30. Furan-Based Adhesives

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IX. MISCELLANEOUS APPLICATIONS OF FURAN RESINS Azimov et al. [65] compared the performances of furan resins with those of conventional phenol–formaldehyde adhesives. They used these binders to assemble aluminumto-aluminum and glass-to-glass structures and showed that furanic resins gave much higher rupture moduli in both systems studied. Rassokha and Avramenko [66] studied similar systems, but gave more information about the furan resin used. They used 5- and6-based adhesives both in the presence and absence of zeolite-based fillers, and assembled aluminum-to-aluminum and glass-to-glass structures. They showed that the use of polyethylene-co-vinylacetate as a filler dispersant gave well dispersed suspensions and consequently the best mechanical properties of the assembly. Nikolaev et al. [67] studied the thermal stability of furan resins produced by the reaction between a furfuryl ether of glycerol and 2,4-toluene diisocyanate. They showed clearly that the incorporation of this resin into conventional adhesives improved their thermal stability. The mechanical properties of steel-to-steel assemblies based on these compositions were found to follow the same tendency as that observed for the thermal properties. Poly(hydroxymethyl furfurylidene-acetone) adhesive resins were synthesized and characterized [68–70] through the 5-formaldehyde adduct (19) and its acid-catalyzed polymerization. The catalysts used were sulfuric, phosphoric, or p-toluenesulfonic acid. The authors postulated that the first condensation products resulted from the condensation of two methylol groups of two 19 molecules (adduct 20). They also proposed a hypothetical structure of the network formed after curing (21). It seems, however, difficult to envisage the acid-catalyzed resinification of 19 without the participation of hydrogen atoms at the C5 position of the furanic ring [2]. Copyright © 2003 by Taylor & Francis Group, LLC
Macro-diisocyanates based on the reaction of an excess of 2,4-toluene diisocyanate with different poly(dimethylsiloxane)diols of different lengths have been prepared by Nikolaev et al. [71]. These macro-diisocyanates were reacted with 2 in stoichiometric proportions and the resulting adduct (22) was cured with a commercial epoxy resin in the presence of what was termed ‘‘poly(ethylene)-poly(amine)’’ at room tempcerature, 80, and 100 C. The mechanical and thermal properties of steel-to-steel assemblies joined by these adhesives were better than those obtained using more common binders. Bowles et al. [72] studied the copolymerization of different methacrylates with NCOethyl methacrylate to obtain dental adhesives. Furfuryl methacrylate (23) was among the monomers tested. The main objective of this investigation was to establish a correlation between the solubility parameter of the copolymers and their shear strength. It was moreover shown that the setting time of the furan-based copolymer was very short compared to that of aliphatic homologues, but its shear strength was relatively low. Dopico et al. [73] prepared 5- and6-based furan resins which, after acid-catalyzed polymerization, were subjected to epoxidation with thiokol in different proportions. In a second series of experiments, 6–7% of 2 was added to the epoxidized resins. They showed that all these resins presented a lower flexure resistance compared to unmodified totally furanic binder. Moreover, the addition of 2 was found to induce negative effects on the mechanical properties of metal-to-metal assemblies. <strong>Furan</strong> resins have also been used as binders in grinding wheels [4]. In this field, 5–20% of phenolic resin in combination with 1 as a special wetting agent is added to the abrasive grains and the resulting wheels thereafter coated onto the surfaces of different substrates. Paper, cloth as well as composites based on glass fiber reinforced films, have been used as grinding wheel supports. Acid-catalyzed poly2 has also been used in the aircraft industry as a low-temperature setting adhesive to bond wood and plastic parts. This adhesive was found to be suitable for assemblies subjected to warping and other deformations at high temperatures [12]. X. CONCLUSIONS AND SUMMARY From the above survey, it appears that the industrial use of furanic monomers such as furfuryl alcohol and furfural, i.e., chemicals based on renewable resources, as binders in foundry molds is highly successful. Similar furan-based resins can also be used as efficient adhesives in wood–particle composites and thus are interesting alternatives to petroleum-based counterparts. The fact that the substitution of formaldehyde by furfural has not yet met with a reasonable industrial success probably stems from the higher cost of the furan aldehyde. The increasing pressure on the reduction of formaldehyde emission and the renewable character of furfural should play in its favor in the near future. This chapter has dealt with the use of furanic derivatives as adhesives and binders. It has been shown that the main industrial applications concern the use of furfural and Copyright © 2003 by Taylor & Francis Group, LLC
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30. Furan-Based Adhesives

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