Scavengers zinc-base

Zinc-Base Scavengers. There are two types of zinc-base scavengers available. These are ... 

Only two types of scavengers are being marketed to the drilling industry currently (1) zinc-based chemicals and (2) iron-oxide (Fe304). Copper compounds, although used in the past, are not considered satisfactory today because of drill pipe corrosion caused by reaction of copper ions with iron. This will be fully discussed in subsequent sections. 

Problems are caused by clay flocculation, but these often can be minimized by adding more deflocculant. In fact, some early commercial zinc-based scavengers contained a lignosulfonate for this purpose. This combination was abandoned because deflocculation is not always desirable in conjunction with a scavenger treatment (Garrett et al. 1979). 

Over the years, there have been searches on getting more environmentally benign, effective, and low-cost scavengers. This actually led to the organic compounds earlier mentioned. However, the search still continues until better and ideal scavengers are found. The common zinc-based compound has been modified into chelates and so forth, but for the fact that zinc is a heavy metal and thus environmentally unfriendly coupled with its adverse effects on mud rheology, the modified products have still not met the requirements of an ideal scavenger. 

Heat stabilisers for PVC act by HC1 scavenging and include organotins, mixed metal salt blends, and lead compounds. The latter account for nearly 64 % of volume (in 1994), followed by barium/cadmium and organotin compounds. Cadmium-based heat stabilisers are rapidly being replaced due to environmental concerns. Barium/zinc and calcium/zinc compounds show a high growth rate. It is expected that methyltin stabilisers will soon dominate the growing PVC pipe market. 

The coupling reaction with the amine usually requires an additional acid scavenger (often a base like triethylamine, DIEA, or NMM) to trap the formed HCl. The reaction can also be accelerated in the presence of a catalytic amount of DMAP (21), pyridine, or metallic zinc (22). 

In many ways, copper-based scavengers fit the criteria previously listed for an ideal scavenger, but copper has one serious limitation—corrosion of metallic iron resulting from spontaneous metallic copper plating. For this reason, the basic copper carbonate that was once widely used has been replaced by its zinc counterpart. 

Acid scavengers neutralize catalyst residues and prevent equipment corrosion. The choice of the acid scavengers can affect the overall acidity/basicity of a resin and influence the reactions of many of the organic additives in the system. Also, the acid scavengers as metallic stearates will impart to the polymer a certain lubricity, mold release or slip properties. As acid scavengers can be used metallic stearates (of sodium, calcium and zinc, zeolites structures (dihydro talcite, both synthetic and natural) CaO and ZnO and other metallic salts based on lactic or benzoic acid. Environmental Products has introduced PTA-210 and PTA-310 for increased thermal stability of PE, EVA, EMA, EVAA, PP, and ionomers. They are used to neutralize acidic catalyst residues. Concentration level is very low (0.2-0.5%). The hydrocaldte L-SSRH introduced by Reheis has a similar effect. 

---