Lead carboxylates

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Schotte B (2007) A study of the electrolytic reduction of corroded lead objects and the apph-cation, characterization and testing of a protective lead carboxylate coating, PhD dissertation Ghent University, Ghent, Belgium (https //archive.ugent.be/handle/1854/8316). 

X-ray diffraction measurements were used to determine the orientation of talc and lead carboxylate fillers in plasticised PVC extrudates. Correlations between the extrusion conditions (draw ratio and temperature), the development of filler particle orientation and the tensile properties of the plasticised PVC were studied. The presence of fillers enhanced Young s modulus and this was predicted well by the model developed by Halpin and Tsai. The extrudates were stretched above and below the gel-liquid transition temperature of PVC (about 205C). Above this temperature, the PVC could be stretched more and the tensile results indicated that the crystallites which were surrounded by more flexible chains were more oriented. 24 refs. 

STRUCTURE OF LEAD STABILIZERS. I. BASIC LEAD CARBOXYLATES... 

IR and NMR spectra of basic lead carboxylates used as heat stabilisers for halogenated polymers, e.g. PVC, indicated that these salts were not complexes, or double compounds of lead oxide, as suggested in most textbooks, but unique compounds of interesting structure. Stabilisers studied included monobasic lead phthalate, tribasic lead maleate, tetrabasic lead fumarate, dibasic lead phthalate and dibasic lead stearate. Lead stabilisers probably functioned in these polymers by converting hydrogen chloride to water, interfering with acid catalysis of the elimination reaction. 6 refs. 

The two reagents (periodic acid and lead tetraacetate) are complementary, since periodic acid is best used in water and lead tetraacetate in organic solvents. Chiral lead carboxylates have been prepared for the oxidative cleavage of l,2-diols. ° When three or more OH groups are located on adjacent carbons, the middle one (or ones) is converted to formic acid. 

The lead compounds used as heat stabilisers include dibasic lead carbonate and phthalate, dibasic and neutral lead stearate, tribasic and tetrabasic lead sulphate, dibasic lead phosphate and phthalate, liquid lead carboxylates and tetrabasic lead fiunarate. Lead stearates are added to other lead compounds because some combinations show a synergistic effect. 

Most lead stabilisers have the side effect of making the products opaque, except for the lead carboxylates. They also react with any sulfur present, producing brown lead sulfide. 

A fresh lead surface slowly oxidizes into a thin, protective lead oxide (PbO) that stops further oxidation of the metal. Lead gives satisfactory resistance to corrosion in rural, marine, and industrial environments. The corrosion rate data for lead is shown as 0.5-0.7 pm/y in industrial (New York, NY), 1.2-2.2 pm/y in marine (Kure Beach, NC), and 1.05-1.85 pm/y in rural (State College, PA) [6]. Lead corrosion products in such environments, in addition to lead oxide, are sulfate, chloride, and carbonate, with lead chloride being the most soluble of all four products (see Table 1). However, lead in outdoor exposures was found to produce sulfate (PbS04) and/or carbonate (PbC03), and indoor exposures lead carboxylates. The primary atmospheric agents responsible for degradation of lead are SO2, CO2, and carboxylic acid [7]. The corrosion rate of chemical lead in Key West, Florida, and La Jolla, California is 0.58 and 0.53 pm/y (0.023 and 0.021 mpy), respectively [2]. 

Very many organolead carboxylates have been investigated for potential commercial uses. Little structural information is available for these compounds, but the trimethyl-lead carboxylates are believed to be coordination polymersii in which planar trimethyl-lead ions are joined by carboxylate groups as in Fig. 8, producing five-coordinate lead. 

Bases utilized for this purpose can be either organic molecules (N,N -diphenylurea, dihydropyridin, polyols, etc.) or salts or metallic oxides such as 3Pb0 PbS04-H20 as well as barium, cadmium, calcium, zinc, lead carboxylates, or thiolates. 

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