Note: Descriptions are shown in the official language in which they were submitted.
583-181-0
TITLE OF THE INVENTION:
HEAT-SEALABLE PLASTIC FILMS
Field of the Invention:
The invention relates to heat-sealable plastic films
comprising a mixture of an impact resistant plastic
polystyrene resin and a methacrylate copolymer. These
films are useful for hermetically sealing plastic container
such as polystyrene containers. In particular, the plastic
films are suitable for sealing foodstuffs into containers
for storage.
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
Encyclopedia of Industrial Chemistry", 25th Ed., Verlag
_106454
-2-
Chemie: Weinheim, 1985, pp. 523-560 and 583-618; the
applicable standards are also discussed therein).
Previously gastight seals composed 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 adjoining the aluminum layer typically
comprises polyethylene, ethylene copolymers, or
polypropylene (Stehle, G. (1991) Neue Verpackung,
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, gas tight 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
_2106454
-3-
PVC has good mechanical strength and good barrier
characteristics with regard to gas permeability.
Customarily an acrylic resin is used as a 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, and a
lubricant, possibly applied to a support layer.
European Patent 0 406 681, 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 10 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
..2106454
-4-
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 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
v
_5_ 2106454
polystyrene-compatible methacrylates comprising a molding
compound with a two-phase structure. The grafting
branches, and the engrafted 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 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).
CA 02106454 2004-02-26
-6-
The present inventors have now found that this object
can be achieved by a heat-sealable plastic film comprising
a mixture of 1-99 parts by weight (pbw) of an impact-
resistant polystyrene resin and 99-1 pbw of a copolymer P
consisting essentially of:
p1) 20=90 weight % methyl methacrylate ethyl
methacrylate or a mixture thereof;
p2 ) 10- 8D weight % of at least one monomer of formula
(I)
I H3
CHZ = C - C - O - Rl ( I ) ,
where Rl represents a C3_z4 alkyl group; and
p3) 0-10 weight %, preferably 1-8 weight %, of a
monomer which is copolymerizable with and different from
the monomers (pl) and (p2).
Preferably, the impact resistant polystyrene resin
(PS) is present in the heat-sealable plastic film in
proportions of 20-80 weight %. The impact resistant
polystyrene resin preferably comprises polymers such as
styrene-butadiene-styrene block copolymers which improve
the impact strength of the plastic films and improve their
processibility in an extruder. The plastic films suitably
have thicknesses between 50 and 500 micron, preferably 80
and 400 micron. Optionally the plastic film also has a
cover layer which prevents adhesion of the sealing film to
the sealing head.
2'06454
-
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 vinylaromatic
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 %, of
the total weight of the two-phase polymer mixture.
Suitable vinylaromatic monomer units useful in the
hard phase include styrene, a-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.
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.
CA 02106454 2002-10-23
-8-
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-Othmer,
"Encyclopedia of Industrial Chemistry", 3rd Ed., Vol. 8,
J. Willey, 1979, pp. 492-500 and Vol. 7, pp. 687 and 693.
Cesca, S., J. Polym. 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, 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
.. 210s~~~
-9-
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 %, 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).
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. (For synthesis of block polymers of styrene and a
second monomer, see, for example, Houben-Weyl, "Methoden
der organischen Chemie", 4th Ed., Vol. E20 Part 2, Georg
Thieme: Stuttgart, 1987, pp. 987-993). Preferably,
styrene-butadiene-styrene 3-block copolymers or star-shaped
e~Z1o6454
-10-
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 l0-30 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.
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
CA 02106454 2002-10-23
-11-
micron, wherewith the polybutadiene content of this
polystyrene resin containing particles of the impact
resistant phase is in general 7-15 weight o, preferably
8-11 weight o (based on the total weight of the polystyrene
resin). The polystyrene 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.
Suitably copolymer P consists essentially of the
above-mentioned components (pl), (p2), and (p3). That is,
the sum of the monomer units (pl), (p2), and (p3) is 100
weight o. Copolymer P is suitably formed from the monomers
(pl), (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. 153-160.
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,
-2los~~~
-12-
peresters such as t-butyl perneodecanoate or t-butyl per-2-
ethylhexanoate, perketals, azo compounds such as
azodiisobutyronitrile, or redox initiators. The initiators
suitably are used i.n 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).
