CA1170026A

CA1170026A - Felt-like layered composite of ptfe and glass paper

Info

Publication number: CA1170026A
Application number: CA000392527A
Authority: CA
Inventors: Joseph P. Fagan
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1980-12-19
Filing date: 1981-12-17
Publication date: 1984-07-03
Also published as: DK158797C; JPS57126655A; US4324574A; EP0055112A1; AU7861581A; DK158797B; DK563481A; DE3169483D1; AU542175B2; JPH0129699B2; EP0055112B1

Abstract

ABSTRACT
A felt-like layered composite of the following structure:
a. a first layer of a predominantly horizontal coplanar superimposed layers of filamentary polytetrafluoro-ethylene components interrupted by occasional interlayered orientation;
b. a thin high density nonwoven layer of fine glass staple fibers;
c. a scrim fabric of polytetrafluoro-ethylene fibers and d. a second predominantly horizontal coplanar superimposed layers of filamentary polytetrafluoroethylene components interrupted by occasional interlayer orientation;
wherein the layers are mechanically bonded together by filamentary polytetrafluoroethylene components extending vertically throughout the composite and a process for making this composite; the composite is useful as a filter fabric and is primarily used for bag filters to remove fly ash from the gaseous combustion products of coal fired furnaces and boilers.

Description

1 ~ 7(~02~
, 1 E~-61~3 TITLE
Felt-like Layered Composite of PTFE and Glaqs_Paper BACKGROtJND OF TEIE INVEN~ION
~
This invention relates to nonwoven felt-like produc~s of filam~ntary polytetrafluoroethvlene (hereinaf~er referred to as PTFE) DescriE~o~ Of The Prior Art ~he formation of felts from polymeric filamentary materials is taught in Lauterbach U.SO Patent 2,910,763 issued Novembex 3, 1959.
The use of such felts for filtering is taught in Lauterbach U.S. Patent 2,933,1 4 issued April 19, 1960. Felt-like products o~ PTFE have become commercial products and are used in filtration.
- The use of PTF~ felts as high efficieny and high porosity filters particularly useful as filters for the removaL of fine par~icles such as fly ash from gaseous combustiorl products o~ coal fired ~urnaces and boiler~ is s;hown in Fagan U.S~
Patent 4,031,283 issued June 21, 1977.
~ To meet new stringent regulations, filters : Z5 which are more efflcient, have a low level o~
leakage o~ particles, are cleanable and are resis-~a~t to blinding are required. ~owever, poro~ity of the filters must still be hlgh to maL~tain : the air pressure drop across the filter at a very ~a low level. Also, the air/fiLter cloth, i.e., the ratio of volume of air which passes through an area of.the ~ilter, must be a~ a high level.
~9~
A fél~-like layered composite has the 35 ~ollowing ~tructure:

"". `'~' .

1 :~'7~)026 a. a first layer of a predominantly horizontal coplanar superlmposed layers of filamentary polytetrafluoro-~ ethylene components interrupted ~y occasional interlayered orientation;
b. a thin high density nonwov~n layer of fine gla3s staple fibers;
c. a scrim fabric of polytetrafluoro~
ethylene fibers and d. a s~cond predominantly horizontal coplanar superimposed laye.s of filamentary polytetrafluoroethylene compo~ents interrupted by occasional interlayer orientation;
15 whexein the layers are mechanically bo~ded together by filamentary polytetra~luoroethylene components extending vertically throughout the composite and a process for making such a composite also is a part of this in~ention.
~ BRIEF DESCRIPTION OF THE DR~WING
The Figure illustrates a cross section o the layered composite structure o:E this invention.
~ ESCRIPTION OF T~E IN~NTION
The Figure shows a cross section of the 25 layered composite structure of th:ls invention. A
filamentary PTFE layer 1 is in superimposed adherence to a thin high density nonwoven layer o fine glass skaple ~x~s (herei~lt~r re~erxed to as glass paper) 2 which in tur~ is in ~irm adherence to a P~EE scrim 30 cloth 3 to which is fixmly adhered a second layex -of ~ilamentary PTFE. The entire composite has been needled to irm1y mechanically adhere the layers together by filametary PTFE componen~s that vertically extend through the composite.
The production of felt-like products ls I ~ 7~026 taugh~ in ~he aforemerl~ioned l.au~erbach U . S ~ Patent

