Electretized Material for a Dust Filter

FIG. 1 is a schematic illustration of a drawing apparatus used for the present invention with a hot plate between the feed roller and the draw roller;

FIG. 2 is a schematic illustration of a drawing apparatus used for the present invention including both a hot plate and a friction plate between the feed roller and the draw roller; and

FIG. 3 is a schematic illustration of a drawing apparatus used for the present invention including a corona charging device employed with a grounded hot plate and friction plate

Preparation of the drawn filament as frictionally charged may be made, for instance, by the use of a drawing apparatus as shown in FIG. 1 or 2 of the accompanying drawings. In these Figures, an undrawn filament (1) obtained by melt spinning a polymer material incorporated with a fatty acid metal salt in an amount of not less than 100 ppm in terms of the metal is delivered through a guide (2) by the aid of a feed roller (3), runs slidingly on a hot plate (4) kept at a temperature of 50.degree. to 150.degree. C., preferably of 100.degree. to 130.degree. C. and is drawn by the aid of a draw roller (5), which is rotating at a higher speed than the feed roller (3). In case of FIG. 2, drawing with a draw ratio of 1.5 to 10 is effected between the feed roller (3) and the draw roller (5) by any optional means. The drawn filament from the roller (5) runs slidingly on a friction plate (8) and is delivered by the aid of a draw roller (7) under a certain tension, which is preferably obtained by keeping the ratio of the circumferential speed of the roller (7) to that of the roller (5) being not less than 0.9, preferably not less than 0.95, more preferably from 1.0 to 1.3 when the drawing condition between the rollers (3) and (5) is so set as attaining 75-80% or more of the maximum draw ratio.

By the above procedure, the undrawn filament is drawn and frictionally charged simultaneously or sequentially to give an electretized drawn filament (6). FIGS. 1 and 2 show the undrawn filament (1) wound in a cheese form. However, the undrawn filament from the metal spinning process may be directly introduced into the drawing apparatus without winding in a cheese form. Further, in FIG. 2, a drawn filament drawn by an optional method and wound in a cheese form may be introduced to the draw roller (5). The draw ratio on drawing of the undrawn filament is not limitative and may be usually from 1.5 to 10. Also, no limitation is not present on the denier of the filament as drawn.

Preparation of the drawn filament as electrified by corona charging may be carried out, for instance, by the use of a drawing apparatus as shown in FIG. 3. This drawing apparatus is the same as that shown in FIG. 2 at the major portion and may be operated in the same manner as above. However, the hot plate (4) and the metal plate (8) are earthed, and the filament running slidingly thereon is subjected to corona charging with corona electrodes (9') and (9) by application of such a high voltage as 5 to 25 KV. In the above drawing apparatus, the corona electrode (9') may be omitted, and in this case, earthing at the hot plate (4) is not required. When corona charging with the corona electrode (9') is effected, the metal plate (8) and the draw roller (7) may be omitted. Still, (10) indicates a direct current high voltage source. The corona charging itself may be performed by the use of a conventional device.

It is well known that a polymer material having a high insulating property is electrified statically by rubbing or corona charging. However, electrification sufficient for a dust filter is not obtainable by rubbing and/or corona charging only. When the polymer material is incorporated with a fatty acid metal salt according to this invention, rubbing and/or corona charging is effective to assure a sufficient electrification for a dust filter. In fact, the polymer material incorporated with a fatty acid metal salt can keep its static charge at a high level even after constructed into a dust filter, and thus high dust removing performances are exerted by the electrostatic field inside the filter.

The electretized dust filter of the invention can maintain the electrification over a long period of time and efficiently catch the dust in the air by the coulomb force or the exciting force. It is suitable to catch the dust having a particle size of about 0.1 to 1 micron such as tobacco smoke, flower powder, sand dust, etc. and also for purifying the air in living rooms or cars. Thus, it is useful as dust preventing masks, cleaning cloths for removing dust deposited on furniture, etc.

Practical embodiments of the present invention will be illustratively shown in the following Examples wherein part(s) are by weight.

Polypropylene as the insulating polymer material was incorporated with magnesium stearate (C.sub.17 H.sub.35 COO).sub.2 Mg as the fatty acid metal salt in an amount of 500 ppm in terms of Mg, and the resultant mixture was melt spun to make an undrawn filament (A). The undrawn filament (A) was drawn with a draw rate of 46 m/min at a draw ratio of 2.7 while rubbing by the use of an apparatus as shown in FIG. 2 to make a drawn rubbed filament (B). Also, the undrawn filament was rubbed on a metal plate at 46 m/min without drawing to make an undrawn rubbed filament (C). Using each of the filaments (A), (B) and (C), a dust filter in a mat form having a fiber size of 7 denier and a weight of 200 g/m.sup.2 was constructed. The dust removing efficiency (Ei) of the dust filter was determined at a passing speed of 10 cm/sec on the atmospheric dust having a particle size of 0.5 to 1.0 micron according to the light scattering counting method. The results are shown in Table 1 wherein the pressure drop as determined is also shown.

