TRANSPARENT, STERILISABLE PACKAGE OF PLASTIC

Abstract: A transparent, sterilisable package of plastic is described. The packaging material is a mixture of about 55-75 % by weight, preferably about 65 % by weight, of low-density polyethylene (LDPE) and about 25-45 % by weight, preferably about 35 % by weight, of high-density polyethylene (HDPE), preferably HDPE of narrow molecular weight distribution. The package which preferably is in the form of a bag, such as a blood bag, suitably has a wall thickness of about 0,2-2mm and stands up to sterilisation of 121 DEG C for 20 min.

Description: TRANSPARENT, STERILISABLE PACKAGE OF PLASTIC The present invention relates to a transparent, sterilisable package of plastic. By "transparent" in the context of this invention is meant thac the plastic is transparent in the sense that it is possible to discern an object through the ma¬ terial. A frequently used practical transparency test for packages, for example bottles, is the so-called "watch test" in which a package in the form of a bottle is filled with water and the face of a digital watch is then viewed through the filled bottle. If it is possible to read the time, the package is deemed to have passed the test and is considered transparent. By "sterilisable" in the context of this invention is meant that the plastic can withstand water vapour sterilisation corresponding to at least 121°C for 20 min. At present, the accepted minimum demands for sterilisa¬ tion are 121°C for 20 min. For sterilisation at higher temperatures the sterilisation time can be reduced cor¬ respondingly. This means that every increase of the ste- rilisation temperature by one degree brings considerable advantages in the form of shorter sterilisation times and increased productivity. In many different applications, objects are requir¬ ed which are, inter alia, transparent and can be steri- Used. One such application is the field of packaging, especially the packaging of drugs where the requirement for viewability of the goods within the package and for sterilisation of the package is especially pronounced. Another application is the field of food packaging. Bags containing infusion solution are sterilised at, for example, a temperature of 121°C for different periods of time, although for at least 20 min. The products are sterilised in an autoclave containing pure saturated water vapour. At start-up, the temperature may rise excessively, and furthermore temperature fluc¬ tuations within the autoclave must be allowed for. Put together, this means that the material should preferably be able to withstand temperatures in excess of 121°C, preferably by a margin as wide as possible, without detracting from any of the other demands (optical cha¬ racteristics, impact strength, etc.). In other words, every additional degree the material can withstand beyond the maximum sterilisation temperature increases the safety of the operation. In packages with sterile contents, such as a ste¬ rilised liquid, it is, furthermore, frequently desired that the package can be emptied of its liquid contents without simultaneously admitting air into the package because this would mean that the sterility were lost. For this reason, the packing material should not be brittle and hard, but tough and deformable so that it can be emptied while being deformed by the sub-pressure produced in the package. Among present-day materials for transparent and sterilisable packages, mention may be made of glass which has excellent transparency and is highly steri¬ lisable, but is heavy, fragile and easily broken. Another prior art material is polypropylene which, although it eliminates the disadvantage of the fragility of the glass, has poor mechanical properties in cold and contains additives which may be precipitated as particles upon sterilisation in an autoclave and thus contaminate the sterile product. Furthermore, it is known to produce packages of polyethylene plastic (PE). In the prior art technique, however, the insistence on transparency or thermal resistance, such as sterilisability, has not been espe¬ cially high. Thus, a package made of low-density poly- ethylene (LDPE) melts or softens at about 100-110°C and has a maximum temperature of use of about 90°C, which means that this plastic does not withstand steri¬ lisation at 121°C for 20 min. On the other hand, a package made of high-density polyethylene (HDPE) melts or softens only at about 130-135°C, but is too brittle. Depending on whether it has a narrow or a wide molecular weight distribution (MWD), HDPE also has other disadvantages in the context of this invention. By wide MWD is meant a value in excess of 10, and by narrow MWD is meant a value not exceeding 10, prefer¬ ably 2-10, most preferred about 4, the value of MWD being the quotient of the weight average value and the number average value of the molecular weight (M /M ) . Although HDPE of narrow MWD is transparent, it is difficult to process. The poor processability can be