BIAXIALLY ORIENTED CONTAINER OF PLASTICS MATERIAL

Abstract: A biaxially oriented container is blow moulded from a preform of polymeric material, wherein the container side wall has a wall thickness which is 6-20 times, preferably about 12 times, less than the wall thickness of the preform body and wherein said polymeric material comprises a modified nitrile-acrylate copolymer, for instance Barex3 210I. The container has excellent oxygen barrier properties and may be designed as a bottle for juice, beer or the like. A method for making the container is disclosed as well, and a use of the polymeric material.

Description: BIAXIA LY ORIENTED CONTAINER OF PLASTICS MATERIAL Technical Field The present invention relates in general to the technique of blow moulding biaxially oriented containers of plastics material, and in particular to a container having excellent oxygen barrier properties and thus being suitable for packaging of beer, juice or other beverages and liquids sensitive to oxygen. Background Art The technique of blow moulding containers from preforms of plastics material, such as polyester, is well known to the skilled person. For instance, reference can be made to applicant's EP-A-521 841 which discloses a method for making a container in particular for beverages and in which polyethylene terephthalate (PET) is mentioned as a preferred material for the preform. In WO 97/32711, further processes for blow moulding plastics containers from preforms are disclosed. In recent years, PET has been the predominating material for producing containers of the present type. When packaging liquids sensitive to oxygen, such as beer and juice, PET is not a fully acceptable alternative since its oxygen barrier properties do not meet the requirements placed on containers for these beverages. In other words, the shelf life is too short for PET bottles containing, for instance, beer or juice. Thus, one has tried to find other plastics materials which would be suitable for packaging oxygen-sensitive liquids. One material which has been considered is polyethylene naphthalate (PEN) which, however, is far too expensive compared with PET and similar materials. Indeed, the oxygen barrier of PEN is about five times better than that of PET, but at the same time PEN is about five times more expensive than PET. This means that PEN is not a realistic alternative since the market is not willing to pay the price for the enhanced oxygen barrier. Therefore, designers of plastics containers have turned to other materials having improved gas barrier properties while being cheaper than PEN. An alternative of this kind is the so-called High Nitrile Resins, in the following referred to as HNRs, which have been known as a packaging material since the late sixties. A major supplier of HNRs is the British company BP Chemicals Ltd which is marketing a commercial HNR under their trademark ® BAREX which is a modified acrylonitrile-methyl acrylate copolymer. The history and the structure of HNRs in general and ® BAREX m particular are described m a paper entitled "High Nitrile Resins for Barrier Packaging" by Mr David L.G. Lainchbury, employed by BP Chemicals. The paper was published in 1991 by RAPRA Technology Ltd under the title "An International Journal Author Reprint", edited by K.M. Watkinson; Vol. 4, No. 1, 1991; (ISSN: 0952-6900) . In the Lainchbury paper, the low gas transmission ® rates of various polymeric materials of the BAREX type are compared with and found better than other packaging materials, among them PET and PEN. The paper also dis- ® cusses briefly how preforms of BAREX material can be blow moulded into containers, and it is mentioned that orientation of the material by stretch blow moulding may lead to an improved gas barrier effect . ® In a pamphlet entitled "Barex high barrier resins - injection processing" issued by BP Chemicals Ltd in 1992 ® and reprinted in 1994, a particular BAREX material is ® described, namely "Barex 210 Injection Grade Resin", in ® the following referred to as Barex 2101. On page 7 of ® this pamphlet, injection stretch blow moulding of Barex 2101 is discussed, a stretch ratio of 4:1 being indicat- ed as appropriate. Thus, the supplier of this material recommends this stretch ratio for obtaining a suitable orientation of the material and thereby the aimed-at oxy- gen barrier. This means that the side wall of the blow moulded container should have a wall thickness that is four times less than the wall thickness of the preform body. ® Thus, it seems that HNR and in particular a BAREX material would be an attractive alternative when it comes to producing a container for packaging liquids sensitive ® to oxygen. For instance, Barex 2101 is believed to have somewhat better oxygen barrier than PEN while the price is about the half. Furthermore, the oxygen barrier of ® Barex 2101 is believed to be about seven times better than that of PET, while the price is only about the double . From the above-mentioned documents published by BP ® Chemicals, it is clear that BAREX materials have an oxygen transmission rate of 0.15-0.8 cm 3 .mm/m 2 .24h.bar, measured in accordance with ASTM standards, which should be compared with the corresponding rate for PET which is 2.4 cm 3 .mm/m 2 .24h.bar . It is appreciated that a container consisting of a BAREX ® material will have a longer shelf life than a common PET bottle. The requirements from the market, however, are still higher, which means that the designers of containers of the present type have to work further with the matter of enhancing the oxygen barrier and prolonging the shelf life while material costs and process costs are kept low. As a further example of prior art, WO 97/10997 should be mentioned. This document discusses the idea ® of using BAREX materials for packaging liquids sensi- tive to oxygen, namely juices based on various fruits and vegetables. The process of blow moulding the material is briefly described, but the parameters of this process are not discussed in detail. For instance, nothing is said about the stretch ratio. Summary of the Invention An object of the present invention is to provide a container which has significantly improved oxygen barrier properties over prior-art containers while having a price that is competitive relative to these known containers. Another object of invention is to provide a method for making such an improved container. A further object of the invention is to provide a use of a particular material for obtaining said improvement . These and other objects, which will appear from the following description, have now been achieved by a con- tainer, a method and a use having the features of the appended independent claims 1, 6, 21 and 22. Preferred embodiments and variants of the invention are set forth in the appended subclaims. Surprisingly and in accordance with the invention, it has been found that a significant increase of the stretch ratio which corresponds to the wall thickness ratio between the preform body comprising the present polymeric material and the container side wall, will lead to a most significant increase of the oxygen barrier pro- perties of the finished container. While the major supplier of HNR materials recommends a stretch ratio of 4:1, the invention shows that an increase of this stretch ratio by at least 50%, corresponding to a wall thickness ratio of at least 6:1, results in a very good oxygen bar- rier fulfilling the objects of the present invention. As used herein, the term "polymer" comprises polymers as well as less polymerised variants thereof, the latter being considered as oligomers or prepolymers thereof. Further, the term "copolymer" as used herein comprises copolymers, graft copolymers as well as higher polymers, such as terpolymers . In a first aspect of the invention, the preform is formed from polymeric material comprising a modified nitrile-acrylate copolymer as defined in appended claim 1. In a preferred embodiment of this aspect, the preform ® consists of Barex 2101 which in the blow moulding step is stretched into a container following a stretch ratio of about 12:1. In its unoriented state, this material has a oxygen transmission rate of 0.3 cm 3 .mm/m 2 .24h. bar, and by stretching the material 12 : 1 this rate is below 0.016 cm 3 .mm/m 2 .24h.bar . Thus, the oxygen barrier is increased at least in the magnitude of about twenty times . In a second aspect of the invention, the preform is formed from polymeric material comprising a modified copolymer of a nitrile monomer and an aromatic compound as defined in appended claim 6. If this preform is blow moulded and stretched in the way discussed with respect to the first aspect above, a significant increase of the oxygen barrier is achieved. In preferred embodiments, substantially the same oxygen barrier is achieved by blow moulding a preform of the second aspect of the invention as in the first aspect. In brief, the general inventive concept is based on the idea that a preform comprising a suitable HNR material should be blow moulded in such a way that the wall thickness of the side wall of the finished blow moulded container is 6-20 less than the wall thickness of the preform body. By this excessive biaxial stretching or orientation of the HNR material far beyond the recommendations of skilled experts and suppliers of the present material, a surprising increase in oxygen barrier of the finished container is achieved. Hence, a container made in accordance with the present invention will meet the requirements from the market as to prolonged shelf life, etc. There is an important difference between PET which at present is the predominant packaging material in this technical field, and the specific HNR materials suggested in accordance with the invention. In the heating and blow moulding process, the PET is crystallised (as is describ- ed in EP-A-521 841 mentioned by way of introduction) , whereas HNR is amorphous throughout this process. In most applications, it is advantageous that the material is maintained amorphous since the finished container will then always be transparent . Containers of PET and the like having crystallised wall and/or bottom portions are sometimes somewhat opaque which may be an