Transparent shrink film based an polypropylene, process for making it and its use for shirkable labels.

0001The present invention relates to a transparent shrinkable film comprising a base layer containing a mixture of polypropylene and hydrocarbon resin. The invention further relates to a method for producing such a film and its use as shrink labels.

0002From "Verpackungsrundschau" 10/1983, 1121-1122, it is already known to completely enclose cylindrical or approximately cylindrical packaging units such as bottles or cans with all-round labels; these all-round labels are referred to in the Anglo-Saxon literature as so-called 'sleeves'. The films for this application are e.g. B. made of polyethylene, polybutylene, polystyrene, polyethylene-polypropylene copolymers, ethylene-vinyl acetate copolymers, various blends, but especially made of polyvinyl chloride. To achieve the shrink properties of the films suitable for the desired application in the circumferential direction of the all-round labels, the films are generally biaxially stretch-oriented, which, for. B. can be accomplished by the bubble, stenter or calender method, with particular preference being given to orientation in the transverse direction. In order to ensure that the wrap-around labels really fit wrinkle-free on the packaging unit, B. transverse shrinkage (see_q) of approximately 20 to 40% and longitudinal shrinkage (see_l) of a maximum of 6 to 8% at a temperature of 90 ° C and a treatment time of 15 min in a convection oven.

0003The shrink films are also z. B. printed in reverse printing and then glued or welded to a tube. Thanks to the controlled cross-shrinkage, the hose is fixed in the shrinking furnace and free of creases around the packaging unit. The label is applied mechanically with the help of brushes or alternatively manually. In addition to the desired shrinkage, the all-round labels must have the following properties: high gloss, crystal-clear, good lubricity and rigidity (= product of modulus of elasticity and thickness³) for the machine application of the label, good printability and good sealability.

0004The entire requirement profile described so far is best achieved according to the prior art according to US Pat. No. 4,352,849 by PVC film. In particular, the high shrinkage capacity and the good optical and mechanical properties cover almost the entire area of ​​application. The change in shape or diameter of the packaging unit, in particular the can or bottle, can make up more than 30% in the area of ​​the label.

0005A disadvantage of the all-round PVC labels is the high price, which mainly results from the high density of 1.39 kg / dm³, which is approx. 50% higher than that of e.g. B. PP, to name, there are also problems with regard to corrosion of the manufacturing and processing units (see. EP-A-0 233 400).

0006Shrink label films based on polyolefin consist primarily of blends based on homopolymers, copolymers and terpolymers. In order to achieve the required shrinkage properties, the films are stretched biaxially in the bubble or in the stenter process.

0007According to the state of the art, shrink labels based on polyolefin films have properties that can be disadvantageous in practice:<ul id="ul0001" list-style="none"><li>1. The mechanical properties are usually very low and / or</li><li>2nd the process for achieving a high transverse shrinkage and a low longitudinal shrinkage is very complex.</li></ul>

0008Low mechanical properties are particularly disadvantageous in automatic shrinking machines. In order to obtain sufficient (same or similar good) stiffness as with PVC, the thickness of the polyolefinic film has to be increased by 50% and more. However, this increases the price of the film to the same extent, and the larger thickness - with the same final shrinkage value - also reduces the shrinking speed. A method according to the prior art for achieving a high ratio between transverse shrinkage and longitudinal shrinkage of polypropylene films is e.g. B. described in EP-A-0 171 733, from which films of the generic type from copolymers of propylene with other alpha-olefins and a resin additive are known. In this process, a biaxially oriented film is produced using the two-stage process. In order to achieve a low longitudinal shrinkage, the film is annealed between the longitudinal stretching and the transverse stretching in an additional step at an elevated temperature (approx. 130 ° C.). The annealing time is 2 to 180 s. Depending on the speed of the production plant - nowadays approx. 200 to 300 m / min - a relatively complex and expensive unit must be arranged between the longitudinal and transverse stretching in this process, because with a tempering time of e.g. B. 60 s and a machine speed of 200 m / min would require a furnace length of 200 m. The method is therefore unsuitable from a process engineering point of view for the production of films using the methods customary today. In addition, the film has mechanical properties that can be improved.

0009The object of the present invention was to develop a transparent, easily glued and easily shrinkable multilayer film for wrap-around labels based on polypropylene, which has very good optical and mechanical properties and thus above all enables economic advantages over films according to the prior art.