remedied by additives, such as fluorine-containing polymers, for example of the Viton ® type, but in many applications, such as foods and drugs, additives are undesirable because they may be hazardous to health. HDPE of wide MWD is more easily processed, but is brittle and has the further disadvantage of an infe¬ rior transparency. A further type of polyethylene polymer is linear medium-density polyethylene (LMDPE), i.e. a density in the range of about 930-940 kg/m 3 . LMDPE has a melt¬ ing point of about 126°C and consequently a maximum temperature of use that lies 5-10°C below this tempera¬ ture. This means that LMDPE can withstand sterilisation at 121°C for 20 min., but not at higher temperatures, such as 127°C. Depending on whether it has a narrow or a wide MWD, LMDPE furthermore has other disadvan¬ tages in the context of this invention. Thus, LMDPE of narrow MWD is difficult to process, and LMDPE of wide MWD is not transparent. Even though there thus is a need for a package of a material which is transparent and sterilisable and which is preferably also deformable, it will be appreciated from what has been said above that prior art materials all suffer from one or more disadvantages which, in some respect or other make them unsuitable for the production of such packages. It has now surprisingly been found, in the con¬ text of this invention, that a specific type of plas¬ tic is capable of satisfying all of the above-mentioned requirements for transparency and sterilisability, simul¬ taneously as the material is impact resistant, easy to process and cheap. Utilising this material, the present invention provides a transparent sterilisable package of plastic, said package being characterised in that the plastic consists of a mixture of about 55-75% by weight of low- 3 density polyethylene (915-930 g/cm ) and of about 25-45% 3 by weight of high-density polyethylene (941-965 g/cm ). Further characteristic features of the invention will appear from the following description and the claims. It should be mentioned that mixtures of polyethy¬ lene for various other purposes are previously known. Thus, US 4,536,549 discloses a polymer composition which consists of a mixture of 40-90% by weight of li¬ near low-density polyethylene (LLDPE) and 10-60% by weight of polypropylene (94-99% by weight of propylene and 1-6% by weight of ethylene) . Furthermore, 2-15% by weight of high-density polyethylene (HDPE) may be admixed. It is natural for this composition to be ther¬ mally resistant since the composition contains polypro- pylene which has a melting point of about 150°C. Thus, the polymer composition according to US 4,536,549 con¬ tains polypropylene as an essential constituent, which is not the case in the present invention. Furthermore, high-density polyethylene (HDPE) is included in the present invention as an essential constituent in an amount of 25-45% by weight, whereas HDPE, when present, constitutes but 2-15% by weight in US 4,536,549. Moreover, US 2,983,704 discloses a method for the preparation of high-density polyethylene (HDPE). Mixing HDPE with low-density polyethylene (LDPE) is also mentioned, but the production of transparent and sterilisable packages is not disclosed. US 4,461,873 refers to a mixture of medium-density polyethylene (LMDPE) and high-density polyethylene (HDPE), not to a mixture of LDPE (density 0.915-0.930 g/cm ) and HDPE, as in the present invention. JP 51034956 discloses a polymer mixture of HDPE with admixture of 0.1-30% of polyolefin, for example LDPE. This is different from the present invention which requires a LDPE proportion of 55-75% by weight. JP 53033259 relates to an adhesive polyethylene which consists of a mixture of LDPE and HDPE. The use for transparent and sterilisable packages is not disclosed. JP 60-55042 discloses a mixture of HDPE with polyethylene of ultra-high molecular weight (LDPE) to afford improved flowability. JP 59-84942 discloses the production of filaments (not packages) of a mixture of 5-75%, preferably 10-50% LDPE and 25-95%, preferably 50-90% of HDPE. LDPE thus preferably is a minor proportion of the composition, and this is different from the present invention in which LDPE is to constitute the major proportion. GB 860,329 discloses a mixture of 50-85 parts of HDPE and 15-50 parts of LDPE, i.e. LDPE constitutes also here a minor proportion of the composition. US 3,375,303 discloses a composition which con¬ sists of a mixture of LDPE and 1-9% of HDPE, which is different from the present invention where 25-45% by weight of HDPE is required. To sum up, the prior art technique does not dis- close the use of mixtures of low-density polyethylene and high-density polyethylene (HDPE) in the proportions specified by the present invention, for the production of transparent and sterilisable packages. In view of the demand for transparency, the pre¬ sent invention prefers to utilise as HDPE a HDPE of narrow NWD. In view of the mechanical characteristics, the transparency and