Note: Descriptions are shown in the official language in which they were submitted.
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COATING COMPOSITION FOR METAL SUBSTRATES
This invention relates to a coating composition that can be used for the
coating
of metal substrates, for example steel substrates. In particular, it relates
to a
coating composition for semi-finished steel products which are subsequently to
be fabricated by heat-intensive processes and overcoated. Such semi-finished
steel products are used in the shipbuilding industry and for other large-scale
structures such as oil production platforms and include steel plates, for
example
of thickness 6 to 75 mm, bars, girders and various steel sections used as
io stiffening members. The most important heat-intensive process is welding;
substantially all such semi-finished steel products are welded. Other
important
heat-intensive processes are cutting, for example oxy-fuel cutting, plasma
cutting or laser cutting, and heat fairing, where the steel is bent into shape
while
being heated. These steel products are often exposed to the weather during
storage before and during construction, and they are generally coated with a
coating called a "shop primer" or "pre-construction coating" to avoid
corrosion of
the steel occurring before the steel construction, e.g. a ship, is given its
full
coating of anticorrosive paint, thereby avoiding the problem of having to
overcoat or remove steel corrosion products. In most big. shipyards, the shop
primer is applied as one of several treatments carried out on a production
line in
which the steel is for example preheated, shot- or grit-blasted to remove
miliscale and corrosion products, shop primed, and passed through a drying
booth. Alternatively, the shop primer can be applied by a trade coater or
steel
supplier before the steel is delivered to the shipyard or other construction
site.
Although the main purpose of the shop primer is to provide temporary corrosion
protection during construction, it is preferred by shipbuilders that the shop
primer does not need to be removed but can remain on the steel during and
after fabrication. Steel coated with the shop primer thus needs to be weldable
without removal of the shop primer and to be overcoatable with the types of
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protective anti-corrosive coatings generally used on ships and other steel
constructions, with good adhesion between the primer and the subsequently
applied coating. The shop primed steel should preferably be weldable without
any significant detrimental effect on the quality of the weld or on the speed
of
the welding process and should be sufficiently resistant to heat for the shop
primer to retain its anticorrosive properties in areas heated during fairing
or
during welding of the opposite face of the steel.
Commercially successful shop primers available today are solvent borne
coatings based on prehydrolyzed tetraethyl orthosilicate binders and zinc
powder. Such coatings contain a large proportion of volatile organic solvent,
typically about 650 grams per litre, to stabilize the paint binder and to
enable the
product to be applied as a thin film, typically of about 20 microns thick.
Release
of volatile organic solvent can be harmful to the environment and is regulated
by
1s legislation in many countries. There is a need for a shop primer which
releases
no, or much less, volatile organic solvent. Examples of such coatings are
described in US-A-4,888,056 and JP07070476.
JP06200188 ' is concerned with shop primer coatings and mentions the
possibility of using an aqueous alkali silicate salt type binder. - Coatings
comprising an aqueous alkali metal silicate and zinc powder are also proposed
in GB-A-1226360, GB-A-1007481, GB-A-997094, US-A-4,230,496, and JP-A-
55-106271. Alkali silicate binders for anticorrosive coatings are also
mentioned
in US-A-3,522,066, US-A-3,620,784, US-A-4,162,169, and US-A-4,479,824. In
EP-A-295 834 coatings containing a mixture of alkali metal silicate with a
minor
amount of colloidal silica, A1203 powder as filler, and metal powder as
toughening agent are mentioned. We have found that primer coatings based on
an aqueous alkali silicate binder containing zinc powder can give adequate
corrosion protection and allow the steel surfaces they cover to be welded, but
give rise to problems when overcoated. The aqueous silicates contain a large
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quantity of alkali metal cations required to keep the silicate in aqueous
solution
and these ions are still present in the coating after it has dried. We have
found
that if primer coatings having these large quantities of alkali metal ions are
overcoated with any conventional organic coating and then immersed in water,
blistering (local delamination of the coating) occurs. We have performed tests
which show that this problem can be reduced if the coating is weathered
outside
for some time after application of the shop primer or washed prior to
overcoating. However, these processes are not compatible with use in today's
high-productivity shipyards.
Aqueous silica sols having a very low alkali metal ion content are available
commercially but coatings based on the conventionally used large sols normally
have very poor (initial) film strength in terms of adhesion, cohesion,
hardness,
and resistance to abrasion and water. These poor physical properties of the
coating make it susceptible to damage during handling or further processing.
