Polymer Durability and Radiation Effects - American Chemical Society

Chapter 7

Lifetime Prediction of Elastomer for Radiation-Induced Degradation by Time Accelerated Method

Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch007

Masayuki Ito Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

The short time test was performed by the irradiation of Co-60y ray up to 10 MGy at 70• in air. Dose rate was 5.0 kGy /h. The samples used were ten kinds of EPDM that have different compounding formula. After irradiation, the mechanical properties were measured at room temperature. "ModulusUltimate elongation plotting" evaluated the ratio of scission and crosslinking occurred during irradiation. The ratio differed greatly on the compounding formula. "Tensile Energy (TE)" was proposed as a criterion for estimating the life time of elastomer. TE is the area of stress-strain curve which decreased with increasing dose. The dose at the life time where ultimate elongation decreased to 100% or TE decreased to 500 %-MPa was 4 MGy for the most radiation resistant EPDM in the experiment.

© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Electric wires and cables used in nuclear power plant are exposed by low dose rate irradiation during the life time of the plant. In addition, loss of coolant accident (LOCA) that is a design basis accident brings about the degradation by heat and radiation on the electric wire and cables. The dose varies from plant to plant. IEEE std.323-1974 (7) estimated the dose 0.5 MGy for the period of the life time and 1.5 MGy for LOCA as one of the example. The standard estimated 40years and one year, for the lifetime of plant and duration of LOCA respectively. The time accelerated aging method for electric wires and cables requires a high dose rate irradiation, but the exposure to polymer in air results only the oxidation of surface when the dose rate is higher. I previously reported the methodology study of time accelerated irradiation of elastomer (2). The study showed two appropriate methods, one was irradiation in pressurized oxygen at room temperature; the other was irradiation at 70°C in air. The article studied the effect of higher dose (up to 10 MGy) irradiation on ethylene-propylene-diene elastomer (EPDM) by using the time accelerated method. Irradiation at 70°C in air was chosen as the time accelerated irradiation condition because of the experimental convenience.

Experimental Materials EPDM is used for the insulating material of electric cable in nuclear power plant. As the properties of EPDM depend on their compounding formula, there are many receipts for improving heat resistant properties, durability against oil, resistance to ozone, and so on. Table I-V shows the compounding receipt of the samples used in this study.In th etable phr means pers per hundred resin (rubber). The thickness of the samples was about 0.5 mm. EPDM-3, 4, 5, 6 and 7 are the series of the compounding formula of the curing by low sulfur. EPDM-2 contains the relatively higher sulfur with Hypalon (Chlorosulphonated polyethylene) that increases tensile strength of elastomer. EPDM-1, 8 and 10 were crosslinked by peroxide. Triallyl isocyanurate in EPDM-1 is a multi-functional compound that increases the concentration of network.


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Table I. Compounding Formulation of EPDM-1 Ingredient Nordel 1070 (Du Pont) EP11 (JSR) Triallyl isocyanurate Dicumyl peroxide Zinc oxide Ulltrafine magnesium silica Hard clay Carbon black (FEF) Process oil 2-Mercaptobenzoimidazole Polymer of 2,2,4-trimethyl-l,2 dihydroqinoline

phr 70 30 2 2.5 5 20 20 20 5 4 1

phr : Parts per hundred resin

Table II. Compounding Formulation of EPDM-2 Ingredient Nordel 1070 (Du Pont) Hypalon Sulfer Zinc oxide Stearic acid N-Cyclohexyl-2-benzothiazolyl-sulfide Tetramethylthiuramdisulfide Nickel dibutyldithiocarbamate Hard clay Ulltrafine magnesium silica Carbon black (FEF) Process oil

phr 100 5 1.5 10 1 1.5 0.5 2 20 20 20 5


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Table III. Compounding Formulations of EPDM-3,4,5,6,7,9 EPDM Ingredient Nordel 1070 (du pont) Tetramethylthiuramamidisulfide 4,-4'-Dithiodimorpholine Zinc dimethyldithiocarbamate Zinc di-n-butyldithiocarbamate Sulfer Selenium Zinc oxide Carbon black (FEF) Clay Ulltrafine magnesium silica Aromatic oil Parafifinic oil Parafine Nickel dibutyldithiocarbamate 2-Mercaptobenzoimidazole Polymer of 2,2,4-trimethyl-l,2 dihydroqinoline

7

4 5 6 3 phr phr phr phr 100 100 100 100 3 3 3 3 2 2 2 2 2 2 2 2 2 2 2 2 0.5 0.5 0.5 0.5 3 5 5 5 5 30 30 30 30

100 3 2 2 2 0.5 3 5 30

15

15 20

20

2

2

9 phr 100 3 2 2 2 0.5 5 30 70 15

20 5 2

2 4

4

1

1

Table IV. Compounding Receipt of EPDM-8 Ingredient Nordel 1070 (du pont) Dicumyl peroxide Zinc oxide Lead peroxide Carbon black (SRF) Silane coated clay Parafinic oil Parafine Vinyl-tris (beta-methoxyethixy) silane. Polymer of 2,2,4-trimethyl-l,2 dihydroqinoline

phr 100 2.5 5 3 4 110 15 5 1 1.5


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Table V. Compounding Receipt of EPDM-10 Ingredient Esplen 301 (Sumitomo Chemical ) Dicumyl peroxide Zinc oxide Sulfer Stearic acid Talc Sonic 2000 p-Quinone dioxme Polymer of 2,2,4-trimethyM,2 dihydroqinoline

phr 100 2.8 5 0.3 100 20 0.5 0.5

Irradiation Conditions The irradiation was performed by Co-60 γ ray at 70°C in an air oven. Dose rate was 5.0 kGy/h; the maximum dose was 10 MGy.

