Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2160026
LIGHT COLORED CONDUCTIVE
SF~LANT MATERIAI ANt) METHOD OF PRODUCING SAME
BACKGROUND OF THE INVENTION
1. Field ofthe Invention
The present invention relates to thermally conductive fibers used in a
variety of applications and especially as packings and seals.
2. Desc. ;~lion of Related Art
A packing is a sealing material used to minimize leakage between two
o components of a fluid container, and especially in containers where the
components undergo motion relative to each other, such as in a pump. A
packing used to seal between components moving relative to one another is
commonly referred to as a "dynamic seal," as opposed to a Ustatic seal," which
is an absolutely fluid tight stationary seal such as that formed by a gasket in a
stationary joint. A good packing material should have a number of properties,
including: fitting correctly in the packing space, being able to withstand inherent
temperature and pressure conditions, being negligibly affected by the fluid
being sealed, and being sufficiently flexible to accommodate varying degrees of
longitudinal and/or radial displacement.
Common packings comprise fibers which are first woven, twisted,
braided or otherwise joined together, and then formed into appropriate shapes
(e.g. coils, spirals, or rings) for insertion around a shaft or other component.For packings of high speed pumps and similar devices, the packing
material should permit the escape of small amounts of liquid to help reduce
2~ friction and heat build-up between the components. Ideally in such
environments, the packing should also have a relatively high thermal
conductivity to assist in dissipating frictional heat generated by the movement
of the component parts.
In order to achieve most of these properties, it is common today to
employ packing and sealant nlaterial made from polytetrafluoroethylene (PTFE)
coated or impregnated with graphite or similar material. The chemical and
biological inertness of this material combined with its exceptional lubricity
makes it a highty desirable packing material, particularly in chemical, food,
drug, and pulp and paper industries.
Regrettably, many of these materials suffer from one or more
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WO 94/26960 ~ PCTIUS94/04911
~ 26002~ 2
major defic~encies. First in those mater~als employing a simple
graphite coating there is a tendency to shed off graphite
particles during use--resulting in significant amounts of dark
contamination around the pumps and often in the chemtcal stream.
Second in those materials which do not employ a coating PTFE
alone is a thermal insulator which tends to be inadequate in
dissipating heat. Both of these deficiencies were significantly
improved by the fiber and process disclosed in United States Patent
4 256 806 issued March 17 1981 to Snyder.
United States Patent 4 256 806 teaches a process for producing
a smudge-free graphite-impregnated expanded PTFE packing material.
In this process a fine powder dispersion of PTFE ts combined with
a liquid lubrtcant and graphite and mixed with suffic~ent shearing
force to form a thermally conductive eYp~nded PTFE material which
is resistant to shedding graphite. Such a mater~al is now
commercially available under the trademark GFO flber from ~. L.
Gore ~ Associates Inc. of Elkton MD.
Although the fiber of United States Patent 4 256 806 is now
the preferred pack~ng material for many applications a packing
which incorporates a dark colored material has caused concern in
some industries. For example for use in the handling of paper
pulp or similar material which must re~ain extremely clean of any
dark particle conta~ination many manufacturers prefer to use a
light-colored packing material to avoid any risk of costly
contamination. Unfortunately none of the l~ght colored packing
material presently available provides suff~cient lubr~city and
thenmal conductivity to achieve the desired level of pump
protection.
A typical example of such light-colored material comprises a
fiber of eYp~ded PTFE dipped in an aqueous dispersion of
tetrafluoroethylene (TFE) and silicone oil. Such a standard grade
white pack~ng material ~s ava~lable from ~. L. Gore ~ Associates
Inc. under the trademark GORE-TEX (prelubricated) fiber. Although
this matertal 1s qu~te acceptable for ltght-colored applications
and provides very good lubric~ty its thermal conductivity is
considerably lower than the material taught in United States Patent
4 256 806.
Accordingly it is a primary purpose of the present invention
n _ ~ r ~ J_.~J _ ~~._~-r~ ~ - T I '
~ 21601~2~
.
to provide a fiber and m~thod for producing it which i~ light-cclored while being
suffi~iently therrnally conductive ~o assure A~e~ te cor"po~-nt pro~e~on
when used as 8 packing material.
