WO1994014889A1

WO1994014889A1 - Barrier material

Info

Publication number: WO1994014889A1
Application number: PCT/AU1993/000658
Authority: WO
Inventors: Gigy Jacob Philip; Eugene Pawliw
Original assignee: Aci Operations Pty. Ltd.
Priority date: 1992-12-22
Filing date: 1993-12-20
Publication date: 1994-07-07

Abstract

A barrier material suitable for use in packaging which comprises an homogenous blend of a styrene polymer, at least one polyolefin and a compatibilizing agent.

Description

BARRIER MATERIAL

This invention relates to a plastics blend suitable for use as a packaging material or as a barrier layer in a packaging laminate. It has particular application in producing packages and containers for foodstuffs.

When packaging foodstuffs or other products which have a high moisture content, it is obviously important to utilize a material which is a good moisture barrier. In addition, where the material is a plastics one, the material is preferably one which is relatively easy to thermoform. Various polyolefins have been used in the past as moisture barrier materials (e.g. high density polyethylene and polypropylene) . Whilst these materials have very good moisture barrier properties, they are difficult to thermoform. Polyolefins generally have low melt strength and unless thermoformed in a narrow temperature band will sag and this precludes satisfactory container formation. Polystyrene is an alternative material which has good structural strength and is adapted for thermoforming over a broader temperature band. However, polystyrenes do not have the same moisture impermeability as do polyolefins and neither are they heat film sealable thus, in some applications, these materials are unsuitable.

Thus, it is an object of the present invention to provide a material which has good moisture barrier properties or which can be heat film sealable but which is also adapted to be thermoformed over a relatively broad temperature range.

The current invention achieves this object by providing a material which is an homogenous blend of a polyolefin and a styrene polymer. In accordance with the present invention, there is provided a barrier material suitable for use ^• in packaging which comprises an homogenous blend of a styrene polymer, at least one polyolefin and a compatibilizing agent.

Preferably, the compatibilizing agent is present in an amount of up to 15% by weight. The polystyrene and polyolefin must be blended to form an homogenous mix so that the barrier material may be extruded without separating out into different phases. By themselves, these materials are incompatible. To enable them to be blended into an homogenous layer or sheet, it is necessary to utilize a compatibilizing agent. Reference in this specification to such an agent is to be understood to relate to a material suitable for enabling the blending of a polyolefin with a styrene polymer. Styrene polymers suitable for use in this invention include any polymer having a styrene component. These include acrylonitrile-butadiene-styrene ("ABS") and polystyrene.

The preferred polyolefins are polyethylenes and polypropylenes. Most preferred are high density polyethylenes, as these materials have particularly low water vapour transmission rates. The high density

3 polyethylenes generally have densities of 900 kg/ or more. The applicants have found that higher levels of polyolefins within the blend result in a material which is more difficult to thermoform. With high levels of polyolefins within the blend, the temperature range within which the material may be easily processed narrows. Accordingly, it is preferred that the barrier material have no more than 80% by volume of polyolefin and most preferably no more than 20% by volume. Most preferably, the polyolefins are present in an amount ranging between 10 to 20% by volume. Higher density polyolefins have lower water vapour transmission rates and by utilizing such materials, it is possible to utilize a lower level of polyolefins within the blend whilst achieving the desired moisture transmission barrier. The ratio of the polyolefin within the blend thus depends on the water vapour transmission rate desired, the type of polyolefin used and the wall thickness of the material when extruded to form a package layer or wall.

The compatibilizing agent may be any suitable material as is known in the art. There are a number of products available which are suitable for achieving homogenous blends of styrene and olefinic polymers. Suitable compatiblizing agents include graft copolymers such as ethylene styrene graft copolymers and ethylene - methylmethacrylate graft copolymers. Block copolymers such as styrene/butadiene may also be effectively utilized to homogenise a blend of these immisible polymers. Preferably, the compatibilizer has a structure which provides more than interfacial adhesion between the styrene and polyolefin but in fact creates a blend with a single Tg. Most preferably, it is an anhydride modified EVA based adhesive which is used. It is believed by the inventor that such a compatibilizing agent becomes entangled in the styrene and polyolefin polymers to produce a particularly homogenized blend. Suitable adhesives are available commercially (e.g. the EVA based "Modic" or "Chemplex" products) . Particularly useful compatibilizing agents for the practice of this invention are the Du Pont EVA adhesives sold under the trade mark VYNEL, CXAE 361 and 410. The barrier material may comprise additional materials apart from the polyolefin, polystyrene and compatibilizing agent. Preferably, however, it comprises only these materials. It may be a blend of virgin polyethylene with virgin polystyrene, a blend of virgin polyethylene with re-grind polystyrene or a blend using all or part re-grind of the barrier material itself with additional polyolefin (and materials as required) .

