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The Journal of Adhesion

ISSN: 0021-8464 (Print) 1545-5823 (Online) Journal homepage: http://www.tandfonline.com/loi/gadh20

Micro-Morphological Features of CuredUrea-Formaldehyde Adhesives Detected byTransmission Electron Microscopy

Arif Nuryawan, Adya P. Singh, Byung-Dae Park & Valerio Causin

To cite this article: Arif Nuryawan, Adya P. Singh, Byung-Dae Park & Valerio Causin (2016)Micro-Morphological Features of Cured Urea-Formaldehyde Adhesives Detected byTransmission Electron Microscopy, The Journal of Adhesion, 92:2, 121-134

To link to this article: http://dx.doi.org/10.1080/00218464.2014.1003545

Published online: 06 Oct 2015.

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The Journal of Adhesion, 92:121–134, 2016Copyright # Taylor & Francis Group, LLCISSN: 0021-8464 print/1545-5823 onlineDOI: 10.1080/00218464.2014.1003545

Micro-Morphological Features of CuredUrea-Formaldehyde Adhesives Detected by

Transmission Electron Microscopy

ARIF NURYAWAN1, ADYA P. SINGH1, BYUNG-DAE PARK1, andVALERIO CAUSIN2

1Department of Wood and Paper Sciences, Kyungpook National University,Daegu, Republic of Korea

2Department of Chemical Sciences, University of Padova, Padova, Italy

This work examined micro-morphological features responsible forthe crystallinity of cured urea-formaldehyde (UF) adhesives, usingtransmission electron microscopy (TEM) to identify and character-ize distinctive crystalline structures in resins obtained with differ-ent formaldehyde to urea (F/U) mole ratios and hardener levels.The TEM examination of cured UF resin adhesives impregnatedinto wood cell lumen revealed the presence of spherical particleswith variable diameter and number per unit area. The diameterand number/area of the spherical particles increase for decreasingF/U mole ratio and decrease with an increase in the hardenerlevels, an effect which is closely related to their crystallinity. There-fore, the present findings suggest that the spherical particles areresponsible for the crystallinity of cured UF resin adhesives. Theresults also indicate that crystalline structures represent aninherent feature of cured UF resin adhesives, particularly for lowF/U mole ratios, even though these resins are usually classified asamorphous and cross-linked thermosetting polymers.

KEYWORDS Adhesive for wood; Crystallinity; Cured urea-formaldehyde resin adhesives; Microscopy; Mole ratio; Wood

Received 28 November 2014; in final form 29 December 2014.Address correspondence to Byung-Dae Park, Department of Wood and Paper Sciences,

Kyungpook National University, 80 Daehak-Ro, Buk-gu, Daegu 702-701, Republic of Korea.E-mail: byungdae@knu.ac.kr

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1. INTRODUCTION

Formaldehyde-based thermosetting adhesives are obtained by the reaction offormaldehyde with urea, melamine, phenol, resorcinol, or a mixture of ureaand melamine and are widely used as binders for manufacturing wood pro-ducts. In particular, urea-formaldehyde (UF) adhesives represent the largestclass of aminoplastic thermosetting resins and are predominantly used forwood-based composite panel’s binder such as plywood, particleboard, orfiberboard. UF resin adhesives possess several advantages, such as fast cur-ing and good performance in wood panels. Furthermore, low productioncosts, nonflammability, and colorlessness promoted the continued use of thisresin in the wood industry. The main disadvantages of UF resin adhesives areformaldehyde emission from UF adhesive-bonded products and low resist-ance to moisture-induced hydrolysis.

UF resin adhesives are synthesized by a two-step alkaline-acid pro-cedure [1]: (1) first, hydroxymethylation or methylolation in aqueous alkalinesolution allows formaldehyde to react with urea to produce mono-, di-, andtri-hydroxymethyl ureas as the main reaction products, together with somesecondary products containing methylene–ether bonds; (2) the hydroxy-methyl ureas then react by acid condensation, forming methylene linkagesby eliminating water until a target viscosity (adjusted for a specific appli-cation) is reached. In this polymerization step, some hydroxymethyl groupsalso split off as free formaldehyde through the reverse of the methylolationreaction (1). A second or third urea addition is used to remove the excessof free formaldehyde and reduce formaldehyde emissions.

