Rotary viscometer / Rotary rheometer

Rheometer RHEOTEST^® RN - Pharmaceutics and Cosmetics

Measuring task in research and development

In pharmaceutics and cosmetics it is, predominantly, a question of emulsions such as, e.g. creams and lotions, as well as gels and solutions that contain surface-active agents often with complicated rheological properties. That is why, determination, optimization, management and control of flow properties play a decisive part during new products development. In addition, one takes into account and assesses such the most important rheological characteristics:

	determination, optimization and control of yield point, first of all of creams in interralation with conditions during prepackaging, storage and use
	assessement, optimization and control of flow properties corresponding to very different shear stresses during the complete production cycle, to begin with production and processing (mixing, sedimentation, dispersion, homogenization, pumping) through end product production (prepackaging, extrusion, coloring), transport and storage till the use by end consumer (pressing from tube, rubbing)
	assessement, optimization and control of time-dependent flow properties (thixotropy)
	assessement, optimization and control of structure changes in product (viscoelastic properties)

The possible shear rates during complete production cycle are the following:

Storage:			0 s^-1
Flow,sedimentation:	0,001	...	1 s^-1
Pumping, mixing:	1	...	1 000 s^-1
Dispersion, extrusion, roller coating, coloring:	1 000	...	100 000 s^-1
Rubbing:	10 000	...	100 000 s^-1
High pressure homogenization:	10 000	...	1 000 000 s^-1

First of all, one should pay attention to product stability during storage and transport. Sedimentation and mixing processes are to be prevented. At that, optimal set of yield point and optimal viscosity in rest condition play a decisive part. Yield point and optimal viscosity in rest condition are also important criteria during product use (pressing and rubbing).

Production assessement and quality control

First of all, quality control serves for reproducible and metrologically correct data acquisition on flow properties. Determined with the help of rheometer flow characteristics must have good comparability in production range of a producer, as well as be metrologically significant for suppliers and quality control authorities.

To characterize flow properties of pharmaceutics and cosmetics products it is necessary to take flow curves in determined wide range of shear rate and to measure exactly the yield point of end product. Approximate determination of yield point by means of deformation measurements and rotational measurements with consequent extrapolation could lead to great distortions and to quality defects and could be recommended only in sound, exceptional cases. Moreover, structure strength (thixotropy) and viscoelastic properties must be measured.

Important parameters to characterize a product are yield point, course of viscosity change, degree and rate of structure destruction and formation, as well as duration of elastic recovery and modulus. Standard measurement methods should find implementation to assure good reproducibility and comparability. Also they should take into account thermal and mechanical preparation of sample (prehistory) and special algorithms of assessement for the given product.

Possible measurement methods:

•	Measurement procedure to determine shear rate-dependent viscosity
	Controlled Rate Tests - (CR-tests)

	Equilibrium flow curve in the shear rates range of 0.04 ... 20,000 s^-1 for investigation of shear rate-dependent flow properties
	Example: see Figure 1

	Linear ramps with controlled shear rate in forward and backward direction to research shear rate-dependent and time-dependent flow properties (thixotropy).
	Example: see Figure 2

•	Measurement procedure to determine kinetics of structure destruction or formation processes
	Controlled Rate Tests - (CR-tests)

	Step-change tests with conrolled shear rate to determine structure formation or destruction
	Example: see Figure 3

Controlled Stress Tests - (CS-tests)

	Step-change tests with controlled shear stress to determine structure formation or destruction
	Example: see Figure 4

•	Measurement procedure to determine yield point and zero viscosity
	Controlled Stress Tests - (CS-tests)

	Linear ramps with controlled shear stress to measure yield point or zero viscosity
	Example: see Figure 5

•	Measurement procedure to determine viscoelastic properties
	Controlled Stress Tests - (CS-tests)

	Creep tests and creep-recovery tests with controlled shear stress to determine material's viscoelastic properties
	Example: see Figure 6

Measuring task:

Study of shear rate-dependent and time-dependent flow properties.
Influence determination of receipt components, production conditions and aging on viscosity change depending on shear rate (equilibrium curves) and load duration (thixotropy).
Equilibrium data of flow curves or viscosity have a special meaning during processing (pumping, mixing). They correlate also with sensitive perception of a consumer (secondary sensation on skin).
Equilibrium data obtained from the linear ramps with controlled shear rate in forward and backward direction are of special interest for characterization of inner structure and stability of emulsions and gels.

Figure 1: Equilibrium flow curves

Figure 2: Linear ramps with controlled rate in forward and backward direction (hysteresis area)

Notes:

High temperature constancy is required to determine viscosity as function of shear rate. Moreover, time of measurements has decisive significance. It is necessary to set equilibrium for every shear rate between the latter and resultant force. It means that longer mesurement time is necessary for small shear rates and shorter measurement time is necessary for big gradients. Time of measurements and shear rate-dependent effects are registered by means of hysteresis curves determination by setting of shear rate ramps in forward and backward direction.

Top

Measuring task:

Influence determination of receipt components, production conditions and aging on structure destruction (viscosity reduction) under load (application) or structure formationin rest conditions or at small loads (storage) with the help of step-change-tests with controlled rate (CR-tests) and step-change-tests experiments with controlled stress (CS-tests). One could obtain notion about conditions of emulsions and gels with the help of the measurements described. Thus, one could realize purposeful influence upon structure strength and modification.

Figure 3: Controlled rate step-change measurements (CR-Step-Change Tests)

Figure 4: Controlled stress step-change measurements (CS-Step-Change Tests)

Notes:

Destruction of structure by forced shear (CR) gives information about desired viscosity decrease during transportation, processing and packaging of interim and end products.
Formation of structure in rest conditions and at low shear rates is typical for stability during storage. Destruction of structure under load at definite shear stress give information about product stability during transportation, processing and packaging of interim and end products. During measurement with controlled stress (CS), product stability (in contrast to measurements with controlled rate) correlate directly with shear stress. Formation of structure in rest conditions and at low shear rates is typical for stability during storage. In contrast to controlled rate measurements, influence on yield point is measured directly.

Top

Measuring task:

Influence determination of receipt components, production conditions and aging on yield point. For that, one rises continuously in limits of time set shear stress and in discrete time intervals one measures resulting shear rates. After this, one calculates viscosity using them.

Figure 5 Control of yield point of end product

Notes:

It became useful to represent "viscosity as function of shear stress" in praxis of data assessment. Shear stress corresponding to maximum viscosity is designated as a critical shear stress that is necessary to initiate flowing of the system. The critical shear stress correlates with primary sensation on skin that embraces the sensitive perception at initial applying of cosmetics emulsion. Besides, yield point measured gives important evidences about stability during storage and inclination to lamination.

Top

Measuring task:

Determination of viscous and elastic constituents of total deformation and of critical values such, e.g., as recovery time that is necessary to sustain in order to obtain reproducible results. Required for this measurement of creep-recovery describes at the first stage deformation properties at constant shear stress and at the second stage it describes possible back deformation of a completely discharged sample.

Figure 6 Creep-recovery test

Notes:

Over-structure is built in emulsions due to intermolecular interactions that could be destroyed partially or completely during vessel filling (corresponds to the preliminary shear). This destruction of the structure is in general reversible, that is why one should take into account sample recovery. Relations of elastic and viscous deformation and recovery time reflect important consuming features. Pronounced elastic properties and short recovery time correlate, e.g., with good properties during make-up application.

Top

Back to Rheometer RHEOTEST^® RN