Note: Descriptions are shown in the official language in which they were submitted.
iO!.~OOZ8
FLUOROPOLYMER PRIMER COMPOSITIONS
BACKGROUND OF THE_INVENTION
Fie~d of the Invention
This invention relates to fluoropolymer primer
compositions useful for coating substrates, particularly copolyme~
based on ethylene and a comonomer selected from the group
consisting of chlorotrifluoroethylene, tetrafluoroethylene and
mixtures thereof.
DESCRIPTION OF THE PR_OR ART
Copolymers of about 40 to about 60 mol percent ethylene
and correspondingly from about 60 to about 4Q mol percent of
chlorotrifluoroethylene, tetrafluoroethylene and mixtures thereof,
are known in the art as disclosed, for example, in Mueller et al.
U.S. Patent 3,847,881, issued November 12, 1974. As described
therein, such copolymers may contain 3,3,3-trifluoro-2-trifluoro-
methyl propene to improve their stress-crack resistance. Also,
as described in Car]son U.S.P. 3,624,250,issued November 30, 1971,
such copolymers may contain certain vinyl monomers in order to
improve their stress-crack resistance. Such copolyrners have been
suggested as being useful for coating articles by powder coating
techniques. However, it has been found that the adhesion between
such copolymer coatings and the underlying 9ubs-trate, particularly
in the case of metal substrates, is too weak for certain applica-
tions, especially those wherein a severe environment is encoun-
tered. Robertson et al. U.S.P. 3,989,873, issued November 2, 1976,
describes primer coatings for such copolymers which include about
1 to 40 percent by weight of an oxide of cobalt, nickel, manganese,
chromium, tin, molybdenum, tungsten or mixtures thereof. Although
such primer coatings are effective for bonding such copolymers
to metal and other
; 30
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substrates, it has been found that upon long term exposure to
high temperature wet environments, the bond strength of coated
articles substantially decreases. It would be desirable to pro-
vide a primer coating composition and coated articles based on
- such copolymers which have improved high temperature wet
environment bond strengths.
è SUMMARY OF THE INVENT ON
In accordance with one embodiment of the present
invention, there are provided copolymers of ethylene, a halo-
genated comonomer selected from the group consisting of tetra-
, fluoroethylene, chlorotrifluoroethylene and mixtures thereof
-~ and 0 to 10 mol percent o~ an additional monomer selected
from the group consisting of 3,3,3-trifluoro-2-trifluoromethyl
propene, a vinyl monomer which is free of telogenic activity
and which provides a side chain having at least two carbon
atoms, said side chain being aromatic or having its elements
bonded together by a single bond only, or mixtures thereof,
` and containing from about 5 to 60 percent by weight of the -~
total composition of an epoxy resin and about 1 to 25 percent
by weight of the total composition of an oxide of cobalt, nickel,
`~~ manganese, tungsten or mixtures thereof. Such compositions
. .
are particularly useful as a primer coating for substrates
such as metals, preferably in conjunction with an overlying
layer of such copolymers. In accordance with another embodiment
of the present invention, coated articles are provided which
. .,
~ comprise a substrate, a primer coating fused thereon and com-
,
prising a copolymer of ethylene, a halogenated comonomer
selected from the group consisting of tetrafluoroethylene,
; chlorotrifl~oroethylene and mixtures thereof and 0 to 10
mol percent of an additional monomer selected from the group
consisting of 3,3,3-triflyoro-2-trifluoromethyl propene, a
vinyl monomer which is free of telogenic activity and which pro-
.
1' - 2 -
, . .
