Note: Descriptions are shown in the official language in which they were submitted.
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~ This inv0ntion relates to the polymerization of
pH sensitive and ethylenically unsaturated carboxylic acid
monomers into ~he same latex system. More specifically this
invention relates ~o coatings for interior can llnings, a
process o forming said coatings and articles having said
coatings, notably beverage containersO
Coatings used for interior can linings have been
compcsed mostl~ o~ epoxy and polyvinyl chlor;de copolymers.
The can linlngs for food, beer and general beverage containers
presently in use are applied from organic solvents which con-
tribute to air pollution of the surrounding communities. In
oxder to meet local and federal pollution regulations, expen-
sive and energy~con~sum~ng incinerators mus~ be installed to
combust the solvents emitted from the present can coatings. In
an attempt to save energy and c~rcumvent the purchase of costly
incinerators, considerable effort has been expende~ by coatings
manufacturers to develop water-borne coatings which meet the
physical performance characteristics required for interior can
coatings and still remain within the federal air pollution guide-
lines~ At present,~United States government regulations allow
20% organic solvent by volume in the volatile phase~
The principal aspect of the present invention is a
water-borne interior can coating which possesses excellent
physical properties, meets or exceeds present air pollution
limitation, and which is prepared by the method of emulsion
polymerization.
24~i
BRIEF DESCI~IPI'ION OF THE INVEN-ION
The em~lsion polymers prepared by the procedure of the present
invention possess superior performance and spray application properties when
compared to water-borne polymers synthesized by standard solution and
emulsion polymerization techniques. These properties are accomplished by
polymerizing the monomers by a two step process in which the unsaturated
carboxylic acid and other f mctional monomers are incorporated into the
interpolymer in the first stage of the polymerization for the purpose of pH
control of the reaction, adhesion and emulsion stability.
According to the present invention; there is provided a latex of a
two-stage interpolymer exhibiting a glass transition temperature of -6C. to
50C. and comprising: (a) lO to 35% of a first stage seed latex comprising
the following reactants in percent by weight; 15-60% of an aromatic vinyl
monomer; 25-50% of acrylate ester monomers selected from a group consisting
of methyl methacrylate, ethyl acrylate, n-butyl acrylate or methacrylate,
isobutyl acrylate or methacrylate, and 2-ethyl hexyl acrylate; 5-30% of an
unsaturated carboxylic acid monomer selected from the group consisting of
acrylic or methacrylic acid, itaconic acid, maleic or fumaric acid and their
half esters; and; 1-25% of polar monomers selected from the group consisting
of hydroxy ethyl or propyl acrylate or methacrylate, acrylamide methacrylamide,
acrylonitrile, methacrylonitrile, t-butyl-amino ethyl acrylate or methacrylate,
dimethyl amino ethyl acrylate and methacrylate; and (b) 65 to 90% of a
second stage polymer interpolymerized in the presence of the first stage seed
latex comprising the following reactants in percent by weight; 0-97% of an
aromatic vinyl monomer; 3-30% of a functional monomer selected from the group
of N-Methylol acrylamide or methacrylamide and 1 to 6 carbon containing
alkoxy ethers of the same; and ; 0-97% acrylate ester monomers selected from
a group including methyl methacrylate, N-butyl acrylate or methacrylate,
isobutyl acrylate or methacrylate and 2-ethyl hexyl acrylate.
