U.S. patent application number 14/034753 was filed with the patent office on 2014-01-23 for coating formulation for the interior surfaces of cans.
The applicant listed for this patent is HENKEL AG & CO. KGAA. Invention is credited to Holger Endres, Andreas KUNZ, Patrick Podwoiski, Wolfgang Schneider, Bernd Struck.
Application Number | 20140023782 14/034753 |
Document ID | / |
Family ID | 44508436 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140023782 |
Kind Code |
A1 |
KUNZ; Andreas ; et
al. |
January 23, 2014 |
COATING FORMULATION FOR THE INTERIOR SURFACES OF CANS
Abstract
The invention relates to a water-based can inner coating
comprising a copolymer or a copolymer mixture of at least one
aliphatic and acyclic alkene with at least one
.alpha.,.beta.-unsaturated carboxylic acid in water-dispersed form,
wherein the acid number of the copolymer or of the copolymer
mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g,
and at least one water-dispersed or water-soluble curing agent.
Inventive can inner coatings are characterized in that due to the
good crosslinking of the copolymer or of the copolymer mixture with
the curing agent, the cured film on the inner surfaces of metal
cans possesses excellent properties in regard to hardness, abrasion
resistance and resistance towards hot liquids.
Inventors: |
KUNZ; Andreas; (Remscheid,
DE) ; Podwoiski; Patrick; (Erkrath, DE) ;
Endres; Holger; (Neuss, DE) ; Schneider;
Wolfgang; (Langenfeld, DE) ; Struck; Bernd;
(Bonn, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HENKEL AG & CO. KGAA |
Duesseldorf |
|
DE |
|
|
Family ID: |
44508436 |
Appl. No.: |
14/034753 |
Filed: |
September 24, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2012/053830 |
Mar 7, 2012 |
|
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14034753 |
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Current U.S.
Class: |
427/239 ;
523/100 |
Current CPC
Class: |
B65D 25/14 20130101;
C09D 123/0869 20130101 |
Class at
Publication: |
427/239 ;
523/100 |
International
Class: |
B65D 25/14 20060101
B65D025/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2011 |
EP |
11160695.0 |
Claims
1. A water-based can inner coating comprising, in addition to
water, a) a copolymer or a copolymer mixture of at least one
aliphatic and acyclic alkene with at least one
.alpha.,.beta.-unsaturated carboxylic acid in water-dispersed form,
wherein the acid number of the copolymer or of the copolymer
mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g,
and b) at least one water-dispersed or water-soluble curing agent
selected from an aminoplast, a carbodiimide and combinations
thereof.
2. The water-based can inner coating according to claim 1, wherein
the acid groups of the copolymer or of the copolymer mixture in
water-dispersed form are at least partially neutralized.
3. The water-based can inner coating according to claim 2,
comprising a neutralization agent for neutralizing the acid groups
of the copolymer or of the copolymer mixture in water-dispersed
form, said neutralization agent being selected from the group
consisting of ammonia, amines, metallic Al, metallic Zn,
water-soluble oxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II),
water-soluble hydroxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II) and
combinations thereof.
4. The water-based can inner coating according to claim 3, wherein
the neutralization agent is selected from ammonia, morpholine,
hydrazine, hydroxylamine, monoethanolamine, diethanolamine,
triethanolamine, dimethylethanolamine, diethylethanolamine, and
combinations thereof.
5. The water-based can inner coating according to claim 1, wherein
the copolymer or the copolymer mixture has a glass transition
temperature of not more than 80.degree. C.
6. The water-based can inner coating according to claim 1, wherein
the aliphatic and acyclic alkene is selected from ethene, propene,
1-butene, 2-butene, isobutene, 1,3-butadiene,
2-methylbuta-1,3-diene and combinations thereof.
7. The water-based can inner coating according to claim 1, wherein
the .alpha.,.beta.-unsaturated carboxylic acids are selected from
cinnamic acid, crotonic acid, fumaric acid, itaconic acid, maleic
acid, acrylic acid, methacrylic acid and combinations thereof.
8. The water-based can inner coating according to claim 1, wherein
a weight fraction of the aliphatic and acyclic alkenes in the
copolymer or in the copolymer mixture is at least 40 wt. %, but not
more than 95 wt. %.
9. The water-based can inner coating according to claim 1, wherein
the copolymer or the copolymer mixture additionally comprises
comonomers that are selected from esters of
.alpha.,.beta.-unsaturated carboxylic acids.
10. The water-based can inner coating according to claim 9, wherein
the esters of .alpha.,.beta.-unsaturated carboxylic acids comprise
linear or branched alkyl esters of acrylic acid and/or methacrylic
acid, said esters having an aliphatic group of not more than 12
carbon atoms, wherein the copolymer or the copolymer mixture has an
acid number of less than 100 mg KOH/g.
