- Email: [email protected]
The effect of gamma-irradiation on the electrical properties of two typical epoxy resin systems
0146-5724/91 $3.00+ 0.00 Copyright © 1991 PergamonPress plc
Radiat. Phys. Chem. Vol. 37, No. 4, pp. 581-588, 1991 Int. J. Radiat. Appl. lnstrum., Part C
Printed in Great Britain. All fights reserved
THE EFFECT OF GAMMA-IRRADIATION ON THE ELECTRICAL PROPERTIES OF TWO TYPICAL EPOXY RESIN SYSTEMS R. A. FOUp.xc~ 1, H. M. BANFORD2, D, J. TEDFORDl, S. GEDEON1, X. CAO 3, S. W u 3 and L. F u 4 ~Department of Electronic and Electrical Engineering, University of Strathclyde, George Street, Glasgow GI IXE, U.K. 2Scottish Universities Research and Reactor Centre, Birniehill, East Kilbride, Glasgow G75 0QU, U.K. 3Department of Electrical Engineering, Xian Jiaotong University, Xian, Shaanxi Province, People's Republic of China 4Harbin Institute of Electrical Technology, Harbin, People's Republic of China Almtract--Measurements of the effects of y-irradiation on two epoxy systems have been undertaken upto doses of 2 MGy. Changes have been shown to occur in the ra~sured values of tan 6 and these are related to the dose. In one resin the value of tan 6 measured was increased and attributable to d.c. conductivity. In the other resin, which showed a loss peak in the plot of tan 6 vs temperature, changes were associated with crosslinking and these were dose dependent. A factorial experiment on one resin system has shown that dose rate effects as well as effects due to different hardener to resin ratio occur, as measured by changes in Ts and the onset of d.c. conductivity effects on tan 6 measurements.
1. INTRODUCTION Resin systems are used extensively as electrical insulation and for encapsulation of electronic circuits. In certain applications they can be exposed to ionising radiation which affects the performance of the resin system. This might be either in the form of a serious curtailment of the operational lifetime of the material o r a decreased performance which may have significant operational consequences. Situations in which this could occur are in the superconducting magnets proposed for fusion reactors, the instrumentation in fission reactors which could be exposed to a loss of coolant conditions in the reactor containment vessel and on space satelites. It is therefore important to establish the behaviour of these materials when exposed to radiation at dose levels equivalent to that experienced during the projected useful life o f the material. This can involve doses as great as several MGy, Investigations have therefore been undertaken to study the changes in electrical behaviour of an expoxy resin system after exposure to y-irradiation from a Co e° source for dose levels upto 2 MGy. Two resin systems were investigated. These were based on an epoxy resin of diglycidyl ether of hisphenol A (DGEBA), manufactured by Ciba Geigy as MY750, This was cured using one of two hardeners. The first was a hydroxyalkylated polyamine (Ciba Geigy HY956)used in the proportion of 4 parts of resin to 1 part of hardener by weight, The second was a dodecenyl-succinicanhydride (DDSA) used in conjunction with benzyldimethylamine (lphr) as an accelerator, The proportions were 1:1.3:0,0,1 by weight. 581
The resin material was made by mixing weighed amounts of resin and hardener which had been kept for a period of time under a reduced pressure to remove both dissolved water and atmospheric gases (Gedeon, 1988). The resulting mixture was cured according to a fixed temperature schedule (Table 1). Samples were in the form of 1 or 1.5 crn discs of thickness 0.6 mm. Electrodes were formed by the vacuum deposition of aluminium. Samples used for d.c. conductivity measurements had a guard ring surrounding the low voltage electrode. 2. SAMPLE PREPARATION AND EQUIPMENT Measurements of loss tangent (tan 6) were made using a General Radio bridge type 716-C. d.c. current measurements were made using a Keitldey electrometer type 602 and a Brandenburg photomultiplier d.c. voltage supply type 485R. Specimen temperature control was by using either a hot air oven in the case of the dielectric measurements or an electronically controlled hot stage in the case of the d.c. current measurements. 3. RESULTS 3.1 Resin system M Y 7 5 0 / H Y 9 5 6 : y-irradiation effects
Figure 1 shows the effect of y-irradiation on the loss tangent (tan 6) as a function of temperature. The measurements were made at 200 Hz. Two sets of data are shown, the first is for a relatively low radiation dose of 0.5 MGy, the second for a dose of 2.0 MGy.
582
R. A. FOURACR~ et al. Table 1. Curing schedule Time Temperature (h) (°C) (=) 5-10 22
Epoxy sytem
(b) I (c) 2
MY750/HY956
4O 6O
(d) 10-15
100
(a) 5-10 (b) 1 MY750/DDSA (c) 2 (d) 10 Note: The above procedures are sequential.
22 40 6o 90
Each set of data consists of three curves obtained from the same sample. These represent measurements made prior to irradiation, after irradiation and after irradiation and annealing. Annealing consisted of heating the sample in a hot air oven at 90°C for a period of 3 h with the electrodes connected together electrically (short circuit condition). In the case of the sample exposed to 2.0 M G y it proved impossible to make measurements on the sample after exposure but prior to annealing. This was because the values of tan were outside the range of the bridge used, however measurements were possible after annealing. The feature of all these curves is that after irradiation there is a considerable increase in the measured values of tan ~ at a given measurement temperature. The effect of post-irradiation annealing w a s to lower the value of tan & The value was not reduced to that measured for the same sample prior to irradiation. Owing to the form of the changes in tan ~ as a function of temperature it was necessary to present the data a little differently. This was achieved by plotting the logarithm of the ratio of tan ~ measured after irradiation and annealing to that measured prior to radiation exposure. Figure 2 shows such a plot. The measurements were made at two different frequencies and several different temperatures. Each curve has been derived from measurements made on different samples but at the same temperature. As can be seen
the curves initially take negative values and then rise to large positive values. The increase represents a degradation process for the material but the value of tan ~ cannot be used as a measure of the degree of degradation because it is dependent on the precise measurement conditions. It is thought for example that the initial decrease in tan 8 is linked in part with increased cross-linking in the epoxy resin system. It is by no means obvious from the data presented in Fig. 2 to what exposure level this is an important factor. This is because the point at which the curve crosses the axis is dependent on the measurement frequency. It is possible to extend the measurements described above by making them over an extended temperature range. Figure 3 depicts two sets of data made from samples exposed to two different doses of radiation. Each set consists of three curves for the three different measurement conditions described above. The curves are similar in shape. They consist of two regions; one in which relatively small values of tan ~ occur, this being confined to the lower temperature portion of the curve and the other in which the value of tan increases rapidly with temperature. As can be seen the effect of irradiation is to displace the tan J curve to lower temperatures. Post-irradiation annealing displaces the curve to higher temperatures. If the radiation dose is large, the displacement of the curve due to the annealing is insufficient for it to reach its position prior to irradiation. On the other hand for the lower radiation dose the annealing has the effect of displacing the curve to higher temperatures than originally measured. It is possible to define a temperature TA as the point of intersection of the tangent drawn to the two sections of the tan 8 curve (Fig. 3). The effect of both irradiation and annealing may then be described in terms of T^ in much the same way as effects on polymer structure may be measured in terms of the glass transition temperature Ts (Blythe, 1979).
