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Description  |
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BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to the use of aqueous polymer dispersions based on
acrylate-methacrylate copolymers as lamination adhesives for glossy films.
2) Background Art
The lamination of printed papers or printed board with plastic films for
the purpose of print finishing is a process which has been practiced for
some time. Printed products are in this way protected against mechanical
influences, are given a more attractive appearance, and can be presented
in a visually more appealing manner due to the surface gloss thus
achieved. Practical examples thereof are record sleeves, book covers,
paperbacks, packaging materials for cosmetic articles and advertising
materials.
The process of coating and bonding printed paper products by means of
plastic films is known as lamination with glossy films. The adhesive
employed is accordingly known as a lamination adhesive for glossy films
and has the task of bonding the plastic film on the one hand to the
printed paper on the other hand. At the beginning of this technological
development, exclusively solvent-containing polymer resins were used,
which were used either in one-component polyacrylate-based form or in
two-component form as reactive polyurethanes. More stringent environmental
protection regulations increasingly forced the avoidance of these
solvent-containing systems and favored the use of aqueous polymer
dispersions. In the case of the lamination films used, the trend
increasingly developed in favor of polyolefin films, for example polyfilms
(PE films) or oriented polypropylene films (OPP films). These are either
flame- or corona-pretreated on their surface in order to improve the
adhesion.
The first proposals for bonding flame- or corona-pretreated OPP films to
printed paper or printed cardboard packaging by means of aqueous polymer
dispersions are described in JP-A 77/42532 (Derwent Abstract) and in EP-A
46823 (U.S. Pat. No. 4,377,433). Both publications essentially describe
the use of aqueous vinyl acetate-ethylene copolymer dispersions in which
epoxide and amine functions have been incorporated as reactive components.
At the time of use, the epoxide-containing and amine-containing vinyl
acetate/ethylene dispersions are mixed with one another. These systems
have a relatively short pot life and must be used immediately after mixing
(2-component adhesive).
All other proposals for improving the bond strengths between polyolefin
films and printed paper or board are also characterized by the use of
reactive crosslinking agent systems. For example, in JP-A 60/112874,
ethylene-vinyl acetate-acrylate dispersions are provided with good
adhesion properties with the aid of water-soluble polyamine compounds.
DE-A 3642485 (U.S. Pat. No. 4,980,404) recommends acrylate dispersions
containing polyamino compounds for bonding plastic films to paper or
board. DE-4117487 uses carbonylcontaining polymer dispersions which are
crosslinked with the aid of multifunctional aminooxy compounds and thus
achieve a significant improvement in adhesion on the corona-treated
polyolefin surfaces. Other reactive systems which have been proposed are
lattices containing itaconic acid (EP-A 307050). Water-soluble polymers
have also been proposed for improving the adhesion in the lamination of
glossy films in DD-A 254949 and U.S. Pat. No. 3,974,112.
WO-A 92/12213 describes the use of aqueous dispersions of copolymers of
(meth)acrylates of .alpha..beta.-unsaturated carboxylic acids and
(meth)acrylates of alkylsulfonic acids as adhesives for the lamination of
substrates, where the substrates to be bonded should preferably be
selected from the group consisting of polymer films, printed paper,
metal-coated paper and metal foils. The systems described therein were
intended to replace solvent-containing adhesives or hot-melt adhesives in
the production of laminates. The specific problems of lamination with
glossy films (gloss and transparency of the laminates) are not discussed.
The object of the invention was to develop an aqueous polymer dispersion
which guarantees good bond strength between an optionally corona- or
flame-pretreated polyolefin film on the one hand and printed paper or
printed cardboard packaging on the other hand, even without the
above-mentioned reactive crosslinking agent systems, to give laminates
having excellent gloss and transparency.