11. The water-based can inner coating according to claim 1, wherein
water dispersed polymeric constituents of the water-based can inner
coating have a D90 value of not more than 100 .mu.m and a D50 value
of not less than 1 .mu.m.
12. The water-based can inner coating according to claim 1,
comprising at least 40 wt. % water and a) 4-30 wt. % of the
copolymer or of the copolymer mixture, b) 2-20 wt. % of the at
least one curing agent, c) not more than 5 wt. % of emulsifiers
selected from non-ionic amphiphiles with an HLB value of at least
8; d) not more than 40 wt. % of water-miscible organic solvents; e)
not more than 10 wt. % of auxiliaries selected from wetting agents,
leveling agents, defoamers, catalysts, film-formers, stabilizers
and neutralizing agents.
13. A method of coating inner surfaces of a can comprising applying
the water-based can inner coating according to claim 1 to inner
surfaces of a can and drying said water-based can inner
coating.
14. The method of claim 13, wherein the can is a tin plate can and
the inner coating is deposited in a dry film thickness of at least
5 g/m.sup.2, but not more than 50 g/m.sup.2, on to the inner
surface.
15. The method of claim 13, wherein the can is an aluminum can and
the inner coating is deposited in a dry film thickness of at least
1.5 g/m.sup.2, but not more than 50 g/m.sup.2, on to the inner
surface.
16. The method of claim 13, wherein the water-based can inner
coating is deposited in a spraying process.
Description
[0001] This application is a continuation of PCT/EP2012/053830
filed 7 Mar. 2012, which claims priority to EP11160695.0 filed 31
Mar. 2011.
[0002] The present invention relates to a water-based can inner
coating comprising a copolymer or a copolymer mixture of at least
one aliphatic and acyclic alkene with at least one
.alpha.,.beta.-unsaturated carboxylic acid in water-dispersed form,
wherein the acid number of the copolymer or of the copolymer
mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g,
and at least one water-dispersed or water-soluble curing agent
selected from the group of aminoplasts and/or the group of
carbodiimides. Inventive can inner coatings are characterized in
that due to the good crosslinking of the copolymer or of the
copolymer mixture with the curing agent, the cured film on the
inner surfaces of metal cans possesses excellent properties in
regard to hardness, abrasion resistance and resistance towards
aqueous liquids. The present invention makes available an
alternative to the conventional use of epoxides based on bisphenols
in can inner coatings.
[0003] In the food industry, tin plate strip is valued as a
suitable material for the production of packaging units for
receiving aqueous liquids or preserved foods. This is because, even
over a longer period of time, tin plate strip, due to its
electrochemically noble tin layer, releases only low amounts of
potentially toxic tin salts to the food product that is in contact
with the tin surface. Tin plate strip is therefore an important
starting material for food packaging, for example for the
production of cans for receiving beverages. Aluminum strip, due to
its passive oxide layer, is also a suitable starting material for
the production of cans for filling with beverages. In addition,
aluminum salts that are taken up in small amounts by the liquid are
harmless to health.
[0004] The packaging industry, when producing cans, coats the inner
surface of the can with an organic protective layer or
alternatively uses a strip material pre-coated with an organic
protective layer for producing cans. The organic finish that coats
the inner surface prevents any direct contact of the metallic
interior of the can with the liquid. This achieves first of all a
significantly reduced corrosion of the base material and secondly
minimizes the entry of metal salts, such that the taste of the
foodstuff is not changed for the worse even after a lengthy storage
or stockpiling of the beverage cans.
[0005] Another aspect in regard to the production of cans concerns
the composition of the coating, which conventionally consists of
epoxy resins based on Bisphenol A. Such epoxides with a Bisphenol A
basic structure are suspected estrogens and are teratogens for
males. Cured coating formulations that come into contact with
acid-containing aqueous foodstuffs can release Bisphenol A from the
coating into the stored foodstuff. In practice, the curing of the
coating and the resulting crosslinking of the coating components is
also never complete, such that unreacted Bisphenol A-based epoxides
can also diffuse into the foodstuff. Consequently there exists a
need for Bisphenol A-free formulations for the inner coating of
cans for storing foodstuffs; various national legislation
initiatives, driven inter alia by the EU Directive 2002/72/EU,
exist that define the maximum limits for the migration of Bisphenol
A from packaging into foodstuffs.
[0006] US 2008/0193689 discloses an epoxide-based coating
composition that is suitable for use as a can coating and
comprises, in addition to the epoxy resin, mono and difunctional
low molecular weight organic compounds that can react with the
epoxy resin. The coating is formulated in such a way that after
curing, only very minor amounts of unreacted epoxides based on
Bisphenol A remain in the coating, such that when the composition
is used as a can inner coating only traces of Bisphenol A from the
cured coating can migrate into the stored foodstuff.