(a)
(b) 1.0
200Hz 0.5 MGy
0.100
oO
irradiation ~
200Hz 2.0 MGy
0.8
0.075
After
anneatin9
0.6
0.050
0.4 /
.Before
0.025
0.2(~
0 20
~ / /
0
_ 0--"-"~-" ~ A f t e r annealing
~
I
=
I
40
60
80
0
~
Before irradiation ~"
7
y
i
7
20 40 60 80 Temperature (°C) Fig. 1. y-Irradiation effects on measured loss tangent as a function of temperature. Epoxy resin system: MY750/HY956.
y-Irradiation and electrical properties of epoxy resin systems (a)
583
(b) BI : Before irradiation
2.0
AA: After irradiation and onneoUng 1 : 25"C
i-t
2 40°C ~- S~'C
"~ 1.s
/
4 m "fO"C 5 - 85"C
dO to
5 / jm/
1 - 25°C
/n
5
/
////
o..,
~. 1.o
4
/
o B-
"3
2OkHz
////'
/
o,_
./
j/J./.
O~
I
I
I
I
I
I
I
I
I
I
I
0.4
0.8
1.2
1.6
2.0
0
0.4
0.8
t .2
1.6
2.0
Irradiation dose [MGy)
Fig. 2. 3'-Irradiation effects plotted as the logarithm of the ratio of tan 6 after irradiation and annealing to tan 6 prior to radiation exposure. An explanation for the behaviour described above can be made in terms of the production of ions during the irradiation; the loss of some of these ions through the annealing process as well as radiation induced polymer cross-linking (Wu et al., 1988). The rapid increase in tan 6 with temperature is associated with the d.c. conductivity of the system. The reason for this is that it is, difficult to associate the large values of tan 6 with a dipole loss mechanism and consequently the increase can not be due to a loss peak. Such a dipole loss mechanism is illustrated in Fig. 4 for a different resin system. The tan 6 values are considerably smaller and is typical of such loss mechanisms. Tan 6 for a Debye type process may be described by an equation containing two terms: 6d.c.
.
second term is due to a.c. loss mechanisms. As the frequency decreases th~ relative importance of the d.c. conductivity term increases until at low frequencies it can predominate. It is already known that for the present resin system the d.c. conduction mechanism is ionic in nature and is an activated process (C-edeono 1988). If it is assumed that for the present measurements, made at a frequency of 200 Hz, the d.c. conductivity is predominant for temperatures above T^ then an Arrhenius plot of log tan 6 as a function of 1/T should be a straight line with an activation energy identical to that for the d.c. conduction process. Figure 5 shows such a straight line plot with an activation energy of 1.23 eV. This may be compared with that of 1.0 eV obtained from measurements of d.c.-current as a function of temperature at a fixed d.c. voltage (Fouracre et al., 1991). In view of the limited temperature range over which it was possible to make the relevant tan 6 measurements the two values are not considered to be in disagreement. It is also found that the tan 6 curve is
E"
tan <~= ~ - ~ o + ~7
where 6d~, is the d.c. conductivity and ~o is the angular frequency of the applied voltage, ~' and ~" are the real and imaginary parts of the complex permittivity. The
(a)
(b) 1 Before irradiation 2 After irradiation 3 After anneoUng
0.15
3
0.10 --
0.05 -
/
// A ,
-150
1
f
-100
-50
O
I T,1 50 Temperature
1OO
IT 0
50
TA1 I I TA3 1OO
PC]
Fig. 3. Tan 6 as a function of temperature. Effects of irradiation and annealing. (a) High dose 2.0 MGy; (b) low dose 0.5 MGy.
584
R. A. Fou~cp~ et al.
0'07 t
v D 0 o
ZMGy
B.L A.A, 0.2 kHz B.I. A.A. 20 kHz
oo 0.05 /
0.0~ /
0,0' 40
I
I
I
60
80
1O0
Temperot ure PC)
Fig. 4. 7-Irradiation effects on measured loss tangent as a function of temperature. Epoxy resin system: MY750/DDSA. displaced to higher temperatures as the measurement frequency is increased, as predicted by the equation for tan ~. This is a result of changes in the relative importance of the a . c . and d.c. conductance terms which are frequency dependent. Thus as the d.c. component is reduced in importance with increasing frequency the tan ~ curve will be displaced to higher temperatures. This phenomenon is also the explanation of the frequency dependence seen in Fig. 2 where the higher frequency curves show a lower value of tan ~ for the same dose and temperature. In both cases the ion production during irradiation would be identical and the differences are caused by the frequency dependence of the d.c. conductivity term.
-2.5 ~x -3.0
\
x
\
x
\
8O 8
-3.5
-4.0
-4.5
Activotton
I 2.66
x\ x\ x\
The increased values of tan ~ for the curves measured at higher temperature are due to the increase in d.c. conductivity which is governed by a thermally activated process. Values of tan J and conductance have been determined for a curing resin sample. In this situation both the conductance and tan ~ vary with time (Fouracre and Fu, 1988). The tan J measurements were made at 200 Hz and at a temperature sufficiently high to ensure that the measurements were made above TA. The sample was solid. As is seen in Fig. 6 the tan curve and the conductance curve are of identical form. Thus changes in conductance are reflected in the changes in tan <~for this particular system. From the above considerations it is reasonable to conclude that for the present system the increase in tan ~ above T^ is due to d.c. conduction processes. Since T^ is determined by the onset of the dominance of d.c. conductivity it will also reflect d.c. conductivity changes in response to external factors. The changes in TA produced by the ~-irradiation may therefore be explained in terms of changes in d.c. conductivity produced by that irradiation. During irradiation two types of ion are produced, namely small mobile ions and ions which are attached to the polymer chain and which are not mobile (Wu et al., 1988). After irradiation the d.c. conductivity of the material has been enhanced by the presence of these mobile ions and as a result the tan J curve is displaced to lower temperature; increasing the dose of radiation increases the number of small ions produced resulting in a greater d.c. conductivity with a consequential larger displacement in the tan 5 curve. This is seen in Fig. 3. Annealing in the manner described previously causes the curve to be displaced to higher temperatures. It may thus be inferred that the d.c. conductivity term has been reduced in importance. This can be due to two different reasons. The first is that the mobile ions have been discharged, thus decreasing the number density of the conducting species and hence the specimen conductivity. The second process is that of polymer cross-linking. This reduces the free -8.8 -9.0 ~
x\
energy1.23eV
I Z,70 IO00/T (K)
-1.0 -1.1
-9.2
x\ x\
94
X\x
tom~~~~._
-9.6
=\
-1.4 -1.5
2.75
Fig. 5. Arrhenius plot of tan 6 as a function of I / T for t©mperatures greater than T^. Epoxy rain system: MY750/HY956.
g
-1.3 *"
60
120 160 20,0 300 360 Curing time (rain)
Fig. 6. Measurement of tan 6 and conductance for a curing resin sample. Epoxy resin system: MY750/HY956.
