Gas streams often carry particulate material. In many instances, it is desirable to remove some or all of the particulate material from a gas flow stream. For example, air intake streams to engines for motorized vehicles or power generation equipment, gas streams directed to gas turbines, and air streams to various combustion furnaces, often include entrained particulate material. The particulate material, should it reach the internal workings of the various mechanisms involved, can cause substantial damage. Removal of the particulate material from the gas flow upstream of the engine, turbine, furnace, or other equipment involved is often needed. Another example is the commercial and residential heating, ventilating, and air conditioning (HVAC) filter system. Air streams to HVAC system often carry particulate material such as pollens, spores, atmospheric dust, and other sub-micron particles. Removal of the particulate material is desirable to reduce allergic reaction as well as potential health risks.
Nanofibers and web compositions that comprise nanofibers have improved properties that can be used in a variety of applications including the formation of filter media for filtration applications. See for example U.S. Pat. No. 7,008,465 which describes the use of nanofibers in air filtration applications.
In making fine fiber filter media, a variety of materials have been used including fiberglass, metal, ceramics, and a range of polymeric compositions. A variety of fiber forming methods or techniques has been used for the manufacture of small diameter micro- and nanofibers. One method involves passing the material through a fine capillary or opening either as a melted material or in a solution that is subsequently evaporated. Fibers can also be formed by using “spinnerets” typical for the manufacture of synthetic fiber such as nylon. Electrostatic spinning is also known. Such techniques involve the use of a hypodermic needle, nozzle, capillary or movable emitter. These structures provide liquid solutions of the polymer that are then attracted to a collection zone by a high voltage electrostatic field. As the materials are pulled from the emitter and accelerate through the electrostatic zone, the fiber becomes very thin and can be formed in a fiber structure by solvent evaporation.
In filtration applications, it is commonly known to pleat the filter medium in order to increase the effective surface area available for fluid impact. Nanofiber webs made by conventional techniques result in poor fiber surface stability of the nanofiber web layer. Lower bond strength between scrim and nanofiber web layer have resulted in unacceptable pleating quality for making pleated filter. The nanofiber webs are abraded from pleating rolls and the nanoweb layer is delaminated from the scrim during the pleating process.
To protect from the fiber abrasion and delamination of the nanoweb layer, a second scrim is commonly used to make a “SNS” (i.e. scrim—nanoweb—scrim) structure. The second scrim is bonded by adhesive or ultrasonic and thermo bond which adds cost and thickness to the product. The thicker product may limit pleating density (number of pleats per inch) in a pleated filter.
For example in one commercial embodiment of the prior art, the pleated filters are made from “SNS” structure which is bonded by ultrasonic bond technology.
In a second commercial embodiment, pleated filters are made from “SN” structure, but the basis weight of the nanofiber web layer is typically less than 0.5 grams per square meter (gsm) and its scrim are made from wet-laid paper and spunbond nonwovens.
In a third commercial embodiment, the medium is an “SN” structure in which the basis weight of the nanofiber layer is less than 2 gsm and its scrim is 130 gsm spunbond polyethylene terephthalate (PET).
The filter industry needs a nanoweb plus scrim structure that retains the superior filtration properties of the nanoweb layer while being pleatable with no further modification.
The present inventors have found a way to circumvent the problems associated with pleating simple nanoweb plus scrim structures.