Improving y-Radiation Stability of PVC-A Review

QI WANG and SANDOR NAGY

Occidental Chemical Corporation

Technology Center
2801 Long Road,Grand Island, New York 14072

WC has a significant tendency to degrade under y irradiation. Therefore, consider-

able efforts have been made to improve its y-radiation stability. The purpose of this
review is to summarize recent results in this area, with a special emphasis on the
improvement of color stability of WC-based materials used in medical applications.

1. INTRODUCTION both excitation and ionization should contribute to the

overall degradation of the irradiated material. While
ith increasing use of y irradiation as a method
W of sterilizing medical products as well as meats
and poultry (1-4), the stability of PVC under y radia-
the damage induced by the component of excitation is
more or less similar to those of photodegradation, it is
the ionization mode of y radiation that puts researchers
tion becomes a very important issue, since WC, which
into a less-charted temtory.
has found wide applications in both medical and food
industries, is known to degrade upon its exposure to
2.1 Mechanism of PVC Degradation
such radiation (5).
y Rays are electromagnetic waves with wavelengths The interaction of ionizing radiation with PVC re-
ranging from 0.0001 nm to 0.01 nm and energies from sults in the ejection of electrons from the polymer
0.1 MeV to 100 MeV (6-8). By comparison, ultraviolet atoms, leading to the formation of "geminate" pairs of
rays have wavelengths ranging from 10 nm to 400 nm radical cations and electrons. These radical cations
and carry energies of only 5 eV to 100 eV. Due to and electrons could in turn undergo a cascade of elec-
the high energy, y rays can penetrate deeply into the tron transfer processes (electron or hole scavenging)
material and induce greater damage in the irradiated to form secondary charged intermediates or recom-
materials. bine to generate molecules with highly excited states.
The SI unit for measuring ionizing radiation dose is The energized molecules will undergo homolytic disso-
Gray (Gy). 1 Gy is equivalent to 100 rad, an off-system ciation to yield radicals.
unit which has traditionally been used. For commer- There are only a limited number of publications
cial sterilization of medical devices, a dose of 2.5 Mrad that discuss the mechanism of radiolytic degradation
is normally employed (6).The reported stability tests of PVC.The following reactions illustrate the most often
are usually performed using doses ranging from 2.5 to suggested transformations, which are quite similar to
5.0 Mrad. However, it is suggested that a lower dose those considered to be responsible for photoxidation
could be used for sterilization since now a much cleaner of PVC.
environment has been maintained during the produc-

r-
tion of these medical devices (6).

2. DEGRADATION OF PVC H CI CI CI ?I
UNDER y RADIATION CI + -CH~-~H+~H-CH~-&H-- -CH,-~H~CH-CH,-CH- + HCI
CI H CI H

When interacting with PVC, energy of y radiation

will be absorbed more or less uniformly throughout Fl cl R CI
-CH,-CH~;"-CH~-CH- $1
the bulk of the material instead of being absorbed CI H

only by the surface layers of the material. It will cause

-CHz-CH-C=CH-CHz-CH- F' + CI
both excitation and ionization in the material. The -CHz-CH-i-CH-CHz-CH-
CI H CI
fractions of the absorbed energy spent for ionization
0.7' 7'
and excitation are about the same (5). -CH,-FH+CH-CH~-CH-
While the effects on WC by W radiation have been CI H

relatively well-established, the mechanism of degrada-

tion by y radiation is a rather complex process and is As illustrated by the scheme, a chlorine radical could
much less understood. Common sense dictates that be generated by scission of a carbon-chlorine bond.

