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Title:
NON-WOVEN ELEMENT COMPRISED OF FIBRID BINDER
Document Type and Number:
WIPO Patent Application WO/1991/016119
Kind Code:
A1
Abstract:
Provided is a strong, flexible wet laid non-woven fibrous element comprised of polymeric fibrids as a binder. The polymeric fibrids have a softening point below the softening point of the fibers in the non-woven element, and are preferably cellulose acetate fibrids. Such non-woven elements can exhibit excellent wet web strength and dry strength, and are particularly applicable as filter elements. In a preferred embodiment, the non-woven element is comprised of glass fibers (1), synthetic short fibers (2) and the polymer fibrids (3). Such non-woven element exhibits excellent chemical resistance and a usefulness as a filter element in removing particulates from gases at lower temperatures.

Inventors:
KINSLEY HOMAN B JR (US)
Application Number:
PCT/US1990/002065
Publication Date:
October 31, 1991
Filing Date:
April 16, 1990
Export Citation:
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Assignee:
JAMES RIVER PAPER CO (US)
International Classes:
B01D39/16; B01D39/18; B01D39/20; D04H1/42; D04H1/58; D21H13/06; D21H13/26; D21H13/40; (IPC1-7): B01D46/02; D04H1/58
Foreign References:
US4917714A1990-04-17
Download PDF:
Claims:
What Is Claimed Is:
1. A nonwoven element of high strength and good flex and permeability comprising a nonwoven mat of fibers, and polymer fibrids having a softening point below the softening point of said fibers.
2. The nonwoven element of claim 1, wherein the polymer fibrids comprise cellulose acetate fibrids.
3. The nonwoven element of claim 1, wherein the fibers comprise glass fibers.
4. The nonwoven element of claim 3, wherein said glass fibers comprise a mixture of glass macro fibers having a diameter of 615 microns and glass micro fibers having a diameter of 0.052 microns.
5. The nonwoven element of claim 4, wherein about 5090 per cent of said glass fibers are macro fibers and about 1050 percent of said glass fibers are said micro fibers.
6. The nonwoven element of claim 1, wherein said fibers are comprised of synthetic fibers.
7. The nonwoven element of claim 1, wherein said fibers are comprised of polymeric fibers, graphite fibers or carbon fibers.
8. The nonwoven element of claim 7, wherein said fibers are poly(mphenylene isophthalamide) .
9. The nonwoven element of claim 1, wherein said polymer fibrids are comprised of a polymer fibrid having a melting point of less than about 650°F.
10. The nonwoven element of claim 1, wherein said fibers are comprised of natural fibers.
11. A nonwoven element of high strength and good flex and permeability comprising a nonwoven mat of fibers and polymer fibrids, which fibers are melt bonded by the fibrids.
12. The nonwoven element of claim 11, wherein the polymer fibrids comprise cellulose acetate fibrids.
13. The nonwoven element of claim 11, wherein the fibers comprise glass fibers.
14. The nonwoven element of claim 11, wherein the fibers comprise natural fibers.
15. The nonwoven element of claim 11, wherein the fibers comprise polymeric fibers, graphite fibers or carbon fibers.
16. The nonwoven element of claim 11, wherein the fibers comprise cellulosic fibers.
17. The nonwoven element of claim 12, wherein the fibers comprise cellulosic fibers.
18. A filter element of high strength and good flex useful in the filtration of particulates from a medium, said filter element comprising a nonwoven mat of fibers and polymer fibrids, said fibers being bound to each other by use of said polymer fibrids as a binder.
19. The filter element of claim 18, wherein said polymer fibrids are comprised of cellulose acetate fibrids.
20. The filter element of claim 18, wherein said fibers comprise glass fibers.
21. The filter element of claim 18, wherein said fibers comprise a synthetic fiber.
22. The filter element of claim 18, wherein said fibers comprise a natural fiber.
23. The filter element of claim 18, wherein said fibers are meltbonded by the polymeric fibrids.
24. The filter element of claim 18, wherein said fibers comprise cellulosic fibers.
25. The filter element of claim 19, wherein said fibers comprise cellulosic fibers.
26. A filter element of high strength and good flex and permeability comprising a nonwoven mat of (i) glass fibers, (ii) synthetic short fibers and (iii) polymer fibrids having a softening point below the softening point of said short fibers, said short fibers being bound to said glass fibers by the polymer fibrids.
