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Impact of the synthesis procedure on ureaformaldehyde resins prepared by alkaline-acid process Carolina Gonçalves, João Pereira, Nádia Paiva, João Ferra, Jorge Martins, Fernão D. Magalhães, Ana Barros-Timmons, and Luisa Carvalho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06043 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019
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Industrial & Engineering Chemistry Research
Impact of the synthesis procedure on ureaformaldehyde resins prepared by alkaline-acid process Carolina Gonçalves1,2, João Pereira1,3, Nádia Paiva2, João Ferra2, Jorge Martins1,4, Fernão Magalhães1, Ana Barros-Timmons5, Luísa Carvalho1,4,* 1LEPABE
- Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n
4200-465, Porto, Portugal 2EuroResinas –
3ARCP
Indústrias Químicas SA, 7520-195, Sines, Portugal
- Associação Rede de Competência em Polímeros, Rua Dr. Júlio de Matos
828/882, Porto, Portugal 4DEMad
– Wood Engineering Dept., Instituto Politécnico de Viseu, Campus Politécnico de
Repeses 3504-510, Viseu, Portugal 5CICECO
- Aveiro Institute of Materials and Departamento de Química, Universidade de
Aveiro, 3810-193, Aveiro, Portugal *Luísa Carvalho (E-mail:
[email protected]) 1
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ABSTRACT In this work two synthesis procedures for the preparation of urea-formaldehyde (UF) resins, based on the alkaline-acid process, are studied in order to better understand the chemical reactions involved. For that purpose, the number of urea loads and the methyolation temperature were varied. The molecular weight distribution of the resins was monitored by Gel Permeation Chromatography/Size Exclusion Chromatography and the unreacted oligomers by High-Performance Liquid Chromatography, during the synthesis processes. Chemical modifications were investigated using quantitative analysis by carbon-13 Nuclear Magnetic Resonance of samples taken during the synthesis. The largest difference identified between the procedures regards the methylene linkages and methylol groups determined by 13C-NMR. With the results of this study, it is possible to optimize the process and the properties of particleboards produced. KEYWORDS: urea-formaldehyde (UF) resin, alkaline-acid process, Gel Permeation Chromatography/Size Exclusion Chromatography (GPC/SEC), HighPerformance Liquid Chromatography (HPLC), quantitative carbon-13 Nuclear Magnetic Resonance (13C-NMR) INTRODUCTION Urea-formaldehyde (UF) resins are among the most widely used adhesives in modern wood-based panel industry. The good binding strength, low price, and low pressing time for 2
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full curing are the main reasons for their wide applications. There are also some limitations associated with this type of resins such as low moisture resistance and formaldehyde (F) emission during manufacturing, and service life. 1,2 Trying to achieve a low F content, generally compromises the mechanical properties of the panels is observed. Therefore, a better understanding of the mechanisms of the synthesis process is crucial to improve the performance of these resins. Nowadays, two industrial procedures are mainly used: the alkaline-acid-alkaline three-step procedure, and the alkaline-acid-alkaline two-step procedure. 3,4 Despite of the extensive work available on these resins, comparisons between different synthesis procedures, taking in to account the resin evolution during the synthesis, are limited. 5–8 In a previous work, our group studied five different synthesis procedures and concluded that two of them led to a better performance of particleboards (PB). Keeping this in mind, a deeper study was crucial to understand the chemistry and the reason for the good properties obtained with those two processes. Trying to understand the chemical structure of the polymer formed during the synthesis process, several methods have already been used. 6,9,18,19,10–17 Amongst the many existing methods used to characterize resins, liquid-state 13C-NMR is the most used. This technique provides the most complete information on the chemical structures of F based resins, enabling the identification and quantitative determination of many functional groups. Many experiments indicated that reactions under alkaline conditions formed methylolureas and a small amount of oligomers linked by methylene ether linkages, whereas the 3
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methylene linkages were barely formed. Recently, Liang et al. revealed that the competitive relationship between ether bonds and methylene linkages is affected both by activation energy and steric effects. 16 When the F/U molar ratio was 2.00 the methylene linkages were predominantly formed, whereas the formation of methylene ether linkages was favored when the molar ratio was lowered to 1.00. It is also known that, under acidic conditions, the content of methylene linkages in final structures is much higher than that of ether bonds at the end point of the reactions. Furthermore, some studies reported that a portion of ether bonds formed in the initial step are converted to methylene linkages in the condensation step. With those results, it was possible to infer that the methylene linkages are thermodynamically more favorable than ether linkages. 16,20 Wang et al. studied the structural changes during three-step synthesis of low molar ratio. They concluded that, at the first alkaline stage with F/U molar ratio = 2.00, the condensations that produced polymers linked by ether bonds were notable in addition to the hydroxymethylation. After the reactions were brought to the acidic stage, methylene linkages began to form, and became dominant at the end of this stage. The considerable formation of branched methylene linkages, that had the highest content among all the condensed structures, was the key feature of this stage. Changes were observed in the chemical structures of the resin components after the F/U molar ratio was lowered to 1.20 upon addition of urea at the final alkaline stage. 21 In another study resins (uncured and cured UF) were prepared using two different F/U molar ratios. The hydrolysates of UF cured resins prepared using F/U