It is a fu~her cbjec~ of the present inv~ntion to provide such a flber
s which can be en~ploye~ in a ~farie~y of ~pplic~tions w~ere shedding or any
particulatc ~natter is undesir~ble and ~ o~clin9 of dark pa~ticulate rnatter is
,n3~.cop~ ' e .
These and other pulposes ol' the prcscnl invention will ~ecome eviclent
frwn review of th~ followin~ sp~Gificdlion.
1~ .
SUMMARY OF THE INVENllON
The present inventi~n provides an innpro~ed cornposition and m~thod to
produce a l"at~ l suit~ble for use in pacl~ing and se31in~ which is both
lS themlally conducti~ and light-colored. Whiile contributin3 nec~ss~y lubricityand thermal p.~ cLior, for component parts, the fiber of the prosent in~ar,Lon
avoids risk of datk particulate conLa,ninaffon in light colored manufactured
products such as pap-r, food, ph~,l"4ccuticals, and ch-micals. Additionally,
in c~rtain embodiment~ ~e mat~rial of ~hQ pre~ent in~en~ion has proven to be
20 clc~ ically n~n-conductive, which mak-s in uniquely applicable to for use in
eiectrical insulation and as a non-corrosiYe packing ",dlcnal, such as in marineenvi,4nm4nts to reduce or ~liminat- ~aivanic c~l,u3;0n.
Th~ presQnt inventiGn employs a co" ~.n.,iivn of polyt trstluoroethylene
(PTFE~ and a light-colored therrnally conduc~iw filler material such as boron
~5 nitrid~ ortin powder. These co~ponenb a~e ~ *d to~o~l.cr, prefe~ly in
the presence of a mixing m~dium, to cause shearin~ and encnrs~ on of the
çonduc~iv- material within the PTFE. By subs~qucntly he~ting and expanding
the PTFE, a li~ilt-color~d, durable and slipp-ry fiber i~ j v~id~d w~th sufficient
ll ,c..,.al conduc~Yit~l~ to b~ suitable for ail but ~ mos~ e~Lc"-a l"eci ,~0 conditions. Al~hough the amount of parti~ulate sheddin~ is minimal wi~ this
c ~ ~ ~, the use of li~ht-colored ~hermaily ~ndu~ te m~w~idl assures thzt
li~ht-colored products will not b~ contaminated from oc~t~?n~l sheddin3 of
fii~-r or conductive filler.
AiM~Ni~ED SI~EEr
WO 94/26960 ~ PCT/US94/04911
A further embodiment of the present invention employs the
above described fiber or a fiber of expanded PTFE, preferably a
towed fiber, which is impregnated and/or coated with a dispersion
of tetrafluoroethylene, a light-colored thermally conductive
filler, and a lubricant. Mechanical working of the coated fiber
shears the dispersion and provides a light-colored thermally
conductive fiber.
The present invention can be applied in any suitable manner,
including as a twisted, braided or woven fiber, and shaped for
virtually any form of application, including as sheets, tubes,
rings, spirals, or coils.
DET~ILED DESCRlPTION OF THE INVENTION
The present invention provides an improved l~ght-colored fiber
which is thermally conductive and suitable for use in a variety of
applications, and particularly for use as a packing material to
assist in sealing around component parts to reduce or eliminate
fluid leakage.
In the first embodiment of the present invention, the fiber is
formed by mixing together a fine powder dispersion of
polytetrafluoroethylene (PTFE), a m1xing medium such as a mineral
spirits, and a light-colored thermally conductive flller such as
boron nitride. The component parts are combined in proportions as
described below and mixed in the following general manner.
First, the light-colored conductive filler and water are mixed
to form ~ slurry. A dispersion of fine powder PTFE ~s then added
to the slurry and vigorously agltated, preferably in the presence
of the mixing medium, until the mixture coagulates. Mixing is
complete once the coagulated solids precipitate to the bottom of
the container in the form of a coagulu~, leav1ng a substantially
clear effluent. The coagulum is then thoroughly dried, such as
through use of a convention oven or similar means, to remove the
water.
The dried coagulum formed in this process can then be formed
or worked in any suitable manner, including heated and expanded in
a process such as that disclosed in United States Patent 3,953,566,
2~60~2~ ..--. : :: .. -.::.. ::-.::~
issued April 27,1976, to Gore. Preferably, the coagulum is ram extruded into a
paste or tape. The tape can then be heated to approximately 120-177C t250-
350F) and stretched approximately 2 to 150 times its original dimensions to
form a tape of expanded PTFE (ePTFE). The tape can then be further treated
5 in a variety of manners, including being slit and formed into fibers, driven
through cutting elements to form a tow, etc.
