Sealable films

This invention relates to films based on polypropylene which can be sealed by the application of heat.

Various polypropylene films have been described, e.g. in GB 2 028 168A, EP 135 178 and DE 3 247 988 where the core or substrate of the film is polypropylene. Vanous additives are added for example certain modulus improvers, certain antistatic media, siloxanes and pigments.

Example 10 of GB 2 028 168A uses a core layer of polypropylene admixed with 15 weight % of a polyterpene resin having a softening point of 115°C (Zonarez 7115) and a single component seal layers of a random propylene-ethylene copolymer.

US 4 230 767 discloses an improved seal packaging structure with a core layer or base layer to which lower molecular weight thermoplastic resins may be incorporated which resins may vary widely and have no specific softening points. Sealability is stated to be improved by the incorporation of a low molecular weight thermoplastic resin such as hydrocarbon resins.

The invention aims to provide a film of which the seal strength properties are improved over those previously obtained having a single component seal layer. In this way either the seal strength is improved at the same temperature or the same seal strength can be obtained at lower temperature.

Accordingly this invention relates to the use in heat seal packaging of a film comprising a base layer which comprises 70 to 97 weight %, preferably 70 to 90 weight % of a polyolefin and 3 to 30 weight %, preferably 10 to 30 weight % of a resin having a molecular weight lower than that of the polyolefin and a softening point of from 120 to 180°C, said layer having on at least one surface thereof 1 to 20 weight % based on the weight of the base layer, of a heat sealing film layer consisting of a copolymer of 80 to 99 weight % of propylene and 1 to 20 weight % of ethylene.

These films are capable of being used in the packaging industry and can meet the stringent criteria which are required for such films, such as sufficiently high modulus and excellent clarity. In addition, they have good barrier properties by which we mean low permeability to water vapour and gases, particularly oxygen. The films also have superior seal strength properties and are especially useful in heat seal packaging.

The base layer comprises a polyolefin and a low molecular weight resin. The polyolefin is preferably a polymer of a mono-alpha olefin containing from 2 to 8 carbon atoms and particularly 2 to 4 carbon atoms per molecule. Thus the polymer may be a homopolymer of one of the following olefins: ethylene, propylene, butene-l and 4-methyl pentene-l, or hexene or a copolymer of two or more of these olefins. A particularly suitable material for the base layer of the film is polypropylene, especially a high molecular weight stereoregular predominantly crystalline polymer of propylene. Alternatively, a copolymer of propylene with up to 20 weight per cent of another olefin, e.g. ethylene, could be used. An especially preferred polyolefin is an isotactic polypropylene with a density of from 0.86 to 0.92 g/ml measured at 23°C according to ASTM D 1505 and a melt flow index of from 1 to 15 g/10 min as determined according to ASTM D1238 (conditions 230°C and 2.16 kg). It may be made by Ziegler polymerisation methods using for example AlCl₃ and TiCl₄ as catalyst.

The other component of the base layer is the low molecular weight resin, preferably a hydrogenated resin. This resin has a molecular weight below that of the polyolefin and is usually less than 5000, preferably less than l000, for example, 500 to 1000. The resin can be a natural or synthetic resin and can have a softening point as measured by ASTM E 28 of from 120°C to 180°C, e.g. preferably up to 150°C, or up to 140°C.

Suitable resins which can subsequently be hydrogenated are hydrocarbon resins, ketone resins, polyamide resins, colophonium, coumarone resins, terpene resins, chlorinated aliphatic or aromatic hydrocarbon resins. Examples of hydrocarbon resins are polymers of coke oven gas, cracked naphtha, gasoil and terpene oil.

Particularly preferred hydrogenated resins are hydrogenated petroleum resins. These are usually prepared by catalytically hydrogenating a thermally polymerised steam cracked petroleum distillate fraction, especially a fraction having a boiling point of between 20° and 280°C. These fractions usually are of compounds having one or more unsaturated cyclic rings in the molecule, such as cyclodienes, cycloalkenes and indenes. It is also possible to hydrogenate resins produced by the catalytic polymerisation of unsaturated hydrocarbons.