[0007] On the other hand, EP 2031006 proposes can inner coatings
based on specific alicyclic epoxides, so as to circumvent in this
way the formulations that include epoxides based on Bisphenol
A.
[0008] WO 2006/045017 provides a beverage can coating formulation
that contains latices of ethylenically unsaturated monomers and an
aqueous dispersion of an acid-functional polymer in the presence of
amines, wherein the latices for the crosslinking are constructed at
least partially from monomers having a glycidyl group. Such can
inner coatings can be formulated free of epoxides based on
Bisphenol A.
[0009] The object of the present invention consists in providing
another alternative to an epoxide-based can inner coating, wherein
the coating formulation can be deposited on the inner surfaces of
the can in a spray process and after curing affords thin,
homogeneous, highly flexible coating films with a simultaneously
good coating adhesion and resistance towards aqueous compositions.
Another object consists in being able as far as possible to obviate
the use of organic solvents and emulsifiers in the formulation of
stable and coatable can inner coatings.
[0010] This object is achieved by a water-based can inner coating
comprising, in addition to water, [0011] a) a copolymer or a
copolymer mixture of at least one aliphatic and acyclic alkene with
at least one .alpha.,.beta.-unsaturated carboxylic acid in
water-dispersed form, wherein the acid number of the copolymer or
of the copolymer mixture is at least 20 mg KOH/g, but not more than
200 mg KOH/g, and [0012] b) at least one water-dispersed or
water-soluble curing agent selected from the group of aminoplasts
and/or the group of carbodiimides.
[0013] Cans are inventively understood to mean metallic containers
for filling, storing and holding stocks of foodstuffs, in
particular of beverages.
[0014] In this context, a can inner coating is a coating
formulation that for the formation of a coating layer on the inner
surface of the can is deposited, made into a film and cured in
order to prevent the direct contact of the foodstuff with the
metallic material of the can during filling, storing and holding
stocks of the foodstuff.
[0015] A water-based coating inventively contains a dispersion
and/or emulsion of organic polymers in a continuous aqueous phase,
wherein in the context of the present invention, an aqueous phase
is also understood to mean a homogeneous mixture of water and a
water-miscible solvent. The term "in water-dispersed form"
therefore means that each polymer is dispersed as a solid or liquid
in the continuous aqueous phase.
[0016] According to the invention, mixtures of chemically and/or
structurally different copolymers of at least one aliphatic and
acyclic alkene with at least one .alpha.,.beta.-unsaturated
carboxylic acid constitute a copolymer mixture. Thus, a copolymer
mixture of an inventive coating formulation can for example
comprise in parallel copolymers that comprise different alkenes or
different .alpha.,.beta.-unsaturated carboxylic acids as the
comonomers or have a different number of otherwise identical
comonomers in the copolymer.
[0017] The acid number is inventively an experimentally measurable
characteristic number that reflects the number of the free acid
groups in the copolymer or in the copolymer mixture.
[0018] The acid number is determined by dissolving a weighed
quantity of the copolymer or the copolymer mixture in a solvent
mixture of methanol and distilled water in the volume ratio 3: 1,
and subsequently potentiometrically titrating the mixture with 0.05
mol/l KOH in methanol. The potentiometric measurement is carried
out with a combination electrode (LL-Solvotrode.RTM. from Metrohm;
reference electrolyte: 0.4 mol/l tetraethylammonium bromide in
ethylene glycol). Here, the acid number corresponds to the added
quantity of KOH in milligrams per gram copolymer or copolymer
mixture at the inflection point of the potentiometric titration
curve.
[0019] As a melted on, thin film on metal surfaces the copolymer or
copolymer mixture of the aliphatic and acyclic alkane with an
.alpha.,.beta.-unsaturated carboxylic acid with the abovementioned
acid number already shows a good coating adhesion, in particular on
tin plate and aluminum surfaces. In addition, the acid groups
impart the inherent characteristic to the copolymer or to the
copolymer mixture of being self-emulsifying, such that in the
aqueous phase, even in the absence of emulsifiers, microparticulate
aggregates can be formed by using shear forces. The presence of the
copolymer or copolymer mixture in the form of a microparticulate
aggregate lends thixotropic properties to the inventive coating,
such that a homogeneous wet film of the water-based coating can be
deposited onto the inner surface of the can, the coating remaining
there until a film is formed and cured, and does not run off inside
the can due to the force of gravity.
