Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
: .:
Background of the Invention ~ -
Pulverulent metal has heretofore been blended, for
example in a chromic acid liquid medium, to obtain a coating
for application to metal substrates. This can provide corrosion
resistance for the substrate as has been disclosed ln U.S. Patent
3,687,738. The coating compositions are typically dispersions
of pulverulent metal powder or metal flake in water or t-butanol.
Such compositions that are characterized by being substantially
water based and containing metallic flake can be improved through
blending with a high boiling organic liquid. This will, for
example, enhance coating characteristics.
CQmpositions of typically aluminum flake, a polymeric
.
glycol plus wetting agent have been taught in U.S. Patent No.
3,318,716. These concentrates, usually in paste or liquid form,
may be used in small amounts as anti-foaming and pigmenting
compositions. The paste or liquid is added to coating composi-
tions such as dispersions of resin in water. ~dditional pre-
packaged admixes that may be useful include
.,
,.,~: ,~. ,
^'~
''
.-
~ '
'' '.
~ .
- , ~: .
- ,. , . , . .. ~ .. : , .... ~ .
r~
3,~6~ 8~ ~ :
particulate zinc, water-dispersible organic liquid plus thick-
ening agent. These admixes are useful in the preparation of ;
coating compositions for metal substrates. These zinc-
containing admixes can also be water-based, and have been ;
generally discussed in U.S. Patent 3,849,141.
Summary of the Invention
Pigment concentrates such as the above discussed compo-
sitions disclosed in U.S. Patent 3,318,716, can be useful to
impart minor amounts of pigment to a coating composition.
When the particulate metal is used in the coating composition
in heavy doses, as disclosed in U.S. Patent 3,687,738, such
particulate metal offers a desirable combination of coating
characteristics going beyond pigmentation. To facilitate
ease in handling and storage, it is most desirable to pre-
package ingredients into only a few, storage stable premix-
tures. When the particulate metal is zinc flake, and it -
will be used in heavy doses, such prepackaging can become ;
troublesome, often owing to the potential reactivity of pre-
blended ingredients.
An improved premix is now provided which may be used in
concentrated form and in such form can readily be blended ~;
with other liquid coating composition ingredients while ex-
hibiting excellent storage stability before use. Such a
premix composition will also provide aqueous-based liquid
coating compositions when mixed with substituents that can
themselves be prepackaged into a unified, stable blend.
Thus, the premix affords quick coating compositon preparation
from only two packages. Unexpectedly, the two package
system permits the preparation of liquid coating compositions
that exhibit compositional stability themselves, and this
stability is substantially extended beyond what has been
heretofore acheived.
_3_
: , . . - . , . . ,, :
/~ - `
.
Broadly, the present invention relates to a method of
preparing a pulverulent-metal-containing coating composition,
having enhanced ease of preparation from stable, premixed
components. The method involves the blending of a pulverulent-
metal-free and water-containing chromate component (A), with
an anhydrous and chromate-free pulverulent metal component
(B), to thereby prepare a composition adapted for treating
metal substrates and providing corrosion resistance thereto.
The method comprises preparing the water-containing chromate
component (A) by establishing an aqueous composition containing
hexavalent-chromium-providing substance in aqueous solution.
The method also comprises preparing the anhydrous and chro~ate-
free pulverulent metal component (B) by blending together
water-soluble liquid dispersing agent and water-dispersible
organic liquid in amount sufficient to supply the coating
composition which above about 0.0005 volume percent of the
agent and at least about 5 volume percent of such organic
liquid, both basis total volume of the coating composition.
Next the broad method of the invention comprises admixing
water-soluble cellulose ether to the resultlng blend of
dispersing agent and organic liquid, there being sufficient
of such ether to provide between about 0.01-3 weight percent
of ether in the pulverulent metal component (B), exclusive
of the liquid medium of such component (B); and next this
method comprises mixing pulverulent metal flake to the
resulting ether-containing admix in amount sufficient to
provide a weight ratio of the metal flake to the organic
liquid of from about 1:4 to about 4:1 and sufficient to
supply above about 50 grams per liter of metal flake to the
coating composition. Lastly, the method calls for blending
the components (A) and (B) together to prepare the coating
composition.
-4-
' ~ '
.. . , :,:
~ ` ~L069.~`~9
The present invention is further directed to the prepara-
tion of coating compositions using the above described anhy-
drous and chromate-free premix composition, as well as
directed to such premix composition itself.
