Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGRQlN~ O~ THE INVENTION
This invention relates to the field of container end sealing compounds,
and, in particular, to a method of modif~ing a container end sealing compound
so that formation of rust at the exposed edges of metal container ends is
preventedO
The art of container sealing has an established terminology which
will be used in this specificationO The walls of the container are known
as the "bodyl'. The end closures are known as "ends". The gasket which is
interposed bet~een the ends and bod~ and is responsible for the hermetic or
liquid-tight seal betw-een the parts is known as the "lining". The plastic
or liquid composition ~hich, upon cooling or drying forms the solid gasket, is
known as ''lining''O
The bod~ of the can is a crlindrical piece of sheet metal normally
made by forming a rectangular piece of metal and then joining the ends of
the rectang~le along the side seam. The metal in the ends of the cylinder
are flanged outwardly. The can is completed by placing a circular piece of
metal called the "end" over each end of the cylinder and rolling the outer
edge of the end and flange on the body together in a double seam. The can ends
are made in a separate operation b~ stamping a blank from like material which
ma~ be tin plate, black plate, aluminum, or other sheet metal. That portion
of the end ~hich covers the interior of the can is called the "panel". The
outer periphery of the end is formed into a circular depression called the
"channel" ~hich cooperates with the flange on either end of the body. The outer
edge of the channel called the "curl" is deformed upwardly and inwardl~ to
provide contact of the can end ~ith the inner side of the flange of the can
bod~ during the first stage of the double seaming operation. The "cut edge"
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of a metal can end reers to the exposed edge of the can end located at the
curl area of the can end. The exposed edge comes into being when the can end
is formed by means of the stamping operation. The cut edge is prone to
corrosion because of the lack of protective coating or lacquer over the cut
edge. The remaining portion of the panel is generally resistant to corrosion
because it is covered with a protective coating that has not been damaged
or removed by the stamping operationO The inner wall of the channel, i.e., the
portion between the channel and the panel, is known as the "shoulder". The
plastic or liquid composition which forms the "lining" is placed in the
channel of the can endO
Sealing compounds for can covers ~"ends") are commonly applied to
the covers in liquid formO The machines, called "lining machines" which apply
the compound to the joint area, have, as essential operating elements, a con-
tinuously rotating chuck which receives and rotates the cover, a "nozzle",
essentiallr a squirt gun, controlled by a needle valve which projects the
compound downwardly onto the joint area of the cover, and a quick opening
and closing cam which lifts the needle valve and closes it at the proper
instant.
common type of compound-applying machine removes a blank closure
from a stack; slides the closure along a table; places it on a continuously
revolving chuck, where the compound is applied and placed; pushes the closure
from the chuck; tucks it beneath a stack of finished closures, and places a
new closure on the chuck in a single cycle of operation. After the can ends
have been lined, they are conveyed to an area where the volatile components
of the sealing composition are removed.
Through the operation of the lining machine, a ring of fluid
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compound is formed on the joint area adjacent the periphery of the can end.
This ring of compound, whether in liquid form or later when it is dried, or
fluxed into a solid mass is called the "lining". Its function is to form the
gasket between the can parts and hermetically seal the can.
Ballou et al, United States Patent 3,013,896 and Flaherty, United
States Patent 3,310,196 describe the basic operations employed in lining can
ends and attaching the can ends to the can body.
Water-based can end lining compositions are water dispersions of
special rubbers which, when flowed into can ends and dried, provide a hermetic
seal. The solids portion of a water-based can end lining composition ranges
from about 30 percent to about 85 percent of the total composition.
~: Based upon 100 parts per hundred of rubber (hereinafter referred
to as phr), the solids portion of a typical water-based lining composition has
the following composition (all parts are expressed in weight):
Ingredient ~mount
Rubber 100 phr
Pigment and/or filler 50-200 phr
Resin 0~100 phr
Antioxidant less than 1%
Bacteriocide less than 1%
Surface Active Agents 0.5% to 10%
` The remaining 15 percent to 70 percent of the total composition consists of
water. The conventional method of drying the water-based compositions involves
the use of an oven drier. Air drying with no mechanical assist can also be
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utilized.
