Aging analysis of high voltage silicone rubber/silica nanocomposites under accelerated weathering conditions

2.1 Materials

RTV SiR (RTV 615) was procured from Lanxess Chemicals (Germany), and nanosilica (12 nm) was procured from Degussa (USA).

2.2 Preparation

Nanocomposites of SiR were prepared according to the percentage weight (wt.%) of the fillers and polymer. For example, SNC-5 composites refer to samples that were made by mixing 5 g nanosilica and 95 g RTV 615 SiR [having 86.36 g RTV 615-A (base polymer) and 8.64 g RTV 615-B (curator)]. The ratio of base polymer to curator was kept at 10:1 in a sample. The maximum amount of nanofumed SiO₂ loading was restricted to 5 wt.%. A high shear laboratory mixer with a sonicator was used for the preparation of the SiR composites.

The SiR composite samples were prepared in three steps. In the first step, dry fillers were gently added to RTV 615-A while keeping the mixing speed low to wet out the fillers. After the fillers got properly wet, the mixing speed was gradually increased to the maximum revolution per minute and was kept at the same maximum speed until the mixture did not show any visible signs of lumps. In the second step, after 2–3 min of cooling, RTV 615-B was added to the mixture and mixed at low speed for 2 min by a sonicator and then the mixture was degassed in a vacuum at 27 in Hg until the bubbles were completely removed. In the third step, the SiR mixture was discharged into molds and allowed to cure at room temperature for 24 h. Subsequently, the samples were postcured for 4 h in an oven at 90°C. The list of the prepared samples is given in Table 1. In comparison to microsilica, nanosilica interacts better at much lower concentration due to smaller size and fine dispersion. Many properties of polymeric materials decrease roughly below 1% and above 5%. Therefore, concentrations of nanosilica were taken at 5% and 2.5% to observe its optimum influence on the aging behavior of RTV-SiR, without compromising the other required insulating properties [32], [46], [47], [48].

3.1 Aging chamber

An environmental chamber with dimensions of 1×2×1 m³ was designed and fabricated in which prepared insulating materials were energized with a 2.5 kV voltage through 220 V/2.5 kV transformer having an output current capacity of nearly 100 mA. The accelerated weathering arrangement is shown in Figure 1. Different environmental stresses such as controlled UV-A, temperature, humidity, acid rain, and salt fog were applied timely along with the electrical stress. To include variable heating stress, 1500 W (max.) heater was used. For UV radiation, two UV lights (xenon lamps) were used, each of 15 W (15×2=30 W) to make luminous intensity as per IEC 61109 Annex-C. HCl dilute solution with pH 4.5 was used for acid rain purpose. Two accelerated weather cycles were followed during the experiment (i.e. summer cycle was 17 days and winter cycle was 11 days). During summer cycle, the temperature was kept at 47.3°C, and in winter cycle, the temperature was 35.2°C. The specifications of accelerated weather cycles are shown in Table 2.

Figure 1:

Accelerated multistress environmental chamber.

For the preparation of accelerated aging setup, the ASTM D4762-11a, ASTM D2865M-06, and IEC 62217 ed2.0, 61109 standards were used as guides.

3.2 Analysis techniques

3.3 Attenuated total reflectance-FTIR spectroscopy

FTIR spectroscopy was performed on each sample from 0 h (virgin) to 5000 h. For this purpose, spectrographs of samples were obtained through a Perkin-Elmer Spectrum 2000 FTIR spectrometer after each 1000 h. Thus, a total of six spectrographs were obtained for each sample during the 5000 h aging period. In all tests, potassium bromide (KBr) powder was used as a transparent material.

3.4 SEM

To investigate the surface morphology of virgin and aged samples at the microlevel, SEM was used. Before subjecting to SEM, the materials were gold coated. The images were taken at 1 μm scale by applying 15 kV voltage. SEM-Hitachi (SU-1500) was used for SEM analysis.

4.1 Visual inspection

During periodic visual inspection after each weathering cycle, major degradation or cracking was not recorded. The SiR insulators remained in acceptable conditions. With the passage of time, the transparency of samples was reduced but at a low rate. However, this color fading phenomenon is common for polymeric insulators [12], [40], [41]. Photographs taken from virgin and 5000 h aged samples are given in Figures 2 and 3, respectively.

Figure 2:

Photographs of virgin samples of neat SiR, SNC-2.5, and SNC-5.

Figure 3:

Photographs of 5000 h aged samples of neat SiR, SNC-2.5, and SNC-5.