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 weight average molecular weight of
copolymer P and Mn is the number average molecular weight
of copolymer P.
Preferably copolymer P contains 20-90 weight % of
monomer units (pl) and 10-80 weight % of monomer units (p2)
of the formula (I), where R1 represents a C3_z4 alkyl group,
preferably a CQ_18 alkyl group.
~'~Of454
-13-
Preferably the relative proportion of monomer (p2) in
copolymer P decreases as the number of carbon atoms in R1
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):
Molecular weight of monomer pl
weight% of monomer p2 = x 100
(Molecular weight of monomer pl
+ Molecular weight of monomer p2)
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 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
-2'06454
-14-
(meth)acrylate, and 3-dimethylaminopropyl (meth)acrylate).
Alternatively, (p3) is styrene or a C1_2o 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
a1. "Polymer Handbook", 2nd Ed., Vol. III, Wiley
Interscience, 1975, p. 211). The compatible polymer
mixture suitably has a single index of refraction and a
single glass transition temperature which is between the
glass transition temperatures of the two components,
copolymer P and the polymeric hard phase of the impact
resistant polystyrene resin (PS).
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
_,~~p0645~
-15-
of the present 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 ml/g, preferably J = 20-40 ml/g. (See DIN
51 562 for the 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 % ethyl methacrylate and approximately 50 weight
butyl methacrylate with a J-value of 20-50, preferably
25-40 ml/g.
210454
-16-
Preferably the polymeric mixtures comprise 20-90
weight % of the above-described copolymer 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, a-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
butadiene content of approximately 70 weight %.
In addition to the above-mentioned block copolymers
with a high butadiene content, styrene-butadiene block
copolymers with butadiene contents of < 50 weight % may
also be present in the polymeric mixture.
Mixtures of the above-mentioned SBS block copolymers
with copolymer P are much more impact resistant than
copolymer P itself. Thus, such mixtures display high
extensibility and tear strength. Mixtures comprised of
block copolymers and copolymer P can be sealed to impact
resistant polystyrene at low temperatures (e. g. 140-200°C).
s ~'~~6454
-17-
Particularly advantageous is the good behavior of these
mixtures as sealants when plastic containers sealed with
them are opened. Plastic containers sealed with them can
be peeled opened smoothly and compliantly without jerking
action.
Suitably, mixtures comprised of 75-25 pbw of block
copolymers and 25-75 pbw of copolymer P are used.
Particularly preferred mixtures comprise block copolymers
and copolymer P in the weight ratio range 65:35 to 35:65.
In 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 copalymers.
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
CA 02106454 2002-10-23
-18-
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).
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.
A preferred formulation is a block copolymer
comprlslng:
A) 20-80 weight o of styrene, and
B) 80-20 weight o butadiene, isoprene, or a mixture
thereof.
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-
CA 02106454 2002-10-23
-18a-
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
onto an acrylate rubber (such as butyl acrylate crosslinked
with allyl methacrylate). Such impact strength-modified
copolymers with an elastomer content of 1-65 weight %,
.g1os45~
-19-
preferably 10-50 weight %, are suitable for use as such or
in a mixture with other copolymers, as materials for the
plastic films.
Mixtures produced in this manner, and having the
described composition, have high flowabilities, and may be
used as plastic films to seal a suitable substrate (as a
rule, polystyrene) at low temperatures (130-200°C).
The plastic films according to the present invention
are comprised of the above-described mixtures which contain
at least 1 weight %, preferably at least 10 weight %,
particularly preferably at least 20 weight %, of impact
resistant polystyrene resin, such as, SBS block copolymers.
They can be fabricated by conventional methods, for example
by extrusion.
Plastic films in accordance of the present invention
suitably have thicknesses of 50-500 micron, preferably
80-400 micron.
Particularly suitable proportions of block copolymers
in the polymeric mixtures are 20-90 weight %, preferably
30-80 weight %, particularly preferably 35-65 weight %, for
the heat-sealing plastic films.
The plastic films according to the present invention
can be heat-sealed without problems (see Stehle, G., "Neue
Verpackung", supra) and have good processing reliability.
