Nonconventional bonds

A vast range of materials have been bonded to silica such as phenyl, cyano, nitro, amino, and diol functionalities. The variety of silica-based sorbents are listed in the United States Pharmacopeia (USP) [23] where it is noted that of the 33 column packings, 19 are silica based. The synthesis and characteristics of over 80 nonconventional bonded silicas have been reviewed [24]. In spite of the number of chromatographic packings, 50% of the chromatographers develop methods using a C18 sorbent [1]. Sorbents... 

X HE TERM NONcoNVENTioNAL BONDING has been in use for some time, but its choice can be hardly termed as fortunate. In the first place it is negative, i.e., it is based on concepts outside of the scope of definition. Secondly, it lacks time stability what is nonconventional today might be conventional tomorrow. Thirdly, it is too broad, as it can relate to nonconventional glues, nonconventional practices, or even to nonconventional equipment. Probably the only reason for using this term is the lack of appropriate alternatives. 

Advantages of nonconventional bonding are associated with covalently bonded wood surfaces (external and to some extent internal) and include dimensional stability of the products. Occasionally increased brittleness and a loss in mechanical properties due to acidic degradation of carbohydrates are observed. 

Some nonconventional bonding methods are based on the use of agricultural by-products, i.e., on nonpetroleum-based materials this use constitutes another advantage. Some nonconventionally bonded materials produce reduced amounts of toxic gaseous materials, such as formaldehyde, that make them preferable to phenol-formaldehyde products and urea-formaldehyde resins. Economically, the nonconventional methods do not offer any particular advantages, although they appear to be competitive with the conventional methods. 

Table IV. Properties of Nonconventionally Bonded Full-Size Panels Compared to Laboratory-Made Boards and Phenol-Formaldehyde (PF) Boards...

The situation is simpler with wood preactivated in a separate step (S3, S5), although in this case a question might be raised whether changes on the wood surface (e.g., formation of carboxyls) merely catalyze cross-linking of the polymer or whether covalent bonds form involving surface. Not much experimentation has been done so far aimed at elucidation of the mechanisms of nonconventional bonding methods involving intermediacy of a polymer. Most of the available evidence is restricted to the Philippou process. 

Participation of the wood surface has not been stressed much in nonconventional bonding based on cross-linking of carbohydrates in the presence of weak acids (110), cross-linking of lignosulfonates in the presence of strong acids (111-17), and based on cross-linking of lignosulfonates in the presence of oxidants (106—9). At the same time... 

The mechanism of nonconventional bonding based on mixtures of diamines with PVC was explained (82) on the basis of the reported ability of the two materials to partially cross-link (180) (Scheme 3). Epichlorohydrin, used occasionally as an additive, polymerizes with polyfunctional amines (181) (Scheme 4) and reacts with lignin in the presence of amines or carboxylic acids (182). Even simpler to visualize is the cross-linking of the mixture of maleic anhydride and PVA by formation of ester linkages (118) that attach to the hydroxyls of the wood surface. 

The research in the area of nonconventional wood bonding has not reached its zenith as yet and much applied and fundamental work remains to be done. So far, in spite of an appreciable number of nonconventional bonding systems in existence, only a few are com-... 

Nonconventional Bonding with Acid Activation. Concentrated or spray-dried, spent sulfite liquor, with or without previous ultrafiltration, was used as a binder for waferboards. The press temperatures used were between 210 and 230 °C and the press times between 5 and 10 min, with the amount of resin between 4 and 5%. The boards produced were better or comparable to the boards made using phenol-formaldehyde resin according to IB, MOE, MOR, hardness, and linear expansion tests the cost of the binder was twofold or threefold less (187). 

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