Water-borne fluoroelastomer coatings and related method

A water-borne fluoroelastomer coating composition comprising an aqueous dispersion of a fluoroelastomer polymer; from about 0.1 to about 10 parts by weight of an amino/polyamino-siloxane curative per 100 parts by weight of the polymer; and, from 0 to about 40 parts by weight of an additive filler, per 100 parts by weight of the polymer, wherein there is sufficient water to provide a coating composition having a solids content of from about 10 to about 80 percent by weight. A method of coating a substrate with the fluoroelastomer coating composition of the present invention is also provided. Cured fluoroelastomer films, cured with amino/polyamino-siloxane curatives are also provided.

TECHNICAL FIELD 
The present invention is generally directed toward fluoroelastomer coating 
compositions. More particularly the present invention is directed toward 
water-borne fluoroelastomer compositions having improved pot life as well 
as improved adhesion to a variety of surfaces. Specifically, the 
water-borne fluoroelastomer coating compositions of the present invention 
contain an improved curative which is an oligomeric, 
amino/polyamino-siloxane curative. The present invention is also directed 
toward a method of applying the water-borne fluoroelastomer coating 
compositions and toward cured fluoroelastomer films. 
BACKGROUND OF THE INVENTION 
Fluoroelastomer coating compositions are well known. They typically 
comprise copolymers of vinylidene fluoride and hexafluoropropylene or 
terpolymers of vinylidene fluoride, hexafluoropropylene and 
tetrafluoroethylene. 
Fluoroelastomers are well known for their heat, weather, solvent and 
chemical resistance. Furthermore, cured fluoroelastomer films typically 
exhibit good mechanical properties and adhere to a variety of surfaces. As 
a result of these properties, fluoroelastomer coating compositions are 
applied to protect a variety of surfaces including metals, plastics, 
rubbers, concrete, glass and the like. 
Heretofore in the art, fluoroelastomer coating compositions have been 
applied using organic solvent systems. Typical solvent systems employed 
ketones or ethers. Such solvents, however, are hazardous to use inasmuch 
as they are flammable, toxic, and volatile. Moreover, growing 
environmental concern over the use of such volatile organic compounds 
restricts the use of such compounds in various areas of the country. 
Finally, the pot life of the fluoroelastomer coating compositions 
employing such solvents is generally only a few hours. Pot life, as it 
will be used herein, refers to the time required for the fluoroelastomer 
coating composition to begin gelation. 
Water-borne fluoroelastomer coating compositions are also known. For 
example, DuPont de Nemours Company, of Wilmington, Del. teaches aqueous 
dispersions of their fluoroelastomer, Viton.RTM., with a polyamine curing 
agent. The pot life of such dispersions, however, is relatively short, 
generally only 2 to 5 days. Ausimont U.S.A. also teaches aqueous 
dispersions employing their fluoroelastomer, TECNOFLON.RTM., with 
aliphatic amines as curing agents. As with the dispersions taught by 
DuPont, those employing aliphatic amines also have a relatively short pot 
life, on the order of only 3 to 5 days. 
U.S. Pat. No. 4,399, 553 also teaches a water based fluoroelastomer coating 
composition containing partially or completely hydrolyzed aminosilane 
compounds, with or without additional amine compounds, as curing agents. 
These aqueous dispersions are taught to have a pot life of up to one month 
at 25.degree. C. 
Thus, although water based fluoroelastomer coating compositions are known, 
a need still exists for a water based fluoroelastomer coating composition 
having improved pot life, excellent stability, and which produces cured 
films with excellent adhesion to a wide variety of substrates. 
SUMMARY OF INVENTION 
It is, therefore, a primary object of the present invention to provide 
water-borne fluoroelastomer coating compositions. 
It is another object of the present invention to provide water-borne 
fluoroelastomer coating compositions that have an improved pot life. 
It is yet another object of the present invention to provide water-borne 
fluoroelastomer coating compositions that produce a cured coating or film 
that adheres to a variety of substrates. 
It is a further object of the present invention to provide water-borne 
fluoroelastomer coating compositions containing essentially no volatile 
organic compounds. 
It is still another object of the present invention to provide water-borne 
fluoroelastomer coating compositions that produce a cured coating or film 
that exhibits good abrasion resistance and provides protection against a 
wide variety of solvents and chemicals. 
It is yet a further object of the present invention to provide water-borne 
fluoroelastomer coating compositions that do not contain metallic oxides 
commonly used as acid acceptors in fluoroelastomer compositions. 