1090028
: vides a side chain having at least two carbon atoms, said side
- chain being aromatic or having its elements bonded together by a
single bond only, or mixtures thereof, and from about 5 to 60
, percent by weight of the total composition of an epoxy resin and
about 1 to 25 percent by weight of the total composition of an
oxide of cobalt, nickel, manganese, tungsten or mixtures thereof,
and an outer coating fused to the primer coating and comprising
a copolymer of ethylene, a halogenated comonomer selected from
. the group consisting of tetrafluoroethylene, chlorotrifluoroethy-
- 10 lene and mixtures thereof and 0 to 10 mol percent of an additional
monomer selected from the group consisting of 3,3,3-trifluoro-2-
trifluoromethyl propene, a vinyl monomer which is free of telo-
genic activity and which provides a side chain having at least
:i two carbon atoms, said side chain being aromatic or having its
elements bonded together by a single ~ond only, or mixtures
thereof. Preferably, the substrate is a metal substrate and the
copolymers of the primer and overlying layers are the same. It
has been found that the use of such primer layers, especially
those containing an oxide of cobalt, provides articles which have
excellent bond strength even when exposed to prolonged high
i temperature wet environments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As stated above, the copolymers of the present invention
are known and may be prepared by known polymerization methods,
such as are described in the aforesaid Mueller et al. and Carlson
, patents. Preferably, the copolymers contain from about 40
. to about 60 mol percent of ethylene, from about 60 to about 40 mol
percent of chlorotrifluoroethy].ene, tetrafluoroethylene or mix-
, tures thereof and 0 to 10 mol percent of an additional monomer
selected from the group consisting of 3,3,3-trifluoro-2-trifluoro-
~B -3
1~9~ Z8
methyl propene, a vinyl monomer which is free o~ telogenic acti-
vity and which provides a side chain having at least two carbon
atoms, said side chain being aromatic or having its elements
bonded together by a single bond only, or mixtures thereof.
bout equimolar copolymers of ethylene and chlorotrifluoro-
ethylene or tetrafluoroethylene axe particularly preferred.
The termonomers disclosed in Mueller et al. (i.e.,
3,3,3-trifluoro-2-trifluoromethyl propene) and in Carlson (i.e.,
vinyl monomers) may optionally be present in the copolymer.
Amounts of about 0.1 to about 10 mol percent, based on the total
mols of the other monomers, may be employed. Preferably, about
0.5 to about 5 mol percent of such additional monomers are
employed.
The monomers disclosed in Carlson are copolymerizable
vinyl monomers that are free of telogenic activity and which pro-
vide a side chain having at least two carbon atoms and being
either aromatic or having its elements bonded together only by
single bonds. By "free of telogenic activity" is meant that the
vinyl monomer does not act as a chain transfer agent to an
.,,
extent which undesirably limits the molecular weight of the
copolymer. Examples of such vinyl monomers include fluorinated
alpha-mono-olefins and those of the following formulae:
R-CF=CF2; ROCF=CF2; CF3tCF2)mCH20CF=CF2; CH3~CH2)nOCF=CF2;
R'CH2(X)=CH2 and R20CH2C(X)=CH2,
s; wherein R is an organic group containing 2 to 8 carbon atoms, R'
is a perfluorinated or chlorofluoroalkyl group of 1 to 7 carbon
atoms or a secondary or tertiary halo-alcohol group, R2 is a
perfluorinated or chlorofluoroalkyl of 1 to 7 carbon atoms, X
is H or CH2, m is an integer of 0 to 6 and n is an integer of 1 to
30 7. Especially preferred vinyl monomers are perfluoropropyl per-
fluorovinyl ether, 1,1,1-trifluoro-2-(trifluoromethyl)-4-penten-
, . .
2-ol
- 4 -
. `
,
. .
10900Z8
and allyl heptafluoroisopropyl ether.
; Preferred copolymers are those containing from about 40
to about 60 mol percent of ethylene and from about 60 to about 40
mol percent chlorotrifluoroethylene, especially about equimolar
copolymers of such comonomers, as well as copolymers of ethylene
and chlorotrifluoroethylene of the aforesaid mol content (and
;~ especially about an equimolar content) together with about 0.5
to 5 mol percent of 3,3,3-trifluoro-2-trifluoromethyl propene,
and copolymers of ethylene and tetrafluoroethylene of the afore-
- 10 said mol content (and especially about an equimolar content)
together with about 0.5 to 5 mol percent of perfluoropropyl
perfluorovinyl ether.