A seriQus~problem with ~ater~based can end lining compositions is
that the~ tend to promote rust and/or corrosion in the area of the cut edge
of a metal can end. This rust or corrosion is caused primaril~ by high humidity
conditions which are induced by the environment of the can end lining operation~
~hen a stack of can ends lined ~ith a water-based can end lining composition is
placed in the interior of a pallet containing stacks of lined can ends, the
water present in the lining composition promote~ corrosion in the area of the
cut edge of the can end, because the absence of circulation of air retards the
evapo~ation of water from the lining compositionO The components of the
water_based can sealing composition ~ill also induce rust or corrosion through
various chemical means.
Solvent-based can end lining compositions are primarily solvent
solutions of special rubberO Solvent-based can end compositions may also pro-
mote rust near the cut edge of a metal can endJ but to a lesser extent than
water-based can end lining compositions.
Numerous materials have been employed effectively as rust inhibitors
for metal can ends. These materials include sodium hydroxide ~pH 11),
potassium hydroxide ~pH 11), triethanolamine, diethanolamine, monoethanolamine,
sodium benzoate, and sodium. Each of these materials is effective to some
degree. The effectiveness of these materials is based on their ability to
inactivate either the surface of the subytrate or the components of the lining
compositions.
Although these materials retard the rusting of steel can ends, it
~ would be desirable to essentially eliminate the formation of rust at the
; exposed edges of the can ends.
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This invention seeks to provide a method for preventing formation
of rust on metal can ends.
Also, this invention seeks to provide a method for preventing forma-
tion of rust at the cut edge of any type of metal product susceptible to
corrosion by rust.
We have discovered a method of modifying container end sealing
compound so that formation of rust on steel container ends is virtually elim-
inated. The method involves adding a phosphate compound to a container
sealing compound formulation prior to applying the container sealing compound
to the container end. Hollingshad, United States Patent 3,941,562 discloses
a method of inhibiting the corrosion of metals in a water system by maintaining
in the water of the system at least about 10 ppm of a composition comprising
a polyacrylamide and a source of orthophosphate. This method, however, is
unsuitable for prevention of rust on the cut edge of metal can end, because
product specifications will not always allow manufacturers to incorporate the
polyacrylamide/orthophosphate composition into the product which is to be
stored in the can.
` The terms "can end lining composition" and "can end lining compound",
; and "lining composition" and "lining compound", are used interchangeably in the
specification and claims of this disclosure.
SUMMARY OF THE INVENTION
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This invention involves a method for preventing the formation of
rust and/or corrosion at the panel and the cut edge of a metal container end.
The method comprises treating a container sealing compound with
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a phosphate salt prior to applying sealing compound to the end o~ the container.
The seal thus formed rom the treated sealing compound is effective in
preventing rust formation at the outer edge of the container end.
DETAILED DESCRIPTION
The addition of a suitable phosphate salt to a can end lining
compound formulation will allow-the formulation to inhibit the formation of
rust and/or corrosion at the cut edge of a metal can end.
The can end lining compound formulations contemplated for use in
this invention include both water-based formulations and solvent based
formulations. A suitable water-based formulation is described below. This
formulation contains about 15 weight percent to about 60 ~eight percent water,
and about 40 weight percent to about 85 weight percent solids. The foregoing
percentages are based on total weight of the formulation. The solids component
is further characterized as comprising the following ingredients in the amounts
indicated:
IngredientAmount
~percent, based on total
weight of solids component
only)
. Binder 24 - 65
Resins 0 - 24
Emulsifiers0 - 3
Filler 20 ~ 60
Antioxidant0 - 1
The preferred binder is rubber (SBR)o The preferred resins are resins derived
from hydrocarbons. Suitable fillers include silicates, such as talc, clays,
and sodium silicate. In addition to the foregoing ingredients, a water-based
can end lining compound may also contain thickeners, pH modifiers, processing
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aids, plas*icizers-, oils, anti-microbialogical protectorants, e.g. mildewcides,
biocides, fungicides, and coalescense aids.
