Flocculation measurements, sterically

In studies of steric stabilizers too little attention is generally paid to the dispersion force attractions between particles and the critical separation distance (H ) needed to keep particles from flocculating. Adsorbed steric stabilizers can provide a certain film thickness on each particle but if the separation distance between colliding particles is less than H the particles will flocculate. The calculation of H is not cr difficult and measurements to prove or disprove such calculations are not difficult either. For equal-sized spheres of substance 1 with radius or in medium 2 the Hamaker equation for the dispersion force attractive energy (Uj2i) at close approach is (7) ... 

Incipient flocculation of sterically stabilized suspensions, i.e. the condition when the chains are in a poor solvent condition, can be investigated using turbidity measurements. The suspension is placed in a spectrophotometer cell placed in a block that can be heated at a controlled rate. From a plot of turbidity versus temperature one can obtain the critical flocculation temperature, which is the point at which there is rapid increase in turbidity. 

Viscosities of concentrated suspensions of carbon black in a white mineral oil (Fisher "paraffin" oil of 125/135 Saybolt viscosity) were measured with a Brookfield viscometer as a function of OLOA-1200 content. Figure 13 shows the viscosities of dispersions with 30 w%, 35 w% and 70 w% carbon black. In all cases the viscosity fell rapidly as the 0L0A-1200 content increased from 0 to 1%, then fell more gradually and levelled off as the 0L0A-1200 content approached 2%. In many respects the reduction in viscosity with increasing OLOA-1200 content parallels the conductivity measurements both phenomena are sensing the buildup of the steric barrier, and this steric barrier weakens, softens, and lubricates the interparticle contacts. As evidenced in foregoing sections, the particles are still flocculated but can be easily stirred and separated mechanically. The onset of electrostatic repulsion at OLOA-1200 contents in excess of 2.5% did not affect viscosities. 

Influence of Addition of Electrolyte and Increase of Temperature Addition of electrolyte or increase of temperature at a given electrolyte concentration to a sterically stabilized dispersion may result in its flocculation at a critical concentration or temperature, which in many cases coincides with the theta point for the stabilizing chain. At the theta point the mixing term in the steric interaction is zero and any yield value measured should correspond to the residual van der Waals attraction. The energy arising from van der Waals attraction may be calculated from the following approximate relationship,... 

With a sterically stabilised dispersion, weak flocculation can also occur when the thickness of the adsorbed layer decreases. Again, the value of E can be used as a measure of the flocculation the higher the value of E, the stronger the flocculation. 

Shear-Sensitive Systems. In addition to hydrodynamic effects and simple viscous behavior, the act of pigmentation creates a certain amount of complex behavior (13). If the particles are fine. Brownian movement (14-17) and rotational diffusion (14. 18. 19) are among the phenomena that cause dispersed systems to display complex rheology. The role of van der Waals forces in inducing flocculation (20) and the countervailing role of two electroviscous effects (17. 21. 22) in imparting stability, particularly in aqueous systems, have been noted. Steric repulsions appear to be the responsible factor in nonaqueous systems (23. 24). The adsorbed layer can be quite large (25-28). as detected by diffusion and density measurements of filled systems or by viscometry and normal stress differences (29). 

The foregoing results may be contrasted with those of Croucher and Hair (1981) and Feigin et al. (1981). The former measured the particle concentration dependence of the UCFT of polyacrylonitrile latices sterically stabilized by polyisobutylene in 2-methylbutane (see Fig. 5.7). The UCFT was found to be insensitive to the variation of the latex volume fraction, within the limits of experimental error, over the range studied (2 x 10 -2 x 10 ). Note that in these experiments, the molecular weight of the stabilizing moieties (7-6 X 10 ) was relatively high. Moreover, flocculation did not occur in better than 0-solvents. Rather, there was close agreement between the UCFT (327 K) and the 0i-temperature (325 K). 

Equation (16.16) suggests that Vj should depend approximately upon the molecular weight of the free polymer with an inverse power of ca 0-4-0-5. This is significantly less than the exponent of -0-7 measured experimentally (Sperry et al, 1981) for the flocculation of aqueous latices, sterically stabilized by poly(oxyethylene), using hydroxyethyl cellulose. The latter, however, are extremely stiff chains. 

An interesting example of the application of the concept of steric stabilization is the interpretation of the optical performance of alkyd paints pigmented with TiOi by Franklin et al -. Having established from electrophoresis and opacity measurements that the electric charge on the particles is not the controlling factor in flocculation, a study was made of the adsorption characteristics of the resin using pigments coated with different levels of silica/ alumina such that the surfaces created varied from predominantly silica to mostly alumina. 

The stability of emulsion paints will be affected by the charge surrounding the latex pigment and extender particles. Flocculation will occur if the electric double layer is insufficient to maintain interparticle separation. Therefore, zeta potentials of the individual particles in the paint can give a measure of the tendency to flocculate and hence of the long-term stability of the paint. However, emulsion paints cannot rely solely on electrostatic stabilization because they need to withstand dilution with hard water. This means that some steric stabilization will invariably be used. 

As discussed before, addition of free non-adsorbing polymer can produce weak flocculation above a critical volume fraction of the free polymer, This weak flocculation produces a gel structure that reduces sedimentation. As an illustration, results were obtained for a sterically stabilised suspension [using a graft copolymer of poly(methyl methacrylate) with poly(ethylene oxide) side chains] to which hydroxyethyl cellulose with various molecular weights was added to the suspension. The weak flocculation was studied using oscillatory measurements. Figure 7.52 shows the variation of the complex modulus G with p. 

For sterically stabilized suspensions, one can measure the incipient flocculation when the medium for the chains becomes a 0-solvent. This occurs, for example, on heating an aqueous suspension stabilized with poly(ethylene oxide) (PEO) or poly(vinyl alcohol) chains. Above a certain temperature (the 0-temperature) that depends on electrolyte concentration, flocculation of the suspension occurs. The temperature at which this occurs is defined as the critical flocculation temperature (CFT). This process of incipient flocculation can be followed by measuring the turbidity of the suspension as a function of temperature. Above the CFT, the turbidity of the suspension rises very sharply. For this purpose, the cell in the spectrophotometer that is used to measure the turbidity is placed in a metal block that is connected to a temperature programming unit (which allows one to increase the temperature raise at a controlled rate). 

Rheological characterization of the paste is to measure the relationship between shear stress and shear rate varying harmonically with the time, indicating the level of interparticle force or flocculation in the paste. The ideal paste for thick-film films should have a proper degree of pseudoplastic as well as thixotropic behavior (Wu et al., 2010). The thixotropic effect is a result of aggregation of suspended particles. Aggregation in the system caused by the attraction forces such as Van der Waals and repulsion forces due to steric and electrostatic... 

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