At least one or more of the foregoing objects of the present invention, 
together with the advantages thereof over existing fluoroelastomer coating 
compositions and water-borne fluoroelastomer coating compositions that 
shall become apparent from the specification that follows, are 
accomplished by the invention as hereinafter described and claimed. 
In general, the present invention provides a water-borne fluoroelastomer 
coating composition comprising an aqueous dispersion of a fluoroelastomer 
polymer; from about 0.1 to about 10 parts by weight of an 
amino/polyamino-siloxane curative per 100 parts by weight of the polymer; 
and, from 0 to about 40 parts by weight of an additive filler, per 100 
parts by weight of the polymer, and sufficient water to provide a coating 
composition having a solids content of from about 10 to about 80 percent 
by weight. 
The present invention also provides a method of coating a substrate 
comprising the steps of applying a water-borne fluoroelastomer coating 
composition to a substrate, wherein the water-borne fluoroelastomer 
coating composition comprises an aqueous dispersion of a fluoroelastomer 
polymer and an aminolpolyamino-siloxane curative. 
The present invention further provides a cured fluoroelastomer film 
comprising a layer of fluoroelastomer wherein said fluoroelastomer has 
been cured with an amino/polyamino-siloxane curative. 
PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION 
The present invention is directed toward water-borne fluoroelastomer 
coating compositions. The fluoroelastomer coating compositions are 
water-borne inasmuch as the compositions are based on aqueous dispersions 
or lattices of fluoroelastomer polymers. The fluoroelastomer polymers can 
include any copolymerizable monomers, but preferably include copolymers of 
vinylidene fluoride and hexafluoropropylene, or terpolymers of vinylidene 
fluoride, hexafluoropropylene and tetrafluoroethylene. Other examples of 
fluoroelastomers include those modified with monomers that provide 
enhanced properties, e.g.,copolymerization with 
perfluoro(methylvinylether)to improve low temperature performance. 
Mixtures of the above fluoroelastomers may also be employed. 
The fluoroelastomer coating compositions of the present invention further 
include an improved amino or polyamino-siloxane curative, referred to 
hereinafter as an amino/polyamino-siloxane curative. Unlike the curatives 
employed heretofore in the art, the curative of the present invention is a 
terminated, stabilized, oligomeric, amino/polyamino-siloxane wherein the 
degree of polymerization of the oligomeric siloxane is essentially 
limited. 
It should be appreciated that silanes readily undergo hydrolysis in water 
to produce silanols. The silanol groups will also self condense to form 
siloxane oligomers. The self condensation continues beyond the soluble 
dimer and trimer to yield insoluble tetramers and higher, including 
branched and cyclic species. It is believed that these insoluble compounds 
deleteriously affect the ability of the oligomer to cure the 
fluoroelastomer coating compositions. It has now been found that the use 
of stabilized, oligomeric, amino/polyamino-siloxanes, limited to a trimer 
or smaller, can produce water-borne fluoroelastomer coating compositions 
having improved pot-life and improved adhesion over the water-borne 
fluoroelastomer coating compositions known heretofore in the art. 
Limiting the amino/polyamino-siloxane curative to a trimer or smaller 
provides for excellent adhesion between the fluoroelastomer coating and 
the substrate. This is believed to result from the fact that termination 
and stabilization of each amino/polyamino-siloxane curative molecule 
increases the proportional silanol concentration of each molecule, the 
silanol groups being reactive sites for bonding to various substrates. 
Without wishing to be bound by any particular belief or theory, it is 
nevertheless believed that the hydrolyzed amino/polyamino-siloxanes are 
stabilized, and thus prohibited from further polymerization or 
self-condensation, as a result of other constituents present in the 
aqueous solution containing the siloxanes. For example, hydrolyzed 
alkoxysilanes, which are inhibited from further self-condensation by using 
an emulsifier, are disclosed in U.S. Pat. No. 5,552,476. Also, European 
Published Application 675,128 discloses stable, water-borne silane 
compounds. 
Preferably, the degree of polymerization of the amino/polyamino-siloxane 
curative of the present invention is essentially limited to a trimer. 
Specifically, the curative is a water soluble oligomer that can be defined 
by the formula (I) 
##STR1## 
wherein R.sup.1, R.sup.2.sub.1 and R.sup.3 are the same or different and 
are selected from the group consisting of amino groups, organic moieties 
and hydrogen, with the proviso that at least one of R.sup.1, R.sup.2 and 
R.sup.3 contain an amino group. The functionality, size and/or 
configuration of each of R.sup.1, R.sup.2 and R.sup.3, individually or in 
combination, must be such that the oligomer is soluble in water or aqueous 
mediums (soluble hereinafter). Inasmuch as a multitude of 
amino/polyamino-siloxanes can fall within the parameters set forth in 
formula (I), it should be appreciated that a mixture of siloxanes, all 
generally defined by formula (I), can be employed in the present 
invention. 
