ABSTRACT:We report a new method of polyimide synthesis based on the interaction of dianhydrides with acylated diamines for preparing a melt processable mixture of prepolymer and rare earth magnetic alloy particles in the form of 75-100 m particles. This mixture can be easily converted to useful thermoplastic polyimide bonded magnets by heating at 300°C. It is shown that the prepolymer based on 1,3-bis(3,4-dicarboxyphenoxy)benzene dianhydride and the diacetyl derivative of 2,2-bis(4-(4-aminophenoxy)phenyl)sulfone diamine after removing less than 5% by weight of the volatile components can be melted at 220 -240°C to give a fluid with a viscosity of 10 -20 Pas. This low viscosity of the prepolymer facilitates blending it with magnetic particles at relatively high volume fractions (up to 85 vol %) that are not possible using conventional methods. The resulting polyimide-bonded magnets exhibit excellent thermal stability and a high storage modulus of 10 GPa at 400°C. Magnetic property measurements showed a Ն10% increase in energy products over that of typical commercial bonded magnet materials such as the ones containing thermoplastic poly(phenylene sulfide) or polyamide matrices.
ABSTRACT:We report a novel method of polyimide (PI) synthesis from prepolymers based on dianhydrides and diacetyl derivatives of aromatic diamines that facilitate the preparation of a melt processable mixture at 300 Ϯ 10°C of the prepolymer and magnetic Nd-Fe-B alloy to provide PIbonded magnets with enhanced properties. It is shown that chemical structure of the prepolymers strongly influences viscosity behavior via crystallization of the oligoimide in the melt, leading to formation of PI with rigid-rod like structure. This structural ordering of the prepolymers based on diacetyl derivative of diamine used in this study, if not controlled, leads to exponential increase of melt viscosity with time, making it practically impossible to prepare melt processable mixture of the magnetic particles and the PI prepolymers at elevated temperatures. The results obtained demonstrate that appropriate dianhydrides and diacetyl derivatives of diamines that do not lead to crystallization of oligoimides in prepolymer mixture can be used under controlled processing conditions to prepare melt-processable PI-bonded magnets containing rigid-rod like PI structure that significantly increases thermal stability of the magnets. The temperature dependencies of the magnetic properties of the PI-bonded magnets under conditions that they are likely to encounter during their service life were found to be remarkably similar to that of commercial thermoplastic magnets such as injection-molded nylon magnets.
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