,
Description of the Preferred Embodiments
The water-soluble cellulose ethers, and most especially
those of commercial importance, can be classified into the
ionic type such as sodium carboxymethycellulose, and the ;
anionic. This later includes the hydroxyalkyl ethers as
exemplified by hydroxyethylcellulose, as well as the alkyl
type as exemplified by methylcellulose. Although the ionic
type are the most hydrophylic they may be subject to precipi-
tation from solution in the presence of metal ions. Thus,
- especially in regard to aqueous coating compositions prepared
from chromic acid, the nonionic cellulose ethers are preferred.
Further, the hydroxyalkyl ethers are particularly preferred
owing to their more ready solubility in cold, as well as hot
water.
For economy, the coating composition preferably contains
below about 3 weight percent, based on the total weight of
the coating composition, of water-soluble cellulose ether.
To enchance thickening such composition contains the cellulose
ether in an amount greater than about 0.01 weight percent,
also based on the total weight of the coating composition.
In general then the premix will contain exclusive of its
liquid medium, between about 0.01-3 weight percent of the
water-soluble cellulose ether. Preferably, for efficiency
and economy, the water-soluble cellulose ether is one of -
hydroxethylcellulose, methylcellulose, methylhydroxypropyl-
cellulose, ethylhydroxyethylcellulose, methylethylcellulose
or mixtures of these substances.
' ::'
--5_
.. - , , , :. . . , . ;, : , . , ....... ~:
6~6~ ;
Although the cellulose ether needs to be water soluble
to augment coating composition thickening, it need not be
soluble in the water-dispersible organic liquid. The ether
may be simply dispersed in the liquid, which liquid can
contribute substantially to the make up of the premix, since
up to 50 volume percent of the coating composition based on
the total volume of liquid in the coating composition, can
be supplied by such water-dispersible organic liquid. Such
organic liquid, when present, also supplies substantially
above about 5 volume percent, and advantageously above about
15 volume percent for enhanced coating characteristics, both
on the same basis of total liquid volume of the coating
composition.
It is most important that the organic liquid be a high
boiling organic substance have a boiling point at atmospheric
pressure above 100C. The organic liquid should also be
easily dispersible in water and preferably water-soluble.
Such organic liquids as are used are those that are retained
during baking on the coated substrate in sufficient amount
2~0 and duration to permit participation of the liquid in the
formation of a coating. This partlcipation can be exemplified
in the coating by reduction of chromium in the coating from
hexavalent to the trivalent state.
The organic liquids contain carbon, oxygen and hydrogen
and have at least one oxygen-containing constituent that may
be hydroxyl, or oxo, or a low molecular weight ether group,
i.e., a Cl-C4 ether group. Since water dispersibility and
preferably water solubility is sought, polymeric hydro-
carbons are not particularly suitable and advantageously
serviceable hydrocarbons contain less than about 15 carbon
atoms. Particular hydrocaFbons which may be present in the
- .
.
1~)6~ 39
aqueous coating composition include tri-, and tetraethylene
glycol, di- and tripropylene glycol~ the monomethyl, dimethyl,
and ethyl ethers of these glycols, as well as diacetone
alcohol, the low molecular weight ether of diethylene glycol,
- and mixtures of the foregoing.
The pulverulent metal flake, e.g., zinc flake or mixtures ;
therof as with aluminum flake, is most typically such pulveru-
lent metal having a thickness on the order of 0.1-0.5
micron and most typically a size in the longest dimension of
not substantially above about 150 microns. Aluminum flake,
also sometimes termed leafing aluminum pigment, has been
discussed, for example, in U.S. Patent 2,312,088. Flake may
be blended with pulverulent metal powder, but typically in
only minor amounts of the powder, and such powder shauld
. . .
have particle size so that all particles pass lO0 mesh and a
. .. . .
major amount pass 325 mesh ("mesh" is used herein as U.S. ;~
Standard Sieve Series). The powders are generally spherical
as opposed; to the leafing characteristic of the flake.
The coating compositions of particular interest contain
20 an amount of pulverulent metal that does not exceed about :~ ;
.~ - .