SUMMARY OF THE INVENTION
The invention relates to the use of aqueous polymer dispersions based on
acrylate-methacrylate copolymers as lamination adhesives for glossy films,
where the aqueous polymer dispersions have a solids content of from 20 to
65% by weight, the copolymer has a glass transition temperature of from
-35.degree. to -10.degree. C., a Fikentscher K value of from 40 to 100 and
a mean particle size of less than 200 nm, and the copolymer is obtainable
by free-radical emulsion polymerization of:
a) from 50 to 80 parts by weight of one or more comonomers from
the group consisting of alkyl acrylates of alcohols having 2 to 10 carbon
atoms, and
b) from 10 to 25 parts by weight of one or more comonomers from the group
consisting of alkyl methacrylates of alcohols having 1 to 10 carbon atoms,
and
c1) from 0 to 10 parts by weight of one or more vinyl-aromatic comonomers,
c2) from 0.1 to 4.0 parts by weight of one or more
.alpha..beta.-unsaturated, monoolefinic carboxylic acids,
c3) from 0 to 4.0 parts by weight of one or more .alpha..beta.-unsaturated,
monoolefinic carboxamides,
c4) from 0 to 8 parts by weight of one or more comonomers from the group
consisting of hydroxyalkyl (meth)acrylates made from esters of acrylic
acid or methacrylic acid and diols having 2 to 10 carbon atoms, and
c5) from 0.1 to 4.0 parts by weight of one or more
.alpha.,.beta.-unsaturated, monoolefinic compounds containing sulfonyl or
sulfonate groups.
Examples of suitable alkyl acrylates are ethyl acrylate, propyl acrylate,
n-butyl acrylate and 2-ethyl-hexyl acrylate. The copolymers preferably
contain ethyl-hexyl acrylate and/or n-butyl acrylate.
Examples of suitable alkyl methacrylates are methyl methacrylate, ethyl
methacrylate, propyl methacrylate and n-butyl methacrylate. The copolymers
preferably contain methyl methacrylate, particularly preferably in an
amount of from 15 to 20 parts by weight.
An example of a suitable vinylaromatic compound c1) is styrene. Examples of
.alpha..beta.-unsaturated, monoolefinic carboxylic acids c2) are acrylic
acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid and
fumaric acid. Examples of .alpha.,.beta.-unsaturated, monoolefinic
carboxamides c3) are acrylamide and methacrylamide. Examples of suitable
hydroxyalkyl (meth)acrylates c4) made from esters of acrylic acid or
methacrylic acid and diols having 2 to 10 carbon atoms are hydroxyethyl
acrylate and hydroxypropyl methacrylate. Examples of suitable
.alpha.,.beta.-unsaturated, monoolefinic compounds c5) containing sulfonyl
or sulfonate groups are sodium styrene sulfonate, styrene sulfonic acid,
acrylamidopropanesulfonic acid, vinyl sulfonate and disulfoalkyl esters of
itaconic, maleic or fumaric acid.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a preferred embodiment, the comonomer phase contains:
a) from 50 to 80 parts by weigh of one or more comonomers from the group
consisting of ethylhexyl acrylate and n-butyl acrylate,
b) from 10 to 25 parts by weight of methyl methacrylate,
c1) from 0.5 to 10 parts by weight of styrene, c2) from 0.1 to 4.0 parts by
weight of one or more comonomers from the group consisting of acrylic
acid, methacrylic acid and crotonic acid,
c3) from 0.1 to 4.0 parts by weight of one or more comonomers from the
group consisting of acrylamide and methacrylamide,
c4) from 0.5 to 8 parts by weight of one or more comonomers of the group
consisting of hydroxyethyl acrylate and hydroxypropyl methacrylate, and
c5) from 0.1 to 4.0 parts by weight of one or more comonomers from the
group consisting of acrylamidopropanesulfonic acid, vinyl sulfonate,
styrenesulfonic acid and the corresponding salts.
In particular, the solids content of the aqueous polymer dispersions which
can be prepared by emulsion polymerization of the above-mentioned
comonomer compositions is from 30 to 55% by weight, where the polymer has
a glass transition temperature of from -25.degree. to -15.degree. C. and a
K value of from 70 to 100, and a mean particle size of 150 nm and a
Brookfield viscosity (20 rpm) of 1000 mPas, preferably less than 500 mPas.
