U.S. patent application number 10/379000 was filed with the patent office on 2003-08-14 for polymer-coated metal composites by dip autopolymerization.
Invention is credited to Agarwal, Rajat, Bell, James P., Zhang, Xu.
Application Number | 20030153704 10/379000 |
Document ID | / |
Family ID | 24780892 |
Filed Date | 2003-08-14 |
United States Patent
Application |
20030153704 |
Kind Code |
A1 |
Bell, James P. ; et
al. |
August 14, 2003 |
Polymer-coated metal composites by dip autopolymerization
Abstract
A composition and method is described for providing conformal
protective or decorative polymer coatings on metals such as
aluminum, copper, iron, steel, zinc, and their by dip
autopolymerization. In accordance with the present invention, an
acidic solution of organic monomer undergoes autopolymerization
upon contact with a metal substrate, thereby forming a polymeric
coating on the substrate. The method comprises providing the acidic
monomer solution, dipping the metal substrate to be coated for a
prescribed period of time depending on the thickness of the coating
desired, and then removing the substrate from the solution.
Importantly, the polymerization requires no application of external
driving force, such as thermal or electrical energy. The coatings
thus formed are up to 50 microns thick, and conform to the shape of
the substrate. These coatings further have uniform thickness, and
excellent thermal stability and protective properties. In one
preferred embodiment of the present invention, the composition
comprises an acidic solution of an organic electron acceptor
monomer that undergoes autopolymerization in contact with a metal
substrate, thereby forming a polymeric coating on the substrate. In
another preferred embodiment of the present invention, the
composition comprises an acidic solution of an organic electron
acceptor monomer and an organic electron donor monomer that undergo
autopolymerization in contact with a metal substrate, thereby
forming a polymeric coating on the substrate. Metal-polymer
composites are also described.
Inventors: |
Bell, James P.; (Mansfield,
CT) ; Zhang, Xu; (Storrs, CT) ; Agarwal,
Rajat; (Willington, CT) |
Correspondence
Address: |
FISHMAN, DIONNE & CANTOR
88 Day Hill Road
Windsor
CT
06095
US
|
Family ID: |
24780892 |
Appl. No.: |
10/379000 |
Filed: |
March 4, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10379000 |
Mar 4, 2003 |
|
|
|
08692515 |
Aug 6, 1996 |
|
|
|
5807612 |
|
|
|
|
Current U.S.
Class: |
526/262 ;
427/388.4; 427/435 |
Current CPC
Class: |
H05K 3/0079 20130101;
Y10T 428/31692 20150401; B05D 7/142 20130101; Y10T 428/31699
20150401; H05K 3/3452 20130101; C09D 4/00 20130101; C09D 4/00
20130101; C08F 220/00 20130101 |
Class at
Publication: |
526/262 ;
427/388.4; 427/435 |
International
Class: |
C08F 026/06 |
Claims
What is claimed is:
1. A polymer-coated metal composite consisting of a metal substrate
and a single layer of polymeric coating produced by: providing an
acidic starting solution consisting essentially of at least one
electron acceptor monomer selected from the group consisting of
acrylamides, alkyl, cycloalkyl, aryl, and aralkyl acrylates,
methacrylamide, alkyl, cycloalkyl, aryl, and aralkyl methacrylates,
acrolein, methacrolein, thiocarboxylic acids, thiocarboxamides,
alkyl, cycloalkyl, aryl, and aralkyl thiocarboxylates,
dithioacrylic acids, dithiomethacrylic acids, and alkyl,
cycloalkyl, aryl, and aralkyl esters of dithiocarboxylic acids; at
least one electron donor monomer selected from the group consisting
of 1-alkenes, alkyl, cycloalkyl, or aralkyl-substituted 1-alkenes,
internal olefins, alkyl, cycloalkyl, aryl, or aralkyl-substituted
internal olefins, conjugated dienes, and alkyl, cycloalkyl, aryl,
and aralkyl-substituted conjugated dienes; and at least one solvent
which dissolves or forms an emulsion of the monomers, wherein the
at least one electron acceptor monomer and the at least one
electron donor monomer are polymerizable at the metal substrate
upon contact with the metal substrate in the absence of other
catalyst or catalysts; dipping the metal substrate into the
starting solution, wherein the metal substrate is a metal which
will initiate polymerization of the at least two monomers on the
surface of the metal substrate in the absence of other catalyst or
catalysts; and leaving the metal substrate in the starting solution
for a time effective to form a polymeric coating by polymerization
which occurs at the metal substrate and upon contact with the metal
substrate in the absence of other catalyst or catalysts.
2. The polymer-coated composite of claim 1, wherein the pH of the
starting solution is less than 6.5.
3. The polymer-coated composite of claim 1, wherein the metal
substrate is a metal selected from the group consisting of
aluminum, copper, iron, steel, zinc, transition metals, chromium,
tin, indium, nickel, cobalt, titanium, and alloys thereof.
4. The polymer-coated composite of claim 3, wherein the metal
substrate is aluminum, copper, iron, zinc, or steel.
5. The polymer-coated composite of claim 1, wherein the electron
acceptor monomer is 4-carboxyphenyl maleimide, N-phenyl maleimide,
bis maleimide, or 2-(methacryloyloxy)ethyl acetoacetate.
6. A polymer-coated metal composite produced by: providing an
acidic starting solution consisting essentially of at least one
electron acceptor monomer selected from the group consisting of
acrylamides, alkyl, cycloalkyl, aryl, and aralkyl acrylates,
methacrylamide, alkyl, cycloalkyl, aryl, and aralkyl methacrylates,
acrolein, methacrolein, thiocarboxamides, alkyl, cycloalkyl, aryl,
and aralkyl thiocarboxylates, and alkyl, cycloalkyl, aryl, and
aralkyl esters of dithiocarboxylic acids; at least one electron
donor monomer selected from the group consisting of 1-alkenes,
alkyl, cycloalkyl, or aralkyl-substituted 1-alkenes, internal
olefins, alkyl, cycloalkyl, aryl, or aralkyl-substituted internal
olefins, conjugated dienes, and alkyl, cycloalkyl, aryl, and
aralkyl-substituted conjugated dienes; and at least one solvent
which dissolves or forms an emulsion of the monomers, wherein the
at least one electron acceptor monomer and the at least one
electron donor monomer are polymerizable at a metal substrate upon
contact with the metal substrate in the absence of other catalyst
or catalysts; dipping the metal substrate into the starting
solution, wherein the metal substrate is a metal which will
initiate polymerization of the at least two monomers on the surface
of the metal substrate in the absence of other catalyst or
catalysts; and leaving the metal substrate in the starting solution
for a time effective to form a polymeric coating by polymerization
which occurs at the metal substrate and upon contact with the metal
substrate in the absence of other catalyst or catalysts.
7. The polymer-coated composite of claim 6, wherein the pH of the
starting solution is less than 6.5.