This process can be performed with a broad range of beginning
proportions, such as of 2-75% by dry weight boron nitride filler, 15-85% by dry
weight PTFE, and 10-30% by weight mineral spirits. Through this process, a
10 tape is produced with a boron nitride content of 2-75% and a PTFE content of
2~-98%. The fiber of this composition is preferred for high pressure
applications and in processes which are sensitive to oil contamination.
PTFE fine powder dispersions are obtained by polymerization of
tetrafluoroethylene (TFE) in liquid water containing suitable dispersing agent.
5 The preferred dispersion for use in the pres~ent invention comprises 30% by
weight PTFE solids. Suitable material is available from ICI Americas, Inc. of
Wilmington, DE, under the trademark FLUON (AD-1).
The mixing medium may comprise any substance which can provide
sufficient lubricity in mixing or extruding processes to allow the PTFE dispersion
~o to be sheared. In addition to mineral spirits, other suitable lubricants include
water, silicone oil, kerosene, naptha, propylene, petroleum extractants~ and
other similar lubricants.
The boron nitride is preferably a fine powder. This material is available
from Advanced Ceramics of Cleveland, Ohio, under the trade designation HCP
2s grade. Although boron nitride is the preferred filler for use in the present
inventi.on, a number of other light~colored thermally conductive materials may
also be ~sed. Examples include tin, zinc oxide, calcium oxide, or glass fiber.
For some applications it is desirable to coat and/or impregnate the fiber
with a liquid lubricant to decrease further its coefficient of friction. It should be
,o appreciated that the term "coating" as usèd herein is intended to encompass
any application of the mixture of the present invention onto or into a
.
p~E~ED S~EE~
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WO 94/26960 216 0 0 2 6 PCT/US94/04911
^ 6
substrate, whether merely applied over the sur~ace of the substrate
or impregnated below the sùbstrate's surface. In its simplest
form, this coating process comprises dipping, spraying or otherwise
covering the fiber of the present invention with the lubricant.
The lubricant should have kinematic viscosity of about 50,000
centistokes or less, and preferabiy a kinematic viscosity of 1000
centistokes or less. Among the suitable lubricants are silicone
oil, mineral oil, paraffin wax, or petroleum based o~l.
Preferably, the lubricant comprises polydimethyl siloxane, such as
that sold by Dow Corning Corp. of Midland, HI, under the
designation DOW CORNING 200.
A preferred coating for use with the present invention is a
liquid or paste comprising PTPE dispersion, light-colored thermally
conductive filler, and the lubricant. Th~s coating can be fonmed
by combining 25-75% by weight light-colored thermally conductive
filler (e.g. boron nitride powder), 20-70X by weight lubricant
(e.g. polydimethyl sllica), and 25-80% by we~ght dispers10n of PTFE
(e.g. 60% sollds dispersion available from E.I. duPont de Nemours
and Co., of ~llmington, DE, under the trademark TEFLON). The
components are blended thoroughly to form a relat~vely uniform
mixture.
Once the m~xture is formed, the coating can be applied to a
variety of substrates to produce a thermally conductive fjber.
Among the substrates which can be employed are PTFE, exp~ded PTFE,
PTFE composites (e.g. plated, filled, or ~mpregnated PTFE),
fiberglass, polyi~ides fibers, acryl ks, etc. Preferably, the
coating ls combined with the l~ght-colored ther~ally conductive
tape previously described. This may be accomplished through any
su~table means, including by dip coating the fibers or tape in the
coating, intermix~ng the coating between f~bers, merg~ng the tape
and the coating, or coextruding coating and substrate.
Another embodiment of the present invention employs a
substrate of towed expanded PTFE fiber (either filled in accordance
w~th the present invention or unfilled) which is dipped or
~5 otherwise coated with a composit~on of fluoropolymer (e.g. PTFE or
tetrafluoroethylene (TFE)) mixed with the llght-colored thermally
conductive f~ller. By mechanically worklng the coated substrate,
the composit~on can be sheared on and in the substrate to impart
~ W O 94/26960 21 6 0 0 2 ~ PCTAUS94/04911
suitable thermally conductive properties.