2,910,763~
Lauter~ach di~closes that PTFE filamen~ary materials can be used ~or making feltsO
5 ~~ One process for producing the fel~-like composi~e use~ the following s~epsO
(1) a loose batt o~ fila~aelltary PTFE in super~npo~ed layers is formed on a gla~s pap~r which is placed over a PTFE scrim fa~ric;
(2) the ~tructure formed in step (1) is mecha~ic~lly needled to bond th paper, the batt and ~he scrim fa~ric in~o a fel~ like the stnlc~ure, wherein filamentary PTFE component~ extend ver~ically throughou~ the struc~ure~;
t3 ) a second filamentary batt of super~
. i~posed layers is formed on the scrim fa}3ric side o~ the structure of step (2~;
(4) mechanieally ne~dling ~he bat~ to bond ~he batt ~o the struc~ure of st~p (2) and thQreby forI;Iing a felt-like co}nposite struc~ure, wherein - filamentary PTFE components ex~end ~rertically throughou t th~ struc:~ure;
~5) heat setting th~ felt~lik~ compo5ite structur~ orxned in step ( 4 ) by exposing the compo~ite s~ctur~ t~ a temperature of 230 ~o 315C usually for about 4 to 10 minutes.
r~he f ilamer~tary PTFE used herei~ c~n be any commercially a~railable PITE filamen~ in the full r~ge o d~ier. To obtai~s a product which is the mos~
3S coTmnercially accep~able, a denier ra~ge o 2-10 i~;

.,...~,.,,.~............................................

~ ~ 7~02~

preferred, a range of 5-9 is more preferred, and a range of 6-7 is even more pre~rred.
PTFE filaments are produced by various -- methods, including slitting PTFE film into thin . . , structures and then expandin~ and orien~ing these structures as shown in Gore U.S. Patent 3,664,915, , issued May 23, 1972; or by blendins viscous with a PTFE dispersion, and then extruding the filament and removing the viscous. This extruded filament is dark brown; however, it can be bleached, if desired by ~arious techniques, for example, ~assing it through a nitric acid bath or baking it at high temperatures. The extruded PTFE fil~ment is preferred for use in the invention.
lS The PTFE filaments are cut into a staple length of a~out 1 to 6 inches preferably about 4.5 inches before being formed into a batt in the abo~e process.
The glass paper used herein has a very ~ high level of air permeability, ls of fine glass p fiber of about 0.2-l.0 inches in length and is available from commercial sources. The paper is about 5--3~ mils in thickness~ The glass paper can contain up to 20~ by weight of fi.lamentary P~FE
components. The paper blocks the passage of very fine particles ~hrough the composite structure without causing an excessi~ve pressure drop and decrease in air porosity.
The PTFE scrim ~abric used ~erein can be any of the commercially available fabrics and usually is about S-15 mils in thickness. ~he fabric is usually con~tructed of about 200 to l,000 denier PTFE yarn.
As the Lauterbach reference discloses, there are many nuances allowable in processes for i 11 7~02~
forming felt; however, the last step of any process i5 compacting, i.e., retrac~ion or conde~sation.
Compacting may result from a simple reduction in length (i.e., shriveling) or from a distortion of the S filament in~o an irregular shape (i.e., crimping or curing) or both.
The composite is preferably compacted by placing a roll of uncompacted composite .structure onto a tenter frame and passing the composite structure through an oven. A tenter fram~ is a device commonly known by t~ose skilled in the art. The tenter frame allows the edges of the structure to be attached to it by various means, for example, to provide support to the ~elt during compacting, and to pull the structure through the oven.
The oven will have a means fox heating the composite structure. The means can be air having a temperature of 230-315C, preferably 245-275C, and most preferable, about 255~265C.
Preferably, air jets will be used to blow the hot air against both the upper and lower surfaces of the composite structure.
The composite structure will advance through the oven u~on the tenter frame at a rate above 20 yards per minute, preferably 25-40, and more pxefexably about 28-32 yards per minute.
The dwell time ~or each pass wi~hin the oven should be above 2 minutes, and preferably about 2.5 minutes.
The composite structure will have more than two changes of direction within the oven. The angle o~ the change of direction can vary from abaut 45 to about 240, pre~erably, 135 235, more preferably 160-200~. It i5 preferable to have at least 6 0 2 ~
changes of direction. Rollers can be used to change the direc~ion of the felt. Preferred rollers have a diameter greater ~han about 12 inches more preferabl~, between about 16-~0 inches. The composit~ structure preferably will undergo more than one pass in the oven.
The resulting composite structure is about 40-140 mils thick. The composite filamentary PTF2 layers are each about 15~30 mils thick, the scrtm fabric is about 5-15 mils thick and the glass paper is about 5-30 mils thick.
Another particularly useful composite str~cture ha~ a second glass paper positioned between the PTFE scrim fabric and the second layer of filamentary PTFE. In step (4) of t~e aforementioned process, this second glass paper is also needled into the composite structure.
The composite structure of the present invention, which can be preparecl as suggested above, when compared with the currently commercially avail-able felts, has as high a filtration efficiency while having a high porosity and a siynificant decrease in absolute leakage.
The porosity of the composite structure is determined by t~e Standard Method of Test for Air Permeabilit~ of Textile Fabrics, AS~M-D-737-69, also known as the Frazier Air Porosity Test.
Air porosity or air permeability is the rate of the air flow through a material under a differential pressure between the two fabric surfaces.
Air porosity is expressed in U . S . customary units ~s cubic feet o~ air per minute per square foot (CF~) of fabric at a stated pressur~ dirferential ~etween the two surfaces of the fabric.

