Formaldehyde resins, molecular

Thiourea will react with neutralised formalin at 20-30°C to form methylol derivatives which are slowly deposited from solution. Heating of methylol thiourea aqueous solutions at about 60°C will cause the formation of resins, the reaction being accelerated by acidic conditions. As the resin average molecular weight increases with further reaction the resin becomes hydrophobic and separates from the aqueous phase on cooling. Further reaction leads to separation at reaction temperatures, in contrast to urea-formaldehyde resins, which can form homogeneous transparent gels in aqueous dispersion. 

From this brief discussion it is clear that crosslinking in phenol-formaldehyde resins is complicated and no individual specimen of these materials can be characterised well at the molecular level. Crosslinking is irregular and variable, though it gives rise to a material having sufficiently acceptable properties that it became the first commercially important plastic material indeed, as mentioned in Chapter 1, these resins continue to retain some commercial importance in certain specialised applications. 

Thermosets differ molecularly from thermoplastics in that their individual chains are anchored to one another through crosslinks. The resulting network creates cohesive materials that demonstrate better thermal stability, rigidity, and dimensional stability than thermoplastics. Some examples of traditional thermosets are melamine-formaldehyde resins, which are used to treat fabrics to make them wrinkle-free, and Bakelite (a phenol-formaldehyde resin), a historically important polymer used in many applications, such as costume jewelry, electrical switches, and radio casings. 

Emulsion blocks within the formation can form as a result of various well treatments and are more easily prevented (by using surfactants in conjunction with well treatments, see above) than removed. Aromatic solvents can be used to reduce the viscosity and mobilize oil-external emulsions (167). Low molecular weight urea-formaldehyde resins have been claimed to function in a similar manner in steam and water injection wells (168,169). Water-external emulsion blocks can be mobilized by injection of water to reduce emulsion viscosity. 

Cross-linked Resins It is possible to convert the low Molecular weight urea-formaldehyde resin into the high Molecular weight cross-linked resins by heating the former in acidic conditions. 

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. 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

Molecular Weight Characterization of Resole Phenol—Formaldehyde Resins... 

Dase-catalyzed phenol-formaldehyde resins polymerized with a mole ratio of formaldehyde to phenol greater than one pose an interesting molecular weight characterization problem. This system is a dynamic one with active methylol end groups. Branched and crosslinked structures are formed, and in general, the separation of the resin from the reaction mixture is difficult. Figure 1 illustrates the chemical nature of a resole resin. 

Phenol-formaldehyde resins find numerous applications in such areas as wood composites, fiber bonding, laminates, foundry resins, abrasives, friction and molding materials, coatings and adhesives, and flame retardants (JL). From a specialty chemicals standpoint, they are also used as developer resins in carbonless papers (2.). Conventional methods of preparation involve condensation of a phenol with formaldehyde under either acidic (novolak) or basic (resole) conditions (2). Their typical molecular weight range is from 800-4000 daltons (D) and includes a wide variety of alkyl or aryl substituted phenols (A)- The... 

Figure 2 Creep-recovery tests of chemically treated woods. U, untreated wood Fs, vapor phase formalization F, liquid phase formalization A, acetylation PO, etherification with propylene oxide MG, treatment with maleic acid and glycerol PFl, impregnation with low molecular weight phenol-formaldehyde resin PEG-ICP, impregnation with polyethylene glycol (PEG-IOOO) WPC, formation of a wood- polymer composite (PMMA) WIC, formation of a wood-inorganic material composite.

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