Eur. j. wood prod.
- 1. ORIGINAL Hydroxymethyl furfural-modified urea–formaldehyde resin: synthesis and properties N. Esmaeili1 • M. J. Zohuriaan-Mehr1,2 • S. Mohajeri1 • K. Kabiri1,2 • H. Bouhendi1 Received: 30 June 2015 / Published online: 24 June 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Considering the importance of urea–formalde- hyde (UF) resins in the wood industry, this work reports on a new bio-based modification of UF resins. The use of 5-hydroxymethyl furfural (HMF) is motivated by the cur- rent concerns about the effects of formaldehyde on human health. UF and urea–HMF–formaldehyde (UHF) resins were synthesized by an alkaline-acid method and charac- terized by FTIR, thermogravimetric analysis, and differ- ential scanning calorimetry. The UHF, as a newly modified polymeric resin, was thermally characterized, and it was found that its thermo-stability and char yield was improved. In order to investigate the performance of the UHF, the preparation of particleboards with the UHF as adhesive, as well as its film formation ability have been studied. The UHF films formed on wood panels were uniform without any crack. Film formation ability of the UHF resin was improved due to the presence of more hydroxyl groups as well as furan rings of the HMF moieties resulting in more activated groups to be bonded by wood. Furthermore, formaldehyde release of the particleboards bonded by UHF was significantly lower than that of which bonded by the UF resin. Lab particleboards using the UHF resins showed higher modulus of rupture, modulus of elasticity, and internal bond compared to boards with UF resins, as well as lower water absorption and thickness swelling. Based on these results UHF resin can be con- sidered as a possible candidate as adhesive for wood and wood based panels. 1 Introduction In the past decades, urea–formaldehyde (UF) resins have been used in wood industry for production of wood panels, such as medium density fiberboard (MDF) or others. Other adhesives such as phenol–formaldehyde resins (Fink 2005) have also been used in wood industry. However, because of their very low costs, non-flammability, high curing rate, and the light color, UF resins are still used in vast amounts and as the most important adhesive by far in the production of wood based panels (Zorba et al. 2008; Abdullah and Park 2010; Basta et al. 2011; Patel et al. 2013). UF resins show some disadvantages, for example, low water-sus- ceptibility (Christjanson et al. 2002, 2006), low heat-sta- bility (Dim 2011), non-uniform film formation (Singh et al. 2013) and, most importantly, emission of carcinogenic free formaldehyde (Birkeland et al. 2010). For solving the latter problem, reducing the formaldehyde (F)/urea (U) molar ratio in the synthesis is one the most favorable approaches (Tohmura et al. 2000; Ferra 2010). Some drawbacks in the manufacturing of the UF resin with lower F/U molar ratio occur (Han et al. 2008); for example, the mechanical strength of the resins was deteriorated, due to lack of suf- ficient cross-linkages formed by formaldehyde. However, some approaches for improvement have been reported, such as increasing the polycondensation time in order to create more crosslinks (Park and Kim 2008; Park et al. 2006). A usual approach for reducing free formaldehyde emission is addition of some formaldehyde scavengers to the particleboards (Park et al. 2008). & M. J. Zohuriaan-Mehr [email protected]; [email protected] 1 Adhesive and Resin Department, Polymer Processing Faculty, Iran Polymer and Petrochemical Institute, PO Box 14965-115, Tehran, Iran 2 Biomass Conversion Science and Technology (BCST) Division, Iran Polymer and Petrochemical Institute, PO Box 14965-115, Tehran, Iran 123 Eur. J. Wood Prod. (2017) 75:71–80 DOI 10.1007/s00107-016-1072-8
