Thermomechanical behaviour of stabilized polyethylene irradiated with gamma rays

Radiat. Phys. Chem. Vol. 27, No. I. pp, I-5. 1986 Inl. dnl. Radial. Applic. Inxtrum., Part C

0146-5724/86 $3.00 + .00 © 1986 Pergamon Press Ltd.

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THERMOMECHANICAL BEHAVIOUR OF STABILIZED POLYETHYLENE IRRADIATED WITH GAMMA RAYS't LJ. NOVAKOVIC,V. MARKOVlCand O. GAL Boris Kidric Institute of Nuclear Sciences, P.O.B. 522, 11001 Belgrade, Yugoslavia and V. T. STANNE'IW North Carolina State University, Raleigh, NC, U.S.A. (Received 10 March 1985)

Alastract--The moduli of elasticity at 150"C for irradiated linear low density and low density polyethylenes, pure and with 0.5% antioxidants were determined using the penetration technique. Simultaneously, on similar samples, the gel content was measured. Analysing the radiation parameters and comparing data derived from the two methods the efficiency of radiation crosslinking of different polyethylenes and the effect of antioxidants is discussed.

samples were irradiated with 100 to 400 kGy absorbed dose.

INTRODUCTION IN MANYC^SmSof polymer application it is desirable to obtain a degree of crosslinking which corresponds to at least 60-70% gelation. Satisfactory viscomechanical properties are then reached. This is valid for radiation-induced crosslinking as well. This crosslinking effect is usually measured by determination of the gel content in the treated systems, or by measuring their viscoelastic characteristics. in practice most polymer systems contain additives, most often various types of antioxidants. However, the addition of antioxidant affects crosslinking ability of the irradiated polymer. H'2"'~ In investigating the influence of antioxidants on the radiation-chemical behaviour of PE we are interested in how much it is possible to follow the changes in the crosslinking efficiency of the composition by comparing both methods of gel content determination and viscoelastic parameters. In the present work we chose the determination of the moduli of elasticity above the melting point of polymers by measuring penetration in different irradiated systems: a low-density polyethylene and a linear low-density polyethylene, pure and when 0.5 w% antioxidant is added. In the same systems, even in the same samples, we measured gel content by extraction. Both measurements were made when

EXPERIMENTAL

The basic polymers used were low-density polyethylene (LDPE) Lotrene CD-302, CdF Chimie, M,,. = 103.000, p = 0.922 g/cc, and linear low-density polyethylene (LLDPE) Dowlex 2045, M.. = II0.000, p -- 0.920 g/cc; both polymers were additive free. The antioxidants used were: a secondary sulphur containing (Cyanamid Plastanox LTDP), denoted A3, and a primary phenolic type (Ciba-Geigy, lrganox 1010), denoted A5. Blends from polyethylene and an antioxidant were made by mixing two components in a rbeocord type mixer ( 150oc, 5 min, 150 rpm). The same treatment were applied to the pure polymers as well. ~4~ Sheets of 20 x 10 x 2 mm, made by pressing the blended material were packed between two PE sheets and irradiated in a T-source at the 20 kGy/h dose rate. The samples from the same sheets, irradiated and annealed (2h. 80°C) were taken for both thermomechanical measurements and for gel content determinations. For determination of viscoelastic parameters measurements of penetration at constant temperature (150"C) were used on the thermomechanical t Presented at the Fifth International Meeting on Ra- analyser TMS-2, Perkin Elmer. with a spherically tipped quartz probe of radius 0.456 ram. The gel diation Processing, San Diego, CA. Oct. 1984.

[PC 27:1-A

l

LJ. NOVAKOVIC et

content was determined by extraction with boiling xylene.(3) RESULTS AND DISCUSSION To calculate moduli of elasticity E, we used the F i n k i n ' s equation(5) as has been done in the Gillen's experiment(6)

E-

3(1 - v2)P 4R~/2da/2 ,

where P is the load, Rp the radius of the hemispheric tip of the indentor, d the penetration depth, and v the Poisson's ratio. For each sample penetration was measured at three loadings (0.05, 0.10 and 0.15 N) mostly in triplicate. For the values of penetration we used those reached by 1000 seconds time from the beginning of loadings (Fig. 1) for all six systems and for all absorbed doses. By 1000 s the values of penetration were close to equilibrium. In Table 1 the values of E, derived from d~0oos

TABLE 1.

al.

