Poly (vinyl chloride) (PVC) has properties that make it desirable for use as medical devices and as packaging for medical applications. Articles that are used in the medical field have to be sterilized before they are used.

Gamma radiation is often used for this purpose. It has been found, however, that gamma radiation causes PVC to become yellow and medical articles that have turned yellow may be rejected as being of inferior quality. Articles made of PVC that are re-used and sterilized after each use are especially likely to turn yellow.

A variety of substances have been added to PVC to stabilize it from gamma radiation and prevent yellowing, including organic Ca\Zn soap blends (JP 57,003,653), organo tin compounds (JP 02189352, JP 08073619, and EP 83112278), and sulfur compounds (JP 08151495,07102142, and 01278551).

According to one aspect of the present invention there is provided a method of preparing a sterile article comprising (A) producing a chlorinated polymer selected from polyvinyl chloride and polyvinylidene chloride that contains about 0.0001 to about 5 wt% of a metal complex having at least one anionic ligand that has at least one resonance structure in which the charge is localized on a carbon atom ; (B) fabricating said article from said chlorinated polymer ; and (C) sterilizing said article with gamma radiation.

(D) Preferably said metal complex has the general formula wherein Ri, ? 2, and R3 are each independently selected from R, OR, SR, and N (R) 2, R is hydrogen, alkyl from C, to C, 2, or aryl from C6 to C, 8, and n is the valance of M.

Advantageously R, and R3 are each independently selected from alkyl and alkoxide from C, to C8.

Conveniently R2 is hydrogen.

Preferably M is divalent.

Conveniently M is calcium, barium, zinc, cadmium, lead, tin, or aluminium.

Preferably M is Zn.

Conveniently said metal complex has the formula Preferably said metal complex is selected from Advantageously said chlorinated polymer is poly (vinyl chloride).

Conveniently said chlorinated polymer is polyvinylidene.

According to another aspect of the present invention there is provided a method of preparing a sterile article comprising (A) making poly (vinyl chloride) that contains about 0.001 to about 0.5 wt% of a zinc complex having the general formula where indepen OR, SR, Ri, R2, and R3 are each dently selected from R, and N (R) 2, and R is hydrogen, alkyl from C, to C, 2, or aryl from C6 to Cils ; (B) fabricating said article from said poly (vinyl chloride); and (C) repeatedly sterilizing said article with gamma radiation.

Preferably said zinc complex has the formula where R, and R3 are each independently selected from alkyl and alkoxide from C, to C8.

Advantageously R, and R3 are CH3.

Conveniently said zinc complex is selected from According to a further alternative embodiment of the present invention there is provided a method of preparing a sterile article comprising (A) making poly (vinyl chloride) that contains about 0.001 to about 0.5 wt% of a zinc complex having the general formula where R, and R3 are each independently selected from alkyl and alkoxide from C, to C8; (B) fabricating said article from said poly (vinyl chloride) ; and (C) repeatedly sterilizing said article with gamma radiation.

Preferably R, and R3are methyl.

According to a further preferred embodiment of the present invention there is provided a sterile article prepared according to any of the methods described above.

The article may be a medical article.

The addition of certain metal complexes to PVC and polyvinylidene chloride has been found to stabilize the polymers against gamma radiation.

The metal complexes can be easily added to the polymers during polymerization or processing so that no additional procedural steps are required. The articles can be sterilized with gamma radiation without significant yellowing.

In the method of this invention, a sterile medical article or sterile packaging is prepared by first producing a chlorinated polymer with a metal complex additive. The chlorinated polymer may be either PVC or polyvinylidene chloride. The metal complex can be added to the polymer at any stage, such as, for example, to the monomer, to the partially polymerized monomer, during the drying of the polymer, or during processing of the polymer into the medical article. However, it is preferably added at the earliest possible stage after the monomer has at least partially polymerized in order to avoid any extra procedural steps and to ensure its complete incorporation into the polymer.

The metal complex that is added to the polymer to stabilize it against gamma radiation contains at least one anionic ligand that has at least one resonance structure in which the charge is localized on a carbon atom. That is, the anionic ligand resonates between two structures and, in at least one of those structures, the negative charge is localized on a carbon atom. For example, if the anionic ligand is acetylacetonate the following two structures resonate, where the negative charge is on the carbon atom in one of the two structures.

