WO2016150657A1

WO2016150657A1 - Packaging material comprising polyethylene foam

Info

Publication number: WO2016150657A1
Application number: PCT/EP2016/054239
Authority: WO
Inventors: Renate Francisca Tandler; Maria Soliman; Douwe Wiebe VAN DER MEER; Emanuel Joseph Herman Marie VAN DER VEN; Johan Maria Krist
Original assignee: Sabic Global Technologies B.V.
Priority date: 2015-03-24
Filing date: 2016-02-29
Publication date: 2016-09-29

Abstract

The invention is directed to a packaging material comprising polyethylene foam obtained by foaming a composition comprising low density polyethylene and a cellulose based component. The packaging material shows reduced permeation of oxygen and reduced permeation of carbon dioxide.

Description

Packaging material comprising polyethylene foam

The present invention relates to a packaging material comprising polyethylene foam.

Packaging materials protect food, medicines, and all other products, which should not be wasted or damaged. Plastic packaging can very well protect especially food. Those packaging materials are optimized depending on the type of food they have to protect. Gas barrier properties are optimized for conservation of sensitive food products, commonly known as 'shelf life'. Diffusion of oxygen towards the food has to be reduced in order to keep it fresh and tasteful for a longer period. The amount of material for a package has to be reduced while keeping the functionality. In countries where food is not packed in an optimized way, more than 20% of the valuable food is already wasted before it reaches the consumer, which is unfavorable from a financially and environmental point of view.

It is the object of the present invention to provide polymer compositions, which result in improved shelf life of food packaging materials.

The invention is characterised in that the packaging material comprises polyethylene foam obtained by foaming a composition comprising low density polyethylene and a cellulose based component wherein the foam is produced via a physically blowing process.

The invention results in packaging material having a reduction of permeation of oxygen and reduction of permeation of carbon dioxide.

The carbon dioxide transmission rate decreases more than 20% in the compound which contains a cellulose based component (in comparison with a compound comprising no cellulose based component).

The oxygen transmission rate decrease more than 8% in the compound which contains a cellulose based component (in comparison with a compound comprising no cellulose based component).

The reduction of these permeation values result in improved barrier properties and consequently in an improved shelf life of food packaging.

A further advantage is the possibility to down gauge and to obtain lighter products without loss of the required product properties.

The composition according to the invention can be applied in a broad range of protective packaging for example food packaging, packaging material for medicines and health care uses, seals, closures, -packaging for electronics and egg trays. Generally the LDPE to be applied in the present invention has a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index

MFI(190°C, 2.16kg) between 0.10 and 100 dg/minute (according to ASTM D1 133).

Preferably the melt index MFI (190°C, 2.16kg) ranges between 0, 5 and 8.0 dg/minute and more preferably between 1 , 5 and 5.0 dg/minute.

The high-pressure polymerisation process to obtain LDPE may be an autoclave polymerisation process or a tubular polymerisation process.

According to a preferred embodiment of the invention, the high-pressure polymerisation process is a tubular polymerisation process.

The low-density polyethylene may be obtained for example by a polymerisation process as disclosed in WO2006/094723.

The LDPE applied in the present invention may comprise a small amount of HDPE such as for example disclosed in WO2008/145267 wherein a blend comprising 95.5 - 99.5% by weight low density polyethylene and 0.5 - 4.5% by weight high density polyethylene is used.

The cellulose component has OH functionality. This functionality results in the reduction of permeation of oxygen and reduction of permeation of carbon

dioxide. Furthermore the component plays a role as a filler for modifying mechanical properties.

The cellulose component must have the ability to be dispersed in LDPE.

The L/D ratio of the cellulose component ranges between 1 and 1000. Cellulose powder has L/D of 1 and cellulose fibers have a L/D higher than 1.

The man skilled in the art will select a particle size of the cellulose compound which is suitable to be applied during the compounding process.

Suitable cellulose components include for example cellulose, paper pulp, recycled paper, mycrocristalline cellulose, nanocellulose, cotton, ramie, jute, and aflax (bast fibers), sisal , abaca (leaf fibers), sawdust, alfalfa, wheat pulp, wood fibers, wood chips, wood particles, ground wood, wood flour, wood flakes, wood veneers, wood laminates, paper, cardboard, straw, cotton, peanut shells, bagass, plant fibers, bamboo fiber, palm fibers, bast, leaves, newspaper, coconut shells and seed fibers.

Preferred cellulose components are wood fibers and wood flour.