Since the terms PTFE and TFE are sometimes used
interchangeably, especially when referring to original supplies of
PTFE which contain very short chain homopolymers of TFE, except as
is specifically addressed herein, the term PTFE is intended to
encompass any polymer of TFE regardless of length.
Mechanical working of the substrate may take any appropriate
form, such as sliding it across one or more fixed surfaces,
rotating it around one or more spool surfaces, driving it between
nip rollers, or driving it through ~counter-current~ rollers
actuated in the opposite direction from the direction of material
movement. Alternatively, mechanical working of the composite fiber
can be avoided or limited by shearing the coating prior to
application to the substrate and applying the coating before its
water base evaporates.
In all forms of the present invention, the combined
proportions of components in the substrate and the coating in the
final product should generally comprise: 25-75X by weight light-
colored thermally conductive filler (wtthin a broad range of lO-
95X); 10-4~% by weight lubricant (within a broad range of 10-60X);
and 45-85X by weight ~Ypanded PTFE and/or TFE (within a broad range
of 5-90%). Ideally, the composition comprises 30-60% boron
nitride, 20-30X lubricant, and 50-70X PTFE and/or TFE.
The composit~on of the present invention can be used ~n a
variety of applications. For example, the material can be for~ed
as fibers, sheets, tubes, beading, tapes, etc. As flbers, the
composition can be formed into packing material through any known
manner, such as braids, wovens, composites, twisted ropes, etc.
Preferably, the material is calendered or otherwise formed to
achieve its operative shape.
These fibers are particularly useful as pump packings, valve
stem packings, and similar p,oducts when braided 1nto a square or
round cross section. The combinat~on of the material properties
and the dimensions will form dynamic and static liquid seal inside
a pump stuffing box and valve body. This material can be readily
braided into square sizes ranging fro~ 0.125 lnches (0.32 cm) to 3
inches (7.6 cm). The material can also be used as a filler fiber
w~thin a braid or a jacket fiber over core material.
W094/26960 2~60~2 b 8 PCT/US94/04911
Other possible appl ioations for this material ~nclude as
sealing devices (e.g. as gaskets sealing an opening), heat sinks,
electrical insulation, etc.
The fibers of the present invention have numerous advantages
S over existing packing materials. First, when produced in the
manner described, the thermally conductive filler tends to be
encapsulated within the PTFE and is resistant to shedding or
Hsmudging" off the completed fiber. This resistance to smudging is
further improved by processes such as braiding. Second, even upon
occasional shedding of filler or fiber, the light-colored nature of
the fiber of the present invention assures that no visual
contamination of the chemical stream will occur. This makes the
fiber far more acceptable to customers for use in color sensitive
environments such as pulp and paper production, some food and
pharmaceutical applications, marine applications, light-colored
chemical production, etc. Third, as should be evident from review
of the following examples, unlike presently available light-colored
packing materials, these fibers have demonstrated very good
operational properties, such as lubriclty, thermal conductivity,
and coefficients of thermal expans~on.
One of the more surprlsing and promislng aspects of the
present invention ~s that it produces a f~ber wh kh is highly non-
galvanic and anti-corrosive. Unlike most exist~ng thermally
conducttve f~bers which are also electricilly conductlve, such as
graphite filled f~bers, the f~bers of the present ~nvent~on are
both thermally conductive and electrically non-conductive. As a
resu1t, the fibers of the present tnvent~on can be freely placed in
d~rect contact with metals subject to ox~dat~on (e.g. ~ron or
steel), even in the presence of a corros~ve ~ed~a llke seawater,
without risk of establ~shing a corrosive g~lvanic cell.
Accordingly, the fiber of the present invention is part~cularly
useful as a packing or seal~ng med~a in d~ff~cult corrosive
env~ronments, such as in submerged mar~ne appl~cations.
Without intending to lim~t the scope of the present invention,
the composltion and method of the present invention may be better
understood in ligbt of the following examples
.
