Note: Descriptions are shown in the official language in which they were submitted.
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ACYLOXY S LANE TREATMENTS FOR METALS
l3ackaround of the Invention
Field of the Invention
The present invention relates to silane coatings for metals. More
particularly, the present invention provides coatings which include an acyloxy
silane, and are particularly useful for preventing corrosion andlor promoting
adhesion between a metal substrate and a polymer layer applied to the treated
metal substrate_ Solutions for applying such coatings, compositions as well as
methods of treating metal surfaces, are also provided.
pescription of Related Art
Most metals are susceptible to corrosion, including the formation of
various types of rust. Such corrosion will significantly affect the quality of
such
metals, as well as that of the products produced therefrom. Although rust and
the like may often be removed, such steps are costly and may further diminish
the strength of the metal. In addition, when polymer coatings such as paints,
adhesives or rubbers are applied to the metals, corrosion may cause a loss of
adhesion between the polymer coating and the metal.
By way of example, metallic coated steel sheet such as galvanized steel
is used in many industries, including the automotive, construction and
appliance
industries. In most cases, the galvani2ed steel is painted or otherwise coated
with a polymer layer to achieve a durable and aesthetically-pleasing product.
Galvanized steel, particularly hot-dipped galvanized steel, however, often
develops "white rust" during storage and shipment.
White rust (also called "wet-storage stain") is typically caused by moisture
condensation on the surtace of galvanized steel which reacts with the zinc
coanng_ On products such as GALVALUME~, the wet-storage stain is black in
color ("black rust"). White rust (as well as black rust) is aesthetically
unappealing and impairs the ability of the galvanized steel to be painted or
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otherwise coated with a polymer. Thus, prior to such coating, the surface of
the
galvanized steel must be pretreated in order to remove the white rust and
prevent its reformation beneath the polymer layer. various metnoas are
currently employed to not only prevent the formation of white rust during
shipment and storage, but also to prevent the formation of white rust beneath
a
polymer coating (e.g., paint)-
(n order to prevent white rust on hot-dipped galvanized steel during
storage and shipping, the surtace of the steel is often passivated by forming
a
thin chromate film an the surface of the steel- Wh~le such chromate coatings
do
promde resistance to the formation of white rust, chromium is h~gmy toxic aria
environmentally undesirable. It is also known to employ a phosphate conversion
coating in conjunction with a chromate rinse in order to improve paint
adherence
and provide corrosion protection. It ~s belevec7 tnat the cnrvma~e rinse cwe~
a
the pores in the phosphate coating, thereby improving the corrosion resistance
and adhesion performance. Once again, however, it is highly desiraple to
eliminate the use of chromate altogether. Unfortunately, however, the
phosphate conversion coating ~s generally not very effective without the
chromate rinse.
Recently, various techniques for eliminating the use of chromate have
been proposed. l~hese include coating the galvanized steel with an inorganic
silicate followed by treating the silicate coating mth an organofunct~onal
silane
(U.S. Patent No. 5,108,793).
U.S. Patent No. 5,292,549 teaches the rinsing of metallic coated steel
sheet with a solution containing an organofunctional silane and a crosslmkW g
agent.
U_S. Patent No. 6,071,566 relates to a method of treating a metal
substrate to provide permanent corrosion resistance. The method comprises
applying a solution containing one or more mnyl silanes ~n admixture with one
or more multi-silyl-functional silanes to a metal substrate in order to form a
coating.
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Various other techniques for preventing the formation of white rust on
galvanized steel, as well as preventing corrosion on other types of metals,
have
also been proposed. Many of the proposed techniques described in the prior arc
are, however, ineffective, or require time-consuming, energy-inefficient,
multi-
step processes. Thus, there is a need for a simple, low-cost technique for
preventing corrosion on the surtace of metal
A particular problem associated with the silane treatments of the prior art
~s the rate of hydrolysis of the silane compounds. Such compounds are
generally hydrolysed in water, at a specific pH, prior to application of the
solution
to the substrate to be treated. The rate of hydrolysis varies between silanes,
and the degree of hydrolysis is a priori not known. Generally, it has to be
guessed when the solution is ready for application When the solution has
turned cloudy, this indicates that condensation of the silanes has occurred
and
the effectmeness of the treatment solution is reduced
A further problem with the prior art techniques is the inherent insolubility
in aqueous media of some of the silanes employed in the metal treatments. To
overcome this problem it is commonplace to dissolve the silane with the aid of
an organic solvent, for example, alcohols Thus a final treatment solution
commonly contains up to 60% alcohol- The use of many volatile organic
compounds (VOCs), including solvents, ~s highly undesirable from an economic,
aswell as an environmental perspective. Apart from the cost of such organic
solvents, including the cost of their disposal and methods of treatment
solution
preparation, such compounds present a threat to the environment and are a
hazzard to the premises and personnel handling the materials.
