A new composite carrier based on magnetic particles and a phenol resin has been developed for application to long life electrophotographic developers. The carrier, composed of core particles and a resin shell, is quite spherical in shape with a narrow particle size distribution in the range of 10 – 200 μm in diameter. The carrier core can either be made up of magnetic fine particles such as ferrites and magnetites, or of non-magnetic particles such as hematites with 90 wt% or high loading. The core particles were prepared by dispersing magnetic or non-magnetic fine particles in an aqueous solution of phenol and formaldehyde in the presence of ammonia as condensation catalyst. The surface of the core particles was then coated with a silicone resin. The composite carrier thus prepared exhibited excellent charging and charge-holding properties that greatly exceeded those of commercial ferrite based carriers.
Toshiyuki Hakata, "A Novel Composite Carrier for Electrophotographic Developers" in Journal of Imaging Science and Technology, 2002, pp 591 - 597, https://doi.org/10.2352/J.ImagingSci.Technol.2002.46.6.art00015