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molar ratio of 1.20 had slightly higher molecular weight (Mw) compared to resins prepared using F/U molar ratio of 1.00. The hydrolysate of UF cured prepared using F/U molar ratio of 1.00 contained more linear methylene linkages (28–57%) and tri-hydroxy- methyl ureas (43–66%) than those of the resins prepared using 1.20 F/U molar ratio. However, the latter has more branched methylene linkages (4–73%), hydroxymethyl groups (3–35%), uron species (12–54%), ether linkages (1–15%) and mono-hydroxy-methyl urea (9–20%). Methylene linkages, hydroxymethyl groups, and mono- and tri-hydroxymethyl ureas were the main functional groups, which were also the main contributors to formaldehyde liberation during hydrolysis. 22 The GPC/SEC is another technique for the characterization of the polymer, essentially the polymer structure and size in terms of hydrodynamic volume. The chromatograms obtained can be divided in two essential zones: low and high molecular weights. Some studies have been carried out using this technique. 7 HPLC is a chromatographic technique that allows the separation of a mixture of different molecular weight compounds, specially low molecular weights. 23–25 The use of this technique in the analysis of UF resins allows the separation and identification of unreacted urea (U), monomethylolurea (MMU) and dimethylolurea (DMU). In this article, these techniques will be important to evaluate the synthesis procedure and to understand the growth of the polymer. XRD is often used to determine the crystalline structure of polymers. This technique allows studying the crystallinity and domain size of cured resins as a function of different 5
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parameters. 26–29 In 1986, Stuligross et al.30 reported the crystallinity of a UF resin when the F/U molar ratio was reduced to 1.00 and also showed that the resin formulation did not change the crystalline structure but only the degree of crystallinity. Recently, Park et al. showed that using low molar ratio UF resins, the particles are larger in size and more abundant, which also exhibit crystallinity. 26 Ferg et al. 31 investigated the existence of a correlation between crystallinity and adhesion strength of the hardened resin. The role of crystallinity on reducing bond strength is ultimately linked to the fact that orderly packed structures are not contributing to the formation of the tridimensional network structure in the bond-line. Crystallinity can also be changed in contact with wood, for example, according to Levendis et al. 32, an UF resin loses crystallinity when in contact with wood. In this work two resins were produced using the alkaline-acid process. The synthesis of the resins was off-line monitored by 13C-NMR, GPC/SEC and HPLC. At the end of synthesis, the resins were characterized by standard characterization, the techniques mentioned above, as well as XRD. To assess the effect of the synthesis conditions, PBs were prepared and characterized according to the standards in force. The differences between and along the synthesis procedures are notorious from the different characterization techniques. Therefore, this study allows to better understand the chemical structure of the polymer and to evaluate two different synthesis procedures. Furthermore, the PBs characterization was also performed being the results similar for both procedures. MATERIALS AND METHODS 6
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MATERIALS Formaldehyde (55 wt. % solution), urea, ammonium sulphate, sodium hydroxide (50 wt. % solution) and acetic acid (25 wt. % solution) were provided by EuroResinas – Indústrias Químicas, S.A. (Sines – Portugal). Wood particles and paraffin for the production of PBs were suplied by Sonae Arauco (Oliveira do Hospital – Portugal). METHODS
Resins production The production of the resins was carried out in 2.5 L round bottom reactor, equipped with mechanical stirring and thermometer. The reactor was heated with a mantle and the temperature was controlled with a thermometer. The pH and viscosity measurements were performed offline on samples taken from the reaction mixture (and re-added after). Both resins were produced according to the alkaline-acid process and two final F/U molar ratios were considered. Table 1 presents the main characteristics of the resins synthesized. For Resin A, the synthesis procedure begins with the addition of the first load of U under controlled temperature until the desired condensation F/U molar ratio was achieved. The pH was adjusted to moderate acid and the second load of U was added. The condensation step was monitored by viscosity measurements and pH control. When the desired viscosity was reached, the pH was adjusted to alkaline, stopping the condensation reactions. The
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system was cooled down and the last load of U was added to reach the intended F/U molar ratio. Resin B was synthesized in a way similar to the traditional two-step procedure. The first load of U was added under controlled temperature until attaining the desired F/U molar ratio. The pH was adjusted to moderate acid and the condensation step was activated. As in Resin A, when the desired viscosity was reached, the pH was adjusted to slightly alkaline to stop the condensation reactions. This was followed by cooling, upon which the last load of U was added until the desired final F/U molar ratio. In brief, the main differences between the procedures are related to the methylolation step and the number of U loads additions. As Resin B involves a smaller number of U loads than Resin A, Resin B can be considered easier. Table 1. Production parameters for the synthesized resins.
Resin A
Resin B
Tmethylolation (ºC)
80
Tcondensation (ºC)
>80
>80
Condensation F/U Molar ratio