27. The filter element of claim 26, wherein said glass fibers comprise glass micro fibers having a diameter of about 0.052 microns.
28. The filter element of claim 27, wherein from about 570 weight percent of said element is comprised of said short fibers and polymer fibrids.
29. The filter element of claim 28, wherein about 1050 weight percent of said element is comprised of said short fibers and polymer fibrids.
30. The filter element of claim 26, wherein said glass fiber is a mixture of glass macro fibers having a diameter of 615 microns and glass micro fibers having a diameter of 0.052 microns.
31. The filter element of claim 30, wherein about 5090 per cent of said glass fibers are macro fibers and about 1050 percent of said glass fibers are said micro fibers.
32. The filter element of claim 26, wherein said synthetic short fibers are comprised of an aromatic polyamide, polybenzemidazole, polyphenylene sulfide, nylon, polyester, liquid crystal polymer, polypropylene or carbon fiber.
33. The filter element of claim 32, wherein said short fiber is poly(mphenylene isophthalamide) .
34. The filter element of claim 26, wherein said polymer fibrids are comprised of a polymer fibrid having a melting point of less than about 650"F.
35. The filter element of claim 34, wherein said polymer fibrids are essentially cellulose acetate fibrids.
36. A filter element of high strength and good flex useful in the filtration of particulates from a medium at temperatures below 400°F, said filter element comprising a nonwoven mat of (i) glass fibers, (ii) aromatic polyamide short fibers and (iii) polymer fibrids, said short fibers being bound to said glass fibers by use of said polymer fibrids as a binder, and said polymer fibrids being cellulose acetate.
37. The filter element of claim 36, wherein said glass fiber is a glass micro fiber having a diameter of about 0.052 microns.
38. The filter element of claim 37, wherein about 1050 weight percent of said mat is comprised of said short fibers and polymer fibrids.
39. The filter element of claim 36, wherein said glass fibers are a mixture of glass macro fibers having a diameter of 615 microns and glass micro fibers having a diameter of 0.05 to 2 microns.
40. The filter element of claim 36, wherein the aromatic polyamide short fiber is poly(mphenylene isophthalamide) .
41. A system for removing particulates from a gas comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive said gas, a passage within said housing from said inlet to said outlet and a filter element of claim 26 completely across said passage whereby said gas passing from said inlet to said outlet passes through said filter element.
42. A system for removing particulates from a gas liquid comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive the gas, a passage within said housing from said inlet to said outlet and a filter element of claim 36 completely across said passage whereby said gas passing from said inlet to said outlet passes through said filter element.
43. A system for removing particulates from a gas comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive said gas, a passage within said housing from said inlet to said outlet and a filter element of claim 18 completely across said passage whereby said gas passing from said inlet to said outlet passes through said filter element.
44. A system for removing particulates from a gas comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive said gas, a passage within said housing from said inlet to said outlet and a filter element of claim 23 completely across said passage whereby said gas passing from said inlet to said outlet passes through said filter element.
45. A system for removing particulates from a liquid comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive the liquid, a passage within said housing from said inlet to said outlet and a filter element of claim 18 completely across said passage whereby said liquid passing from said inlet to said outlet passes through said filter element.
46. A system for removing particulates from a liquid comprising a filtration housing having an inlet and an outlet, said inlet being operatively connected to receive the liquid, a passage within said housing from said inlet to said outlet and a filter element of claim 23 completely across said passage whereby said liquid passing from said inlet to said outlet passes through said filter element.
Description:
NON-WOVEN ELEMENT COMPRISED OF FIBRID BINDER

