Silicone gum is too thick to pour, so buy the blend by gum content, not viscosity

Your formulator approves a leave-on hair serum built on a lab sample labeled silicone gum, and purchasing asks three suppliers to quote it. One offers neat gum in a pail, too stiff to pour. The other two offer blends, one in cyclopentasiloxane and one in a light dimethicone fluid, each with a viscosity close to the sample’s. None of the quotes states how much gum is in the drum, and that is the number the formula was built on.

Silicone gum names a class of polymer, not a product. The order line has to close that gap.

What is silicone gum?

Silicone gum is a silicone polymer, such as dimethicone or dimethiconol, at a molecular weight so high that it barely flows at room temperature, so it is supplied as a neat gum, pre-dissolved in a lighter silicone as a blend, or, for rinse-off products, as an emulsion. One dimethiconol CAS number, 31692-79-2, spans grades from a 70 cSt fluid to a 600,000 cSt gum at 25 °C.

Molecular weight sets the class. US Patent 6,719,967 (2004) defines silicone gums as polydiorganosiloxanes with a molecular weight of 200,000 to 1,000,000, a figure the patent gives without a unit, and names dimethiconol gums, dimethicone gums and certain phenyl and vinyl copolymer gums as members. The same patent records that polydimethylsiloxanes with hydroxyl end groups carry the ingredient name dimethiconol. Dimethicone chains end in methyl caps instead. How the two compare grade for grade is covered in what dimethiconol is and how it differs from dimethicone.

Silicon gum is a misspelling, not a different material. Silicon is the element; silicone is the polymer built on a backbone of alternating silicon and oxygen atoms with organic groups such as methyl attached, and only the polymer forms a gum. Silica, silicon dioxide, is an inorganic solid and not a silicone at all.

One identity trap is worth knowing before any quote arrives. PubChem cross-references CAS 31692-79-2 to CID 14014, a record titled dimethylsilanediol at 92.17 g/mol. That is the one-silicon monomer, not the gum, so any molecular weight on a gum specification has to come from the supplier’s grade data.

Where does a silicone fluid end and a gum begin?

The FDA/NCATS Global Substance Registration System (GSRS, records retrieved 2026) carries CAS 31692-79-2 as a family code on six dimethiconol grade records, each with a reported kinematic viscosity at 25 °C and an average molecular weight. Four of them show the climb, set against the patent definition of a gum:

Read the table for two things. First, viscosity climbs far faster than molecular weight at the top. From the 100,000 cSt record to the 600,000 cSt record, molecular weight rises from 140,000 Da to 250,000 Da, less than double, while viscosity rises six-fold. That steep end of the curve is where a fluid stops pouring and a gum starts, and it is why gums are hard to dose neat.

Second, molecular weight is a soft number. GSRS files its 600,000 cSt record under two names, 250000 MW and 260000 MW. Write the specification in viscosity, with unit, temperature and method, because your receiving lab can test it, and add nominal molecular weight only where your formula is defined by it; a gum in the millions of cSt needs a rotational or plasticity test, not a capillary one.

The grades below the gum band are silicone fluids; their viscosity ladder and uses are set out in silicone oil types, properties and applications.

Why silicone gum blends are sold pre-dissolved

A gum that barely flows is hard to pump, meter or disperse into a batch at room temperature, so gums are also sold already dissolved in a lighter silicone. The carrier brings the material down to a viscosity a plant can handle. GSRS carries three such blends of dimethiconol in a lighter carrier, each indexed under CAS 31692-79-2:

Each record describes its blend as approximately 12-20 of an ultra-high viscosity dimethiconol in low viscosity diluents, and none states the unit.

Now read the viscosity column. Two dimethicone blends carrying the same description sit at 3200 cP and 6500 cP, about double. The band is wide enough that the two may differ in gum content, gum grade or both, and the viscosity figure cannot say which. A blend’s viscosity moves with the gum’s chain length and the carrier as well as with the amount of gum, so viscosity alone cannot tell you how much gum you are paying for.