Preferably, the compatibilizing agent is present in amount by weight of between 3 to 15%. Most preferably, it is present in amount between 4 to 8%.

The barrier material may be foamed by normal methods known to those skilled in the art for blowing polystyrene foam. The foam has an increased moisture barrier over foam blown from 100% polystyrene. However, the foam has the added advantage of being able to be heat film sealable. The present invention has particular application for use in multi-layer structures. Multi-layer structures enable the use of various plastic materials having different attributes to yield a final structure which has physical characteristics which would be difficult, costly or impossible to achieve through the use of a single plastics material. In food packaging applications, it is desirable to produce a material which is strong and which provides a good gas barrier to the deleterious effects of oxygen. Other attributes, such as providing a barrier to odours, can also be important. In the past, such multi-layer structures have utilized relatively cheap resilient plastics such as polystyrene or a suitable polyolefin for structural strength. Whilst polystyrene and polyolefins such as polyethylene provide reasonable structural strength and moisture impermeability, they are not in themselves good gas barriers and foodstuffs held within containers formed from such materials quickly deteriorate.

There are a large number of gas barrier materials known in the art which provide good resistance to oxygen transmission. A practical problem that arises through the use of such materials is that they are usually also poor moisture barriers. Barrier properties are dependent on molecular structure. Generally, highly polar polymers containing hydroxy groups are good gas barriers but poor water barriers. Non polar hydrocarbon polymers are good water barriers but poor gas barriers.

Further, the oxygen barrier qualities of many such materials are dependent on the isolation of the material from moisture. Polyvinylalcohols (PVA), which are amongst the most effective synthetic resin gas barrier materials, exhibit rapid loss of gas barrier performance when exposed to water vapour. The absorption of moisture into the gas barrier layer also leads to product loss. For example, prior efforts to use containers formed from ethylene-vinyl alcohol polymers (EVOH) sandwiched between polystyrene (which is a poor moisture barrier material by itself) have been less than satisfactory. Because of the moisture absorbing nature of EVOH, up to 30% product loss over four months has been known to occur with products such as UHT milk. Alternative oxygen barriers are known but many of them have the same characteristics, namely that they are poor moisture barriers. Polyvinylidene chloride (PVDC) is a product which has both characteristics, in that it is a reasonably good oxygen barrier and also an effective moisture barrier. However, PVDC is difficult to handle on conventional co-extrusion equipment. PVDC is highly temperature sensitive and is prone to degradation on heating. It produces hydrogen chloride which is particularly corrosive. If a conventional extrusion line is used in such an enviroment, it quickly becomes unworkable. Accordingly, to extrude with PVDC, it is necessary to utilize an unreactive coating such as nickel through the entire extrusion line. This is, quite obviously, expensive. Furthermore, PVDC is not recyclable and thus it is less favoured (in environmental terms) for the formation of multi-layer structures suitable for use in packaging consumer foodstuff products.

Another alternative is to sandwich the gas barrier material between layers of moisture resistant resins. A thin layer of EVOH sandwiched between two layers of polyethylene has previously been proposed and this yields a total structure having both the desired gas and moisture barrier properties. However, due to the difficulties in thermoforming polyethylene and other polyolefins, such materials are difficult to utilize and often sag precluding satisfactory container formation.

It is thus a further objection of the present invention to provide a multi-layer structure which provides both good oxygen and moisture barrier characteristics and which is relatively easy to utilize in thermoforming and form-fill-seal applications. This object is achieved through the use of an oxygen barrier material protected by a blended polyolefin/polystyrene layer of the type previously described, positioned on at least one side of the oxygen barrier material in a multi-layer structure. In use, the oxygen barrier material should be positioned so that the protective blended polyolefin/polystyrene layer is between it and the inside of the food container package. It has been found that a blended polyolefin/polystyrene layer presents a good moisture barrier and has good melt strength so that it is relatively easy to use in thermoforming operations and provides the necessary moisture shield for the oxygen barrier layer.

