| More than 80% of wood-based adhesives are urea-formaldehyde (UF) resins, due to their advantages of low prices, abundant supply of raw materials, simple synthesis process, and fast curing rate, et al. However, the continuously emitted formaldehyde has already been one of the main pollutants in the indoor environment, which becomes one of the most urgent issues to be solved in nowadays. In present dissertation, A series of methods containing the molecular structure design of polymers, the organic-inorganic intercalation, the microencapsulation, and the photocatalytic degradation had been applied in the thesis to reduce the formaldehyde emission of UF adhesives from itssynthesis to the plywood production. The relationships of formaldehyde emissions (FE) with the structure and performances of UF adhesives had been systematically studied, and environment-friendly UF adhesives products were finally obtained with excellent performances. The main research works are as follows:1 The aminolysis of atyrene-maleic anhydride copolymers for a new modifier used in UF resins.In order to effectively control the FE to acceptable levels, low molar ratios of formaldehyde/urea (F/U) are usually chosen to prepare UF adhesives, but the bonding strength (BS) is deteriorated correspondingly. In the thesis, the alternating copolymers of styrene (St) and maleic anhydride (MA) with different molecular weight (MW) were synthesized via radical copolymerization. The copolymers were subsequently transferred into water-soluble maleic amic acid derivatives (SMAA) via the aminolysis reaction. The synthesized polymers were applied as a new kind of macromolecular modifier and added into the reaction system during the synthesis of urea-formaldehyde (UF) resins via the traditional alkaline-acidic-alkaline three-step process. The macromolecular structures, addition methods, and addition amounts of SMAA used in the modification of UF were studied in detail, and the modified structures, curing behaviors, and application performances of modified adhesives were also characterized. The results showed that the multi-functional groups of SMAA could participate in the polymerization and curing processes of UF to optimize the network structures and reinforce the resin matrix. Meantime, the formaldehyde in the UF could be further scavenged by the residual active groups in SMAA aiming to control and decrease FE. The SMAA modified UF adhesives were finally applied to prepare the plywood, and the effects of molecular weight (MW), addition amounts, and addition methods of SMAA on BS were characterized. The optimum result was shown on the resin sample using 7.5% of SMAA with the MW of 0.98×104 added in the acidic stage of the synthesis, with the corresponding FE (1.20 mg·L-1) and BS (0.80 MPa) of the bonded plywood satisfied the national standards.2 The urea-montmorillonite intercalated composites and the application on UF resinsThe matrix strength of UF is low, and the fractures are usually induced by the big internal stresses existed in resin structures, especially in the UF of low F/U molar ratios. Therefore, the urea-montmorillonite intercalated composites were designed and prepared to reinforce the matrix structures of UF synthesized in the low F/U via also the traditional alkaline-acidic-alkaline three-step process. The structure, urea content, addition methods, and addition amounts of intercalated composites used in the modification of UF were studied in detail, and the modified structures, curing behaviors, and application performances of modified adhesives were also characterized. The results showed that the weak matrix structures of UF could be effectively enhanced by the montmorillonite nanosheets introduced by the organic-inorganic intercalated composites. The urea in the composites could react with the hydroxymethyl groups in UF and adsorb the free formaldehyde aiming to decrease FE. The modified adhesives were finally applied to prepare the plywood, and the effects of addition amounts and methods of the organic-inorganic intercalated composites on BS were evaluated. The optimum result was obtained by the resin sample using 7% of intercalated composites added in the third alkaline steps, which showed the satisfactory performances of FE (1.20 mg·L-1) and BS (0.80 MPa).3 The controlled-release microcapsule-type formaldehyde scavenger and the application in UF resinsThe free formaldehyde residual in UF or modified resins as well as from the degradation of resin structures will both lead FE during the applications. A certain amount of formaldehyde scavengers were usually added in the plywood production to depress FE to the low level via reaction of active agents with formaldehyde. However, the traditional type formaldehyde scavengers always have universal disadvantages of short timeliness with formaldehyde and significant deterioration on BS. In this work, a new kind of long-term effective formaldehyde scavenger in the microcapsule form was prepared by using an intra-liquid desiccation method. The characterizations of the capsule (UC) were performed including the morphologies, the yields, the loading efficiency as well as its sustained-release of urea in aqueous conditions. The prepared UC could be integrated in urea-formaldehyde resins by simply physical blending, and the mixtures were available to be applied as the adhesives for the manufacture of plywood. The bonding strength (BS) and the FE of the bonded plywood in both short (3 h) and long (12 week) period were evaluated in detail. It was found that the FE profile of the plywood behaved following a duple exponential law within 12 week. The addition of UC in the adhesive can effectively depress the FE of the plywood not only in a short period after preparation, but also in a long-term period during its practical application. The slow released urea would continuously suppress the emission of toxic formaldehyde in a sustained manner without obviously deteriorating on the BS of the adhesives. The optimum result was obtained by the resin sample using 15 phr of UC urea-microcapsule type scavengers, which showed the remarkable FE (0.62 mg·L-1) and satisfactory BS (0.70 MPa) comparing with national standards.4 The TiO2@Carbon nanocomposite microspheres and the photocatalytic degradation of formaldehydeNowadays, the photocatalytic degradation of organic pollutants in environment is an effective method and has attracted more and more attentions. Based on this idea, in this thesis, the spherical resorcinol-formaldehyde (RF) hydrogels were firstly prepared and then in-situ loaded with TiO2 via a one-pot two-step sol-gel protocol. Then, the regular morphology of carbon-based TiO2 nano-sphere (TiO2@RFC) comsposites were obtained by the subsequent high temperature calcination process. The above synthesis protocol was simple and controllable, which could be conducted absolutely in the atmospheric pressure condition. The diameter, structure, specific surface area, porosity, and the photocatalysis efficiency of TiO2@RFC were characterized in detail. The results showed that the dispersant types and their addition amounts had obvious influences on the diameter and distribution of organic RF spheres. The prepared TiO2@RFC had synergistic effect of intensive physical adsorption capacity and excellent phtocatalysis efficiency due to the hierarchically micro-mesoporous carbon skeleton structure and high crystalline TiO2 in pure anatase phase, respectively. When the TiO2@RFC were employed to degrade the organic pollutants of methyl orange and chlorobenzene under UV light irradiation, the measured residual rates were 19.5% and 5.6% after 10 h, respectively. Then, the TiO2@RFC were applied to treat the stored plywood via gaseous adsorption-degradation on the emitted formaldehyde in the air under UV light irradiation, and the phtocatalysis efficiency was obtained as high as 94% when only 3% of TiO2@RFC-2 (tetrabutyl titanate,1.5 g) nanocomposites were employed. |
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