8. The polymer-coated composite of claim 6, wherein the metal
substrate is a metal selected from the group consisting of
aluminum, copper, iron, steel, zinc, transition metals, chromium,
tin, indium, nickel, cobalt, titanium, and alloys thereof.
9. The polymer-coated composite of claim 8, wherein the metal
substrate is aluminum, copper, iron, zinc, or steel.
10. A polymer-coated metal composite produced by: providing an
acidic starting solution consisting essentially of at least one
electron acceptor monomer selected from the group consisting of
acrylic acids, acrylamides, alkyl, cycloalkyl, aryl, and aralkyl
acrylates, methacrylic acid, methacrylamide, alkyl, cycloalkyl,
aryl, and aralkyl methacrylates, acrolein, methacrolein,
thiocarboxylic acids, thiocarboxamides, alkyl, cycloalkyl, aryl,
and aralkyl thiocarboxylates, dithioacrylic acids,
dithiomethacrylic acids, and alkyl, cycloalkyl, aryl, and aralkyl
esters of dithiocarboxylic acids; at least one electron donor
monomer selected from the group consisting of, alkyl, cycloalkyl,
or aralkyl-substituted 1-alkenes, internal olefins, alkyl,
cycloalkyl, aryl, or aralkyl-substituted internal olefins,
conjugated dienes, and alkyl, cycloalkyl, aryl, and
aralkyl-substituted conjugated dienes; and at least one solvent
which dissolves or forms an emulsion of the monomers, wherein the
at least one electron acceptor monomer and the at least one
electron donor monomer are polymerizable at a metal substrate upon
contact with the metal substrate in the absence of other catalyst
or catalysts; dipping the metal substrate into the starting
solution, wherein the metal substrate is a metal which will
initiate polymerization of the at least two monomers on the surface
of the metal substrate in the absence of other catalyst or
catalysts; and leaving the metal substrate in the starting solution
for a time effective to form a polymeric coating by polymerization
which occurs at the metal substrate and upon contact with the metal
substrate in the absence of other catalyst or catalysts.
11. The polymer-coated composite of claim 10, wherein the pH of the
starting solution is less than 6.5.
12. The polymer-coated composite of claim 12, wherein the metal
substrate is a metal selected from the group consisting of
aluminum, copper, iron, steel, zinc, transition metals, chromium,
tin, indium, nickel, cobalt, titanium, and alloys thereof. cm 13.
The polymer-coated composite of claim 12, wherein the metal
substrate is aluminum, copper, iron, zinc, or steel.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 08/692,515, filed on Aug. 6, 1996, which is incorporated herein
in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to compositions and methods for
coating metal substrates. In particular, this invention relates to
compositions and methods for forming a polymeric coating on a metal
surface by dip autopolymerization. Importantly, the polymerization
requires no application of external driving force, such as thermal
or electrical energy. The coatings thus formed are uniform,
conformal and pinhole-free. The method is suitable for forming
coatings with a variety of desirable properties, such as corrosion
and erosion resistance, abrasion resistance, and electrical and
thermal insulation.
[0004] 2. Brief Description of the Prior Art
[0005] Surface treatment, i.e., coating, of metals plays an
important role in metal applications. Such coating processes may be
applied for protective or decorative purposes. Conventional coating
processes include, for example, spray painting, electrodeposition,
electrophoresis, powder coating, anodization, and chromate
conversion coatings. Metals ordinarily treated by these methods
include iron, aluminum, copper, and their alloys. Each of these
process technologies are currently used extensively in the metal
finishing industry, and each has certain advantages and
disadvantages.
[0006] For example, U.S. Pat. Nos. 5,232,560 and 5,466,357 to Bell
et al. describe a method for electropolymerization in a
substantially aqueous solution to form copolymer coatings onto
electrically conductive substrates. Similarly, as described in U.S.
Pat. No. 5,238,542 to Bell at al., cyclic N-substituted
methacrylamide monomers may be electropolymerized onto electrically
conductive filler. Electropolymerization is advantageous in that it
allows the deposition of thick, thermally stable coatings. But
while suitable for their intended purposes, electropolymerization
requires the application of electrical energy as a driving
force.
[0007] The practice of the other above-mentioned methods can be
expensive, and result in undesirable waste products that must be
subsequently disposed of. Spray painting may require a separate
prepolymerization step prior to application of the coating.
Electrodeposition and anodizaton require specialized equipment and
thus capital investment. Many technologies require the input of
heat or thermal energy as a driving force for the deposition
processes. For these reasons, industry is in constant search of
less expensive technologies to improve competitiveness in the world
marketplace. In addition, increasing environmental concerns call
for cleaner technologies to meet government regulations and reduce
waste treatment and disposal cost.
[0008] In particular, aluminum is the most widely used metal today
by volume. Areas of application for aluminum range from aerospace
to marine to architectural. In almost all cases where aluminum is
used, a surface treatment and finishing process are applied. The
two most common surface treatments for aluminum is anodization and
conversion coating. Anodization is an energy-intensive process,
using phosphates. Conversion coating uses chromates and other heavy
metal ions. Both of these treatments thus generate environmentally
hazardous waste, which must be properly disposed of. Accordingly,
there remains a need for efficient, inexpensive compositions and
coating methods for aluminum and other metals that generate minimal
waste.
SUMMARY OF THE INVENTION
[0009] The above-discussed and other problems and deficiencies of
the prior art are overcome or alleviated by the composition and
method of the present invention for coating metals by dip
autopolymerization, wherein organic monomer in an acidic solution
undergoes autopolymerization upon contact with a metal substrate at
room temperature, thereby forming a polymeric coating on the metal
substrate. Importantly, the polymerization requires no application
of external driving force, such as thermal or electrical energy.
The coatings thus formed may be up to 50 or more microns thick, and
have molecular weights up to about 250,000 or more.
[0010] In one preferred embodiment of the present invention, the
composition comprises an acidic solution of an organic electron
acceptor monomer that undergoes autopolymerization in contact with
a metal substrate, thereby forming a polymeric coating on the
substrate. In another preferred embodiment of the present
invention, the composition comprises an acidic solution of an
organic electron acceptor monomer and an organic electron donor
monomer that undergo autopolymerization in contact with a metal
substrate, thereby forming a polymeric coating on the substrate. By
electron acceptor monomer it is meant a monomer or small polymer
having at least one electron withdrawing group, capable of further
polymerization with the electron donor monomer. By electron donor
monomer it is meant a monomer or small polymer having at least one
electron donating group, capable of further polymerization with an
electron acceptor monomer. Other metals suitable for use in the
practice of the present invention include copper, iron, and
zinc.
[0011] In another embodiment of the present invention, there is
described metal-polymer composites comprising a metal such as
copper, aluminum, iron, zinc, steel, or alloys thereof, and a
polymeric coating.
