0
⁢
0

formula
⁢

I

wherein

- T_n−2is the moment calculated according to the formula II:

T

n
-
2

=

∑

w
⁡
(
i
)

T
⁡
(
i
)

2

∑

w
⁡
(
i
)

T
⁡
(
i
)

3

formula
⁢

II

and

- T_z+2is the moment calculated according to the formula III:

T

z
+
2

=

∑

w
⁡
(
i
)

·

T
⁡
(
i
)

4

∑

w
⁡
(
i
)

·

T
⁡
(
i
)

3

formula
⁢

III

wherein

- w(i) is the sampled weight fraction in wt % with regard to the total sample weight in a-TREF analysis of a sample (i) taken at temperature T(i), where T(i)>30° C., the area under the a-TREF curve being normalised to surface area=1 for T(i)>30° C.; and
- T(i) is the temperature at which sample (i) is taken in a-TREF analysis, in ° C.

In the multilayer film of the present invention, the second ethylene-based polymer may for example have a chemical composition distribution broadness (CCDB) of ≥5.0 and ≤25.0, preferably ≥5.0 and ≤12.5 or ≥15.0 and ≤25.0.

In the context of the present invention, it is to be understood that a bi-directionally oriented film wherein the orientation in both directions is introduced in the solid state is nota blown film. In a blow film, the orientation is introduced in molten state. It is preferred that the orientation of the film of the present invention is performed at a temperature of at least 10° C. below the melting temperature of the film.

The invention will now be illustrated by the following non-limiting examples.

The following materials were used in the examples according to the present invention:

HDPE1
A high-density polyethylene homopolymer

with properties as in the table below

HDPE2
A high-density polyethylene homopolymer

with properties as in the table below

LLDPE1
SABIC LLDPE BX202, a linear low-density polyethylene

LLDPE2
SABIC SUPEER 8112, a linear low-density polyethylene

LLDPE3
SABIC COHERE 8402, a linear low-density polyethylene

CA1
CON-X FL 826 PE, a cavitating masterbatch

comprising 60.0 wt % calcium carbonate in

polyethylene carrier, obtainable from Constab

CA2
Granic D105, a cativating masterbatch comprising

70.0 wt % calcium carbonate in polyethylene

carrier, obtainable from Granic

In the table below, key properties of the polyethylene materials are presented.

Material
HDPE1
HDPE2
LLDPE1
LLDPE2
LLDPE3

MFR2
0.7
8.0
2.1
1.0
3.5

Density
961
967
921
912
902

T _pm
134
134
124
108
100

T _C
118
118
111
95
81

Ethylene units
100.0
100.0
89.0
88.1
81.5

content

Comonomer
0.0
0.0
11.0
11.9
18.5

units content

Comonomer type
0.0
0.0
C6
C8
C8

M_n
9
11
18
32
24

M_w
105
72
109
95
70

M_z
670
324
463
205
145

M_w/M_n
11.3
6.3
6.0
3.0
2.9

M_z/M_w
6.3
4.5
4.2
2.2
2.1

M_z/M_n
74
28.4
25.4
6.4
6.4

a-TREF < 30
0.0
0.0
13.5
0.9
2.9

a-TREF 30-94
0.0
0.0
50.9
99.1
97.1

a-TREF > 94
100.0
100.0
35.6
0.0
0.0

CCDB

8.6
18.6

Wherein:

- the MFR2 is the melt mass flow rate as determined in accordance with ASTM D1238 (2013) at a temperature of 190° C. under a load of 2.16 kg, expressed in g/10 min;
- the density is determined in accordance with ASTM D792 (2008), expressed in kg/in³;
- T_pmis the peak melting temperature as determined using differential scanning calorimetry (DSC) in accordance with ASTM D3418 (2008), expressed in ° C.;
- T_cis the crystallisation temperature as determined using differential scanning calorimetry (DSC) in accordance with ASTM D3418 (2008), expressed in ° C.;
- the ethylene units content indicates the weight quantity of units present in the polymer that are derived from ethylene, also referred to as the quantity of moieties derived from ethylene, with regard to the total weight of the polymer, expressed in wt %;
- the comonomer content indicates the weight quantity of units present in the polymer that are derived from the comonomer, also referred to as the quantity of moieties derived from the comonomer, with regard to the total weight of the polymer, expressed in wt %;
- the comonomer type indicates the type of comonomer used in the production of the polymer, where C6 is 1-hexene and C8 is 1-octene;
- the content and type of comonomer were determined using ¹³C Nuclear Magnetic Resonance on a Bruker Avance 500 spectrometer equipped with a cryogenically cooled probe head operating at 125° C., whereby the samples were dissolved at 130° C. in C₂D₂Cl₄containing DBPC as stabiliser;
- M_nis the number average molecular weight, M_wis the weight average molecular weight, and M_zis the z-average molecular weight, wherein M_n, M_w, and M_zare each expressed in kg/mol, and determined in accordance with ASTM D6474 (2012);
- a-TREF<30 indicates the fraction of the polymer that is eluted in a-TREF according to the method presented above in the temperature range ≤30.0° C., expressed in wt %, and represents the amorphous fraction of the polymer, calculated by subtracting the a-TREF 30-94 and the a-TREF>94 fraction from 100.0 wt %;
- a-TREF 30-94 indicates the fraction of the polymer that is eluted in a-TREF in the temperature range of >30.0 and ≤94.0° C., expressed in wt %, and represents the branched fraction of the polymer;
- a-TREF>94 indicates the fraction of the polymer that is eluted in a-TREF in the temperature range of >94.0 and <140° C., expressed in wt %, and represents the linear fraction of the polymer;
- the CCDB is the chemical composition distribution broadness calculated according to the method described herein above.

