Curing, lead oxides

MHX-Ni batteries 219 crystallography 365 cubic close packing 47 cubic packed arrays 293 curing, lead oxides 167... 

The tubular positive plate uses rigid, porous fiber glass tubes covered with a perforated plastic foil as the active material retainer (Fig. 2). Dry lead oxide, PbO, and red lead, Pb O, are typically shaken into the tubes which are threaded over the grid spines. The open end is then sealed by a polyethylene bar. Patents describe a procedure for making a type of tube for the tubular positive plate (90) and a method for filling tubular plates of lead—acid batteries (91). Tubular positive plates are pickled by soaking in a sulfate solution and are then cured. Some proceed directiy to formation and do not requite the curing procedure. 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

The production of tubular positive plates is in principle similar to that of pasted plates. A number of manufacturers use the same gray oxide as the basic filling substance. Sometimes the share or red lead or minium (Pb304) is increased above 25 or even to 100wt.%. The latter is more economic when the manufacturer runs his own minium plant then the expense of the chemical oxidation of lead oxide (PbO) to minium (Pb304) may be compensated by reduced formation cost. Furthermore, curing is not required, because of the high oxidation state, and the battery starts with full capacity when formed. 

Curing is the process of exposing plates pasted positive and negative to a regime of (a) controlled time (minimum 32h), (b) temperature (30-35°C), and (c) relative humidity (>90%). This process converts the free lead into lead oxide, using oxygen from the surrounding air. The plates are allowed to cure for a minimum of 32 h. Care is also taken to ensure that the maximum temperature of the plate does not exceed 60°C. The cured plates are then parted. 

Lead oxide (PbO or Pb304) up to levels of 20 phr can be used to improve resistance to water as the chloride formed during cure is insoluble. 

In addition to the use of peroxides for crosslinking, metal oxide, polyfunctional alcohols, amines and epoxide resin cure systems can be used with CSM rubbers. In the metal oxide based cure systems it is usual to add a weak acid, such as stearic acid, and accelerators, such as MBT, MBTS or TMTD magnesium or lead oxides are generally used. 

Propellants include both rocket and gun propellants. Most rocket propellants are either Hazard Class 1.3 composites, which are based on a rubber binder, and ammonium perchlorate (AP) oxidizer, and a powdered aluminum (Al) fuel or Hazard Class 1.1 composites, which are based on a nitrate ester, usually nitroglycerine (NG), nitrocellulose (NC), HMX, AP, or polymer-bound NC. If a binder is used, it usually is an isocyanate-cured polyester or polyether. Some propellants contain combustion modifiers, such as lead oxide. 

Cure Conditions 5.0 1.0 Red lead NBC Eagle-Plcher Co. DuPont > 90% red lead oxide Active ingredient, nickel... 

Filled liquid neoprene sealants are cured at room temperature by the addition of lead oxide, tertiary amines or epoxy resins. Neoprene sealants covered by a top coat of white chlorosulfonated polyethlyene are being used on solar collectors.21 Mixtures of carboxyl terminated acrylonitrile elastomers (CTBN) and liquid epoxy resins cure readily at room temperature in the presence of bis phenol A and amines.22 Coal tar-epoxy resin, two component epoxy resin, and epoxypolyester resin systems have been used as sealants.22... 

The second parameter that affects the rate of lead oxidation is temperature. This effect is illustrated in Fig. 8.16 presenting the amount of unoxidized lead in the paste after 1 h of curing at different relative humidities and temperatures [14]. The initial lead content in the paste is 21%. 

With increase of temperature in the curing chamber the rate of lead oxidation increases, irrespective of the RH. However, at RH >81% this increase is insignificant, whereas at RH = 55—75% it is substantial. For 1 h of curing at 50 °C in air atmosphere with RH between 55 and 75%, the content of residual free lead in the paste diminishes from 21 to 11—12%. 

The rate of lead oxidation is influenced also by the moisture content of the paste prior to curing. The amount of water in the paste pores should be reduced so as to open the pores for access of oxygen from the air needed to oxidize the residual free lead in the paste. The rate of water evaporation from the paste pores depends on the RH in the curing chamber. This dependence is illustrated in Fig. 8.17 for plate curing at 25 °C and various relative humidities [14]. 

When the moisture content of the paste is 11.8%, the pores with radii larger than 11 pm are evacuated. At 8.6% moisture content, the pores with diameter > 1.5 pm are open for air access, and at 3.9% moisture content of the paste, this happens also to the pores with diameter d > 0.24 pm. On drying the paste after curing, the pores with the largest radii are evacuated first and then the smaller ones. Such behaviour indicates that evaporation of moisture from the paste follows a pattern opposite to the process of wetting the lead oxide powder. 

SLI plates were kept in contact with air for different periods of time. The obtained kinetic curves of oxidation of the plates by the air are presented in Fig. 13.6. After reaching a definite critical temperature and moisture content, a vigorous oxidation or burning of the plates commences. This phenomenon is quite similar to the processes observed during curing of the pasted plates. At the end of this period of rapid oxidation, the active mass contains 50—60% sponge lead and up to 40—50% lead oxide and lead sulfate. This composition of the negative active mass yields only a very low capacity, and often none at all. 

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