¦ The copolymers and terpolymers may include a high
molecular weight fraction as is disclosed in U.S. Patents
4,020,253, issued April 26, 1977, and 4,033,939, issued ~uly 5,
1977. For example, the copolymers and terpolymers may contain
about 0.5 to 60 weight percent of a high molecular weight
component which has a melt index per ASTM D-]238 at 275C. of
about 0.0001 to 1.0, preferably about 0.005 to 0.5, and about
99.5 to 40 weight percent of a low molecular weight component
, which has a melt index of about 3 to 3000, preferably about 5
,., ~o 1000.
The copolymer primer compositions include from about
5 to 60 weight percent, based on the total weight of the composi-
tion, of an epoxy resin, and preferably from about 25 to 50
; weight percent of such resin. The epoxy resins useful herein
may be any of the known adhesive epoxy resins. Such resins may
be prepared by the condensation of epoxy compounds, such as epi-
chlorohydrin and g]ycerol dichlorohydrin, with polyhydric organic
~compounds ~uc as alcohole, e.g., pentaerythritol: dihydric
-5-
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alcohols, e.g., glycerol; dihydric phenols, e.g., bisphenol A;
`~ and trihydric phenols. For example, epoxy resins prepared by the
condensation of bisphenol A and epichlorohydrin may be employed.
Other types of epoxy resins include diglycidyl ether of bisphenol
A, the epoxidized novolac resins such as epoxy cresol novolac
and epoxy phenol novolac. Still other epoxy resins include the
cycloaliphatic resins in which the epoxide groups are attached
directly to the cycloaliphatic portions of the molecule rather
than on the alkyl chain.
The above epoxy resins include hardeners and/or
accelerators which permit their curing at elevated temperature.
-- Such catalysts include Lewis bases such as tertiary amines (e.g., ~-
benzyldimethylamine) and imidazoles, and Lewis bases such as
boron triflyoride complexes with monoethylamine. Hardeners
` (cross-linking agents~ include primary and secondary amines
- (e.g., diethylene triamine and triethylene tetramine), polyamides,
~l polyesters, carboxylic acids, anhydrides (such as maleic anhy-
'.'t' dride), phenolic compounds and alcohols. The concentration of
~` such additives may vary widely depending upon the desired proper-
ties in the cured material.
Such epoxy resins are commercially available as liquids
or solids. When solid epoxy resins are employed they are pre-
ferably in the form of aqueous emulsions or as solutions in suit-
able volatile solvents such as ketonic solvents/ e.g., methyl
ethyl ketone, methyl isobutyl ketone and the like; cyclic ethers
such as tetrahydrofuran, etc. It has been ~ound desirable to
utilize the epoxy resin as a solution in a volatile solvent since
better bonding to substrates such as metals results.
The copolymer primer compositions also include from
about l to 25 weight percent, preferably from about 5 to 15
weight percent, of an oxide of cobalt, nickel , manganese, tung-
- 6 -
,. ,
: ,
10~0028
sten or mixtures thereof. Cobalt oxide is preferred and is
employed in the form of cobaltic oxide (Co2O3), although
cobaltous oxide (CoO) and cobalto-cobaltic oxide (Co3O4) may
also be used as well as mixtures of any two or more forms of
cobalt oxide. The cobalt and other oxides are preferably em-
ployed in powder form.
~ It has been found that such composltions, especially
: compositions in which the copolymer is an approximate equimolar
copolymer of ethylene and chlorotrifluoroethylene and contain- -
ing 3,3,3-trifluoro-2-trifluoromethyl propene, are effective
primer coatings for metal and other substrates. The incorpor-
ation of the epoxy resins and cobalt oxides in the primer
composition provides coated articles in which the outer coat-
ing is firmly adhered to the substrate.