Thus, in accordance with this further aspect of the invention, there is provided a multi-layer laminate suitable for use in producing packaging for foodstuffs including an oxygen barrier layer and a moisture barrier layer positioned on at least one side of said oxygen barrier layer; wherein said moisture barrier layer includes an homogenous blend of a styrene polymer, at least one polyolefin and a compatibilizing agent. The moisture barrier layer may be made in accordance with any of the preferments previously described with respect to the barrier material per se.

As it is only one side of the laminate that will be exposed to high moisture levels (i.e. the inside in contact with the foodstuff), it is usually only necessary for the multi-layer laminate to have one moisture barrier layer, that being located on the inside of the oxygen barrier layer.

For applications where the laminate will be exposed to high levels of moisture on both sides (e.g. high humidity locations) , it is desirable to sandwich the oxygen barrier layer between two such moisture barrier layers.

Preferably, the moisture barrier layer or layers are positioned immediately adjacent to the oxygen barrier layer. Depending on the composition of these layers, it may be necessary to also utilize an adhesive interposed between the oxygen barrier layer and the moisture barrier layers to ensure integrity of the multi-layer structure. Suitable adhesives will depend on the respective compositions of the oxygen barrier layer and the moisture barrier layer. A thermoplastic EVA based adhesive similar to that described as one of the preferred compatibilizing agents is preferred. In cases where the same material is utilized for both the adhesive and the compatibilizing agent, an additional benefit accrues in that scrap material produced during container formation may be utilized in the production of further moisture barrier material. The applicants have found that the small level of oxygen barrier material in the scrap material does not substantially effect the moisture barrier properties of the styrene/olefin blend. Preferably, where scrap is used in this way, the total amount of polyolefin is reduced by 5%, the total amount of styrene polymer is similarly reduced by about 5% and replaced by a similar amount of scrap. The amount of compatibilizing agent utilized in the moisture barrier material would remain about the same. In most thermoforming operations, the proportion of scrap can be up to 40%, more commonly 30%, depending on the geometry of the articles being thermoformed. The applicant has found that all the scrap can be reground and used to form a moisture barrier with addition of other materials as necessary. The structures described above can be formed in accordance with known methods. Preferably, they are produced by co-extrusion.

The wall thickness of the moisture barrier material will depend on the particular application, the type of product being packaged and the storage conditions of the package (i.e. the humidity and temperature in which the package is to be stored) , but would normally be between 5% to 50% of the total sheet thickness. Generally, multilayer barrier materials made in accordance with this invention are between 0.2 mm to 5 mm in thickness.

In a still further embodiment of this invention, the multi-layer laminate comprises an additional structural layer or layers. Preferably, such layers are positioned on either side of the laminate to form outer and inner walls of a multi-layer structure.

Where an additional structural layer or layers are used, the material from which the layers are formed should preferably have a melt strength at least as high as the moisture barrier layer material so that the processing temperature range for thermoforming the material is not compromised. Preferably, such structural layers are formed from polystyrene which has a broad forming temperature range and has high melt strength. Polya ides might also be utilized for such layers with the use of appropriate adhesive layers. In preferred embodiments of the invention where there are protective layers on both sides of the multi-layer structure, the protective layers are each of a thickness preferably between 10% to 45% of the total multi-layer structure thickness. In the case where the blended moisture barrier is provided on only one side of the oxygen barrier layer within the laminate, it is particularly desirable to utilise a structural layer of this type on at least the other side of the oxygen barrier material to provide some level of protection to this layer. It is normally necessary in such cases to utilize a suitable adhesive tie layer between the oxygen barrier layer and the structural layer.

The oxygen barrier material may be selected from a number of known materials in the art such as PVA resins, polyamides or acrylonitriles. Most preferred are PVA barrier resins such as EVOH. The thickness of the oxygen barrier material is not narrowly critical. Largely, it depends on the type of product being packaged and the shelf life requirements of the product. Generally, it would be between 2% to 20% cf the total sheet thickness.

In accordance with a further aspect of this invention, there is provided a packaging container which is thermo-formed from a multi-layer laminate of the type hereinbefore described.

In a preferred form, the walls of such a container comprise the following layers:

inside container

polystyrene moisture

styrene barrier

In applications where the package is likely to be exposed to moisture from the outside, a further moisture barrier layer should be interposed between the adhesive and polystyrene layers adjacent the outside of the container wall.