BACKGROUND OF THE INVENTION This invention relates to the field of non- woven elements, and, in particular, non-woven fibrous elements composed of polymeric fibrids. The production of a papery product using a binder, such as a suitable resin, is known.For example, U.S. Patent No. 3,573,158 discloses the formation of icroporous sheets which are useful as filters. A binding agent can be employed in order to strengthen the adhesion between fibers, which binding agent can be liquid or solid. Liquid condensation polymer resins are noted as being particularly advantageous binding agents, with representative resins including phenol- formaldehyde resins, urea-formaldehyde resins, melamine-formaldehyde resins, polyester resins and polyepoxide resins. Solutions of solid condensation polymer resins in suitable solvents such as polyamides, cellulose acetate and ethyl cellulose are also suggested, as are thermoplastic solid binders comprised of polyethylene, polypropylene, poly ethylenε, polybutylene, polyisobutylene, copolymers of vinyl chloride and vinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polyvinyl butyrol and polytetrafluoroethylene. In addition, polymers derived from natural products are appropriate, such as lignin-sulfonate resins, starch binders and casein binders, among others.

A combination of glass fibers and aromatic polyamide fibers and fibrids is disclosed in U.S. Patent No. 3,920,428. This patent relates to a filter element adapted to remove particulate from the exhaust of an internal combustion engine comprising a non-woven

mat of glass fibers fused with aromatic polyamide fibers. More specifically, the aromatic polyamide fibers may be mixtures of fibrids and short fibers as described in U.S. Patent No. 3,756,908. However, while the materials described in U.S. Patent No. 3,920,428 may be satisfactory for high temperature applications, it is not a very economically suitable filter under the contemplated circumstances. Moreover, such a filter element is found to exhibit decreased permeability and a low void fraction.

U.S. Patent No. 4,398,995 relates to a papery product composed of a fibrous web, at least part of which is made up of wholly aromatic polyamide fibers having a readily soluble skin layer and a sparingly soluble or insoluble core layer. In forming the web, the skin layer is softened and fuses the polyamide fibers. The papery product may, optionally, include fibers of glass and cellulose acetate. The method of forming papery products in accordance with this patent involves complicated treatment steps to form the core/shell polymer fiber used in the formation of the papery product. This renders the product extremely expensive, and hence impractical, to manufacture. There remains a need, therefore, for an inexpensive and practical non-woven element which exhibits good wet web strength and dry strength, and which requires low amounts of energy to provide a strong, yet flexible filter element which has a relatively high void fraction. It is therefore an object of the present invention to provide a novel strong, yet flexible, non- woven element.

Still another object of the present invention is to provide a flexible, non-woven element, useful as

a filter, which is of increased permeability, and which is strong and ideally suited for use at lower temperatures.

Yet another object of the present invention is to provide an inexpensive, practical filter element having minimal energy requirements for its manufacture.

These and other objects will become apparent to the skilled artisan upon a review of the detailed description of this invention, the drawing and the claims appended hereto.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a wet-laid non-woven element of high strength and good flexibility and permeability. The element of the present invention is comprised of a wet- laid non-woven mat of fibers and polymer fibrids having a softening point below the softening point of the fibers contained in the wet-laid non-woven mat. The fibers are bound to one another by the polymer fibrids, which act as a binder. The resulting non-woven element exhibits good strength and flexibility, improved permeability and can be manufactured in an efficient and cost effective manner. The non-woven elements are suitable as filter elements which are particularly well suited for use in lower temperature filtering applica¬ tions, i.e., at temperatures lower than the softening point of the fibrids.

In a preferred embodiment, there is provided a non-woven element which is particularly applicable as a filter element for use at temperatures below about

400°F. The non-woven element is comprised of a wet- laid non-woven mat of (i) glass fibers, (ii) synthetic short ' fibers, e.g., aromatic polyamide short fibers and

(iii) polymer fibrids having a softening point below the softening point of the short fibers. The synthetic short fibers are believed bound to the glass fibers by the polymer fibrids, which act as a binder.

