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Viscosity (resin) - A203

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Viscosity (resin)
Foundational knowledge article
Material properties-UakV3h9hweWZ.svg
Document Type Article
Document Identifier 203
Relevant Class



Introduction[edit | edit source]

The viscosity (\(\mu\)) of a fluid represents its resistance to deformation at a given rate. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations. By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Scope[edit | edit source]

This page defines resin viscosity and explains its significance to composites processing. Typical values for various polymer matrices and everyday encountered fluids are provided. Finally, a brief introduction to viscosity measurement is provided.

Significance[edit | edit source]

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing.

Resin viscosity is significant for:

  • Fibre wetout
  • Controlling how well/long the resin holds in place (e.g. critical in adhesives, control of resin bleed, laminate gas pathways for laminate gas evacuation, etc.)

Prerequisites[edit | edit source]

Recommended documents to review before, or in parallel with this document:

Definition[edit | edit source]

The viscosity (\(\mu\)) of a fluid represents its resistance to deformation at a given rate, and in simplest form is:

\(\mu = \frac{\tau}{\dot{\gamma}}\)


\(\tau = \) Shear stress

\(\dot{\gamma} = \) Shear rate

Whereas the viscosity of a thermoplastic drops exponentially but monotonically with increasing temperature, thermoset resins behave in a more complex fashion. Initially, viscosity drops in the same manner as a thermoplastic, but once cure kicks off, the increasingly higher molecular weight and cross-linked polymer structure increases the viscosity rapidly, until gelation occurs, and viscosity becomes unmeasurably large. Past gelation, the material is no longer a fluid and should be treated as a viscoelastic solid.

Typical Property Values[edit | edit source]

The following table, reproduced from Hoa [1], shows some typical values of various polymer matrix materials, and other common fluids encountered in everyday life.

Material 20 °C T °C
(mPa·s) (mPa·s)
Air 0.0187
Water 1
Polyester (thermoset) 100-300
Vinyl ester (thermoset) 100-300
#10 Motor oil 500
Golden syrup 2500
Epoxy (Shell Epon 828 14phr MPDA, 15 phr BGE) (thermoset) 600
Epoxy (Shell 826 16 phr MPDA, 10 phr BGE) (thermoset) 750
Epoxy (Dow 332 16phr MPDA, 10 phr BGE) (thermoset) 500
Molasses 105 @ 25 °C
Epoxy 5208 (thermoset) 100 @ 177 °C
BMI (thermoset) 1000 @ 150 °C
Ryton (thermoplastic) 107 @ 313 °C
PEEK (thermoplastic) 106 @ 400 °C
Ultem (thermoplastic) 108 @ 305 °C
Torlon (thermoplastic) 109 @ 350 °C

The viscosity of thermoplastics is orders of magnitude greater than the short monomers that make up thermosetting polymers prior to the thermoset curing process. Thermoplastics require elevated processing temperature in order to flow. Even heated, they still require substantial applied pressure to accommodate their relatively high viscosity.

Process Dependencies[edit | edit source]

Resin viscosity ranges considerably across different resin chemistries and formulations. Additionally, resin viscosity is directly influenced by the following factors:

  • Temperature (increase temperature = viscosity decrease)
  • Degree of cure – thermoset polymers (increase DOC = viscosity increase)
  • Resin fillers and tougheners (increase particulate content = viscosity increase)

Thermal management of viscosity[edit | edit source]

Two of the above listed viscosity influencing factors can be controlled during processing through thermal management – control of temperature. The effects of temperature and its downstream linked degree of cure (DOC) are illustrated below for a thermoset resin system.

Example of resin viscosity behaviour against temperature and degree of cure (DOC) effects.

Resin viscosity drops as a function of increasing temperature. Countering this resin viscosity drop, the thermoset crosslinking reaction takes place with elevated temperature and sees increasing molecular weight and restriction in mobility of the polymer – resulting in a corresponding increase in resin viscosity as the thermosetting reaction progresses. During a heated cure cycle, the resin therefore experiences the following sequence:

  1. An initial lowering of resin viscosity as the resin is being heated, this is quickly countered and overtaken with,
  2. An increasing resin viscosity due to the thermoset reaction effects.
  3. Resin viscosity will continue to increase until the resin reaches the gelation point, where it takes on the behaviour of a semi-solid.

Viscosity relationship to temperature and degree of cure[edit | edit source]

The resin viscosity relationship to temperature and its degree of cure can be described using an empirical model for an Arrhenius-type equation shown below:

\(\mu = Ae^{\frac{\Delta E}{RT}f(x)}\)


\(A = \) A constant

\(\Delta E = \) Activation energy

\(RT = \) Kinetic energy as a function of temperature

\(f(x) = \) Dependency on the degree of cure

These two competing factors (temperature, and DOC – resin reaction) affecting resin viscosity must be balanced for setting the resin’s processing window. This is of particular importance in resin infusion process steps, where liquid resin must fully infuse the fibre preform – sometimes with flow lengths in the range of metres. Increasing temperature lowers the resin viscosity which aids in resin/fibre wetout. This is the fundamental motive behind the practice of performing heated resin infusion. However, in doing so, there is also an associated risk that the elevated temperature kicks off and accelerates the resin reaction. This can quickly become the dominating effect by rapidly and irreversibly increasing the resin viscosity. If the resin’s degree of cure progresses too quickly, this will quickly inhibit the resin’s ability to flow and ultimately reduce the time available for the resin to flow. Insufficient resin infusion may occur, and result in dry spots with a partially resin infused fibre preform.

