Urea-formaldehyde resins, surface

Urea—formaldehyde resins are also used as mol ding compounds and as wet strength additives for paper products. Melamine—formaldehyde resins find use in decorative laminates, thermoset surface coatings, and mol ding compounds such as dinnerware. 

Uses The urea formaldehyde resins are used for domestic electrical fittings, bottle caps. These also find use for wood adhesives, surface coatings and textile finishings. 

For the preparation of the foam, a solution of 1 g technical sodium diisobutyl naphthalene sulfonate in 50 ml of 3% orthophosphoric acid is prepared. 20 ml of this solution are poured into a 11 beaker and air is stirred in with a fast running mixer until the cream-like dispersion has reached a volume of 300-400 ml.Then,20 ml of the prepared urea/formaldehyde resin are mixed in, whereby the resin must be evenly distributed. After 3-4 min the introduced resin gellifies into a molded article permeated with many water/air pores under the influence of the acidic catalyst. After 24 h,the crosslinking is completed. Drying for 12 h at 40 °C in a circulating air dryer yields a brittle thermoset foam.The foamed plastic obtained is hydrophobic and has a large internal surface. It can take up about 30 times its own weight of petroleum ether. 

Fire-retardant paints for wood have been developed using urea-formaldehyde resins and ammonium phosphate. Heat causes this compound to intumesce and develop an insulating layer for the surface upon which it is applied. Oil-based fire-retardant paints usually contain organic materials such as resins, paraffins, or chlorinated rubbers combined with inorganic vehicles, or they may be compounded of inorganics such as ammonium compounds. 

Dost in 1971 reported on a study where redwood bark fiber was used in three-layer particleboard (44). Amount of bark in the furnish, by weight, was 0, 10, 20, and 30% hammemilled disk flakes or Pallmann flakes of redwood wood made up the remainder of the furnish. Urea formaldehyde resin was applied at three percentages. Test results showed surface smoothness and strength properties (MOR, MOE, and IB) decreased with increasing bark content in the boards. Water absorption decreased, but thickness swelling and linear expansion increased as the amount of bark increased. 

Urea is used as a solid fertilizer, a liquid fertilizer and miscellaneous applications such as animal feed, urea, formaldehyde resins, melamine, and adhesives. Presently, the most popular nitrogen fertilizer is a urea-ammonium nitrate solution. Urea-formaldehyde resins have large use as a plywood adhesive. Melamine-formaldehyde resins are used as dinnerware and for extra-hard surfaces (Formica ). The melamine is synthesized by condensation of urea molecules. 

Immerse for 5-10 min at 90°C in potassium permanganate (saturated solution), acidified with sulfuric acid (96%). Rinse in water and distilled water. Dry in warm air. 4. Prime surface with a lacquer based on urea formaldehyde resin diluted with carbon tetrachloride. Coatings (dried) offer excellent bonding surfaces without further treatment... 

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes sympy, then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, then show how further condensation leads to cross-linking. (Hint The carbonyl group lends acidity to the N—H protons of urea. A first condensation with formaldehyde leads to an inline, which is weakly electrophilic and reacts with another deprotonated urea.)... 

Results of a 2-year paint study indicate acetylated wood is a better painting surface (37) than untreated wood. UV radiation darkens unacetylated wood, but there is no change or a slight bleached effect with acetylated wood (37). In general, acetylation reduces the adhesive strength of wood (48). Adhesive strength is reduced with urea-formaldehyde resins (54, 55) and casein glues (55), but there is very little effect with resorcinol-formaldehyde resins (55). 

Molded articles from cellular polysulfone, coated with polystyrene, epoxy resin, or vinyl resin, are reported to have improved hardness and surface rigidity. They can also be used as insulating or packaging materials, or, when modified with urea-formaldehyde resin, as flowerpots that are permeable to air and water. These foams have excellent selfextinguishing and low anoke-generating properties (21). 

Formaldehyde release from pressed wood products is due to latent formaldehyde. During the pressing process, hot steam from moist wood particles transfers heat, formaldehyde, and other volatiles from the surface of the mat to the core of the board where un reacted urea-formaldehyde resin components accumulate. The resulting formaldehyde concentration in the core is approximately twice that of the surface. Release of formaldehyde is diffusion-controlled and gradually decreases over time (Meyer and Hermanns 1985). Formaldehyde can also be produced by hydrolytic cleavage of unreacted hydroxymethyl groups in the formaldehyde resins. Melamine formaldehyde resins generally are more stable, and the amounts of formaldehyde emitted from them are much lower (WHO 1989). 

Figure 2. Scanning electron photograph of urea-formaldehyde resin. This surface was exposed by simple fracture of a solid plug of solid resin.

One of the important parameters in producing urea formaldehyde resins with a low formaldehyde level, is the so called molar ratio. Table III shows that the parameter Cg is closely related to the molar ratio, which varies from 0.70 to 1.30. The mass transfer coefficient is not related to the molar ratio, while this parameter in principle is only related to the nature of the surface. 

Unmodified urea-formaldehyde resins are unsuitable for use in surface coatings formulations, due to their lack of solubility in common solvents and incompatibility with other resins. This limitation can be substantially overcome when the resins are modified by alcohols, such as n-butanol. Very commonly, butylated urea-formaldehyde resins are blended with alkyd resins to provide good flexibility and adhesion in coatings. Furfuryl alcohol-modified urea resins are used to bind foundry cores formed in preheated pattern boxes, due to their capability of rapid core production. 

Surface binding agent urea-formaldehyde resin, 2.5 mg/m. Low bleaching agent content (8HR TWA formaldehyde)... 

Urea-formaldehyde resins for surface coatings are commonly modified for solubility in oiganic solvents by reacting them with alcohols to form ether groups. Usually n-butyl alcohol is used. The reaction is carried out under basic conditions, before acidification o o... 

Optimization of the Synthesis of Urea-Formaldehyde Resins using Response Surface Methodology... 

Urea-formaldehyde resins, internal bond strength, formaldehyde emission, response surface methodology, optimization... 

The flammability of polyamide fibres is usually reduced in the finishing process. Rigid yams are dressed by applying urea, thiourea, or melamine/formaldehyde resins. On the surface of flexible polyamide fibres, about 10 per cent of ammonium bromide is fixed by a urea/formaldehyde resin. Specific flame-retardant finishing is not usual. ... 

Prime surface with lacquer based on urea-formaldehyde resin diluted with carbon tetrachloride... 

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