Opaque film for twist wrapping sweets

The invention relates to an opaque, biaxially stretch-oriented film made of thermoplastic material for wrapping candy.

A special way of closing full wraps for the purpose of packaging packaging goods is turning or wrapping. This method of packaging is known in particular for the full wrapping of smaller packaging goods, in particular round or nearly round objects such as candies, bottles, candles, drops rolls, chocolate and marzipan bars or the like are often packaged in this way.

A prerequisite for the use of turning is the properties of the film, which must not tear in or tear off at the turning points, but on the other hand must be so stiff that there is no shrinking or shrinking during the turning process. According to the state of the art, cellulose, ie regenerated cellulose, non-oriented polypropylene or PVC film, is predominantly used for candy wrappings, see "Packaging with plastics" by Günther Kühne, published in 1974, Carl-Hanser-Verlag, Munich, page 63. From the For example, US-A-3,663,488 discloses α-olefin copolymer films for wrapping candy. Furthermore, packaging films made of polypropylene / styrene graft polymers are described in GB-A-2 063 899.

Within the framework of the further development of the known packaging films, the task for the present invention was to create an opaque film especially for the wrapping of small packaging goods, in particular candies, which, thanks to its particularly high rigidity in the twisted (twisted) shape, has an improved twist behavior and which also guarantees optimal machine operation on the candy wrapping machines and, thanks to its very good sliding properties, also enables the bags to be filled smoothly with the wrapped candies.

In the context of the present invention, the twist behavior of the film is to be understood as the property of the film that after it has been screwed in laterally (twisted) in the screwed-in (twisted) form, it has such great stability (rigidity) that the package contents during transport and storage is not removed from the packaging by automatically opening the film. The twist behavior of the film can be expressed numerically by the twist rate. The twist rate is defined as the percentage of residual twist remaining:<maths id="math0001" num=""><img file="EP0244614B1_D0001.tif" /></maths>

This means, for example, with an assumed total rotation of 1.5 (≙ 540 °) when twisting and a return angle of 100 °:<maths id="math0002" num=""><img file="EP0244614B1_D0002.tif" /></maths>

The above object is achieved according to the invention by means of a film of the type mentioned at the outset, the distinguishing features of which are that it consists of a polymer mixture comprising 40 to 60% by weight of polypropylene and 35 to 55% by weight of polystyrene, based on the total weight of the polymer mixture , and that the mixture also contains 5 to 15 wt .-% fillers, based on the total weight of the mixture.

The film according to the invention can be a single-layer (monofilm) or a multilayer film. The monofilm consists entirely of the specified mixture, while in the case of a multilayer film the base layer consists of the specified mixture and is covered on one or both sides by functional cover layers. The base layer preferably has a thickness of more than 50% of the total thickness of the multilayer film.

In a preferred embodiment, the polymer mixture contains polypropylene in an amount of 45 to 55% by weight and polystyrene in an amount of 35 to 45% by weight, based in each case on the total weight of the polymer mixture. The polypropylene should in particular represent the predominant polymer component.

In the context of the present invention, an isotactic propylene homopolymer or a copolymer which is predominantly composed of propylene units is used as the preferred polypropylene for the mixture of the monofilm or the base layer. Such polymers usually have a melting point of at least 140 ° C., preferably in the range from 150 to 170 ° C. Isotactic polypropylene with an n-heptane soluble content of less than 15% by weight, copolymers of ethylene and propylene with an ethylene content of less than 10% by weight and copolymers of propylene with other alpha olefins with 4 to 8 carbon atoms and with a content of these Alpha olefins of less than 10% by weight are typical examples of the preferred thermoplastic polypropylene of the blend.

The preferred thermoplastic polymers expediently have a melt flow index in the range from 0.5 g / 10 min to 8 g / 10 min at 230 ° C. and 2.16 kg load (DIN 53 735), in particular from 1.5 g / 10 min to 4 g / 10 min. In principle, any styrene polymer can be used as polystyrene. Copolymers of unsubstituted and substituted styrenes such as alkylstyrene or alkoxy styrene can also be used.