This brings the potential requirement of significant coating repair with major
cost
implications. Suggested improvements to silica sol coatings are described in
US-A-3,320,082, which adds a water-immiscible organic amine, GB-A-1541022,
which adds a water-soluble acrylamide polymer, GB-A-1485169, which adds a
quaternary ammonium or alkali metal silicate, and JP-A-55-100921, which adds
clay materials and/or metal oxides such as AI203, and aluminium biphosphate
and/or ethyl silicate. However, such coatings have not achieved physical
properties similar to those of coatings based on alkali metal silicates.
Coatings
based on silica sols show low levels of blistering when overcoated/immersed.
Although the water-soluble salt content and osmotic pressure are low,
blistering
can still occur, as due to its poor physical properties the coating exhibits
little
resistance to blister initiation/growth.
There is a need for a water based shop primer of low alkali metal ion content
which has improved adhesion to substrates and improved film strength in terms
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of the properties discussed above to resist blister initiation and growth.
Further,
there is a need for a blister-free water based shop primer showing fast
development of the physical coating properties after application to enable
handling and further processing of the substrate without the risk of damaging
the coating.
It has been found that the ratio of the pigment volume concentration (PVC) to
the critical pigment volume concentration (CPVC) has a significant impact on
the film properties. In addition, the speed of property development of the
film
can be adjusted by altering the PVC / CPVC ratio.
The pigment volume concentration (PVC) is the volume percentage of pigment
in the dry paint film. The critical pigment volume concentration (CPVC) is
normally defined as the pigment volume concentration where there is just
sufficient binder to provide a completely adsorbed layer of binder on the
pigment surfaces and to fill all the interstices between the particles in a
close-
packed system. The critical pigment volume concentration can be determined
by wetting out dry pigment with just sufficient linseed oil to form a coherent
mass. This method yields a value known as the "oil absorption", from which the
critical pigment volume concentration can be calculated. The method for
determining the oil absorption is described in British Standard 3483 (BS3483).
US-A-3,721,574 suggests coatings containing a mixture of alkali metal silicate
with a minor amount of colloidal silica, said colloidal silica preferably
being A1203
modified. Also coatings containing a mixture of alkali metal silicate with a
minor
amount of colloidal silica, and zinc dust are mentioned. In the zinc-modified
coatings, preferably a mixture of alkali metal silicate with a minor amount of
non-modified colloidal silica is employed: In the Examples, the zinc dust is
employed in extremely high proportions. This results in the formation of films
containing about 95 percent by weight of zinc in the dried coating. However,
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such a high zinc level has a detrimental effect on the weldability of the,
coating.
Thus, such coatings are not suitable for use as shop primers for semi-finished
steel products which are subsequently to be faired or welded and overcoated.
US-A-3,721,574 does not make reference to the PVC / CPVC ratio of the
5 coatings, neither for the zinc-free, nor for the zinc-comprising coatings.
The
document does not disclose that the PVC / CPVC ratio has a significant impact
on the film properties and on the speed of property development of the film.
WO 00/55260 discloses a coating composition comprising a silica or silicate
binder and zinc powder and/or a zinc alloy. The binder has a Si02/M20 mole
ratio, wherein M represents the total of alkali metal and ammonium ions, of at
least 6:1. The document teaches that the pigment volume concentration of the
coating should be at least equal to the critical pigment volume concentration.
It
has now been found that the film properties of the coating composition and the
speed of property development of the film can be improved by using the coating
composition according to the present invention when the binder comprises
silica
or silicate particles having an average size larger than 10 nm.
The composition according to the present invention, which can be used for
coating a metal substrate which is intended to be fabricated and overcoated,
has a PVC / CPVC ratio smaller than 1. The coating comprises a silica binder
comprising an aqueous silica sol and, optionally, a minor amount of alkali
metal
silicate, with the silica and/or silicate particles in the composition having
an
average size larger than 10 nm. Said binder has a Si02/M20 mole ratio of at
least 6:1, wherein M represents the total of alkali metal and ammonium ions.
For the purpose of the present application, a minor amount of alkali metal
silicate means that the weight ratio of alkali metal silicate to silica sol in
the
composition is smaller than 0.5, preferably smaller than 0.25, more preferably
smaller than 0.1.
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The composition according to another aspect of the present invention for
coating
a metal substrate which is intended to be fabricated and overcoated, said
composition comprising a silica binder, characterized in that the ratio of the
pigment volume concentration to the critical pigment volume concentration of
said
composition is smaller than 1, and in that the binder comprises an aqueous
silica
sol and, optionally, alkali metal silicate, wherein the weight ratio of alkali
metal
silicate to silica sol is smaller than 0.5, with the silica and/or silicate
particles
having an average size larger than 10 nm and less than 22 nm, and in that said
binder has a Si02/M20 mole ratio of at least 6:1, wherein M represents the
total
of alkali metal and ammonium ions, and wherein the coating composition further
comprises zinc powder, zinc alloy or a combination thereof.