4 JOURNAL OF VINYL 81ADDITIVE TECHNOLOGY,MARCH 1999, Voi. 5, No. 1

 Improving y-Radiation Stability of W C - A Review

The generated chlorine atom will abstract a hydrogen 2.2 Degradation of W C Additives
from other PVC chains preferentially at the methylene
Considering that the PVC resin is usually formu-
group, leading to the formation of additional carbon-
lated with large quantities of additives such as plasti-
centered polymer radicals (8, 9). The resulting alkyl
cizers, thermal stabilizers, antioxidants, etc. for proc-
radicals could undergo a number of transformations,
essing, it is important to recognize the importance
such as loss of a chlorine atom to form a double bond,
of the radiation stability of these additives. Unfortu-
or reaction with an oxygen molecule to form a peroxy
nately, there exists only a limited number of studies
radical. They can also recombine with other radicals
in the literature focusing on this aspect. Even among
to form chain-terminating products. Most of the proc-
the few published studies, discussion generally cen-
esses in the scheme, however, involve formation of tered on the migration behaviors of polymer additives
new radicals, which will further propagate the degra- in connection with the issue of food safety (21).
dation process. In a series of papers published by Allen and co-
It is worthwhile to further point out that oxygen is workers, the effects of y irradiation on hindered phe-
apparently one of the most important factors in any nol antioxidants in poly(viny1 chloride) have been in-
potential mechanism for PVC degradation. Experi- vestigated (22-24). In these studies, the hindered
ments have demonstrated that the presence of oxygen phenol antioxidants octadecyl 3-(3,5-di-tert-buty1-4-
during y irradiation can result in the formation of per- hydroxypheny1)propionate (Irganox 1076) and pen-
oxides, carbonyl groups, and hydroxyl groups in PVC taerythritol tetrakis-3-(3,5-di-tert-4-hydrowhenyl)
polymers, which can adversely affect the physical propionate (Irganox 1010)were first incorporated into
properties of the polymers (10-12). In addition, irradi- W C polymers at appropriate levels, and the resulting
ation of oxygen or air was reported to produce an acti- polymers were then subjected to progressive doses of
vated oxidative species, which was claimed to be a y-ray irradiation. It was found that the antioxidants
stronger oxidizing agent than 0,. This active agent were progressively destroyed with the increase of the
was reported to react with WC, yielding detectable radiation doses. With a dose of only 10 kGy, 30-40?!
amounts of free chlorine (13).Further evidence regard- of the antioxidant activity in W C was lost. When a
ing the adverse oxygen effect comes from a patent dose of 50 kGy was used, the loss of antioxidants in-
which claimed that colors of polymeric materials were creased to 61-66%. In addition, the authors could not
much better after y irradiation when oxygen was ex- detect the presence of any low molecular weight degra-
cluded from the radiation environments (14). dation products derived from the antioxidants. There-
Various physical characteristics of PVC, such as fore, it was suggested that these phenols became co-
heat resistance, tensile strength, and elongation, as valently bonded to the polymers as a result of radical
well as optical properties, can be affected by the deg- coupling processes (23).
radation process (5, 8, 15, 16).The change in mechan- In the 1970s, Krylova and co-workers investigated
ical properties of PVC is usually not very severe at the effect of y irradiation on the stability of WC plasti-
sterilizing doses of y radiation. However, higher doses cizers (25). Both alkyl phthalates and adipates were
can produce dramatic changes in the material, in- evaluated as a function of the nature of the carboxylic
cluding the chemical composition. For example, after acid and alkyl moieties. In the phthalate series, the
PVC was irradiated at 20 MGy, the material showed a stability decreased with increasing length of the alkyl
structure consisting mainly of carbon, which in some group, especdly at large irradiation doses (100 Mrads).
cases was crystalline (17, 18).At lower doses, the The most stable plasticizer was dimethyl phthalate.
most salient feature of the degradation is discolora- Butyl phthalyl butyl glycolate was less stable than
tion, or color drift (5, 8, 19). Since the color change, dioctyl phthalate, and the least stable plasticizer was
usually yellowing, gives an impression of inferior qual- dioctyl adipate. The protective influence of the phtha-
ity, the color problem is probably the most pressing late moiety was attributed t o the energy dissipation
issue facing PVC manufacturers, especially when they process via the n-electron system. These conclusions
deal with the quality conscious medical industry. were further supported by additional results presented
Traditionally, the discoloration is considered to be by the same authors in a later study (26).
caused by formation of conjugated double bonds in The same authors have also investigated the effect
PVC chains during the degradation process. Some of various stabilizers on the degradation of alkyl phthal-
polyenyl radicals have been identifed experimentally ate and adipate plasticizers (27). That study demon-
by ESR spectra during a y-irradiation study of WC strated that metal stearate heat stabilizers protected
(20). Consequently, the major efforts to prevent color the plasticizers from degradation and could be used to
formation utilized radical scavengers as additives to obtain stable and colorless WC compositions. Phenolic
trap the active species that can potentially lead to the stabilizers also had a protective influence on both plas-
formation of the conjugated double bonds. Other ef- ticizers during radiolysis. but color intensification was
forts included the addition of basic additives to poly- observed during the process. The greatest stabiliza-
meric compounds to protect them from hydrogen chlo- tion of dioctyl phthalate occurred with zinc stearate.
ride released from WC upon degradation, which is The effectiveness of other salts decreased in the order
also believed to contribute to the overall yellowing of Ca > Ba > Cd > Mg > Pb. The most effective stabilizers
WC during and after y irradiation. for dioctyl adipate were dibasic lead stearate and tri-

