous to add known-in-the-art plasticizers to the various polymers described above, in order to reduce the T g  of the fiber polymer. Suitable plasticizers will depend upon the polymer to be electrospun or electroblown, as well as upon the particular end use into which the nanoweb will be introduced. For example, nylon polymers can be plasticized with water or even residual solvent remaining from the electrospinning or electroblowing process. Other known-in-the-art plasticizers which can be useful in lowering polymer T g  include, but are not limited to aliphatic glycols, aromatic sulphanomides, phthalate esters, including but not limited to those selected from the group consisting of dibutyl phthalate, dihexl phthalate, dicyclohexyl phthalate, dioctyl phthalate, diisodecyl phthalate, diundecyl phthalate, didodecanyl phthalate, and diphenyl phthalate, and the like. The  Handbook of Plasticizers , edited by George Wypych, 2004 Chemtec Publishing, incorporated herein by reference, discloses other polymer/plasticizer combinations which can be used in the present invention. 
         [0020]    The as-spun nanoweb (and scrim) can be calendered prior to the needling process, in order to impart desired improvements in physical properties. In one embodiment of the invention the as-spun nanoweb is fed into the nip between two unpatterned rolls in which one roll is an unpatterned soft roll and one roll is an unpatterned hard roll, and the temperature of the hard roll is maintained at a temperature that is between the T g , herein defined as the temperature at which the polymer undergoes a transition from glassy to rubbery state, and the T om , herein defined as the temperature of the onset of melting of the polymer, such that the nanofibers of the nanoweb are at a plasticized state when passing through the calendar nip. Further, the nonwoven web can be stretched, optionally while being heated to a temperature that is between the T g  and the lowest T om  of the nanofiber polymer. The stretching can take place either before and/or after the web is fed to the calender rolls, and in either or both of the MD or CD. 
         [0021]    In the needlepunching process of the invention, the diameter of the needles used in the needlepunching operation is at least 500 times the average diameter of the nanofibers of the nanowebs, and preferably at least 1000 times the average diameter of the nanofibers. 
         [0022]    According to the present process, rather than the intermingling of coarse and fine fibers that is typical of prior art needled fiber structures, the coarse fibers are preferentially pushed through the nanoweb structure as though it were a solid sheet being perforated by the needles. The coarse fibers remain anchored in the coarse fiber web while having a portion of their length pushed through the nanoweb, such that they protrude beyond the surface of the nanoweb. The coarse fibers act to fill the holes left in the nanoweb by the needles, thereby reducing the impact of the needling on the pore structure of the fine fiber web. In this manner, the mean pore size of the bonded, composite sheet can be equal to or less than the mean pore size of the nanoweb and the coarse fiber web before needlepunching. 
         [0023]    The amount of needling is not limited in the current invention. As in other needling operations, however, numerous factors must be optimized to provide the desired pore structure and amount of bonding between the nanoweb and the coarse fiber web. Those factors include the size and type of the needles, the amount of needling, the depth of needling, selection of appropriate coarse fibers in terms of fiber type, length, denier and web density. 
         [0024]    The process of the present invention can further include heat treating of the composite sheet after needlepunching, such as by hot roll calendering or heating in an oven. 
       EXAMPLES 
     Example 1 
       [0025]    A 24% solution of polyamide-6,6 in formic acid was spun by electroblowing as described in WO 03/080905 into a nanoweb. The number average fiber diameter was about 422 nm. Nominal basis weight of the nanoweb was 28.5 grams per square meter (gsm), and thickness was 60 microns. 
         [0026]    Four layers of a backing material made of 80% Kevlar® fibers and 20% polyester fibers, with a basis weight of approximately 2 oz per square yard, were needled using a hand needle loom to the polyamide nanoweb at various levels of penetrations per square inch. Needles in the loom were 38 gauge (0.5 mm diameter) which is approximately 1185 times the average fiber diameter of the nanoweb. Measurements were then made of the pore structures using standard capillary flow porometry instruments manufactured by PMI (Porous Materials Incorporated) 
         [0027]    Optional calendering was carried out by delivering the hand sheet laminate samples to a two steel roll calender nip. The calender was set to a gap of 0.045 inches, a nip pressure of 850 pounds per linear inch and was operated at room temperature. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Mean Flow 
               