Before hydrogenation occurs the polymerised resin is usually dissolved in a saturated hydrocarbon solvent such as heptane. The hydrogenation catalysts used may be nickel, reduced nickel, or molybdenum sulphide. Hydrogenation can take place in a single stage at a temperature of 200°C to 330°C, preferably from 210°C to 230°C at a pressure of 20.26 to 121.56 bar (20 to 120 atmospheres), more preferably from 30.39 to 91.17 bar (30 to 90 atmospheres) for a period of 5 to 7 hours. After filtering off the catalyst, the solvent is removed by distillation and recovered for recycling. An improved hydrogenation process leading to increased yields of high quality hydrogenated hydrocarbon resins is described and claimed in EP-A 0 082 726.

The ratio of polyolefin to the lower molecular weight resin in the base layer is 70 to 97, preferably 70 to 90 weight per cent of polyolefin and 3 to 30, preferably 10 to 30 weight percent of lower molecular weight resin. More preferably the relative amounts are respectively 75 to 85 weight per cent and 15 to 25 weight per cent, for example about 80 weight per cent and 20 weight per cent.

The base layer or core of the film has on one or preferably both surfaces 1 to 20 weight per cent, preferably l to 10 weight per cent and particularly about 5 weight per cent for the or each surface based on the weight of the base layer, of a copolymer of 80 to 99 weight per cent of propylene and 1 to 20 weight per cent of ethylene, such copolymers ing advantageously random in nature and frequently referred to as random copolymers. This randon copolymer preferably comprises 90 to 99 weight per cent, especially 94 to 98 weight per cent, e.g. about 95.5 weight per cent of propylene, the remainder being ethylene and is especially useful to provide heat-sealability.

This random copolymer of propylene and ethylene is usually an isotactic propylene-ethylene copolymer with a density of from 0.86 to 0.92 g/cc measured at 23°C according to ASTM D 1505 and a melt flow index of from 2 to 15 g/10 min as determined according to ASTM D 1238 (conditions 230°C and 2.16 kg). It may be made by well-known polymensation methods using a Ziegler catalyst, for example, AlCl₃ and TiCl₄.

The films useful is this invention, i.e. multilayer films are suitably formed by combining the components (which may if desired include other components such as an antistatic medium, blocking agents and slip aids) of the base layer and surface layer(s) in a conventional manner, preferably by a simultaneous co-extrusion technique.

Multiple-layer films useful in the invention may be unonentated or uniaxially orientated, but are preferably biaxially orientated by drawing in two mutually perpendicular directions in the plane of the film to impart strength thereto. Orientation of flat film may be effected by a stenter technique, while orientated tubular film is suitably produced by coextruding the polymeric materials in the form of a multilayer tube from an annular die, cooling the extruded tube (the cast tube), reheating and inflating the tube by the so-called "bubble" process to introduce transverse orientation, and simultaneously elongating the tube longitudinally to orientate the film in a lengthwise direction. The film is then preferably "heat-set", i.e. dimensional stability of the film is improved by heating the film, while restrained against thermal strinkage, to a temperature above the glass transition temperature of the polymer from which the film is formed but below the melting point thereof.

The films may vary in thickness depending on the intended application, but films of a thickness of from 2 to 150 µm are usually suitable. Films intended for packaging are usually from 10 to 60 µm thick. The thickness of the or each outer layer is usually from 0.05 to 2.5 µm.

Example

In this Example coextruded films of different composition were made and compared as to their sealability, stiffness, optical properties and barrier properties.

These films were biaxially orientated by drawing in two materially perpendicular directions in the plane of the film to impart strength thereto. The stenter technique was used to impart an orientation ratio of 500% in machine direction and of 900% in transverse direction. Each of the biaxially orientated films had two seal layers of 1 µm thickness while the total thickness was about 22 µm.

The first film which was for comparison purposes consisted of a core of isotactic polypropylene having a density of about 0.90 g/ml measured at 23°C according to ASTM D l505 and a melt flow index of 2.8 g/10 min as determined according to ASTM D1238 (conditions 230°C and 2.16 kg). On both faces of the core there was a surface film of a random copolymer of about 95.5 weight per cent propylene and 4.5 weight per cent of ethylene. This was an isotactic copolymer with a density of 0.90 g/ml measured at 23°C according to ASTM D l505 and a melt flow index of 6.5 g/10 min as determined according to ADTM D1238 (conditions 230°C and 2.16 kg).