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1999, Vol. 5, No. 1 5

 Qi Wang and Sandor Nagy

basic lead sulfate. The effectiveness of other stearate 75%. In a related patent, the same inventors stated
salts decreased in the order Zn > Ca > Pb > Cd > Ba. that besides the phenolic and phosphite-type antioxi-
In a short summary, the mechanism of PVC degra- dants, P-diketones could also be beneficially added to
dation in the presence of y radiation is quite compli- the polymerization process for improving the y stabil-
cated. The degradation may proceed by many different ity of the produced PVC resins (30).
routes. Variables such as radiation dose strength, A patent (31) indicated that polymerization at re-
acidity, and presence of oxygen all have to be con- duced temperature could improve the y-radiation re-
sidered. In addition, attention should also be paid to sistance of PVC. When PVC obtained from a reaction
the y-radiation stability of added materials, such as mixture consisting of vinyl chloride with 2,4,4-tri-
plasticizers and antioxidants, as well as their impacts methylpentyl peroxy-2-neodecanoatein water at 48°C
on the overall stability of PVC-based items during the was compounded with plasticizers and stabilizers and
radiation. irradiated with 5-Mrad y rays, it showed a yellowness
index of 36.1 vs. 54.9 for control samples similarly
3. IMPROVING ?-RADIATION prepared except at 55°C with diisopropyl peroxydicar-
RESISTANCE OF W C bonate as initiator.
There are two major approaches to improve y-radia-
3.1.2 lrnproving y Resistance via Copolymerization
tion resistance of PVC. One approach is to improve the
intrinsic stability of PVC resins. This can be achieved The y stability of PVC resins has been improved by
either by improving or m o d m g the polymerization copolymerization of vinyl chloride with various mono-
process or by incorporation of certain chemical moi- mers (32-37).
eties into WC chains. The incorporation of chemical Copolymers of vinyl chloride with vinyl ethers and
functionalities can be carried out via copolymerization a-olefins have been reported to have improved y-radi-
or post-polymerization chemical treatment. The sec- ation resistance (32, 33).Vinyl ethers with alkyl groups
ond approach is to add stabilizing substances to PVC containing up to twenty-four carbons can be used for
resin during its processing. These additives have to the polymerization. The data presented for the propyl-
have the ability of protecting PVC molecules without ene copolymer, however, indicate that these improve-
adversely affecting the overall performance of fabri- ments are marginal. For example, the colorimetric val-
cated items. Since the most salient feature of PVC ues for copolymer made from 15 g of propylene and
degradation is yellowing at lower y-radiation doses 3000 g of vinyl chloride were 4.2 initially and 9.1 after
(5, 8, 19),we will put a special emphasis on color re- irradiation with 5-Mrad y rays, compared to 3.2 and
tardation in the following discussion of improving the 10.7, respectively, for pure PVC.
y-radiation resistance of PVC. Graft copolymerization has also been reported to
enhance the y stability of PVC (34-37). The grafting
3.1 Improving Intrinsic Stability of PVC Resins can be carried out by polymerization of 5-20 parts of a
mixture containing aromatic vinyl monomers, meth-
3 . 1 . 1 Irnprouing y Resistance by Modifiatbn of the
acrylate, and acryfate with 100 parts of PVC. For in-
Polymerization Process
stance, a polymer was obtained by reacting 2000 g of
A Japanese patent described a suspension polymer- W C with 10 parts of 20:30:50 styrene/Me methacry-
ization process for manufacturing radiation resistant late/2-ethylhexyl acrylate mixture, 2 g of Bisphenol A-
vinyl chloride polymers (28).In this process, peroxides modified diacrylate, 0.2 part of dioctyl peroxydicar-
containing one or more benzene rings were used as bonate, and 0.2 part of lauryl peroxide in 5 liters of
initiators. More interestingly, phenolic antioxidants water at 60°C for 5 h and at 80°C for 3 h. When the ob-
with molecular weights larger than 400 were added to tained polymer (100 phr) was compounded with DOP
the reaction mixture during polymerization. For exam- (55 phr), epoxidized soybean oil (5 phr), Zn stearate
ple, vinyl chloride in water was polymerized in the (0.2 p h ) , Ca stearate (0.2 phr), and stearic acid (0.1
presence of partially saponified poly(viny1 alcohol), phr) and pressed into a sheet, it gave a b value of 1.8
cumyl peroxyneodecanoate, and octadecyl 3-(3,5-di- after 2.5 W a d y-ray irradiation (34).
tert-butyl-4-hydroxypheny1)propionate at 50°C to give Similar results have also been reported by Russian
WC. When a sheet made from mixing the resulting researchers (38).They found that substantial improve-
PVC (100 phr) with dioctyl phthalate (55 phr), epoxi- ment in radiation stability can be achieved by gmfting
dized soybean oil (5 phr), Zn stearate (0.2 phr), Ca approximately 50% of PVC with styrene and a &we
stearate (0.2 phr), and stearic acid (0.1 phr) was sub- of styrene and acrylonitrile. However, some physical
jected to y radiation, it showed no discoloration. properties of the polymers were adversely affected by
In a more recent patent, the researchers further the modification. It was reported that even 10% of
claimed that addition of a phosphite-type antioxidant these monomers rendered the graft polymers opaque.
containing one or more benzene rings during the sus- On the other hand, graft copolymers of PVC with
pension polymerization process also produced PVC methyl methacrylate had a higher radiation stability
resins capable of resisting yellowing during y steriliza- than PVC without sigmficant reduction of transpar-
tion (29).The addition of the phosphites should be car- ency. Random vinyl chloride-acrylate polymers were
ried out after the monomer conversion reaches 15- more radiation stable than those with methacrylates,

