Abstract - The recovery of hydrophobicity of ethylene propylene diene monomer (EPDM) insulator after aging by long exposure to a stress of saline water at different temperatures is investigated. The hydrophobicity is determined by measuring the contact angle of a droplet of distilled water on EPDM rubber. The aging of EPDM rubber was done by immersing it for up to 576 h in saline water solutions in the range 5.0 - 105 mS/cm. The aging temperatures were 0, 25, 50, 75 and 98 °C.
After aging, the specimens
were kept in air at room temperature for up to 2,500 h during
which the recovery of hydrophobicity and weight changes were measured.
From these results, surface free energy and diffusion coefficient
of EPDM rubber were evaluated.
1. Introduction
Hydrophobicity is a ability of the solid insulator surface to resist the formation of a continuous film of water. It is often used to evaluate the aging process of the surface of insulating materials. Degradation of the surface of polymers causes a loss of hydrophobicity which is usually accelerated with increasing temperature and contamination. Hydrophobicity exhibits a time dependent recovery after removal of the aging stress. Therefore, it is important to study the recovery of hydrophobicity.
Studies on the loss of hydrophobicity
and its recovery in XLPE [1], Nylon [2,3], EPDM and silicone rubber
[4,5] have been reported. EPDM rubber which has shown excellent
arc resistance in laboratory test is widely used for weathershed
of outdoor non-ceramic insulators and the cable terminators [6].
A study of the recovery of hydrophobicity of EPDM rubber aged
by a combined high salinity water and high temperature has not
been reported yet. In the present paper the recovery of hydrophobicity
of EPDM rubber is determined after aging at different salinity
of water over a wide temperature range. The contact angle q
is used to evaluate the hydrophobicity of the surface. Recovery
of the original weight of EPDM rubber after immersion in a saline
solution is also measured to evaluate the desorption (drying)
process of absorbed saline water.
2. Specimens and Experimental Method
Specimens of EPDM were cut from the weathershed of a 46 kV outdoor insulator which had been in service for 6 years. During the aging specimens were immersed in saline solutions having different conductivity and at different temperatures for up to 576 h. The temperatures of the solution were 0, 25, 50, 75 and 98 °C. The saline solution was obtained by adding table salt (NaCl) to distilled water, which had a typical conductivity of less than 5 mS/cm. Salinity levels of 5, 103, 104 and 105 mS/cm were used. The 105 mS/cm saline solution was saturated and some salt deposits appeared at the bottom of the solution. Other details on aging test may be found in [7].
After aging, the specimens were kept
in air at 24.5 ±
3.5 °C
and humidity of 50 ±
22% for up to 2,500 h during which the recovery of hydrophobicity
and the change in weight were measured. Contact angle q
was measured on a horizontal surface with a goniometer using sessile
drops of distilled water (2.5 ±
0.5 mS/cm)
of about 6.8 ml.
3. Results and Discussion
3.1 Changes of Weight of EPDM Rubber During Aging and Recovery
During the aging of immersion in solutions of different salinity and temperatures the sample weight had increased by the absorption of saline water. The water absorption of EPDM rubber was larger at higher temperatures. However, it was smaller at higher salinity of solution. At higher temperatures the absorption of saline water seemed to saturate during the aging period for all salinity levels. At 75 °C and higher temperature the sample weight showed gradual decrease during the aging especially at higher salinity. This means the solution of undefined material from EPDM bulk to the saline water. Simultaneously, the change of color of sample surface from original gray to white was also recognized and specimens were becoming hydrophobic [7].
Figs. 1 to 3 show the recovery of the weight when the samples are aged at 0, 50 and 98 °C, respectively. The time variation of percentage increase of the weight of EPDM rubber is plotted during the dry process in air at room temperature. The original weights recovered completely within 48 h for all temperatures and salinity. This quick recovery of weight seems to require longer time for the samples aged at higher temperatures.
Table 1 shows the percentage change in the weight during 192 h of recovery. These values indicate the amount of absorbed water in the specimen during the 576 h of aging in saline water and in air. The amount of absorbed water was larger at higher temperature but smaller at higher salinity.
To determine the diffusion coefficient
following equations were used to the linear part of figures, where
the recovery time (t) was plotted by square root scale.
M(t)/M = 4 ( D t / p
l2 )1/2
, (1)
i.e., D = 0.049 / ( t / l2
)1/2 (2)
where M(t) is the change in mass after time t, M is the change in mass after infinity time, t is time in s, D is diffusion coefficient in m2/s, l is the sample thickness as assumed 0.005 m and (t/l2)1/2 is the (t/l2) at M(t)/M =1/2 ( i.e., t is the time when the change in weight of absorbed water is the half of its final value ), respectively [8]. From equation (2) the diffusion coefficient of 5.0 mS/cm solution of EPDM rubber during the aging at 25 to 98 °C and the recovery aged at 98 °C are evaluated. The results are shown in Table 2.
The diffusion coefficient of water during aging and recovery at room temperature should be almost same [8]. Whereas Table 2 shows the value for aging is 3.9 10-12 m2/s and for recovery is 2.2 10-11 m2/s, the diffusion coefficient of absorption process is smaller than the one of drying process at room temperature. On the other hand, Nylon required longer recovery time compared with its aging time[2,3]. A part of these differences may be caused by the shapes of the specimens are not thin plane.