The aqueous polymer dispersions are prepared by emulsion polymerization,
using the batch or feed method or with initial introduction of a portion
of individual components, the remainder being metered in during the
polymerization. The polymerization temperature is between 0.degree. and
100.degree. C.
In order to initiate the polymerization, the thermal or redox initiator
systems which are conventional in emulsion polymerization can be used. In
general, the initiation is carried out by means of water-soluble
free-radical formers, which are preferably employed in amounts of from 0.4
to 3.0% by weight, based on the total weight of the monomers. Examples of
these are ammonium and potassium persulfate and peroxodisulfate; hydrogen
peroxide; azo compounds, such as azobisisobutyronitrile or
azobiscyanovaleric acid. In the case of thermal initiation, the
polymerization is preferably carried out at between 60.degree. and
100.degree. C. The free-radical formation can be accelerated at lower
temperatures, preferably below 60.degree. C., with the aid of reducing
agents, such as alkali metal formaldehydesulfoxylates, alkali metal
sulfites, bisulfites and thiosulfates, and ascorbic acid.
Dispersants which can be used are all anionic and nonionic emulsifiers
conventionally used in emulsion polymerization. From 1 to 6% by weight,
based on the total weight of the monomers, of emulsifier are preferably
employed. Examples of suitable emulsifiers are anionic surfactants, such
as alkyl sulfonates having a chain length of from 8 to 18 carbon atoms,
alkyl and alkylaryl ether sulfates having 8 to 18 carbon atoms in the
hydrophobic radical and up to 40 ethylene oxide or propylene oxide units,
alkyl or alkylaryl sulfonates having 8 to 18 carbon atoms, esters and
monoesters of sulfosuccinic acid with monohydric alcohols or alkylphenols.
Examples of suitable nonionic surfactants are alkylpolyglycol ethers and
alkylaryl polyglycol ethers having 8 to 40 ethylene oxide units.
The pH range desired for polymerization, which is generally between 2.5 and
10, preferably between 3 and 8, can be established in a known manner by
acids, bases or conventional buffer salts, such as alkali metal phosphates
or alkali metal carbonates. In order to adjust the molecular weight (K
value), conventional regulators, for example mercaptans, aldehydes and
chlorinated hydrocarbons, can be added during the polymerization.
A preferred procedure involves initially introducing a portion of the
comonomer mixture and the emulsifier, heating the batch to the
polymerization temperature, and the polymerization is subsequently
initiated by metering in a small amount of initiator. The remainder of the
comonomers and initiator are added by the feed method from a preemulsion
prepared in advance.
A particularly preferred procedure involves initially introducing from 2 to
10% by weight of the acrylate, methacrylate and emulsifier, heating the
reaction batch to a temperature of from 60.degree. to 100.degree. C.,
initiating the polymerization by addition of from 1.0 to 2.5% by weight of
the initiator, and metering in the remainder of the comonomer mixture,
emulsifier and initiator as a preemulsion at the rate at which they are
consumed.
In the use according to the invention of the aqueous polymer dispersions as
binders in the lamination of glossy films, printed cardboard packaging and
papers are generally adhesively bonded over the entire area to plastic
films, preferably polyolefin films, which may have been flame- and/or
corona-pretreated, polyester films and cellulose acetate films. To this
end, the aqueous polymer dispersion is generally applied, in a manner
known to a person skilled in the art, to the pretreated side of the
plastic film, preferably in a thickness of from 5 to 50 .mu.m, and is
subsequently dried. To produce the laminates, the dried polymer film is
covered with the paper or cardboard sheet to be laminated and is
heat-sealed at a suitable temperature and under pressure, preferably at a
temperature of from 50.degree. to 100.degree. C. and under a pressure of
from 2 to 6 bar.
