For many years nonwoven fibrous webs have been used for filtration and other purposes. Some of such webs have been made from polypropylene using melt-blowing techniques of the type described in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled "Manufacture of Super Fine Organic Fibers" by Van A. Wente et al. Such melt-blown microfiber webs continue to be in widespread use for filtering particulate contaminants, for example, as face masks and as water filters, and for other purposes, such as sorbent webs for removal of oil from water, as acoustic insulation and as thermal insulation.
The aerosol filtration efficiency of nonwoven fibrous webs can be improved by imparting an electrical charge to the fibers, forming an electret material. A number of methods are known for forming such electret materials. Such methods include, for example, bombarding melt-blown fibers as they issue from the die orifices, as the fibers are formed, with electrically charged particles such as electrons or ions, charging fibers by means of a corona discharge after fiber formation or imparting a charge to a fiber mat by means of carding and/or needle tacking (tribocharging). Recently, a method in which jets of water or a stream of water droplets impinge on a nonwoven web at a pressure sufficient to provide filtration enhancing electret charge has been described.
Other types of nonwoven fibrous webs useful for filtration purposes have been prepared by fibrillating films of polyolefin to form a fibrous material. Such fibrillated materials may be charged as the film, for example, by corona discharge and then fibrillated, collected and processed into a filter.
Resins used in preparing the filtration material are generally required to be substantially free of materials which could increase the electrical conductivity or otherwise interfere with the ability of the fibers to accept and hold electrostatic charge. For example, certain polystyrene-type polymers have better properties if they have not more than 1% by weight of electron-donor impurities and not more than 0.5% by weight of electron-acceptor impurities.
Additives, however, are known which provide improved electret properties when blended with the resin. Electret materials prepared by compounding 4-methyl-1-pentene polymer with at least one compound selected from compounds which have a phenol group, compounds of higher aliphatic carboxylic acids and metal salts thereof, compounds of ester thiocarboxylates, phosphorus acid group containing compounds, and ester group containing compounds and providing a charge by exposure to high voltage have been disclosed. Also disclosed is an electret material for a dust filter which is a blend of an insulating polymer with a fatty acid metal salt in an amount of not less than 100 ppm in terms of the metal with charging carried out by a conventional procedure such as rubbing or corona charge treatment.
Also known are polypropylene, including blends and copolymers, electret materials containing at least one stabilizer selected from hindered amines, nitrogen-containing hindered phenols, and metal-containing hindered phenols. The electret may further contain an additional stabilizer selected from phenol-, sulfur-, and phosphorous-containing stabilizers and/or an ultraviolet light absorber with charging being carried out in a high voltage field at room temperature. Electret filters prepared from a resin whose angle of contact upon wetting with pure water is no less than 95.degree. or has been adjusted to no less than 95.degree. by addition of silicone oil have been disclosed. The electret resin may optionally contain other additives, including heat stabilizers, weathering agents, anti-blocking agents, and inorganic or organic fillers. Charging may be carried out in various ways. Further disclosed are electret filter media with a melt processable fluorochemical additive having a melting point of at least 25.degree. C. and a molecular weight of about 500 to 2500. Charging involves subjecting the material to corona discharge or pulsed high voltage.