Gamma radiation resistant carbonate polymer compositions containing linolenic compounds

Carbonate polymer compositions are rendered gamma ray resistant by the incorporation of 100 to 50,000 ppm of one or more linolenic compounds. Compared to the controls, the compositions of the invention have a reduced yellowing after exposure to cobalt 60 radiation.

BACKGROUND OF THE INVENTION 
This invention relates to a gamma radiation resistant carbonate polymer 
composition wherein the polycarbonate polymer is rendered radiation 
resistant by the incorporation of an effective amount of one or more 
linolenic compounds. 
There is a distinct need for polycarbonate moldings that are resistant to 
gamma radiation so that they can be sterilized without substantial loss of 
clarity and physical characteristics. 
It is known that polyolefins can be rendered radiation resistant by the 
addition of benzhydrol, hydrocarbon oils, phthalic esters, and 
benzaldehyde acetals. See for example U.S. Pat. Nos. 4,431,497, 4,460,445; 
and 4,467,065. 
It is also known that blends of polycarbonate resins and polyethylene 
terephthalate are resistant to gamma rays. Modern Plastics January 1984 
page 104: Plastics World December 1983 pages 68 and 69. 
The use of various stabilizer compounds such as esters, alcohols, 
thioesters, thiadiazoles, thiourea, phosphonates, phosphonites, and 
diphosphonites is disclosed in EP-0152,012. 
SUMMARY OF THE INVENTION 
The present invention is a carbonate polymer having improved gamma 
radiation stability due to the incorporation of one or more linolenic 
compounds in an amount sufficient to improve the gamma radiation 
resistance. 
In general, this effective amount has been found to be in the range from 
100 to 50,000 parts per million. A preferred range is 2500 to 15,000 ppm 
and the most preferred range is 5000 to 10,000 ppm. 
DETAILED DESCRIPTION OF THE INVENTION 
The carbonate polymers employed in the present invention are advantageously 
aromatic carbonate polymers such as the trityl diols carbonates described 
in U.S. Pat. Nos. 3,036,036, 3,036,037, 3,036,038 and 3,036,039, 
polycarbonates of bis(ar-hydroxyphenyl)-alkylidenes (often called 
bisphenol-A type diols) including their aromatically and aliphatically 
substituted derivatives such as disclosed in U.S. Pat. Nos. 2,999,835, 
3,038,365, and 3,334,154, and carbonate polymers derived from other 
aromatic diols such as described in U.S. Pat. No. 3,169,121. 
It is understood, of course, that the polycarbonate may be derived from (1) 
two or more different dihydric phenols or (2) a dihydric phenol and a 
glycol or a hydroxy- or acid-terminated polyester or a dibasic acid in the 
event a carbonate copolymer or interpolymer rather than a homopolymer is 
desired. Also suitable for the practice of this invention are blends of 
anyone of the above carbonate polymers. Also included in the term 
carbonate polymer are the ester/carbonate copolymers of the types 
described in U.S. Pat. Nos. 3,169,121, 4,287,787, 4,156,069, 4,260,731 and 
4,105,633. Of the aforementioned carbonate polymers, the polycarbonates of 
bisphenol-A and derivatives, including copolycarbonates of bisphenol-A, 
are preferred. Methods for preparing carbonate polymers for use in the 
practice of this invention are well known, for example, several suitable 
methods are disclosed in the aforementioned patents which are hereby 
incorporated by reference in their entirety. 
The linolenic compounds used in this invention are commercially available 
or can be made from the commercial materials by known techniques. These 
linolenic compounds have the formula 
EQU CH.sub.3 --CH.sub.2 --CH:CH).sub.3 (CH.sub.2).sub.7 C(O)--X 
where X is --OH, --NR.sub.1 R.sub.2 or --OR and R is an alkyl group of 1 to 
18 carbons, R.sub.1 and R.sub.2 are independently hydrogen or alkyl groups 
of 1 to 4 carbons. 
A specific example of the useful linolenic compounds is linolenic acid. 
This acid has the IU or system name of 9, 12, 15-octadecatrienoic acid. 
Also useful in this invention are the esters of linolenic acid with methyl 
alcohol and its homologs up to an including stearyl alcohol. 
The linolenic acid can be converted to amides by reacting the acid with a 
substituted or unsubstituted amine. The amines can be primary or secondary 
amines substituted with alkyl groups of one to four carbon atoms. 
The following examples are presented to further illustrate but not limit 
the invention.

EXAMPLES 1 and 2 
Samples were prepared by adding 5 gram amounts of unsaturated fatty acids 
and esters to 1400 grams of polycarbonate resin pellets followed by 
agitation to disperse said additive. Each sample was then extruded on a 
1.5 inch single-screw extruder. The resulting strand-chopped pellets were 
injection molded to yield test discs of 2 inch diameter by 1/8 inch thick. 
Each sample is listed below along with the corresponding amounts of 
additive, beginning yellowness index, and final yellowness index after 
exposure to 3.2 Mrad of Cobalt-60 gamma radiation. ASTM Yellowness Index 
Test D-1925 was used to measure the relative yellowing between the sample 
containing the additive and the control. The results are shown in Table I 
where the percent reduction in yellowness over the control resin is 
tabulated. 
TABLE 1 
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Additive 
Amount Yl Yl % 
Sample (ppm) Initial Final 
.DELTA.Yl 
Reduction 
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Control 1 0 4.7 26.0 21.3 0 
Control 2 3,571 5.2 29.3 24.1 (13.1% 
(oleic acid) increase) 
Control 3 3,571 5.7 27.2 21.5 0 
(linoleic acid) 
Ex. 1 3,571 7.3 26.3 19.0 11.6 
(linolenic acid) 
Ex. 2 3,571 5.5 23.5 18.0 16.3 
(methyl 
linolenate) 
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