- 2. Another way for minimizing the content of free formaldehyde is its replacement by other aldehydes par- tially or completely (Zhang et al. 2014, Li et al. 2009). Though abundance of lab results is available in technical and patent literature, the breakthrough in industrial appli- cation is still missing. For many applications, the weakness of aminomethylene linkages against water and hydrolytic degradation of resin structure limits the use of UF resins (Paiva et al. 2012). The main reason for the low water resistance of UF resins is the reversibility of the linkage in aqueous media. By introducing some relatively hydrophobic monomers (e.g., melamine) in the UF resin structure for decreasing the possibility of water attack to the resin chains, the stability of the resin against water is improved (No and Kim 2007). The better chemical resistance of the C–N linkages in MUF resins is due to the quasi-aromatic behavior of the p electrons as well as the pH value in the hardened resin, which is still in the acidic range but higher compared to hardened UF resins. In this work, hydroxymethyl furfural (HMF) was used as co-monomer for synthesizing urea–HMF–formaldehyde (UHF) resins by alkaline-acid method. According to Gan- dini (2010) furan polycondensates are now of great interest of both, fundamental and industrial research. To the best of the authors‘ knowledge, HMF-based modification of UF resin has not been reported so far. HMF was introduced to the structure for its good thermo- and hydrolytic stability. Furthermore, replacing the carcinogenic formaldehyde by HMF with higher safety and less volatility was considered. Another important issue is that formaldehyde is originated petro-chemically, but HMF is a bio-based monomer pre- pared from biomass resources such as fructose (Van Putten et al. 2013). The structure of the UHF resin and curing process were studied by FTIR spectroscopy. The thermal behavior of the resins was analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Other experiments were performed for further investigations, such as gelation time and water durability. 2 Experimental 2.1 Materials Fructose, urea, paraformaldehyde, glacial acetic acid, sodium hydroxide, hydrochloric acid, ammonium chloride, and methyl-isobutyl ketone (MIBK), all of analytical grades, were purchased from Merck and used as received. 2.2 HMF synthesis from fructose The monomer 5-HMF was synthesized by conventional heating method (Van Putten et al. 2013; Esmaeili et al. 2016). Briefly, fructose, distilled water, HCl and MIBK were added to a flask and magnetically stirred for 2.5 h at 80 °C. After all, HMF was extracted by MIBK and then recrystallized in MIBK to yield purified product. 2.3 Synthesis of UF resin The UF resin was synthesized by alkaline-acid method (Table 1) (Ferra et al. 2012). Paraformaldehyde and dis- tilled water were added into a 250 mL two-necked glass flask equipped with a magnetic stirrer, thermometer and reflux condenser. In the first step, NaOH solution (0.1 M) was added dropwise into the flask to adjust pH *10, heated to 80 °C and kept for 0.5 h to dissolve paraformaldehyde completely. Then, 7.7 g urea was added to the flask and kept for 1 h. As the second step, glacial acetic acid solution (25 %) was added for adjust- ing pH *4, second urea (5.5 g) was then added and polymerization was carried out during 0.5–4 h. In the third step, the mixture was neutralized with NaOH solu- tion (0.1 M) and final urea (4.8 g) was added and mixed until dissolving the total urea, then cooled to ambient temperature. UF1–UF5 distinguish by the duration of the acidic condensation step, hence in the degree of conden- sation. For further analysis and characterization sample UF2 (Table 1) was chosen. 2.4 Synthesis of UHF resin The UHF resin synthesis procedure was the same method as for the UF resin, but in the first step, HMF and paraformaldehyde were added to the flask simultaneously. In the second step, the polymerization time was 2 and 4 h, respectively (Table 1). Sample UHF3 with the highest replacement rate of paraformaldehyde by HMF was chosen for further analysis and characterization. In this resin, approx. 