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It has been found that improved early coating properties can be obtained using
coatings with a PVC between 35% and 65%, more preferably between 40% and
55%. In a coating having a PVC below 35% and comprising only zinc as
pigment, there is insufficient zinc to provide effective corrosion protection
on
outdoor exposure when more than 6 months' protection is required. When using
coatings with a low zinc level, for instance between 10 and 40%, acceptable
corrosion protection can be obtained when one or more secondary corrosion
inhibitors are added or when a conductive extender, such as di-iron phosphide,
is added.
The primer coating preferably contains zinc powder which preferably has a
volume averaged mean particle size of 2 to 12 microns, and most preferably
such zinc powder is the product commercially obtainable as zinc dust having a
mean particle size of 2 to 8 microns. The zinc powder protects the steel by a
galvanic mechanism and may also form a protective layer of zinc corrosion
products, enhancing the corrosion protection given by the coating.
All or part of the zinc powder can be replaced by a zinc alloy. The amount of
zinc powder and/or alloy in the coating generally is at least 10% and can be
up
to 90% by volume of the coating, on a dry film basis. The zinc powder and/or
alloy can be substantially the whole of the pigmentation of the coating or can
for
example comprise up to 70%, for example 25 to 55%, by volume of the coating,
on a dry film basis, with the coating also containing an auxiliary corrosion
inhibitor, for example a molybdate, phosphate, tungstate or vanadate, as
described in US-A-5246488, ultrafine titanium dioxide as detailed in KR
8101300, and/or zinc oxide and/or a filler such as silica, calcined clay,
alumina
silicate, talc, barytes or mica. The amount of zinc powder and/or alloy in the
coating preferably is between 35 and 60%, more preferably between 40 and
50%.
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Other pigments can be used in conjunction with zinc-based pigments. Examples
of these other non-zinc pigments include conductive extenders such as di-iron
phosphide (Ferrophos ), and micaceous iron oxide. Use of these conductive
non-zinc pigments may allow a reduction of the zinc level while maintaining
effective corrosion protection. To obtain optimum coating properties,
extenders
are preferably sufficiently dispersed in the coating composition. The types
and
sizes of the extenders used can be adjusted to obtain an adequate state of
dispersion. For example, when the extender pigment Satintone (ex Lawrence
Industries) is selected, a mean particle size below 3 pm, preferably below 2
pm
io can be used.
The binder is most preferably based on an aqueous silica sol. Such sols are
available from Akzo Nobel under the Registered Trademark "Bindzil" or from
DuPont under the Registered Trademark "Ludox", although the literature
concerning them emphasizes that conventional grades of colloidal silica are
not
good film formers. Various grades of so[ are available having various
colloidal
silica particle sizes and containing various stabilizers. The particle size of
the
colloidal silica can for example be in the range 10 to 100 nm; particle sizes
towards the lower end of this range, for example 10 to 22 nm, are preferred.
In a
composition according to the present invention, the binder more preferably has
colloidal silica particles with an average particle size between 10 nm and 20
nm,
even more preferably between 10 nm and 16 nm.
The silica sol preferably has a Si02/M2O mole ratio of at least 10:1, more
preferably of at least 25:1, even more preferably of at least 50:1, and can
have a
Si02/M2O mole ratio of 200:1 or more. Further, it is possible to use a blend
of two
or more silica sols having a different Si02/M2O mole ratio, wherein the
Si02/M2O
mole ratio of the blend is at least 25:1. The sol can be stabilized by alkali,
for
example sodium, potassium, or lithium hydroxide or quaternary
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ammonium hydroxide, or by a water-soluble organic amine such as
alkanolamine.
The silica sol can be blended with a minor amount of an alkali metal silicate,
for
example lithium silicate, sodium-lithium silicate or potassium silicate, or
with
ammonium silicate or a quaternary ammonium silicate. Other examples of
suitable sol-silicate blends or mixtures can be found in US 4,902,442. The
addition of an alkali metal or ammonium silicate may improve the initial film-
forming properties of the silica sol, but the amount of alkali metal silicate
should
be low enough to have a Si02/M2O mole ratio of the binder sol of at least 6:1,
preferably of at least 8:1, and most preferably above 15:1. For the purpose of
the present application, a minor amount of alkali metal silicate means that
the
weight ratio of alkali metal silicate to silica sol in the composition is
smaller than
0.5, preferably smaller than 0.25, more preferably smaller than 0.1.