585
T-Irradiation and electrical properties of epoxy resin systems volume and consequently makes it more difficult for the ions to pass through the material. The crosslinking process occurs during the annealing since segments of the molecular chain are capable of rotation at the annealing temperature which is close to Ts. This enables adjacent chemically active portions o f the resin structure produced by the radiation to cross link. Both these processes can occur concurrently during annealing. For the material exposed to the lower level of irradiation the curve is displaced to a higher temperature than that measured for the non-irradiated sample and needs to be explained. The d.c. conductivity in the unexposed sample is thought to be due to ionised NaCI, present as a residual contaminant as a result of the manufacturing process (Gedeon, 1988). The process of irradiation and annealing will not affect the concentration of these ions. Thus the conductivity decrease indicated by the curve shift to higher temperature cannot be due to the discharge of these ions. The most likely cause is the cross,linking effect. This is supported by the increase in measured Tg after annealing. For the more heavily irradiated material the curves do not recover after annealing to their un-irradiated state (Fig. 3). The explanation of this is two-fold. In the first place ~radiation causes both chain scission, which increases the free volume thus enabling more rapid transport of ionic material and results in increased condactivity, and additional cross-linking which has the reverse effect. The predominant process is determined by the radiation conditions. Annealing enables some cross-linking to occur which results in decreased conductivity as well as discharge of some of the mobile ions. The recovery is not complete as is evident from T, measurements and thus the tan curve would not be expected to recover completely. Secondly, ions are produced which are attached to the polymer chain. These are not mobile and cannot be discharged at the electrodes during the annealing process. They can act as trapping centres for highly polar water molecules. These in turn would cause increased dissociation of any ionic material present because of the high dielectric constant of water. Annealing can cause some of this trapped water to be lost from the material thus decreasing the conductivity. Even prolonged annealing at high temperature does not cause the tan 6 curve to be displaced sufficiently to return to its original position and even a relatively short exposure to normal laboratory air produces a shift of the curve to lower temperatures (Wu et al., 1988). The material has become more sensitive to the presence of water vapour in the surrounding environment. It should be realised that the two processes, chain scission and ion production, are not mutually exclusive.
3.2 Resin system MY750/DDSA: T-irradiation effects Figure 4 shows the results of tan 6 measurements taken as a function of temperature at several different RFC37/4..-D
fixed measurement frequencies. Also shown is the effect of exposure to I MGy of irradiation. The curves show that there is a loss peak the position of which depends on the measurement frequency. This is the case for both irradiated and unirradiated samples. The effects of the irradiation causes the loss peak observed at a given frequency to be displaced to higher temperatures. If the loss peak is assumed to be due to a Debye type loss mechanism then each frequency is related to an activated relaxation time z (Blythe, 1979). By plotting z as a function of I/T=, where T= is the temperature of occurrence of the maximum value of the tan 5, an Arrhenius type plot is obtained (Fig. 7). The activation energy measured from this graph for non-irradiated samples is 2.36 eV. This is the mean value for three different samples. It will be argued in Section 3.3 that measurements of activation energy as a measure of the effects of T-irradiation are not reliable. For this reason it is 0nly possible to compare measurements between irradiated and non-irradiated values of activation energy made on the same sample. Figure 8 shows the effect of the dose of gamma irradiation on the change in activation energy for the loss peak process. This has been derived from three separate samples; the measurements were made before and after irradiation. The activation energy change increases with increasing dose. Both this and the peak shift in temperature is consistent with a radiation induced cross.linking. The loss peak is assumed to be associated with the rotation of polar groups attached to the main polymer chain. Ts measurements have shown that after irradiation the value of Ts is higher which is again consistent with such cross-linking effects.
3.3 T-irradiation effects: factorial regression experiments, MY750/HV956 system In order to investigate the influence on epoxy resin system exposed to T-irradiation of changes in some of the radiation parameters a three factorial experiment -6, Dose (MGy) 1.0 0.5 0
"y
2.0
-10
/ -12 2.6
/
2.8
I 3.0
I 3.2
I O 0 0 / T (K)
Fig. 7. Arrhenius plot of relaxation time ~ as a function of I/T. Epoxy resin system: MY750/DDSA.
586
R.A.
FOtaACRE et al.
40
30
2o
k
-
hl
IO
0
I 0.4
] [ o.e 1.7, Dose (MGy)
l 1.6
I 2.0
Fig. 8. Changes in activation energy as a function of ?-dose for the MY750/DDSA system. was designed. The design was based on the following principles. It was assumed that the response of the resin system, as measured by some physical parameter Y, could be described by a second degree polynomial of the form:
r=Bo+B,X,+B2X2+B3X3+B,X~X, + B~X,X~ + B6X2X3 + B7X~ + BsX22 + B9X32 where Xj, X2 and X3 are the values of the physical factors influencing Y. This assumption is equivalent to saying that the value of Y in the region of some point (Xt, X2, X3) may be determined by expanding Y as a Taylor series about that point and terminating the expansion at the second degree terms. This may always be done provided that the range of the factors X, are sufficiently small and that there are no discontinuities in Y or its derivatives up to the second order terms over the region considered. The polynomial describes a surface in X space and whether this
surface fits measured data over the range considered s a y be determined statistically. The technique had been used successfully to determine the relationship between some factors which affected the cure of an epoxy resin (Fu et al., 1991). In the present case the factors chosen were: the dose rate X1, which covered the range 0.5-2.5 kGy/h; the length of time of exposure to the radiation X2 over the range 225-1075 h; and the ratio of hardener to resin X3 over the range 22-28% of hardener. Various factors can be used to measure the response of the epoxy resin to the radiation conditions. In the present experiments the factors used were the glass transition temperature Ts; the activation energy E of the d.c. conduction process either measured above or below Ts and the point at which the curve of tan 6 measured at a low frequency increases rapidly, this being defined by TA as described above. Obviously these parameters may be presented either directly or by the change in those values compared to the cured but unirradiated samples. Tg was obtained by measuring the d.c. current through the sample for a fixed applied d.c. voltage as a function of temperature and plotting the results as an Arrhenius plot. From this graph the activation energies above and below Tg were also obtained. In order to define the parameters B]_9, nine separate experiments are required. In addition in order to establish the validity of the polynomial equation as a description of Y(X~, X~, X3) by an analysis of variance a further six experiments are needed. From the variance analysis it is also possible to determine the influence of the factors and their cross-product terms on the parameter chosen to measure the state of the sample. The values of the factors X, employed in the present measurements to determine the polynomial coefficients are shown in Table 2. Once the equation has been defined much data may be extracted just by