500 grams of metal per liter of coating composition liquid
medium. More than this can add expense without a significant
ncrease in protection for the coated substrate.~ Advanta-
geously, for augmented coating protection, the coatlng com- ;
position contai~s above about 50 grams of metal per liter.
Thus the premix contains sufficient metal flake to supply ;~
above about 50 grams per liter to the coating composition.
~, .. .
The premix ls prepared to contain a weight ratio of the
water-dispersible organic liquid to the pulverulent metal
flake of between about 1:4 - 4:1. Within this ratio the
premix will exhibit desirable stability whlle providing
ready mixing with further compositional constituents to ;
yield the coating composition.
-7-
.- . . . . . . .. . . .. :: . .. ~i . .. ~
~L()6~!L6~
The premix should contaln some water-soluble liquid
dispersing agent. Water-solubillty of the agent provides
enhanced stability for the later prepared coating composition.
However, the agent need not be soluble in the water-dispersible
organic liquid for such premix to exhibit desirable stability.
Such agent can be present in the premix in amount sufficient
to supply the coating composition with as little as about
0.0005 volume percent of agent, basis total coating composition
liquid. It is generally contemplated to employ a dispersing
agent that is a nonionic surfactant which may be an ethoxylated
alkylphenol such as a nonyl or octyl phenol. It is also
contemplated to employ the nonionic ethoxylated aliphatic
alcohols, representatives of which include the oleyl, lauryl,
and stearyl alcohols. Other suitable nonionic surfactants
that are also readily commercially available and are contem-
plated for use in the present invention include, for example,
carboxylic esters that encompass the glycerol esters and the
anhydrosorbitol esters, as well as the polyoxyethylene
esters of fatty, rosin and tall oil acids. ~It is also
further contemplated to use carboxylic amide nonionic surfac-
; tants for dispersing the pulverulen~ metal and these are
meant herein to include the polyGxyethylene fatty acid
amides. The preferred nonionic liquid dispersing agents are
- polyethoxy adducts, exemplified by the alkylphenoxypolyethoxy-
alkanols, and derivatives thereof, some of which are described
,
in U.S. Patent 3,281,475. Such agents are nonionic and have
between about 7 and 50 oxyethylene units in the molecule.
For economy the agent is generally present in the coating
composition in an amount not above 1-2 volume percent, on a
total volume of the coating composition.
,~ ,
. ., '
, . ' .
~96~6~9 - ~
. . .
The liquid dispersing agent, cellulose ether, pulverulent
metal flake, and water-dispersible organic liquid are the
key ingredients for the premlx composition. This mixture of
ingredients will exhibit extended storage stability. To
enhance such stability, the premix should be chromate-free.
Also for best extended stability, the premix composition
should be anhydrous. By the use of this term it is meant
that the premix may contain some very minor amount of water,
as for example, less than 0.5 weight percent water. Water
will typically be contributed from commercially available
materials used in preparing the premix composition and not
from the deliberate addition of water to the premix. For
example, commercially available water-dispersible liquids
can include very minor amounts of water, generally on the
order of a few tenths of a weight percent or less, and it is
not meant to exclude the use of such materials in the premix. -
The critical components of the premix may be blended
-
together in any mixing order. Thus it is not critical that -
:. . .
the water-soluble liquid dispersing agent and water-dispersible
20 organic liquid be first mixed with one another, although this
is preferred for efficiency. After preparation, often
followed by storage and/or shipping, the premix is ready for
blending with additional ingredients to form an aqueous
coating composition. Preferably, this blending will be
carried out by simply mixing together the premix composition
with an additional package of substituents. Further, this
additional package of constituents will typically itself
exhibit excellent storage stability. Thus, by means of the
premix composition, a coating composition exhibiting excellent
coating characteristics including corrosion protection for
coated metal substrates, which composition itself further
exhibits highly-desirable, extended useful life, will be
prepared simply from two packages.
_9_ ;
It is, however, contemplated that the premix composition
may be blended with additional coating composition ingredients -
that are added individually or in packages other than one
packaye. In addition to water, these additional ingredients
include a chromate component. Furthermore, for extended
storage life, it will be a pulverulent-metal-free component.
