Filter element having an inherently stable, permeably porous plastic body

A filter element for separating solid particles from a fluid medium. The filter element comprises a permeably porous, substantially inherently stable shaped body made substantially of polyethylene. The polyethylene is made from polyethylene grains combined and heated to form the shaped body. The polyethylene grains comprisean ultrahigh molecular polyethylene component including fine grains and having an average molecular weight of more than 10.sup.6. The polyethylene component further has, in an initial state before heating, a grain size distribution of at least 95% by weight of fine grains being larger than 63 microns and less than or equal to 250 microns. In addition, the polyethylene grains comprise a further polyethylene component including fine grains in an initial state before heating, and which has an average molecular weight of less than 10.sup.6. The fine grains of the further polyethylene component are adapted to be combined and heated with the fine grains of the polyethylene component for forming the shaped body. The filter element further includes a fine pored coating of a fine grained material disposed on the afflux surface of the shaped body for filling at least a considerable depth of surface pores present on the afflux surface, the coating having an average grain size less than an average grain size of the shaped body.

FIELD OF THE INVENTION 
The present invention relates to a filter element, in particular for 
separating solid particles from air. 
BACKGROUND OF THE INVENTION 
The document EP-B-0 177 521 discloses a filter element whose substantially 
inherently stable, permeably porous shaped body is made of fine-grained 
polyethylene with a higher molecular weight, and polyethylene which is 
fine-grained in the initial state with a lower molecular weight, these 
polyethylene components being combined into the shaped body by the action 
of heat, and a surface-pore coating of fine-grained 
polytetrafluoroethylene being provided. In actually produced filter 
elements of this kind the higher-molecular polyethylene has a molecular 
weight of more than 10.sup.6. Due to the surface-pore coating, the filter 
element can filter on the principle of surface filtration. Even fine and 
extremely fine particles of the medium to be filtered are already held on 
the afflux surface of the filter element and can be cleaned off of this 
surface very easily, for example by the backflow principle. 
These filter elements were hitherto produced using an ultrahigh-molecular 
polyethylene starting material, in which just under 10% of grains are 
larger than 250 microns and smaller than or equal to 63 microns. These 
filter elements are used quite successfully in practice. However the 
invention achieves a further improvement in known filter elements. 
It is an object of the invention to solve the technical problem of 
developing a permeably porous filter element having a substantially 
inherently stable shaped body made substantially of polyethylene and 
bearing a fine-pored coating on its afflux surface, so as to reduce flow 
resistance and improve the formation of the coating. 
SUMMARY OF THE INVENTION 
The above is fulfilled by the instant invention, according to a first 
embodiment of which a filter element has a permeably porous, substantially 
inherently stable shaped body, wherein: 
(a) the shaped body is made substantially of polyethylene; 
(b) the shaped body is built up of ultrahigh-molecular, fine-grained 
polyethylene with an average molecular weight of more than 10.sup.6 and a 
further polyethylene component which is fine-grained in the initial state 
and has an average molecular weight of less than 10.sup.6 ; 
(c) the grains of ultrahigh-molecular polyethylene and the further 
polyethylene component are combined into the shaped body by the action of 
heat; 
(d) the shaped body is provided on its afflux surface for the medium to be 
filtered with a fine-pored coating of a fine-grained material having a 
smaller average grain size than the shaped body and filling at least a 
considerable depth of the surface pores thereof on the afflux surface; and 
(e) the ultrahigh-molecular polyethylene has in the initial state a 
grain-size distribution with at least 95% by weight of grains in the range 
of&gt;63 to .ltoreq.250 microns. 
This first inventive solution and novel sharp definition of the grain-size 
range of the ultrahigh-molecular polyethylene for the shaped body to be 
coated means that the uncoated shaped body has a very uniform pore 
distribution in which particularly very small pores which increase flow 
resistance are virtually fully absent. The surface pores of the uncoated 
shaped body also have a very uniform pore-size distribution. The coating 
therefore also has a very uniform pore-size distribution with a smaller 
average pore size, based on the same coating material. This results in a 
virtually uniform filter load and a virtually perfect surface filtration 
over the total afflux surface of the filter element. 
