influence of gamma radiation on hindered phenols in ldpe, paraffin oil and paraffin wax

Radiation Physics and Chemistry 68 (2003) 925–931

Influence of gamma radiation on hindered phenols in LDPE,paraffin oil and paraffin wax

Shamshad Ahmed*, Tariq Yasin, Abdul Ghaffar

Polymer Processing and Radiation Technology Lab, ACL, PINSTECH, Nilore, Islamabad, Pakistan

Received 21 August 2002; accepted 30 December 2002

Abstract

Extraction and high performance liquid chromatography procedures have been developed for the determination of

two hindered phenols—Irganox-1010 and Santonoxs, incorporated in low density polyethylene (LDPE), paraffin oil

and paraffin wax at varied concentrations. These blends were gamma irradiated from 25 to 400 kGy and the radiation

and thermal stability of the antioxidant under the experimental conditions has been determined. Upon increase in

radiation dose from 25 to 400 kGy, a gradual diminution in the extractable level of each antioxidant was observed. The

fate of both the antioxidants in various matrices has been compared. It has been shown that both the antioxidants can

influence the polyethylene network formation and the radical yield in different ways resulting in retardation in the rate

of crosslinking, as determined by gel-content analysis.

r 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

Antioxidants are of great importance in the manu-

facture of thermoplastic materials, but primarily in their

stabilization as required by the intended use of the final

product. In polymer blends exposed to ionizing radia-

tion, for modifying their mechanical properties, the

antioxidants have to satisfy some requirements such as

stability in radiation field and minimum inference with

the crosslinking process. Several authors have discussed

why certain antioxidants are more efficient stabilizers

than the others and to correlate the structure of an

antioxidant with its activity in blends (Gugumus, 1985;

Shimada et al., 1974; Layer, 1985). Some problems

related to irradiated polyethylene-antioxidant systems

such as thermo-oxidative degradation of irradiated

blends and the influence of stabilizers on the yield of

crosslinking have been previously investigated (Nova-

kovic et al., 1985; Gal et al., 1983; Jaworska et al., 1991).

Most studies are related to the effects of concentration

and the influence of only a narrow range of radiation

dose.

We have been engaged in the development of a

radiation crosslinkable thin wire insulation based on

LDPE using different additives including Irganox 1010

as primary antioxidant (Shamshad et al., 1995). In this

work a comprehensive study has been carried out to

investigate comparative fate of a phenolic antioxidant

(Irganox-1010) and a thio-phenolic antioxidant, Santo-

noxs admixed with LDPE at various concentrations

and exposed to wide range of radiation doses ranging

from 25 to 400 kGy. Molecular formulae of antioxidants

used are shown in Table 1. The antioxidants used in this

work are high molecular weight hindered phenols, which

have one or more t-butyl substituents at ortho positions.

The work has also been extended to include the

monitoring of the fate of the antioxidants in paraffin

oil and wax for comparing the behavior of antioxidant

in closely related polymeric matrices. These investiga-

tions permit us to calculate the actual amount of

carryover available for stabilization during post-irradia-

tion storage. This, in turn, may allow us to determine the

optimum amount of an antioxidant required to stabilize

LDPE or other matrices being irradiated.

ARTICLE IN PRESS

*Corresponding author. Fax: +92-51-9290275.

E-mail address: shamshad [email protected] (S. Ahmed).

0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0969-806X(03)00010-0

2. Experimental

2.1. Materials

The phenolic antioxidants used in the present work

were of commercial grade. Both the antioxidants were

used without further purification. LDPE was obtained

from Neste (USA), kept in n-hexane (72 h) to remove

any additives/stabilizers. Solvents used for extraction

and chromatography were HPLC grade. Paraffin

wax (m.p. 73–78�C) and paraffin oil (density 0.862)

were from Aldrich (USA). Xylene used for gel

fraction determination was a commercial grade isomeric

mixture.

2.2. Preparation of LDPE, paraffin oil and paraffin wax

formulations

The additives were coated on LDPE granules by

spraying the powder and then tumbled in a drum.