By use o the term "chromate component", it is most typically
contemplated to use chromic acid. However, it is also
contemplated to use its equivalent in aqueous medium, for
example, chromium trioxide or chromic acid anhydride. But it
is contemplated that the component can be supplied in whole
or in part by a salt such as calcium, barium, magnesium,
zinc, cadmium or strontium dichromate. Additionally, a
minor amount such as 20 percent or less of the component
might be a mixed chromium compound, i.e., include trivalent
chromium compounds. Although the coating composition might
contain only a small amount, e.g., 5 grams per liter of
hexavalent chromium, expressed as CrO3, and may contain as
much as about l00 grams per liter of composition of hexavalent
chromium, expres-sed as CrO3, it will typically contain
between about 20-60 grams.
For supplying the liquid medium of the aqueous coating
composition, without considering the contribution by the
water-dispersible organic liquid, water virtually always
supplies the whole amount. Other liquids may possibly be
used, but preferably only a very minor amount of the aqueous
medium, basis the water content of the medium, is such other
liquid material. Such other liquids that might be contem-
plated include alcohols, most notably t-butanol, and halo-
genated hydrocarbon liquid, some of which have been discussed
in U.S. Patents 2,762,732 and 3,437,531. ; ` `-
., , ,' '' ' ', . '
- 1 0 ~
,`:':,`;'"~ '.
31 ~616~9
.An additional ingredient generally used in preparing
the coating composition is an inorganic pH adjustment agent.
Such agent is basic, i.e., will yield a solution pH above 7.0
when dissolved in water alone, and has sufficient solubility
in chromic acid solution to provide for pH adjustment. Repre-
sentative pH adjusting agents are the inorganic metallic oxide,
carbonate and hydroxide of lithium. The higher metals in
Group lA, i.e., sodium and potassium, can be initially
adequate for pH adjustment. ~owever, the subsequent coatings
on metal substrates have been found to be water soluble and
thus such agents are not suitable. Other metal oxides,
carbonates and hydroxides can be supplied by metals in Group ;
IIA, e.g., calcium oxide or calcium carbonate, or metals in
groups above IIA, i.e., to the right of the IIA Group in the ~-
periodic table, such as zinc oxide as a represantative of
Group IIB.
Other ingredients may be present in the aqueous coating
aomposition but, even in combination they are most generally
present in very minor amounts. Since the adherence for the
particulate metal to the metal substrate might be achieved
by the ostensible interaction of the chromate component with
the water-dispersible organic liquid during curing of the
coating, these coating compositions need not contain resin
and are preferably resin-free, and such coatings that will
be subsequently topcoated are virtually always pigment~free.
These other ingredients further include inorganic salts and
acids as well as organic substances, often typically employed
in the metal coating art for imparting some corrosion resis-
tance or enhancement in corrosion resistance for metal
surfaces. Such materials include zinc chloride, magnesium
chloride, molybdates, glutamic acid, succinic acid, zinc
-11- '. ' '. .~.
' ' , .
~L~6:~6~39
.
nitrate, and succinimide and these are all preferably avoided,
but if present, are most usually employed in the liquid
composition in a total maximum amount of less than 5 grams
per liter.
For the metal substrates containing applied coating,
the preferred temperature for the subsequent heating, which
is also often referred to as curing and which may be preceded
by drying such as air drying, is within the range from about
400F. at a pressure of 760 mm. ~g up to not e~sentially
above about 1,0~0F. Such an elevated substrate temperature
may be attained by preheating the metal prior to application
of the liquid composition. However, such curing temperatures
do not often exceed a temperature within the range of about
450F-700F. At the elevated curing temperatures the heating
can be carried out in as rapidly as about a few seconds but
is often conducted for several minutes at a reduced temperature.
Before coating with the aqueous coating composition it
is, in most cases advisable to remove foreign mattex from
the metal sur~ace by thoroughly cleaning and degreasing.
