Filter element and method for the manufacture thereof

A filter element (10) comprises a porous, thick-walled, integral, self-supporting, resin impregnated and bonded fibrous, tubular filter structure (11) having a hollow core (15); an inner shell (20) of a first large pore size porous media adjacent the hollow core and forming the majority of the filter structure; and an outer shell (21) of a second small pore size porous media, finer than the first porous media and adjacent the inner shell, the first and second porous media being resin impregnated and bonded. A method for the manufacture of a porous, thick-walled, integral, self-supporting, resin impregnated fibrous tubular filter element comprises the steps of forming a hollow inner shell of a first large pore size porous media; forming an outer shell of a second small pore size porous media, onto the inner shell, finer than the first porous media; impregnating the inner shell and the outer shell with a resin binder; and curing the resin to form an integral, self-supporting, resin impregnated and bonded fibrous, tubular filter element.

TECHNICAL FIELD 
The present invention relates to filter elements and a method for their 
manufacture. More particularly, a filter element is provided comprising 
resin impregnated and bonded fibrous materials and having a higher 
efficiency than known filter elements comprising resin impregnated and 
bonded fibrous material. 
BACKGROUND ART 
Filter elements which are self-supporting resin impregnated and bonded 
fiber structures are well known in the art. Preferred embodiments of such 
filters are described in U.S. Pat. Nos. 2,539,767 and 2,539,768 to 
Anderson are produced and sold by the Assignee herein under the trademark 
MICRO-KLEAN.RTM. (Cuno, Incorporated, Meriden, Conn.) wherein the bonding 
systems are water soluble thermosetting resins. Broadly, these filter 
elements are relatively rigid, self-supporting, porous thick-walled, 
tubular members composed entirely of a resin impregnated and bonded 
fibrous material. The filter elements are used for filtering liquids and 
gases by flowing radially inwardly under a differential pressure. 
Typically the filter elements are designed to obtain maximum contaminant 
capacity consistent with their filtration efficiency by providing a 
fibrous structure of a graded porosity, with the size of the pores 
progressively increasing rapidly outwardly toward the outer surface. By 
virtue of such graded porosity, or density, as the fluid flows inwardly 
through progressively smaller pores, the particulate contaminant to be 
filtered out penetrates to varying depths according to its size. Thus, the 
filter elements can accommodate more solids without effecting flow, with a 
consequently longer, effective life before the elements need replacing. 
For example, in Anderson, the graded porosity in the filter elements is 
accomplished by vacuum accreting resin-impregnated fibers from an aqueous 
uniform dispersion of such fibers under controlled conditions as to the 
amount of vacuum used in effecting such accretion and as to the 
composition and characteristics of the fibrous stock used. 
This approach to producing rigid, self-supporting, porous, thick-walled 
tubular filter elements has been usefully employed for over 40 years. It 
has, however, presented distinct limitations to the development and 
production of higher efficiency filter elements capable of removing 
ever-smaller contaminant particles. As is well known to the art, achieving 
such high filtration efficiencies requires the use of very small diameter 
fibers such as glass microfibers. Aqueous dispersions containing 
significant amounts of such fine fibers exhibit very slow formation rates 
during the required vacuum accretion. In many cases, it has proven 
impossible to form, by vacuum accretion, the required thick-walled tubular 
filter elements from such fine fiber dispersions. Similarly, it has proven 
difficult, if not impossible, to impregnate, dry, and cure such filter 
elements because of the high resistance to liquid or gas flow. Using the 
current state-of-the-art MICRO-KLEAN.RTM. process, the highest efficiency 
cartridge that can be produced is practically limited to an 8 micrometer 
nominal (90 percent particle removal efficiency) rating. There are other 
approaches to producing high efficiency filter cartridges but all involve 
the use of expensive raw materials and/or processes. Accordingly, there is 
a well-defined industrial need for low-cost, economical rigid, 
self-supporting resin impregnated and bonded filter elements with nominal 
filtration ratings of less than 8 micrometers, that the prior art has not 
been able to satisfy. 
DISCLOSURE OF THE INVENTION 
It is therefore an object of the present invention to provide a filter 
dement, comprising resin bonded fibers, having a higher efficiency than 
existing filter elements of this type. 
It is another object of the present invention to provide a higher 
efficiency filter element comprising an inner shell of a first porous 
media and an outer shell of a second porous media, more fine than the 
first porous media. 
It is yet another object of the present invention to provide a method for 
the manufacture of a higher efficiency filter element comprising two 
layers of resin impregnated and bonded fiber materials. 