Polypropylene as the insulating polymer material was incorporated with magnesium stearate (C.sub.17 H.sub.35 COO).sub.2 Mg as the fatty acid metal salt in an amount as shown in Table 2, and the resultant mixture was melt spun to make an undrawn filament. The undrawn filament was frictionally drawn on a hot plate set at a temperature of 110.degree. C. with a draw rate of 42 m/min at a draw ratio of 3.0 by the use of an apparatus as shown in FIG. 2 to make an electretized drawn filament having a fineness of 6 denier. The electretized drawn filament was blown onto a mat by the use of air jetting to make a dust filter in a mat form having a weight of 200 g/m.sup.2. The dust removing efficiency of the dust filter was measured and shown in Table 2 wherein the pressure drop is also shown.

These dust filters showed no material change in dust removing efficiency even after 4 months from their manufacture.

Polypropylene as the insulating polymer material was incorporated with aluminum palmitate (C.sub.15 H.sub.31 COO).sub.3 Al as the fatty acid metal salt in an amount as shown in Table 3, and the resultant mixture was melt spun to make an undrawn filament. In the same manner as in Example 1 but using the undrawn filament as hereinabove prepared, there was prepared a dust filter in a mat form having a weight of 200 g/m.sup.2. The dust removing efficiency of the dust filter was measured and shown in Table 3 wherein the pressure drop is also shown.

These dust filters showed no material change in dust removing efficiency even after 3 months from their manufacture.

Polypropylene as the insulating polymer material was incorporated with magnesium stearate (C.sub.17 H.sub.35 COO).sub.2 Mg as the fatty acid metal salt in an amount as shown in Table 4, and the resultant mixture was melt spun to make an undrawn filament. Using an apparatus as shown in FIG. 2, the undrawn filament was frictionally drawn on a hot plate and a friction plate set at a temperature of 115.degree. C. with a draw rate of 56 m/min at a draw ratio of 4.0 under a ratio of the circumferential speeds of the draw rollers (7) and (5) of 1.1 to make an electretized drawn filament having a fineness of 5 denier. In the same manner as in Example 1 but using the electrized drawn film as hereinabove prepared, there was prepared a dust filter having a weight of 200 g/m.sup.2. The dust removing efficiency of the dust filter was measured and shown in Table 4 wherein the pressure drop is also shown.

These dust filters showed no material change in dust removing efficiency even after 3 months from their manufacture.

Polypropylene as the insulating polymer material was incorporated with magnesium stearate (C.sub.17 H.sub.35 COO).sub.2 Mg as the fatty acid metal salt in an amount of 500 ppm in terms of Mg, and the resultant mixture was melt spun to make an undrawn filament. Using an apparatus as shown in FIG. 3 (the corona electrode (9') being not present), the undrawn filament was frictionally drawn on a hot plate set at a temperature of 115.degree. C. with a draw rate of 42 m/min at a draw ratio of 3.0 and subjected to corona charge treatment while applying a voltage of -13 KV to the corona electrodes and keeping the ratio of the circumferential speeds of the draw rollers (7) and (5) before and after the corona charge at 1.0 to make an electretized drawn filament having a fineness of 6 denier. The electretized drawn filament was blown onto a net by the use of air jetting to make a dust filter in a mat form having a weight of 200 g/m.sup.2. In the same manner as above, there was prepared (i) a dust filter which was not subjected to corona charge treatment, (ii) a dust filter which was not incorporated with magnesium stearate and (iii) a dust filter which was not incorporated with magnesium stearate and not subjected to corona charge treatment. Their dust removing efficiency was determined at a passing speed of 10 cm/sec on the atmospheric dust having a particle size of 0.5 to 1.0 micron according to the light scattering counting method and shown in Table 5 wherein the pressure drop is also shown.

These dust filters showed no material change in dust removing property even after 4 months from their manufacture.

Polypropylene as the insulating polymer material was incorporated with aluminum palmitate (C.sub.15 H.sub.31 COO).sub.3 Al as the fatty acid metal salt in an amount of 500 ppm in terms of Al, and the resultant mixture was melt spun to make an undrawn filament. In the same manner as in Example 5 but using the undrawn filament as hereinabove prepared, there was prepared a dust filter in a mat form having a weight of 200 g/m.sup.2. The dust removing efficiency of the dust filter was measured and shown in Table 6 wherein the pressure drop is also shown.

These dust filters showed no material change in dust removing property even after 4 months from their manufacture.