Tribasic 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. 

Tribasic Lead Sulfate. Tribasic lead sulfate [12202-17 ] 3PbO PbSO 2 hue white powder, mol wt 890.93, sp gr 6.9, refractive... 

Tribasic Lead Silicate. Ttibasic lead siHcate [12397-06-7] 3PbO -Si02, is a reddish yeUow powder, spatingly soluble ia water. Its compositioa... 

Stabilizers. Lead stabilizers, particularly tribasic lead sulfate, is commonly used in plasticized wire and cable compounds because of its good nonconducting electrical properties (122). 

For this reason tribasic lead sulphate, a good heat stabiliser which gives polymer compounds with better electrical insulation properties than lead carbonate, has increased in popularity in recent years at the expense of white lead. Its weight cost is somewhat higher than that of lead carbonate but less than most other stabilisers. This material is used widely in rigid compounds, in electrical insulation compounds and in general purpose formulations. 

Paste mixing means the addition of sulfuric acid and water. The result is a fairly stiff paste with a density between 1.1 and 1.4gcm 3 containing 8-12wt% of lead sulfate. The water content of thus mix determines the porosity of the active material achievable later (cf. "curing" below). In the paste, a mixture of lead sulfate and basic lead sulfate is formed (cf. Table 1). In the usual mixing process between room temperature and 50 °C, tribasic lead sulfate is formed. The generation of the tetrabasic... 

Tribasic acid, 18 817 Tribasic lead silicate, 14 796 Tribasic lead sulfate, 14 790 for PVC polymers, 25 671 Tribenzoate chiral stationary phase, 6 88t Tribenzylbismuthine, 4 27 Tribenzylether chiral stationary phase, 6 88t... 

Tribasic lead picrate-lead propionate, Pb(C6H2N30T)2.3Pb(OH)2.3Pb(C3H502)2,255oC Monobasic lead picrate-lead lactate, Pb(C6H2N307)3.Pb(0H)2.Pb(C3Hfl03)2, 276°C... 

Lead compounds are generally added to polyvinyl chloride in electrical formulations in order to stabilize them against thermal decomposition 7 p.h.r. of National Lead Tribase XL modified tribasic lead sulfate was used throughout the present study. Since the stabilizer itself is an ionic impurity, it is remarkable to note that it actually increases volume resistivity (Table IV). 

Figure 5.11 Infrared spectra of 50/50 PVC/NBR blends after mixing (a) for 5 minutes, (b) for 60 minutes (stabilised with tribasic lead sulfate) at 180 °C, (c) at 160 °C, (d) at...

Thermal Stability. The ABS-type polymers containing sufficient DBPF to pass the Underwriter s test were notably less prone to discoloration during processing than unstabilized PVC. On the other hand, they discolored when subjected to processing conditions normally used for conventional ABS polymers. Tribasic lead silicate was as effective as any of the common PVC stabilizers in preventing this discoloration during milling. 

Lead compounds, usually based on a mixture of lead salts, e.g., tribasic lead sulfate or dibasic lead phosphite, and lead soaps, e.g., dibasic or normal lead stearates, have been very cost effective heat stabilisers for many years, and function as HC1 absorbers (236, 365,438). 

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. 

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