Table 2. Values of XL to determine the polynomial coefficients Factor Results Experimental plan
Run No. 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19
TA(°C)
x,
x2
x3
(T°C)
t(min)
R(%)
Ti(°C)
I
2
3
Average
T(°C)
95 95 95 95 65 65 65 65 98 62 80 80 80 80 80 95 95 65 65
480 480 120 120 480 480 120 120 300 300 519 81 300 300 300 480 120 480 120
27.5 22.5 27.5 27.5 27.5 22.5 27.5 22.5 25.0 25.0 25.0 25.0 28.0 22.0 25.0 25.0 25.0 25.0 25.0
91 103 91 96 89 92 83 81 104 92 100 90 85 90 97 104 99 95 87
94 97 84 77 86 84 83 68 89 84 92 76 77 78 83 99 86 86 78
90 98 87 81 88 83 86 64 93 80 91 75 83 72 87 97 84 87 77
92 99 86 78 85 85 83 68 94 80 89 79 83 77 88 98 88 88 80
92 98 86 79 86 84 84 67 92 81 91 77 81 76 86 98 86 87 78
-1 5 5 18 3 8 -l 14 12 II 9 13 4 14 II 6 13 8 9
587
7-Irradiation and electrical properties of epoxy resin systems considering the polynomial. F o r example the variation of Y as a function of ~l"twith X2 and ~I"3 constant may be derived directly, or constant factor curves may be drawn. It was found that for most of the parameters used the polynomial was a good description of the measured data. This was concluded both from the variance analysis, Table 3 and from additional experiments in which the measured values were compared to those predicted by the polynomial. Figure 9 shows curves of changes in T^ as a function of exposure time for three different dose rates. AT^ is the difference between TA measured before irradiation and after both irradiating and annealing. As can be seen for the lowest dose rates the curve passes through a maximum. The effect may be explained in terms of two competing processes initiated by the irradiation. These are ion production, which leads to an increased conductivity and causes a lowering of the value of TA and cross-linking which gives rise to a reduced free volume with a consequential reduced conductivity. The former process may also be associated with chain scission. F o r short exposure times cross-linking predominates, for longer periods of exposure a combination of ion production and chain scission is more important. F o r larger dose rates the same trend is apparent but the maximum if it occurs would appear to be at much shorter exposure times. This is in agreement with the measurements discussed previously. Figure 10 is a plot of the value of the glass transition temperature Ts as a function of the ratio of hardener to resin. Two curves are shown, the first is that produced prior to irradiation, the second is the curve obtained after the specimen has been exposed to a dose of 1MGy. The pre-exposure curve shows a maximum value of Ts which occurs at a hardener/ resin ratio of 25% and this value is designated the Table 3. Variance analysis for polynomial fit to data. System: MY750/HY956 Response B, and F,
TA
AT
B0 Bt B2 B3 a4 Bs B6 B~ Bs B9
92.3 4.62 3.30 - 2.20 -I.25 -2.00 -1.50 1.14 - 0.89 - 5.97
Ti
84.0 4.32 5.57 2.38 0.75 -2.25 -3.50 2.65 0.96 - 2.77
8.3 0.38 -2.36 - 4.67 -2.12 0.12 2.12 - 1.45 - 1.78 - 3.4
Fj F2 F3 F4 F5 F6 F7 Fs F9 F
187.8 95.9 42.4 10.0 25.7 14.7 4.6 2.8 124.8 56.6
83.5 138.5 25.3 1.8 16.5 39.9 12.5 1.6 13.6 37.0
*** *** *** ** ** ** * *** ***
*** *** *** *** *** ** ** ***
0.7 26.6 104.0 15.7 0.1 15.7 4.0 6.0 18.7 21.3
-10
300
500
I 7'00 Rodiation time ( h )
\1 900
Fig. 9. Changes in ATA as a function of exposure time to 3'-irradiation. stoichiometric ratio. This result confirms that found previously by Wu et al: (1988). After irradiation, the curve is of a somewhat different shape. Those samples which were depleted of hardener showed an increase in Ts to values above the previously measured maximum value and those samples which were rich in hardener show a decrease in the value of Ts below the maximum value. The value of Ts was greater than that for the corresponding un-irradiated sample. This phenomenon may be explained in terms of the irradiation causing further cross-linking of either the excess resin present or the excess hardener. In previous work the hardener/resin ratio was fixed at 25%. Figure 11 shows curves of equal change in T^ plotted as a function of dose rate and radiation time. Three sets of such curves are shown, each curve being produced for a different hardener/resin ratio. Also shown are curves representing a constant dose. If a
11o
lO5 ~
-IO0
*** *** *** *** * * ** ***
B, are regression coefficients and F, are variance. *** Excellent fit; ** very good fit; * good fit.
irrodiotino to IMGy
95
9O
I 23
I 24
I 25
I 26
i 27
Rotio of hordener to resin (%)
Fig. 10. Glass transition temperature Tz as a function of hardener to resin ratio.
588
R.A. FOURACREet al.
11oo \ \ \ "%',,, hor.dlmer/ \ ~ resln ~
0::
700
~ThA---r~!~ener/
~
/ ~ ~ ~ " ~
1.0MGy constan'¢~ dose curve ~ -" 500 I t ~ I 1.1 1.5 1.9 2.1 Dose rate (KGy/h) Fig. 11. Curves of equal change in TA as a function of dose rate and radiation time. change in the dose rate has no effect on the value of T^ for samples exposed to the same radiation dose then the curves of equal change in AT^ should be parallel to the constant dose curves. This is not the case and indicates that a dose rate effect occurs. An examination was made of the results obtained using the activation energy obtained from d.c. current measurements as the parameter Y. This showed by an analysis of variance that it was not possible to fit a second degree polynomial to the data and was attributed to both sample and sample variation and the difficulty of measuring accurately the gradient from an Arrhenius plot. It is thought that such difficulties also occur in the MY750/DDSA resin system. 4. CONCLUSIONS
The effects of ~-irradiation upto doses of 2 M G y has been investigated for two different epoxy resin systems. The two systems, which are based on a single resin but use different hardeners, behave differently. The system with a Tg value of I08°C shows degradation at high doses as measured by changes in Ts and in the form of the tan ~ curve as a function of temperature measured at a fixed frequency. For low doses, after annealing the material recovers,
for high doses complete recovery does not occur. The other resin system has a Ts of 35°C and the tan 6-temperature curve exhibits a loss peak whose position is both frequency dependent and dependent on radiation dose. In addition the activation energy for the process derived from such peaks shows an increase with increasing dose. The postulated mechanisms causing such changes are for the MY750/MY956 system the production of some crosslinking at low dose levels after annealing, but permanent degradation in the form of ion production and chain seission at higher dose levels. In the case of the lower Ts material crosslinking is the predominant mechanism. In order to investigate further the parameters influencing radiation damage a three factor regression experiment were undertaken on the MY750/HY956 system. The three factors were dose rate, exposure time and hardener resin ratio. It was found that there were dose rate effects and that the system response was sensitive to the hardener-resin ratio. In addition it was found that the temperature T^ at which there was an upturn in the tan 6-temperature curve depended on dose. The activation energy for d.c. conductivity could not be made sufficiently accurately or was sufficiently reproducible between samples to make it a meaningful measure of radiation effects. REFERENCES
Blythe A. R. (1979) Electrical Properties of Polymers. Cambridge University. Fouracre R. A. and Fu L. (1988) 2ndlnt. Conf. on Properties and Applications of Dielectric Materials, Beijing, pp. 539-542. Fouracre R. A., Banford H. M., Tedford D. J., Cad X. and Gedeon S. (1991) Conduction mechanisms in epoxy resin systems: influence of gamma-irradiation. Radiat. Phys. Chem. 37, 589-597. Fu. L., Fouracre R. A., Banford H. M. and Tedford D. J. (1990) A statistical investigation of structural change in an epoxy resin system during the curing process. Trans. IEEE. Gedeon S. (1988) The effect of gamma irradiation on the electrical properties of a epoxy resins. Ph.D. Thesis, University of Strathclyde. Wu S., Gedeon S., Fouracre R. A. and Tedford D. J. (1988) J. Nucl. Mater. 151, 140-150.