Compounding of LDPE with Irganox-1010 and Santo-

noxs was achieved by heating at 220�C for 15min at

60RPM in mixing chamber of Brabender mixer in all

cases.

Dissolution of Irganox-1010 in paraffin oil was

achieved by heating the mixture in oven at 100�C for

12 h. As the additive did not separate out from the

solution on standing for 2 days, the formulated material

was considered to contain uniformly distributed addi-

tive. In case of Santonoxs, uniform solutions of only

0.2% and 0.3% (w/w) in paraffin oil could be achieved.

In case of paraffin wax, uniform formulations of both

the additives were obtained by heating the mixtures of

additive containing varying concentrations with paraffin

wax at 120�C for 16 h. On cooling, solidified mixtures of

the wax and the additives were then pulverized to ensure

uniform formulations.

2.3. Irradiations

Irradiations were carried out in air at room tempera-

ture (B25�C) to simulate factual conditions prevalent

during commercial irradiation of cable insulation. Dose

rate was 4.23 kGyh�1. The doses of irradiation ranged

from 25 to 400 kGy. All the samples were irradiated seal

packed in polyethylene bags.

2.4. Chromatography

A Varian Liquid Chromatograph, equipped with a

Rheodyne 7125 injection valve, 20ml loop and variable

wavelength UV-Visible detector was used. A C-18

precolumn was set before the analytical column ODS-

Zorbax (25 cm� 4.6mm) Du-Pont USA, to retain any

impurities. Flow rate was 0.75ml/min in all cases, unless

otherwise specified.

In view of sharp chromatographics peaks obtained

under the conditions and the fact that the reproduci-

bility was satisfactory, the peak heights were measured

directly for quantitative determination. Average of three

determinations has been quoted. Calibration curves for

both the additives were obtained by injecting known

amounts of individual compounds to obtain chromato-

grams and plotting peak heights against the concentra-

tion. In view of maxima of absorption of the additives

around 280 nm, this wavelength was selected for all the

determinations. This set-up allowed high sensitivity and

though, by-products of reticulation and oxidation were

observed, their higher retention times at this wavelength

eliminated the interferences due to their peaks.

ARTICLE IN PRESS

Table 1

Molecular formulae of antioxidants used for this study

Antioxidant Structure

Tetrakis [methylene-(3-(30,50-di-tert-butyl-4 hydroxyphenyl) propionate] methane

(Irganox-1010)

4,40-Thiobis-(6-t-butyl-3-methyl phenol)

Santonoxs

S. Ahmed et al. / Radiation Physics and Chemistry 68 (2003) 925–931926

3. Results and discussions

3.1. Radiation stability of antioxidants

Since the objective of the present work was to assess

the amount of stabilizer lost or utilized while preventing

the oxidation of the polymer (or other matrices) during

irradiation, it was important to study the losses of

the additive through degradation, etc. incurred by the

exposures to gamma rays alone and to asses the

contribution to the overall losses. To check the radiation

stability of antioxidants both the antioxidants were

irradiated upto 300 kGy in air. Then unirradiated and

irradiated samples of both Irganox-1010 and Santonoxs

(1 g each) were extracted in methanol and amount of

antioxidant in each sample was determined by reverse

phase HPLC (from the calibration curves), performed

under the conditions recorded in Table 2. The difference

between quantity of antioxidants determined in the

irradiated and the unirradiated material is less than 3%

indicating that both these antioxidants do not decom-

pose in the radiation field in any appreciable way.

3.2. Quantification of antioxidants

In the published literature, methods for the quantita-

tive determination of stabilizer in polymer matrix are

reported which are based on the in situ spectroscopic

analytical schemes proposed or adopted to overcome the

difficulties caused by insoluble polymer matrix. The

approach adopted in this work consisted of preparation

of a thin film of the polymer, followed either by UV or

IR analysis. Although both the antioxidants absorb

strongly in the UV spectrum, the absorption bands of

degradation products were found to overlap and thus

this method was not applicable to multi-component

system: IR analysis is more functional group specific

than UV but quantification was made difficult by the

presence of unique interference bands for each degrada-

tion product. These observations prompted us to adopt

a more sensitive analytical technique to allow analysis in

the presence of degradation products, also present in the

extractions of stabilizers from the polymer, paraffin oil

or wax matrices.

