Nuclear Instruments and Methods in Physics Research B 236 (2005) 130–136

www.elsevier.com/locate/nimb

Studies on polyethylene pellets modified by low doseradiation prior to part formation

Song Cheng a,*, Frederique Dehaye b, Christian Bailly b, Jean-Jacques Biebuyck b,Roger Legras b, Lewis Parks a

a Sterigenics, Advanced Applications, 7695 Formula Place, San Diego, CA 92121-2418, USAb Unite de Physique et de Chimie des Hauts Polymeres, Universite Catholique de Louvain, Croix du Sud 1,

1348 Louvain-la-Neuve, Belgium

Available online 23 May 2005

Abstract

When it is combined with other processing steps, radiation modification of polyethylene pellets prior to conversion

into end products (formed parts) has brought about significant improvement of various properties of the polymers and

products made from them despite the low cross-linking degree. The physical and chemical changes of the polymers after

the radiation modification by electron beam (EB) and gamma ray at low dose levels are studied using various charac-

terizations. Fourier Transform Infrared Spectroscopy (FTIR) showed the formation of carbonyl groups and changes of

unsaturated bonds. Gel permeation chromatography (GPC) results indicated broadening of the molecular weight dis-

tribution. Rheological analysis in linear visco-elasticity regime showed increased dynamic viscosity and large amplitude

oscillatory shear (LAOS) analysis showed increased hysteresis. It is proposed that the radiation at low dose levels and

under ambient conditions induces various reactions on the polymer chains including long chain branching, oxidation

and changes of unsaturated bonds.

� 2005 Elsevier B.V. All rights reserved.

PACS: 61.82.Pv; 81.05.Lg; 83.60.Df

Keywords: Polyethylene; Radiation; Long chain branching; Oxidation; Rheology; LAOS

1. Introduction

It has been well known for many years that irra-

diating parts formed from PE leads to cross-

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.nimb.2005.03.272

* Corresponding author.

E-mail address: scheng@sterigenics.com (S. Cheng).

linking of the polymer and hence improvementsof mechanical properties and thermal stability,

etc., of the parts. Generally, a high degree of

cross-linking (e.g. 60–75% gel content) is imparted

by such modification and the radiation doses

required for such processes are typically in the

range of 50–150 kGy. However, there has been

ed.

S. Cheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 130–136 131

significantly less research and development on

radiation modification of polyethylenes prior to

part formation in the forms of pellets and pow-

ders, etc. High levels of cross-linking would drasti-

cally decrease the melt flow of the polymers andhigh gel contents would make it very difficult or

impossible to process the polymers and convert

them into parts.

A family of polyethylene pellets and powders

were modified by radiation prior to part formation

at lower doses (<25 kGy) and under ambient con-

ditions. The modified resins have low gel content

(<3%). The processibility of the polymers is main-tained with the low dose modification while signif-

icant improvements of practical properties are

achieved [1,2].

Ionizing irradiation of PE polymers may induce

various reactions such as cross-linking, chain scis-

sion, chain branching, oxidation and gas formation.

The radiochemical yield of each event depends on,

among other things, the radiation conditions (typeand energy of the irradiator, atmosphere, tempera-

ture, dose and dose rate, etc.) and the chemical

nature and morphology of the polymer. In order

to better understand the physical and chemical

changes in the polymers after the radiation modifi-

cation, studies were carried out on some of the

irradiated PE polymers with comparison to un-irra-

diated ones. This paper reports the results of someof the studies on an HDPE polymer.

2. Experimental

2.1. Materials

An HDPE homopolymer in white pellet formwith a narrow molecular weigh distribution

(DMDA-8007 from Dow) was used. It has a melt

flow index (MFI) of about 8.0 g/10 min measured

at 190 �C and under a load of 2.16 kg, and a den-

sity of 0.963 g/cm3 or higher. The resin was used as

received.

2.2. Irradiation

For electron beam (EB) irradiation, the HDPE

resin was irradiated using an EB accelerator under

ambient atmosphere and temperature. The EB

accelerator�s beam energy was 12 MeV and the

beam power was 8 kW. The surface dose was

targeted at 8, 16 and 24 kGy. Dose mapping was

carried out using Far-West radiochromic film dosi-meters and the actual average absorbed doses were

determined to be 8.8, 17.6 and 26.4 kGy, respec-

tively, with a dose uniformity ratio (max/min

ratio) of 1.3. The target surface doses will be used

in this paper to refer to ‘‘the EB radiation doses’’

unless otherwise specified. For gamma irradiation,

the HDPE resin was irradiated with a gamma

irradiator under ambient atmosphere and temper-ature. The source activity was 2.2 MCi. The mini-

mum irradiation dose was targeted at 16 kGy.