At least one or more of the foregoing objects, together with the advantages 
thereof over known methods which shall become apparent from the 
specification which follows, are accomplished by the invention as 
hereinafter described and claimed. 
In general, the present invention provides a filter element comprising a 
porous, thick-walled, integral, self-supporting, fibrous, tubular filter 
structure having a hollow core; an inner shell of a first large pore size 
porous media adjacent the hollow core and forming a significant portion of 
the filter structure; and an outer shell of a second small pore size 
porous media, finer than the first porous media and adjacent the inner 
shell, the first and second porous media being resin impregnated and 
bonded. 
The present invention also provides a method for the manufacture of a 
porous, thick-walled, integral, self-supporting, fibrous, tubular filter 
element comprising the steps of forming a hollow inner shell of a first 
large pore size porous media; forming an outer shell of a second small 
pore size porous media, onto the inner large pore size shell finer than 
the first porous media; impregnating the inner shell and the outer shell 
with a water soluble thermosetting resin binder; and curing the resin to 
form an integral, self-supporting, fibrous, tubular filter element.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION 
Apart from the novelty of the filter elements taught by the present 
invention, filter elements of this type are well known in the art as is 
their method of manufacture. As previously indicated these are produced, 
for example, as described in Assignee's U.S. Pat. Nos. 2,539,767 and 
2,539,768 to Anderson, the entire disclosures of which are incorporated 
herein by reference. In the current MICRO-KLEAN.RTM. production process, 
the Anderson process has been modified so that the fibers are vacuum 
accreted from a uniform aqueous dispersion and then, subsequently, vacuum 
impregnated with a water soluble thermosetting resin. Accordingly, the 
filter elements of the present invention generally comprise a relatively 
rigid self-supporting, porous, thick-walled, tubular member composed 
entirely of resin-impregnated and bonded fibrous materials. 
Typically, filter elements of this type are sealingly arrayed within a 
filter housing which allows for the ingress of a fluid, liquid or gas, to 
be filtered and the egress of a filtrate. Within the housing or cartridge, 
means are provided to direct the fluid to the radially outermost surfaces 
of the element where the fluid then flows radially inwardly under pressure 
and is filtered, to exit axially via a central hollow core. It is to be 
understood that the environment for such use is well known and does not 
constitute novelty of the present invention. Accordingly, such housings 
and the passage of the fluid have not been depicted. 
In similar fashion, to provide tighter, more efficient filter elements of 
this type, it has become necessary to seal the ends of the filter elements 
in order to prevent the by-pass of the filter by the unfiltered fluid or 
contamination by the filtered particles. One particularly useful means of 
effecting such sealing is to employ a gasket at each end comprising a 
polyethylene closed cell foam. Such foams are well known in the art, a 
preferred brand being sold by Volteck of Lawrence, Mass. under the 
registered trademarks VOLARA and MINICEC, the former being preferred. The 
use of such gaskets for sealing filter elements of this type is described 
in U.S. Pat. Nos. 5,015,316 and 5,028,327 owned by the Assignee of record, 
the subject matter of which is incorporated herein by reference. 
With reference to the drawings, the filter element of the present invention 
is depicted in FIG. 1, indicated generally by the numeral 10. The filter 
element 10 preferably comprises a cylindrical structure or body 11 having 
an outer surface 12, opposed ends 13 and 14 and a hollow axial core 15. 
The outer surface 12 may provide grooves 16, to provide increased surface 
area and contaminant capacity. 
The element 10 is intended to be used for filtering liquids and gases which 
are caused to flow radially inwardly under a differential pressure. 
Heretofore, in order to obtain the maximum life consistent with the 
filtering efficiency, the fibrous structure of the filter element 10 has 
comprised a graded porosity, with the size of the pores progressively 
increasing radially outwardly toward the outer surface 12. By virtue of 
such graded porosity, or density, as the fluid flowed inwardly through 
progressively smaller pores, the particulate contaminants to be filtered 
penetrated to varying depths according to their size. 
The filter element 10 of the present invention employs an incrementally 
graded porosity; however, in contradiction to prior art the porosity 
gradation has been reversed, that is, the pores are most numerous and 
their sizes the smallest at the outer surface 12 of the body 11. More 
particularly, the body 11 actually comprises two components, an inner 
shell 20 of generally large pore size and an outer shell 21 of small pore 
size. The inner shell 20 is bounded at its inner diameter or wall 22 by 
the hollow axial core 15 and an outer diameter or wall 23, forming a 
thickness which comprises approximately 30 to about 70 percent of the 
overall element thickness. Similarly, the inner diameter or wall 24 of the 
outer shell 21 is continuous with the outer diameter wall 23 of inner 
shell 20 while the radially outer wall 25 is continuous with the outer 
surface 12. As depicted in the drawings, particularly FIG. 4, the outer 
surface 12 may provide grooves 16, which are individual annular rings. The 
bottom 26 of each groove or ring 16 is located within the outer shell 21. 
