WO2008031540A2

WO2008031540A2 - Food packaging

Info

Publication number: WO2008031540A2
Application number: PCT/EP2007/007855
Authority: WO
Inventors: Marcellinus Guilliame Marie Neilen; Jacobus Christinus Josephus Franciscus Tacx
Original assignee: Saudi Basic Industries Corporation
Priority date: 2006-09-11
Filing date: 2007-09-10
Publication date: 2008-03-20
Also published as: EP2069145A2; WO2008031540A3

Abstract

The invention is directed to a food packaging material having a multi layered structure comprising a low density polyethylene containing inside layer. The inside layer contains a blend comprising low density polyethylene and Ziegler-Natta catalysed linear low density polyethylene. The blend may comprise - 20-80% by weight of a low density polyethylene having a density between 910 kg/m3 and 935 kg/m3 (according to ISO 1183) and a melt index between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and - 20-80% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m3 and 935 kg/m3 (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100% by weight.

Description

FOOD PACKAGING

The invention is directed to a food packaging material having a multi layered structure comprising a low density polyethylene containing inside layer.

As disclosed in the article "On the fracture of thin laminates" by Sharon Kao-Walter (Blekinge Institute of Technology; Dissertation Series No. 2004:07; ISSN 1650-2159; ISBN 91-7295-048-X; Published 2004; Karlskrona 2004) food packages have become part of our daily life. Thanks to developments in packaging industries, it has become possible to buy well- stored food produced thousands of miles away. At the same time, packaging companies as well as consumers pay more and more attention to environmental and sustainability aspects. An example of a food packaging material is an aseptic liquid food package. Such a package may provide a shelf life-time up to one year and a typical laminate structure of the package comprises layers of for instance low density polyethylene (LDPE) -paperboard-LDPE-aluminium foil-adhesive-LDPE.

LDPE outside layer

Paperboard

LDPE intermediate layer

Al-foil

Adhesive

LDPE inside layer

Each of the layers in this laminated structure contributes certain properties to the structure. The upper outside layer of LDPE is used to protect the paperboard from direct contact with water and to avoid moisture from penetrating into the paperboard. The paperboard layer mainly contributes to grip stiffness of the package. The outside layer may also function as the decor layer. A second intermediate LDPE layer is used as laminate layer to laminate the paperboard and aluminium foil (Al-foil) together. The Al-foil prevents oxygen and light from reaching the packed food product. The last layer of the LDPE is laminated to the Al-foil by an adhesive layer, which increases the adhesion to a desired level. This inner LDPE layer is the layer that is actually in contact with the food. The coating weight of the outside LDPE layer may range between

10 and 20 g/m², the coating weight of the intermediate LDPE layer between the paperboard and the foil may range between 15 and 20 g/m², the coating weight of the inside LDPE layer ranges between 30 and 50 g/m² and the average coating weight of the inside LDPE layer is about 40 g/m². The packaging companies as well as the consumers pay attention to environmental and sustainability aspects. Using fewer raw materials, reducing costs and developing the functionalities of the packages have become goals for research and development. There exist a lot of theoretically options such as for example in the field of using thinner layers of different materials, using less material by changing the shape of the package, using recycled material and using refillable packages. However these solutions often do not result in a packaging that fulfils all requirements for providing a good protection to the packaged products.

It is an object of the present invention to provide a package which has improved material efficiency and which is suitable for packaging liquid and dry food. In case of savings on packaging material these savings may not result in a less efficient product and the packaging requirements needed for a good protection of the packaged products should still be met.

The invention is directed to a food packaging material having a laminated structure comprising a low density polyethylene containing inside layer wherein the inside layer contains a blend of low-density polyethylene and Ziegler-Natta catalysed linear low-density polyethylene.

The use of the blend comprising LDPE and Ziegler-Natta catalysed linear low-density polyethylene (ZNLLDPE) results in a reduction of the inside layer thickness as compared with an inside layer made from LDPE.

The coating weight of the inside LDPE layer comprising the blend according to the invention ranges between 15 and 30 g/m². The average coating weight of the inside layer comprising the blend according to the invention is about 20 g/m². The down gauging from about 40 g/m² to about 20 g/ m² is a considerable improvement with respect to material efficiency.

Surprisingly the improvement regarding down gauging is obtained while maintaining also the required performance regarding for example sealability, hot tack, organoleptics, puncture resistance and processing characteristics such as for example neck- in, web width variation, draw down and motor load.

