Filter medium

A method and apparatus of producing a filter medium of a relatively strong substrate and a fibrous felt layer bonded to the substrate by needle punching the felt through the substrate and fusing the fibers protruding through the perforations to the substrate and each other.

This invention relates to a filter medium for fluids. 
More particularly, the invention relates to a filter medium which includes 
a relatively strong perforated substrate and a fibrous felt layer bonded 
to the surface of the substrate upstream to the flow of fluid to be 
filtered. The bond is achieved by punching needles through the fibrous 
felt layer and the substrate to form filtration perforations in the 
substrate and by fusing the fibers protruding through the perforations on 
the downstream surface of the substrate. 
The filter medium of the present invention is unaffected by water and 
highly resistant to degradation from acid and alkaline fluids and can be 
used in pneumatic filtration systems to separate moisture and dust in 
liquid filtration systems to separate desirable or undesirable solids. 
More importantly, the filter medium can be used in either type of system 
to separate, trap and retain materials for which the fibrous felt layer 
has a strong affinity or attraction. For example, in a preferred 
embodiment of the filter medium the fibrous felt layer has excellent oil 
wetability characteristics and an attraction and affinity for and an 
ability to soak up undesirable high viscosity tramp oil. Thus, the filter 
medium is highly useful for filtering the higher viscosity tramp oil from 
a lower viscosity coolant oil in a circulating cooling system. 
A particularly useful application of the filter medium of the present 
invention is in connection with filtering the oil coolant of the 
circulating cooling system for machines for making beverage cans from 
aluminum and steel. In such machines the metal particles and tramp oil, 
along with other undesirable dirt and debris, find their way into the 
coolant. The filter medium of the present invention has been found to have 
superior filtration capabilities in removing the metal particles, dirt, 
debris, but particularly the tramp oil, from the coolant over conventional 
filter media, such as nonwoven cellulose sandwiched between woven cotton 
fabrics heretofore used as a filter medium.

