Collation shrink films

Abstract

A process for collation shrink wrapping an object that may include a number of individual product containers, preferably substantially identical product containers. The process may include the steps of (i) obtaining a collation shrink film that may include a multimodal linear low density polyethylene (LLDPE), the film being a stretched film that is uniaxially oriented in the machine direction (MD) in a draw ratio of at least 1:3, (ii) wrapping the object in the film, and (iii) heating the object wrapped in the film in order to collation shrink the film around the object.

Claims

1. A process for collation shrink wrapping an object which comprises a plurality of individual product containers, preferably substantially identical product containers, comprising: (i) obtaining a collation shrink film comprising a multimodal linear low density polyethylene (LLDPE), said film being a stretched film which is uniaxially oriented in the machine direction (MD) in a draw ratio of at least 1:3; (ii) wrapping said object in said film; and (iii) heating said object wrapped in the film in order to collation shrink said film around said object.

2. A process as claimed in claim 1 comprising: (i) obtaining a collation shrink film comprising a multimodal linear low density polyethylene (LLDPE), said film being a stretched film which is uniaxially oriented in the machine direction (MD) in a draw ratio of at least 1:3 on a spool; (ii) dispensing film from said spool and cutting the film into an appropriate length to wrap around said object; (iii) wrapping said object in said film; and (iv) heating said object wrapped in the film in order to collation shrink said film around said object.

3. A process as claimed in claim 1, whereas said collation shrink film has a thickness of 15 to 40 microns.

4. A process as claimed in claim 1, wherein the object wrapped in the film is heated to a temperature of 170 C. or less.

5. A process as claimed in claim 1, wherein said multimodal polyethylene is formed from a lower molecular weight homopolymer and a higher molecular weight copolymer or a lower molecular weight copolymer and a different higher molecular weight copolymer.

6. A process as claimed in claim 1, wherein said collation shrink film is free of HDPE and LDPE.

7. A process as claimed in claim 1, wherein said collation shrink film comprises a unimodal LLDPE.

8. A process as claimed in claim 1, wherein the collation shrink film is a monolayer film.

9. A process as claimed in claim 1, wherein said collation shrink film comprises a multilayer film having at least two layers A and B.

10. A process as claimed in claim 9 wherein layer (A) comprising a unimodal LLDPE and a multimodal LLDPE.

11. A process as claimed in claim 9, wherein layer (B) comprises a multimodal LLDPE.

12. A process as claimed in claim 1, wherein said collation shrink film is free of LDPE.

13. A process as claimed in claim 1, wherein said collation shrink film comprises a multilayer film having at least three layers ABC.

14. An object which comprises a plurality of individual product containers collation shrink wrapped by a collation shrink film comprising a multimodal linear low density polyethylene (LLDPE), said film being a stretched film which is uniaxially oriented in the machine direction (MD) in a draw ratio of at least 1:3.

15. A collation shrink film comprising a multimodal linear low density polyethylene (LLDPE), said film being a stretched film which is uniaxially oriented in the machine direction (MD) in a draw ratio of at least 1:3 and which shrinks by at least 50% in the machine direction at a temperature of 170 C. or less and preferably has a thickness of 50 microns or less.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 shows the relative machine direction shrinkage properties of the films of the invention relative to a non oriented reference film at various different temperatures.

(2) FIG. 2 shows the cold shrink force as a function of shrink rate at different temperatures (100-200 C.).

DETERMINATION METHODS

(3) Density of the materials is measured according to ISO 1183:1987 (E), method D, with isopropanol-water as gradient liquid. The cooling rate of the plaques when crystallising the samples was 15 C./min. Conditioning time was 16 hours.

(4) Melt Flow Rate (MFR) or Melt Index (MI)

(5) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190 C. for PE and at 230 C. for PP. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR.sub.2 is measured under 2.16 kg load, MFR.sub.5 is measured under 5 kg load or MFR.sub.21 is measured under 21.6 kg load.
Molecular Weights, Molecular Weight Distribution, Mn, Mw, MWD
The weight average molecular weight Mw and the molecular weight distribution (MWD=Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) is measured by a method based on ISO 16014-4:2003. A Waters 150CV plus instrument, equipped with refractive index detector and online viscosimeter was used with 3HT6E styragel columns from Waters (styrene-divinylbenzene) and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 140 C. and at a constant flow rate of 1 mL/min. 500 L of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 10 narrow MWD polystyrene (PS) standards in the range of 1.05 kg/mol to 11 600 kg/mol. Mark Houwink constants were used for polystyrene and polyethylene (K: 1910.sup.3 dL/g and a: 0.655 for PS, and K: 3910.sup.3 dL/g and a: 0.725 for PE). All samples were prepared by dissolving 0.5-3.5 mg of polymer in 4 mL (at 140 C.) of stabilized TCB (same as mobile phase) and keeping for 2 hours at 140 C. and for another 2 hours at 160 C. with occasional shaking prior sampling in into the GPC instrument.
Comonomer Content (% wt and % mol) was determined by using .sup.13C-NMR. The .sup.13C-NMR spectra were recorded on Bruker 400 MHz spectrometer at 130 C. from samples dissolved in 1,2,4-trichlorobenzene/benzene-d.sub.6 (90/10 w/w). Conversion between % wt and % mol can be carried out by calculation.
Haze is measured according to ASTM D 1003.
Cold shrink forces have been measured according to standard ISO 14616:1997 in Machine (MD) direction in the following way. Specimens of 15 mm width and 115 mm length are cut out from the film sample MD. The samples are tightly mounted into the jaws of the load cell in such a way that the distance between the jaws is 100 mm and the actual force is zero. The samples are then exposed to hot air at given temperature. After closing the chamber the shrinkage temperature is reached at the maximum force. This is recorded and it represents the hot shrink force. The hot air chamber is removed after force has reduced 15-30% from maximum while continuing to record the force. The maximum force is again recorded and this second maximum represents the cold shrink force.

