Urea-formaldehyde resins adhesive strength

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). 

The work emphasized shredded carpet selvage to which various amounts of cut waste fibers such as nylon 6, nylon 6,6, polyester, and cotton were added. Fabric bits of waste denims and cotton-polyester fabrics were also used. The waste carpet blend was then coated with phenolic or urea formaldehyde resins that were dispersed in a water base. The composites contained various amounts of different fibers or fabrics and 7.5-20 wt% adhesive solids with respect to the fiber content. The fibers were spray coated and molded in a heated press at 150-200°C and 3.4 MPa. Test results show that one can achieve high flexural moduli of 2.4-2.S GPa with face yam, that is, fibers that bind to the matrix snch as nylon, polyester, and cotton. These values together with flexural strengths of 34-48 MPa make the products suitable for many outdoor and transportation applications. 

Urea- and melamine-formaldehyde resins are used as moldings, lacquers, and adhesives (for wood), also as textile additives (increased crease resistance) and paper additives (improved wet strength). 

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... 

The addition of the nitrogen-containing crosslinking agents, melamine, urea, and urea-formaldehyde (UF) resin, resulted in a substantial increase in strength (Table III, entries 7-12). The best results were obtained by the melamine-crosslinked adhesives. The addition of 33% melamine resulted in an adhesive with strength values well within exterior-grade adhesive specifications. 

This chapter reports work on two aspects of this adhesive system 1) tests on the strength of panels bonded with phenol/carbohydrate/urea/formaldehyde (P/C/U/F) adhesive compositions outside the ranges previously reported (9,10) and 2) analysis of chemical reactions in this resin system. 

Furane adhesives are suitable for gap-filhng applications because their strength is maintained with thick glue lines. For this reason, the resins are used as modifiers for urea-formaldehyde adhesive to improve gap-filling and craze resistance. As furanes are compatible with a variety of other resins, they are used in mixtures with silicates and carbonaceous materials for chemically resistant grouting compositions. ... 

Melamine formaldehyde and urea formaldehydes. Melamine formaldehyde resins are colorless adhesives for wood. Because of high cost, they are sometimes blended with urea formaldehyde. Melamine formaldehyde is usually supplied in powder form and reconstituted with water a hardener is added at the time of use. Temperatures of about 200°F are necessary for cure. Adhesive strength is greater than the strength of the wood substrate. 

Urea-formaldehyde systems are used in products that require more demanding properties. These systems are used on both paper and cloth products. Urea bonds impart better heat resistance and more strength than either glue or varnish adhesives. The urea resins are usually obtained in a liquid form, and are catalyzed by additions of ammonium chloride or other acid catalyst. The fillers must not interfere with the acid catalyst. Typically, calcium carbonate is avoided, but other fillers such as calcium sulfate are acceptable. 

---