A further problem is that the silane systems used in treatment solutions
have to have their pH maintained in specific ranges by the initial and
continuous
addition of adds or bases.
It would therefore be desirable to provide an effectme treatment method
for metal surfaces, especially to prevent corrosion, and/or improve adhesion.
It would also be desirable to provide a treatment solution useful in
preventing corrosion, andlor adhesion promotion of metal surtaces, for
example,
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steel, aluminium, aluminium alloys, zinc, zinc alloys, magnesium, magnesium
alloys, copper, copper alloys, tin and tin alloys, particularly zinc, zinc
alloys, and
other metals having a zinc-containing coating thereon
It would additionally be desirable to promde a metal surface hamng
improved corrosion resistance and/or improved adhesion characteristics.
Summary of the Invention
The present invention provides a method of treating a metal surface,
comprising the steps of:
(a) promdmg a metal substrate; and
(b) applying a solution to said metal substrate, said solution
compnsmg
(i) at least one acyloxy silane which comprises at least one
acyloxy group, wherein said silane has been at least
partially hydrolysed; and
(ii) at least one basic compound;
wherein the acyloxy silane and the basic compound are present in
concentrations to provide a solution pH of between about 3 and about 10, more
preferably between about 4 and about 8, most preferably 4 to 5 and wherein the
solution is substantially free of acid other than acid produced upon
hydrolysis of
the acyfoxy silane.
The present invention also provides a composition comprising at least
one acyloxy silane and at least one basic compound, wherein the at least one
acyloxy silane ~s at least partially hydrolyzed. A metal surface having
improved
corrosion resistance andlor adhesion and a composition concentrate is also
provided.
Detailed Description of the Invention
The acyloxy silane(s) utilised in the present invention may comprise one
or more s~lyl groups and the solution may contain a mixture of acyloxy silanes
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S
Where the acyloxy silane comprises a single silyl group the silicon atom
is tetrasubstituted, wherein the substituents are mdimdually selected from the
group consisting of alKyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl, vinyl,
amino,
ure~do, glycidoxy, epoxy, hydroxy, alKoxy, aryloxy and acyloxy, or any of the
group alkyl, alkenyl, alkynyl, aryl, alkaryl and aralkyl substituted by a
group
selected from the group consisting of vinyl, amine, ureido, glycidoxy, epoxy,
hydroxy and alkoxy, v~rith the proviso that at least one of the substituents
on the
silicon atom is an acyloxy group.
Where more than one acyloxy group is attached to the silicon atom of the
syn group, the acyloxy groups are preferably all the same. The acyloxy
groups) are preferably selected from the group consisting of Cz_,2
alkanoyloxy,
C3_~Z alkenoyloxy, C3_,2 alkynoyloxy and C~_~e arenoyloxy, preferably C2_6
alkanoyloxy, C3~ alkenoyloxy, C3~ alkynoyloxy and Cy_,Z arenoyloxy. Most
preferably the acyloxy groups are all the same and are ethanoyloxy (acetoxy)
or
methanoyloxy groups.
Where the acyloxy s~lane comprises a single silyl group, preferably three
of the substituents on the silyl group are acyloxy groups and the fourth
substituent is preferably selected from a the group consisting of vinyl or
vinyl
substituted group, amine or amine substituted group, ureido or ureido
substrtuted group and glycidoxy or glycidoxy substituted group.
In a particularly preferred embodiment, the acyloxy silane is selected from
the group consisting of
H
OCOR OCOR
~X-S''OCOR HzN-Y'Si-ocoR
H OCOR OCOR
OII OCOR O OCOR
H2N~N~Z-S~-OCOR ~W'S~-OCOR
OCOR OCOR
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wherein W, X, Y and Z are selected from the group consisting of a C-Si bond,
substituted aliphatic groups, unsubstituted aliphatic groups, substituted
aromatic
groups and unsubstituted aromatic groups; and
R is selected from methyl, ethyl and propyl, preferably ethyl.