[0012] In accordance with the method of the present invention, an
acidic monomer solution is provided, and a clean metal substrate is
submerged (dipped) into the monomer solution for a prescribed
period of time. Polymerization occurs on the metal surface in the
monomer solution spontaneously without application of any external
driving force, such as thermal or electrical energy. Rather,
polymerization is initiated at the metal-solution interface due to
the interaction of the metal surface with the monomer in solution.
The monomer converts directly to a polymer coating on the metal
surface and thus obviates the need for the prepolymerization step
sometimes required in conventional coating methods.
[0013] The present invention results in the formation of a uniform,
conformal and pin-hole free polymeric organic coatings on metal
surfaces. Physical and chemical properties such as corrosion and
erosion resistance, electrical or thermal insulation, adhesion, and
scratch resistance of the coating may be varied simply by varying
the composition of the monomer solution. The polymer coatings
formed may also be used as a primer for further surface finishing
treatments or as a final coating.
[0014] The process can be very easily applied in various industrial
settings. Relative to other polymerization techniques, the method
is robust, being insensitive to moisture and other impurities. No
special metal treatment is required other than cleaning of the
metal surface. In the practice of the method, the low cost of
operation relative to other coating processes will result in
considerable energy savings. Sophisticated equipment is not
required, so capital equipment cost is also minimal. Solvent use is
minimal, and no heavy metals are required to be present. Also,
water is frequently used a co-solvent with a suitable solvent to
dissolve the monomer, resulting in a more environmentally
acceptable coating composition and method.
[0015] The above-discussed and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Referring now to the drawings wherein like elements are
numbered alike in the several FIGURES:
[0017] FIG. 1 is a transmission IR spectrum of a typical
poly(4-CPMI/styrene) coating on steel obtained according to the
method of the present invention;
[0018] FIG. 2 is transmission IR spectra of (a) poly(4-CPMI), (b)
polystyrene, and (c) a blend of the two hompolymers;
[0019] FIG. 3 is a plot showing the absorbance ratio of 1510
cm.sup.-1 to 1493 cm.sup.-1 vs. the ratio of 4-CPMI to styrene in a
blend of polystyrene homopolymer and poly(4-CPMI);
[0020] FIG. 4 is a plot showing the effect of varying the 4-CPMI to
styrene feed ratio on the 4-CPMI to styrene ratio in a
poly(4-CPMI/styrene) according to the present invention;
[0021] FIG. 5 is a plot showing the effect of varying the 4-CPMI
content in the feed on the yield of poly(4-CPMI/styrene) according
to the present invention;
[0022] FIG. 6 is a plot showing the effect of varying
concentrations on yield of poly(4-CPMI/styrene) according to the
present invention;
[0023] FIG. 7 is a plot showing the effect of varying the pH of the
monomer solution on the yield of poly(4-CPMI/styrene) according to
the present invention;
[0024] FIG. 8 is a TGA thermogram of a poly(4-CPMI/styrene) coating
obtained on steel by the method according to the present invention
as measured under (a) nitrogen atmosphere and (b) oxygen
atmosphere;
[0025] FIG. 9 is a plot showing the weight average molecular weight
(M.sub.w) of a poly(NPMI/styrene) coating formed by the method
according to the present invention as a function of total monomer
concentration for solutions of varying pH;
[0026] FIG. 10 is an FTIR overlaid spectra showing formation of
poly(NPMI/styrene/MEA) polymer in accordance with the present
invention;
[0027] FIG. 11 is a plot showing the effect of polymerization time
on the coating thickness for various monomer feed compositions in
the practice of the present invention;
[0028] FIG. 12 is a plot showing the effect of polymerization
temperature over time on the measured coating thickness of
poly(NPMI/styrene/MEA/BMI) aluminum composite according to the
present invention;
[0029] FIG. 13 is a plot showing the effect of varying 4-CPMI
content in the feed on the glass transition temperatures of
poly(4-CPMI/MMA) coatings in accordance with the present invention;
and
[0030] FIGS. 14(A)-(C) are photographs showing corrosion resistance
of poly(NPMI/styrene/MEA) aluminum composites according to the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] The present invention is directed to a composition and
method for producing polymeric coatings on clean metal substrates,
wherein organic monomer in an acidic solution undergoes
autopolymerization upon contact with a metal substrate at room
temperature, thereby forming a polymeric coating on the metal
substrate. The coating solution and method is environmentally
benign. The method is particularly advantageous in that it requires
no external application of thermal or electrical energy.
[0032] The composition of the present invention comprises an acidic
solution of organic monomer, capable of undergoing
autopolymerization upon contact with a metal substrate, thereby
forming a polymeric coating on the metal substrate. As used herein,
and as is shown in the accompanying Examples 1-5,
"autopolymerization" refers to a polymerization process whereby
polymerization of monomers occurs upon exposure of the monomer
solution to the metal substrate in the absence of other catalyst or
catalysts. Water may be used as a co-solvent with a suitable
solvent to dissolve the monomers, resulting in a more
environmentally acceptable coating composition and method.
Polymerization ordinarily proceeds at room temperature. However,
heat may be applied to the solution in order to increase the rate
of polymerization, or to effect a variation in the properties of
the final coating.
[0033] Although the preferred embodiments of the present invention
contemplate at least a two-component monomer solution, a
one-component monomer solution is also with the scope of the
present invention. Furthermore, the term "monomer" as used herein
is intended to comprise both monomers and small polymers, e.g.,
small molecules with one or more repeating units. Such polymers
must be soluble in the solvents suitable for use with the present
invention, and capable of undergoing autopolymerization upon
exposure to a metal substrate.
[0034] In one preferred embodiment of the present invention, the
composition comprises an acidic solution of organic electron
acceptor monomer that undergoes autopolymerization in contact with
a metal substrate, thereby forming a polymeric coating on the
substrate. Thus, one particularly preferred embodiment of the
present invention comprises an acidic solution of methyl
methacrylate (MMA) and 4-carboxyphenyl maleimide (4-CPMI).
Introduction of a steel substrate into this solution induces
autopolymerization of the MMA and 4-CPMI, thereby forming a
poly(MMA/4-CPMI) coating onto the steel. Polymeric coatings thus
formed may be up to about 50 microns or more thick. They are even,
conformal, and pinhole-free. Other particularly preferred
embodiments of this type comprise an acidic solution of 4-CPMI and
acrylonitrile (AN). Preferably, these monomer solutions are used to
coat steel, although other metals might also be used. Without being
bound by theory, it is hypothesized that when used to coat steel,
polymerization is initiated by a redox process on the surface of
the steel substrate. It is unlikely that the process is Lewis-acid
catalyzed, as the addition of Fe.sup.+3 to the acidic monomer
solution failed to induce polymerization.
