Antimicrobial nonwoven filters composed of different diameter fibers and method of manufacturing the same

This application claims the benefit of provisional application 61/069,000 filed on Mar. 11, 2008.

Prior art filter methods include, for example, mechanical filtration—a physical retention of particles larger than the pores of the filter media; electrostatic filtration—adhering particles to fibers in the filter without killing/deactivating the particles. When it is desired to kill micoorganisms or deactivate other particles an agent has been incorporated in a filter. One such filter system is taught in U.S. Pat. No. 5,980,827 which issued to the inventor hereof on Nov. 9, 1999 and is entitled “Disinfection Of Air Using An Iodine/Resin Disinfectant.” Another is taught in U.S. Publication No. 2006/0251879 (the \'879 publication). The entire contents of U.S. Pat. No. 5,980,227 and the \'879 publication are incorporated herein by reference. It has been determined that improved iodinated resin filtration occurs in a thin media when the product is incorporated to a media with a convoluted pathway. By forcing the microorganism/toxin to pass through a circuitous route, the microorganism/toxin is eventually killed/deactivated. One method for providing a circuitous route is to employ an electrostatically charged nonwoven media.

Various techniques of incorporating an antimicrobial agent in a fiber are known in the art. One such method involves physically entrapping the active agent within the three-dimensional structure of the nonwoven material. The active agent must have the appropriate size to be entrapped within the matrix structure of the nonwoven web. For instance, U.S. Publication No. 2006/0144403 (the \'403 publication), to Messier, describes several methods of physically entrapping an active agent such as iodine demand disinfectant resin in a three-dimensional nonwoven matrix. The \'403 publication is hereby incorporated by reference. One such method involves making use of a meltblown system where the desired active agent is provided in a cloud at the location closest to the extrusion point of polypropylene fibers. The cloud of active agent envelops the extruded fibers exiting a spinerett. Upon cooling, the active agent becomes physically entrapped within the fibers on the collecting web.

In addition to physically entrapping the active agent, the active agent may be incorporated directly into the fiber. Methods of incorporating an antimicrobial agent into a nonwoven material are also known in the art. Generally, the active agent is blended with the polymer prior to extrusion so that it is present throughout the polymer. Upon solidification of the polymer, the active agent is dispersed throughout the resultant fiber. The active agent may diffuse to the surface of the nonwoven, where it exerts is toxic effect on the microorganism/toxin. For example, the \'403 publication describes a method in which polymer granules are placed in a hopper along with active agent in dust form, preferably an iodine/resin disinfectant, prior to extrusion. The two components are then heated, extruded and attenuated to form fibers having the active agent incorporated thereto. The resulting fibers having the active agent embedded can be air laid, vacuum laid or water laid. Nonwoven materials generated from this process may be utilized as antibacterial filters in various applications.

Despite the advantages of incorporating an active agent in a nonwoven material, there are several problems associated with the manufacture of such materials with limit their utility. For example, using a conventional meltblown or spunbond process to manufacture fibers or microfibers for nonwoven filters requires high temperatures. In a meltblown process typically used to generate filter-grade fibers, molten polymer extruded from the die holes is hit with high velocity hot air streams to reduce the diameter of the fiber. If certain antimicrobial agents, such as an iodinated rein, is incorporated into the molten polymer, iodine may vaporize from the molten polymer. Besides the associated environmental hazards, the effectiveness of the resultant nonwoven material will be mitigated. Moreover, the chemical integrity and stability of the antimicrobial agent may be compromised by subjecting the active agent to such high temperatures.

To serve as an antimicrobial filter, a nonwoven web must have the ability to trap and retain particulate matter. Generally, these filters can be made using a meltblown process where the extruded polymers can be drawn into very thin microfibers. The nonwoven webs formed from these microfibers have a large surface area, a small pore size and are lightweight, characteristics which are important for a high degree of filtering efficiency. Moreover, the density of the web formed from these microfibers imparts excellent filtering characteristics to the web.

The manufacture of an antimicrobial nonwoven material is thus limited to the use of filtration grade materials produced from polymers that can be formed into very fine fibers, in particular polypropylene. As discussed above, owing to the high temperature needed to generate these fine fibers, extruding an active agent with polypropylene followed by attenuation of the fibers is highly problematic, leading to vaporization of the active agent and/or thermal decomposition. Moreover, the size of active agent limits the amount of active ingredient that can be incorporated into the fiber. We have found that this is particularly problematic when using active agents such as iodinated resins as antimicrobial agents. At the present time, iodinated resins with particulate sizes of approximately 5 microns is as small as can be uniformly manufactured. The loading capacity of these iodinated resin particles on polypropylene microfibers is limited. Furthermore, producing very thin fibers ideal for filtering applications is a significant problem when using iodinated resin particles as an active agent. Because the diameter of the resin may be larger than the diameter of the fiber, the resin incorporated fibers may be unstable. For instance, we have found that such fibers are prone to breaking during the manufacturing process. As a result, the efficacy of the filter formed from these may be compromised to a significant degree.

Given the shortcomings of the prior art, it is advantageous to develop an antimicrobial filter that is easy to manufacture while at the same time being highly efficacious. The present invention comprises a nonwoven antimicrobial fiber that is manufactured from a composite of different fibers each optimized for a different property, one for excellent filtering capability and the other antimicrobial activity.

The present invention provides a method of manufacturing a nonwoven material, and the fibers that make the same, with both a high degree of filtering capacity and antimicrobial activity. The process involves using two different extruders to generate fibers of different diameters. Fibers generated in the first extruder are ideally drawn into very thin microfibers or nanofibers that when bonded, display excellent filtering capacity. Fibers generated in the second extruder will be drawn into fibers that have a larger diameter than fibers formed from the first extruder. The second fibers are formed by extruding a polymer together with an antimicrobial agent such that the antimicrobial agent becomes embedded within the fiber. The two different size fibers exiting the respective extruders are elongated and solidified as they move in the direction of a collecting belt. In the process, the two different size fibers become physically entangled. Subsequently, the entangled, composite fibers are bonded to form a three-dimensional nonwoven web.

The nonwoven composite structures of the present invention have excellent strength, flexibility and filtering capacity. Moreover, because the antimicrobial agent need not be incorporated into very thin microfibers, a large variety of different types of fibers can be used to incorporate the antimicrobial agent. An advantage of this methodology is that polymers with significantly different melting points can be used to form the composite fiber. Accordingly, the size and chemical nature of the fiber incorporating the active agent can be varied depending on the desired filtering application.

In certain embodiments, the thicker fiber is a melt-extrudable polymer with embedded iodinated resin particles. The iodinated resin particles are mixed with polymer granules to form an emulsion. Following extrusion and quenching of the fibers, the iodinated resin is embedded in the fiber. It is believed that iodine (I₂) released from the resin migrates to the surface of the fiber where it can exert its antibacterial effect. The rate of migration of iodine to the surface of the fiber can be controlled by the appropriate selection of fiber and/or the addition of other additives. Hence, the nonwoven articles of the present invention have significant advantages over antimicrobial nonwoven materials in the prior art.