The refractive index will not identify the carrier either. The dimethicone blend reads 1.396 and the cyclopentasiloxane blend 1.397, a difference in the third decimal place, and GSRS flags both values as approximate. The carrier has to be named on the Certificate of Analysis (CoA), not inferred from a refractometer. If the carrier is cyclopentasiloxane, check its status in each market the finished product ships to; the cyclopentasiloxane safety brief covers the EU rules.

Watch the units as well. GSRS files the blend figures in cP, a dynamic viscosity, under a kinematic label, while the neat grades are in cSt. A quote comparing a blend in cP with a neat gum in cSt is comparing two different measurements.

The carrier also decides what stays behind. Cyclopentasiloxane (CAS 541-02-6) is a volatile carrier: in a leave-on product it evaporates and leaves the gum as a film. A non-volatile dimethicone stays in the film with the gum and changes the after-feel; ask which kind a dimethicone blend uses, because the lightest dimethicone grades are volatile too. Both components appear in the blend’s name, and both reach your ingredient list.

What to write on the order line for a silicone gum blend

1. Gum identity: dimethiconol (hydroxyl ends, CAS 31692-79-2) or a dimethicone gum, with the gum’s own viscosity or nominal molecular weight from the supplier’s grade data.

2. Gum content: weight percent of the blend, stated on the Technical Data Sheet (TDS). For a volatile carrier, ask for non-volatile content per lot, since in a two-component blend that residue is the gum you keep.

3. Carrier: by name and CAS number, and whether it is volatile.

4. Blend viscosity: a window with unit, temperature and test method, and the measured lot value on each CoA. Use it as a lot-to-lot consistency check, not as a measure of gum content.

5. Documents: a CoA per lot, the TDS, and the current Safety Data Sheet (SDS) for the exact blend. Take hazard and handling information from that SDS.

What are silicone gums used for?

In cosmetics, the Cosmetic Ingredient Review’s 2017 safety assessment, published in the International Journal of Toxicology, states that dimethiconol and its esters and reaction products are used as skin-conditioning agents or hair-conditioning agents. PubChem’s household-products section (retrieved 2026) lists 277 consumer products containing dimethiconol of any grade, not gums alone, filed under Personal Care and Inside the Home.

In leave-on hair and skin products, the gum is bought for feel and film. A blend in cyclopentasiloxane spreads as a liquid, the carrier evaporates, and a thin film of high-molecular-weight gum remains. That film gives the slip and smooth after-feel formulators specify the gum for, and a heavier gum or a higher gum content changes it.

Rinse-off products take the gum as an emulsion, and two patents and a patent application mark out the design space. US Patent 6,274,130 (2001) claims a rinse-off conditioner with emulsion-polymerized dimethiconol at 0.01-10 wt% and a viscosity of 1 to 20 million cSt, and states in its description that higher viscosity increases the conditioning effect obtainable. US Patent 6,719,967 (2004) states in its description that silicone gums with a slight degree of cross-linking impart body, volume and stylability to hair.

Patent application WO1998018434A1 (1998) places a dimethiconol-type emulsion silicone in a conditioning shampoo at 0.005-10 wt%, with 0.2-3 wt% most preferred. These ranges are published claims and examples, not evidence of current practice and not a recommendation to work inside them.

Beyond conditioning, the PubChem record this CAS number points to, titled dimethylsilanediol, lists antifoaming among its cosmetic functions, drawn from the EU cosmetic ingredient inventories of 2000 and 2006; the entry does not name the ingredient it was filed for. Outside personal care, high-molecular-weight silicone gums are the base polymer of heat-cured silicone rubber, compounded with silica and crosslinked, and they are combined with MQ silicone resin in silicone pressure-sensitive adhesives. Those gums are built for compounding and are a separate purchase with their own specification.

Methodology: grade and blend data from the FDA/NCATS GSRS records for CAS 31692-79-2 (retrieved 2026); identity cross-references from PubChem (CID 14014), CAS Common Chemistry and ECHA; the gum definition and formulation ranges from US Patents 6,719,967 (2004) and 6,274,130 (2001) and PCT application WO1998018434A1 (1998), cited as published data; cosmetic function from the Cosmetic Ingredient Review (2017).