The compositions of the present invention may be pre-
pared by any suitable technique known fox preparing blended
thermoplastic compositons. Preferably, the epoxy resin in a `
suitable solvent is blended with pellets or powder of the co-
polymer in a ribbon blender, v-cone blender or like apparatus
and the solvent is removed by evaporation, vacuum or other
means. Thereafter, the cobalt oxide, also preferably in pow-
der form, is blended into the mixture on the same equipment.
If necessary, the final mixture may be milled to the desired
size depending upon the desired method of application.
The copolymer compositions are preferably in a powder
form and any of the well-known powder coating techniques may
be employed to coat the compositions onto the substrate.
These techniques include electrostatic deposition and fluid-
ized bed and similar techniques. As these methods are con-
ventional they are not specifically described herein. It is
preferred to use an electrostatic deposition coating process.
. .
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1~)91~Z8
As is well known, this process provides initial adherence of
a powder composition to a substrate and the substrate is there-
; after subjected to a heat treatment above the melting point of
the powder to fuse or coalesce ("flow out") the particles of
the powder into a continuous layer. In forrning the coated
articles of this invention, it is preferred to employ flow out
temperatures in the range of about 400 to 700F., preferably
about 500 to 550F. for copolymers based on ethylene and
chlorotrifluoroethylene and about 450 to 800F., preferably
about 575 to 650F. for copolymers based on ethylene and
:.
tetrafluoroethylene or mixtures with chlorotrifluoroethylene.The duration of this heating step is dependent upon a number
of factors, including the temperature employed, the coating
thickness, the type of substrate, etc. In general, flow out
times may conveniently be in the order of 1 or 2 minutes to -
1 or 2 hours or more. Upon exposure to the elevated tempera-
ture, the epoxy resin crosslinks to provide firm adhesion to
the substrate.
After a continuous primer coating is formed over the
substrate, a layer of particles of the outer coating may be
likewise deposited on the primer coating and then heated to
flow out such particles into a continuous coating which is
fused to the surface of the primer layer. The composition
used to form the outer coating obviously need not include the
epoxy resin or the cobalt oxide. Preferably, the copolymer
used in the primer and outer coatings is the same. In certain
cases, it may be desirable to omit the outer coating. Alter-
. .,
natively, the outer layer may be deposited on the primer layer
before the latter is flowed out, so that only one flow out
step need be employed.
The primer composition and the outer copolymer com-
position may include any of the conventional addi-tives usually
.
~9~
incorporated into such copolymer compositions. Such additives
include inert fillers, stabilizers, pigments, reinforcing
agents, lubricants and the like. For example, the four com-
ponent stabilizing system described in U.S. Patent 3,773,698
to Khattab (1973), or the three component stabilizing system -described in U.S. Patent 3,745,145 to Khattab et al. (1973),
for about equimolar ethylenechlorotrifluoroethylene copolymer
compositions may be employed herein.
The various substrates capable of being coated in
accordance with this invention include, for example, a metal
surface such as surfaces of steel, aluminum, iron, zinc, cad-
mium, magnesium, brass, bronze, Monel, Inconel; fabrics that
- will withstand the baking temperatures required, such as
fiber glass fabrics, asbestos fabrics, metal fabrics and the
, . .
like, as well as glass and any plastic surface which will
withstand high temperatures required herein without degrading.
Examples of such plastic surfaces include thermosetting resins,
~.
polyurethanes, silicones, fluoroelastomers and the like.
The type of metal or other material employed as the
base layer is not critical and is dictated by the desired end
use of the coated article. Especially preferred substrates
are those formed of brass, bronze, aluminum, stainless steel
and carbon steel. Likewise, the thickness of the substrate is
not critical and is dependent upon design factors.
It is preferred to pretreat metal substrates prior to
coating with the primer layer to obtain a roughened surface in
order to achieve a stronger adherence of the primer coating to
the substrate. Any suitable surface treatment can be employed
for this purpose such as sand or grit blasting, etching, etc.
