Note: Descriptions are shown in the official language in which they were submitted.
21flfl~22
5go-ia3-o
TITLE OF TFiE INVENTION:
MULTILAYERED HEAT-SEALABLE PLASTIC FILMS
Field of the Invention:
The invention relates to heat-sealable plastic films
comprised of at least two layers, a support layer and a
sealing layer, which mutually adhere. The support layer
comprises an impact resistant polystyrene resin, The sealing
layer, which adheres to the support layer, comprises 20-x.00
weight v of a methacrylate copolymer. The plastic films are
suitable for hermetically sealing plastic containers, such as
polystyrene containers. In particular, the plastic films are
suitable for sealing containers in which foodstuffs are
stored.
Discussion of the Background:
Plastic containers are currently more popular than
containers composed of wood or inorganic materials such as
metal, glass, or ceramics, for storage especially for food
storage. An important factor in food storage, whether the
food is prepared for storage by dehydration, freezing, or
sterilization, is the complete inhibition of microbial growth.
This can be achieved by sealing foodstuffs into containers
with gastight seals.
Important factors critical for preserving foodstuffs in
containers with gastight seals include the mechanical
strength, durability, ability to maintain water and ability to
minimize the effects of the atmosphere and light on the
preserved foodstuff of the gastight seals (see "Ullmann's
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Encyclopedia of Industrial Chemistry", 25th Ed., Verlag Chemie: Weinheim,
1985, pp. 523-560 and 583-618; the applicable standards are also discussed
therein).
Previously gastight seals were composed of a layer of aluminum
coated with a sealing coating have been used to seal plastic containers
holding
food, particularly dairy products such as yogurt. Aluminum seals are typically
comprised of a three-layered laminate. The outer layer frequently comprises
biaxially oriented polyethylene terephthalate (0-PET), biaxially oriented
polypropylene (0-PP), biaxially oriented polyamide (0-PA) or cellulose. The
middle layer comprises aluminum. The heat-sealable inner layer adj oining the
aluminum layer typically comprises polyethylene, ethylene copolymers, or
polypropylene (Stehle, G. (1991) Neue Verpackun~, 9:94-101). U.S. Patent
4,753,708 describes heat-sealable coatings for metal foils which are suitable
for sealing various substrates, such as polystyrene substrates. The coatings
comprise a film-forming dispersion of a graft polymer based on an olefin and
a (meth)acrylate, in an organic solvent. However, the use of aluminum for
packaging has recently met with ecological and economic objections.
Accordingly, gastight seals composed of plastic films with sealable
coatings are being used. Hard polyvinylchloride (PVC) increasingly is widely
used as a relatively inexpensive material for sealable films. Hard PVC has
good mechanical strength and good barrier characteristics with regard to gas
permeability. Customarily an acrylic resin is used as a
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sealing coating layer. The adhesiveness and melting point of
the acrylic resin can be modified with additives.
Unfortunately the high permeability of certain plastics
to gases and vapors can lead to problems in food preservation
when the plastics are used as packaging materials. Multilayer
films have been suggested to overcome this problem (see German
Patent 35 31 036 and European Patents 0 406 681 and 0 437
745).
German Patent 35 31 036 describes plastic films produced
by coextrusion comprising a sealable layer of an impact
resistant polystyrene, a block copolymer, arid a lubricant,
possibly applied to a support layer.
European Patent 0406 681, discusses the problems of
using heat-sealable plastic films instead of aluminum
laminates. As a rule, plastic seals require much narrower
processing ranges (usually between l0 and 20°K) than aluminum
seals. The processing temperature must be continuously
monitored in order to ensure problem-free production and use
of the sealed package. When the containers being sealed
consist of a plurality of cavities which must be
simultaneously filled, such as cups or the like, processing
requirements are often difficult to meet. To solve these
problems, European Patent 0 406 681 describes a plastic film
produced by coextrusion or roll-lamination of two or three
layers (optionally separated by intermediate layers), wherein
each layer contains an adhesive for binding the layers
together. The film comprises 1-50% of a layer of a
heat°sealable impact resistant polystyrene, up to 95% of a
21~~4~2
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support layer, and 1-99% of a high melting plastic layer,
wherein the sum of the thicknesses or weights of all layers is
100%.
European Patent 0 437 745 describes a sealable
thermoplastic molding compound comprising at least four
components: an impact resistant polystyrene resin, a block
copolymer, a lubricant, and at least one homo- or copolymer of
an aliphatic olefin. The sealable molding compound is applied
to conventional support films, preferably comprised of
polystyrene. The films are useful for sealing polystyrene or
polyolefin (such as polyethylene or polypropylene) containers.
Unfortunately, multilayer films are expensive, difficult
to dispose of properly and cannot be recycled. A
heat-sealable film which is suitable for gastight sealing of
plastic containers, particularly polystyrene containers, in a
homogeneous layer and without additional surface treatment, is
described in German Patent Applications P 41 42 691.6 and P 41
42 692.4. These films are directly sealable to polystyrene
with the use of ordinary apparatus. These heat-sealable
plastic films are based on polystyrene-compatible
methacrylates comprising a molding compound with a two-phase
structure. The grafting branches, and the ungrafted parts, of
the impact resistant phase, are compatible with polystyrene.
German Patent Applications P 41 42 691.6 and P 41 42 692.4
also relate to multilayer composite films wherein the above-
mentioned molding compounds are processed to form support
films and in a second step are coated with
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polystyrene-compatible molding compounds which have a
two-phase structure.