In another aspect, the invention provides a process of forming a
latex of a t~o-stage interpolymer of pH sensitive monomers comprising a first
stage of polymerizing in an aqueous emulsion in the presence of an initiator:
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15-60% of an aromatic vinyl monomer; 25-50% o acrylate ester monomers
selected from a group consisting of methyl methacrylate, ethyl acrylate,
n-butyl acrylate or methacrylate and 2-ethyl hexyl acrylate; 5-30% of an
unsaturated carboxylic acid monomer selected from the group consisting of
acrylic or methacrylic acid, itaconic acid, maleic or fumaric acid and their
half esters; and; 1-25% of polar monomers selected from the group consisting
of hydroxy ethyl or propyl acrylate or methacrylate, acrylamide, methacrylamide,
acrylonitrile, methacrylonitrile, t-butyl amino ethyl acrylate or methacry-
late, dimethyl amino ethyl acrylate and methacrylate to thereby form a first
stage polymer latex; neutralizin~ said first stage polymer latex to a pH of
from 6 to 10 with a base, subsequently incorporating into said Eirst stage
polymer latex, and polymerizing in an aqueous emulsion in the presence of a
radical initiator, a mixture monomer components to form, in the latex product,
a second s~age polymer comprising: 0-97% of an aromatic vinyl monomer; 3-30%
of a functional monomer selected from the group consisting of N-Methylol
acrylamide, and alkoxy ether derivatives thereof; and; 0-~7% of at least one
acrylate ester selected from a group consisting of methyl methacrylate,
ethyl acrylate, n-butyl acrylate or methacrylate, iso-butyl acrylate or metha-
crylate and 2-ethyl hexyl acrylate; wherein said second stage polymer is formed
in a reaction medium comprising: 70-100% deioni~ed water; and; 0-30% of an
organic solvent or mixture thereof comprising ketones, hydrocarbons, esters,
long chain aliphatic alcohols and glycol ethers.
In a preferred embodiment the invention provides a coating com-
prising the latex hereinbefore defined and an organic solvent. Thus, the
first stage of the reaction comprises from 10 to 35% by weight of the total
monomers and is neutralized to a pH of from 6 to 10 before the second stage
of the reaction is carried out. During the second stage, pH sensitive and
reactive functional monomers such as n-Methylol acrylamide9 and isobutoxy
methylacrylamide, as well as other ethylenically unsaturated monomers, are
copolymeri~ed in~o ~he initial first stage "seed" latex. It is of advantage
to incorporate functional reactive monomers and a N-Methylol acrylamide and
its ether derivatives into the interpolymers to provide crosslinking of the
film into a tough, insoluble and chemically resistant coating.
- 2a -
Mhen incorporating pH sensitive monomers such as N-Methylol
acrylamide into an emulsion polymer, inorganic buffering agents are employed
to moderate the pH of the system. The buffering agents, as well as the levels
of surfactants conventionally used in polymerization, interfere with the
final film properties. ln particular, adhesion, water and beverage
- 2b -
Z465
spotting and the barxier properties to iron ion migration are
- effected. By the use of the two step polymerization processof the present invention, the inorganic buffering agents and
much of the emulsifiers can be eliminated from the reaction.
DETAILED DESCRIP~I~N
In the present invention, a water-borne interior
can coating is provided. The coating is prepared by emulsion
polymerization without the formation of coagulation of the latex,
in the presence of low levels of emulsifiers. The latex is
1 also prepared without the use of water sensitive inorganic
' buffering agents. These aspects of the invention are accomplished
by preparing the latex in a two stage process in which the first
stage contains most of the ethylenlcally unsaturated carboxylic
I! acid monomer. The unsaturated acid in the first stage is
i incorporated by copolymerization with vinyl monomers such as
! vinyl aromatic monomers and acrylic or methacrylic acid esters.
¦ For an interior can coating, the first stage "seed"
¦l latex exhibits the following composition by weight:
¦~ 15 to 60 percent of an aromatic viny~ monomer. ~
25 - 50% of acrylate ester monomers selected from a
group consisting of methyl methacrylate,
ethyl acrylate, n-butyl acrylate or
methacrylate and 2-ethyl hexyl acrylate.
5 - 30% of an unsaturated carboxylic acid monomer
selected from the group consisting of acrylic
or methacrylic acid, itaconic acid, maleic
-or fumaric acld and their half esters.
_ 3 _
11~2465
1 - 25~ of polar monomers selected from the group
consisting o hydroxy ethyl or propyl acrylate
of methacrylate~ acrylamide, methacrylamide,
acrylonitrile r methacrylonitrile, t-butyl amino
ethyl acrylate or methacrylate, dimethyl amino
ethyl acrylate and methacrylate.
