Light-tight shrink wrapping film

Abstract

The invention relates to a heat-shrinkable film comprising a first polymer ply A comprising at least one dark pigment, a second polymer ply B comprising at least one white pigment, wherein the thickness ratio of the first ply A is in the range of 5:95 to 50:50, the film has a thickness of 20 μm bis 100 μm, the light transmission of the film is not more than 12%, and the film after 15 seconds in a water bath having a temperature of 95° C. exhibits shrinkage in a main shrinking direction in the range of 20% to 85%. The invention further relates to a hose comprising such a heat-shrinkable film and to a process for producing such a heat-shrinkable film.

Claims

1. Heat-shrinkable, uniaxially stretched film consisting of: a first polymer ply A which comprises at least one dark pigment; a second polymer ply B which comprises at least one white pigment; and at least one pigment ply consisting of pigments applied to the second polymer ply B, wherein: the first polymer ply A and the second polymer ply B, independently of each other, comprise one or more polymers which are selected from the group of the polyesters, polyolefins, polystyrenes and polyvinyl halides, the film has a thickness of 20 μm to 100 μm, the thickness ratio of the first polymer ply A to the second polymer ply B is in the range 5:95 to 50:50, the light transmission of the film is not more than 12%, and the film after 15 seconds in a water bath having a temperature of 95° C. exhibits shrinkage in a main shrinking direction in the range of 20% to 85%.

2. Heat-shrinkable film according to claim 1, wherein the second polymer ply B has an L* value of at least 85.

3. Heat-shrinkable film according to claim 1, wherein at least one of the surfaces which face away from each other of the first polymer ply A and of the second polymer ply B is smooth.

4. Heat-shrinkable film according to claim 1, wherein the second polymer ply B can be printed on.

5. Heat-shrinkable film according to claim 1, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyester.

6. Heat-shrinkable film according to claim 1, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyolefin.

7. Heat-shrinkable film according to claim 1, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polystyrene.

8. Heat-shrinkable film according to claim 1, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyvinyl halide.

9. Heat shrinkable film according to claim 1, wherein the second polymer ply B has a roughness Rz of more than 2.00 μm, preferably a roughness Rz in the range of 4 μm to 6 μm.

10. Heat-shrinkable film according to claim 9, wherein the second polymer ply B has a roughness Rz of more than 2.00 μm, preferably a roughness Rz in the range of 4 μm to 6 μm.

11. Heat-shrinkable, un axially stretched film consisting of a first polymer ply A which comprises at least one dark pigment and at least one white pigment; and a second polymer ply B which comprises at least one white pigment, wherein: the first polymer ply A and the second polymer ply B, independently of each other, comprise one or more polymers which are selected from the group of the polyesters, polyolefins, polystyrenes and polyvinyl halides, the film has a thickness of 20 μm to 100 μm, the thickness ratio of the first polymer ply A to the second polymer ply B is in the range 5:95 to 50:50, the light transmission of the fila is not more than 12%, and the film after 15 seconds in a water bath having a temperature of 95° C. exhibits shrinkage in a main shrinking direction in the range of 20% to 85%.

12. Heat-shrinkable film according to claim 11, wherein the second polymer ply B has an L* value of at least 85.

13. Heat-shrinkable film according to claim 11, wherein at least one of the surfaces which face away from each other of the first polymer ply A and of the second polymer ply B is smooth.

14. Heat-shrinkable film according to claim 11, wherein the second polymer ply B can be printed on.

15. Heat-shrinkable film according to claim 11, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyester.

16. Heat-shrinkable film according to claim 11, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyolefin.

17. Heat-shrinkable film according to claim 11, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polystyrene.

18. Heat-shrinkable film according to claim 11, wherein the first polymer ply A and the second polymer ply B, independently of each other, comprise a polyvinyl halide.

19. Heat-shrinkable film according to claim 11, wherein the second polymer ply B has a roughness Rz of more than 2.00 μm, preferably a roughness Rz in the range of 4 μm to 6 μm.