BRIEF DESCRIPTION OF THE DRAWING

The Figure of the Drawing is a plan view of a portion of a non-woven fiber mat of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a non-woven element of high strength and good flex. Such non-woven elements have many conventional applications, and are particularly useful as a filter in the filtration of particulates from a medium at lower temperatures. The non-woven element generally is comprised of a non-woven mat of fibers and polymer fibrids having a softening point below the softening point of the fibers. In the non-woven mat, the fibers are bound to each other by use of the polymer fibrids as a binder. This binding effect is evident even when the fibrids are present in a non-melted state, but most substantial improvements are realized when the fibrids are melted, i.e., heat- bonded or melt-bonded. It should be noted, however, that the contribution of the fibrids is believed to be more than just that of a binder. The polymer fibrids also add to the unique structural and physical characteristics of the fibrous mat of the instant invention.

The fibers contained in the non-woven element of the present invention can be any useful fiber. Synthetic fibers, including polymeric fibers and graphite fibers, carbon fibers, natural fibers, glass fibers, are all contemplated. Mixtures of fibers,

including mixtures of different types of fibers, can also be used. It is only important that the polymeric fibrids employed as a binder have a softening point below that of all the fibers used in the non-woven element.

Illustrative of useful polymeric fibers are the polyamides, polyesters, polyethylene, polypro¬ pylene, polyaramids, polybenzi idazole, polyphenylene sulfide, liquid crystal polymers, and polyfuorocarbonε. Useful natural organic fibers include, but are not limited to, those of wood, flax, cotton, bamboo, grass (esparto) , ramie, silk and wool. Useful manmade inorganic fibers include those of glass, metal, silica carbide, polyphosphates and mineral wool. Illustrative of natural inorganic fibers are asbestos fibers.

The fibers of the present invention can have about any conventional fiber diameter and length. Mixtures of fibers having different diameters and/or lengths can also be used. The preferred polymeric fibers used in the non-woven element of the present invention are those conventionally used to make synthetic papers.

Glass fibers which are useful in the practice of the present invention generally can have any diameter, but preferably have a diameter of from about 0.05-15 microns, and lengths from about 1/4 to 3 inches. The glass fibers can comprise macro fibers, micro fibers or a mixture thereof.

In one preferred embodiment, the glass fibers comprise a mixture of glass macro fibers having a diameter of about 6-15 microns, and glass micro fibers having a diameter of about 0.05-2 microns, and most preferably about 0.6 micron. The macro fiber can be of any length up to about 3 inches or more, while the

micro fibers are preferably less than one inch, and more preferably less than 1/4 inch long. For example, good results are obtained using micro fibers of about 0.6 micrometers in diameter and about 1/16 through 1/4 inch in length. During blending they may became chopped up into random lengths. Mixtures containing about 50-90 weight percent macro fibers and 10-50 weight percent micro fibers relative to the total glass fiber content are preferred. Other inorganic fibers besides glass fibers, as discussed above, can also be successfully used.

The fibrids according to the present invention are small, non-granular, non-rigid, fibrous or film-like particles. Two of their three dimensions are on the order of microns. Their smallness and suppleness allows them to be deposited in physically entwined configurations such as the fiber mat. The polymer fibrids of the present invention also have a softening point below the softening point of the fibers used in the non-woven element. Thus, when heat is applied to a mixture of fibers and the polymer fibrids, the fibrids soften and surround the fibers thereby binding the fibers together and forming a mat. The fibrids may be composed of any polymeric material, e.g., a nitrile polymer, nylon, polyester or cellulose acetate, so long as its softening point is below the softening point of the fibers used. In a preferred embodiment of the present invention, the polymer fibrids are comprised of cellulose acetate fibrids. While it is most preferred that heat-bonding or melt-bonding of the fibrids be employed in formulating the non-woven element of the present invention, the fibrids can be employed in the non- melted state. Heat or melt-bonding occurs when heat is

applied to a mixture of fibers and fibrids so that the fibrids soften and surround the fibers. In the non- melted state, the fibrids have also been found to act as a binder due to their high surface area. It is this binding effect in the non-melted state which permits the formation of the non-woven web when using many synthetic fibers. Melt-bonding of the fibrids, however, is most preferred in that, generally, added web strength is realized. As well, the porosity of the non-woven element is increased upon melt-bonding the fibrids.