29 % of the paraformaldehyde was replaced by HMF based on the amount of moles. 2.5 Curing of resins An aqueous ammonium chloride solution (15 %) was used as curing agent. Thus, 0.50 g of the solution was added to 10.0 g resin, gently mixed and put in an oven at 105 °C for 3 h (Bono et al. 2003). 2.6 Characterization The viscosity of the resins was analyzed by a Brookfield viscometer (Visco Star R, Selecta Co., Spain) by LCP spindle at 23 °C, in the range of 1–50 rpm. The spectral and thermal characterization of the resins was performed by FTIR spectroscopy, TGA, and DSC, respectively. 72 Eur. J. Wood Prod. (2017) 75:71–80 123
- 3. FTIR spectroscopy (Bruker, IFS48, Germany) was used for characterization of functional groups of the uncured and cured resins. The samples were dried (1.0 g resin, 105 °C, 3 h, without hardener) and prepared as KBr pellets with 1 wt% of the dried material. Thermal analysis was carried out with TGA (Mettler Toledo, TGA 1500, England) and DSC (Netzsch, 200-F3 Maia, Germany). For the TGA, cured samples were placed in alumina crucibles. Samples were heated from ambient temperature to 600 °C under N2 flow rate of 50 mL min-1 and heating rate of 10 °C min-1 . For the DSC, powdered but still uncured samples were prepared by drying at 105 °C for 3 h, and then mixed with 0.01 g ammonium chloride and put into the high pressure steel crucibles for analysis. After all, samples were heated from -20 to 220 °C, with a heating rate of 10 °C min-1 . 2.7 Solid content Solid content of the resin was determined by heating at 105 °C for 3 h according to the procedure described by Bono et al. (2003). 2.8 Film formation properties For the investigation of the film formation ability, the resins were mixed with the hardener solution and then applied onto the surface of particleboards (10.0 9 10.0 9 0.5 cm3 ); the amount of resin mix was selected in order to form a film with 500 lm thickness; the coated particleboard with the film was then heated at 105 °C for 3 h. The structure of the final film on the sub- strate was visualized for probable cracks on the surface. 2.9 Determination of formaldehyde release from particleboards The content of formaldehyde release from the lab parti- cleboards was determined by the flask method (based on EN 717-3:1996). 2.10 Particle board preparation and testing One-layer laboratory particleboards having 13 mm thick- ness were prepared by adding 10 % total resin solids on dry wood particles. The boards were pressed at a maximum pressure of 1.5 MPa at a press temperature of 180 °C for 12 min. This remarkably long press time was selected in order to guarantee full curing of the experimental resin UHF3. All tests were carried out in accordance with the relevant test methods as described in the various EN standards on a universal testing machine (STM-20, Santam, Iran). The tests performed on the specimens were: internal bond strength perpendicular to the plane of the board (IB; EN 319), modulus of rupture (MOR, EN 310), modulus of elasticity (MOE, EN 310), water absorption (WA, EN 317), and thickness swelling (TS, EN 317). The WA and TS samples were fully immersed in distilled water at 25 °C for Table 1 Reaction conditions for preparation of the UF and UHF resins Resin code 1st stepa (F ? HMF)/U molar ratio 2nd stepb , time (h) (F ? HMF)/U molar ratio 3rd stepc , viscosity (cP) (F ? HMF)/U molar ratio UF1 10 g para-F 2.59 0.5 1.51 325 1.11 UF2 10 g para-F 2.59 1 1.51 449 1.11 UF3 10 g para-F 2.59 2 1.51 478 1.11 UF4 10 g para-F 2.59 3 1.51 523 1.11 UF5 10 g para-F 2.59 4 1.51 826 1.11 UHF1 d 9 g para-F, 3.8 g HMF 2.57 4 1.50 456 1.10 UHF2 e 8 g para-F, 8 g HMF 2.57 4 1.50 457 1.10 UHF3 f 7 g para-F, 12.2 g HMF 2.57 2 1.50 459 1.10 a Para-F indicates paraformaldehyde; 20 g water, 7.7 g urea, 1 h, 80 °C, pH *10 b 5.5 g urea, 80 °C, pH *4 c 4.8 g urea, 0.5 h, 25 °C, F/Utotal = 1.11, pH *10, viscosity measured by a Brookfield viscometer at 1 rpm, 23 °C, LCP spindle d 9 % replacement of formaldehyde by HMF (mole by mole) e 19 % replacement of formaldehyde by HMF (mole by mole) f 29 % replacement of formaldehyde by HMF (mole by mole) Eur. J. Wood Prod. (2017) 75:71–80 73 123
