Acid scavenging

Acid scavengers are an important type of stabilizers used in the stabilization of polyfvinyl chloride), although they are used in other types of polymers as well. Therefore, this class of substances deserves a separate chapter. 

Acid scavengers are also addressed as acid absorbers, antacids, or still less precisely as costabilizers. The addition of acid scavengers is necessary because catalyst residues from processing and manufacture may contribute to imdesired properties. This does not necessarily cause a diminished stability of the polymeric base material. Instead, residues from Ziegler-Natta catalysts in poly(olefine)s may contain traces of halogen that could cause corrosion reactions to metals that are in contact with the polymer. 

For example, acid scavengers have been proposed as additives in poly(ester)-poly(carbonate) compositions (1). 

Calcium lactate compounds may form complexes with trace metals, such as titanium or aluminum. In propylene polymer compositions, potassium citrate as an acid scavenger has been proposed (3). In hydrotalcite type acid scavengers, the chloride anions are inter- 

One example in which industrial chemists have to deal with suspensions is add scavenging. Browsing organic textbooks one will find numerous reactions which liberate acids that have to be scavenged in order to prevent decomposition of the product or other side reactions. Equations (1) to (6) show some common acid -liberating reactions, namely esterfication, elimination, phosphorylation, sul-furylation, silylation, and deprotonation, respectively. 

In some cases, acid scavenging is not used to stop further reactions of the acid, but to shift an existing equilibrium to the favored side. This is true for elimination reactions or the formation of ylids that are further used in Wittig reactions. Hydrochloric acid (HCI) is the acid most commonly formed. Usually, tertiary amines such as triethylamine are added to scavenge the acid. With the acid, these substances form solid salts which turn the reaction mixture into a suspension. The solid byproduct must then be removed. This can be done via a filtration step which requires expensive equipment, especially for precipitates vrith high filter resistances or continous filtration. If the product is inert to and insoluble in water, one can dissolve the solid by adding water to the reaction mixture, and remove the organic phase by a simple liquid-liquid separation. If the product is sensitive to acids and moisture, until now only filtration was left as a suitable means. 

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Acid scavenging stabilizeis aie typically recommended for use with HBCD in themioplastic appHcations. Stabilized grades are available from the manufacturers. 

THPC—Amide Process. The THPC—amide process is the first practical process based on THPC. It consists of a combination of THPC, TMM, and urea. In this process, there is the potential of polymer formation by THPC, melamine, and urea. There may also be some limited cross-linking between cellulose and the TMM system. The formulation also includes triethanolamine [102-71-6J, an acid scavenger, which slows polymerization at room temperature. Urea and triethanolamine react with the hydrochloric acid produced in the polymerization reaction, thus preventing acid damage to the fabric. This finish with suitable add-on passes the standard vertical flame test after repeated laundering (80). 

Figure 5 illustrates the type of encapsulation process shown in Figure 4a when the core material is a water-immiscible Hquid. Reactant X, a multihmctional acid chloride, isocyanate, or combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydroly2ed poly(vinyl alcohol) or a lignosulfonate. Reactant Y, a multihmctional amine or combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase thereby initiating interfacial polymerisation and formation of a capsule shell. If reactant X is an acid chloride, base is added to the aqueous phase in order to act as an acid scavenger. 

In some instances, the presence of the acid scavenger affects the nature of the product. 

Phosphite esters are formed readily by the reaction of phosphoms haUdes and isopropyl alcohol. Eor example, triisopropyl phosphite [116-17-6] is prepared from phosphoms trichloride [7719-12-2] and isopropyl alcohol at low temperatures ia the presence of an acid scavenger, eg, pyridine [110-86-1]. 

Magnesium oxide is a typical acid scavenger for chlorinated mbbers. Compounds containing zinc oxide or magnesium oxide may tend to swell upon immersion in water. These inorganic salts have some water solubiHty and osmotic pressure causes the vulcanizates to imbibe water to equalize pressure (8,9). As such, vulcanizates tend to sweU more in fresh (distilled) water than in salt water. To minimize water sweU, insoluble salts such as lead oxides can be substituted. Because of the health concerns associated with lead, there is much mbber industry interest in other acid acceptors, such as synthetic... 

Oxidation of aHoocimene in the presence of a catalyst produces a polymeric peroxide, which can be thermally isomerized to produce aHoocimene diepoxide [3765-28 ] (56) in 70—75% yield (99). The diepoxide has been used in the manufacture of resins and as an acid scavenger for halogenated solvents (100). 

A significant use of butylene oxide [26249-20-7] is as an acid scavenger for chlorine-containing materials such as trichloroethylene. Inclusion of about 0.25—0.5% of butylene oxide, based on the solvent weight, during preparation of vinyl chloride and copolymer resin solutions minimizes container corrosion which may be detrimental to resin color and properties. 

Tosylates are generally formed from an amine and tosyl chloride in an inert solvent such as CH2CI2 with an acid scavenger such as pyridine or triethylamine. 

HgCl2, CaCO, MeCN, H2O. The calcium carbonate is used as an acid scavenger for acid-sensitive substrates. 

Upon carefully controlled hydrolysis with hydrochloric acid at room temperature, the corresponding serine methyl esters 4 are obtained in reasonable yields. Higher yields of 4 arc obtained by hydrolyzing with dilute trifluoroacetic acid5. In some cases, the diastereomeric ratio of 4 does not exactly correspond to the d.r. of the adduct 3, which is attributed to different kinetics in the hydrolysis of the diastereomers 4. Subsequent treatment of the methyl ester with excess 5 N hydrochloric acid and methyloxirane as an acid scavenger results in the free amino acid 54,7. 

If nitrite is present in excess by mistake, it has to be destroyed. Traditionally urea has been employed as a nitrous acid scavenger. The reaction yields gaseous products, as shown by Scheme 2-2, but as already mentioned by Zollinger (1961, p. 16) this reaction is slow. The recommendation has been to replace urea by sulfamic acid, which reacts according to Scheme 2-3. In strongly acidic solution,... 

Many procedures for the formation of carboxylic acid amides are known in the literature. The most widely practiced method employs carboxylic acid chlorides as the electrophiles which react with the amine in the presence of an acid scavenger. Despite its wide scope, this protocol suffers from several drawbacks. Most notable are the limited stability of many acid chlorides and the need for hazardous reagents for their preparation (thionyl chloride, oxalyl chloride, phosgene etc.) which release corrosive and volatile by-products. Moreover, almost any other functional group in either reaction partner needs to be protected to ensure chemoselective amide formation.2 The procedure outlined above presents a convenient and catalytic alternative to this standard protocol. 

Grisham, M.B., Ryan, E. and Von Ritter, C. (1987). 5-Aminosalicylic acid scavenges hydroxyl radical and inhibits myeloperoxidase activity. Gastroenterology 92, 1416. 

Acid scavengers - Acid neutralisers - Antiacids Activators... 

Neutral 2,3-dihydro-l,3,2-diazaphosphinines are straightforwardly accessible from condensation of 4-amino-1-azabutadienes with appropriately substituted dichlorophosphines or PC13 (Scheme 16). The reactions are generally carried out in benzene in the presence of triethylamine as acid scavenger, and the products are isolated in excellent yields of 86-96% [86, 87],... 

Biphasic Acid Scavenging Utilising Ionic Liquids (BASIL ) process, 26 899, 900 Biphenyls... 

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