The relative softening point of the fibrids as compared to the fibers can be realized in any environment, whether it be air or an inert environment. For example, in the case of cellulosic fibers, the fibers do not actually have a softening point, but begin to destructively oxidize in air at high temperatures. However, temperatures of 600°F and higher can be safely reached in a superheated steam environment or some other inert atmosphere for brief periods. It is considered within the purview of the present invention to include such combinations of fibers and fibrids, e.g., cellulosic fibers and fibrids which have a softening point below that of the cellulosic fibers in an inert atmosphere. Thus, for example, the melt-bonding of a non-woven web comprised of cellulosic fibers and a particular fibrid can occur in a superheated stream environment or under a nitrogen atmosphere. The ratio of fibers to polymeric fibrids in the non-woven element can vary widely. A preferred range, however, is from about 5 to about 75 weight percent of polymer fibrids and from about 25 to about 95 weight percent of fibers, and more preferably from

about 15 to about 50 weight percent of polymer fibrid and about 50 to about 85 weight percent of the fibers.

In one preferred embodiment of the present invention, the non-woven element is a three-component element comprised of a mixture of different types of fibers. The mixture contains glass fibers and synthetic short fibers, most preferably aromatic polyamide short fibers. The polymeric fibrids employed are most preferably comprised of cellulose acetate. Such a non-woven element is particularly applicable as a filter element for use at temperatures below about 400°F. Excellent chemical resistance is exhibited by such a filter element.

The glass fibers employed in the above- described three component non-woven element can generally be any glass fiber, but are preferably comprised of a mixture of macro and micro glass fibers as hereinbefore described.

Among the useful synthetic (man made) short fibers are the aromatic polyamide short fibers, as well as short fibers of polybenzimidazole, polyphenylene sulfide, polyamides (nylons) , polyesters, liquid crystal polymers, polypropylene or carbon short fibers. The most preferred short fiber, however, is that of an aromatic polyamide short fiber. The term aromatic polyamide includes wholly aromatic polyamides as well as non-wholly aromatic polyamids, both of which are contemplated. Examples of suitable aromatic polyamides are described in U.S. Pat. No. 3,094,511 and British Pat. No. 1,106,190, which are incorporated herein by reference.

The preferred wholly aromatic polyamide contains repeating units of formulae (I) and (II) ,

-NR 2 - Ar 2 - NR 3 - IC - Ar 3 - (II) wherein Ar l Ar and Ar 3 respectively represent, independently from each other, an unsubstituted or substituted divalent aromatic radical which comprises a single aromatic ring, or two or more aromatic rings that are condensed together, or are linked together by a single bond, or by a bridging atom or radical, and which is oriented either eta or para, and R^, R 2 and R 3 respectively represent, independently from each other, a hydrogen atom or an alkyl radical having 1 to 3 carbon atoms. In the formulae (I) and (II) , it is preferable that r lf Ar and Ar 3 be respectively selected, independently from each other, from the group consisting of the radicals of the formulae:

wherein R represents a member selected from the group consisting of lower alkyl radicals having 1 to 6 carbon atoms, lower alkoxy radicals having 1 to 6 carbon atoms, halogen atoms and a nitro radical, n represents zero or an integer of from 1 to 4 and X^ represents a member selected from the group consisting of:

-C-, -0-, -S- , -N- and -C-

wherein Y 2 represents a member selected from the group consisting of a hydrogen atom and lower alkyl radicals having 1 to 6 carbon atoms.

Also, in the formulae (I) and (II) , it is more preferable that Ar 1; Ar 2 and Ar respectively represent, independently from each other, a member selected from p-phenylene radical, m-phenylene radical, biphenylene and radicals of the formulae:

-o-o

wherein X 2 represents a member selected from

-CH -, in which Y 2 represents a hydrogen atom or an alkyl radical having 1 to 3 carbon atoms.

