Note: Descriptions are shown in the official language in which they were submitted.
~9 ~
BAC~crO~No ~F 5E~ INVENTION
The well kno~, three-piece, sheet metal, food container is fa~
being supplanted by a two-piece container having a seamless body
with one end wall integral with the body and the other end wall
secured to the ~dy by means of a double seam after filling the
container. Such containers for beer and soft drinks are made by the
drawin~ and ironing process; whereas containers for fruits and
vegetables which require a heavier and sturdier side wall are
beginning to be made by a newer proces- known as draw/redraw.
Although the production of sheet metal containers by means of
multiple draws i5 not new, the draw/redraw process is an improvement
over the older drawing process in that it is done at higher speeds
with greater draw ratios in each drawing stepO Achieving this
higher productivity-rate requires special machines and, inter alia;
special lubr.icants for th~ metal~working operation; and to maintain
this higher productivity-rate, subsequent processing steps to the
formed con~iner must also be at high speed or eliminated where
possible.
U.S. Patent 4,287,741 describes a class of citric acid esters
which are efective as lubricants in the drawing and ironing
process. ~e have found that the same citric acid esters are
effective lubricants in the draw/redraw process described in
relation to the present invention~
The present inventio~ involves the discovery that the citric
acia esters d.isclosed in U.S~ Patent 4,287~741 not only act as
effective lubricants in the drawing and redrawing of ferrous metal
stock to form a container, bu~ that these lubricants, unlike
metal-working lubricants used heretofore~ need not be removed from
the surface of the formed container, but may be made to react with a
s~bsequently applied syn~hetic resin top coat by heating to form a
hardened barrier layer over the interior surface of the container.
~ he citric acid ester~ useful in the present invention are
dlsclosed in U~S. Patent 4,287,7410~ e~u~r~
h Y~i~ ~ This '741 patent gives the following
structural formula for these esters:
CH2-~C--O----R
O~C~ O~R2
CH2 - --R3
wherein Rl, R~ and R3 are selected from the group consisting
o hydrogen and alcoholic residues containing 1-10 carbon atoms,
R4 is selected from the group consisting of hydrogen and
carboxylic acid radical~ containing 1-10 carbon atoms, and at leas
one of Rl, R2 and R3 is an alcoholic residue~
We have found that the methcd of the present invention is
egually applicable to tinplated ferrous metal and to tin-free, low
.. ..
carbon sheet ~teeln Although the comTnon prior art tinplate referred
to in column ~, lines 50~56, of U.S. Patent 4,287t741 is usable in
the present inYention, non-reflowed9 matte finish tinplate described
in U.S. Patent 3,3~0,157 is preferred for the reasons set forth in
this ' 157 patent in respect of the drawing and ironing process fsr
making containers. The tin weight on the steel basis metal may Yary
fr~m 0.10 lbs. to 1.0 lbs~ per base box.
The preferred tin-free steel has a chromium surface treatment.
Wh ile it is s~ill in flat sheet orm prior bo its formation into 2
contailler, a ~ynthetic resin base coat i5 applied and adhered to
this treated surface. The most usual compositions for application
as a base coat are those containing an epoxy resin or vinyl resin or
polye 5 ter r e s in .
DETAILED DESCRIPTION OF T~E INVENTION
IN LUDING P_EFERRED EMBODIMENTS
The tin-free steel preferred for use in the presen~ invention is
aluminum-killed, continuous cast steel with a chromium/chromium
oxide surace treatment. The chromium in the oxide is present at
about 0.5 to 2.0 mg per square feet and the chromium metal at about
3 to 13 mg per sguare foot. The material described is known in the
art a~ TTS~CT for tin-free steel, chromium type~ The treatment is
described in a paper published in the Journal of the Electrochemical
So~iety, ~ol. 116, No. 9, pp 1299-1305.
The preferred tinplate ha the same composition of steel as set
forth above and at the steel mill in a well known manner has tin
applied to it~ surface electrolytically in various amounts, for
example, .25 lb. per base box. As mentioned previously, this
ti~plate is left in a matte condition, i.e., is not flow brightened,
is oiled for rust inhibition and coiled for shipment to a
container-making installatîon~
Thereafter tbe tin-free steel when received in the can-making
plant has applied to its surfaces a base coating~ The preferred
coating contains an epoxy resin based on bisphenol A with a~ epoxide
equivalent weight of 2300 to 4000, a urea formaldehyde crosslinker,
2 sulfonic acid ca~alyst, and a high-melting polyethylene internal
lubricant. The coating has a solids content of 28 percent. Both
vinyl and polyester resin compositions have also been used as base
coats.
