Crystalline Formaldehyde resin

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. 

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. 

A large number of commercially important condensation polymers are employed as homopolymers. These include those polymers that depend on crystallinity for their major applications, such as rylons and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and urea-formaldehyde resins. In many applications, condensation polymers are used as copolymers. For example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been incorporated into the urea-formaldehyde resin structure to enhance its stability. Copolyesters find application in a fairly broad spectrum of end uses. 

In woody materials, the long crystalline fibrils of cellulose are bound into a composite structure by lignin, a macromolecule based on polyphenols. Lignin, which is present to the extent of 25-30% in most woods, is a cross-linked polymer rather similar to man-made phenol-formaldehyde resins and may be looked upon as a glue which gives wood its permanent form (Figure 1.1). 

The story starts some fourteen years after Baeyer had initiated the first study of phenol-formaldehyde condensation chemistry (which led to the study of phenol-formaldehyde resins and subsequently to calixarene chemistry)" - with the publication of his paper on the condensation of pyrrole and acetone. Baeyer mixed pyrrole, acetone and hydrochloric acid and obtained a white crystalline material, which later proved to be the tetra-pyrrolic macrocycle, me.so-octamethylcalixpyrrole 1. 

Dimethylol urea n. A colorless crystalline material resulting from the combination of urea and formaldehyde in the presence of salts or alkaline catalysts, the first or A-stage of urea-formaldehyde resin. 

Urea yii- re-o [NL, fr. F uree, fir. urine] (1806) n. NH2CONH2. Mp, 132°C Sp gr, 1.323. n. A white, crystalline powder derived from the decomposition of ammonium carbonate. It is used in the production of urea-formaldehyde resins. Syn carbamide. 

Most of these polymers are regarded as highly stable in water because they are insoluble, often semi-crystalline and absorb only very low levels of water. However, most of them will undergo surface attack in acids or alkalis and are generally unsatisfactory for long-term use in these media. The same is true of most cross-linked materials epoxy resins, formaldehyde resins and unsaturated polyesters all suffer hydrolytic scission over long periods, especially at extremes of pH. 

The main raw materials used for the synthesis of ureo-formaldehyde resins are urea and formaldehyde. Urea is a solid, crystalline, water-soluble substance. It has a weak basic character. Formaldehyde is used in solution form at 30-55% concentration. It is of advantage to use either high-concentration formaldehyde solutions or precondensates. Both processes provide a satisfactory economic efficiency and diminish residual water content. Precondensates can be obtained through the reaction between urea and formaldehyde (molar ratio 1 1.1-6), at T = 45 C and pH = 6.8-8.0 [19]. 

Urea was first discovered in urine in 1773 by the French chemist Hilaire M. Rouelle [12]. Pure urea (CO(NH2)2) is a white, at room temperature, crystalline compound, which represents a bulk chemical with an annual production of about 129 million ton. Urea plays a significant role in soil and leaf fertilization (more than 90% of total use) [13]. Because of its high nitrogen content (>46 wt%) and its nonhaz-ardous character, it is today the dominant nitrogen source in agriculture. Further applications of urea are the production of urea/formaldehyde resins and melamine as well as its usage as reactant for the reduction [13]. 

Specific Heat Capacity. Representative values of specific heat capacity are shown in Tables 3 and 6. The range of values is only about 850 to 2400 J/(kgK) or barely a factor of three. As a general rule, differences are usually associated with the molecular composition of the polymer and less with molecular architecture, although crystallinity may be important. For example, a comparison of three forms of polyethylene (Table 6) reveals little difference in heat capacity the high density, and hence more crystalline, form has a somewhat lower value. Similarly, no differences are observed between two grades of phenol-formaldehyde resin, or between them and phenol-furfural resin. However, in comparing isotactic and atactic (amorphous) polypropylene shown in Table 3 with values of 1790 and 2350 J/(kg K), respectively, a fairly substantial difference is observed the more ordered, denser isotactic form has the lower heat capacity, as is to be expected. However, comparable values of isotactic and atactic polystyrene have been reported to be 1264 and 1227 J/(kg-K), respectively (65) here the difference is small. 

Trioxane and Tetraoxane. The cycHc symmetrical trimer of formaldehyde, trioxane [110-88-3] is prepared by acid-catalyzed Hquid- or vapor-phase processes (147—151). It is a colorless crystalline soHd that bods at 114.5°C and melts at 61—62°C (17,152). The heats of formation are — 176.9 kJ/mol (—42.28 kcal/mol) from monomeric formaldehyde and —88.7 kJ/mol (—21.19 kcal/mol) from 60% aqueous formaldehyde. It can be produced by continuous distillation of 60% aqueous formaldehyde containing 2—5% sulfuric acid. Trioxane is extracted from the distillate with benzene or methylene chloride and recovered by distillation (153) or crystallization (154). It is mainly used for the production of acetal resins (qv). 

Urea is a white crystalline compound with a melting point of 132.6°C and is highly soluble in water. It is substantially cheaper than the other intermediate (formaldehyde) used in the resin preparations. 

Polymerized formaldehyde (trioxane) is a ring compound of anhydrous formaldehyde with the formula (HCHO)j. See also Acetal Resins. Trioxane is a colorless crystalline solid with a pleasant odor, mp 62°C, bp II5°C. sp gr 1.17. This compound is used as a tanning agent and solvent and as a source of dry HCHO gas. Because trioxane ignites readily at I I3°C and bums with an odorless, hot flame, it has been furnished in tablet form as a replacement for solidified alcohol in portable heating applications. 

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