Thermal Silicone Fluids for Mechanical and Heat Transfer Applications (Aromatic siloxanes)
The thermal silicone fluids are described in chemical notation as aromatic siloxanes due to the presence of phenyl groups. High phenyl-content fluids are utilized as heat-exchange fluids, dielectric coolants, impregnants for sintered metal bearing, and base oils for high temperature fluids. Low phenyl-content fluids are utilized at lower temperatures than high phenylsilicones and find extended temperature service applications as lubricating oils for critical devices such as timers and systems involving rubber, plastic, and aluminum matting surfaces.
Phenylsiloxane-dimethylsiloxane copolymer
Diphenylsiloxane-dimethylsiloxane copolymer
At elevated temperatures, and in the presence of oxygen, silicone polymers are subject to two types of degradation:
Phenyl groups provide enhanced thermal properties by two mechanisms:
- Better protection of the chain Si—O—Si—O by steric hindrance.
- The lower susceptibility of the phenyl group to oxidative attack.
As phenyl groups replace methyl groups in a polysiloxane, several changes occur. Oxidation resistance, thermal stability, and shear resistance are enhanced. For polyphenylmethylsiloxanes, the service temperature is -55 °C to 290 °C. The gel time of several fluids is provided in the accompanying table. In closed oxygen-free systems the polyphenylmethylsiloxanes are stable for thousands of hours at 250 °C. The materials are used in heating baths.
Diphenylsiloxane-dimethylsiloxane copolymers, CAS: [68083-14-7]
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % (diphenylsiloxane) | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PDM-0421 | 100 | 29 | 0.62 | -73 | -- | 150-200 | 0.98 | 1.422 | 22.6 | 2.75 | 280 | 484 | 4-6 | 3,500-4,000 |
| PDM-0821 | 100-125 | 30-32 | 0.63 | -70 | -- | 1,200-1,500 | 0.99 | 1.436 | 22.8 | 2.78 | 280 | 484 | 7-10 | 2,800-3,200 |
| PDM-1922 | 150-250 | 26-28 | 0.78 | -40 | -- | 1,500-2,000 | 1.05 | 1.49 | 24.5 | 2.83 | 285 | 488 | 18-22 | 1,600-2,400 |
Phenylmethylsiloxane-dimethylsiloxane copolymers CAS: [63148-52-7]
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PMM-1015 | 50 | 14 | 0.61 | -70 | -121 | 220-260 | 1.00 | 1.425 | 25 | 2.77 | 275 | 482 | 8-12 | 1,500-1,600 |
| PMM-1021 | 100 | 35 | 0.62 | -70 | -121 | 210-230 | 1.00 | 1.425 | 24.5 | 2.79 | 280 | 482 | 8-12 | 3,000-4,000 |
| PMM-1025 | 500 | 180 | 0.62 | -70 | -121 | 180-200 | 1.00 | 1.425 | 24.4 | 2.8 | 285 | 482 | 8-12 | 9,000-11,000 |
| PMM-1043 | 30,000 | 5,500 | 0.63 | -65 | -121 | < 100 | 1.00 | 1.425 | 24.8 | 2.82 | 285 | 482 | 8-12 | 40,000-50,000 |
| PMM-5021 | 125 | 20 | 0.78 | -51 | -100 | 1,000-1,400 | 1.07 | 1.5 | 24.5 | 2.87 | 296 | 482 | 45-50 | 2,000-2,200 |
| PMM-6025 | 500 | 60 | 0.79 | -34 | -- | 1,500-1,900 | 1.08 | 1.507 | -- | 2.89 | 285 | 482 | 58-62 | 3,500-4,000 |
Phenylmethylsiloxane homopolymers CAS: [9005-12-3]
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PMM-0011 | 10-20 | <5 | -- | -55 | -- | >flashpoint** | 1.01 | 1.47 | -- | -- | 220 | -- | -- | 350-450 |
| PMM-0021 | 100-200 | 14-20 | 0.79 | -- | -- | 1,600-2,100 | 1.09 | 1.52 | -- | 2.93 | 280 | 484 | -- | 700-1,200 |
| PMM-0025 | 500 | 35 | 0.88 | -20 | -86 | 1,500-2,000 | 1.11 | 1.533 | 28.5 | 2.95 | 300 | 487 | -- | 2,500-2,700 |
**Unsafe operating temperature in air.
Phenylmethylsiloxane-diphenylsiloxane copolymers CAS: [308073-01-0]
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % (phenylmethylsiloxane) | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PMP-5025 | 300-600 | -- | -- | -10 | -- | -- | 1.1 | 1.543 | -- | -- | 300 | -- | 45-55 | 600-800 |
Phenylmethylsiloxane oligomers - diffusion fluids
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1,1,5,5-Tetraphenyl-1,3,3,5-tetramethyltrisiloxane CAS: [3982-82-9 PDM-7040 | 35-40 | <5 | -- | -35 | -- | >flashpoint** | 1.07 | 1.556 | 37.3 | -- | 221 | 425 | -- | 485 |
| 1,1,3,5,5-Pentaphenyl-1,3,5-trimethyltrisiloxane CAS: [3390-61-2] PDM-7050 | 170-175 | 6 | -- | -15 | -- | >flashpoint** | 1.09 | 1.588 | 36.5 | -- | 245 | 440 | -- | 547 |
**Unsafe operating temperature in air.
Tetrachlorophenylsilsesquioxane-dimethylsiloxane copolymers CAS: [68957-05-1]
| Product code | Viscosity 25 °C (cSt) | Viscosity 99 °C (cSt) | Viscosity temperature coefficient | Pour point (°C) | Transition temperature (Tg °C) | Gel time (hours) 250 °C in air* | Density | Refractive index | Surface tension | Dielectric constant | Flashpoint (°C) | Ignition temperature (°C) | Comonomer mole % | Molecular weight |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PTT-1117 | 70 | 18 | 0.68 | -73 | -- | 270-280 | 1.05 | 1.428 | 21 | 2.9 | 300 | 480 | -- | 1,600-3,000 |
The phenyl group also introduces rigidity in the silicone chain. When substitution exceeds 75 mole %, the polymers are solid. Refractive index also increases with phenyl concentration. At 15-16 mole % phenyl concentrations, the refractive index matches that of optical fibers and amorphous silica allowing “invisible” connections and transparent blends.
Low viscosity phenyl fluids, PDM-7040 and PDM-7050, are used as diffusion pump fluids. Chlorinated aromated siloxanes provide superior lubrication for metal-metal contact. The polyphenylmethylsiloxanes also exhibit good radiation resistance, remaining serviceable up to 200 megarads exposure.
The compressibility of phenyl-containing siloxanes is reduced in comparison to dimethyl fluids. The compressibility of selected thermal fluids at 20,000 psi are as shown in the accompanying table.
| Thermal Expansion, (25-150 °C), cc/cc/°C | 7.5-9.4 x 10-4 |
| Thermal Conductivity, cal/(sec.)(cm-2)(°C/cm) | 3 x 10-4 |
| Specific Heat, 38 °C, cal/gm/°C | 0.34-0.39 |
| Volume Resistivity, Ω-cm | at 25 °C: 1-4 x 1014 at 50°C: 1-5 x 1012 |
| Dielectric Strength, kV/mm | 14 |
| Dissipation Factor | at 102 Hz: 1.1-5.1 x 10-4 at 106 Hz: 0.1-1.1 x 10-4 |