Urea-formaldehyde resin discussion

Urea-formaldehyde resin, like phenol-, or furfuryl alcohol-formaldehyde resins, is typically thought of as resulting from simple condensation chemistry. The ultimate hardening of the resin is thought to be the result of the formation of a cross-linked network brought about by acid catalysis. Current reviews are available (1, 2) which discuss this traditional preception of UF resin chemistry. 

Fillers in Thermosets. Sixty-five years ago, in a paper presented before the American Chemical Society, L. H. Baekeland (55) discussed the usefulness of phenol-formaldehyde resins that, when compounded with wood flour, could be molded. Wood flour, ground nut shells, a-cellulose, or paper add bulk to phenolics, melamine, or urea-formaldehyde resins and make them easier to fabricate and less expensive. 

Discuss the chemistry of urea-formaldehyde resins, their preparation and uses. 

Urea-formaldehyde resin and melamine-formalde-hyde resin are used as glues in the wood industries to make furniture press plates. Despite a low constant release of formaldehyde from these plates into the indoor air, the health effect for individuals living or working in the room is way overestimated in our opinion. Construction workers are also exposed to formaldehyde resins in modern building materials. Textile finishes are another use for these formaldehyde resins, but this does not fit into our discussion in this chapter (Fowler et al. 1992). Even cosmetics may contain PTBP-FR as Angelini and others have shown previously (Angelini et al. 1993). Both resins are currently available from Chemotechnique, Sweden, urea-FR as a 10% petrolatum and melamine-FR as a 7% preparation in the textile colour and finishes series. 

Urea. Urea (carbamide) CH4N2O, is the most important building block for amino resins because urea—formaldehyde is the largest selling amino resin, and urea is the raw material for melamine, the amino compound used in the next largest selling type of amino resin. Urea is also used to make a variety of other amino compounds, such as ethyleneurea, and other cyclic derivatives used for amino resins for treating textiles. They are discussed later. 

Urea is sufficiently important as an additive to PF resins for OSB to warrant some discussion. It has had a large favorable economie impact on the OSB industry. When used, it is generally added after the polymerization is complete. Thus, it is not part of the polymer and does not have any direet effect on polymer resistance to hydrolysis, as might be expected if it was part of the polymer backbone. Under alkaline pH conditions, urea-formaldehyde adducts do not polymerize at a rate that is significant compared to the PF polymerization therefore, the urea does not participate signifieantly in the euring proeess of the PF, despite the faet that it is present during the cure. Since urea is not present in the cured PF polymer per se, it does not detract from the durability of the polymer. Despite this, it is possible to see redueed OSB durability as a result of formulated urea if its use has led to actual PF polymer application rates that are too low. 

In the following discussion, only the most widely used adhesive types are described. These are the urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins, phenol-formaldehyde (PF) resins, diisocyanates, polyisocyanates, polymers and copolymers of vinyl acetate, and polyamides. These are all predominantly thermosetting resin systems. 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

In Chapter 2 we indicated that the formation of a polymer requires that the functionality of the reacting monomer(s) must be at least 2. Where the functionality of one of the monomers is greater than 2, then a cross-linked polymer is formed. Thermosets like phenol-formaldehyde, urea-formaldehyde, and epoxy resins develop their characteristic properties through cross-linking. In this section our discussion is confined to those polymeric systems designed with latent cross-linkability that under appropriate conditions can be activated to produce a polymer with desirable properties. 

Figure Q13.12 shows the mechanical properties of two urea-formaldehyde (UF) resins cured with NH4CI (bottom) variation of the shear strength of bonded wood joints with cyclic wet-dry treatments of joints (middle) development of internal stress with duration of resin cure at room temperature (top) dynamic mechanical properties of resins. Discuss the interrelationships between the observed mechanical properties. 

Table 5.5 shows that some polymers have a particularly high dielectric constant that is, these polymers have greater ability to store power in a given volume of polymer. These are polyesters, alkyd resins, phenol-formaldehyde, nylon 6,6, nylon 6,10, urea formaldehyde, and plastized polyvinyl chloride (PVC), all of which have dielectric constant values of >5 at 1 kHz. Tsuchiya and coworkers [14] discussed... 

In 1927, Schiebler et ai, [14] found that in acid solution the methylolureas are converted to insoluble substances similar to Goldschmidt s compound. Today the polymerization mechanisms involved are similar to those discussed for other methylol compounds such as phenolic resins (Chapter 2) or melamine resins (see Section 3 of this chapter). It is interesting to note that because urea has four active hydrogens and three sites for polymerization, linear, branched, and cyclic structures are possible. In fact, Kadowaki [15] has isolated several low-molecular-weight condensation products of urea-formaldehyde and has described their properties. The cyclic structures commonly called urones, such as dimethylolurone (iV,N -dimethyloltetra-hydro-4//-l,3,5-oxidiazin-4-one), have also been prepared by Kadowski. 

Urea-formaldehyde (UF) resins are mainly used as adhesives for wood. Laminated sheets (tables and counter tops) are a major application for melamine resins, which stay in the outer decorative surface. Molding compounds, their first big application, is still a major market, taking advantage of their extreme hardness and heat resistance. Coatings, textile finishing, paper additives, leather tanning and foundry binders, for which methanol- or butanol-etherified resins are usually employed, are important markets discussed in Ref 203. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

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