Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
2000-09-05
2004-08-17
Jackson, Monique R. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S349000, C428S484100, C428S515000, C428S516000, C428S910000
Reexamination Certificate
active
06777067
ABSTRACT:
The invention relates to a polypropylene film having improved barrier properties with respect to the passage of water vapor and improved mechanical properties.
The improvement in the mechanical properties of films, in particular of films for the packaging sector, has recently increased in importance. For cost and environmental reasons, the packaging industry desires ever thinner films which still pass through the machine as smoothly and have the same or improved barrier properties, in particular with respect to the passage of water vapor.
However, thinner films have over-proportionately impaired rigidity in the machine direction and consequently significantly worse machine running behavior, in particular in today's high-speed wrapping machines. In addition, the barrier properties likewise worsen over-proportionately with the reduction in film thickness. As a consequence of the poorer barrier properties of thin films, the protective action of the film against drying-out and spoilage of the contents is greatly restricted.
The increase in the modulus of elasticity (E-modulus) in the machine direction has been the subject of intense efforts for some time since this mechanical property is directly related to the technical suitability and thus determines the processing behavior directly.
The barrier action of BOPP films to water vapor (WVBA) and oxygen (OBA) decreases with the film thickness. In the usual thickness range of BOPP films (from 4 to 100 &mgr;m) there is, for example, an approximately hyperbolic relationship between the water vapor barrier (WVBA) and the thickness (d) (WVBA=const.). The constant depends essentially on the raw material composition and the stretching conditions. For BOPP packaging films in accordance with the prior art, the constant has a value of approximately: const.=28 g.&mgr;m/m
2
.d. The water vapor permeability here has been measured in accordance with DIN 53 122.
It is known that the modulus of elasticity of BOPP films in the machine direction can be increased either via the process technology or via raw material modifications or a combination of the two possibilities.
The modification of polypropylene films with various hydrocarbon resins is known from the prior art. A raw material modification of this type enables the production of polypropylene films whose mechanical strength in the longitudinal direction is significantly improved compared with films made from unmodified raw materials, but does not achieve the values of films which have been subjected to subsequent longitudinal stretching, and whose shrinkage in the longitudinal direction is likewise relatively high.
U.S. Pat. No. 4,921,749 (=EP-A-0 247 898) describes a heat-sealable BOPP film having improved mechanical and optical properties. The heat-sealability of the film and the water vapor and oxygen permeability are likewise improved. All the improvements result from the addition of a low-molecular-weight resin to the base layer. The resin content here is between 3 and 30% by weight. The resin has a molecular weight of significantly less than 5000, preferably less than 1000, and is, for example, 600. The softening point of the resin is from 120 to 140° C.
EP-A-0 645 417 describes a biaxially oriented polypropylene film whose n-heptane-insoluble content has a chain isotacticity index, measured by means of
13
C-NMR spectroscopy, of at least 95%. The base layer contains from 1 to 15% by weight of a natural or synthetic resin which has a softening point of from 70 to 170° C.
U.S. Pat. No. 5,155,160 describes the improvement in the barrier properties by the addition of wax to unoriented polypropylene films. The waxes described are paraffin waxes and polyethylene waxes having a molecular weight of from 300 to 800. The barrier action is said to be less than 0.2 g/100 square inches/24 hours.
There is a continuous demand for a further improvement in the water vapor barrier action of biaxially oriented packaging films made from polypropylene. None of the methods disclosed hitherto reduces the water vapor barrier action to the desired extent and all impair other essential film properties in an unacceptable manner.
The object of the present invention was therefore to provide a biaxially oriented polypropylene film which is distinguished by a good water vapor barrier action and has good mechanical properties. It must be possible to produce the film with reliable running and operation at production speeds of up to 400 m/min. Other physical film properties required in view of their use as packaging film must not be adversely affected. The film should have high gloss, no optical defects in the form of fisheyes or bubbles, good scratch resistance, fault-free running on high-speed packaging machines at low film thickness, and low film haze. In addition, the heat-sealing properties must not be adversely affected.
This object is achieved by a multilayer biaxially oriented polypropylene film comprising a base layer and at least one heat-sealable top layer, wherein the base layer comprises a combination of resin and wax, and the resin has a mean molecular weight Mw of from 600 to 1500, and the wax has a mean molecular weight Mn of from 200 to 700. The sub-claims indicate preferred embodiments of the invention.
The base layer of the film generally comprises at least 85% by weight, preferably from 85 to <100% by weight, in particular from 90 to 95% by weight, in each case based on the base layer, of propylene polymer.
In general, the propylene polymer comprises at least 90% by weight, preferably from 94 to 100% by weight, in particular from 98 to 100% by weight, of propylene. The corresponding comonomer content of at most 10% by weight or from 0 to 6% by weight or from 0 to 2% by weight generally consists, if present, of ethylene. The data in percent by weight in each case relate to the propylene homopolymer.
Preference is given to isotactic propylene homopolymers having a melting point of from 140 to 170° C., preferably from 155 to 165° C., and a melt flow index (measurement DIN 53 735 at a load of 21.6 N and 230° C.) of from 1.0 to 10 g/10 min, preferably from 1.5 to 6.5 g/10 min. The n-heptane-soluble content of the polymer is generally from 1 to 10% by weight, preferably 2-5% by weight, based on the starting polymer.
The molecular weight distribution of the propylene polymer can vary within broad limits, depending on the area of application. The ratio between the weight average molecular weight M
w
and the number average molecular weight M
n
is generally between 1 and 15.
In a preferred embodiment of the film according to the invention, the ratio between the weight average molecular weight M
w
and the number average molecular weight M
n
is from 2 to 10, very particularly preferably from 2 to 6. Such a narrow molecular weight distribution of the propylene homopolymer of the base layer is achieved, for example, by peroxidic degradation thereof or by preparing the polypropylene by means of suitable metallocene catalysts.
A measure of the degree of degradation of the polymer is the degradation factor A, which gives the relative change in the melt flow index, measured in accordance with DIN 53 735, of the polypropylene, based on the starting polymer.
A
=
MFI
2
MFI
1
MFI
1
=melt flow index of the propylene polymer before addition of the organic peroxide
MFI
2
=melt flow index of the peroxidically degraded propylene polymer.
In general, the degradation factor A of the propylene polymer employed is in the range from about 1 to 15, preferably from about 1 to 10.
Particularly preferred organic peroxides are dialkyl peroxides, where the term alkyl radical is taken to mean a conventional saturated, straight-chain or branched lower alkyl radical having up to six carbon atoms. Particular preference is given to 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide.
In a preferred embodiment of the invention, the polypropylene employed is highly isotactic. For highly isotactic polypropylenes of this type, the chain isotacticity index of the n-heptane-insoluble cont
Ernst Frank
Hansohn Robert
Schumann Detlef
Speith-Herfurth Angela
Dickinson Wright PLLC
Hoechst Trespaphan GmbH
Jackson Monique R.
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