According to a second embodiment, the present invention relates to a filter 
element, in particular for separating solid particles from air, which has 
a permeably porous, substantially inherently stable shaped body, together 
with the abovementioned features (a) to (d), 
wherein: 
the ultrahigh-molecular polyethylene has in the initial state a grain-size 
distribution with at least 60% by weight of grains in the range of&gt;125 
to.ltoreq.250 microns. 
This second inventive solution to the stated technical problem achieves 
effects along the lines of the effects described above in connection with 
the first solution. While clearly more than half of the grains of 
ultrahigh-molecular polyethylene in the grain-size range of 63 to 250 
microns were hitherto in the grain-size range of 63 to 125 microns and the 
part by weight of grains in the grain-size range of 125 to 250 microns was 
clearly under 50%, it has surprisingly been found that the second 
inventive solution results in reduced flow resistance of the shaped body 
and homogenizing effects in accordance with those described above. 
According to a third embodiment, the present invention relates to a filter 
element, in particular for separating solid particles from air, which has 
a permeably porous, substantially inherently stable shaped body, together 
with the above-mentioned features (a) to (d), wherein: 
(a) the shaped body is made substantially of polyethylene; 
(b) the shaped body is built up of ultrahigh-molecular, fine-grained 
polyethylene with an average molecular weight of more than 10.sup.6 ; and 
(c) the shaped body is provided on its afflux surface for the medium to be 
filtered with a fine-pored coating of a fine-grained material having a 
smaller average grain size than the shaped body and filling at least a 
considerable depth of the surface pores thereof on the afflux surface, 
characterized in that 
(d) the ultrahigh-molecular polyethylene has in the initial state a 
grain-size distribution with at least 70% by weight of grains in the range 
of&gt;63 to.ltoreq.315 microns; and 
(e) the grains of ultrahigh-molecular polyethylene are combined directly 
into the shaped body by the action of heat. 
The filter element according to the third aspect of the invention is not 
provided with a further polyethylene component having an average molecular 
weight of less than 10.sup.6 ; the grains of ultrahigh-molecular 
polyethylene are combined directly into the shaped body by the action of 
heat. The third inventive solution achieves effects along the lines of the 
effects described in connection with the first inventive solution, without 
the virtual elimination of grains with a size under 63 microns and grains 
with a size of more than 250 microns (see the description of the first 
inventive solution above) or the relatively high concentration on the 
grain-size range of 125 to 250 microns (see the description of the second 
inventive solution above) being as crucial, although these measures, taken 
alone or jointly, are also preferred as developments of the third 
inventive solution. 
Alternatively, it is possible to use a further polyethylene component with 
an average molecular weight of less than 10.sup.6 in the third inventive 
solution, but in a clearly lower proportion than hitherto provided. One 
can use a part by weight of the further polyethylene component of under 
15%, preferably under 10%, based on the sum of ultrahigh-molecular 
polyethylene and further polyethylene component. 
Reduced flow resistance for the medium to be filtered flowing through the 
filter element wall means that smaller delivery, for example of the blower 
or pump, is required for transporting the medium to be filtered through 
the filter element. In other words, for a given filtering apparatus with a 
given filtering surface and given delivery, one obtains a higher 
throughput of the medium to be filtered through the filtering apparatus. 
The shaped body can be made solely of the polyethylene components described 
above. However it is also possible for the shaped body to have further 
components which do not significantly impair the permanent combination of 
the components by the action of heat. These additional components, if any, 
are generally additives in a relatively small percentage amount. An 
example is carbon black as an antistatic additive. 
The inventive filter element is suitable in general for separating 
particles from liquid or gaseous media to be filtered. Particularly 
preferred areas of application are the separation of solid particles from 
air and the separation of solid particles from liquids such as water or 
oil. 
It is pointed out that the stated coating can also cover the total afflux 
surface of the shaped body, i.e. it need not be limited to part or all of 
the depth of the surface pores present on the afflux surface of the shaped 
body. What is primarily important functionally is that at least a 
considerable depth of the surface pores be filled, preferably the part of 
the depth beginning at the surface but also a part of the depth recessed 
from the surface. 
In the third inventive solution the proportion of grains in the range of 63 
to 315 microns is preferably at least 80% by weight, more preferably at 
least 90% by weight, and even more preferably at least 95% by weight. 
