Process for the Preparation of Blown Film

BRIEF DESCRIPTION OF THE FIGURES

The invention is elucidated by reference to the appended figures of which

FIG. 1A is a schematic representation of X-ray scattering from a non-oriented film;

FIG. 1B is a photograph of X-ray scattering of non-oriented polyethylene;

FIG. 2A is a reprersentation defining the angle ".alpha.";

FIG. 2B is a representation of angle "a" at 0.degree..

FIG. 3 is a diagram of an X-ray photograph;

FIG. 4 is a graphic representation of changes in intensity during a densitometer scan as a function of diffraction angle;

FIGS. 5, 6 and 7 are photographs of X-ray scattering of the film of Examples 21, 22 and 23 respectively.

The degree and direction of orientation of a film can be determined by X-ray diffraction. This method yields information on the orientation of the crystalline fraction of the film. In non-oriented film the X-rays are scattered equally in all directions, resulting in annular bands without fluctuations in intensity. This is represented schematically in FIG. 1A, while FIG. 1B is a photograph of non-oriented polyethylene. In oriented polyethylene the scattering intensity is direction dependent, so that there are fluctuations in intensity. In polyethylene, the intensity is largest at a diffraction angle of the X-ray beam (2.theta.) of ca. 21.degree. (110 reflection of rhombic PE). The intensity fluctuations of this band are characteristic of the crystallite orientation and thus of the orientation of the crystallized molecules. The molecule direction is perpendicular to the associated intensity maximum. The average direction of orientation is characterized by the angle .alpha. which is the angle between the average orientation direction of the polymer molecules and the film extrusion direction (FIG. 2A). The more the average molecule direction and the direction of extrusion coincide, the nearer .alpha. will approach 0.degree. (FIG. 2B).

The distribution of the molecule direction (degree of orientation) manifests itself in the extensiveness of the intensity maximum. So the ratio intensity maximum: intensity minimum is an index of the degree of orientation.

FIG. 3 is a diagram of an X-ray photograph. X is the direction of the X-ray beam, 2.theta. is the diffraction angle of the X-ray beam, M is the direction of film extrusion (machine direction), .alpha. is the angle between the average direction of orientation of the polymer molecules and the direction of film extrusion.

It has now been found that films that have, when subjected to X-ray diffraction analysis, an intensity ratio max./min. of the 110 reflection band of at least 20 and an angle .alpha. between the average orientation direction of the polymer molecules and the film extrusion direction of at most 20.degree., have special mechanical properties, such as good tear resistance, tensile strength, high stiffness, etc. Especially films with an intensity ratio max./min. .gtoreq.30 and an angle .alpha..gtoreq.15.degree., and in particular .alpha..gtoreq.10.degree., possess these properties to a high degree.

A degree of orientation this high has not previously been found in known films on the basis of low-density polyethylene.

Polymer compositions that are suitable for being processed into blown film in accordance with the invention generally contain at least 50 wt. % polyethyene homopolymer and/or one or more copolymers with a density of 910-940 kg/m.sup.3. Low-density polyethylene homopolymer, copolymers of ethylene with one or more C.sub.3 -C.sub.15 alkenes, copolymers of ethylene and one or more polar comonomers with a comonomer content of at most 10 wt. %, relative to the ethylene (e.g. vinyl acetate, methacrylate) can be applied with good results. Also mixtures are suitable, like for instance mixtures of polyethylene homo- and/or copolymers with a density of 910-940 kg/m.sup.3, or mixtures of polyethylene homo- and/or copolymers having a density of 910-940 kg/m.sup.3 with low-density polyethylene copolymers (with a density less than 940 kg/m.sup.3), high density polyethylene (with a density in excess of 940 kg/m.sup.3) and or polypropylene. In these mixtures preferably at least 70 wt. %, relative to the total polymer content, of the homo- and/or copolymer with a density of 910-940 kg/m.sup.3 is present. For the preparation of films having good optical properties, the use of a polymer composition on the basis of polyethylene homopolymer or a copolymer of ethylene and one or more polar comonomers with a comonomer content of at most 10 wt. % relative to the ethylene, is to be recommended. The process according to the invention is particularly suitable for the preparation of films having good mechanical and optical properties from low-density polyethylene homopolymer or ethylene vinyl acetate copolymer.