Furthermore, in the formulae (I) and (II) , it is still more preferable that Ar^, Ar 2 and Ar 3 be respectively a p-phenylene or m-phenylene radical. Moreover, it is preferred that the aromatic polyamide contain the repeating units of the formula (II) in which Ar 2 and Ar 3 are respectively a p- phenylene or m-phenylene radical, most preferably, a m- phenylene radical. The aromatic polyamide may contain 30 molar % or less of one or more comonomers, for example, aliphatic diamines, such as hexamethylene diamine and piperazine, and aliphatic dicarboxylic acid, such as adipic acid, based on the entire molar amount of the comonomers contained in the polyamide. The preferred aromatic polyamides are those having a softening temperature above about 350°C. Examples include: poly(4,4*-diphenylene terephthala ide) poly(3,3'-dimethyl-4,4'-diphenylene terephthalamide) poly(ethylene terephthalamide) poly(4,4 '-methylenediphenylene terephthalamdie) poly(4,4 '-diphenylene isophthalamide) poly(4,4 '-methylenediphenylene isophthalamide) poly(tri ethylene terephthalamide) poly(m-phenylene isophthalamide) poly(ethylene-N,N'-dimethylterephthalamide) poly(3,3'-dimethy1-4,4 '-methylenediphenylene tereph¬ thalamide) poly(p-xylene terephthalamide) poly(3,3 '-dimethyl-4,4 '-diphenylene isophthalamide)

The most preferred aromatic polyamide is poly(m- phenylene isophthalamide) , which is commercially available under the name "Nomex", (Reg. trademark E.I. du Pont Company) .

The synthetic short fibers used in this, or any other embodiment of the present invention, can have about any conventional fiber diameter and length. Generally, these short fibers are referred to as "floe" and comprise fibers less than one inch in length, and generally about 0.25 inch in length.

It has been found that when the glass fibers are heated with the synthetic short fibers and the polymer fibrids of the present invention, even at temperatures generally below 500 β F, the fibrids soften and deform around the glass fibers and the short fibers, thereby binding them to each other. The resulting element exhibits good strength and flexibility, and also exhibits improved permeability after the heat treatment.

The ratio of glass fibers to synthetic fibers in the three-component non-woven element can vary widely. A useful range is from about 30-95 weight percent glass fiber and from about 5-70 weight percent of a mixture of synthetic short fibers and polymer fibrids.

Excellent filters can be made in a preferred embodiment of the three-component non-woven element using a mixture of glass micro fibers, poly(m-phenylene isophthalamide) short fibers and cellulose acetate fibrids. Preferably the glass micro fibers are in the 0.05-2 micron range, and more preferably in the 0.6 through 1.6 micron range. From about 5 to about 70 weight percent of the mat can be poly(m-phenylene

isophthalamide) short fibers and cellulose acetate fibrids, and the balance glass micro fibers. More preferably, from about 10-50 weight percent of the mat is poly(m-phenylene isophthalamide) short fibers and cellulose acetate fibrids and the balance glass micro fibers having a diameter of about 0.6-1.6 microns.

Referring to the Figure of the Drawing, the filter element is seen to be a random nonwoven mat of glass micro fibers 1, aromatic polyamide short fibers 2, and polymer fibrids 3, e.g., cellulose acetate fibrids. The polyamide short fibers 2 are bonded to the glass micro fibers 1 by the melting fibrids 3, thus binding the mat into a unitary flexible filter element of high strength. Generally, a non-woven element in accordance with the present invention can be made by (a) forming an aqueous slurry of the fibrids and fibers, or mixture of fibers to be used, (b) filtering the aqueous slurry to form a non-woven mat, and preferably, (c) then heating the web for a short period of time to a temperature which is at least the softening point of the fibrids and is preferably at or above the melting temperature of the polymeric fibrids. With cellulose acetate, good results are obtained by heating the dried web or pad in an oven set at about 525 to 575 °F. A mechanical device with more positive means of heat transfer than an oven will, of course, bond the sheet more rapidly and at a lower temperature. Once the sheet is heated to the melting temperature of the fibrid, the melting occurs almost instantaneously.