This base coating applied to both sides of the steel may be
applied while the s~eel is still in coil form or the steel may be
cut into sheets and the coating applied to individual sheets, which
coating is subsequently baked to form a tough, adherent base coat on
the tin-free steel~
~s~
The citric acid ester of the present invention i.q dip coated
onto both ~urace~ of individual sheets of the base coated tin-free
steel, the excess ester is removed and the sheets are then fed into
a blanking and cupping press which cuts from the sheet one or re
circular discs of 7.947 inches in diameter, and draws the disc into
a cup of 5.007 inche~ in diameter and 1.850 inches in side wall
height. In two subsequent operations, the cup is successively
reduced in diameter with concurrent lengthenin~ of its side wall,
i,.e., drawn; and simultaneously this side wall is slightly thinned,
iOe~ to about 10% less than the starting gauge~ and further
elongated, i.e., ironed, in the manner similar to that described in
.S. Patent 3,360~157~ The final diameter and side wall height
accomplished in the drawing plus ironing are 3.060 in~ and 4.450
in. respectively and are accomplished in a few seconds. The
diameter of the starting blank and the height to diameter ratios,
draw ratios, in the ensuing metal working process may be varied
depending upon the desired size of the finished can. Also, as
between different draw/redraw systems, the amount of draw in each
step may be varied provided the cumulative effeGt of the plural
draws produces the can of desired height and diameter.
It is readily apparent that a draw/redraw system with ironing is
a more severe metal working process than a draw/redraw system
without ironing. The citrate ester lubricant of the present
. . .
invention performs equally well in both systems~
The particular citric acid compound used in the abcve example is
acetyl t~ibutyl citrate. It may be applied by any suitable means
such as roller coating, immersion coating with excess suitably
removed, electrostatic deposition which is accurate in application
both as to weight and placement~ or by hot spraying from an oxy-dry
unit or cold spraying if the ester i~ reduced with a suitable
solvent.
7~
The amount of lubricant applied over the base coat can vary from
10 to 40 and preferrably 10 to 20 milligrams per square foot of
total usface, i.e., both sides, of the sheet being fed into the
draw~redraw apparatus It has been found that the lubricating
efect fall~ off appreciably below 10 mgs/ft2 and for most
operations 20 mgs/ft2 is sufficient to achieve the high speed,
trouble free, multiple draws from flat blank to formed container.
Due to the severity of the metal-working operation, i.e., the
appreciable draw rati~s and draw speed plus ironing~ substantial
heat is generated on the surfaces being worked. ~hile not wishing
to be bound by any partieular theory, it is possible that this
generated heat causes ~t least a partial decomposition of the
lubricant on the worked surfaces, the~eby liberating reactive
~ubstances s~ch as those having carboxyl or acetyl functional
groups. For example, in the case of the preferred acetyl tributyl
citrate, acetic acid would be liberatedO These functional groups
are belie~d to attach themselves to the base coat and/or be
available for reaction with the postsprayed top coating.
At the completion of the draw/redraw operation, the container is
beaded to impart strength to the side and bo~tom wal3.s before being
ea into a device for applying a top coat to the container' s inside
surface. Most usually this device involve~ a turret which revolves
the container past a r~ciprocating spray gun which enters the
interior of the container which is spun on it~ longitudinal axis
while ~he spray gun, as it is retracted from the container ~ody~
emits a 360 degree spray of a synthetic resin solution to coat the
entire inter ior surface of the container.
7~
After completion of the top coating operatlclll, the container is
then subjected to a temperature of 400Fo for 4 minutes to harden
and cure the top coat. Unlike pr ior art procedures which require
the metal-working lubricant to be removed before application of the
top coat to avoid cont~mination and improper curing o the top coat,
the procedure of the present invention not only eliminates the
time-consuming st~p of removing the lubricant and permits the
applica~ion of the top coat directly to a still-lubricated internal
sur~ace of the container but assists in firmly adhering the top coat
to the base coatO Again, not wishing to be bound by any particular
theory, it is believed that the reactive groups liberated frQm the
applied lubricant previously and/or during the above mentioned
bakins operation react with or at least anchor themselves into the
top coat as well as the base coat thereby forming a strong adhering
medium between the base coat and top coat.
Tbe citric acid ester lubricant and what remains thereof after
th~ draw/redraw operation are soluble in organic solvent.s such as
butanol~ butyl Ce1losolYe, di-isobutyl ketone, Cellosolve acetate
and Solvesso 150~ Therefore, resins for top coats which are also
soluble in these same solvents and provide inert, continuous, resin
films upon thermal curing are preferred such as epoxy resins and
acrylic resins~ Vinyl resins are usable if they are applied over a
base other than a thermoset epoxy resin. The citric acid ester may
also be used in water base coating compositions provided the liquid
system contains a solvent for the citrate ester ~hich is miscible
with the water of the system~
Evaluation of top coats applied over an epoxy-UF base coat is
done by testing process resistance as well as intercoaS: adhesion
bets~een the base coat and the top coat. The follo~iny table
summarizes the results of these tests:
~e
37~
Eval~ation of Postsprayed Top Coats.
--~5'L~L ~G#bU~ t- Lse Coat
Process Intercoat
ToP Coat _ Resistance Adhesion
acryllc passed passed
vinyl passed failed
epoxy-phenolic passed passed
epoxy-acrylic passed passed
epoxy-phenolic passed passed
ep~xy-phenolic passed passed
epoxy-phenolic pa~sed passed
epoxy passed passed
The epoxy-phenolic resin systems in the above table differ from
each other in . the propc:rtion of epoxy to phenolic.