- 4. 2 and 24 h, respectively. For accelerated aging tests (water resistance test), specimens were boiled in water for 2 h, dried at 105 °C for 16 h, and then tested for IB. 3 Results and discussion 3.1 Resin synthesis Urea–formaldehyde and HMF-modified urea–formalde- hyde (UHF) resins were synthesized through known alka- line-acid procedure (Ferra et al. 2012) (Fig. 1). In order to achieve a viscosity range of 400–460 mPa s, the duration of the acidic polycondensation was varied (Table 1). For the UF resin, necessary duration of the condensation was 1 h, but it was 2 h for the UHF resin in order to produce a resin with similar viscosity. Therefore, as the intended viscosity range was 400–460 mPa s, the UF2 sample with viscosity of 449 mPa s and UHF3 sample with viscosity of 459 mPa s were chosen for further analyses. The difference of the polycondensation time implies that the reactivity of the HMF is much lower than that of formaldehyde due to (1) higher steric hindrance near the carbonyl group in HMF compared to the formaldehyde, and (2) decreased electrophilicity of the carbonyl group due to its resonance with the aromatic furan ring (Gancarz 1995). Regarding the steric hindrance, nucleophilic attack of the nitrogen atom of urea on formaldehyde is easy, but such attack on the carbonyl group of HMF is not favored by the neighboring bulky furan ring. The carbonyl group in the HMF is also involved in the resonance by the furan ring with the result, that the nucleophilic addition to it will be disfavored. Further, the furan group of HMF has an inductive electron donor property, so that the elec- trophilicity of its carbonyl group is decreased; this will impede the nucleophilic attack of the NH2 group of urea. Therefore, longer condensation time was needed to achieve the UHF resin with the targeted viscosity. The main properties of four selected resins (UF2 sample as well as three UHF samples) are summarized in Table 2. As the reactivity of HMF aldehyde group is lower than that of the formaldehyde (Gancarz 1995), the majority of formaldehyde is consumed in the condensation reactions before HMF will contribute to the reaction. The existence of a furan ring near the aldehyde functional group in HMF makes the nucleophilic attack to be difficult. So, a con- siderable proportion of the total nucleophilic addition had taken place with formaldehyde instead of HMF during the first period of reaction. Furthermore, it should be consid- ered that the methylol group of HMF could contribute to the condensation reactions during resin synthesis. In the first stage at basic pH (Fig. 1c), urea reacts with aldehyde group but not with methylol group of HMF; so there is no competition between aldehyde and methylol group of HMF for reacting with urea. In the second stage of the reaction at acidic pH, the methylol group can condense with urea or other methylol groups (see Fig. 1d). Though the HMF molecule owns two active functional groups, the aldehyde group is considered as the reactive group during formaldehyde replacement calculations. 3.2 Resin properties The various solid contents of the resins according to their recipe (64 % for UF2 vs. 60–61 % for the UHF resins) were not corrected prior to further analysis. The gelation time was 105 s for the UF resin and 240–270 s for the UHF resins (Table 2). The gelation time increased significantly by replacing even only a smaller part (9 %) of the formaldehyde by HMF in the structure of the resin. The curing process is, as it is the continuation of the condensation process in the reactor, based on the reaction of the aldehyde groups with the amino groups. As already noticed during the acidic condensation step, the reactivity of HMF is lower than that of formaldehyde due to the lower reactivity of its aldehyde group in comparison with that