However, in the case of multilayer composite films
comprised of a support film produced from a
polystyrene-compatible molding compound with a two-phase
structure and a sealing layer comprised of a
polystyrene-compatible readily flowable molding compound, a
large difference in viscosity between the molding compound of
the support layer and the molding compound of the sealing
layer exists which creates problems in processing. Further,
these (meth)acrylate films do not always have adequate tear
strength under mechanical load.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to
provide a heat-sealable plastic film which has good
recyclability, mechanical stability, processibility, and
manufacturability (particularly by coextrusion).
The present inventors have now found that this object can
be achieved by a heat-sealable plastic film comprising at
least two layers, a support layer and a sealing layer, which
mutually adhere. The support layer comprises an impact
resistant polystyrene resin (PS), and the sealing layer, which
adheres to the support layer, comprises 20-100 weight % of a
copolymer (copolymer P) consisting essentially of:
p1) 30-90 weight %, based on the total weight of
copolymer P, of methyl methacrylate, ethyl methacrylate or a
combination thereof;
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p2) 10-70 weight %, based on the total weight of
copolymer P, of at least one monomer of formula I:
CH3 O
CHZ == C - C - O - Rl ( I ) r
where Rl is a C3_za alkyl group; and
p3) 0-10 weight %, preferably 1-8 weight %, based on the
total weight of copolymer P, of a monomer which is
copolymerizable with and different from the monomers (p1) and
(p2); such that the sum of the proportions of the monomers
(p1), (p2) and (p3) iS 100 weight %.
The sealing layer, Where not comprised of 100 weight % of
copolymer P, comprises up to 80 weight % of one or more
additional polymers, such as styrene-butadiene-styrene block
copolymers, which, in particular, improve the impact strength
of the sealing layer and its processibility in an extruder.
The support layer may comprise copolymer P, or other
components of the sealing layer, in proportions of l-50 weight
%. Likewise, the sealing layer may comprise components of the
support layer in proportions up to 80 weight %, preferably
0.1-60 weight %. It is preferred that the content of
copolymer P in the sealing layer is greater than the content
of copolymer P in the support layer, by at least 20 weight
units, preferably 40 weight % units. This is particularly
important from the standpoint of reusability of punching
wastes.
In general the thickness o~ the plastic film is in the
range 80-500 micron; the thickness of the support layer is
5-495 micron layer and the thickness of the sealing layer is
2-495 micron. If an impact-strength-modified sealing layer,
for example a sealing layer comprising 50 weight % of
copolymer P and 50 weight % of styrene-butadiene-styrene block
copolymer, is used, the sealing layer may be much thicker,
because in such a case there is no embrittlement of the whole
system by an excessiWely thick, brittle sealing layer. The
sealing layer may also contain a third component which
inhibits adhesion of the sealing layer to the sealing head.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Suitable impact resistant polystyrene resins (PS) in
accordance with the present invention are two-phase polymer
mixtures comprising (i) a polymeric hard phase which forms a
matrix and which preferably contains ~inylaromatio monomer
units and (ii) a polymeric impact resistant phase formed by
0.01-20 micron particles which are finely dispersed in the
hard phase matrix. The polymeric hard phase is preferably
50-95 weight %, more preferably 60-95 weight %, and
particularly preferably 80-95 weight o, of the total weight of
the two-phase polymer mixture.
Suitable vinylaromatic monomer units useful in the hard
phase include styrene, c-methylstyrene, p-methylstyrene, or
other substituted styrenes and mixtures thereof. Preferably
styrene is used. The weight average molecular weights of the
hard phase polymers are in the range 50,000-500,000 Dalton,
preferably 100,000-350,000 Dalton.
I
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Suitable polymers useful for the impact resistant phase have glass
transition temperatures < 10°C, preferably < -10°C, and
ordinarily are
classified as "elastomers" or "rubbers". Crosslinked and uncrosslinked
polymers of polysiloxanes, ethylene-vinyl acetate copolymers, polyacrylates,
or polyolefins are suitably used. Preferably, polyolefins are used,
particularly
preferably polydienes.
Suitable polyolefins useful in the impact resistant phase include homo-
or copolymers of ethylene, propylene, or isobutylene (see "Ullmanns
Enzyklopaedie der technischen Chemie", 4th Ed., Vol. 19, Verlag Chemie,
1980 pp. 167-226). In general, the weight average molecular weight of the
uncrosslinked polyolefins is of from 50,000-1,000,000 Dalton (determined, for
example by gel permeation chromatography as described by Mark et al.,
"Encyclopedia of Polymer Science and Engineering", 2nd Ed., Vol. 10, J.
Wiley, 1987, pp. 1-19). Preferably, ethylene-propylene-dime (EPDM)
terpolymers, wherein the dime component is dicyclopentadiene,
ethylidenenorbornene, or hexadiene, are used (see Ullmann, supra 4th Ed.,
Vol. 13, pp. 619-621; Kirk-4thmer, "Encyclopedia of Industrial Chemistry",
3rd Ed., Vol. 8, J. Wiley, 1979, pp. 492-500 and Vol. 7, pp. 687 and 693.
Cesca, S., J. Poem. Sci., Macromol. Rev., 1975, 10: 1) EPDM terpolymers
can be produced as described in the above references.
Particularly preferred polydienes comprise the well known rubber types
including polybutadiene, poly-2-chlorobutadiene,
_ , 21~6~2~
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or polyisoprene (see Ullmann, supra, 4th Ed., Vol. 13, pp.
595-635). Preferably the impact resistant phase comprises
polybutadiene, particularly preferably grafted with styrene
monomer units. In this case, medium-cis or high-cis
polybutadienes with weight average molecular weights of
70,000-450,000 Dalton are preferably used. The impact
resistant phase is finely dispersed in the hard phase matrix.
