PROPYLENE COPOLYMER COMPOSITIONS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/723,523 filed October 4, 2005, the entire disclosure of which is incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to improved propylene copolymer compositions. More particularly, the present disclosure relates to improved fast cycling, propylene-ethylene copolymer compositions which are sterilisable by high energy irradiation, e.g., gamma- irradiation, from a cobalt-60 source.

BACKGROUND

It has proven extremely difficult to develop polymeric compositions which can meet the fast injection molding cycles required in current thermoforming operations and withstand gamma irradiation at levels necessary to effect sterilization. This is especially true with respect to injection molding of products with very little tooling draft angles and those products bearing large cylindrical surface areas such as encountered when injection molding syringe barrels. To date, success has only been achieved using non-clarified, gamma resistant homopolymer polypropylene. Currently available gamma grade clarified random copolymers have not been able to meet the fast cycling requirements encountered in the fabrication of syringe barrels with little or no taper.

Since syringes are generally sterilized prior to use by gamma irradiation, it is also necessary that the compositions used for syringe fabrication be resistant to gamma irradiation. Gamma irradiation, especially at the levels used for sterilization, e.g., up to about 40 kilograys (4 megarads), can result in molecular weight breakdown and deterioration of the product, e.g. embrittlement with resultant loss of ductility.

SUMMARY

Accordingly, it is an object of the present disclosure to provide random propylene copolymer compositions which meet the fast injection molding cycle times currently required.

It is another object of the present disclosure to provide improved propylene copolymer compositions able to resist gamma irradiation in doses up to about 40 kGy, and exhibit subsequent resistance to autoclaving at about 132°C for about 8 minutes.

It is yet another object of the present disclosure to provide improved propylene copolymer compositions able to resist yellowing, i.e., Yellowing Index less than about 7.5, caused by gamma irradiation at doses up to about 40 kGy.

It is still another object of the present disclosure to provide improved propylene copolymer compositions with acceptable clarity for use in the medical industry, typically not greater than about 25% haze value.

These, as well as other objects and advantages are achieved by the present disclosure which provides fast cycling, gamma resistant propylene copolymer compositions comprising:

a propylene/ethylene copolymer comprising from about 2 to about 3.5 wt % ethylene; from about about 800 ppm to about 1200 ppm of one or more light stabilizers; from about 300 ppm to about 1200 ppm of one or more acid scavengers; from about 1600 ppm to about 2200 ppm aluminum, hydroxybis[2,4,8,10 tetrakis (l,l-dimethyl(ethyl)-6-hydroxy-12H dibenzo[d,g][l,3,2] dioxaphoshocin 6-oxidato]; and sufficient amount of one or more viscosity modifiers to break down the resulting polymer viscosity to the range of from about 23 to about 31g/10 minutes measured at about 230°C/2160g.

DETAILED DESCRIPTION

The propylene copolymers employed in the present disclosure are random propylene copolymers comprising from about 2 to about 3.5 wt % ethylene and, in embodiments, from about 2.6 to about 3.2 wt % ethylene. The random propylene-ethylene copolymers of the present disclosure may be produced in the presence of a Ziegler-Natta catalyst employing known polymerization methods to obtain copolymers exhibiting a melt index of less than about 3 gms/10 minute measured at about 230°C/2160g. It is also preferred that the molecular weight distribution of the copolymers be less than about 5.5 ± 10%.

The propylene copolymer compositions of the present disclosure can also include light stabilizers to quench the effects of gamma rays, ultraviolet light, and the like. These stabilizers are also useful in controlling the thermal stability of the melt. Typical light stabilizers useful in the present disclosure include, for example, polymeric hindered amines, such as CHIMASSORB 994 (poly [[6-[( l,l,3,3-tetramethylbutyl)amino-l,3,5 triazine-2,4- diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-l,6-hexanedi yl[(2,2,6,6-tetramethyl-4- piperidinyl)imino]] (sometimes referred to herein as C944, which is commercially available from Ciba-Geigy); TINUVIN 622 (a combination of dimethyl succinate and tetramethyl hydroxy- 1-hydroxyethyl piperidine polymer) (sometimes referred to herein as T622, which is commercially available from Ciba-Geigy); FIBERSTAB 410, a non-phenolic processing stabilizer system composed of oxidized bis (hydrogenated tallow alkyl) amines and a high molecular weight hindered amine (CHIMASSORB 944) in a 1 : 1 weight ratio (sometimes referred to herein as FS410, which is commercially available from Ciba Specialty Chemicals), combinations thereof, and the like. Light stabilizers can be present in the compositions of the present disclosure in amounts from about 800 ppm to about 1200 ppm (parts per million of polymer) and in embodiments, from about 900 ppm to about 1100 ppm.