of formaldehyde (Gancarz 1995); hence, the curing reaction of the UHF resin (as the continuation of the acidic condensation step in the resin preparation) also was slower than UF resin. Surprisingly already a small replacement rate in UHF1 by 9 % HMF (means replace- ment of 9 % of the moles of formaldehyde by HMF) increased the time of curing already to more than the double value. Higher HMF contents, however, did not show further significant increase in the gelation time (samples UHF2 and UHF3). Figure 1e proposes a structure of the cured UHF resin. A block-like copolymeric structure is more probable rather than a random-like one. It might be similar to the structure reported for a urea–CH2O–furfural resin system (Zhang et al. 2014). 3.3 FTIR spectroscopy FTIR spectroscopy showed characteristic bands (Kandel- bauer et al. 2007) proving the synthesis of resins (Fig. 2). During the first step of the addition reaction occurring between formaldehyde and urea, dimethylolurea (1,3- bishydroxymethyl urea) is produced. The specific bands proving the structure of both UF and UHF resins are as follows: 3,370 cm-1 (alcohol O–H stretching, broad; hydrogen bonded with water and methylol groups), 3,030 cm-1 (C–H aromatic stretching), 1,655 cm-1 (C=O stretching of primary amide), 1,644 cm-1 (secondary amide, appearance as a shoulder of the C=O primary amide 74 Eur. J. Wood Prod. (2017) 75:71–80 123
- 5. (a) Methylolation step in the synthesis of UF resin H2N NH2 O + H H O H2N N H O OH pH 10 (b) Condensation step in the synthesis of UF resin. H2N N H O OH NH2 N H O HO + H2N N H O O N H NH2 O pH 4 H2N N H OH O + H2N NH2 O pH 4 H2N N H N H O NH2 O (c) Methylolation step between urea and HMF in the synthesis of UHF resin. H2N NH2 O + O OHC CH2OH O CH2OH N H O H2N OH pH 10 (d) Condensation step of HMF methylols in the synthesis of UHF resin O CH2OH N H O H2N OH 2 O N H O H2N OH O N H O NH2 OH O pH 4 (e) Proposed structure of the UHF resin after the curing step Fig. 1 Main reactions in the synthesis of UF and UHF resins (a–d) and structure of the cured UHF resin (e) Eur. J. Wood Prod. (2017) 75:71–80 75 123
- 6. band), 1,547 cm-1 (N–H bending amide), 1,513 cm-1 (N– C–N of a methylene bridge), 1,509 cm-1 (C=C of furan ring), 1,380 cm-1 (C–N bending vibration of amide), 1,292 cm-1 (CH2 methylol groups of urea moieties), and 1,251 cm-1 (C–N bending of amide). In the UHF samples, three bands appeared at 1,010, 1,505, and 3,020 cm-1 and can be attributed to C–O–C ether, aromatic C=C bond, and C=C–H of furan ring, respectively. The absence of C–H stretching vibration of the aldehyde group (at 2,830 cm-1 ) proved successful incorporation of HMF into the UHF resin structure (Taylor et al. 2013). C–O–C ether linkages were observed in the structure UF resins as well (ether bridges from the reaction of 2 methylol groups), but these linkages appear at 1,100–1,104 cm-1 (Ahamad and Alshehri 2014). FTIR spectroscopy was also used for the investigation of the cured resins. As shown in Fig. 2, the curing results in decreased intensity of the O–H alcohol stretching at 3,370 cm-1 . This spectral observation verifies the reduc- tion of the O–H groups. During the curing process, the – CH2–OH groups react further to form either –CH2–O– CH2– ether bridges or –CH2– methylene bridges (Kan- delbauer et al. 2007). In the curing process of the UHF resin, intensity of the C=C aromatic furan ring in the 1,505 cm-1 was decreased. This fact can be attributed to possible Diels–Alder reac- tions of the furan rings during the curing (Bobrowski and Grabowska 2012). 