MXPA98001608A - Polypropylene tolerant to radiation and its articles uti - Google Patents

Abstract

Packaging material or an article or medical device, prepared for radiation sterilization of itself, its contents, or combinations, or which has been exposed to sufficient radiation for such sterilization, comprising a physical mixture of about 99 to about 50% by weight of homo or copolymerized polypropylene including from about 1 to about 50% by weight of polyethylene produced by catalysis in a single site. A process for making such radiation-tolerant packaging material, article or medical device is also provided.

Description

POLYPROPYLENE TOLERANT TO RADIATION AND ITS USEFUL ITEMS Field of the Invention This invention relates generally to olefinic polymers. More particularly, this invention relates to a process for imparting tolerance to radiation to polypropylene, and to uses of such radiation tolerant polypropylene. BACKGROUND OF THE INVENTION Polypropylene is an excellent material for use in a variety of applications, particularly medical and food packaging applications. Polypropylene is resistant to crushing, resistant to most chemical agents, inexpensive, easily formed, easily handled, and can be incinerated or recycled. However, currently available polypropylene is subject to certain limitations. For example, polypropylene materials tend to be somewhat nebulous or translucent, rather than clear. Also, the typical polypropylene tends to soften and deform when sterilized at high temperatures by water vapor, or become yellow and / or brittle when treated with high energy radiation, particularly beta and gamma radiation. Beta radiation, such as that from an electron beam, or gamma radiation, such as that from a cobalt-60 source, is often used to sterilize medical equipment. It is a particularly convenient means of sterilization as the items can be packaged in bulk, or in clean packages individually sealed, and irradiated after packing. Such treatments render the instruments and devices sterile without the need for special handling or re-packaging after sterilization. In this way, sterility and improved patient safety are ensured. However, because polypropylene tends to degrade when exposed to sterilizing levels of radiation, such treatment is generally inappropriate for medical devices that incorporate polypropylene components. But except for this limitation, polypropylene would be extremely useful for making a large number of useful items, including syringe barrels, culture dishes, tissue culture bottles, intravenous catheters and tubing, and bags or bottles, surgical probes, surgical material, suture, and other items. The potential usefulness of polypropylene has been recognized for some time. Others in the field have tried to overcome property limitations by numerous means. In US Pat. No. 4,110,185, for example, Williams, Dunn and Stannett describe the use of a non-crystalline mobilizing agent in polypropylene formulations to increase the free volume of the polymer and prevent radiation brittleness. In US Pat. No. 4,845,137, Williams and Titus describe a polypropylene composition that is stable to sterilizing radiation, comprising polypropylene of a narrow molecular weight distribution (MWD), a liquid mobilization additive, a hindered amine compound, and a clarifying agent. Although these additives generally appear to improve radiation tolerance, mobilization additives tend to be oily or greasy. This can contribute to processing difficulties and product defects. Other inventions that attempt to stabilize polypropylene against the effects of high energy radiation employ syndiotactic polypropylene. EP-A2-0 431 475 describes making a radiation-resistant polypropylene resin composition suitable for the preparation of molded articles in which the physical properties "are poorly deteriorated during sterilization by radiation", using substantially syndiotactic polypropylene. The composition may also include an anti-oxidant containing phosphorus, an anti-oxidant containing amine, and a nucleating agent. JP 04-214709 apparently describes ethylene / propylene copolymers with at least 50% syndiotacticity, which have improved tolerance to radiation. Such copolymers are produced by specific chiral metallocene type catalysis and are preferably combined with anti-oxidants containing phosphorus or amine for better radiation tolerance. US Patent 5,340,848 discloses a radiation resistant polypropylene resin composition comprising a polypropylene having a substantially syndico-tactical structure with optional anti-oxidants and / or nucleating agents. WO 92/14784 describes physical mixtures of 30 to 40% by weight of an ethylene-based copolymer, with 70 to 30% by weight of a propylene-based copolymer for use in thermal seal applications. The prior art clarifies that a simple, cost-effective system for providing radiation-tolerant polypropylene has been sought for a long time. Ideally, such a polypropylene composition would provide products that are clear and dimensionally stable at elevated temperatures. Such products may optionally be sterilized by means other than radiation without suffering softening or deformation or significant deterioration of the optical properties. It would additionally benefit manufacturers of polypropylene articles that the physical polymer blend used for training would not tend to fail the molding equipment with oil or grease. Users of the final formed products, as well as the manufacturers of such articles, would benefit if such polymeric compounds do not exude oil or grease from the surface of the molded parts. Such articles would be particularly attractive to the medical and food packaging industries. The present invention provides a simple and commercially practical system. SUMMARY OF THE INVENTION This invention provides polypropylene material tolerant to radiation which is a physical mixture of polypropylene and a polymer produced using single site catalysis (SSC). Such a polypropylene material is resistant to the effects of softening the elevated temperature and is resistant to degradation when exposed to gamma and beta radiation sterilization doses. In a preferred embodiment, such material exhibits sufficient optical properties for a wide variety of end uses. A process for producing such polypropylene material is also provided. Another embodiment of the invention concerns articles of manufacture that are produced from such polypropylene material. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 provides a graph of the secant flexure modulus against the polyethylene content produced by SSC in polypropylene for Examples 1-8. Figure 2 provides thermal deflection temperatures plotted against the polyethylene content produced by SSC in polypropylene for Examples 1-8. Figure 3 provides a graph of the secant flexure modulus against the polyethylene content produced by SSC in polypropylene for Examples 23-32.

