Cadmium-barium systems

Mention has already been made of epoxide stabilisers. They are of two classes and are rarely used alone. The first class are the epoxidised oils, which are commonly employed in conjunction with the cadmium-barium systems. The second class are the conventional bis-phenol A epoxide resins (see Chapter 22). Although rarely employed alone, used in conjunction with a trace of zinc octoate (2 parts resin, 0.1 part octoate) compounds may be produced with very good heat stability. 

Figure 15 illustrates the effect of a 45-minute dwell time on two plastisols containing 75 p.h.r. of DOP with different stabilizer systems. The upper example is a liquid cadmium-barium system and the lower a thiotin-dibutyltin maleate system. The organotin system is obviously the more effective. The leading yellow zone, which is followed by a dark zone, may indicate two different breakdown mechanisms. 

Somewhat better results have been obtained with octoates and benzoates but these still lead to some plate-out. The use of liquid cadmium-barium phenates has today largely resolved the problem of plate-out whilst the addition of a trace of a zinc salt helps to improve the colour. Greater clarity may often be obtained by the addition of a trace of stearic acid or stearyl alcohol. Thus a modem so-called cadmium-barium stabilising system may contain a large number of components. A typical packaged stabiliser could have the following composition ... 

Barium cadmium based systems (may also include zinc) have been available for many years due to their cost effectiveness in combination with good initial colour and long-term stability. However, in the European area, then-use was voluntarily phased out by the PVC industry in 2001, due to severe restrictions for environmental and toxicity reasons concerning cadmium. 

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 ... 

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. 

The basic metal salts and soaps tend to be less cosdy than the alkyl tin stabilizers for example, in the United States, the market price in 1993 for calcium stearate was about 1.30— 1.60, zinc stearate was 1.70— 2.00, and barium stearate was 2.40— 2.80/kg. Not all of the coadditives are necessary in every PVC compound. Typically, commercial mixed metal stabilizers contain most of the necessary coadditives and usually an epoxy compound and a phosphite are the only additional products that may be added by the processor. The requited costabilizers, however, significantly add to the stabilization costs. Typical phosphites, used in most flexible PVC formulations, are sold for 4.00— 7.50/kg. Typical antioxidants are bisphenol A, selling at 2.00/kg Nnonylphenol at 1.25/kg and BHT at 3.50/kg, respectively. Pricing for ESO is about 2.00— 2.50/kg. Polyols, such as pentaerythritol, used with the barium—cadmium systems, sells at 2.00, whereas the derivative dipentaerythritol costs over three times as much. The P-diketones and specialized dihydropyridines, which are powerful costabilizers for calcium—zinc and barium—zinc systems, are very cosdy. These additives are 10.00 and 20.00/kg, respectively, contributing significantly to the overall stabilizer costs. Hydrotalcites are sold for about 5.00— 7.00/kg. 

Some metals, such as cadmium, cobalt, and lead, are selectively car-diotoxic. They depress contractivity and slow down conduction in the cardiac-system. They may also cause morphological alterations, e.g., cobalt, which was once used to prevent excessive foam formation in beers, caused cardiomyopathy among heavy beer drinkers. Some of the metals also block ion channels in myocytes. Manganese and nickel block calcium channels, whereas barium is a strong inducer of cardiac arrhythmia. 

Boyle EA(1981) Cadmium, zinc, copper, and barium in foraminifera tests. Earth Planet Sci Lett 53 11-35 Brugger J, McPhail DC, Black J, Spiccia L (2001) Complexation of metal ions in brines application of electronic spectroscopy in the study of the Cu(II)-LiCl-H20 system between 25 and 90°C. Geochim Cosmochim Acta 65 2691-2708... 

II elements invariably are not used singly, but in Subgroup A-B couples. Aside from lead compounds, the elements currently used in most applications are barium, cadmium, calcium, zinc, and tin—e.g., the calcium-zinc soap systems for nontoxic (food contact) applications, the general purpose barium-cadmium soap stabilizers, and the organotin stabilizers. The objective of this paper is to consider the rheological consequences of... 

The system containing barium-cadmium-zinc stabilizer significantly outperformed the corresponding system based on tin mercaptide. The addition of the phosphite did not alter relative performance in Ba-Cd-Zn systems (Table VII). 

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