Investigation of combination catalyst system for UF resin in particleboard and MDF manufacturing. Part III. FTIR and DMA analyses of UF and MUF resin curing behaviors in the presence of twenty-four catalysts
In North America, ammonium chloride is one of the catalysts commonly used to catalyze urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) resin curing in particleboard and medium density fiberboard (MDF) manufacturing. However, there are some limitations with this catalyst. A major limitation is that its catalytic impact depends on the availability of free formaldehyde in the resin. This is because the interaction of the catalyst with formaldehyde generates a strong acid, which accelerates the resin curing rate. Another major limitation is the possibility of polychlorinated dioxine compounds being formed during the recycling of particleboard made with UF resins catalyzed by ammonium chloride, which are classified as toxic materials.
To explore a suitable catalyst system as a substitute for ammonium chloride, a total of 24 catalyst systems were prepared from 12 chemicals, including aluminum chloride, aluminum sulfate, ammonium chloride, ammonium hydroxide, ammonium nitrate, ammonium persulfate, ammonium sulfate, hexamethylenetetramine, phosphoric acid, triethanolamine, triethylamine and urea. The 24 catalyst systems included 4 systems with 1 component (D-1, D-2, D-3, D-4), 6 with 2 components (E-1, E-2, E-3, E-4, E-5, E-6), 9 with 3 components (F-2, F-3, F-4, F-5, F-6, F-7, F-8, F-9, F-10), 3 with 4 components (F-1, G-3, G-4), and 2 with 5 components (G-1, G-2).
All catalyst systems were characterized for pH at room temperature, gel time, pot life, free formaldehyde (FF) release of cured resin, hydrolysis resistance (HR), variation of functional groups (before and after hydrolysis) characterized by Fourier transform infrared (FTIR) spectroscopy and strength development measured by dynamic mechanical analysis (DMA). Five to six hours was considered a reasonable pot life for UF and MUF resins when making adjustments to catalyst performance. Ammonium chloride, coded D-1 in this study, was used as a reference to set the other criteria for evaluating catalyst performance.
In the presence of formaldehyde in catalyst solutions, potential catalysts include D-2, D-3, E-1, F-1, F-4, F-5, F-9, F-10, G-1, G-2, G-3 and G-4. These catalyst systems caused a significant decrease in pH values when heated up (100oC). This result indicated that the formaldehyde may have reacted or interacted with some components of the catalyst systems.
In terms of gel time and pot life, potential catalyst systems seemed to be D-2, D-3, D-4, E-1, E-4, E-5, F-4, F-5, F-9, F-10, G-1 and G-2 for a low F/U UF resin; and D-2, D-3, D-4, E-1, E-2, E-3, E-5, F-9, F-10, G-1 and G-3 for low F/(U+M) MUF resin. It was observed that the curing of MUF resin needed a more acidic catalyst system than the curing of UF resin did. Thus, ammonium persulfate would be a key component in the catalyst system for a low F/(U+M) MUF resin.
In terms of free formaldehyde release and hydrolysis resistance of cured resins, most of the catalyst systems (except D-2, D-4, E-6, F-1, F-2 and F-9) seemed to be suitable for UF resin, while D-3, D-4, E-5, F-1 and F-5 appeared to be the systems suitable for MUF resin.
In terms of the change in relative concentrations of functional groups and the stability of resin chemical structure after hydrolysis, potential catalyst systems include E-1, E-6, F-1, F-3, F-5, F-7, F-8, G-3 and G4 for UF resin, and D-3, D-4, E-5, E-6, F-1, F-2, F-4, F-7, G-2 and G-4 for MUF resin.
In terms of the maximum modulus, or strength property, of completely cured resins, potential catalyst systems seemed to be series D, E (except E-4 and E-5), F (except F-1 and F-4) and G for UF resin, and series D (except D-2 and D-3), F (except F-6 and F-9) and G for MUF resin.
With respect to the estimated costs of catalyst systems, some systems cost less than the control of ammonium chloride (D-2, E-2, F-4, F-5, F-6, F-7), some are comparable to the control (D-4, E-1, E-3, E-4, E-5, F-9, F-10) and some cost more than the control but with costs that are considered reasonable (F-1, F-2, G-1, G-2, G-3, G-4). Only the cost of four catalyst systems (D-3, E-6, F-3, F-8) was considered much higher than the control.
It seems unlikely to be able to base the selection of an optimal catalyst that would satisfy all criteria in catalyzing UF and MUF resins. Thus, it is recommended that selection of a suitable catalyst be made on the basis of resin type and the important requirements of resin characteristics and board performance, as well as actual board manufacturing conditions.