Dibasic lead

By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

Rigid Applications. The use of the lead stabilizers is very limited in the United States but, they are stiU used in several rigid PVC appHcations in Europe and Asia. The highest use of lead stabilizers in rigid PVC is for pipe and conduit appHcations. Tribasic lead sulfate is the primary heat stabilizer with lead stearates included to provide lubrication. The lead products are typically fully formulated, usually including lubricants and pigments for pipe extmsion appHcations. These lead one-packs, when used at about 1.8—2.5 phr, provide all of the stabilizer and lubrication needed to process the polymer. A lead one-pack contains tribasic lead sulfate, dibasic lead stearate calcium stearate, polyethylene wax, paraffin wax, ester wax, and pigments. 

Dibasic Lead Sulfate. Dibasic lead sulfate [12036-76-9] 2PbOPbSO, is a white powder, mol wt 749.70, mp 961°C. The dibasic compound can be prepared by fusion of the two components. It has been sold as a PVC stabilizer in Japan and is sold in Europe in combination with dibasic lead phosphite. 

Dibasic Lead Phosphite. Dibasic lead phosphite [12141 -20-7] 2PbO PbHPO pfrac l 2 H2O, is a white crystalline powder, mol wt... 

Dibasic Lead Phthalate. Dibasic lead phthalate [17976-43-17, 2PbO Pb(02C)2CgH Xphacjl 2 H2O, is a white, crystalline powder, mol... 

For viayl compouads, the geaeral range of dibasic lead phthalate stabiHzet usage is betweea 4 and 4 phr resia. In 105°C electrical iasulation PVC stocks, approximately 7 phr is required. For vinyl plastics and foams, between 3 and 5 phr of lead stabiHzet is recommended. 

AppHcatioas of dibasic lead phthalate, coated grade, iaclude 90 and 105°C rated PVC electrical iasulation, plastisols, profile extmsions, calendered sheet, and molded products. The recommended range of usage ia viayl electrical iasulatioa is 5—7 parts pet huadred resia, depending oa the particular iasulatioa classiftcatioa to be met. Ia geaeral-purpose extmded and molded PVC stocks, approximately 3—6 phr of coated dibasic lead phthalate is suggested. 

Lead Whites. Basic lead carbonate, sulfate, siHcosulfate, and dibasic lead phosphite are commonly referred to as lead whites. Usage is limited because of environmental restrictions placed on the use of lead-containing compounds. 

Normal and dibasic lead stearate have a stabilising effect but their main uses are as lubricants (see section 12.5.4). Lead silicate is sometimes used in leathercloth formulations but is today of little importance. Other lead compounds now of negligible importance are coprecipitated lead orthosilicates and lead salicylate. 

In Britain calcium stearate has been most commonly used with nontransparent products and stearic acid with transparent compounds. In the United States normal lead stearate, which melts during processing and lubricates like wax, is commonly employed. Dibasic lead stearate, which does not melt, lubricates like graphite and improves flow properties, is also used. 

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

The main method of crosslinking the homopolymer and copolymer is by use of thioureas, and, as the cure reaction requires basic conditions, an acid acceptor is also added. Litharge, red lead, magnesium oxide and dibasic lead phosphite are commonly used acid acceptors, and the most commonly used thiourea is ethylene thiourea, but this has a tendency to promote mould fouling. 

Zinc oxide/dibasic lead phosphite Least tendency to scorch, intermediate acid resistance. 

Dibasic lead picrate-lead acetate, Pb(C6H2N307)2.2Pb(0H)2.2Pb(C 302)2, 276°C... 

PKC6H2N307)2.3Pb(0H)2.3Pb(C2Hs02)2, 277°C Monobasic lead picrate-lead formate, Pb(C6H2N307)2.Pb(OH) jjPbttiCO.jJa, 258°C Dibasic lead picrate-lead formate, Pb(C6H2N307)2.2Pb(0H)2.2Pb(HC02)2, 277°C... 

---