In the use according to the invention of the aqueous polymer dispersions
with the above-mentioned copolymer compositions, an adhesive film which
gives high-quality, stable adhesive bonds for very thin adhesive layers is
formed after the drying step, even at very low application rates. When
used in the lamination of glossy films, where the optical properties of
the laminate are particularly important, products are obtained which are
distinguished by excellent transparency and gloss.
In contrast to 2-component adhesives for the lamination of glossy films,
which contain reactive water-soluble or water-emulsifiable crosslinking
agent systems, the aqueous polymer dispersions are distinguished by a
virtually unlimited shelf life and pot life. The omission of reactive
crosslinking agent systems also results in superior rheological properties
of the aqueous polymer dispersions, which enable both manual processing
and processing on high-speed machines.
The examples below serve to further illustrate the invention.
Preparation of the polymer dispersions:
EXAMPLE 1
Three metering solutions were prepared:
Initiator solution I: 0.06 g of potassium persulfate were dissolved in 8.5
ml of water.
Initiator solution II: 5.1 g of potassium persulfate were dissolved in 100
ml of water and 0.5 g of NH.sub.3 (12% strength).
Preemulsion: 69.3 parts by weight (577 g) of n-butyl acrylate, 19.0 parts
by weight (158 g) of methyl methacrylate, 1.75 parts by weight (14.6 g) of
styrene, 4.7 parts by weight (39.8 g) of hydroxypropyl methacrylate, 0.65
part by weight (5.4 g) of acrylic acid, 0.65 part by weight (18.1 g of 30%
strength solution) of acrylamide, 0.27 part by weight (2.23 g) of sodium
styrenesulfonate and 27.1 g of a 35% strength solution of a nonylphenol 25
ethylene oxide sulfate (Rewopol NOS25) were emulsified in 274 ml of water.
3.7 parts by weight (29.8 g) of n-butyl acrylate, 1.0 part by weight (8.2
g) of methyl methacrylate, 0.43 part by weight (3.6 g) of styrene, 0.2
part by weight (2.0 g) of hydroxypropyl methacrylate, 0.09 part by weight
(2.4 g in 30% strength solution) of acrylamide, 0.02 part by weight (0.18
g) of sodium styrenesulfonate and 17.8 g of a 35% strength solution of a
nonylphenol 25 ethylene oxide sulfate (Rewopol NOS25) were introduced,
along with 441 ml of water, into a stirred reactor fitted with reflux
condenser and internal thermometer, and the mixture was warmed to
80.degree. C. When the temperature equilibrium had been reached, the two
initiator solutions were commenced simultaneously, with the metering rates
being selected corresponding to a metering time of 10 minutes for
initiator solution I and of about 3.5 hours for initiator solution II.
Immediately after the noticeable commencement of an exothermic reaction,
the preemulsion was introduced at a constant metering rate; the metering
time was 3 hours. When the metering was complete, the polymerization was
completed by stirring for 30 minutes at 80.degree. C.
A speck-free dispersion having a solids content of 50.3% by weight and a
mean particle size of 90 nm was obtained. The K value (measured in THF) of
the polymer was 93.1, and the glass transition temperature was Tg=-16
2.degree. C. A solids content of 47% by weight was established by
dilution.
EXAMPLE 2
The procedure was analogous to Example 1, using the amounts of comonomer,
emulsifier and initiator given in Table 1. The solids content, K value,
particle size and glass transition temperature are likewise given in Table
1.
COMPARATIVE EXAMPLE 1
The procedure was analogous to Example 1, using the amounts of comonomer,
emulsifier and initiator given in Table 1. In contrast to Example 1, only
about 0.4 times the amount of initiator was employed, so that a polymer
having a K value of >100 was obtained.
COMPARATIVE EXAMPLE 2
The procedure was analogous to Comparative example 1, using the amounts of
comonomer, emulsifier and initiator given in Table 1.