6 JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1999, Vol. 5, No. 1

 Improving y-Radiation Stability of PVC-A Review

and their stability decreased with increasing length of soybean oil. The potential of such effects in stabiliza-
the 1- group in the methacrylate. tion of PVC should be further explored.
Alkyl adipates were reported to increase the radia-
3.1.3 Improving y Resistance via Post-Polymerization tion resistance of bio-compatible PVC compositions
'keatment (521, especially those used for making blood tubings.
This work further confirmed earlier studies (53,54)
Post-polymerization treatment of PVC resins with
which demonstrated that dioctyl adipate was a much
certain chemicals was demonstrated to be an effective
more efficient plasticizer in retarding color drift of PVC
way to enhance their y-radiation resistance (39-43).
compounds after exposure to a y source, especially
Nakagawa and co-workers reported that the reaction
when epoxidized oils were excluded from the formula-
of PVC with sodium N,N-diethyldithiocarbamate in
tion. These findings seem to contradict earlier results
DMF at 50°C under nitrogen gave a polymer having
(25) which indicated that dioctyl adipate was much
resistance to y rays. Analyses indicated that the re-
less stable than dioctyl phthalate under the radiation.
sulting polymer contained N,N-diethyldithiocarba-
The only logical explanation for this seeming contra-
mate functional groups. This effect was further found
diction is that dioctyl adipate must have played a bet-
to be a general phenomenon for vinyl polymers con-
ter sacrificial role in the radiation degradation process
taining CH,CH(S&NRR') groups, where R and R' were of W C .
alkyl or aryl groups. Blending the treated polymers or
Use of epoxy plasticizers, otherwise called epoxy co-
poly(viny1 N,N-diethyldithiocarbamate)with regular
stabilizers, is essential in the stabilization of y-steril-
PVC also improved the stability of the resulting mate-
ized PVC compounds (53-59). The improvement is
rials on irradiation.
generally attributed to the ability of epoxidized oils to
Incorporation of phenyl-ring-containing molecules
act as acid scavengers by absorbing the HC1 gener-
into W C chains also improves the stability of the ma-
ated during PVC degradation. Both epoxidized soy-
terial (44). I t is well known that aromatic compounds
bean oil and epoxidized tall oil were effective, although
are generally much more stable to high-energy radia-
epoxidized soybean oil was preferred, primarily due to
tion than alkanes, and that their presence enhances
its lower price (53, 54). Formulations with 5 or 10 phr
the y resistance of alkanes (45, 46). In a related study
of the plasticizer did not affect outcomes of the experi-
(47). it was demonstrated that the rate of dehydro-
mental results. On the other hand, other researchers
chlorination of WC, used as a measure of its stability,
reported that the presence of 15 to 20 phr of epoxi-
decreased by about a factor of 3 after the polymer was
dized soybean oil worked better in their y-resistant
irradiated in the presence of chlorobenzene.
formulation (55).Other epoxy compounds such as sty-
rene oxide and butyl cis-9,lO-epoxystearatehave also
5.2 Improving Stability of PVC by Using Additives
been used to enhance the y stability of PVC (56-59).
The second frequently used approach to improve
the y resistance of WC involves additives. These mate- 3.2.2 H e a t Stabilizers, Antioxidants, and
rials could be plasticizers, various stabilizers, fillers, or Related Compounds
any other compounds that can enhance the y stability
3.2.2.1 Ca/Zn Stabilizers: Various heat stabilizers
of PVC without adversely affecting its properties.
and antioxidants have also been evaluated for the pur-
pose of enhancing y resistance of PVC compounds.
3.2.1 Addition of Plasticizers
Ca/Zn stabilizers are generally good in controlling the
Plasticizers are known to improve the y stability of color drift after exposure to y radiation, especially when
PVC compounds and, as a result, the flexible PVC high zinc concentration is present in the compounds
compounds normally resist y radiation better than the (53, 54). The performance of these stabilizers is more
rigid ones (8, 48, 49). It was observed that with in- or less equivalent whether a solid, liquid, or paste form
creasing concentration of plasticizers in the formu- is used.
lation, color deterioration tends to decrease after the The phenomenon of better color control by using
radiation (50).The observed improvement of color sta- Ca/Zn stabilizers has also been documented in sev-
bility is believed to be due to the dilution effect. In other eral other studies. A patent claimed that Zn and Ca
words, as plasticizer loading increases, the percentage salts of fatty acids were needed in formulating clinical
of WC resin in the given volume of the compound de- WC materials suitable for y sterilization (60). Exam-
creases. However, synergistic effects have been re- ples cited included using zinc palmitate and calcium
ported for certain plasticizer-stabilizer systems (51). palmitate. Samples compounded with the fatty acid
During a study on stabilizing W C hard foils against y salts did not show any appreciable changes in their
radiation, Szynxmski and Smietanska found that strong properties after being subjected to sterilization by y
synergistic effects existed in the systems ethylene gly- rays at 2.5 and 5.0 Mrads. On the other hand, the
col bis(p-aminocrotonate)/Ca-Zn laurate/ epoxidized controls prepared without the fatty acid salts had
soybean oil/DOP, ethylene glycol bis(P-aminocroto- increased hardness and were discolored after irradia-
nate)/Ca-Zn laurate/epoxidized soybean oil/dioctyl tion.
sebacate, and ethylene glycol bis(P-aminocrotonate)/ 3.2.2.2 Tin Stabilizers: Tin stabilizers have been
Mg-Zn stearate/dioctyl sebacate/sorbitol/epoxidized widely studied as potential candidates for the preven-