In a preferred development of the first and third solutions at least 60% by 
weight of grains are in the range of&gt;125 to.ltoreq.250 microns, as in the 
second solution. 
In all three solutions it is preferable for at least 70% by weight of 
grains to be in the range of&gt;125 to.ltoreq.250 microns. 
In the second and third solutions it is preferable for at least 80% by 
weight, more preferably at least 90% by weight, and even more preferably 
at least 95% by weight, of grains to be in the range of&gt;63 to.ltoreq.250 
microns. 
In all three solutions it is preferable for at least 97% by weight of 
grains to be in the range of&gt;63 to.ltoreq.250 microns. 
In the first and second solutions the part by weight of the further 
polyethylene component is preferably 3 to 70%, more preferably 5 to 60%, 
and even more preferably 20 to 60%, based on the sum of 
ultrahigh-molecular polyethylene and further polyethylene component. 
When the further polyethylene component is present it preferably has an 
average molecular weight in the range of 10.sup.3 to 10.sup.6. According 
to a first alternative the average molecular weight of the further 
polyethylene component is preferably in the range of 10.sup.4 to 10.sup.6, 
more preferably in the range of 10.sup.5 to 10.sup.6, even more preferably 
in the range of 2.times.10.sup.5 to 10.sup.6. The latter is normally 
termed a high-molecular polyethylene but in the instant description any 
polyethylene component with an average molecular weight over 
5.times.10.sup.4 will be termed high-molecular for the sake of simplicity. 
According to a second alternative the average molecular weight of the 
further polyethylene component is in the range of less than 
5.times.10.sup.4, more preferably in the range of 10.sup.3 to 
5.times.10.sup.4 even more preferably in the range of 5.times.10.sup.3 to 
5.times.10.sup.4. These ranges are all in the area of low-molecular 
polyethylene. Low-molecular polyethylene in the stated molecular weight 
ranges is frequently also referred to as polyethylene wax. It is possible 
to use a mixture of the two materials described in the above paragraph as 
the first alternative and second alternative, so that the further 
polyethylene component is composed of a first subcomponent with a higher 
average molecular weight and a second subcomponent with a lower average 
molecular weight. The part by weight of the second subcomponent is 
preferably 2 to 50%, more preferably 5 to 20% based on the total further 
polyethylene component. 
As far as the grain-size distribution of the high-molecular polyethylene 
component of the further polyethylene component, if any, is concerned, in 
the initial state the grain-size distribution is preferably such that at 
least 95% by weight of grains are under a grain size of 1000 microns and 
at most 15% by weight of grains are under a grain size of 63 microns, 
preferably at least 99% by weight of grains are under a grain size of 1000 
microns and at most 5% by weight of grains are under a grain size of 63 
microns. 
As far as the grain-size distribution of the low-molecular polyethylene 
component of the further polyethylene component, if any, is concerned, the 
grain-size distribution in the initial state is preferably such that at 
least 95% by weight of grains are under a grain size of 500 microns and at 
most 15% by weight of grains are under a grain size of 63 microns. 
Alternatively, a so-called microwax is preferred in which at least 95% by 
weight of grains are under a grain size of 63 microns in the initial 
state. 
When the expression "in the initial state" is used in the present 
description, it refers to the state of the polyethylene components before 
the action of heat for combining them into the permeably porous shaped 
body. 
If the further polyethylene component has an average molecular weight of 
less than 5.times.10.sup.4 or contains a subcomponent with this molecular 
weight, one observes a particularly high adhesive power of the coating 
material grains in the surface pores of the shaped body. Pictures of the 
afflux surface of the filter element taken by scanning electron microscope 
before application of the coating show that the cause of this effect is 
presumably that the low-molecular polyethylene component forms curved-stem 
projections on the walls of the surface pores which evidently promote a 
particularly firm anchoring of the coating material grains. 
The ultrahigh-molecular polyethylene preferably has a grain-size 
distribution in the initial state such that a graph of "cumulative 
percentage of pores over pore diameter" with a substantially linear course 
at least in the range of 20 to 75% results for the uncoated shaped body. 
More details on the above are found below in the example section of the 
description. 