in the corresponding experimental curves and calculated according to the above equation, were presented. For further considerations the average value of E from three loadings (0.05, 0.10 and 0.15 N) were used; the discrepancies in all cases were less than --. 15%. That the values of E average obtained in these measurements and given in Table 1 can be valid for the comparison of the observed systems is shown by the diagram on the Fig. 2, where the variation of the indentation d with the expression p2/3/E2/3R I/3 for all loadings even for all absorbed doses for L L D P E fits into the same straight line.(7) The same is the case for each system studied in this work. This means that in the present experiments, the Young's modulus is constant over the range of the loadings used. In Fig. 3 the values of E for all systems observed in the present work are presented as a function of absorbed dose. It is clear from the shape of the curves that there is a distinctive difference between materials, not only caused by the presence of two (different) antioxidants, but also between two poly-

O F E L A S T I C I T Y , E(MPa), D E R I V E D F R O M P E N E T R A T I O N 1000 s T1ME(150"C) MEASUREDBYVARIOUSLOADINGS

MODULUS

P(N)

100 kGy

150 kGy

0.050 0.100 0.150

0.627 0.650 0.608

0.732 0.865 0.690

200 kGy

AT T H E

300 kGy

350 kGy

400 kGy

1.207 1.302 !.318

1.082 I. 178 0.862

1.596 1.302 1.693

1.492 1.190 1.011

1.595 1.407 1.584

0.650 1.190 1.130

0.605 1.073 1.063

1.004 1.525 1.397

0.487 0.643 0.495

0.676 0.746 0.569

0.839 1.092 1.015

0.439 0.379 0.400

0.426 0.428 0.429

0.439 0.517 0.482

0.650 0.614 0.536

LLDPE 1.027 1.147 1.063

LLDPE + 0.6% A-3 0.050 0.100 0.150

0.297 0.313 0.258

0.426 0.517 0.494

0.050 0.100 0.150

0.404 0.436 0.440

0.564 0.631 0.609

0.657 0.617 0.679

1.002 1.147 0.921

LLDPE + 0.5% A-5 0.536 0.766 0.615 LDPE 0.050 0.100 0.150

0.0769 0.0520 0.0620

0.137 0.0907 0.0843

0.269 0.344 0.353

LDPE + 0.5% A-3 0.050 0.100 0.150

m

0.020 ---

0.050 0.100 0.150

B

0.0325 --

0.103 0.106 0.073

0.210 0.222 0.219

LDPE + 0.5% A-5

I

0.121 0.093 0.091

0.276 0.293 0.275

Thermomechanical behaviour of stabilized polyethylene irradiated with ~ rays

LLDF~ I - 1 5 0 kGy :~ - 3.~OkGy 3 - 4001~y

load

3

0.tm I

i

i

I t

'6

1oos I ~OOOs

I

timllllc}

Fig. 1. Typical thermomechanicai curves, penetration vs time at a constant loading (0.1 N) recorded at 150~C.

mers themselves, LLDPE and LDPE. Although the

moduli increase with increasing absorbed dose in the observed dose range the modulus is higher for LLDPE than for LDPE, and addition of any of both antioxidants causes a decrease of the moduli in the basic polymers. The same general conclusion could he drawn from the experimental data on the gel formation in the same systems, Fig. 4. It is interesting to compare the data obtained by two methods. From Table 2, Which contains the radiation parameters of the systems studied, the different hehaviour of two PE's as well as the influence of the antioxidant is evident: LLDPE shows a high initial rate of gel formation (gel%/kGy) in the dose range Din, - 100 kGy.

The addition of either antioxidant causes (a) an increase in the Ds~ values calculated from the Chariesby-Pinner equation ~s) from the data presented in Fig. 4; and (b) a lowering of the rate of gelation leading to lower moduli of elasticity. With further increases in dose the gel content still increases, but at a slower rate. At 200 kGy the gel content for LLDPE reaches a value of more than 75~. In the 100-200 kGy range the AE per dose is 4.2 Pa/Gy. The effect of the antioxidants is still felt both by lower gel contents and lower moduli of elasticity compared with pure LLDPE. However, the rate of gelation is higher (0.24 and 0.27) compared with pure LLDPE (0.16~'ogel/kOy) whereas the rate of increase in the modufi is still higher with pure LLDPE.

0.50.4.

|

03.

j

,~o

02. 0.1.

Fig. 2. Variation of indentation d with ~ / E 2 n R ~ n.

L J . NOVAKOVlC e t a l .