[0020] If the acid number of the copolymer or copolymer mixture of
alkenes and .alpha.,.beta.-unsaturated carboxylic acids is less
than 20 mg KOH/g, then a cured coating formulation according to the
art of the present invention does not have sufficient adhesion to
metal surfaces and consequently is not suitable as a film-forming
component of can inner coatings. Conversely, an acid number of the
copolymer or copolymer mixture of alkenes and
.alpha.,.beta.-unsaturated carboxylic acids greater than 200 mg
KOH/g as the film-forming component in can inner coatings only
brings about an inadequate barrier effect against corrosively
acting ions in aqueous media and furthermore a coating that is
comparatively less resistant against water at temperatures above
60.degree. C.
[0021] The weight fraction of the aliphatic and acyclic alkenes in
the copolymer or in the copolymer mixture is preferably at least 40
wt %, particularly preferably at least 60 wt %, but preferably not
more than 95 wt %. This ensures that the ion-permeability of the
cured coating on the can inner surface and the swelling of the
coating in contact with aqueous media, with at the same time an
adequate wettability and adhesion of the coating to the material of
the can, is reduced as much as possible.
[0022] Preferred aliphatic and acyclic alkenes of the inventively
obtained copolymer or copolymer mixture are selected from ethene,
propene, 1-butene, 2-butene, isobutene, 1,3-butadiene and/or
2-methylbuta-1,3-diene, particularly preferably ethene.
[0023] Preferred .alpha.,.beta.-unsaturated carboxylic acids of the
inventively obtained copolymer or copolymer mixture are selected
from cinnamic acid, crotonic acid, fumaric acid, itaconic acid,
maleic acid, acrylic acid and/or methacrylic acid, particularly
preferably acrylic acid and/or methacrylic acid, in particular
acrylic acid.
[0024] Further comonomers that may be an additional component of
the copolymer or the copolymer mixture in an inventive can inner
coating are selected from esters of .alpha.,.beta.-unsaturated
carboxylic acids, preferably linear or branched alkyl esters of the
acrylic acid and/or methacrylic acid containing not more than 12
carbon atoms in the aliphatic group. Such comonomers improve the
adhesion of the cured inner coating of the can to metal surfaces
due to an increased mobility of the polymer backbone which again
facilitates the orientation of the acid groups that have a surface
affinity to the metal surface. This effect is ensured in particular
with low acid numbers of the copolymer below 100 mg KOH/g. It is
generally the case that low acid numbers of the copolymer or
copolymer mixture improve the barrier properties of the cured
inventive coating formulation when exposed to aqueous media.
Accordingly, copolymers or copolymer mixtures that additionally
comprise the above described comonomers are inventively preferred
with acid numbers below 100 mg KOH/g, particularly below 60 mg
KOH/g.
[0025] The copolymer or the copolymer mixture of the inventive can
inner coating preferably comprises less than 0.05 wt %,
particularly preferably less than 0.01 wt of epoxidically bonded
oxygen.
[0026] A good film-formation when curing the can inner coating
requires that the water-dispersed copolymer or the water-dispersed
copolymer mixture of the can inner coating is converted into a
melted state after the aqueous phase has been driven off. This
requirement is satisfied when copolymers or copolymer mixtures are
preferred that as such have a glass transition temperature of not
more than 80.degree. C., particularly preferably not more than
60.degree. C. Copolymers or copolymer mixtures with a weight
average molecular weight M.sub.w of not more than 20 000 u and
which are based on alkenes and .alpha.,.beta.-unsaturated
carboxylic acids usually have glass transition temperatures that
are significantly below 100.degree. C., such that copolymers or
copolymer mixtures with a weight average molecular weight of not
more than 20 000 u, in particular not more than 15 000 u, are
preferred in inventive can inner coatings.
[0027] In a preferred formulation of the inventive can inner
coating, the acid groups of the water-dispersed copolymer or the
water-dispersed copolymer mixture are at least partially
neutralized. This measure increases the ability of the copolymer
for self-emulsification in the aqueous phase, such that more stable
coating formulations result with lower particle sizes of the
dispersed copolymers. Accordingly, the can inner coating preferably
additionally comprises a neutralizing agent. Preferred suitable
neutralization agents that are additionally comprised in such a
preferred formulation are ammonia, amines, metallic aluminum and/or
zinc, preferably in powdered form, as well as water-soluble oxides
and hydroxides of the elements Li, Na, K, Mg, Ca, Fe(II) and
Sn(II). The person skilled in the art is aware here that the
neutralization agents, corresponding to their function, enter into
neutralization reactions with the components of the inventive
coating, and therefore in these preferred formulations are
optionally detectable as such only indirectly in the form of their
reaction products. For example, metallic aluminum powder or zinc
powder reacts in the aqueous phase, giving off hydrogen, to afford
the corresponding hydroxides that again neutralize the acid groups
of the copolymer or copolymer mixture, such that in the inventive
coating finally only the cations of the elements aluminum or zinc
can be detected. The neutralization agents are therefore understood
to be solely as formulation aids of the inventive can inner
coating. Ammonia and amines are particularly preferred
neutralization agents, as they pass into the gas phase when the
coating is cured at elevated temperatures and therefore do not
remain in the cured can inner coating. Preferred amines that can be
employed as the neutralization agent in inventive can inner
coatings are morpholine, hydrazine, hydroxylamine,
monoethanolamine, diethanolamine, triethanolamine,
dimethylethanolamine and/or diethylethanolamine.