The silica sol preferably has a low level of agglomeration. This can be
determined by asserting the S-value of the sol. The S-value can be measured
and calculated as described by Iler & Dalton in J. Phys. Chem. Vol. 60 (1956),
pp. 955-975. The silica content, the volume of the dispersed phase, the
density,
and the viscosity of the silica sol affect the S-value. A low S-value can be
considered to indicate a high degree of particle aggregation or inter-particle
attraction. The silica sol used in the coating composition according to the
present invention can have an S-value of 20-100%, preferably 30-90%, even
more preferably 50-85%.
It has now been found that a silica sol having a low level of agglomeration
gives
also very good results in the systems and processes described in WO
.00/55260, WO 00/55261, WO 02/22745, WO 02/22746. For these systems and
processes, the silica sol can have an S-value of 20-100%, preferably 30-90%,
even more preferably 50-85%.
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The silica sol can alternatively or additionally contain a dissolved or
dispersed
organic resin. The organic resin preferably is a latex, for example a styrene
butadiene copolymer latex, a styrene acrylic copolymer latex, a vinyl acetate
ethylene copolymer latex, a polyvinyl butyral dispersion, a silicone/siloxane
dispersion, or an acrylic based latex dispersion. Examples of suitable latex
dispersions that can be used include XZ 94770 and XZ 94755 (both ex Dow
Chemicals), Airflex 500, Airflex EP3333 DEV, Airflex CEF 52, and Flexcryl
SAF34 (all ex Air Products), Primal E-330DF and Primal MV23 LO (both ex
Rohm and Haas), and Silres MP42E, Silres M50E, and SLM 43164 (all ex
Wacker Chemicals). Water-soluble polymers such as acrylamide polymers can
be used but are less preferred. The organic resin is preferably used at up to
30% by weight, more preferably 10-20% by weight, based on solid binder.
Higher amounts of organic resin may cause weld porosity during subsequent
welding. It was found that the addition of an organic resin improves the
adhesion/cohesion as measured in the cross hatch test.
Alternatively, the silica sol can contain a silane coupling agent which
contains
alkoxysilane groups and an organic moiety containing a functional group such
as an amino, epoxide or isocyanate group. The silane coupling agent preferably
is an aminosilane such as gamma-aminopropyl triethoxy silane or gamma-
aminopropyl trimethoxy silane, or a partial hydrolyzate thereof, although an
epoxy silane such as gamma-glycidoxypropyl trimethoxy silane can also be
used. The silane coupling agent preferably is present at up to 30% by weight,
for example 1-20% by weight, based on solid binder.
The binder of the primer coating can additionally comprise an aqueous solution
of an alkali metal or ammonium silicate stabilized by a siliconate substituted
by
at least one anionic group of lower pKa than silicic acid, such as a
carboxylate
or sulphonate group. Such a binder preferably is a solution having a Si02/M20
mole ratio in the range 8:1 to 30:1 and a pH in the range 7 to 11.5 prepared
by
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lowering the pH of a solution of silicate and siliconate by cation exchange.
Thus
the siliconate can be added at relatively low levels, for example at a mole
ratio
of 1:2 to 1:20, to a conventional 3.9:1 Si02/K20 alkali silicate. The solids
may
then be reduced to improve ease of processing and to further improve
stability.
5 At this stage the solution has a pH of 12-12.5. The solution is ion-
exchanged
using a standard ion-exchange resin. K+ ions are replaced with H+ ions,
reducing both the alkali content of the binder and the pH. Without the
presence
of the siliconate the silicate would gel on reducing the pH. Clear, stable
solutions with a pH as low as 8 have been obtained. The resultant binder has a
10 Si02/K20 mole ratio typically in the range 8-20:1 and can be concentrated
if so
desired to increase the solids. The binder is a clear, stable solution and is
stable
in the presence of zinc, but coatings based on these ion-exchanged binders
have relatively poor film strength compared to coatings based on alkali
silicate
binders.
Preferably, a binder having a pH of 9 to 11.5 is used, more preferably in the
range 9.5 to 11. While we do not wish to be bound by any theory explaining the
pH effect on the film properties, it appears that an increased pH results in
an
increased amount of silica ions and/or silicate ions in solution. This seems
to
have the potential for effecting in situ gel reinforcement after the
application of
the coating composition. Additionally, pH adjustment can have a minor pot life-
extending effect. When a commercially obtainable silica sol is used, a sol
with a
high pH can be selected and/or the pH- of the sol can be adjusted. The pH can
be adjusted, for example, by adding pH-influencing pot life extenders such as
dimethyl amino ethanol (DMAE) or dilute sulphuric acid, or by adding sodium
hydroxide. For example, commercially obtainable 22 nm silica sols normally
have a pH of about 8.5-9. Increasing the pH of these sols to 10-11 markedly
improves the rate of coating property development.
