V. JANOSTIK et al.: LOCAL MECHANICAL PROPERTIES OF IRRADIATED CROSS-LINKED POLYPROPYLENE
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LOCAL MECHANICAL PROPERTIES OF IRRADIATED
CROSS-LINKED POLYPROPYLENE
LOKALNE MEHANSKE LASTNOSTI RADIOAKTIVNO
OBSEVANEGA PRE^NO VEZANEGA POLIPROPILENA
Vaclav Janostik, Lenka Hýlová, David Manas, Miroslav Manas, Lenka Gajzlerova,
Ales Mizera, Michal Stanek
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.2017.097
Using high doses of beta radiation for isotactic polypropylene (iPP) and its influence on the changes in the micromechanical
properties of the surface layer has not been studied in detail so far. Specimens of isotactic polypropylene (iPP) were made with
the injection-moulding technology and irradiated with high doses of beta radiation (0, 45, 66 and 99) kGy. The changes in the
micromechanical 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 with
different doses on the ultra nanohardness of the surface layer of isotactic polypropylene (iPP) and compare these results with
those of non-irradiated samples. The study was carried out due to the ever-growing use of this type of polymer, isotactic
polypropylene (iPP).
Keywords: isotactic polypropylene (iPP), surface layer, mechanical properties, ultra nanohardness
Avtorji prispevka so raziskovali vpliv mo~nega radioaktivnega sevanja  na mikromehanske lastnosti povr{inskih plasti
izotakti~nega polipropilena (iPP), kar do sedaj {e ni bilo natan~neje raziskano. Vzorci iPP so bili izdelani s tehnologijo injekcij-
skega brizganja in obsevani z visokimi dozami  radioaktivnega sevanja (0, 45, 66 in 99) kGy. Mikromehanske lastnosti
povr{inskih plasti obsevanih vzorcev so dolo~ili z in{trumentiranim merilnikom ultrananotrdote. Rezultati meritev so pokazali
znatno zvi{anje mikromehanskih lastnosti (nanotrdote in modula elasti~nosti) radioaktivno obsevanih vzorcev. Namen tega
prispevka je prikazati vpliv radioaktivnega sevanja  razli~nih jakosti na povr{inske plasti iPP 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, ultrananotrdota
1 INTRODUCTION
Isotactic polypropylene (iPP) is a commodity
polymer with a semi-crystalline structure, which is very
complex and depends strongly on the thermal history and
processing conditions. Isotactic polypropylene can
crystallize into three phases: the 
-phase is the most
stable and the most common. The crystals are mono-
clinic. The -phase is metastable and its crystals are
hexagonal. The -phase is mainly found in block PP
copolymers and can be generated by adding specific
nucleating agents. This phase was discovered by Padden
and Keith in 1953 and can be improved with a cry-
stallization between 130 °C and 132 °C or an orientation
with high shear or through additions of specific
nucleating agents. The presence of the -phase in PP
homopolymer generally increases the ductility of
finished parts. The maximum effect is observed at 65 %
of the -phase. The 
-phase is also metastable, with
triclinic crystals. This form is not very common; it
appears mainly in low-molecular-weight polypropylene
due to the crystallization at a very high pressure and very
low cooling rate.1–2
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. Stan-
dard polymers that are easy obtainable 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
production of all polymers is as high as 90 %.
The present work deals with the influence of beta
irradiation on the mechanical properties of the surface
layer of injection-moulded isotactic polypropylene
(iPP).3–6
Materiali in tehnologije / Materials and technology 52 (2018) 1, 31–33 31
UDK 67.017:620.179.15:62-4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)31(2018)
2 EXPERIMENTAL PART
For this experiment, PTS-Crealen EP-2300L1-M800,
PTS Plastics Technologie Service, Germany (unfilled,
iPP+TAIC, MFR–230 °C /2, 16 kg–6 g/10 min) was
used. The material already contained a special cross-
linking agent, TAIC – triallylisocyanurate (6 % of volu-
me fractions), which enabled the subsequent cross-
linking with ionizing -radiation. 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, 45, 66 and 99) 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
A nanoindentation test was done using an ultra
nanoindenation tester (UNHT), CSM Instruments
(Switzerland), according to the CSN EN ISO 14577.
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 maxi-
mum applied force, and Ap is the projected area of the
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 isotactic polypropylene (iPP) was
characterized with a test of the ultra nano-hardness (HIT),
as can be seen in Figure 3. The lowest value (47 MPa) of
the indentation hardness was found for the isotactic
polypropylene (iPP) irradiated with the dose of 99 kGy,
while the highest value of the indentation hardness was
V. JANOSTIK et al.: LOCAL MECHANICAL PROPERTIES OF IRRADIATED CROSS-LINKED POLYPROPYLENE
32 Materiali in tehnologije / Materials and technology 52 (2018) 1, 31–33
Figure 3: Indentation hardness HIT
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 –
Figure 5: Indentation creep CIT
Figure 2: Dimensions of sample
Figure 4: Indentation elastic modulus EIT
found for the isotactic polypropylene (iPP) irradiated
with the dose of 45 kGy (95 MPa). The increase in the
indentation hardness at the 45 kGy radiation dose was
92 %, compared to the non-irradiated isotactic polypro-
pylene (iPP).
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
isotactic polypropylene (iPP) (0.77 GPa) irradiated with
the dose of 0 kGy, while the highest value was found for
the isotactic polypropylene (iPP) irradiated with 45 kGy
(1.19 GPa). A significant increase in the indentation
elastic modulus (54 %) was recorded at the radiation
dose of 198 kGy, compared to the non-irradiated isotac-
tic polypropylene (iPP).
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
pause at the maximum force. The creep is a relative
change of the indentation depth when the test force is
kept constant. The measurements of the ultra nano-hard-
ness showed (Figure 5) that the highest creep value was
obtained for the sample irradiated with the 66 kGy dose
(13.7 %), while the lowest creep value was found for the
isotactic polypropylene (iPP) irradiated with the 45 kGy
dose (7.9 %). The creep decreased by 21 % 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 the tested isotactic polypro-
pylene (iPP) surface layer modified with beta radiation.
Injection-moulded test bodies were irradiated with beta
radiation using doses of (0, 45, 66 and 99) 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 ultra nano-hard-
ness of the isotactic polypropylene (iPP) surface layer
irradiated with the 45 kGy dose increased by 92 %. The
rigidity of the tested surface layer represented by the
modulus of elasticity increased by 54 % for the sample
irradiated with the dose of 45 kGy. The creep of the
tested surface layer decreased from 10 % for the
non-irradiated sample to a value of 7.9 % for the sample
irradiated with the dose 45 kGy.
Acknowledgment
This paper was supported by an internal grant of the
TBU in Zlin, No. IGA/FT/2017/010, funded from the
resources for 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 go to Dr. Michal Danek (BGS Beta-Gamma-
Service GmbH & Co, KG, Germany) for his kind
assistance during the sample preparation.
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V. JANOSTIK et al.: LOCAL MECHANICAL PROPERTIES OF IRRADIATED CROSS-LINKED POLYPROPYLENE
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