The impact resistant particles are present in the hard phase
in proportions of 5-50 weight %, preferably 5-40 weight ~,
particularly preferably 5-20 weight o, based on the total
weight of the two phase polymer mixture. The mean particle
sizes of the dispersed impact resistant phase, are in the
range 0.01-20 micron, preferably 0.3-10 micron, and can be
determined, for example, by electron microscopy.
Suitable impact resistant polystyrene resins (PS) in
accordance with the present invention can be produced by
conventional methods. High impact polystyrene resins can be
manufactured, for example, by polymerization in bulk,
suspension polymerization or emulsion polymerization (see
Kirk-Othmer, supra, Vol. 17, 1982, pp. 470-471, and Vol. 21,
1983, pp. 811-816). Preferably the monomers comprising the
hard phase are polymerized in the presence of the impact
resistant particles by radical initiation.
The impact resistant polystyrene resin (PS) can also
contain styrene-butadiene block copolymers and/or styrene-
isoprene block copolymers, wherein multiblock copolymers such
as 2-block-, 3-block-, and star-block copolymers can be used.
(Fc~r synthesis of block polymers of styrene and a second
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monomer, see, for example, Houben-Weyl, "Methoden der
organischen Chemie'°, 4th Ed., Vol. E20 Part 2, Ceorg Thieme:
Stuttgart, 19x7, pp. 987-993). Preferably,
styrene-butadiene-styrene 3-block copolymers or star-shaped
styrene-butadiene copolymers with a high content of butadiene,
> 50 weight %, are suitable for modifying the polystyrene
resins, particularly for modifying the tear strength. In a
preferred embodiment the polystyrene resins comprise at least
2 weight % of a styrene-butadiene-styrene block copolymer, and
in a particularly preferred embodiment 5-20 weight % of a
styrenebutadiene-styrene block copolymer comprising 50-80
weight % of butadiene components, based on the total weight of
the polystyrene resin.
Alternatively, the impact resistant polystyrene resin
(PS) can be comprised entirely of block copolymers. In this
case, a smaller proportion of the butadiene in the block
copolymer is used, so that the overall content of butadiene in
the polystyrene resin is < 50 weight %, preferably < 40 weight
%, and particularly preferably in the range 10-30 weight %.
The polystyrene resin (PS) can also contain additional
polymer components, such as polybutylene, in amounts of 2-6.
weight % based on the total weight of the polystyrene resin,
or can contain the components of the sealing layer in
proportions of up to 50 weight % based on the total weight of
the polystyrene resin.
The polystyrene resin (PS) can also comprise a mixture of
the described multiblock polymers, such as polymers having >
50 weight % polybutadiene and polymers having < 50 weight
23.~~~~~d
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polybutadiene (for example, up to 15 weight ~); and further
can comprise impact resistant styrene polymers produced by
radical polymerization, which polymers have the characteristic
particle structure of impact resistant polystyrene. Another
component of the mixture can be polystyrene which contains no
polybutadiene.
Particularly preferred polystyrene resins (PS) are impact
resistant polystyrene types which contain particles of an
impact-resistant phase -- obtained, for example, by radical
polymerization of styrene in the presence of polybutadiene.
As a rule, the particles of the impact resistant phase have
sizes of 1-5 micron, preferably 2-4 micron, wherewith the
polybutadiene content of this polystyrene resin containing
particles of the impact resistant phase is in general 7-15
weight %, preferably 8-11 weight % (based on the total weight
of the polystyrene resin). The palystyrene resin can also
contain the usual additives employed in polymer processing for
including lubricants (such as paraffin oil), stabilizers (e. g.
radical scavengers), and/or pigments.
The sealing layer comprises 20-100 weight o, in general
at least 30 weight %, preferably 40-90 weight %, and
particularly preferably 45-75 weight %, based on the tota-1
weight of the sealing layer, of copolymer P.
Other components of the sealing layer, other than
copolymer P, can be present in proportions up to 80 weight %,
and in the preferred composition up to 50 weight %. These
other components modify the impact strength of the sealing
layer', or modify the melt rheology during processing.
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Suitably copolymer P consists essentially of the abovementioned
components (p1), (p2), and (p3). That is, the sum of the monomer units (p1),
(p2), and (p3) is 100 weight %. Copolymer P is suitably formed from the
monomers (p1), (p2), and optionally (p3) by conventional methods such as
radical or anionic polymerization (see Rauch-Puntigam, H., and Voelker, T.,
1967, "Acryl- and Methacrylverbindungen", Springer-Verlag, Heidelberg; and
Houben-Weyl, 1961, 4th Ed., Vol. XIV/1, Georg Thieme, pp. 1010; and/or by
group transfer polymerization (see, Houben-Weyl, 1987, supra. Vol. E20,
pp. 15 3 -16 0 . Copolymer P can suitably be polymerized in bulk, in
suspension, in emulsion, or in solution.
In the case of radical polymerization, suitable initiators include
peroxide compounds, particularly organic peroxides such as dibenzoyl
peroxide or lauroyl peroxide, peresters such as t-butyl perneodecanoate or t-
butyl per-2ethylhexanoate, perketals, azo compounds such as
azodiisobutyronitrile, or redox initiators. The initiators suitably are used
in
amounts of 0.01-5 weight % (based on the total weight of the monomers).
Radical polymerization can alternatively be initiated by high energy
radiation. Suitable polymerization regulators include sulfur compounds such
as mercapto compounds, in amounts of 0.1-5 weight % (based on the total
weight of the monomers).
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The weight average molecular weight of colpolymer P is
suitably 2,000-1,000,000 Dalton, preferably 10,000 200,000
Dalton, particularly preferably 20,000-100,000 Dalton,
(determined, for example, by GPC).