[0035] Another preferred embodiment of the present invention
comprises an acidic solution of an electron acceptor monomer and an
electron donor monomer. By electron acceptor monomer it is meant a
monomer or small polymer having at least one electron withdrawing
group, capable of further polymerization with the electron donor
monomer. By electron donor monomer it is meant a monomer or small
polymer having at least one electron donating group, capable of
polymerization with an electron acceptor monomer.
[0036] The molar ratio of the electron donor monomer to the
electron acceptor monomer will generally range from about 5:95 to
about 95:5. The preferred concentration of electron donor monomer
in solution is generally between about 0.01-5 moles per mole of
electron acceptor monomer, and preferably about 0.05-1 moles of
electron donor monomer per mole of electron acceptor monomer.
[0037] Typical electron withdrawing groups for electron acceptor
monomers include, but are not limited to, carbonyl, carboxyl,
carboxylate, carboxamide, nitrile groups and the like. Thio
analogues of the oxygen-containing functional groups are also
effective groups for electron acceptor monomers.
[0038] Illustrative electron acceptor monomers include, but are not
limited to, acrylic acid, acrylamide, acrylonitrile, and alkyl
acrylates in which the alkyl moiety contains 1 to 40 carbon atoms.
Preferably, the alkyl moieties have 1 to 22 carbon atoms, including
methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl
acrylate, butyl acrylate, isobutyl acrylate, sec-butyl acrylate,
hexyl acrylate, 2-ethylhexyl acrylate, heptyl acrylate, octyl
acrylate, isooctyl acrylate, nonyl acrylate, decyl acrylate,
undecyl acrylate, dodecyl acrylate, tridecyl acrylate, pentadecyl
acrylate, hexadecyl acrylate, octadecyl acrylate and the like.
[0039] Other electron acceptor monomers include but are not limited
to aryl and aralkyl acrylates such as phenyl acrylate and p-tolyl
acrylate, cycloalkyl acrylates such as cyclohexyl acrylate,
methacrylic acid, methacrylamide, methacrylonitrile, and alkyl
methacrylates in which the alkyl moiety contains 1-40 carbon atoms.
Preferably, the alkyl moieties contain 1 to 22 carbon atoms,
including methyl methacrylate, ethyl methacrylate, propyl
methacrylate, isopropyl methacrylate, butyl methacrylate, isobutyl
methacrylate, sec-butyl methacrylate, amyl methacrylate, hexyl
methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl
methacrylate, nonyl methacrylate, decyl methacrylate, undecyl
methacrylate, dodecyl methacrylate, tridecyl methacrylate,
pentadecyl methacrylate, hexadecyl methacrylate, octadecyl
methacrylate, and the like.
[0040] Other electron acceptor monomers contemplated for use with
the present invention include, but are not limited to, aralkyl
methacrylates such as benzyl methacrylate, aryl methacrylates such
as phenyl methacrylate and p-tolyl methacrylate and cycloalkyl
methacrylates such as cyclohexyl methacrylate, as well as various
ketones, such as methyl vinyl ketone, ethyl vinyl ketone, methyl
isopropenyl ketone, acrolein, methacrolein and the like.
[0041] Sulphur-containing compounds which are effective electron
acceptor monomers include thiocarboxylic acids such as thioacrylic
acid and thiomethacrylic acid, thiocarboxamides such as
thioacrylamide and thiomethacrylamide, as well as alkyl, aryl,
aralkyl and cycloalkyl thiocarboxylates such as methyl
thioacrylate, methyl thiomethacrylate, phenyl thioacrylate, benzyl
thiomethacrylate, cyclohexyl thiomethacrylate and the like.
Dithioacrylic acid, dithiomethacrylic acid and the esters of these
dithiocarboxylic acids are also useful electron acceptor
monomers.
[0042] Electron donor monomers are typically acyclic and cyclic
monoolefins and conjugated dienes. The monoolefins may be
alpha-olefins or internal olefins, including cycloolefins, and may
be unsubstituted or may contain alkyl, aryl or aralkyl
substituents. The effective alpha-olefins include, but are not
limited to, 1-alkenes such as ethylene, propylene, 1-butene,
1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene,
1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene and other
1-alkenes containing up to 40 carbon atoms.
[0043] 1-Alkenes having substituents further removed from the
double bond are also effective, including but not being limited to
3-methyl-1-pentene, 4-methyl-1-pentene, 3-methyl-1-hexene,
5-methyl-1-hexene and the like.
[0044] Aromatic substituted alpha-olefins are particularly
effective electron donor monomers and include styrene,
alpha-methylstyrene, p-methylstyrene, o-methylstyrene,
m-methylstyrene, p-ethylstyrene, p-propylstyrene,
p-isopropylstyrene, p-butylstyrene, p-nonylstyrene,
p-chlorostyrene, and other 1-alkenes containing alkyl substituted
aromatic moieties. Similarly, alpha-olefins containing aromatic
substituents further removed from the double bond such as
3-phenyl-1-butene, 4-p-methylphenyl-1-pentene and the like are
effective electron donor monomers.
[0045] Internal olefins which are useful electron donor monomers in
the present invention may be unsubstituted alkyl or aryl
substituted acyclic or cyclic monoolefins, including 2-butene,
2-pentene, 3-hexene, 2-heptene, 3-heptene, 2-octene, 3-octene,
4-octene, 2-nonene, 3-nonene, 4-nonene, 2-methyl-2-butene,
2-methyl-2-pentene, 4-methyl-2-pentene, 2-methyl-2-hexene,
4-methyl-2-hexene, 5-methyl-2-hexene, 2,5-dimethyl-3-hexene,
1-phenyl-3-pentene, cyclopentene, cycloheptene, cyclooctene,
cyclononene, cyclodecene, cycloundecene, cyclododecene,
1-methylcycloheptene, 5-methylcycloheptene and the like.
[0046] Illustrative conjugated dienes useful as electron donor
monomers according to this invention include butadiene, isoprene,
2-chloro-1,3-butadiene, 2,3-dichloro-1,3-butadiene,
2,3-dimethylbutadiene, propylene, 2,4-hexadiene,
2-methyl-1,3-pentadiene, 2-ethyl-1,3-butadiene,
2-propyl-1,3-butadiene, 2-phenyl-1,3-butadiene,
3-methyl-1,3-pentadiene, 2-ethyl-1,3-pentadiene,
2-methyl-1,3-hexadiene and the like. Cyclic conjugated dienes such
as 1,3-cyclohexadiene, 1,3-cycloheptadiene, 1,3-cyclooctadiene,
1,3-cyclononadiene and the like are also effective electron donor
monomers.
[0047] Preferred electron acceptor monomers are 4-carboxyphenyl
maleimide (4-CPMI), N-phenyl maleimide (NPMI), bis-maleimide (BMI),
and 2-(methacryloyloxy)ethyl acetoacetate (MEA), and acrylonitrile
(AN). A preferred electron donor monomer is styrene.