0010This object is achieved by a film of the type mentioned at the outset, the characteristic features of which are that a base layer comprises 60 to 95% by weight of a propylene polymer and 5 to 40% by weight of a hydrogenated hydrocarbon resin with a softening point in the range from 60 to 125 ° C, where the percentages are based on the total weight of the mixture, and that on both surfaces of the base layer there are polypropylene cover layers containing 60 to 100% by weight of a propylene polymer and 0 to 40% by weight of a hydrogenated hydrocarbon resin with a softening point in the range from 90 to 145 ° C, the percentages being based on the total weight of the cover layers.

0011The propylene polymer of the base layer is an isotactic polypropylene with an n-heptane-soluble fraction of 15% by weight or less, with isotactic polypropylenes with an n-heptane-soluble fraction of 2 to 6% by weight being particularly preferred. Suitable propylene polymers expediently have a melt flow index of 0.5 g / 10 min to 8 g / 10 min at 230 ° C. and 2.16 kp load (determined in accordance with DIN 53 735), in particular from 1.5 g / 10 min to 4 g / 10 min.

0012The propylene polymer of the cover layers is an isotactic polypropylene with an n-heptane-soluble content of 15% by weight or less, with isotactic polypropylenes with an n-heptane-soluble content of 2 to 6% by weight being particularly preferred. The melt flow index of the propylene polymer of the cover layer should advantageously be higher than that of the propylene polymer for the base layer. Propylene polymers suitable for the cover layers should therefore have a melt flow index in the range from 5 to 20 g / 10 min.

0013The hydrocarbon resin which is also present in the base layer of the film according to the invention is a low molecular weight synthetic resin which has a softening point in the preferred range from 70 to 90 ° C., determined in accordance with ASTM-E 28. Such hydrocarbon resins are usually made from resin-forming compounds such as styrene, methyl styrene, vinyl toluene, indene, pentadiene, cyclopentadiene and the like. Ä. formed. According to the invention, hydrogenated resins, in particular hydrogenated cyclopentadiene resins, are particularly preferred. The Saybold color number (according to ASTM D 158) is greater than 20, preferably greater than 25. The resin contained in the amounts indicated in the cover layers can in principle be the same resin as is also contained in the base layer, but it has been found that resins which have a softening point at higher temperatures can be used advantageously for the cover layers, preferably in the range of 110 ° C to 135 ° C. The chemical composition of such resins is the same as that of the softening point resins in a lower temperature range.

0014The cover layers preferably additionally contain an inorganic or organic antiblocking agent. Suitable antiblocking agents are inorganic additives, e.g. B. silicon dioxide and calcium carbonate or the like. It is essential for the antiblocking agent that the average particle size is between 1.5 and 3 µm and its form factor is &lt;3. The refractive index of the antiblocking agents is between 1.4 and 1.6. SiO₂ and calcium carbonate are preferred as antiblocking agents. The amount added is 0.1 to 0.5% by weight. The form factor of the antiblocking agent is the ratio of the largest surface area to the thickness of the particles. According to this definition, a cube-shaped antiblocking agent has e.g. B. a form factor of 1.

0015In order to further improve certain properties of the shrink film according to the invention, in particular with regard to improvements in the running behavior of the film in the manufacturing or processing process, the layers can contain suitable active ingredient components in each case in effective amounts, preferably antistatic agents and / or lubricants.

0016Preferred antistatic agents are essentially straight-chain and saturated aliphatic, tertiary amines with an aliphatic radical having 10 to 20 carbon atoms, which are substituted by 2-hydroxyalkyl (C₁-C₄) groups, including N, N-bis (2-hydroxyethyl) -alkylamines with C₁₀-C₂₀, preferably C₁₂-C₁₈, are particularly suitable as alkyl groups. The effective amount of antistatic is in the range from 0.05 to 1% by weight, based on the layer.

0017It has proven particularly expedient to provide the top layer with 0.1 to 0.7% by weight of an N, N-bis-ethoxyalkylamine with an aliphatic radical with 10 to 20 carbon atoms.

0018Examples of lubricants are higher aliphatic acid amides, higher aliphatic acid esters, waxes and metal soaps as well as polydimethylsiloxane. The effective amount of lubricant is in the range of 0.1 to 2% by weight based on the weight of the layer. The addition of higher aliphatic acid amides (eg erucic acid amide) with 0.15 to 0.25% by weight in the base layer and / or the top layers has proven to be particularly suitable. Very good results are achieved by adding polydimethylsiloxane to one or both top layers. The amount added is expediently in the range from 0.5 to 1.5% by weight, the viscosity of the polydimethylsiloxane being between 1,000 and 100,000 mm 2 / s.

