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Tech Tip # 3: Viscosity and the Thixotropic IndexViscosity and the Thixotropic Index (T.I.) are two common components in an epoxy that can have a great effect on how the product will work for a particular application. Viscosity is a measurement of a fluid’s resistance to flow. Viscosity is measured in Poise, and the centipoise (cP) is the common unit used. One cP is defined as the viscosity of water. From this base, all other viscosities are derived. A product like honey would have a much higher viscosity—around 10,000 cPs—compared to water. Honey would flow much slower out of a tipped glass than water would. Here is a list of common material’s and their viscosities: Approximate Viscosities of Common Materials (At Room Temperature-70°F) Material Viscosity in Centipoise Water 1 cps Milk 3 cps SAE 10 Motor Oil 85-140 cps SAE 20 Motor Oil 140-420 cps SAE 30 Motor Oil 420-650 cps SAE 40 Motor Oil 650-900 cps Castrol Oil 1,000 cps Karo Syrup 5,000 cps Honey 10,000 cps Chocolate 25,000 cps Ketchup 50,000 cps Mustard 70,000 cps Sour Cream 100,000 cps Peanut Butter 250,000 cps Thixotropic Index is a ratio of a material’s viscosity at two different speeds, generally different by a factor of ten. A thixotropic material’s viscosity will reduce as agitation or pressure is increased. This index indicates the material’s ability to hold its shape. Mayonnaise is a great example of this. It holds its shape very well, but when a shear stress is applied, the material easily spreads. The viscosity of a material can be decreased with an increase in temperature in order to better suit an application. The viscosity of most materials will drop in a common fashion, exponential fashion. Below, the graph of 353ND shows how a material will react to heat.353ND Mixed Viscosity versus Temp0100020003000400023 25 27 29 31 33 35 45 55 65 70Temp ( C )Viscosity ( cps )viscosity The viscosity of the 353ND drops very quickly, and then levels out at increased temperature. This is a good method to use if an epoxy is very good for a certain application, but the viscosity is slightly high. It must be noted that by heating up the mixture, some of the pot life is used up during the heating, so it will gel faster than if left at room temperature. Still have questions? Call Technical Services at 1-978-667-3805 Or E-mail: [email protected] Temp (°C) Viscosity (cPs) 23 3153 25 2252 27 2147 29 1728 31 785 33 737 35 737 45 409 55 163 65 143 70

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