Preirradiated ultrahigh molecular weight polyethylene (UHMWPE) was subjected to other cycle of γ-exposure in order to explain the behavior of this material after radiation processing. The oxygen uptake investigation was selected for the characterization of thermal stability of multisession irradiation. The long life radicals promote crosslinking or oxidative degradation depending on the second irradiation dose and on the environmental conditions. The main kinetic parameters: oxidation period and oxidation rate were evaluated depicting the availability of radiochemical processing of UHMWPE for further long term applications. Irradiation of UHMWPE in salt solution reveals the improvement in the thermal strength for low exposure doses

C Oros

E Silva

L Andrade

Mihai Adrian

S Jipa

T Zaharescu

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JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 10, No. 4, April 2008, p.  826 - 829 
 
The assessment of thermal and radiation stability of 
UHMWPE 
 
 
MIHAI ADRIANa, T. ZAHARESCUa,*, S. JIPAa,b, L. ANDRADE E SILVAc, C. OROSb 
aINCDIE ICPE CA, 313 Splaiul Unirii 313, Bucharest 030138, Romania 
b”Valachia” University of Targoviste, Bd. Unirii 18, Romania 
cIPEN, Radiation Technology Centre, São Paol, Brasil 
 
 
Preirradiated ultrahigh molecular weight polyethylene (UHMWPE) was subjected to other cycle of γ-exposure in order to 
explain the behavior of this material after radiation processing. The oxygen uptake investigation was selected for the 
characterization of thermal stability of multisession irradiation. The long life radicals promote crosslinking or oxidative 
degradation depending on the second irradiation dose and on the environmental conditions. The main kinetic parameters: 
oxidation period and oxidation rate were evaluated depicting the availability of radiochemical processing of UHMWPE for 
further long term applications. Irradiation of UHMWPE in salt solution reveals the improvement in the thermal strength for low 
exposure doses. 
 
(Received February 25, 2008; accepted April 2, 2008) 
 
Keywords: UHMWPE, Irradiation, Thermal resistance, Oxygen uptake 
 
 
1. Introduction 
 
The high resistance of ultrahigh molecular weight 
polyethylene (UHMWPE) solves the requirements for 
production of high performances materials. It means that 
this material provides a convenient answer to the action of 
hard service conditions. On this basis, UHMWPE is largely 
used in the manufacture of prostheses and special medical 
wear or equipments [1-5]. 
The quality of materials used for the fabrication of 
implant articles is stated in ISO 5834-4:2005, where the 
standard conditions for the oxidation state of materials are 
imposed. The process of oxidative chain scission starts 
predominantly in the amorphous phase of materials, which 
initiates the degradation process in the entire material, 
either in crystalline or amorphous zones [6]. 
The oxidation of UHMWPE is in reality a complex 
sequence of various cascade reactions, which is not fully 
understood. The main aspect of ageing is related with the 
maximum oxidation (region of embrittlement) that often 
occurs 1-2 mm below the surface of UHMWPE 
components. Perhaps the most convincing explanation for 
the formation of subsurface maximum in oxidation comes 
from the work of Yeom et al. [7], who described the 
cascade of events occurring at various depths within 
gamma irradiated UHMWPE. At the surface of the 
UHMWPE the O2 concentration is highest and most 
primary alkyl radicals (-CH2. .CH2-) and secondary alkyl 
radicals (-CH2-CH.-) produced by gamma irradiation will 
react to form peroxyl radicals (-CH-O-O  or >CH-O-O., 
respectively). Other secondary alkyl radicals can combine 
to form crosslinks. A large concentration of peroxyl 
radicals exists at the surface and they can combine with O2 
to form ketones, a reaction which does not produce any 
further free radicals. In the subsurface region, on the other 
hand, fewer peroxyl radicals form, due to the lower O2 
concentration in this region. Because these peroxyl radicals 
will be further apart to each other, there will be a lesser 
tendency to recombine and greater tendency to abstract H 
atoms from the polyethylene chains, thereby forming 
hydroperoxides and new free radicals. This initiates a 
sequence of reactions, where carbonyl groups, chain 
scission and further free radicals are formed. As oxygen 
diffuses into the UHMWPE, further reaction with the newly 
created free radicals can occur, eventually leading to higher 
levels of oxidation in this inner region relative to material 
surface. Deeper into the UHMWPE there will be less 
oxygen available for reaction. Most of the radicals will 
recombine to produce crosslinks. In fact, the oxidation of 
this polymer takes place according with the mechanism 
comprehensively analyzed by Singh [8]. The irradiation of 
UHMWPE brings about the possibility to test the endurance 
of material under special conditions. 
The radiation crosslinking of UHMWPE exhibits a 
deep regularity of all σ bonds (Fig. 1), which increases the 
stability of this polymer. 
 
