Cadmium, stabilizers

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

Antimony tris(isooctylthioglycolate) has found use in pipe formulations at low levels. Its disadvantage is that it cross-stains with sulfide-based tin stabilizers (122). Barium—zinc stabilizers have found use in plasticized compounds, replacing barium—cadmium stabilizers. These are used in mol dings, profiles, and wire coatings. Cadmium use has decreased because of environmental concerns surrounding certain heavy metals. 

PolyCvinyl butyrate), 10 492-493 Polyvinylcarbazole (PVK), 22 207t PolyCvinyl carbonates), 25 601 PolyCvinyl chloride) (PVC), 23 407 24 170 25 657-691. See also PVC entries Vinyl chloride polymers antioxidant applications, 3 123 asbestos substitute, 3 315 cadmium stabilizers with, 4 501 coatings, 7 39... 

Burton, D.J., Takei, R., and Shin-Ya, S., Preparation, stability, reactivity and synthetic utility of a cadmium stabilized complex of difluoromethylene phosphonic acid ester, J. Eluorine Chem., 18, 197, 1981. 

Metal Soaps—Barium and cadmium stabilizers are less toxic than lead and have lower cost on the basis of use levels, better sulfide stain resistance, and good clarity. They are a major class of heat stabilizers. Calcium and zinc soaps are also included in this classification because of their similar performance. The soaps used are based on a wide variety of anions such as octoates (2-ethyIhexanoic acid derived), benzoates, laurates, stearates, and substituted phenolates. 

Auxiliary Organic Compounds—These are of major significance in formulating barium-cadmium stabilizers and include the following ... 

The market for heat stabilizers has kept pace with the growth of the PVC industry. Current annual consumption is about 80 million lb with the major part of the market made up of barium-cadmium stabilizers. The overall make-up of the stabilizer market in 1981 is shown in Table X (32). 

PVC has been under attack from environmental groups on a variety of grounds for a number of years. Because PVC contains chlorine, it may lead to the formation of chlorinated dioxins if it is disposed by incineration. While evidence suggests that the presence or absence of PVC is not very significant in dioxin emissions from well-controlled incineration, less important than combustion conditions, there is reason for concern about its presence in poorly controlled incineration systems. PVC has also been attacked because of concerns related to the carcinogenicity of its vinyl chloride monomer. Increasingly, lead and cadmium stabilizers used in some PVC resins are being restricted or banned, due to concerns about toxic effects of these heavy metals. 

Cadmium Pigments and Cadmium Stabilizers Need Substitutes... 

Figu re 5.76 Properties of non-weathered and outdoor weathered (according to DIN 53386 at Oftringen, Switzerland, 400 m above sea level 45° southward) rigid PVC window profiles (90 parts S-PVC 70 10 parts chlorinated polyethylene (PE-C, 36% chlorine) 2.2 parts barium-cadmium stabilizer system 1.5 parts lubricant combination 1.5 parts epoxy plasticizer ... 

Batzer H (1983) Use and possibilities for substitution of cadmium stabilizers. Ecotoxicol Environ Saf 7(1) 117-121... 

PVC can be protected with a variety of stabilizer systems. The choice of stabilizer depends on the probable decomposition mechanisms involved. Primary and secondary stabilizers are generally used in combination. The primary stabilizers are mainly acid acceptors such as metallic soaps. Typical examples are, metallic soaps such as cadmium, barium, and zinc stearates which also act as lubricants. These chemicals simply react with HCl and are able to reduce further degradation. The mechanism of stabilization and properties of various metal soaps are reviewed (39,40). Calcium and zinc stabilizers are low in toxicity and they are often prepared in the form of a dispersion in epoxidized oil. Barium and cadmium stabilizers are mainly solid products those which are liquid contain phosphite groups. The compatibility of PVC stabilizers containing heavy metals (41) and synergetic effects of metal stabilizers (42) are reviewed. 

At present, there are no good substitutes for lead stabilizers in PVC electrical applications, although the search goes on (see, however. Chapter 4). Cadmium stabilizers, widely used for flexible and semirigid vinyl applications, appear to be phasing out in favor of barium/zinc, calcium/zinc, and zinc/polyphosphite systems. Performance capabilities of many of these packages leave something to be desired. 

---