Siloxane particles

Precrosslinked poly(organosiloxane) particles are composed of crosslinking trifunctional and linear difunctional siloxane units (T and D units, respectively) [5]. The molar ratios of D and T units can be varied without restrictions thus, hard spheres (fillers) as well as soft, elastic silicone particles are accessible. In this study, the siloxane particles were synthesized in emulsion. The particle size was controlled by emulsifier concentration and crosslink density highly crosslinked particles were obtained with particle diameters ranging from 20-50 nm the size of elastic particles could be varied between 70 and 150 nm. The composition of precrosslinked poly(organosiloxane) particles is summarized in Scheme 1 further, organic radicals R which can be incorporated into the partieles are listed [6,7]. 

Graft copolymers are obtained by subsequent radical emulsion polymerisation of olefinic unsaturated monomers in the presence of functionalized siloxane particles. Scheme 3 illustrates that the graft... 

Simultaneously, the molecular weight of the particles is reduced from M 10 g mol to M 10 g mol [9]. Fig. 2a and 2b illustrate the monodispersity of precrosslinked poly(organo-siloxane) particles and of PMMA grafted core/shell particles, respectively. 

The diameters of the particles, prepared from dispersion, are approximately 120 nm each. The powder morphology of spray-dried PMMA grafted siloxane particles is illustrated in Fig. 3a the spherical agglomerates are approximately 1-20 pm in size. The microstructure of the graft copolymer after processing is shown in Fig. 3b. 

Fig. 3a. Powder morphology (scanning electron micrograph) of spray-dried PMMA grafted siloxane particles (obtained from the dispersion of Fig. 2b)...

Fig, 4a. Pressed sheets consisting of PMMA and of PMMA grafted elastic siloxane particles (powder from Fig. 3a transmission electron micrographs, ultramicrotome sections) 10 wt. % core/shell particles... 

As expected for two-phase polymer systems, two glass transitions are detected for thermoplast grafted elastic siloxane particles by means of thermal analysis (DSC, DMTA) [10]. In Table 1, glass transition temperatures of elastic siloxane graft copolymer particles with different crosslink densities (1-20 mol% T units) and distinct thermoplastic shells (PMMA, PS) are listed. 

Partially crystalline behavior at low temperature (endothermic melting peaks in DSC at approximate -50 °C) is observed for weakly crosslinked siloxane particles and the corresponding graft copolymers. 

Perry, R. J., Applications for Cross-Linked Siloxane Particles. Chem. Tech. 

By this way the structure of the final material is completely different. In the acidic preparation long shaped siloxane particles are obtained, while using an anunonia catalyst round-shaped particles are produced. 

Under acidic conditions long shaped siloxane particles were produced and microporous amorphous silicas with high surface area were obtained. The form of isotherms suggests the presence of rather regular channels having a very small diameter (about l.S nm). The base-catalyzed samples exhibited features of colloidal gel. Round-shaped particles were produced and mesoporous amorphous materials woe obtained. The structure is characterized by low bulk density and relatively low surface area with a mean pore diameter of the ortter of 20 nm. 

Additives used in final products Fillers aluminum, barium sulfote, aluminum hydroxide, glass fiber, mica, montmorillonite, Ni-BaTi03, nickel, silica, titanium dioxide, titanium fiber Plasticizers di-(2-ethylhexyl) phthalate, 2-hydroxyethyl methacrylate, 4-cyanophenyl 4-heptylbenzo-ate Antistatics copper dimethacrylate, glycerol monolaureate, indium tin oxide, lauramide diethanolamide, polyaniline, polypyrrole Antiblocking crosslinked siloxane particles Release magnesium stearate, methylpolysilsiquioxane, silicon nitride, stearic add Slip emcamide, slip ... 

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