Dose mapping was carried out using Far-West

radiochromic film dosimeters and the actual aver-

age absorbed dose was determined to be 21.4 kGy.

The minimum absorbed dose will be used in

this paper to refer to ‘‘the gamma radiation dose’’

unless otherwise specified.

2.3. Characterizations

2.3.1. Fourier transform infrared (FTIR)

spectroscopy

FTIR spectra were performed in transmission

on polymer films with a Fourier Transform

Spectrometer, Perkin–Elmer FT-IR 2000. The res-olution was 4 cm�1 or 1 cm�1 and 10 scans were

signal averaged. Films of polymers (about

30 cm2) were prepared by molding them on a press

using heated plates at 200 �C for 30 s with a pres-

sure of 10 ton. The molten sample was rapidly

quenched in water. The film thickness was be-

tween 50 and 100 lm. Spectra are normalized on

1368 cm�1 (absorbance = 0.1).

2.3.2. Gel permeation chromatography (GPC)

The molecular weight distributions (MWD) for

the resin samples were determined using a GPC

2000 V Waters chromatograph instrument with a

set of three columns (HT6E, HT6E, HT2). The

analysis temperature was 135 �C in trichloro-

benzene (TCB) and the injection volume was215.5 lL. The universal calibration was performed

with PS standards. The MWD was verified with

PE NBS 1475 standard.

132 S. Cheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 130–136

2.3.3. Capillary rheometry

An Instron 3211 capillary rheometer coupled to

a Servogo 310 recorder was used. The analysis was

performed at 190 �C.

2.3.4. Cone-to-cone viscosimetry

All experimental work was performed using the

RPA2000 (Rubber Process Analyser) commercial-

ized by Alpha Technologies. The key components

of the device, the die and the test cavity, are illus-

trated in Burhin�s paper [3]. Two types of rheolog-

ical tests have been performed with this equipment

in the linear and non-linear visco-elastic regimes:(1) Linear visco-elasticity: small amplitude oscilla-

tory shear (SAOS): the strain amplitude (c0)was kept below 30% to stay in the linear visco-

elastic regime. The angular frequency was varied

between 200 rad/s and 0.1 rad/s. (2) Non-linear

visco-elasticity: large amplitude oscillatory shear

(LAOS): To investigate the polymer behavior in

the non-visco-elastic regime, the chosen strain

0.015

0.010

0.005

0.000

Abso

rban

ce

1800 1750 1700 1650

Wavenumber (cm-1)

1716

1743

1699

Un-irradiated (control)Gamma irradiatedElectron beam irradiated

Fig. 1. FTIR spectra of un-irradiated, gamma and EB irradi-

ated HDPE samples (carbonyl range).

amplitude was 1000%. The frequency was kept at

0.1 Hz. In all tests, a total of 17 cycles were digi-

tally acquired. The first 10 cycles were discarded.

The last 7 cycles were used for Lissajou figures or

dynamic viscosity calculation.

3. Results and discussion

3.1. FTIR

FTIR spectroscopy for gamma and EB irradi-

ated HDPEs and for the un-irradiated controlwas taken. Fig. 1 shows the carbonyl range of

the FTIR spectra. We can clearly see the appear-

ance of new bands after the EB and gamma irradi-

ation. The two bands located around 1743 cm�1

and 1716 cm�1 are assigned to carbonyl stretch-

ing vibration in ester groups and in ketones

0.010

0.005

0.000

Abso

rban

ce

950 900

Wavenumber (cm-1)

966

908

889

Un-irradiated (control)Gamma irradiated Electron beam irradiated

Fig. 2. FTIR spectra of un-irradiated, gamma and EB irradi-

ated HDPE samples (unsaturated bonds range).

S. Cheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 130–136 133

respectively. The small band that appears around

1699 cm�1 may be assigned to acid end-groups.

