Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2'(~
VOC COMPLIANT PRETREATMENT PRIMER~
William ~. Lesney
Frank M. Jachim
~ACRGROUND OF T~E INVENTION
Pretreatment primers which are used in the industrial and
automotive industries traditionally are very high in volatile
organic content (VOC). Typical VOC levels are upwards of 5.0
lbs/gallon (packaged) and 6.0 lbs/gallon (reduced to sprayable
viscosity). Many recent developments in environmental laws and
regulations severely limit the VOC content of paint ~and coating
products. For example, the California South Coast Air Quality
Management District has proposed VOC restrictions for pretreatment
primers of 3.5 lbs/gallon.
In light of the severe VOC restrictions now coming into force,
our efforts have focused on developing a low VOC, water reducible
pretreatment primer composition which will perform comparably to
higher VOC pretreatment primer compositions. This invention
relates to a film-forming pretreatment primer for use over clean,
untreated metal surfaces having a VOC of less than 3.5 lbs/gallon.
The coatings of this invention are particularly useful as
pretreatment primers to serve as tiecoats between metal substrates
and topcoats in the industrial and automotive industries.
BUMMA~Y OF T~E INVENTION
Thl~ invention relates to a two-component pretreatment primer
sy~tem for application to clean metal surfaces. The pretreatment
primer serves as the tiecoat between the substrate and a suitable
2~ f;
topcoat. The pretreatment primer of this invention, fully
described herein, comprises:
a) an aqueous first component comprising
1) a resin,
2) a zinc chromate pigment,
3) a zinc phosphate pigment, and
4) a package stabilizing agent; and
b) an aqueous second component comprising a catalyst solution for
said firs~ component. -
Accordingly, it is an object of this invention to teach a lowVOC pretreatment primer composition which performs comparably to
conventional higher VOC pretreatment primers.
This and other objects will become readily apparent from the
detailed description of the invention, below.
DETAILED DE8C~IPTION OF T~E INVENTION
As stated above, this invention relates to a low VOC
pretreatment primer. This primer comprises:
a) an aqueous first component comprising
1) a resin,
2) a zinc chromate pigment,
3)- a zinc phosphate pigment, and
4) a package stabilizing agent; and
b) an aqueous second component comprising a catalyst solution for
said first component.
The concept behind the compogitions of this invention is
2r~ s
similar to that of conventional pretreatment primers. The
aomposition, when dissolved in organic solvent and applied over a
metal surface will react with the metal surface to form an
adherent, complex surface coating that is composed of metal
components from the active ingredients in the primer as well as the
metal components from the metal surface. These pretreatment
primers can advantageously serve as the replacement phosphatizing
process at facilities where the size or adaptability of the metal
part makes it unattractive to treat the metal via a separate
phosphatizing process.
Contrary to conventional, higher VOC pretreatment primers, the
compositions of the present invention are aqueous compositions
which combine an aqueous resin with a zinc phosphate pigment in the
same component. This pigment contributes phosphate ion to the
composition, which in combination with the zinc chromate pigment,
enables the composition to readily adhere to clean metal surfaces.
Contrary to conventional pretreatment primers wherein the phosphate
ion source is contributed by a separate catalyst solution, the
present invention incorporates the phosphate ion source in the same
solution as the film forming resin. In the present invention, the
separate catalyst serves predominantly to acid etch the metal.
Suitable resins for use in the first component of this
invention include the aqueous latex resins widely known in the
paint and coating art. They are typically the result of the
emulsion polymerization of acrylic and vinyl monomers and
prepolymers. Suitable monomer~ can be selected from the group
z~
consisting of acrylonitrile, methacrylonitrile, acrylic acid,
methacrylic acid, itaconic acid, and esters of these acids,
especially hydroxy ethyl acrylate and methacrylate, hydroxy propyl
acrylate and methacrylate, methyl acrylate and methacrylate, ethyl
acrylate and methacrylate, butyl acrylate and methacrylate, lauryl
acrylate and methacrylate, and the like, trimethylol propane
triacrylate and trimethacrylate, hexanediol diacrylate, acrylamide,
methacrylamide, vinyl chloride, vinylidene chloride, styrene,
divinyl benzene, vinyl toluene, vinyl naphthalene, multifunctional
epoxides, melamines and isocyanates, and mixtures thereof. In
addition to pure monomers, preformed polymers and polymeric
intermediates can be used.
The resin of this invention can be manufactured according to
the well-known processes for producing emulsion polymers, or, the
resin can be purchased from readily available sources.
In selecting a resin system for this invention, one need be
concerned with preparing or utilizing a resin which air dries by
film coalescence, has a Tg sufficiently low enough to provide good
film formation under various humidities, generally below about 30
degrees C, exhibits some degree of solvent resistance (for prevent
lifting when used with solvent-borne topcoats), and, importantly,
exhibits good adhesion over untreated metal surfaces.
one particularly preferred resin system for use in the first
component of this invention is commercially available from Rohm and
~aas Company under the designation "MV9". MV9 is a latex based
upon an acrylonitrile-acrylic copolymer- having a Tg of
approximately 20 degrees c. MV9 has been ~ound to be capable of
undergoing crosslinking with the zinc chromate pigment of this
invention provided there is a water soluble source of phosphate
ions in the composition to impart acidity to the solution. This
phosphate ion source is contributed by the zinc phosphate pigment
present in the solution.