Degreasing may be accomplished with known agents, for instance,
with agents containing sodium metasilicate, caustic soda,
carbon tetrachloride, trichlorethylene, and the like. ~ `
Commercial alkaline cleaning compositions which combine
washing and mild abrasive treatments can be employed for
cleaning, e.g., an aqueous trisodium phosphate-sodium hydro-
xide cleaning solution. In addition to cleaning, the sub- ` ~;
strate may undergo cleaning plus etching. i
After heating, the resulting coated substrate can be ~; ;
further topcoated with any suitable paint, i.e., a paint,
primer, including electrocoating primers, and weldabLe
primers such as the zinc-rich primers that can be applied
before, typically, electrical resistance welding, and paints
-12-
. ': -: ', ' ': ' , ,, ' '.' , `, . ' : . .,.';. i ' 1 ' . ' ' ,: ' ' '`, ~: , ' : ,; . ' `
6~9
such as enamel, varnish, or lacquer. Since the coated metal
surfaces can exhibit a desirable upgrading in topcoat adhesion
when compared, for example, to the uncoated substrate metal,
paints are often applied over such coated substrates. Such ;
paints may contain pigment in a binder or can be unpigmented,
e.g., generally cellulose lacquers, resin varnishes, and oleo-
resinous varnishes, as for example tung oil varnish. The paints
can be solvent reduced or they may be water reduced, e.g.,
latex or water-soluble resins, including modified or soluble
alkyds, or the paints can have reactive solvents such as in
the polyesters or polyurethanes. Additional suitable paints
which can be used include oil paints, including phenolic
resin paints, solvent-reduced alkyds, ipoxys, acrylics,
vinyl, including polyvinyl butyral and oil-wax-type coatings
such as linseed oil-paraffin wax paints. The paints may be
applied as mill finishes.
The following examples show ways in which the invention
has been practiced but should not be construed as limiting
the invention. In the examples the following procedures
-
have been employed:
Preparatlon of Test Parts
Test parts are typically prepared for subsequent treat-
ment by immersing in water which has incorporated therein 2-
.
5 ounces of cleaning solution per gallon of water. The
cleaning solution is typically 75% by weight of potassium .
hydroxide and 25 weight percent tripotassium phosphate. The
bath is maintained at a temperature of about 150-180F. ~ ~ `
After the cleaning treatment the parts are rinsed with warm
water and may be dried.
Application of Coating to Test Parts and Coating Weight
Clean parts are typically coated by placing in a wire
basket and dipping the basket into coating composition,
-13~
.
~)6~16~9
removing the basket and draining excess composition therefrom
with a mild shaklng action and then immediately baking or
air drying at room temperature until the coating is dry to
the touch and then baking. saking proceeds under infrared
lamps or in a hot air convection oven, attaining substrate
temperatures that can exceed about 450F., with times of
several minutes, generally as specified in the examples.
Coating weights for parts, generally expressed as a
weight per unit of surface area, are determined by selecting
a random sampling of parts of a known surface area and
weighing the sample before coating. After the sample has
been coated, it is reweighed and the coating weight per
selected unit of surface area, most always presented as
milligrams per square foot (mgms./sq.ft.), is arrived at by
straightforward calculation.
Corrosion Resistance Test (ASTM B-117-64) and Rating
Corrosion resistance of coated parts is measured by
means of the standard salt spray (fog) test for paints and
varnishes ASTM B-117-64. In this test, the parts are placed
20 in a chamber kept at constant temperature where they are ~-
exposed to a fine spray (fog) of a 5~ salt solution for ~ ;
speciied periods of time, rinsed in water and dried. The
extent of corrosion on the test parts is determined by
.
comparing parts one with another, and all by visual inspection.
In the following examples the efficacy of the corrosion ~ ~ ~
resistance obtained on coated parts is, in part, quantatively ;~ -
evaluated on a numerical scale from 0 to 10. The parts are
visually inspected and compared with one another and the
system is used for convenience in the reviewing of results.
30 In the rating system the following numbers are used to cover ~ ;
the following results:
(10) retention of film integrity, no red rust;
:.:
-14-
~ ~ ~()6~L613~ ~
.
(8) initial coating degradation, pinpoints of red ^
rust;
(6) less than 3~ red rust basis total surface area of
the part; ;
(4) 3 to 10% red rust, i.e., a significant amount of
rust;
(2) 10 to 25 percent surface area red rust; r, ',
(0) greater than 25 percent red rust.
EXAMPLE 1
: .