Useful wet-laid, non-woven elements can be prepared using a handsheet mold or, for example, a Fourdrinier machine. The element thickness is determined by the thickness of the web laid down on the

wire. The porosity can be varied by varying the amount and content of the fibers used.

The resulting non-woven elements exhibit excellent wet web strength and dry strength, and are useful in many different applications depending on the composition of the web, with a particular usefulness being realized in filtration applications. The use of cellulose acetate fibrids has been found to provide particularly useful and strong non-woven elements for low temperature applications, which elements are cost- effectively made.

The following examples are provided in order to further illustrate the present invention and the advantages thereof. The examples are in no way meant to be limitative, but merely illustrative.

EXAMPLES 1-13 A series of filter elements can be made from an aqueous slurry of glass macro fibers (6.5 microns, 0.25 inch long), glass micro fibers (about 0.6 microns) and a mixture of poly(m-phenylene isophthalamide) short fibers and cellulose acetate fibrids. The short fibers can be prepared, for example, by disintegrating a synthetic poly(m-phenylene isophthalamide) paper in a Waring blender. All the ingredients can then be combined in the slurry and blended further.

The filter sheets can be made from the furnish by filtering the furnish on a 150 mesh wire. The wet pad can be couched with dry blotters, removed from the wire, and oven dried. The resultant pad can then be heat-bonded by placing in a convection oven at 550°F for about 10 minutes. (If an efficient positive heat transfer means is employed, the heat-bonding may be substantially instantaneous.)

The following table exemplifies various proportions of the foregoing components (in percentage by weight) useful in preparing filter elements in accordance with the present invention.

A series of handsheets were made from an aqueous slurry using different fibers and fibrids. A handsheet mold was used to filter the aqueous slurries and form the handsheets. Melt-bonding of the fibrid was employed only for the handsheets of Example 17 (17A, 17B and 17C) , while the handsheets in the remaining examples were heated only to the drying temperature. Different fibers and fibrids were used in making the series of handsheets, as were different proportions of the fiber and fibrid. The physical properties of the handsheets were measured, as reflected in the Table below:

TARTft

Fiber Basis

Example No. Fiber

14 *polyester

15A mercerized **nitrile 100/0 2.8 71 25.4 135 southern pine polymer (Buckeye HPZ)

17B 17C

Fiber/ Basis

Fibrid Apparent Wt. Caliper Permeability Tensile Heating

Example No. Fiber Fibrid t. % Density fib/ream, .mils. (Frazier. fibs/inch) Step 18A mercerized cellulose 100/0 2.8 71 25.4 74 drying w/o

-southern pine acetate compression

(Buckeye HPZ) at 250"F.

18B mercerized cellulose 90/10 3.8 74 19.5 22 drying w/o southern pine acetate compression (Buckeye HPZ) at 250 β F.

EXAMPLE 19 In order to demonstrate the significance between using a polymeric fibrid in accordance with the present invention as a binder vis-a-vis a particulate powder or saturated solution, several experimental runs were made. Briefly, handsheets were made, tested and photographed to demonstrate the significant differences between using a saturated solution and a particulate powder, vis-a-vis a fibrid. The fibers used were glass, although any fiber could have been used having a softening point greater than the fibrid, and cellulose acetate fibrids were used. The compositions of the handsheets were as follows:

COMPOSITION OF HANDSHEETS

The handsheets were formed from water at a pH of 3 to obtain good formation. In Sheet Nos. 2 and 3, the cellulose acetate polymer was added to the glass fiber slurry prior to the sheet being formed. In Sheet No. l, the dry handsheet was saturated with a solution of cellulose acetate and acetone. The same cellulose acetate polymer was used for the fibrid, the powder (60 mesh) and the solution. Handsheet No. 4 used the same

mass of glass fibers as the other handsheets, except that no cellulose acetate was added thereto.

Physical properties of the handsheets prior to a heat treatment were then measured. The two physical properties were permeability and tensile strength. The physical properties measured are recorded in the following table.