[0048] A particularly preferred embodiment of the electron
donor/electron acceptor monomer solution comprises styrene and
N-phenyl maleimide. Without being bound by theory, it is
hypothesized that in this system, the driving force for
autopolymerization in the electron donor/electron acceptor systems
on aluminum substrates is derived from the metal surface where
Lewis acids such as Al.sup.3+ are generated when immersed in an
acidic solution. The Al.sup.3+ ion is classified as a hard Lewis
acid and therefore has a strong affinity for an electron pair.
Lewis acids may increase the electrophilicity of the electron-poor
monomer by complexing to its lone pair electrons. This in turn
increases the electron disparity between the reaction partners. It
is proposed here that these transition metal Lewis acids interact
with an electron acceptor monomer such as NPMI, thereby increasing
the electrophilicity of the monomer and resulting in the
spontaneous formation of an NPMI-styrene tetramethylene diradical.
The polymerization may then proceeds by alternate addition of the
electron donor monomer, e.g., styrene, and the electron acceptor
monomer, e.g., NPMI, to the growing radical. This kind of
alternating addition of monomers is also known as
cross-propagation.
[0049] Where two monomers are present in the monomer solution, the
polymer formed therefrom may be an essentially alternating
copolymer, that is, a copolymer in which the comonomer units are
present in essentially equimolar quantities and are situated
alternately along the copolymer chain. However, random polymers may
also be formed by the autopolymerization method of the present
invention. Furthermore, a third and even a fourth monomer (or more)
may be introduced into the monomer solution. Variation in the
number of monomers, the monomer ratios, and their composition
allows for variation in the final properties of the formed coating.
Other process parameters which may be used to affect the final
properties of the coatings include monomer concentration, metal
surface pretreatment, polymerization time, and drying
temperature.
[0050] A single solvent may be used to dissolve the monomers, or a
mixture of several hydrocarbon, halogenated, aromatic, or oxygen
donating solvents. Appropriate solvents include, but are not
limited to, benzene, toluene, chloroform, methylene chloride,
hexane, acetone, tetrahydrofuran, acetonitrile, dimethyl formamide,
dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide.
In many instances water may be used as a co-solvent, especially
where other polar solvents, such as acetone, tetrahydrofuran,
acetonitrile, dimethyl formamide, dimethylacetamide,
N-methyl-2-pyrrolidone, or dimethyl sulfoxide are used. Such
aqueous systems are preferable because the process cost is lowered.
Where polymerization proceeds via a free radical mechanism, aqueous
systems would favor such a mechanism, thus allowing high molecular
weight polymer coatings to be easily obtained.
[0051] The solvent or solvent mixture used in the method of the
present invention must be compatible with the monomer(s) used. The
solvent or solvent mixture should have reasonable solubility for
the monomer(s) used in the process, i.e, able to dissolve each
monomer to a concentration of about 0.01-5 molar, and preferably to
a concentration of about 0.1-1 molar. Alternatively, the monomers
may be present in solution as an aqueous emulsion. However, such
solvent or solvent mixture preferably has significantly lower
solubility for the formed polymer, as the polymer should be
insoluble to prevent it from dissolving into the solution. Using
the proper solvent or solvent mixture thus allows the formation of
a uniform coating on the metal substrate.
[0052] Importantly, the pH of the monomer solution must be acidic,
having a pH of preferably less than about 6.5. The optimal pH value
is within range of about 1 to about 6.5, and preferably from about
2.5 to about 5.5. Solutions having an essentially neutral pH failed
to undergo autopolymerization, even in the presence of a metal
substrate. When the monomer solution is itself acidic as prepared,
pH adjustment may be unnecessary. Otherwise, the pH may be adjusted
using acids such as dilute sulfuric acid, hydrochloric acid, dilute
nitric acid, acetic acid, phosphoric acid, and citric acid.
[0053] While not required, a accelerator may be added to the
monomer solution to increase the reaction rate. Preferably, the
accelerator is a metal salt. The salt most likely interacts with
the electron acceptor monomer to increase its reactivity. Possible
salts may include, but are not limited to, halides of aluminum,
zinc, nickel, magnesium, cerium, tin zirconium, chromium, vanadium,
titanium and molybdenum, and boron trifluoride, ethyl aluminum
dichloride or ethyl aluminum sesquichloride. The amount of
accelerator used may vary over a wide range. However, the preferred
concentration is generally between about 0.01-5 mole of accelerator
per mole of electron acceptor monomer, and most preferably, about
0.05-1 mole of accelerator per mole of electron acceptor
monomer.
[0054] Metal substrates which may be coated by the method of the
present invention include a number of economically important
metals, including aluminum, iron, copper, steel, zinc, and alloys
thereof. Other metals may be possible, for example, other
transition metals and their alloys, such as chromium, tin, indium,
nickel, cobalt, and titanium and their alloys. Metals such as
magnesium, or inert substances such as silica or glass are not
coated by the autopolymerization method of the present
invention.
[0055] In accordance with the method of the present invention, the
above-described acidic monomer solution is provided, and a clean
metal substrate is submerged (dipped) into the monomer solution for
a prescribed period of time. Polymerization occurs on the metal
surface in the monomer solution spontaneously without application
of any external driving force, such as thermal or electrical
energy. Rather, polymerization is apparently initiated at the
metal-solution interface due to the interaction of the metal
surface with the monomers in solution, converting the monomers
directly to polymer coatings on metal surface and obviating the
prepolymerization step in the conventional coating methods.
[0056] No special surface treatments are necessary for the
autopolymerization method. The only requirement is a clean surface.
Furthermore, simple cleaning processes such as detergent washing,
grit blasting, and trichloroethylene degreasing are sufficient to
remove dirt and organic contaminants on the metal surface.
[0057] The method of the present invention may be used for various
applications requiring coatings on metals, such as automobile
bodies and parts, circuit board heat sinks, electric motor stators,
air conditioner fins, and any other type of metal parts or fittings
requiring an organic coating. The process can be scaled up to
industrial requirements without apparent complications. Process
scale-up involves simply larger solution volume formulations and
use of larger polymerization tanks.
[0058] Thus, still another embodiment of the present invention is a
polymer-metal composite comprising a metal substrate and a
polymeric coating. In this embodiment, the metal substrate
comprises aluminum, copper, steel, iron, or zinc, and alloys
thereof. Preferably, the polymeric coating comprises a coating of
poly(4-CPMI/styrene), poly(4-CPMI/MMA), poly(4-CPMI/AN),
poly(NPMI/styrene), poly(NPMI/BMI/styrene), and
poly(NPMI/BMI/MEA/styrene).
[0059] The composition and method for autopolymerization on metals
of the present invention allows polymer coatings to be synthesized
directly onto the metal surface. The method may be conducted at
room temperature, and no external driving force, i.e., thermal or
electrical energy, is required. The key difference between this
process and other conventional coating processes, such as dip
coating or electrophoresis, is that here, the polymer chains "grow"
at the metal surface, allowing formation of uniform coatings on
objects with complex topographies. The composition and method also
results in better adhesion of the metal substrate to the polymer
coating, because wetting of the metal surface is easier by
monomers, compared with polymers. Furthermore, it is not necessary
to evaporate large quantities of solvent during the drying of the
polymer coating, as is required by conventional coating
methods.
