Note: Descriptions are shown in the official language in which they were submitted.
7~..5~7~
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TITLE
PRO~UCTS AND PROCESS
This invention relates to aqueous
dispersions of ~etrafluoroethylene polymers and to
coatings of such dispersions on glass fabric~
BACKGROUND OF THE INVENTION
Fabric made of glass fiber is useful in
filter applications to separate particles from a gas
stream. Commonly the glass fabric i5 coated with a
tetrafluoroethylene polymer coating, usually
polytetrafluoroethylene (PTFE), to increase the flex
life, i.e., resis~ance to breaking on repeated
flexing, of the fabric. Such a coating i5 especially
useful when the glass fabric is employed as a filter
bag to filter out particulate solid impurities such
as carbon black or fly ash which may be present in
flue gase~. Due to the passage of hot flue gases
through the glass fabric and due to back flushing or
pulse fl~shing during cleaning, the fabric is
subjec~ed to flexural stresses which weaken the glass
fibers of the fabric and ultimately cause breakage.
While the tetrafluoroethylen~ polymer
coating increases ~he flex life of the glass fabric,
resistance of the coated glass fabric to attack by
acids present in hot flue gases could be improved.
~any hot 1ue gases contain oxides of sulfur and
~ater vapor which combine to form an acidic
environment. Glass fabric coated with
tetrafluoroethylene poly~er coatings heretofore were
subject to attack by acids in ~he flue gas which
weakened the fa~ric and shortened the fabric's flex
life.
AD 503~ 35
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It would be desirable to have a
tetrafluoroethylene polymer coating composition which
imparts greater resistance to attack by acids than
tetrafluoroethylene polymer coating compositions
heretofore available.
SUMMARY OF THE INVENTION
It has now been discovered that
tetrafluoroethylene polymer coating dispexsions which
impart resistance to attack by acids to glass fabric
coated with the dispersion can be obtained by
employing in the coa~ing dispersion, a water-soluble,
polyhydrolyzable silane, a fluorinated acrylate
water-repellant additive, and a siloxane.
Specifically, the composition of this
invention is an aqueous dispersion consisting
essentially of
a) water,
b) 5-65% by weight tetrafluoroethylene
polymer, said weight based on weight of water and
polymer, said polymer being of film-forming molecular
weight,
c) 2-60% by weight of a polyhydrolyzable
silane of the formula
R~Si~OR)3
wherein R is lower alkyl, preferably alkyl of 1-3
carbon atoms, and Rl is phenyl or substituted lower
alkyl, pre~erably of 1-3 carbon atoms, in which the
substituents are selected from halogen, quaternary
ammonium, or -NR'R" in which Rl and R" are each ~,
lower alkyl, lower alkoxyalkyl, amino lower alkyl,
hydroxyl lower alkyl, or substituted (amino lower
alkyl), said weight based on weight of
tetrafluoroethylene polymer,
~'7
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d) 1-20% by weight of a hydrocarbyl
siloxane, said weight based on weight of
tetrafluoroethylene polymer,
e) 1-20% by weight of a polymer of a
fluorinated ester of an acrylic acid, said weight
based on weight of tetrafluoroethylene polymer.
DESCRIPTION OF THE INVENTION
The tetrafluoroethylene polymer in the
compositions of this invention can consist of the
homopolymer, polytetrafluoroethylene, or can be a
copolymer of tetrafluoroethylene with a minor
proportion, e.g. up to 35% by weight based on weight
of copolymer, of another copolymerizable
ethylenically unsaturated monomer. For example, the
homopolymer can include small amounts of comonomer
modifier, wherein the homopolymer still retains its
non-melt fabricable character, such as up to two
percent by weight of polymer units derived by
copolymerization with tetrafluoroethylene of
perfluoroalkyl or oxyperfluoroalkyl trifluoroethylene
of 3 to 10 carbon atoms, and preferably
hexafluoropropylene, as disclosed in U.S. Patent No.
3,142,665 to Cardinal, Edens, and Van Dyk or
perfluoro(alkyl vinyl ether). Larger amounts of
these comonomers or other comonomers render the
resultant copolymer melt fabricable. Examples of
such copolymers include copolymers of
tetrafluoroethylene with such monomers as
hexafluoropropylene, as disclosed in U.S. Patent No.
3,946,763 to Bro and Sandt, higher perfluoroalkenes
such as those containing from 4 to 10 carbon atoms,
perfluoro(alkyl vinyl ethers) such as perfluoroethyl
or perfluoropropyl vinyl ether, disclosed in U.S.