From the same studies of recovery
in air at room temperature of Nylon and silicone rubber, the diffusion
coefficients aged at 98 °C
of 5.0 mS/cm
solution are 1.9 10-13 m2/s and 3.7 10-11
m2/s, respectively. EPDM
rubber and Nylon have larger absorption of water compared with
silicone rubber, however, the recovery of weight of Nylon requires
longer period compared with rubbers.
Table 1 Percentage decrease in the weight of EPDM rubber during 192 h of recovery (0.2%).
Table 2 Diffusion coefficient of water ( 5.0 mS/cm ) of EPDM rubber during the aging at different temperatures and the recovery in air at room temperature aged at 98 °C.
Fig. 1 Time variation of percentage
increase of weight of EPDM rubber during the 2,500 h of recovery.
Samples were aged by the immersion in saline solutions of different
conductivity and leaving in air for 576 h at 0 °C.
Fig. 2 Time variation of percentage
increase of weight of EPDM rubber during the 2,500 h of recovery.
Samples were aged by the immersion in saline solutions of different
conductivity and leaving in air for 576 h at 50 °C.
Fig. 3 Time variation of percentage increase of weight of EPDM rubber during the 1,416 h of recovery. Samples were aged by the immersion in saline solutions of different conductivity and leaving in air for 576 h at 98 °C.
From t = 0 of Fig. 3, after keeping the specimen in air at 98 °C the sample weight decreased 0.8 %. This decline of weight implies that the sample before aging had at least 0.8 weight % of water or other material which can evaporate during the aging period of 576 h in air at 98 °C. The reduction of weight at higher temperature air is also recovered gradually by the absorption of moisture from the environmental air.
Small weight loss of all samples
aged in saline solution and after long recovery is related to
the reduction of moisture in the sample when the humidity of air
is decreased in winter.
3.2@Changes of Contact Angle during Aging
The contact angle of EPDM rubber
which had been in service for 6 years was 129 ±5
at room temperature in air. Despite the accumulation of surface
contamination, the contact angle showed a excellent hydrophobicity.
After cleaning the sample surface by 5% acetic acid and distilled
water and thereafter dried naturally in room temperature air the
contact angle decreased to 98 ±3.5.
This value was usually reduced after immersion in saline water.
A larger decrease in the contact angle for higher salinity solution
was observed at lower temperature, however at higher temperature,
q aged
at higher salinity not always decreased more but showed small
or no decline [7]. A summary of the results showing the q
after aging is given in Table 3.
Table 3 Contact angle ( in degrees) of droplet of water on EPDM rubber after aging in air and in saline water at different temperatures for 576 h.
3.3 Recovery of Contact Angle of EPDM Rubber after Removal of the Stress
Figs. 4, 5 and 6 show the recovery of the contact angle as a function of time after immersion in different saline solutions and leaving in air at 0, 50 and 98 C, respectively. The contact angle recovered and increased quickly during the first few days and gradually thereafter. The former quick recovery seems to relate to the reduction of weight due to the desorption of saline water. The later gradual recovery of q seems to relate to not only the change of gradual decrease of surface moisture but also the migration of low surface energy material to the sample surface.
Excellent hydrophobicity of the samples
which aged at higher salinity and temperature and recovered in
air at room temperature may be related to the increment of surface
roughness by saline deposit which is covered by low surface energy
material. These samples changed the surface color and became the
fractal
Fig. 4 Recovery of contact angle
in air at room temperature of EPDM rubber after immersion in saline
solutions of different conductivity and leaving in air. Aging
conditions: temperature, 0°C; time of stress,
576 h.
Fig. 5 Recovery of contact angle
in air at room temperature of EPDM rubber. Aging conditions: temperature,
50°C; time of stress, 576 h.
Fig. 6 Recovery of contact angle in air at room temperature of EPDM rubber. Aging conditions: temperature, 98°C; time of stress, 576 h.
surface. The fractal surface enhances the property of water repellency.[9]
q of the
samples aged at lower temperatures showed the full recovery, however,
after shaking the sample which had water droplet on it, the water
film was still remain on the sample especially aged at lower salinity
solutions. q of
all samples aged in air showed almost no change during aging and
gradual increase during recovery. The surface was hydrophobic
and there was no water film after shaking the sample to remove
the water droplet. q of
the sample aged at higher temperature and higher salinity was
increasing more than 130 even the sample surface was covered by
saline deposit.
3.4 Changes of Surface Energy of EPDM Rubber during Aging and Recovery
Absorption of the saline solution and adhesion of saline deposits increase the surface energy of EPDM rubber and therefore decrease the contact angle [7]. The contact angle q is related to the surface free energy [10-12]. Using the known values of surface energies of water and methylene iodide (MI) and the measured values of contact angle of water and MI on EPDM, the surface energies of EPDM are calculated and are shown in Table 4. The adhesion energy of the liquids to EPDM rubber WSL and the interfacial free energy gSL are also listed in Table 4.
During aging, the decline
of contact angle mainly relate to the change of gSH,
the non-dispersion part of surface energy of EPDM rubber [4, 12].
Therefore, during recovery, the increase of contact angle is related
to the decline of gSH.
The surface energy of EPDM lower than 23×10-3
J/m2 may be related to the migration of low surface
energy material.
Table 4 Used values of surface energies (×10-3 J/m2) of water and MI, measured values of contact angle and evaluated surface energies for EPDM rubber. Contact angles ( in degrees) are the values before aging.
4. Summary
Acknowledgments
This research is supported by the
Ministry of Education, Science and Culture of Japan and by the
Natural Science and Engineering Research Council of Canada.
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