A new technology for production of 4BS pastes for the positive (lead dioxide) plates of lead-acid batteries has been developed based on an Eirich Evactherm(R) mixer. The basic principle of this new technology is that 4BS crystals with dimensions between 20 and 25 mum are formed first from a semi-suspension at a temperature higher than 90 degreesC and then the excess water is removed from the semi-suspension under vacuum until the desired paste density is obtained. During the vacuum treatment the temperature of the paste decreases and small 4BS and PbO crystals are formed. During the paste formation procedure, the large 4BS crystals build up the PbO2 skeleton of the PAM, whereas the small crystals form the energetic PbO2 structure, which participates in the charge-discharge processes on cycling of the battery. It has been found, through XRD and thermogravimetric analysis, that the 4BS particles comprise crystal and amorphous zones. The crystal zones contain water molecules, part of which can be easily removed from the particles under vacuum treatment and curing as a result of which the crystallinity of the 4BS particles decreases. Another part of the bound water remains in the 4BS particles after curing of the pastes and can leave them only after heating to 250 degreesC. The ability of water to leave the particles depends on the density of the semi-suspension used for preparation of the paste. Experimental tests have shown that the highest battery performance is obtained when the paste is prepared under the following conditions: degree of lead oxidation in the leady oxide (LO) 85% PbO/LO, H2SO4/LO ratio 5-6%, liquid content (H2SO4 + H2O) in the semi-suspension 240-260 ml/kg LO, temperature of the semi-suspension equal to or higher than 90 degreesC, duration of paste mixing about 15 min. The new semi-suspension technology of 4BS paste preparation facilitates the formation of stable PAM structure that ensures high capacity and long cycle life of the positive plates of lead-acid batteries. (C) 2001 Elsevier Science B.V.All rights reserved.