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Description  |
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This invention relates to the irradiation ofpolymeric material, and more
particularly to irradiation sterilization of polymeric materials.
Semi-crystalline polymeric materials, such as polypropylene, are often
employed in articles where it is necessary to subject the article to
irradiation sterilization. Such materials, however, degrade during or
subsequent to such irradiation; i.e., during shelf storage time, and as a
result of such degradation, the articles become embrittled. To date, it
has not been generally possible to produce an irradiation sterilized
polypropylene article which is not severely limited in its applications as
a result of such embrittlement.
An object of the present invention is to provide an irradiation sterilized
polymer.
A further object of the invention is to provide an irradiation sterilized
polymer which is not embrittled during the irradiation or during storage
subsequent to the irradiation.
Another object of the invention is to provide irradiation sterilized
articles of polypropylene.
These and other objects of the invention should be apparent from reading
the following detailed description thereof.
In accordance with the present invention, there is provided a
semicrystalline polymer article which is sterilized by irradiation, with
such polymer having incorporated therein during the irradiation a
mobilizing amount of a non-crystalline mobilizer.
More particularly, the mobilizer is a low molecular weight noncrystalline
substance, which is miscible with the polymeric material and is also
compatible therewith; i.e., the mobilizer does not adversely affect the
properties of the polymer. The mobilizer is a substance which increases
the free volume of the polymer and, therefore, also lowers the density of
the polymer. The mobilizer functions to mobilize the amorphous portion of
the polymer, and as a result, increases the radical termination reactions
which prevent or miminize degradation during and subsequent to the
irradiation.
The mobilizer can be any one of a wide variety of liquids which increase
the total free volume of the polymer. The term liquid is used herein
includes highly viscous substances, commonly referred to as greases. In
general, such mobilizers have a density of from 0.6 to 1.9 g/cm.sup.3, and
preferably of from 0.6 to 1.1 g/cm.sup.3. The mobilizer has a low
molecular weight, with the average molecular weight, generally being in
the order of from 100 to 10,000 grams/mole, and preferably from 100 to
5,000 grams/mole.
As representative examples of suitable mobilizers, there may be mentioned;
hydrocarbon oils, halogenated hydrocarbon oils, phthalic ester oils,
vegetable oils, silicone oils, low molecular weight non-crystalline
polymer greases, such as hydrocarbon polymer greases, low molecular weight
polyester greases, polyarylether greases, etc. It is to be understood that
the above examples are only illustrative and the use of other mobilizers
should be apparent to those skilled in the art from the teachings herein.
The preferred mobilizer is a liquid mobilizer which is not highly viscous,
and in particular, a hydrocarbon oil or phthalic ester oil.
The polymers employed in the present invention are semi-crystalline
polymers, with such polymers having a crystalline content in the order of
from 20 to 90, and preferably of from 40% to 80%. The polymer may be
comprised of one, two or more monomers, and the term polymer generically
refers to both homopolymers and copolymers comprised of two or more
monomers. As representative examples of suitable polymers, there may be
mentioned: polymers of propylene, ethylene, oxymethylene, butylene, etc.
The preferred polymer is polypropylene.
The mobilizer is incorporated into the polymer in a mobilizing amount, with
such mobilizer generally being present in an amount of from 0.01% to 50%
and preferably of from 0.1% to 20%, all by weight.
The polymer may also include other additives which are conventionally used
in the art, such as antioxidants, preservatives, fillers, etc.
Although we do not intend that the present invention be limited by any
theoretical reasoning, it is believed that irradiation degrades a polymer,
such as polypropylene, by both chain scission and oxidation, as
represented by the following equations:
(1) R .fwdarw. R.multidot.
(2) r.multidot. + o.sub.2 .fwdarw. ro.sub.2 .multidot.
(3) ro.sub.2 .multidot. + rh .fwdarw. rooh + r.multidot.