0019The thickness of the film according to the invention is in the range from 15 to 50 μm, preferably from 20 to 45 μm, the cover layers having a thickness of 0.5 to 1.0 μm.

0020The film according to the invention described above with regard to its chemical composition is characterized in particular by very particularly desirable shrink properties. It has a shrinkage capacity of more than 20% at 90 ° C and more than 40% at 120 ° C in the transverse direction and at the same time a shrinkage capacity of less than 8% at 90 ° C and less than 16% at 120 ° C in Longitudinal direction, the percentages being based in each case on the respective linear expansion of the film before the shrinking process. The specified shrinkage values ​​were determined in a convection oven over a period of 15 minutes in accordance with DIN 406 34. The shrinkability of the film according to the invention is preferably in the transverse direction in a range from 20 to 30% at 90 ° C. and from 40 to 60% at 120 ° C. and in the longitudinal direction in a range from a maximum of 4 to 8% at 90 ° C. and a maximum 10 to 16% at 120 ° C, each based on the linear expansion of the film before the shrinking process.

0021In addition to the excellent shrink properties already described, the film according to the invention also has particularly high desirable mechanical properties. The modulus of elasticity is determined using a tension-stretching device from Zwick in Ulm-Einsingen of type 1445 in accordance with DIN 53 455. The film according to the invention then has a modulus of elasticity in the longitudinal direction of more than 2,000 N / mm², preferably from 2,200 to 2,600 N / mm², and an elastic modulus in the transverse direction of more than 4,000 N / mm², preferably of 4 500 N / mm² to 5 500 N / mm².

0022Another physical parameter that clarifies the mechanical properties of the film according to the invention is the tensile strength, which is also determined in accordance with DIN 53 455. The shrink film according to the invention has a tensile strength in the longitudinal direction of more than 130 N / mm², preferably in the range from 145 to 185 N / mm², and in the transverse direction of more than 220 N / mm², preferably in the range from 235 to 290 N / mm².

0023In particular, the optical properties of the film according to the invention are very excellent. The gloss value is in the range from 110 to 130, determined in accordance with DIN 67 530 or ASTM-D 523, and the haze of the film is less than 20%, preferably 10 to 15%, the haze of the film based on ASTM-D 1003-52 is measured. Instead of a 4 ° perforated diaphragm, a 1 ° slotted diaphragm is used, and the haze in percent is given for four layers of film lying one above the other. The four positions were chosen because this enables the optimal measuring range to be used. The particularly high gloss value of the film according to the invention has a particularly effective advertising effect, and therefore the area of ​​application of the film according to the invention can be seen above all where the labeling process (folding and shrinking) takes place automatically and is high on the product (can / bottle / dispenser) visual requirements.

0024The object of the invention mentioned at the outset is also achieved by a method for producing the film described above. The process according to the invention consists of firstly producing a pre-film in a slot die by means of extrusion or coextrusion, which is then solidified on a chill roll and then oriented in the longitudinal and transverse directions by stretching. According to the invention, the conditions in the longitudinal stretching are selected so that the longitudinally stretched film is only slightly oriented. The prerequisites for achieving a high transverse shrinkage and a low longitudinal shrinkage are then particularly favorable. A common measure for assessing the extent of the orientation of the elongated film is the birefringence Δ n. The film according to the invention is characterized in that the birefringence of the longitudinally stretched but not yet transversely stretched film does not exceed a value of Δn = 10 to 13 · 10 bis³. The birefringence Δ n should preferably be &lt;8 · 10⁻³. The longitudinal stretching is carried out at a temperature of more than 140 ° C., preferably in the range from 145 to 165 ° C., and with a stretching ratio of less than 4.5, preferably in the range from 3 to 4.

0025Surprisingly, it has been found that the transverse stretching temperature T_q can be selected to be much lower than is otherwise customary and nevertheless very good process reliability is guaranteed. According to the invention, the transverse stretching is carried out at a temperature of less than 120 ° C., preferably less than 110 ° C. According to the invention, the stretch ratio in the transverse direction is more than 8, preferably in the range from 8.5 to 11.

0026After the film has been stretched in the transverse direction, a final fixing step follows. The film is continued at a temperature of 20 to 40 ° C below the stretching temperature, in particular at a temperature below 110 ° C, preferably below 80 ° C, in the stretching frame, possibly slightly converging. The convergence during the fixing stage is preferably 5 to 15%.