The nonuniformity of copolymer P is suitably in the range
0.1-3. The nonuniformity is calculated according to the
formula:
U = Mw/Mn - 1,
where Mw is the mean molecular weight of copolymer P and Mn is
the number average molecular weight of copolymer P.
Preferably copolymer P contains 20-90 weight % of monome~c
units (p1) and 10-80 weight % of monomer units (p2) of the
formula (I), where R1 represents a C3_24 alkyl group, preferably
a Cf_ze alkyl group.
Preferably the relative proportion of monomer (p2) in
copolymer P decreases as the number of carbon atoms in R~
increases. Quantitatively, the proportion of monomers (p2) in
copolymer P may be expressed as follows (see German Patent 37
30 025 also U.S. Patent 4,952,455):
2 0 Molecular weight of monomer p1
weight% of monomer p2 ~ x 100
(Molecular weight of monomer p1,
+ Molecular weight of monomer pz)
Suitable monomers (p2) according to formula (I) are
methacrylic acid esters wherein R1 represents propyl, n-butyl,
isobutyl, amyl, isoamyl, n-pentyl, n-hexyl, n-octyl,
2-ethylhexyl, n-decyl, n-dodecyl, n-tetradecyl, n-hexadecyl,
n-stearyl or the alkyl group of a tallow fatty alcohol. R1 can
also represent a substituted or unsubstituted cycloalkyl group
such as cyclopentyl, cyclohexyl, or cycloheptyl. Suitable
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substituents include methyl, ethyl or butyl. Preferably R1 is
cyclohexyl.
Suitable comonomers (p3) are present in copolymer P in
amounts of 0-10 weight %, preferably 1-8 weight %. Suitable
comonomers (p3) include (meth)acrylic acid, salts of
(meth)acrylic acid, hydroxyalkyl esters of (meth)acrylic acid
(such as 2-hydroxyethyl (meth)acrylate, or 2-hydroxypropyl
(meth)acrylate), alkoxyalkyl esters of (meth)acrylic acid
(such as 2-butoxyethyl (meth)acrylate or 2-methoxyethyl
(meth)acrylate) and aminoalkyl esters of (meth)acrylic acid
(such as 2-dimethylaminoethyl (meth)acrylate,
2,2,6,6-tetramethyl-4-piperidyl (meth)acrylate, and
3-dimethylaminopropyl (meth)acrylate). Alternatively, (p3) is
styrene or a C1_zo ester of acrylic acid. Suitably, the
proportion of acrylic acid esters in copolymer P is limited to
< 5 weight %, preferably < 1 weight %, and particularly
preferably zero.
In accordance with the present invention, copolymer P
forms a compatible polymeric mixture with 'the impact resistant
polystyrene resin (PS). The polymeric mixture can be
characterized according to recognized criteria (see
Kirk-othmer, supra, Vol. 18, pp. 457-460; and Brandrup et al.
"Polymer Handbook", 2nd Ed., Vol. III, Wiley Interscience,
1975, p. 211). The compatible polymer mixture suitably hay a
single index of refraction and a single glass transition
temperature which is between th.e glass transition temperatures
of the two components, copolymer P and the polymeric hard
phase of the impact resistant polystyrene resin (PS).
l
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As a further indication of the compatibility of the
polymeric mixture, there is the occurrence of an LCST. This
phenomenon occurs when upon heating a clear, transparent
polymeric mixture separates into different phases and becomes
optically cloudy. This is unambiguous evidence that the
original polymer mixture comprised a single phase in
thermodynamic equilibrium (see, Paul, D.R., "Polymer Blends
and Mixtures°', Martinus Nijhoff, Dordrecht and Boston, 1985,
pp. 1-3). Although the polymer mixture of the present
l0 invention is not complete compatibility in the sense of a
polymer blend with only a single glass temperature which is
dependent upon composition, the plastic film of the present
invention adheres well to polystyrene substrates when the
composition of copolymer P is strictly observed. Further, the
total assembly can be recycled easily, as a consequence of the
good compatibility of the plastic film and the polystyrene
substrate which is to be sealed.
According to the invention, copolymer P can be 40-60
weight % methyl methacrylate and 60-40 weight % butyl
methacrylate. Alternatively, copolymer P can be 70 weight %
methyl methacrylate and 30 weight % n-decyl methacrylate.
Preferably, the number of carbon atoms in the substituent
RZ of monomer component (p2) should exceed the number of carbon
atoms in the methyl or ethyl substituent of monomer component
(pi) by >_ 2, particularly preferably >_ 3. Preferably,
copolymer P comprises 50 weight % methyl methacrylate and 50
weight % butyl methacrylate with a J-value of 15-70 mi/g,
preferably J = 20-40 ml/g. (See D1N 51 562 for the
N
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determination of the J-value, which is performed in chloroform
at 25°C and is a measure of the molecular weight of the
copolymer) or approximately 50 weight o ethyl methacrylate and
approximately 50 weight o butyl methacrylate with a J-value of
20-50, preferably 25-40 ml/g.
Preferably the sealing layer comprises polymeric mixtures
comprised of 20-90 weight % of the above-described copolyiaer P
and 80-10 weight % of one or more block copolymers comprised
of at least one block of one or more monomers selected from
styrene, c-methylstyrene, and alkyl substituted styrene, and
at least one block selected from isoprene and butadiene. The
block copolymers can be linear, branched, or star-shaped.
Particularly preferred block copolymers in the polymeric
mixture comprise 2 or more polystyrene blocks such as linear
styrene-butadiene-styrene (SBS) 3-block copolymers, and radial
or star-shaped SBS block copolymers. Particularly preferred
are SBS block polymers which contain at most 50 weight % of
styrene blocks; and still more preferred are linear SBS
tri-block copolymers having a 'outadiene content of
approximately 7o weight %.
