DE2351923B2 - Heat sealable, oriented plastic film - Google Patents

Description

The invention relates to a heat sealable, oriented Plastic film made of an oriented polypropylene film, which on at least one surface with a heat-sealable layer made of an ethylene-containing copolymer, which together with the polypropylene film has been stretched in at least one direction and a method for the same Manufacturing.

In recent years, polypropylene films have been used extensively as wrapping and packaging materials because of their excellent clarity and strength, which are greatly improved when the film is stretched in one or more directions under stretch-orientable conditions. As a result, the molecular structure is at least partially oriented in the stretching direction. Under "stretch-orientable conditions" for polypropylene films is to be understood that the stretching measure is carried out when the film is kept within a temperature range between 2 ^C C below the crystalline melting point of the polypropylene and its transition temperature of the second order.

For effective packaging purposes, especially on high speed packaging machines, is

it is necessary that the polypropylene film at all times and

can be easily sealed to itself. While Polypropylene film is inherently heat sealable to itself and an oriented film will shrink

ϊ forms unwanted bumps or distortions in the Temperatures required to obtain a seal of adequate strength (i.e. about 16 (FC).

The invention is based on a plastic film of the introduction

ίο described type addressed with the appropriate heat seals can be achieved at a temperature at which there is insignificant heat distortion of the oriented polypropylene film takes place. The plastic film of the invention is characterized in that as a heat-sealable layer, a predominantly linear, disordered copolymer of ethylene and up to 12% another -olefin having at least 3 carbon atoms is used.

The term "polypropylene film" includes one

2ü film made of polypropylene, to which a small proportion of a compatible polyolefin, which does not significantly affect the physical properties of the film, is mixed in.

Although AU-PS 2 92 070 are heat-sealable,

r> oriented plastic films made from an oriented polypropylene film and one applied to it heat-sealable layer made of polyethylene, ethylene-propylene polyallomer and vinyl acetate-ethylene copolymers can exist, known.

In It becomes polyethylene because of its' desirable Heat sealing properties referred to as particularly suitable, which means that the properties of the Polyethylene can only be deteriorated if another monomer is incorporated into it.

ίο Also DE-OS 21 04 898, which consists of a composite film an oriented polypropylene film and a non-oriented film made from a random propylene-ethylene copolymer with an ethylene content of about 1 to 5% does not give any suggestion

■ do development of the plastic film of the invention.

The copolymer used in the plastic film of the invention is made by mixing the monomers can react under conditions under which the formation of linear chains of ethylene units with

4 ^r > few branches and a disordered distribution of the comonomers along the chains is favored, e.g. B. under catalytic low pressure conditions, such as those used in the polymerization of ethylene for the production of high density polyethylenes.

w Under these conditions the reaction takes place at relatively low pressures in the presence of a coordination or organometallic catalyst, e.g. B. a so-called Ziegler catalyst, a transition metal oxide catalyst or similar catalysts instead.

If the copolymer has greater than about 12 mole percent additional alpha-olefin content, it shows a tendency to become sticky. This leads to the fact that the plastic films difficult in the form of rolls and can be handled on film converting machines.

The proportion of the further alpha-olefin in the disordered copolymer is preferably in the range between 2 and 8 mol%, especially between

h5 2.5 and 5 mol% of the copolymer. The other Alpha-olefin can preferably be propylene or butene-1 be.

The copolymers of the invention are by one

Melting point below 135 ° C, determined by differential thermal analysis (D.TA.), characterized copolymers particularly suitable for the invention have DXA. melting points below 125 ° C. The D.TA. melting point is determined by means of a differ- ~> The scanning calorimeter from Perkin-EImer, Norwalk, Connecticut, USA, determines which one Has heating rate of 16 ° C per minute.

The heat sealable layer can be by means of any one of known method can be applied to the polypropylene film. You can z. B. a prefabricated film from the disordered copolymer by means of a suitable adhesive or adhesive on the polypropylene film layers. One can also put the prefabricated film or preferred films in intimate contact with it stretch the polypropylene film when the combination unites to form a single film One can also melt extrude the disordered copolymer onto one surface or both of a polypropylene film, followed by cooling before stretching the polypropylene film under stretch orientable conditions or is stretched.

The heat-sealable plastic film is preferably formed by coextruding the polypropylene film with a layer or two of the disordered copolymer 2 ^r > through a mold with a slit-shaped or ring-shaped opening, whereupon it is cooled, erased or injected flat (quenching) and then stretched under conditions capable of stretching or stretches. i <>

Since the disordered copolymer is compatible with polypropylene, it is possible to use a small amount of heat-sealable waste film returned to the extruder for the polypropylene film, o'jne that the properties of the film can be significantly impaired as a result.

The heat-sealable plastic film according to the invention can be placed twice, that is, to form a laminate having useful properties can be übereinandergelegl two films, where they are combined with w a copolymer on the other.

