Formaldehyde-urea ratio

The natural abundance 15NNMR spectra of urea-formaldehyde and melamine-formaldehyde adducts and resins (recorded with approximately 20000 pulses) can be used for direct determination of different types of amino groups. The spectra of a urea-formaldehyde resin (1 1 formaldehyde/urea ratio) after 15 and 60 min of reaction are shown in Fig. 3. On the basis of chemical shifts of urea based model compounds (monomethylol-, Af,AT-dimethylol- and N,N,N -trimethylol-ureas) the following assignment of nine 15N resonances is proposed... 

The behavior of this adhesive is identical to that of adhesive 2, although larger amounts of resorcinol are used. These adhesives have a higher formaldehyde/urea ratio. 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Urea—Formaldehyde Reaction Products. Urea—formaldehyde (UF) reaction products represent one of the older controlled release nitrogen technologies. An early disclosure of the reaction products of urea [57-13-6] and formaldehyde [50-00-0] was made in 1936 (1) (Amino resins and plastics). In 1948, the USDA reported that urea (qv) and formaldehyde (qv) could react to produce a controlled release fertilizer at urea to formaldehyde mole ratios (UF ratio) greater than one (2). 

The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0 1 to 2.4 1. Condensation of the methylolureas from the methylolat ion reaction is at atmospheric reflux with a pH of 4 to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7-3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6 1 to 1.8 1. 

Phenol/carbohydrate/urea/formaldehyde molar ratios = 1 1 0.25 2. 

Figure 10. Formaldehyde loss ratios at 20 percent relative humidity for various materials. (Formaldehyde removed from a material divided by that removed from urea-formaldehyde particleboard. Board elution by nitrogen. Resin liberation by weighing bottle test. PF = phenol-formaldehyde) (ML85 5437)...

Figure 11. Formaldehyde loss ratios at 80 percent relative humidity (RH) and in water. (Loss ratio = CH2O liberated relative to that from urea-formaldehyde particleboard in same test.

Fia 3a b. 15NNMR spectra (with expansions) of a urea-formaldehyde resin made with a 1 1 fonnaldehyde/urea ratio a after 15 min reaction b after 1 h reaction. Spectra were referenced with respect to an external standard consisting of the NH ion in a solution of ammonium nitrate (m d[Pg.201]

The kinetics of the formation and condensation of mono- and dimethylolureas and of simple UF condensation products has been studied extensively. The formation of mono-methylolurea in weak acid or alkaline aqueous solutions is characterized by an initial fast phase followed by a slow bimolecular reaction [4,5]. The first reaction is reversible and is an equilibrium which proceeds to products due to the uptake of the products, the methylolureas, by the second reaction. The rate of reaction varies according to the pH with a minimum rate of reaction in the pH range 5 to 8 for a urea/formaldehyde molar ratio of 1 1 and a pH of 6.5 for a 1 2 molar ratio [6] (Fig. 1). The 1 2 urea/formaldehyde reaction has been proved to be three times slower than the 1 1 molar ratio reaction [7]. 

UF resins for particleboard with urea/formaldehyde molar ratios of 1 1.45, 1 1.32, and 1 1.25 have free formaldehyde contents of 0.8%, 0.3%, and less than 0.2%, respectively [15]. While the current tendency internationally is to use UF resins that have a urea/formaldehyde molar ratio lower than or much lower than 1 1.2, which release much less formaldehyde, these resins perform less well in the production of UF-bonded particleboard [15,17]. In particular, they do not allow as much flexibility in particleboard production as do resins with higher formaldehyde/urea molar ratios. This fact stresses the need for greater control and supervision of the production at particleboard plants where UF resins of low molar ratio are used. An example of the variation in properties between particleboard manufactured with different molar ratio resins is given in Table 1. 

Urea/ formaldehyde molar ratio Approximate density (g/cm ) Internal bond (MPa) Percent water swelling (2 h) Percent HCHO released, perforator method (mg HCHO/lOOg board)... 

The resins are either phenol-formaldehyde (PF), or urea-formaldehyde/phenol-formaldehyde copolymers (UF/PF), both being "resols", with a formaldehyde/phenol ratio higher than one. 

NOTE The urea-formaldehyde resin is one prepared from a formaldehyde urea molar ratio of 1 1 to 2 1 and is about 70% solids. However, a solid resin may be used to give similar results. 

The goal is to reduce formaldehyde emissions [222], while keeping the properties of products (such as wood panels) at an acceptable level. Since the formaldehyde/ urea molar ratio had to be decreased, the process has also become more difficult to control. 

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]. 

The main outlet for urea formaldehyde resins is in adhesives for plywood and chipboard. The ratio of urea to formaldehyde used will influence the properties of the adhesive. For example, a resin with a high formaldehyde to urea ratio will give an adhesive having the best clarity, water resistance and mechanical properties. The main disadvantage of this type of adhesive is its poor water resistance. Hence, it cannot be used for exterior applications where it is superseded by adhesives based on phenolic resins. 

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

Urea reacts with formaldehyde compounds such as monomethylolurea, NH2CONHCH2OH, dimethylolurea, HOCH2NHCONHCH2OH, and others, depending on the mol ratio of formaldehyde, to urea and upon the pH of the solution. Hydrogen peroxide and urea give a white crystalline powder, urea peroxide, CO(NH2 (2 -H2 02, known under the trade name of Hypersol, an oxidizing agent. 

Manufacturing and Processing. Urea—formaldehyde fertilizers are prepared from the reaction of formaldehyde with excess urea (U/E mole ratio 1). The reaction proceeds in two steps. In Step 1, urea and formaldehyde are combined to form monomethylol [1000-82-4] and dimethylolurea [140-95-4] intermediates. 

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