I .
The amount and kind of initiator used can be varied
depending on the particular polymerization desired. Any compound
1 which supplies a source of free radicals such as peroxides,
ii hydroperoxides, perbenzoates, and persulfates can be employed.
i If a persulfate is used, ammonium persulfate is preferred since
ik contains a fugitive cation that will not interfere with the
I final film properties. The polymerization in bo~h the first and
second stages of this present invention can be initiated by either
15 ¦I the thermal decomposition of the free radical initiators or by a
redox system. The amount of initiator normally used is from .2
~i to 1%, although the reaction can adequately be conducted with
I higher and lower amounts~
l The polymerization c2n be carr-ed out at a t~mperature
of from 50~C to 100C over a 6 to 20 hour period. However~ ¦
~' in practice, the temperature of the overall reaction changes
from 60C to BSC over a 10 hour perlod.
The first stage copolymer latex comprises from 10~ ¦
l to 35% o the total polymer composition and is formed by
I polymerizing said monomers in an aqueous medium in the presence
1 of emulsifiers and radical initiating compounds.
', I
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; After the fi-rst stage of the reaction is completed
and before the second stage of the reaction can begin, the initial
seed latex must be neutralized with base. The bases used in this
l~process are those which are reactive with the carhoxylic acid
S ~group on the seed polymer to form the corresponding salts.
Suitable bases are monovalent inorganic and organic bases.
~Examples, o~ basesr one or more of which may be employed in the
process, are ammonium hydroxide (which may be provided by the
~ addition of ammonia to the aqueous reaction mixture), water
I soluble alkali metal hydroxides such as sodium and potassium
~hydroxide, primary, secondary and tertiary amines in which the
groups attached to the amino nitrogen atom are cyclic and acyclic
alkyl, hydroxyalkyl and alkyl ether groups, e.g~, ethylamine,
I diethylamine, triethylamine, dimethylamine, trimethylamine,
¦I N, Nl N , N - tetramethyl methylene diamine, N-hexylamine,
¦, dimethyl ethanolamine, N-dimethyl isopropanolamine and morpholine
l! and quaternary ammonium bases such as benzyl trimethyl ammonium
hydroxide and dodeoxytrimethyl ammonium hydroxide.
i Ammonia or organic bases are normally preferred in
I order to reduce -later spotting of the final film. The pH of the
system can readily be adjusted from 6 to 10 pH units Dy neutral-
izing the polymer in such a manner.
In the second stage of the polymerization, a pH
1 sensitive compound can be incorporated into the seed latex.
! The N-methylol derivates as well as N-methylol ether derivatives
of acrylamide and methacrylamide can be easily incorporated into
the second stage of the present invention. The reactive function-
al monomers are copolymerized with vinyl monomers and the polymer
110~65
strUCtULe of the second stage reaction can comprise the following
composition:
; 0 - 79~ of an aromatic vinyl monomer, preferably
styrene.
3 - 30% of a functional monomer such as N-Methylol
acrylamide or its alkoxy ether derivakes
such as isobutoxy methyl acrylamide.
0 - 97% acrylate esters selec~ed from a group
including methyl methacrylate, ethyl acrylate,
n-butyl acrylate or methacrylate, iso-butyl
acrylate or methacrylate and 2-ethyl hexyl
acrylate.
; The second stage react~on can be performed in the
i presence of organic solvent without reduction of the yield
Il of the reaction. The composition of the reaction medium in
; I the second stage can be composed o~:
¦ 30 - 100% deionized water
0 - 30% of an organic solvent ox mixture thexeof
comprising ketones, hyarocarbons, esters,
l long chain aliphatic alcohols and glycol
¦ ethers. -
¦ In the finished coating, from 5% to 30% organic
I solvent is desirable for good spray application, improved wetting
li of the interior can substrate and good coalescence of the film
Il upon application. If the organic solvent is added before or
during polymerization of the second stage r~action, lower vis-
cosities of the final co~ting are obtained. It is therefor-
- I - 6
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ll~}Z~65
Il of advantage to polymerize the second stage reaction in a
¦I solvent-water blend. The final viscosities of the coating
obtained by this technique will be ~ - 4 times lower than if
,I the polymerization were conducted in deionized water and the
S 1~ solvent post-added after completion of the radical polymeriza-
I! tion.
