Formaldehyde 568 structure

Acetal resins n. High molecular weight, stable, linear polymers of formaldehyde structurally, an oxygen atom joins the repeating units in an ether rather than ester-type link. These also include butyrals. Strong AB (2000) Plastics materials and processing. Prentice-Hall, Columbus, OH. 

Phenol-formaldehyde resins resemble a group of chemicals that contain the formaldehyde structure but are not necessarily associated with formaldehyde allergy. Resols and novolacs are distinguished within that group. While the chemical curing of novolacs requires the presence of formaldehyde to react with the phenol terminate group, resols do not. Resols are intermediates, which, in turn, need heat to cure (Malten 1984). Para-substituted phenol resins do not crosslink but adhere readily under pressure. Among... 

The only example of stepwise polymerization to appear in the literature is the electrosynthesis of an open pore urea-formaldehyde structure at the anode from an aqueous solution of a non etherified urea-formaldehyde resin. The structure has similar characteristics to those of a foam obtained by acidification. ... 

Open pore urea-formaldehyde structures have unique properties, and their spherical and pore sizes can be controlled to make them suitable for many applications. Filtration structures, chromatographic columns, porous urea-formaldehyde pigmented polystyrene, smog dispersal agents, moisture retentive fertilizers, fruit coatings, and porous polymer-bound multicomponent corrosion inhibitors have been prepared. Development of technologies based on open pore urea-formaldehyde structures is a distinct possibility. 

Highly dispersed crosslinked melamine-formaldehyde structures have been prepared by Reimer [37]. In a typical open pore melamine-formaldehyde preparation, a melamine resin that has a low memelamine fonnaldehyde molar ratio (1 1.6) and that contains 15% sucrose as an additive was converted into an open pore melamine—formaldehyde oligomer by acidification with 7% phosphoric acid (based on resin solid) at 50-60°C. 

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl - phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byproduct. 

LW Hill, S Lee. Effect of melamine-formaldehyde structure on cure response of thermoset coatings. J Coahngs Technology 71(897) 127-133, 1999. 

Hippel, P.H. von, Wang, K.-Y. Dynamic aspects of native DNA structure kinetics of the formaldehyde reaction with calf thymus DNA. J. Mol. Biol. 61 (1971) 587-613... 

In the mid 1970s, Ugi and co-workers developed a scheme based on treating reactions by means of matrices - reaction (R-) matrices [16, 17]. The representation of chemical structures by bond and electron (BE-) matrices was presented in Section 2.4. BE-matrices can be constructed not only for single molecules but also for ensembles of them, such as the starting materials of a reaction, e.g., formaldehyde (methanal) and hydrocyanic add as shown with the B E-matrix, B, in Figure 3-12. Figure 3-12 also shows the BE-matrix, E, of the reaction product, the cyanohydrin of formaldehyde. 

Another method for the hydroxylation of the etliylenic linkage consists in treatment of the alkene with osmium tetroxide in an inert solvent (ether or dioxan) at room temperature for several days an osmic ester is formed which either precipitates from the reaction mixture or may be isolated by evaporation of the solvent. Hydrolysis of the osmic ester in a reducing medium (in the presence of alkaline formaldehyde or of aqueous-alcoholic sodium sulphite) gives the 1 2-glycol and osmium. The glycol has the cis structure it is probably derived from the cyclic osmic ester ... 

Alkynyl)oxiranes also react with carbon nucleophiles to afford furan derivatives. Furanes of different types are obtained depending on the structure of the substrates. 7-Methyl-2-ethynyloxirane (95) reacts with acetoacetate to give the furan 97 by the elimination of formaldehyde from the cyclized product 96. The hydroxy ester of the alkylidenefuran 98 and the corresponding lactone 99 are obtained by the reaction of i-methyl-2-(2-propynyI)oxirane[40, 42]. 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

In, for example, the planar asymmetric rotor molecule formaldehyde, IT2CO, shown in Figure 5.1(f), it is possible by obtaining, say, and B in the zero-point level and in the V = 1 level of all six vibrations to determine and B. Two rotational constants are insufficient, however, to give the three structural parameters rg(CFI), rg(CO) and (ZFICFI)e necessary for a complete equilibrium structure. It is at this stage that the importance of... 

Two substituents on two N atoms increase the number of diaziridine structures as compared with oxaziridines, while some limitations as to the nature of substituents on N and C decrease it. Favored starting materials are formaldehyde, aliphatic aldehydes and ketones, together with ammonia and simple aliphatic amines. Aromatic amines do not react. Suitable aminating agents are chloramine, N-chloroalkylamines, hydroxylamine-O-sulfonic acid and their simple alkyl derivatives, but also oxaziridines unsubstituted at nitrogen. Combination of a carbonyl compound, an amine and an aminating agent leads to the standard procedures of diaziridine synthesis. 

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

Although phenolic resins have been known and widely utilised for over 60 years their detailed chemical structure remains to be established. It is now known that the resins are very complex and that the various structures present will depend on the ratio of phenol to formaldehyde employed, the pH of the reaction mixture and the temperature of the reaction. Phenolic resin chemistry has been discussed in detail elsewhere and will be discussed only briefly here. 

These materials will then slowly react with further formaldehyde to form their own methylol derivatives which in turn rapidly react with further phenol to produce higher polynuclear phenols. Because of the excess of phenol there is a limit to the molecultir weight of the product produced, but on average there are 5-6 benzene rings per molecule. A typical example of the many possible structures is shown in Figure 23.11. 

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much... 

In general, the reaction between a phenol and an aldehyde is classified as an electrophilic aromatic substitution, though some researchers have classed it as a nucleophilic substitution (Sn2) on aldehyde [84]. These mechanisms are probably indistinguishable on the basis of kinetics, though the charge-dispersed sp carbon structure of phenate does not fit our normal concept of a good nucleophile. In phenol-formaldehyde resins, the observed hydroxymethylation kinetics are second-order, first-order in phenol and first-order in formaldehyde. 

Polymerization and curing rates of novolacs depend strongly on the acidity of the reaction mixture. Fig. 16 depicts the general pH dependence. Fig. 17 shows a partial structure for a hexa-cured novolac. Incorporation of amine is widely, though not universally, reported in hexa-cured novolac structures. In addition to the structure shown in Fig. 17, A, A -dibenzyl and A, A, A -tribenzylamine linkages have been reported [185-192]. The main by-products of hexa-curing conditions are water and ammonia, though formaldehyde is also produced. The structure and abundance of the amino portions of the cured polymer vary considerably with conditions. 

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