Synchrotron X-Ray Diffraction Studies on Crystalline Domains in Urea–Formaldehyde Resins at Low Molar Ratio

Formalin (37 wt%) and extra pure grade urea (99 wt%) were obtained from Daejung Chemical, Korea. Extra pure grade NH₄Cl (99 wt%), NaOH (93 wt%), and aqueous formic acid (85 wt%) were purchased from Duksan Chemical, Korea. The SnakeSkin^® pleated dialysis tubing, 7000 MWCO, was purchased from Thermo Scientific (Waltham, MA, USA).

UF resins with F/U molar ratio of 1.0 was synthesized via an established alkaline−acid−alkaline three-step reaction. Fig. 1 summarizes the UF resins synthesis procedure. In brief, formalin solution was placed into a four-neck glass reactor, pH was adjusted to 8.0 using NaOH (20 wt%), and the temperature was raised to 40°C. Urea was then added to the reactor, yielding in an initial F/U molar ratio of 2.0. To allow the addition process to occur, the liquid was heated to 90°C and the pH was maintained at 8.0 for 1 h. Subsequently, the temperature and pH were adjusted to 80°C and 4.6, respectively, to promote the condensation reaction. When a target viscosity of 'J–K' scale was obtained based on readings using a standard bubble viscometer (VG 900, Gardener-Holdt, Columbia, SC, USA), the reaction was terminated, and the pH was adjusted back to alkaline (8.0–8.2). A certain amount of urea was also added to the glass reactor to reach a final F/U molar ratio of 1.0, and the temperature was maintained at 60°C for 20 minutes. The resins were then cooled to 25°C and the pH was adjusted to 8.2. Prior to analysis, the resins were stored at room temperature for 24 h. Fig. 1 shows the position where the liquid resin has been sampled and labeled as samples #1 and #2. Sample #1 was taken at the end of the addition stage and the sample #2 was taken at the final stage after adjusting the pH to 8.0–8.2.

Prior to synchrotron XRD analysis, sample #1 and #2 were dried in an oven at 50°C and crushed to a fine powder. Afterwards, the powdered sample was purified using a dialysis process. Approximately, 50–100 mg of each sample was put in the SnakeSkin pleated dialysis tubing, which was filled with roughly 40–50 mL distilled water. The tubes were sealed on both sides and submerged in 1,000 mL of distilled water. The dialysis was performed for 7 days, after which the residue was dried to provide a pure sample. Furthermore, synchrotron XRD data were collected at the 7A beamline of the Pohang Accelerator Laboratory (PAL, Pohang, Korea), Pohang Light Source II (wavelength of 0.9793 Å, beam size of 100 μm), under nitrogen cold stream (100 K). Data were collected using the oscillation method in intervals of 1° step on an ADSC Quantum 270 CCD detector (Area Detector Systems, Poway, CA, USA) with a crystal-to-detector distance of 120 mm (Kim et al., 2020). As a comparison, unpurified sample #1, unpurified sample #2, and cured sample #2 (prepared by adding 3% NH₄Cl and put in an oven at 120°C for 1 h) were characterized using laboratory XRD (D/Max-2500, Miniflex II, Rigaku, Tokyo, Japan) with a Cu Kα radiation source (λ = 0.15406 nm). The samples were scanned at room temperature from 0 to 50° with a step of 0.02°/min. Furthermore, the crystallinity values of UF resins were determined from XRD patterns using the peak deconvolution method using a Gaussian function (Wibowo and Park, 2020). The crystallinity of each sample was calculated as follows:

where, S_c and S_a represent the area of the crystalline domain and the area of amorphous domain, respectively.

TEM investigation was performed according to the published method to observe the morphology of crystalline domains in UF resins (Singh et al., 2015). To prepare the sample for TEM investigation, a small piece of radiata pine (Pinus radiata D. Don) veneer with dimensions of 25 mm × 35 mm × 6 mm was impregnated in a glass beaker filled with cured neat UF resins containing 0.1% NH₄Cl as hardener. Furthermore, the sample was placed in a vacuum drier for impregnation at ambient temperature under a pressure of 67.7 kPa for 24 h. Then, the sample was pre-cured in a vacuum drier for 24 h at 60°C and 67.7 kPa. The ultrathin sections were obtained using an ultramicrotome and a diamond knife. The slices were stained for 7 min with a 2% aqueous uranyl acetate solution before being examined with an H-7100 Hitachi TEM at 75 kV and a detector X Max.

