Micro-morphological Features of Liquid Urea-Formaldehyde Resins during Curing Process at Different Levels of Hardener and Curing Time Assessed by Transmission Electron Microscopy

(1)

http://dx.doi.org/10.14518/crals.2014.32.3.019 Print ISSN 2287-271X Original Article

Micro-morphological Features of Liquid Urea-Formaldehyde Resins during Curing Process at Different Levels of Hardener and Curing Time Assessed by Transmission Electron Microscopy

Arif Nuryawan and Byung-Dae Park*

Department of Wood and Paper Sciences, Kyungpook National University, Daegu 702-701, Korea

Received: June 10 2014 / Revised: June 30 2014 / Accepted: July 9 2014

Abstract This study used transmission electron microscopy (TEM) to investigate the micro-morphological features of two formaldehyde to urea (F/U) mole ratio liquid urea-formaldehyde (UF) resins with three hardener levels as a function of the curing time. The micro-morphological features of the liquid UF resins were characterized after different curing times. As a result, the TEM examination revealed the presence of globular/nodular structures in both liquid UF resins, while spherical particles were only visible in the low F/U mole ratio resins. The high F/U mole ratio liquid UF resins also showed extensive particle coalescence after adding the hardener, along with the appearance of complex filamentous networks. When the resins were cured with a higher amount of hardener and longer curing time, the spherical particles disappeared. For the low mole UF resins, the particles tended to coalesce with a higher amount of hardener and longer curing time, although discrete spherical particles were still observed in some regions. This is the first report on the distinct features of the crystal structures in low F/U mole ratio UF resins cured with 5%

hardener and after 0.5 h of curing time. In conclusion, the present results indicate that the crystal structures of low F/U mole ratio UF resins are formed during the curing process.

Keywords: liquid urea-formaldehyde resin, curing condition, micro-morphology, transmission electron microscopy

Introduction

Since the commercialization of urea-formaldehyde (UF) resins in 1931, UF resins have prevailed as thermosetting wood adhesives as the raw materials (urea and formalin) are inexpensive and easy to handle for mass production (Dunky, 1996). In addition, UF resins are colorless and form stable bonds in the finished products.

Thus, due to these advantages, UF resins have been extensively applied in wood-based composite products. One of the main parameters affecting the properties of UF resins is the formaldehyde to urea (F/U) mole ratio for the synthesis of UF resin. In general, the higher the F/U mole ratio, the stronger the bond strength. In the 1970’s, UF resins with an F/U mole ratio of 1.6 were applied in the wood-based panel industry (Dunky, 1998), including plywood, particleboard, and fiberboard. However, concerns over the influence of formaldehyde emissions (FE) on human health and resultant FE regulations on national and international standards required a lower F/U mole ratio for UF resin. A comprehensive literature review on the effect of the F/U mole ratio on FE as well as wood based panel properties has already been carried out by Myers (1984), and according to the review, the gel time used as an indication of resin reactivity increases when decreasing the F/U mole ratio. In general, a lower F/U mole ratio causes lower FE from products due to a loss of physical and mechanical properties, particularly thickness swelling after water immersion for 24 h, as well as the internal bonding strength. Furthermore, it has been reported that a close F/

U mole ratio for UF resins produces an almost identical performance in the finished products, leading to the conclusion that the most important factor in the synthesis of UF resin is the F/U mole ratio (Christjanson et al., 2002). Moreover, an F/U mole ratio of 1.0 was reported to give either an acceptable strength requirement or low E1 grade FE results (Pizzi et al., 1994).

To understand the impact of lowering the F/U mole ratio to reduce the FE and improve the bond strength, extensive research

*Corresponding author: Byung-Dae Park Tel: 82-53-950-5797; Fax: 82-53-950-6751 E-mail: byungdae@knu.ac.kr

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.

org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2014 Institute of Agricultural Science and Technology, Kyungpook

National University

(2)

on UF resins with different F/U mole ratios has already been done, not only in the fundamental aspects, such as the chemical structures (Tomita and Hatono, 1978; Chuang and Maciel, 1992;