(4) ro.sub.2 .multidot. + r.multidot. .fwdarw. roor
(5) r.multidot. + r.multidot. .fwdarw. r-r
where R represents the long polymeric chain composing the polymer. The
ability of steps (2) and (3) to repeat themselves many times before
termination by either steps (4) or (5) results in an auto-oxidative
reaction. Also, the stability of the radicals formed during irradiation
allow this reaction to continue for long periods of time even after
radiation has ceased. This post-degradation is very severe since a product
can embrittle on the shelf although it was acceptable immediately after
irradiation. In accordance with the present invention, it is believed that
the presence of the mobilizing additive increases the radical termination
reactions shown in steps (4) and (5). Consequently, the oxidative steps
(2) and (3) are minimized during irradiation. Equally important, however,
the increased termination rates brought about by the mobilizing additive
prevent and/or minimize the severe post-oxidative reaction. Therefore, the
product does not embrittle during normal shelf time of several years.
The polymer, preferably polypropylene, including the liquid mobilizer can
be employed to produce an article which is to be sterilized by procedures
known in the art. As representative examples of such articles, there may
be mentioned: syringes, tube assemblies, tissue culture flasks, needles,
package film, etc.
The polymer having the mobilizer incorporated therein, either as the
polymeric material per se, or as an article, e.g., a syringe or package
film, can be sterilized by subjecting the polymer to a sterilizing amount
of high energy radiation. The high energy radiation can be provided by any
one of a variety of sources, including cobalt 60, high energy electrons
and X-rays. In general, the sterilizing radiation doses are in the order
of from 0.5 to 6 megarads, with the typical dose being 2.5 megarads.
It has been found that by effecting the radiation sterilization of a
crystalline polymer having incorporated therein a mobilizer, the sterlized
or irradiated polymer is not embrittled, and moreover, does not become
embrittled subsequent to the irradiation (no embrittlement with age);
i.e., the polymer retains its flexibility. Thus, for example, prior to
irradiation, such polymers have a bending angle of at least 90.degree.,
and in accordance with the present invention, the irradiated polymer
subsequent to irradiation and even after storage for a long period of time
has a bending angle of at least 90.degree..
The invention will be further described with respect to the following
examples; however, the scope of the invention is not to be limited thereby
.
EXAMPLE 1
Polypropylene containing 4% of an aliphatic hydrocarbon oil having an
average molecular weight of 1200 grams/mole (mobilizing additive) was
irradiated to 2.5 megarads in air. Following irradiation, the sample could
be bent through an angle of 90.degree.. Even after 7 months of aging at
ambient conditions, the sample could be bent through an angle 90.degree..
A corresponding control polypropylene sample containing no mobilizing
additive was severely embrittled after 2.5 megarads. In fact, this sample
only bent 45.degree. before snapping. When this sample was aged for 6
months, it was further embrittled and "snapped" at only a 20.degree.
bending angle.
EXAMPLE 2
Polypropylene containing 2% dioctyl phthalate was irradiated (2.5 megarads)
in air. Following irradiation the sample was flexible and could bend
through an angle of 90.degree.. The irradiated control sample containing
no mobilizing additive would only bend through an angle of 20.degree.
before snapping.
EXAMPLE 3
Polypropylene containing 3.6% of a hydrocarbon oil as in Example 1 was
irradiated to 2.5 megarads in air. Following irradiation the sample was
still flexible and would easily bend through an angle of 90.degree.
without breaking. A corresponding control sample containing no mobilizing
additive was severely embrittled after 2.5 megarads. In fact, the control
sample only bent 45.degree. before snapping. When this sample was aged 6
months, it was further embrittled and "snapped" at only a 20.degree.
bending angle.
EXAMPLE 4
The polypropylene sample as described in Example 3 except containing 2.4%
of a hydrocarbon oil. As in Example 1, the sample was still flexible after
2.5 megarads and would bend through an angle of 90.degree. without
breakage.
EXAMPLE 5
The polypropylene sample as described in Example 3 except containing 1.2%
of a hydrocarbon oil. As in Example 1, the sample was still flexible after
2.5 megarads and would bend through an angle of 90.degree. without
breakage.
The present invention is particularly advantageous in that semicrystalline
polymer articles can be irradiation sterilized without detrimentally
affecting the flexibility of such articles even after the articles have
been stored over a period of time.
Numerous modifications and variations of the present invention are possible
in light of the above teachings and, therefore, within the scope of the
appended claims, the invention may be practiced otherwise than as
particularly described.
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