Hydrolytic degradation of a cured UF resin adhesive has been known asa major factor affecting long-term formaldehyde emission in service [2–4]. Inthe hydrolysis reaction, the chemical species go through reversible reactionwhen they are exposed to moisture. Particularly, the hydrolysis of curedUF resin adhesives in the wood-based composite panel seems unavoidabledue to wood substance in a hygroscopic material. Further, as a binder, UFresin adhesives penetrated not only in cell lumens but also into tracheid walls[5]; thus interaction between wood and UF adhesives occurs continuously.Many studies have investigated the hydrolysis of UF resin adhesives to under-stand the mechanism of formaldehyde release, not only from pure cured UFresin adhesives [2,6–12] but also from UF resin adhesive-bonded wood panelproducts [13–16]. The tendency toward hydrolytic degradation of a cured UFresin adhesives depends on its chemical structure and the degree of cross-linking and could be accelerated by high temperatures and strong acidic con-ditions [15]. The degree (or density) of cross-linking of UF resin adhesivesalso plays a crucial role in their adhesion performance. For example, a resinwith high F/U mole ratio cures much faster, forming a highly cross-linkednetwork structure that leads to stronger bonding performances.

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However, UF resin adhesives with high F/U mole ratios are less resistantto hydrolytic degradation than those with low F/U [12]. This behavior hasbeen ascribed to the presence of inherently crystalline structures in the lowF/U resins [17], although UF resin belonged to thermosetting polymer. Otherthermosetting polymers such as epoxy resin (ER) and polyurethanes (PUs)showed only heterogeneity [18–20] because of higher cross-linking density[18], hard and soft clusters [19], hard domains, aggregates, and micro-domains [20]. Furthermore, its examination using small-angle X-ray scattering(SAXS) did not exhibit sharp peak [20] and did not provide direct informationon domain size distribution and connectivity [21]. However, after intention-ally inserting semicrystalline polymer such as polycaprolactone (PCL), ERsexhibited crystallinity as shown by Groeninckx’s [22–24] and Hartwig’sgroups [25,26]. As the PCL content increased, the crystallinity of ERs alsoincreased. In addition, Lützen et al. [25,26] reported that the morphology ofthese blends showed crystallinity formed by the nucleation of PCL molecules.For PUs, they showed crystalline regions when they were produced only byhigh molecular weight of polyols [27]. In fact, our group previously reportedthe nature of crystallinity within cured UF resin thermosetting adhesive usingSAXS on the spatial arrangement of the crystalline domains and of their aver-age distance that the crystallinity of cured UF resin adhesives increases as theF/U mole ratio decreased from 1.6 to 1.0, the average distance betweendomains in cured UF resins decreased from 39 to 34 nm while the crystallitesize increased from 1.3 to 5 nm. Moreover, wide-angle X-ray diffraction(WAXD) also confirmed that crystallinity increases from 26% to 48% whenthe F/U mole ratio decreases from 1.6 to 1.0, suggesting that crystallinity isan inherent feature of the cured UF resin adhesives. Furthermore, the pres-ence of crystals in the cured UF resin adhesives was also confirmed bynanoindentation measurements, which showed a relatively large hardnessvariation for resins with a low F/U mole ratio [28].

In a recent work [29], we showed that morphology of some crystallinestructures was confined to specific regions of the cured UF resin adhesiveswith low (i.e., 1.0) F/U mole ratio containing mature and candidate crystals.The former exhibited truly fan-shaped crystal structures and the latter wereassociated with globular/nodular particles, and the real crystal appeared tobe protruding from them. In other words, globular/nodular particlesappeared to be nucleation sites for the emerging crystals. These featureswere not common like crystalline in thermoplastics polymer, such as lamellain polyethylene or PCL [30–32]. The particles within cured UF resin could beassociated with the colloidal character of the resin [33,34], which has beenlinked to the crystalline regions [12,35–38]. Larger and more abundant parti-cles are found in low F/U resins [10,39], which also show crystallinity[12,28,29,34–37,40–43].

The aforementioned reports suggested that nodular, globular, colloidal,and spherical structures contribute to and are responsible for the crystallinity

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of cured UF resin adhesives. However, to the best of our knowledge, thenature of these crystal particles still remains to be clarified. Therefore, in thiscontribution, we investigated micro-morphological features of particlesresponsible for crystallinity in cured UF resin adhesives using transmissionelectron microscopy (TEM), which allowed us to quantify the diameter andnumber per unit area of crystal particles in cured UF adhesives prepared withdifferent F/U mole ratios and hardener levels.