The present invention results in the required rheological properties to ascertain good web width variation, neck in (shrinkage in width of the LDPE comprising web) and draw down (the maximum line speed at which the LDPE comprising web breaks).

Surprisingly the present invention has also advantages in comparison with a co extruded layer consisting of two separate layers of LDPE and LLDPE. Preferably the blend according to the invention comprises

- 20-80% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index (MFR) between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and

20-80% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100% by weight.

More preferably the blend comprises

- 30-70% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index (MFR) between 1 dg/minute and 20 dg/minute (according to ASTM

D 1133) and

- 30-70% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1 183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100% by weight. According to a further preferred embodiment of the invention the blend comprises

^' 45-55% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index (MFR) between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and

- 45-55% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100% by weight.

Preferably the MFR ratio of the low density polyethylene and the Ziegler-Natta catalysed copolymer ranges between 1 :3 and 3:1. Preferably the density of the low density polyethylene ranges between 915 kg/m³ and 925 kg/m³.

Also other layers of the packaging may comprise the blend comprising LDPE and Ziegler-Natta catalysed LLDPE.

If desired the inner LDPE layer may also be composed of for example a layer of LDPE and a layer of the blend comprising of LDPE and Ziegler-Natta catalysed LLDPE.

The blend according to the invention may also comprise for example other polymers to improve specific characteristics of the end product. These polymers may be present in an amount of for example between 0.1 and 15 % by weight.

The packaging material obtained with the blend according to the present invention is preferably obtained with extrusion coating. Liquid packaging is the largest market segment for extrusion coating in Europe. Today LDPE produced by using high-pressure autoclave technology is the commercially applied polyethylene for use in extrusion coating applications. LDPE obtained with an autoclave process is suitable to be applied in extrusion coating for reasons of processing (for example web stability, draw-down and neck-in) in relation to the molecular composition (broad molecular weight distribution, long chain branching) of the polymer. In the extrusion coating process polymers and substrates are combined to form products with specific synergetic characteristics. The increasing processing and product requirements and quality demands may result in several different problems that can occur in the extrusion coating process. Examples of these problems are edge waving, edge tear, web break, gels, streaks, lacing, transfer thickness variation, machine thickness variation and die deposits. In extrusion coating, the thin molten polymer film is coated on the substrate. At high extrusion coating speed, even a minor disturbance on the melt web causes major quality problems that can very rapidly lead to large quantities of waste. Therefore polymers are required with high and consistent quality to avoid waste due to polymer edge instability and web breaks.

The production processes of polyethylene such as low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) are summarised in Handbook of Polyethylene by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43-66. The catalysts can be divided in three different subclasses including Ziegler Natta catalysts, Phillips catalysts and single site catalysts. The latter class is a family of different classes of compounds, metallocene catalysts being one of them. As elucidated at pages 53-54 of said Handbook a Ziegler-Natta catalysed polymer is obtained via the interaction of an organometallic compound or hydride of a Group l-lll metal with a derivative of a Group IV-VIII transition metal. An example of a (modified) Ziegler-Natta catalyst is a catalyst based on titanium tetra chloride and the organometallic compound triethylaluminium. A difference between metallocene catalysts and Ziegler Natta catalysts is the distribution of active sites. Ziegler Natta catalysts are heterogeneous and have many active sites. Consequently polymers produced with these different catalysts will be different regarding for example the molecular weight distribution and the comonomer distribution.

The LDPE applied in the blend may be produced by use of autoclave high pressure technology and may be produced by tubular reactor technology.

According to a preferred embodiment of the invention LDPE has been produced by polymerising ethylene in a tubular reactor.

The polymerisation may take place in the presence of a di-or higher functional (meth) acrylate co monomer such as for example 1 ,4-butane diol dimethacrylate (BDDMA), hexane diol dimethacrylate (HDDMA)₁ 1 ,3- butylene glycol dimethacrylate (1 ,3-BGDMA), ethylene glycol dimethacrylate (EGDMA) , dodecanediol dimethacrylate (DDDMA), trimethylol propane trimethacrylate (TMPTMA) and/or trimethacrylate ester (TMA ester). A preferred comonomer is 1 ,4-butane diol dimethacrylate (BDDMA).

As disclosed in WO 2006/094723 the polymerisation in the tubular reactor may take place at a peak temperature between 29O⁰C and 35O⁰C wherein the di-or higher functional (meth) acrylate is present in an amount between 0.008 mol % and 0.200 mol % relative to the amount of ethylene copolymer.