The fillers are inorganic or organic, powdery materials incompatible with the mixture of polypropylene and polystyrene. Inorganic fillers are preferred. Suitable inorganic fillers are aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates such as aluminum silicate (kaolin clay) and magnesium silicate (talc), silicon dioxide and / or titanium dioxide, among which calcium carbonate, silicon dioxide, titanium dioxide or mixtures thereof are preferably used. Calcium carbonate (chalk) is particularly preferred. Suitable organic fillers are, for example, particles made from polymers such as polymethyl methacrylate, polyethylene acrylate or polyethylene, from polyamides or from polymers based on polyesters. According to the invention, the amount of filler is 5 to 15% by weight, based on the weight of the mixtures of polymers plus fillers. According to the invention, the (average) particle size of the powdery filler is in the range from 2 to 5 μm, preferably in the range from 3 to 4 μm. The mixture for the monofilm or the base layer can additionally also contain useful additives such as antioxidants, antistatic agents, dyes and / or stabilizers in effective amounts.

The density of the film is lower than the calculated density from the type and proportion of the individual components.

With regard to its intended use, the film with the above-mentioned composition has a very advantageous twist rate of greater than or equal to 85%, measured at a total rotation rate in the range from 360 to 900 °.

As already mentioned, the film according to the invention can in principle be in the form of a monofilm. A monofilm is produced by the extrusion process, in which the thermoplastic material is melted in an extruder, extruded through a die and quenched into a pre-film. The pre-film is then stretch-oriented in mutually perpendicular directions (lengthways and crossways) and then heat-set. The thickness of the finished film is preferably in the range from 15 to 30 μm, particularly preferably from 20 to 25 μm.

In a preferred embodiment, however, the film according to the invention consists of a base layer and functional cover layers applied thereon on one or both sides.

The cover layers applied to the base layer from the polymer mixture on one or both sides can be both heat-sealable and cold-sealable layers. It can also be non-sealable layers. All of these layers can be on one or both sides. A heat seal layer preferably consists of an ethylene homopolymer (high density polyethylene or low density polyethylene), a copolymer of propylene as the main component and ethylene, preferably in an amount of at most 10% by weight (based on the copolymer), of a copolymer of propylene as the main component and butene (1), preferably in an amount of 10 to 15% by weight (based on the copolymer), a terpolymer of propylene, Ethylene and an alpha olefin having 4 to 10 carbon atoms, preferably one composed of 93.2 to 99.0% by weight of propylene, 0.5 to 1.9% by weight of ethylene and 0.5 to 4.9% by weight % of an alpha olefin having 4 to 10 carbon atoms, or a mixture of these polymers. The comonomers are essentially statistically distributed in the polymers.

The cover layer or the cover layers can also consist of a propylene homopolymer, the melt index of the polymer used for the cover layer preferably having a value which is 0.5 to 1.0 g / 10 min higher than the melt index of the polymer mixture used for the base layer. Suitable cold seal layers consist of polymers based on natural or synthetic rubber.

A lubricant has been added to the cover layer (s) to improve the lubricity with a view to satisfactory machinability and with a view to smoothly filling the packaged goods into bags. A polydialkylsiloxane, preferably a polydialkylsiloxane, which contains 1 to 4 carbon atoms in the alkyl group, is expediently used as the lubricant, polydimethylsiloxane being particularly preferred. The polydialkylsiloxane has a kinematic viscosity of 1,000 to 100,000 mm² / sec at 25 ° C, preferably from 5,000 to 50,000 mm² / sec at 25 ° C. The amount of polydialkylsiloxane used in the cover layer or layers is 0.4 to 1.5% by weight, preferably 0.5 to 1% by weight, based on the weight of the cover layers.

The thickness of the multilayer film according to the invention is likewise 15 to 30 μm, preferably 20 to 25 μm, the thickness of the cover layer (s) being 0.5 to 5 μm, preferably 0.5 to 2 μm, based on the respective cover layer.

In order to further improve certain properties of the film according to the invention, both the base layer and the top layer (s) can contain corresponding additives in a respectively effective amount, preferably antistatic agents, stabilizers and nucleating agents. In particular to improve the ability to be assembled, the cover layer or 0.1 to 1% by weight, in particular 0.2 to 0.5% by weight, of an organic or inorganic antiblocking agent are added to the cover layers. Suitable antiblocking agents are, for example, incompatible organic polymers such as polyamide, polyester, polycarbonates and the like or inorganic substances such as silicon dioxide and silicates, aluminum silicate is particularly suitable.