2. MATERIALS AND METHODS

2.1. Materials

A commercial urea granules (purity 99%) and formalin (37%) produced byDuksan Pure Chemicals Co. Ltd. (Ansan, South Korea) were mixed in propor-tions suitable to prepare UF adhesives, with different F/U mole ratios in theresin mix. Aqueous solutions of both formic acid (20wt%) and sodiumhydroxide (20wt%) produced by DC Chemicals Co. Ltd. (Seoul, South Korea)were used to adjust the pH level during synthesis. An aqueous solution of theammonium chloride (NH4Cl) hardener (DC Chemicals Co. Ltd.) was added tocure the resin on the basis of the nonvolatile resin solids of UF resinadhesives prepared.

2.2. Preparation of Urea-Formaldehyde Adhesives

All UF resin adhesives examined in this study were prepared in the labora-tory, following traditional alkaline-acid two-step synthesis, involving secondurea addition. Formalin (405.8 g) was placed in the reactor, adjusted to pH7.8 with sodium hydroxide, and then heated up to 40°C. Subsequently, a cer-tain amount of first urea (151.7 g) was added, and the mixture was heated to90°C under reflux for 1 hr to allow the addition reaction. Then, in order toallow the condensation, the pH of the reaction mixture was reduced to 4.6by adding formic acid, and the condensation reaction was made to proceeduntil a viscosity between J and K, as measured using a bubble viscometer(VG-9100, Gardner Holdt Bubble Viscometer, Pompano Beach, FL, USA),was reached. When the target viscosity was obtained, different amounts ofsecond urea, namely 37.9, 65.0, 101.1, and 151.7 g, were added to obtainUF resin adhesives with F/U mole ratios of 1.6, 1.4, 1.2, and 1.0, respectively.The UF resin adhesives produced were then cooled to ambient temperatureand adjusted to a final pH of 8.0.

2.3. Measurement of Resin Properties

About 1 g of UF resin adhesives was poured into a disposable aluminum dishand then dried in a convective oven at 105°C for 3 hr. The content of

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nonvolatile resin solids was determined by weighing the UF resin adhesivesprior to and after drying. The results were averaged over three replications.In order to compare the reactivity of the synthesized UF resin adhesives, theirgel time was determined from three replications for each F/U mole ratio, byadding a 3% ammonium chloride solution at 100°C using a gel time meter(Davis Inotek Instruments, Philadelphia, PA, USA). The viscosity measure-ments at ambient temperature were carried out using a cone-plate viscometer(DV-IIþ, Brookfield, Middleboro, MA, USA) with a No. 2 spindle at 60 rpm.

2.4. Preparation of Samples for TEM Analysis

Because a cured UF resin adhesives is hard, brittle, and impossible to preparein ultrathin sections without embedding it in a medium (as the resin turnspowdery upon trimming), we used a specific technique for holding a smallquantity of UF resin adhesives in the capillaries of the wood cells. We filledwood cell lumens with the resin that was then cured for a certain time. UFresin adhesives with different F/U mole ratios (1.6, 1.4, 1.2, and 1.0) weremixed with 0.1% NH4Cl (based on the resin solid content) prior to using themfor gluing wood veneers (see below). This low hardener amount wasemployed in order to clearly understand the impact of F/U mole ratio onthe resin morphology. Similarly, for evaluating the effect of the hardener,3% NH4Cl was added and mixed with the UF resin adhesives prior to gluingwood veneers, but only for the highest and lowest F/U mole ratios con-sidered (i.e., 1.6 and 1.0). Commercially sourced radiata pine (Pinus radiata)veneers were glued with the UF resin adhesives in parallel pairs, with thesame wood grain orientation. The mixture of UF resin adhesives and hard-ener was spread thinly over the surface of a veneer, and the other veneerwas placed over it (in contact with the resin). The paired veneers were thenpressed at ambient temperature with a 288 kg/m2 weight applied for 24 hr, inorder to allow the resin to penetrate into the wood tissues from the adhesivebond line. The paired veneers were then placed in a convection oven, withthe same weight applied, for curing at 60°C for 24 hr. To avoid direct contactof the resin with water, the paired veneers were sectioned dry (withoutsoftening the wood with water) on a sliding microtome (Yamato KOHKI,Saitama, Japan) in the longitudinal direction of the wood grains, to obtainsections about 70 µm thick. The sections containing cured UF resin adhesiveseither on the bond line or within wood cell lumens were clamped betweentwo glass slides to avoid curling.