Radiat. Phys. Chem. Vol. 37, No. 4, pp. 581-588, 1991 Int. J. Radiat. Appl. lnstrum., Part C
Printed in Great Britain. All fights reserved
THE EFFECT OF GAMMA-IRRADIATION ON THE ELECTRICAL PROPERTIES OF TWO TYPICAL EPOXY RESIN SYSTEMS R. A. FOUp.xc~ 1, H. M. BANFORD2, D, J. TEDFORDl, S. GEDEON1, X. CAO 3, S. W u 3 and L. F u 4 ~Department of Electronic and Electrical Engineering, University of Strathclyde, George Street, Glasgow GI IXE, U.K. 2Scottish Universities Research and Reactor Centre, Birniehill, East Kilbride, Glasgow G75 0QU, U.K. 3Department of Electrical Engineering, Xian Jiaotong University, Xian, Shaanxi Province, People's Republic of China 4Harbin Institute of Electrical Technology, Harbin, People's Republic of China Almtract--Measurements of the effects of y-irradiation on two epoxy systems have been undertaken upto doses of 2 MGy. Changes have been shown to occur in the ra~sured values of tan 6 and these are related to the dose. In one resin the value of tan 6 measured was increased and attributable to d.c. conductivity. In the other resin, which showed a loss peak in the plot of tan 6 vs temperature, changes were associated with crosslinking and these were dose dependent. A factorial experiment on one resin system has shown that dose rate effects as well as effects due to different hardener to resin ratio occur, as measured by changes in Ts and the onset of d.c. conductivity effects on tan 6 measurements.
1. INTRODUCTION Resin systems are used extensively as electrical insulation and for encapsulation of electronic circuits. In certain applications they can be exposed to ionising radiation which affects the performance of the resin system. This might be either in the form of a serious curtailment of the operational lifetime of the material o r a decreased performance which may have significant operational consequences. Situations in which this could occur are in the superconducting magnets proposed for fusion reactors, the instrumentation in fission reactors which could be exposed to a loss of coolant conditions in the reactor containment vessel and on space satelites. It is therefore important to establish the behaviour of these materials when exposed to radiation at dose levels equivalent to that experienced during the projected useful life o f the material. This can involve doses as great as several MGy, Investigations have therefore been undertaken to study the changes in electrical behaviour of an expoxy resin system after exposure to y-irradiation from a Co e° source for dose levels upto 2 MGy. Two resin systems were investigated. These were based on an epoxy resin of diglycidyl ether of hisphenol A (DGEBA), manufactured by Ciba Geigy as MY750, This was cured using one of two hardeners. The first was a hydroxyalkylated polyamine (Ciba Geigy HY956)used in the proportion of 4 parts of resin to 1 part of hardener by weight, The second was a dodecenyl-succinicanhydride (DDSA) used in conjunction with benzyldimethylamine (lphr) as an accelerator, The proportions were 1:1.3:0,0,1 by weight. 581
The resin material was made by mixing weighed amounts of resin and hardener which had been kept for a period of time under a reduced pressure to remove both dissolved water and atmospheric gases (Gedeon, 1988). The resulting mixture was cured according to a fixed temperature schedule (Table 1). Samples were in the form of 1 or 1.5 crn discs of thickness 0.6 mm. Electrodes were formed by the vacuum deposition of aluminium. Samples used for d.c. conductivity measurements had a guard ring surrounding the low voltage electrode. 2. SAMPLE PREPARATION AND EQUIPMENT Measurements of loss tangent (tan 6) were made using a General Radio bridge type 716-C. d.c. current measurements were made using a Keitldey electrometer type 602 and a Brandenburg photomultiplier d.c. voltage supply type 485R. Specimen temperature control was by using either a hot air oven in the case of the dielectric measurements or an electronically controlled hot stage in the case of the d.c. current measurements. 3. RESULTS 3.1 Resin system M Y 7 5 0 / H Y 9 5 6 : y-irradiation effects
Figure 1 shows the effect of y-irradiation on the loss tangent (tan 6) as a function of temperature. The measurements were made at 200 Hz. Two sets of data are shown, the first is for a relatively low radiation dose of 0.5 MGy, the second for a dose of 2.0 MGy.
582
R. A. FOURACR~ et al. Table 1. Curing schedule Time Temperature (h) (°C) (=) 5-10 22
Epoxy sytem
(b) I (c) 2
MY750/HY956
4O 6O
(d) 10-15
100
(a) 5-10 (b) 1 MY750/DDSA (c) 2 (d) 10 Note: The above procedures are sequential.
22 40 6o 90
Each set of data consists of three curves obtained from the same sample. These represent measurements made prior to irradiation, after irradiation and after irradiation and annealing. Annealing consisted of heating the sample in a hot air oven at 90°C for a period of 3 h with the electrodes connected together electrically (short circuit condition). In the case of the sample exposed to 2.0 M G y it proved impossible to make measurements on the sample after exposure but prior to annealing. This was because the values of tan were outside the range of the bridge used, however measurements were possible after annealing. The feature of all these curves is that after irradiation there is a considerable increase in the measured values of tan ~ at a given measurement temperature. The effect of post-irradiation annealing w a s to lower the value of tan & The value was not reduced to that measured for the same sample prior to irradiation. Owing to the form of the changes in tan ~ as a function of temperature it was necessary to present the data a little differently. This was achieved by plotting the logarithm of the ratio of tan ~ measured after irradiation and annealing to that measured prior to radiation exposure. Figure 2 shows such a plot. The measurements were made at two different frequencies and several different temperatures. Each curve has been derived from measurements made on different samples but at the same temperature. As can be seen
the curves initially take negative values and then rise to large positive values. The increase represents a degradation process for the material but the value of tan ~ cannot be used as a measure of the degree of degradation because it is dependent on the precise measurement conditions. It is thought for example that the initial decrease in tan 8 is linked in part with increased cross-linking in the epoxy resin system. It is by no means obvious from the data presented in Fig. 2 to what exposure level this is an important factor. This is because the point at which the curve crosses the axis is dependent on the measurement frequency. It is possible to extend the measurements described above by making them over an extended temperature range. Figure 3 depicts two sets of data made from samples exposed to two different doses of radiation. Each set consists of three curves for the three different measurement conditions described above. The curves are similar in shape. They consist of two regions; one in which relatively small values of tan ~ occur, this being confined to the lower temperature portion of the curve and the other in which the value of tan increases rapidly with temperature. As can be seen the effect of irradiation is to displace the tan J curve to lower temperatures. Post-irradiation annealing displaces the curve to higher temperatures. If the radiation dose is large, the displacement of the curve due to the annealing is insufficient for it to reach its position prior to irradiation. On the other hand for the lower radiation dose the annealing has the effect of displacing the curve to higher temperatures than originally measured. It is possible to define a temperature TA as the point of intersection of the tangent drawn to the two sections of the tan 8 curve (Fig. 3). The effect of both irradiation and annealing may then be described in terms of T^ in much the same way as effects on polymer structure may be measured in terms of the glass transition temperature Ts (Blythe, 1979).
