Scope:

ASTM D789, D4878 tests are used to determine the relative viscosity of polyamide (PA) by Brookfield method. The viscosity of a substance is its resistance to flow and change shape at a given shear rate. The flow behavior of a substance indicates an indirect measure of material consistency and quality. To ensure good quality, the product consistency of the material needs to be the same in different batches. During molding, the viscosity of a polymer may get reduced due to degradation and reduced molecular weight. One can determine the change by comparing the original polymeric resin with the finished product. 

Test Procedure:

For the ASTM D789 and ASTM D4878 test method an 8-ounce bottle with a screw cap is filled with 22.0 +/- 0.01 grams of polymer, and to this, formic acid is added. The whole system is agitated using a shaker to dissolve the sample polymer, and then the bottle is placed in a bath maintained at 25 °C for at least 1 hour. The resulting solution is placed in a Brookfield Viscometer where the spindle is set at a suitable speed according to the viscosity of the solution. This instrument measures the force to turn the spindle at a given rate in the solution and then determines the viscosity of the solution.  

The video given below explains the test procedure

Specimen size:

About 22 grams of polymer.

Data:

Relative Viscosity in this method is calculated by dividing the absolute viscosity of the polymer/formic acid solution by the absolute viscosity of the formic acid used.

R_v = relative viscosity of the polymer-formic acid solution.

M_f = absolute viscosity of formic acid.

M_s = absolute viscosity of the polymer-formic acid solution.
R_v = (M_s/M_f)

Conclusion:

ASTM D789 and ASTM D4878 is a standard test method for determination of relative viscosity of polyamide (PA).