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The solids content of the primer coating generally is at least 15% by volume
and
preferably in the range of 20 to 35% by volume. The volume solids'content is
the theoretical value calculated on the basis of all the components present in
the coating composition. The coating preferably has a viscosity such that it
can
easily be applied by conventional coating applicators such as spray
applicators,
particularly airless spray or high volume low pressure (HVLP) spray
applicators,
to give a coating having a dry film thickness of less than 40 microns,
preferably
between 12 and 25 to 30 microns.
Optionally, the coating composition may comprise further additives well-known
to the skilled person, e.g., thixotropes and/or rheology control agents
(organo
clays, xanthan gum, cellulose thickeners, polyether urea polyurethanes,
(pyrogenic) silica, acrylics, etc.), defoamers (in particular when latex
modifiers
are present), and, optionally, secondary pot life extenders, such as chromates
(for example sodium dichromate) or tertiary amines (for example triethylamine
or dimethyl aminoethanol). Preferred thixotropes and/or rheology control
agents
include Bentone EW (ex Elementis), which is a sodium magnesium silicate
(organo clay), Bentolite WH (ex Rockwood), which is a hydrous aluminium
silicate, Laponite RD (ex Rockwood), which is a hydrous sodium magnesium
lithium silicate, HDK -N20 (ex Wacker Chemie), which is a pyrogenic silica,
and
Rheolate 425 (ex Elementis), which is a proprietary acrylic dispersion in
water.
Preferred defoamers include Foamaster NDW (ex Cognis), Tego Foamex 88
(ex Tego Chemie), and Dapro 1760 (ex Elementis). It was found that other
compounds which may be present in the coating composition for other reasons
can also act as secondary pot life extenders. For example, the addition of
molywhite anticorrosive pigments or styrene butadiene latex can lead to a
minor
extension of the pot life. Preferred secondary pot life extenders are tertiary
amines, which offer a chromate-free option for pot life extension.
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A longer pot life is also found for systems further comprising alumina. In
this
application, the concentration of alumina in the coating composition is given
as
the percentage by weight of A1203 based on the silica sol or silicate
particles
present in the composition. To obtain optimum properties, preference is given
to
the use of alumina-stabilized silica sols, for example an alumina-modified
silica
sol. In alumina-modified sols, the surface of the particles is modified by
sodium
aluminate bound to the particles. Preferably, the silica sol is modified with
0.05
to 2.5 wt.% of alumina, more preferably with 0.05 to 2.0 wt.% of alumina.
Normally, the coating system is provided as a two- (or more) component system
where the components are thoroughly mixed prior to application of the coating.
It is also possible to prepare the coating composition just prior to
application of
the coating, for example by adding and thoroughly mixing all components of the
coating composition shortly before application. Such a process can also be
referred to as on-line mixing of the components in the coating composition.
This
process is particularly suitable for coating compositions that have a limited
pot
life.
The development of properties can be accelerated by a post-treatment process
in which the substrate can be treated with a solution which increases the film
strength of the primer coating. Preferably, a metal substrate is primer coated
with a coating according to the invention, and after the primer coating has
dried
to the extent that it is touch dry, it is treated with a film strengthening
solution.
Such a solution, which increases the film strength of the primer coating, can
in
general be an aqueous solution of an inorganic salt or a solution of material
having reactive silicon-containing groups.
The development of properties can alternatively be accelerated by immersion of
the optionally post-treated coated substrate in water, or by conditioning the
optionally post-treated coated substrate in an atmosphere with a relative
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humidity of at least 50%, preferably at least 80%. Preferably, a metal
substrate
is primer coated with a coating according to the invention, and after the
primer
coating has dried to the extent that it is touch dry, it is immersed in water
or
alternatively kept in an atmosphere with a relative humidity of at least 50%,
more preferably at least 80%. More preferably, a metal substrate is primer
coated with a coating according to the invention, and after the primer coating
has dried to the extent that it is touch dry, it is first treated with a film
strengthening solution and then it is immersed in water or alternatively kept
in
an atmosphere with a relative humidity of at least 50%, more preferably at
least
80%.
When fast drying is not an issue, it is possible to let a non-post-treated
coating
dry at low relative humidity, for instance between 25 and 50% relative
humidity.
The development of the coating properties will proceed less fast, but
eventually
good coating properties are obtained.