Amino groups, as stated above, include primary and secondary amino groups, 
as well as primary and secondary amine substituted organic moieties. Amino 
groups also include polyamino groups, which can include both primary amino 
and secondary amino groups. Polyamino groups typically include primary and 
secondary amine substituted organic moieties. Preferably, the amino groups 
are primary amine groups, primary amine substituted organic moieties or 
polyamino substituted organic moieties. 
Organic moieties refer to alkyls, alkenes and alkynes, which can be 
straight or branched. It is contemplated that these moieties can also be 
cyclic and aromatic. It should further be understood that the organic 
moieties can include hetero atoms, such as oxygen or sulfur, so long as 
the presence of these atoms does not have a deleterious affect on the 
oligomer or the composition of the present invention. Preferably, the 
organic moieties are alkyls. 
Regarding the proviso that at least one of R.sup.1, R.sup.2, and R.sup.3 
contain an amino group, it is preferred that the amino/polyamino-siloxane 
curative contain at least two reactive amine functionalities so as to best 
achieve crosslinking of the fluoroelastomer monomers. This is achieved 
through at least two amino groups on separate side-chains or through at 
least two amino groups on at least one side-chain, i.e. a polyamino group. 
The use of reactive amino functionalities refers to primary or secondary 
amino groups, with primary amino groups being preferred. Accordingly, it 
is preferred that at least two of R.sup.1, R.sup.2 and R.sup.3 contain 
amino groups or at least one of R.sup.1, R.sup.2 and R.sup.3 contain a 
polyamino group. 
For example, substituents R.sup.1, R.sup.2, and R.sup.3 can be defined 
according to the following formula (II) 
##STR2## 
wherein R.sup.4 is a divalent organic moiety as defined hereinabove; 
R.sup.5 is selected from hydrogen, organic moieties as defined 
hereinabove; and n is 0 or 1. It is preferred that n is 1 and that R.sup.4 
is an alkyl. It should be understood that where R.sup.1, R.sup.2, or 
R.sup.3 are hydrogen, formula (II) is not representative. 
It should be appreciated that those skilled in the art, without undue 
experimentation, can readily determine the maximum size and/or appropriate 
configuration of substituents R.sup.1, R.sup.2, and R.sup.3 permissible 
without rendering the oligomer insoluble in aqueous media. It should be 
understood that the size of the substituent refers to the number of carbon 
atoms therein. 
Without wishing to be bound by any particular size and/or molecular weight, 
it is believed that each of R.sup.1, R.sup.2 and R.sup.3 can include up to 
about 6 carbon atoms without rendering the molecule insoluble. The skilled 
artisan will appreciate that solubility will improve with fewer carbon 
atoms, as well as with the addition of substituents such as nitrogen and 
oxygen atoms. The amount of branching will also affect the solubility. 
Thus, the skilled artisan may be able to synthesize larger molecules by 
adding other substituents and/or changing the chemical structure of 
R.sup.1, R.sup.2 or R.sup.3. Furthermore, although the size of any given 
substituent, e.g. R.sup.1, can fluctuate based on the size of the 
complementary substituent groups, e.g. R.sup.2 and R.sup.3, it is 
believed, and thus preferred that the oligomer employed in the present 
invention contain less than about 20 carbon atoms, more preferably less 
than 15 carbon atoms, and even more preferably less than 12 carbon atoms. 
Again, it should be understood that larger molecules, so long as they are 
soluble, are contemplated. 
One such oligomeric siloxane is the diamino-siloxane sold by Huls America, 
Inc. under the name Hydrosil.RTM. 2776. This curative is hydrolyzed, 
terminated and stabilized to contain essentially diamino-siloxanes that 
are trimers or smaller. It should be appreciated that although Hydrosil 
2776 is an example of a preferred embodiment, any stabilized amino or 
polyamino-siloxane meeting the above criteria can be used. 