               
                   
                   
                   
                 Minimum Pore 
                 Maximum Pore 
                 Pore 
               
               
                   
                 Material 
                 Extent of 
                 Diameter, 
                 Diameter, 
                 Diameter, 
               
               
                 Sample 
                 Construction 
                 Needling 
                 microns 
                 Microns 
                 microns 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 Single backing 
                 none 
                 4.321 
                 246.5 
                 46.5 
               
               
                   
                 layer 
               
               
                 B 
                 4 layers of 
                 800 ppsi 
                 1.3794 
                 455.0 
                 29.0 
               
               
                   
                 backing 
               
               
                 C 
                 4 layers backing + nanoweb 
                 none 
                 0.7708 
                 14.5 
                 5.9 
               
               
                 D 
                 4 layers backing + nanoweb 
                 200 ppsi 
                 0.6617 
                 216.9 
                 5.3 
               
               
                 E 
                 4 layers backing + nanoweb 
                 800 ppsi 
                 0.957 
                 110.8 
                 15.4 
               
               
                   
                   
                 calendered. 
               
               
                   
               
             
          
         
       
     
         [0028]    It can be seen from Samples A and B, that the mean flow pore diameters of the materials made only from the coarser fibers are well above the diameters required to meet the conditions for the small pore layers of the desired composite sheet material. Simply stacking the coarse and fine fiber materials without needling (Sample C) establishes the pore structure contributed by the fine fiber material. Sample D illustrates that despite needling, the mean flow pore diameter of the unbonded composite can be essentially sustained. Note, however, that the maximum pore diameter is increased, although not to the extent that is found without the fine fiber material. Sample E illustrates that with additional needling, the mean flow pore diameter begins to increase. However, calendering of the material successfully limits the maximum pore diameter. 
         [0029]    Example 1 illustrates that, contrary to expectations, needles sized for the coarse fiber material may be used to laminate a nanoweb to one or more webs of coarser fibers without negatively impacting the pore structure of the nanoweb and without requiring a dense amount of needling to close up the felt. 
       Example 2 
       [0030]    Nanoweb with a basis weight of 10 gsm was spun from polyamide-6,6 using the process of World Patent Publication No. WO 03/080905 onto a 33.9 gsm polyester spun lace (Sontara®, Du Pont, Wilmington, Del.). Mean fiber diameter was 400 nm. The nanoweb was bonded to a 14 oz polyester partially consolidated felt (Southern Felt) by needlepunching. 
         [0031]    Needlepunching entailed bringing the felt and the scrim+nanoweb structure together with the nanoweb on the inside against the felt and needlepunching from the felt side. Line speed was 1.5 meter/min. The number of penetrations per inch (PPI) was 383. The air permeability of the laminate was 32 cubic feet per minute (cfm). 
       Example 3 
       [0032]    Nanoweb with a basis weight of 5 gsm was spun from polyamide-6,6 using the process of World Patent Publication No. WO 03/080905 onto a 33.9 gsm polyester spun lace (Sontara®). Mean fiber diameter was 400 nm. The nanoweb was bonded to a 14 oz partially consolidated polyester felt (Southern Felt) by needlepunching. 
         [0033]    Needlepunching entailed bringing the felt and the scrim+nanoweb structure together with the nanoweb on the inside against the felt and needlepunching from the felt side. Line speed was 1.5 meter/min. The number of penetrations per inch (PPI) was 383. The air permeability of the laminate was 37 cubic feet per minute (cfm). 
       Example 4 
       [0034]    Nanoweb with a basis weight of 10 gsm was spun from polyamide-6,6 using the process of World Patent Publication No. WO 03/080905 onto a 33.9 gsm polyester spun lace (Sontara®). Mean fiber diameter was 400 nm. The nanoweb was bonded to a 14 oz fully consolidated polyester felt (Southern Felt) by needlepunching. 
         [0035]    Needlepunching entailed bringing the felt and the scrim+nanoweb structure together with the nanoweb on the inside against the felt and needlepunching from the felt side. Line speed was 1.5 meter/min. The number of penetrations per inch (PPI) was 383. The air permeability of the laminate was 26.1 cubic feet per minute (cfm).