21~002 ~: : :-- .. .. .. .. ... ~
EXAMPLE 1 ~
'~`
A slurry of 4.38 Kg of boron nitride and 55.0 1 of de ionized H2O was
prepared in a 115 I baffled stainless steel container. The boron nitride was .-s grade HCP obtained from Advanced Ceramics, Inc. While the slurry was
agitating, 4.32 Kg. of PTFE in the form of a 15.7% dispersion was rapidly
poured into the vessel. The PTFE dispersion was an aqueous dispersion
obtained from ICI Americas Inc. The mixture coagulated and after 2 minutes
the mixer was stopped. The solids settled to the bottom of the vessel and the
10 effluent was clear.
The coagulum was dried at 165C in a convection oven for 24 hours.
The material dried in small cracked cakes and was chilled to below O`C. The
chilled cake was hand ground using a tight circular motion with minimal
downward force through a 0.635 cm stainless steel screen, then 0.267 Kg of
lS mineral spirits per Kg of powder was addea. The mixture was chilled, again
passed through a 0.635 cm screen, tumbled for 10 minutes, then allowed to sit
at 18C for 24 hours.
A pellet was formed in a cylinder by pulling vacuum and pressing at
59.8 kgslsq cm (850 psi). The pellet was heated to 49C and then extruded
20 into tape form.
The tape was then calendered through heated rolls to 0.043 cm. The
lubricant was dried and the tape was stretched by running it across heated rollsat 275C maximum temperature, at a stretch rate of 5.9-1 ratio and 48.7
meters/min. output speed.
EXAMPLE 2
The composition from Example 1 was employed. The tape was
stretched across a heated plate at 385C, at a stretch rate of 3.4: 1 ratio and 34
;0 meters/min output speed. The tape was then stretched across heated plate at
365C at a ratio of 1.2:1 and 4i meters/min. output speed. The tape was
sintered across a heated plate at 405C at 42 meterslmin. The tape was then
slit into a fiber 1.5 inches (3.8 cm) wide and the f!ber was converted into a
AMENDE~ S~IEEl
WO 94/26960 2~6 Q ~ PCT/US94/04911
- 10
0.375 inch (o.9s cm) square brald. The braid 1ncorporated 196
fibers.
EXAMPLE 3
Braid was made as per Examples 1 and 2. The braid was dipped
into a 1000 centistoke silicone oil to form a lubrication coating
thereon.
EXAMPLE 4
Tape was made as per Examples 1 and 2. The tape was
calendered through heated rolls to 0.0008 ~nches (0.002 cm) thick.
The tape was then slit into a fiber 1.5 inches (3.8 cm) wide and
the fiber was converted into a 0.375 inch (0.95 cm) square braid.
The braid incorporated 108 fibers.
EXAMPLE 5
Tape was made as per Examples 1 and 2. The tape was
calendered through heated rolls to 0.0012 ~nches (0.003 cm) th~ck.
The tape was then slit into a fiber 1.5 ~nches (3.8 cm) wide. The
fiber was then convo ~ed 1nto a .0375 inch (O.9S c~) square braid.
The braid incorporated 108 fibers.
EXA~PLE 6
Tape was made as per Examples 1 and 2. Multiple pieces of
tape were merged w1th a thermally conductive llquid co~prised of
13.2% by weight of a 60% aqueous PTFE dlspersion, 29.~X by weight
of HCP grade boron nitride, 28% by wetght water, 27.5X by weight of
a 1000 centistoke silicone oil, 1.8X by weight on a non-ionic
surfactant, and 0.1% by weight of a 5X ammonium hydrox~de. The
compos~te was dried in an oven at 190C for 18 hours. The
composite was slit into a fiber approximately 0.6 inches (1.5 cm)
2 S
wide. The fiber was converted into a 0.375 inch (0.95 cm) square braid. The
braid incorporated 42 fibers
EXAMPLE/ THERMAL THEORETICAL
MATERIAL CONDUCTIVllY COEFFICIENT ~.=
(W/m-K) OF ;~
EXPANSION ~.
(ppm)
EXAMPLE 2 0.34 85
E>~AMPLE 3 0.48 85
EXAMPLE 4 0.51 85
EXAMPLE 5 0.53 85
EXAMPLE 6 0.90 57
GFO FIBER 1.47 55
100% PTFE fiber 0.24 159
G-2 (graphite
filled fiber) 0.85 83
Thermal conductivity was measured and calculated under the following
~o parameters. Thermal conductivity is the material property that determines theamount of heat that will flow through an object when a temperature difference
exits across the object. Thermal conductivity is a steady state property; it canonly be directly measured under conditions in which the temperature
distribution is not changing and all heat flows are steady. In the instant case,~s thermal conductivity was determined in a manner similar to that described in
ASTM (American Society for Testing and Materials) Standard Test Method E
1225 (Standard Test Method for Thermal Conductivity of Solids by Means of
the Guarded-Comparative -Longitudinal Heat Flow Technique).