1994), but also in the application in wood products (Irleand Bolton, 1988; Ebewele et al., 1994; Park et al., 2006). Similarly, many studies have also investigated the properties between liquid and cured UF resins. For example, liquid UF resins have been studies as regards their thermal curing behavior (Dongbin et al., 2006; Popovic et al., 2011), penetration characteristics on wood tissue (Gavrilovi -Grmuša et al., 2010a; 2010b; 2012; Nuryawan et al., 2014), and their morphology in relation to the colloidal behavior in low F/U mole ratio UF resins (Pratt et al., 1985;

Stuligross and Koutsky, 1985; John and Dunker, 1986; Motter 1990; Depres and Pizzi, 2006; Ferra et al., 2010). In contrast, the studies on cured UF resins have mainly focused on the hydrolytic degradation of UF resins (Myers and Koutsky, 1990; Chuang and Maciel, 1994; Tohmura et al., 2000; Park and Jeong, 2011a) and their crystal structures (Park and Jeong, 2011b; 2011c).

To elucidate the crystal features of UF resins, various studies have reported on the crystal structure present in UF resins. For example, John and Dunker (1986) and Motter (1990) stated that the colloidal dispersion of UF resins is partially crystalline.

Furthermore, the current authors provided evidence on the crystallinity inherent within cured UF resins using scanning electron microscopy (SEM) (Park and Jeong, 2011a), atomic force microscopy (AFM) (Park and Jeong, 2011c), small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXD) (Park and Causin, 2013). Crystalline structures have also been reported by the current authors in UF adhesives in contact with wood, and SEM images provided of crystal aggregations and small developing crystals, as well as globular/nodular structures that appear to be the site of emerging crystals (Singh et al., 2014).

Yet, despite evidence of crystal structures in cured UF resins, there is still an information gap as to whether these crystal structures are also formed in the liquid phase of UF resin.

Therefore, this study prepared high and low F/U mole ratio liquid UF resins and then rendered them in a partially cured state by adding different amounts of a hardener. After certain curing times, the liquid UF resins were dissolved in dimethyl sulfoxide (DMSO) as a solvent to understand the micro-morphological features during the process of curing liquid UF resins.

Materials and Methods

Technical grade urea granules (99%) and formalin (37%) were used for the synthesis of the UF resins. Aqueous solutions of both formic acid (20 wt%) and sodium hydroxide (20 wt%) were used to adjust the pH level during and at the end of the UF resin synthesis. An aqueous solution (20%) of ammonium chloride

(NH

₄

Cl) (20 wt%) was used as the hardener.

Preparation of liquid UF resins

UF resins with extremely different F/U mole ratios (1.6 and 1.0) were prepared in the laboratory, following the conventional alkaline-acid two-step reaction with a second addition of urea. The formalin was placed in a glass cooking reactor and the 1

^st

amount of urea added with the stirrer speed kept constant until the temperature reached 40

^o

C. The mixture was then adjusted to pH 7.8~8.0 using aqueous sodium hydroxide and formic acid to control the conditions. The reaction was heated in 90

^o

C for 60 min to allow methylolation reactions, and then adjusted to 80°C. When the temperature reached 83

^o

C, the pH was adjusted to 4.6, and the condensation reactions continued until reaching a target viscosity of JK, which was measured using a bubble viscometer (VG-9100, Gardner-Holdt Bubble Viscometer, USA). Once the condition was attained, the 2

^nd

amount of urea was added. After all the urea had dissolved, the UF resin was cooled to room temperature, and the pH adjusted to 8.0.

Measurement of UF resin properties

About 1 g of liquid UF resin was poured into a disposable aluminum dish and then dried in a convective oven at 105

^o

C for 3 hours. The solid content was determined by measuring the weight of the UF resin before and after drying. An average of three replications is presented. To compare the reactivity of the synthesized UF resins, the gel time was measured using three replications for each UF resin with a different F/U mole ratio based on adding 3 wt% NH

₄

Cl as a hardener in boiled water using a gel time meter (Davis Inotek Instrument, Charlotte, NC). The viscosity of the UF resin adhesives at ambient temperature was measured using a rotational viscometer (DV-II+, Brookfield, USA) with a no.2 spindle at 60 rpm.