Figure 4 provides thermal deflection temperatures plotted against the polyethylene content produced by SSC in polypropylene for Examples 1-8 and 23-32. Figure 5 provides a superfi cial response curve relating nebulosity, melt index and density for physical blends of homopolypropylene with polyethylene produced by SSC. Detailed Description of the Invention It has been found that improved physical blends of polypropylene can be obtained for use in medical packaging applications, food applications and related applications by incorporating catalyzed polymers at a single site, most preferably polyethylene catalyzed at a single site. The resulting physical blends have improved tolerance to radiation and heat and have better clarity than conventional physical blends of polypropylene. The polyethylene catalyzed in a single site is preferably produced using metallocene catalysis. Such metallocene materials are commercially available from Exxon Chemical Company of Houston, Texas, United States, under the name "Exact". These materials can be made in a variety of processes (including grout, solution, high pressure and gas phase) using metallocene catalysts. The processes for making a variety of polyethylene materials with metallocene catalyst systems are well known. See, for example, U.S. Patent No. 5,064,802. Generally, polymers produced with metallocene catalysis exhibit a narrow molecular weight distribution, which means that the ratio of the weight average molecular weight to the number average molecular weight will be equal to or less than 4, more typically in the range of 1.7 to 4.0. Preferably, they will also exhibit a narrow compositional distribution, which means that the fractional content of co-monomer from molecule to molecule will be similar. A useful method of measuring the composition distribution is by using the "Compositional Distribution Amplitude Index" (CDBI), which is defined as the weight percentage of the copolymer molecules having a co-monomer content within of 50% (ie, 50% on each side) of the total molar average content of co-monomer. CDBI measurements can be made using the "elution fraction that raises the temperature" (TREF), as is well known in the art. The technique is described by Wild and collaborators in the Journal of Polymer Science, edition of Polymer Physics, vol. 20, p. 441 (1982). From the curve of fractions by weight versus distribution of the composition, the CDBI is determined by establishing that percentage by weight of the sample has a content of co-monomer within 50%, on each side, of the average content of co-monomer. Additional details relating to the determination of the CDBI of a copolymer are known to those skilled in the art. For example, PCT patent application WO 93/03093, published February 18, 1993, and incorporated herein by reference, provides improved means of measuring CDBI by recognizing and treating with low molecular weight fractions. The invention provides processes for making polypropylene compositions that are radiation tolerant. The term "radiation tolerant" generally means resistant to deterioration in mechanical properties, clarity and color typically experienced by certain materials, such as polypropylene, when subjected to radiation. Of course, the acceptable level of radiation tolerance depends, at least in part, on the application or end use of the irradiated material. For example, in applications that require very rigid, clear and visually attractive items, a small deterioration in properties can render such items useless, while other applications may be more tolerant. The present invention provides commercially useful means of imparting radiation tolerance to polypropylene compositions without significantly affecting the clarity or processability of such polypropylene compositions. As is well known by the technicians in the field, the physical mixtures of polypropylene and traditional polyethylene, produced by Ziegler-Natta catalysis, tend to produce nebulous films and articles. The present invention, on the other hand, allows the production of radiation tolerant films and articles that exhibit excellent optical properties. In fact, it is surprising that SSC polyethylene can be added in significant amounts without severely reducing the optical properties of the polypropylene composition. Also, ideally, the compositions of the present invention are highly resistant to the softening effects of elevated temperature. The processes comprise physically mixing homo or propylene copolymer with about 1 to about 50% by weight of ethylene homo- or copolymer produced by catalysis in a single site, having a molecular weight distribution between about 1 and about 4, and a higher CDBI of around 45%. In a preferred aspect, the invention provides food or medicine packaging materials or articles, or medical devices, which may be clear and / or resistant to high temperature softening. These are suitable for sterilization by high energy radiation by themselves, with their content, or have been exposed to sufficient radiation for such sterilization. Such products or articles comprise physical mixtures of propylene homo or copolymers with from about 1 to about 50% by weight of ethylene homo- or copolymer produced by catalysis in a single site, having a molecular weight distribution between about 1 and about of 4, a CDBI greater than about 45%, or combinations thereof. Most preferably, enough polyethylene SSC is incorporated to provide tolerance to radiation doses of up to 10 MRad. However, in some applications a lower radiation dose, such as 2.5, 5.0 or 7.5 MRad, can be used, allowing less SSC polyethylene, such as 1 to 30% or 15% by weight, to be used to achieve a desired level of tolerance. to radiation. Thus, the amount of SSC polyethylene required to produce a radiation-tolerant propylene composition depends on a variety of factors, such as the crystallinity of the polypropylene (or the level of extractables with hexane), the radiation environment to which it is subjected. submits the composition, and the application or final use of the composition. The present invention provides a balance of physical properties, clarity and resistance to radiation, and any or all of these can be optimized for a wide variety of commercial applications. A preferred embodiment of this invention is packaging material or medical article or device, prepared for radiation sterilization thereof, its content, or combinations, or which has been exposed to sufficient radiation for such sterilization, comprising: a) homopolypropyleneNu. ; b) about 1 to about 50% by weight of polyethylene produced by catalysis in a single site; c) about 0.01 to about 0.5% by weight of a hindered amine stabilizer; and, optionally, d) up to about 1.0% by weight of secondary anti-oxidant of the thiodipropionate type, or up to about 0.5% by weight of the phosphite type; e) up to about 0.5% by weight of clarifier or nucleating agent; f) up to about 0.5% by weight of neutralizing acid scavenger; or g) their combinations. In