20. Heat-shrinkable film according to claim 19, wherein the second polymer ply B has a roughness Rz of more than 2.00 μm, preferably a roughness Rz in the range of 4 μm to 6 μm.

Description

EXAMPLES

(1) The examples are based on the following measurement methods, where necessary:

(2) Determining the Shrinkage

(3) In the context of the present invention, the shrinkage S[%] in percent denotes the difference between the length of the film in the respective

(4) shrinking or stretching direction before shrinking and after shrinking, in relation to the length before shrinking:
S[%]=((length before shrinking−length after shrinking)/(length before shrinking))*100.

(5) Shrinking is triggered by introducing a rectangular film with a known starting length, for example with an edge length of 100 mm×100 mm, the edges of which run parallel to the machine direction and to the transverse direction, into a water bath at a temperature of 95° C. and determining the length of the film by means of a ruler after 15 seconds in this water bath and subsequent removal.

(6) Determining the Thickness

(7) The thickness is determined in accordance with DIN 53370 by means of a scanning device manufactured by Mitutoyo, Japan, type 543/250 B. For the point measurements, the film was placed between the open measurement faces of the scanning device, which were then closed in a shock-free manner. The thickness was read by the scanning device.

(8) Transmission

(9) The transmission is determined by means of the “BYK-Gardner haze-gard plus” measurement instrument (manufacturer: BYK-Gardner, Germany). The light source D65 was used and the total transmission according to standard ASTM 1003 was measured based on a spectral range of 400 nm to 750 nm, and an integral transmission value for the visible range was obtained. To measure the spectral transmission both in the visible range and in the UV range (360 nanometres to 750 nm), the spectrophotometer “color i7” (X-Rite, Grand Rapids, USA) was used in transmission mode to verify that the necessary low transmission values were maintained across the entire spectral range.

(10) Determining the Degree of Whiteness

(11) To determine the degree of whiteness, the L* value of the CIE L*a*b* colour space was determined according to the DIN 5033 standard family. To this end, the colour spectrophotometer “color i7” (manufacturer: X-Rite) was used in the “reflection” measurement mode within a measurement wave range of 400 nm to 750 nm, with the sample placed on a standard white background.

(12) Roughness

(13) The roughness was measured according to DIN EN ISO 4287. A Hommel T8000 tester manufactured by Hommelwerke GmbH (Villingen-Schwenningen, Germany) and a TKT 100/17 probe, or a type S2 Perthometer and two styluses RT 50 and RT 250 manufactured by Mahr GmbH, Germany were used. Dust- and grease-free, as flat as possible samples of a size of approximately DIN A6 were measured without any further pretreatment. The average roughness Rz was determined as an average from five individual roughness depths according to standard EN ISO 4287.

(14) Determining the Gloss

(15) Gloss values at 60° were determined according to DIN 67530.

(16) Cavity Content (Pore Content)

(17) The percentage of cavities in films according to the invention in which cavities form only after stretching is established by determining the density of a film before and after stretching. Since the mass of the film does not change as a result of stretching, the volume of the cavities can be calculated using the change in the density, according to the following formula:
Cavity volume[%]=(1−(density after stretching/density before stretching))*100

(18) To determine the density according to DIN EN ISO 1183-1 (Part 1: “Immersion method”, April 2013), three samples are taken, which can have any shape and have a mass in the range of 1 to 2 g, as standard. The samples are immersed at a temperature in the range of 20-23° C. in distilled water which contains 0.1% of a surface-active agent (for example detergent) to remove the air bubbles on the surface of the samples (for example, film folded in any way). The samples are checked to ensure they are free of air bubbles. To determine the mass and temperature, measurement instruments having a weighing accuracy of less than 0.001 gram and ±0.5° C., respectively, are used; the reproducibility is approximately 0.005 g/cm.sup.3.

