Note: Descriptions are shown in the official language in which they were submitted.
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. BACKGROUND OF THE INVENTION
This invention relates to compositions of propylene
polymers suitable for high energy radiation treatment. One aspect
.5 of the invention relates to propylene polymer compositions of
specific characteristics which have been subjected to sterilizing
dosages of high energy radiation, such as gamma radiation.
Propylene polymers, and specifically those of a substan-
tial crystalline content have been applied to many new uses, which
O ~ uses have required that the polymer be processed or treated in a
particular manner. For example, propylene polymers generally have
the contact clarity, heat distortion resistance arld low chemical
?~ reactivity required by manufacturers of medical single-use items
such as syringes, forceps, surgical clamps and various instrument
trays, etc., used in the operating room. Obviously, before any
such articles can be safely used, it is required that it be steri-
lized.
Ethylene oxide has the sterilizing effectiveness needed
by these same manufacturers, and until recently was regarded as
,g - 1 - ~k
~! generally safe. Recently, however, ethylene oxide has come under
¦increasing scrutiny by the authorities, since it is believed to be
a mutagen and possibly a carcinogen. Acceptable limits of resid-
ual ethylene oxide and its by-products have therefore been lowered.
In addition, the cost of ethylene oxide is rising, and the steril-~
izing costs (because of a 14-day holding requirement and the indi-
vidual batch testing requirement) are becoming non-competitive.
High energy radiation, on the other hand, leaves no res-
idue and has a dose-related sterilization rate which permits
immediate release upon certification of the dose. However, unlike
ethylene oxide, radiation, especially r -radiation, damages the
polymer, in that it causes either embrittlement or discoloration
or both of these effects occur simultaneously: As disclosed in
U.S. Patent No. 3,537,967 and in its Canadian counterpart Canadian
Patent No. 811,766, the discoloration is attributable to the use
of phenolic antioxidants in the compositions to prevent or minimiz
the radiation induced degradation of the polymer. Even worse dis-
coloration is obtained with additive systems containing a phenolic
antioxidant and a thiodipropionic ester synergist. The patents
teach that discoloration resistance upon high energy radiation is
achieved by the incorporation into the polymer of the ester of
thiodipropionic ester as the sole stabilizer. However, these com-
positions have been found not to be entirely satisfactory espe-
cially for use in the fabrication of syringes, in that the strengt~
retention (flexural strength) of the irradiated polymer is minimal
even at high levels of thiodipropionic ester incorporation, and
the sterilized article becomes so embrittled, that it is prone to
breakage in use. Also, because of the known lesser efficiency of
thiosynergists in providing long term stability a~ainst oxidative
¦degradation of propylene polymers as compared to that of phenolic
¦antioxidants, the shelf life of articles manufactured from the
compositions of U.S. Patent No. 3,537,967 and Canadian Patent No.
811,766 is not as long as would be desired.
~lirao in U.S. Patent No. 3,940,325 teaches that the dis-
advantages of the compositions of the aforementione~ patents can
2~77
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~lbe overcome provided that as phenolic antioxidant an addi~ive
jselected only from either octadecyl 3,5-dl-t-butyl-4-hydroxyhydro-
¦,cinnamate or/and tetrakis[methylene(3l5-di-~-butyl-4-hydroxyhydro-
¦¦cinnamate)] methane be used. Many other propylene polymers con-
l¦tainin~ combinations of various antioxidant thiosynergist and lu-
¦bricant additives were tested by Hirao at various concentration
levels and were found to be deficient, including those containing
phenolic isocyanurate stabilizers.
According to Gilles in U.S. Patent No. 3,644,277, such
isocyanurate compounds provide stability against oxidative, therma
and photochemical degradation, but nothing is said regarding the
effectiveness of the additives during high energy radiation.
Gilles also teaches that synergistic activity is obtained when his
specific isocyanurates are combined with thiosynergists such as
the well known diesters of ~ -thiodipropionic acid of the afore-
¦mentioned U.S. Patent No. 3,537 r 967. In order to achieve this
beneficial synergistic effect the weigh~ ratio of thiosynergist to
l isocyanurate stabilizers should be between about 1:1 and up to
! about 5:1. This ratio range is typical for most other antioxidant
~0 ¦ thiosynergist combinations, e.g. those shown and discussed by
¦IHirao in U.S. Patent No. 3,940j325.
It is an object of the present invention to provide
l¦novèl compositions of matter of polymers of propylene which are
;Iresistant to discoloration and to degradation of physical proper-
¦¦ties upon high energy radiation treatment.
It is a further object of this invention to provide
¦¦shaped articles of polymers of propylene which can be used for
medical or food packaging purposes, which shaped articles have
~been subjected to a sterilizing dose of high energy radiation.