Optionally, the fluoroelastomer coating compositions of the present 
invention may further include various fillers. These fillers can include 
carbon black, mineral fillers (clays, synthetic silicates, whiting, 
barytes, and the like), color pigments (preferably inorganic and heat 
resistant), glass micro beads and short, chopped fibers, as well as 
materials to modify resistivity, such as metal powders, graphite and the 
like. Other examples include those materials used as fillers in rubber, 
plastic and coating formulations. These fillers are well known and 
documented in the art. In essence, any material that does not adversely 
affect the chemical and physical performance of the coating can be used as 
a filler. It should be appreciated that the presence or absence of any 
fillers and pigments typically does not affect the performance of the 
coating. Of course, the total amount of filler added will be limited by 
its effect on viscosity, film formation capabilities and other properties 
of the fluoroelastomer coating. Additional ingredients such as 
surfactants, viscosity modifiers and the like may be added if deemed 
necessary to control the liquid coating properties. 
The solids content of the fluoroelastomer coating compositions of the 
present invention is typically in the range from about 10 to about 80 
percent by weight based on the total weight of the coating composition, 
and preferably from about 60 to about 70 percent by weight based on the 
total weight of the coating composition. By solids is meant the 
fluoroelastomer polymers, fillers, curatives and any other solid component 
of the coating composition. 
The solids component of the coating composition of the present invention 
typically contains 100 parts by weight polymer or rubber, from 0 to about 
40 parts by weight filler per hundred parts by weight polymer or rubber 
(phr) and from about 0.1 to 10 parts by weight curative phr. Accordingly, 
the fluoroelastomer coating composition of the present invention will 
typically include from about 25 to about 1,350 parts by weight water phr, 
from 0 to about 40 parts by weight filler phr, and from about 0.1 to about 
10 parts by weight curative phr. Preferably, the fluoroelastomer coating 
composition of the present invention will include from about 50 to about 
95 parts by weight water phr, from 0 to about 40 parts by weight filler 
phr, and from about 0.1 to about 1.8 parts by weight curative phr. 
The coating compositions may be prepared by mixing the fluoroelastomer 
aqueous dispersion, curative(s), filler(s) and other desired additive(s) 
in a ball mill or other suitable mixing equipment. The conditions of 
mixing are dependent upon the coating composition ingredients and can be 
readily determined by those with skill in the art, without undue 
experimentation. The coating composition mixture is preferably filtered to 
remove any undispersed particles. The water-borne fluoroelastomer coating 
compositions of the present invention may be sprayed, dipped, brushed or 
applied in any similar fashion to form a film on the desired substrate 
which can include metal, rubber, plastic, concrete or other such surfaces. 
The film is typically dried at ambient temperatures, or in an oven at 
about 60.degree. C. to about 70.degree. C., and then cured at higher 
temperatures. Preferably the curing takes place at about 100.degree. C. 
for about one hour.

In order to demonstrate a practice of the present invention, the following 
examples are provided. It should be appreciated that the examples are not 
to be viewed as limiting the invention as disclosed herein, the claims 
serving to define the invention. 
EXAMPLES 
Three water-borne fluoroelastomer coating compositions having varying 
amounts of curative were prepared according to the present invention. The 
composition of each coating is represented in Table I. 
TABLE I 
______________________________________ 
WATER-BORNE FLUOROELASTOMER COATING 
COMPOSITIONS 
Examples 
(parts by weight) 
Constituent 1 2 3 
______________________________________ 
Water-borne fluoroelastomer dispersion, 
153.8 153.8 153.8 
65% solids content 
Medium Thermal Carbon Black 
20.0 20.0 20.0 
Hydrosil .RTM. 2776, 23-25% active 
7.5 10.0 15.0 
Water (to maintain solids) 
6.4 4.8 1.6 
Triton .RTM. X-100 (nonionic surfactant 
0.5 0.5 0.5 
manufactured by Rohm and Haas Co.) 
______________________________________ 
Upon preparation of each fluoroelastomer coating composition, a film was 
prepared using a portion of the coating composition by pouring the coating 
onto a Mylar.RTM. film, drawing the coating down to a uniform thickness 
and allowing the film to dry at ambient temperatures. The dry films were 
then cured in an oven for one hour at 100.degree. C. From these films, 
original tensile strength data was obtained as represented in Table II. 
The remainder of the coating compositions prepared above were placed in 
closed containers and aged at room temperature for up to four months or 
longer. During this time, the coating compositions were examined on a 
regular basis to detect any signs of gelation. Upon the first indication 
of gelation, a film was prepared as above by pouring the coating 
composition onto a Mylar.RTM. film, drawing the coating down to uniform 
thickness and allowing the film to dry at ambient temperatures. These 
films were also cured in an oven for an hour at 100.degree. C. The films 
prepared from the aged coating material had appearance and properties 
similar to films from the freshly prepared coatings as generally 
represented from the tensile strength as displayed in Table II. Also 
represented in Table II is the time in months in which it took the 
fluoroelastomer coating compositions prepared above to begin gelation. 