[0060] The present invention is further illustrated by the
following non-limiting examples.
[0061] Materials
[0062] Styrene and methyl methacrylate were obtained from Fisher
Scientific Company. Inhibitor-free styrene was obtained by vacuum
distillation at 40.degree. C. Methyl methacrylate was also purified
by vacuum distillation. N-methyl-2-pyrrolidone (NMP) were used as
received from Fisher Scientific Company. 4-Carboxyphenyl maleimide
(4-CPMI) was prepared according to the method described by B. S.
Rao, in Journal of Polymer Science: Part C: Polymer Letters, Vol.
26, p. 3 (1988). Monomer purity was confirmed by nuclear magnetic
resonance (NMR) and differential scanning calorimetry (DSC). The
DSC thermogram showed only one sharp melting peak at 241.degree. C.
N-Phenyl maleimide (NPMI) and bis-maleimide (BMI) were purchased
from Mitsui Toatsuo Chemical Co., Japan. NPMI was recrystallized
from cyclohexane and BMI was used as obtained.
2-(Methacryloyloxy)ethyl acetoacetate (MEA) from Aldrich Chemical
Co. was purified by passing through DHR-4 inhibitor removal columns
from Scientific Polymer Products.
[0063] Metal substrates were SAE 1010 carbon steel coupons from
Q-Panel Company, and aluminum 1100, 2024, 6061, or 7071 alloy. The
steel surface was cleaned by washing with detergent, rinsing with
an ample amount of distilled water, and oven-drying at 90.degree.
C. Aluminum was cleaned by one of two methods: (i) degreasing with
5% aqueous Micro.RTM. (an alkaline soap) solution in an ultrasonic
bath for 5 min., rinsing with distilled water, treating with 5%
hydrofluoric acid for 15 sec., and rinsing with distilled water; or
(ii) grit blasting using 170 mesh silica, then rinsing with
distilled water.
[0064] Characterization
[0065] A Nicolet 60SX Fourier transform infrared spectrometer
(FT-IR) was used to characterize the composition of the polymer
coating at a resolution of 4 cm.sup.-1, by pressing KBr pellets
from mixtures of about 150 mg KBr powder and 10 mg polymer. A
differential scanning calorimeter (DSC) model 2920 and
thermogravimetric analyzer (TGA) model 2950 from TA Instruments
were used for thermal analyses, using a heating rate of 10.degree.
C./min for the DSC and 20.degree. C./min for the TGA. The molecular
weight of the polymers were obtained by GPC on a Waters 150C gel
permeation chromatograph using polystyrene standards. The
measurements were conducted at 35.degree. C. in tetrahydrofuran.
Dielectric constant measurements of the coating were obtained using
a Time Domain Dielectric Spectrometer (TDDS) from IMASS, Inc.
Corrosion protection properties of the polymer coatings were
studied by exposing samples to a 5% NaCl salt fog, following the
ASTM B-117 test method.
EXAMPLE 1
Poly(4-CPMI/Styrene) on Steel
[0066] General Polymerization Procedure: A monomer solution was
prepared by first dissolving the styrene and 4-CPMI in NMP on a 1:1
mole basis, then mixing with water to a final concentration of 0.2
M in each monomer. Polymerization was initiated by dipping a
cleaned substrate in the solution. Simultaneous polymerization of
the monomers and deposition of the formed polymers occurred at the
metal surface. After a prescribed period of time, ranging from 1
minute to 1 hour, the metal substrate was withdrawn from the
solution, throughly rinsed with distilled water, and dried in an
oven at 50 to 150.degree. C. for 1-24 hours. Further drying was
carried out at 250.degree. C. under vacuum to remove any trapped
solvent.
[0067] Results and Discussion: The IR spectrum of a typical
poly(4-CPMI/styrene) polymeric coating obtained by the above
procedure is shown in FIG. 1. The symmetric and asymmetric C.dbd.O
stretching of the carbonyl groups in the imide ring absorb at 1776
cm.sup.-1 and 1714 cm.sup.-1, respectively. The strong absorption
at 1383 cm.sup.-1 is due to symmetric C--N--C stretching of the
imide ring. The peak at 1512 cm.sup.-1 is attributed to the
para-substituted phenyl ring in 4-CPMI, while the aromatic C--C
stretching at 1608 cm.sup.-1 has contributions from both 4-CPMI and
styrene. The characteristic absorption of polystyrene at 1493 and
1452 cm.sup.-1 is assigned as the semi-circle stretching and mixed
C--H bending of a mono-substituted phenyl ring. The C--H bending of
the vinyl group at 991 cm.sup.-1 and 908 cm.sup.-1 of styrene, and
the C--C stretching of the imide ring at 949 cm.sup.-1 of 4-CPMI
are absent in the spectrum, confirming that the polymerization
reaction occurred via opening of the double bond.
[0068] To determine the composition of the coating, a calibration
was obtained using homopolymers of 4-CPMI and styrene. FIGS. 2(a)
and 2(b) are transmission IR spectra of polystyrene and
poly(4-CPMI), respectively, obtained by solution polymerization in
THF. The spectrum of the blend of the homopolymers agrees with that
of the copolymer with no additional peaks appearing, as shown in
FIG. 2(c).
[0069] A calibration plot was then constructed using blends of
these two homopolymers at various ratios. The absorption peaks at
1510 cm.sup.-1 for styrene and that at 1493 cm.sup.-1 for 4-CPMI
were chosen at the characteristic peaks for 4-CPMI and styrene,
respectively. When the absorbance ratio of these two peaks is
plotted vs. the molar ratio of poly(4-CPMI) to polystyrene present
in the blend, as shown in FIG. 3, very good linearity is obtained.
The polymerization was controlled in such a way as to proceed at
low monomer conversion levels, in which case the monomer
concentrations can be assumed to be constant during the reaction
course. Using the above calibration, the ratio of 4-CPMI and
styrene incorporated into the polymer obtained were then determined
to be 1:1. As shown in FIG. 4, as the ratio of 4-CPMI to styrene in
the feed changes, the ratio of 4-CPMI to styrene in the polymer
coating remains essentially constant.
[0070] The molecular weight of the polymer formed was determined to
be fairly high, with the weight average molecular weight, M.sub.w,
of 146,000 by GPC, and a polydispersity index, M.sub.W/M.sub.N, of
2.4. These results are typical for free radical polymerization, in
that high molecular weight polymer can usually be obtained
relatively easily.