The second stage can comprise from 65% to 90~ of the
total monomer composition. Although the polymerization process
Il can be conducted in a number of different techniques commonly
1l known to those versed in the art, it is preferable to add the
¦I monomers with stirring to the neutralized first stage and then
initiate the system by the use of radical generating compounds.
In practice, a hydroperoxide is used in the presence of a reducing
~ agent such as ascorbic acid, isoascorbic acid, glucose, fructose,
j, dihydroxyacetone, hydrazine,~sodium formaldehyde sulfoxa;late, -
polyamines and the like. A small trace of a complexed transition
metal ion catalyst is also employed. The second stage of the
, reaction can~be conducted at 40C to 100C depending on the
li polymeric properties desired and the time for complete conversion ¦
I preferredO
¦ Emulsions used in the synthesis of the above polymers
¦ include both nonionic and anionic surfactants and their mixtures
commonly employed for such purposes. In the first stage of the
1, reaction from .2 to 2.5~ surfactant based on monomers in the first
stage may be employed. The stability of the second stage and the
amount of emulsifier requlred for stabilization depends on the
composition of the first stage of the reaction. When high levels
~ 5
~of functional monomers such as acrylic acid, acrylamide or
I hydroxy ethyl acrylate are employed in the first stage '~seed"
I I latex the surfactant can be reduced or eliminated in the second
I stage of the reaction.
The advantages and utilîty of this invention are
demonstrated below in the following examples. The examples are
given only by way of illustration of the invention and not by
limitation. All par~s are expressed by weight unless otherwise
stated.
Example 1
; ~ In accordance with the method stated above, the polymer
coating was prepared in a two step process.
I ¦l (A) First-Stage of Reaction - An interpolymer of -the following
¦ composition was prepaxed:
Styrene ~ 26.0
Ethyl Hexyl Acrylate 32.8
Acrylic Acia 7.7
~1 Hydroxy Ethyl Acrylate 4c4
Methyl Methacrylate 29.1
Deionized Wa~er 150
(NH4)~2S208
Ammonium Salt of Sulfate
Nonylpkenoxy poly (ethyleneoxyl)
ethanol 2.5
~ The ammonium persulfate and deionized water were deaerated under
' a nitrogen stream and heated to 75C. The emulsifier was sub-
i sequently added. 10~ of the monomer mixture was added and the
ll~ remaining monomer was added over a 3 hour period. The reaction
¦~ was then heated to 85C and held for one hour to complete
1¦ polymerization~ Yield was 99% to 100%~ viscosity 15 seconds
~4 Ford cup and pH = 2.1.
- 8 -
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(B) Second Stage - The following interpolymer composition
is prepared:
1) The resin of Example 1
Part A 250 (40% solids~
2) Styrene 160
Ethyle Hexyl Acrylate160
Ethyl Acrylate 32
Isobutoxy
Methylacrylamide 48
3) Deionized Water 755.0
Ascorbic Acid 2.0
Dimethyl ethanol 13.0
Amine
FeSO4 Trace
4) t-Butyl Hydroperoxide 2.0
Deionized Water 122
. 2-Butoxy ethanol 122
250 parts of Resin A (1) is diluted with Part (3) and deaerated
¦l and the monomer mixture added ~2) L The reaction mixture is then
li heated to 60C and one-half of the initiator-solvent mixture
! added o~er 15 minutes. The temperature increases to 75C and
lS held at 75C throughout the reaction. The remaining initiator
solvent mixture is added over 1 hour. Total reaction time is~
4 hours. Solids 30%, pH 8.4 J and viscosi~y is 30 seconds
#4 ~ord cup. ~ I
~, Example 2
j . , . _
¦¦ 1) The resin of ~xample 1
I Part A 172.6
Dimethy~ ethanolamine8.0
Ascorbic Acid 1.2
Fe Citrate Trace
2) Styrene 119.4
Ethyl Hexyl Acrylate90.0
Methyl Methacrylate126.9
. Isobutoxymethylacrylamide 53.0
3) 2-Butoxy ethanol 5
Cu~l~ne hydroperoxide 5
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., . .