Alternatively, for other embodiments within the scope of the present 
invention, the outer surface 12 can be free of grooves 16. 
In order to manufacture the filter element 10 according to the present 
invention, a fibrous material is mixed with water or other suitable 
dispersant to form a slurry. Subsequently, one or more perforate formers 
or dies are immersed in the slurry in a felting tank holding the aqueous 
dispersion of fibers and the fibers are caused to be accreted upon the 
formers by application of a vacuum suction imposed upon the interior of 
the formers. By the control of the degree of vacuum and the length of time 
over which the vacuum is applied, in conjunction with the proper selection 
and control of the characteristics of the fiber, a filter carcass is 
produced of the depth, or thickness and porosity that is desired. 
This procedure is terminated when an adequate volume of fibers have been 
accreted to form the inner shell 20. After formation thereof, a 
substantial portion of the water or dispersing agent is removed by drawing 
hot air through the former and inner shell 20. The former is then immersed 
in a second aqueous dispersion of fiber media selected to form the 
tighter, smaller pore size of the outer shell 21. This step of overfelting 
the inner shell 20, is controlled by the degree of vacuum and continued 
for a duration of time adequate to produce an outer shell having the 
intended thickness. After formation of the shell 21, a substantial portion 
of the water or dispersing aid is again removed by drawing hot air through 
the former, inner shell 20 and outer shell 21. 
In order to impart to the filter element strength and rigidity, as well as 
to waterproof the fibers so that they will not become soggy, or soft in 
the presence of water or other fluids, a resin is used to impregnate the 
fibers and to bond them together in a relatively fixed relationship. The 
amounts of resin used may vary between 30 percent and 60 percent by total 
weight of the filter element. Various resins including thermosetting 
resins such as phenol formaldehyde condensation products, urea 
formaldehyde condensations products and the melamine resins may be used. 
Thermoplastic resins may also be employed, such as polystyrene. Preferred 
resins are melamine and phenolic resins. 
Resin impregnation is conducted by immersing the dried composite of inner 
shell 20 and outer shell 21 in a tank of resin and applying a vacuum 
through the former for a sufficient period of time for all of the fibers 
to be contacted. Generally, about 10 minutes will suffice but, of course, 
time is a function of the vacuum as well as the size and density of the 
filter element and hence, the method of the present invention should not 
be limited thereby. 
After impregnation, the filter element is cured by heat under temperature 
and time conditions appropriate for the curing of the particular resin 
used. In the final step, the filter cartridge is sized to accurate 
dimensions by cutting or trimming mechanically as by means of a knife, saw 
or grinder (see the U.S. Pat. No. 2,539,767 to Anderson) 
The fibers employed to manufacture the inner shell 20 can comprise acrylic, 
nylon, polyester, cellulose and mixtures thereof. These fibers have 
average diameters ranging from about 10 to 40 micrometers and provide a 
pore size ranging from about 5 to about 50 micrometers, with 10 to 20 
micrometers being preferred. 
The fibers employed in the overfelting step to form the outer shell 21 are 
selected to provide a very tight, small pore size second porous media. As 
such, the fibers can comprise acrylic, nylon, polyester, cellulose and 
mixtures thereof to which a relatively high percentage of glass 
microfibers have been added. Amounts of the latter range from about 5 to 
about 30 weight percent. Average glass microfiber diameters can be 
selected ranging from about 0.5 mm to 5 mm. 
Because of the unique structure of the filter element made according to the 
present invention, it is now possible to remove substantially finer 
particles than via the use of a conventional fibrous filter element. A 
comparison is presented in Table I hereinbelow between a MICRO-KLEAN.RTM. 
filter element, characterizing the known art, and a filter element 
according to the present invention. 
TABLE I 
______________________________________ 
TICLE SIZE FOR INDICATED INITIAL TICLE 
REMOVAL EFFICIENCY 
50% 90% 95% 98% 99% 
______________________________________ 
Prior Art.sup.a 
4.6 7.9 8.9 9.9 11.6 
(micrometers) 
High Efficiency.sup.b 
(c) 3.1 3.4 3.8 4.3 
(micrometers) 
______________________________________ 
.sup.a tighest prior art MICROKLEAN .RTM. efficiency 
.sup.b tighest efficiency for filter element according to the 
present invention 
(c) could not be measured 
By reviewing the data presented in Table I, it should now be appreciated 
that the relatively open inner shell formed by the first porous media and 
overfelted outer shell of the second porous media, being much finer than 
the first porous media, results in a unique filter element which can 
filter much finer particles with high efficiency. 
