Stabilization suspensions

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Kulkarni, S.S., Hasse, D.J., and Kratzer, D.W. (2006) Novel method of making mixed matrix membranes using electrostatically stabilized suspensions. US Patent Application 2006/0117949. 

Soil consists mainly of particulate, greasy matter. Detergents are supposed to keep the soil suspended in the solution and restrict redeposition. Tests also show that detergents stabilize suspensions of carbon or other solids such as manganese oxide in water. This suggests that detergents adsorb on the particles. Detergents add redeposition controllers such as carboxymethylcellulose. 

Similarly to the solubility of active drugs, the solubility of surfactants that were used in CFC systems has significantly changed. Surfactant solubility in HFA 134a ranges from 0.005% to 0.02% w/v, much lower than the concentration required to stabilize suspensions (0.1-2.0% w/v) (24,42). The surfactants can be solubilized with the addition of cosolvents such as ethanol. However, it is most likely that cosolvents will be incompatible with suspension formulations because drug solubility will also be promoted and crystal growth will occur. 

A combination consisting of 2 % of Kollidon 90 F and 5-9% of Kollidon CL-M has proved to be an effective system for stabilizing suspensions. 

The polyoxamers, Lutrol F 68 and Lutrol F 127, in concentrations of 2 - 5 %, expressed in terms of the final weight of the suspension, offer a further opportunity of stabilizing suspensions. They also do not increase the viscosity when used in these amounts and can be combined with all other conventional suspension stabilizers. 

The kinetics of stabilization must match the kinetics of aggregation. While ion diffusion and adsorption is fast (electrostatic stabilization), the diffusion and adsorption of polymers (steric stabilization) is slow and may be a limiting factor which can only be overcome by applying high polymer concentrations. The stability of the electrostatic stabilized suspension in moderately fast processes can for example be monitored and controlled in-line by modem electro-acoustic techniques [13]. 

In the simplest example of colloid stability, suspension partides would be stabilized entirely by the repulsive forces created when two charged surfaces approach each other and their electric double layers overlap. The repulsive energy VR for spherical particles, or rigid droplets, is given approximately as ... 

There are also several water-soluble mixtures of polysaccharides, including those derived from seaweeds and marine animals, such as gum arabic, agar, algin, carrageenan, and chitin. These are hydrophilic. Their suspensions in water can be quite viscous and can readily form gels, and they can form viscous interfacial films around dispersed droplets and bubbles [821], They are used to stabilize suspensions, foams, and emulsions and are used in many different foods and medicines including, for example, ice cream [428,430], See also Ref. [822],... 

Adhesive interaction of one particle toward another surface is closely related to the various phenomena associated with colloid chemistry such as colloidal stability, suspension, flocculation, and so forth. For instance, solid particles in water are maintained in suspension, in other words, they may be protected against flocculation due to the van der Waals attraction, if their surfaces possess electrical charging or some protective substances such as hydrophilic... 

Determination of the sulfate content requires the preliminary hydrolysis of galac-tans by the action of 1 M HC1 at 100 °C for 6 h (hydrolysis in 2 M trifluoroacetic acid can also be used for simultaneous determination of monosaccharides and sulfate). Quantitative estimation of sulfate in hydrolyzates can be performed by many methods known from inorganic analytical chemistry,218,219 but the most popular procedure in the case of galactans is turbidimetry of stabilized suspensions of BaSO.4. It was suggested in the early 1960s220 and is widely used at present in the slightly modified form.221... 

Figure 11.6 is a plot of the osmotic pressure versus volume fraction for a suspension with different widths, tr = In a-g, of the log-normal particle size distributions where Og is the geometric mean size and a-g is the geometric standard deviation. As the width of the size distribution increases, the osmotic pressure decreases for a particular volume fraction. With this osmotic pressure, we can evaluate the order—disorder transition for an electrostatically stabilized suspension, which is discussed next. 

This section on concentrated suspensions discusses the rheological behavior of sj tems which are colloidally stable and colloidally unstable suspensions. For stable sj tems, the rheology of sterically stabilized and electrostatically stabilized systems wiU be considered. For sterically stabilized suspensions, a hard sphere (or hard particle) model has been successfid. Concentrated suspensions in some cases behave rheologically like concentrated polymer solutions. For this reason, a discussion of the viscosity of concentrated polymer solutions is discussed next before a discussion of concentrated ceramic suspensions. 

Soft Spheres at High Concentration At the maximum volume fraction, of the electrostatically stabilized suspension low and high... 

In any multiphase liquid, stability is a paramount concern. Thermodynamics drives clumping of dispersed components, and this is sometimes enhanced by flow. However, tricks for stabilizing suspensions are as old as the inks of Egypt. Electrostatic and steric stabilization are the most common. By matching the dielectric prc effie s, sdrrie pafticle-fluid combinations can be found that are inherently stable. A more detailed discussion of suspension stabilization is deferred to Chapter 7. 

In Brownian suspensions, as (p increases, the slope of the viscosity-shear rate curve in the shear-thickening regime typically increases, and for electrostatically stabilized suspensions at high-volume fractions, it can even become a discontinuous jump. At shear rates above the shear-thickening regime, there is typically a second shear-thinning regime (see... 

This chapter deals almost exclusively with neat, or pure, diblock copolymer melts. Polymer blends are discussed in Chapter 9, micellar solutions in Chapter 12, and stabilized suspensions in Chapter 6. In the following, Section 13.2 briefly reviews the thermodynamics of block copolymers, and Section 13.3 describes the rheological properties and flow alignment of lamellae, cylinders, and sphere-forming mesophases of block copolymers. More thorough reviews of the thermodynamics and dynamics of block copolymers in the liquid state have been written by Bates and Fredrickson (1990 Fredrickson and Bates 1996). The processing of block copolymers and mechanical properties of the solid-state structures formed by them are covered in Folkes (1985). Biological applications are discussed in Alexandridis (1996). 

The properties of such apparently soluble materials differed greatly from those containing stabilized polyacetylene crystallites. Attempts to dope the soluble copolymers yielded materials with low conductivities, and chemistry typical of solution chemistry (bromination) was observed rather than the formation of a stable bromine-doped polyacetylene phase. A poly(isoprene-fo-acetylene) copolymer oxidized with iodine gave conductivities as high as 1-10 S/cm, but the characterization of the copolymer was insufiScient to unambiguously identify it as a soluble copolymer. On the basis of previously reported work, this material is likely to correspond to a stabilized suspension rather than a solution. 

Poloxamers and other polymeric materials such as albumin may coat the micro- or nano particle, alter their surface characteristics and reduce their phagocytosis and opsonization by the reticuloendothelial system following IV injection. Such surface modifications often result in prolongation in the circulation time of intravenously injected colloidal dispersions.Poloxamers also have been used to stabilize suspension such as NanoCrystal . ... 

Food products As thickeners, to provide specific rheology, to form gels, to inhibit syneresis, to stabilize emulsions, to stabilize suspensions, to emulsify, to bind/hold water, to form films and coatings, to inhibit crystallization, to improve texture/mouthfeel, as processing aids, as whipping agents, to stabilize foams, as extrusion aids, as binders, to stabilize proteins, to encapsulate, to flocculate... 

Suspension stabilizer Suspensions, instant granules, dry syrups... 

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