GB1569112A - Heat-sealable polypropylene films - Google Patents

Description

(54) HEAT-SEALABLE POLYPROPYLENE FILMS (71) We, BRITISH CELLOPHANE LIMITED. a British Company of Bath Road, Bridgwater, Somerset do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention is concerned with heat-sealable polypropylene films.

Polypropylene film is well known as a packaging material having high clarity. excellent barrier properties in respect of water vapour and excellent strength particularly when the film has been oriented in one or preferably two directions at right angles to each other by stretching while subjected to a temperature in the range between 2"C below the crystalline melting point of the polypropylene and its second order transition temperature.

However, for efficient wrapping of goods, particularly upon high speed packaging and wrapping machinery it is necessary for the polypropylene film to be readilv heat-sealable to itself so forming seals which have adequate strength for the particular packaging purpose.

Polypropylene film is inherently heat-sealable to itself and at temperatures of about 1600C will form seals of adequate strength, but at such temperatures and down to about 140"C oriented polypropylene will shrink and produce undesirable buckling in the region of the heat-seal.

It is known to attempt to overcome this problem by forming on one or more of the surfaces of oriented polypropylene film a layer of a heat-sealable polymer or copolymer having a heat-seal temperature at which seals of adequate strength are formed below the temperature at which any significant heat-distortion takes place in the base oriented polypropylene film.Such known heat-sealable layers include low density polyethylene, propylene/ethylene copolymers in which the ethylene content lies in the range between about 2% and 6% by weight and propylene/butene-l copolymers in which the butene-1 content lies in the range between about 5% and 30% by weight It is desirable that the heat-sealable polymers or copolymers of the heat-sealing layer will heat-seal and provide seals of adequate strength at temperatures as low as possible without being tacky at ambient temperatures.There is then provided a wide temperature range between the optimum heat-sealing temperature and heat-distortion temperature of the oriented polypropylene base film which allows for temperature drift and other variations which occur in the heat-sealing elements of high speed packaging machines and also allows for transfer of sufficient heat through several layers of film to bring about adequate heat-sealing of the inner layers without heat-distortion of the outer layers. Although the heat-sealable layers of polymers and copolymers at present employed are useful in this respect providing seals of adequate strength down to about 1()() C. there is a residual haze in the heat-sealing layers which detracts from the desirable high degree of transparency found in oriented polypropylene base film without the heat-sealable layers.

According to the present invention a low haze transparent heat-sealable oriented polypropylene film comprises an oriented base polypropylene film as hereinafter defined having on at least one surface a heat-sealable layer comprising a heat-sealable composition of a random copolymer of propylene and butene-l in which the butene-1 content lies in the range between 5% and 30% by weight blended with between 15% and 85% by weight of the blended mixture of a random copolymer of propylene and ethylene in which the ethylene content lies in the range between 2C/c and 15% by weight.

It has been found, surprisingly, that the haze value (as hereinafter defined) of the heat-sealable oriented polypropylene film in accordance with the invention is lower than the haze value of similar heat-sealable polypropylene film having a heat-sealable layer or layers of either one of the copolymers in the blend.

Preferably, the copolymer of propylene and butene-t is blended with between 20arc and 60% by weight of the blended mixture of the copolymer of propylene and ethylene.

Further, the ethylene content of the copolymer of propylene and ethylene preferably lies in the range between 2% and 6% by weight.

By "base polypropylene film" is meant a film of a homopolymer of polypropylene a copolymer of propylene with one or more other copolymerisable monomers in which the other monomer or monomers does not exceed 20eye by weight or a blend of the homopolymer with not more than 20cue by weight of a further polymer or copolymer.

By "haze value" is meant the percentage of transmitted light which in passing through heat-sealable oriented polypropylene film deviates from the incident beam by forward scattering by more than an average of 2.5 to the incident beam.

The heat-sealable composition may be applied to one or more surfaces of the base polypropylene film by any of the known methods such as by lamination of a preformed film of the heat-sealable composition to one or both sides of the base polypropylene film, by stretching a preformed film or films in intimate contact with the base polypropylene film under conditions in which the preformed film or films form a unitary film with the base film, by melt extrusion of the heat-sealable composition upon the base film or by coextrusion of the heat-sealable composition with the base polypropylene film.

Example A melt of polypropylene was coextruded through a flat die with outer layers of a blend of equal quantities by weight of a random propylene/butene-1 copolymer in which the butene-1 content was about 22% by weight and a random polypropylene/ethylene copolymer in which the ethylene content was about 3% by weight. The resulting extruded composite film was quenched and then biaxially oriented by stretching first in the longitudinal direction by 600% at a temperature of 1000C and then in the transverse direction by 1000% at a temperature of 1500C. The resulting composite film was about 25 microns in total thickness with the blended copolymer layers on each surface being each about 1 micron in thickness.

Sample strips 38 millimetres wide were cut from the composite film and were heat-sealed together in pairs across one end between crimp heat-sealing jaws. at temperatures ranging from 97l/2 C to llO"C under a pressure of 64 pounds per square inch for a dwell time of one second. The heat-seal strength of the seals were then determined in the weight in grams required to peel the pairs of strips apart at the heat-seal.