3~

7 1 11 70~6 A filter made from the compesite structure of this invention has an efriciency greater than 98~, preferably 99, and even more preferably 99.75.
The composite structures have high ef~ici~
while having an air porosity, measured at 0.5-inch w~ter gauge (W.G.), of greater than 10 cubic feet ~er minute (CFM) and preferablY qreater ~h~.n 25 ~F~..
The following example illustrates the in~ention. All parts and percentages are on a weight basis unless otherwise indicated.
~ EXAMPLE
Ex~rud~d PTFE filaments having 6.67 denier are skeined and cut into 4.S-inch staple~ The staple is garnetted to comb and orient the staple.
The combed staple is deposited and cross-lapped on~o a ~hin high densi~y nonwoven layer of fine glass .staple fibers hereinafter referred to as a glass paper. The glass paper is placed onto a PTFE scr~n f~bxic. A scrim fabric is used-to 2~ psovide additional suppor~ to ~he composite structure.
The glass paper is about 15 mils thick and is formed o about` 0.3-de~ier ine (~5 mic~on-d~E~er) g ~ ~ staple ~ibers a~out 1/4 inches in len~h and con~ains about 4% by weight of a polyvi~yl - - ~
alcohol bi~der which is driven off during the sub=
se~ue~t he~ g s~ep.
The scrim-is a--PTFE fabric weighin~-0-,~
pounds/ya~d length/77 inches wide. The-one-slde coated sc~im is~lightly nee-d-led t~ facilitate-ha~dling.
The result~ng s~ruc~ure is th~n p~sed- -~hrough a tac~r loom wit-h-regu-lar barbed nee~-es-.to punch PTFE staples into and through- the batt, paper and scrim fabri~ in a ~irection substan~ia~ - per-pendicular to the structure 3 5 The one ~ide coated structure is -then .

8 ~

turned o~er and a second glass paper is placed in contact with the scrim fabriG and combed staple is deposited and crosslapped onto the second paper to form a ~att.
The resulting structure is again passed S t~rough the tacker loom and ~hen the structure is passed through a needle loom and needled about 1,500 tim~s per square inch. ~,-.e ~site structure is compacked by heat shrinkage to provide further str~ngth a~d higher density and to increase its heat stability.
The composite structure is compacted in a Kenyon Drye~ sold ~y Kenyon Company. First, the composite is placed onto the tenter ~rame of the Kenyon Dryer. Then the composite structure while on lS the tenter rame, ~ravels through the oven of the Kenyon ~ryer. The distance tra~eled on the tenter frame within ~he oven is 16 yards. The composite structure tr~v~ls at a speed of 30 yards per minute and has a dwell ~i~e within of the o~en of 2-1/2 ~0 minu~es.
While in the oven, the composite structure changes direction seven tLmes by goi~g roun~ seven rollers, ~ach having an 18-inch diameter. The an~le of ~he change of direction i 180.
The oven has hot air nozzles which blow hot ; air ~at 260~C) directly onto ~he upper a~d lower surfaces o~ the felt. The nozzles are holes placed along the le~gth of tubes and positioned to allow aix passing through the holes to blow directly onto the felt. The roll of co~posite structure is passed through th~ o~en ~wice.
~ he following is ~ summary of phy~ical proper~ie~ of co~posite s~ructures:
* Denotes trade mark ~ ~ 7~2~
Weight - 29 (Oz. pex Square Yard) Frazier Air Porosi~y ~ 13 CFM per square foot at 0 . S" WoG~