 
 
Fig. 1. Spatial structure of radiation crosslinked UHMWPE. 
 
This paper presents the post-irradiation stability of 
e-beam exposed UHMWPE. These results will be 
considered for the characterization of life time of 
prostheses. 
The assessment of thermal and radiation stability of UHMWPE 
 
827
2. Experimental 
 
UHMWPE tested in this work was provided by 
Ultra-Hi Plasticos Industriais Ltda (São Paulo, Brazil) as 
two sorts: UTEC 3041 (weight average molecular weight 
3.106 kg/kmol, impact strength: 170 kJ/m2), and UTEC 
6541 (weight average molecular weight 8.106 kg/kmol, 
impact strength: 80 kJ/m2). The preirradiation (electron 
beam, maximum energy: 1.5 MeV, dose rate: 22.61 kGy/s) 
treatment of samples was performed at IPEN, São Paolo, 
and the second irradiation was carried out in a γ-source 
(137Cs, dose rate: 0.4 kGy/h) in ICPE CA, Bucharest. 
During the accelerated electron irradiation of our sampled 
three different total doses (100, 300 şi 500 kGy) were 
applied. For γ-irradiation each e-beam irradiated 
UHMWPE sample was devided into four specimens which 
received further 0, 10 20 and 50 kGy. 
The thermal stability of all kinds of samples was 
performed by oxygen uptake method [9], which establishes 
the resistance of material against oxidation by the values of 
oxidation induction time (the period when the oxidation 
takes place at the unmeasurable rate), and the oxidation rate 
– the value of uptaken oxygen on propagation stage of 
oxidation). The conditions of exidability experiments were: 
temperature: 2000C, pressure of the oxidation environment 
(air): 1 atm, sample weight: 20 mg of chips. The oxygen 
uptake was done as isothermal and isobaric investigations. 
The extension of investigation was done on the two 
types of samples: one kind was native materials (marked 
with N) and the second kins consisted of recycled 
UHMWPEs (marked with R). For simulation of human 
body condition related to the salinity of implant 
environment, two paralel γ-exposures were accomplised: in 
air, in distilled water and in salt solution containing NaCl 5 
%. 
 
3. Results 
 
The raw materials were qualified by the evaluation of 
oxidation induction times and oxidation rates in accordance 
with the nature of samples and the received dose (Table 1). 
 
Table 1. Kinetic parameters of raw and recycled UHMWPEs. 
 
Dose 
(kGy) 
UTEC 
3041 N 
UTEC 
3041 R 
UTEC 
6541 N 
UTEC 
6541 R 
Oxidation induction time (min) 
0  170 205 138 85 
100  84 109 55 46 
300  44 42 52 19 
500  22 38 30 9 
Rate of oxidation (mmol O2/min) 
0  3.2 2.8 3.9 4.2 
100  3.8 3.5 4.5 4.8 
300  4.6 4.7 4.8 5.5 
500  5.2 5.0 5.1 6.5 
 
The Table 2 presents the kinetic parameters obtained 
for the irradiated UHMWPE samples in air for the different 
γ-doses received by all e-beam preirradiated materials. 
These data concerns the UTEC 3041 N samples, which are 
representative for all other UHMWPE specimens. Similar 
results were obtained for the other sorts of ultrahigh 
molecular weight polyethylene, that demonstrates the 
similarity in the effect of radiation processing on studied 
materials. 
 
Table 2. Oxidation parameters for UTEC 3041 N. 
 
γ-Irradiation doses (kGy) Preirradiation 
dose 
(kGy) 10 20 50 
Oxidation induction time (min) 
0  140 122 191 
100  120 130 160 
300  140 112 142 
500  160 153 123 
Rate of oxidation (mmol O2/min) 
0  3.7 3.9 2.8 
100  3.5 3.5 3.4 
300  3.4 3.8 3.5 
500  3.2 3.6 3.7 
 
The irradiation of UTEC 3041N samples in various 
environments provided their stability state (table 3). 
 
4. Discussion 
 
The first irradiation of UHMWPE samples with 
accelerated electrons causes the generation of free radicals, 
which participate further to other reactions in the function 
of treatment condition. 
The initial materials present a decrease in their thermal 
stability, which is due to the scission of C-C and C-H bonds. 
According to the mechanism reported by Bolland and Gee 
[10], if oxygen is present, peroxyl radical intermediates 
would be formed and the rate of oxidation increases 
significantly. From Table 1 it may be remarked that the 
oxidation induction times decrease sharply for the first 100 
kGy. Similar behavior was reported for irradiated other 
polyolefins [11]. The next doses bring about a lighter 
alteration in the values of induction periods. However, the 
modification in the oxidation rate measured on the 
propagation step does not get large differences between 
samples, which were irradiated at increasing doses. 
Consequently, it may be assumed that the long life 
radicals that remains in the e-beam irradiated UHMWPE 
remained as trapped entities. The low rate diffusion of 
oxygen into these materials does not react with free radicals, 
and they can survive in the exposed specimens. 
The second irradiation (γ-exposure) had allowed 
radicals to react with each other or with oxygen that is 
dissolved or diffused into processed samples. The most 
important results were obtained on the samples that they 
were not previously subjected to electron beam. They 
emphasize the availability of radicals to recombine. They 
rebuild the 3D structure by crosslinking. The kinetic 
parameters, oxidation induction time and oxidation rate, 
attend very promising values, which demonstrate the 
possibility of the delay of significant degradation, if two 
Mihai Adrian, T. Zaharescu, S. Jipa, L. Andrade E Silva, C. Oros 
 
