3.1. Characterization of the PF resins
The pH and viscosity values and free formaldehyde content of all the PF resins are given in Table 3. As shown in Table 3, When the amount of the OMW increased from 0%to 40 the pH value of PF resin decreased from 11.90 to 10.67. The OMW contains organics mainly phenolic compounds, and it has low pH (4.8). These compounds and the low pH value of the OMW decreased the pH value of the PF resins synthesized with OMW. All the PF (PF10-PF40) resins synthesized with OMW exhibited a higher viscosity than the reference resin. The free formaldehyde content of PF resins increased from 0.12 to 0.38% as the amount of the OMW substitution rate increased from 0 to 40%. These results were not unexpected due to OMW composed of high phenol and organic acid than clean water. These findings were good consistent with the reports on the basic properties of green PF resins (Danielson and Simonson, 1998; Yu et al., 2018; Mao et al., 2018; Yi et al., 2012; Chen et al., 2019).
Table 3
Basic properties of the PF resins
Resin type | OMW substitution level (%) | pH (20°C) | Viscosity (25°C, cPs) | Free formaldehyde (%) |
PF (reference) | 0 | 11.90 | 320 | 0.11 |
PF10 | 10 | 11.74 | 365 | 0.12 |
PF20 | 20 | 11.60 | 390 | 0.19 |
PF30 | 30 | 11.04 | 420 | 0.23 |
PF40 | 40 | 10.67 | 480 | 0.38 |
The FT-IR spectra of PF, PF10, PF20, PF30 and PF40 resins are illustrated in Fig. 1. It was observed that the replacement of the OMW had no the distinctive effect on the resin in terms of the chemical structure. Except for the PF40, the spectrum of the PF40 resin was different from others, mostly aromatic rings were observed. The characteristic bands at about 1600, 1500, 1250, 1150, 990, and 750 cm− 1 could be attributed to the phenolics for the IR spectra of all PF resin. In addition, with the increase of OMW substitution level the band at about 990 cm− 1 became weak up to, it vanished finally. FT-IR analysis results were consistent with previous reports prevıous reports on the IR analysis results of the PF resin and green PF resins (Poljansek and Krajnc, 2005; Zhang et al., 2013; Yu et al., 2018; Li et al., 2017; Cui et al., 2017).
The thermal behaviour of the PF and OMW containing PF resins was characterised by TGA analysis (see in Fig. 2). Generally, thermal degradation of phenolic resins occurred in three main steps, where the mass loss in the first stage might be caused by the release of free-phenol, formaldehyde, oligomer and water, due to the further cross-linking or condensation reaction between methyl groups (post-curing). The second stage occurred in the temperature between 350 to 450°C. The huge mass loss in this stage might be due to the decomposition of bridged methylene (thermal reforming). In the last decomposition stage, the mass loss could be referred to as the further degradation of phenol to a carbonaceous structure and the generation of carbon monoxide and methane linked by the degradation of methylene bridges (ring stripping) (Li et al., 2016; Alma and Kelley, 2000; Wang et al., 2009: Lee et al., 2012; Yun et al., 2018). TGA curves of reference PF resin and PF resins containing different amounts of OMW are shown in Fig. 2. As it can be seen in Fig. 2, the weight losses of the PF resins containing different amounts of OMW showed a similar trend and the PF resin had a higher thermal stability than that of PF resins containing OMW. The PF resins with a lower content of OMW (PF10 and PF20) exhibited similar thermal behaviours to the reference PF resin. The weight loss of the PF resins with 30% and 40% OMW replacements dramatically increased at the temperature of 250°C. This relatively low thermal stability can be explained by the presence of low molecular weight compounds in OMW. These compounds may be lead to the decomposing more easily at high temperatures of the PF resins containing different amounts of the OMW.
3.2. Bonding Performance Of The Pf Resins
The data of bonding strength of PF and PF with OMW resins are given in Table 4. The bonding strength of the wood samples bonded with the reference resin was higher than that of the samples bonded with the resins containing 10–40 wt.% of the OMW. As the OMW substitution rate was reached up to %40, the strength was dramatically decreased under dry conditions. However, the specimens bonded with PF resins containing OMW with 10%, 20%, and 30% comply with met the minimum requirements, which was the durability class C1(≥ 10) according to EN 12765 standard under dry conditions. The highest bonding strength with a value of 8.8 N/mm2 was found in the reference resin specimens, followed by PF10 resin (8.2 N/mm2) under wet conditions (pre-treatment 2). For the pre-treatment 3, a similar trend was observed with treatment 2. The bonding strength decreased dramatically after the pre-treatment 3, as matched with pre-treatment 1 and pre-treatment 2. The bond quality was negatively affected by the presence of the OMW more or less. When the amount of OMW in the PF resin was increased up 30wt%, the bonding strength of the samples decreased from 6.1 to 2.9 N/mm2. However, the samples bonded with PF, PF10, PF20 and PF30 exceeded the minimum requirements for class C3 specified in EN 12765 standard. According to the bonding test results, it was recommended that OMW can be partially replaced by clean water for the production of the green PF resin.
Table 4
The bonding strength data of PF resins (N/mm2).
Type of the PF resin | Pre-treatment 1 | Pre-treatment 2 | Pre-treatment 3 |
Mean1 | SD2 | Scale strength3 (N/mm2) | Mean1 | SD2 | Scale strength3 (N/mm2) | Mean1 | SD2 | Scale strength3 (N/mm2) |
PF | 12.5 | 0.64 | ≥ 10 | 8.8 | 1.27 | ≥ 7 | 6.1 | 2.41 | ≥ 4 |
PF10 | 12.0 | 0.72 | ≥ 10 | 8.2 | 1.66 | ≥ 7 | 5.3 | 1.65 | ≥ 4 |
PF20 | 11.6 | 1.10 | ≥ 10 | 7.5 | 1.48 | ≥ 7 | 4.6 | 1.56 | ≥ 4 |
PF30 | 10.9 | 1.55 | ≥ 10 | 6.1 | 1.54 | ≥ 7 | 4.1 | 2.78 | ≥ 4 |
PF40 | 8.6 | 1.74 | ≥ 10 | 3.2 | 1.86 | ≥ 7 | 2.9 | 2.52 | ≥ 4 |
1, Each value represents an average from 20 specimens; 2, Standard deviation; 3, Requirements for durability value (N/mm2) |
Pre-treatment 1 (Dry condition, Pre-treatment 2 (24-h submersion in water), Pre-treatment 3 (3-h boiling and then 24-h submersion in water) |