[0028] The degree of neutralization of the acid groups in the
copolymer or copolymer mixture in the inventive can inner coating
is such that at least 20%, particularly preferably at least 30% of
the acid groups are neutralized. High degrees of neutralization
above 70%, preferably above 60%, are to be avoided in a preferred
embodiment of the can inner coating, as the almost completely
neutralized copolymers are already dissolved in significant amounts
in water, thereby resulting again in a high viscosity of the
coating and average particle sizes of the dispersed copolymer or
copolymer mixture in the sub-micrometer range, such that these
kinds of formulations are less suitable as the can inner coating
due to their rheological properties.
[0029] In this context, the neutralization agent to the can inner
coating is preferably to be formulated in such an amount that,
based on 1 g of copolymer or copolymer mixture, at least 4/z
.mu.mol, preferably at least 6/z .mu.mol, each multiplied by the
acid number of the copolymer or copolymer mixture, are comprised as
the neutralization agent, but preferably not more than 12/z
.mu.mol, particularly not more than 10/z .mu.mol, multiplied by the
acid number of the copolymer or copolymer mixture. The divisor z is
a natural number and corresponds to the equivalent number of the
neutralization reaction. The equivalent number represents how many
moles of acid groups of the copolymer or copolymer mixture are
neutralized by one mole of the neutralization agent.
[0030] The inventive can inner coating comprises a water-dispersed
or water-soluble curing agent from the group of aminoplasts and/or
the group of carbodiimides. The curing agent enables the copolymer
or the copolymer mixture to crosslink in a condensation reaction
and thus to form a cured coating film on the inner surface of the
can. The barrier properties of the cured inventive can inner
coating as a film are comparable with those of cured epoxide-based
coating films.
[0031] In the inventive coating the curing agent must have the
characteristic that it crosslinks the copolymer or copolymer
mixture through a condensation reaction only at temperatures above
the glass transition temperature, preferably only above 100.degree.
C., as otherwise, curing would already occur before the dispersed
polymeric components of the coating could form a complete film on
the inner surface of the can, thus producing very heterogeneous
coating films.
[0032] Particularly suitable aminoplast curing agents are based on
melamine, urea, dicyandiamide, guanamines and/or guanidine. In
inventive can inner coatings, the aminoplast curing agents are
particularly preferably melamine-formaldehyde resins with a molar
ratio of formaldehyde: melamine that is preferably greater than
1.5.
[0033] Alternatively or in addition, the curing agent of the
inventive can inner coating is a carbodiimide. According to the
invention, carbodiimides possess at least one diimide structural
moiety of the --C.dbd.N.dbd.C-- type. However, they are preferably
polyfunctional with a diimide equivalent weight in the range of
300-500 grams of the polyfunctional compound per mole of diimide
groups. Particularly preferred carbodiimides result from
isocyanates with at least two isocyanate groups by decarboxylation,
in particular those of the general Formula (I):
##STR00001##
with n: a whole natural number between 1 and 20; [0034] R.sub.1 an
aromatic, aliphatic or alicyclic residue with not more than 16
carbon atoms.
[0035] The isocyanate groups are additionally preferably blocked
with hydrophilic protective groups that as such lend an improved
water-dispersibility or water-solubility to the carbodiimide. The
use of these preferred carbodiimides furnishes the additional
advantage that the can inner coating can be formulated to be almost
free of organic solvents as these carbodiimides are highly
water-soluble without already crosslinking the copolymer or
copolymer mixture in the aqueous formulation. In a preferred
embodiment of an inventive can inner coating that at least
partially comprises carbodiimides as the curing agent, the content
of organic solvents is therefore less than 10 wt %, particularly
preferably less than 4 wt %, in particular the can inner coating
preferably comprises no solvent. Exemplary suitable protective
groups with hydrophilic character are hydroxyalkyl sulfonic acids,
hydroxyalkyl phosphonic acids, hydroxyalkyl phosphoric acids,
polyalkylene glycols as well as quaternary aminoalkyl alcohols and
aminoalkylamines. In a particularly preferred embodiment of the can
inner coating, the curing agent is therefore selected from
carbodiimides with blocked terminal isocyanate groups according to
the general structural Formula (II):
##STR00002##
with n: a whole natural number between 1 and 20; [0036] R.sub.1: an
aromatic, aliphatic or alicyclic residue with not more than 16
carbon atoms. [0037] X: --NH--R.sub.1--N(R.sub.1).sub.2,
--O--R.sub.1--N(R.sub.1).sub.2, --NH--R.sub.1--N(R.sub.1).sub.3Y,
--O--R.sub.1--N(R.sub.1).sub.3Y, --O--R.sub.1--SO.sub.3Z,
--O--R.sub.1--O--PO.sub.3Z, --O--R.sub.1--PO.sub.3Z,
--O--(C.sub.2H.sub.4).sub.p--OH, --O--(C.sub.3H.sub.6).sub.p--OH
[0038] with Y: hydroxide, chloride, nitrate, sulfate [0039] with Z:
hydrogen, ammonium, alkali metal or alkaline earth metal [0040]
with p: a whole natural number between 1 and 6
[0041] Preferred diisocyanates that are afforded by decarboxylation
of the corresponding carbodiimides are for example hexamethylene
diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate,
isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate,
methylcyclohexane diisocyanate and tetramethylxylylene
diisocyanate, 1,5-naphthylene diisocyanate, 4,4-diphenylmethane
diisocyanate, 4,4-diphenyldimethylmethane diisocyanate,
1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate,
2,4-toluenylene diisocyanate, 2,6-toluenyfene diisocyanate.