Generally, the metal complex can have the general formula where M is a metal, Ri, R2, and R3 are each independently selected from R, OR, SR, and N (R) 2, R is hydrogen, alkyl from C, to C, 2, or aryl from C6 to C18, and n is the valence of M. Preferably, R, and R3 are each independently selected from alkyl and alkoxide from C, to C8 and are the same as that simplifies synthesis of the complex. Preferably, R2 is hydrogen as those compounds are more readily available. Examples of metals that can be used include calcium, barium,-zinc, cadmium, lead, tin, and aluminum. Divalent metals (n=2) are preferred as the Lewis acidity of those complexes is in the desirable range and adjustments to acidity can be avoided. Zinc is the preferred metal as zinc complexes have less toxicity and are less expensive.

Examples of more preferred zinc complexes inclue.

The amount of metal complex added can be about 0.0001 wt% to about 5 wt%, based on the weight of the polymer; the preferred range is about 0.001 to about 0.5 wt%.

In addition, plasticizers, heat stabilizers, epoxidized soybean oil, lubricants, and other additives can be mixed in with the PVC as is well known in the art.

The following examples further illustrate this invention.

Examples 1 to 5 To mixtures of 150.00 g PVC (sold by Occidental Chemical Corp. as "Oxy 240"), 0.30 g stearic acid, 0.23 g of a soap thermal stabilizer sold by Witco as"Mark 152 S,"97.50 g dioctyl phthalate, and 15.00 g epoxidized soybean oil sold by Witco as"Drapex 6.8" was added various amounts of different zinc salts of ß-biscarbonyl compounds. The mixtures were thoroughly blended and hot milled at 300°F (149°C) for 5 minutes. The resulting PVC sheets were pressed at 330°F (166°C) and cut into 4"x3"x1/4" (10 cm x 8 cm x 0.6 cm) plaques. The plaques were divided into two smaller pieces. One piece was saved for the purpose of comparison and the other one was subjected to 5.0 Mrad of y radiation at 50 kGy. The irradiated piece was again divided into two pieces and one piece was oven aged at 50°C for 48 hours. All three pieces were measured for yellowness index with a Mascbeth 2020 Plus Color Eye Spectrometer. The control materials were similarly prepared, but without a zinc complexe of a ß-biscarbonyl compound.

The following table gives the results : Yellowness Example Metal Complex With Metal Complex Without Metal Complex 1 0.35 g Initial 11.3 12.4 Zn acetylacetonate After y 29.4 45.0 Aged 37.2 72.0 2 0.60 g Initial 21.6 19.8 Zn acetylacetonate After y 34.1 52.8 Aged 38.8 73.3 3 0.91 g Initial 34.5 19.8 Zn acetylacetonate After y 35.1 52.8 Aged 37.4 73.3 4 0.35 g Initial 16.4 12.4 Zn bis (2,2,6,6-tetramethyl- 3,5-heptanedionate After y 39.5 45.0 Aged 57.7 72.0 5 0.31 g Initial 11.6 12.4 Zn hexafluoroacetylacetonate After y 36.8 45.0 Aged 55.0 72.0 The table shows that the PVC that contain zinc salts of p-biscarbonyl compounds had significantly less discoloration after y radiation compared with the control samples, which did not contain those compounds.

Examples 6 to 9 Example 1 was repeated using various zinc salts as y radiation stabilizers or the same molar equivalent amount of the corresponding ketone (see JP 02263853). The following table gives the stabilizers tested and the results: Zinc Salt (g) Yellowness Example Ketone Initial After y After Rays Aging Zinc Acetylacetonale (0.40) 19.1 38.6 45.9 6 2,4-pentanedione 16.9 55.8 76.3 Zinc 1-benzoylacetonate (0.59) 18.9 39.4 44.9 7 1-benzoylacetone 15.6 50.9 72.6 Zinc (methoxyethoxy) carbonyl 19.0 40.6 49.1 8 acetonate (0.58) 2-methoxyethoxy acetoacetate 18.0 54.0 73.3 Zinc 3- (2'-ethylcarboxyethyl)- 21.9 45.0 52.9 9 2,4-petanedionate (0.70) 1 Ethyl 4-acetyl-5-oxohexanoate 17.1 49.6 61.5 The above table shows that the zinc salts are more effective in reducing yellowness than the corresponding ketones.

In the present specification the word"comprise"or"comprises"means "includes or consists of'and"comprising"means"including or consisting of'.