The changes indicate that oxidation was induced

by both gamma and EB irradiation and new car-

boxylic functional groups were introduced ontothe polyethylene. Fig. 2 shows the unsaturated

bond range of the spectra. The three bands

annotated in Fig. 2 at 966 cm�1, 908 cm�1 and

(a) HC CH (b) CH2HC CH2 (c)

C

CH2

Scheme 1. Vinylenes (a), vinylic end-groups (b) and vinylidenes

(c) groups.

0.006

0.003

0.000

Abso

rban

ce

3020100Average Dose (kGy)

VinylenesVinylic end groupsVinylidenes

Fig. 3. Dose effect on absorbance of vinylene (965 cm�1),

vinylic end (908 cm�1) and vinylidenes (889 cm�1) groups.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

23456789

Log MW

dw/d

(Log

M)

un-irradiated 16kGy EB irradiated 16kGy gamma irradiated

Fig. 4. GPC graphs for irradiated and un-irradiated HDPE

samples.

889 cm�1 are assigned to vinylenes (a), vinylic

end-groups (b) and vinylidenes (c) respectively [4]

(see Scheme 1). The FTIR spectrum of the un-

irradiated HDPE resin shows two bands related

to vinylic end-groups (b) and vinylidenes (c).Gamma and EB irradiations induce the creation

of vinylene (a) groups and the destruction of some

of the vinylic end-groups (b). The changes of inten-

sities of the three bands are plotted against the

average EB dose in Fig. 3. Fig. 3 shows that with

the increase of the dose the content of the vinylene

(a) groups increases while the content of the vinylic

end-groups (b) decreases. The vinylidene group (c)content remains the same. It is necessary to point

out here that the FTIR results have not provided

2

3

4

567891 0 3

2

3

4

5

67891 04

2

Dyn

amic

vis

cosi

ty (P

a.s)

0.1 1 10 100 1000

Angular frequency (rad/s)

U n -irradiated 3 min 4 hCap.Rheom.

γ - irradiated 3 min 4 hCap. Rheom.

E B irradiated 3 min 4 hCap.Rheom.

Fig. 5. Dynamic viscosity versus angular frequency at 190 �Cwith a strain amplitude c0 < 30%.

Table 1

Molecular weights of the HDPE before and after EB irradiation

Average EB dose (kGy) Mn (Da) Mw (Da) Mw/Mn

0 15,452 112,093 7.25

8.8 13,098 122,368 9.34

17.6 11,073 158,473 14.3

26.4 13,035 241,912 18.6

134 S. Cheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 130–136

sufficient information about the branching on the

polymer chain.

3.2. GPC

GPC graphs of some of the irradiated samples

and the un-irradiated sample of the HDPE are

shown in Fig. 4. A shoulder in the high molecular

mass region can be observed for the irradiated

samples. Table 1 lists the results obtained from

GPC for the number average molecular weight

(Mn), weight average molecular weight (Mw) and

the polydispersity index (Mw/Mn) for the un-irradiated resin and EB-irradiated resins at various

0.1

2

46

1

2

46

10

2

46

100

G(k

Pa)

0.1 1 10 100Angular frequency (rad/s)

Un-irradiatedG' 3 min 4hG" 3 min 4h(theor. slope 1 2)

0.1

2

46

1

2

46

10

2

46

100

G(k

Pa)

0.1 1Angular freq

EB irradiatedG' 3 min 4hG" 3 min 4h(Theor. slope 1 2)

(a) (

(c)

Fig. 6. Storage moduli (G 0) and loss moduli (G00) at 190 �C with a strai

gamma-irradiated HDPE sample and (c) EB irradiated HDPE sampl

dose levels. The polydispersity index (Mw/Mn) of

the resin increases with the increase of radiation

dose, indicating increasingly widened molecular

weight distribution that is probably a result of long

chain branching (LCB). The slight decrease of theMn and the increase of the Mw with the increase of

dose indicate the simultaneous occurrence of chain

scission and branching.

3.3. Rheological analysis

3.3.1. Linear visco-elasticity

Fig. 5 shows the frequency dependence of thedynamic viscosity at 190 �C for the un-irradiated

0.1

2

46

1

2

46

10

2

46

100G

(kPa

)

0.1 1 10 100Angular frequency (rad/s)

γ - irradiatedG' 3 min 4hG" 3 min 4h(theoretical slope 1 2)

10 100uency (rad/s)

b)

n amplitude c0 < 30% of (a) the un-irradiated HDPE sample, (b)

es.