¦I THE INVENTION
¦ The above objects are unexpectedly accomplished by in-
corporating into a propylene polymer (a) from about 200 to about
400 ppm of a phenolic antioxidant containing an isocyanurate group
- 3 -
Il .
77
in its molecular structure, (b) a thiosynergist in an amount which
¦is at least 6 times the amount of the antioxidant of (a), and sub-
jecting the resulting composition to a dose of high energy radia-
tion.
Illustrative examples of phenolic antioxidants contain-
ing an isocyanurate group in the molecular structure are those
having the general formula:
R3
~2 _ O~ (1)
wherein Rl and R2 independently from each other are either hydro-
gen, an alkyl group of from 1 to 5 carbon atoms, or a group repre-
sented by the formula:
R3
x Q OH (2)
5 4
wherein R3, R4 and R5 independently from each other are either
hydrogen or a lower alkyl group containing from 1 to 5 carbon
atoms, and X is a hydrocarbon linking group.
Preferably, Rl and R2 are both groups represented by the
~formula (2) given above. R3 and R4 are preferably alkyl groups
and most preferably tertiary alkyl groups such as t-butyl and t-
amyl.
The linking group -X- can generally be any hydrocarbon
group providin~ the necessary linkage between the isocyanurate
group and the phenolic group.
For instance, X can be a group represented by the formu-
la: .
I' R6
i l
CH2 (3)
, R7
Il
¦Iwherein R6 and R7 independently from each other can be either
¦¦hydrogen or an alkyl group having from 1 to 6 carbon atoms. Phen-
i~olic antioxidant compounds containing such linking groups are known
,!in the art and can be prepared e.g. by the condensation reaction
¦io a phenol ~containing the R3 - R5 groups as defined above), an
appropriate aldehyde and isocyanuric acid (con~taining the Rl and
R2 groups defined above) with the formation of water as byproduct.
¦ Other suitable X groups include those represented by
l,the formula:
1 O
- (CH2)a O C (CH2)b
~wherein a and b independently from each other are each at least 2
l¦and a + b ranges from 4 to 18.
il Compounds containing these linking groups are also well
ilknown in the art. Generally, the compounds can be prepared by
, first reacting a phenol, (as defined before), with an appropriate
ester having the formula CH2 = CH (CHz)a 2 COOR8 wherein R8
j,is a lower alkyl group such as methyl, ethyl, propyl or butyl (the
¦¦base catalyzed Michael reaction).
', In a separate step an isocyanuric acid (as defined be-
jfore) is reacted with a halogen-substituted alkanol or thiol of
¦¦the general formula Y (CH2)b Z wherein Y is a halogen and Z
~¦is either -OH or -SH. After removal of byproduct HY, e.g. HCl when
`Ithe halogen is chlorine, the reaction products from the above de-
l¦scribed two steps are reacted with each other forming the desired
Icompound and the corresponding R8 alcohol or thiol as byproduct.
1! 5 _
i I .
Analogous procedures can obviously ]~e used in preparing
many other phenolic antioxidant compounds having additional groups
~present in the linking group, for instance, by the use of other
'halogen substituted alkanols or thiols such as
'¦ y (CH2)d - S ~ (CH2)e
Rg o Rlo
Y (CH2)d - ~ - C - ~ ~ (CH2)e Z (6)
Y (CH2) d ~ C (CH2) e
¦wherein d and e each are an integer of at lea:,t 2
, d + e ranges from 4 to 6,
and a + d + e ranges from 6 to 18,
Rg and Rlo each are either hydrogen or a hydroxyl group.
Il The phenolic compounds preferably employed in this inven-
lS !¦tion are those having the linking groups - X - defined by formula~
¦(3) and (4) above, e.g. tris (3,5-di-t-butyl-4-hydroxybenzyl) iso-
jlcyanurate and 3,5-di-t-butyl-4-hydrocinnamic acid triester with
il1,3,5-tris (2-hydroxyethyl)-S-triazine-2,4,6 (lH, 3H, 5H)-trione.
,¦These compounds are commercially available.
1l In addition to the above specified phenolic antioxidants
, the propylene polymer composition also contains a thiosynergist in
¦ amounts at least 6 times the weight of the antioxidant and gener-
~ally in concentrations ranging from about 1201) ppm to about 3500
ppm. The thiosynergist is a dialkyl ester of thiodipropionic acid
wherein the alkyl radicals-contain from 4 to 22 carbon atoms. Par-
ticularly suitable diesters are those having alkyl groups contain-
ing 18 carbon atoms namely distearyl thiodipropiorate and 22 car-
bon atoms namely dilauryl thiodipropionate.