Sample preparation for TEM

To observe the curing process of the UF resins, two liquid UF resin samples with different F/U mole ratios (1.6 and 1.0) were cured by adding different amounts of the hardener (0%, 0.1%, 3%, and 5%). These samples were then kept at room temperature for different curing times, such as 0.5 h, 3 h, and 23 h. After the curing periods, a certain amount of each liquid UF resin was diluted in DMSO at a 5% concentration based on the UF resin solid content.

Drops of either the neat or cured liquid UF resins were placed

on a parafilm. A carbon-coated 100 mesh copper grid was then

placed on the drops. After 10 seconds, the grid was removed using

fine forceps, and the excess neat or cured UF resin was blotted

using a filter paper. The grids were then kept in a convection oven

at 50

^o

C for 24 h to remove the water in the sample. The same

procedure was repeated for other samples after 3 h and 23 h to

investigate the influence of the curing time on the micro-

c í

(3)

morphological features of the liquid UF resins.

Examination under TEM

The TEM grids containing either the neat or cured UF resin with different amounts of hardener and after different curing times were kept in a petri dish prior to staining. The specimens were then negatively stained with 2% uranyl acetate for 10 seconds, immediately taken out using forceps, and washed twice with filtered water (water was filtered using a disposable syringe 0.45 µm prior to use). Thereafter, the specimens were viewed using a TEM micrograph (located at Korea Basic Science Institute, Daegu, South Korea) operated at 75 kV. For complementary data, image analysis software (i-solution, Image and Microscope Technology, Vancouver, Canada) was used to measure the particle diameters of the UF resins observed by the TEM micrograph. The average of at least 25 particle measurements is provided.

Results and Discussion

The general properties of the liquid UF resins used in this study are given in Table 1. As the F/U mole ratio decreased, the solid content increased. This may have been due to the adjustment of the final F/U mole ratio of the UF resin adhesive according to the added amount of the second urea. However, the viscosity of the resin adhesive decreased as the F/U mole ratio decreased. As expected, the gelation time increased as the F/U mole ratio decreased. In other words, a decrease in the F/U mole ratio reduced the reactivity of the resin adhesive. This may have been due to the free formaldehyde in the UF resin adhesives, as less free formaldehyde makes the curing condition less acidic. It has already been reported that the amount of free formaldehyde in UF resin decreases with a decrease in the F/U mole ratio (Myers, 1984).

For the micro-morphological features of the high and low F/U mole ratio liquid UF resins, presented in Figure 1, both exhibited globular/nodular structures, although the number and morphology were variable. The high F/U mole ratio UF resins (Figure 1a) showed extensive interconnected globular/nodular structures (arrows), as well as a complex filamentous network, indicating that the 1.6 F/U mole ratio liquid UF resins had a much more branched network, likely due to the greater reactivity of the resin in the initial stage, as confirmed by the gel time shown in Table 1.

Meanwhile, Figure 1b shows the features of the 1.0 F/U mole

ratio UF resins, which exhibited a less branched network structures (arrows) and a number of single spherical structures (arrow heads), indicating that the low F/U mole UF resins contained more or less linear structures and discrete particles, which is also compatible with other published results (Chuang and Maciel, 1992). In previous studies, it has been suggested that the particles may either be a part of the crystal structure (Park and Jeong, 2011a) or the site of crystal structure growth (Singh et al., 2014). Thus, the discrete spherical particles were seemingly formed during the curing process of the UF resin prior to the building of the three dimensional networks.

Figure 2 shows the micro-morphological features of the high F/

U mole ratio liquid UF resins according to the amount of hardener and curing time, and after mixing with the hardener during the early stage (0.5 h). The high mole ratio liquid UF resins exhibited globular/nodular structures, indicating the formation of a globular/

nodular structure during the curing process, as previously reported (Park et al., 2013; Park and Jeong, 2011a). These results also suggest that the globular/nodular structures were correlated with the crystalline formation in the cured UF resins.

Thus, when compared with the low F/U mole ratio UF resins, the high F/U mole ratio UF resins were more extensively cross- linked during the curing process. After a longer curing time (3 h), the globular/nodular structures transformed into a lamellar shape (Figures 2b and h), especially after the longest curing time of 23 h (Figures 2c, f, and i). Therefore, as a result of coalescence, the globular/nodular structures disappeared and became lamellar in shape. In other words, the globular/nodular structures in the high F/U mole ratio UF resins decreased after being exposed to the acidic conditions related to the hardener and an extended curing time. These conditions are also consistent with a greater amount of free formaldehyde in the high F/U mole ratio UF resins. It has already been reported that the amount of free formaldehyde in UF resin increases when increasing the F/U mole ratio (Myers, 1984).

