Urea-formaldehyde resin formation

Formaldehyde Scavenging. The formation of oxazoHdines from alkanolamines and formaldehyde is rapid at room temperature and provides a method for the elimination of excess formaldehyde from products such as urea—formaldehyde resins. AEPD and TRIS AMINO are the products of choice for this purpose because one mole of each will react with two moles of formaldehyde (22). 

Urea-formaldehyde resins are usually prepared by a two-stage reaction. The first stage involves the reaction of urea and formaldehyde under neutral or mildly alkaline conditions, leading to the production of mono and dimethylol ureas (Figure 24.1). The ratio of mono to dimethylol compounds will depend on the urea-to-formaldehyde ratio and it is important that there should be enough formaldehyde to allow some dimethylol urea formation. 

Thiourea will react with neutralised formalin at 20-30°C to form methylol derivatives which are slowly deposited from solution. Heating of methylol thiourea aqueous solutions at about 60°C will cause the formation of resins, the reaction being accelerated by acidic conditions. As the resin average molecular weight increases with further reaction the resin becomes hydrophobic and separates from the aqueous phase on cooling. Further reaction leads to separation at reaction temperatures, in contrast to urea-formaldehyde resins, which can form homogeneous transparent gels in aqueous dispersion. 

The synthesis of urea-formaldehyde resin takes place in two stages. In the first stage, urea is hydroxymethylolated by the addition of formaldehyde to the amino groups of urea (Figure 19.1). This reaction is in reality a series of reactions that lead to the formation of mono-, di-, and trimethy-lolureas. Tetramethylolurea does not appear to be produced, at least not in a detectable quantity. The addition of formaldehyde to urea takes place over the entire pH range, but the reaction rate is dependent on the pH. 

The difference between the pH profiles of the two stages of urea-formaldehyde resin synthesis is taken advantage of in the production of these resins (Figure 19.2). In general, the commercial production of urea-formaldehyde adhesive resins is carried out in two major steps. The first step consists of the formation of methylolureas under basic conditions (pH 8 to 9), to allow the methylo-lation reactions to proceed in the absence of reactions involving the condensation of the methylolureas. 

Emulsion blocks within the formation can form as a result of various well treatments and are more easily prevented (by using surfactants in conjunction with well treatments, see above) than removed. Aromatic solvents can be used to reduce the viscosity and mobilize oil-external emulsions (167). Low molecular weight urea-formaldehyde resins have been claimed to function in a similar manner in steam and water injection wells (168,169). Water-external emulsion blocks can be mobilized by injection of water to reduce emulsion viscosity. 

Figure 7.24 Formation of urea-formaldehyde resins from 1,3-dihydroxymethylurea.

Fig. 10.3 The formation and structure of a urea-formaldehyde resin (UF), a thermosetting resin.

Fig. 7.26. Mechanism for the formation of a urea/formaldehyde resin from methylol urea (R1 = H in formula A possible preparation Figure 7.25) or dimethylol urea (R1 = HO CH2 in formula A possible preparation Figure 7.25). The substituents R1, R2, and R3 represent the growing —CH2—NH—...

The physicochemical features of the processes of formation, stabilisation and solidification of foams are best studied for a polymer foam from urea-formaldehyde resins. That is why the urea polymer foams are used here below to exemplify the principles of optimisation of the technology for production of polymer foam materials. 

While catalysts are also used in the production of other types of polymers, the properties of most of these materials are not particularly dependent on the type of catalyst employed. Many poly condensation reactions, e. g. the formation of polyesters, polyamides or urea-formaldehyde resins, are speeded up by addition of some Bronsted or Lewis acids. Since relevant properties of these polymer products, such as their average chain lengths, are controlled by equilibrium parameters, primarily by the reaction temperatures and molar ratios of the monomers employed, and since their linkage patterns are dictated by the functional groups involved, addition of a catalyst has little leverage on the properties of the resulting polymer materials. 

Urea-formaldehyde resin solutions are shown to be dominated by physical associations rather than primary chemical bonding. These physical associations, or colloidal dispersions, are directly related to the thermodynamic balance of secondary bond formation between resin and solvent systems. Steric and entripic evaluations of molecule configuration have shown that linear urea-formaldehyde oligomers resemble polypeptides, and have the potential to form both 3-sheets and n-helixs, While the exact configuration of the associations is not known, their presence has been confirmed by x-ray analysis, which shows that urea-formaldehyde resins are crystalline in solid form. 

Urea-formaldehyde resin, like phenol-, or furfuryl alcohol-formaldehyde resins, is typically thought of as resulting from simple condensation chemistry. The ultimate hardening of the resin is thought to be the result of the formation of a cross-linked network brought about by acid catalysis. Current reviews are available (1, 2) which discuss this traditional preception of UF resin chemistry. 

The total world production of urea is about 100 million tons per year. By far the largest part of it is used as a nitrogen fertilizer both in solid form and in solution this consumes approximately 87% of all urea production. It is also a livestock feed additive (5%) and a raw material for urea-formaldehyde resins (6%) and melamine (1%). Other applications (1%) include its use as deicing agent, raw material for fine chemicals (cyanuric acid, sulfamic acid), formation of crystalline clathrates, and so on. 

Acid Catalyzed Condensation Polymerizations. The strong protonic acids produced by the photolysis of onium salts I-III can also be employed to catalyze the condensation of phenolic, melamine, and urea formaldehyde resins. Very durable photoresists based on these inexpensive and readily available resins can be made. Such resists generally require a postbake prior to development to complete the condensation and to enhance image formation. 

Chemical reactions in sources are well known from hydrolysis of urea formaldehyde resins in chipboard resulting in the emission of formaldehyde and from the hardening processes of many sealants, glues, and varnishes (Roffael, 1993). The emission of volatile compounds from these products may in some cases be limited by formation. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

FIGURE 4.26 Reactions in the formation of urea-formaldehyde resins. 

In the past, it was believed by some that further condensations that take place at pH below 7 include formations of cyclic intermediates. This, however, was never demonstrated. NMR spectra of urea-formaldehyde resins show that condensations under acidic conditions proceed via... 

The ethers cleave upon acidification and network structures form. For metholylated melamines that are not etherified, acidification is not necessary and heating alone is often adequate for network formation. Melamine-formaldehyde resins have the reputation of being harder and more moisture resistant than the urea-formaldehyde resins. 

Formic acid propyl ester. See Propyl formate Formic acid sodium salt. See Sodium formate Formic aldehyde. See Formaldehyde Formic ether. See Ethyl formate Formimidic acid, 1-carbamoyl-. SeeOxamide Formol 55. See Urea-formaldehyde resin Formol. See Formaldehyde Formonitrile. See Hydrogen cyanide Formosa camphor. See Camphor Formosa camphor oil Formose oil of campor. See Camphor (Cinnamomum camphora) oil Formosol. See Sodium formaldehyde sulfoxylate... 

Resin chemists are familiar with the reaction sequences common to commercial condensation resins such as PF or urea formaldehyde resins. These sequences involve prolonged reaction times with little or no change in the viscosity of the resins, followed by a period when the viscosity increases rapidly. If unchecked by rapid cooling, changes in pH or ionic content, or die addition of solvent, this reaction will lead to the formation of a solid resin in the reaction kettle in a matter of minutes. 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

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