Viscosity Measurement for R&D Labs

To measure flow behavior properties, pharmaceutical R&D labs test new formulations for viscosity. In addition to formulation stability during storage, transport, and consumer use, viscosity has an effect on processing decisions for mixing in manufacturing and filling containers with consumer products.

Instruments known as “viscometers” and “rheometers” are utilized to measure the viscosity of semi-solid materials like creams, ointments and pharmaceutical liquids.

Viscosity flow curves shown on the instrument display characterize common behavior of pharmaceutical products (see Figure 1). Viscosity decreases as rotational speed of the spindle escalates. R&D will select a data point on this curve and direct QC to test for that value when qualifying production consignments for delivery.

Brookfield DV3T 1 cP = 1mPa*s. Rheometer with Cylindrical Spindle.

Figure 1. Brookfield DV3T 1 cP = 1mPa*s. Rheometer with Cylindrical Spindle.

QC Test Technique

R&D makes three decisions when arranging the QC test method. The first is to determine the manufactured material’s viscosity range. Scientific units of centipoise (cP) are typically used in North America, while other regions of the world also use milli-Pascal seconds (mPa*s).

Luckily, the reference material is water, possessing a viscosity of 1 cP at 20 ºC. Cough syrups and other medicinal liquids that are ingested are usually less than 100 cP,  rubbing ointments could be 1,000 cP or over. Thick creams start at around 10,000 cP. The selection of viscometer with suitable torque measurement range is dependent on knowing which material(s) will be analyzed.

For the majority of pharmaceutical products, two standard torque ranges are utilized. “RV” is selected for “regular” or medium viscosity products, whilst “LV” denotes low viscosity materials. There are choices for higher viscosity, for example HA and HB, but these are not regularly utilized to test creams/ointments and pharmaceutical liquids.

The instrument model selection is predicated on the operating features which are necessary for the measurement after the torque range has been fixed. R&D determines the type of spindle and rotational speed to employ when establishing viscosity.

Spindles, at large, are either cylindrical in shape, as shown in Figure 1, or may contain a disc at the bottom as shown in Figure 2. In QC labs, disc spindles are the most common type used. Yet, there are other types too, such as T-bar and cone.

Disc-type Spindle for Viscosity Measurement.

Figure 2. Disc-type Spindle for Viscosity Measurement.

Some other variables involved in the choice of viscometer include an integrated clock to record the length of time the spindle rotates before the viscosity data point is noted. If there is a temperature requirement for conditioning the sample, then viscometer should be supplied with an integrated temperature probe for the confirmation of sample temperature.

Each feature will increase the cost of attaining the viscometer, so it is important that R&D clearly determines all of the specifics essential to making the viscosity measurement accurate.

The last requirement for QC is awareness of the standard parameters for the viscosity measurement to approve the product for shipment. During the validation process R&D will test numerous samples to establish cP values for the QC test. When measured, viscosity falls between these two and shipment takes place. If the measured viscosity drops outside, then it is possible that the instrument is not reading correctly.

To establish that the instrument is not flawed a quick test is performed with a viscosity standard fluid (see Figure 3). The fluid is certified to have a certain viscosity value. To determine the instrument is in proper working condition, measurement with the QC instrument must come sufficiently near to the mentioned viscosity value. QC must report back the viscosity test at that point.

Calibration Test Kit with Viscosity Standard Fluid.

Figure 3. Calibration Test Kit with Viscosity Standard Fluid.

Conclusion

In addition to viscosity standard fluid, follow these three simple steps to choose the right viscometer, and your QC Lab will be in excellent condition. It is advised to revert back to R&D for verification of test method and correct method for conducting an effective viscosity test If there is any doubt.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Brookfield.

For more information on this source, please visit AMETEK Brookfield.

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