Process resistance i~ checked by filling the container with
ae~Oni2ed water followed by steam processing at 265F. for 90
minutes. After water cooling and standing overnight, the cans are
emptied, cut open, cross hatched, and taped to test intercoat
adhesion.
Other suitable base coat/top coat combinations include vinyl and
polyester resins as base coats r examples of which are in the table
below:
Intercoat Adhesion ~fter Process
Base Coa~
V,~
polyester passed passed passed
vinyl passed passed passed
epoxy passed failed passed
epoxy passed failed passed
vinyl passed passed passed
epoxy passed failed passed
The failures noted in the above table result from an attempt to
adhere a thermoplastic, vinyl resin top coat to a thermoset, epoxy
r e s in ba æ c oat . The se f a il u re~ are independent of the cit ra te
ester lubricant of the present invention and would have occurred
were the citrate ester not present.
Although the inven~ion described above is in respect o~ a base
coated tin-~ree steel, it is also applicable to the utilization of
matte-~inish, electrolytic tinplate as the metal starting material.
As set Eorth in U.S. Patent 3,360,157, the matte finished tinplate
and a liquid lubricant act synergistically with each other and an
analogous situation obtains in the instant invention between the
matte tin and citrate ester to augment the lubricant system during
the draw~redraw process for forming the container. Further, as
theorizea above, it i believed that functional groups, e.g~,
carboxyl and/or acetyl, formed either during the metal-working
process or during the thermal hardening of the top coat, or i~ both
instances, form in effect anchoring chains to hold the top coat
firmly adhered to the tinplate surface.
Consistent with this theory are the.results of gas
cbromatographic and mass spectrometric analyses which indicate that
acetic aeid and butanol are formed and liberated from thermal
degradation of acetyl tributyl citrate when it is heated to 195
C., the approximate temperature attained in the thermal cure of th~
top coat.
Furthermore, acetic and citric acids and butanol are produced by
hydrolysis of the acetyl tributyl citrate with an amine, such as
2-amino-2-methyl propanol; a solvent generally ~sed as a component
o water-base coatings
Comparative tests were run forming a base coated tin-free steel
inbo containers by means of the draw/redraw with ironing proc~ss
using various metal working lubricants. In each instance except for
the acetyl tributyl citrate (ATBC) o the instant invention, the
lubricant either was inferior to the acetyl tributyl citrate as a
metal-working lubricant as exemplified by the required longer time
to produce the container or it was incompatible with the top coat
preventing the top coat from forming a continuous protective film.
The table set forth immediatedy bel~w shows the results of these
comparative tests:
4~
EFFECT OF LUBRICANT TYPE ON DE~AW/REDRAW AND IRONING
TI N--E REE STEEL AND EIECTROLYTIC TINPLATE
LUBRICANT FABRICATIONADHESION OF TQP COAT
Petrolatum (hTP) Good, Very poor
Butyl Stearate~ ~ETP) - C;ood Good
Dioctyl Stearate~ (ETP~ Good Good
Neoderle (o~-olefin) Cracked domes; Good
ITF5) Broken radi i
Phos pha te E s ter Fa il u re on --
(TFS) Second Operation
Ac~tyl Tributyl Good Good
Citrate ~ETP)
ATBC with Carnauba (;ood Good
wax ( ~S)
Acetyl trihexyl Good Good
citrate (TFS)
Note~:
1. ~o adhesion before or after process.
20 Application problems due to rapid visc05ity changes as
solidi ~ication point (65F. ) is approached.
3. ~ower stability due to volatilization and oxidation (Ref: U.S.
Patent 3,923,471)~
We believe another factor contributing to the effectiveness of
acetyl tributyl citrate is itS ability to increase the wettability
of the TFS or ~TP surface by the top coating as shown by the contact
angle measurements in the table below. The measurements were taken
by applying a coating of the listed lu~ricant to the metal surface,
placing a drop of water on the lubricated surface and then measuring
the angle between the lubricated surface and an intersecting line
tangent to the curved surface of the water droplet. The greater the
wettability of the lubricant, the flatter the water droplet and the
less the angle.
CONTAC~ ~NGLES
L~ ICANT PLATE CONTACT ANGLE
.
None TFS 83.5
Petrolatum TFS 96 . 7
ATJ3C TFS 7 4 . 5
None ErP . 7 2 . 4
Pe trol atum ESTP 8~ . 5
ATBC E:rP
ATBC with ET}? 7 0 . 6
Carnauba Wax
But yl S tea ra teETP 77 . O
Dicctyl Stearate ETP 73.5
Nes~dene EqlP 79 . S
7~
~ igh melting waxes can be added to the citrate es'cer lubricant
in concentrat ions of up b~ about 2~ of ~he ester -olids to
improve lubricity without adverse effects on intercoat adhesion.
It is understood that the invention is not confined to any
particular embodiment described herein as illustrative of the
invention but embraces all such modifications thereof as may come
w;thin the scope of the following claims.
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