828 
steps irradiation processing are included in the processing 
of bone prostheses. 
 
Table 3. Kinetic parameters of irradiation of UTEC 3041 N in 
various irradiation environments. 
 
Dose 
(kGy) Distilled water 
5 % NaCl 
aqueous solution 
10 kGy of γ-dose 
Oxidation induction time (min) 
0  233 464 
100  193 189 
300  171 112 
500  62 74 
Rate of oxidation (mmol O2/min) 
0  2.4 1.9 
100  2.8 2.8 
300  3.0 3.3 
500  3.7 4.1 
20 kGy of γ-dose 
Oxidation induction time (min) 
0  399 350 
100  170 295 
300  68 138 
500  14 27 
Rate of oxidation (mmol O2/min) 
0  2.2 2.1 
100  2.9 2.8 
300  3.6 3.6 
500  4.5 4.6 
50 kGy of γ-dose 
Oxidation induction time (min) 
0  236 499 
100  133 219 
300  168 60 
500  82 36 
Rate of oxidation (mmol O2/min) 
0  2.8 2.0 
100  3.4 3.1 
300  3.3 3.8 
500  3.0 4.5 
 
These second irradiated samples present higher 
thermal resistance after irradiation in aqueous salt solution. 
It means that the contribution of environment would favor 
the recombination of radicals. This process was depicted as 
charge transfer process, which involves chloride ions from 
the interface between polymer and solution. It was also 
reported in the case of the irradiation of ethylene-propylene 
copolymers [12]. The difference between the oxidation 
parameters for the irradiation of UHMWPE in water and 
aqueous salt solution demonstrates the possibility of 
enhance in the thermal strength of polyolefins by their 
radiation treatment at low doses in the presence of salts. 
The diffusion of oxygen is hindered by the existence of a 
water layer behind the contact surface of polymer sample. 
This kind of “protection” is a physical way on which neither 
additive, nor radical scavengers would offer. 
The recycled UHMWPE and UTEC 3041R exhibit 
higher thermal resistance in comparison with the native 
corresponding material. This aspect would be explained by 
the presence of a stabilizer, which retards the radiation 
oxidation of processed samples. The lack of stabilizer in the 
formulation of UHMWPE, UTEC 6541R, causes the faster 
degradation of samples in comparison with the native sort 
of polyethylene. 
These results are in a good agreement with other papers 
which are concerned with the thermal degradation of 
various sorts of polyethylene [13 – 15]. The durability of 
radiation conditioning materials depends strictly on the 
sequence of applied operations that involve the reactions of 
free radicals: crosslinking and oxidation [16 – 18]. 
 
5. Conclusions 
 
The post irradiation treatment by γ-exposure brings 
about an improvement in the thermal resistance of the 
investigated UHMWPE materials. The presence of salt 
(NaCl) in a low concentration allows decreasing the 
weariness of materials during the retention in human body. 
The modification in the kinetic parameters of 
degrading UHMWPE depicts the availability of free 
radicals formed in the first step of irradiation for the 
recombination with each other. 
The testing conditions influenced the level of results. It 
means that a sequence of the two types of irradiations, 
accelerated electrons treatment followed by 
electromagnetic radiation exposure is more favorable for 
the increase in the thermal stability of UHMWPE. This 
order involves the formation of high amount of long life 
radicals, which can react further under the energetic action 
of γ-radiation. 
For medical purposes, the radiation treatment of 
UHMWPE brings about not only an advanced crosslinking 
state, but also a sterilization of prostheses and medical 
wear. 
This study confirms the large potential of oxygen 
uptake method in the characterization of engineering 
polymer materials. 
 
Acknowledgments 
 
The authors express their thanks to IAEA for 
supporting this research according with the program 
“Controlling of Degradation Effects in Radiation 
Processing of Polymers”. 
 
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______________________ 
*Corresponding author: zaharescu@icpe-ca.ro 
 
 
 
 
 


The assessment of thermal and radiation stability of UHMWPE

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The assessment of thermal and radiation stability of UHMWPE

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