Molecular weight and
A silicone’s molecular weight is a function of its degree of polymerization
(DP) – the number of repeating Si-O-Si units in the polymer.
The greater the degree of polymerization, the higher the molecular
The higher the molecular weight, the longer the polymer
The longer the polymer, the higher the viscosity
The higher the viscosity, the more slowly the polymer will flow
Silicone fluids range in viscosity from 0.65 cSt (thinner than water) to
more than 20,000,000 cSt (thicker than chewing gum).
Effect of shear forces
At low molecular weights, linear poly(dimethylsiloxane), or PDMS, polymers
behave essentially as Newtonian liquids (their viscosity is not influenced by
shear rate). This is true to shear rates in excess of 1,000 s-1.
At molecular weights above 10,000 Da, PDMS polymers become entangled and
exhibit a viscoelastic response and a decrease in viscosity (or shear-thinning
behavior) at modest shear rates. Learn more about how molecular weight and
shear rate impact the viscosity of PDMS polymers.
Silicone polymers can also be engineered to be:
Thixotropic – Thixotropic silicone formulations may appear to be
solid at rest. However, they are easy to mix and spread. Once the mixing or
spreading force is removed, they return to their original solid-like state.
This is a benefit in hand creams, cosmetics, auto and furniture polishes,
paints, sealants, and two-part moldmaking rubbers.
Dilatant (shear thickening) – Dilatant silicone formulations appear
flexible, but harden when subjected to shear deformation.
Silly Putty® is an example of a dilatant silicone.
Putty® is a registered trademark
of Binney & Smith, Inc.
Viscosity and performance
Viscosity has relatively little effect on the chemical properties of a
silicone fluid, but it does affect flow behavior and solubility.
High-viscosity silicones exhibit more resistance to flow than
low-viscosity silicones. But they do flow – and both are capable of forming
very thin films.
High-viscosity silicones are less soluble in some solvents than
| ||Did you know ... silicones give you more
viscosity choices than hydrocarbon fluids, making it easier|
to “dial in” the exact viscosity you need!
Effect of temperature
Think about what happens to the viscosity of honey if you put it in the
freezer or in the oven.
Question: What do you think would happen if you subjected a
polydimethylsiloxane fluid to the same conditions?
Answer: Not much! The viscosity of silicones changes significantly
less with temperature than does the viscosity of organic materials.
How silicone viscosity is modified to meet
By blending two different viscosity fluids together
By diluting the silicone in solvent or emulsifying it in water to create
solvent or aqueous delivery systems
By compounding with fillers or crosslinking to create more viscous, almost
solid, delivery systems that produce thicker films
All of these methods will change the flow behavior and solubility of the
silicone, enabling you to use it in an ever-widening range of applications,
formulations, and manufacturing processes. Explore the many ways
silicones can be tailored to meet your needs.