JOURNAL OF VINYL &ADDITWE TECHNOLOGY, MARCH 1999, Vol. 5, No. 1 7

 Qi Wang and Sandor Nagy

tion of discoloration of PVC by y radiation (53, 54, The discoloration of PVC packaging materials by ra-
6 1-69), and various organotin-based compounds are diation sterilization could not be prevented by adding
reported to be effective. It is apparent that most of conventional organ0 Sn and Sb stabilizers, even though
these active compounds contain sulfur moieties. many of them contain thiol moieties, such as esters of
Alkyltin salts of thiols were the key ingredients in thioglycolic acid (75).However, according to the study,
patented formulations (63, 64)for retarding the dis- a dramatic improvement could be achieved with addi-
coloration of PVC compounds caused by y irradiation. tion of additional mercaptan esters containing one S H
The additives were added at a level of 0.1-5 phr. The group for every 3- 10 carbon atoms.
effectiveness of the tin-based materials in the formu- 3.2.2.4 Alcohols, Phenols, and Related Compounds:
lation was illustrated by using dibutyltin bis(l-dode- The presence of alcohols during y sterilization was re-
canethiolate) as an example. When a sheet made of ported to prevent yellowing of W C products such as
W C (100 phr), Ca stearate (1.0 phr), dibutyltin bis(1- blood transfusion bags and tubes (76).According to
dodecanethiolate) (1.0 phr), and sorbitan laurate (3 the patent, after a 1-mm resin sheet made from PVC
phrl was y irradiated to a dose of 5 m a d , the light (100 phr), epoxidized soybean oil (10 phr), DOP (60
transmission (400 mp) was 74%. On the other hand, phr), and a Ca-Zn stabilizer (5 phr) was irradiated
only 5% of light transmission was observed for a simi- with y rays at 2.0 Mrad and subjected to aging at
larly irradiated film containing Bu2Ti maleate in place 60°C for 14 days in the presence of 1.0 part EtOH, it
of dibutyltin bis(1-dodecanethiolate). showed only slight yellowing.
Similar results were reported by other researchers Similarly, certain partially esterified polyols were
(65).They found that radiation resistant vinyl halide also reported to improve the y resistance of PVC (77).
resin compositions could be formulated with sulfur- These compounds usually contained 10-4OVo of OH
containing organotin compounds such as dioctyltin groups by weight. The amount of additives used in the
bis(isoocty1 mercaptoacetate). The same dioctyltin formulation was generally 0.1-10 phr relative to W C .
bis(isoocty1 mercaptoacetate) has also been used in For example, after a sheet made from Sumilit SX- 11F
another patent to improve the y stability of PVC (66). PVC (100 phr), di-2-ethylhexyl phthalate (30 phr),
A difference was that thiols were used together with epoxidized soybean oil (8 phr), Zn stearate (0.5 phr),
the tin compound in the Japanese case. hydrotalcite (0.5 phr), dipentaerythntol terephthalate
Beside the ddky1t.b materials, the monoalkyl de- (OH content 25.1Yo) (0.5 phr), and Neulizer 4B [ 1.4-
rivatives of tin stabilizers have also been found to be butanediol bis(P-aminocrotonate)] (0.5phr) was irradi-
effective in improving the y resistance of PVC com- ated with y-rays at a dose of 2.5 Mrad and left at 60°C