In a preferred embodiment, the coating composition according to the present
invention is a water based shop primer for the coating of steel substrates
which
are intended to be fabricated and overcoated, said composition having a solid
content of 20 - 40% by volume, and wherein the ratio of the pigment volume
concentration to the critical pigment volume concentration is smaller than 1,
comprising:
- an aqueous silica sol binder having a Si02/M20 mole ratio of at least 6:1
and a pH between 9.5 and 11, wherein M represents the total of alkali metal
and ammonium ions and wherein the optionally alumina modified silica
particles have an average size between 10 nm and 16 nm,
- 10 - 55% by volume of the coating on a dry film basis of zinc powder and/or
a zinc alloy having a mean particle size in the range 2 to 12 pm,
- 0 - 35% by weight, based on solid binder, of an organic resin,
- 0 - 30% by weight, based on solid binder, of a silane coupling agent,
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- optionally non-zinc pigment(s), and
- optionally a pot life extender.
The invention will be elucidated with reference to the following examples.
These
are intended to illustrate the invention but are not to be construed as
limiting in
any manner the scope thereof.
The compounds used as starting material in the examples have the following
origin:
Ludox SM a silica sol of concentration 30% by weight, average particle
size 7 nm, Si02/Na2O mole ratio 50:1, ex DuPont, pH 10.3
Ludox HS-40 a silica sol of concentration 40% by weight, particle size. 12
nm, Si02/Na2O mole ratio 95:1, ex DuPont, pH 9.8
Ludox TM-40 a silica sol of concentration 40% by weight, average particle
size 22 nm, Si02/Na2O mole ratio 225:1, ex DuPont, pH 8.8
Bindzil 40/170 a silica sol of concentration 40% by weight, average particle
size 20 nm, Si02/Na2O mole ratio 160:1, ex Akzo Nobel
(Eka Chemicals), pH 9.4
Nyacol a silica sol of concentration 40% by weight and average
particle size 16 nm, Si02/Na2O mole ratio 105:1, ex Akzo
Nobel (Eka Chemicals), pH 9.8
Nyacol Al an alumina-modified version of Nyacol, pH 9.9
XZ 94770 a styrene/butadiene organic latex of 50 vol.% solids, ex
Dow Chemicals.
Minex 20 a sodium potassium aluminum silicate extender pigment of
2.95 m mean particle size, ex North Cape Minerals
Zinc dust a 7 pm mean particle size metal powder, ex Trident Alloys
Molywhite 212 calcium zinc molybdate, an anticorrosive pigment of particle
size 4.1 m, ex Sherwin Williams
Bentone EW a sodium magnesium silicate thixotrope, ex Elementis
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In the experiment, the silica sols were used as obtained, that is, having a pH
as
listed above, unless stated otherwise. When a pH is indicated that is
different
from the pH as listed above, the pH adjustment was carried out as follows:
(i) pH 9 was achieved by adding dilute sulphuric acid, having a pH of 1.5, to
5 a stirred sol
(ii) pH 10 was achieved by adding sodium hydroxide, having a pH of 14, to a
stirred sol
(iii) pH 11 was achieved by adding sodium hydroxide, having a pH of 14, to a
stirred sol.
10 In another aspect of the invention, there is provided a process for primer
coating
a steel substrate wherein the metal is primer coated with a coating
composition of
the invention which is prepared using a silica sol of which the pH is adjusted
to
9.5-11.
In still another aspect of the invention, there is provided a process for
primer
15 coating a steel substrate wherein the metal is primer coated with a coating
composition of the invention wherein after the primer coating has dried to the
extent that it is touch dry, it is optionally treated with a film
strengthening
solution.
In yet another aspect of the invention, there is provided a process for primer
coating a steel substrate wherein the metal is primer coated with a coating
composition of the invention wherein after the primer coating has dried to the
extent that it is touch dry, the coated substrate is immersed in water or
alternatively kept in an atmosphere with a relative humidity of at least 50%.
The invention is illustrated by reference to the following Examples:
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Example 1
To determine the effect of varying the PVC in coatings comprising a 12 nm
silica
sol and having 40% zinc by volume in the dry film, several compositions having
a solids concentration of about 28% by volume were prepared.
The composition used in Example I c was prepared from the following
ingredients.
Component % by weight
Ludox HS-40 41.43
Water 14.77
Zinc dust 39.91
Bentone EW 0.20
Molywhite 212 2.11
Minex 20 1.58
For examples 1 a, 1 b, 1 d, and 1 e compositions were prepared with varying
PVC
by adding or removing Molywhite 212 and Minex 20 to or from the composition
of Example 1c.
The obtained primer coatings were applied to 15 cm x 10 cm steel panels at a
dry
film thickness of 15-20 m at 35 C and 30% relative humidity. The primers were
allowed to dry at 23 C, 60% RH and were tested for their physical
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properties 1 hour and 1 day after application. The results of the tests are
shown
in Table 1.