all aspects of this invention, the physical polymer blends comprise 1 to 50% by weight of ethylene homo- or copolymer produced by single-site catalysis, the remainder being polypropylene. Copolymerized polypropylene can also be used. Preferred ranges of polyethylene produced by SSC would be in the range of about 5 to about 15% by weight. Preferred ranges of amine stabilizers would be in the range of about 0.02 to about 0.25% by weight. The preferred range for the preferred secondary anti-oxidant, a phosphite, would be from about 0.02 to about 0.25% by weight; the preferred ranges for the clarifier / nucleating agent would be from about 0.1 to about 0.4% by weight; while the preferred range for the neutralizing acid scavenger, such as stearates or basic inorganic compounds, would be from about 0.05 to about 0.15% by weight. The ability of the physical mixture to withstand high-energy radiation without brittleness increases as the amounts of the ethylene polymer increase. The modulus, the chemical resistance, the resistance to softening at high temperature and other properties directly related to the crystallinity of the polymer are reduced by increasing the amount of the ethylene polymer in the physical mixture. Although an ethylene polymer can be chosen for the physical mixture, which has minimal impact on the nebulosity of the physical mixture, generally the amounts of the increasing ethylene polymer can increase to a certain extent the haze of the physical mixtures. The ethylene polymer produced by catalysis in a single site and used as a partner for polypropylene in the physical blends of this invention can be chosen from the range of homopolymer ethylene polymers with a density of about 0.965 g / cc to high-density copolymers. Co-monomer content and density of about 0.85 g / cc. The molecular weight of the ethylene polymer used in the physical mixtures of this invention can be any value that allows the physical mixtures to function properly in the conversion processes and the intended use of the articles made therefrom, and can be measured using any suitable technique. known, such as gel permeation chromatography. The molecular weight ranges that are preferred for physical mixtures can be specified by the use of the correlated measurement known as the melt index ("MI"), measured according to ASTM method D-1238, condition E. Polymers of ethylene which are preferred due to their greater improvement in radiation tolerance of the physical mixtures herein are those which vary in density from 0.865 to 0.920 g / cc. More preferred by their combination of good improvement in radiation tolerance and low impact on the appearance (low increase in haze) of the physical mixtures are the ethylene polymers that vary in density from 0.880 to 0.915 g / cc and MI of 2- 11 degree / minute. More preferred for these same reasons are the resins in the density range of 0.895-0.910 g / cc and MI of 3-9 degrees / minute. The ethylene-based copolymers of this invention may contain any single type or combinations of co-monomer units of a wide range of types that effectively co-polymerize with ethylene in the presence of the single-site catalysts. Usually, any co-monomer or combinations of co-monomers having Ziegler-polymerizable bonds will be useful in the practice of this invention. The polypropylene, the partner in the physical mixture, or both, can themselves be physical mixtures of polymers. The invention is useful both for homo-polypropylene and propylene copolymers. Similarly, the invention relates to polypropylene materials produced either by conventional catalysts such as Ziegler-Natta systems or by metallocene catalysts, and comprising molecules of any of the types of tacticity (atactic, isotactic, or syndiotactic) combined in any proportions . Useful for their naturally superior tolerance to high energy radiation, and their high impact qualities and clarity are relatively low crystallinity polypropylenes such as random copolymers containing relatively high proportions of co-monomer units and isotactic and syndiotactic homo and copolymers with relatively high occurrence of atacticity present along the molecular chains of the polymer, as occurs in the polymerization reactor. Also, materials that have useful properties and wide utility in food packaging applications, medical packaging, and medical devices that require the ability of a material to tolerate sterilization treatments at elevated temperatures, such as by means of high water vapor pressure or in an autoclave, are the most highly crystalline polypropylenes such as highly isotactic or syndiotactic homopolymers containing a minimum of component or atactic character. However, isotactic material is preferred based on its availability. To obtain a useful balance of properties of impact resistance, clarity and tolerance to radiation and high temperature sterilization treatments, it will often be desirable to use as the polypropylene member of the physical mixture a random homo or copolymer exhibiting both tacticity and crystallinity. intermediate between the ends described above. In this way, a homopolymer or a random copolymer with low level of co-monomer incorporation and with a moderate level of isotacticity would be particularly useful. A moderate level of syndiotacticity would also be useful. Homopolymers having a level of insolubles in heptane of about 88 to 99% are preferred, and homopolymers having a level of insolubles in heptane of about 90 to 97% are more preferred. The co-monomer referred to in the above description can be any of the monomers known to combine with propylene to produce the class of polymers commonly known as random propylene copolymers. Some representative but non-limiting examples of this class of materials are C2 to C20 alpha-olefins, such as ethene, butene, pentene, hexene, heptene, octene and so on. The ethylene is the most common and the preferred representative of this class, but to impart special properties to a copolymer, any of the others, particularly butene, hexene and octene, which are also readily available and economically attractive, may be useful. Other monomers of interest may be styrene, cyclohexene, other cyclic olefins, linear or cyclic dienes such as 1,3-butadiene, 1,5-hexadiene, ethylidene norbornene, etc. Larger and longer alpha-olefins, such as 1-undecene and 1-octadecene, can also be used. The list of possible co-monomers is too large to list. The foregoing list provides some examples that represent the amplitude of the co-monomers that may be present in the propylene polymer without limiting the scope of the invention in any way. The polypropylene partner of the physical mixtures herein can be further characterized by any molecular weight and any molecular weight distribution (MWD) in a wide range thereof., as long as a particular embodiment of the polypropylene member of the physical mixture exhibits a molecular weight and MWD suitable for the process required to convert the physical resin mixture into a useful