Physical lead oxides

Some physical and chemical properties of the lead oxides are compiled in Table 2. 

Studies of the formation, chemical composition, and properties of deposits have shown that they consist of partially oxidized organic material, including more or less nitrogen, sulfur, and phosphorus. Compounds of iron, silicon, calcium, and other metals are present in small quantity, together with substantial amounts of lead oxides, sulfates, and halides from combustion of the antiknock fluid. The effects of these deposits are both physical and chemical in nature they may physically interfere with lubrication, heat transfer, gas flow, operation of valves and spark plugs chemically, they may bring about corrosion and oxidation. 

Table 4 Selected physical, thermal, and mechanical properties of various lead oxides...

Lead oxide (PbO) exists in two modifications (polymorphs) (1) red tetragonal lead oxide (tet-PbO) (also known as a-PbO or litharge) and (2) yellow orthorhombic lead oxide (orthorhomb-PbO) (also known as P-PbO or massicot). Tet-PbO is stable at low temperatures and low pressures. The transition temperature of tet-PbO to orthorhomb-PbO is 486—489 °C and the thermal effect of the transition is 1.35 kJ mol When orthorhomb-PbO is cooled rapidly, it may remain unchanged and continue to exist at low temperatures. Eventually, it is slowly transformed into tet-PbO under external physical action. Lead oxide exists also in amorphous form. The latter s amount depends on the method of manufacture of PbO. 

The above investigations indicate that the physical and chemical properties of the lead oxide used as precursor material for the production of battery plates, though the lead oxide is only a starting compound for a number of chemical processes (paste preparation and plate formation, whereby Pb02 and Pb are formed), exert an influence on the energetic and capacity performance parameters of lead-acid cells and batteries. Hence, it is essential to produce leady oxides with optimum and stable physico-chemical properties which would guarantee high battery performance. 

Uncontrolled oxidation of rubber is detrimental to its physical properties. Oxidation reactions take place readily at unsaturated groups in polymers and are often referred to collectively as epoxidation however, oxidation under controlled conditions can lead to useful products such as the epoxidized natural rubber introduced by the Malaysian Rubber Producers Association (Schults etal., 1983 Cunneen and Porter, 1965 Ceresa, 1965 Avery and Watson, 1956). Natural rubber in the latex form is treated with hydrogen peroxide dissolved in acetic acid. This gives 50% epoxidized natural rubber. This rubber shows very interesting physical properties and excellent carbon black dispersion. Similarly, nonaqueous epoxidizations of synthetic polyisoprene can be achieved... 

If soda is replaced by potassium carbonate in glass manufacture and lead oxide is added, the light refraction properties of the glass are improved. The whole crystal glass contains at least 24% lead oxide, while the semi-crystal glass has at least 10% in all of lead, potassium and zinc oxides. Some characteristic physical properties of the different glass types are shown in Table 43.5. 

TABLE 23.5 Physical and Chemical Properties of Lead and Lead Oxides (Pb02>... 

Lead oxide PbO (plumbous oxide) can exist in two different modifications. Red lead oxide PbO precipitates in tetragonal crystals. Yellow lead oxide precipitates in rhombic crystals. Yellow Iccid oxide is also called Litharge (= Stone Silver ) this is because this oxide precipitates as slags when plumbiferous silver is refined. According to Atkins Physical Chemistry, these modifications of lead oxide have the following thermodynamic standard values ... 

Zinc oxide is the major vulcanization agent required for Thiokol FA however, the particular grade is important as certain impurities may affect the cure rate and final properties. A French process zinc oxide, such as Midwest AZO 66L or equivalent, is recommended. Other oxides such as lead oxide or dioxide, cadmium oxide, and zinc hydroxide may also be used with varying rates of cure and physical properties. The amount of zinc oxide over or under 10 phr is not critical and will not affect the rate of vulcanization. Note that the MBTS and DPG are peptizing agents, not accelerators. 

Table 4 presents solubilities and other physical properties of inorganic lead compounds. Inorganic lead (II) salts of lead sulfide, and lead oxides are poorly soluble in water. Notable exceptions are lead nitrate, lead acetate, lead chlorate, and, to a lesser degree, lead chloride. Inorganic lead (II) salts have relatively high melting points, with corresponding low vapor pressures at room temperature. 

Lead, like tin, forms only one hydride, plumbane. This hydride is very unstable, dissociating into lead and hydrogen with great rapidity. It has not been possible to analyse it rigorously or determine any of its physical properties, but it is probably PbH4. Although this hydride is unstable, some of its derivatives are stable thus, for example, tetraethyllead, Pb(C2Hj)4, is one of the most stable compounds with lead in a formal oxidation state of + 4. It is used as an antiknock in petrol. 

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