As such, the present results showed a correlation between the presence of globular/nodular structures in the micro-morphology Table 1. Solid content, viscosity, and gel time of liquid UF resins used in

this study F/U mole

ratio

Solids content (%)

Viscosity

(mPa ⋅s) Gel time (s)*

1.6 55.4 385 26

1.0 62.2 205 80

*Measured values at 3% NH

₄

Cl level.

Figure 1. TEM micrographs of liquid UF resin with different F/U mole

ratios: (a) 1.6 F/U mole ratio, and (b) 1.0 F/U mole ratio. Both F/U mole

ratio UF resins showed interconnected globular/nodular structures with a

filamentous network, which was extensive in the high F/U mole ratio

resin, yet less dominant in the low F/U mole ratio resin (arrows). Discrete

spherical particles not connected with the network only appeared in the

low F/U mole ratio resin (arrow heads).

(4)

of the high F/U mole UF resins and their crystallinity.

As presented in Figure 3, the TEM observation of the low F/U mole ratio UF resins after mixing with the hardener revealed the presence of spherical particles with all the amounts of hardener and curing times (Figure 3a~i). Yet, the particles tended to coalesce with a higher amount of hardener and longer curing time.

Figure 3f shows the development of extensive coalescence of the spherical particles in the low F/U mole ratio UF resins when increasing the amount of hardener and curing time. Thus, the TEM results provide supporting evidence of the crystalline characteristics of the low F/U mole ratio UF resins. Our previous X-ray study also confirmed the crystalline characteristics of UF resins with different mole ratios (Park and Jeong, 2011b; Park and Causin, 2013). Thus, the current results confirm the inherent existence of crystalline phases in UF resins with a 1.0 F/U mole ratio. Therefore, the physical association of emerging crystals with globular/nodular structures suggests that the particles function as nucleation sites for the formation of crystals, which is also consistent with the finding that the colloidal particles of UF resins

represent crystalline or semi-crystalline phases (Pratt et al., 1985).

Furthermore, for the first time, distinct crystal structures were found in the 1.0 F/U mole ratio liquid UF resin after mixing with 5% hardener and 0.5 h of curing time, as shown in Figure 3 g.

While the presence of crystals and crystal aggregates with

morphologically distinct forms is uncommon in liquid resins, this

finding strongly supports the results of previous studies on the

crystal phases in cured UF resins, a thermosetting resin. A recent

study showed that the crystallinity of 1.0 F/U mole ratio UF resins

reached almost 50% (Park and Causin, 2013). Plus, the images of

the UF resins during the early stage of curing after mixing with

the hardener enabled quantitative measurements of the diameters

of the spherical particles in the UF resins. Based on these

measurements, the diameters of the spherical particles were

50.3 ± 5.7 nm. However, in previous studies using SEM, 1.0 mole

UF resins have been reported to consist of relatively large globular

particles (148.1-704.0 nm size range) and smaller substructures

(28.0-39.6 nm size range) (Park et al., 2013). Therefore, this

suggests that the spherical particles observed in this study were

Figure 2. TEM micrographs of 1.6 F/U mole ratio liquid UF resins with different amounts of hardener and curing times: 0.5h, 3 h, and 23 h. The

presence of globular/nodular structures (arrows) was visible with 0.1% hardener, while the filamentous networks became more extensive with a longer

curing time (a~c). When increasing the amount of hardener up to 3% (d~f), globular structures were still observed during the early stage and 0.5 h after

adding the hardener (d), yet disappeared after a longer curing time (3 h and 23 h) (e~f). The globular structures were transformed into lamellar

structures. The globular structures tended to coalescence with a higher amount of hardener up to 5% (g~h), and concomitant with a longer curing time

they became lamellar structures (i).

(5)

within the substructure nano-architecture of the 1.0 F/U mole ratio UF resins.