article of food packaging, medical packaging, or medical device. . Preferred for their improved resistance to high energy radiation are those resins with MWD in the lower end of the range that is suitable for the conversion process. For example, in the case of injection molded medical devices, the polypropylenes of this invention can vary reasonably in average heavy molecular weight from 10,000 to 400,000 and in MWD from 1 to 9, measured as the ratio of the heavy average molecular weight to the molecular weight numerical average, as determined using well-known means, such as gel permeation chromatography. Polypropylenes with heavy average molecular weight between 40,000 and 300,000 and MWD less than 6 are preferred from this range. Most preferred are polypropylenes with heavy average molecular weight in the range of about 50,000 to about 200,000, with MWD less than 4. The final molecular weight achieved in the polypropylene formulation can be achieved by any of several means: by minimum rupture of a resin during the formulation and the combination in the presence of minimum mechanical shear stress and oxygen; by uncontrolled mechanical and oxidative degradation, more severe during formulation and combination; or by the process often referred to as the controlled rheology process in which a measured amount of organic peroxide is used to reproducibly obtain the breakdown of a polymer of molecular weight greater than the desired lower molecular weight. Materials and processes of "controlled rheology" are generally described by Branchesi and Balbi, in "Mechanical and Structural Properties of As-Spun Polypropylene Filaments in Relation to Resin Rheology", published, on pages 27 / 1-9, by the conference International of the Plastics and Rubber Institute (November 1989). The articles made from the physical blends of this invention exhibit improved resistance to discoloration and brittleness upon exposure to sterilizing doses of high energy radiation, as compared to otherwise identical formulations of polypropylene alone. Also, notably, the articles of these compositions resist softening and high temperature distortion. Certain other aspects of the physical mixtures of the present may be cited as contributing to the high overall level of radiation resistance they exhibit. The polypropylene or polyethylene physical blend components, or the physical mixture itself, may also contain a powerful chemical solubilizing additive useful for providing polypropylene with radiation tolerance such as a hindered amine light stabilizer (HALS). Preferred examples of this additive are 2, 2, 4, 4-tetramethylpiperidine derivatives such as N, N'-bis (2, 2, 6, 6-tetramethyl-4-piperidinyl) -1,6-hexanediamine, bis (2) , 2,6,6-tetramethyl-4-piperidinyl) decanedioate, and the reaction product of dimethyl succinate plus 4-hydroxy-2, 2,6,6-tetramethyl-1-piperidine-ethanol sold by Ciba-Geigy Corporation under the denominations Chimassorb 944LD, Tinuvin 770, and Tinuvin 622LD, respectively. The HALS is used at a ratio of 0.01 to 0.5% by weight of the formulation, preferably 0.02 to 0.25% by weight, and most preferably 0.03 to 0.15% by weight. Resistance to oxidative degradation of the formulations is also enhanced by the presence of a secondary antioxidant such as those of the thiodi-propionate and phosphite ester types. Preferred examples of thiodipropionates are distearyl thiodipropionate (DSTDP) and dilauryl thiodipropionate (DLTDP), commercially available from Deer Polymer Corporation. Preferred embodiments of the phosphites are tris (2,4-di-t-butylphenyl) phosphite and bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite available as Irgafos 168 from Ciba-Geigy Corporation and Ultranox 626 from General Electric Specialty Chemicals, respectively. Additives of this kind may optionally be included in the physical mixtures herein at a ratio of 0.01 to 0.50% by weight of the formulation. Preferably, if used, they would be added to 0.02-0.25% by weight of the formulation, most preferably at a rate of 0.03-0.15% by weight of the formulation. The additives included for the purpose of providing clarity to the physical mixtures of this invention are taken from the general class of compounds known as organic nucleating agents. In this class there is a wide variety of chemical compositions, including but not limited to salts of benzoic acid and other organic acids, salts of partially esterified phosphoric acid, and dibenzylidene sorbitols. Dibenzylidene sorbitols are preferred for their powerful clarification effects. Most preferred are bis-4-methylbenzylidene sorbitol and bis-3,4-dimethylbenzylidene sorbitol, which are available from Milliken Chemical Company under the names Millad 3940 and Millad 3988, respectively. When included in the formulations of the materials herein, these clarifying nucleating agents are used from 0.05 to 1.0% by weight of the composition, preferably from 0.1 to 0.5% by weight, and most preferably from 0.15 to 0.35% by weight. In all of the above cases, the described additives can be incorporated into the physical blends of this invention as part of either the larger polymer components of the physical mixture or as an additional component added to the physical mixture itself. With respect to the physical process of producing the physical mixture, sufficient mixing must take place to ensure that a uniform physical mixture is produced before conversion into a finished product. Thus, in the cases of injection molding of medical devices, emptying and insufflation of packaging films, extrusion of pipes and profiles, etc., simple physical mixtures in the solid state of the beads serve equally well as physical mixtures in the same state. melted, pelletized, raw granules of polymers, granules with pearls, or pearls of the two components, since the formation process includes a step of re-melting and mixing raw materials. However, in the compression molding process of medical devices, little mixing of the melted components occurs, and a melted, pelletized physical mixture would be preferred over simple physical mixtures in the solid state of the beads and / or the constituent granules. Those skilled in the art will be able to determine the appropriate procedure for physically mixing the polymers to balance the need for intimate mixing of the component ingredients with the desire for economy of the process. Useful applications of the processes and materials, articles and devices include food packaging material, comprising: film and a multi-layer structure, self-sustainable, which includes: 1) metal sheet, 2) cellulosic material, 3) opaque plastic film , or their combinations. This of course includes simple wrapping film, film useful for bubble or blister packaging, and materials useful for producing containers known as "liquid boxes", as well as other useful hybrid bags, bottles or containers. Useful food packaging materials can be formed by extrusion, blowing, rolling, or combinations thereof. Also provided by the processes and