Tospearl particles

Tospearl particles are crosslinked siloxanes made by the controlled hydrolysis and condensation of methyltrimethoxysilane. Their spherical nature, narrow particle size distribution and chemical and thermal stability make them ideal for use in wear resistance, antiblocking and light diffusing applications. 

Tospearl particles are also readily dispersed in a variety of solvents. Table 2 shows the dispersion viscosity of Tospearl 120 in polar and nonpolar solvents. In polar ketone, alcohol and ether solvents, the viscosities are quite low. When nonpolar or chlorinated solvents are used, the dispersion viscosities rise by an order of magnitude. The polar solvents more readily associate with the residual silanol groups on the Tospearl particles effectively isolating them from self-association. In non-polar solvents, self-association is possible and the viscosity rises accordingly. 

All applications described thus far have utilized untreated Tospearl particles. Although extensive condensation occurs during the synthesis of these particles, some silanol groups remain on the surface. The following two examples illustrate the ease with which the Tospearl surface can be modified. 

A fine-particle silicone additive has been developed by GE Silicones to improve the surface and surface Interaction characteristics of polymers, rubbers and similar materials. A key application for the additive (Tospearl) is an antiblocking agent for thin plastics film. Added to the resin during compounding, the fine particle spheres (0.5 12 pm) protrude from the surface of the blown film to form a uniformly uneven surface that prevents adhesion between film layers. The particle size is selected depending on film thickness. The spherical shape of the particles, combined with good inherent lubricity of silicone, also improves the slip characteristics of the film. 

Figure 17.1. With an average particle diameter of 4.5 fini. Tospearl is an advanced silicone anti-blocking...

A unique, amorphous T resin particle has been reported (541). This material, known by the trade name Tospearl (Tospearl is a trademark of the General Electric Co.), is a spherical resin particle made by the controlled, base-catalyzed hydrolysis and condensation of CHsSilOCHsla (eq. 43). 

Highly cross-linked spheres with an average particle size from 0.5 to 12.0 /rm have been produced. Extremely narrow particle size distributions are possible. The particle size and degree of monodispersivity are controlled by kinetic factors during the hydrolysis. Applications for Tospearl powders include wear resistance, antiblocking properties, light-diffusion control, and lubrication (542). 

Tospearl is a spherical silicone resin particle made by the controlled hydrolysis and condensation of alkyl trialkoxysilanes (equation 1) and produced by Toshiba Silicones, a joint venture of GE Silicones. The first report of these materials appeared in 1985 when the aqueous amine catalyzed condensation of methyl trimethoxysilane was described (1). Since the initial disclosure, a number of other patents have issued in this area (2-8). The product is a tluee-dimensional network that is intermediate between inorganic and organic particles. This paper describes the physical and chemical properties of this network as well as its utility in a number of applications. 

As expected for a material that is primarily inorganic, the thermal stability of the silicone resin spheres is extremely high, especially when compared to fine organic particles, as shown in Figure 7. Tospearl 120 was stable to 400+°C in air and at 800°C, there was only 10% weight loss. The weight loss seen at 450 0 is attributed to further condensation within the particle with loss of water and/or methanol. 

Figure 7. Thermogravimetric Analysis of Tospearl 120 and Organic Resin Fine particles.

Tospearl has also been used to increase the abrasion resistance of weather stripping (Table 4). Untreated EPDM base rubber abraded easily. When treated with a curable silicone, the abrasion resistance increased by an order of magnitude and the friction was halved, but a glossy rather than the desired matte finish was obtained. Addition of Tospearl 120 or 145 improved the abrasion resistance another 6-8 fold and the friction coefficient decreased 4-8 times. In addition, the rubber took on a matte appearance. If the particles were too large, as with Tospearl 3120, the abrasion resistance decreased. 

The surface of Tospearl 120 can be rendered more hydrophobic by treatment with hexamethyldisilazane (HMDZ) or trimethylchlorosilane (TMSCl) as shown in Table 9. Hydrophobicity is determined by the amount of material that is wetted by the solvent mixture and settles out during centrifugation. 45% of the untreated particles are wetted by a 60/40 methanol/water mixture. After hydrophobizing the surface, less than 5% were wetted (19). 

Tospearl GE Silicones, USA Fine particle, silicone fully crosslinked, for thin films... 

This is a very diverse group of polymeric materials. Particles have mostly spherical form. Spherical silicone resin (commercial name - Tospearl) is produced in several grades, most of which are spherical (some are elongated and irregular). Controlled hydrolysis and condensation of alkyl trialkoxysilane permits tight control of properties." Particles of some grades are monodisperse, containing only a small admixture of particles of other sizes than attempted in s mthesis. 

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