(19) If the density of the sample is >1.0 g/cm.sup.3 and it therefore sinks on immersion in the distilled water, the density is determined as follows:

(20) The density is determined as an average of 3 measurements according to the following equation: DI=(m1*DIFI)/(m1−m2) [g/cm.sup.3]

(21) Where:

(22) m1 stands for the determined mass of the sample in g in air,

(23) m2 stands for the determined mass of the sample in g immersed in the test liquid (according to the measurement method already reduced by the buoyancy of the sample holder), and

(24) DIFI stands for the density of the test liquid (water, 20° C., DIFI=0.998 g/cm.sup.3)

(25) If the density of the sample is ≤1.0 g/cm.sup.3 and it therefore floats on immersion in the distilled water, the density is determined as follows:

(26) The density is then—corrected by the weight—determined as an average of 3 measurements according to the following equation:
DI=(m1*DIFI)/(m1+m3−m4)[g/cm.sup.3]

(27) Where:

(28) m1 stands for the determined mass of the sample in g in air,

(29) m3 stands for the determined mass of the weight in g in the test liquid,

(30) m4 stands for the determined mass of the sample (incl. the weight) in g immersed in the test liquid (see note for m2 above), and

(31) DIFI stands for the density of the test liquid (water, 20° C., DIFI=0.998 g/cm.sup.3).

(32) The individual values of the three measurements are averaged and statistically evaluated. Determining the crystallisation half-time

(33) The crystallisation half-time of the copolyester used in the first polymer ply A is determined with the aid of a differential scanning calorimeter (DSC). The differential scanning calorimeter (DSC) is a standard method for measuring thermal properties, in particular the phase transition temperatures of solids. In the present invention, the crystallisation half-time is established by heating 15 mg of the polyester to be measured to 290° C. then cooled to a predetermined temperature of 180 to 210° C. at a rate of 320° C. per minute in the presence of helium, and the time span necessary until reaching the isothermal crystallisation temperature or the crystallisation peak of the DSC curve is detected. The crystallisation half-time is determined using the plot of the crystallisation over time. The crystallisation half-time corresponds to the time necessary, at the predefined temperature of 180 to 210° C., after the initial phase of crystallisation to achieve 50% of the maximum achievable crystallinity in the sample.

Example 1—Comparison of Different Comparative Films and Four Films According to the Invention

(34) The following abbreviations are used in the examples below: CE: Comparative example LV: Embrace LV (manufacturer: Eastman Chemical Company) HY: Embrace HY 1000 (manufacturer: Eastman Chemical Company) AB: Antiblock masterbatch (manufactured by Clariant) TiO.sub.2-MB: Masterbatch containing titanium dioxide (procured from Sukano AG, Schindellegi, Switzerland or from Clariant Produkte (Deutschland) GmbH, Frankfurt/Main, Germany) CB-MB: Masterbatch containing carbon black

(35) Titanium dioxide was added in the form of a PETG masterbatch with 70 wt % TiO.sub.2 to the compositions indicated in the tables below for the comparative examples and the examples according to the invention, carbon black was added in the form of a PETG masterbatch with 25 wt % carbon black.

(36) First, two single-ply films which are obtainable as specimens, have an identical composition and differ only in thickness were tested as comparative example 1 (CE1) and comparative example 2 (CE2). The mixture of 75% LV and 25% HY resulted after stretching in cavities which caused opacity of the heat-shrinkable film. The details of the film compositions and the results are shown in Table 1 below.

(37) TABLE-US-00001 TABLE 1 CE1 CE2 Ply 1 75% LV 75% LV 25% HY 25% HY Ply 2 — — Ply 3 — — Thickness 110 μm 50 μm Transmission 13.3% 28.3%

(38) For comparative example 2, the L* value was also determined, which was 96. The measured transmissions were more than 13% even at film thicknesses of 110 μm and therefore did not meet the requirements of light-tightness which must be met in particular for packaging light-sensitive foodstuffs. It was concluded therefrom that the necessary transmission values could not be achieved in single-ply, cavity-containing films without having to resort to film thicknesses which are unsuitable in practice.

(39) Then, several tests were carried out with 3-ply films, in which different combinations of the use of HY in different concentrations and/or in different plies and of the use of titanium dioxide as a pigment providing a white colour and thus blocking light were tested.