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Part 1 was added to the reaction flask and heated to 60C. Then
~ Part (2) monomer mixture was added. The reaction was initiated
Il by the addition of one-half of Part 3. The remainder of Part 3
~ was added over one hour. Total reaction time three hours~
j~ Viscosity 14 seconds r pH a 8 ~1 and solids 34~O
¦¦ Although this invention has been illustrated by
reference to specific embodiments, modifications thereof which
¦ are clearly within the scope of the invention will be apparent
Ii to those skilled in the art.
ll The coating prepared by the methods stated above are
1~ ideally suited for interior of containers. The coatings can be
¦I applied by both air and airless spray equipment and when tested
Il against a commercially used vinyl chloride copolymer, the coatings
li of the presen~ invention show excellent performance characteris-
j' tics. The standard tests conducted on these coatings are
I ~I summarized belowa
1) Adhesion
¦¦ The coatings were applied to tin free steel, tin plate,
¦¦ treated and untreated aluminum, blac]~ plate as well as
¦¦ commercialiy used can coating organic basecoats. The
¦¦ materials were coated at approximately 5 mg/sq. inO and
¦' baked at 320F for 5 minutes~ The cured coatings were
scored with a knife edge and the scored area covered with
ill Scotch Tape (3M Corporation3~which was removed. The
¦~ coating was then examined for removal of material proximate
¦, to the scored area by the Scotch Tape. The amount of
!l coating material removed by the Scotch Tape is indicative
I of coating adhesion.
I
~ ~-10- 1
1, !
~2~6~;
2) Pasteurization
The coating substrate is placed in beer having a temperature
of 160F for one-half hour. The blush resistance of the
coating is measured by the degree of milkiness or less of
gloss o the coating.
3) Flavor Test
Aluminum foil is coated on both sides with the subject
coat}n~. The coating weight is approximately 200 mg and
l the total area exposed to the beer is 40 sq. inches. The
¦! coated aluminum foil is placed in a 12 oz. bottle of beer
¦1 and sealed tightly~ The bottles are stored for a period
jl of time at room temperature, chilled in a refrigerator
overnight and tasted by a trained flavor panel. The coatings~
are compared with a co~mercial control for any flavor or odor
,, which they may lmpart to the beer.
l 4) Turbidity Test
I ¦~ The test measures any change in the clarity of the beer
~i which may be imparted by the coating variable. The test is
1 conducted by coating A0 sq. in. o~ aluminum foil with 200 mg.
1 of the ~est coating and baking the ~oating at ~20F for
l S minutes. The coated foil is then placed in a ~2 oz. bottlei
¦~ of beer, tightly seale~ and stored at an elevated temperature
for a predetermined period of time. The turbldity is
Il measured using a Hach Turbidity Meter and is expressed in
11 Forma~in Turbidity Units (FTU). A value of IS - 30 FTU
is normal for an acceptable bottle of beer.
! I
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The coatings of the present invention pass or exceed all of the
above tests standardly employed in the evaluation of interior can
liners and are summarized in the table below.
TABLE I
PHY5ICAL I'EST COMPARISON .
Turbidity
3 Weeks Flavor Tin Epoxy Blush
Coating @ 100F_ Test TFS Plate Phenolic Resi~tan~e
Vinyl Control 23 FTU Pass Poor Poor Good Good
Example 1 21 FTU Pass Good Good Good Good
Example 2 24 FTU Pass Good ~ Good Good Good
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