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Terminal silanol groups render polydimethylsiloxanes susceptible to condensation under both mild acid and base conditions. They are intermediates for most room temperature vulcanizable (RTV) silicones. Low molecular weight silanol fluids are generally  produced by kinetically controlled hydrolysis of chlorosilanes. Higher molecular weight fluids can be prepared by equilibrating  low molecular weight silanol fluids with cyclics, equilibrium polymerization of cyclics with water under pressure or methods of polymerization that involve hydrolyzable end caps such as methoxy groups. Low molecular weight silanol fluids can be condensed  to higher molecular weight silanol fluids by utilization of chlorophosphazene (PNCl2) catalysts.

Condensation cure one-part and two-part RTV systems are formulated from silanol-terminated polymers with molecular weights  ranging from 15,000 to 150,000. One-part systems are the most widely used. One-part systems are crosslinked with moisture-sensitive multifunctional silanes in a two stage reaction. In the first stage, after compounding with fillers, the silanol is reacted with an excess of multifunctional silane. The silanol is in essence displaced by the silane. This is depicted below for an acetoxy system.

The silicone now has two groups at each end that are extremely susceptible to hydrolysis. The silicone is stored in this form and  protected from moisture until ready for use. The second stage of the reaction takes place upon use. When the end groups are exposed to moisture, a rapid crosslinking reaction takes place.

The most common moisture cure systems are:

The crosslinking reaction of alkoxy systems are catalyzed by titanates, frequently in combination with tin compounds and other metal-organics. Acetoxy one-part systems usually rely solely on tin catalysts. The tin level in one-part RTV systems is minimally about 50 ppm with a ratio of ~2500:1 for Si-OR to Sn, but typical formulations have up to 10x the minimum. Other specialty crosslinking systems include benzamido and mixed alkoxyamino. The organic (non-hydrolyzable) substituents on the crosslinkers influence the speed of cure. Among the widely used crosslinkers, vinyl-substituted is the fastest: vinyl > methyl > ethyl >> phenyl.

Two-part condensation cure silanol systems employ ethylsilicates (polydiethoxysiloxanes) such as PSI-021 as crosslinkers and dialkyltincarboxylates as accelerators. Tin levels in these systems are minimally 500 ppm, but typical formulations have up to 10x the minimum. Two-part systems are inexpensive, require less sophisticated compounding equipment, and are not subject to
inhibition.

The following is a starting point formulation for a two-part RTV. (Flackett, D., “One Part Silicone Sealants in Silicon Compounds: Silanes and Silicones”, 433-439, 2004)

10:1 ratio of A to B

Part A:

Part B:

This low tear strength formulation can be improved by substituting fumed silica for silica powder.

Incorporation of hydride functional (Si-H) siloxanes into silanol elastomer formulations results in foamed structures. The blowing agent is hydrogen which forms as a result of silanol condensation with hydrosiloxanes. Foam systems are usually two components which are compounded separately and mixed shortly before use.

Condensation Cure Catalysts
Condensation Cure Crosslinkers

Silanol-terminated diphenylsiloxane copolymers are employed to modify low temperature properties or optical properties of silicone RTVs. They are also utilized as flow control agents in polyester coatings. Diphenylsiloxane homopolymers are glassy materials with softening points > 120 °C that are used to formulate coatings and impregnants for electrical and nuclear applications.

The reactivity of silanol fluids is utilized in applications other than RTVs. Low viscosity silanol fluids are employed as filler treatments and structure control additives in silicone rubber compounding. Intermediate viscosity (1,000-10,000 cSt) fluids can be applied to textiles as durable fabric softeners. High viscosity silanol-terminated fluids form the matrix component in tackifiers and pressure-sensitive adhesives.

Silanol-Terminated Polydimethylsiloxanes, CAS: [70131-67-8], TSCA

Product CodeViscosity (cSt)Molecular WeightWt% OHOH (eq/kg)Refractive IndexDensity
DMS-S1216-32400-7004.5-7.52.3-3.50.951.401
DMS-S1435-45700-1,5003.0-4.01.7-2.30.961.402
DMS-S1545-852,000-3,5000.9-1.20.53-0.700.961.402
DMS-S2190-1204,2000.8-0.90.47-0.530.971.402
DMS-S27700-80018,0000.20.11-0.130.971.403
DMS-S311,00026,0000.10.055-0.0600.981.403
DMS-S322,00036,0000.090.050-0.0550.981.403
DMS-S33*3,50043,5000.080.045-0.0500.981.403
DMS-S355,00049,0000.070.039-0.0430.981.403
DMS-S4218,00077,0000.040.023-0.0250.981.403
DMS-S4550,000110,0000.030.015-0.0170.981.403
DMS-S5190,000-150,000139,0000.020.010-0.0150.981.403
*also available as an emulsion [button skus="DMS-S33M50"]DMS-S33M50[/button]

Silanol-Terminated Diphenylsiloxane-Dimethylsiloxane Copolymers, TSCA

Product CodeMole % DiphenylsiloxaneViscosity (cSt)Molecular WeightWt% OHRefractive IndexCAS
PDS-03382.5-3.56,000-8,00050,0000.4-0.71.420[68083-14-7]
PDS-161514-1850-60900-1,0003.4-4.81.473[68951-93-9]
Employed as gloss enhancing additive for organic coatings and color stabilizers in sintered PTFE composites.

Silanol-Terminated Polydiphenylsiloxane, CAS: [63148-59-4], TSCA

Product CodeMole % DiphenylsiloxaneViscosity (cSt)Molecular WeightWt% OHRefractive Index
PDS-9931100glassy solid1,000-1,4002.4-3.41.610
Tm: 142-155 °C; contains cyclics

Silanol-Terminated Polytrifluoropropylmethylsiloxane, CAS: [68607-77-2], TSCA

Product CodeMole % TrifluoropropylmethylsiloxaneViscosity (cSt)Molecular WeightWt% OHRefractive IndexDensity
FMS-992110050-160550-8004-71.3791.28
FMS-9922100150-250800-1,2003-51.3791.28

Silanol-Trimethylsilyl-Modified Q Resins, CAS: [56275-01-5], TSCA

Product CodeWt% Q ResinMolecular WeightWt% OHBase ResinSolvent
SQO-2991003,000-4,0001.7-2.0----
SQD-255503,000-4,000----50% D5
SQT-221603,000-4,000----40% toluene
SQS-26135-403,000-4,000--DMS-S61*40% toluene
*300,000-400,000 MW silanol-terminated polydimethylsiloxane
Silanol-Trimethylsilyl-Modified Q resins are often referred to as MQ resins. They serve as reinforcing resins in silicone elastomers and tackifying components in pressure-sensitive adhesives.

For silanol-terminated vinylmethylsiloxane copolymers, see Vinylmethylsiloxane-Dimethylsiloxane Copolymers, silanol-terminated