4.2 Hydrophobicity analysis

The hydrophobicity class of neat SiR and nanocomposites varied with the increase of aging period. This variation in hydrophobicity was also in unpredictable manner. Virgin samples of neat SiR, 2.5 wt.% nanosilica (SNC-2.5), and 5 wt.% nanosilica (SNC-5) showed hydrophobicity classes HC-2, HC-3, and HC-1, respectively, as shown from the photographs taken at the start of the experiment in Figure 4. However, both recovery and loss were seen during the 5000 h aging, where neat SiR remained at HC-2 most of the time and also touched HC-3 at nearly 2000 h aging. SNC-2.5 though had HC-3 at start but remained at HC-2 until 3000 h and then showed further increase in hydrophobicity, showed HC-1 at the second last test, and ended with HC-2 after 5000 h aging. SNC-5 remained at HC-1 most of the time and also showed HC-2 alternatively with HC-1 until 4000 h. However, the hydrophobicity of SNC-5 remained at HC-1 during the last four tests. Photographs taken for all three insulators after 5000 h aging are given in Figures 4 and 5, whereas the hydrophobicity variation in neat SiR, SNC-2.5, and SNC-5 with aging is presented in Figure 6.

Figure 4:

Photographs at the start of experiment for STRI hydrophobicity classification.

Figure 5:

Photographs taken after 5000 h for STRI hydrophobicity classification.

Figure 6:

Hydrophobicity class variation in neat SiR, SNC-2.5, and SNC-5 with aging.

The unpredictable loss and recovery for SiR was reported by many authors previously for SiR. This nature of SiR may be due to the rotation of backbone chain and transfer of hydrophobic molecular groups from the material bulk to the surface. The abundance of methyl groups on the surface of the SiR can be attributed to this unique nature, which can be further validated through FTIR analysis. The combined variation graph of neat SiR, SNC-2.5, and SNC-5 given in Figure 6 shows that neat SiR is relatively more hydrophilic compared to nanocomposites and that SNC-5 has better hydrophobic behavior than SNC-2.5. However, the change in this behavior among these insulators was less and the hydrophobicity of all three samples remained in the acceptable limit.

4.3 LC analysis

Both nanocomposites in virgin form showed lesser LC than neat SiR. However, nanocomposite with SNC-2.5 showed a slightly greater value of LC than SNC-5. All three samples respond in pattern variation during the entire aging period. Decrease and increase in LC were recorded for all samples. Besides continuous increase in complex form, all samples showed decrease to initial values at 3624 h aging. For the next consecutive cycle, the LC of all samples showed sudden jump, and a maximum value of 1 μA was recorded for each sample. However, after 4000 h, LC recorded for all samples was <1 μA. The cause of this abrupt rise at some points in LC is not known and termed as saturation current by the authors [44]. At the end of experiment, neat SiR, SNC-2.5, and SNC-5 showed LC of 0.9, 0.8, and 0.6 μA, respectively. All these samples have LC in acceptable limit and none of them crossed the unsafe limit of LC for an insulator. Figure 7 presents the combined graph of LC for neat SiR, SNC-2.5, and SNC-5.

Figure 7:

LC variation in neat SiR, SNC-2.5, and SNC-5 with aging.

4.4 FTIR spectroscopy

To investigate the structural changes in neat SiR and its nanocomposites and hybrid composites, FTIR spectroscopy in absorption form was used after each weather cycle. The important groups in SiR and relevant wave-numbers are given in Table 3.

Applications of cyclic stresses for 5000 h caused scission of main polymer groups, loss of molecules of low molecular weight (LMW), and oxidation. Thus, variations in all samples were observed at the structural level after each 1000 h aging.

In neat SiR, loss in absorption peaks of symmetric C-H stretching of CH₃ and symmetric bending of Si-CH₃ occurred at ~2963–2960 and ~1280–1255 cm^-1, respectively. After 1000 h, absorption peak representing symmetric C-H stretching of CH₃ decreased by 17.7% and remained at the same intensity until the end of test. Si-CH₃ symmetric bending also showed decrease, and at the end of 2000 h, a maximum reduction of 27.1% was recorded. However, after 2000 h, this peak increased intensity to almost 86.5% of virgin and maintained this absorption until 5000 h. Similarly, absorption peaks representing other important groups also decreased. At the end of the test, absorption peaks of Si-O-C stretching at ~1110–1050 cm^-1, Si-O-Si asymmetric stretching at ~1130–1000 cm^-1, and Si-O of O-Si(CH₃)₃ decreased by 14.9%, 18.75%, and 25%, respectively. Table 4 shows the absorption peaks of different groups in neat SiR at relevant wave-numbers from 0 (virgin) to 5000 h.

Not only decrease at different wave-numbers occurred in neat SiR, but bond shifting was also recorded (i.e. after aging, molecules changed their vibrational frequency as can be observed from the absorption form spectrographs given in Figure 8A–F).

