Stannous activator

The aolventa, enumerated under 10 and 11, may contain traces of peroxides. These can be removed either by refiuxing over anhydrous stannous ofiloride (see concluding paragraph of Section VI, 12) or by filtration under slight pressure through a column of activated alumina. 

Many kits contain the indicated biologically active ingredient in a lyophilized form with stannous chloride. A Tc-labeled radiopharmaceutical, which can be used for six hours, is formed when mixed with Tc pertechnetate. Preparation of the agent is at room temperature, unless otherwise stated. Technetium-99m. Available Tc kits are Hsted below. 

Catalysis is done by an acidic solution of the stabilized reaction product of stannous chloride and palladium chloride. Catalyst absorption is typically 1—5 p-g Pd per square centimeter. Other precious metals can be used, but they are not as cost-effective. The exact chemical identity of this catalyst has been a matter of considerable scientific interest (19—21,23). It seems to be a stabilized coUoid, co-deposited on the plastic with excess tin. The industry trends have been to use higher activity catalysts at lower concentrations and higher temperatures. Typical usage is 40—150 ppm of palladium at 60°C maximum, and a 30—60-fold or more excess of stannous chloride. Catalyst variations occasionally used include alkaline and non-noble metal catalysts. 

Acceleration modifies the surface layer of palladium nuclei, and stannous and stannic hydrous oxides and oxychlorides. Any acid or alkaline solution in which excess tin is appreciably soluble and catalytic palladium nuclei become exposed may be used. The activation or acceleration step is needed to remove excess tin from the catalyzed surface, which would inhibit electroless plating. This step also exposes the active palladium sites and removes loose palladium that can destabilize the bath. Accelerators can be any acidic or alkaline solution that solubilizes excess tin. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

In plain tinplate cans for acid foods, tin provides cathodic protection to steel (3,4). The slow dissolution of tin prevents steel corrosion. Many investigators (5-1I) have defined this mechanism in detail and have shown that the tin dissolution rate is a function of the cathodic activity of the base steel, the steel area exposed through the tin and the tin-iron alloy layers, and the stannous ion concentration. Kamm et al. showed that control of the growth of the tin—iron alloy layer provides a nearly continuous tin-iron alloy layer and improves the corrosion resistance of heavily coated (over 45 X 10"6 in. tin) ETP for mildly acid food products in which tin provides cathodic protection to steel (12). The controlled tin-iron alloy layer reduces the area of steel exposed to the product. ETP with the controlled alloy is designated type K, and since 1964, 75 type K ETP has been used to provide the same protection as 100 ETP provided previously (13). 

The active part of the molecule is the fluoride ion, which is why two other fluorine-containing compounds, stannous fluoride (tin fluoride) and sodium monofluorophosphate, are also used. 

Dioxan develops appreciable quantities of peroxides upon exposure to air or upon keeping. These can be eliminated by refluxing over anhydrous stannous chloride (compare Section VI,12) or by filtration through a column of activated alumina. 

The success of this type of reaction depends on the quality of the catalyst. The most active and dependable form of anhydrous stannous chloride 1 is prepared as follows In a 600-cc. beaker is placed 204 g. (189 cc., 2 moles) of acetic anhydride (99-100 per cent) and, while the liquid is stirred by hand, 226 g. (1 mole) of commercial c.p. crystalline stannous chloride dihydrate is added. This operation should be performed in a hood, for the heat of the reaction is sufficient to cause the acetic anhydride to boil. After about one and a half hours, the anhydrous stannous chloride is filtered on a large Buchner funnel, rinsed with two 50-cc. portions of dry ether, and dried overnight in a vacuum desiccator. The yield is quantitative (189 g.). The product may be kept in a tightly closed bottle until it is wanted. The product secured by dehydrating crystalline stannous chloride in an oil bath at 195-200° is satisfactory in many instances but is not dependable. 

The oxidation of aromatic hydroxylamines with peracids in the presence of cupric ions produces nitroso compounds. In the rigorous absence of metallic ions, azoxy compounds are formed [32]. On the other hand, the air oxidation is strongly accelerated by metals, the approximate order of activity based on a kinetic study being cupric s ferric > manganous > nickel chromic > cobaltous ions. Silver and stannous ions appear to have no effect [33]. 

Catalysts Catalysts are widely used for PU manufacture. Sometimes a combination of two or three catalysts is required to obtain the desired balance of reaction rates between compounds of differing active hydrogen activity. Metal compounds, especially organotin compounds, are much more efficient catalysts than tertiary amines for the -OH/NCO reaction. In addition to more commonly used dibutyltin(IV) dilaurate, dibutyltin(IV) diacetate, dialkyltin(IV) oxide or salts of divalent fin with a variety of carboxylic acids such as stannous octoate, hexoate and naphthenate etc. are available for this purpose. Combination of tin catalysts with tertiary amines has been reported to lead to a synergistic increase in catalytic activity. 

---