Flocculation tendencies

In solvent-containing systems with several pigments, but also in full-shade systems, color changes (rub-out effect) can take place due to flocculation, especially if the systems are subjected to mechanical stress during application and drying. The flocculation tendency can be determined by means of the rub-out test . 

The pigment properties of chromium oxides can be modified by precipitation of hydroxides (e.g., of titanium or aluminum), and subsequent calcining. This treatment changes the color to yellow-green, and decreases the flocculation tendency [3.54], Aftertreatment with organic compounds (e.g., alkoxylated alkylsulfon-amides) is also used [3.55]. 

Higher A/S ratios appear to be more effective at higher overall chemical dosages or higher coagulation-flocculation tendencies. This correlation suggests that there might be support for air-bubble attachment to floes by means of chemicals. 

FIGURE 10.12. In a sterically stabilized system cxjntaming low-molecular-weight or weakly adsorbed polymer (a), as two particles approach, the loosely bound polymer may desorb, leaving bare spots on the approaching surfaces, leading to an enhanced flocculation tendency (b). That process is referred to as depletion flocculation. ... 

The action of plasticizers consists of increasing the mobility of cement particles by reduction of the interfacial tension and increasing electrokinetic potential. That way the flocculation tendency of cement particles, and also of other fine particles in the fresh mix, is considerably reduced and thin water cover appears around these particles. 

Browarzik et al calculated asphaltenes flocculation at high pressures for methane + crude oil - - 2,2,4-trimethylpentane [i-octane] using continuous thermodynamics where 2,2,4-trimethylpentane acts as a precipitant. The asphaltene flocculation was considered to be a liquid -b liquid equilibrium. Browarzik et al applied the van der Waals equation of state. The polydispersity of the crude oil was considered to be described by the solubility parameter of the Scatchard-Hildebrand theory. Within this distribution the asphaltenes represent the species with the highest solubility parameters. The calculated results were compared to experimental data. For oils with a very low content of asphaltenes the model describes the experimental flocculation data reasonably well. However, on contrary to the experimental results, the model predicts the asphaltenes to show a higher flocculation tendency with increasing asphaltenes content of the crude oil. Based on these comparisons further work was undertaken by Browarzik et al and the associates formed... 

Camphor, dibutyl phthalate [84-74-2], and other Hpidic solvents are common plastici2ers. Nad lacquers requite the presence of a suspending agent because pigments have a tendency to settle. Most tinted lacquers contain a suitable flocculating agent, such as stearalkonium hectorite, a reaction product of hectorite [12173 6-6] and stearalkonium chloride [122-19-0],... 

This angle plate gravity separator removes suspensions of solids from a dilute liquid. The unit is more compact than a box-type settler due to the increased capacity achiev ed by the multiple parallel plates. The concept is fairly standard (U.S. Patent 1,458.805—year 1923) but there are variations in some details. For effective operation, the unit must receive the mixture with definite particles having a settling velocity. The units are not totally effective for flocculants or coagulated masses that may have a tendency to be buoyant. 

Great care must be exercised when using graduated cylinders because decreases in the diameter of small containers can produce a wall effect, which often affects the settling rate or ultimate sedimentation volume of flocculated suspensions. Such small containers have a tendency to hold up the suspensions due to adhesive forces acting between the container s inner surface and the suspended particles. 

PVA Particles. Dispersions were prepared in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. PVA particles prepared with a block copolymer having M PS) x 10000 showed a tendency to flocculate at ambient temperature during redispersion cycles to remove excess block copolymer, particularly if the dispersion polymerization had not proceeded to 100 conversion of monomer. It is well documented that on mixing solutions of polystyrene and poly(vinyl acetate) homopolymers phase separation tends to occur (10,11), and solubility studies (12) of PS in n-heptane suggest that PS blocks with Mn(PS) 10000 will be close to dissolution when dispersion polymerizations are performed at 3 +3 K. Consequently, we may postulate that for soft polymer particles the block copolymer is rejected from the particle because of an incompatibility effect and is adsorbed at the particle surface. If the block copolymer desorbs from the particle surface, then particle agglomeration will occur unless rapid adsorption of other copolymer molecules occurs from a reservoir of excess block copolymer. 

In the earlier literature on steric stabilization there was a tendency to ignore the importance of anchoring sites for steric stabilizers. Without strong anchors the adsorbed molecules are easily swept aside in collisions, as has been so well illustrated with polystyrene adsorbed onto carbon blacks in hydrocarbon solutions in these systems particles flocculate in every collision.(11)... 

Figure 5 shows the variation of Xg with temperature at two C values (0.20 and 0.25 mol dm 3). In both cases Xg is essentially zero until a critical temperature is reached, above which xg increases rapidly with increasing temperature reaching a maximum above which there is a tendency forXg to fall again with further increase in temperature. The critical temperature corresponding to the abrupt increase inTg is 20 and 25°C for C equal to 0.25 and 0.20 mol dm, respectively This temperature may be identified with the critical flocculation temperature (CFT) of the concentrated dispersion. 

The surface character of fibres influences their affinity towards various chemical additives by, for example, their adsorption properties and also in their tendency to flocculate. Cellulosic fibres, because of the presence of acidic groups which are introduced during chemical pulping and bleaching, are mildly anionic. These acidic groups may be carboxylic (COOH) or in some cases sulfonic acid (S03H) and they are able to dissociate to leave a net negative... 

This isoindoline nickel complex pigment is mainly recommended for the coloration of metallics and effect coatings, especially for water based systems. P.R.271 affords yellowish to medium red shades, providing a bright flop in metallics. Alkyd/melamine resin systems may safely be overcoated and withstand exposure of 140°C for 30 minutes. The flow properties of P.R.271 in these systems and in polyester cellulose acetobutyrate base coat systems are good, but there is a certain tendency to flocculate. 

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