(40) The mixtures provided for the respective plies were supplied to an extruder in which they were melted, homogenised and conveyed to a feedblock at a corresponding pressure and at an extrusion temperature in the range of 240° C. to 180° C. In the feedblock, the respective melt streams are merged in a defined manner and extruded via a slot die to form a flat melt film. This melt film was transferred to a cooling roll and pulled out of the gap at the speed predefined by the rotation speed of the cooling roll, as a result of which a pre-film was obtained which was cooled to below the glass transition temperature.

(41) The cooled pre-film was heated again to a defined stretching temperature in the range of 75° C. to 100° C. while still inside the same production plant, supplied to a stretching unit, and stretched with a stretching factor in the range of 4 to 6 according to the stretching parameters specified in each case. Stretching took place in the transverse direction. After stretching and cooling, the now heat-shrinkable films was used for the other tests.

(42) The compositions of the film and the plies thereof according to comparative tests 3 to 7 and results of the tests are shown in Table 2 below.

(43) TABLE-US-00002 TABLE 2 CE2 CE4 CE5 CE6 CE7 Ply 1 99.5% LV 80% LV 89.5% LV 99.5% LV 99.5% LV 0.5% AB 20% HY 10% HY 0.5% AB 0.5% AB 0.5% AB Ply 2 70% LV 80% LV 75% LV 70% LV 70% LV 30% HY 20% TiO.sub.2-MB 25% HY 15% HY 30% TiO.sub.2-MB 15% TiO.sub.2-MB Ply 3 99.5% LV 80% LV 89.5% LV 99.5% LV 99.5% LV 0.5% AB 20% HY 10% HY 0.5% AB 0.5% AB 0.5% AB Thickness 12.5 μm 4 μm 4 μm 4 μm 4 μm 25 μm 32 μm 32 μm 32 μm 32 μm 12.5 μm 4 μm 4 μm 4 μm 4 μm Transmission 30.3% 35.1% 32.4% 31.6% 37.1%

(44) The figures relating to titanium dioxide given in Table 2 relate to the weight percentage of the corresponding masterbatch, which contains 70 wt % titanium dioxide (TiO.sub.2-MB).

(45) A transmission of not more than 15% was not achieved with any of the compositions.

(46) Then, films according to the invention having two plies were tested, of which one ply contained carbon black by addition of the above-indicated, carbon-black-containing masterbatch and thus corresponded to the first polymer ply A comprising at least one dark pigment, in the form of a first polyester ply A. To cover the black colour caused by the carbon-black-containing ply, the other ply contained titanium dioxide by addition of the above-indicated, titanium-dioxide-containing masterbatch and thus corresponded to the second polymer ply B comprising at least one white pigment, in the form of a second polyester ply B. When the second polyester ply B was viewed from above, a perfectly printable, white surface could be seen, which completely covered the underlying first polyester ply A in terms of colour. The exact composition and the obtained results are shown in Table 3.

(47) TABLE-US-00003 TABLE 3 Inventive Inventive Inventive Inventive example 1 example 2 example 3 example 4 Ply 1 70% LV 73% LV 70% LV 70% LV (corresponding 20% HY 17% HY 30% TiO.sub.2-MB 20% HY to ply B) 10% TiO.sub.2-MB 10% TiO.sub.2-MB 10% TiO.sub.2-MB Ply 2 67% LV 62% LV 77% LV 71% LV (corresponding 12% HY 17% HY 10% TiO.sub.2-MB 17% HY to Ply A) 9% TiO.sub.2-MB 9% TiO.sub.2-MB 12% CB-MB 9% TiO.sub.2-MB 12% CB-MB 12% CB-MB 1% AB 3% CB-MB Ply 3 — — Thickness 32 μm 32 μm 32 μm 32 μm Ply B Ply A 8 μm 8 μm 8 μm 8 μm Transmission 0.7% 0.9% 1.3% 6.3%

(48) The films according to the invention exhibited a light transmission of less than 10% and in some cases even less than 5%; even values below 1% could be achieved with a low film thickness of 40 μm in total. The measurements were carried out using the “BYK-Gardner haze-gard plus” measurement instrument; the film according to inventive example 2, for example, was measured with the “color i7” spectrophotometer, and transmissions of well below 0.1% were established for the range of 360 to 400 nm. The films were therefore also usable for critical applications in the food industry in which light-perishable foodstuffs must be protected. For inventive example 2, the L* value was also determined, which was 90.