pounds (68).For instance, a 0.7-mm PVC sheet pre- and relative humidity 90?! for 7 days, the sheet showed
pared with a 50:50 mixture of R’Sn[S(CH2)mC02R2], less discoloration than a control prepared similarly
and (R’)2Sn[S(CH2),C02R2]2, where R’ = octyl, R2 = but without dipentaerythritol terephthalate. A similar
2-ethylhexyl, and m = 1 or 2, showed no coloration effect was also found when glycerol was used as an
immediately after irradiation with 3.5 Mrad and a slight additive (78).
coloration after storage at 60°C for 60 days. Mercap- The role of alcohols in the stabilization of W C is not
totin esters have also been used recently to improve well understood. One possible explanation might be
the resistance of plasticized PVC to radiation steriliza- that alcohols act as active agents to react with the
tion (691. defects on PVC chains generated during irradiation,
In a study on y-irradiation-induced degradation of preventing the further catastrophic dehydrochlorina-
organotin-stabilized WC, the tin stabilizers were found tion. A n earlier study showed that when vinyl chloride
to decompose during the irradiation, and several tin- resins were mixed with aliphatic alcohols, aliphatic
containing intermediates derived from the stabilizers ethers, or alicyclic ethers and exposed to radioactive
could be detected (70). For instance, the y irradiation, rays at a dose range of 0.5-5 Mrad, the dehydrochlo-
in the range 0-200 kGray (0-20 Mrads), of PVC food rination rate of the stabilized polymers was only half
packagmg stabilized by dibutyltin bis(isoocty1thiogly- of that for unstabilized polymers (79).
colate) ( 1.2%) or dibutyltin bis(isoocty1maleate) (2.OYo) Epoxy-resin-modified PVC was used for surgical
showed varying amounts of monobutyltin intermedi- goods (80).In this case, 1-20 phr of Bisphenol A-epi-
ates and a steady increase in SnC14 release as irradia- chlorohydrin copolymer was mixed with 100 phr of vinyl
tion progressed. By contrast, thermally treated (180°C chloride polymers. The resulting materials had good
for 120 min) WC films showed only a trace of SnCl, strength and did not undergo discoloration upon irra-
and approximately 5% of monobutyltin species, with diation. For example, when a mixture of W C (100 phr),
most of the tin remaining in the dibutyl state. This Kaneace B22 (5 phr), Bisphenol A-epichlorohydrin co-
type of study was also carried out by other workers, polymer (7 phr), and other additives (3.5phr) was cal-
and similar results were reported (71). endered to a sheet and subjected to 5 Mrad radiation,
3.2.2.3 Thwls and Related Compounds: Thiols and the yellowness index of the sample changed from 0.51
related compounds are another group of potential y to 0.56. On the other hand, the change would have
stabilizers (72-74). Alkyl mercaptans, including 2-mer- been from 0.50 to 4.95 if the Bisphenol A-epichlorohy-
captoethanol, were reported to stabilize PVC under y drin copolymer had been absent from the formulation.
radiation. In these cases, 0.1-10 parts of thiol are Phenolic antioxidant butylated hydroxytoluene (BHTJ
normally used for every 100 parts of W C . was reported to be a better additive compared to buty-