the processability, the proportion of LDPE is about 55-75% by weight and the proportion HDPE about 25-45% by weight of the mixture. Especially preferred is a mixture of about 65% by weight of LDPE and about 35% by weight of HDPE to provide an optimal combination of processability, transparency, mechanical characteristics and sterilisability. The melting points of LDPE and HDPE as utilised in the present invention lie within the above-mentioned temperature ranges and preferably at about 107°C for LDPE and at about 133° for HDPE. It must be considered surprising that it is pos¬ sible to produce a package satisfying the above-men¬ tioned requirements, from the polymer mixture accord- ign to the invention, especially a polymer mixture containing predominantly LDPE in view of the low melt¬ ing point of this polymer. One would expect the melting or softening temperature of the package to be dominated by the main constituent of the material, i.e. LDPE, but this is not the case. Although the polymer mix¬ ture has two melting points corresponding to the melt¬ ing points of the polymers included in the mixture, it still can withstand sterilisation at 121°C for 20 min. The especially preferred polymer mixture of 65% by weight of LDPE and 35% by weight of HDPE even withstands sterilisation at 127°C. It is also surprising that the polymer mixture of HDPE and LDPE according to the invention has bet¬ ter characteristics than LMDPE having a corresponding density. As mentioned above, a 65/35% by weight mixture of LDPE/HDPE according to the invention, which has 3 a density mean value 935 kg/m , can withstand sterili¬ sation at 127°C, in spite of the fact that the LDPE included has a melting point of about 107°C. A LMDPE 3 having the density 935 kg/m , which should be expected to have better characteristics, has a melting point of 126°C and thus does not withstand sterilisation at 127°C. The reason why the sterilisation characteris¬ tics of the polymer mixture are better than those of the directly polymerised medium-density polymer (LMDPE) is not clear. Also the processability of the polymer mixture according to the invention is better. The package according to the invention may, in principle, be any type of package whatsoever, but pre- ferably is in the form of bottles, cans or bags. The package is preferably manufactured by blow moulding, film blowing or injection moulding of the plastic in per se known manner. Sterilisation of the package may take place in connection with the production of the package, or at a later stage when the package has been filled with its contents. As has also been mentioned before, drugs are one application where the requirement for sterilisability of packages is especially pronounced, and the package according to the invention thus is especially useful for this application. The products to be sterilised can be solid or liquid and packaged both in bottles and in cans. Another advantageous package are bags, for example for packaging blood, which are heat-sealed after filling. Because of its excellent heat-sealing properties, the plastic according to the invention is well suited for this type of packages. The wall thickness of the package according to the invention is per se not critical , but in order to make the package sufficiently deformable to allow it to be emptied of its liquid contents without si¬ multaneous admission of air, the wall thickness should not exceed 2.0 mm. On the other hand, the wall thickness preferably should not be less than 0.02 mm because this may jeopardise the function of the package and make it too fragile. To facilitate understanding of the invention the following nonrestrictive Examples are given. EXAMPLE 1 65% by weight of a long chain-branched LDPE poly¬ mer of Mw=140,000, Mw/Mn=9, melting point 107°C, melt index (2 kg weight) = 0.3 g/10 min. and density 922 (ISO 1183 D) was mixed with 35% by weight of a linear HDPE polymer having 0.3 ethyl side branches/1000 carbon atoms, melting point = 132°C, Mw=130.000, Mw/M=4 and melt index (2 kg weight) = 0.85. 3 The density of the resulting mixture was 935 kg/m (ISO 1183 D). The LD polyethylene and the HD polyethylene were commercial products, the former produced in a high- pressure tube reactor, the latter in a low-pressure fluidised bed reactor by means of organometallic cata¬ lysts of the Ziegler-Natta type. The polymers were mixed in the molten state at 210°C so that a homogeneous mixture was obtained, whereupon a 100 μm film was produced by conventional film blowing technique. Operating conditions were as follows: Blowing ratio = 2rl Temperature of melt = 230°C Frost line level = 2 x die diameter The film was converted after extrusion into 1000 ml bags, filled with water and sealed by impulse heat- sealing. Vapour sterilisation was carried out by placing the bags in an autoclave and then subjecting the bags to vapour treatment at a 121-127°C for 20 min. with a supporting pressure of 1.8 bar. After sterilisation, the bags were inspected