In additian to the abovementioned block copolymers with a
high butadiene content, styrene-butadiene block copolymers
with butadiene contents of < 50 weight % can also be present
in the polymeric mixture, in fact as low as 15 weight %,
particularly 30-18 weight ~:
Polymeric mixtures of the abovementioned SBS block
capolymers with copolymer P are much more impact resistant
than copolymer P itself. Thus, such mixtures display high
~l~fi~~2
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extensibility and tear strength. The mixtures comprising
block copolymers and copolymer P on impact resistant support
materials (PS) can be sealed to impact resistant polystyrene
at low temperatures (e. g. 140-200°C). Particularly
advantageous is the good behav~.or of these mixtures as
sealants when the plastic containers sealed with them are
opened. Plastic containers sealed with them can be opened
smoothly and compliantly (peeled) without jerking action.
Further, the flow behavior of the block copolymer -
copolymer P mixture matches that of the impact resistant
support material, so that simple coextrusion of the sealing
layer on the support is possible.
For example, if one starts with a low molecular weight
copolymer P (such as a copolymer of 50 parts methyl
methacrylate and 50 parts butyl methacrylate, with J = 23
ml/g), then the torque measured for this material in a
measuring kneader at 190°C is close to 0 N-m. By mixing 50
parts of this extremely flowable material with 50 parts of
CARTFLEX~R~ TR 1102 (an SBS block copolymer with approximately
70~ butadiene which is available from Shell), one obtains a
flowable coextrusion compound which gives a torque on the same
measuring kneader of 3.1 N-m.
If the proportion of the SBS block copolymer is
inoreased, the torque is increased (for example, torque 4.6
N-m for a mixture of 70 parts C.~RIFLEX~R~ TR 1102 and 30 parts
of a copolymer of 50 parts methyl methacrylate units and 50
parts butyl methacrylate units, with J -- 23 ml/g).
_18_
In general, mixtures of 75-25 parts of block copolymers)
and 25-75 parts copolymers) (P) are used. Particularly
interesting are mixtures in the weight ratio range 65:35 to
35:65.
Tn addition to the above-described SBS block copolymers
with high (> 50 weight %) butadiene content, block copolymers
with, 15-40 weight % butadiene can be used. These latter are
generally highly transparent, impact resistant polystyrene
molding compounds. These block copolymers are also very
suitable to use in the sealing layer for modifying the
rheology of the copolymers. In general, however, the
pulling-away (peeling) behavior (upon opening) of containers
sealed with mixtures with SBS block copolymers having
butadiene content > 50% is better than that for containers
sealed with these sealing layers. Preferably the sealing
comprises at least 3 components, (i) copolymer P (in the
amount of 45-65 weight %), (ii) the block copolymers with
butadiene content > 50 weight percent (in the amount of 10-35
weight %), and (iii) the block copolymers with butadiene
content 15-50 weight percent (in the amount of 10-35 weight
%). The weight average molecular weights of the SBS block
copolymers used are in the range 50,000-500,000 Dalton,
preferably 80,000-300,000 Dalton, and particularly preferably
100,000-250,000 Dalton. The melt-flow index of the SBS-block
copolymers (without addition of copolymers) is generally in
the range 4-20 g/10 min, preferably 5-10 g/10 min, at 200°C
(for 5 kg).
2~~6~~Z
Beside these particularly preferred styrene-butadiene
block copolymers, the corresponding block copolymers based on
isoprene, or the corresponding hydrogenated block copolymers
comprising styrene-(ethylene-butylene)-styrene block
copolymers and/or styrene-(ethylene-propylene)-styrene block
copolymers, can also be employed. The synthesis of the
styrene-butadiene or styrene-isoprene block copolymers is
carried out in general by means of anionic polymerization (see
Houben-Weyl, supra, 4th Ed., Vol. E20/2, p. 989), usually with
alkyllithiums as initiators.
In addition to the above-mentioned modification of the
copolymers by mixing with block copolymers based on SBS,
modification is possible by means of elastomer-copolymer graft
products. Of particular interest are emulsion polymers with a
core-and-shell structure wherein a shell comprised of
copolymer is at least partially grafted owto an acrylate
rubber (such as butyl acrylate crosslinked with allyl
methacrylate). Such impact strength-modified copolymers with
an elastomer content of 1-65 weight %, preferably 10-50 weight
%, are suitable for use as such or in a mixture with other
copolymers, as the sealing layer. Also the coextrudability,of
the sealing layer can be improved in this manner (see
Examples).
Sealing layers produced in this manner and with this
composition have high flowabilities and can be used to seal a
suitable substrate (as a rule, polysty~°ene) at quite law
temperatures (130-200°C).
~1~~~N~
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The plastic films of the present invention comprise at
least two mutually adhering layers and can be fabricated by
conventional methods, for example, by extrusion of the
individual layers followed by lamination of the layers
together (see Mark, H.F., et al., 1988, supra, Vol. 11, pp.
269-271, and Vol. 4, p. 816; 1988, '°Ullmann's Encyclopedia of
Industrial Chemistry", supra, Vol. All, pp. 85-111; and
Hensen et al., 1986, "Handbuch der
Kunststoff-Extrusionstechnik°', Vol. II
("Extrusionsanlagen'°),
l0 pub. Carl Hanser).
The thickness of the film is suitably in the range 80-500
micron, preferably 120-350 micron. The thickness of the
support layer is suitably 5-495 micron, preferably 50-300
micron. The thickness of the sealing layer is suitably 2-495
micron, preferably 5-100 micron. The particularly preferred
sealing layer comprises a mixture containing 20-90 weight % of
the impact resistant polystyrene resin (PS), which preferably
is one of the above-mentioned styrene-containing block
copolymers, and has a thickness of 5-495 micron, preferably
20-150 micron. When the sealing layer comprises 100 weight
of copolymer P, the thickness of the sealing layer is
preferably 2-100 micron, particularly preferably 4-40 micron.