The invention will be explained in more detail below with the aid of exemplary embodiments.

Two examples according to the invention (examples 1 and 2) and three comparative examples (examples 3, 4 and 5) were produced. The twist rate was determined uniformly for the films of all examples using the method given above. All of the foils were rotated 1.5 times (= 540 °) and then released and the return angle measured. From this, the specified twist rate is calculated in percent. The machine running behavior was assessed by simply comparing the films produced with one another.

example 1

A monofilm was extruded through a flat die at a temperature of about 280 ° C. The polymer mixture consisted of 50% by weight of polypropylene and 40% by weight of polystyrene. The polymer mixture additionally contained 10% by weight of powdered calcium carbonate with an average particle size of 3.8 μm, all of the% by weight refer to the total weight of polymers plus filler. After the extruded mono film had cooled to about 30 ° C with a chill roll, the film was stretch-oriented with the aid of rolls driven at different speeds at a temperature of 125 ° C in the longitudinal direction with a stretch ratio of 6.0 and then in a transverse stretching frame at a temperature of 165 ° C with a stretch ratio of 9.0. The biaxially stretch-oriented film was then heat-set at a temperature of 160 ° C. and for a period of 5 s. The resulting opaque film had a density of 0.8 g / cm³ and a thickness of 23 µm. Information about the twist rate and the machine running behavior of the film can be found in the table at the end of the examples.

Example 2

According to the coextrusion process, a three-layer film was produced with the aid of a flat die, comprising a base layer as specified in Example 1, which additionally contained 0.1% by weight Armostat® 300 = N, N-bis-ethoxy-alkyl-amine and two as an antistatic Cover layers made of polypropylene and an addition of 1.0% by weight of dimethylpolysiloxane with a kinematic viscosity of 30,000 mm² / s. The film was produced as indicated in Example 1, the three-layer film had a total thickness of 25 μm, the outer layers each being about 1 μm thick. The twist rate and machine running behavior of the film of Example 2 are also listed in the table at the end of the examples.

Example 3 (comparative example)

According to DE-OS 28 14 311, a 25 µm thick polypropylene film with heat-sealable cover layers of 1 µm each was co-extruded at about 270 ° C with a flat die. The polypropylene melt contained 8% by weight of finely divided calcium carbonate particles with an average particle size of 2.4 μm. The cover layer consisted of a random copolymer of propylene and 4% by weight of ethylene. After cooling to about 30 ° C. with a cooling roll, the film was stretched at 125 ° C. in the longitudinal direction, stretch ratio 5.5, and then at 165 ° C. in the transverse direction, stretch ratio 9.0. The subsequent heat setting was carried out at 160 ° C. The density of the film was 0.75 g / cm³. Information about twist rate and machine running behavior are listed in the table below.

Example 4 (comparative example)

A polypropylene film with heat-sealable cover layers present on both sides was coextruded through a flat die at about 270 ° C. The melt of the film forming the base layer consisted of 80% by weight of propylene homopolymer and 20% by weight of powdered calcium carbonate with an average particle size of 3.8 μm. The melt of the polymer forming the outer layers consisted of a random copolymer of propylene and 4% by weight of ethylene. After cooling the coextruded film with a chill roll to about 30 ° C, the film was stretched at 125 ° C in the longitudinal direction, stretch ratio 6.0, and then at 165 ° C in the transverse direction, stretch ratio 9.0. The subsequent heat setting was carried out at 160 ° C. for 5 s. The opaque polypropylene multilayer film thus obtained had a density of 0.46 g / cm³ and a thickness of 25 μm, the thickness of the heat-sealing layers was 1 μm in each case. For information on twist behavior and regarding machine running behavior, see table.

Example 5 (comparative example)

By extrusion and subsequent stretch orientation and heat fixation analogous to the previous examples, a monofilm was produced, consisting of a mixture of 75% by weight of propylene homopolymer and 25% by weight of a low molecular weight hydrocarbon resin with a softening point in the range from 80 to 120 ° C was the hydrocarbon resin sold under the name "Arkon P 125®" by the company Arakawa, Japan. The monofilm had a thickness of 25 µm. See table below for twist rate and machine running behavior.<tables id="tabl0001" num="0001"><img file="EP0244614B1_D0003.tif" /></tables>