Table 1
Mechanical Mechanical
properties 1 hour properties 24 hours
after application after application
Ex. PVC A MW Minex Wet Pencil Wet Pencil
No. 212 20 double Hardness double Hardness
rubs rubs
la 40 0.56 0 0 3 <2B 4 <2B
lb 45 0.65 5 0 12 <2B 15 2B
1c 50 0.72 5 5 12 <2B 28 B
1d 55 0.80 5 10 22 2B 60 2H
1e 60 0.88 5 15 17 B 50 2H
if 70 1.04 5 25 13 HB 30 2H
A represents the ratio of the pigment volume concentration to the critical
pigment
volume concentration, i.e. PCV/CPCV ratio.
Example 2
To determine the effect of latex XZ 94770 in coatings comprising a 12 nm
silica
sol and having 40% zinc in the dry film, several compositions having a solids
concentration of about 28% by volume were prepared. The shop primer
compositions were prepared analogous to Examples 1a to If. The latex level in
all compositions prepared for Example 2 was 20% by volume based on silica sol.
The composition used in Example 2c was prepared from the following
ingredients.
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Component % by weight
Ludox HS-40 34.99
Water 15.64
Zinc dust 42.19
XZ 94770 3.08
Bentone EW 0.20
Molywhite 212 2.23
Minex 20 1.67
The obtained primer coatings were applied to 15 cm x 10 cm steel panels at a
dry film thickness of 15-20 m at 35 C and 30% relative humidity. The primers
were allowed to dry at 23 C, 60% RH and were tested for their physical
properties 1 hour, and 1 day after application. The results of the tests are
shown
in Table 2.
Table 2
Mechanical Mechanical
properties 1 hour properties 24 hours
after application after application
Ex. PVC A MW Minex Wet Pencil Wet Pencil
No. 212 20 double Hardness double Hardness
rubs rubs
2a 40 0.56 0 0 14 HB >>100 H
2b 45 0.65 5 0 26 2H >>100 3H
2c 50 0.72 5 5 >100 2H >>100 6H
2d 55 0.80 5 10 >100 2H >>100 6H
2e 60 0.88 5 15 65 2H >>100 6H
2f 70 1.04 5 25 25 2H >100 4H
The results in Table 2 compared with those of Table 1 show that improved film
properties can be obtained by adding latex to the composition. The fastest
development of coating properties was obtained at PVC 50-55%.
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Example 3
To determine the effect of increasing latex levels in coatings comprising a 12
nm silica sol and having 40% zinc in the dry film, several compositions having
a
solids concentration of 28% by volume were prepared. The primer coatings had
a pigment volume concentration of 50%, which is 0.72 times the critical
pigment
volume concentration.
The composition used in Example 3a was prepared from the following
ingredients.
Component % by weight
Ludox HS-40 41.43
Water 14.77
Zinc dust 39.91
Bentone EW 0.20
Molywhite 212 2.11
Minex 20 1.58
For Examples 3b to 3d, compositions were prepared by reducing the amount of
silica sol and adding latex XZ 94770 in increasing amounts.
The application and cure conditions were as used in the aforementioned
examples. The primers were tested for their physical properties 1 hour and 1
day after application. The results of the tests are shown in Table 3. The
amounts of silica sol and latex XZ 94770 given refer to the volume percentage
in the dry film.
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Table 3
Mechanical properties Mechanical properties
1 hour after 24 hours after
application application
Ex. Vol% Vol% Wet Pencil Wet Pencil
No. Silica XZ double Hardness double Hardness
Sol 94770 rubs rubs
3a 50 0 12 <2B 28 B
3b 40 10 >100 2H >>100 6H
3c 30 20 75 2H >>100 6H
3d 25 25 90 3H >>100 6H
Examples 4-7
Several compositions having sol sizes above 12 nm and a pigment volume
concentration of 50% (A=0.72) were prepared. All compositions contained 40%
zinc, 5% Molywhite 212, 5% Minex 20, and 20 vol.% latex XZ 94770 based on
silica sol. Additionally, one comparative example, Example 41, was carried out
with 70% PVC (A=1.06).
The application and cure conditions were as used in the previous examples.
The results of the tests are shown in Table 4.