If the thickness exceeds 100 microns, the sealing layer can
become brittle and separate from the suppart layer.
Coextrusion is a particularly suitable method for producing
the heat-sealable plastic films of the present in~rention,
because in general the difference in flow behavior between the
2~~6~2~
-21-
impact resistant polystyrene resin forming the support layer
and the sealant containing copolymer is very small.
For this reason, very uniform layer thicknesses can be
attained for the support layer and sealing layer, with the
desired thicknesses. In particular, for a thin sealing layer
< 20 micron, it is necessary to have similar flow behaviors
for both the support layer material and the sealing layer
material for homogeneous application of the sealing layer.
The sealing layer can also be applied to the support
material by lacquering techniques. In such cases, care must
be taken such that the solvent of copolymer P only dissolves
the support layer on the surface when the lacquer formulation
is applied: In other words, the solvent of copolymer P is a
weak solvent for the impact resistant support materials(PS).
In this instance, pure copolymer P can also be used for the
sealing layer.
The plastic films of the present invention can be heat-
sealed without problems (see Stehle, G., "Neue Verpackung",
supra), and have goad processing reliability. The plastic
films are deep-drawable, stampable, punchable, and pressable.
They can be successfully colored by the conventional
coloration methods for plastics (see Becker-Braun, 1990,
"Kunststoff-Handbuch", Vol. 1, Carl Hanser, pp. 539-540).
The plastic films are particularly advantageously used
for sealing plastic containers, especially containers
comprised of polystyrene and impact-strength-modified
polystyrene. Plastic containers sealed with the plastic films
according to the present invention satisfy very well the
2~~54~2
-22-
above-stated requirements for mechanical and chemical
stability, thermal behavior, and processibility. The sealing
conditions (such as the temperature of the sealing coating, or
the pressure) can be varied within wide limits.
Of particular interest is the fact that the films are
easy to seal, allowing sealing in 0.5 sec at 140°C, even using
a film 100 micron thick. Thicker films require
correspondingly higher temperatures or longer sealing times.
This is principally a consequence of the extremely good
heat-sealability of the sealant.
The low sealing temperatures frequently render
unnecessary an antiblock coating to impede adhesion of the
film to the hot sealing head.
In general, however, it is advantageous if the plastic
film contains an additional protective layer which impedes
baking to the sealing head. This layer can be comprised of
the protective coating material used to mark the container; or
the plastic film can have an antiblock layer which is 2-50
micron, preferably 5-20 micron thick, comprised of a high
melting plastic which does not adhere to the sealing head at
temperatures up to 200°C (preferably up to 250°C). The
additional protective layer is preferably applied on the face
associated with the support material. Suitable high melting
plastics include polyamides (such as Polyamide 6), or
polyterephthalic acid esters (such as polybutylene
terephthalate), or impact-strength-modified polyphenylene
ethers (PP~s), or any polymers with softening point > 160°C.
2~~~4~2
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Further, the plastic film can contain additional layers,
such as an intermediate layer between the support and sealing
layers, which preferably comprise recycled film material.
A particular advantage of the inventive multilayer
plastic films, however, is that the individual layers of the
film adhere to each other directly without any primer.
In addition, the plastic film wastes can be entirely
recycled, for example recycled film material can generally be
used directly as a component in the production of the support
layer or the sealing layer. If the sealing layer S is
suitably thin, recycle proportions of up to 100% are possible.
Further, the following properties should be noted:
--The films are suitable for stamping, punching, and
pressing;
--In the case of punching,,the wastes can be reprocessed
to produce new films;
--The films accept printing;
--The plastic films, and if necessary the support films,
can be produced to have high impact strength, so as to be
usable under high load-bearing and high stacking conditions;
--Copolymer P, the impact resistant polystyrene resin
(PS), and the (optionally impact resistant) polystyrene of the
container are completely compatible, so that one can recycle
the containers and covers together;
--The inventive films can be sealed directly to
polystyrene. In general, the cover films are sealed to
containers comprised of impact resistant polystyrene, which
is, as a rule, extrusion polystyrene, such as VESTYRC~N~R~ 638.
~1~6~~~~
-24-
Frequently the impact resistant polystyrenes used for the
containers contain additional highly transparent polystyrene,
for example, a container can be produced from a mixture of
VESTYRON~R~ 638 and VESTYRON~R~ 224 (both products of Huels AG);
and
--The films can be sealed on apparatus customarily used
for heat sealing (examples of conditions: sealing pressure > 2
bar, time 0.1-2 sec, sealing temperature 130-220°C).
Preferably sealing heads are used which have a coating of
Teflon or another material which impedes blocking. If the
sealing film itself is provided with a non-blocking final
coating or has an antiblock layer Z, one can employ a metallic
sealing head, such as an aluminum sealing head.
The inventive films can be adjusted such that punching
wastes or other residues of the film are used in their
entirety to produce a new plastic film, or a new support film;
in the latter case the sealing layer must be comprised only of
fresh material, because the support and sealing layers are
miscible in arbitrary proportions. Obviously, all of the
components of the plastic film which are subject to come into
contact with foods have minimum contents of residual monomers
and other components which can detract from the usability of
a
the f i lm .
Having generally described this invention, a further
understanding can be obtained by reference to certain specific
examples which are provided herein for purposes of
-25-
illustration only and are not intended to be limiting unless
otherwise specified.