The formation of crystal domains in UF resins with low molar ratios has been extensively studied, and it has been revealed that such crystal formation decreases the adhesive strength of UF resins while compensating for decreased formaldehyde emission (Despres and Pizzi, 2006; Keith Dunker et al., 1986; Park et al., 2011; Pratt et al., 1985; Stuligross and Koutsky, 1985). Their adhesive strength deteriorates due to a decrease in cohesive force caused by the restriction of crosslinking formation. In fact, the molecules organized in the crystal lattice cannot contribute in the construction of the UF resin crosslinked structure, but are instead held together by relatively weak intermolecular interactions (Ferg et al., 1993). The cured samples of thermosetting polymers typically have a strong three-dimensional network or cross-linking that is covalently bonded that holds the polymer chains together. Thus, any crystalline domain in the polymer structure may obstruct the establishment of a three-dimensional cross-linking network (Wibowo et al., 2022). Moreover, in low molar ratio UF resins, crystalline domains are constructed by linear structures that are held together by hydrogen bonding (Wibowo and Park, 2021), which is not as strong as covalent bonding and hence leads to a weak cohesive force. It is expected that by understanding the definite crystalline structure of UF resins, it would be possible to simply modify the structure of UF resins to prevent the degradation of their cohesive force. Fig. 2 depicts a comparison of UF resins isolated at different stages of synthesis and treated under various conditions. Fig. 2(a) and (b), for example, compare unpurified samples of #1 and #2. There are clear differences, primarily in the shape of the spectra and the number of the peaks, in sample #1, several peaks represent the crystalline domains are possibly belong to the crystal structures of unreacted urea or small crystals composed of mono-, bis-, and tris(hydroxymethyl) urea (Park and Jeong, 2011; Wibowo et al., 2020b). This is due to the fact that, as previously reported (Wibowo et al., 2020b), the crystalline peaks of UF model compounds (mono-, bis-, and tris(hydroxymethyl) urea) matched with the crystalline peaks of sample #1. In contrast, the spectrum of sample #2 shows a clear combination of crystalline and amorphous characteristics, as indicated by the sharp crystalline peak with multiple minor peaks and a noticeable amorphous underneath it. This is an indication of a semicrystalline nature. Another significant difference could be seen in their crystallinities and crystallite sizes. It has been reported that the crystallinity of sample #1 differed from that of sample #2, which is around 57% and 42% for sample #1 and #2, respectively (Wibowo et al., 2020b). In addition, in the XRD patterns of samples #1 and #2, the average crystallite sizes associated with the crystalline peaks were around 6.5 nm and 34.4 nm, respectively (Wibowo et al., 2020b). These findings revealed that the crystallite size in sample #1 was smaller than that in sample #2, implying that the crystalline domains of sample #2 may be made up of longer and more regular polymers than those of sample #1. Furthermore, Fig. 2(c) and (d) display the synchrotron XRD spectra of samples #1 and #2 following dialysis purification. The XRD spectra before [Fig. 2(a) and (b)] and after purification clearly show a significant transformation [Fig. 2(c) and (d)]. For example, the intensity of several crystalline peaks in sample #1 [Fig. 2(c)] decreased dramatically, suggesting that certain crystal components were eliminated. Interestingly, the second peak from the left was the most decreased peak in the XRD spectra; this peak might be attributable to unreacted urea or mono(hydroxymethyl) urea crystals. This is because the XRD pattern in Fig. 2(a) was most likely a combination of mono-, bis-, and tris(hydroxymethyl) urea XRD patterns, whereas the XRD pattern in Fig. 2(c) was very similar to the XRD pattern of the bis(hydroxymethyl) urea model compound (Park and Jeong, 2011; Wibowo et al., 2020b), indicating that the majority of the bis(hydroxymethyl) urea crystal was preserved. As illustrated in Fig. 3(a), the crystalline structure of bis(hydroxymethyl) urea might be stronger than that of mono(hydroxymethyl) urea due to the greater number of hydrogen bonds that can be formed. As a result, mono(hydroxymethyl) urea was easy to separate from the primary structure, dissolve in water, and pass through the dialysis membrane [Fig. 3(b)]. Surprisingly, the most striking XRD pattern alteration was discovered in Fig. 2(d). After purification, sample #2 was completely different from the unpurified one; the pattern was more similar to the cured sample #2 depicted in Fig. 2(e). During dialysis, it seems that most of the crystals and some amorphous parts of unpurified sample #2 were eliminated, leaving the original crystalline domain of UF resins as illustrated in Fig. 3(b). The removed crystals were primarily composed of unreacted urea and small crystal domains assigned to mono(hydroxymethyl) urea and short linear oligomers. Meanwhile, the amorphous components that were liberated were attributable to the oxymethylene formaldehyde and short methylene ether linkages that formed during synthesis (Steinhof et al., 2014; Wang et al., 2018; Wibowo et al., 2020b). The fact that the XRD patterns in Fig. 2(d) and (e) were so identical suggested that the original crystalline domain of UF resins was present even before curing. The curing process simply improves the development of crystalline domains by using an acid catalyst. The hardener appears to promote linear structural elongation, hence enhancing hydrogen bonding formation and resulting in a more pronounced crystal structure and sharper XRD pattern. This is supported by prior study, which shown that the crystallinity value of cured UF resins rose as the percentage of hardener increased (Wibowo and Park, 2020). The typical crystalline domain of UF resins, as aforementioned, was constituted of linear oligomers stacked together by hydrogen bonding force [Fig. 3(a)]. This type of structure was strong enough to be maintained in the primary structure of the solid sample because of the high number of hydrogen bonds, as well as because these crystalline domains were covalently bonded to the surrounding amorphous structure of UF resins (Wibowo and Park, 2021). Thus, even after dialysis, the crystalline domain remains in sample #2, as demonstrated in [Fig. 3(b)].

