L. HYLOVA ET AL.: LOCAL MECHANICAL PROPERTIES OF IRRADIATED CROSS-LINKED HDPE
27–29
LOCAL MECHANICAL PROPERTIES OF IRRADIATED
CROSS-LINKED HDPE
LOKALNE MEHANSKE LASTNOSTI PRE^NO VEZANEGA HDPE
PO MO^NEM RADIOAKTIVNEM OBSEVANJU 
Lenka Hýlová, David Manas, Miroslav Manas, Lenka Gajzlerova, Ales Mizera
Tomas Bata University in Zlin, T. G. Masaryk Square 5555, 760 01 Zlin, Czech Republic
hylova@utb.cz
Prejem rokopisa – received: 2017-07-01; sprejem za objavo – accepted for publication: 2017-10-11
doi:10.17222/mit.2017.096
Using high doses of beta radiation for high-density polyethylene (HDPE) and its influence on the changes in the micromecha-
nical properties of the surface layer has not been studied in detail so far. Specimens of HDPE were made with the injection-
moulding technology and irradiated with high doses of beta radiation (0, 132, 165 and 198) kGy. The changes in the micro-
mechanical properties of the surface layer were evaluated using an ultra nano-hardness test. The results of the measurements
showed a considerable increase in the micromechanical properties (indentation hardness, indentation elastic modulus) when
high doses of beta radiation are used. The aim of this paper is to study the effect of ionizing radiation at different doses on the
ultra nano-hardness of the surface layer of HDPE and compare these results with those for non-irradiated samples. The study
was carried out due to the ever-growing use of this type of polymer HDPE.
Keywords: high-density polyethylene (HDPE), surface layer, mechanical properties, ultra nano–hardness
Avtorji prispevka so raziskovali vpliv mo~nega radioaktivnega sevanja  na mikromehanske lastnosti povr{inskih plasti
polietilena z veliko gostoto (HDPE), kar do sedaj ni natan~neje raziskoval {e nih~e. Vzorce HDPE so izdelali s tehnologijo
injekcijskega brizganja in jih obsevali z visokimi dozami (0, 132, 165 in 198) kGy radioaktivnega sevanja . Mikromehanske
lastnosti povr{inskih plasti obsevanih vzorcev so dolo~ili z merjenjem ultra nanotrdote. Rezultati meritev so pokazali znatno
povi{anje mikromehanskih lastnosti (nanotrdote in elasti~nega modula vtiskovanja) mo~no obsevanih vzorcev. Namen tega
prispevka je prikazati vpliv radioaktivnega sevanja  razli~nih jakosti na povr{inske plasti HDPE in primerjavo z neobsevanim
vzorcem. Raziskava je bila izdelana zaradi vse ve~je uporabe tega polimernega materiala v pogojih radioaktivnega sevanja.
Klju~ne besede: polietilen z veliko gostoto (HDPE), povr{inska plast, mehanske lastnosti, ultra nanotrdota
1 INTRODUCTION
The irradiation cross-linking of thermoplastic mate-
rials via an electron beam or cobalt 60 (gamma rays)
proceeds separately after the processing. The cross-link-
ing level can be adjusted with the irradiation dosage and
often by means of a cross-linking booster.
The main deference between - and 
-rays (Figure 1)
is in their different abilities of penetrating the irradiated
material; 
-rays have a high penetration capacity. The
penetration capacity of electron rays depends on the
energy of the accelerated electrons.
Thermoplastics used for the production of various
types of products have very different properties.
Standard polymers that are easily obtained at favourable
price conditions belong to the main class. The disadvan-
tage of standard polymers relates to both the mechanical
and thermal properties. The group of standard polymers
is the most considerable one and its share in the produc-
tion of all polymers is as high as 90 %.
Engineering polymers are a very important group of
polymers, exhibiting much better properties in compa-
rison to those of standard polymers. Both mechanical
and thermal properties are much better than in the case
of standard polymers. The production of these types of
polymers accounts for less than 1 % of all the poly-
mers.1–6
High-performance polymers have the best mechani-
cal and thermal properties but their share in the produc-
tion and use of all the polymers is less than 1 %.
The present work deals with the influence of beta
irradiation on the mechanical properties of the surface
layer of injection-moulded HDPE.
2 EXPERIMENTAL PART
For this experiment, high-density polyethylene
(HDPE) DOW – HDPE 25055E, the DOW Chemical
Company, USA (unfilled, HDPE) – was used. Irradiation
was carried out at the company BGS Beta-Gamma-
Service GmbH & Co, KG, Germany, using electron rays,
an electron energy of 10 MeV, and doses of (0, 132, 165
and 198) kGy in air at ambient temperature.