3.2.1. Extraction of antioxidants

The extraction and measurement of stabilizer is a

difficult task due to the following three factors, firstly,

the presence of stabilizer in a relatively insoluble

polymer matrix, secondly the high reactivity and low

stability of the stabilizer and finally the low stabilizer

concentrations. Keeping in view these factors, different

procedures for extraction of stabilizer were adopted.

3.2.1.1. Extraction procedure for LDPE. Chloroform

was found to be the most suitable solvent for LDPE and

the antioxidant blends. Boiling of chopped fine pieces of

LDPE with analar grade chloroform for 5–6 h was

carried out, followed by cooling and filtration of the

polymer and subsequent evaporation of the solvent

under vacuum. The residue thus obtained was dissolved

in methanol, filtered and finally chromatographed under

the conditions described elsewhere in the paper.

3.2.1.2. Extraction procedure for paraffin oil. Extraction

of additives from paraffin oil was achieved in much

more facile manner by shaking the oil–antioxidant

mixture thrice with progressively decreasing amounts

of CH3OH at room temperature, followed by separation

of oily layer by centrifuging. At higher doses, some

particulate matter was observed. Filtration of the extract

was found necessary to obtain clear solution for

performing HPLC.

3.2.1.3. Extraction procedure for wax. In case of ex-

traction of additives from wax, a technique of facile

conversion of wax into powdery material by super-

cooling it in liquid nitrogen was adopted. The flakes so

obtained were ground. Powdery material obtained from

wax on boiling with iso-propanol for 3–4 h gave

quantitative recoveries and the results were reproduci-

ble. Extract was further cooled to separate out any

residual amount of wax, which was removed by

filtration. On further prolonged cooling, no wax was

found to separate.

3.2.2. Quantification of the extracted antioxidants by

chromatography

Analyses were performed by reverse phase chromato-

graphy by using ODS column under isocratic conditions

using pure methanol as well as methanol–water as eluent

at 0.8mlmin�1 flow rate. The injection volume was

20 ml.In case of both the antioxidants, the analysis time

for the extracts was less than 5min. The retention

volumes VR were found to be in the decreasing order

ARTICLE IN PRESS

Table 2

Radiation stability of antioxidant Irganox-1010 and Santo-

noxs

Sample Amount

taken (mg)

Calculated

from curve

(mg)

Loss (%)

Irganox-1010 (control) 33.0 32.2 2.5

Irganox-1010

(300 kGy)

33.0 31.4 5.0

Santonoxs (control) 20.0 20.0 0.0

Santonoxs (300 kGy) 19.5 19.2 1.03

All solution in 20ml of CH3OH.

Flow rate: 0.75ml/min, Mobile phase: CH3OH, lmax: 280 nm.

S. Ahmed et al. / Radiation Physics and Chemistry 68 (2003) 925–931 927

Santonoxs>Irganox-1010 in consistency with the

relatively more non-polar nature of the latter compound

as compared to the former. In the case of Santonoxs,

better resolution of the additive and the degradation

products was observed when a mixture of water and

methanol (CH3OH: H2O=90:10) was used as eluent.