In addition the haze value (as hereinbefore defined) of samples of the composite film was determined using the Gardner Haze Meter manufactured by Gardner Laboratory Inc. of Bethesda, Maryland, United States of America.

The procedure was repeated as described except that the quantity of the propylene/ ethylene copolymer in the blend was reduced to 20% by weight.

In both runs the oriented heat-sealable polypropylene film was very clear with little or no haze observable by eye.

For comparison purposes the procedure was repeated twice as described but with the copolymer blend replaced first by the propylene/butene-l copolymer alone and second by the propylene/ethylene copolymer alone. In the first run the resulting heat-sealable polyprgpylene film had appreciable haze whereas in the second run the haze was less apparent but still readily observable by eye.

The heat-seal strength and the haze value results obtained were as shown in the following table: 78:22 97:3 Heat-seal Strength in grams/38mm Propylene/ Propylene/ Width at Sealing Temperatures Haze Butene-1 Ethylene Value Copolymer Copolymer % % by weight % by weight 97.5 C 100 C 102.5 C 105 C 107.5 C 110 C 50 50 230 340 450 600+ 600+ 600+ 1.8 (GOOD) 80 20 200 450 480 600+ 600+ 600+ 1.9 (GOOD) 100 - 270 280 470 600+ 600+ 600+ 4.0 (POOR) - 100 0 0 230 400 450 600+ 2.5 (FAIR) By the use of the heat-sealable compositions of the present invention upon the base polypropylene film, the film can be sealed at 100"C and above to provide heat-seals of adequate strength for packaging purposes, that is, strength of from about 340 grams/38 millimetres width of seal as well as having distinctly less haze than base polypropylene film coated with either of the copolymers present in heat-sealable compositions. At sealing temperatures at and above 105"C the heat-seal strengths were more than 600 grams/ 38 millimetres width of seal.

Since the random copolymer blend in accordance with the invention is compatible with polypropylene it is possible to return a small quantity of waste heat-sealable polypropylene film, for example edge trim, to the polypropylene feed stock from which the base polypropylene film is formed.

Specification No. 1352439 is concerned with synthetic papers possessing excellent whiteness and opacity and discloses in one form a base layer comprising a copolymer of propylene with up to 10% by weight of butene-l plasticised with a copolymer of propylene with up to 5% by weight of ethylene laminated to a paper-like layer of a polypropylene resin containing between 0.2 and 30% by volume of a fine filler having an average particle size of from 0.5 and 30 microns.

The present invention is not concerned with such white opaque synthetic papers.

WHAT WE CLAIM IS: 1. A low haze transparent heat-sealable oriented polypropylene film comprising an oriented base polypropylene film as hereinbefore defined having on at least one surface a heat-sealable layer comprising a heat-sealable composition of a random copolymer of propylene and butene-1 in which the butene-1 content lies in the range between 5% and 30% by weight blended with between 15who and 85% by weight of the blended mixture of a random copolymer of propylene and ethylene in which the ethylene content lies in the range between 2% and 15% by weight.

2. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 in which the random copolymer of propylene and butene-1 is blended with between 20% and 60% by weight of the blended mixture of the random copolymer of propylene and ethylene.

3. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 or claim 2 in which the ethylene content of the copolymer of propylene and ethylene lies in the range between 2% and 6% by weight.

4. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 substantially as described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. By the use of the heat-sealable compositions of the present invention upon the base polypropylene film, the film can be sealed at 100"C and above to provide heat-seals of adequate strength for packaging purposes, that is, strength of from about 340 grams/38 millimetres width of seal as well as having distinctly less haze than base polypropylene film coated with either of the copolymers present in heat-sealable compositions. At sealing temperatures at and above 105"C the heat-seal strengths were more than 600 grams/ 38 millimetres width of seal. Since the random copolymer blend in accordance with the invention is compatible with polypropylene it is possible to return a small quantity of waste heat-sealable polypropylene film, for example edge trim, to the polypropylene feed stock from which the base polypropylene film is formed. Specification No. 1352439 is concerned with synthetic papers possessing excellent whiteness and opacity and discloses in one form a base layer comprising a copolymer of propylene with up to 10% by weight of butene-l plasticised with a copolymer of propylene with up to 5% by weight of ethylene laminated to a paper-like layer of a polypropylene resin containing between 0.2 and 30% by volume of a fine filler having an average particle size of from 0.5 and 30 microns. The present invention is not concerned with such white opaque synthetic papers. WHAT WE CLAIM IS:

1. A low haze transparent heat-sealable oriented polypropylene film comprising an oriented base polypropylene film as hereinbefore defined having on at least one surface a heat-sealable layer comprising a heat-sealable composition of a random copolymer of propylene and butene-1 in which the butene-1 content lies in the range between 5% and 30% by weight blended with between 15who and 85% by weight of the blended mixture of a random copolymer of propylene and ethylene in which the ethylene content lies in the range between 2% and 15% by weight.

2. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 in which the random copolymer of propylene and butene-1 is blended with between 20% and 60% by weight of the blended mixture of the random copolymer of propylene and ethylene.

3. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 or claim 2 in which the ethylene content of the copolymer of propylene and ethylene lies in the range between 2% and 6% by weight.

4. A low haze heat-sealable oriented polypropylene film as claimed in claim 1 substantially as described.