[0071] The effect of various process parameters on yield of
poly(4-CPMI/styrene) are shown in FIGS. 5, 6, and 7. FIG. 5 is a
plot of yield based on mole percent of 4-CPMI in the feed at a
polymerization time of five minutes. As shown in FIG. 6, yield
generally increases both with increased reaction time and increased
molar concentration of both monomers. Yield decreases with
increasing pH (FIG. 7).
[0072] The incorporation of the rigid five-membered ring of 4-CPMI
into the polymer backbone significantly hindered segmental rotation
and stiffened the chain, resulting in much improved thermal
stability as compared with polystyrene homopolymer. As shown in the
TGA thermogram in FIG. 8, when measured under nitrogen atmosphere,
the copolymer is stable up to 450.degree. C., after which it
undergoes a one-stage decomposition. The thermal stability is still
excellent in an oxygen atmosphere, where the onset temperature for
degradation is only lowered by about 50.degree. C., to 400.degree.
C. It is likely that the significant improvement in thermal
properties of the poly(4-CPMI/styrene) obtained by the method of
the present invention, as compared to polystyrene, is due to the
large proportion of 4-CPMI incorporated into the polymer chain.
[0073] A low dielectric constant is very critical for insulation
applications. Dielectric constant measurements on the coating at
selected frequencies are listed in Table 1 below. The
poly(4-CPMI/styrene) coating has a dielectric constant of about 2.6
in the frequency range investigated, comparable to or slightly
lower than that of one of the widely used commerical polyimides.
Thus the coating of the present invention will be an attractive
candidate for insulation applications.
1TABLE 1 Dielectric Constant of Poly(4-CPMI/styrene) Frequency (Hz)
Dielectric Constant 1 2.68 10 2.65 60 2.64 100 2.64 600 2.63 1000
2.63 6000 2.60 10000 2.55
EXAMPLE 2
Poly(NPMI/styrene) on Aluminum
[0074] General Polymerization Procedure: NPMI and styrene were
dissolved in NMP in a equimolar ratio to obtain the final
concentrations shown in FIG. 9. Dilute aqueous sulfuric acid (0.025
M) was then added slowly to the solution while stirring, until a
57/43 volume ratio of NMP/water was obtained.
[0075] The monomer solution was then purged with nitrogen.
Dissolved oxygen content was kept to less than 2 ppm. When the
pretreated aluminum was immersed in the monomer bath, a white,
swollen polymer coating began to form. Polymerization time for
samples in the bath varied from 10 minutes to 120 minutes. The
coated sample was then immersed in a gently stirred 10% NMP
solution for 1 hour, to remove excess NMP, and was oven dried at
150.degree. C. for 1 hour and then at 225 to 250.degree. C. for 4-6
hours.
[0076] Results and Discussion: The solvent quantity of the 57/43
NMP/water solution is close to the solubility limit of the
monomers. The polymer formed on the aluminum surface is insoluble
in the solution but is in a swollen state. The swollen nature of
the polymer coating permits the diffusion of monomers through the
polymer to reach the aluminum surface and also to react with the
propagating chain ends. Thus, coatings up to 50 .mu.m thick can be
obtained. Coating thickness can be controlled by varying the
polymerization time, or the monomer concentration.
[0077] FIG. 9 shows the weight average molecular weight (M.sub.w)
of the coatings formed by autopolymerization plotted as a function
of total monomer concentration for solutions of varying pH as
determined by GPC measurement. At low pH it is expected that a
greater number of Lewis acid sites should be generated at the
aluminum metal surface, resulting in a high rate of initiation.
This would give a relatively low molecular weight polymer, in
keeping with classical free radical polymerization equations. As
the pH increases, the number of Lewis acids formed should be less,
and thus the molecular weight of the polymer obtained should be
higher. It was further found that the polymerization reaction was
quenched by the addition of 2,2-diphenyl-1-picrylhydrazyl hydrate
(DPPH), confirming the free radical nature of the reaction of NPMI
and styrene autopolymerization on aluminum.
[0078] Furthermore, use of higher monomer concentration yielded
higher molecular weight polymers. At any given concentration,
molecular weight increased with solution pH. Polydispersity index
(PDI) ranged from 2.5 to 3.5 in all cases.
EXAMPLE 3
Poly(NPMI/MEA/styrene) on Aluminum
[0079] The polymer coatings of poly(NPMI/styrene) formed on
aluminum exhibited a tendency to crack while drying when the
coating thickness was greater than 10 microns. To solve this
problem, another monomer was incorporated into the polymer. The MEA
monomer is an electron acceptor due to presence of electron
withdrawing carbonyl groups next to the C.dbd.C bond, similar to
NPMI, and has been previously used in adhesives and anti-corrosion
primer compositions.
[0080] Thus, MEA was introduced in the feed solution and the
NPMI/MEA ratio was varied while maintaining a constant styrene
concentration. In this way, an equimolar ratio of acceptor and
donor monomers was maintained in the solution. Coatings of
poly(NPMI/MEA/styrene) were synthesized according to the general
procedure of Example 2, wherein NPMI and styrene were dissolved in
NMP. Dilute aqueous sulfuric acid (0.025 M) was then added slowly
to the solution while stirring, until a 57/43 volume ratio of
NMP/water was obtained. MEA was then added dropwise, yielding a
clear, yellow-colored solution of pH 3.3.
[0081] Inclusion of MEA in the polymer resulted in crack free
coatings and a significant improvement in the adhesion of the
swollen coating to the aluminum substrate. Corrosion resistance of
the coated samples was also improved by addition of MEA (see FIG.
14).
[0082] To detect the inclusion of MEA in the polymer coating, FTIR
spectra were collected on a series of polymer coatings made by
varying the NPMI/MEA ratio from 0.10M/0.00M to 0.00M/0.10M in
increments of 0.01M. The total concentration of NPMI and MEA was
kept at 0.10M. Styrene concentration was 0.10M. Spectra for the
series are shown in FIG. 10. As the concentration of MEA in feed is
increased, a carbonyl peak in the 1625-1635 cm.sup.-1 region
appears, and increases in intensity, relative to the 1774 cm.sup.-1
imide peak from NPMI. The low wave number from the MEA peak is due
to the hydrogen bonded enol form to the beta-diketone linkage. Also
a general broadening of the carbonyl bands in the 1720 cm.sup.-1
region occurs, due to the overlapping from MEA and NPMI. The 1598
cm.sup.-1 peak is due to the phenyl ring from NPMI. It also
decreases in intensity as MEA content relative to NPMI increases in
the resulting polymer.
EXAMPLE 4
Poly(NPMI/MEA/BMI/styrene) on Aluminum
[0083] A series of poly(NPMI/MEA/BMI/styrene) coatings on aluminum
were prepared by the method of Example 4, wherein the concentration
of BMI was 0.0025 M. Similar FTIR results were obtained as those
for Example 3. The addition of BMI to the comonomer solution also
illustrates the versatility of the autopolymerization process, in
that different monomers, or different monomer ratios may be used to
adjust the properties of the final coating. After the metal
substrate is coated with polymer, it is withdrawn from the bath and
then dried. During the drying process, water evaporates faster due
to its lower boiling point (the boiling point of NMP is 210.degree.
