Stress Creep Measurements

A constant stress a is applied on the system (that may be placed in the gap between two concentric cylinders or a cone and plate geometry) and the strain (relative deformation) y or compliance J =Y/ff, Pa ) is followed as a function of time for a period of t. At t = t, the stress is removed and the strain y or compliance J is followed for another period t [1]. 

Creep is the sum of a constant value J oq (elastic part) and a viscous contribution 

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Constant stress (creep) measurements A constant is stress is applied to the system and the strain y or compliance J (y/a) is followed as a function of time. By measuring creep curves at increasing stress values, it is possible to obtain the residual (zero-shear) viscosity ri 6) and the critical stress that is, the stress above which the structure starts to break down. <7 is sometimes referred to as the true yield value. 

Figure 21.6 Constant stress (creep) measurements for PS latex dispersions as a function of EH EC concentration.

The most useful method for predicting creaming is to use constant stress (creep) measurements, from which the residual (zero shear) viscosity j (0) can be obtained. 

To characterize the rheology of a cosmetic emulsion, one needs to combine several techniques, namely steady state, dynamic (oscillatory) and constant stress (creep) measurements. A brief description of these techniques is given below. 

In constant stress (creep) measurements, one applies the stress (that is kept constant at each measurement) in small increasing increments. If the stress applied is below the yield stress, the system behaves as a viscoelastic solid. In this case, the strain shows a small increase at zero time and this strain remains virtually constant over the duration of the experiment (near zero shear rate). When the stress is removed, the strain returns back to zero. This behaviour will be the same at increasing stress values, provided the applied stress is stiH below the yield stress. Any increase in stress will be accompanied by an increase in strain at zero time. However, when the stress exceeds the yield stress, the system behaves as a viscoelastic liquid. In this case, the strain rapidly increases at zero time, giving a rapid elastic response characterised by an instantaneous compliance Jo (the compliance is simply the ratio between the strain and applied stress, Pa ). At time larger than zero, the strain shows a gradual and slow increase with time. This is the region of retarded response (bonds are broken and reformed at different rates). Ultimately,... 

Three different rheological measurements may be applied to study the bulk properties of suspension concentrates [118-120] (i) Steady state shear stress-shear rate measurements (using a controlled shear rate instrument), (ii) Constant stress (creep) measurements (carried out using a constant stress instrument), (iii) Dynamic (oscillatory) measurements (preferably carried out using a constant strain instrument). These... 

Another method for studying flocculation is that of constant stress (creep) measurements that was described before. This allows one to obtain the residual viscosity q(o) and critical stress a . The values of q(o) and may be used to assess the flocculation of the suspension on storage. If flocculation occurs on storage (without... 

Two transient methods can be applied to study paint rheology [49] (i) Stress relaxation after sudden application of strain, (ii) Strain relaxation after sudden application of stress (creep measurements). In the stress relaxation case, a constant strain y is applied within a very short period (that must be much smaller than the relaxation time of the sample) and the stress a is followed immediately as a function of time. For a viscoelastic liquid (that is the case with many paint systems), the stress decreases exponentially with time t and reaches zero at infinite time. If the stress is divided by the applied constant strain, one obtains the stress relaxation modulus G(t) which is related to the instantaneous modulus by the following expression. 

Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

Bohlin also markets a computer-driven controlled stress rheometer, which determines viscosity as a function of stress and measures creep and... 

Tensile Testing. The most widely used instmment for measuring the viscoelastic properties of soHds is the tensile tester or stress—strain instmment, which extends a sample at constant rate and records the stress. Creep and stress—relaxation can also be measured. Numerous commercial instmments of various sizes and capacities are available. They vary greatiy in terms of automation, from manually operated to completely computer controlled. Some have temperature chambers, which allow measurements over a range of temperatures. Manufacturers include Instron, MTS, Tinius Olsen, Apphed Test Systems, Thwing-Albert, Shimadzu, GRC Instmments, SATEC Systems, Inc., and Monsanto. 

Another resonant frequency instmment is the TA Instmments dynamic mechanical analy2er (DMA). A bar-like specimen is clamped between two pivoted arms and sinusoidally oscillated at its resonant frequency with an ampHtude selected by the operator. An amount of energy equal to that dissipated by the specimen is added on each cycle to maintain a constant ampHtude. The flexural modulus, E is calculated from the resonant frequency, and the makeup energy represents a damping function, which can be related to the loss modulus, E". A newer version of this instmment, the TA Instmments 983 DMA, can also make measurements at fixed frequencies as weU as creep and stress—relaxation measurements. 

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described eadier, it can be operated as a forced-vibration instmment. It is fully computerized and automatically determines G, and tan 5 as a function of temperature at low frequencies (10 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstmctures of polymer blends (285) and latex films (286). 

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

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