[0042] Basically, the weight average molecular weight M.sub.w of
the curing agent in the inventive can inner coating is preferably
not more than 2500 u, particularly preferably not more than 1500 u,
in order to ensure an adequate crosslinking with the copolymer or
the copolymer mixture.
[0043] The flow properties of the inventive can inner coating are
preferably to be adjusted, such that on the one hand to enable the
coating to be applied in a spray process and especially in the
airless process (which illustrates an airless atomization spray
process) that is usually used in the beverage can industry. On the
other hand, the wet film deposited on the inner surface of the can
must not immediately run off due to gravitational forces, thereby
causing an inhomogeneous coating. Optimum flow properties with good
film formation of the dispersed constituents are obtained for
inventive can coatings, whose dispersed polymeric constituents of
the water-based coating preferably have a D.sub.90 value of not
more than 100 .mu.m, particularly preferably not more than 60
.mu.m, wherein the D.sub.50 value is preferably not less than 1
.mu.m, particularly preferably not less than 10 .mu.m. The
D.sub.90-value, respectively the D.sub.50-value, means that 90 vol
% respectively 50 vol % of the dispersed particles of the can inner
coating are smaller than the specified value.
[0044] The D.sub.90-value, respectively the D.sub.50-value, can be
determined from volume weighted cumulative particle size
distributions, wherein the particle size distribution curve can be
measured with the help of light scattering methods.
[0045] The viscosity of the can inner coating is preferably such
that a flow time between 20 and 40 seconds results, when measured
with a 4 mm DIN flow cup of DIN EN ISO 2431. If the viscosity,
measured as the flow time from the normalized flow cup, is in this
range, then the coating, present as a thin film on the inside of
the can, has a flow behavior that reduces any run off of the wet
film and simultaneously ensures that the can inner coating is able
to be applied in spray processes.
[0046] Emulsifiers that support the dispersion of the copolymer or
the copolymer mixture can be added as an auxiliary to the inventive
can inner coating. At least 0.1 wt % of emulsifiers are preferably
added for this purpose. Preferably, non-ionic amphiphiles with an
HLB value of at least 8 can be additionally comprised as the
emulsifiers in the can inner coating.
[0047] According to the present invention, the HLB value is
calculated by the following formula and can assume values of zero
to 20 on an arbitrary scale: [0048] HLB=20 (M.sub.l/M) [0049] with
M.sub.l molecular weight of the lipophilic group of the amphiphile
[0050] M: molecular weight of the amphiphile
[0051] The content of this added auxiliary emulsifier in the can
inner coating is preferably not more than 5 wt %, particularly
preferably not more than 2 wt %. However, the copolymer or the
copolymer mixture used in the inventive can inner coating is
characterized in that it already possesses self-emulsifying
properties as a result of its acid groups. Moreover, it has been
shown that the addition of emulsifiers frequently causes a
decreased adhesion of the cured can inner coating to the tin plate
and aluminum surfaces. Accordingly, in a preferred embodiment of
the can inner coating, in the case that the acid number of the
copolymer or copolymer mixture is greater than 60 mg KOH/g,
preferably greater than 80 mg KOH/g, or the degree of
neutralization of the copolymer or copolymer mixture with an acid
number below 100 mg KOH/g is at least 30%, then less than 0.1 wt %,
particularly preferably less than 0.01 wt % and especially
preferably no emulsifiers are comprised that are based on the
non-ionic amphiphiles with an HLB value of at least 8.