3.4 Thermal characterization DSC was used for monitoring the curing behavior of the resins. Curing of the UF resin started at lower temperature in comparison with the UHF resin, due to the higher reactivity of formaldehyde rather than that of HMF and therefore the higher reactivity of the resin as such. Steric hindrance of the aldehyde group by the furan ring in the HMF molecule causes delayed curing of the UHF resin compared to the UF resin (Fig. 3). In addition, no glass transition temperature (Tg) is observed, probably due to Table 2 Characteristics of the synthesized UF and UHF resins Resin codea Initial HMF (g) Gelation time (s) Solid content (%) pH WSb Film formation Fc (mg kg-1 ) UF2 (0 %) 0 105 64 10.1 0.559 Weak 14 UHF1 (9 %) 3.8 240 61 10.0 0.315 – – UHF2 (19 %) 8.0 260 61 10.2 0.276 – – UHF3 (29 %) 12.2 270 60 10.0 0.245 Good 8 a HMF: mol % based on original number of moles of formaldehyde in the UF recipe; numbers are given in parenthesis b Water solubility (resin:water, g g-1 ) c Formaldehyde emission from lab particleboard Fig. 2 FTIR spectra of typical UHF (upper curves) and UF resins (lower curves) before and after curing Fig. 3 DSC thermograms of resins UF2 and UHF3 76 Eur. J. Wood Prod. (2017) 75:71–80 123
- 7. hiding by the broad curing peak. It can be confirmed by the endothermic shift of the base line of both resins (Ebewele 1995). As the curing peak of the UHF sample appears at much higher temperature (130 °C for UHF vs. 80 °C for UF), the possible Tg of the UHF sample seems to be higher than that of the UF sample. It can be attributed to the existence of the aromatic groups in the UHF resin structure (Gao et al. 2008; Sperling 2005). TGA and differential TGA (DTG) diagrams for the cured resins are shown in Fig. 4. The UF resin thermogram showed a three-step weight-loss but a four-step pattern for the UHF resin. For both resins, the weight-loss observed in the range of 50–110 °C is attributed to adsorbed moisture. In the temperature range of 100–180 °C formaldehyde is released from dimethylene ether bridges (–CH2–O–CH2–) to form more stable methylene (–CH2–) bridges (Liu et al. 2008). For the UF resin, the major thermo-degradation steps occurred above 180 °C. In this stage, chain splitting hap- pened and the C–C bonds were homolitically disconnected to form free radicals causing additional thermo-degradation (Ahamad and Alshehri 2014). At even higher temperatures (400 °C), the methylene bridges started to be quickly decomposed, causing the major weight loss of the UF resin. The char yield of the UF resin was only 11 % (Roumeli et al. 2012; Taylor et al. 2013). The UHF resin was decomposed in four stages. After releasing the absorbed moisture at around 70–110 °C, the second step, attributed to the formaldehyde emission, also happened at 100–180 °C. These two steps were similar to those mentioned for the UF resin. The UHF resin structure is more complicated due to the presence of the furan rings; one part of the structure is formed by the reaction of urea with formaldehyde and another part from the reaction of urea with HMF. Also, as it has been mentioned before, due to the lower reactivity of the carbonyl group in HMF in comparison to formalde- hyde, the UHF resin is assumed to bear a block-like copolymeric structure (Fig. 1e). There are some chains formed only by urea and formaldehyde; after all formaldehyde has reacted, HMF molecules will be attacked by urea to form the HMF-urea structure. The latter part induces heat stability to the UHF resin. Whereas the decomposition of UF moieties takes place at 180–320 °C (the 3rd decomposition stage) (Jiang et al. 2010), there is another step of the UHF resin decomposition, appearing at 380 °C; this temperature stage is not observed for the UF resin and leads to a high char yield (34 %). This is attrib- uted to formation of some stable carbonic cyclic species originated from furan rings which can tolerate more heat than linear structures do. These cyclic structures increase the char yield of the UHF resin during combustion (Tu- molva et al. 2009). 