According to a preferred embodiment of the invention the composition comprises a compatibiliser. The compatibiliser or coupling agent such as for example maleic anhydride-g-PE may be applied to improve the adhesion between LDPE and the hydrophilic fiber. The classification and application of thermoplastic foams are closely related to the structure of foams (cell size, cell density, foam density or volume expansion ratio, closed or open cell). Therefore, the structural characterization of foams is very important. For example for barrier properties a foam with essentially closed cells is required. For example for optical properties a small cell sized foam is necessary.

The polyethylene foam has a density of between 10 and 800 kg/m³.

According to a preferred embodiment of the invention the polyethylene foam has a density of between 50 and 600 kg/m³.

Generally the physically blown polyethylene and cellulose based component based foam has a highly regular, fine cellular foam structure wherein the cell size may range between 30 and 1200 micrometres.

dso may range between 250 pm and 350 pm.

The amount of cellulose based component in the foam composition may range between 0.1 and 30 % by weight of the total composition.

Preferably, the amount of cellulose based component in the foam composition ranges between 10 and 25 % by weight of the total composition.

The amount of polyethylene in the foam composition is at least 70 % by weight of the total composition to be foamed. Preferably, the amount of polyethylene in the foam composition ranges between 75 and 90 % by weight of the total composition.

The polyethylene and cellulose based component based foam has a

substantially closed cell foam structure. The amount of closed foam cells will be higher than 90 %, preferably higher than 98 %.

The foam is produced via a non-crosslinkable physically blowing process wherein all steps take place in one extruder. The preferred physical foaming process is a continuous extrusion process in which the blowing gas, which forms cells in the complete mixture of PE, cellulose based component, and other additives for example nucleating agents, cell stabilizers is injected directly into the PE melt and

homogeneously mixed and dissolved in the molten polyethylene. At or just before the die exit the gas demixes and the PE melt is foamed.

The composition to be foamed contains at least the polymer, the cellulose based component, the nucleating agent, a physical blowing agent and a cell stabilizer. The composition may additionally contain other additives such as for example flame retardants, pigments, lubricants, antistatic agents, processing stabilizers, chemical blowing agents and/or UV stabilizers. A nucleating agent is distributed homogeneously. The nucleating agent may be an organic or an inorganic nucleating agent. Besides the cellulose based component other nucleating agents for example talc, silicium oxide, titanium oxide and alumium trihytate can be apllied.

Suitable organic nucleating agents include an amide, an amine and/or an ester of a saturated or unsaturated aliphatic (C10-C34) carboxylic acid.

Examples of suitable amides include fatty acid (bis)amides such as for example stearamide, caproamide, caprylamide, undecylamide, lauramide, myristamide, palmitamide, behenamide and arachidamide, hydroxystearamides and alkylenediyl-bis- alkanamides, preferably (C2-C32) alkylenediyl-bis-(C2-C32) alkanamides, such as for example ethylene bistearamide, butylene bistearamide, hexamethylene bistearamide, and/or ethylene bibehenamide.

Suitable amines are for instance (C2-C18) alkylene diamines such as for example ethylene biscaproamine and hexamethylene biscaproamine.

Preferred esters of a saturated or unsaturated aliphatic (C10-C34) carboxylic acid are the esters of an aliphatic (C16-C24) carboxylic acid.

It is possible to apply a nucleating agent for example in an amount of between

0.1 and 4.0 wt. % relative to polyethylene. A more preferred amount ranges between 0.5 and 1 wt. %.

Suitable physical blowing agents include for example isobutane, CO2, pentane, butane, nitrogen and/or fluorohydrocarbons. Preferably, the physical blowing agent is isobutane or CO2. In order to keep the gas dissolved in the PE melt, a minimum pressure, which is dependent on the gas used and the prevailing melt temperature, is needed in the molten polyethylene.

Suitable cell stabilizers include for example glycerol monostearate (GMS), mixtures of GMS and glycerol monopalmitate (GMP) and/or amides such as for example stearyl stearamide and/or stearamide. Preferably, the cell stabiliser is GMS.

The composition is extruded at a temperature just above the crystallization temperature of the polyethylene. The exit temperature from the extrusion opening usually is maximum 10°C and preferably maximum 5°C higher than the crystallization temperature. The temperature at which the viscosity increase begins due to the crystallization of polyethylene corresponds with the crystallization onset temperature from a DSC curve. In order to achieve on the one hand the maximum viscosity and hence the desired fine cellular structure and, on the other, to prevent the melt from "freezing" (crystallizing too rapidly) in the outlet, the melt temperature is maintained at about 5°C and preferably about 2°C above the crystallization onset temperature so as to obtain the desired fine cellular foam. The end product is completely foamed. The end product is recyclable and can be processed as any thermoplastic polymer.