According to a preferred development of the invention one uses grains of 
ultrahigh-molecular polyethylene that have a shape with bump-like raised 
areas above the otherwise substantially spherical grain shape. It has been 
found that the above configuration results in particularly good adhesion 
of the coating material grains to the shaped body or in its surface pores. 
One has also observed a tendency toward reduced flow resistance of the 
shaped body. 
With respect to the average molecular weight of the ultrahigh-molecular 
polyethylene an upper limit of 6.times.10.sup.6 is preferred; a range of 
2.times.10.sup.6 to 6.times.10.sup.6 is particularly preferred. 
The grains of ultrahigh-molecular polyethylene preferably have a bulk 
density of 300 to 550 g/l, whereby 350 to 500 g/l is particularly 
preferred. The bulk density of the high-molecular polyethylene component 
in the initial state is preferably 200 to 350 g/l. The melting temperature 
of the low-molecular polyethylene component is preferably 100.degree. C. 
to 150.degree. C. 
The fine-grained material for coating the surface pores is preferably 
polytetrafluoroethylene. The fine-grained coating material preferably has 
an average grain size of under 100 microns, most preferably under 50 
microns. 
Another object of the invention is to provide a method for producing the 
filter elements or shaped bodies described above, including the steps of: 
pouring the grains of ultrahigh-molecular polyethylene, optionally mixed 
with the further polyethylene component, into a mold; 
heating the content of the mold to a temperature of 170.degree. C. to 
250.degree. C. for a sufficient time to combine the grains into the shaped 
body (generally 10 to 180 min); 
cooling the shaped body in the mold (not necessarily down to room 
temperature); 
removing the shaped body from the mold; and 
applying the coating to the afflux surface of the shaped body removed from 
the mold. 
The coating can be applied particularly favorably in the form of a 
suspension which is then dried, preferably by blowing hot air thereon. The 
suspension can be applied particularly well by the spray and brush method. 
The content of the mold is preferably shaken into the mold by vibration. 
When the content of the mold is heated the low-molecular polyethylene 
component, if present, melts first. As the temperature of the mold content 
increases further the high-molecular polyethylene component also starts to 
melt, and the ultrahigh-molecular polyethylene grains can soften somewhat 
on their surface in an inherently stable fashion. The high-molecular 
polyethylene component forms a binding skeleton between the 
ultrahigh-molecular polyethylene grains, while the low-molecular 
polyethylene component, if present, is deposited on the high-molecular 
binding skeleton and the ultrahigh-molecular polyethylene grains. If the 
further polyethylene component is not present the ultrahigh-molecular 
polyethylene grains bond together when the content of the mold is heated 
due to the inherently stable softening on their surface. 
The polyethylene components described above are commercially available, for 
example from Hoechst AG and BASF AG, apart from the grain-size 
distribution according to the first inventive solution and the grain-size 
distribution according to the second inventive solution. 
It is explicitly pointed out that filter elements with one or more of the 
features stated above are technically useful and inventive even without 
the features of the first, second or third inventive solutions. 
The invention shall be explained in more detail in the following with 
reference to examples.

DETAILED DESCRIPTION OF THE INVENTION 
Filtering apparatus 2 illustrated in FIG. 1, often referred to for short as 
a "filter," comprises housing 4 in which four filter elements 6 are spaced 
apart parallel to one other in the embodiment example shown. Filter 
elements 6 have--roughly speaking--the shape of a narrow right 
parallelepiped at their outlines, and include bellows like walls extending 
in zigzag fashion on the long sides (see FIG. 3). Such filter elements 6 
are also referred to as lamellar filter elements. Pocket-shaped filter 
elements 6 are hollow, are open on the top and the underside, and have a 
substantially constant wall thickness all around. It is pointed out that 
the filter elements can also have a different shape, for example a tubular 
shape. 
Each filter element 6 comprises outwardly coated shaped body 22 whose 
material composition will be described more exactly below. Each body 22 is 
provided in the upper head area with edge area 10 protruding all around, 
and provided there with a fastening and stiffening plate so that it can be 
fastened more easily in housing 4 of filtering apparatus 2. The four 
filter elements 6 are fastened from below to strong perforated plate 12 
disposed transversely in housing 4, with interior space 14 of each filter 
element communicating with the space above perforated plate 12 via a 
plurality of holes. In the lower foot area of each filter element 6 
skirting 15 is fastened to shaped body 22, sealing body 22 from below and 
rising above it at both ends. Skirting 15 rests with each end on 
projections 17 of housing 4. Body 22 shown is divided into three cavities 
following one another in the longitudinal direction of skirting 15. 