TO 55 milliliters (mls.) of dipropylene glycol (DPG) there
,
is blended with moderate hand agitation 0.5 ml. of wetter which
.
i5 a nonionic, modified polyethoxy adduct having a viscosity
in centipoises at 25C. of 180 and a denslty at 25C. of 8.7
pounds per gallon. The blending is seen to prepare a solution ~;
of the wetter and the DPG. To this solution there is then
blended with moderate hand agitation 0.5 gram (gm.) of
hydroxyethyl cellulose thickener. The thickener is a cream
to white colored powder having a specific gravity of 1.38-
1.40 at 2~0/20~C., an apparent density of 22-38 pounds~cu.ft.,
and all particles paS5 through 80 U. S. mesh. During agitation, ;; -~ -
the thickener is seen to readily uniforml~ disperse into the
blend of the wetter and DPG. -
To this thickener mixture there is then added 84 grams of
zinc flake and the addition is accomplished with manual agita- -
tion. The zinc flake has particle thickness of about 0.1-0.5
micron and a longest dimension of discrete particles of about ;
80 microns. The resulting dispersion is seen to have a desir~
able uniformity. ~-~
Separately there is added to 88 ml. of deionized water,
containing sufficient chromic acid to provide 50 grams per
liter (g./l.) of CrO3, an additional 88 ml. of deioni`zed water.
.. . . .
','',:''', :'
-15-
~0~:i16~
To this chromic acid solution is added 0.5 gm. of zina oxide.
The resulting chromic acid solution is slowly added to the '
metal flake dispersion to form a coating composition. From
observation and analysis, the resulting coating bath is `~
deemed to be a highly acceptable bath, comparing favorably
to coating baths prepared by a different, standardized method.
~.
EXAMPLE 2
In the manner of Example 1 the following ingredients, all
as described in Example 1, are blended together, with the pro-
portions being in grams for a liter of subsequent coating compo-
sition: 213 gms. of DPG, 3 gms. of wetter, 3 gms. of cellulose
thickener, and 336 gms. of zinc flake; this prepares an admix
package. Separately, to prepare an additional package, there ~ -
is added to deionized water sufficient chromic acid to provide ~ ~ ;
about 50 g./l. of CrO3 for a subsequent coating composition,
and zinc oxide~ to provide 16 g./1. of same for the coating
- composition. The metal flake dispers~ion (admix) package is
slowly added to the chromic acid solution package to form a i~
coating composition from just the two packages.
~` To prepare a three-package bath, into 216 mls. of DPG
plus 3 mls. of the above described wetter, there is blended ;`
. .
336 gms. of the above discussed zinc flake. This prepares ~ -
the first package. Separately there is added to 355 mls. of ! ' ,
deionized water, 50 gms. of chromic acid plus 16 gms. of
zinc oxide with moderate mechanical sitrring. This formulates
the second package.
To the zinc flake slurry (first package) there is slowly ~
added, during maderate mechanical agitation, a third package ~ -
that contains 355 mls. of a predissolved solution of water
30 containing 3 gms. of the cellulose thickener. Subsequently, ;
the chromic acid solution (second package) is slowly added ~`
-16-
- ~ID616~9 -:
to the zinc flake/ thickener blend accompanied by moderate
mechanical agitation. This prepares a bath from three- `
packages.
The coating composition from the two packages, as well
as that from the three packages, are each used to coat ten
number 12d mild steel nails and ten 1/4" X 1/2" mild steel
bolts. All parts are coated by dipping and draining in the
manner described hereinbefore and the coated parts are cured
in an oven for 30 minutes at an oven air temperature of 320C. ;
10 This coating process is repeated so that each of the 20 nails ~ ~;
and each of the 20 bolts contains three coatings. All bolts :r
are determined to have about 1400 mgms./sq.ft. of coating.
All parts are subjected to the corrosion resistance salt
spray test and after 500 hours of such testing, all parts -
., . ~
are seen to have the same excellent rating. ~his demonstrates
that the two-package coating composition procedure of the
present lnvention will provide compositions yielding highly
desirable coatings offering excellent corrosion protection
equal to that obtained from baths prepared from the three
packages.
A similar two-package bath is made in the above described
manner, except that the chromic acid solution package is
slowly admixed in to the metal flake dispersion package. Next, ~ -
using the same substituents and proportions a three-package
bath is prepared as discussed hereinabove. In this preparation,
and as mentioned above, first a zinc flake/thickener blend
is prepared from two packages and then the chromic acid solution
is slowly admixed to the zinc flake/thickener blend. All
,. . . .
packages used for the preparation of each bath are formulated -- ~
30 to have excellent shelf stability. In shelf stability testing ~;
for the resulting baths, which can likewise be important, the
:., . ~ .
three-package bath is stable for only about 60~ of the shelf
life of the two~package bath.
'"''
-17-