The coating itself may be deposited upon the substrate
in any desired -thickness. In forming thic]s coatings (e.g. 1
_ g _
.
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. ~090(~
mil or above) it may be desirable to employ a plurality of
thin layers as is conventional in order to improve the uni-
.::
formity and integrity of the coating. This of course re-
quires additional exposure of the underlying coatings to high
temperatures and for this reason it is preferred to include
- stabilizing agents in the coating compositions.
Substrates which are coated according to this inven-
tion have coated surfaces which are continuous and pinhole-
free. The coatings may be employed as protective as well as
~; 10 antistick surfaces. Exemplary of articles which may be coated
include wire and cable; cookware, industrial tubing, pipes,
pumps and tanks, and dried rollers as well as other articles
which require coatings having high strength at elevated tem-
peratures, resistance to chemical attack, good electrical
insulating properties, non-sticking surfaces, abrasion and
scratch resistance and firm adherence of the coating.
The coated articles of this invention preferably in-
clude the substrate, a layer of the primer composition of the
thickness of about 0.5 to 20 mils, preferably about 1 to 10
mils, and an overlying layer of the copolymer composition
having a thickness of about 0.5 to 500 mils, preferably about
3 to 50 mils.
The present invention provides a single primer layer
which includes both cobalt oxide and an epoxy resin. It has
~.~
- been found that such primer layer provides excellent high tem-
perature wet strength properties to coatings of such copoly-
mers in an economic manner. This is especially surprising
since it has been indicated in the prior art that two separate
primer layers are necessary in order to provide adequate ad-
hesion of perfluorohalo-olefin polymers such as polychlorotri-
fluoroethylene. U.S.P. 2,979,418 to Dipner (1961) suggests
-- 10 --
., .
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.
a primer system for such perfluorohaloolefin polymers (which
;~ do not include the subject copolymers~ including a first primer
layer comprising the polymer and an adhesive resin such as an
` epoxy and/or phenolic resin and a second primer layer which
. ~ ., .
includes the polymer and a metal oxide of a metal above the
fourth group of the periodic table, such as cobalt oxide.
However, it has been found, as indicated in Example 12 below,
that in contrast to the present invention, such a primer
system does not provide adequate adhesion properties for the
copolymers of this invention.
-~ U.S.P. 2,961,341 to Long (1960) also suggests primers
., .
; for perfluorohaloolefins (as opposed to the present copolymers)
:;
but such primer layer only includes cobalt oxide. U.S. P.
~ 3,019,206 to Robb (1962) suggests latexipolyblends of poly-
- tetrafluoroethylene and fluorine-containing elastomers as
useful coating materials. Among the heat stabilizers suggest-
ed for such blends is cobalt oxide. Although copolymers are
also suggested, all of the copolymers are fluorinated or are
perfluorohaloolefins. It is quite clear that this patent
does not suggest the incorporation of cobalt oxide and an
epoxy resin into a copolymer based on ethylene and trifluoro- ;~
` ethylene, chlorotrifluoroethylene or mixtures thereof as
- adhesion promoters to provide a primer coating composition.
In order to further describe the present invention,
the following non-limited examples are given. All parts and
percentages are by weight.
EXAMPLES 1-10
., .
An equimolar ethylene-chlorotrifluoroethylene copoly-
mer containing about 2 mol percent of hexafluoroisobutylene,
and having a melt index of 8 to 1~, was blended in powder form
with a liquid eposy resin sold by M & T Chemicals as M & T
.
-- 11 --
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primer 252 P in a weight ratio of 65% epoxy solution to 35%
fluoropolymer. The epoxy solution contained about 25 weight
percent solids so that after drying the composition contained
about 32 weight percent epoxy resin and 68 weight percent
fluoropolymer. The mixture was dried in air for 48 hours and
then dried in a vacuum oven for 16 hours at 160F. to remove -
the solvents. The resultant cake was milled on a micro-
pulverizer after which 10 weight percent cobalt oxide powder,
based on the total weight of the composition, was added and
the composition remilled to a particle size of about 60
mlcrons .
