Urea formaldehyde resin adhesives

Urea is largely used as a fertilizer (ISy ), and as a non-protein feed supplement for sheep and cattle. The most important chemical use, which however accounts for only a small part of urea production, is in the manufacture of urea-formaldehyde resins. U is also used in the manufacture of adhesives, pharmaceuticals, dyes and various other materials. U.S. production 1981 7 0 megatonnes urea resins 1983 6 megatonnes. 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

Urea-formaldehyde resins can be cured with isopropylbenzene production wastes containing 200 to 300 g/liter of AICI3 as an acid hardener [189]. Isopropylbenzene is formed as an intermediate in the Hock process by a Friedel-Crafts reaction from propene and benzene. The mixture hardens in 45 to 90 minutes and develops an adhesion to rock and metal of 0.19 to 0.28 MPa for 0.2% AICI3 and 0.01 to 0.07 MPa for 0.4% AICI3, respectively. A particular advantage is the increased pot life of the formulation. 

Dead burned magnesium oxide is suitable as an expanding additive [368-370]. The expansion occurs by a hydration mechanism. The additive is particularly effective when used at setting temperatures greater than approximately 150° C. Enhanced adhesion of expanded cements can be achieved by the addition of urea-formaldehyde resins [1720]. 

White pigments such as calcium carbonate, aluminum hydroxide, silica, kaolin, or urea-formaldehyde resin are used as filler. The filler functions as an absorbent of melted components to prevent their adhesion on the thermal head. Thus, the filler is required to be high in oil absorption and not to wear the thermal head. 

Approximately one million metric tons of urea-formaldehyde resin are produced annually all over the world. More than 70% of this urea-formaldehyde resin is consumed by the forest products industry. The resin is used in the production of an adhesive for bonding particleboard (61% of the urea-formaldehyde used in the industry), medium-density fiberboard (27%), hardwood plywood (5%), and as a laminating adhesive (7%) for bonding furniture case goods, overlays to panels, and interior flush doors, for example. 

Urea-formaldehyde resins are used as the main adhesive in the forest product industry because they have a number of advantages, including low cost, ease of use under a wide variety of curing conditions, low cure temperatures, water solubility, resistance to microorganisms and to abrasion, hardness, excellent thermal properties, and a lack of color, especially in the cured resin. 

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. 

An acidic-cure catalyst is added to the urea-formaldehyde resin before it is used as an adhesive. Ammonium chloride and ammonium sulfate are the most widely used catalysts for resins in the forest products industry. A variety of other chemicals can be used as a catalyst, including formic acid, boric acid, phosphoric acid, oxalic acid, and acid salts of hexamethylenetetramine. 

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. 

Other applications for phenolics are switchgears, handles, and appliance parts, such as washing machine agitators (that s why they re usually black). Phenolics are widely used to bond plywood, particularly exterior and marine grades. Although urea-formaldehyde resins are cheaper for this purpose, they were not nearly as water-resistant and have been limited to interior grades. Abrasive wheels and brake linings also are bonded with phenolic adhesives. 

Another use of urea is for resins, which are used in numerous applications including plastics, adhesives, moldings, laminates, plywood, particleboard, textiles, and coatings. Resins are organic liquid substances exuded from plants that harden on exposure to air. The term now includes numerous synthetically produced resins. Urea resins are thermosetting, which means they harden when heated, often with the aid of a catalyst. The polymerization of urea and formaldehyde produces urea-formaldehyde resins, which is the second most abundant use of urea. Urea is dehydrated to melamine, which, when combined with formaldehyde, produces melamine-formaldehyde resins (Figure 96.2). Melamine resins tend to be harder and more heat-resistant than urea-formaldehyde resins. Melamine received widespread attention as the primary pet food and animal feed contaminant causing numerous cat and dog deaths in early... 

Today s Post-Its are made with a very sophisticated technology. We can actually reuse them, because their adhesive is contained within thousands of little bubbles of urea formaldehyde resin. These bubbles break under pressure, but they don t all break at the same time. So, how many times can a Post-It be reused I don t know. I should do an experiment. I ll leave myself a note to do just that. On a Post-It, of course. 

Adhesive. Urea-formaldehyde water-based dispersions are the most widely used particleboard binders. The low-cost, rapid curing, and colorless properties of urea-formaldehyde adhesives make them the adhesive of choice for most interior particleboard. These adhesives have been continuously improved by the resin manufacturers, resulting in reduced press times without detrimental effects on their storage life or handling characteristics. 

In the April, 1975, issue of Plywood Panel magazine, we find reported square feet, quarter inch basis, domestic hardwood plywood manufactured in this country was approximately 3.5 billion and softwood plywood was 27.5 billion. This means 89 percent of the total plywood produced was softwood plywood, and 97 percent of this production was glued with phenol-formaldehyde resin adhesives, according to the American Plywood Association. This leaves 11 percent domestic hardwood plywood which was 95 percent glued with urea-formaldehyde resin adhesives,... 

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. 

Urea is used in Liquid and solid fertilizers, urea-formaldehyde resins that make adhesives and binders mostly for wood products, livestock feeds, melamine for resins, NOx control from boilers and furnaces, and numerous chemical applications113. Urea uses in the United States are listed in Table 11.6230. 

Most furfural is produced from corncobs and oat and rice hulls, primarily by the Quaker Oats Company. The product is used in the chemical industry as a solvent and in wood rosin refining. A large amount of furfural is treated further to give furfuryl alcohol. The furfuryl alcohol is added to urea-formaldehyde resins in applications for adhesives and foundry core binders. 

The amounts of lignins, tannins, and carbohydrates available as residues from processing of forest trees dwarf the commodity adhesive market. At the same time, the forest products industry is especially reliant on adhesives, since over 70% of all wood products are bonded, and their production consumes about 45% of all phenolic and 85% of all urea-formaldehyde resins produced in the United States. 

Application Urea-formaldehyde resins are used as adhesives in the wood-working industry and are typically used in the production of plywood and particle board. They are available as concentrated solutions or in powder form as a result of the spray-drying process. 

Although WLs due to biological attack decreased in treated MDF blocks, the results were unsatisfactory when compared with those of solid wood (Tables 5 and 6). Excessive reaction time (24 h) caused a rather detrimental effect on dimensional stability and on biological resistance of treated boards. This can possibly be explained by a thermal degradation of urea-formaldehyde resin as an adhesive. 

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). 

---