The linear low density polyethylene component of the composition is a low density polyethylene copolymer comprising ethylene and a C₃- C₁₀ alpha-olefin co monomer. Suitable alpha-olefin co monomers include butene, hexene, 4-methyl pentene and octene. Preferably, the alpha-olefin co monomer is present in an amount of about 5 to about 20 percent by weight of the ethylene-alpha olefin copolymer, more preferably an amount of from about 7 to about 15 percent by weight. Preferred co monomers are butene and hexene.

The technologies suitable for the LLDPE manufacture include gas- phase fluidized-bed polymerization, polymerization in solution, polymerization in a polymer melt under very high ethylene pressure, and slurry polymerization.

According to a preferred embodiment of the present invention the LLDPE has been obtained with gas phase polymerisation. The blend according to the present invention comprises Ziegler-Natta catalysed LLDPE. A blend comprising metallocene catalysed LLDPE instead of Ziegler-Natta catalysed LLDPE is excluded. Applying these different blends will result in different products.

The invention also relates to a process for producing a food packaging material having a multi layered structure comprising a low density polyethylene containing inside layer, said process comprising a step of extrusion coating the inside layer from a blend of low-density polyethylene and Ziegler-Natta catalysed linear low-density polyethylene. Preferred processes of the invention are analogous to the discussion on preferred blends as above.

The invention will be further elucidated by the following non- restrictive examples.

Examples and Comparative Experiments

A packaging was produced on the SABIC Pilot Extrusion Coating Line. This line is disclosed in the presentation "Statistical Models to describe the correlations between the molecular mass distribution and the extrusion coating process ability" by Marcel Neilen on the 2003 TAPPI 9^th European PLACE Conference, May 12-14, 2003 in Rome.

The packaging comprised as layers a LDPE outside layer, a paperboard substrate, an adhesive and an inside layer comprising LDPE and LLDPE wherein

- the paperboard substrate was Kraft paper 60 g/m², width 800 mm,

- the adhesive tie layer was Escorene ®5020 (EAA) of Exxon

- the LDPE applied in the outside layer and in the inside layer was SABIC ®LDPE 1808AN00 of SABIC (density 919 kg/m³; melt index 7.5 dg/minute) and

- the LLDPE applied in the inside layer was SABIC ®LLDPE 6318B of SABIC (density 921 kg/m³; melt index 2,8 dg/minute ; polydispersity 5).

Comparative Example C is directed to the inner layer of a co extruded product consisting of two layers wherein the first layer is a LDPE layer with a thickness of 10 g/m² and wherein the second layer is a LLDPE layer with a thickness of 10 g/m².

The extrusion coating process was performed on the above- described extrusion coating line. The composition of the inner layer applied in the Examples I-IV according to the invention and the comparative Experiments A-C are given in Table 1.

The evaluation of the processing and the end properties of the inner layer are summarized in Table 2 and in Table 3. Preferred values for the characteristics are:

Motor load: as low as possible. The acceptable level is determined by the power of the extruder. T melt: maximum 305⁰C.

Neck-in: lower than 150 mm; this is finally determined by the die width substrate combination at the customer. Web width variation: between 1 and 3 mm. Draw down: as high as possible.

Friction coefficient: as low as possible; preferred to be maximum at pure LDPE level.

Seal strength: higher than 18 N/mm; minimum at pure LDPE level. Trouser test: as high as possible.

Puncture resistance: as low as possible, however the customer has the possibility to provide solutions in case this value has not the initial desired value lower than 300 mJ/mm.

TABLE 1

TABLE 2

TABLE 3

The web stability , the neck in and the draw down were determined using the SABIC Pilot Extrusion Coating Line as disclosed in the presentation "Statistical Models to describe the correlations between the molecular mass distribution and the extrusion coating processability " by Marcel Neilen on the 2003 TAPPI 9^th European PLACE Conference, May 12-14, 2003 in Rome. The neck-in is the shrinkage in width of the web in comparison with the internal die width.

The applied conditions of the coating equipment were as follows: - Screw diameter 11.4 mm - Die width 1100 mm

- Die gap 0.6 mm

- Die temperature 290⁰C

- Air gap 280 mm - Chill roll temperature 11⁰C

The drawdown was determined by increasing the line speed in increments of 50m/min. during the draw down determination the extruder throughput was kept constant and fixed on kg/ m² at a starting velocity of 200 m/min. The samples were taken under the following conditions: -Line speed 400 m/min

-Temperature extruder die 29O⁰C -Chill roll mat

-Chill roll temperature set at 1 1⁰C

The other coating line processing variables such as for example die gap and air gap were kept constant. The neck-in was determined at a line speed of 200 m/min.