Resin fillers and tougheners[edit | edit source]

Whether as resin fillers or matrix toughers, the addition of solid particles to a liquid resin results in a viscosity increase to the combined mix. At low particle concentrations, and for spherical particles greater than 1 \(\mu\)m in size, this relationship is described Einstein’s equation that can predict the effective resin viscosity of the mix.

Einstein's equation for dilute suspensions:

\(\mu_{eff} = \mu_0 (1 + 2.5 \phi)\)


\(\mu_{eff} = \) Resulting effective mix viscosity

\(\mu_0 = \) Viscosity with no particle content

\(\phi = \) Volume fraction of particles

The viscosity increase is more prevelent for higher particle concentrations and smaller particle sizes, where particle-to-particle interactions result in even greater viscosity increases than the relationship predicted by Einstein’s equation. This viscosity increase needs to be considered for infusion processes where high particle loading is desired, or when small particles such as nano-scale particle tougheners are intended to be added to the liquid resin.

The following knowledge base article by Anton Paar (equipment manufacturers of rheometers) discusses the influences of particles on suspension rheology [2].

Composite Viscosity[edit | edit source]

Link to Link to main Viscosity (composite) page

While it is common practice to isolate and examine the viscosity of the resin matrix component on its own, it is important to consider the influence of the other composite constituent materials when the entire combined system of the matrix and the reinforcement materials are considered together as a whole viscous material.

When examining the composite material system as a whole, the addition of the reinforcement fibres has an enormous influence viscoelastic response of the material system. The fibres are able to resist shear deformation in proportion to the shear modulus of the fibre bed due to the waviness of the fibres, with this effect being higher for high volume fraction composites [3]. The fibre bed’s ability to absorb and dissipate the applied shear forces acts to essentially “thicken” the viscous behavioural response of the fibre/resin composite material. As a result, the effective viscosity of the composite material is much higher when compared to examining solely the resin viscosity on its own.

Click here to visit the Viscosity (composite) page and learn more.

Measurement[edit | edit source]

Laboratory rheometer testing instrument.

Resin viscosity is typically measured using a rheometer with parallel plate geometry, and disposable aluminum plates are used since the material cures to the plates. A rheometer is a specialized laboratory device that examines the response of a liquid, suspension, or soft viscous solid when subjected to applied forces. In particular, the method described here concerns the use of rotational shear type rheometers, examining a liquid’s response to shear forces.

Viscosity measurements can be made using two different plate rotational motions:

  • Continuous rotation
  • Oscillation

Rheometer rotational motions: (left) continuous rotation, (right) rotational oscillation. Reproduced from [4] .

Rheometers can operate with either continuous rotation or rotational oscillation motion of the plate, cone, etc. setup. The information that can be obtained differs between the two motions, and each has its advantages and disadvantages.

Continuous Rotational Motion Oscillating Rotational Motion
  • More closely simulates fluid flow processes dependent on flow velocity or volume flow rate [4]
  • Can provide information past gelation point - when sample becomes solid
  • Can only measure up to gelation point, can't determine when resin is completely cured
  • Nature of shear forces imposed on resin may not be representative of resin flow process (compared to continuous rotation)
Illustration of a typical resin viscosity measurement rheometer setup using parallel plates.

To approximate the resin’s gelation point, both rheometer motions can be used. However, for curing polymers, oscillating motion is generally considered preferable as it can measure and provide information past the gelation point until the sample becomes a solid [5].

To learn more about viscosity and viscosity measurement, the following rheology knowledge base document by Anton Paar (manufacturer of rheometer equipment) [4] is a good resource.

Related pages

Page type Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents
Case Studies
Perspectives Articles


  1. [Ref] Hoa, S V (2018). Principles of the Manufacturing of Composite Materials. DEStech Publications, Incorporated. ISBN 9781605954219.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  2. [Ref] Empty citation (help)
  3. [Ref] Thorpe, Ryan J. (2013). Experimental Characterization of the Viscoelastic Behavior of a Curing Epoxy Matrix Composite from Pre-Gelation to Full Cure. MASc thesis (Thesis). doi:10.14288/1.0073820.CS1 maint: uses authors parameter (link)
  4. [Ref] Anton Paar. "Basics of rheology". Retrieved 21 January 2021.CS1 maint: uses authors parameter (link)
  5. [Ref] Anton Paar. "Time-dependent behavior with gel formation or curing". Retrieved 21 January 2021.CS1 maint: uses authors parameter (link)

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