applications are medical devices that are suitable for 1) intravenous (IV) use, 2) transport, storage, dispensing or combinations of medications, 3) surgical use, 4) medical examination, 5) development of crops, preparation, examination or their combinations, 6) other laboratory operations, or 7) their combinations. Such medical devices include items such as 1) intravenous catheter probe, expansive device such as an arterial "balloon", or combinations thereof, 2) intravenous fluid container or dispenser, intravenous line, intravenous valve, intravenous injection gate, unit dose, syringe or barrel of syringe, or combinations thereof, 3) forceps, handle or fastener for surgical instruments, surgical probe, curette, clamp or clamp device, retractor, biopsy sampler, robes, curtains, masks, filters, membranes filter, caps, plugs, or their combinations, 4) speculum, probe, retractor, forceps, stripper, sampler, or combinations thereof, 5) culture dish, culture bottle, cuvette, object holder, sample container or objects, or their combinations. Additional specific examples of useful medical devices that can be made by practicing the invention include disposable and reusable hypodermic syringes, particularly barrel and plunger parts. This would include, of course, pre-filled hypodermic syringes for packaging and delivery of medications as well as auxiliary parts of syringes, including caps and needle liners. This will also include parts for parenteral kits including valves, cannula lids, connectors and cannula lining. Parts for catheters are also included, particularly cannula lids, connectors and cannula lining. Useful laboratory material can also be produced, including test tubes, culture tubes, and centrifuge tubes as well as tubes for vacuum blood collection and auxiliary parts, including needle adapters / fasteners, and liners as well as medicine bottles, caps and stamps. Measuring devices such as drippers, eye droppers, pipettes and graduated feeding tubes, cylinders and burettes, as well as infant and disabled bottles and bottle holders can also be usefully made by practicing the invention. Other useful articles and products can be formed economically by practicing the invention, including: laboratory equipment, such as rolling bottles for tissue culture and media bottles, instrumentation sample holders, and sample windows; liquid storage containers such as bags, receptacles and bottles for storage and intravenous infusion of blood or solutions; packaging material including that for any medical device or medication, including unit-dose or blister or bubble packaging, as well as for wrapping or preserving food preserved by radiation. Other useful items include medical tubing and valves for any medical device, including infusion kits, catheters and respiratory therapy, as well as packaging materials for medical devices or foods that are irradiated, including trays, as well as stored liquids, particularly containers for water, milk or juices, including containers for unitary portions and bulk storage as well as transfer means such as pipes, tubes and the like. These devices can be made or formed by any useful means of forming polyolefins. These will include, at least, molding, including compression molding, injection molding, blow molding, and transfer molding; blowing or forging of film; extrusion, and thermoforming, as well as lamination, pultrusion, protrusion, reduction of diameter, rotational molding, link by rotation, spinning in molten state, blowing in melted state, or combinations thereof. The use of at least thermoforming or film applications allows the possibility of and the derivation of benefits of uniaxial or biaxial orientation of the radiation tolerant material. Those skilled in the art will recognize other applications and processes not named that fall within the scope of this invention. It is not intended to exclude such applications and processes that are apparent in the light of the present disclosure, but merely provide a useful exemplification of the present invention. In an effort to further clarify the present invention, a brief history and examples of the tests themselves are provided. These are provided as exemplification and not for limitation. EXAMPLES Four series of experiments were run to evaluate the radiation tolerance, clarity and resistance to high temperature softening of the physical mixtures of this invention. The crystallinity of the propylene homopolymers was estimated from the known relative parameter with insolubles in heptane or Hl. Hl is a measurement of the portion of a finely divided polypropylene sample that is insoluble in boiling heptane (boiling time of 1.5 hours). As Hl approaches 100%, the crystallinity of the resin approaches a maximum level in commercially available polypropylenes, some of which are formulated specifically for resistance to radiation, such as those available from Exxon Chemical Company of Houston, Texas, United States . As is well known to those skilled in the art, the stabilization of polypropylene to high energy radiation becomes more difficult as the Hl value increases. The equipment used in the experimentation included various instruments and machines. The deflection to the peak bending load and the secant modulus were determined in a universal test machine (Instron), model 1122 or 1123, available from the Instron Instrument Co. of Clinton, Massachusetts, United States. The deflection temperature was measured with a five-bar automatic deflection temperature tester, Tinius Olsen, from Tinius Olsen Testing Machine Company of Willow Grove, Pennsylva-nia, United States. The color was measured in a Hunter Ultrascan colorimeter SN 7557 (Hunter Associates Laboratories, Inc., of Reston, Virginia, United States). The actual irradiation of the samples was carried out by Isomedix, Inc. of Morton Grove, Illinois, United States. The nebulosity was measured using a Gardner photometric unit, model PG 5500 (Pacific Scientific, Gardner Laboratory Division, Bethesda, Maryland, United States). Examples 1-8 Stabilization of Physical Properties of Irradiated Homopolymer For this series, granular, isotactic propylene homopolymer, with nominal melt flow rate of 1.0 ° / min. (MFR, determined with the method ASTM D-1238, condition L), produced by conventional catalysis Ziegler-Natta with high crystallinity represented by Hl of more than 95.5% was first processed with organic peroxide in order to increase its MFT to 25 ° / min. and reduce your MWD to less than 4.0. Then, seven physical mixtures corresponding to Examples 1 to 7 were combined and extruded from a bead-forming extruder from the homopolymer with MFR of 25 ° / min. , with a variable amount of Exact 4017 and the following additives: 0.08% calcium stearate, 0.05% Tinuvin 622LD (Ciba-Geigy Corporation, Hawthorne, New York, United States), 0.08% Ultranox 626 (General Electric Co., Parkesburg, West Virginia, United States), and 0.25% of Millad 3940 (Milliken and Co., Spartanburg, South Carolina, United States). Exact 