The acyloxy silane may comprises more than one silyl group. Although
the term acyloxy silane generically refers to such a compound, it may be
referred to as a multi-silyl-acyloxy silane. More than one multi-silyl-acyloxy
s~lane may be employed in a mixture with one or more other mule-silyl-acyloxy
silanes or one or more acyloxy silanes containing a s~ngie silyl group as
described above.
The acyloxy groups bound to the silicon atoms of the s~lyl groups of the
multi-silyl-acyloxy silane are preferably all the same and are preferably
selectea
from the group consisting of C2.,2 alkanoyloxy, C9_,z alkenoyloxy, C3.,2
alkynoyloxy and Cz_,e arenoyloxy, preferably C2~ alkanoyloxy, C3_6
a~kenoyloxy,
C3_6 alkynoyloxy and C~.,2 arenoyloxy. Most preferably the acyloxy groups are
all the same and are ethanoyloxy or methanoyloxy groups.
Preferably the multi-silyl-acyloxy silane utilised in the present invention
has the structure
OC OR 1
Q Si - OC OR 1
OC OR 1
2
wherein Q is selected from the group consisting of either a bond, an aliphatic
or
aromatic group; and
R' is selected from methyl, ethyl and propyl.
Preferably Q is selected from the group consisting of a bond, C,-C6
alkylene, Cz-CB alkenylene, C,-C6 alkylgne subst~tutea mth at least one amino
group, CZ-C6 alkenylene substituted with at least one amino group, C,-C6
alkylene substituted with at least one sulfide group containing 1 to 6 sulfur
atoms, C2-Cs alkenylene substituted with at least one sulfide group containing
1 to 6 sulfur atoms, arylene and alkylarylene. In the case where Q is a bona,
the
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multi-functional silane comprises two trisubstituted silyl groups which are
bonded
directly to one another. Preferred multi-silyl-acyloxy silane are
bis-(triacetoxysilyl)alkane, bls-(triacetoxysilylalkyl)amine and
bis-(triacetoxysilylalkyl)tetrasulfide, most preferably bis-
(triacetoxysilyl)ethane,
bas-(triacetoxysilylpropyl)amine and bis-(triacetoxysilylpropyl)tetrasulfide.
In an especially preferred embodiment, the acyloxy silane utilised in the
present mvent~on is vinyltriacetoxysilane.
Acyloxy silanes utilised in the present invention generally dissolve and
hydrolyze readily and completely in water to produce organic acids. For
example, where an acetoxy silane is used, acetic acid is produced. Unlike the
analogous alkoxy silanes commonly utilised in the prior art which produce
alcohols upon hydrolysis, the acyloxy silanes utilised m the present invention
produce substantially none or small amounts of VOCs depending on the level
of non-acyloxy group substitution in the silanes_
Depending on the level of substitution of acyloxy groups in the silanes
utilised in the present invention, the pH of the resultant solution can be
predetermined and manipulated. Commonly, high degrees of acyloxy group
substitution are present, for example ~ 100% substitution, and this can result
in
a pH as low as 1 or 2 At these low levels of pH, the hydrolysed acylvxysilanes
tend to condense, therefore reducing their efficacy. It ~s therefore necessary
to
add a base to maintain the pH in an optimal range.
Preferably, where a single silyl group-containing silane is used as the
acyloxy silane, 3 of the groups attached to the silicon atom of the silyl
group are
acyloxy groups, preferably methanoyloxy or acetoxy.
Preferably, where a multi-silyl-acyloxy silane is used, 3 of the groups
attached to the each silicon atom of each silyl group are acyloxy groups,
preferably methanoyloxy or acetoxy.
The pH of the silane mixture is between about 3 and about 10, more
preferably between about 4 and about 8, most preferably 4 to 5 and should be
maintained. The pH may be adjusted by the addition of one or more basic
compounds or addition of acyloxy silane(s).
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During preparation of the treatment solution, a pH of above 2, more
preferably above 3, most preferably between 4 and 5 should be maintained.
In order to maintain an optimal pH during preparation of the treatment
solution, a basic compound is applied to the treatment solution. The identity
of
the bask compounds) is not critical but it is highly beneficial to provide a
compound which complements the acyloxy silane. °Complements" means that
the basic compound aids, or at least does not substantially detract from the
formation of the silane coating on the metal substrate or from the coatings
effectiveness in improving corrosion resistance andlor adhesion promotion.