TABLE II 
______________________________________ 
PHYSICAL PROPERTIES 
Examples 
Property 1 2 3 
______________________________________ 
Gelation (months) 
&gt;4.0 &gt;3.0 &gt;1.0 
Tensile Strength (psi) 
Original 1101 1054 1010 
Aged 1048 894 1100 
Elongation at Break (%) 
Original 677 604 577 
Aged 691 515 374 
______________________________________ 
Generally, the pot life of a solvent based or water-borne fluoroelastomer 
coating composition is related to the amount of curative present. The 
above examples illustrate that this relationship also occurs for the 
stabilized oligomeric diamino-siloxane, Hydrosil.RTM. 2776. Surprisingly, 
however, the use of an oligomeric diamino-siloxane, such as Hydrosil.RTM. 
2776, provided a pot life ranging from 5 to more than 16 weeks in length. 
Specifically, Examples 1 and 2 show that the curative levels employed had 
a pot life of more than 3 to 4 months and produced cured films with more 
than adequate tensile properties. 
Comparative examples employing 
N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 1,6-hexanediamine, and 
triethylenetetramine as curatives were prepared and tested. The 
composition of each coating is represented in Table III. 
TABLE III 
______________________________________ 
COMISONS WITH OTHER CURATIVES 
Examples 
(parts by weight) 
Constituent 1 2 3 
______________________________________ 
Water-borne fluoroelastomer 
153.8 153.8 153.8 
dispersion, 65% solids content 
Medium Thermal Carbon Black 
20.0 20.0 20.0 
1,6-hexanediamine 2.0 -- -- 
Triethylenetetramine -- 5.0 -- 
N-(2-aminoethyl)-3-aminopropyltri- 
-- -- 5.0 
methoxysilane 
Water (to maintain solids) 
12.2 13.8 13.8 
Triton .RTM. X-100 (nonionic surfactant 
0.5 0.5 0.5 
manufactured by Rohm and Haas Co.) 
______________________________________ 
Water-borne fluoroelastomer coating compositions prepared using 
1,6-hexanediamine produced gelation within 2 to about 5 days, and those 
prepared using triethylenetetramine produced gelation within 2 to about 5 
days. Gelation time for N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was 
not obtained in view of the following findings. 
It is believed that N-(2-aminoethyl)-3-aminopropyltrimethoxysilane is sold 
under the tradename Z-6020 by Dow Corning of Midland, Mich. This curative 
was mixed into an aqueous dispersion of fluoroelastomer and within one day 
a film was cast, dried and cured for one hour at 100.degree. C. Three days 
later another film was cast from the same catalyzed coating and cured for 
one hour at 100.degree. C. The tensile properties of the cured coatings 
are represented in Table IV. 
TABLE IV 
______________________________________ 
COMATIVE PHYSICAL PROPERTIES 
Property Z-6020 
______________________________________ 
Aging (day) &lt;1 3 
Tensile Strength (psi) 
1150 490 
Elongation at Break (%) 
300 960 
______________________________________ 
As is clearly indicated by the data in Table IV, the cured fluoroelastomer 
coating composition prepared using a three day old composition 
demonstrated tensile properties that were inferior to those obtained 
employing the fluoroelastomer coating compositions of the present 
invention. Indeed, the tensile properties of the cured coatings or films 
prepared using the fluoroelastomer coating composition of the present 
invention were superior even though the coating composition of the present 
invention was allowed to age for up to four months. 
Thus it should be evident that the composition of the present invention is 
improved over known water-borne fluoroelastomer coating compositions. 
Based upon the foregoing disclosure, it should now be apparent that the 
water-borne fluoroelastomer coating compositions described herein will 
carry out the objects set forth hereinabove. It is, therefore, to be 
understood that any variations evident fall within the scope of the 
claimed invention and thus, the selection of specific component elements 
can be determined without departing from the spirit of the invention 
herein disclosed and described. In particular, the water-borne 
fluoroelastomer coating compositions according to the present invention 
are not necessarily limited to those including a specific fluoroelastomer 
or a specific stabilized, oligomeric, amino/polyamino-siloxane curative. 
Furthermore, it should be understood that the ranges specifying carbon 
content or amount of constituent or reagent, necessarily include the 
members or amounts within those ranges. Thus, the scope of the invention 
shall include all modifications and variations that may fall within the 
scope of the attached claims.