Specifically, thermal conductivity was measured at a nominal
;o temperature of 200C using the guarded comparative longitudinal heat flow
technique. The samples were submitted as braided lengths of material. For
testing, the samples were cut into 5.1 cm (2 inches~ lengths; these were laid
side-by-side to produce test samples 5.1 cm square (2 inches square). No
thermocouples were placed in the test samples. Surface temperatures of the
3s test samples were obtained by extrapolation from thermocouples in the
reference samples. PYREX material type-7740 was used as the thermal
conductivity reference material.
The fundamental equation that governs steady-state heat flow in a slab
geometry is: -
NDED SHE~T
~ 21 6002~
Q = ~A x ~T x A)J~cwhere C~ ~ the r~te of heat flow ~ou~h ~ slab
(W or Btu/h~
= the ~ermal conductiYity of the slab material
(Wtrn K or BtLlh ft ~F~
~ T ~ ~e temperatura Jiff~ ~ncc across the slab
(C o~ F)
~ x = the ~iclcne~s (m or ft)
A = the c~oss sectionaJ ars~ 2 or f~2)
o Male,idls that hav- Iow valucs o~ thermal conductivi~ ~llow only a smail
amount of h~at flow and are calied thonnal insulators. ~aterials with larg~
Yalues o~ therm~l conductivi~ allow rnore he~t to flow across the slab w~ith thessme temperature diflerence. Thermal ccnductivity is a material proporty and
does not depend upo~ the ~co-"~t~ of the sample. In Qsn-ral, therrnal
1~ conductivity is a funcbon of the nean sample tempsratur .
A Ll .ær,- ,~1 heat f~ow circuit was us~d whTch is generally ana~o~ous to an
ele~ical circuit with rssi~b" - r s in series. The PYREX 7740 ma~rial was
chosen be~ause it has a therrnal conducti~ ose to that c~i"...ted for the
s~r, r 'e Ref~rence slandar~s ~also known ~s heat meters~ havin~ ~e same
~o cross-sec~onal dimensions as th~ sample were plac~d abov~ and below the
s~mple. An upper hoator, a l~wer heater, and a Eleat sink wor~ added to the
"sta~ f -;ple~ the he~t flow urGuit.
The t-mp~ratur~ ~rL~;enls (an~lo~ous to potenti~l differenc~s~ alon~ the
stack ~ere measured with typ~ K (L,~.r~".. I~alumet~ the~mocoupbs placsd at
2~ known separ~ffons. ~he tllermocouples were placad into hol~s or ~rooYes in
th~ r~lferenc~ mat~rial and also in the sample ~; .~nc~r the sample was thick
enou~h to a~.~,r"~dste thern~
The stack was clar-~ -d with a reproducible to~d to insura intimate
contact ~ a~ n the c~l"por,~nts. In or~er to produc~ ~ linear flou~ of heat
30 ~xially ffuou~h th~ stack and reduce the amount of he~ that flows radialiy, a~uard tube was pl~u~d a~ound th~ stack and ~e intervening space was filled
with insuiabn~ grainB of ve~ i - u'~t or zeolite. The ~ F . a~ radient in ~e
guar~ was ~ to that in the stack to ~educe radial heat flow further.
O'
~ND~D SHEET
2l 60026
WO 94/26960 PCT/US94/04911
1 3
The comparative method is a steady state method of measuring
thermal conductivity. When equilibrium was reached, the heat flux
(analogous to current flow) down the stack was determined from the
references. The heat into the sample is given by the following
formula:
Qin ~ ~top(dT/dX)top
and the heat out of the sample is given by:
Qout ~ ~bottom(dT/dX)bottom
where ~ ~ thermal conductivity
dT/dx ~ temperature gradient
and "top" refers to the upper reference while ~bottom~ refers to
the lower reference. If the heat were confined to flow just down
the stack, then Qin and Qout would be equal. If Qin and Qout are
in reasonable agreement, the average heat flow is calculated from:
Q ~ (Qin + Qout)/2-
The sample thermal conductivity is then found from
~sample - Q/(dT/dx)sample.