Conclusions

This study identified the presence of globular/ nodular structures in both high and low F/U mole ratio liquid UF resins. When compared with the low F/U mole ratio UF resins, the high F/U mole ratio UF resins were more interconnected and exhibited more branched globular/nodular structures. Meanwhile, the low F/

U mole ratio UF resins revealed a more single spherical structure, indicating the presence of linear structures. When increasing the amount of hardener and curing time, the high F/U mole ratio resins showed a more branched network and filamentous network structures. The micro-morphological changes in the liquid UF resins were found to depend on the amount of hardener and curing time. When increasing the amount of hardener and curing time, the network quickly changed to a gel, indicating that the amount

of hardener and curing time had a significant impact on the micro- morphological features of the liquid UF resins. This study also confirmed the micro-morphological features of the crystal structures. As a result, the crystallinity of the 1.0 F/U mole ratio liquid UF resins was not only observed in the globular/nodular structures, but also found in the spherical structures. This is the first report of a distinctive crystal structure in a 1.0 F/U mole ratio liquid UF resin with 5% hardener and a 0.5 h curing time. Therefore, the micro-morphological features observed by TEM revealed that the 1.0 F/U mole UF resin formed crystalline structures during the curing process in the presence of the hardener, indicating that distinctive crystals are inherent in the liquid state of UF resins during the early stage of curing in the presence of a hardener.

Acknowledgments

This work was supported by the Basic Science Research Program

through the National Research Foundation (NRF) of Korea

Figure 3. TEM micrographs of 1.0 low mole UF resins with different amounts of hardener and curing times: 0.5 h, 3 h, and 23 h. With a low amount of

hardener (0.1%) (a~c), spherical particles (arrows) were observed, although coalescence became extensive with a longer curing time. When increasing

the amount of hardener up to 3% (d~f), globular structures were still observed, yet coalescence became extensive after a longer curing time (f). With a

higher amount of hardener up to 5% (g), masses of crystals were observed (h), indicating that the crystal structures resided elsewhere, and distinct

crystals and globular/nodular/spherical structures are both inherent. The globular structures tended to coalescence when increasing the curing time (g~i).

(6)

funded by the Korean Ministry of Education, Science and Technology (2011-0022112).

References

Christjanson P, Siimer K, Pehk T, Lasn I (2002) Structural changes in urea- formaldehyde resins during storage. Holz als Roh- und Werkstoff 60:

379-384.

Chuang IS, Maciel GE (1992) Carbon-13 CP/MAS NMR study of the structural dependence of urea-formaldehyde resins on formaldehyde- to-urea molar ratios at different urea concentrations and pH values.

Macromolecules 25: 3204-3226 .

Chuang IS, Maciel GE (1994) NMR study of the stabilities of urea- formaldehyde resin components toward hydrolytic treatments. J Appl Polym Sci 52: 1637-1651.

Despres A, Pizzi A (2006) Colloidal aggregation of aminoplastic polycondensation resins: urea-formaldehyde versus melamine- formaldehyde and melamine-urea-formaldehyde resins. J Appl Polym Sci 100: 1406-1412.

Dongbin F, Jianzhang L, An M (2006) Curing characteristics of low molar ratio urea-formaldehyde resins. Journal of Adhesion and Interface 7:

45-52.

Dunky M (1996) Urea-formaldehyde glue resin. In: Polymeric Materials Encyclopedia, ed Salamone JC (CRC Press, Florida), pp. 8502-8511.

Dunky M (1998) Urea-formaldehyde (UF) adhesive resins for wood. Int J Adhes Adhes 18: 95-107.

Ebewele RO, River BH, Myers GE (1994) Behavior of amine-modified urea-formaldehyde-bonded wood joints at low formaldehyde/urea molar ratios. J Appl Polym Sci 52: 689-700.

Ferra JMM, Mendes AM, Costa MRN, Carvalho LH, Magalhães FD (2010) A study on the colloidal nature of urea-formaldehyde resins and its relation with adhesive performance. J Appl Polym Sci 118:

1956-1968.

Gavrilovi -Grmuša I, Dunky M, Miljkovi J, Diporovi -Mom ilovi M (2010a) Radial penetration of urea-formaldehyde adhesive resins into beech (Fagus moesiaca). J Adhes Sci Technol 24: 1753-1768.