Samples (Figure 2) were made using the injection-
moulding technology on an injection-moulding machine
Arburg Allrounder 420C. The processing temperature
was 245–295 °C, the mould temperature was 85 °C, the
injection pressure was 80 MPa and the injection rate was
45 mm/s.7–11
Materiali in tehnologije / Materials and technology 52 (2018) 1, 27–29 27
UDK 67.017:620.179.155.1:62-4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)27(2018)
A nanoindentation test was done using an ultra
nanoindentation tester (UNHT), CSM Instruments
(Switzerland), according to the CSN EN ISO 14577. The
load and unload speed was 1000 N/min. After a holding
time of 90 s at the maximum load of 500 μN, the speci-
mens were unloaded. The specimens were glued onto
metallic sample holders (Figure 2).7–11
HIT = Fmax/Ap (1)
Here HIT is the indentation hardness, Fmax is the
maximum applied force, and Ap is the projected area of
contact between the indenter and the test piece deter-
mined from the force-displacement curve and the
knowledge of the area function of the indenter.7–11
3 RESULTS
4 DISCUSSION
The development of the micromechanical properties
of the irradiated high-density polyethylene (HDPE) was
characterized with a test of the ultra nano-hardness (HIT),
as can be seen in Figure 3. The lowest value (45 MPa) of
the indentation hardness was found for the high-density
polyethylene (HDPE) irradiated with the dose of 165
kGy, while the highest value of the indentation hardness
was found for the non-irradiated high-density polyethy-
lene (56 MPa). The increase in the indentation hardness
at the 198 kGy radiation dose was 4 % compared to the
non-irradiated high-density polyethylene (HDPE).
A similar development was recorded for the micro-
stiffness of the specimens represented by the indentation
elastic modulus (EIT) illustrated in Figure 4. The results
of the measurements show clearly that the lowest value
of the indentation elastic modulus was measured for the
high-density polyethylene (HDPE) (1.24 GPa) irradiated
with the dose of 165 kGy, while the highest value was
found for the high-density polyethylene (HDPE)
irradiated with 198 kGy (1.4 GPa). A significant increase
in the indentation elastic modulus (7 %) was recorded at
the radiation dose of 198 kGy, compared to the non-
irradiated high-density polyethylene (HDPE).
Very important values were found for the indentation
creep. For the materials, which creep as polymers, the
basic calculation of the creep can be measured during a
L. HYLOVA ET AL.: LOCAL MECHANICAL PROPERTIES OF IRRADIATED CROSS-LINKED HDPE
28 Materiali in tehnologije / Materials and technology 52 (2018) 1, 27–29
Figure 4: Indentation elastic modulus EIT
Figure 3: Indentation hardness HIT
Figure 2: Dimensions of a sample
Figure 1: a) design of gamma rays and b) electron rays: 3 – secondary
electrons, 4 – irradiated material, 5 – encapsulated Co-60 radiation
source, 6 – gamma rays, b) 1 – penetration depth of electrons, 2 –
primary electrons, 3 – secondary electrons, 4 – irradiated material
Figure 5: Indentation creep CIT
pause at the maximum force. The creep is a relative
change of the indentation depth when the test force is
kept constant. The results of the test of the ultra nano-
hardness, Figure 5, show that the highest creep value
was measured at the 0 kGy radiation dose (17 %), while
the lowest creep value was found for the high-density
polyethylene (HDPE) irradiated with the 165 kGy dose
(14 %). The creep decreased by 13 % because of the
radiation, which is a considerable increase in the
surface-layer resistance.
5 CONCLUSIONS
This article deals with the measurements of the
mechanical properties of a high-density polyethylene
(HDPE) surface layer modified with beta radiation.
Injection-moulded test bodies were irradiated with beta
radiation using doses of (0, 132, 165 and 198) kGy. The
measurements of the mechanical properties were realized
with an ultra nano-hardness tester.
The measurement results show an improvement in
the chosen mechanical properties. The micro-hardness of
the high-density polyethylene (HDPE) surface layer
irradiated with the 198 kGy dose increased by 4 %. The
rigidity of the tested surface layer represented by the
modulus of elasticity increased by 7 % for the sample
irradiated with the 198 kGy dose. The creep of the tested
surface layer decreased from 17 % for the non-irradiated
sample to a value of 14 % for the sample irradiated with
the 165 kGy dose.
Acknowledgment
This paper was supported by an internal grant of the
TBU in Zlin, No. IGA/FT/2017/010, funded from the
resources of the specific university research and by the
Ministry of Education, Youth and Sports of the Czech
Republic within the National Sustainability Programme,
project No. LO1303 (MSMT-7778/2014) and also by the
European Regional Development Fund under project
CEBIA-Tech, No. CZ.1.05/2.1.00/03.0089. Special
thanks goes to Dr. Michal Danek (BGS Beta-Gamma-
Service GmbH & Co, KG, Germany) for his kind
assistance during the sample preparation.
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