A suitable solvent-based formulation contains non-volatile components
which are similar to those of conventional water~based formulations~ The
composition of the particular can end lining composi~ion is not critical to
this invention.
Phosphate salts which are suitable for this invention include mono-
alkali, di-alkali, and tri-alkali metal phosphate salts. These alkali metal
phosphate salts may be anhydrous or hydrated. Examples of these salts are
monosodium phosphate, monopotassium phosphate, and trisodium phosphate~ It
should be understood that the suitable alkali metal phosphate salts are not
limited to the foregoing examplesD
In order to utilize the phosphate salt as a rust preventative, it
may first have to be treated with a suitable base to raise the pH of the salt
to the pH level of the can end lining compound. The addition of the base is
unnecessar~ if the pH of the phos.phate salt is reasonabl~ close to or com-
patible with the pH of the can end lining composition.
To a water-based can end lining composition should be added about
0.1 weight percent to about 3 weigh~ percent phosphate salt, based on the weight
of the lining composition. A higher concentration of phosphate salt may be
emplo~ed; however, no appreciable improvement in rust inhibition is noted at
phosphate concentrations above about 3 weight percent phosphate salt. The
preferred concentration of salt in the lining composition is about 0.7 weight
percent.
It may be desirable to add alkali to the phosphate treated container
sealing composition The purpose of the alkali addition is to prevent fl~ccul-
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ation during formulation of the composition.
The following specific examples are illustrative but not limitativeof the invention; it should be understood that similar results are obtainable
~ith other combinations of differen~ ingredients. All such variations which
do not depart from the basic concept of this invention and composition disclosed
herein are intended to come within the scope of the appended claims.
EXAMPLES 1-15
T~o parts by ~eight of ammonia ~NH3) ~as added to 100 parts by ~eight
of a 33 percent aqueous solution of monosodium phosphate. The pH of the mono-
sodium phosphate solution reached 7.
The pH-adjusted monosodium phosphate ~as added to a ~ater-based can
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end lining composition at concentration varying from 0.5 parts of phosphate per
- hundred parts of rubber to 4 parts of phosphate per hundred parts of rubber.
The lining composition contained the following components in the
amounts indicated ~all parts are expressed in ~eight~:
Solids Portion ~73 percent of total composition)
100 parts rubber
200 parts pigment and/or filler
1 part antioxidant
1 part bacteriocide
0.5_4 parts rust inhibitor
10 parts surfactants, thickeners
Liquid Portion ~27 percent of total composition)
100 parts ~ater
The .specific gravity of the lining composition ~as 1.62.
~` Tests were conducted on a variety of substrates, e.g., can ends,
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flat plate. Test conditions specified exposure of the substrate to 97 percent
relative humidity for a minimum of 72 hours. To evaluate tin plate~ whether
in sheet or in can end form, the substrate was scored to expose the steel
underbody. This action resulted in a much more demanding test with increased
sensitivity.
The following Table shows the results of tests which demonstrate
the effectiveness of the method of this invention in preventing corrosion
of the end of metal containers and of metal plate in general.
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; The foregoing Table shows that increaslng the concentration of phos-
phate in the can end llning compound decreases the level of rust formation
on the lined can end, tinplate, and blackplate. As little as 1 part phosphate
per hundred parts rubber ~ill prevent the formation of rust on a scored can
end ~hich is subjected to 97 percent relative humidit~ for at least 72 hours.
As little as 4 parts phosphate per hundred parts rubber will prevent the
formation of rust on scored flat plate which is subjected to 97 percen~
relative humidity for at least 72 hours. Higher concentrations of phosphate
salt may be employed; however, no appreciable improvement in rust inhibition
is noted at phosphate salt concentrations above 4 parts phosphate per hundred
parts rubber,
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