The medium to be filtered flows inside housing 4 through afflux port 16, 
then from the outside to the inside of filter elements 6, from there into 
space 19 above perforated plate 12, and leaves filtering apparatus 2 
through exit port 18. Below filter elements 6 housing 4 is funnel-shaped 
so that particles separated from the medium to be filtered and dropping 
off filter elements 6 due to cleaning can be removed from time to time 
through particle discharge port 20. 
Shaped body 22 of each filter element 6 may be made of ultrahigh-molecular 
polyethylene grains and a further polyethylene component. These components 
were fine-grained when poured into the production mold but only the 
ultrahigh-molecular polyethylene in granular form exists in finished body 
22. The stated components are combined by the action of heat into the 
substantially inherently stable, permeably porous shaped body by the 
production method described above. 
FIG. 4 shows schematically the structure of finished shaped body 22 
including fine-pored surface-pore coating 32. High-molecular polyethylene 
component 26 forms a binding skeleton between the grains of 
ultrahigh-molecular polyethylene under the action of heat during 
production. The low-molecular components are additionally deposited on the 
high-molecular and the ultrahigh-molecular polyethylene material. 
Ultrahigh-molecular polyethylene grains 24 have virtually not changed 
their shape during production. Altogether the structure of shaped body 22 
is highly porous. If no high-molecular and no low-molecular polyethylene 
component are used the resulting shaped body structure is as in FIG. 5. 
Ultrahigh-molecular grains 24 are sintered together at their points of 
contact. 
Coating 32 comprises small polytetrafluoroethylene grains. Coating 32 fills 
at least part of the depth of pores 36 present on outer surface 34 (i.e. 
the afflux surface for the medium to be filtered) of shaped body 22. 
FIG. 6 shows the grain shape of preferred ultrahigh-molecular polyethylene 
component 24. This grain shape can be described--roughly speaking--as 
substantially spherical with bump-like or wart-like raised areas 38. The 
advantageous effects of using ultrahigh-molecular polyethylene with this 
grain shape have been described above. 
EXAMPLES 
Three examples of inventive shaped bodies will be described more precisely 
with respect to their material structure and the grain-size distribution 
of the ultrahigh-molecular polyethylene grains, and compared with a 
standard example according to the prior art. 
Example 1: 
A shaped body is produced by the described method from about 60% by weight 
of ultrahigh-molecular polyethylene with an average molecular weight of 
2.times.10.sup.6 and about 40% by weight of fine-grained high-molecular 
polyethylene with an average molecular weight of about 3.times.10.sup.5. 
The grain-size distribution of the ultrahigh-molecular component in the 
initial state is: 
______________________________________ 
under 63 microns: 1% 
63 to 125 microns: 59% 
125 to 250 microns: 40% 
over 250 microns: 0% 
______________________________________ 
A graph of "cumulative percentage of pores over pore diameter" as in FIG. 7 
is determined on the shaped body. Pore diameter d.sub.50, i.e. that pore 
diameter of the body in which 50% of pores are larger than d.sub.50 and 
50% smaller than d.sub.50, is about 20 microns. The graph of FIG. 7 is 
substantially linear in the range of about 10 to 83%. There are virtually 
no pores sized over 40 microns. 
After applying a coating of small polytetrafluoroethylene grains one 
determines the graph of pore-size distribution as in FIG. 8, like the 
graph of FIG. 7. Now d.sub.50 is about 8.5 microns. Only about 36% of 
pores are larger than d.sub.50 and only about 10% of pores are smaller 
than d.sub.50. Pores with a size above 30 microns are virtually absent. 
After 200 working hours in a filtering apparatus according to FIG. 1 one 
measures a pressure loss of 225 millimeters head of water on the filter 
element. 
Example 1 is within the definitions of the first inventive solution. 