2.5. TEM Imaging

The sections were first dehydrated with acetone, then infiltrated with andembedded in the Spurr’s low-viscosity ER, according to the method describedin detail elsewhere [44]. Ultrathin sections with 70 nm thickness obtained on

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an ultramicrotome using a diamond knife were stained for 7min with 2%aqueous uranyl acetate. The sections were then viewed with an H-7100transmission electron microscope (Hitachi, Ibaraki, Japan) operating at75 kV in Korea Basic Science Institute (Daegu, Korea).

The diameter of spherical particles found in the UF resin adhesives wasdetermined by an image analysis software (i-solution, Image and MicroscopeTechnology, Vancouver, Canada). The diameter measurements wereundertaken on at least 40 particles for each micrograph. We used at leasttwo micrographs for determination the diameter measurement.

3. RESULTS AND DISCUSSION

The properties of UF resin adhesives with different F/U mole ratios are shownin Table 1. As the F/U mole ratio decreased, the nonvolatile solid contentincreased. This could depend on the fact that the final F/U mole ratio of UFresin adhesives had been controlled by varying the amount of second ureaadded. However, the viscosity of the resin decreased as the F/U mole ratiodecreased. As expected, the gel time increased as the F/U mole ratiodecreased: in other words, a decrease in the F/U mole ratio reduced the reac-tivity of the resin. This could be due to a lower amount of free formaldehydein the lower F/U resins, which makes the curing conditions less acidic andthus less effective. In fact, it had already been reported that the amount of freeformaldehyde in a UF resin adhesives decreases for lower F/Umole ratios [45].

Figure 1 displays TEM images of cured UF resin adhesives filled in woodcell lumens. The images highlight the presence of spherical particles in allcured UF resin adhesives. It has been proposed that these spherical particlesare positively stained by uranyl acetate because the uranyl group reacts withamide groups of the resin [44]: in fact, it has been proved that uranyl acetatereacts with the amide group of proteins [46]. It is also believed that veryordered and crystalline particles expose more terminal hydroxymethyl(CH2OH) groups on their surface that can react with uranyl groups, thusresulting in relatively dark spherical particles under TEM. Cured UF resinadhesives with greater F/U mole ratios (i.e., 1.6 and 1.4) are less crystallineand thus show relatively lower particle amounts compared to resins of lowF/U mole ratios (i.e., 1.2 and 1.0).

TABLE 1 Properties of UF Resin Adhesives With Different F/U Mole Ratios

F/U mole ratio Non-volatile solid content (%) Viscosity (mPa · s) Gel time (s)*

1.6 55.4� 0.1 385 261.4 58.5� 0.5 342 431.2 59.6� 0.2 275 731.0 62.2� 0.2 205 80

*Measured at 3% NH4Cl.

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In cured UF resin adhesives with higher F/U mole ratios, sphericalparticles had relatively uniform sizes and shapes and were observed lessfrequently [Figs. 1(a) and 1(b)]. By contrast, cured UF resin adhesives withlow F/U mole ratios contained higher amounts of spherical particles, alsowith a wider size distribution [Figs. 1(c) and 1(d)]. These results indicatethat the F/U mole ratio has a great impact on the diameter and amountof spherical particles found within a UF resin: the particles tend to bemore abundant, more clearly visible, and more variable in size as theF/U mole ratio decreases. These results are compatible with previousreports [17] that crystallites in cured UF resin adhesives are more closelypacked with each other when the F/U mole ratio decreases. In addition,spherical structures could be arranged in a very ordered way within resinsof low F/U mole ratios, contributing to their greater crystallinity. Thesespherical particle structures are deemed responsible for the formation ofcrystalline domains that provide a greater resistance to the hydrolytic degra-dation of cured UF resin adhesives. The spherical particles appear to formtightly packed crystalline domains that prevent hydrolytic degradation inthe presence of water [17].

FIGURE 1 Typical TEM micrographs of cured UF resin adhesives with 0.1% NH4Cl anddifferent F/U mole ratios. (a) 1.6 F/U mole ratio, (b) 1.4 F/U mole ratio, (c) 1.2 F/U mole ratio,and (d) 1.0 F/U mole ratio. Arrowheads and full arrows indicate spherical particles and woodcell walls, respectively.