Patent No. 3,132,123 to Harris and McCane,
7i ~7 ~
perfluoro-(2-methylene-4-methyl-1,3-dioxolaneJ
disclosed in U.S. Patent No. 3,308,107 to Selman and
Squire, and the highly fluorinated monomers in which
a single hydrogen is present which does not change
the fluorocarbon character of the copolymer, such
monomers being for example 2-hydroperfluoroalkene
containing 1 to 3 carbon atoms such as
2-hydropentafluoropropene, the omega
hydroperfluoroalkenes containing from 3 to 10 carbon
atoms, and the omega-hydroperfluoro(alkyl vinyl
ethers) in which the alkyl group contains from 1 to 5
carbon atoms. The tetraEluoroethylene polymers can
be of the non-melt-fabricable type, having an
extremely high molecular weight, as evidenced by a
specific melt viscosity of 1 X 109 poise or more
measured at 380C at a shear stress of 6.5 psi or can
be of the melt fabricable type having a melt
viscosity of from 1 X 103 to 1 X 106 poise under
the same conditions. Preferably, the polymer is
polytetrafluoroethylene.
The polymers are preferably prepared by the
aqueous dispersion method for preparing
tetrafluoroethylene polymers in which sufficient
ionic dispersing agent is present in the
polymerization mixture to maintain the polymer
particles in dispersion. The tetrafluoroethylene
polymer dispersion can be used as prepared or
redispersed in water using a suitable dispersing
agent. A suitable dispersing agent in an amount of
up to about 6% based on polymer may be present.
The water-soluble, polyhydrolyzable silane
is preferably one in which Rl is aminoalkyl or
substituted amino alkyl. Representative of these
S~;~
amino alkyl groups are -CH2-CH~CH2NH2,
-CH2- (CH2) 2N~1 (CH2) 2NH2'
-CH2-(CH2~3N(CH3)2, and the like. The term
lower alkyl means alkyl of 1-6 carbon atom5, as used
herein. Representa~ive silanes include y-aminopropyl
triethoxy silane; (N,N-dimethyl-3-amino propyl
trimethoxy silane; N-trimethoxy silylpropyl-N,N,N-
trimethyl ammonium chloride; 3(N-styryl
methyl-2-aminoethyl) aminopropyl tr imethoxy-silane
hydrochloride; and the like. Preferably the silane
will be present in an amount between 3-12%.
The siloxane is preferably one of the formula
R'
R3 ~ Si-O ~ SiR3
wherein R' and R" are each independently a
hydrocarbyl group of 1-20 carbon atoms and one of R'
and R" can be hydrogen, n is an integer of between
about 5 and 5000, preferably between 10 and 2000 and
most preferably between 10 and 100 and R is lower
~i.e., 1-4 carbon atoms~ alkyl or phenyl, and wherein
the polysiloxane can be a homopolymer of a copolymer
with another polysiloxane having different
hydrocarbyl R' and R" substituents. More preferably
R' and R" are each independently alkyl of 1-10 carbon
atoms, aryl of 6-10 carbon atoms, alkaryl of 7-11
carbon atoms, or aralkyl of 7-11 carbon atoms.
Preferably the siloxane is present in an amount of
between 3-12~
The polymers of a fluorinated estex of
acrylic acid used herein are water-repellant
additives and are commonly referred to as fluorinated
acrylate polymers (including methacrylate). Such
~ ~'7~7 ~
polymers include ZEPEL* fluorinated water-repellant
polymers, and the like. The fluorination of the
ester group is generally in the form of a
perfluoroalkyl yroup containing from ~ to L2 carbon
atoms. The polymers can be homopolymer or
copolymers, including segmented copolymers, with
other copolymerizable monomers, with the ester repeat
unit which supplies the water repellancy to the
polymer being generally represented by the formula
lQ - CH2 - C~ -
Lo
CO(CH2)sQ
wherein J is H or CH3, s is an integer of 1 to 12,
and Q is an organic group which contains a
perfluoroalkyl group of 3 to 12 carbon atoms.
Examples o~ monomers from which this repeat unit is
dervied by polymerization ~or copolymerization) are`
as follows:
2 C COOcH2cH2(cF2)2cF3t
C~2=cHcoocH2cH2(cF2)4cF3~
CH2=CHCOO~H2CEI2 (CF2) 8CF3 '
~2 CHCOO(CH2)11(CF2)7CF3,
C~2=C(CH3)COOCH2C~2N(CH3)S02(CF2)7CF3,
C~2=cHcoocH2cH2N)cH2cH2cH3)so2(cF2)
and
CH~=C(CH3)cooc~2cH2(cF2)5cF3.