The packaging may be composed completely of low-density polyethylene (LDPE) foam in a monolayer system and the packaging may comprise a one of the layers the LDPE foam in a multi-layer system wherein the packaging material may comprise several layers such as a foamed support layer of low density polyethylene (LDPE) and layers of a non-foamed, solid polymeric material or a solid metal, called lamination films.

Lamination films can be present at one or both sides of the foamed support layer. In between the lamination film and support foam an adhesive or extrusion coated layer maybe present.

The thickness of the foamed support layer can be between 0.5 mm and 4 mm, whereas the individual thickness of the other layers may be between 15 micrometres and 200 micrometres. The thickness of the adhesive or extrusion coated layer is usually small in comparison to the dimensions of other layers.

Suitable materials for the outer-layers are for example bi-oriented polypropylene (BOPP), bi-oriented polyester terephthalate (BOPET), ethylene vinyl alcohol and an aluminium layer and multilayer filmic structures comprising for example LDPE, LLDPE, HDPE , PP and other material (i.e. barrier layer). Those film layers may be used as such (transparent or opaque) or may be printed possible combined with coated with an additional barrier material or slip material (silicones), or vacuum deposited with a metal like aluminium. The foamed support layer consists mainly of a low density

polyethylene copolymer. The outer layers are laminated to the foamed support layer after the production of the foamed support layer via heat or extrusion lamination process or with adhesives. This can be done in-line or in a second process step.

GB1353041 discloses a batch mould process for producing foamed composite articles. The method to produce composite articles comprises placing in a mould crosslinkable powdered plastic materials and crosslinkable granular plastic materials in specific particle sizes containing a chemically blowing agent and next rotating and vibrating the mould to separate the two kinds of plastics materials from each other and heating the mixture.

US2014/272229 discloses a polyethylene foam and a multi-layer structure. The end product is not completely foamed because the structure comprises a foam and a solid composition. The solid composition may comprise a huge amount of additives for example fillers for example wood powder. CN 103360674 is directed to a crosslinked foam composition based on less than 50 % by weight LDPE which is produced with azodicarbonamide as chemically blowing agent.

CN103554626 is directed to a batch process to produce molded articles wherein the composition to be foamed comprises azodicarbonamide and a crosslinking agent.

The invention is elucidated by means of the following non-restrictive

experiments and examples. Examples.

The applied materials are:

-LDPE: LDPE 1905 UMS of SABIC with a melt flow rate MFR (190°, 2.16kg) of 5.0 dg/min and a density of 920 kg/m³.

- Wood fiber pellets: Woodforce GWFB00003 of Sonae Industria.

- Maleic anhydride modified HDPE: Polybond 3029 (Addivant)

Production of compounds.

-Compound I was produced by compounding of 88.5 parts by weight LDPE, 10 parts by weight wood fibers and 1.5 parts by weight maleic anhydride modified HDPE (Polybond 3029) on a screw extruder.

The extruder had a temperature profile of the screw as follows: zone 1 : 30°C, zone 2: 155°C, zone 3 to 5: 160°C, zone 6 to 12: 165°C and die: 180°C.

The compound was dried 4 hours at 80°C.

-Comparative Compound A was produced via the same process with the exception that no wood fibers and no maleic anhydride modified HDPE were applied.

-Comparative Compound B was produced via the same process with the exception that no wood fibers and no maleic anhydride modified HDPE were applied and additionally 10 % by weight talcum were present. Production of foam.

Before foaming to

- Compound I: 1 .0 % by weight (relative to the total amount of the compound) glycerol mono stearate were added

- Compound A: 1 .0 % by weight (relative to the total amount of the compound) glycerol mono stearate and 1.0 % by weight talc (relative to the total amount of the compound) were added and - Compound B: 1.0 % by weight (relative to the total amount of the compound) glycerol mono stearate were added.

Isobutane was blowing agent during foaming of said compounds at a foam extrusion line comprising a direct-gassed single screw extruder with fluid cooled extension barrel with a metering unit for blowing agents, fluid cooled static mixer and melt pump. Temperature setup of the foam extrusion line was 50-160-180-180-180- 167-147-140-115 °C.

Table 1 shows the densities of the foam based on Compound I, Compound A and Compound B.

TABLE 1

Density kg/m³

Compound I 196

Compound A 180

Compound B 163

The density of the foam was determined by the method wherein sample weight was divided by sample volume. Sample volume was calculated by the formula: height x π (diameter)².