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Table 4
Mechanical properties Mechanical properties
hour after application 24 hours after
application
Ex. Silica sol Sol size Wet Pencil Wet Pencil
No. double Hardness double Hardness
rubs rubs
4 Nyacol 16 nm 35 HB >>100 3H
41 Nyacol 16 nm 16 HB 47 HB
5 Nyacol Al 16 nm 30 HB >>100 H
6 Bindzil 40/170 20 nm 10 HB 60 HB
7 Ludox TM-40 22 nm 7 <2B 40 H
Comparative example
The examples show that for 16 nm sols fast development of coating properties
5 can be obtained at PVC 50-55%. Additionally, the examples show that coating
properties fall off on increasing the sol size.
Example 8 and 9
Two primer coatings having a solids concentration of 28% by volume were
10 prepared using blends of sols. Both primer coatings had a pigment volume
concentration of 50%, which is 0.72 times the critical pigment volume
concentration.
The primer coating used in Example 8 was prepared from the following
ingredients, resulting in a coating with an average sol size of 10 nm.
Component % by weight
Ludox SM ( 7 nm) 5.5
Ludox HS-40 (12 nm) 29.6
XZ 94770 3.1
Water 15.5
Bentone EW 0.2
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Zinc 42.2
Molywhite 212 2.2
Minex 20 1.7
The primer coating used in Example 9 was prepared from the following
ingredients, resulting in a coating with an average sol size of 10 nm.
Component % by weight
Ludox SM ( 7 nm) 6.8
Nyacol (16 nm) 30.0
XZ 94770 3.1
Water 13.9
Bentone EW 0.2
Zinc 42.1
Molywhite 212 2.2
Minex 20 1.7
The obtained primer coatings were applied to 15 cm x 10 cm steel panels in a
dry film thickness of 15-20 m and allowed to dry at 35 C, 30% RH. Within 1
hour the primed substrates were stored at 60% RH. Subsequently, the coatings
were tested for their physical properties 1 hour and 1 day after application.
The
results of the tests are shown in Table 5.
Table 5
Mechanical properties I Mechanical properties 24
hour after application hours after application
Example Sol sizes in Wet double Pencil Wet double Pencil
No. the blend rubs Hardness rubs Hardness
8 7 nm / 12 nm 42 HB >>100 H
9 7 nm / 16 nm 28 HB >>100 H
The results in Table 5 show that good film properties can be obtained using a
blend of sols.
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Examples 10-13
Several compositions with varying pH and having a pigment volume
concentration of 50% (A=0.72) were prepared. All compositions contained 40%
zinc, 5% Molywhite 212, 8% Minex 20, and 20 vol.% latex based on silica sol.
The application and cure conditions used were the same as in Example 1. The
results of the tests are shown in Tables 6, 7, 8, and 9.
Table 6
Mechanical properties 1 Mechanical properties
hour after application 24 hours after
application
Ex. pH Silica Sol Wet Pencil Wet Pencil
No. sol size double Hardness double Hardness
rubs rubs
10a 9 Ludox 12 9 2B 9 2B
HS-40 nm
11 a 9 Nyacol 16 5 2B 5 2B
nm
12a 9 Bindzil 20 5 2B 5 2B
40/170 nm
13a 9 Ludox 22 7 <2B 7 <2B
TM-40 nm
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Table 7
Mechanical properties 1 Mechanical properties
hour after application 24 hours after
application
Ex. pH Silica Sol Wet Pencil Wet Pencil
No. sol size double Hardness double Hardness
rubs rubs
10b 10 Ludox 12 40 B >>100 2H
HS-40 nm
11b 10 Nyacol 16 35 HB >>100 3H
nm
12b 10 Bindzil 20 75 H >>100 3H
40/170 nm
13b 10 Ludox 22 6 B 60 HB
TM-40 nm
Table 8
Mechanical properties 1 Mechanical properties
hour after application 24 hours after
application
Ex. pH Silica Sol Wet Pencil Wet Pencil
No. sol size double Hardness double Hardness
rubs rubs
10c 11 Ludox 12 >100 HB >>100 2H
HS-40 nm
11c 11 Nyacol 16 55 H >>100 3H
nm
12c 11 Bindzil 20 30 HB >>100 HB
40/170 nm
13c 11 Ludox 22 15 HB 100 H
TM-40 nm
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Table 9
Mechanical properties Mechanical properties
1 hour after application 24 hours after
application
Ex. pH Silica Sol Wet Pencil Wet Pencil
No. sol size double Hardness double Hardness
rubs rubs
10d >11 Ludox 12 >100 2H >>100 4H
HS-40 nm
13d >11 Ludox 22 15 HB 60 HB
TM-40 nm
From Tables 6-9 it becomes clear that reducing the pH of sols with an average
particle size of 12-20 nm has an adverse effect on the (development of)
coating
properties. On the other hand, increasing the pH of the 22 nm sol improves the
(rate of development of) coating properties.