-20.000

-10.000

0

10.000

20.000

Shea

r stre

ss (P

a)

-8 -6 -4 -2 0 2 4 6 8

Shear rate (s-1)

Un-irradiated 190 °C 3 min 4h // 220 °C 20 min16 kGy gamma irradiated 190 °C 3 min 4h // 220 °C 20 min16 kGy EB irradiated 190 °C 3 min 4h // 220 °C 20 min

Fig. 7. LAOS (Lissajou figures) at 190 �C and 220 �C with a

strain amplitude c0 = 1000% and a frequency of 0.1 Hz for

irradiated and un-irradiated HDPE samples.

S. Cheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 130–136 135

sample and samples irradiated by gamma ray and

EB. The capillary and cone-to-cone viscosimetry

results are presented on the same graph. With

the cone-to-cone viscosimeter, the measurements

were performed with a strain amplitude c0 of<30% on the samples that had been stabilized for

3 min and 4 h respectively under small amplitude

oscillatory shear with a c0 of 0.56% and an angular

frequency of 12.57 rad/s. With the capillary rhe-

ometer, the measurements have been performed

once on all the three samples immediately after

melting without any stabilization. When the poly-

mer is gamma or EB irradiated, we observe an in-crease of the dynamic viscosity at low angular

frequencies regardless of the stabilization time.

Resulting experimental storage moduli (G 0) and

loss moduli (G00) are plotted as a function of the

angular frequency in Fig. 6(a)–(c). The graphs

show that the melt elasticity of the HDPE is signif-

icantly increased when it is irradiated. For the

gamma irradiated samples, G 0 and G00 run almostparallel between 1 and 100 s�1. This is indicative of

a ‘‘gel-like’’ behavior in this frequency range and

demonstrates a highly elastic behavior. EB irradia-

tion has a comparable but slightly lower influence

on elasticity and viscosity than gamma irradiation.

Both types of irradiation induce a very large in-

crease in elasticity (G 0), which should translate to

higher melt strength.

3.3.2. Non-linear visco-elasticity

The rheological observations in the linear visco-

elastic regime suggest that the irradiated HDPE

polymers have long chain branching (LCB). To

confirm the occurrence of LCB, we analyzed the

samples in the non-linear visco-elastic regime

employing the large amplitude oscillatory shear(LAOS) method [3]. The experiments were carried

out at 190 �C and at 220 �C with a c0 of 1000%

and a frequency of 0.1 Hz. Before the measure-

ment the samples were stabilized at 190 �C for

3 min or 4 h and at 220 �C for 20 min with a c0of 25% and a frequency of 2 Hz. The results (Lis-

sajou curves) are plotted in Fig. 7. Fig. 7 shows

that in the irradiated samples the loading part ofthe stress signal is significantly more separated

from the unloading part resulting in increased

loop area. The broadening of the loop after irradi-

ation is a signature indication that long chain

branching has occurred after the radiation modifi-

cation [3].

4. Conclusions

We have shown that electron beam and gamma

radiation of the HDPE resin at low dose levels and

under ambient conditions have induced long chain

branching, oxidation and formation of unsatu-rated bonds on the polymer chains. FTIR analysis

shows that oxidation products were formed and

changes of the unsaturated bonds occurred after

the irradiation. The occurrence of long chain

branching after the irradiation is indicated by the

increase of polydispersity index (Mw/Mn), the

increase of dynamic viscosity and the broadening

of the LAOS loading–unloading loop, etc. Thedynamic viscosity of the polymer is increased as

a result of the long chain branching.

References

[1] D. Kerluke, S. Cheng, G. Forczek, RaprexTM: A New Family

of Radiation Pre-Processed Polymers, SPE Polyolefins

Conference, Houston, 2004.

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[2] G. Forczek, D. Kerluke, S. Cheng, H. Suete, T.A. du Plessis,

A Novel Material for Plastic Pipe Applications, Plastic Pipes

XII Conference, Milan, 2004.

[3] H.G. Burhin, in: Progress in Rheology: Theory and Appli-

cations, Grupo Espanol De Reologia, Sevilla, 2002, p. 89.

[4] M. Palmlof, T. Hjertberg, Polymer 41 (2000) 6481.