Fig. 2. X-ray diffraction spectra of 1.0 F/U UF resins isolated at different stages of synthesis and treated under various conditions: (a) unpurified sample #1, (b) unpurified sample #2, (c) purified sample #1, (d) purified sample #1, and (e) cured sample #2. UF: urea–formaldehyde.

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Fig. 3. Schematic representations of (a) chemical structures of possible crystalline domains in UF resins and (b) the dialysis process and crystal structure visualization of UF resins. UF: urea–formaldehyde.

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Interestingly, the crystallinity of purified samples #1 and #2 calculated from the deconvoluted XRD spectra of Fig. 4 reduced to 52% and 40%, respectively, when compared to the unpurified ones, corroborating the hypothesis that some crystals were removed during dialysis. Further, the crystallinity of purified sample #2 is 40%, which is lower than that of cured sample #2 [i.e., 52% (Wibowo and Park, 2021; Wibowo et al., 2020a)]. This finding indicates that, although the purified sample #2 contains the original crystalline domain of UF resins, its crystallinity is smaller than that of cured sample #2. Additionally, based on the similarities in XRD pattern, it is possible to assume that their crystal structure is the same [Fig. 3(b)]. Furthermore, Fig. 5 depicts the two-dimensional (2D) diffractogram of UF resins isolated at different stages of synthesis. The 0° angle denotes parallel radiation of the incident X-ray beam to the sample, whereas the 90° angle denotes perpendicular radiation of the incident X-ray beam to the sample. Fig. 5(a) demonstrates that sample #1 has a greater number of concentric circles (Debye rings). Meanwhile, sample #2 has less concentrated rings, which are roughly four circles at most [Fig. 5(b)]. This suggests that sample #2 comprised fewer crystalline domains than sample #1, as evidenced by the lower number of crystalline peaks in the XRD spectra and degree of crystallinity. A radial slice of a 2D diffractogram is known to offer the traditional XRD pattern of intensity vs. $\overrightarrow{q}$. The angular slice of the 2D diffractogram, on the other hand, offers information on the texture of the sample. For example, a perfect polycrystalline sample exhibits full Debye rings, whereas single crystals show as dots, and an amorphous material has no distinctive features (Widjonarko, 2016). In the case of UF resins, the 2D diffractograms of samples #1 and #2 exhibit perfect circles, indicating that the crystals in the sample are isotropic polycrystals with no preferred (random) orientation relative to the incident electron beam (Singh et al., 2014). The crystalline domains of UF resin with no specified orientation was illustrated in Fig. 3(b).

Fig. 4. Deconvoluted synchrotron X-ray diffraction spectra of purified UF resins isolated at different stages of synthesis: (a) sample #1, (b) sample #2. UF: urea–formaldehyde.

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Fig. 5. Two-dimensional synchrotron X-ray diffraction pattern of purified UF resins isolated at different stages of synthesis (a) sample #1, (b) sample #2. UF: urea–formaldehyde.

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The morphology of cured neat UF resins (sample #2) was observed using TEM (Fig. 6). The resins were impregnated in a wood veneer to visualize the morphology of resins present in the lumen of tracheid. Fig. 6 shows a high-resolution TEM image with a magnification of 400k. Remarkably, several lamellar structures representing a cluster of individual polymer chain were observed at some specified sites (e.g., yellow circle), whereas other areas comprised of irregular amorphous structures. The well-ordered lamellae were formed as a result of hydrogen bonding between the C = O and N−H groups of linear oligomers of UF resins as previously reported in literature (Wibowo and Park, 2021, 2022). Moreover, these lamellar crystalline domains clearly have a random orientation (as indicated by the red arrow), which is consistent with the 2D synchrotron XRD pattern that shows full Debye rings (Fig. 5). Furthermore, the TEM image (Fig. 6) shows that UF resins have a typical semicrystalline morphology, with a random orientation of crystal lamellae domains surrounded by amorphous regions, as previously illustrated by the Fig. 3(b). Surprisingly, the lamellae of cured UF resins are similar to those found in polypeptide crystals. A recent study used TEM to demonstrate an individual chain of the particular protein-like polymers (Drnovšek et al., 2016). They discovered polypeptide lamellae coupled with a β-sheet structure, which is comparable to what was discovered in this study. Moreover, TEM images of different types of semicrystalline polymers reported by many research published in the literature were remarkably similar to our findings (Kuei et al., 2020; Libera and Egerton, 2010; Tosaka et al., 2005).

Fig. 6. A TEM image of cured neat UF resins (sample #2) demonstrating the random orientation of crytsallin domains. TEM: transmission electron microscopy, UF: urea–formaldehyde.

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