Comparative QC Test Methods for Toothpaste Consistency

 Figure 1 – Brookfield viscometer with T-bar spindle (AMETEK Brookfield, Middleboro, Mass.).

Important quantitative data can be obtained in the study of viscosity by measuring the consistency of toothpaste, i.e., thickness, ease of handling and ability to retain position, and observing the squeezing or pumping action that expels the product from its container. A rotational viscometer with T-bar spindle (Figure 1) is traditionally used to test for consistency. The instrument is mounted on a laboratory stand with motorized drive that moves the viscometer head in a downward direction into the toothpaste sample. This motion allows the rotating T-bar spindle to cut a spiral path through fresh material with each revolution. The measured viscosity value may fluctuate slightly, but in general provides a stable reading that is recorded by a QC technician to verify product acceptability (see Figure 2). Provided the average viscosity value falls within established limits, the product passes the QC check and can be shipped.

R&D runs this test when evaluating new formulations. It can take up to 2 minutes for the spindle to travel down into the sample and then back up.

The goal of the QC technician is to minimize test time if possible and obtain a faster answer that confirms proper viscosity value. Toothpaste is tested by QC in its packaging container. Tubes are cut in half and mounted in a vertical position, with the open end facing up. The T-bar spindle cross-bar is usually less than half as wide as the tube diameter. The weight on the spindle connector assembly causes it to rotate vertically downward into the tube while measuring viscosity. It is possible to reduce travel distance by half since measurement during downward movement provides sufficient data. Once the technician records the measured viscosity value, the viscometer head can be raised quickly, thereby ejecting the T-bar spindle from the tube. Total test time is 1 minute for sample setup and 1 minute to run the test and record data.

 Figure 2 – Viscosity data from T-bar test.
 Figure 3 – Texture Analyzer with ball probe.

A new test method uses the Texture Analyzer (AMETEK Brookfield) with ball probe to evaluate product consistency (see Figure 3). The analyzer measures the force required to compress or pull apart a test sample. Probes include cylinders, cones, rods and spheres, depending on the nature of the material to be measured. Spheres and cylinders are good choices for a soft, solid material like toothpaste in a tube.

The probe penetrates the tube at a defined speed and measures the material’s resistance to movement in units of grams-force. Toothpaste is pushed out from beneath the probe and is squeezed upward in the restricted space between the sphere and tube wall. Figure 4 shows the texture data for a test that plots force on the y-axis versus distance traveled by the probe in a downward direction. The force increases as the probe enters the tube until it reaches a steady-state peak value with the probe fully immersed in the tube.

Area under the curve from start of test to peak force value is another analytical parameter that can be used to quantify the consistency of toothpaste. This calculation is called “work done” and is equivalent to the energy required to cause the material to flow. This can be correlated to squeezing or pumping action to expel toothpaste from a container.

QC obtains two separate data values from this test method utilizing the Texture Analyzer: peak force and work done. Consider the second parameter bonus information. Consequently, the test method has potentially higher value to the manufacturer than the viscosity test, which is a single data point.

Additional benefits of this test method are ease of setup (requires only the attachment of the probe) and reduced time to run the test (typically under 15 seconds). Cleanup is relatively quick since the probe can be cleaned in place. Total time can thus be under 1 minute, half the time of a viscosity test.

 Figure 4 – Texture test data using ball probe.

A side-by-side comparison of the two methods demonstrates that the Texture Analyzer is a useful adjunct to test procedures for toothpaste characterization (in R&D) and pass/fail testing in manufacturing (in QC). While the cost of the Texture Analyzer is almost double that of the viscometer with T-bar spindle, ROI is seen in the first year of operation due to the time saved per test. Some manufacturers use both methods because the T-bar data gives a long-term picture of how toothpaste consistency has evolved. In the competitive world of personal-care products, it is critical to implement meaningful tests that can answer consumer demand for user-friendly toothpaste.

Robert McGregor is director, Global Marketing & High End Lab Instrument Sales, AMETEK Brookfield, Instrumentation & Specialty Controls Division, 11 Commerce Blvd., Middleboro, Mass. 02346, U.S.A.; tel.: 508-946-6200; e-mail: [email protected]www.brookfieldengineering.com

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