PHYSICAL PROPERTIES PRIOR TO HEAT TREATMENT Sheet No. Permeability. Frazier* Tensile, lb/inch 1 7.0 10.2

2 22.1 2.4

3 0.9 12.1

4 19.1 2.3

* Frazier is cubic feet of air per square foot of surface area per minute at 0.5 inch water pressure drop.

One significant advantage immediately seen from using fibrids is the superior dry tensile of the paper after it is formed. Sheet No. 3 possessed a tensile strength of 12.1 pounds per inch width while Sheet No. 2 made from the powder was significantly weaker with a tensile of only 2.4 pounds per inch. A handsheet which would be saturated with cellulose acetate polymer (Handsheet No. 1) possessed a tensile of 2.3 pounds per inch prior to saturation. Only the handsheet made with the fibrid was sufficiently strong to be handled on a commercial paper machine. Once the Handsheet No. 1 was saturated with an acetone solution of cellulose acetate and dried, it achieved a tensile strength of 10.2 pounds per inch.

In an attempt to study the strength of the various sheets, scanning electron photomicrographs of Sheet Nos. 1-3 were prepared. It was observed in the

photomicrographs that the fibrid dominated the structure, and that the very high surface area of the fibrid was apparently responsible for the glass fiber bonding. Prior to using any of the sheets as filters, the handsheets were treated at 550"F for about 10 minutes. The saturated sheet increased in permeability from 7.0 to 18.9 Frazier (Sheet No. 1), the powder bonded sheet increased from 22.1 to 25.8 Frazier (Sheet No. 2) , and the fibrid bonded sheet increased from 0.9 to 13.3 Frazier (Sheet No. 3). In addition to the changes in permeability, a change in tensile strength was also observed. The fibrid sheet (Sheet No. 3) possessed by far the highest tensile strength after the heat treatment, as can be seen from the following table:

PHYSICAL PROPERTIES AFTER HEAT TREATMENT

Sheet No. Tensile, lb/inch Permeability, Frazier

1 4.6 18.9 2 4.2 25.8

3 7.5 13.3

Photomicrographs were also taken after the heat treatment. These photomicrographs reveal a structural change in the sheets. The fibrid bonded sheet (Sheet No. 3) was observed to be the most uniform in fiber distribution.

In the photomicrographs where the cellulose acetate was added to the paper as a solution, a polymer deposit on the surface of the web itself was observed upon evaporation of the solvent. Very little difference in appearance was observed between the

unheated and heated sheets. Once the polymer was deposited on the surface area of the sheet, there was very little redistribution upon melting during the heat treatment. A more uniform distribution of the polymer was observed for the addition of the polymer in particulate form. There was no evidence in the photomicrographs of the formation of a surface film of polymer when the cellulose acetate was added as a particle. Upon heating, the polymer particles melted. However, the sheet was weak and permeable.

In the photomicrographs showing the addition of cellulose acetate fibrids, the fibrid appeared to be one of the dominant structural components, even though it comprised only 20% of the mass of the sheet. The ubiquitous appearance of the high surface area fibrid explains the high resistance of air flow of this structure. The polymer distribution uniformity was also observed to be exceptionally good, explaining the high tensile of the structure made with the polymer in fibrid form. Once this structure was heated and the polymer was melted, it was difficult to see the polymer in the photomicrographs. The fibrids flowed on to the surface of the glass fibers and reinforced the fiber to fiber bonds. The sheet made with fibrid was the strongest structure after heating, relevant evidence of the polymer uniformity. The disappearance of the fibrid upon melting resulted in a large increase in sheet permeability. Little formation of a surface film on the web either before or after heat bonding was observed in the photomicrographs.

In summary, the structure reinforced with a polymeric fibrid is unique in strength and uniformity. Prior to the heat treatment, the fibrid dominated the

sheet structure and reinforced it. After the heat treatment (which melted the polymer) , the sheet made with fibrid was the most uniform in structure and was the strongest. The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular embodiments disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others without departing from the spirit and scope of the invention.