The filter medium of the present invention, as shown schematically in the 
drawings, includes a relatively strong substrate 10 and a fibrous felt 
layer 11 on the surface of the substrate upstream to the flow of fluid to 
be filtered. The fibrous felt is bonded to the substrate by needles 
punched through the fibrous layer and the substrate to form filtration 
perforations 12 in the substrate. The substrate perforating operation 
punches fibers of the fibrous felt layer 11 into and through the 
perforations 12, and the fibers penetrating and protruding through the 
perforations are fused on the downstream surface of the substrate, as 
indicated by the fused fibers designated 13 in the drawings, to interlock 
the layers together. 
The filter medium is preferably made of materials unaffected by water and 
chemically resistant to degradation from both acid and alkaline liquid and 
pneumatic fluids. In a preferred embodiment of the invention, the 
substrate 10 is a spun bonded polypropylene sheet or web 14 laminated to a 
melt blown polypropylene sheet or web 15 having a fibrous polypropylene 
felt 11 needle punched into the substrate. The spun bonded polypropylene 
forms the upstream side of the substrate and imparts the necessary 
strength, stability and integrity to the filter medium, and has the 
ability to deform and retain the discrete, defined minute perforations 
formed therein for the free passage of fluids while preventing the flow 
therethrough of oversize particles. The melt blown polypropylene forms the 
downstream side of the substrate, and it provides a thin, fibrous 
downstream layer capable of retaining and filtering fine particles, 
particularly fine metal particles, which manage to pass through the 
perforations 12. 
The fibrous polypropylene felt 11 embodies a fibrous mass to collect, trap 
and retain a substantial quantity of dirt and debris, but more 
particularly the polypropylene fibers have a high affinity and attraction 
for the higher viscosity oil molecules of the undesirable tramp oil. 
A preferred polypropylene substrate is made by Kimberly-Clark Corp. of 
Neenah, Wis. under the designation Evolution Two for the home furnishing 
market. It is a laminated material about 2.1 oz. per square yard and 
composed of a spun bonded polypropylene web, about 1.5 oz. per square 
yard, laminated with a melt blown polypropylene web, about 0.6 oz. per 
square yard. The spun bonded web and the melt blown sheet are fed in web 
form through heated embossed rolls which bond and fuse the webs together 
forming indentations 16 in the spun bonded polypropylene layer and 
indentations 17 in the melt blown layer to form the laminate. 
The fibrous polypropylene felt 11 is preferably made from a web of 
polypropylene batting needled together, for example, by subjecting it to a 
needle punching operation of about 500 fine gauge needles per square inch 
to produce a felt of about 4 oz. per square yard. A felt having these 
properties is manufactured by West Point-Pepperell, Inc. of West Point, 
Ga. 
The fibrous felt 11 is bonded to the substrate 10 by needle punching 
through the fibrous felt layer to form the filtration perforations 12 
through the substrate while at the same time filling the perforations with 
polypropylene fibers, some of which penetrate and protrude through the 
perforations 12 on the downstream surface of the filter medium. The needle 
punching operation should be carried out with very fine needles, for 
example, 41 gauge, to produce an array of uniform minute, discrete 
perforations in the range of from 50 to 1000 per square inch, preferably 
in the order of about 250 per square inch. The needle punching operation 
forms the filtration perforations in the substrate, introduces felt fibers 
into the perforations to improve the filtration and serves to bond the 
filter to the substrate. 
After the needle punching operation, the downstream surface of the filter 
medium is subjected to heat of a temperature high enough to fuse or at 
least soften the felt fibers protruding through the perforations in the 
substrate, thus fusing or adhering the protruding fibers to the substrate 
and to each other, thereby strengthening the bond between the felt and the 
substrate and to some extent partially decreasing the size of the 
perforations. The application of heat must not be so intense as to close 
the perforations entirely. Although the heat can be applied in various 
ways, the heat fusing or softening step has been carried out by passing 
the downstream surface of the substrate across a gas flame with the 
temperature controlled so that the heat generated melts, fuses or softens 
the protruding fibers to each other and to the surface of the substrate. 
A typical filter medium of the present invention manufactured by the method 
described herein has the following properties: weight, 6.3 oz. per square 
yard; thickness, 70 mils; grab tensile stength, 76.5 lbs. lengthwise 
direction, 83.1 transverse direction; Mullen burst, 137 psi; Frazier air 
permeability, 40 cfm; particle retention, 5 microns and over. 
The filter medium of the present invention has been used successfully and 
found to have superior filtering capabilities in the circulating cooling 
systems used in machines for manufacturing aluminum and steel beverage 
cans in which a 6% low viscosity oil and 94% water emulsion is used as the 
coolant. The circulating machinery of this type is manufactured by J. R. 
Schneider Company of Tiberon, Calif. In this type of equipment metal 
particles, debris and dirty higher viscosity tramp oil find their way into 
the low viscosity coolant and the filter medium has effectively filtered 
out the debris and particles 5 microns and over, as well as the more 
viscous tramp oil with the coolant operating at pressures in a range from 
about 2 to 30 psi, depending on the condition of the filter medium. The 
filter medium has not only been used successfully, but it has been found 
to have a longer life than conventional cellulose filter media and to do a 
more effective job of filtering out the tramp oil. 
Although the present invention has been described in a single preferred 
form and by way of example only, many variations and modifications can be 
made therein within the spirit of the invention. For example, for certain 
applications nylon, polyester or polyethylene felts may be used, but do 
not have as strong an affinity and attraction for retention of the high 
viscosity tramp oil as does the filter medium of the present invention. 
They will, nevertheless, provide a filter medium made from a fusable 
synthetic fiber structure capable of being bonded to a substrate. Also, 
the felt fibers protruding through the perforations in the substrate can 
be bonded to each other and the downstream surface of the substrate by 
various means other than by heat fusing, such as by chemical fusing. The 
invention is not intended to be limited to any specific form or embodiment 
except in so far as such limitations are expressly set forth in the claims 
.