For the determination of the liquid packaging characteristics experiments with and without substrates were applied. Coated substrate samples were used to determine seal curve and friction coefficient.

For the determination (on the SABIC Pilot Extrusion Coating Line) of the puncture resistance and the trouser test, an isolated coating was used. For this, a mylar film (12 μm and a width of 300 mm) was positioned at the centre of the Kraft paper via a secondary unwinder, downstream of corona Station 1. Station 2 and 3 were switched off. The coating at this centred PET film was peeled off for analysis. These samples were stored at 4-6°C.

The other characteristics mentioned in Table 2 and in Table 3 were evaluated as follows:

Puncture resistance Indicates: packaging integrity (toughness; creating hole; elongation)

ISO method 2389

Diameter ball: 2.35 mm

Test speed: 5 mm/min

Trouser test Indicates: toughness (propagation of an existing hole/tear)

ISO method 2570

Specimen width: 24 mm

Tear length: 75 mm

Test speed: 250 mm/min

Machine direction and transverse direction

Seal strength Indicates: seal quality during storage

DSM method 2711

Temperature range 90-140⁰C with increments of 5°C.

Seal pressure: 3 bar

Seal time: 3 sec

Specimen width: 24 mm

Friction coefficient Indicates: packaging speed

DSM method 2385

Test speed: 15 cm/min

Sledge size: 150^*100mm

68O g

Static and dynamic. Besides the down gauging of the food packaging by applying an inner layer with a blend according to the Examples I-IV these blends also resulted in an acceptable combination of packaging characteristics such as for example the motor load, neck-in, web width variation, draw-down, friction coefficient, seal strength, trouser test and puncture resistance.

Comparative Experiment A resulted in a product having a coating weight of the inner layer of 40 g/m² and consequently there is no down gauging. Comparative Experiment B resulted in a product having an inner layer which is not processable.

Comparative Experiment C resulted in a product having a neck- in of 241 mm. This value is unacceptable because the neck-in is preferably less than 150 mm. Furthermore the motor load was very high compared with the values according to Examples I-IV. A high motor load is disadvantageous because of the high energy consumption. A high motor load may also give throughput problems during the extrusion coating process.

Claims

1. A food packaging material having a multi layered structure comprising a low density polyethylene containing inside layer characterised in that the inside layer contains a blend of low density polyethylene and Ziegler- Natta catalysed linear low density polyethylene.

2. A packaging material according to Claim 1 characterised in that the blend comprises

- 20-80% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and

- 20-80% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1 183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100 % by weight.

3. A packaging material according to any one of Claims 1-2 characterised in that the blend comprises

- 30-70% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1 183) and a melt index between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and - 30-70% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100 % by weight.

4. A packaging material according to any one of Claims 1-3 characterised in that the blend comprises

- 45-55% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and

- 45-55% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100 % by weight.

5. A packaging material according to any one of Claims 1-4 characterised in that the melt index ratio of the low density polyethylene and the Ziegler- Natta catalysed copolymer ranges between 1:3 and 3:1.

6. A packaging material according to any one of Claims 1-5 characterised in that the coating weight of the inside LDPE layer ranges between 15 and 30 g/m².

7. A process for producing a food packaging material having a multi layered structure comprising a low density polyethylene containing inside layer, comprising a step of extrusion coating the inside layer from a blend of low density polyethylene and Ziegler-Natta catalysed linear low density polyethylene.

8. A process according to Claim 7 characterised in that the blend comprises

- 20-80% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100% by weight.

9. A process according to any one of Claims 7-8 characterised in that the blend comprises

- 30-70% by weight of a low density polyethylene having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index between 1 dg/minute and 20 dg/minute (according to ASTM D 1133) and

- 30-70% by weight of a Ziegler-Natta catalysed copolymer of ethylene and an alpha olefin having from 3 to 10 carbon atoms having a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a polydispersity between 4 and 10 wherein the sum of the low density polyethylene and the Ziegler-Natta catalysed copolymer is 100 % by weight.

10. A process according to any one of Claims 7-9 characterised in that the blend comprises

11. A packaging material according to any one of Claims 1-6 and a process according to any one of Claims 7-10 characterised in that the low density polyethylene is produced in a tubular reactor.