It may be desirable that the syringe barrels fabricated from the compositions of the present disclosure be substantially transparent. It has now been found that when the clarifier, NA-21 (aluminum, hydroxybis [2,4,8,10 tetrakis (l,l-dimethyl(ethyl)-6-hydroxy-12H dibenzo [d,g][l,3,2] dioxaphoshocin 6-oxidato]) (available from Askai Denka Kogyou Kiki), is incorporated in the composition in amounts ranging from about 1600 ppm to about about 2200 ppm, and in embodiments, from about 1800 ppm to 2000 ppm, not only are

substantially transparent syringe barrels obtained but also the injection molding cycle time may be significantly reduced.

Polyolefin polymerization effected in the presence of Ziegler-Natta catalyst systems can result in acidic residues in the polymer. Consequently, it is considered advantageous to incorporate acid scavengers in the polymeric composition to prevent the formation of or neutralize any acidic residues therein. Suitable acid scavengers include calcium stearate (CaSt), synthetic hydrotalcite, e.g., DHT-4V (available from Kyowa Chemical Industry, Co., Ltd.), and the like. The acid scavenger can be incorporated in the polymeric composition in amounts ranging from about 300 ppm to about 1200 ppm. In embodiments, when acid scavengers such as calcium stearate are employed, they may be employed in amounts advantageously ranging from about 800 ppm to about 1200 ppm; whereas, when synthetic hydrotalcites are employed, they may be employed in amounts ranging from about 300 ppm to about 500 ppm.

In order to meet the currently required fast injection molding cycle times, i.e., a cycle time generally less than or equal to about 18 seconds, it is considered advantageous to break down the viscosity of the resulting polymeric composition from an initial melt index of about 3 g/10 minute or less to a melt index ranging from about 23 to about 31 gms/10 minutes (measured at about 230° C/2160 g). To effect such viscosity breakdown, organic peroxides such as LUPERSOL 101 (2,5-dimethyl 2,5-di(tert-butyl peroxy) hexane (available from the Lucido Division of Pennwalt Corp.) can be added to the polymeric composition post polymerization as may be needed to achieve the desired viscosity breakdown.

It has been found in accordance with the present disclosure that when the foregoing additives are admixed with the propylene copolymer compositions employing methods well known to those skilled in the art, such as through use of a Brabender plastograph, a Banbury mixer, or the like, and then injection molded to form syringe components such as syringe barrels and/or plungers, fast injection molding cycles may be achieved despite these products exhibiting very little tooling draft angles and large cylindrical surface areas. Moreover, the compositions of the present disclosure may be non-toxic, substantially transparent i.e., exhibit acceptable clarity for use in the medical industry, typically not greater than about 20% haze value, and exhibit gamma irradiation resistance up to about 40 kGy. Other gamma grade

random copolymers have not been able to meet the fast cycling requirements for syringe barrels with little or no taper.

The propylene copolymer compositions of the present disclosure can also include other additives, if desired, such as antioxidants, nucleating agents, fillers, reinforcing agents, plasticizers, lubricants, pigments, rheology additives, flow-control agents, optical brighteners, antistatic agents, and the like.

The following examples further illustrate the present disclosure but should not be construed in limitation thereof. AU percentages and parts are by weight unless otherwise stated.

EXAMPLES 1 TO 6

Using a high speed powder mixer, random propylene copolymer base flake was admixed with various amounts of powdered additives to yield the respective formulations specified in the accompanying Table 1 below. Each formulation was heated to melt the polymer, and for the viscosity-broken grades, the requisite amount of organic peroxide was added to produce pellets at 30MF. The pellets were molded into custom plaques and irradiated using a controlled research loop in a commercial cobalt-60 gamma irradiator.

Table 1

M3988 = 3,4 dimethylbenzylidine sorbitol available from Milliken Chemicals, Spartanburg, SC.

