The spindle attaches to the instrument head and is brought down into contact with a flat plate on the base of the instrument where the sample has been placed. Small sample volumes, typically 1.0 mL or less, are needed for testing therefore, temperature equilibrium is achieved within a very short time interval prior to start of testing. Measuring the yield stress of the material can answer questions including: How much force does it take to initiate movement and start the material flowing? Does the applicator exert sufficient force on the adhesive/sealant to cause flow to begin? Once flow begins, does it sustain until the material is in place in its intended position? Will it then hold position and not ooze away from its targeted destination (i.e., creep)? Modern low-cost controlled-stress rheometers can perform all of these tests directly on the material in one or two minutes of total test time.Ĭone/plate geometry is the preferred choice for busy labs where lots of samples are tested daily (see Figure 1, p. Other test parameters of interest are the yield stress of the material and its creep behavior. Testing flow behavior over a range of rotational speeds is the smart approach in today’s QC lab because it mimics the way that customers will try to use the adhesive or sealant in actual practice. Low rotational speeds of the viscometer correlate with how the adhesive/sealant settles into position after application, while high rotational speeds simulate spraying, rolling, squeezing or scraping the material onto the substrate surface. Assessing how viscosity changes when the material is tested at different rotational speeds gives a complete picture of flow behavior. Increased capability designed into today’s generation of benchtop viscometers and rheometers has created opportunity for more comprehensive viscosity measurements in roughly the same amount of test time. Failure requires a repeat of the test on a fresh sample or, if there is concern that the instrument might be the problem, a calibration check on the viscometer is performed with a viscosity standard fluid.

Using a rotational viscometer with disc-type spindle or T-bar spindle (or perhaps even a cone/plate model), the technician runs the instrument at one speed, records the viscosity number, and verifies that it falls between maximum and minimum limits for acceptance. Traditional quality control methods for viscosity measurement of adhesive and sealant materials focus on the single point test.

Testing viscosity