Formaldehyde, reactions with starch

Various aminoalkyl and alkylaminoalkyl halides react with starch in alkaline media and are readily available by the reaction of epichlorohydrin with primary, secondary, and tertiary amines and even aqueous ammonia. Prior to the reaction with starch, the quaternary ammonium compounds were either decomposed to free amines, or they reacted as amminium compounds to give cationic aminium starches. Cationic starches were produced by the reaction of starch with arylalky-laminoalkyl epoxides. In the presence of formaldehyde, inorganic ammonium salts provided the source of amino groups.1316... 

Earlier it was considered that formaldehyde adsorbs onto starch,719 but later papers have shown that chemical reaction takes place with the formation of acetals and hemiacetals.720,721... 

It was reported that the reaction of starch with phenol in the presence of a Lewis acid such as AICI3 resulted in resins of controlled melt viscosity.252 Probably, the product results from the hydrolysis of starch to glucose with conbversion of the latter into 5-(hydroxymethyl)-2-furaldehyde, which subsequently condensed with phenol.253 The reaction of starch with phenol without any catalyst required temperatures between 200 and 260 °C, and the resultant resinous product was then hardened by condensation with formaldehyde.254 Mastication of either glycerol or phenol with starch and water was said not to involve alcoholysis, but instead results in the formation of polymeric products.255 Reactions with gossypol256,257 and propylene glycol258 that were performed in the presence of a basic catalyst were in fact polymerization reactions and not alcoholyses. 

Since aldehydes behave as bifunctional reagents, crosslinking of starch occurs. For example, Syniewski1275 reported the reaction of starch with formaldehyde. 

A wide variety of reaction conditions have been proposed for the reaction of starch with aldehydes. Early studies entailed use of alkaline solutions, perhaps in order to gelatinize starch.1276 Because of the known behavior of aldehydes in alkaline media (Cannizzaro disproportionation and aldol condensations of aldehydes with active active a-methylene groups), this reaction has no simple outcome. However, a process of acetalation in alkaline solutions was subsequendy patented.1293 An exothermic reaction was observed within 3 4 min using an excess of formaldehyde. It was also reported that a,(3-unsaturated aldehydes reacted readily in slightly basic media.1294... 

The reaction in water at pH 7 required either 24 h at room temperature or 12 h at the boiling point.1290 These observations were later confirmed by reaction of aliphatic aldehydes with starch over the temperature interval from room temperature to below the gelatinization temperature.1303 The properties of the products obtained under identical reaction conditions were dependent on the starch variety. Aromatic aldehydes reacted with difficulty under these conditions, as the reactions lasted up to one week at 37 °C.1303 The reaction may be performed without any catalyst, however, it required heating in a sealed tube at 160 °C for 3 h.1304 At lower temperatures, an extended heating time was required and the proportions of water, formaldehyde and starch also affected the outcome of this reaction. As expected for this reversible reaction, an increase in water content shifted the reaction in favor of the reactants. An excess of formaldehyde stopped the reaction at the stage of hemiacetal formation. 

A cationic starch can be prepared by blending a condensate of formaldehyde plus an inorganic ammonium salt with starch and heating at 80 °C for up to 1 h.1316 A great interest was developed with urea-formaldehyde resins that were made to react with such polyalcohols as starch.1317-1319 It was reported that the reaction of starch with urea-formaldehyde resin required an acidic catalyst592,1320-1325 and the use of such dispersants as polyphosphates.1326 Ammonium metaphosphate catalyzed the reaction without the addition of any other acid catalyst.1327 A product that formed a paste was produced by controlling the proportion of starch to aldehyde (either up to 20% of formaldehyde or up to 30% of acetaldehyde) and urea (up to 15%) at pH 2 1.1328... 

Reaction of starch dialdehyde with urea gave a condensation product.442,2633 Carboxyamides,2624 including acrylamide,524,585 were also allowed to react with starch dialdehyde. Condensation can occur between starch dialdehyde and macromolecules containing amido groups, for instance, epichlorohydrin-crosslinked polyamide resin,598 ammonia-diethylamine-epichlorohydrin copolymer,588 dicyanamide-formaldehyde resins,565,566 quaternary alkylam-monium compounds,596,610,611 amino acids,2634,2635 protein,626,2636-2639 and aminoalkylated starch.2515 The reactions are favored by low pH. The guanidino moieties of proteins entered the condensation the most readily. 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

The effect of microwave radiation on maize, potato, and cassava starch (all either air-dried, in slurry, or pregelatinized) has been studied by Muzimbaranda and Tomasik.295a Starch readily dextrinized under such treatment. The sensitivity to microwaves increased in the order com > cassava >> potato starch. Micro-wave radiation also allowed facile cross-linking of starch with formaldehyde. The reaction took place for 5-15 min. and did not require any catalyst. Extended periods of reaction led to dextrins. In the presence of hydrogen peroxide, starch could be cross-linked with acetylene.2953... 

There are also patents for solid, water-insoluble products resulting from such a reaction.1276,1277 In addition there are reports of the strengthening of artificial silk, starches, and cellulose after contact with formaldehyde.1278 On the other hand model studies carried out by Tomasik and Schilling1279 on the reaction of maltodextrins with formaldehyde and pyruvaldehyde suggested that neither of these, in contrast to glyoxal and glutaraldehyde, acted as crosslinker. 

Early patents reported formation of acetals with formaldehyde at pH < 2.1295,1296 Interestingly, even when starch was pretreated with alkali,1295 the reaction is complete within 3-28 h at pH 1.6-2.5 and at a temperature below gelation. The concentration of either formaldehyde or acetaldehyde was 0.075-0.5 wt.% of starch in suspension. It was reported that this reaction carried out in acidic media could be shortened to 1 h and that the viscosity of the final product depended on the... 

A thixotropic starch was produced in reactions of POCl3-crosslinked starch with unsaturated aliphatic aldehydes.1361 Starch phosphates have been crosslinked by various aliphatic and aromatic aldehydes in the presence of urea, melamine, and similar compounds. Among several aldehydes tested[formaldehyde, propanal (propionaldehyde), glyoxal, glutaraldehyde, 2-hydroxyadipaldehyde (2-hydroxy -1,6-hexanedial), and some aromatic aldehydes], only those products that had reacted with glyoxal were insoluble in water.1322 Significant increases in viscosity occurred after starch phosphates having a low DS were treated with urea.1590... 

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