The plastic films are deep-drawable, stampable, punchable,
and pressable. They can be successfully colored by the
21oe~5~
-20-
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 covers
comprising the plastic films of the present invention
satisfy the above-stated requirements for mechanical and
chemical stability, thermal behavior, and processibility.
The sealing conditions (for example, the temperature of the
sealing coating, or the pressure) may 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 mixtures.
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 to employ an antiblock coating if the plastic
film contains an additional protective layer which impedes
baking to the sealing head. Often this layer comprises a
protective coating material used to mark the container.
~1~6~54
-21-
Suitably, 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). suitable antiblock coatings comprise high
melting plastics such as polyamides (such as Polyamide 6),
or polyterephthalic acid esters (such as polybutylene
terephthalate), or impact-strength-modified polyphenylene
ethers (PPEs), or in general polymers with softening point
> 200°C.
Plastic films of the present invention have the
following properties:
-- 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 are suitable for printing;
-- The plastic 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, and the (optionally impact resistant) polystyrene of
the container are completely compatible, so that one may
recycle the containers and covers together;
-- The inventive films can be sealed directly to
polystyrene. In general, the plastic films are sealed to
~1os454
-22-
containers comprising impact resistant polystyrene, which
is as a rule an extrusion polystyrene, such as VESTYRON'R'
638. Frequently the impact resistant polystyrenes contain
additional highly transparent polystyrene, such a container
may be produced from a mixture of VESTYRON'R' 638 and
VESTYRON'R' 224 (both products are available from Huels AG);
and
-- The films may 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, one may employ a
metallic sealing head, such as an aluminum sealing head.
The inventive films may be adjusted such that punching
wastes or other residues of the film are used in their
entirety to produce a new plastic film.
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 the film.
Having generally described this invention, a further
understanding can be obtained by reference to certain
specific examples which are provided herein for purposes of
~10045,~
-23-
illustration only and are not intended to be limiting
unless otherwise specified.
EXAMPLES
Example 1: Production of a copolymer pi by
polymerization in mass:
To a mixture of 500 g methyl methacrylate, 500 g butyl
methacrylate, and 14 g dodecyl mercaptan was added 1.5 g
t-butyl perneodecanoate and 0.5 g t-butyl
per-2-ethylhexanoate. The mixture was charged to a plastic
jar (HOSTAPHAN'R', available from Hoechst AG), and was
polymerized in a water bath 24 hr at 45°C, followed by 10
hr at 80°C.
The resulting copolymer was comminuted in a mill, and
the mill granules were then further comminuted, and
degassed in an extruder.
The product obtained was a highly transparent, very
readily flowing copolymer, J = 20 ml/g.
Example 2: Production of the mixture comprised of
copolymer pl and impact resistant polystyrene resin; and
production of the plastic film.
The copolymer pi according to Example 1, in the amount
of 45 weight %, was mixed in a drum mixer with 55 weight %
of an impact resistant polystyrene resin (CARIFLEX'R' TR
1102, available from Shell), and the resulting mixture was
then granulated twice with an extruder, and degassed. The
~,l ~ ~10645~
-24-
result was a white granulate which was extruded on a film
extruder at 200°C to form a film KF1 of thickness 270
micron and width 120 mm.
Example 3: Production of covers from the plastic film;
and heat-sealing of polystyrene cans.
From the plastic film produced according to Example 2,
covers were stamped, and were sealed to polystyrene cans
(purchased from Knauer, having capacity 200 ml, and cover
diameter 75 mm) at a sealing temperature of 180°C, a
sealing pressure of 0.8 bar, and a sealing time of 0.5 sec.
The cans were sealed well and had good opening
characteristics ("peeled" very well).
Also, strips of polystyrene (POLYSTYROL 466 J,
available from BASF AG) 1.5 cm wide were sealed with the
plastic films produced according to Example 2, 1.5 cm wide
(sealing surface 1.5 cm x 1.0 cm), at sealing temperatures
of 230°C sealing pressure 2.5 bar, and sealing time 0.5
sec. The thus sealed strips had a separation strength
(peeling strength) of 6.6 N.
Having now fully described the invention, it will be
apparent to one of ordinary skill 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.