To maintain the pH in the preferred range, the acyloxy s~lane and the
basic compound are preferably mixed together prior to the addition of water
and
subsequently dissolved in water. Exemplary basic compounds include the
carbonates, hydrogen carbonates and hydroxides of the alkali and alkaline
earth
metals, organic amines, ammonia, amides and the like. A rn~xture of different
basic compounds may be added to the treatment composition
In a preferred embodiment, the basic compound is a basic s~lane
compound. For example, amino silanes are particularly preferred. In one
embodiment, ammo silanes which may be employed m the present invention
each have a single trisubst~tuted silyl group in addition to the basic amine
moiety, wherein at least on of the substituents is an alkoxy group. Thus, the
amino silanes which maybe used in the present invention have the general
structure
oR2
,N-x'-s.-o~
R~ oR2
Rz is chosen from the group consisting of hydrogen and C,-C2, alkyl,
preferably
C,-C6 alkyl and each RZ may be the same or different. Preferably RZ is
individually chosen from the group consisting of hydrogen, ethyl, methyl,
propyl,
iso-propyl, butyl, iso-butyl, sec-butyl and ter-butyl.
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X' is a group selected from the group consisting of a bond, a substituted
or unsubstituted aliphatic or aromatic group. Preferably X' is selected from
the
group cons~stmg of a bond, C,-C6 alkylene, CZ-Cg alkenylene, C,-C6 alkylene
substituted with at least one amino group, CZ-C6 alkenylene substituted with
at
least one amino group, C6-,e arylene and C~-C,e alkylarylene;
R3 is a group indwidually selected from the group consisting of hydrogen, C,-
C4
alKyl, C2-C6 alkenyl, C,-C6 alkyl substituted with at least one amino group,
C2-C6
alkenyl substituted with at least one amino group, arylene and afkylarylene_
Preferably R3 ~s individually selected from the group consisting of
hydrogen, ethyl, methyl, propyl, ~so-propyl, butyl, iso-butyl, sec-butyl ter-
putyl
and acetyl.
Particular preferred ammo silanes employed in the method of the present
mvent~on are y-ammopropyltriethoxysilane and y-aminopropyl trimethoxysilane.
In another embodiment, the amino silane may be a bis-silyl
am~nosilane(s). Such a compound comprises an aminosilane having two
tnsubst~tuted silyl groups, wherein the substituents are individually selected
from
the group consisting of hydroxy and alkoxy. Preferably, the bis-silyl
aminosilane
compnses_
OR° OR°
R°O- ~ i R5 XZ RS- ~ ~-OR°
R4 ~ R°
wherein each Rq is individually selected from the group consisting of:
hydrogen
and C, - C24 alkyl;
each RS is individually selected from the group consisting of: substituted
aliphatic
groups, unsubstituted aliphatic groups, substituted aromatic groups, and
unsubstituted aromatic groups; and
Xz ~s either:
Rs Ra Rs
or ~N~R~ N-
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wnerein each RB is individually selected from the group consisting of:
hydrogen,
substituted and unsubstituted aliphatic groups, and substituted and
unsubstituted aromatic groups; and
R' is selected from the group consisting of: substituted and unsubstituted
aliphatic groups, and substituted and unsubstituted aromatic groups
Particularly preferred bis-silt' aminosilanes which may be used in the
present invention include:
bis-(trimethoxysilylpropyl)amine (which is sold under the tradename A-
1170 by Witco):
OCH3 H OCH3
CH30--SI-CjH6-N--C3H6-SI-OC H3
OCH3 OCHg
bis-(triethoxysilylpropyl)amme:
OC2Hs H OC2H5
C2H50-~I-C3H6-N-C3H6-Si-OC2H5
OC2Hs OCzHs
and bis-(triethoxysilylpropyl)ethylene d~amine:
OCH3 H H OCH3
I I I I
CH30-SI-C3H6-N-C2Hp-N-C3H6-SI-OCH3
OCH3 OCH3
Particularly preferred combinations of acyloxy silanes and basic compounds
are:
vinyltriacetoxysilane and bis-(trimethoxysilylpropyl)amine;
1,2-bis-(tri~thoxysilyl)ethane and bis-(tnmethoxysilylpropyl)amme;
v~nyltriacetoxysilane and aminopropyltriethoxysilane;
vinyltriacetoxysilane and bas-(triethoxysilylpropyl)amine;
1,2-bis-(triethoxysilyl)ethane and bis-(triethoxysilylpropyl)amine;
vinyltriacetoxysilane and am~nopropyltrimethoxysilane.
