Curing urea-formaldehyde adhesives

Cured urea-formaldehyde adhesive is characterized by the presence of methylene bridges between strongly hydrogen-bonded urea linkages. Consequently, cured UF adhesives are inherently stiff and brittle. Incorporation of DDDU with its 12 methylene groups into the resin structure results in cured UF adhesive with a more flexible network. The increased flexibility decreases internal stress and the associated flaws, and hence the fracture energy increases. 

The cutting unit cuts the paper on hne. The speed of such a laminating line may reach 40 m/min by using fast curing urea-formaldehyde adhesives (refer to the Chapter 5, Woodworking and Furniture Adhesives , of Volume 3 of this Handbook). 

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. 

Lumber banding consists of gluing lumber strips, 1/2 to 2 inches in width, on the particleboard edges. These strips are normally used in applications where the particleboard is to be covered with wood veneers. The solid wood strip can be machined to decorative edges and, with the veneer surfaces, the panel is fully as functional and attractive as a solid wood panel, but at a lower cost. The lumber bands are normally bonded to the particleboard with polyvinyl acetate or urea-formaldehyde adhesives, cured rapidly by either contact or high frequency heating. 

A novel concept of the structure of cured urea-formaldehyde resin. J. of Adhesion. Vol 17(x), page xxx. 

Figure 5 Smooth (glassy) fracture surface of a brittle urea-formaldehyde adhesive layer fractured by stress developed in the adhesive layer as it cured. Note the tensile rupture of the cells at the wood surface (arrow) caused by the cure-shrinkage crack in the adhesive. 

Figure 6 Fracture surfaces of an amine-modified urea-formaldehyde adhesive showing three distinct types of fracture surface (A) cure-shrinkage crack surface (B) vacuum-pressure soak-dry crack surface (C) crack surface created during loading to failure.

Figure 8 Fracture surface pattern produced by cleavage of a bondline weakened by precracking (arrow) of the adhesive layer as it shrinks during cure (unmodified urea-formaldehyde adhesive). 

Figure 9 Overview of fracture surface of phenol-formaldehyde bonded joint showing exposed adhesive layer (A) without preexisting cure-shrinkage cracks and surface of lower adherend (B). The sloped test fracture surfaces (arrow) characteristic of phenol-formaldehyde and toughened urea-formaldehyde adhesive layers show where the crack jumped from one interphase to the opposite as the crack traveled in the fiber direction.

Comparatively little attention was given to the use of tannin-bonded particleboards for interior use until restrictions on formaldehyde emissions from urea-formaldehyde bonded products became a critical concern (178). Wattle tan-nin/urea-formaldehyde adhesive formulations for interior particleboards do reduce the large, short-term formaldehyde release normally observed soon after pressing, but not the slow emission that is common after extended storage (33). The scavenging effects of wattle tannins are limited to comparatively small amounts of formaldehyde because of the highly condensed nature of the tan-nin/urea-formaldehyde polymer after curing, as was also observed by Marutzky and Dix (136). 

The adhesive is applied to both sides of the alternate ply veneers. They are passed through a glue spreader and then stacked between the uncoated veneers to form the plywood assembly. A product made with urea-formaldehyde adhesive might be pressed at 125 °C and a pressure of 150-300 pounds per square inch for 5-10 minutes to achieve a satisfactory cure. 

Urea-formaldehyde adhesives came under scmtiny in the 1990s due to suspected emission of formaldehyde when the bonded wood was exposed to high-humidity conditions. It was found that the emission of formaldehyde could be reduced substantially if the molar ratio was reduced from the standard 1.55 to -1.85 (formaldehyde to urea) to levels as low as 1.02 to -1.1. This change could be accomplished by careful addition of urea to the cooking process. Another way in which the emission of formaldehyde could be lowered was by the addition of melamine to the resin cook. Addition of melamine to the resin causes an improvement in the hydrolytic resistance of the cured resin. ... 

Both melamine—formaldehyde (MF) and resorcinol—formaldehyde (RF) foUowed the eadier developments of phenol—, and urea—formaldehyde. Melamine has a more complex stmcture than urea and is also more expensive. Melamine-base resins requite heat to cure, produce colorless gluelines, and are much more water-resistant than urea resins but stiU are not quite waterproof. Because of melamine s similarity to urea, it is often used in fairly small amounts with urea to produce melamine—urea—formaldehyde (MUF) resins. Thus, the improved characteristics of melamine can be combined with the economy of urea to provide an improved adhesive at a moderate increase in cost. The improvement is roughly proportional to the amount of melamine used the range of addition may be from 5 to 35%, with 5—10% most common. 

The primary adhesive used ia hardwood plywood is urea—formaldehyde (UF) mixed with wheat flour as an extender to improve spreadabiUty, reduce penetration, and provide dry-out resistance. A catalyst may also be added to UF resias to speed the cure or to cause the UF to cure. Scavengers also may be added to reduce formaldehyde emissions from finished panels. If more water-resistance is requited using a UF bond, small amounts of melamine maybe added, producing a melamine—urea—formaldehyde (MUF) adhesive. 

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. 

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. 

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. 

Phenol-formaldehyde adhesives are the only other adhesive system used in significant quantity in particleboard production. The increased durability of this class over that of the ureas results in phenolics as the adhesive of choice for exterior particleboard. However, phenolic adhesives are only used where the additional durability is required since they are more expensive and require longer curing times. 

Particleboards are composed of discrete particles of wood bonded together by a synthetic resin adhesive, most commonly urea-formaldehyde or phenol-formaldehyde. The material is consolidated and the resin cured under heat and pressure. The strength of the product depends mainly upon the adhesive and not upon fiber... 

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