Table 2 shows the open cell content of the foam based on Compound I, Compound A and Compound B.

TABLE 2

Open Cell content %

Compound I < 1

Compound A < 1

Compound B < 1 The open cell content was determined by the method wherein a (weight) piece of foam is submerged in a solution of 95 wt % water / 5 wt% ethylene glycol at a pressure of 540 mbar. After 10 minutes samples were taken out of the solution and submerged for 5 seconds in methanol to remove the excess of water, followed by 5 min at 60°C in an oven. The weight of the piece of foam was determined again. Open cell content was calculated by (weight start - weight end) / (weight start/density)^*100%. Table 3 shows the results of the permeability tests which were performed by cutting foamed sheets. The results were calculated for 1 mm foam.

TABLE 3

Density Carbon dioxide Oxygen transmission rate

transmission rate ASTM D3985

DIN 53380-4

23.0°C/0 % r.h. 23°C/0% r.h. 23°C/50% r.h. cm³/(m².d.bar) cm³/(m².d.bar) cm³/(m².d.bar)

Compound I 196 3768 1007 1021 Compound A 180 4737 1090 not tested Compound B 163 5042 1414 not tested Table 3 shows that the carbon dioxide transmission decreases more than 20% in the compound which contains wood fibers.

Table 3 shows that oxygen transmission rate decrease more than 8% in the compound which contains wood fibers.

The combination of a decrease of carbon dioxide- and oxygen transmission rate is excellent.

Figure 1 shows the results of the tensile strength tests (according to ISO 1926) on foamed sheets based on Compounds I, A and B.

The foam based on Compound I shows a maximum force of 1.3 MPa at an elongation of 32%.

The foam based on Compound A shows a maximum force of 0.9 MPa at an elongation of 64%.

The foam based on Compound B shows a maximum force of 1.1 MPa at an elongation of 83%.

SEM images of Fig 2, Fig 3 and Fig 4 are the results of samples cut out of the foamed sheets with a razor blade (at room temperature) and fixed onto a SEM sample holder. The samples were coated with a conductive Au/Pd-layer (5 nm) before imaging.

Figure 2 shows the cell structure of foam based on Compound I,

Figure 3 shows the cell structure of foam based on Compound A and

Figure 4 shows the cell structure of foam based on Compound B.

In comparison with foam based on Compound I, the foam based on Compound A and Compound B have more compression of the cells. The cells are oval shaped. Foam based on Compound I has a cell size d50 of 300 pm and foam based on Compound A has a cell size d50 of 200 pm (determined via Image analyzing).

Claims

1. Packaging material comprising polyethylene foam obtained by foaming a

composition comprising low density polyethylene and a cellulose based component wherein the foam is obtained is produced via a physically blowing process.

2. Packaging material according to Claim 1 wherein low density polyethylene has a density between 910 kg/m³ and 935 kg/m³ (according to ISO 1183) and a melt index between 0.10 and 100 dg/minute (according to ASTM D1133).

3. Packaging material according to any one of Claims 1-2 wherein the low density polyethylene is obtained via a high pressure polymerisation process in a tubular reactor.

4. Packaging material according to any one of Claims 1-3 wherein the L/D ratio of the cellulose component ranges between 1 and 1000.

5. Packaging material according to any one of Claims 1-4 wherein the cellulose component is selected from cellulose, paper pulp, recycled paper,

mycrocristalline cellulose, nanocellulose, cotton, ramie, jute, and aflax (bast fibers), sisal , abaca (leaf fibers), sawdust, alfalfa, wheat pulp, wood fibers, wood chips, wood particles, ground wood, wood flour, wood flakes, wood veneers, wood laminates, paper, cardboard, straw, cotton, peanut shells, bagass, plant fibers, bamboo fiber, palm fibers, bast, leaves, newspaper, coconut shells and seed fibers.

6. Packaging material according to Claim 5 wherein the cellulose component is

selected from wood fiber and wood flour.

7. Packaging material according to any one of Claims 1-6 wherein the composition comprises a compatibiliser.

8. Packaging material according to any one of Claims 1-7 wherein the cellulose based component is present in an amount between 0.1 and 30 % by weight of the total composition.

9. Packaging material according to any one of Claims 1-8 wherein the amount of polyethylene in the foam composition is at least 70 % by weight of the total composition.

10. Packaging material according to any one of Claims 1-9 wherein the carbon

dioxide transmission rate decreases more than 20% and the oxygen transmission rate decrease more than 8% in comparison with a compound comprising no cellulose based component.