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Where basic silanes are used as the basic compound, additional basic
compounds may be used, for example, the inorganic bases referred to above.
The solutions and methods of the present invention may be used on a
vanety of metals, including steel, aluminium, aluminium alloys, zinc, zinc
alloys,
magnesium, magnesium alloys, copper, copper alloys, tin and tin alloys. In
particular, the present method is particularly useful on zinc, zinc alloy, and
metals having a zinc-containing coating thereon, as well as aluminium or
aluminium containing substrates. For example, the treatment solutions and
methods of the present invention are useful in preventing corrosion of steel
having a zinc-containing coating, such as: galvanized steel (especially not
dipped galvanized steel), GALVALUME~ (a 55%-AI/43.4%-Zn/1.6% - Si alloy
coated sheet steel manufactured and sold, for example, by Bethlehem Steel
Corp), GALFAN~ (a 5%-Al/95%-Zn alloy coated sheet steel manufactured and
sold by Weirton Steel Corp., of Weirton, WV), galvanneal (annealed hot dipped
galvanized steel) and similar types of coated steel. Zinc and zinc alloys are
also
particularly amenable to application of the treatment solutions and methods of
the present invention. Exemplary zinc and zinc alloy materials include:
titanium-
zinc (zinc which has a very small amount of titanium added thereto), zinc-
nickel
alloy (typically about 5% to about 13% nickel content), and zinc-cobalt alloy
(typically about 1 % cobalt).
The solutions of the present invention may be applied to the metal prior
to shipment to the end-user, and provide corrosion protection during shipment
and storage (including the prevention of wet-storage stain such as white
rust).
If a paint or other polymer coating is desired, the end user may merely apply
the
paint or polymer (e.g., such as adhesives, plastics, or rubber coatings)
directly
on top of the silane coating provided by the present invention. The silane
coatings of the present invention not only provide excellent corrosion
protection
even without paint, but also provide superior adhesion of paint, rubber or
other
polymer layers. Thus, unlike many of the currently-employed treatment
techniques, the silane coatings of the present invention need not be removed
prior to painting (or applying other types of polymer coatings such as
rubber).
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Suitable polymer coatings include various types of paints, adhesives
(such as epoxy automotive adhesives), and peroxide-cured rubbers (e.g.,
peroxide-cured natural, NBR, SBR, nitrite or silicone rubbers). Suitable
paints
include polyesters, polyurethanes and epoxy-based paints. Plastic coatings are
also suitable including acrylic, polyester, polyurethane, polyethylene,
polyimide,
polyphenylene oxide, polycarbonate, polyamide, epoxy, phenolic, acrylonitrile-
butadiene-styrene, and acetal plastics. Thus, not only do the coatings of the
present invention prevent corrosion, they may also be employed as primers
and/or adhesive coatings for other polymer layers.
Tne solutions of the present invention do not require the use or addition
of silicates-
The compositions may optionally comprise other silane compounds to the
acyloxy silanes or the basic silanes disclosed herein.
The treatm~nt solution is aqueous, and may optionally include one or
more compatible solvents (such as ethanol, methanol, propanol or isopropanol)
although their presence is not normally required. Where an organic solvent is
required, ethanol is preferred. Preferably, solutions of the present invention
are
substantially free of organic solvents and VOCs.
As mentioned above, the silane(s) in the solution of the present invention
are at least partially, and preferably are substantially fully hydrolyzed in
order to
facilitate the bonding of the silanes to the metal surface and to each other.
During hydrolysis, the alkoxy groups in the case of the non-acyloxy silanes
and
the acyloxy in the case of the acyloxy silanes are replaced by hydroxyl
groups.
Hydrolysis of the silanes may be accomplished, for example, by merely mixing
the silanes in water, and optionally including a solvent (such as an alcohol)
in
order to improve silane solubility and solution stability.
In order to accelerate silane hydrolysis and avoid silane condensation
during hydrolysis, the pH may be maintained below about 8, more preferably
between about 4 and about 6, and even more preferably between about 4 and
about 5.
