Urea-formaldehyde, structure

The only example of stepwise polymerization to appear in the literature is the electrosynthesis of an open pore urea-formaldehyde structure at the anode from an aqueous solution of a non etherified urea-formaldehyde resin. The structure has similar characteristics to those of a foam obtained by acidification. ... 

Open pore urea-formaldehyde structures have unique properties, and their spherical and pore sizes can be controlled to make them suitable for many applications. Filtration structures, chromatographic columns, porous urea-formaldehyde pigmented polystyrene, smog dispersal agents, moisture retentive fertilizers, fruit coatings, and porous polymer-bound multicomponent corrosion inhibitors have been prepared. Development of technologies based on open pore urea-formaldehyde structures is a distinct possibility. 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

The smoking salons of the Hindenburg and other hydrogen-filled dirigibles of Ihe 1930s were insulated with urea-formaldehyde polymer foams. The structure of this polymer is highly cross-linked, like that of Bakelite (Section 31.5). Propose a structure. 

While the condensation of urea and formaldehyde was described in 1884, urea-formaldehyde (UF) resins were not patented until 1918. Comparable products, based on the condensation of formaldehyde and melamine (2,4,6-triamino-1,3,5-triazine), were not patented until 1939. The term MF (structure 4.83) is used to describe these products. 

In far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

These pre-condensates are soluble in water and alcohol they are transformed by further condensation with elimination of water, first into high-molecular-weight, poorly soluble materials and finally into crosslinked insoluble products. The structure of the crosslinked (hardened) urea-formaldehyde resins is not yet entirely understood. 

First step (a) represents the initial system - solution of the poly(acrylic acid) (urea and formaldehyde are not shown). Then, growing macromolecules of urea-formaldehyde polymer recognize matrix molecules and associate with them forming polycomplex. This process leads to physical network formation and gelation of the system (step b). Further process is accompanied by polycomplex formation to the total saturation of the template molecules by the urea-formaldehyde polymer (step c). Chemical crosslinking makes the polycomplex insoluble and non-separable into the components. In the final step (c), fibrilar structure can be formed by further polycondensation of excess of urea and formaldehyde. 

Properties of composites obtained by template poly condensation of urea and formaldehyde in the presence of poly(acrylic acid) were described by Papisov et al. Products of template polycondensation obtained for 1 1 ratio of template to monomers are typical glasses, but elastic deformation up to 50% at 90°C is quite remarkable. This behavior is quite different from composites polyacrylic acid-urea-formaldehyde polymer obtained by conventional methods. Introduction of polyacrylic acid to the reacting system of urea-formaldehyde, even in a very small quantity (2-5%) leads to fibrilization of the product structure. Materials obtained have a high compressive strength (30-100 kg/cm ). Further polycondensation of the excess of urea and formaldehyde results in fibrillar structure composites. Structure and properties of such composites can be widely varied by changes in initial composition and reaction conditions. 

The chemical structures of thermosets are generally much more diverse than the commodity thermoplastics. The most common types of thermosets are the phenol-formaldehydes (PF), urea-formaldehydes (UF), melamine-formaldehydes (MF), epoxies (EP), polyurethanes (PU), and polyimides (PI). Appendix 2 shows the chemical structure of these important thermosetting polymers. 

Two 15N-enriched urea-formaldehyde resins with different crosslink density were studied by tfie solid state CP MAS 15N NMR. Despite at least six expected 15N chemical shifts arising from tertiary, secondary and primary amides in the different structural moieties, both resins exhibit only two major peaks. The lower field resonance is more pronounced in the highly cured resin, suggesting its origin in the tertiary amides. A DD experiment, which would confirm this assumption, does not result in clearly separated secondary and tertiary amides. Thus, from the analytical point of view, it seems that 13C NMR spectra are more useful than 15N NMR spectra, although 1SN resonance data provide a useful supplement 252). 

Both melamine—formaldehyde (MF) and resorcinol—formaldehyde (RF) followed the earlier developments of phenol—, and urea—formaldehyde. Melamine has a more complex structure than urea and is also more expensive. Melamine-base resins require heat to cure, produce colorless gluelines, and are much more water-resistant than urea resins but still 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. 

Fig. 10.3 The formation and structure of a urea-formaldehyde resin (UF), a thermosetting resin.

Biblos and Coleman investigated another type of potential structural composite product (53). They made and tested panels consisting of a particleboard core from sawdust and bark and faces of veneer. All material was southern pine, and 9% urea formaldehyde served as binder. Strength tests indicated the composite panels were superior to conventional two-layer floor systems of 1/2-inch plywood plus 5/8-inch particleboard underlayment. 

Some report that over 50 percent of the urea-formaldehyde resins consumed went into particleboard. This is brought out because there may be a shift away from urea resin for certain types of oriented particleboard used in structural plywood constructions. Historically, particleboard has been used for inner plies as previously mentioned in some hardwood plywood. There is now one plant in production in Idaho which produces mechanically oriented strand particleboard for use specifically as core for softwood plywood production. It is anticipated that this trend to some degree will increase in the future, and phenolic resins appear to be the mechanism with which this particleboard will be bonded. 

The types of adhesives suitable for laminating beams are restricted by the conditions of application and by their end-use requirements. A wider choice of adhesives for plywood depends on whether softwoods or hardwoods are used, whether they are required for internal or external exposures, or whether they are to be used for ornamental or structural purposes. Thus phenol-formaldehyde types would be used for marine or exterior construction uses urea-formaldehyde types would be advantageous for cold pressing, or melamine-urea adhesives might be preferred for hardwood plywood, or lumber-core panels used in furniture production. 

Urea-formaldehyde foams are usually brittle structures with low compressive strength (under 50 psi or 0.34 MPa). The term "frangible" may be applied to them. They are open-cell, sponge-like foams that can absorb large quantities of water. Hiese foams also exhibit thermal and acoustical insulating properties common to low-density foams. For example, their thermal conductivities range within the values quoted for polystyrene foam (0.24 - 0.33). This is the result of their low density and snail cell size (5). 

Figure 3. A comparison of the structures of polyglycine, a simple protein, and urea-formaldehyde resin.

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

Rammon, R.M. 1984. The Influence of Synthesis Parameters on the Structure of Urea-Formaldehyde Resins. Ph.D. Thesis, Washington State University, Pullman, Washington. 

The reactions of melamine (the structure of which is shown in Scheme 1.16) are similar to those of urea except that there are three amino groups per molecule, so it is hexafunctional with respect to formaldehyde and it is possible to separate the different reaction products with formaldehyde up to hexamethylol melamine. The crosslinking reactions are identical to those of urea-formaldehyde resins. 

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