These water-repellant additives are soluble in some
organic solvents and are generally available as an
aqueous dispersion which can conveniently be added to
the PTFE aqueous dispersion to form a co-dispersion.
Pre~erably the fluorinated acrylate polymer will be
present in an amount of between 3-12%.
* denotes trade mark
a.~7~
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To prepare the coating dispersions of this
invention, the silane, siloxane and fluorinated
acrylate polymer are generally added to an aqueous
dispersion of the tetrafluoroethylene polymer.
Neither temperature nor pressure are critical during
the preparation.
The glass fabric to which coating
dispersions of the present invention are applied can
be made of any glass such as soda-lime-silica,
aluminosilicate or borosilicate, but will usually be
the glass from which commercially available glass
yarn is made. Typically, the glass fabric will have
a sizin~, such as starch, on the surface thereof.
Preferably, however, the glass fabric can be cleaned
of sizing such as by conventional heating procedures
prior to coating, such as passing a web of glass
fabric through an oven heated at about 700C in order
to burn off the sizins, or batch heating in an oven.
To coat the glass fabric, the coating
dispersion is adjusted to a solids content of 5-30
by weight, if necessary, and the fabric is
conveniently dipped into the dispersion and then
excess liquid is removed by passing through rollers
or blades. Alternatively the dispersion can be
sprayed on one or both sides of the fabric. The
coated fabric is then heated at between lOO and 340C
to cure and dry the coating The amount of coating
on the cured and dried fabric can be between 3 and
20~ by total weight and preferably is between 6 and
15%.
If desired, the coated glass fabric can be
topcoated with a dispersion of the water-repellant
fluorinated acrylate polymer.
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~ :ll 7 ~ 5~7~
EXAMPLES
"PTFE dispersion" means an aqueous
dispersion of polytetrafluoroethylene containing
nominally 60~ solids and stabilized with octaphenoxy
polyethylene oxide dispersing agent having an average
of 10 ethylene oxide units and neutralized with
ammonium hydroxide J
"Silane" means an y-aminopropyl triethoxy-
silane (H2NCH2CH2CH2Si~OCH2CH3]3)-
"Siloxane'l means 35~ methyl phenyl siloxane
polymer emulsion.
"Water Repellant" means fluorinated acrylate
copolymer dispersion.
"Dispersion A" means a copolymer of
tetrafluoroethylene/hexafluoropropylene (89.5/10.5)
containing 55% solids and stabilized with octaphenoxy
polyethylene oxide dispersing agent having an average
of 10 ethylene oxide units and neutralized with
ammonium hydroxide.
"Dispersion B" means a copolymer of
tetrafluoroethylene/perfluoro propyl vinyl ether
(97/3) containing 55% solids and stabilized with
octaphenoxy polyethylene oxide dispersing agent
having an average of 10 ethylene oxide units and
neutralized with ammonium hydroxide.
The coating dispersions were prepared byadding water to the PTFE dispersion, and then adding
the other ingredients. Amounts of ingredients of the
coating dispersion are listed in each Example, as are
% pickup of dried coating on glass fabric.
Six inch x six inch pieces of glass fabric
were immersed in the coating dispersion and wrung
partially dry through rollers. The fabric was then
cured at 250C for ten minutes, unless otherwise
- 9
noted. The percent pick up was determined by
weighing the fabric after heat cleaning and again
after curing and drying.
Samples were tested for flex life in the
5 warp direction using an MIT flex tester Model 2
(Tinius Olson Co).Strips 1/2-inch wide were used
and fabric tension was supplied by a four or five
pound weight, Samples were conditioned prior to
testing as shown below. Five or six strips were
tested at each condition listed as follows unless
otherwise noted:
Condition
.
As made - samples were conditioned 24 hours at 72F
and 50% RH for at least 24 hours prior to
testing.
Acid treated - samples were heated four hours at 450F
or 500F in an air circulating oven (unless
they had been subjected to prior longer term
heating). The samples were then immersed
for five minutes in l.ON sulfuric acid
maintained at 80C. The samples were
removed from the acid and dried five minutes
at 450F. The acid immersion was repeated
for a total of 4 times. The fabric was then
heated one hour at 450~F or 500F and allowed
to remain at 72F, 50~ RH prior to testing.
EXAMPLE 1
Si~ inch x six inch pieces of B~RLINGTON 484*
fabric were heat cleaned by placing them in an air
circuiating oven at 350~C for 10 minutes. The
samples were dipped in the coating dispersions shown
below, wrung dry and cured and dried 10 minutes at
250C. Six fabric samples were coated with each
*denotes trade mark
,l,~
~t~ 7~
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formulation. Three samples were tested a$ made an,d
three after acid treatment. Six 1/2" stxips we~e
tested from each piece giving a total of 18 strips
per composition per conditioning method, using a fiYe
pound weight.