2

I0

0

0.5:S t3

l

o,t-

0.05

-

tfl

0.02.

Oow (kGy)

Fig. 3. Dependence of moduli of elasticity with absorbed dose for: LDPE pure (O) and with 0.5 w% antioxidant A3 (A) and A5 (El); LLDPE pure (O), and with antioxidant A3 (&) and A5 (1). <.

Irradiations were carried out up to 400 kGy. The gel contents are above 65% in all systems and begin to approach asymptotically the appropriate values of gm~. The moduli show a nearly linear increase with dose in this region. Faster rates of increase of both gel content and moduli are found in the blends than with pure L L D P E . The values of gm~ calculated by the Inokuti relationship (9) from the data shown in Fig. 4 are decreased when either antioxidant is added. These values are quite close to each other for all three systems. It might be explained by decreases of the effect of antioxidant at the prolonged irradiations. The gel-dose curves converge as is similar in the case with the changes of moduli, E.

The same trends are manifested in the pure L D P E and its blends. Only all corresponding values are lower. L D P E is less susceptible to radiation crosslinking and its blends even less. The different behaviour of two P E ' s and their blends over irradiations to a 400 kGy dose is also illustrated through the changes of Me, the average molecular weight between crosslinks. Using the theoretical formula for rubberlike elasticity E, (l°) E

=

3pRTIM,,,

where p is the polymer density, R is the gas constant, and T the absolute temperature; Mc values were derived from our experimental data, Fig. 3.

9O

~6o

z6o

36o

~oo

D~e(kGy)

Fig. 4. Gel content as a function of absorbed dose for PE, pure and with 05 w~ antioxidant: LDPE (O); LDPE + A3 (A); LDPE + A5 (V1); LLDPE (O); LLDPE + A3 (&); LLDPE + A5 (I).

Thermomechanical behaviour of stabifized polyethylene irradiated with "y rays TABLE 2. RADIATIONPARAMETERSOF PE SYSTEMSSTUDIED D = 100 kGy

Ds~l gel kGy %

System

gmz

gel E Agel AE % MPa %/kGy Pa/Gy

gel %

Agel AE %/kGy Pa/Gy

%

0.95 0.84 0.80

76 1.02 0.16 66 0 . 7 5 0.24 63 0 . 6 3 0.27

4.2 3.3 3.5

83 80 78

0.07 0.14 0.15

2.5 3.8 4.0

91 88 86

0.74 0.67 0.57

63 0.31 0.20 52 0.11 0.25 44 0.08 0.29

2.5 ---

74 68 66

0.11 0.16 0.22

3.0 2.6 2.0

82 69 68

LDPE LDPE + 0.5% A5 LDPE + 0.5% A3

42 60 74

43 0.05 27 unmeasurable 15 "

VALUES, DERIVED FROM THE M O D U L I O F ELASTICITY E AT 150°C I00 kGy 200 kGy 300 kGy 400 kGy

171143 30355 16218 -85724 29451 -117661 39220 15654 22357 33532

200-400 kGy

0.60 0.42 0.28

60 41 36

LLDPE LLDPE + 0.5% A5 LLDPE + 0.5% A3

D = 400 kGy

Agel %/kGy

37 51 55

LDPE LDPE + 0.5% A5 LDPE + 0.5% A3

100-200 kGy

E, MPa

LLDPE LLDPE + 0.5% A5 LLDPE + 0.5% A3

TABLE 3. THE ~

D~t - 100 D = 200 IGy kGy

9211 6957 12523 8538 1 4 9 0 4 9295

10346 14703 19202 6137 6659 6857

of the viscoelastic retardation spectra at various temperatures would be extended to give additional insight into the problems raised in this investigation.

Acknowledgment--Tiffs work was supported by the Fund of Yugoslav-U.S. Joint Board on Scientific and Technological Cooperation through U.S. NSF; grant number YOR84/079. REFERENCES

Table 3 contains the Mc values for all systems studied over the given dose intervals. L L D P E exhibits a much higher ability for radiation crosslinking. The Mc after 100 kGy absorbed dose is lower than for LDPE at 300 kGy. L L D P E shows an initial rapid increase whereas LDPE crosslinks more slowly. With both polymers A3, the secondary antioxidant, decreases crosslinking more than the primary phenolic antioxidant, AS. Both antioxidants are more effective with LDPE than L L D P E making the differences in the crosslinking efficiencies even larger. In this paper a considerable amount of new data has been presented. It is clear that further studies are needed to explain some of the results. A study

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