Adhesives aluminum-oxide-filled

Thermal-transfer adhesives that are electrically insulating also exhibit wide ranges of thermal conductivities, depending on the filler type and amount. The thermal conductivities of epoxies filled with boron nitride or diamond are approximately 4 W/m K and 12 W/m K, respectively, while those of the more common aluminum-oxide-filled adhesives range from 1 to 2 W/m K. 

Fillers (calcium carbonate, calcium sulfate, aluminum oxide, bentonites, wood flour) increase the solid content of the dispersion. They are added up to 50%, based on PVAc. The purpose of the addition is the reduction of the penetration depth, provision of thixotropic behavior of the adhesive, gap filling properties and the reduction of the costs. Disadvantage can be the increase of the white point and a possible higher tool wear. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Theoretically, boron nitride is an optimum filler for thermally conductive adhesives. However, it is difficult to fill systems greater than 40 percent by weight with boron nitride. Beryllium oxide is high in cost, and its thermal conductivity drops drastically when it is mixed with organic resins. Therefore, aluminum, aluminum oxide, and copper fillers are commonly used in thermally conductive adhesive systems. 

The best current 100% solids epoxy adhesives contain about 70% aluminum oxide by weight and give thermal conductivities in the range of 0.8-1 in the English units shown in Table 2. For convenience, a conversion chart is included in Table 2 to permit conversion to any other set of units. The k values for the best alumina-filled epoxies are 10-12 times greater than for unfilled epoxy resins, but are still much lower than for pure metals or solders. Nevertheless, heat flow is adequate for bonding most components. For example, an adhesive with a thermal conductivity of 0.91 and a bond thickness of 3 mils would be able to transfer about 20 W/cm of surface area, with a AT only about 10 C above the heat sink temperatures ... 

Adhesives are poor conductors of heat and electricity, but both can be increased by filling with powdered metals, especially silver. To increase thermal conductivity alone, metal oxide fillers can be used. The most effective of these is beryllium oxide, which is both toxic and expensive aluminum oxide is a practical alternative. Some values of thermal conductivity are collected in 0 Table 18.6. 

Tape adhesives can be made thermally conductive by the dispersion of small articles of a conductive filler such as Saint-Gobain boron nitride (BN) PCTH3MHF and spherical aluminum oxide (AI2O3) available from Denka Corp. [42]. For example, 3M Corp offers pressure-sensitive adhesive (PSA) tapes filled with thermally conductive ceramic particles and flame retardant fillers. This product is designed with a thin polyester (PET) film and a soft acrylic polymer. It conforms to surfaces to which it adheres thus providing contact surface area for heat transfer [43]. 

A film of. colloidal silica on the surface of fibers, as in carpets, greatly reduces the pickup of dirt and leaves a cleaner appearance after vacuuming. It has been postulated that the silica forms a smooth adherent film to which soil particles do not cling, especially because it fills crevices in the fiber surface that would otherwise be filled with dark particles of dirt. Similar effects are reported on painted surfaces, plastic fabrics, window shades, and wallpaper (575). This type of use for colloidal silica was patented by Cogovan and Frederic (576). The advantage of using two hydrous colloidal metal oxides together is claimed by Florio and Rainard (577). Soluble aluminum phosphate is claimed to improve the adhesion of the silica to the fiber (578). 

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