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It should be noted that the various s~lane concentrations discussed and
claimed herein are all defined in terms of the ratio between the amount (by
volume) of unhydrolyZed silane(s) employed to prepare the treatment solution
(i.e., prior to hydrolyzation), and the total volume of treatment solution
components (i.e., acyloxy silanes, basic compound, water, and optional
solvents- In the case of acyloxy silane(s), the concentrations herein (unless
otherwise specified) refer to the total amount of unhydrolyzed acyloxy sifanes
employed, since multiple acyloxy silanes may optionally be present. The basic
compounds concentrations herein are defined in the same manner.
As for the concentration of hydrolyzed silanes in the treatment solution,
beneficial results will be obtained overawide range of silane concentrations
and
ratios It is preferred, however, that the solution have at least about 0.1
acyloxy silanes by volume, more preferably at least about 1 % acyloxy silanes
by
volume, most preferably between about 2% and about 5% by volume. Lower
vinyl silane concentrations generally provide less corrosion protection.
Higher
concentrations of acyloxy sifanes (greater than about 10%) should also be
avoided for economic reasons, and to avoid silane condensation (which may
limit storage stability).
The concentration of the basic compound required in the treatment
solution varies strongly with the type of acyloxy silane employed and the type
of
basic compound. Obviously, a strongly acidic solution produced by a highly
acyloxy group-substituted acyloxy silane will require an appropriate amount of
basic compound to result in a treatment solution with a pH in the pre-
determined
range. Once the pH of the acyloxy silane in solution is known, an appropriate
amount of a basic compound (with a known pN value in solution) can be added
to the solution. The relative acidity and basicity of the acyloxy silane and
the
basic compound may be established before the solution is made up and are
commonly presented in standard tables reciting physical properties of known
compounds. However, the concentration of the basic compound is generally in
the range of about 0.1 °~ and about 10% by volume.
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Where a basic silane is used as the basic compound, the solution should
have at least about 0.1 % basic silanes by volume, more preferably at least
about
1 % basic s~lane by volume, more preferably between about 2% and about 10%,
most preferably between about 2% and about 5% by volume.
As for the ratio of acyloxy silanes to basic compound, a wide range of
ratios may be employed, and the present invention is not limited to any
particular
range of silane ratios.
The mixture of the acyloxy and basic compound may be provided to the
user m a pre-mixed, unhydrolysed form which improves shelf life as
~ 0 condensation of the siiane is limiteo. Sucn a mixture can then be made up
into
a treatment solution as defined herein Such a pre-mixed, unhydrolysed
compositions should preferably be substantially free of water but may include
one or more organic solvents (such as alcohols). The composition may also
include other components such as stabilizers, pigments, desiccants, and the
like.
Such a pre-mixed composition can be made up with a pre-determined
amount of acyloxy silane and bask compound so that the addition of the mixture
to water results in a pH within the preferred range. Such pre-mixing prevents
or
Imits a drop in pH, due to the acyloxy silane alone being present in solution,
to
levels which promote condensation of the silanes in solution- However, the
composition can be presented in a "two-pack" kit, wherein one part of the kit
comprises the acyloxy silane, while another part of the kit provides the basic
compound.
In ether of the above presentation embodiments, the acyloxy silane and
basic compound, along with the other components of the composition are
provided in a concentrated form as a powder or liquid mixture. In either case,
the concentrate is substantially free of water and may be presented in a
hermetically sealed container or kit. Preferably, substantially no organic
solvent
~s present in the composition.
The concentration of the acyloxy silane and basic compound in the pre-
mixed concentrate composition is generally in the range 10-100%, preferably 15-
80%, most preferably 25-70%. The concentrate may contain numerous
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additional components such as stabilisers, pigments, anti-oxidants, basic pH
adjusters, desiccants, adhesion promoters, corrosion inhibitors and the like.
The treatment method itself is very simple. Where the solution is to be
made up of separately presented components, the unhydrolyzed acyloxy silane,
water, basic compound, solvent (~f desired), are combined with one another.
The solution is then stirred at room temperature in order to hydrolyze the
silanes. The solution generally goes clear when hydrolyses is complete In this
embodiment it is beneficial to maintain the pH of the solution above 2 to
limit any
condensation of the silanes in solution, particularly the acyloxy silanes_
Where the composition is presented as a pre-mixed kit, the composition
is simply added to a pre-determined amount of water and mixed until the
solution
is substantially clear.