Furthermore, various additives, such as stabilizers, slip agents, fillers, colorants, waxes and the like, may be present. The amount by weight of additives usually will not exceed 20% of the amount by weight of polymer.

The melt index of these polymer compositions, determined according to ASTM D 1238, may have the usual values, for instance between 0.3 and 30 dg/min. In view of the low processing temperature, however, it is to be recommended to choose a melt index of not lower than 2 dg/min. The favorable effect that low-temperature processing in combination with the large die gap has on the mechanical properties, decreases when the melt index exceeds 25 dg/min, and is most pronounced at a melt index of at most 20 dg/min, in particular at most 17 dg/min.

The advantage of the invention is that also polymer compositions having a relatively high melt index can be processed into blown film with good mechanical properties.

Often, packaging film must not only satisfy requirements with respect of its mechanical properties, but also requirements relating to its optical properties. The optical properties of the film improve when use is made of a polymer composition having a melt index of at least 5 dg/min, in particular at least 8 dg/min.

There already is advantage in applying the invention at die gaps of 1 mm, but larger die gaps, for instance of 5 or 10 mm, can also be applied; in addition, the geometry of the die may be varied. It has now been found that the mechanical properties of the film improve when the die gap used is at least 2 mm and at most 5 mm, in particular at least 3 mm, while the optical properties are also improved.

In combination with the low processing temperature, these larger die gaps result in even better mechanical properties, notably in good stiffness and tear properties.

In blown film extrusion, the polymer composition exits the annular die in molten condition and is subsequently blown up. The resulting balloon is cooled by contacting it with relatively cool air or water.

Balloon diameter divided by annular die diameter is called the blow-up ratio. This may vary between 1 and 6, but generally is 1.5 to 4. These values can also be applied in the process according to the invention, yielding good results.

The process according to the invention is suitable especially for the preparation of monolayer film, but can also be used for multilayer film.

The invention will now be elucidated on the basis of some examples.

Blow film was prepared under conditions as shown in Tables 1-3. Use was made of a Schwabenthan extrusion equipment with a screw diameter of 30 mm and a length/diameter ratio of 24:1. The diameter of the annular die was 5 cm.

The melt temperature at the die gap was determined by infrared measurement. The film output was held constant at 50 g/min. Of the film, the following properties were determined:

modulus of elasticity according to ASTM D 1922;

strength and tensile strength according to ISO R 527;

puncture resistance, determined from the energy required to puncture the film with a plunger at a rate of 50 mm/min;

notched tear resistance according to a method derived from DIN 53363, but with the notch at the center of the film, at the drawing rate of 2.5 cm/min;

gloss according to ASTM D 523;

opacity according to ASTM D 1003;

transparency according to the Electro Evans Ltd. method.

For examples 15 and 19-23, the X-ray diffraction measurement was performed using CuK.alpha. radiation (50 kV, 35 mA, Ni-filter) and a Statton camera. Parallel film radiation was employed (see FIG. 3). The sample-photograph distance 2 as 5 cm. Densitometry of the photograph as carried out along the diameters of the circular bands for the directions M and .alpha.. For one densitometer scan the changes in the intensity are recorded as a function of the diffraction angle of the X-ray beam (2.theta., FIG. 2 4) for densitometrical purposes use was made of an Enraf Nonius microdensitometer, model 1, and the method described in C.G. Vonk & A.P. Pipers, J. Appl. Cryst., 14, 8, (1981).

The correction for the amorphous background radiation is carried out as indicated in FIG. 4, taking into account the characteristic shape of the scattering curve for amorphous PE.

The shaded part of FIG. 4 represents the crystalline fraction of the 110 reflection band.