undesired pro- perty. Detailed Description of Preferred Embodiments In the following, some preferred but non-limiting embodiments of the present invention will be described by way of example . As indicated above, the first aspect of the present invention relates to a biaxially oriented container blow moulded from a preform of polymeric material, wherein the container side wall has a wall thickness which is 6-20 times less than the wall thickness of the preform body, the polymeric material comprising a modified nitrile-acrylate copolymer. In an embodiment of the first aspect, the polymeric material comprises a copolymer of a nitrile monomer and an acrylate monomer having the general formula I and II, respectively: wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms ; wherein R x is hydrogen or an alkyl group having from 1 to 4 carbon atoms ; and R 2 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . In a preferred embodiment of the first aspect, the polymeric material comprises a first component being a copolymer of the nitrile monomer I and the acrylate monomer II admixed or polymerised with a second component selected from the group consisting of a monomer, a polymer and a copolymer. In a more preferred embodiment, the polymeric material comprises a copolymer of the nitrile monomer I and the acrylate monomer II admixed with a rubber selected from the group consisting of: i ) a polymer of a diene monomer having the general formula III : H 2 C=C CH^CH 2 III R 3 wherein R 3 is hydrogen or an alkyl group having from 1 to 4 carbon atoms ; ii ) a polymer of an aromatic compound having the general formula IV: wherein R 4 is hydrogen or an alkyl group having from 1 to 4 car- bon atoms; iii ) a copolymer of the nitrile monomer I and the diene monomer III; iv) a copolymer of the nitrile monomer I and the aromatic compound IV; v) a copolymer of the diene monomer III and the aromatic compound IV . In an alternative embodiment of the first aspect, the polymeric material comprises a terpolymer of the nitrile monomer I, the acrylate monomer II and a monomer selected from the group consisting of: i ) the diene monomer III, or a polymer thereof; ii ) the aromatic compound IV, or a polymer thereof; Hi ) a mixture of the diene monomer III and the nitrile monomer I , or a copolymer thereof . As mentioned above, the second aspect of the present invention relates to a biaxially oriented container blow moulded from a preform of polymeric material, wherein the container side wall has a wall thickness which is 6-20 times less than the wall thickness of the preform body, the polymeric material comprising a modified copolymer of a nitrile monomer and an aromatic compound having the general formula I and IV, respectively: H 7 C=C- ■ CN R wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms; wherein R 4 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . In an embodiment of the second aspect, the polymeric material comprises a terpolymer of the nitrile monomer I, the aromatic compound IV and a diene monomer having the general formula III: wherein R 3 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . In each of the two aspects above, R is preferably H in the nitrile monomer I . Furthermore, R x is preferably H in the acrylate monomer II in the first aspect. Moreover, R 2 is preferably a methyl group in the first aspect. In the second aspect as well as in particular embodiments of the first aspect, R 4 is preferably H in the aromatic compound IV. In preferred embodiments of both the first aspect and the second aspect, R 3 is either H or a methyl group in the diene monomer III. In the most preferred embodiment of the first aspect, the polymeric material comprises a terpolymer of acrylonitrile, methyl acrylate and butadiene. In the most preferred embodiment of the second aspect, the polymeric material comprises a terpolymer of acrylonitrile, styrene and butadiene. Practical tests of the invention have shown excellent results when it comes to increasing the oxygen barrier. In order to illustrate this, an example is describ- ed in the following. Example ® An injection moulded preform of Barex 2101, which has been described by way of introduction, is used. Basically, the preform is of the general type shown in appli- cant's EP-A-521 841. The preform comprises a tubular body having a closed hemispherical end and one open end defining a mouth portion having outer threads for engagement with corresponding inner threads of a closure cap. The tubular preform body has a cylindrical portion between the mouth portion and the closed end portion. This cylindrical portion defines the side wall of the preform, and in the blow moulding step this preform side wall is transformed to an expanded cylindrical body defining the side wall of the container. In this example, the container to be produced is a transparent bottle for juice or beer having a volume of 0.5 litre. ® The weight of the Barex preform is 47g which should be compared with corresponding known preforms of PET and ® PEN which have a weight of 54g. Since the Barex material has a lower density (1.11 kg/m 3 ) than