Examples 1 to 4 were compositions placed in a screening process for gamma irradiation stability at about 29 kGy to pick the best compositions for further higher dose testing. Examples 5 and 6 correspond to the best of these compositions but the

clarifier/nucleator used therein was changed to NA-21. The compositions of Examples 5 and 6 were then subjected to gamma irradiation at about 40 kGy. The plaque bend test is a screening technique useful to measure embrittlement of propylene polymers, however, it is a relative unit of measurement and no absolute numbers or angles to break are considered as having failed gamma irradiation.

EXAMPLE 1

A reactor grade propylene copolymer at 2.2% ethylene level having the composition set forth in the above Table was gamma irradiated to 29 kGy in air. Following irradiation and 9 months of ambient aging, the plaque samples were bent to 135° with average angle to break recording 42°. Non-irradiated samples bent to 135°.

EXAMPLE 2

A reactor grade propylene copolymer at 3.1% ethylene level having the composition set forth in the above Table was gamma irradiated to 29 kGy in air. Following irradiation and 9 months of ambient aging, the plaque samples were bent to 135° with average angle to break recording 85°. Non-irradiated samples bent to 135°.

EXAMPLE 3

A reactor grade propylene copolymer at 2.1% ethylene level having the composition set forth in the above Table was viscosity-broken to a melt index of 30 and gamma irradiated to 29 kGy in air. Following irradiation and 9 months of ambient aging, the plaque samples were bent to 135° with average angle to break recording 104°. Non-irradiated samples bent to 135°.

EXAMPLE 4

A reactor grade propylene copolymer at 2.7% ethylene level having the composition set forth in the above Table was gamma irradiated to 29 kGy in air. Following irradiation and 6 months of ambient aging, the plaque samples were bent to 135° and none of the specimens broke. Non-irradiated samples bent to 135°.

EXAMPLE 5

A reactor grade propylene copolymer at 2.7% ethylene level having the composition set forth in the above Table was viscosity-broken to a melt index of 30 and gamma irradiated to 40 kGy in air. Following irradiation and 9 months of ambient aging, the plaque samples were bent to 135° with an average angle to break recording 58°. Non-irradiated samples bent to 135°. The Yellowness Index (ASTM E313 using a BYK Gardner "Color View" machine) measured at 5.18 and the % Haze (ASTM D1003— plaque sample at 0.040" thickness measured using a BYK Gardner "Haze-Gard Plus" model) was 16.4%.

EXAMPLE 6

A reactor grade propylene copolymer at 2.8% ethylene level having the composition set forth in the above Table was gamma irradiated to 40 kGy in air. Following irradiation and 9 months of ambient aging, the plaque samples were bent to 135° with an average angle to break recording 43°. Non-irradiated bent to 135°. The Yellowing Index measured at 4.96 and the Haze Value was 13.4%.

EXAMPLE 7

Propylene copolymer compositions as described in Examples 4, 5 and 6 were tested for high speed moldability using a 32 cavity production mold to produce 35ml syringe barrels in an automatic cycle. The resin composition as described in Example 4 would cycle at 21.1 seconds, while the resin compositions of Examples 5 and 6 would cycle at 17.7 seconds and 18 seconds, respectively. The injection molding cycle dictated by production was less than or equal to 18 seconds. It was quite evident that the resin composition of Example 5 would produce barrels without any molding disruptions or hang-ups of parts in the mold cavity with longer production runs, i.e., all 32 parts were demolded and ejected out of the cavities at each shot. Resin compositions of Example 4, however, had difficulty in maintaining a fast cycle as parts hung-up (remained behind) in the cavity and were not ejected. This led to scuffing of the hung-up barrels when the next molding cycle brought the cores back into the cavities. This phenomena lead to poor quality and unacceptable barrels and risked entire lot rejection as scuffed barrels could be on their route to full assembly stations. The composition with the sorbitol base clarifϊer had to be cycled slower to 21.1 seconds to maintain high quality and

produce clean barrels. Maintaining a fast cycle of 18 seconds or less would amounts to an increase in productivity up to 5 millions parts per year.

It will be understood that the present disclosure, while described in reference to the fabrication of syringe components, may be used for injection molding and other thermo- forming operations to prepare other specific forms of molded products, sheets or films without departing from the scope or spirit of the present disclosure. The presently disclosed embodiments, therefore, are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details set forth herein.