0 JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1999, Vol. 5, No. 1

 Improving y-RadiationStability of PVC-A Review

lated hydroxyanisole (BHA) for the protection of rigid PVC compounds, a number of commercial nontoxic
vinyl articles against gamma radiation (81). Clear organic phosphite stabilizers were reported not to
vinyl articles with a yellowness index increase less show good stabilizing effects (55). In other publica-
than 10 during 5 Mrad y-irradiation sterilization can tions (53, 54), use of tris(nonylpheny1)phosphite was
be manufactured from formulations containing BHT, found not to prevent the discoloration of PVC com-
adipate or glutarate esters, and alkyltin compounds. pounds after y irradiation when calcium and Ca/Zn
These formulations generally contained 2-20 phr of heat stabilizers were used in the formulations. How-
adipate or glutarate with molecular weight less than ever, phosphites appeared to work better when only
2000, 0.2-5 phr of BHT, and optionally, an alkyltin zinc stabilizers were present. It appears that organo-
compound such as Me,Sn(SCH,CO,C,H,,),. For ex- phosphites are formulation dependent. To answer the
ample, a sample containing PVC (100 phr), acrylic question of whether organophosphite co-stabilizers
processing aid (3 phr), Me2Sn(SCH,CO2C8H,,), (Mark can improve the color stability of PVC compounds,
1984) (2 phr), lubricant (0.2 phr), DOA (15 phr), and further studies are needed.
BHT (1 phr) exhibited a yellowness index of 4.70 and 3.2.2.7 Aromatic Hydrocarbons: As mentioned ear-
7.85 before and after 5 Mrad irradiation, compared lier, aromatic hydrocarbons can efficiently dissipate
with 2.15 and 20.16, respectively, for a similar sample the energy of irradiation. They can, in principle, act as
containing BHA instead of BHT. additives for enhancing radiation stabilization of PVC,
Use of Irganox 1010, a phenolic antioxidant, was as they have done with other polymers (45, 46). It was
also reported to improve the color stability of PVC- indeed demonstrated that these aromatics could serve
based material during y irradiation (82). this purpose (87, 88).For example, methylfluoranthene
3.2.2.5 Ketones and Other Carbonyl-Containing and methylpyrene were used to stabilize PVC materi-
Compounds:2-Hydroxybenzophenone derivatives were als geared for applications in the cable industry. How-
found to help control color drift during y irradiation of ever, aromatic hydrocarbons are generally not appro-
PVC compounds (83).When a 3-mm sheet comprised priate additives if the color of the PVC is the primary
of PVC (100 phr), DOP (50 phr), epoxidized soybean concern, since many of these aromatic compounds
oil (5 phr), Ca stearate (0.1 phr), Zn stearate (0.3 phr), are colored by themselves or can produce colored mol-
and 2,4-dihydroxybenzophenone(0.1 phr) was irradi- ecules after irradiation.
ated with 2.5 Mrad y rays, the sheet showed discol-
oration (color difference) of 40.3 initially and 35.2 after 3.2.3 Inorganic Fillers
being stored for 10 days at 65"C, compared with 52.0 3.2.3.1 Zeolites and Related Compounds: Zeolites
and 63.0, respectively, for a control sample without and other metal silicates such as hydrated Mg sili-
the ketone. cate were reported to help stabilize PVC compounds
P-Diketones have also found application as stabiliz- toward radiation degradation (89, 90). For example,
