Note: Descriptions are shown in the official language in which they were submitted.
CA 02831808 2013-09-30
COATING FORMULATION FOR THE INTERIOR SURFACES OF CANS
[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 a,$-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, aluminium 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
CA 02831808 2013-09-30
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.
[0008p/0 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 gylcidyl group. Such can inner coatings can be formulated free of
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CA 02831808 2013-09-30
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,
a) a copolymer or a copolymer mixture of at least one aliphatic and acyclic
alkene with at least one a,-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
the
group of aminoplasts and/or the group of carbodiimides.
[0011] Cans are inventively understood to mean metallic containers for
filling,
storing and holding stocks of foodstuffs, in particular of beverages.
[0012] 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.
[0013] 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
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solid or liquid in the continuous aqueous phase.
[0014]According to the invention, mixtures of chemically and/or structurally
different copolymers of at least one aliphatic and acyclic alkene with at
least
one a,$-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
ad61-
unsaturated carboxylic acids as the comonomers or have a different number of
otherwise identical comonomers in the copolymer.
[0015]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.
[0016]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 moth 1 KOH in methanol. The potentiometric
measurement is carried out with a combination electrode (LL-Solvotrode from
Metrohm; reference electrolyte: 0.4 mo1/1 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.
[0017]As a melted on, thin film on metal surfaces the copolymer or copolymer
mixture of the aliphatic and acyclic alkane with an a,/.?-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
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film is formed and cured, and does not run off inside the can due to the force
of
gravity.
[0018] If the acid number of the copolymer or copolymer mixture of alkenes and
aft-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 aft-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 C.
[0019] 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 'Yo. 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.
[0020] 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.
[0021] Preferred aft-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.
[0022] Further comonomers that may be an additional component of the
copolymer or the copolymer mixture in an inventive can inner coating are
CA 02831808 2013-09-30
selected from esters of a,$-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.
[0023]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.
[0024]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 C, particularly preferably not more than 60 C. Copolymers or
copolymer mixtures with a weight average molecular weight M, of not more
than 20 000 u and which are based on alkenes and aft-unsaturated carboxylic
acids usually have glass transition temperatures that are significantly below
100 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.
[0025] 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
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CA 02831808 2013-09-30
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.
[0026]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
7
CA 02831808 2013-09-30
range, such that these kinds of formulations are less suitable as the can
inner
coating due to their rheological properties.
[0027] 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 pmol, preferably at least 6/z pmol, 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 pmol,
particularly not more than 10/z pmol, 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.
[0028]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.
[0029] 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 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.
[0030] 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
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CA 02831808 2013-09-30
preferably greater than 1.5.
[0031]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=N=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):
OftesCameN¨Rif-NlimeCui.N¨Ri-Nom.C...0 (I)
with n: a whole natural number between 1 and 20;
R1 an aromatic, aliphatic or alicyclic residue with not more than 16
carbon atoms.
[0032]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
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CA 02831808 2013-09-30
to the general structural Formula (II):
---X
(II)
0 - n
with n: a whole natural number between 1 and 20;
R1: an aromatic, aliphatic or alicyclic residue with not more than 16
carbon atoms.
X: -NH-R1-N(R1)2, -0-R1-N(R1)2, -NH-R1-N(R1)3Y, -0-R1-N(R1)3Y,
-0-R1-S03Z, -0-R1-0-PO3Z, -0-R1-PO3Z, -0-(C2H4)p-OH,
-0-(C3H6)p-OH
with Y: hydroxide, chloride, nitrate, sulfate
with Z: hydrogen, ammonium, alkali metal or alkaline earth metal
with p: a whole natural number between 1 and 6
[0033] 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-toluenylene diisocyanate.
[0034] Basically, the weight average molecular weight Mw 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.
[0035] 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
CA 02831808 2013-09-30
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
Dgo value of not more than 100 pm, particularly preferably not more than 60
pm, wherein the D50 value is preferably not less than 1 pm, particularly
preferably not less than 10 pm. The D90-value, respectively the D50-value,
means that 90 vol % respectively 50 vol % of the dispersed particles of the
can
inner coating are smaller than the specified value.
[0036]The D90-value, respectively the D50-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.
[0037]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.
[0038] 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.
[0039]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:
HLB = 20 (1-M1/M)
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with MI molecular weight of the lipophilic group of the amphiphile
M: molecular weight of the amphiphile
[0040] 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.
[0041]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 C.
[0042] 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 %.
[0043] 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
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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%.
[0044]A preferred formulation of an inventive can inner coating comprises, in
addition to at least 40 wt `)/0 water,
a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the copolymer
mixture in dispersed form,
b) 2-20 wt.%, preferably 4-12 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 %, preferably at least 1 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/or
neutralizing agents, preferably not more than 12/z pmol 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.
[0045]A particularly preferred reduced-solvent formulation of an inventive can
inner coating comprises, in addition to at least 40 wt % water,
a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the copolymer
mixture in dispersed form,
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,
c) not more than 5 wt % of emulsifiers selected from non-ionic amphiphiles
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CA 02831808 2013-09-30
with an HLB value of at least 8;
d) not more than 10 wt %, preferably not more than 1 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/or
neutralizing agents, preferably not more than 12/z pmol 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.
[0046] 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.
[0047]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-1 at
temperatures in the range of 80-200 C and a pressure of 1-6 bar, wherein the
energy input is preferably in the range of 103-105 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
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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 Dgo value of not more
than 100 pm, wherein the D50 value is preferably not below 1 pm, particularly
preferably not below 10 pm.
[0048]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
C and 200 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.
[0049] 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 a,fl-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.
[0050] 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/m2, but preferably of not more than 50 g/m2, on to the inner surface of a
tin
plate can and in a dry film thickness of at least 1.5 g/m2, but preferably not
more than 50 g/m2, on to the inner surface of an aluminum can.
CA 02831808 2013-09-30
Examples:
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 C to a dry coating with
a coating weight of 6-7 g/m2.
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 at 95 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.
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.
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.
16
Table 1 Exemplary formulations of inventive can inner coatings
Constituents in wt A) (rest is water)
Constituent Compound
B1 B2 B3
Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralization degree
17.1
50% (dimethylethanolamine)
c)
Copolymer
Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralization degree
0
- 21.3 21.5 I.)
0
30% (ammonia)
UJ
H
CO
0
Melamine/formaldehyde resin partially methylated of the imino type
9.0 - 4.5 0
Curing agent
I.)
0
Polycarbodiimide with diimide eq. wt of 445 g/mol
- 4.0 4.0 H
UJ
I
Monopropylene glycol monomethyl ether
11.0 - 6.0 0
i
Solvent
UJ
Butyl glycol
9.0 - 1.7 0
Defoamer Polyether siloxane copolymer
0.63 0.5 0.5
Wetting agent Bis(2-ethylhexyl)sulfosuccinate, Na salt
- 0.75 0.75
Table 2 Properties of the cured can inner coatings of Table 1 in tin plate
cans
Property Test
B1 B2 B3
T-bend acc. DIN ISO 17132
0 0 0
Coating adhesionl Cross-hatch test acc. DIN 53151 (24h)
0 0 0
Boil test acc. DIN 53151 (30 min at 85 C)
0 0 0
Coating hardness Pencil hardness acc. DIN ISO 15184
HB B B0
Solvent resistance MEK Test acc. DIN EN 13523-11
90 90 100 UJ
CO
0
1
CO
Classification according to coating dissolution in % based on the tested
surface
0
0: no dissolution; 1: 5%; 2:15%; 3: 35%; 4: <65%; 5: >65%
UJ
0
If
UJ
0
18