Figure 8:

FTIR spectrographs of neat SiR: (A) virgin, (B) 1000 h aged, (C) 2000 h aged, (D) 3000 h aged, (E) 4000 h aged, and (F) 5000 h aged.

The absorption peak of symmetric stretching of methyl increased at 4000 h in nanocomposite with SNC-2.5 but came down to same point of virgin sample, and 20.4% reduction in Si-CH₃ was recorded after 5000 h aging. The absorption peaks of other important groups were also reduced. At the end of the test, the absorption peaks of Si-O-C stretching at ~1110–1050 cm^-1, Si-O-Si asymmetric stretching at ~1130–1000 cm^-1, and Si-O of O-Si(CH₃)₃ decreased by 11.8%, 25%, and 10.6%, respectively. Table 5 shows the absorption peaks of the different groups in SNC-2.5 at relevant wave-numbers from 0 (virgin) to 5000 h.

Unlike neat SiR, less reduction in many absorption peaks occurred and no bond shifting took place, as given in the overlayered form FTIR spectrographs of SNC-2.5 in Figure 9A–F.

Figure 9:

FTIR spectrographs of SNC-2.5: (A) virgin, (B) 1000 h aged, (C) 2000 h aged, (D) 3000 h aged, (E) 4000 h aged, and (F) 5000 h aged.

Despite the initial decrease during 1000–3000 h, the absorption peak of symmetric C-H stretching of CH₃ at wave number ~2963–2960 cm^-1 remained intact in SNC-5 until the end. Symmetric bending of Si-CH₃ at wave number ~1280–1255 cm^-1 showed an increase of 6.75% after 5000 h aging. This increase and intactness of methyl group on the surface shows the transfer of methyl group from bulk to surface, which also contributed the excellent hydrophobic behavior to SNC-5, which was recorded during STRI hydrophobicity analysis. The peak representing Si-O-Si and Si-O of O-Si(CH₃)₂-O₃ also increased in absorption, which may be due to the formation of 2D/3D Si-O bonds. This increase may be attributed due to the cross-linking of SiR either by Si-CH₂-Si or by Si-O. The other absorption peaks also reduced lesser compared to SNC-2.5 and neat SiR. Si-O-C stretching at ~1110–1050 cm^-1, Si-O of O-Si(CH₃)₃ at ~870–850 cm^-1, and Si- of Si-(CH₃)₃ at ~700 cm^-1 reduced only by 4.3%, 10%, and 4.8% at the end of 5000 h. Table 6 shows the absorption peaks of the different groups in SNC-5 at relevant wave-numbers from 0 (virgin) to 5000 h. The spectrographs of SNC-5 obtained after each 1000 h are given in Figure 10A–F.

Figure 10:

FTIR spectrographs: of SNC-5: (A) virgin, (B) 1000 h aged, (C) 2000 h aged, (D) 3000 h aged, (E) 4000 h aged, and (F) 5000 h aged.

The reason for the least changes in SNC-5 is the optimum loading of SiO₂. The nanosilica not only provides strength and improves other required properties but also the presence of silanol groups in SiO₂ that interact with polymer matrix at the nanolevel. Therefore, scission and bond shifting occurred much lower in nanocomposites compared to neat RTV-SiR.

4.5 SEM

The surface study of an aged polymer insulating material at the microlevel is important to analyze the extent of degradation. Figure 11A shows the SEM images of virgin samples and Figure 11B shows the SEM images of 5000 h aged samples. The effect of surface degradation can be observed from these images. The surfaces of all samples were affected by the stresses applied. However, the level of degradation was much less in nanocomposites compared to neat SiR. After 5000 h aging, the surface of neat SiR was found more irregular and nonhomogenous accompanied with holes in comparison to virgin sample. From the SEM image taken after 5000 h aging in Figure 11B, loss of materials, erosion, and cracking were observed in neat SiR. At the end of the aging test, loss of materials and minor holes at microscale were also observed in SNC-2.5. As can be observed in Figure 11B, there were no prominent signs of cracking at the 1 μm scale. In Figure 11A and B, respectively, it is evident that SNC-5 showed less changes having better surface topography compared to neat SiR and SNC-2.5 at the microlevel, having only risings at some points.

Figure 11:

SEM images virgin of 5000 h aged samples of neat SiR, SNC-2.5, and SNC-5: (A) virgin and (B) 5000 h aged.

The SEM results also almost matched with FTIR and hydrophobicity classification. The reason for better surface topography of SNC-5 is the presence of abundant silanol groups on the surface, which is the reason of the intactness/bonding of main polymer molecules. This is also evident from FTIR results in which the absorption peaks of methyl groups on the surface do not reduce with aging.