3.2.2.1. Survival of antioxidants in LDPE. The decrease

in extractable antioxidant content expressed as a percent

of the original content (percent recovery) as a function

of absorbed doses are shown in Tables 3 and 4. A

progressive reduction in the level of antioxidants can be

seen from these tables, corresponding to the increase of

dose of gamma-radiation given to the LDPE antioxidant

blend. The antioxidant thus appears to play a sacrificial

role to protect LDPE against radiolytic oxidation or

crosslinking. In both the cases, in general, the antiox-

idants were found to disappear at a faster rate up to a

dose of 100 kGy and thereafter losses were less. The rate

of decrease in the amount of the extractable antioxidant

was found to be dependent upon the initial level of

additives present in LDPE. Additive Santonoxs was

found to disappear at a faster rate when compared to

Iraganox-1010 indicating a dependence of reactivity on

the chemical structure of each stabilizer. The observed

faster rate of disappearance of Santonoxs as compared

to Irganox-1010 may presumably be attributed to its

having unsubstituted sites ortho to the hydroxyl group,

thus enabling ortho coupling to the polymer. In case of

Irganox-1010, on the other hand, as it has no ortho sites

available to couple, the product produced by removal of

the hydroxyl hydrogen either forms an ether link to the

polymer chain, or a semiquinone structure (degraded

structure). This corresponds well with the observed

lower gel content in case of blends of Santonoxs with

LDPE than in case of blends of Irganox-1010 with

LDPE, as described later.

Prior to irradiation, percentage recoveries observed in

case of LDPE-antioxidant blends clearly show that a

reasonable part in case of the additive Santonox is lost

due to the volatilization rather than thermal decom-

position of antioxidant during the heat mixing process

carried out at 220�C. Here again the rate of loss of the

antioxidants was found to be dependent upon their

initial concentration in the sample.

Effect of storage on the concentrations of the

antioxidant Irganox-1010 was also studied by analysis

of irradiated and unirradiated LDPE+Irganox-1010

blends, by storage at room temperature (range from 300

to 311K) and the results are shown in Table 5.

Prolonged storage of LDPE blends shows the loss of

antioxidants. It was observed that with the increase in g-irradiation dose, there was a corresponding increase in

the intensity of discoloration produced. The origin of

discoloration can be attributed to the generation of

highly conjugated, oxidative transformation products

including dimer, trimer and various other structures in

this process. Their likely presence is hinted at by the

presence of peaks in chromatograms appearing at higher

retention times. The area of peaks was found to increase

with corresponding increase in irradiation dose. These

finding have helped us in avoiding the use of these

discoloring phenolic additives while developing the

radiation resistant PP for radiation sterilizable syringes

in which case discoloration or yellowing is a major

disincentive. Instead hindered amines Tinuvin-622 was

used (Shamshad and Basfar, 2000). However, no effort

was made to confirm the structure of the radiolytic

products of degraded materials from both the antiox-

idants. Iso-proponol was used for extractions of

additives from paraffin oil and the same was injected

to obtain chromatograms.

3.2.2.2. Survival of antioxidants in paraffin oil and

wax. The extractable amounts of antioxidants Irga-

nox-1010 in paraffin oil showed progressive diminution

up to 200 kGy as already seen in case of LDPE, and

ARTICLE IN PRESS

Table 3

Influence of g-radiation on extractable amount of Irganox-1010

in LDPE+Irganox-1010 blend (storage period after irradia-

tion=9 months; storage temperature from 300 to 311K)

Irradiation dose (kGy) Recovery (%)a

2.00b 1.00b 0.20b

0 54.40 38.00 9.60

100 35.79 10.00 ND

200 29.40 2.00 ND

300 23.10 ND NA

400 19.95 ND NA

aAverage of three determinations is quoted, standard

deviation70.25%.bAntioxidant added at this level.

ND: Not detectable.

Table 4

Influence of g-radiation on extractable amount of Santonoxs in

LDPE+Santonoxs blend (storage period after irradiation=9

months; storage temperature from 300 to 311K)


2.00b 1.00b 0.2b

0 62.50 43.58 12.50

100 21.57 9.48 4.50

200 17.50 7.6 3.75

300 12.00 6.0 2.50

400 10.25 5.6 ND




thereafter losses were less pronounced (Table 6). On

weight/weight basis, rate of disappearance of Irganox-

1010 was faster in paraffin oil than in LDPE. Since

oxidation is a phenomenon also dependent on area of

exposure to air, the observed difference could be

attributed to the enhanced mobility of the antioxidant

in oil as compared to the LDPE and thus relatively more

exposed surface for oxidation is available as compared

to less exposed surface in LDPE. The antioxidant

presumably also stays locked within the network of

LDPE whereas in case of oil it moves freely, enhancing

the area of contact with matrix, oil. As expected, due to

the absence of losses through thermal degradation,

initial levels of additions in case of antioxidant-oil

blends were found to be almost approaching 100%.