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" ~ A metal stearate such as calcium stearate or zinc stear-
ate, is advantageously also incorporated as a processing aid,
¦iusually in amounts ranging from about 300 to about 700 ppm.
l The polymers of propylene applicable for use in this in-
¦ vention are homopolymers of propylene and random or block copoly-
mers of propylene with other mono ~-olefins such as ethylene,
butene-l and higher homologues containing up to 10 carbon atoms.
Blends of such propylene polymers with other polymers such as
polyethylene are also included within the scope of this invention.
Generally the proportion of polymerized propylene in the total
¦resin phase of the composition should be at least 60~ by weight.
¦ The propylene polymer compositions can be pigmented, if
~so desired. No significant distortion of color is observed after
lexposure to sterilizing doses of high energy radiation, e.g.
phthalocyanine blue pigmented articles of manufacture retain their
brilliant colors without development of green tones.
In general, the radiation treatment can be applied to
polymers of propylene useful particularly for medical purposes;
Ihowever, this should not be understood to be a limiting factor as
¦~the radiation treatment can be applied to the propylene polymers
,ifor any use for which such a treatment is required, such as for
l meat packaging, preserving food in retort packages and other uses.
! The high energy radiation is conveniently provided by a
¦cobalt 60 source. Other radiation treatment, however, can be used
~5 1! such as high energy X-rays or high energy electrons ( ~ -radia-
¦tion). In general, radiation dosages that can be applied range
ilup to about 5 megarads. For sterilization purposes, it has been
i¦found that an article of manufacture such as a syringe can be
¦¦effectively sterilized by applying 2 megarads under gamma radia-
¦I tion.
The following examples illustrate the invention without
limiting it.
7'7
i EXAMPL~S 1 - 3
il . .
A propylene homopolymer resin having a melt flow of
¦4-5 g/10 min. was melt blended with 300 ppm by weight of a commer-
l cially available phenolic isocyanurate Good-rite~ 3125 (which is
¦ the 3,5-di-t-butyl-4-hydroxy-hydrocinnamic triester of 1,3,5-tris
I (2-hydroxy ethyl)-S-triazine-2,4,6 (lH, 3H, 5l~ -trione), 2000 ppm
of either dilaurylthiodipropionate (DLTDP) or distearylthiodipro-
pionate (DSTDP) and 500 ppm of either calcium stearate or zinc
stearate. Tensile bar specimens (2 L/2" x 1/2" x 60 mil) were
prepared from each of these blends and were then subjected to gamma
irradiation with doses of 0, 1, 3 and 5 megarads respectively em-
ploying a cobalt 60 source. Subsequently the.specimens were tested
¦¦for brittleness using a 2 x 180 manual flex test, in which the
grip tab of the test bar is first bent 180 to one side and then
bent back to the other side at 180~. The test is passed if neither
a complete break nor a fibrous hinge results from the two bendings.
The yellowness index (ASTM D- 1925) the tensile properties
¦¦(ASTM D- 1708) were also determined. The pertinent data are shown
in Table 1 below
~/
I! "r~f ~''
,"..-~-"'
f r- '
. ... ............ ..... ......... . .... . ..
8 ~ ~ ~e
~1
~.2~
TABLE 1
!i .
!~ Ex. No. 1 2 _ 3
!
~ Additives - ppm
! Good-rite~ 3125 300 300 300
1 DLTDP - 2000 2000
¦I DSTDP 2000 - . -
i Ca Stearate 500 - 500
. Zn Stearate - 500 500
! Properties
i¦ 2 x 180 flex
0 Mrad ~ Passed
1 Mrad ( ----- Passed ~ .
3 Mrad Ç -~ Passed
5 Mrad ~ - Passed - -- ---~
Yellowness Index
0 Mrad -0,3 ~0.4 -0.4
1 Mrad
3 Mrad - 1.8 2.0
l 5 Mrad 3.2 .5.0 5.1
1 Tensile Yield - psi
0 Mrad 5381 5685 5228
1 Mrad 5282 5436 5456
3 Mrad 5448 5137 5430
5 Mrad ~ 5290 5541 5230
~5 ¦ Tensile ~ail - psi
0 4796 4338 4401
1 4096 4338 4483
. 3 3851 4176 3961
Il 5 - 3680 3659 3680
! % Elongation
I 0 Mrad 248 240 267
1 Mrad 218 208 236
3 Mrad 215 106 198
,¦ 5 Mrad 189 41 175
'i .
ii The above compositions compared favorably with polypro-
',pylene compositions stabilized solely with either D1TDP or DSTDP
jat 1400 to 3500 ppm incorporation in that the very pale and accept
l¦able color developments (3-5YI) after irradiation at sterilizing
i¦doses of 5 megarads were about the same or at most slightly higher
',while the physical properties were considerably improved over the
'ilcomparison compositions, which were brittle and failed the manual .
flex test.
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j Resort can be made to modifications and equivalents
,Ifalling within the scope of the appended claims.
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