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Early studies proposed that the formation of the spherical structurescould be linked to the colloidal character of the UF adhesives[33,34,36,40,41]. However, our recent work proved that various types ofcrystals, such as spherical, globular, or nodular structures, were present inclose proximity of the bond line of cured UF adhesives [29]. Thus, thespherical particles represent one of various different shapes of crystalstructures that had been transversely cut in the ultrathin sectioning. Thisresult provides further important evidence that the spherical particles notonly participate in the main mechanism of crystal growth [33,47], but alsoserve as nucleation sites for the crystallization process [29].

We previously reported that the X-ray diffractograms of cured UFresin adhesives with low F/U mole ratio were quite similar to those of di-hydroxymethyl ureas, thus suggesting that the crystalline structures couldbe composed of di-hydroxymethyl ureas [12]. However, the chemical natureof the spherical particles deserves further investigation.

Since the hardener level influences the reactivity of the resin, weemployed TEM analysis to investigate the micro-morphological features ofresins cured with two different hardener levels, corresponding to the twoextreme F/U mole ratios (1.6 and 1.0) considered in this work. The TEMimages are shown in Fig. 2.

As expected, the cured UF resin adhesives with 1.6 F/U mole ratio [Fig. 2(a)] showed a smaller amount of spherical particles and a less variation intheir size, compared to the resin with F/U¼ 1.0 [Fig. 2(c)]. The 1.6 F/U resin[Fig. 2(b)] also displayed a lower particle amount compared to F/U mole ratioof 1.0 [Fig. 2(d)] when the hardener level increased from 0.1% to 3%.

A poor color contrast appears to affect the spherical particles associatedto the resin with 1.6 F/U mole ratio. As described before, this could reflect thelower crystallinity of high F/U resins. Another reason could be the differentreactivity of resins with high and low F/U mole ratio. The resin with 1.6 F/U mole ratio is more reactive and cures faster than that with 1.0 F/U at thesame hardener level, which consequently results in a tighter and morebranched network structure, leading to the formation of smaller and lessabundant spherical particles in the cured resin. In addition, the NH4Cl addedto the resin releases chloride, which subsequently reacts with free formalde-hyde to produce hydrochloric acid, hexamethylenetetramine, and water, asshown in Equation (1). The more NH4Cl is added, the more hydrochloric acidis produced, accelerating the curing process of the UF resin adhesives.

4NH4Clþ 6CH2O ! 4HClþ CH2ð Þ6N4 þ 6H2O ð1Þ

In order to quantitatively compare the properties of the spherical particles,their measured diameter and amounts are presented in Figs. 3 and 4, respect-ively. The particle diameter increases with a decrease in the F/U mole ratio(Fig. 3), which is fully consistent with the micro-morphological observations

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by TEM, as shown in Fig. 1. In fact, the particle diameter increases from about229� 33 to 331� 57 nm when the F/U mole ratio decreases from 1.6 to 1.0.This result is compatible with previous data on the crystallinity of cured UFresin adhesives based on WAXD [17], and also confirms that the particlediameter is closely related to the crystallinity of the cured UF resin adhesives.In other words, it is reasonable to conclude that particles with larger diametercontribute to the greater crystallinity of cured UF resins with low F/U moleratio.

By contrast, the spherical particle diameter decreases when thehardener level increases from 0.1% to 3% (Fig. 3). In particular, the particlediameter decreases from 229� 33 nm to about 154� 28 nm for F/U 1.6 andfrom 331� 57 to 232� 36 nm for F/U 1.0, respectively.

Figure 4 illustrates the number of spherical particles per unit area as afunction of F/U mole ratios and hardener levels. The number of particlesincreased from 3.4� 1.7/100 to 24.0� 3.8/100 µm2 as the F/U mole ratiodecreased from 1.6 to 1.0. A rapid increase in the particle number, from5.7� 1.9/100 to 24.6� 3.3/100 µm2, was observed when the F/U mole ratio

FIGURE 2 TEM micrographs of cured UF resin adhesives with different F/U mole ratios andhardener levels. (a) 1.6 F/U mole ratio and 0.1% NH4Cl, (b) 1.6 F/U mole ratio and 3% NH4Cl,(c) 1.0 F/U mole ratio and 0.1% NH4Cl, and (d) 1.0 F/U mole ratio and 3% NH4Cl.