3.5 Film formation In order to study the film formation property, the ability of the resins to generate uniform coatings on wood surfaces was investigated. UF resins cannot form a non-cracked and uniform coating due to high brittleness. Singh et al. (2013) have also mentioned the brittleness of UF resin and their problems for making a smooth and uniform surface to investigate the morphology of UF resin by SEM and TEM. In the current work, the UF resin did not generate such a film on a wood plate too. On the contrary, the UHF resin could obtain a uniformed film without cracks (Fig. 5). The capability of the UHF resins in the forming of films on wood panel may be partially attributed to the existence of more hydroxyl groups (as methylols) in its structure, so that hydroxyl groups can intra-molecularly interact (e.g., via hydrogen bonding) with lignocellulose, i.e., wood surface. Fig. 4 TG/DTG thermograms of resins UF2 and UHF3 Fig. 5 Appearance of the UF2 and UHF3 resins applied onto wood panels Eur. J. Wood Prod. (2017) 75:71–80 77 123
- 8. 3.6 Formaldehyde emission The formaldehyde release content of particleboards is usu- ally determined by the chamber test method (EN 717-1), gas analysis (EN 717-2), or flask method (EN 717-3) (Ferra et al. 2012; Sperling2005). In this work, the flask method was used to measure the formaldehyde release from the lab particle- boards; the results were 14 and 8 mg kg-1 for UF and UHF bonded particleboards, respectively. This means that the incorporation of HMF reduced formaldehyde emission. It can be attributed to replacing remarkable parts of formaldehyde by HMF, which simply reduces the content of formaldehyde in the system. The usual method for decreasing the content of free formaldehyde in UF resin is the reduction of the formalde- hyde/urea molar ratio. Beside the direct reduction of F/U this aim can be achieved by replacing the formaldehyde by HMF. So it could be expected that UHF resin should have a lower amount of extractable formaldehyde than that of the UF resin, though more parameters than only the content of free formaldehyde in the liquid resin determine the extractable formaldehyde from cured resins, like curing conditions and achieved degree of curing. 3.7 Mechanical properties of particleboards The results are shown in Fig. 6 and demonstrate the influences of the resin type on the properties of experi- mental particleboards. As can be seen, particleboards bonded with UHF resin exhibit higher mechanical prop- erties than those made with UF resins. The IB for parti- cleboards bonded by UHF3 was 0.55 MPa and higher than that of UF2 with 0.39 MPa. Water resistance analysis was performed and the IB strength decreased to 0.38 and 0.1 MPa for particleboards bonded by UHF3 and UF2, respectively. After the same test, the average MOR and MOE for UHF3 resin decreased from 18.7 to 18.0 MPa and from 3,120 to 2,980 MPa, respectively. Moreover, by performing this resistance test on particleboards bonded with UF2, the average MOR and MOE values decreased from 14.2 to 11.3 MPa and from 2,650 to 2,010 MPa, respectively. Due to more hydrophobicity of the UHF resin in comparison with the UF resin, the aging process was retarded for the UHF and a minor decrease was observed during the test. Based on European Standards (EN), MOR, MOE, and IB values of 16, 2,300, and 0.40 MPa, respectively, are the minimum requirements of structural particleboards for use under dry conditions (P4). According to the results, parti- cleboards in this work produced by UHF3 showed prop- erties that are higher than the EN requirements. However, the long press time used for the production of the lab particleboards has to be considered for such comparison. 3.8 Physical properties Incorporating a hydrophobic monomer like HMF to the resin structure decreased hydrophilicity of the system resulting in higher insolubility and resistance to water. Fig. 6 Average values of IB, MOR, and MOE of particleboards bonded with UF and UHF resins, tested dry and after boiling and re- drying 78 Eur. J. Wood Prod. (2017) 75:71–80 123