E~ficiency - 99 . 93~
Ab~olute Leakage - 0 . 002 grains/~ubic feet per minute of air l'o determine % e f :Eiciency a g~a~rimet~ic 10 method is used. In this test, known amount.s of test fly ash are f2d con~inuously a a reasorlably controlled .
ra~e usia~g a mechanical eed devlce ~o feed ~ly ~sh to an air stream. ~fficierlcy is de~ermined by positioning ~ pre-weighed "absolu~e" filter (pore 15 si2~ - O . 8 mi~roz~) downstream of the test specimen.
~he weight gain~d by the "absolu~e" filter after the test îs corapleted coa2s~itutes ~he ar~ount of dust tha~ has penetra~ad through the ~est specimen~ `
- To de~rmine absolute~ leakage, an ai~
~- ~0 stréa~:at 1~5C containing 2.5 grains of dust per cubic ~: foot :o air is-~ontinuously passecl throu~h~a s~ple~ôf the composite ~truc~ur~ over an 8 hours pèri~d~ usi~g , and air ~ro~ ume to sample ratio of 7 . 5 cubic fe~t of air ~?er } s~uare-foot of-~llter-me~ia. -Abso!u~e . .
- 25 leakage is the~quantitative amoun~: ~f-dust ~at~passés ~P~ thro~gh khe s~p~and~is-expresse3d'a~ gralns~për cubic f oot p~r `~iinù~ o~ air . ~ ~ - ~ ~ - ~ ~ - ~ ~ ~

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3 ~ -- -- -- - ---- . .
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Claims

1. A felt-like layered composite comprising the following:
a. a first layer of a predominantly horizontal coplanar superimposed layers of filamentary polytetrafluoro-ethylene components interrupted by occasional interlayered orientation;
b. a thin high density nonwoven layer comprising fine glass staple fibers;
c. a scrim fabric of polytetrafluoro-ethylene fibers and d. a second predominantly horizontal coplanar superimposed layers of filamentary polytetrafluoroethylene components interrupted by occasional interlayer orientation;
wherein the layers are mechanically bonded together by filamentary polytetrafluoroethylene components extending vertically throughout the composite.

2. The felt-like composite of claim 1 having a thickness of 40-140 mils.

3. The-felt-like composite of claim 2 having a filter efficiency of greater than 98%.

4. The felt-like composite of claim 3 having an air porosity measured at 0.5 mils W.G. of greater than 10 cubic feet per minute.

5. The felt-like composite of claim 4 in which the filamentary polytetrafluoroethylene components have a denier of about 2-10.

6. The felt-like composite of claim 4 in which the glass-staple fibers have a denier of about 0.1-1.

7. The felt-like composite of claim 4 in which the filamentary polytetrafluoroethylene layers are each about 15-30 mils in thickness;
the thin high density nonwoven layer of glass staple is about 5-30 mils thick and the scrim fabric is about 5-15 mils thick.

8. The felt-like composite of claim 4 in which a second layer of the thin high density nonwoven layer of fine staple glass fibers is between the scrim fabric and the second filamentary polytetrafluoroethylene layer.

9. The felt-like composite of claim 4 in which the thin high density nonwoven layer of staple glass fibers contains up to 20% by weight of filamentary polytetrafluoroethylene components.

10. The felt-like composite of claim 8 in which the thin high density nonwoven layer of staple glass fibers contains up to 20% by weight of filamentary polytetrafluoroethylene components.

11. A bag filter for a coal fired furnance to remove fly ash from the gaseous combustion products comprising the felt-like layered composite of any one of claim 8, claim 9 and claim 10.

12. A process for producing the felt-like composite of claim 1 which comprises the following steps:
a. forming filamentary polytetra-fluoroethylene in superimposed layers into a loose batt on the thin high density nonwoven layer of fine glass staple fibers and a polytetrafluoroethylene scrim fabric;
b. mechanically needling the batt to mechanically bond the batt to the paper and the scrim fabric and form a felt-like structure.

c. forming a second batt of filamentary polytetrafluoroethylene of super imposed layers of the scrim fabric side of the structure formed in step (b);
d. mechanically needling the batt to mechanically bond the batt to the structure formed in step (b) and thereby forming a felt-like composite structure;
e. heat setting the felt-like composite structure formed in step (d) by exposing the composite to 230-315°C.

13. The process of claim 12 in which a second layer of the thin high density nonwoven layer of glass staple fibers is positioned between the scrim fabric and the second batt of filamentary polytetra-fluoroethylene.