Optimising the viscosity test

Robert G McGregor, General Manager, Global Marketing, Brookfield Engineering Laboratories, gives an outlook about the type of instruments to be used while evaluating ‘thicker’ materials that do not flow readily

20141215ep66

Robert G McGregor

R&D has the responsibility for developing test methods that qualify materials for acceptability in manufacturing. The physical property that addresses flow behaviour is viscosity, which is a measurement of resistance to flow. For liquids this seems reasonably obvious, but for semi-solid materials, like margarine, butter, jams, etc. the word ‘spreadability’ may seem more appropriate. Observing and quantifying the way in which materials change shape during deformation is all part of the viscosity measurement world. The question is what type of instruments to use when evaluating these ‘thicker’ materials that do not flow readily.

20141215ep68

Figure 1: Brookfield DV3T Rheometer uses rotating spindle to measure viscosity

Standard bench top viscometers and rheometers use a rotating spindle immersed in the fluid to make the viscosity measurement. (See Figure 1).

Resistance to rotation at different speeds is measured as discreet torque values and converted mathematically into scientific units called ‘centipoise’. The resulting graph from this test looks like the display on the instrument in Figure 1. Viscosity values are recorded on the y-axis vs. rotational speeds on the x-axis. Non-Newtonian materials exhibit a decrease in viscosity as rotational speed increases. This type of flow behaviour is referred to as ‘pseudoplastic’ or ‘shear thinning.’

Non-flowing materials present more of a challenge when measuring viscosity because the spindle digs a hole when rotating in place. Once the material moves away from the spindle, it does not recover and leaves a void in the space adjacent to the spindle. Viscosity readings decrease quickly and there is no practical way to record data that is meaningful. Choosing a different spindle geometry is the solution.

20141215ep67

Figure 2: Brookfield RS Soft Solids Tester with Vane Spindle for Non-flowing Materials

The vane spindle shown on the instrument in Figure 2 offers several advantages. It can be immersed directly into the material without disturbing the sample structure. The material that is trapped between the vanes represents fresh sample that can be tested without prior shearing history.

The type of instrument shown in Figure 2 is a controlled stress rheometer. It offers the dual capability of either applying a defined torque to the spindle or rotating the spindle at a defined speed. The former mode of operation is useful in determining the force to initiate flow while the latter characterises both the initial force and the viscosity or resistance to movement once flow begins. Using the latter method provides two pieces of comparative data in a quick test that can be used to qualify products.

In a controlled speed test at low rpm, say 0.5 rpm, the spindle measures increasing resistance to movement as rotation commences. The recorded stress climbs to a peak value called the ‘yield stress’ because it represents the initiation of flow behaviour for the material. Figure 3 shows the graph for this type of test on traditional butter and a newer spreadable version of the same product. The slope of the lines after flow commences give an indication for the viscosities of the respective products since it indicates their different resistance to movement at the same rate of spindle rotation.

20141215ep69

Figure 3: Stress vs. time graph for two butters tested with Vane Spindle at 0.5rpm

The same test can be used to compare butters to margarine for spreadability. Figure 4 shows the same test on three different products. The behaviour of the butters exhibits a gradual flow transition around the point of yield stress whereas the margarine has a sharp drop off after start of flow. This reflects a more brittle structure in the margarine which manifests as less resistance to spreading the margarine once movement begins. Butter 1 clearly shows higher yield stress and viscosity values compared to butter 2 and might therefore represent a premium brand.

20141215ep70

Figure 4: Stress vs. time graphs for comparison of spreadable butters to margarine

If the controlled torque capability of the rheometer is used and held at a constant value close to the yield stress of the material, the creep behaviour can be measured. This characterises the ability of materials, such as jams and icings, to hold their shape under constant load, such as gravity. This type of test is important for applications where the appearance of the material is important to the customer. Another example is maintaining structural integrity of jams and icings used as fillers in cakes and pastries, so that oozing does not occur. Holding shape is the desired property in all cases.

20141215ep71

Figure 5: Brookfield CT3 texture analyser with cylinder probe measuring gelatin sample

An alternative approach for making these types of measurements is with a texture analyser as shown in Figure 5. This device moves a probe into the test sample at a defined rate of penetration. Measurement of spreadability is accomplished using the fixture shown in Figure 6 with the cone-shaped probe moving at 1mm/sec into the container of test material. The resistance to penetration is recorded as a force measurement in units of grams by the instrument, both as a function of time and displacement. The distance which the probe penetrates into the sample is defined by the user.

There are several advantages to this method:

  • The test sample can be kept in its original package or container if desired.
  • Material is not disturbed prior to testing.
  • Work expended to penetrate or flow the sample is quantifiable.
  • Removal of the probe can measure the adhesive property of the material, i.e. its ability to cling to the probe as it retracts.
20141215ep72

Figure 6: Spreadability test fixture used with texture analyser

Figure 7 shows the graphical data for a typical spreadability test on butter and margarine. The peak force quantifies the hardness of the sample. The slope of the curve up to the peak force is a measurement of ‘stiffness’ and will change if the speed of penetration for the probe is adjusted. The area under the curve leading up to the peak force is a measurement of the work done to penetrate the sample. The margarine exhibits softer behaviour compared to the butter throughout the test.

Given the two available methods for measuring highly viscous, non-flowing materials — rheometer with (cone) VANE spindle and texture analyser with cone probe — is one better than the other? The data from both types of tests are valid and represent quantifiable ways to measure hardness/ firmness and spreadability. The bottom line is that both methods work acceptably, are easy to execute, and require similar prep and cleanup. The ability to mathematically calculate the work done with the texture analyser gives an added piece of information. Repeatability may be comparable with either method, but is best confirmed through testing of the material in question.

20141215ep73

Figure 7: Force vs. distance graph for spreadablility of butter and margarine

Of importance to the lab manager is the cost to purchase the equipment and the training of personnel. Both instruments have been available in the market place for many years and are used by general industry. While R&D may use the software for initial material characterisation, the QC lab can use either instrument in standalone mode and report values for yield stress or hardness respectively.

So it really boils down to preference. In labs with sufficient resource, it is normal to find both devices employing test methods similar to the above. Perhaps now is the time for your lab to investigate these test techniques and improve the QC testing on your ‘soft-solid’ product.