Panels of various metal substrates measuring 8 x 8 x
1/4 inch were grit blasted with 60 grit aluminum oxide. The
substrates were preheated to 600F. and coated with the primer
composition by electrostatic deposition. The primer coated
panels were then powder coated by the same technique with the
same fluoropolymer used to prepare the primer coat (but with-
out the epoxy and oxide). The coated panels were vertically
supported in an electrically heated air oven and the powder
coatings were flowed out at 500 to 530F. for about 10 minutes.
The coated panels were subjected to a high pressure
steam environment to determine the resistance and adherence
of the coating. The coated plaques were exposed to 50 psig
steam with a six inch diameter circle of the coating in con-
taat with the steam. Following the test, the samples were
examined for blisters or lift off of the coating. The results
are shown in Table 1, below. In the Table, E-CTFE refers to
the fluoropolymer.
- 12 -
1~90028
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~i As can be seen from Table 1, the primer composition
of this invention provide good bond strength upon long term
- exposure to steam. Comparative Example 5 demonstrates the
; poor adhesion with primer compositions that do not contain
:..,
epoxy resin. The incorporation of 7~ of epoxy (Example 4)
provided better adherence than the comparative example and
.~ the incorporation of 14% epoxy (Example 3) provided further
improvement. Examples 1 and 2 demonstrate the excellent
results obtained with 29% and 43% epoxy, respectively. The
`- 10 knife test referred to in the Table was conducted by cut-ting
an x shape into the coating of a 2 x 4 x l/4 inch panel and
attempting to separate the coating from the metal substrate
with a knife. Example 6 demonstrates that the primer com-
position is not the limiting factor for coating integrity.
EXAMPLES_11-12
In Example 11, Example 7 was repeated on an 8 x 8 x
1/4 inch brass panel which was then subjected to the 50 psig
v~ steam test. The adhesion was good after 24 hours. In Example
12 (comparative), Example 7 was repeated with two primer coats
as suggested in the aforementioned U.S.P. 2,979,418. The first
primer coat contained 32% by weight of the epoxy resin and no
~. ~
cobalt oxide and was applied to a thickness of about 3-5 mils.
The second primer coat contained 20% by weight cobalt oxide and
no epoxy and was applied to about the same thickness. An outer
coating was employed as before. In the knife test, the coating
; could be completely lifted off from the brass, which indicates
.,
that the prior art primer system would not provide adequate
adhesion in high temperature wet environments. These examples
,.,
demonstrate the unexpected improvement in adhesion resulting
- 30 from the present invention.
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EXAMPLES 13 to 16
- Example 1 is repeated except that the primer composi- ;
tion includes, at 10 weight percent levels, nickel oxide, manga-
~ .
nese oxide, tungsten oxide in Examples 13 to 15, respectively
and a mixture of 5 weight percent cobalt oxide and 5 weight
percent nickel oxide in Example 16. Similar results are noted.
EXAMPLE 17
Example 1 repeated except that the copolymer was an
approximate equimolar copolymer of ethylene and tetrafluoro-
F lo ethylene. Similar results are noted.
EXAMPLE 18
Example 17 is repeated except that the copolymer
included 3 mol percent of perfluoropropyl perfluorovinyl
ether. Similar results are obtained.
EXAMPLE 19
Example 1 is repeated except that the copolymer was
an approximate equimolar copolymer of ethylene and chlorotri- --
fluoroethylene. Similar results are noted.
It is to be understood that variations and modifica-
tions of the present invention may be made without departing
from the scope of the invention. It is also to be understood
that the scope of -the invention is not to be interpreted as
limited to the specific embodiment disclosed herein, but only
in accordance with the appended claims when read in light of
the foregoing disclosure.
.. . .
- 15 -
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