0027It must be emphasized that, because of the production process according to the invention, the top layers can be equipped with low molecular weight additives (carboxamides, N, N-bis-ethoxyalkylamines) without major technical problems. Evaporation problems and deposit problems in transverse stretching frames do not occur. Because of the process conditions according to the invention, there is also no annoying resin evaporation.

0028The printability or bondability of the film is achieved by one of the usual surface treatments before rolling up, e.g. B. flame treatment or electrical corona treatment.

0029For corona treatment, which can be carried out according to one of the known methods, the procedure is expediently such that the film is passed between two conductor elements serving as electrodes, such a high voltage, usually alternating voltage (approximately 10,000 V and 10,000 Hz.) Between the electrodes ), is designed that spray or corona discharges can take place. The air above the film surface is ionized by the spray or corona discharges and combines with the molecules on the film surface, so that polar inclusions arise in the essentially non-polar polymer matrix.

0030The treatment intensities are in the usual range. Treatment intensities of 38 to 42 mN / m are preferred.

0031For printing, the film is preferably printed in reverse printing.

0032The shrink film produced in this way has a combination of properties that make it particularly suitable for its intended use as a film for all-round labels. Your preferred area of ​​application can be seen above all where the shape change of the can or bottle in the area of ​​the labeling accounts for less than 25%.

0033The film according to the invention described in detail above is to be explained more clearly below by means of an exemplary embodiment.

example

0034A multilayer film consisting of a base layer of 90 wt .-% isotactic polypropylene and 10 wt .-% hydrogenated cyclopentadiene resin with a softening temperature of 85 ° C and outer layers of isotactic polypropylene, the 0.3 wt .-% of a CaCO₃ with an average particle size of 2 µm and a form factor of 1, 0.5% by weight of N, N-bis-ethoxyalkylamine (^(R)Armostat 300) and 0.7% by weight of polydimethylsiloxane (viscosity 30,000 cSt) were added via the process steps of coextrusion, cooling, longitudinal stretching, transverse stretching and fixing. The total thickness of the film was 30 µm, the cover layers each being approximately 0.8 µm thick. The manufacturing conditions in the individual process steps were:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" morerows="1" align="left">Extrusion:</entry><entry namest="col2" nameend="col4" align="left">Temperature of the melts</entry><entry namest="col5" nameend="col5" align="right">270 ° C</entry></row><row><entry namest="col2" nameend="col4" align="left">Pull roller temperature</entry><entry namest="col5" nameend="col5" align="right">60 ° C</entry></row><row><entry namest="col1" nameend="col1" morerows="1" align="left">Longitudinal stretch:</entry><entry namest="col2" nameend="col4" align="left">Temperature T = 155 ° C</entry><entry namest="col5" nameend="col5" /></row><row><entry namest="col2" nameend="col4" align="left">Longitudinal stretch ratio λ = 4.0</entry><entry namest="col5" nameend="col5" /></row><row><entry namest="col1" nameend="col1" morerows="1" align="left">Transverse stretching:</entry><entry namest="col2" nameend="col4" align="left">Temperature T = 105 ° C</entry><entry namest="col5" nameend="col5" /></row><row><entry namest="col2" nameend="col4" align="left">Transverse stretching ratio λ = 10</entry><entry namest="col5" nameend="col5" /></row><row><entry namest="col1" nameend="col1" morerows="1" rowsep="1" align="left">Fixation:</entry><entry namest="col2" nameend="col2" align="left">Temperature T</entry><entry namest="col3" nameend="col3" align="right">=</entry><entry namest="col4" nameend="col4" align="right">70 ° C</entry><entry namest="col5" nameend="col5" /></row><row rowsep="1"><entry namest="col2" nameend="col2" align="left">Time period t</entry><entry namest="col3" nameend="col3" align="right">=</entry><entry namest="col4" nameend="col4" align="right">1 s</entry><entry namest="col5" nameend="col5" /></row></tbody></tgroup></table></tables>

0035The film produced in this way had the properties listed in the table (last line). Before being rolled up, the film was subjected to a corona treatment in order to ensure printability / stickability. The treatment intensity was 39 mN / m.

0036In the table, the film according to the invention was compared with a PVC film, a film according to EP-A-0 171 733 and a polypropylene film produced under conditions similar to the ones according to the invention without the addition of resin. A comparison of the reported properties shows that the film according to the invention has proven to be superior to all known films, in particular with regard to the combination of properties sought in the task.<tables id="tabl0002" num="0002"><img file="EP0348749A2_D0001.tif" /></tables>