Example 1

(6) Two 25 micron uniaxially stretched films were produced from two different multimodal LLDPEs. Grade 1 was a multimodal Ziegler Natta linear low density polyethylene having a density of 923 kg/m.sup.3 and an MFR.sub.2 of 0.4 g/10 min. Grade 2 is a multimodal Ziegler Natta linear low density polyethylene having a density of 931 kg/m.sup.3 and an MFR.sub.2 of 0.2 g/10 min.

(7) The inventive films were made in a first step on a blown film line at a thickness of 150 m, BUR 1:3 and die gap 1,4 mm. The film structure is a monolayer film consisting of Grade 1 or Grade 2 (IE1 or IE2 respectively).

(8) The film is then stretched on an MDO unit with a stretch ratio of 6 at a temperature of 110 C. This is done by unwinding the film and feeding it into the MDO unit. First the heating rolls get the film to up to a temperature of 110 C., then the film is stretched in between the stretching rolls due to 6 times higher speed of the second stretching roll over the first one. In an annealing and cooling step the orientation in the film is fixed.

(9) This film was compared to a 30 micron non oriented monolayer film formed from a blend of multimodal znLLDPE (45 wt % of density 931 kg/m.sup.3 and MFR.sub.2 0.2 g/10 min)/HDPE (40 wt % of density 958 kg/m.sup.3 and MFR.sub.2 of 0.7 g/10 min) and/LDPE (15 wt % of density 920 kg/m.sup.3 and MFR.sub.2 of 0.3 g/10 min).

(10) Shrinkage in Oil:

(11) Comparative determination of shrinkage was performed in oil according to internal Borealis method. 50 mm50 mm samples are placed in oil (Polydimethylsiloxan) at given temperature for 10 seconds. After this the samples are removed, and conditioned at room temperature for 1 hour. Finally the shrinkage, i.e. change in dimension is measured. Results are presented in table 1. Shrinkage values are calculated as:
Shrinkage=(LoLm)*100/Lo
wherein Lo is the original length (i.e. 50 mm), and Lm is the length measured after thermal exposure.

(12) If the measured value increases (e.g. in the TD direction), then shrinkage is negative.

(13) TABLE-US-00001 TABLE 1 Shrink Non Temperature oriented (in Oil) Parameter Unit film Film IE1. Film IE2 80 C. Length Md After mm 50 46.3 45.5 80 C. Length Td After mm 50 50.2 50 80 C. Shrinkage Md % 0 7.4 9 80 C. Shrinkage Td % 0 0.4 0 100 C. Length Md After mm 50 39.2 37 100 C. Length Td After mm 50 51.6 51.2 100 C. Shrinkage Md % 0 21.6 26 100 C. Shrinkage Td % 0 3.2 2.4 120 C. Length Md After mm 48.7 9.5 12.1 120 C. Length Td After mm 50 53.5 52.9 120 C. Shrinkage Md % 2.6 81 75.8 120 C. Shrinkage Td % 0 7 5.8 140 C. Length Md After mm 11.5 4 4 140 C. Length Td After mm 46.6 44.2 44 140 C. Shrinkage Md % 77 92 92 140 C. Shrinkage Td % 6.8 11.6 12

Example 2

(14) Mechanical data from the films are presented in Table 2 along with further shrink ratio data. Table 2 shows shrink data occurring the shrink force tests. Due to thermic conductivity, the shrinkage is different than at the same temperature in oil.

(15) TABLE-US-00002 TABLE 2 Shrink Parameter Non oriented Film Film Temp. (MD always) Unit film IE1 IE2 100 C. Cold Shrink Force N nd 1.86 1.93 100 C. Shrink Ratio % nd 15.11 18.73 120 C. Cold Shrink Force N 0.18 2.44 2.59 120 C. Shrink Ratio % nd 47.78 44.63 150 C. Cold Shrink Force N 1.62 0.91 3.17 150 C. Shrink Ratio % 12.87 80.7 80.6 180 C. Cold Shrink Force N 2.09 nd nd 180 C. Shrink Ratio % 71.2 87.43 90.3 200 C. Cold Shrink Force N 2.03 nd nd 200 C. Shrink Ratio % 71.23 93.17 91.67 Nd not determined

(16) Shrink ratios % length that the film is shorter after heat application.