A preferred embodiment of the invention is hereafter described with reference to the accompanying figure which is a schematic representation of a multi-layer structure made in accordance with the invention. In the figure, a multi-layer laminate 1 is generally illustrated. This laminate is intended for use in form-fill-seal production of a container for holding a beverage such as UHT milk. The total laminate thickness is in the order of 0.5 mm to 1.0 mm.

Inner layer 2 is formed from polystyrene. It is usually approximately the same thickness as outer layer 8. Together, these layers preferably constitute between 35-70% of the total laminate thickness. Adjacent to the polystyrene layer 2 is a moisture barrier layer of the invention 3 formed from a blend of high density polyethylene and polystyrene. The blend is homogenously dispersed through use of a thermoplastic EVA based compatibilising agent. The agent used is the Du Pont

"VYNEL" brand EVA adhesive in an amount of 5% by weight.

The blend comprises 20-30% virgin high density polyethylene (having a density of approximately 1000

₃ kg/m ) blended with regrind scrap laminate comprising polystyrene and polyethylene. Moisture barrier 3 is preferably between about 20 to 50% of the total laminate thickness. Moisture barrier 3 is adhered to an oxygen barrier material layer 4 by an adhesive layer 5. Adhesive layer 5 may be any adhesive compatible with both the blended polyethylene/polystyrene layer and the oxygen barrier material layer 4. Preferably, it is the same EVA based adhesive as used as the compatibilizing agent in the moisture barrier layer. The oxygen barrier layer is formed from EVOH and preferably makes up between 5 to 7 % of the total laminate thickness.

The laminate shown in the figure has a similar structure on the other side of the oxygen barrier material layer 4. This includes an adhesive layer 6, a moisture barrier layer 7 formed from a blended polystyrene/high density polyethylene and a polystyrene outer layer 8. Alternatively, as previously disclosed the moisture barrier layer 7 may be omitted from the structure provided that moisture barrier 3 is used towards the inside of any package thermo-formed from the laminate.

It has been found that the moisture barrier layer in the structure is suitable for thermoforming over a broad temperature range and exhibits good moisture barrier properties. The laminate incorporating this material has comparable performance to laminates formed using PVDC but allows one to avoid the use of this material, which for reasons previously detailed, is not favoured.

The applicants have tested the moisture loss performance of a preferred multilayer structure of the present invention and compared this performance with a prior art product including PVDC as the moisture/oxygen barrier material.

The respective materials were of the following structure:- 1. Prior Art : Polystyrene/adhesive/PVDC/adhesive/

Polystyrene

The total sheet thickness of this product was 0.8 mm and comprised a layer of PVDC constituting 5% of the laminate thickness, layers of adhesive on either side constituting together about 5% of the laminate thickness and inner and outer layers of polystyrene constituting the remainder of the laminate structure.

Multi-Laver : Polystyrene/moisture barrier/adhesive/ Material of EVOH/adhesive/moisture barrier/ the Invention polystyrene

The total laminate thickness was again 0.8 mm and the respective layers were as follows: Polystyrene : 0.4 mm (total)

Moisture Barrier: 60% polystyrene 40% polyethylene

(EVA adhesive compatabilizing agent added in amount 5% by weight) Total thickness: 0.3 mm Adhesive : EVA based adhesive 0.05 mm (total) EVOH : 0.05 mm

Cups were formed from both materials and then filled with UHT milk and sealed. The sealed cups were stored for four months under three sets of conditions. The average weight loss per 18 ml was:-

Whilst the PVDC product performed marginally better than the multi-layer material of the invention, it will be appreciated that the percentage differences are marginal making the product of the invention an effective substitute for the PVDC material.

The structure 1 set out hereinbefore was made again using 30% of the laminate product as re-grind to make the moisture barrier layer 3 (with the addition of additional HDPE and compatibilizing agent) . The composition of the moisture layer was 53% polystyrene, 33% HDPE, 7% EVA and 7% EVOH. The resulting laminate was tested and had substantially unchanged barrier properties.

