Aging Silicone Oils

Table 4-187 Thermogravimetry of oven-aged silicone oils (aged with 2 wt % wood chips) in air.

Preparation of a highly conductive polyacetylene was achieved when Ziegler-Natta catalyst was used by Shirakawa in aged silicone oil at 150°C. It is believed that this reaction results in formation of polymers with less defects in the structures. The conductivity of these materials, when doped, actually approaches that of copper [62]. 

As the coefficient of friction can be reduced by the use of a lubricant, it is generally beneficial to use a suitable oil or grease when required. The addition of a modifier to the prepolymer itself must be done with care, as the material will reduce the overall properties including aging and may influence the bond ability of the material. Additives that have been employed in this application include molybdenum disulfide, graphite, and silicone oil. They must be used at the lowest level possible. 

As described in Section II.B.l above, doping causes a drastic change in the electrical properties of polyacetylene. The initial values of electrical conductivity were of the order of 10 S cm" for unoriented materials d24-i30 when doped by iodine and AsFs, were enhanced to the order of 10 S cm, which was obtained in the parallel direction of the doped films oriented by mechanical stretching 31 Improvements in polymerization methods and in the catalyst systems also enhanced the electrical conductivity. Highly oriented films prepared in liquid crystal solvents (Section II.A.l.d.iii) exhibited a conductivity higher than 10 S cm, as did also a well stretch-oriented film prepared by Ti(OBu)4-EtsAl dissolved in silicon oil and aged at 120°C. In further studies Naarmann and Theophilou and Tsukamoto and coworkers attained a conductivity of ca 10 S cm k... 

Shen et al. [43] prepared silica gel from polysiloxane E-40 precursor. After aging and solvent replacement, the silica gel was placed in TMCS solution (10%) for surface modification for 3 days, followed by washing with silicone oil repeatedly, and dried under ambient pressure to obtain Si02 xerogel. Its pore size was 10.4 nm, the specific surface area 969 m /g, and shrinkage ratio 9%. 

An acetylene polymerization reaction catalyzed by an aged mixture of Ti(0C4Hg)4/Al(C2H5)3 in a silicone oil reaction medium yields a homogeneous, defect-free polyacetylene film that can be stretched mechanically by up to 600% corresponding to stretching rates of 7. After we had doped with iodine, the film displayed electrical conductivity of ca. 11 000 S/cm. Both washed (catalyst-free) and unwashed (catalyst-containing) film was stretched. 

Oriented polyacetylene films have been synthesized by using a nematic liquid crystal as a polymerization solvent. The maximum value of parallel electrical conductivity of the films doped with AsFs was 11,000 S cm [17]. The use of an aged mixture of Ti(OC4Hc,)4-A1(C2H0. in silicone oil that has been mixed with various quantities of n-butyllithium, as the catalyst for the polymerization of acetylene, yields polyacetylene that is highly regular, compact, and crystalline, in well-defined parallel planes. When it is stretched and doped, the polyacetylene film shows a dramatic increase in conductivity to a value greater than lO S cm [18]. 

Whilst exhibiting the excellent low-temperature flexibility (with a Tg of about -80°C) and very good heat resistance (up to 200°C) typical of a silicone rubber, the fluorosilicones also exhibit good aliphatic oil resistance and excellent aging resistance. However, for some applications they have recently encountered a challenge from the polyphosphazenes (see Section 13.10). 

Hermetically sealed electrical devices must be verified by a testing laboratory to meet mechanical abuse and to withstand aging and exposure to expected chemicals. Devices potted with common silicones and similar materials by an end user or even a manufacturer, without testing, and devices merely provided with O-rings seldom meet acceptable criteria. Normally, hermetically sealed devices must be sealed through metal-to-metal or glass-to-metal fusion. Many electrical relays, switches, and sensors are available as hermetically sealed devices for common oil and gas producing facility applications. Hermetically sealed devices are often desirable to protect electrical contacts from exposure to salt air and other contaminants. 

Silicone rubber in combination with asbestos fibre can be used in pressure reducers in receivers. In comparison tests all diaphragms from organic rubbers and asbestos cease operation due to aging after 200-360 thousand cycles on the other hand, diaphragms from silicone rubber are still in perfect state after 1 million cycles. We should also mention the use of silicone rubbers in industrial furnaces and various apparatuses operating at high temperatures (oil cracking towers, gas pipelines, recuperation installations). 

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