Hollow microspheres production

Narayan P, Marchant D, Wheatley MA. Optimization of spray drying by factorial design for production of hollow microspheres for ultrasound imaging. J Biomed Mater Res 56(3) 333-341, 2001. 

In another specific application that of hot melt adhesives, the choice of filler has an effect on adhesion (Figure 8.54). Four types of spheres (K-20, S-22, Z-light, and ML 3050) increase adhesion compared with unfilled adhesive. The first two (K-20 and S-22) are hollow microspheres of very low density. The remaining two have a lower density than glass but have thicker walls. Three fillers (CaCOa, Zeospheres, and aluminum) are solid products. All fillers which decrease adhesion have substantially higher thermal conductivity than the fillers which increase adhesion. From the studies of crystallization profiles, it is apparent that longer crystallization times allow the network to orient itself on the substrate, resulting in better adhesion. ... 

Hollow Microspheres. These may be glass or plastic (polyolefin, VDC/AN, phenolic). They may contain air or a volatile liquid. They can be stirred into a liquid system, such as epoxy, which is then cured. This is interesting for deep-sea and low-dielectric products. 

These foams can be defined as composites consisting of hollow microspheres and a polymeric matrix. This one is made of a thermosetting (PU, PIR, PF, EP, silicone or unsaturated polyester) or of a thermoplastic (PE, PP, PVC, PS, polyimide) [56]. The microspheres can be made of silica, glass, carbon, ceramics or polymers such as PS, PE, PP, polyamide (PA), polymethyl methacrylate (PMMA), divinyl benzene (DVB)-maleic anhydride, and so on [56-58]. The diameter of the tiny hollow spheres is 300 mm or less [35]. They contain an inert gas such as nitrogen or a CFC. The properties of these syntactic foams depend on matrix type, microsphere type (and the contained gas), ratio matrix to microspheres, curing process, production technology. Syntactic foams can be made in combination with the conventional ones. Such a complex composite can be formnlated into a mouldable mass then shaped or pressed into cavities. 

F. Caruso, R. A. Caruso and H. Moehwald, Production of Hollow Microspheres from Nanostructured Composite Particles, Chemistry of Materials, 11(11), 3309-3314 (1999). 

Porous spheres can be prepared by leaching one glassy phase from a two-phase spherical product, or by processes similar to those used for hollow microspheres whereby the surface layer never forms in a fuUy continuous manner, i.e., the blowing bubbles are exposed at the surface. 

F. Caruso, R. A. Caruso, and H. Mdhwald, Production of hollow microspheres from nanostructuted composite particles, Chem. Mater. 11,3309-3314(1999). 

Mandal T. K., Fleming M. S., Walt D. R., Production of hollow polymeric microspheres by surface-confined living radical polymerization on silica templates, Chem. Mater. 2000 12 3481-7. 

The RF thermal plasmas are suitable tools for making oxide ceramic microspheres, either dense or hollow. The microstructure of the product can principally be influenced by the structure of feedstock materials and to a less extent by the plasma operating conditions. Highly porous raw materials and/or the presence of blowing agents facilitate formation of hollow spheres. By varying the process conditions, one can primarily affect the thermal history of particles, hereby the degree of evaporation. 

Inexpensive, finely ground minerals like barium sulfate (barytes), dolomite, limestone (whiting), clays, and silica are widely used to provide bulk and reduce the cost of friction material formulations. These materials also act as friction modifiers and alter the performance of the end product. Other less commonly used fillers are hollow and solid organic and inorganic microspheres and fly ash. 

Room temperature ionic liquids (RTILs) are molten salts whose melting points are below room temperature. RTILs are formed when the constituent ions are sterically mismatched, thereby hindering crystal formation [17]. As polar solvents, RTILs have unique applications as tunable and environmentally benign solvents with very low volatility, high fire resistance, excellent chemical and thermal stability and wide liquid temperature range and electrochemical windows [17-19]. Solvent applications of RTILs include, for example, organic synthesis [17,20, 21], separations [22, 23], storage and transportation of hazardous chemicals [24], polymeric electrolytes [25, 26], dissolution of natural products [27] and synthesis of hollow metal oxide microspheres [28]. 

Syntactic foam is made by dispersing hollow microballoons into a liquid polymer and then solidifying it. Microballoons are typically hollow glass or hollow phenolic microspheres, and the most common liquid polymer is an epoxy prepolymer, which is then cured. Although some products are notably woodlike in their properties and machinability, primary applications are high-performance products such as deep-sea instrumentation. 

Method of piastisoi manufacture and use determines final product density. Making a piastisoi composition comprising a thermoplastic resin, a plasticizer resin, inert filler, and hollow thermoplastic microspheres should consider the following points ... 

A biomolecule aided solvothermal synthesis procedure was used to obtain cobalt sulfide hollow spheres.In a typical method L-cysteine, anhydrous cobalt chloride and different surfactants were dissolved in ethylenediamine. Cysteine was incorporated as a sulfur souree and also to facilitate the microsphere formation. The reaetion mixture was autoclaved at 180 °C for different times after stirring for 15 minutes. The resulting products were washed with deionised water and ethanol several times and placed in a vacuum oven at 60 °C for 4 hours. The hollow... 

Glass-ceramic microspheres (solid or hollow, 1-100 pm) are finding increasing usage in a range of industrial products, which include plastics, sealants, adhesives, paints and buoyancy materials [65]. 

Although phosphates have usually been absent from most reported products, fired hollow ceramic spheroids made from AlP04/Na2Si03/Kaolin/Al203 compositions have been patented [66]. Microcellular siliceous materials have been synthesised and phosphates may prove to have important application in this field. Very-low-density (-0.003 g/cc) silica aerogels may have their AIPO4 counterparts. Phosphate-bonded silica microspheres are promising materials [67,68]. 

Thermosets also benefit from the foam structure, as evidenced by improved thermal insulation, sound dampening and mechanical stress absorption responses to temperature changes or impact. Hollow spheres with an already set volume are normally used, that is, pre-expanded microspheres. The reason is that the curing reactions often interfere with any expansion before a sufficient volume increase has been obtained. Hollow organic spheres are found in products such as sealants, adhesives, putties, pipes, cultured marble, body fillers, model-making materials, and pastes [2, 3, 19]. Common suitable matrix materials are epoxies, PUR, and polyesters. 

Weight reduction is increasingly important in transport applications. Most fillers increase the weight of a plastics product, but hollow glass microspheres such as 3M s Scotchlite can reduce it without foaming. They reduce a polymer s density by as much as 30%. 

Syntactic foams are made by using a resin matrix to which has been added hollow spheres. The spheres can be made from many different materials including glass, plastic, ceramic, and naturally occurring substances. The most common matrix resins are epoxy and polyester. The foam is made by simply mixing the microspheres into the catalyzed resin, casting a product, and finally allowing it to cure either at room temperature or at an elevated temperature. 

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