Chapter 1
Materials

1.1 History

The history of silicone rubber, its development and technological progress has been described (1-3). Its first commercial development was in 1944. Pioneering research on silicon opened the door to the development of silicone polymers and silicone rubber. The substitution of two methyl groups on silicon was present in the first examples of silicone rubber and still is the predominant organic group in commercial silicone rubber today (1).

Silicone rubbers have filled a need in the marketplace because of their combination of unusual properties not found in other rubbers. The alternating inorganic main-chain atoms of silicon and oxygen, and the two pendant organic groups, primarily methyl, provide strong chain bonds, backbone flexibility, ease of side-group rotation, and low inter- and intramolecular forces (1).

Also, industrial organosilicone materials, their environmental entry and their predicted fate have been described in a monograph (4). Furthermore, a comprehensive overview of the issues of organosilicon compounds has been presented. Commercial products, such as sealants, adhesives, and coatings are reported (5,6).

1.2 Properties

1.2.1 Tracking and Erosion Resistance

Silicon rubber (SR) also offers excellent electrical performance under contaminated environments. However, a pristine silicone rubber has low thermal properties and this may cause tracking and erosion failure due to severe dry band arcing and ohmic heating on the insulating surface (7).

The effect of material thermal characteristics on the tracking and erosion resistance of silicone rubber filled with micron-sized alumina trihydrate, aluminum nitride and boron nitride particles was investigated (7). Composites with different loading were synthesized by dispersing the particles in pristine room temperature vulcanized (RTV) silicone rubber and an IEC 60587 inclined plane test (8) was conducted to evaluate tracking and erosion resistance. Apart from physical parameters and leakage current, an infrared thermal imager was used to measure the surface temperature distribution during the course of the inclined plane test. Experimental results showed tracking and erosion resistance is significantly enhanced with addition of boron nitride particles followed by alumina trihydrate.

Aluminum nitride composites exhibit a poor tracking and erosion resistance, similar to pristine silicone rubber. It has been concluded that the addition of boron nitride composites improves the ability to impede the tracking and erosion process, the reasons being better thermal stability and enhanced thermal conduction in the discharge region. On the other hand, infrared analysis revealed that thermal accumulation is remarkably higher in aluminum nitride composites, which promotes dry band arcing and results in tracking and erosion failure (7).

To improve the tracking and erosion resistance performance of addition-cure liquid silicone rubber (LSR) without alumina trihydrate, addition-cure LSR samples with different fumed silica mass fraction and platinum catalyst concentration were prepared (9).

The tracking and erosion resistance performance of the samples were evaluated. Also, the thermal decomposition products were detected. Based on the comparative analysis of thermal decomposition characteristics, the morphology, crystal structure, and improvement mechanism were assessed.

The results of this study showed that both increasing the concentration of the platinum catalyst and the mass fraction of fumed silica can improve the tracking and erosion resistance performance, whereas the former was more effective. Further analysis indicated that the platinum catalyst played a very important role in the thermal decomposition process of addition-cure LSR. Increasing the platinum catalyst content can not only promote the crosslinking reaction between the methyl groups, but also can suppress backbone decomposition (9).

1.2.1.1 Impact of Various Fillers

The influence that various fillers having different sizes, from 0.3 µm to 18 µm, and surface modifications (unmodified, modified by the material supplier and in-situ modified during compounding) have on the erosion resistance of high temperature crosslinked silicone rubber composites was analyzed. The particles used were aluminium trihydrate, alumina (Al2O3) and silica (SiO2). The main focus was on aluminium trihydrate fillers since they have the ability to release water at elevated temperature.

By a simple water storage test under defined conditions the water uptake for different composites was analyzed to assess the effectiveness of an in-situ modification. The inclined plane test according to IEC 60587 (8), and the high voltage, low current dry arc test according to IEC 61621 (10), were used to determine the erosion resistance of the different samples.

For the inclined plane test, an adapted evaluation model was applied. This test is known to have a wide scatter in the case of material formulations, which are on the borderline of passing or failing the test and therefore show substantial erosions on certain samples. The scatter could be reduced by evaluating the eroded volume by using samples only, which showed a limited erosion length. It was found for aluminium trihydrate, that larger particles show slightly better results than smaller particles. This can be explained by the formation of boehmite AlO(OH) for the larger particles, which causes a release of the bound water over a wider temperature range.

This effect could be confirmed by thermogravimetric analysis (TGA). The surface modification of the particles with vinyltrimethoxysilane and methyltrimethoxysilane, c.f. Figure 1.1, did not improve the erosion resistance significantly, but reduces the water uptake to a large extent, which is advantageous for the retention of the hydrophobicity. In order to achieve a low erosion rate, high filler loadings are essential (11).

Figure 1.1 Methyltrimethoxysilane and Vinyltrimethoxysilane.

1.2.2 Enhancing Strength

Silica is the most widely used filler to reinforce LSR, but the high viscosity of a LSR/silica suspension significantly limits its processing flexibility (12).

To balance the processability and the reinforcing efficiency of LSR/silica systems, two kinds of enols, i.e., propenol and 1-undecylenyl alcohol, and a saturated alcohol, 1-undecylic alcohol, were employed to modify the silica surface. The compounds used here are shown in Figure 1.2.

Figure 1.2 Alcohols.

Modified silica samples were prepared via surface esterification of silica with alcohols by the reflux method. The calculated amount of precipitated silica (Rhodia Z-142), xylene, and alcohol were added into a reaction flask. Then a solid-liquid extractor containing sufficient CaH2 and a condenser with a CaCl2 drying tube were mounted on the reaction flask. The mixture was heated to 160°C and refluxed for 24 h. Finally, the samples were filtered and washed with ethyl acetate at least five times and dried in a vacuum oven at 120°C for 8 h.

The carbon contents, alkoxyl surface densities, and bound rubber contents of various silica samples are shown in Table 1.1.

Table 1.1 Properties of various silica samples (12).

Obviously, the bound rubber content is largely reduced after silica surface modifications, suggesting that silica poly(dimethyl siloxane) (PDMS) interaction is effectively lowered by silica surface modification. Comparing different modifiers, there is not much difference between 1-undecylic alcohol modified silica and 1-undecylenyl alcohol modified silica, while bound rubber content for propenol modified silica is a little higher. It seems that modifiers with higher carbon chain lengths are more effective in lowering the PDMS silica interactions. Though propenol has a little higher density on the silica surface, it is the combined effect of grafting density and modifier chain length that determines the effect of silica surface modifications.

Various rheological tests were carried out to investigate the processability as well as filler networking and the crosslinking processes of the modified systems.

Polymer/filler composites are known to exhibit a complex rheological behavior that reflects interactions among components in the system. In PDMS/hydrophilic silica suspensions, there are strong interactions between PDMS and silica and among silica, mainly via hydrogen bonding, which would cause a drastic increase in viscosity or reduction in the mobility of PDMS, consequently deteriorating the processing properties of LSR.

The effect of silica surface modification on viscosity of the suspension is believed to be determined by the combined effects of the type of grafting enol (saturated or...