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decreased from 1.4 to 1.2. These changes are similar to the trend of thecrystallinity reported in a previous work [17]. In other words, the numberof spherical particles is proportional to the crystallinity of the cured UF resinadhesives. Thus, it seems reasonable to conclude that, besides the particlediameter, the number of particles also contributes to the greater crystallinityof cured UF resin adhesives with low F/U mole ratio.

FIGURE 4 Number of spherical particles in cured UF resin adhesives as a function of F/Umole ratio and hardener level.

FIGURE 3 Diameter of spherical particles contained within cured UF resin adhesives as afunction of F/U mole ratio and hardener level.

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When the hardener level increased from 0.1% to 3%, however, thenumber of spherical particles per unit area decreased to 3.0� 1.6/100 µm2

for the 1.6 F/U resin and to 15.7� 3.0/100 µm2 for the 1.0 F/U resin, respect-ively. Again, this result mirrors the changes in particle diameter shown inFig. 3. This effect might be ascribed to the lower crystallinity of UF resinadhesives cured at higher hardener level (i.e., 3%). Based on Equation (1),higher hardener levels produce more hydrochloric acid, which acceleratesthe curing of the resin. A greater cross-linking in the fast-cured resin corre-sponds to a more branched network structure, which in turn results in alower crystallinity.

UF resin adhesives belong to the class of formaldehyde-based thermo-setting polymers. In general, it is assumed that thermosetting polymers areamorphous and contain no crystal structures, because of the formation ofcross-linked network structures upon curing, clustering, aggregating or evenmicro-domain which exhibits inhomogeneity, such as in epoxy and PU[18–20]. Although major structural units of semicrystalline polymer are theplatelet-like crystallites, or lamella, and the dominant feature of meltcrystallized specimens is the spherulite [48], however, in this work wedemonstrated the presence of crystalline structures in cured UF resin adhe-sives [12,17,29]. This issue has been extensively discussed in several studiespublished in the mid-1980s [35,40,41,45,47–49], which indicated that thecolloidal character of liquid UF resin adhesives is responsible for the crystal-line structure. In a comparison of chemistry between protein and UF resin,Dunker et al. [47] also reported that UF resin contained colloidal regionsof semicrystalline nature and ascribed the origin of the crystal structure toa high degree of order due to hydrogen bonding. They also mentionedanother possibility that the crystalline regions could have arisen from thecrystallization of some minority components of UF resin, like urons. How-ever, those studies were unable to examine the micro-morphological fea-tures of the cured UF resin adhesives, because the resins were too brittleto obtain ultrathin sections suitable for TEM observation. We solved thebrittleness issue by impregnating the liquid UF resin adhesives into softwood tissues, acting like capillaries holding the cured UF resin adhesivesinside [44].

The work presented here is the first extensive TEM-based micro-morphological characterization of cured UF resin adhesives prepared withdifferent F/U mole ratios and hardener levels. Based on the consistent trendsobserved in the properties of the spherical particles as a function of F/U moleratio and hardener level, we conclude that spherical structures formed in UFresin adhesives cured within wood cell lumen are responsible for theobserved crystallinity. Additional investigations using other methods, likeelectron diffraction on the ultrathin sections examined by TEM, will helpus to understand further details about the orientation and nature of thecrystalline structures in cured UF resin adhesives.

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4. CONCLUSIONS

The present article reports a comprehensive TEM characterization of micro-morphological features of cured UF resin adhesives prepared with differentF/U mole ratios and hardener levels. TEM observations highlighted the pres-ence of spherical particles in the resins, whose measured diameter andamount per unit area increased with a decrease in the F/U mole ratio, anddecreased with an increase in the hardener level. These results show thatthe spherical particles are closely related to, and presumably responsiblefor, the crystallinity of cured UF resin adhesives. The TEM observations alsoindicated that the spherical particles were either part of crystals or served asnucleation sites for the crystalline structures, something like a spherulite, andprovided supporting evidences of the existence of crystalline structures.These results also indicate that crystalline structures were inherently presentin the cured UF resin adhesives, and particularly in low F/U mole ratio ones,even though these resins are normally classified as amorphous and cross-linked thermosetting polymers.

FUNDING

This work was supported by the This work was supported by the BasicScience Research Program through the National Research Foundation(NRF) of Korea funded by the Ministry of Education, Science and Technology(2011-0022112).

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