Tensile Testing After irradiation, sample sheets were cut into a dumbbell shape (Japan Industrial K6301 Tensile testing standard, No.4) by using new type of cutter (Dumbbell co.). Tensile test were performed using Shimazu Model SC-500 Test Machine equipped with pneumatic grips. Samples were strained at ambient temperature at 500 mm/min using an initial jaw distance of 50 mm. No sample was cut near the grips in the tensile test, because the cutting procedure didn't give the defect on the cut surface. At each aging condition four samples were tested and averaged to obtain the results.

Result and Discussion Ultimate Elongation There were two types of the changes in ultimate elongation by irradiation. In the first type, ultimate elongation decreased almost straight with increasing dose during earlier period of irradiation as shown in Figure land 2 . The change was not the same by the samples after irradiation of 4MGy shown in Figure 1 and irradiation of 2MGy in Figure 2. In the second type, ultimate elongation


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decreased rapidly by irradiation, but it remained the same value despite the increasing dose as shown in Figure 3. This type was not durable for irradiation. The latter section shows a detailed discussion of the reasons.


Tensile Strength The relationship between tensile strength and dose depended largely on the compounding formula. Irradiation increased tensile strength of EPDM-4, 6 and 7 except the earlier stage as shown in Figure 4. Figure 5 shows one of the typical changes in tensile strength by irradiation. Tensile strength decreased significantly by relatively low dose irradiation, but the value started to increase from a certain dose. EPDM-8, 9 and 10 were the other types that decreased the tensile strength with increasing dose as shown in Figure 6. Tensile strength is an important mechanical property of elastomer. Five kinds of EPDM in the experiment kept the higher tensile strength at lOMGy than initial values showing the durability of the samples for irradiation. On the view point of the life time prediction, tensile strength is not the appropriate factor, because there are three types of the relationship between the stress (dose) and the response (values of tensile strength) as shown in Figure 4, 5 and 6.

Relationship between Modulus and Ultimate Elongation We studied the effect of quantitative addition of scission and crosslinking on the changes of the mechanical properties of elastomer and showed how the ratio of scission and crosslinking occurred during aging affects the mechanical properties (4). On the basis of the study, "Modulus-Ultimate elongation plotting" was proposed to calculate the changes of the ratio during the aging (5). Figures 7, 8 and 9 show Modulus-Ultimate elongation plotting of ten kinds of EPDM. The simplest case is shown in Figure 7. Crosslinking was predominant in EPDM-2 throughout the period of irradiation. Figure 8 shows the plotting for EPDM-3 to 7. During the earlier stage of the aging, ModulusUltimate elongation plotting of the samples went down along Y axis showing the ratio of scission and crosslinking was the same. Crosslinking became to predominate from the certain period in all of five samples. It is pointed out in the thermal degradation of polymer that as the radical concentration increases with passing the aging time, crosslinking reaction increases by the recombination of the radicals. A certain type of radiation induced degradation of elastomer may have the same tendency. Figure 9 shows the other types of modulus-ultimate elongation relationship. Irradiation raised


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1000 —D-EPDM-2 -0-EPDM-4 -A-EPDM-6 -V-EPDM-7

500" •

C


g "ία σ> o looUJ

Β I 5

so;

\ γ 100

2

M

b

fi

10

Dose / MGy

Figure 1. Decrease in ultimate elongation of EPDM-2,4,6,7.

10-Ή—ι—ι 0 2

1—ι—ι—ι—ι M b

1 6

1

r~ 10

Dose / MGy

Figure 2. Decrease in ultimate elongation of EPDM-1,3,5,9.

18

Dose / MGy

Figure 3. Decrease in ultimate elongation of EPDM-4,6,7.

Dose / MGy

Figure 4. Changes of tensile strength of EPDM-8,10.


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EPDM-8 - • - EPDM-9 EPDM-10

1

1

1

1

τ—•—ι——ι—·—ι——ι——ι—•—ι—— 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.

0

2

Dose / MGy

4

6

8

Figure 6. Changes of tensile strength of EPDM-1,2,3,5.

Figure 5. Changes of tensile strength ofEPDM-8,9,10.