Gavrilovi -Grmuša I, Miljkovi J, Ðiporovi -Mom ilovi M (2010b) Influence of the degree of condensation on the radial penetration of urea-formaldehyde adhesives into Silver Fir (Abies alba, Mill.) wood tissue. J Adhes Sci Technol 24: 1437-1453.

Gavrilovi -Grmuša I, Dunky M, Miljkovi J, Ðiporovi -Mom ilovi M (2012) Influence of the viscosity of UF resins on the radial and tangential penetration into poplar wood and on the shear strength of adhesive joints. Holzforchung 66: 849-856.

Irle MA, Bolton AJ (1988) Physical aspects of wood adhesive bond formation with formaldehyde based adhesives. Part II. Binder physical properties and particleboard durability. Holzforschung 42: 53-58.

John WE, Dunker AK (1986) Urea-formaldehyde resin. In: Formaldehyde release from wood products, eds Meyer B, Andrew BAK, Reinhardt RM (The American Chemical Society), pp. 76-86.

Motter WK (1990) The formation of the colloidal phase in low mole ratio urea-formaldehyde resins. PhD Dissertation. Department of Mechanical and Materials Engineering. Washington State University.

Myers GE (1984) How mole ratio of UF resin affects formaldehyde emission and other properties: a literature critique. Forest Prod J 34:

35-41.

Myers GE, Koutsky JA (1990) Formaldehyde liberation and cure behavior of urea-formaldehyde resins. Holzforschung 44: 117-126.

Nuryawan A, Park BD, Singh AP (2014) Penetration of urea-formaldehyde resins with different formaldehyde/urea mole ratios into softwood tissues. Wood Sci Technol 48: 889-902.

Park BD, Kang EC, Park JY (2006) Effects of formaldehyde to urea mole ratio on thermal curing behavior of urea-formaldehyde resin and properties of particleboard. J Appl Polym Sci 101: 1787-1792.

Park BD, Jeong HW (2011a) Influence on hydrolytic degradation on the morphology of cured urea-formaldehyde resins of different formaldehyde/

urea mole ratios. J Korean Wood Sci Technol 39: 179-186.

Park BD, Jeong HW (2011b) Hydrolytic stability and crystallinity of cured urea-formaldehyde resin adhesives with different formaldehyde/urea mole ratios. Int J Adhes Adhes 31: 524-529.

Park BD, Jeong HW (2011c) Effects of acid hydrolysis on microstructure of cured urea-formaldehyde resins using atomic force microscopy. J Appl Polym Sci 122: 3255-3262.

Park BD, Causin V (2013) Crystallinity and domain size of cured urea- formaldehyde resin adhesives with different formaldehyde/urea mole ratios. Eur Polym J 49: 532-537.

Park BD, Singh AP, Nuryawan A, Hwang K (2013) MRT Letter: High resolution SEM imaging of nano-architecture of cured urea- formaldehyde resin using plasma coating of osmium. Microscopy Research and Technique 76: 1108-1111.

Pizzi A, Lipschitz L, Valenzuela J (1994) Theory and practice of the preparation of low formaldehyde emission UF adhesives. Holzforchung 48: 254-261.

Popovi M, Budinski-Simendi J, Jovi i M, Mursics J, Ðiporovi - Mom ilovi M, Pavli evi J, Risti I (2011) Curing kinetics of two commercial urea-formaldehyde adhesives studied by isoconversional method. Hemijska Industrija 65: 717-726.

Pratt TJ, Johns WE, RammonRM, Plagemann WL (1985) A novel concept on the structure of cured urea-formaldehyde resin. J Adhesion 17:

275-295.

Singh AP, Causin V, Nuryawan A, Park BD (2014) Morphological, chemical and crystalline features of urea-formaldehyde resin cured in contact with wood. Eur Polym J 56: 185-193.

Stuligross J, Koutsky JA (1985) A morphological study of urea- formaldehyde resins. J Adhesion 18: 281-299.

Tohmura SI, Hse CY, Higuchi M (2000) Formaldehyde emission and high- temperature stability of cured urea-formaldehyde resins. J Wood Sci 46: 303-309.

Tomita B, Hatono S (1978) Urea-formaldehyde resins. III. Constitutional characterization by

¹³

C fourier transform NMR spectroscopy. J Polymer Sci Polymer Chem Ed 16: 2509-2525.