(a)
(b) 1.0
200Hz 0.5 MGy
0.100
oO
irradiation ~
200Hz 2.0 MGy
0.8
0.075
After
anneatin9
0.6
0.050
0.4 /
.Before
0.025
0.2(~
0 20
~ / /
0
_ 0--"-"~-" ~ A f t e r annealing
~
I
=
I
40
60
80
0
~
Before irradiation ~"
7
y
i
7
20 40 60 80 Temperature (°C) Fig. 1. y-Irradiation effects on measured loss tangent as a function of temperature. Epoxy resin system: MY750/HY956.
y-Irradiation and electrical properties of epoxy resin systems (a)
583
(b) BI : Before irradiation
2.0
AA: After irradiation and onneoUng 1 : 25"C
i-t
2 40°C ~- S~'C
"~ 1.s
/
4 m "fO"C 5 - 85"C
dO to
5 / jm/
1 - 25°C
/n
5
/
////
o..,
~. 1.o
4
/
o B-
"3
2OkHz
////'
/
o,_
./
j/J./.
O~
I
I
I
I
I
I
I
I
I
I
I
0.4
0.8
1.2
1.6
2.0
0
0.4
0.8
t .2
1.6
2.0
Irradiation dose [MGy)
Fig. 2. 3'-Irradiation effects plotted as the logarithm of the ratio of tan 6 after irradiation and annealing to tan 6 prior to radiation exposure. An explanation for the behaviour described above can be made in terms of the production of ions during the irradiation; the loss of some of these ions through the annealing process as well as radiation induced polymer cross-linking (Wu et al., 1988). The rapid increase in tan 6 with temperature is associated with the d.c. conductivity of the system. The reason for this is that it is, difficult to associate the large values of tan 6 with a dipole loss mechanism and consequently the increase can not be due to a loss peak. Such a dipole loss mechanism is illustrated in Fig. 4 for a different resin system. The tan 6 values are considerably smaller and is typical of such loss mechanisms. Tan 6 for a Debye type process may be described by an equation containing two terms: 6d.c.
.
second term is due to a.c. loss mechanisms. As the frequency decreases th~ relative importance of the d.c. conductivity term increases until at low frequencies it can predominate. It is already known that for the present resin system the d.c. conduction mechanism is ionic in nature and is an activated process (C-edeono 1988). If it is assumed that for the present measurements, made at a frequency of 200 Hz, the d.c. conductivity is predominant for temperatures above T^ then an Arrhenius plot of log tan 6 as a function of 1/T should be a straight line with an activation energy identical to that for the d.c. conduction process. Figure 5 shows such a straight line plot with an activation energy of 1.23 eV. This may be compared with that of 1.0 eV obtained from measurements of d.c.-current as a function of temperature at a fixed d.c. voltage (Fouracre et al., 1991). In view of the limited temperature range over which it was possible to make the relevant tan 6 measurements the two values are not considered to be in disagreement. It is also found that the tan 6 curve is
E"
tan <~= ~ - ~ o + ~7
where 6d~, is the d.c. conductivity and ~o is the angular frequency of the applied voltage, ~' and ~" are the real and imaginary parts of the complex permittivity. The
(a)
(b) 1 Before irradiation 2 After irradiation 3 After anneoUng
0.15
3
0.10 --
0.05 -
/
// A ,
-150
1
f
-100
-50
O
I T,1 50 Temperature
1OO
IT 0
50
TA1 I I TA3 1OO
PC]
Fig. 3. Tan 6 as a function of temperature. Effects of irradiation and annealing. (a) High dose 2.0 MGy; (b) low dose 0.5 MGy.
584
R. A. Fou~cp~ et al.
0'07 t
v D 0 o
ZMGy
B.L A.A, 0.2 kHz B.I. A.A. 20 kHz
oo 0.05 /
0.0~ /
0,0' 40
I
I
I
60
80
1O0
Temperot ure PC)
Fig. 4. 7-Irradiation effects on measured loss tangent as a function of temperature. Epoxy resin system: MY750/DDSA. displaced to higher temperatures as the measurement frequency is increased, as predicted by the equation for tan ~. This is a result of changes in the relative importance of the a . c . and d.c. conductance terms which are frequency dependent. Thus as the d.c. component is reduced in importance with increasing frequency the tan ~ curve will be displaced to higher temperatures. This phenomenon is also the explanation of the frequency dependence seen in Fig. 2 where the higher frequency curves show a lower value of tan ~ for the same dose and temperature. In both cases the ion production during irradiation would be identical and the differences are caused by the frequency dependence of the d.c. conductivity term.
-2.5 ~x -3.0
\
x
\
x
\
8O 8
-3.5
-4.0
-4.5
Activotton
I 2.66
x\ x\ x\
The increased values of tan ~ for the curves measured at higher temperature are due to the increase in d.c. conductivity which is governed by a thermally activated process. Values of tan J and conductance have been determined for a curing resin sample. In this situation both the conductance and tan ~ vary with time (Fouracre and Fu, 1988). The tan J measurements were made at 200 Hz and at a temperature sufficiently high to ensure that the measurements were made above TA. The sample was solid. As is seen in Fig. 6 the tan curve and the conductance curve are of identical form. Thus changes in conductance are reflected in the changes in tan <~for this particular system. From the above considerations it is reasonable to conclude that for the present system the increase in tan ~ above T^ is due to d.c. conduction processes. Since T^ is determined by the onset of the dominance of d.c. conductivity it will also reflect d.c. conductivity changes in response to external factors. The changes in TA produced by the ~-irradiation may therefore be explained in terms of changes in d.c. conductivity produced by that irradiation. During irradiation two types of ion are produced, namely small mobile ions and ions which are attached to the polymer chain and which are not mobile (Wu et al., 1988). After irradiation the d.c. conductivity of the material has been enhanced by the presence of these mobile ions and as a result the tan J curve is displaced to lower temperature; increasing the dose of radiation increases the number of small ions produced resulting in a greater d.c. conductivity with a consequential larger displacement in the tan 5 curve. This is seen in Fig. 3. Annealing in the manner described previously causes the curve to be displaced to higher temperatures. It may thus be inferred that the d.c. conductivity term has been reduced in importance. This can be due to two different reasons. The first is that the mobile ions have been discharged, thus decreasing the number density of the conducting species and hence the specimen conductivity. The second process is that of polymer cross-linking. This reduces the free -8.8 -9.0 ~
x\
energy1.23eV
I Z,70 IO00/T (K)
-1.0 -1.1
-9.2
x\ x\
94
X\x
tom~~~~._
-9.6
=\
-1.4 -1.5
2.75
Fig. 5. Arrhenius plot of tan 6 as a function of I / T for t©mperatures greater than T^. Epoxy rain system: MY750/HY956.
g
-1.3 *"
60
120 160 20,0 300 360 Curing time (rain)
Fig. 6. Measurement of tan 6 and conductance for a curing resin sample. Epoxy resin system: MY750/HY956.