C.), and the relative content of NMP in the polymer coating
increases. Consequently, the swollen polymer coating is plasticized
and some flow of the coating occurs before all of the solvent
evaporates. This phenomenon occurs frequently when
poly(NPMI/MEA/styrene) coatings are dried.
[0084] To arrest this flow, BMI was incorporated to cross-link the
coating and increase the molecular weight of the coating. Addition
of 0.0025M BMI in the NMP/water monomer feed solution arrested the
flow of the coatings during drying and gave coatings with uniform
thickness.
[0085] FIG. 11 shows a plot of coating thickness obtained as a
function of polymerization time for various monomer compositions in
feed. The ordinate axis is plotted as the square root of time,
which results in linear fits of the measured coatings thickness.
This indicates that such a polymerization process is limited by
rate of diffusion of monomers to the propagating radicals. These
observations are consistent with the equations obtained using a
flat plate surface reaction model, where the rate limiting step is
the process of diffusion of monomers in the swollen polymer coating
being formed on the aluminum surface. According to this model, the
thickness of the polymer coating is proportional to the square root
of reaction time. As the ratio of MEA to NPMI in the feed was
increased, the rate of polymerization decreased. This suggests a
lower activity for MEA monomer compared to NPMI monomer.
Sand-blasting and HF etching pretreatments gave similar kinetic
results.
[0086] FIG. 12 shows the coating thickness obtained at different
polymerization temperatures and times. The increase in the
polymerization rate is due to the increase in diffusion
coefficients of the monomers in the solution, consistent with the
diffusion limited model. Regression fits show that the rate of
polymerization increases with temperature. The effect, however, is
not very dramatic. This also indicates that the polymerization
process is limited by the monomer diffusion, rather than reaction
rate limited.
EXAMPLE 5
Poly(4-CPMI/MMA) on Steel
[0087] Coatings comprising poly(4-CPMI/MMA) on steel were obtained
using the same general procedure of Example 1. Thus, 4-CPMI and MMA
were dissolved in NMP on a 1:1 mole basis, then mixed with water to
form a solution of 0.2 M in each monomer. Polymerization was
initiated by dipping a cleaned substrate in the solution.
Simultaneous polymerization of the monomers and deposition of the
formed polymers occurred at the metal surface. As shown in FIG. 13,
the change in composition of the coatings (as indicated by change
in Tg) with change in 4-CPMI content of the feed indicates that a
random, rather than an alternating copolymer is formed.
Poly(4-CPMI/acrylonitrile) may also be formed on steel by this
method.
[0088] Physical Properties
[0089] Thermal Properties: Glass transition temperatures were
measured using differential scanning calorimetry (DSC). A single
T.sub.g was observed for all poly(NPMI/styrene) polymers and all
other compositions of poly(NPMI/MEA/styrene) and
poly(NPMI/MEA/BMI/styrene). A separate T.sub.g of poly(MEA)
(25.degree. C.) was not observed. This indicates that MEA-styrene
acceptor-donor monomers were included in the NPMI/styrene copolymer
in random units. As shown in Table 2 below, the T.sub.g decreased
only slightly with increasing MEA content in the polymer coatings,
both in presence and absence of BMI. All transition temperatures
were greater than 200.degree. C. Glass transition temperatures of
the coating also increased with addition of BMI in feed. This very
significant outcome demonstrates that the autopolymerization
process is suitable for making high temperature resistant coatings
by polymerization at room temperature. Thermal stability of the
coatings was found to be very good. As shown in FIG. 14, onset of
degradation for poly(NPMI/styrene) on aluminum occurred at more
than 350.degree. C. under nitrogen atmosphere and was not affected
by incorporation of MEA.
2TABLE 2 Glass Transition Temperatures. Monomer Feed Composition
Monomer Feed Composition NPMI/MEA/styrene NPMI/MEA/styrene/BMI
(moles/lit) Tg (.degree. C.) (moles/lit) Tg (.degree. C.)
0.10/0.00/0.10 216.2 0.10/0.00/0.10/0.0025 219.8 0.08/0.02/0.10 --
0.08/0.02/0.10/0.0025 218.4 0.07/0.03/0.10 207.7
0.07/0.03/0.10/0.0025 215.8 0.06/0.04/0.10 -- 0.06/0.04/0.10/0.0025
215.5 0.05/0.05/0.10 205.5 0.05/0.05/0.10/0.0025 -- 0.04/0.06/0.10
197.2 0.04/0.06/0.10/0.0025 215.1 0.03/0.07/0.10 200.4
0.03/0.07/0.10/0.0025 210.3 0.02/0.08/0.10 200.6
0.02/0.08/0.10/0.0025 -- 0.01/0.09/0.10 198.6 0.01/0.09/0.10/0.0025
201.8
[0090] Electrical Resistance: The coatings exhibited good
resistance to DC potential. Samples coated with 20 .mu.m thick
polymer, spontaneously polymerized from a feed solution of
NPMI/MEA/BMI/styrene ratio of 0.05/0.05/0.0025/0.10 M, did not show
any significant current leakage up to an applied potential of
1800V.
[0091] Corrosion Studies: FIG. 11A shows an unexposed 6061 aluminum
sample coated with 20 .mu.m thick poly(NPMI/MEA/styrene) polymer
coating on the right half. FIG. 11B shows the sample after 1500
hours of exposure to salt fog as measured by ASTM B-117. The coated
side is unaffected, while the uncoated side is corroded. No
propagation of corrosion is seen occurring at the polymer-metal
interface. No corrosion is seen around any of the edges of the
sample, illustrating the uniformity of the polymer coating. Similar
results are seen for sample exposed for 3000 hours (FIG. 11C). This
is an inherent property and advantage of the dip autopolymerization
process. Since polymerization is initiated only at the surface,
metal samples with complex surface topographies can be uniformly
coated. Coating also forms inside holes and on edges.
[0092] In summary, the method of the present invention is easy,
efficient and environmentally friendly. It requires minimal
equipment, and has the advantage of being economical. Polymer
coatings up to 50 microns in thickness may be formed, and very high
molecular weights in the range of M.sub.w=250,000 and over may be
obtained. The coatings are of uniform thickness and conformal, with
excellent adhesion to the substrate. Coatings are readily formed
even on complex surfaces. Furthermore, the coatings may be readily
formulated to provide properties such as high temperature
resistance and stability, good electrical breakdown resistance, low
dielectric constant, and abrasion and corrosion protection by
appropriate monomer selection and ratio. Such coatings may be used
in a variety of applications, ranging from coating of circuit board
components to automotive components.
[0093] While preferred embodiments have been shown an described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
* * * * *