By Alain Michout @ M51 Resources, Inc.

An important factor to take into consideration when thinking about matrix material is its viscosity. Viscosity is the measure of a fluid’s resistance to deformation at a given rate a.k.a. a fluid’s resistance to flow under shear stresses.

Thereby viscosity (η) can be expressed as the ratio of the shearing stress to the velocity gradient in a fluid and can be formulated as:

or

Another way to understand this relationship would be to use Newton’s Equation stating that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. A clear reference to Newton’s second law of motion (F=m*a).

The SI unit of viscosity is the Pascal second but is rarely used in practice.

Today most fluid engineers use dyne second per square centimeter (dyne s/cm^2) also known as the Poise.

The common unit used to express absolute viscosity is “centipoise”. A centipoise is 1/100 of a Poise and 1/1,000 of a Poiseuille. It is named after the French physician Jean Louis Poiseuille based on his 1846 paper documenting his research into the flow resistance of liquids.

The centipoise is the preferred measure for industry because water happens to have a viscosity of 1.002 centipoise (cP) at 20˚C, making it easier for mental comparisons between fluids.

Here are some other fluids cP at 68˚F/20˚C:

Acetone 0.3
Water 1.002
Pancake Syrup 2,500
Ketchup 50,000
Peanut Butter 250,000
Tar 3x 10^10

As we can see from these numbers, the lower the cP, the more easily the material will flow but it is important to know that temperature has a major influence on a fluid’s viscosity as well.

When heated, fluids exhibit lower viscosities. This relationship between heat and viscosity is critical when computing molding parameters in composites manufacturing (paradoxically, it is interesting to note that the viscosity of gases does rise with temperature).

The viscosity of thermoplastics at room temperature is very high as they are solid (usually in the form of pellets), giving them a massive advantage compared to thermosets when it comes to shelf life and storage.

Thermoplastics can be softened with heat, melted, reshaped (post formed) as many times as desired without any major loss in their properties. For composites manufacturers, this heat/pressure (flow) relationship is very useful when the properties of a resin does not yield the right viscosity outright. They can increase temperature and pressure parameters to lower the resin’s viscosity to fit the application or in this case the required resin flow. This also explains why so many thermoplastics manufacturers advertise the heat (processing temperature) and pressure capabilities of their equipment to demonstrate their process flexibility.

Meanwhile, thermosets have a low viscosity to start with as they are liquid. That viscosity can be lowered with heat but because of the internal chemical reactions in the material they reach their “gel point” and by end of the cure, they reach extremely high viscosity value because they turned into a solid mass that will not be recyclable (post formed).

A more in-depth discussion of viscosity can be found in the
M51 Training Course ENC103 Design and Manufacture of Composite Structures.