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Fascinating Silicone™ Chemistry Corner – Rheology

Silicone rheology

Molecular weight and viscosity

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 weight
  • 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 advertisement

Silly Putty® is an example of a dilatant silicone.

Silly 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 low-viscosity silicones.
 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.

Honey

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 your needs

  • 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.

Did you know ... the relationship between silicone molecular weight and viscosity is linear, up to a point – the 10,000 Da molecular weight point! Learn why.

Viscosity vs. Molecular Weight

Expand your understanding

Learn about:
Silicone vs. organics
Typical performance properties
Commercial benefits
Surface & interface applications

For chemists and engineers:
Viscosity vs. molecular weight comparison graph

Explore your material options

Get answers

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