The second film was the same as the first film except that the core was a blend of 80% by weight of the isotactic polypropylene used as the core of the first film and 20% by weight of a hydrogenated petroleum resin. This resin was prepared by catalytically hydrogenating a thermally polymerised steam cracked petroleum distillate fraction having a boiling point between 20° and 280°C. The hydrogenation process was carried out as described in EP-A 0 082 726. The resulting resin was characterised by a Ring and Ball softening point of l25°C.

The accompanying Fig 1 to 7 show the results obtained for the seal strength, hot tack, haze, gloss, stiffness and barrier properties for films I (comparative) and 2 (according to the invention)

These tests were carried out as follows:

Seal strength

This is the "cold" heat seal strength of the film which is measured after the seal has cooled to ambient temperature and the full potential strength of the seal has developed.

A Packforsk Hot Tack Tester (model 52-B) from Design & Test Consult AB (Bromma-Sweden) has been used to make the heat seals. A strip of l5 mm wide and some 280 mm long is thereto folded over upon itself and sealed together between the heated jaws of this instrument.

The formed seal has an area of some 15 x 5 mm. Dwell conditions on the seal have been kept constant at a pressure of 5 bar and a dwell time of 0.5 seconds. To measure the seal strength, the instrument is used in a mode whereby upon opening of the sealing jaws the automatic peeling action used for hot tack measurements is not activated. The seal is instead cooled to ambient temperature. The unsealed ends of the strip are then attached to the jaws of a tensile testing machine. The force required to break the seal at a rate of 508 mm/min is recorded by the tensile tester and expressed as the seal strength in kg/15 mm.

Hot tack

This is the strength of the heat seal measured just after the seal has been made and before the thermal energy employed to form the heat seal has dissipated. The hot tack has been determined using the same Packforsk instrument. The heat seal is made as described above but just after the opening of the sealing jaws a peeling action is now automatically started and the force to break the seal is automatically recorded and expressed as the hot tack in kg/15 mm. The peeling operation takes places at a controlled delay time (= time between opening of the jaws and start of the peeling operation) and controlled peel rate. Following constant conditions have been used for all hot tack determinations : delay time = 0.9 seconds, peel rate = 100 mm/s.

Haze and gloss

These optical properties of the film have been determined with a Gardner Hazemeter, haze according to ASTM D1003 and gloss according to ASTM D523.

E-modulus

The modulus of elasticity which is a measurement of film stiffness has been determined from the tensile test of l5 mm wide film samples according to DIN 53 457

Barrier Properties

Humidity and oxygen barrier properties were determined by measuring the water vapour transmission rate (WVTR) and the oxygen transmission rate (OTR). WVTR has been measured according to ASTM F 372 at 37.4°C and l00 % relative humidity using a Permetran W5 instrument from Modem Controls Incorporated. OTR was measured according to ASTM D 3985 at 27°C using an Oxtran Twin instrument also from Modem Controls Incorporated. <ul id="ul0001" list-style="none" compact="compact"><li>From Fig I it can be seen that the seal strength (kg/l5 mm) for film 2 is much higher at temperatures of between 100° and 110°C and above 130°C than for film 1. Between 110° and 130°C the seal strength is comparable or higher than for film 1.</li><li>From Fig 2 it can be seen that the hot tack (kg/15 mm) for film 2 is much better than for film l for temperatures up to about 110°C.</li><li>From Fig 3 and Fig 4 it can be seen that there is less haze and more gloss for film 2 than with film 1.</li><li>From Fig 5 it can be seen that the stiffness of film 2, as measured by the E-modulus, is significantly higher than the stiffness of film 1.</li><li>From Fig. 6 and Fig. 7 it can be seen that the barrier properties of film 2 are about twice as good as those of film 1.</li></ul>

It is quite surprising that with the presence of the relatively small amount (20 weight % ) of hydrogenated resin in the base layer one achieves such better results compared with the pnor art film.