4017 is a clean ethylene-butene copolymer with a density of 0.885 g / cm3 and having an MI of 4.0 ° / min. Eight polymer samples were tested together; These are described later. Each polymeric material tested was injection molded into sets of ASTM test parts in a molding tool. Different portions of the tension bars in the shape of dog bone (165 mm long x 12.7 mm wide x 3.18 mm thick), Gardner discs (88.9 mm diameter x 3.18 mm thick), and bending bars (127 mm in length x 12.7 mm in width and 3.18 mm in thickness) were exposed to 0.0 and 7.5 MRad of irradiation with Co60 at approximately a ratio of 1 MRad / hour. These samples were then exposed to accelerated aging at 60 ° C for 21 days, an aging protocol recognized to represent approximately at least 24 months of aging in real time. The samples were then examined by the method known informally as "bending to failure", a test that is favored by the inventors for the determination of the brittleness of the polymers after irradiation. This method is fully described in "Method for Evaluating the Gamma Radiation Tolerance of Polypropylene for Medical Device Applications", by R.C. Portnoy and V.R. Cross, a document published in Proceedings of the Society of Plastics Engineers, 1991 annual technical conference (Society of Plastics Engineers, Brookfield, Connecticut, United States). In short, the test involves flexing a standard test sample derived from the ASTM strain bar in a three-point bend mode as used in the determination of the flexural modulus (ASTM method D 790-86). The test is continued until a peak load is recorded. The deflection at which this peak load occurs is characteristic of the ductility of the sample. For almost all non-irradiated polypropylenes, this deflection is approximately 0.95 to 10.5 cm. The lower the deflection registered in the irradiated samples, the greater the brittleness that results from the irradiation and the aging protocol. Another test applied to the irradiated and aged portions of the eight samples was the determination of the color on the Gardner discs. The test was run in accordance with the protocol developed by the instrument manufacturer, using the C illuminant, 10 ° light source, and having the gate in the closed position. Other color scales and measuring instruments can be substituted in works of this type with comparable, relative results. Color is an important feature of a polymer used in food and medical packaging applications. Generally, white, bluish polymers are more desirable than yellowish polymers. In the Hunter scale b used in the present, 0.0 is considered as pure white. Negative values are more blue. Positive values are more yellow. The deviation of pure white increases with the absolute value of the Hunter measurement b. The level of use of Exact 4017 in each of the physical polymer blends for Examples 1-7 is given in Table IA as the percentage of the total polymer by weight. Example 8, labeled as the "standard", is commercially available 9074MED polypropylene that was used as received from Exxon Chemical Company of Houston, Texas, United States. This material is specifically made as useful for resistance to radiation, particularly in medical applications. The results of the measurements of flexion to failure and color of the irradiated and non-irradiated samples are also listed in Table IA. Table ÍA The results of Table IA show that by increasing the amount of ethylene-based polymer produced by SSC, physical mixtures are increasingly resistant to brittleness and discoloration by high-energy radiation. Note that even low amounts of ethylene-based polymer produced by SSC in physical mixtures results in significant reductions in radiation sensitivity. The physical mixture of propylene homopolymer with 10% ethereal polymer produced by SSC was almost as resistant to brittleness by high radiation dose as the commercial radiation tolerant random copolymer, PP 9074MED. Physical mixtures with higher levels of ethylene polymer were more resistant to brittleness than PP 9074MED. All physical mixtures started less yellow and remained less yellow after irradiation than PP 9074MED. Physical mixtures with high levels of Exact 4017 remained extremely white after irradiation. Globally, physical mixtures were highly resistant to radiation degradation. To demonstrate the rigidity of the physical mixtures and their resistance to softening at elevated temperatures, the results of two additional measurements were achieved on the non-irradiated and non-aged portions of the samples of Examples 1-8. These are recorded in Table IB. The secant flexural modulus at 1% bending deflection is a measure of the stiffness of the polymers, which was determined in the same bending-to-failure test procedure and using the same test samples and equipment used for the test. deflection measurement at peak bending load. The heat deflection temperature was measured on the flex bar samples according to the ASTM method D648-82 at a flex load of about 455 kPa (66 psi).

Table IB As expected, the increase in polyethylene produced by SSC in the mixtures The physical properties of Examples 1-7 caused reductions in stiffness at room temperature and resistance to softening at elevated temperature. But the trend lines plotted from these data in Figures 1 and 2 predict that physical mixtures can be identified with excellent utility for resistance to brittleness and discoloration by high energy radiation and that they have secant modulus at room temperature and temperatures. of deflection characteristic of propylene homopolymers susceptible to being autoclaved, not clarified, for example those physical mixtures in the range of 5 to 15% polyethylene. EXAMPLES 9-15 Effect of the Physical Mixing Agent on the Nebulosity of Polypropylene Physical mixtures in the form of a bead, in the solid state, of PP 9374MED, another commercially available propylene copolymer formulation, sold by Exxon Chemical Company for use in applications that require clarity and tolerance to the sterilizing doses of high energy radiation, and three ethylene polymers produced by SSC, were prepared in 90:10 weight ratios (PP 9374MED: ethylene polymer). The three ethylene polymers used in these physical mixtures were Exact 4033, with a density of 0.880 g / cc and MI of 0.8 ° / min. , Exact 3035, with a density of 0.903 g / cc and MI of 3.5 ° / min. , and Exact 4028 having a density of 0.880 g / cc and MI of 10.0 ° / min. , all commercially available products from Exxon Chemical Company and used as received from the manufacturer. Half of each physical mixture in pearls was physically mixed in a melted state and re-shaped into pearls. Each of the six samples and a control sample, consisting of PP 9374MED clean by itself, as standard, were then molded into nebulae plates about 76.2 mm in length x 50.8 mm in width x 1.02 mm in thickness. The plates were conditioned for 48 hours and the nebulosity was measured according to the method ASTM 1003-92. The results of this work are shown in Table 2.