[0052] Alternatively or in addition to the added emulsifiers, the
inventive can inner coating may comprise water-miscible organic
solvents that decrease the polarity of the aqueous phase, so as to
induce the emulsification of the copolymer or copolymer mixture.
For this purpose, at least 1 wt % of water-miscible organic
solvents are added. In this regard, the boiling point of the
water-miscible solvent under standard conditions is preferably not
more than 150.degree. C.
[0053] Suitable solvents are glycol ethers, alcohols and esters.
The content of the solvent in the can inner coating is preferably
not more than 40 wt %, particularly preferably not more than 20 wt
%.
[0054] Inventive can inner coatings may comprise wetting agents,
leveling agents, defoamers, catalysts, film-formers, stabilizers
and/or the already mentioned neutralizing agents as additional
constituents. These kinds of auxiliaries are generally known to the
person skilled in the art of coating objects, wherein film-formers
in the present invention are understood to mean organic polymers
that can crosslink with the curing agent present in the can inner
coating. The content by weight of film-formers based on the
copolymer or copolymer mixture is at most 20%, preferably at most
10%.
[0055] A preferred formulation of an inventive can inner coating
comprises, in addition to at least 40 wt % water, [0056] a) 4-30 wt
%, preferably 10-20 wt %, of the copolymer or of the copolymer
mixture in dispersed form, [0057] b) 2-20 wt. %, preferably 4-12
wt. %, of the at least one curing agent, [0058] c) not more than 5
wt % of emulsifiers selected from non-ionic amphiphiles with an HLB
value of at least 8; [0059] d) not more than 40 wt %, preferably at
least 1 wt %, of water-miscible organic solvents; [0060] e) not
more than 10 wt. % of auxiliaries selected from wetting agents,
leveling agents, defoamers, catalysts, film-formers, stabilizers
and/or neutralizing agents, preferably not more than 12/z .mu.mol
of neutralization agent multiplied by the acid number of the
copolymer or copolymer mixture are comprised per gram of the
copolymer or copolymer mixture where z is the equivalent number of
the relevant neutralization reaction.
[0061] A particularly preferred reduced-solvent formulation of an
inventive can inner coating comprises, in addition to at least 40
wt % water, [0062] a) 4-30 wt %, preferably 10-20 wt %, of the
copolymer or of the copolymer mixture in dispersed form, [0063] b)
2-20 wt %, preferably 4-12 wt % of at least one resin, of which at
least 40 wt % of a carbodiimide with terminal, blocked isocyanate
groups based on the total content of the curing agent, [0064] c)
not more than 5 wt % of emulsifiers selected from non-ionic
amphiphiles with an HLB value of at least 8; [0065] d) not more
than 10 wt %, preferably not more than 1 wt %, of water-miscible
organic solvents; [0066] e) not more than 10 wt % of auxiliaries
selected from wetting agents, leveling agents, defoamers,
catalysts, film-formers, stabilizers and/or neutralizing agents,
preferably not more than 12/z .mu.mol of neutralization agent
multiplied by the acid number of the copolymer or copolymer mixture
are comprised per gram of the copolymer or copolymer mixture where
z is the equivalent number of the relevant neutralization
reaction.
[0067] Inventive can inner coatings are characterized in that due
to the good crosslinking of the copolymer or of the copolymer
mixture with the curing agent, the cured film on the inner surfaces
of metal cans possesses excellent barrier properties. The metallic
base material is consequently firstly effectively protected against
corrosion and secondly the liquid stored in the can will not take
up any extraneous substance. Therefore, the present invention makes
available an alternative to the conventional use of epoxides in can
inner coatings, in particular epoxides based on Bisphenol A.
Consequently, the content of epoxidically bonded oxygen in
inventive can inner coatings is preferably not more than 0.1 wt %,
particularly preferably not more than 0.01 wt %. An inventive can
inner coating particularly preferably comprises no organic
constituents with epoxide groups.
[0068] Inventive can inner coatings can preferably be produced in
closed processes in pressure reactors using shear forces, wherein
all constituents of an inventive can inner coating are transferred
into a pressure reactor, in order to be subsequently subjected to a
shear rate of at least 1000 s.sup.-1 at temperatures in the range
of 80-200.degree. C. and a pressure of 1-6 bar, wherein the energy
input is preferably in the range of 10.sup.3-10.sup.5 J per second
per liter of coating formulation. Alternatively, the solid
constituents together with the usual components of the can inner
coating are also dispersed in an open process, in which the melted
copolymer or the melted copolymer mixture under the action of the
abovementioned shear force is transferred into the aqueous
composition of the usual can inner coating composition. However,
the shear rate and residence time in each dispersion process is
preferably adjusted such that the dispersed constituents of the can
inner coating have a D.sub.90 value of not more than 100 .mu.m,
wherein the D.sub.50 value is preferably not below 1 .mu.m,
particularly preferably not below 10 .mu.m.