EXAMPLES
Example 1: Production of a copolymer P1 by bulk
polymerization:
1.5 g t-butyl perneodecanoate and 0.5 g t-butyl
per-2-ethylhexanoate were added to a mixture of 500 g methyl
methacrylate, 500 g butyl methacrylate, and 10 g dodecyl
mercaptan. The mixture was charged to a plastic jar
(HOSTAPHAN «~, produced by Hoechst AG), and was polymerized in
a water bath far 24 hours at 45°C, followed by 10 hr at 80°C.
The resulting copolymer block was comminuted in a mill,
and the mill granules were then further comminuted, and
degassed, in an extruder.
A highly transparent, very readily flowing copolymer was
obtained. ,7 = 23 ml/g.
Example 2: Production of a copolymer P2 by emulsion
polymerization:
Tn a 100 L stirred, heated vessel under a protective gas
(argon), 15 kg methyl methacrylate, 15 kg butyl methacrylate,
and 100 g 2-ethylhexyl thioglycolate were emulsified in a
solution of 150 g sodium tetradecylsulfonate in 44 kg water.
The mixture was then heated to 30°C. 6 g of a 1% iron(II)
sulfate solution and 15 g ammonium peroxydisulfate (dissolved
in 1000 g water) were added, and the polymerization was then
started by addition of 3 g sodium bisulfite (dissolved in 100
g wader): When the polymerization temperature reached its
maximum point (c. 70°C), stirring was continued an additional
210~42~
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1 hr at 70°C. Then the mixture was cooled to room
temperature, under stirring.
Examples 3 to 8: Production of copolymers P3 to P8 by bulk
polymerization:
The polymerization was performed using the method described in
Example 1, except that 500 g ethyl methacrylate and 500 g
butyl methacrylate was used.
Dodecyl mercaptan was used as a regulator. In Examples
3-8 the concentration of the regulator was varied, thereby
changing the J-value (see Table 1).
Table 1:
Example Amount of J (ml/g)
(Polymer no.) dodecyl mercaptan (g)
3 (P3) 3 51
4 (P4) 4 4Z
5 (P5) 5 36
6 (P6) 7 2g
7 (P7) 8 25
8 (P8) 9 23
In each case a highly transparent polymer was obtained
which was comminuted to a powder.
Example 9: Production and characterization of two--phase,
heat-sealable plastic films KF1 and KF2:
A 100 micron thick wet film of a solution of 10 g of a
methyl methacrylate-butyl methacrylate copolymer P1 according
to Example I in a mixture of 79.5% 2-propanol, 20% acetone,
and 0.5% 4-hydroxy-4-methyl-2-pentanone was applied with the
aid of a grooved doctor to a 250 micron thick impact resistant
polystyrene film (VESTYRON~R~ 624-30, available from Huels AG)
containing 10 weight % of the additive CARIFLEX~R~ TI3 1102.
~i06~~~2
-27-
The result was a 260 micron thick plastic film KF1 with a
sealing layer 10 micron thick.
This film was used to seal polystyrene strips 1.5 cm wide
(sealing surface 1.5 x 1.0 cm, sealing pressure 2.5 bar). The
following sealing result was obtained, which was dependent on
temperature and sealing time:
Sealing temperature 220°C
Sealing time 0.5 sec
Good seal strength.
Also, a 120 micron thick impact resistant polystyrene
film (VESTYRON'R' 624-30, containing 10 weight % of the
additive CARIFLEX'R' TR 1102) was provided with a 10 micron
thick layer of copolymer P1 of Example 1, by the method
described above. The result was a film KF2 of total thickness
120 micron which could be sealed to impact resistant
polystyrene even at low temperatures, because of its thinness.
Sealing pressure 2.5 bar.
Sealing temperature 180°C.
Good seal strength (peeling strength 6.8 N).
Example 10:
The copolymer P8 according to Example 8, (comprised of,50
parts ethyl methacrylate and 50 parts butyl methacrylate -(,I =
23 ml/g)) was dissolved in a mixture of 80% 2-propanol, 15%
acetone, and 5% 4-hydroxy-4-methyl-2-pentanone, and applied,
with the aid of a grooved doctor, as a 10 micron thick
copolymer layer on a 100 micron thick polystyrene film
(VESTYRON~R' 624-30, containing 10 weight % of CARIFLEX'R' TR
1102) (see also Example 9). The result was a 120 micron thick
2:~~~42~
-28-
plastic film which could be sealed to a polystyrene strip
(sealing surface 1.5 x 1.0 cm) with good seal strength, at
160°C, pressure 2.5 bar, and time 0.5 sec. (Peeling strength
was c. 6 N.)
Example 11:
A 10 micron thick layer of a copolymer P1 (Example 1) was
applied to a 140 micron thick polystyrene-copolymer blend film
as a support material, by the method of Example 9.
Composition of the film material:
-- 75% K-~2ESIN~R~ KR05, a product of Phillips Petroleum
Co., Bartlesville, OK 74004, U.S.A., and
-- 25% copolymer P3 (Example 3), which is a copolymer
comprised of 50 weight % of ethyl methacrylate and 50 weight
of butyl methacrylate, J = 51 ml/g.
The result was a composite film which could be sealed
within 0.5 sec at 180°C and 2.5 bar.
Example 12: Production of an impact-strength-modified
copolymer P9 by two-stage emulsion
polymerization:
The following monomer emulsions in water with 0.3% Na
salt of tetradecanesulfonic acid were charged successively to
a heated polymerization vessel equipped with a stirrer,
thermometer, and reflex condenser (according to U.S.
4,613,118):
-- 200 g of a mixture of 99.4% methyl methacrylate and
0.6% allyl methaarylate;
-- 400 g of a mixture of 98% butyl acrylate and 2% allyl
methacrylate; and
CA 02106422 2003-02-19
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-- 400 g of a mixture of 49.5% ethyl methacrylate and 49.5% butyl
methacrylate, along with 1 % dodecyl mercaptan. A three-phase emulsion
polymer resulted.