The stabilizer Santonoxs also demonstrated similar

behavior. In this case, however, due to its relatively less

solubility in paraffin oil, Santonoxs-oil blends contain-

ing 0.5% and 1% of the antioxidant were found to

precipitate out on standing for 2–3 days. Only solutions

of 0.2% and 0.3% Santonoxs in paraffin oil were

therefore studied. Results are recorded in Table 7.

Progressive diminution of Santonoxs in wax containing

additives at 2% and irradiated from 25 to 100 kGy was

also studied and results are reported in Table 8. Slow

diminution of the antioxidant in wax compared to oil is

observed and could presumably be attributed to its more

radiation tolerance as compared to oil and thus less

generation of free radicals.

3.3. Retardation influence on gel fraction

It is expected that the interaction of free radicals

(generated upon irradiation) and the antioxidants will

ARTICLE IN PRESS

Table 5

Effect of storage on unirradiated and irradiated LDPE-Irganox-1010 blends

Sample Recovery (%)a

Storage period

2 Months 6 Months 9 Months

Irganox-1010 (2%)+LDPE Unirradiated 93 60 54.50

Irganox-1010 (1%)+LDPE Unirradiated 62 38 10

Irganox-1010 (2%)+LDPE Irradiatedb 62 50 38

Irganox-1010 (1%)+LDPE Irradiatedb 15 10 9.50

aAverage of three determinations is quoted, standard deviation70.25%.b Irradiation doseB100 kGy.

Table 6

Effect of g-irradiation on antioxidant Irganox-1010 present in

paraffin oil


2.00b 0.99b 0.21b

0 90 105 100

25 74 50 13.33

50 59 26 ND

100 36 12 ND

200 12 3 NA

300 2.7 NA NA

400 2.7 NA NA



Table 7

Effect of g-irradiation on antioxidant santonoxs present in

paraffin Oil


0.21b 0.30b

0 86.00 96.00

25 NA 25.33

50 NA 14.00

100 0.50 1.11

200 0.50 NA

300 NA NA



Table 8

Effect of g-irradiation on antioxidant Santonoxs (2%) in

paraffin wax

Irradiation dose (kGy) Expected Founda Recovery (%)

0 0.002 0.0020 100

25 0.002 0.00155 77.5

50 0.002 0.0142 71.0

100 0.002 0.0012 60.0


deviation70.25%.

S. Ahmed et al. / Radiation Physics and Chemistry 68 (2003) 925–931 929

result in retardation of gel fraction of LDPE. From the

viewpoint of selection of an ideal antioxidant, the less

the retarding influence of the antioxidants on the

crosslinking yield, the more suitable is the antioxidant.

This aspect thus has been studied in detail in case of

both the antioxidants (Figs. 1 and 2).

The influence of antioxidants on the gel contents of

the LDPE was studied by comparison of the gel contents

found in pure LDPE irradiated at varying doses from

100 to 400 kGy with the gel percentages observed in

LDPE-antioxidants blends of varying proportions and

irradiated accordingly. The gel contents were deter-

mined by continuous extraction of the samples with

xylene using the standard procedure. Both antioxidants

were found to retard crosslinking and the retardation

rates were quite pronounced at initial doses of 100 to

200 kGy whereas at subsequent highest doses 300,

400 kGy, less influence was observed (see Figs. 1 and

2). The fast diminution of the antioxidants in the

polymer matrix seems to correspond well with the

pronounced retardation of crosslinking at the initial

doses of 100 and 200 kGy. It suggests an interaction

between free radicals and the antioxidants and the fast

consumption of the later at early doses.