(49) In view of this degree of whiteness, the B ply of the film was sufficiently white to allow a printing ink to be applied without being influenced by the substrate.

Example 2—Further Parameters of the Film According to the Invention

(50) In the obtained film according to the invention, the gloss at 60° according to test specification ASTM D-523, the roughness and the peak count (number of profile peaks per 10 mm reference length) were determined. The results are shown in comparison with paper, standard copy paper being used, in Table 4 below.

(51) TABLE-US-00004 TABLE 4 Film according to inventive example Paper 2 of Table 3 Gloss 60° 3.8 5.0 RZ 19.3 2.3 Peaks per cm 144 300

(52) The film according to the invention did not have a smooth surface, which can result in slipping, in particular if an object wrapped in such a film is grasped with moist or wet hands. Rather, the haptics were perceived as paper-like, which was attributable to the peaks present on the surface of the film according to the invention, which were in turn caused by the proportion of cavity-forming additive “HY”. Ply 1 of film 2 according to Table 3 corresponds to the second polymer ply B of the heat-shrinkable film. Ply 2 of inventive film 2 according to Table 3 corresponds to the first polymer ply A owing to its content of dark pigment in the form of carbon black. Since the first polymer play A and the second polymer ply B contained the same proportion of cavity-forming additive “HY”, the porosity of the two plies was identical. The shrinkage of the film after 15 seconds in a water bath at a temperature of 95° C. was 67%.

(53) Table 5 shows further property parameters of inventive film 2 from Table 3:

(54) TABLE-US-00005 TABLE 5 Measurement Typical Parameter standard Unit value Thickness ISO 4593 μm 40 Thickness tolerance % ±10 Density DIN EN ISO 1183-2 g/cm.sup.2 1.22 Density tolerance % ±2 Material yield at 40 μm DIN EN ISO 2286-2 m.sup.2/kg 20.5 Shrinkage in transverse DIN 53377 % 67 direction 15 sec. in water at Shrinkage tolerance in 95° C. pp ±2 transverse direction Shrinkage in machine % −1 direction Shrinkage tolerance in pp ±2 machine direction Tensile strength in DIN EN ISO 527 MPa 100 transverse direction V = 50 mm/min Tensile strength in MPa 40 machine direction E-modulus in transverse MPa 3400 direction E-modulus in machine MPa 1500 direction

(55) Although the invention has been illustrated and explained in detail by means of preferred exemplary embodiments, the invention is not limited by the examples disclosed, and other variations can be derived therefrom by a person skilled in the art without departing from the scope of the invention. It is therefore clear that a large number of variation possibilities exists. It is likewise clear that embodiments mentioned by way of example are really only examples which should not be understood in any way as a limitation of the scope of protection or of the possibilities for using the invention. Rather, the above description enables a person skilled in the art to implement the exemplary embodiments in practice; in so doing, a person skilled in the art with knowledge of the disclosed concept of the invention can make various modifications, for example in terms of the function or the arrangement of individual elements mentioned in an exemplary embodiment, without departing from the scope of protection as defined by the claims and the legal equivalents, such as more detailed explanation in the description. It is likewise possible to combine different embodiments as described herein or individual features of embodiments.

INDUSTRIAL APPLICABILITY

(56) The heat-shrinkable film and a hose comprising such a heat-shrinkable film are suitable, for example, for wrapping objects in a light-tight manner. Furthermore, an object wrapped in such a film or such a hose is suitable for accommodating light-sensitive foodstuffs.