The metal surface to be coated with the solution of the present invention
may be solvent and/or alkaline cleaned by techniques well-known to those
skilled in the art prior to application of the treatment solution of the
present
invention. The silane solution is then applied to the metal surface (i.e., the
sheet
is coated wrth the silane solution) by, for example, dipping the metal into
the
solution (also referred to as "rinsing''), spraying the solution onto the
surtace of
the metal, or even brushing or wiping the solution onto the metal surface.
Various other application techniques well-known to those skilled in the art
may
also be used When the preferred application method of dipping is employed,
the duration of dipping is not critical, as it generally does not
significantly affect
the resulting film thickness. It is merely preferred that whatever application
method is used, the contact time should be sufficient to ensure complete
coating
of the metal. For most methods of application, a contact time of at least
about
2 seconds, and more preferably at least about 5 seconds, will help to ensure
complete coating of the metal.
As the treatment solution is used up, the acyloxy silane concentration is
reduced and the acetic acid concentration remains approximately constant as
long as no further acyloxy silane is added to the solution. As further acyloxy
silane is added to maintain their concentration, acetic acid is built up in
the
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solution. To maintain the pH in the preferred range pH adjusters may be added
such as basic compounds as hereinbefore described, buffers and the like. In
one embodiment, a basic compound may be added along with the addiuonat
acyloxy silane which forms a salt with the acid m solution This may form an
insoluble salt which can be removed from the process.
The treatment solution may also be heated when applying the treatment
solution. Where the treatment solution is heated, the temperature of the
treatment solution is generally in the range 20°C to 80°C,
preferably 30°C to
50°C .
After coating with the treatment solution of the present mvenuon, the
metal sheet may be air-dried at room temperature, or, more preferably, placed
into an oven for heat drying. Preferable heated drying conditions include
temperatures between about 20°C and about 200 °C with drying
times of
between about 30 seconds and about 60 minutes (higher temperatures allow far
shorter drying times) More preferably, heated drying is performed at a
temperature of at least about 90°C, for a time sufficient to allow the
silane
coating to dry. While heated drying vs not necessary to achieve satisfactory
results, it will reduce the drying time thereby lessening the likelihood of
the
formation of white rust during drying. Once dried, the treated metal may be
shipped to an end-user, or stored for later use.
The examples below demonstrate some of the superior and unexpected
results obtained by employing the methods of the present invention.
Exameles
Example 1
Salt Spray test (SST)(Lakebluff) was carried out on
A1170Ninyltriacetoxysilane (1/1, 5°r6, natural pH--4) treated AA5005
panels.
Alkaline cleaned blank and chromated AA5005 panels were chosen as controls
The treated panels were cured at 100°C for 10 min, and then exposed to
SST for
29 days, along with the control panels. Four replicates were made for each
treatment The results are presented in Fig. 1.
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1. A1170MAS treated panels showed original surface after 29 days
of exposure to SST, i.e. no corrosion occurred during testing.
2. The blank panels corroded heavily, white the chramated ones
pitted apparently.
Example 2
Salt Spray test (Lakebluff) was carried out on A1170/VTAS (1.511.5%,
natural pH=4) treated A12024-T3 panels. Alkaline cleaned blank and chromated
A12024-T3 panels were chosen as controls. The treated panels were cured at
100°C far 10 min, and then exposed to SST for 7 days, along with the
control
panels. Three replicates were made for each treatment. The results are
presented in Fig. 2.
3. A1170NTAS treated panels showed almost original surface after
7 days of exposure to SST, ~.e , only slight edge corrosion
occurred during testing.
4. The blank panels corroded heavily, while the chromated ones
pitted slightly.
Example 3
In order to investigate the paintability of A1170IVTAS water-based silane
film on metal substrates, A1170IVTAS (1.5/1,2%, pH'-5) water-based silane film
was applied on A12023-T3 and HDG, respectively. The treated panels were
then powder-painted at Lakebluff with Polyester and Polyurethane powder
paints. After that, the panels were put into salt spray chamber for some
times,
along with the control panels, the blank and the chromated. Three replicates
were made for each treatment The results are shown in Fig. 3
1. As for A12024-T3 painted with both powder paints (1000hrs in
SST), the corrosion performance and paint adhesion improved
significantly, which was equal to the chromated and much better
than the blank.
2. As for powder-painted HDG (336 hrs in SST), the corrosion
performance improved apparently, compared with the chromated
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and the clank. The paint adhesion improved somewhat, which
was better that the control panels-