• ο Δ V Ο C Ο

ο

10

Dose / MGy

EPDM-3 EPDM-4 EPDM-5 EPDM-6 EPDM-7

200

Ο LU

LU Φ

% 100Ε S 50 3

Ζ

3 «» 50% Modulus / MPa

Figure 7. Modulus-Ultimate elongation correlation of EPDM-2.

ι

ίο 50% Modulus / MPa

Figure 8. Modulus-Ultimate elongation correlation of EPDM-3 to 7.


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almost the same amount of scission and crosslinking in the earlier aging stage of EPDM-1, but scission reaction predominated in the latter part of aging. EPDM8, 9 and 10 had the similar form in Figure 9. Crosslinking predominated in the low dose region and scission predominated from the certain dose. The scission lost the network concentration of elastomer and generated the low molecular chain. The large amount of scission breaks crosslinking structure of network chain in the elastomer and the sample become the mixture of noncrosslinked molecular chain and the small gel that was the part of crosslinking structure. EPDM-8 and 10 kept the values of ultimate elongation during the period from 1.0 to 2.5 MGy as shown in Figure 3, but the modulus decreased remarkably and the sample lost elasticity. In this case, the elongation in combination with modulus information may be useful for life time prediction.

Tensile Energy Ultimate elongation of elastomer commonly decreases with increasing the period of aging time. Therefore, ultimate elongation is considered to be the factor that evaluate the life time of elastomer. Both scission and crosslinking of molecular chain resulted in decrease of ultimate elongation but the rate of decrease in ultimate elongation was slower in scission predominant condition than in crosslinking predominant condition (4). Considering the evaluation of the life time of elastomer, Samay et al. pointed out that tensile strength has the same importance as ultimate elongation has. They proposed "Braking Energy (BE)" to estimate the life time of elastomer (5). Equation 1 defined BE. BE= (Ultimate elongation) χ (Tensile strength)

(1)

As BE dose not contain modulus in the concept, the losing of elasticity of elastomer is not accounted. Hence, we proposed "Tensile Energy (TE)" as a factor to estimate the life time of elastomer (6). TE is the area under the stressstrain curve. Figure 10a and 10b suggest the difference between BE and TE. For both stress-strain curves, BE is 350 MPa-% where TE in Figure 10a is 125 MPa% and TE in Figure 10b is 200MPa%. The stress-strain curve of the elastomer shown in Figure 10b looks like it is tougher and more durable than that shown in Figure 10a. Therefore, TE is adequate to evaluate the degree of degradation of elastomer. Figure 11, 12, and 13 show the changes of TE by irradiation. TE of all samples used in the experiment decreased with increasing dose. The result suggested that TE could be a factor for estimating the life time. There were three types of the relationship between TE and logarithm of dose. TE of EPDM-9 shown in Figure 11 accelerated the decay ratefromabout 3 MGy. On



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Figure 9. Modulus-Ultimate elongation correlation of EPDM-1,8,9,10.

the other hand, four samples shown in Figure 12 slowed down the rate of decrease in TE from a certain dose. In the third type, the relationship between TE and logarithm of dose was the linear as shown in Figure 13. Only the third case can define the rate of decrease of TE by irradiation from the slopes. There are two considerable methods to estimate the life time by observed TE. The one takes the dose that the TE retained reaches a certain value. The other takes the dose that TE decrease a certain value. The former notices the change of the relative value in TE, the latter takes note of the value of TE decreased by irradiation. The article takes the life time of elastomer when TE decrees to 500 MPa-% at which the elastomer is tough enough, considering the stress-strain curve in Figure 10b. Figure 14 shows the map of the dose at which TE decreased to 500 MPa-% and the dose when ultimate elongation (Eb) decreased to 100% for all of the samples. Conclusion It was determined that the most radiation resistant sample in this study was EPDM-6, that kept 500 % MPa tensile energy at about 4 MGy and kept 100 %



MD

bO

60 100

ISO

140

Figure 10a. Relationship between BE and TE, BE=350 MPa-% TE=125 MPa%

Strain / %

0

M -

M -

0

S

s

20

MO

00

Strain / %

t.0

100

150

1M0

Figure 10b. Relationship between BE and TE, BE=350MPa% TE=200MPa%

SO



81


82 ultimate elongation at about 6 MGy which is twice the dose of the highest reported value (7).

References 1. IEEE Std.323-1974, IEEE standard for qualifying Class 1E equipment for nuclear power generating stations, IEEE, 1974. 2. Ito, M . Nucl Instr. and Meth. in Phys. Res. 2005, B236, 229. 3. Ito, M . Nucl Instr. and Meth. in Phys. Res. 2005, B236, 231. 4. Ito, M . J. Soc. Rubber Ind. Japan 1985, 59, 169. 5. Samay, G.; Palotas, L.; Seregely, Zs. 4 International Seminar on Elastomers (Kurume, Japan). 1990, 333. 6. Ito, M . Material life, 1995, 7, 195. 7. Seguchi, T.; Morita, Y. Radiation resistance of plastics and elastomer, Polymer Handbook, 1998, VI 583.


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Polymer Durability and Radiation Effects - American Chemical Society

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