--Theoretical Coefficlent of Expansion was der1ved by determining
the coefficient of thermal expansion of each of the components from
product literature or through established sources. The overall
coefficient of thermal expansion was determined by multiplying each
of the individual component's coefficient of thermal expansions by
its weight percentage in the mixture and taking the sum of this
amount for the combined components in the ~ixture.
EXAMPLE 7
An expanded PTFE fiber, from W. L. Gore ~ Associates, Inc., of
Elkton, MD, was formed into a tow material by passing it through a
series of rotating cutting elements. The towed eYpanded PTFE fiber
was then dipped into an aqueous tetrafluoroethylene (TFE)
homopolymer dispersion including a doping of about lOX by weight
tin powder. The TFE dispersion comprised a 60% solution of TFE
solids suspended in deionized water. The dispersion was acquired
from ICI Americas, Inc., of Wilmington, DE, under the trademark
FLUON AD-l. The tin powder was acquired fro~ AEE of Bergenfield,
~ 2I6002~ ~
~14 , ~ r ~
-t -~
.
NJ, under the trade designation Powdered Tin (1-2 mm particle size). The TFE
dispersion was sheared in place on the fiber, encapsulating the tin particles inand on the towed fiber, by pulling the coated fiber across tNo 3.2 mm (1/8 inch)diameter stationary bars at a rate of 55 ft/min (16.8 m/min). The fiber was thenheated to 1 80C for about 45 seconds to drive off the water. The final product
fiber contained approximately 3-4% by weight tin. Thermal conductivity was
measured at 0.45 W/m-K.
Tin is thermally conductive and has a silvery tinge to it. It is a good
lubricant with excellent corrosion resistant qualities.
10
EXAMPLE 8
A towed expanded PTFE fiber similar to that employed in Example 7 was
dipped into an aqueous TFE homopolymer dispersion including a doping of
s 20% by weight boron nitride powder acquired from Aldrich Chemical Co., Inc.,
of Milwaukee, Wl. The dipped fiber was then mechanically worked by agitating
in the following manner of stirring the aqueous bath by hand with a 9.5 mm (3/8
inch) diameter stirring rod and then running the material through a series of two
3.2 mm (1/8 inch) diameter stationary ba!s at a rate of 30 ft/min (9.1 m/min) tocause the TFE to shear. The shearing of the TFE dispersion encapsulated the
boron nitride particles in and on the towed fiber. The fiber was heated to
1 80C for about 1 to 2 minutes to drive off the water. The dipped fiber
contained approximately 7% by weight boron nitride. This fiber was formed into
a braid. The braided material tested to have a thermal conductivity of 0.60
2s Wlm-K, considerably better than a conventional packing fiber.
EXAMPLE 9
A test of the non-corrosive properties of the present invention was
performed. Bars of 3/6 stainless steel measuring 1.3 cm x 12.7 cm x 6.4 cm
(1l2 x 5 x 1/4 inches) were fixed to one of a number of braided packing materialand placed in an aqueous solution of 5% NaCI heated to 95F and having a pH
of 6.5-7.2. After 35 days, each of the bars
~EI~ S~EET
WO 94/26g60 21 ~ O 0 2 S PCT/US94/04911
were removed, stripped of packing material and visually examined.
The following observations were made:
Type of Packinq Material Condition of the Bar
(1) Graphite Filled Fiber Some .discoloration
and pitting
(2) G-2 graphite filled fiber No discoloration,
little pitting
(3) Stainless steel alone Brown discoloration,
no pitting
(4) ePTFE coated with tri- Brown discoloration,
phosphate corrosion some pitting
inhibitor
(5) White PTFE fiber obtained Some brown
from Lensing of Austria discoloration, small
~amount of pitting
(6) Boron Nitride filled tape No discoloration,
made per Examples 1 and 2, no pitting
above
While particular embodiments of the present invention have
been illustrated and described herein, the present invention should
not be limited to such illustrat~ons and descriptions. It should
be apparent that changes and modifications may be incorporated and
embodied as part of the present invention within the scope of the
following claims.