The a-TREF analyses were carried out using a Polymer Char Crystaf TREF 300 device using a solution containing 4 mg/ml of sample in 1,2-dichlorobenzene stabilised with 1 g/l Topanol CA (1,1,3-tri(3-tert-butyl-4-hydroxy-6methylphenyl)butane) and 1 g/l Irgafos 168 (tri(2,4-di-tert-butylophenyl)phosphite) at a temperature of 150° C. for 1 hour. The solution was further stabilised for 45 minutes at 95° C. under continuous stirring at 200 rpm before analyses. For analyses, the solution was crystallised from 95° C. to 30° C. using a cooling rate of 0.1° C./min. Elution was performed with a heating rate of 1° C./min from 30° C. to 140° C. The set-up was cleaned at 150° C.

Using the above polymers, three-layer bi-directionally oriented films were produced. The bi-directionally oriented films were produced using a cast film production line with subsequent tenter frame type sequential biaxial orientation. A set-up comprising three melt extruders was used, where an extruder A supplied material for a first skin layer A, an extruder B supplied material for inner layer B, and an extruder C supplied the material for the second skin layer C. The extruders were positioned such that the molten material was forced through a t-shaped die, so that the arrangement of the layers in the obtained cast film was A/B/C. Each of the extruders A, B and C was operated such to supply molten polymer material at a temperature of 265° C. The throughput was 135 kg/h.

The film as extruded through the t-shaped die was cast onto a chill roll having a temperature of 70° C. to form a cast film.

The chilled cast film was subjected to stretching in the machine direction using a set of stretching rolls at a temperature of 90-105° C., followed by an annealing at 80° C., to induce a degree of stretching in the machine direction of 5.

Subsequently, the film was stretched in the transverse direction to a degree of stretching of 7 by subjecting the film to heat whilst applying a stretching force, wherein the film was passed through an oven through which the film was continuously transported, wherein the temperature was 160° C. at the entering zone of the oven, decreasing to 115° C. towards the exit of the oven. The skin layer A was subsequently subjected to a corona treatment of 25 W·min/m².

For each example, bi-directionally oriented 3-layer films were obtained.

The composition of the experimental films is presented in the table below. The skin layers A and C in all examples contained 3.0 wt % antiblock agent CON-X AB 664 and 5.0 wt % slip agent CON-X SL 577, both available from Constab.

Example
Layer
Material composition
Layer weight

E1
A
92.0% LLDPE1
10.0

B
55.0% HDPE1, 30.0%
80.0

HDPE2, 15.0% LLDPE1

C
92.0% LLDPE2
10.0

E2
A
92.0% LLDPE1
10.0

B
50.0% HDPE1, 50.0% LLDPE1
80.0

C
92.0% LLDPE3
10.0

E3
A
92.0% LLDPE1
10.0

B
55.0% HDPE1, 30.0%
80.0

HDPE2, 15.0% CA1

C
92.0% LLDPE2
10.0

CE1
A
92.0% LLDPE1
15.0

B
80.0% HDPE1, 20.0% LLDPE1
70.0

C
92.0% LLDPE1
15.0

CE2
A
92.0% LLDPE1
10.0

B
70.0% HDPE1, 30.0% HDPE2
80.0

C
92.0% LLDPE1
10.0

CE3
A
92.0% LLDPE1
10.0

B
85.0% HDPE1, 15.0% CA2
80.0

C
92.0% LLDPE1
10.0

Wherein the percentage in the material composition relates to the quantity of the particular material, in wt % with regard to the total weight of the material of that given layer, and wherein the layer weight indicates the percentage of the weight of the given layer with regard to the total weight of the given experimental film. In the above table, examples E1-E3 are according to the invention, CE1-CE3 are comparative.

Of the thus obtained films, a set of properties were determined as indicated in the table below.

Example

E1
E2
E3
CE1
CE2
CE3

Thickness
25
25
37
18
23
39

Haze
23
14

11
26

Film density

703

585

TM-MD
1281
765
925
1164
1681
609

TM-TD
1393
897
1491
1744
2353
1041

TS-MD
99
98
57
52
121
58

TS-TD
95
69
67
183
113
93

EL-MD
236
163
201
290
198
105

EL-TD
96
117
14
11
41
15

Thermal Shrinkage-MD
2.0
1.1
1.0
0.6
1.0
0.6

Thermal Shrinkage-TD
0.0
0.2
3.4
1.1
2.3
0.7

Wherein

- Thickness is the thickness of the film as obtained after the bi-directional orientation, expressed in μm;
- Haze is determined in accordance with ASTM D1003 (2013), expressed in %;
- Film density of the density of the film as obtained after the bi-directional orientation, expressed in kg/m³; the density of the cavitated films of examples E3 and CE3 was determined by weighing of a sample of 100×100 mm²film, and calculating the density by dividing the weight of the sample by the volume of the sample;
- TM is the tensile modulus, determined in the machine direction (MD) and transverse direction (TD) of the film, expressed in MPa, determined as 1% secant modulus in accordance with ASTM D882-18, using an initial sample length of 250 mm and a testing speed of 25 mm/min, at room temperature, using preload of 1 N;