Example 2: 
A shaped body is produced by the described method from about 54% by weight 
of ultrahigh-molecular polyethylene with an average molecular weight of 
about 4.times.10.sup.6, about 35% by weight of fine-grained high-molecular 
polyethylene with an average molecular weight of 3.times.10.sup.5 and 
about 11% by weight of fine-grained low-molecular polyethylene with an 
average molecular weight of about 2.times.10.sup.4. The grain-size 
distribution of the ultrahigh-molecular component in the initial state is: 
______________________________________ 
under 63 microns: 1.5% 
63 to 125 microns: 23% 
125 to 250 microns: 73% 
250 to 315 microns: 3% 
over 400 microns: 0% 
______________________________________ 
A graph of "cumulative percentage of pores over pore diameter" as in FIG. 9 
is determined on the shaped body. Pore diameter d.sub.50 is about 23 
microns. The graph of FIG. 9 is substantially linear in the range of about 
8 to 75%. There are virtually no pores sized over 45 microns. 
After 200 working hours in a filtering apparatus according to FIG. 1 one 
measures a pressure loss of 240 millimeters head of water on the filter 
element coated with small polytetrafluoroethylene grains. 
Example 2 is within the definitions of both the first and the second 
inventive concepts. 
Example 3: 
A shaped body is produced by the described method from 100% by weight of 
ultrahigh-molecular polyethylene with an average molecular weight of about 
4.times.10.sup.6. The grain-size distribution of the starting material is: 
______________________________________ 
under 63 microns: 1.5% 
63 to 125 microns: 23% 
125 to 250 microns: 73% 
250 to 315 microns: 3% 
over 400 microns: 0% 
______________________________________ 
A graph of "cumulative percentage of pores over pore diameter" as in FIG. 
10 is determined on the shaped body. Pore diameter d.sub.50 is about 17.5 
microns. The graph of FIG. 10 is substantially linear in the range of 
about 4 to 81%. There are virtually no pores sized over 35 microns. 
After 200 working hours in a filtering apparatus according to FIG. 1 one 
measures a pressure loss of 205 millimeters head of water on the filter 
element coated with small polytetrafluoroethylene grains. 
Example 3 is within the definitions of the first, the second and the third 
inventive concepts. 
Comparative example: 
A shaped body is produced by the described method from about 60% by weight 
of ultrahigh-molecular polyethylene with an average molecular weight of 
about 2.times.10.sup.6 and about 40% by weight of fine-grained 
high-molecular polyethylene with an average molecular weight of about 
3.times.10.sup.5. The grain-size distribution of the ultrahigh-molecular 
component in the initial state is: 
______________________________________ 
under 63 microns: 4% 
63 to 125 microns: 48% 
125 to 250 microns: 45% 
over 250 microns: 3% 
______________________________________ 
A graph of "cumulative percentage of pores over pore diameter" as in FIG. 
11 is determined on the shaped body. Pore diameter d.sub.50 is about 24 
microns. There are virtually no pores sized over 55 microns. 
After applying a coating of small polytetrafluoroethylene grains one 
determines the graph of pore-size distribution according to FIG. 12, like 
the graph of FIG. 11. Now d.sub.50 is about 14.5 microns. About 40% of 
pores are larger than d.sub.50 and about 15% of pores are smaller than 
d.sub.50. 
After 200 working hours in a filtering apparatus according to FIG. 1 one 
measures a pressure loss of 270 millimeters head of water on the filter 
element. 
The shaped body in the comparative example is outside the definitions of 
the first, second and third inventive solutions. 
In all four examples the coating was of course applied from the same 
starting material and the pressure loss measured in the same filtering 
apparatus subjected to air with the same content of solid particles. 
In case of a coating of polytetrafluoroethylene particles it is generally 
preferred to apply the coating in the form of a suspension which in 
addition contains an adhesive. Particularly suitable are adhesives known 
as disperse adhesives, in particular adhesive dispersions on the basis of 
polyvinylacetate, such as MOWILITH (registered trademark of Hoechst AG) 
being an aqueous copolymer dispersion of vinylacetate, ethylene and 
vinylchloride. Typically, the suspension to be applied to the afflux 
surface of the filter element for forming the coating has the following 
composition: 
20 weight % polytetrafluoroethylene particles 
6 weight % MOWILITH 
74 weight % water.