Organic expanders

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

During the first trials with synthetic separators around 1940 it had already been observed that some of the desired battery characteristics were affected detrimentally. The cold crank performance decreased and there was a tendency towards increased sulfation and thus shorter battery life. In extended test series, these effects could be traced back to the complete lack of wooden lignin, which had leached from the wooden veneer and interacted with the crystallization process at the negative electrode. By a dedicated addition of lignin sulfonates — so called organic expanders -— to the negative mass, not only were these disadvantages removed, but an improvement in performance was even achieved. 

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The length of surfactant side chain determines dimensions of the hydrophobic domain and consequently unit cell of a liquid crystal. Pores of different diameter can be obtained by changing surfactant size. This initially allowed MCM-41 pore adjustment from about 20 A up to 50-60 A. Further expansion was possible based on another phenomenon, i.e. micelle swelling by solubilized organic compounds. MCM-41 with pores up to 120 A was obtained with mesitylene as an auxiliary organic (expander) in a hexadecyltrimethylammonium tcmplated preparation [4]. Triblock copolymers were found to afford hexagonal structure, SBA-15, with a pore diameter up to 300 A [33]. Pore dimensions can also be tailored by adjusting the synthesis temperature and post-synthesis treatment with amines 118]. 

The secondary lead structure of the completely formed plates with or without expander is shown in Fig. 3.57. In NAM without expander, the secondary lead structure covers the skeleton in the form of a smooth layer, whereas the secondary lead structure in the NAM of expander-containing plates comprises individual Pb crystals which are located over the skeleton structure. Hence, organic expanders regulate the processes involved in the formation of both types of structure in the lead active-mass during the formation of negative plates. 

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Influence of the organic expander component on the processes ofnucleation and growth of Pb and PbS04 crystals... 

When a maximum current value /max is reached, passivation of the electrode starts due to the formation of a PbS04 layer (curve A, Fig. 7.2). The presence of organic expander in the solution leads to a multifold increase in the quantity of electricity required for electrode passivation (curve B) and shifts the passivation potential at imax to more positive values. A shift of the Pb dissolution processes to more positive potentials is suggested by the appearance of an additional overvoltage in the Pb dissolution processes. These phenomena have been related... 

It follows from the above said that the organic expander improves the capacity of Pb PbS04 plates in lead—acid cells, but it has a negative effect on battery charge... 

It has been established that the organic expander component forms a three-dimensional netlike (reticulate) layer on the metal surface and this layer is colloid-Uke [16]. Lead ions pass through this reticulate layer, though their transfer is impeded (Fig. 7.4b,c) [14]. Lead sulfate crystals form over this layer (rather than directly on the lead surface), thus no compact PbS04 layer forms on the electrode surface to passivate it. Consequently, the quantity of oxidized lead increases and hence the electrode capacity increases, too. 

The above results indicate clearly that grid alloying additives interact with the organic expander. The thus formed organometallic compounds exert an influence on the performance characteristics of negative battery plates. 

Changes in discharge time (capacity) at (a) +30 °C (b) —18 °C, on cycling cells with negative plates containing (1) no additives (2) 0.3% BaSO (3) 1.0% humic acid (the organic expander component) and (4) 0.3% BaS04 + 1.0% humic acid. Fig. 7.25a presents the test results for cells... 

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PbO and Pb), water, sulphuric acid and organic expander and other constituents dependent upon whether the paste is used for positive or negative plates [1]. The plates are then dried, welded together and placed within a case. The battery is shipped in this dry form and is only subsequently filled with sulphuric acid and charged. 

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