585
T-Irradiation and electrical properties of epoxy resin systems volume and consequently makes it more difficult for the ions to pass through the material. The crosslinking process occurs during the annealing since segments of the molecular chain are capable of rotation at the annealing temperature which is close to Ts. This enables adjacent chemically active portions o f the resin structure produced by the radiation to cross link. Both these processes can occur concurrently during annealing. For the material exposed to the lower level of irradiation the curve is displaced to a higher temperature than that measured for the non-irradiated sample and needs to be explained. The d.c. conductivity in the unexposed sample is thought to be due to ionised NaCI, present as a residual contaminant as a result of the manufacturing process (Gedeon, 1988). The process of irradiation and annealing will not affect the concentration of these ions. Thus the conductivity decrease indicated by the curve shift to higher temperature cannot be due to the discharge of these ions. The most likely cause is the cross,linking effect. This is supported by the increase in measured Tg after annealing. For the more heavily irradiated material the curves do not recover after annealing to their un-irradiated state (Fig. 3). The explanation of this is two-fold. In the first place ~radiation causes both chain scission, which increases the free volume thus enabling more rapid transport of ionic material and results in increased condactivity, and additional cross-linking which has the reverse effect. The predominant process is determined by the radiation conditions. Annealing enables some cross-linking to occur which results in decreased conductivity as well as discharge of some of the mobile ions. The recovery is not complete as is evident from T, measurements and thus the tan curve would not be expected to recover completely. Secondly, ions are produced which are attached to the polymer chain. These are not mobile and cannot be discharged at the electrodes during the annealing process. They can act as trapping centres for highly polar water molecules. These in turn would cause increased dissociation of any ionic material present because of the high dielectric constant of water. Annealing can cause some of this trapped water to be lost from the material thus decreasing the conductivity. Even prolonged annealing at high temperature does not cause the tan 6 curve to be displaced sufficiently to return to its original position and even a relatively short exposure to normal laboratory air produces a shift of the curve to lower temperatures (Wu et al., 1988). The material has become more sensitive to the presence of water vapour in the surrounding environment. It should be realised that the two processes, chain scission and ion production, are not mutually exclusive.
3.2 Resin system MY750/DDSA: T-irradiation effects Figure 4 shows the results of tan 6 measurements taken as a function of temperature at several different RFC37/4..-D
fixed measurement frequencies. Also shown is the effect of exposure to I MGy of irradiation. The curves show that there is a loss peak the position of which depends on the measurement frequency. This is the case for both irradiated and unirradiated samples. The effects of the irradiation causes the loss peak observed at a given frequency to be displaced to higher temperatures. If the loss peak is assumed to be due to a Debye type loss mechanism then each frequency is related to an activated relaxation time z (Blythe, 1979). By plotting z as a function of I/T=, where T= is the temperature of occurrence of the maximum value of the tan 5, an Arrhenius type plot is obtained (Fig. 7). The activation energy measured from this graph for non-irradiated samples is 2.36 eV. This is the mean value for three different samples. It will be argued in Section 3.3 that measurements of activation energy as a measure of the effects of T-irradiation are not reliable. For this reason it is 0nly possible to compare measurements between irradiated and non-irradiated values of activation energy made on the same sample. Figure 8 shows the effect of the dose of gamma irradiation on the change in activation energy for the loss peak process. This has been derived from three separate samples; the measurements were made before and after irradiation. The activation energy change increases with increasing dose. Both this and the peak shift in temperature is consistent with a radiation induced cross.linking. The loss peak is assumed to be associated with the rotation of polar groups attached to the main polymer chain. Ts measurements have shown that after irradiation the value of Ts is higher which is again consistent with such cross-linking effects.
3.3 T-irradiation effects: factorial regression experiments, MY750/HV956 system In order to investigate the influence on epoxy resin system exposed to T-irradiation of changes in some of the radiation parameters a three factorial experiment -6, Dose (MGy) 1.0 0.5 0
"y
2.0
-10
/ -12 2.6
/
2.8
I 3.0
I 3.2
I O 0 0 / T (K)
Fig. 7. Arrhenius plot of relaxation time ~ as a function of I/T. Epoxy resin system: MY750/DDSA.
586
R.A.
FOtaACRE et al.
40
30
2o
k
-
hl
IO
0
I 0.4
] [ o.e 1.7, Dose (MGy)
l 1.6
I 2.0
Fig. 8. Changes in activation energy as a function of ?-dose for the MY750/DDSA system. was designed. The design was based on the following principles. It was assumed that the response of the resin system, as measured by some physical parameter Y, could be described by a second degree polynomial of the form:
r=Bo+B,X,+B2X2+B3X3+B,X~X, + B~X,X~ + B6X2X3 + B7X~ + BsX22 + B9X32 where Xj, X2 and X3 are the values of the physical factors influencing Y. This assumption is equivalent to saying that the value of Y in the region of some point (Xt, X2, X3) may be determined by expanding Y as a Taylor series about that point and terminating the expansion at the second degree terms. This may always be done provided that the range of the factors X, are sufficiently small and that there are no discontinuities in Y or its derivatives up to the second order terms over the region considered. The polynomial describes a surface in X space and whether this
surface fits measured data over the range considered s a y be determined statistically. The technique had been used successfully to determine the relationship between some factors which affected the cure of an epoxy resin (Fu et al., 1991). In the present case the factors chosen were: the dose rate X1, which covered the range 0.5-2.5 kGy/h; the length of time of exposure to the radiation X2 over the range 225-1075 h; and the ratio of hardener to resin X3 over the range 22-28% of hardener. Various factors can be used to measure the response of the epoxy resin to the radiation conditions. In the present experiments the factors used were the glass transition temperature Ts; the activation energy E of the d.c. conduction process either measured above or below Ts and the point at which the curve of tan 6 measured at a low frequency increases rapidly, this being defined by TA as described above. Obviously these parameters may be presented either directly or by the change in those values compared to the cured but unirradiated samples. Tg was obtained by measuring the d.c. current through the sample for a fixed applied d.c. voltage as a function of temperature and plotting the results as an Arrhenius plot. From this graph the activation energies above and below Tg were also obtained. In order to define the parameters B]_9, nine separate experiments are required. In addition in order to establish the validity of the polynomial equation as a description of Y(X~, X~, X3) by an analysis of variance a further six experiments are needed. From the variance analysis it is also possible to determine the influence of the factors and their cross-product terms on the parameter chosen to measure the state of the sample. The values of the factors X, employed in the present measurements to determine the polynomial coefficients are shown in Table 2. Once the equation has been defined much data may be extracted just by