In addition to the resin, the first component must further
comprise a pigment which is capable of complexing with the metal
substrate. In the present invention, a combination o~ two separate
pigments is used. The first pigment comprises a zinc chromate
pigment. Preferably, this first pigment comprises finely divided
discrete particles of a zinc tetraoxychromate pigment. A
particularly suitable zinc tetraoxychromate pigment is commercially
available from Mineral Pigments Corporation. The second pigment
comprises a zinc phosphate pigment. Preferably, this second
pigment comprises a zinc silica phosphate hydrate comprising zinc,
BaS04, sio2 ~ MgO and P04. A particularly suitable zinc phosphate
pigment is commercially available from Heubach under the tradename
Heucophos ZBZ (tm). The combined pigmentation used herein produces
an anodic mechanism which complexes with other metal cations on the
substrate surface to provide adhesion of the coating to the
8ubstrate. In order to function properly, the pigmentation used
herein must be slightly water soluble for proper ionization. This
water solubility, however, can adversely impact package stability;
and therefore, the level of pigmentation is critical. The
-pigmentation level in the fir~t component for the zinc chromate
pigment is generally less than or equal to about 4.5% Pvc,
preferably about 3% PVC. The pigmentation level in the first
component for the zinc phosphate pigment is generally less than or
equal to about 6~ PVC, preferably about 3.5% PVC.
In addition to the above plgments, one or more inert pigments
can optionally be included in the first component of the
composition. Preferably, one such pigment is a relatively low
level of micronized talc, up to about 4% PVC, preferably up to
about 2% PVC. Micronized talc can favorably impact the
composition's resistance to transmission of moisture through the
film and improve the dry time of the coating.
In addition to the above, the first component of this
invention further preferably comprises an adheSion promoting
ingredient. The preferred adhesion promoting ingredient comprises
a stabilized casein solution. Casein is a well known
phosphoprotein composition. It is sparingly soluble in water and
nonpolar organic solvents, but is soluble and will form salts in
alkali solutions. The use of a casein solution in the first
component of this invention improves adhesion of the coating to the
substrate, improves the pigment wetting, and thus helps to improve
package stability. One particularly preferred casein solution is
commercially available from Ultra Additives under the tradename
Ulasein 15 (tm). Such solutions can be added to the first
component of this invention in an amount up to about 5% by weight,
preferably up to about 1% by weight.
The first component of this invention must further comprise
a dispersing agent for package ~tability. Preferred package
stability agents include the sodium salts of carboxylated
polyelectrolytes. Such agents are known to impart stability to
latex systems and are useful herein. One particularly preferred
stabilizing agent is Tamol 850 (tm) commercially available from
Rohm and Haas Company. It can be added in an amou~t from about
0.1% (based upon total pigment weight) to about 4.0% (based upon
total pigment weight), preferably in an amount of between about
2.0% and 3.0% (based upon total pigme~t weight). - -
In addition to the above stability agent, the first componentof this invention generally further comprises a thickening agent.
One particularly preferred thickening agent i8 the alkali swellable
acrylic polymer emulsion commercially available from Rohm and Haas
Company under the tradename Acrysol TT-615 (tm). If used, it is
preferably contained in the pigment dispersion and thus added to
the first component during the grind phase. However, it can be
added to the first component during the letdown phase provided that
care i8 ta~en to avoid destabilizing the composition. When used,
suitable amounts of thickening agents can range from between about
2.0% by weight ~based upon polymer solids) to about 10.0% by weight
(based upon polymer solids).
- The first component also generally further comprises
coalescing solvents selected from the group consisting of water
miscible glycol ethers such as butyl cellosolve and butyl carbitol.
Th~ amount of ~utyl cellosolve (weight % butyl cellosolve to
polymer-601ids) is generally between about 35% and about 60%,
preferably about 45%. The amount of butyl carbitol ~weight % butyl
carbitol to polymer solids) is generally between about 10% and
about 20%, preferably about 15%. The butyl cellosolve is
incorporated to provide shorter tack free dry times to the coating
composition whereas the butyl carbitol is incorporated to upgrade
the coatings film formation and adhesion characteristics.
Ethylene glycol can additionally be added to the first
component to fine tune the application characteristics of the
coating and to aid in the milling of the composition~
The second component of this invention comprises an aqueous
catalyst solution for said first component. It comprises an
a~ueous solution of a naturally occurring organic acid, preferably
tannic acid, and tetrachlorophthalic anhydride preneutralized to
a pH of about 11. Generally, ammonia is used to adjust pH. A
preferred catalyst solution comprises between about 0.4% and about
3.0% by weight of tannic acid, between about 0.1% and about 1.0
by weight of tetrachlorophthalic anhydride, in water.
.
