Silicones sit in an awkward spot for buyers. The fluid grades have no assay that means anything, the cyclic grades are regulated for how fast they evaporate, and the coupling agents are priced per kilogram but judged per hydrolyzed bond. Four checks reorganize everything: what the polymer distribution is, what the chain ends are, how much volatile cyclic is in the drum, and how many alkoxy groups are left to react. Walk a spec sheet in that order and most quotes become comparable.

One CAS Number, No Single Molecular Weight

Polydimethylsiloxane fluid is sold under CAS 63148-62-9, a UVCB entry: a family of chain lengths, not a single compound with one formula. That is why a serious silicone oil specification leads with kinematic viscosity in cSt at 25 °C and little else. It also explains a catalog oddity: some listings titled silicone oil carry a molecular weight of 162.38, a melting point of −59 °C and a boiling point of 101 °C. Those constants describe hexamethyldisiloxane, the disiloxane capping agent, not the 350 cSt drum. Neither number is wrong; both belong to the low end of the distribution. Compare quotes on the viscosity grade and its tolerance, the volatiles content and the flash point, and treat any molecular weight printed beside a UVCB CAS as a clue about the oligomer mix, not a purity figure.

The Chain Ends Decide What the Fluid Can Do

Trimethyl-blocked PDMS is chemically inert, the default for defoaming and lubrication. Hydroxyl-terminated fluid is the opposite: the terminal silanol groups condense with alkoxysilane crosslinkers and with filler surfaces, the basis of RTV sealants and adhesive grades. A listing of carbinol terminated polydimethylsiloxane under CAS 67674-67-3 shows a density of 1.02 g/cm³ and, buried in the synonym line, names like heptamethyltrisiloxane ethoxylated with a 35–40 cSt viscosity. Those details describe a polyether-modified trisiloxane, a superwetting surfactant, not neat silanol PDMS, which runs near 0.97 g/cm³. The surfactant drops the surface tension of aqueous sprays far below water's 72 mN/m and is used in tank mixes with actives such as glyphosate. Silanol fluid builds networks.

Cyclics: Volatility Is the Product and the Problem

D4 and D5 are how the industry buys volatility. D4 (CAS 556-67-2, C8H24O4Si4, 296.62 g/mol) melts at 17.5 °C, boils at 175 °C and shows a refractive index of 1.397. D5 (CAS 541-02-6, C10H30O5Si5, 370.77 g/mol) has a specific gravity of 0.959 and boils at 90 °C under 10 mmHg. In antiperspirants and hair sprays, decamethylcyclopentasiloxane carries the active, then evaporates and leaves the film. The same volatility drives the regulatory side: the EU limits D4 and D5 to 0.1 percent in wash-off cosmetic products, and D4 carries a PBT classification. In industrial use the cyclics are the impurity of record inside linear fluids, because volatile rings can outgas and recondense on relay contacts and optical surfaces. So the volatiles line on a PDMS sheet predicts both the regulatory lane your finished product lands in and its outgassing behavior.

Verify the CAS Against the Structure, Not the Title

Catalog data is transcribed by hand, and transcription errors survive into live listings. Two entries for octamethylcyclotetrasiloxane are in circulation: one correctly under CAS 556-67-2, and one sitting under CAS 506-59-2, a number that belongs to dimethylamine hydrochloride. The synonym block of that mislabeled page still lists amine salts, yet its physical constants, a melting point of 17–18 °C, a boiling point of 175–176 °C and an n20/D of 1.396, are pure D4, and its CAS-database field points to 556-67-2. The lesson generalizes across this whole family: cross-check the CAS against the molecular formula and the physical constants. A colorless liquid with formula C8H24O4Si4 cannot be dimethylamine hydrochloride, a white crystalline solid.

From Fluid to Resin: the Silsesquioxane Family

Between fluid and filler sits the T-resin family, where each silicon carries one methyl and 1.5 oxygens. Poly(methylsilsesquioxane) (CAS 68554-70-1) is the workhorse: a methyl grade runs about 4–6 percent residual hydroxyl, density 1.08 g/cm³, refractive index 1.42, and it cures into hard, thermally stable films for release coatings and high-temperature paint systems. The same Si-O-Si backbone explains the filler side of the family. Unfilled silicone gum has negligible tensile strength, and it is reinforcing silica, CAS 112926-00-8, that turns it into a rubber. There the specification that matters is surface area and surface treatment, because the silanol density on the filler surface sets both reinforcement and the viscosity penalty during compounding.

Silane Coupling Agents: Count the Alkoxy Groups

Trialkoxysilanes are bought for what happens after hydrolysis. Si-OR becomes Si-OH, which condenses with hydroxylated surfaces such as glass, metal oxides and mineral fillers, and with itself, releasing an alcohol. Methoxy silanes release methanol, ethoxy silanes release ethanol; the choice is a handling question: methoxy versions react faster and forgive substrate moisture less. Three grades cover most inquiries.

Triethoxyoctylsilane (CAS 2943-75-1, 276.49 g/mol, bp 84–85 °C at 0.5 mmHg) is a penetrating water repellent for concrete and masonry. It wicks into the pore structure, hydrolyzes in place and leaves a polysiloxane network inside the wall rather than a film on top, which is why depth of penetration, not dosage, is the figure acceptance tests measure.

Vinyltrimethoxysilane (CAS 2768-02-7, 148.23 g/mol, bp 123 °C, flash point 26–28 °C) is grafted onto polyethylene in the presence of peroxide, then moisture-cures into a crosslinked Si-O-Si network. That two-step chemistry is the basis of crosslinked cable insulation and heat-shrink tubing, and the spec sheet that matters is one for grafting efficiency, not just GC purity.

Ethyl silicate (TEOS, CAS 78-10-4, 208.33 g/mol, bp around 168 °C) hydrolyzes all the way to silica. It is the binder in sol-gel coatings, investment casting molds and zinc-rich primers, where the content worth comparing is the silica the liquid will become, and every mole of it releases four moles of ethanol on the way there.

What to Confirm Before You Compare Prices

Put the same eight lines in front of any silicone quote. First, the viscosity grade and its tolerance, because a 350 cSt fluid with no tolerance is not a specification. Second, the end group: blocked, silanol, or polyether-modified, with a titrated hydroxyl figure where it matters. Third, volatiles and D4/D5 content, which decide both regulatory lane and outgassing. Fourth, the CAS checked against formula and constants, not the title. Fifth, flash point and transport classification, since some cyclic-rich and silane grades ship as flammable liquids. Sixth, for coupling agents, the alkoxy count per molecule and the alcohol byproduct. Seventh, storage terms, because moisture-curing starts the moment the seal breaks. Eighth, a sample tested in your own formulation, because in this family the name on the drum tells you less than in any other category you source.