A comparison of the retardation influence exerted by

Irgonox-1010 and Santonoxs shows the latter to be

more effective (Figs. 1 and 2). This corresponds well

with the relatively accelerated disappearance/diminution

of the extractable amount of this additive. As expected

in case of both the antioxidants, retardation influence

progressively increased with the increase in quantity of

the antioxidants incorporated in LDPE. A concentra-

tion of 0.2% however was found to exert a little

influence in case of both the antioxidants.

4. Conclusions

It may be concluded that the fate of any of the

antioxidants studied here in LDPE, oil or wax upon

irradiation by gamma rays is dependent:

(a) Upon the history of thermal processing of sample.

(b) On the chemical nature of the antioxidant itself.

(c) The storage period of the sample after irradiation.

(d) On the levels of the initial concentration of the

additive in the matrix and on the nature of the

matrix itself in terms of its reactivity.

The studies also reveal that the presence of antiox-

idants results in a decrease in crosslinking, which is

dependent on the nature of antioxidant and its amount.

Therefore, adding antioxidants in quantities higher than

needed to stabilize against oxidative degradation, etc. is

going to result in undesirable decrease of crosslinking.

This is important from the viewpoint of achieving a

required gel content in cable and wire insulation.

Acknowledgements

One of the authors gratefully acknowledges the

financial support given by Commission of European

Community (Grant No. B/CI1*-900843) and to RIS+National Laboratories Denmark.

References

Gal, O., Novakovic, L.I., Markovic, V., Stannett, V.T., 1985.

The effect of the nature of the antioxidant on the radiation

crosslinking of polyethylene. Radiat. Phys. Chem. 26, 325.

ARTICLE IN PRESS

0 100 200 300 400

0

20

40

60

80

0.0 %

2.0 %

1.0 % 0.2 %

Gel

Fra

ctio

n (%

)

Irradiation Dose (kGy)

Fig. 1. Influence of antioxidant Irganox-1010 on gel fraction

percentage of LDPE during crosslinking by gamma radiation.

0 100 200 300 400

0

20

40

60

80

0.0 %

2.0 %

1.0 %

0.2 %

Gel

Fra

ctio

n (%

)

Dose (kGy)

Fig. 2. Influence of antioxidant Santonoxs on gel fraction

percentage of LDPE during crosslinking by gamma radiation.


Gugumus, F., 1985. Aspects of the stabilization mechanisms of

phenolic antioxidants in polyolefins. Angew. Makromol.

Chem. 137, 189.

Jaworska, E., Kaluska, J., Strzelczak, G., Burlinska, G.,

Michalik, J., 1991. Irradiation of polyethylene in the

presence of antioxidants. Radiat. Phys. Chem. 37, 285.

Layer, R.W., 1985. Developments in polymer stabilisation-4.

Applied Science, London, p.146 (Chapter 5).

Novakovic, L.I., Gal, O., Markovic, V., 1985. Thermogravi-

metric studies of the thermo-oxidative stability of irradiated

and unirradiated polyethylene-II Combined antioxidants.

Radiat. Phys. Chem. 26, 331.

Shamshad, A., Basfar, A.A., 2000. Radiation resistant

polypropylene blended with mobilizer, antioxidants and

nucleating agent. Radiat. Phys. Chem. 57, 447–450.

Shamshad, A., Riffat, T., Tariq, Y., 1995. Thermal stability

studies of gamma irradiated LDPE: improvement by

Irganox 1010. J. Anal. Environ.-Chem. 3, 9.

Shimada, S., Kashiwabara, H., 1974. Decay reaction of free

radicals in irradiated polyethylene and diffusion controlled

processes. Polym. J. 6 (5), 480.

Further readingShamshad, A., Ruimin, Z., 1999. Development of Formulations

of Polyethylene Based Flame-Retardant, Radiation-

Resistant Wires. IAEA TECDOC-1062, IAEA, Vienna,

pp. 129–134.

ARTICLE IN PRESSS. Ahmed et al. / Radiation Physics and Chemistry 68 (2003) 925–931 931

influence of gamma radiation on hindered phenols in ldpe, paraffin oil and paraffin wax

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