Table 2. Values of XL to determine the polynomial coefficients Factor Results Experimental plan
Run No. 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19
TA(°C)
x,
x2
x3
(T°C)
t(min)
R(%)
Ti(°C)
I
2
3
Average
T(°C)
95 95 95 95 65 65 65 65 98 62 80 80 80 80 80 95 95 65 65
480 480 120 120 480 480 120 120 300 300 519 81 300 300 300 480 120 480 120
27.5 22.5 27.5 27.5 27.5 22.5 27.5 22.5 25.0 25.0 25.0 25.0 28.0 22.0 25.0 25.0 25.0 25.0 25.0
91 103 91 96 89 92 83 81 104 92 100 90 85 90 97 104 99 95 87
94 97 84 77 86 84 83 68 89 84 92 76 77 78 83 99 86 86 78
90 98 87 81 88 83 86 64 93 80 91 75 83 72 87 97 84 87 77
92 99 86 78 85 85 83 68 94 80 89 79 83 77 88 98 88 88 80
92 98 86 79 86 84 84 67 92 81 91 77 81 76 86 98 86 87 78
-1 5 5 18 3 8 -l 14 12 II 9 13 4 14 II 6 13 8 9
587
7-Irradiation and electrical properties of epoxy resin systems considering the polynomial. F o r example the variation of Y as a function of ~l"twith X2 and ~I"3 constant may be derived directly, or constant factor curves may be drawn. It was found that for most of the parameters used the polynomial was a good description of the measured data. This was concluded both from the variance analysis, Table 3 and from additional experiments in which the measured values were compared to those predicted by the polynomial. Figure 9 shows curves of changes in T^ as a function of exposure time for three different dose rates. AT^ is the difference between TA measured before irradiation and after both irradiating and annealing. As can be seen for the lowest dose rates the curve passes through a maximum. The effect may be explained in terms of two competing processes initiated by the irradiation. These are ion production, which leads to an increased conductivity and causes a lowering of the value of TA and cross-linking which gives rise to a reduced free volume with a consequential reduced conductivity. The former process may also be associated with chain scission. F o r short exposure times cross-linking predominates, for longer periods of exposure a combination of ion production and chain scission is more important. F o r larger dose rates the same trend is apparent but the maximum if it occurs would appear to be at much shorter exposure times. This is in agreement with the measurements discussed previously. Figure 10 is a plot of the value of the glass transition temperature Ts as a function of the ratio of hardener to resin. Two curves are shown, the first is that produced prior to irradiation, the second is the curve obtained after the specimen has been exposed to a dose of 1MGy. The pre-exposure curve shows a maximum value of Ts which occurs at a hardener/ resin ratio of 25% and this value is designated the Table 3. Variance analysis for polynomial fit to data. System: MY750/HY956 Response B, and F,
TA
AT
B0 Bt B2 B3 a4 Bs B6 B~ Bs B9
92.3 4.62 3.30 - 2.20 -I.25 -2.00 -1.50 1.14 - 0.89 - 5.97
Ti
84.0 4.32 5.57 2.38 0.75 -2.25 -3.50 2.65 0.96 - 2.77
8.3 0.38 -2.36 - 4.67 -2.12 0.12 2.12 - 1.45 - 1.78 - 3.4
Fj F2 F3 F4 F5 F6 F7 Fs F9 F
187.8 95.9 42.4 10.0 25.7 14.7 4.6 2.8 124.8 56.6
83.5 138.5 25.3 1.8 16.5 39.9 12.5 1.6 13.6 37.0
*** *** *** ** ** ** * *** ***
*** *** *** *** *** ** ** ***
0.7 26.6 104.0 15.7 0.1 15.7 4.0 6.0 18.7 21.3
-10
300
500
I 7'00 Rodiation time ( h )
\1 900
Fig. 9. Changes in ATA as a function of exposure time to 3'-irradiation. stoichiometric ratio. This result confirms that found previously by Wu et al: (1988). After irradiation, the curve is of a somewhat different shape. Those samples which were depleted of hardener showed an increase in Ts to values above the previously measured maximum value and those samples which were rich in hardener show a decrease in the value of Ts below the maximum value. The value of Ts was greater than that for the corresponding un-irradiated sample. This phenomenon may be explained in terms of the irradiation causing further cross-linking of either the excess resin present or the excess hardener. In previous work the hardener/resin ratio was fixed at 25%. Figure 11 shows curves of equal change in T^ plotted as a function of dose rate and radiation time. Three sets of such curves are shown, each curve being produced for a different hardener/resin ratio. Also shown are curves representing a constant dose. If a
11o
lO5 ~
-IO0
*** *** *** *** * * ** ***
B, are regression coefficients and F, are variance. *** Excellent fit; ** very good fit; * good fit.
irrodiotino to IMGy
95
9O
I 23
I 24
I 25
I 26
i 27
Rotio of hordener to resin (%)
Fig. 10. Glass transition temperature Tz as a function of hardener to resin ratio.
588
R.A. FOURACREet al.
11oo \ \ \ "%',,, hor.dlmer/ \ ~ resln ~
0::
700
~ThA---r~!~ener/
~
/ ~ ~ ~ " ~
1.0MGy constan'¢~ dose curve ~ -" 500 I t ~ I 1.1 1.5 1.9 2.1 Dose rate (KGy/h) Fig. 11. Curves of equal change in TA as a function of dose rate and radiation time. change in the dose rate has no effect on the value of T^ for samples exposed to the same radiation dose then the curves of equal change in AT^ should be parallel to the constant dose curves. This is not the case and indicates that a dose rate effect occurs. An examination was made of the results obtained using the activation energy obtained from d.c. current measurements as the parameter Y. This showed by an analysis of variance that it was not possible to fit a second degree polynomial to the data and was attributed to both sample and sample variation and the difficulty of measuring accurately the gradient from an Arrhenius plot. It is thought that such difficulties also occur in the MY750/DDSA resin system. 4. CONCLUSIONS
The effects of ~-irradiation upto doses of 2 M G y has been investigated for two different epoxy resin systems. The two systems, which are based on a single resin but use different hardeners, behave differently. The system with a Tg value of I08°C shows degradation at high doses as measured by changes in Ts and in the form of the tan ~ curve as a function of temperature measured at a fixed frequency. For low doses, after annealing the material recovers,
for high doses complete recovery does not occur. The other resin system has a Ts of 35°C and the tan 6-temperature curve exhibits a loss peak whose position is both frequency dependent and dependent on radiation dose. In addition the activation energy for the process derived from such peaks shows an increase with increasing dose. The postulated mechanisms causing such changes are for the MY750/MY956 system the production of some crosslinking at low dose levels after annealing, but permanent degradation in the form of ion production and chain seission at higher dose levels. In the case of the lower Ts material crosslinking is the predominant mechanism. In order to investigate further the parameters influencing radiation damage a three factor regression experiment were undertaken on the MY750/HY956 system. The three factors were dose rate, exposure time and hardener resin ratio. It was found that there were dose rate effects and that the system response was sensitive to the hardener-resin ratio. In addition it was found that the temperature T^ at which there was an upturn in the tan 6-temperature curve depended on dose. The activation energy for d.c. conductivity could not be made sufficiently accurately or was sufficiently reproducible between samples to make it a meaningful measure of radiation effects. REFERENCES
Blythe A. R. (1979) Electrical Properties of Polymers. Cambridge University. Fouracre R. A. and Fu L. (1988) 2ndlnt. Conf. on Properties and Applications of Dielectric Materials, Beijing, pp. 539-542. Fouracre R. A., Banford H. M., Tedford D. J., Cad X. and Gedeon S. (1991) Conduction mechanisms in epoxy resin systems: influence of gamma-irradiation. Radiat. Phys. Chem. 37, 589-597. Fu. L., Fouracre R. A., Banford H. M. and Tedford D. J. (1990) A statistical investigation of structural change in an epoxy resin system during the curing process. Trans. IEEE. Gedeon S. (1988) The effect of gamma irradiation on the electrical properties of a epoxy resins. Ph.D. Thesis, University of Strathclyde. Wu S., Gedeon S., Fouracre R. A. and Tedford D. J. (1988) J. Nucl. Mater. 151, 140-150.