for structural stability, and it was found that the above- mentioned materials and packages stood up to a steri¬ lisation temperature of 126°C for 20 min. without any effect upon the bags. EXAMPLE 2 (Comparison Example) A film was produced from 100% linear medium-density polyethylene (LMDPE) having the same density as the mix¬ ture in Example 1, i.e. 935 kg/m (ISO 1183 D) . The other characteristics of LMD polyethylene were as follows: 8 ethyl side branches/1000 carbon atoms Mw = 130,000 Mw/Mn = 4 Melt index = 0.85 (ISO 133) Melting point = 126°C The LMD polyethylene was homogenised in the molten state at 210°C. A film was blown under the extrusion conditions according to Example 1, whereupon water- filled bags were produced. The vapour sterilisation test revealed a tendency toward deformation of the bag top sides at 124°C for 20 min., and complete melting has occurred when treated at 125°C for 20 min. EXAMPLE 3 The same mixture as in Example 1 was used. The bottles were blow-moulded in a conventional blow moulding machine. The bottles had a wall thick¬ ness of 0.5 mm, a weight of 10 g, and a volume of 125 ml. The bottles had excellent transparency equivalent to that of linear polyethylene of corresponding density. The bottles were filled with water and autoclaved at 121 and 124°C for 20 min. without any effect upon the material. Upon autoclaving at 127°C, the bottles were slightly deformed. EXAMPLE 4 (Comparison Example) A linear polyethylene having the same characteris¬ tics as in Example 2 was used for blow moulding of bottles in a conventional blow moulding machine. The bottles had a wall thickness of 0.5 mm, a weight of 10 g, and a volume of 125 ml. The bottles were filled with water and autoclaved at 121°C for 20 min. without any effect upon the ma¬ terial. Upon autoclaving at 124°C for 20 min., the bottles were slightly deformed. Upon autoclaving at 127°C, the material melted and the bottles collapsed. EXAMPLE 5 55% by weight of a LDPE having the same charac¬ teristics as in Example 1 was mixed with 45% by weight of a HDPE which also had the characteristics stated in Example 1. 3 The density of the resulting mixture was 940 kg/m (ISO 1183 D). Mixing, film production, bag production and va¬ pour sterilisation were carried out in accordance with Example 1. After sterilisation, the bags were inspected for structural stability, and it was found that the above¬ mentioned materials and packages stood up to a steri¬ lisation temperature of 127°C for 20 min. without any mechanical effect upon the bags. However, the 45% by weight HDPE film had an opacity which is a borderline case of what is acceptable. A higher HDPE content gives a film which is unacceptable in respect of transparency. EXAMPLE 6 75% by weight of a LDPE having the same charac¬ teristics as in Example 1 was mixed with 25% by weight of a HDPE which also had the characteristics according to Example 1. 3 The density of the resulting mixture was 930 kg/m (ISO 1183 D) . Mixing, film production, bag production and vapour sterilisation were carried out in accordance with Exam- pie 1. After sterilisation, the bags were inspected for structural stability, and it was found that the above¬ mentioned materials and packages stood up to a steri¬ lisation temperature of 124°C for 20 min. without any effect upon the bags, while higher temperatures caused deformation.

Claims: CLAIMS 1. Transparent, sterilisable package of plastic, c h a r a c t e r i s e d in that the plastic consists of a mixture of about 55-75% by weight of low-density polyethylene (915-930 g/cm ) and of about 25-45% by weight of high-density polyethylene (941-965 g/cm ). 2. A package as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the high-density polyethylene has a narrow molecular weight distribution, the value of the quotient between the weight average value and the number average value of the molecular weight being 2-10, preferably about 4. 3. A package as claimed in claim 1 or 2, c h a ¬ r a c t e r i s e d in that the proportion of low- density polyethylene is about 65% by weight, and that the proportion of high-density polyethylene is about 35% by weight. 4. A package as claimed in any one of the preced¬ ing claims, c h a r a c t e r i s e d in that the low-density polyethylene has a melting point of about 107°C, and that the high-density polyethylene has a melting point of about 133°C. 5. A package as claimed in any one of the preced¬ ing claims, c h a r a c t e r i s e d in that it has a wall thickness of about 0.02-2.0 mm. 6. A package as claimed in any one of the preced¬ ing claims, c h a r a c t e r i s e d in that it is in the form of a bag. 7. A package as claimed in any one of the preced¬ ing claims, c h a r a c t e r i s e d in that it is in the form of a bottle.

Inventors: BAECKMAN MATS, BUSCHKA ANETTE, MAARTENSSON HANS, BAECKMAN, MATS, BUSCHKA, ANETTE, MAARTENSSON, HANS
Applicants: NESTE OY
Application year: 1988
Main classification: C08J5
IPC codes: B65D65, C08J5, C08L23