Rubber magnet is a general term for elastomeric materials with magnetic properties. It is made from raw rubber, magnetic powder (magnetic filler) and other auxiliaries. Magnetic properties are the basic properties of rubber magnet. Rubber magnet exhibits different magnetic grades under the external magnetic field, and the magnet with strong magnetic grade has higher practical value.
Since the rubber itself is not magnetic, its magnetism comes from a large number of magnetic powders (2~8 times the mass of the raw rubber), so magnetic powder is indispensable for the manufacture of rubber magnet. According to the composition, magnetic additives can be divided into two categories: metal magnetic powder and ferrite powder, collectively called magnetic powder. Metal magnetic powder includes iron cobalt powder, iron nickel powder, iron strontium powder and iron barium powder, among which iron cobalt powder is the most widely used. Metal magnetic powder is seldom used because of high price and difficulty in adding. At present, iron oxide powder is more widely used. It consists of iron oxide and some two valence metal. Even the same ferrite magnetic powder, the final magnetic strength may be different because of the different crystalline morphology, particle size and uniformity.
The manufacturing process of rubber magnet is similar to that of other products. For the formulation, we must first determine the variety, size and dosage of magnetic powder. Next, the rubber’s kind should be determined according to the requirements. For example, natural rubber should be selected for wear resistance. During processing, before and after vulcanization, it is necessary to add magnetization procedures which is not required during other products’ processing to give permanent magnetism to the product. The advantage of magnetizing before vulcanization is that the direction of the external magnetic field will not change, and it is more smooth to go through the rubber network; if being magnetizing after vulcanization the rubber’s permeability gets poor, but the operation is more convenient. In a word, each has its own advantages and disadvantages.
The physical quantities used to characterize magnetic properties include magnetic intensity, residual magnetic susceptibility, magnetic energy product and so on. The most commonly used is magnetic intensity whose measurement unit is Gauss (GS). The rubber magnet’s intensity is generally required to reach 100~600 GS.