Table 2 The results of the nebulosity measurements of Examples 9-15 demonstrate that the effect of a moderate amount of ethylene-based polymer produced by SSC on the nebulosity of physical blends with clarified polypropylene can vary from negligible, as in Examples 11 and 12, to very significant, as in the case of Examples 9 and 10. The difference in this effect on the physical mixtures that were processed in the melted state before molding, and those that were mixed only from pearls in solid state, It seems to be very small. EXAMPLES 16-22 Effect of Etiiene Polymers Produced by SSC on Clarified Polypropylene Nebulosity Physical blends in solid state, of the propylene homopolymer used in Example 1 and each of the various Exact resins were produced with a 90:10 weight ratio of polypropylene to polyethylene. Samples of each of these physical mixtures were then injection molded into nebulae plates about 1.02 mm thick, and the nebulosity was measured on the conditioned plates, all as described for Examples 9-15. These Examples 16-21 are fully described in Table 3, where the results of the nebulosity measurements for these physical mixtures are also given. Table 3 Table 3 demonstrates a wide range of haze and suggests caution in selecting the physical mixing agent if high clarity is desired. As a general observation, it does not appear that the high cloudiness is associated with a co-monomer for the polyolefin produced by SSC, but rather the co-monomer content, which generally increases with the reduction in density, and MI can be the most important factor. The results of Table 3 further demonstrate that physical mixtures of a substantial amount of polyethylene produced by SSC with clarified polypropylene can be extremely clear. Examples 23-32 Confirmation of the Ability to Achieve Clarity, Rigidity and Resistance to both Radiation and High Temperature Softening in a Single Physical Mix An experiment similar to that implied in Examples 1-8 was conducted with physical mixtures of a single homopolymer of propylene, clarified, and three levels of each of three different polyethylenes produced by SSC. The purpose of this work was to verify the capacity to produce a single physical mixture of polypropylene and polyethylene produced by SSC incorporating the four desirable characteristics of high clarity, resistance to sterilizing doses of high energy radiation, rigidity similar to propylene homopolymers, and resistance to softening at elevated temperature. An isotactic propylene homopolymer, granular, with Nominal MFR of 1.3 was converted into a physical blend compound by treating it in a pearlizing extruder with organic peroxide to raise its MFR to around 25 ° / min. and reducing its MWD to less than about 4.0 and then physically mixing it in the melted state with 0.03% DHT4A (synthetic hydrotalcite, Kyowa Chemical Industry Co., Ltd., Kagawa, Japan), 0.06% GMS-11 (glycerol monostearate, Lonza, Fairlawn, New Jersey, United States), 0.25% of Millad 3988 (Milliken and Co., Spartanburg, South Carolina, United States), 0.10% Tinuvin 622LD (Ciba-Geigy Corporation, Hawthorne, New York, United States), and 0.08% of Ultranox 626 (General Electric Co., Parkesburg, West Virginia, United States). This propylene homopolymer formulation is denoted as Example 23. Then nine physical mixtures of polypropylene beads in solid state were prepared, corresponding to three levels of each of three different ethylene-butene copolymers produced by SSC. Physical blends with ethylene-butene copolymers are fully described in Table 4 as Examples 24-32. Of the polyethylenes produced by SSC referred to in Table 4, Exact 3024 is a pure polymeric product, commercially available from Exxon Chemical Company with a density of 0.905 g / cc and MI of 4.5 ° / min.; SLP 9043 is an ethylene copolymer produced by unmodified single-site catalyst, which is commercially available from Exxon Chemical Company, having a density of 0.887 g / cc and an MI of 2.2 ° / min. EXP is an unmodified ethylene copolymer resin, produced by catalyst in a single site, having a density of 0.905 g / cc and MI of 9.0 ° / min. The used ethylene-butene copolymer EXP is identical to Exact 3022, except that the ethylene-butene copolymer EXP does not contain the anti-oxidant additive contained in Exact 3022. Both Exact 3022 and the ethylene-butene copolymer EXP are commercially available from Exxon Chemical Company, of Houston, Texas, United States. The 10 materials of Examples 23-32 were molded into ASTM test parts, irradiated, aged, and tested exactly as described for Examples 1-8. The results of the tests are recorded in Tables 4A and 4B. Table 4A Table 4B In order to deduce the inherent tendencies in the response of the heat deflection temperatures and the modulus to the increase in the amount of ethylene polymers produced by SSC in the physical blends of Examples 23-32, the lines were plotted. of regression of Figures 3 and 4. The results of the experiments involving Examples 23-32 are shown in Tables 4A and 4B and Figures 3 and 4 confirm that 5-15% of any of a wide variety of polymers The etiieno produced by SSC will similarly provide to the physical mixtures significant stabilization of the physical properties and color of the polypropylene before the effects of treatment with high energy radiation. They further demonstrate that in addition to their color stabilizing effect, the ethylene polymers produced by SSC provide a bleaching effect in the physical mixture, even in the absence of any irradiation. They also demonstrate again that the effect of the polyethylene component on the clarity of the physical mixture can vary from negligible to substantial, depending on the polyethylene used. They reconfirm that physical mixtures of special polyethylenes and nucleated homopolymers have a relatively high rigidity and resistance to softening at high temperatures, higher than for random copolymers of propylene with resistance comparable to degradation by irradiation. Finally, they show that the polymers produced by SSC generally contribute to a physical mixture excellent resistance to brittleness and discoloration caused by high energy radiation without having almost any effect on the nebulosity of the material. Examples 33-42 A Statistically Designed Experiment for Systematization of the Effects of Ethylene Polymers Produced by SSC on the Nebulosity of Physical Mixtures with Polypropylene To systematize the understanding of the effects of the