TABLE 1
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Recipe [parts Comp. Comp.
by weight] Example 1 Example 2 ex. 1 ex. 2
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BuA 73.0 77.0 73.0 77.0
MMA 20.0 16.0 20.0 16.0
STY 2.18 2.18 2.18 2.18
HPMA 4.90 4.90 4.90 4.90
AA 0.74 0.74 0.74 0.74
AS 0.65 0.65 0.65 0.65
NaSS 0.29 0.29 0.29 0.29
Solids content
47.0 47.0 49.7 49.6
[% by wt.]
K value 93.1 93.1 101 103
Particle size
90 102 106 113
[nm]
Glass transition
-16.2 -21.4 -16.8 -21.4
temp. [.degree.C.]
Viscosity [mPas]
<500 <500 220 480
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BuA: nbutyl acrylate, MMA: methyl methacrylate, STY: styrene, HPMA:
hydroxypropyl methacrylate, AA: acrylamide, AS: acrylic acid, NaSS: sodiu
styrene sulfonate.
Applicational testing:
The following substrates were used to prepare the laminates:
F1: PE film (polyethylene), 50 .mu.m thick, surface tension
(untreated)=28-30 mN/m, in-house corona treatment using a Softal
Electronic type HR-1R AB300 Corona
F2: OPP film (oriented polypropylene), 12 .mu.m thick, pretreated on both
sides, manufacturer: LONZA, type Ultralen K
F3: OPP film, 30 .mu.m thick, pretreated on one side, manufacturer: type
Biafol
P1: High-quality paper and board, printed with Reflex Blue No. 62105, Epple
Druckfarben
P2: Multicolor print cardboard packaging
The adhesive coating was carried out on the pretreated side of the film in
a thickness of 12 .mu.m (wet), corresponding to 5.7 g/m.sup.2 (dry), dried
for 3 minutes at 80.degree. C. and subsequently covered with the print
sheet. In order to produce the laminates, the test specimens were
heat-sealed at 80.degree. C. and 3 bar for 3 seconds. After appropriate
storage of the laminates, the bond strength (peel strength/180.degree.) of
the laminates was determined by means of a Zwick 1445 material testing
machine at a peel rate of 300 mm/min.
The laminates are assessed from the results obtained using the material
testing machine: if the bonding is poor, the laminated film can be peeled
off using a measurable force (peel strength)--corresponding numerical
values are given in Table 2. Good bonds are distinguished by the fact that
the laminated film cannot be peeled off. In these cases, either printing
ink transfer from the paper to the film, draw-out of printing ink from the
paper or tearing-out of the paper occurs during the peel test in the
testing machine; in addition, tearing of the laminated film can occur.
Results of applicational testing are shown in Table 2 for the individual
laminates.
TABLE 2
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Laminate/ Comp. Comp.
storage time
Example 1 Example 2 ex. 1 ex. 2
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F1-P2/5 min
IT/PT IT/PT 1.32/AT 1.39/AT
F2-P2/5 min
FT FT -- --
F3-P2/5 min
IT/PT IT/PT -- --
F1-P2/4 h
IT/PT IT/PT 1.50/AT 1.42/AT
F2-P2/4 h
FT FT -- --
F3-P2/4 h
IT/PT IT/PT -- --
F1-P2/2 d
IT/PT IT 1.30/AT 1.33/AT
F2-P2/2 d
FT FT -- --
F3-P2/2 d
IT/PT IT -- --
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IT: printing ink transfer;
FT: film tear;
PT: paper tear;
AT: adhesive transfer from film to print sheet
The novel dispersions of lamination adhesives for glossy films of Example 1
and Example 2 are peel-resistant immediately after production of the
laminate; in the tensile testing machine, either printing ink transfer
from a print sheet to adhesive-coated films was observed or paper tearing
or film tearing occurred. By contrast, the laminates containing adhesives
of Comparative examples 1 and 2 were easy to peel, and in addition
adhesive transfer from the film surface to the print sheet occurred.
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Description |
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