[0069] The application of a wet film of the inventive can inner
coating is preferably carried out in a spray process, particularly
preferably in the "Airless Process", in which the can inner coating
is airlessly atomized and thus deposited onto the material surface.
In this spray process, a defined quantity of the can inner coating
is introduced into the cleaned and dried can interior by means of
spray guns, while the can is rotated about its own Longitudinal
axis in order to form a homogeneous film. The wet film on the can
inner surface is then cured to a coating film in a drying oven at
temperatures ranging between 120.degree. C. and 200.degree. C.
(object temperature). The curing process includes the
volatilization of the aqueous phase as well as the film formation
and crosslinking of the polymeric constituents.
[0070] In another aspect, the present invention relates to the use
of a copolymer or a copolymer mixture of at least one aliphatic and
acyclic alkene with at least one .alpha.,.beta.-unsaturated
carboxylic acid in water-dispersed form, wherein the acid number of
the copolymer or of the copolymer mixture is at least 20 mg KOH/g,
but not more than 200 mg KOH/g, and the acid groups of the
copolymer or of the copolymer mixture in the water-dispersed form
are at least 20%, but not more than 70% neutralized, as a
constituent of water-based can inner coatings, wherein preferred
uses can be realized by above described corresponding embodiments
of the copolymer or of the copolymer mixture.
[0071] In another aspect, the present invention relates to the use
of an above described can inner coating that is deposited in a dry
film thickness of at least 5 g/m.sup.2, but preferably of not more
than 50 g/m.sup.2, on to the inner surface of a tin plate can and
in a dry film thickness of at least 1.5 g/m.sup.2, but preferably
not more than 50 g/m.sup.2, on to the inner surface of an aluminum
can.
EXAMPLES
[0072] Table 1 lists the compositions of the inventive can inner
coatings that were deposited as a wet film on to the inner surfaces
of tin plate cans by means of spray processes, and then cured for
40 seconds at 180.degree. C. to a dry coating with a coating weight
of 6-7 g/m.sup.2.
[0073] The water-based can inner coatings were manufactured in an
open reactor by continuously metering the melted copolymer to an
aqueous composition of the remaining constituents under a shear
stress of 1500 s.sup.-1 at 95.degree. C. After metering in the
copolymer, the homogenization was continued in the open reactor
until a constant viscosity of the coating formulation was achieved.
The viscosity of the coating formulations, measured as the flow
time from a DIN 4 mm flow cup according to DIN EN ISO 2431 lay in
the range 25-28 seconds.
[0074] The coating formulations homogenized in this way were then
deposited onto the inner surfaces of the tin plate can in a
two-step spray process, wherein the tin plate can was rotated about
an axis and initially the bottom of the can and lower part of the
body was coated and then the can body and end were sprayed. The wet
film was then cured.
[0075] From Table 2 it can be seen that the tin plate cans coated
with the inventive coating possess an excellent flexibility (T-bend
test) and water resistance (Koch test). Solely the hardness and
solvent resistance tests showed differing results, which, however,
all met the requirements of the beverage can industry.
TABLE-US-00001 TABLE 1 Exemplary formulations of inventive can
inner coatings Constituents in wt % (rest is water) Constituent
Compound B1 B2 B3 Copolymer Ethylene-acrylic acid; acid no. 37-44
mg KOH/g; Neutralization degree 17.1 50% (dimethylethanolamine)
Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralization
degree -- 21.3 21.5 30% (ammonia) Curing agent
Melamine/formaldehyde resin partially methylated of the imino type
9.0 -- 4.5 Polycarbodiimide with diimide eq. wt of 445 g/mol -- 4.0
4.0 Solvent Monopropylene glycol monomethyl ether 11.0 -- 6.0 Butyl
glycol 9.0 -- 1.7 Defoamer Polyether siloxane copolymer 0.63 0.5
0.5 Wetting agent Bis(2-ethylhexyl)sulfosuccinate, Na salt -- 0.75
0.75
TABLE-US-00002 TABLE 2 Properties of the cured can inner coatings
of Table 1 in tin plate cans Property Test B1 B2 B3 Coating
adhesion.sup.1 T-bend acc. DIN ISO 17132 0 0 0 Cross-hatch test
acc. DIN 0 0 0 53151 (24 h) Boil test acc. DIN 53151 0 0 0 (30 min
at 85.degree. C.) Coating hardness Pencil hardness acc. DIN HB B B
ISO 15184 Solvent resistance MEK Test acc. DIN EN 90 90 100
13523-11 .sup.1Classification according to coating dissolution in %
based on the tested surface 0: no dissolution; 1: 5%; 2: 15%; 3:
35%; 4: <65%; 5: >65%
* * * * *