- 9. WA and TS for the lab particleboards are presented in Fig. 7. WA for UHF3 and UF2 were 19 and 56 % for 2 h immersion, and 26 and 75 % for 24 h immersion, respec- tively. TS for UHF3 and UF2 were 2 and 8 % for 2 h immersion, and 3 and 12 % for 24 h immersion, respec- tively. In addition, according to EN 312 (board type P4, [10–13 mm), the maximum TS for 24 h requirement is 16 %. An important problem of UF resins is their insta- bility against water. It is mainly originated from hydrolytic cleavage of methylene and especially ether bridges. 4 Conclusion An alkaline-acid method was used for the synthesis of UF and UHF resins. For the UHF resins, HMF was partly used as alternative aldehyde replacing up to 29 % of the formaldehyde (calculated as moles) in the recipe. Some important targets for improving the features such as water durability, film formation capability, and subsequent formaldehyde emission from lab particleboards have been investigated. Improved hydrolytic stability of the HMF containing resin (UHF) was achieved, as well as superior heat stability of the resin with char yield of 34 % compared to the value of 11 % for the UF resin. Film formation of the UHF resin on wood panels was uniform without any crack, which could not be achieved with the UF resins. The gelation time of the UHF resin was significantly longer due to incorpo- ration of HMF into the resin structure. Improved mechan- ical strength was observed in particleboards bonded with UHF resin instead of UF resin. The formaldehyde release from the particleboards bonded with UHF was lower than that bonded with UF. MOR, MOE, IB, WA, and TS of the lab particleboards were also improved with UHF resin compared to UF resins based on the adjusted sufficiently long press times in order to achieve full curing of the UHF resin. Despite its lower reactivity, but owing to the above mentioned improvements, the HMF-modified UF resin can be seen as a potential alternative to be used as adhesive in the wood based panels industry. Acknowledgments The authors are very much obliged to one of the reviewers due to his/her highly informative and deep comments causing evolutionary improvement of this article. References Abdullah ZA, Park BD (2010) Influence of acrylamide copolymer- ization of urea formaldehyde resin adhesives to their chemical structure and performance. J Appl Polym Sci 117:3181–3186 Ahamad T, Alshehri SM (2014) Thermal degradation and evolved gas analysis: a polymeric blend of urea formaldehyde (UF) and epoxy (DGEBA) resin. Arab J Chem 7:1140–1147 Basta AH, El-Saied H, Winandy JE, Sabo R (2011) Preformed amide- containing biopolymer for improving the environmental perfor- mance of synthesized urea–formaldehyde in agro-fiber compos- ites. J Polym Environ 19:405–412 Birkeland MJ, Lorenz L, James M, Frihart CR (2010) Determination of native (wood derived) formaldehyde by the desiccator method in particleboards generated during panel production. Holz- forschung 64:429–433 Bobrowski A, Grabowska B (2012) The impact of temperature on furan resin. Metall Foundry Eng 38:73–80 Bono A, Beng YK, Siambun NJ (2003) Melamine–urea–formalde- hyde (MUF) resin: the effect of the number of reaction stages and mole ratio on resin properties. J Teknol 38:43–52 Christjanson P, Siimer K, Pehk T, Lasn I (2002) Structural changes in urea–formaldehyde resins during storage. Eur J Wood Prod 60:379–384 Christjanson P, Pehk T, Siimer K (2006) Hydroxymethylation and polycondensation reactions in urea–formaldehyde resin synthe- sis. J Appl Polym Sci 100:1673–1680 Dim PE (2011) Application of keratin-modified urea–formaldehyde resin for bonding particleboard. Aust J Basic Appl Sci 5:196–200 Ebewele RO (1995) Differential scanning calorimetry and dynamic mechanical analysis of amine-modified urea–formaldehyde adhesives. J Appl Polym Sci 58:1689–1700 Esmaeili N, Zohuriaan-Mehr MJ, Bouhendi H, Bagheri-Marandi G (2016) HMF synthesis in aqueous and organic media under ultrasonication, microwave irradiation and conventional heating. Korean J Chem Eng 33:1964–1970 Fig. 7 Average values of WA and TS for 2 and 24 h immersion of particleboards in water Eur. J. Wood Prod. (2017) 75:71–80 79 123
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