Following cooling, a dispersion was obtained with solids content 50%.
The dispersion particles had mean diameter 398 nm.
To recover the polymer solid, the dispersion was freeze-coagulated by
freezing at -16°C and thawing in warm water. After washing, filtering,
and
drying, a solid was obtained which could be processed in a measuring kneader
(HAAKE RHEODIVE 5000* ) at 200°C (torque 4.0 N-m).
Example 13: Production of a coextrudable sealant S 1:
750 g of the impact-strength-modified copolymer P9 obtained
according to Example 12 was mixed with 250 g of the low molecular weight
copolymer P6 according to Example 6, and these were granulated together,
and degassed. The sealant S obtained was appreciably more flowable than the
sealant according to Example 12 (torque 2.2 N-m at 200°C).
Example 14: Production of a coextruded sealant S2:
50 parts of the multistage emulsion polymer P9 obtained according to
Example 12 were mixed with 50 parts of the low molecular weight copolymer
according to Example 6, resulting in a yet more flowable sealant (torque
0.6 N-m at 200°C).
Example 15: Production of a two-layer sealing film by coextrusion:
In a coextrusion apparatus a 10-20 micron thick layer of the
coextrudable sealant S2 according to Example 14 was produced on a 200
micron thick layer of an impact resistant
* Trade-mark
21~~~2~
-30-
polystyrene (mixture of 90 parts VESTYRON~R~ 624-30 and 10
parts CARIFLEX~R~ TR 1102).
Product dimensions:
Length 50 m, wound.
Width 120 mm.
Thickness 210-220 micron.
Main extruder material: Mixture of
90 parts VESTYRON~R~ 624-30 and
parts CARIFLEX~R' TR 1102.
10 Main extruder temperature 200°C (nozzle).
Coextruder material: Coextrusion compound according to
Example 14.
Coextruder temperature 220°C (nozzle).
Withdrawal speed 1.8 re/min.
A uniform coextrudate was obtained with a very uniform
distribution of the sealant S on the polystyrene support
Example 1G: Production of a two-layer sealing film by
coextrusion:
The procedure was as in Example 15, except that other
layer thicknesses were chosen:
Overall film thickness 300 micron.
Thickness of the sealing layer S 40-60 micron.
Example 17: Heat sealing tests with the sealing films
according t~ Examples 15 and 16:
Tine film according to Example 15 was stamped out into the
form of a yogurt cup cover and was sealed onto a yogurt cup
comprised of impact resistant polystyrene (300 ml, diameter 75
-31-
mm). A sealing temperature of 160 OC was needed for a
hermetic seal of the cup at 0.75 bar in 0.5 sac.
When the thicker film according to Example 16 was used,
under the same sealing conditions a sealing temperature of
180°C was needed.
Example ~.8: Production of a sealant S3:
50 parts by weight (pbw) CARIFLEX~R~ TR 1102 and 50 pbw
of the copolymer P1 according to Example 1 were mixed at
190°C. A torque of 3.0 N-ra was measured.
Example 19: Production of a two-layer film with improved
peeling behavior after sealing to impact
resistant styrene:
The sealant S3 obtained according to Example 18 was
pressed onto a 60-70 micron thick film at 200°C. Then the
thus obtained film was pressed onto 130 micron thick films
comprised of impact resistant polystyrene (VESTYRON~R~ 624-30,
containing l00 of CARIFLEX~R~ TR 1102). The two-layer film
produced had good handling properties.
Polystyrene strips 1.5 em wide (sealing surface
1.5 x 1.0 cm) were sealed with this film. Sealing pressure
was 2.5 bar. At a sealing temperature of 180°C and sealing,
duration 0.5 sec, a good seal was obtained. Particularly-
noteworthy was the good peeling behavior when the seal was
opened.
Example 20: Production of a two-layer film with improved
peeling behavior after sealing to impact
resistant polystyrene:
J
-32-
30 pbw CARIFLEX~R~ TR 1102 and 70 pbw copolymer P5
according to Example 5 were mixed in a kneader at 191°C.
Torque was 0.9 N-m. This flowable mixture was pressed at
190°C onto a 120 micron thick film, as described in Example
19. Then a 130 micron thick film of impact resistant
polystyrene (VESTYRON~R' 624-30, containing 10% of CARIFLEX~R' TR
1102) was pressed on. The result was a two-layer film with
goad handling properties which could be sealed to impact
resistant polystyrene. This seal also had good, compliant,
elastic peeling behavior upon opening.
Example 21: Production of a two-layer film:
In a coextrusion apparatus according to Example 15, a
sealant mixture S4 comprised of
30 weight % CARIFLEX~R' TR 1102;
50 weight % copolymer according to Example 1 (with a
higher J-value of 25 ml/g); and
weight % K-RESIN~R' KK38 (product of Phillips
Petroleum)
was coextruded onto a polystyrene block copolymer (K-RESTN~R'
20 KK38) as a support material T. The thickness of the
coextrusion layer was 20 micron. Total thickness of the film
was 280 micron.
Covers were punched out of this film. These covers ware
sealed onto 200 ml polystyrene cups (diameter 75 mm), at
pressure 0.8 bar, temperature 180°C, in 0.5 sec. The seal on
the cups was good. This seal also had good, compliant,
elastic peeling behavior upon opening.
* * * *
21fl6~~t z
-33-
Having now fully described the invention, it will be
apparent to one of ordinary sill in the art that many changes
and modifications can be made thereto without departing from
the spirit or scope of the invention as set forth herein.
