Bimodal dispersions

The viscosity of a bimodal dispersion is a strong function of the particle size ratio of the two particle fractions. 

BUR = Build up Ratio, Calcul. = Calculated DIAM from BUR, TEM Estimated D from TEM, S.D. = Standard Deviation, MONO/BIM = Monomodal or Bimodal Dispersion, SEC Part = Secondary Particle Approximate D, COAL. = Coalescence. 

FIGURE 1 Typical courses of molar mass dispersity functions, a, b, c—differential, and d- integral representation a- broad molar mass dispersity polymer b-narrow molar mass dispersity polymer c- polymer with bimodal dispersity. 

Progress in Organic Coatings 29, Nos. 1-4, Sept.-Dec. 1996, p. 183-94 BIMODAL DISPERSIONS IN COATING APPLICATIONS... 

Polymers Paint Colour Journal 186, No.4380, May 1996, p.38/42 BIMODAL DISPERSIONS IN COATINGS APPLICATIONS... 

Dames B, WiUenbacher N (2001) An empirical model predicting the viscosity of highly concentrated, bimodal dispersions with colloidal interactions. Rheol Acta 40 434 40... 

Our initial studies (23) were performed in toluene, and Table I shows the results from the polymerization of a number of representative monomers. The data reported in Table I are for direct addition of the monomer to the sodium dispersion. Inverse addition often leads to higher molecular weights, although the overall polymer yields are usually lower (15,23). The results in Table I show that, under these reaction conditions, a bimodal molecular molecular weight distribution is normally obtained. Furthermore, it is obvious that the crude polymer yields drop precipitously as the steric hindrance in the monomer increases. 

The figure shows U >. S L in this region and Da is predominantly small. At the highest Reynolds numbers the region is entered only for very intense turbulence, U > SL. The region has been considered a distributed reaction zone in which reactants and products are somewhat uniformly dispersed throughout the flame front. Reactions are still fast everywhere, so that unbumed mixture near the burned gas side of the flame is completely burned before it leaves what would be considered the flame front. An instantaneous temperature measurement in this flame would yield a normal probability density function—more importantly, one that is not bimodal. 

A strong similarity is found for the present blends with a PPE/PS ratio of 50/50, as reflected by a similar bimodal cell size distribution for all SAN contents. Small differences can be related to the distinct foaming kinetics of the PPE/PS blend phase. Compared to the PPE/PS 75/25 blend phase, the higher content of PS in the PPE/PS 50/50 phase leads to a cell nucleation and growth kinetics close to the SAN phase. Nevertheless, the PPE/PS phase still appears to restrict the cell growth and expansion in the SAN phase to some extent, and smaller cells are found within the cell walls. Independent of the SAN content, cell growth within the dispersed SAN phase proceeds under the constraints of the continuous, higher Tg PPE/PS phase. 

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