For use in high efficiency filter applications, it is necessary that the 
filter element 10 employ an extremely effective sealing means. Referring 
to FIGS. 1 and 3, thermally bonded to each end 13 and 14 of element 10 is 
a thermoplastic polymer closed cell foamed sealing gasket 29. Each gasket 
29 provides an effective sealing surface between the ends of the cartridge 
10 and the sealing edge of the filter housing (not shown). 
The gaskets are typically in the form of a donut shaped disc circle which 
is adhered to the filter ends 13, 14 concentrically with the hollow axial 
core 15. Typically the gasket may be of a diameter somewhat smaller than 
the outside diameter of the filter element 10, and have an inside diameter 
somewhat larger than the internal diameter of the filter element. 
Typically the discs are 1/16 to 3/32 of an inch thick. This dimension 
could be increased if necessary to compensate for troublesome sealing 
configurations that require more resiliency or depth to provide sufficient 
sealing. The foam is obtained in sheet form and cut into discs of the 
desired size and shape. 
The gaskets 29 are applied to the filter element 10 by heating the ends 13, 
14 of the element 10 to a temperature sufficiently high to thermally melt 
bond the gaskets 29 to the cartridge end when the gasket is contacted with 
the hard filter element surface. Such temperature may be determined 
readily and is empirically derived but is below the temperature at which 
the element starts to deteriorate, melt and/or fuse and is also below a 
temperature that completely melts the gasket. However, it has been found 
that the cells inside the gasketing material insulate the heated lower 
surface in contact with the heated ends of the filter 10 from the 
remaining portion of the gasketing material and thus, only the lower 
surface of the gasket is sufficiently heated to melt bond itself to the 
ends of the filter element 10. Such heating can be accomplished with a hot 
plate, infrared energy, hot air, etc. A number of techniques are available 
for heating the end of the element which are simple to accomplish and to 
automate. 
The closed cell configuration of the polymer is also desirable because it 
provides resiliency or spring back that allows compensation for out of 
alignment or out of flatness of the end of the cartridge. Additionally, 
the closed cell configuration provides sealing between cells through which 
the fluid cannot seep or flow. The use of a solid polymeric gasket would 
be inadequate because although it might bond to the filter element by the 
mere heating of the end of the cartridge, it would either completely melt 
and deform and/or would not provide sufficient resiliency for the sealing 
edges of the filter housing to embed therein. For a more complete 
description of these sealing gaskets, reference can be made to U.S. Pat. 
Nos. 5,015,316 and 5,028,327, noted hereinabove. 
The filter housings used in conjunction with the filter element of this 
invention are well known in the art. The cartridges may be used in varying 
lengths or multiples of a single length, stacked one on top of another. In 
such arrangements all the cartridges in multiple height stack arrangements 
are thermally bonded with a hot melt polymer, e.g., polypropylene, to 
assure alignment and permanent bonding for positive sealing against 
bypass. 
The filter elements may be used for removing particulate contaminants which 
are fibrous, abrasive or gelatinous from fluids such as gas, alcohol, 
glycols, coolants, fuels, oils, lubricants, cosmetics, paints and 
varnishes, syrups, compressed air, water or sensitive process liquids, 
e.g., demineralized water, food products, beverages, photographic 
solutions and, particularly, magnetic oxide slurries for producing 
magnetic recording media. 
Based upon the foregoing disclosure, it should now be apparent that the 
filter element of the present invention will carry out the objects set 
forth hereinabove. It should also be apparent to those skilled in the art 
that the method of the present invention can be practiced to manufacture a 
high efficiency filter element having an outer shell of smaller pore size 
porous media than that of the inner shell. Similarly, the selection of 
fibers and bonding resins which may be employed to prepare the filter 
element can readily be determined by those skilled in the art, depending 
upon the filtration desired. 
It is, therefore, to be understood that any variations evident fall within 
the scope of the claimed invention and thus, the overall structure and 
size of the filter element, as well as the pore size can be varied to suit 
the ultimate application and can be determined without departing from the 
spirit of the invention herein disclosed and described. Moreover, the 
scope of the invention shall include all modifications and variations that 
may fall within the scope of the attached claims.