significant characteristics of the ethylene polymers produced by SSC on the nebulosity of the physical mixtures made with them, a statistically designed experiment was carried out in Examples 33-42. This experiment was carried out exactly as that involving Examples 16-22, which are physical mixtures of 10% by weight of polyethylenes with propylene homopolymer, but using the propylene homopolymer corresponding to Example 23 and the ethylene polymers produced by SSC (described in Table 5) that was dictated by the statistical design. The chosen design was based on three levels each of density and melt index, the two variables in the range of ethylene polymers produced by commercially available SSC under the name Exact that have been shown to impact the nebulosity of the physical mixtures. A total factorial design gave nine experiments, but a combination of factors did not exist in a readily available resin. After eliminating this nonexistent combination, adding two additional runs from the central point of the design for good estimation of the error, and randomizing the design, the experimental plan shown in Table 5 was produced. The nebulosity results of the experiment are similarly included in the Table 5 Table 5 Processing these results by a multiple regression technique provided in the computer program Statgraphics, version 3.0, available from Statistical Graphics Corporation, yielded this empirical equation for the dependence of the nebulosity on the physical mixtures on the density (rho) and the melt index (MI) of the polyethylene component with a correlation coefficient (R2) of 0.91: Nebulosity,% = 1185.5 + 1.3210 (MI) 2-139.64 (MI) -1255.6rho + 137.15 [(MI) rho] Figure 5 is a graph of response surface of this relationship, which graphically emphasizes that the physical mixtures of the homopolymer of 25 ° / min. with the least cloudiness occur with polyethylenes produced by SSC having melt indices between 4 and 8 ° / min. at densities greater than 0.89. The results of this experiment and those of Examples 9-22 further suggest that optimal selections of ethylene polymers produced by SSC can be established to provide high clarity physical mixtures with a wide range of propylene polymers using the techniques described herein. It is evident that this physical mixing system of polyolefins produced by SSC offers opportunities to create numerous useful physical blends of polypropylene with polyolefins produced by SSC, particularly polyethylenes, which combine the useful characteristics of clarity, resistance to softening at high temperature, and tolerance to radiation, without introducing problems with oily components or with yellowing and brittleness after sterilization with high energy radiation.

Claims (14)

1. CLAIMS 1. An article of manufacture suitable for radiation sterilization of itself or its contents or that has been exposed to sufficient radiation for such sterilization, comprising a physical mixture of 50 to 99% by weight of polypropylene with 1 to 50% by weight. polyethylene weight, said polyethylene having a MWD between 1 and 4, and a CDBI greater than 45%.
2. 2. The article of claim 1, wherein said article comprises a film, a packaging material, or a medical device.
3. 3. The article of claim 2, wherein said packaging material comprises packaging material for medical devices or medications, unit dose or blister or bubble packages; wrapping material or food containers preserved by irradiation; or packing materials for stored liquid.
4. The article of claim 2, wherein said packaging material further comprises a multi-layered, self-sustaining structure, wherein said multi-layered structure comprises metallic sheet, cellulosic material, or plastic film.
5. The article of claim 2, wherein said medical device is formed by extrusion, blowing, rolling, blow molding, transfer molding, injection molding, pultrusion, protrusion, reduction, rotational molding, spin linkage, spinning in state melted, or insufflated in melted state.
6. 6. The article of any of the preceding claims, wherein said article has been subjected to radiation in an amount of up to about 10 MRad.
7. The article of any of the preceding claims, wherein said polypropylene is a random copolymer comprising propylene and at least one other alpha-olefin.
8. 8. The article of any of the preceding claims, wherein said polypropylene is a homopolymer.
9. The article of any of the preceding claims, wherein said polypropylene has an average molecular weight of from 10,000 to 400,000, preferably from 40,000 to 300,000, more preferably from 50,000 to 200,000, and a MWD from 1 to 9, preferably from 1 to 6, more preferably from 1 to 4.
10. The article of any of the preceding claims, wherein said polyethylene has a density of 0.865 to 0.920 g / cc, preferably 0.880 to 0.915 g / cc, with greater preference from 0.895 to 0.910 g / cc, and a melt index of 2 to 11 degrees / minute, preferably from 3 to 9 degrees / minute.
11. 11. An article, prepared for radiation sterilization of itself or its contents or that has been exposed to sufficient radiation for such sterilization, comprising: a) from 50 to 99% by weight of polypropylene homopolymer; b) from 1 to 50% by weight, preferably from 5 to 15% by weight of polyethylene produced by catalysis in a single site; and c) 0.01 to 0.5% by weight, preferably 0.02 to 0.2% by weight, more preferably 0.03 to 0.15% by weight hindered amine stabilizer; and, optionally, d) 1% by weight of a thiopro-pionate secondary antioxidant; e) up to 0.5% by weight, preferably 0.02 to 0.2% by weight, more preferably 0.03 to 0.15% by weight, of a secondary anti-oxidant of thiopropionate; f) up to 0.5% by weight, preferably from 0.05 to 0.15% by weight, more preferably from 0.05 to 0.1% by weight of an acid-neutralizing stripping agent; g) up to 0.5% by weight, preferably from 0.1 to 0.4% by weight of a clarifying nucleating agent; or h) their combinations.
12. 12. The article of claim 11, wherein said article comprises packaging material.
13. 13. The article of claim 11, wherein said article comprises a medical device. A process for producing a sterilized article, comprising the steps of physically mixing from about 50 to about 99% by weight of polypropylene with from 1 to 50% by weight of polyethylene, wherein said polyethylene is produced using a catalyst in a single site, produce an article from said physical mixture, and subject said article to radiation, preferably in an amount of up to 10 MRad.