PET and PEN (1.33 kg/m 3 ) , a lighter preform can be used, which makes it possible to reduce the material costs. ® The cylindrical body portion of the Barex preform has a wall thickness of about 3.6 mm, and the wall thickness of the side wall of the finished container should be about 0.3 mm, taking into account strength requirements placed on the container. The container in this example is of the disposable type and blow moulded basically in accordance with a known single-stage process, for instance, of the type described in the introductory portion of the before-mentioned document WO 97/32711. ® The Barex preform is first heated to a temperature of about 180°-200°C, then cooled to a temperature of about 100°C, and finally transferred to a blow mould in which it is blow moulded into the container in a manner known per se using a so-called stretching rod. The blow mould cavity has an inner shape corresponding to the outer shape of the finished container. In this blow moulding step, the material is stretched and biaxially oriented in such a way that the wall thickness of the container side wall is about 12 times less than the wall thickness of the cylindrical portion of the preform. By this excessive orientation, the container obtains excellent oxygen barrier properties. The oxygen barrier of the finished container is tested in oxygen transmission rate measuring equip- ment referred to as "OX-TRAN 2/20", which is marketed by the US company Modern Controls, Inc. and which is well known to people skilled in the art. This test shows that the container has an oxygen transmission rate below 0.016 cm 3 .mm/m 2 .24h.bar, which is extremely good. This rate may be compared with the correspond- ® ing rate of unoriented Barex 2101, which is about 0.3 cm 3 . mm/m 2 24h.bar. Thus, in the above comparison the oxygen barrier is improved at least in the magnitude of about twenty times . The oxygen transmission rate is also significantly im- proved (lowered) over oriented Barex ® 2101 which accord- ing to details given in the BP Chemicals pamphlets has a rate of 0.15 cm 3 .mm/m 2 .24h.bar . In this comparison, the oxygen barrier is improved at least in the magnitude of ten times. Furthermore, the oxygen barrier may be even better than described herein since the OX-TRAN equipment can hardly measure such low rates. Similar tests have shown very good results also when other biaxial orientations have been used. Low oxygen transmission rates fulfilling the present requirements from the market have been obtained for stretching of the polymeric material in accordance with wall thickness ratios preform-container in the range 10-15, preferably 11-13. The amount of nitrile in the polymeric material is also believed to be a parameter of interest when it comes to obtaining low oxygen transmission rates. The tests have shown that this amount of nitrile, as calculated from nitrile monomer (s) used, should be about 60-80% by weight, preferably about 70% by weight. Still another parameter which is believed to have an effect on the process is the glass transition temperature of the polymeric material. This temperature (Tg) should be about 70-90°C, preferably about 75-85°C, and more preferably about 82 °C. The container may be either of the monolayer type consisting of said polymeric material, or of the multilayer type in which said polymeric material is a barrier layer included in the multilayer structure. For instance, a multilayer container may have a side wall comprising a central core or barrier layer of said polymeric material and layers of another thermoplastic material, such as PET, on each side of the barrier layer. Further, there may be so-called tie layers between the barrier layer and the two PET layers. Preferably, said polymeric material comprises a terpolymer of acrylonitrile, methyl acrylate and butadiene. In a variant, methyl acrylate may be replaced by styrene . A preferred embodiment of the inventive method and a preferred variant of the inventive use are clear to the skilled person from the example given above. Finally, it should be pointed out that the invention is by no means limited to the embodiments and examples above, but modifications are feasible within the scope of the appended claims. In particular, it should be mentioned that the inventive concept is applicable to both injection blow moulding processes and injection stretch blow moulding processes, as is appreciated by the skilled person. Further, it should be emphasised that the inventive concept is applicable to both disposable one-way containers and reusable or returnable containers. Of course, this includes modern refillable containers for various liquids. The blow moulding process in which the invention is practised may be either of the so-called single-stage type, that is forming of the preform and the container "in line", or of the so-called two-stage type in which the preform is first injection moulded and then stored before the blow moulding step. These two processes are well known to the skilled person and described in WO 97/32711 mentioned in the introductory portion of this description.