ers for improving y resistance of PVC (84).When used when 100 parts of PVC, 50 parts of DOP, 3 parts
together with a hydrotalcite-type compound, the re- of ESO, 0.1 part of Ca stearate, 0.3 part of Zn rici-
sulting materials were reported to have increased y noleate, 0.2 part of tris(nonylpheny1) phosphite, and
stabilities. For example, when PVC (100 phr), DOP (50 0.15 part of NaA zeolite were roll-kneaded and hot-
phr), methyl epoxystearate (5 phr), Ca stearate (0.25 pressed to give a 3-mm sheet, it showed coloration of
phr), Zn stearate (0.25 phr), dibenzoylmethane (0.1 16.0 initially, 29.7 after 2.5 Mrad y-irradiation, and
phr), a n d Mg0.7AlO.3(OH),(C0,)0. 15.0.57Hz0 (0.1 38.7 after 5.0 Mrad y-irradiation, respectively (89).On
phr) were roll-milled at 170°C and heat-pressed into a the other hand, control samples made from the same
3-mm sheet, it showed yellowness 15.8 initially, 42.8 composition except without the zeolite showed col-
directly after y irradiation at 5.0 Mrad, and 32.6 after oration of 15.4, 44.3, and 59.1, respectively, for the
the irradiated sample was aged at 65°C for 10 days. three corresponding tests. More remarkably, the col-
On the other hand, a control without dibenzoylmeth- oration of the two irradiated samples with zeolites
ane showed yellowness values of 17.4, 64.8, and 69.0, dropped to 25.6 and 34.3, respectively, after storage
respectively . for 10 days at 65°C. In contrast, the storage had a
Aminocrotonic stabilizers were mentioned in several worsening effect on the two control samples, with col-
formulations which can help withstand y radiation (51, oration increased to 56.1 and 64.7, respectively. The
85). As discussed earlier, strong synergistic effects ability of zeolites and other metal silicates to improve
were observed between plasticizers and ethylene gly- the stability of PVC was attributed to their ability to
col bis(P-aminocrotonate), as well as other stabilizers. absorb HC1, reducing the further acid-catalyzed dehy-
However, their particular roles were not detailed in drochlorination of PVC (91).
these studies. However, different results were obtained when zeo-
In addition, it was reported that radiation-resistant lites such as NaY and MgO were used as fillers for
insulation materials can be prepared by mixing PVC PVC (92). These two materials produced a sensitizing
with carbamates (86). effect in the radiation crosslinkmg of PVC at doses of
3.2.2.6 Organophosphites: Organophosphites are 2-100 Wad, resulting in an increase in the radiation-
commonly used co-stabilizers of PVC polymers. Even induced dehydrochlorination a n d the formation of
though they are expected to improve the stability of conjugated double bonds. It was suggested that the

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1999, Vol. 5, No. 1 9

 Qi Wang and Sandor N q y

highly developed surface of zeolite NaY enhanced the DEFINITION OF ABBREVIATIONS

formation of additional conjugated bonds and polyene
WC: Poly(viny1 chloride)
radicals through absorption of HC1. In light of the two
DOP: Dioctyl Phthalate
contradictory experimental results, it is reasonable to
ESO: Epoxidized Soybean Oil
conclude that the nature of the aluminosilicate mate-
phr: Parts Per Hundred of Resin
rials has a critical effect on their performance as sta-
bilizing agents of PVC. Care should be exercised when
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