It has also been found by the applicants that the multi-layer structure of the type described is relatively easy to thermo-form over a broad temperature range. Untreated polyolefins such as polypropylene or polyethylene have sufficient melt strength for container formation over a range of only about 4 C. The product of the present invention may be processed over a broader operating temperature range. The laminate described in the preferred embodiment can be effectively thermo-formed over a temperature range of between about 100 C to 115 C. In this range, the laminate has sufficient resistance to sag so that the thermo-forming operation can be conducted on convention equipment. Optimumly, it is processed at a temperature of about 110°C.

It will be appreciated that additional layers may be incorporated within a multi-layer structure having the essential characteristics of the laminate herein described without departing from the spirit or ambit of this invention. Additionally, the moisture barrier material itself may incorporate further polymeric materials to achieve particular desired physical characteristics without departing from the present invention.

Claims

1. A barrier material suitable for use in packaging which comprises an homogenous blend of a styrene polymer, at least one polyolefin and a compatibilizing agent.

2. A barrier material as claimed in claim 1 wherein said compatibilizing agent is present in an amount of up to 15% by weight.

3. A barrier material as claimed in either one of claims 1 or 2 wherein said compatibilizing agent is an anhydride modified EVA based adhesive.

4. A barrier material as claimed in any one of claims 1 to 3 wherein said styrene polymer is polystyrene or ABS or a mixture thereof.

5. A barrier material as claimed in any one of claims 1 to 4 wherein said polyolefin is a high density polyethylene.

6. A barrier material as claimed in claim 5 wherein said high density polyethylene has a density of 900 ₃ kg/m or greater.

7. A barrier material as claimed in any one of claims 1 to 6 wherein said polyolefin is present in an amount ranging between 10 to 20% by volume.

8. A barrier material as claimed in any one of claims 1 to 7 wherein the barrier material comprises only polyolefin, polystyrene and compatibilizing agent.

9. A barrier material as claimed in any one of claims 1 to 8 wherein said compatibilizing agent is present in an amount by weight of between 3 to 15%.

10. A multi-layer laminate suitable for use in producing packaging for food stuffs including an oxygen barrier layer and a moisture barrier layer positioned on at least one side of said oxygen barrier layer; wherein said moisture barrier layer includes an homogenous blend of a styrene polymer, at least one polyolefin and a compatibilising agent.

11. A multi-layer laminate as claimed in claim 10 wherein moisture barrier layers are positioned on both sides of the oxygen barrier layer.

12. A multi-layer laminate as claimed in either one of claims 10 or 11 wherein the moisture barrier layer or layers are positioned immediately adjacent to the oxygen barrier layer.

13. A multi-layer laminate as claimed in any one of claims 10 to 12 wherein each moisture barrier layer is adhered to the oxygen barrier layer with a compatible adhesive.

14. A multi-layer laminate as claimed in claim 13 wherein said moisture barrier layer is adhered to the oxygen barrier layer by an adhesive which is the same as the compatibilizing agent used in the moisture barrier layer.

15. A multi-layer laminate as claimed in claim 14 wherein said moisture barrier layer includes re-grind scrap of said multi-layer laminate.

16. A multi-layer laminate as claimed in claim 15 wherein said moisture barrier layer comprises about 10% by volume of re-grind scrap of said multi-layer laminate.

17. A multi-layer laminate as claimed in any one of claims 10 to 16 wherein said moisture barrier layer includes at least 10% by volume of one or more polyolefins.

18. A multi-layer laminate as claimed in claim 17 wherein the moisture barrier layer includes at least 10% by volume polyethylene or polypropylene or a mixture thereof.

19. A multi-layer laminate as claimed in claim 18 wherein said moisture barrier layer includes between 10% to 20% by volume of high density polyethylene having a

₃ density of 900 kg/m or greater.

20. A multi-layer laminate as claimed in any one of claims 10 to 19 wherein said moisture barrier layer includes a moisture barrier material as claimed in any one of claims 1 to 9.

21. A multi-layer laminate as claimed in any one of claims 10 to 20 including additional structural layers positioned on either side of the laminate so to form outer and inner walls of the multi-layer structure.

22. A multi-layer laminate as claimed in claim 21 wherein said additional structural layers are formed from a styrene polymer.

23. A multi-layer laminate as claimed in any one of claims 10 to 22 wherein said oxygen barrier layer is EVOH.

24. A packaging container which has been formed from a multi-layer laminate which is adapted to be thermoformed in a temperature range of between 100°C to 115°C and which is as claimed in any one of claims 10 to 23.

25. A multi-layer laminate substantially as hereinbefore described with reference to Figure 1.