Claims: CLAIMS 1. A biaxially oriented container blow moulded from a preform of polymeric material, c h a r a c t e r i s e d in that the container side wall has a wall thickness which is 6-20 times less than the wall thickness of the preform body, said polymeric material comprising a modified nitrile-acrylate copolymer. 2. A container according to claim 1, said polymeric material comprising a copolymer of a nitrile monomer and an acrylate monomer having the general formula I and II, respectively : H 2 C=C CN I R wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms ; H 2 C=C COOR 2 II R, wherein R- L is hydrogen or an alkyl group having from 1 to 4 car- bon atoms; and R 2 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . 3. A container according to claim 2 , said polymeric material comprising a first component being a copolymer of said nitrile monomer I and said acrylate monomer II admixed or polymerised with a second component selected from the group consisting of a monomer, a polymer and a copolymer. 4. A container according to claim 3, said polymeric material comprising a copolymer of said nitrile monomer I and said acrylate monomer II admixed with a rubber selected from the group consisting of: i ) a polymer of a diene monomer having the general formula III: wherein R 3 is hydrogen or an alkyl group having from 1 to 4 carbon atoms; ii ) a polymer of an aromatic compound having the general formula IV: wherein R 4 is hydrogen or an alkyl group having from 1 to 4 carbon atoms; Hi ) a copolymer of said nitrile monomer I and said diene monomer III; iv) a copolymer of said nitrile monomer I and said aromatic compound IV; v) a copolymer of said diene monomer III and said aroma- tic compound IV. 5. A container according to claim 3 , said polymeric material comprising a terpolymer of said nitrile monomer I, said acrylate monomer II and a monomer selected from the group consisting of: i ) said diene monomer III, or a polymer thereof; ii ) said aromatic compound IV, or a polymer thereof; Hi ) a mixture of said diene monomer III and said nitrile monomer I , or a copolymer thereof . 6. A biaxially oriented container blow moulded from a preform of polymeric material, c h a r a c t e r i s e d in that the container side wall has a wall thickness which is 6-20 times less than the wall thickness of the preform body, said polymeric material comprising a modified copolymer of a nitrile monomer and an aromatic compound having the general formula I and IV, respectively: H,C=C- -CN R wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms; wherein R 4 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . 7. A container according to claim 6, said polymeric material comprising a terpolymer of said nitrile monomer I, said aromatic compound IV and a diene monomer having the general formula III: wherein R 3 is hydrogen or an alkyl group having from 1 to 4 carbon atoms . 8. A container according to any one of the preceding claims, wherein R is H in said nitrile monomer I. 9. A container according to any one of claims 2-5 and 8, wherein R x is H in said acrylate monomer II. 10. A container according to claim 9, wherein R 2 is a methyl group. 11. A container according to any one of claims 4-10, wherein R 4 is H in said aromatic compound IV. 12. A container according to any one of claims 4-5 and 7-11, wherein R 3 is either H or a methyl group in said diene monomer III. 13. A container according to any one of claims 5, 8-10 and 12, said polymeric material comprising a ter- polymer of acrylonitrile, methyl acrylate and butadiene. 14. A container according to any one of claims 6-12, said polymeric material comprising a terpolymer of acrylonitrile, styrene and butadiene. 15. A container according to any one of the preced- ing claims, wherein the amount of nitrile in said polymeric material, as calculated from nitrile monomer (s) used, is about 60-80% by weight, preferably about 70% by weight . 16. A container according to any one of the pre- ceding claims, wherein the glass transition temperature (Tg) of said polymeric material is about 70-90°C, preferably about 75-85°C, and more preferably about 82°C. 17. A container according to any one of the preceding claims, wherein the ratio between the wall thick- ness of the preform body and the wall thickness of the container side wall is within the range 10-15, preferably 11-13. 18. A container according to claim 17, wherein said wall thickness ratio is about 12. 19. A container according to any one of the preceding claims, wherein the container is of the monolayer type consisting of said polymeric material . 20. A container according to any one of claims 1-18, wherein the container is of the multilayer type, said polymeric material being a barrier layer included in the multilayer structure. 21. A method of making a biaxially oriented container which is blow moulded from a preform of polymeric material, c h a r a c t e r i s e d in that the preform in the blow moulding step is expanded in such a way that the wall thickness of the container side wall is 6-20 times, preferably 10-15 times, most preferably 11-13 times and in particular about 12 times, less than the wall thickness of the preform body, said polymeric material comprising either a modified nitrile-acry- late copolymer as claimed in claim 1, or a modified copolymer of a nitrile monomer and an aromatic compound as claimed in claim 6. 22. Use of polymeric material comprising either a modified nitrile-acrylate copolymer as claimed in claim 1 , or a modified copolymer of a nitrile monomer and an aromatic compound as claimed in claim 6, for blow moulding a biaxially oriented container from a preform comprising said polymeric material.

Inventors: LJUNGQVIST NILS, QUASTERS MIKAEL, LJUNGQVIST, NILS, QUASTERS, MIKAEL
Applicants: LJUNGQVIST NILS, PLM AB, QUASTERS MIKAEL, LJUNGQVIST, NILS, QUASTERS, MIKAEL
Application year: 1999
Main classification: C08L33
IPC codes: B29C49