Coating Dispersion (gm)~ WarP
Flex Life
For-
mu- ~ater Aci,d
la- PTFE Sil- $il~ Repel- Pick- As Trea~ted
tion Disp. H2O ane oxa'ne lant Up ~ade (450F)
1 100 289 3.6 3~6 3~6 10.7 10,000 4616
A 100 293 3.6 3.6 - 10.5 6,817 930
B 100 293 3.6 - 3.6 10.6 11,795 1678
The results of these tests were analyzed
using the Student's t test to establish confidence
leads on the relati~e ranking~ The results showed
that the composition containing Silane, Si,loxane and
Water Repellant had signi,ficantly better warp flex
life after acid treatment than the other two compo-
sitions. The confidence le,vel is greater than 99%.
EXAMPLE 2
The same procedure and fabric of Example 1
were used. Coating composition formulations and
results are shown in the following tables.
For- Compo'sit'ion~
mu- Water
la- Repel~ %
tion ~Fp, H2O Silane lant Siloxane Pick-Up
A90 210 - - - 11.0
B9Q 2073~2 - - 9~7
C18Q 4046,~ _ 6~4 9S6
190 2003~2 3~2 3~2 9~3
D90 2003~2 3~2 _ 1027
- lQ -
~ ,t~
5'~
Warp Flex Life (5 pound weight)
As Acid
Sample Made Treated _(450F)
A 7146
B 4014 534
C 2755 1801
1 3045 2405
D 4438 973
Comparisons ~, B, C and D did not exhibit as
good flex li~e after acid treatment as Sample 1 which
is a sample of this invention.
EXAMPLE 3
The procedure of Example 1 was followed
except that the fabric employed was CLARK SCHWEBEL
STY~E 6758* in which the sizing had been removed by
heat cleaning. Coat composition formulations and
results are shown in the following tables.
Composition (gm)
Formu- PTFE Disp. Disp.Sil- Sil- Water
lation Disp. A B ~2 ane oxane Repellant
1 - 60 - 135 l.g 1.8 1.8
2 40 12 12 143 1.8 1.8 1.8
3 - - 53 142 1.8 1.8 1.8
4 40 - 10.6 144 1.8 1.8 1.8
- 10.6 144 108 1.8 1.8
6 90.4 - - 261 3.2 3.2 3.2
Dispersions A and B are melt-processible polymers.
The coated fabric was cured for 10 minutes at the
temperatures shown below and tested for warp flex
life using a four pound weight.
*denotes trade mark
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.~
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Formu- Cure ~ Flex Life
lation_ TemptC) ~s Made reated (500F)
l 250 3gO0 3400
2 280 50000 13000
3 250 2600 2900
4 250 24000 14700
310 3500Q 15600
6 250 23000 15300
Formulations 1 and 3 were cured below the melt
temperature of the polymers in Dispersions A and B
and thus have low warp flex life to begin with.
Formulations 1 and 3 show good acid resistance after
acid treatment even though the initial values are
low. Formulations 2, 4, 5 and 6 show the advantage
in employing PTFE along with the melt-processible
polymers, as the warp flex life as made is improved
several fold.
EXAMPLE 4
The procedure of Example l was followed
except that the fabric employed was Clark Schwebel
Style 6758 in which the sizing had been removed by
heat cleaning and except that after dipping the
coating dispersion, wringing dry, and cure-drying lO
minutes at 250C, the coated fabric was dipped into
a second dispersion containing only water repellant and
then wrung dry and cured for 10 minutes at 250C.
Coating composition formulations and results
are shown in the following tables.
1st Coating Composition (gm)
Formu- PTFEWater
lation D~2 Silane Slloxane Repellant
l 90~4 2613.2 3.2 3.2
2 50 1461~8 1.8
3 50 1461.8 l.~ -
4 50 1461.8 1.8
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..
7 a.s72
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2nd Coating Formulation (gm)
Formu- Water % Pickup
lation Repellant H2O 1st 2nd
1 - None ~ 10
2 50 50 9.8 0.58
3 25 75 9.9 0.30
10 . 1 0 . 09
Warp Flex Life
As Made Acid Treated
10 Formulation~4 Eound Weight) (500F)
1 23000 15300
2 28000 45000
3 35000 31000
4 36000 26000
Formulation 1 did not undergo a second coating of the
water-repellant and its as made warp flex life is
seen to be the poorest of the four. Formulation 2 in
which the second coating contained the most
water-repellant of Formulations 2-4 is seen to have
the best warp flex life after acid treatment.
