Лако-красочные материалы — производство

A. Synthesis and Characterization

Within the forest products industry, polymeric MDI competes against the form­aldehyde based thermosets such as urea-formaldehyde, melamine-urea-formaldehyde, and phenol-formaldehyde. Interestingly, the isocyanate wood binders are also based upon formaldehyde. Their preparation begins with the HCl catalyzed condensation of form­aldehyde with aniline, as shown below [2,3].

The scheme above belies the complexity of the reaction; see Twitchett’s discussion for the detailed mechanism [2]. The overall reaction between formaldehyde and two moles of aniline produces the 4,40-, 2,40-, and 2,20-isomers of methylenedianiline. These diamine isomers will react to form the higher methylene bridged polyphenylene polyamines. It has been stated that 2,40- and 2,20-methylenedianiline are preferentially consumed for the production of the higher polyamines [4,5]. The author is unaware of any published work to this effect, but such a preferential reactivity is expected from the steric influence on electrophilic aromatic substitution. It is certainly true that under typical industrial conditions the resulting monomer fraction is mostly 4,40-methylenedianiline, with 2 to 7% of the 2,40-isomer and lesser quantities of the 2,20-isomer. The 2,40- and 2,20-isomer levels may be dramatically increased with elevated reaction pressures and heterogeneous catalysis [6,7]. Otherwise, the composition of the polyamine mixture can be altered through several variables including the aniline to formaldehyde ratio, the aniline to HCl ratio, and temperature [2,5].

The acidic polyamine mixture is neutralized and dried in preparation for reaction with phosgene. The phosgene reaction (phosgenation) occurs in high boiling aromatic solvents such as chlorobenzene or 1,2-dichlorobenzene [2,4]. Phosgenation converts the amino groups into isocyanate; so the isocyanate molecular structure is a reflection of the polyamine. Large stoichiometric excesses of phosgene are used to help avoid undesirable side reactions [2,4]. These side reactions may lead to the formation of sub­stituted ureas, biurets, polyurets, carbodiimides, and related chlorine containing com­pounds [2,4]. Afterwards, the solvent is stripped away and the isocyanate mixture is subjected to distillation for the isolation of the 4,40-MDI monomer. The remaining non­volatile residue is the wood binder known as polymeric MDI; it is a mixture of MDI monomer and the related methylene bridged polyphenyl polyisocyanates. In the forest products industry the binder is commonly referred to as MDI or simply as “isocyanate.” This confusion of names is further complicated by the fact that polymeric MDI is not at all polymeric. Approximately one half of the resin is diisocyanate monomer, while the rest is a complex oligomeric mixture of polyisocyanates with degree of polymerization less than 12, as depicted below.

Since pMDI is not distilled, phosgenation side reactions not only reduce yield but they may also impact reactivity, as all impurities remain. The best measure of purity is through the determination of the isocyanate content, or percentage-NCO. This is a simple reaction with excess dibutylamine followed by back-titration of the residual [8,9]. The isocyanate content is typically 31 to 32% for industrial pMDI. Other notable wet analy­tical methods are the determination of hydrolyzable chlorine, and the determination of acidity [9-11]. Both methods quantitate acidic impurities which could create problems in subsequent chemical transformations, for example acid catalysis of isocyanate reactions, or neutralization of the amine catalysts that are often used in the polyurethane industry. The isomer ratio of the monomer fraction (the relative proportions of 4,40-, 2,40-, and 2,20- MDI) is readily determined with gas chromatography. Gel permeation chromatography may be performed on the underivatized resin if a dedicated system is available. Otherwise, cross-reactions between analytes may be avoided by analyzing the N, N-dibutyl urea derivative of the isocyanate. Molecular weights of typical pMDI wood binders are from about 255 to 280 g/mol number average, and about 470 to 550 g/mol weight average. Typical viscosities are correspondingly low, approximately 0.175-0.25Pas (175-250cP). Finally, pMDI surface tension is approximately 41-46 mN/m. In other words, the 100% organic resin is very low in molecular weight, low in viscosity, and low in surface tension. It is no surprise that this binder wets readily and penetrates deeply into wood, as we shall discuss later.