The laminate can be formed by allowing heat and pressure act or stretched, the two films or enforced if this is ^r in Oberflächenkon- 4> stroke are together. The invention also relates to a method for producing a heat-sealable, oriented plastic film by applying a heat-sealable layer of an ethylene containing v) the copolymer to at least one surface of an oriented propylene film, which layer has been stretched together with the propylene film in at least one direction, The process is characterized in that the copolymer layer is applied to the polypropylene film during the coextrusion of a melt of the copolymer and the polypropylene through a mold with a slit-shaped or ring-shaped opening, which is followed by a cooling process prior to stretching in the orientable state ω

The invention is explained in more detail below, reference being made to the examples.

example 1

A polypropylene melt was made at one speed of 91.44 m / min with outer layers of a melt of a substantially linear disordered Copolymers to form a three-layer molten laminate having a thickness of coextruded approximately 300 microns through a slot die. The polypropylene makes up 90% and each co-polymer layer 5% of the thickness. The copolymer has a D.TA. melting point of 114 ° C and a density of 0.940 on. It consists of ethylene, which under catalytic low pressure conditions contains about 2.5 mol% of butene-1 is copolymerized

After extrusion, the melted laminate is immediately quenched in cold water, is then heated to 100-115 ° C and around 20% stretched or stretched between a row of rollers in the working direction The rollers at the delivery end were rotated at a higher peripheral speed than the rollers at the entrance end.

The monoaxially stretched film was then placed in a hot air device where it was heated to about 150 ° C was heated and stretched in the transverse direction to eight times its original width became. The now biaxially oriented film was cooled at the edges and cut straight and into a roll wrapped The film had high clarity and was about 30 microns thick.

It could be shown that samples of the film seal with one another easily at 125 ° C. under a pressure of 0.98 kg / cm ² in one second, with a heat seal strength of 600 »/ 38 mm film width being achieved. Examination of the sealed strips showed that there was no warping of the film during the sealing operation.

Example 2

The procedure of Example 1 was repeated with the exception that the copolymer by a predominantly linear disordered ethylene-propylene copolymer was replaced, whose propylene content 4 Mol%. The copolymer was prepared under low catalytic pressure conditions and had a density of 0.930 and a D.TA. melting point of 115 ° C.

That of the film edges after the biaxial orientation, but before winding into a roll The cut waste was crushed, ground and returned to the extruder for the polypropylene melt.

The thickness of the biaxially oriented film was approximately 30 microns. The film could be easily sealed to itself at 118 ° C. under a pressure of 0.98 kg / cm ² in one second, giving a heat seal strength of 700 g / 38 mm of film width. The film did not warp during the sealing measures.

Example 3

A predominantly linear, disordered copolymer of ethylene and 1-butene, the 1-butene content of which is 3 Mol% was cast into a sheet with a thickness of 0.0763 mm. The copolymer was under catalytic low pressure conditions and had a density of 0.930 as well as a D.T.A. melting point of 117 ° C.

The cast copolymer film was then laminated onto a cast polypropylene film 1.016 mm thick, the assembly being heated to 140 ° C. and subjected to ^{a pressure of 17.5 kg / cm 2.}

The resulting laminate was then heated to a temperature of 160 ⁰ C and simultaneously stretched by 700% in two orthogonal alternate directions, wherein a biaxially orieniiertes laminate was obtained.

The laminate could be easily sealed with one copolymer side to the other with itself at 120 ° C under a pressure of 0.98 kg / cm ³ in one second to give a heat seal strength of 1000 g / 38 mm film width There was no warping of the film as a result of the heat sealing operations.

Claims (5)

Patent claims: 1. Heat-sealable, oriented plastic film made of an oriented polypropylene film, which on at least a surface: with a heat-sealable layer of an ethylene-containing copolymer is equipped which is stretched together with the polypropylene film in at least one direction has been, characterized in that the heat-sealable layer is a predominantly linear one disordered copolymer of ethylene and up to 12% of another at least 3 carbon atoms containing «olefins is used. 2. Plastic film according to claim 1, characterized in that the content of «-olefin des Copolymers in the range between 2 and 8 mol% Hegt 3. Plastic film according to claim 1 or 2, characterized in that the further α-olefin is propylene or butene-1. 4. Plastic film according to one of claims 1 to 3, characterized in that the D.TA. melting point of the copolymer is below 125 ° C. 5. A method for producing a heat-sealable, oriented plastic film according to claim 1, by placing a heat sealable Layer of an ethylene-containing copolymer applied, which together with the polypropylene film has been stretched in at least one direction, characterized in that the copolymer layer on the polypropylene film during coextrusion of a melt of the copolymer and the polypropylene is applied through a mold with a slot-shaped or ring-shaped opening, which is followed by a cooling process before stretching in the orientable state.