In Situ Polymerization of Phenolic Methylolurea in Cell Wall and

Article pubs.acs.org/IECR

In Situ Polymerization of Phenolic Methylolurea in Cell Wall and Induction of Pulse−Pressure Impregnation on Green Wood Heyu Chen, Xinwei Miao, Zifeng Feng, and Junwen Pu* College of Material Science and Technology, Beijing Forestry University, No. 35 Tsinghua East Road, Beijing 100083, China ABSTRACT: Impregnation of the cell wall with various chemicals continues to attract interest. For most studies, the method of impregnation is the vacuum-pressure process which is limited by the specimens’ size in thickness and the requirement for low moisture content. In the present study, we impregnated fast-growing poplar (Populus euramericana) green wood by pulse− pressure at 0.7−0.8 MPa for 50 min with phenolic methylolurea, and then the modifier in situ polymerized within the wood cell wall by kiln drying. The microscopy analysis showed that the pulse−pressure impregnation method could be feasible because of the presence of a wide-bore capillary column system in the poplar xylem. As a result, the chemical treatment reduced the hygroscopicity and increased the dimensional stability of wood. Reactions mainly happened within the interfibrillar amorphous region of the cell wall. Among the reactions were etherification and esterification since the covalent bonds formed between modifier methylol groups, wood hydroxyl, and acetic acid derived from hemicelluloses.

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INTRODUCTION Poplar (Populus euramericana) is a fast-growing tree with short cultivation time, its early and late wood display little variation in properties. However, there are also disadvantages for poplar such as its low density and low dimensional stability. The cell wall is largely made up by cellulose and hemicelluloses, and in the amorphous region the ample hydroxyl groups lead to the hygroscopic nature whereby wood cell walls shrink or swell upon changes in humidity.1−3 Considering that nonrenewable resources are decreasing, new applications are sought for trees.4−6 Impregnation modification, in general, is a promising approach aimed at wood durability against fungi and insects, flammability, dimensional stability, strength, reducing moisture sorption, fixation during the densification, and electrochemical performance.7−14 For the past so many years, most of the early study areas in wood impregnation modification are resin treatments. Some of the achievements have been applied in industry such as Impreg and Compreg. 15 Among different types of thermosetting polymers, aqueous phenol formaldehyde (PF) resin is widely used.16 Ohmae and his co-workers treated wood with a low-molecular weight PF resin in water and obtained an antiswelling efficiency (ASE) value as high as 74%.17 Recently, for the purpose of reducing the cost and improving the curing behavior of the resins, phenol−urea−formaldehyde (PUF) has been applied in practice to form co-condensed (PUF) resins.18−21 In most studies, the synthesis temperature of PUF resin will be up to 90 °C. However, the behavior of wood treated with PUF modifier which is synthesized at relatively low temperature is still unknown so far. Also, to understand in more detail about the reactions involved will be an important factor in improving the competitiveness of renewable materials usage. Furthermore, the most common method for impregnation is a vacuum-pressure process which is effectively applied on both softwood and hardwood. However, compared with that of longitudinal, wood permeability is quite weak on the tangential. Hence, high pressure is necessary to conduct the wood impregnation process, and that is the reason for device © 2014 American Chemical Society

manufacturers having to develop ever more powerful equipment which could provide even higher pressure for impregnation. Even then, the vacuum-pressure impregnation method is still limited by the size of the thickness of the specimen. In addition, the specimens’ requirement for relatively low moisture content will lead to a preliminary drying which costs a big amount of energy and time. Seeking a way to treat green wood directly will be of interest. In former studies, our group has conducted a series of experiments on green wood impregnation. There is a considerable body of evidence showing that chemical modification with certain reagents leads to an improvement in the mechanical properties of the modified wood, although the mechanism by which this happens is not fully understood.22,23 The concept of the present study includes the preparation of modifier by mixing phenol, urea, and formaldehyde. Then, poplar green wood will be impregnated with the modifier by pulse−pressure (0.7−0.8 MPa) for 50 min. After soaking, the modifier situated now in the cell wall should be cured by heating (in situ polymerization). The expectation is that this pulse−pressure impregnation method could be feasible on fastgrowing poplar green logs, and the properties of treated wood will be more favorable than those of brittle wood after the UFresin treatment or of costly wood after the PF-resin treatment. Thus, the analysis of the modifier flow behavior in the xylem anatomical structure and the characteristics of modified wood will be in focus.

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MATERIALS AND METHODS

Materials. The poplar (Populus euramericana cv. “I-214”) was collected in Beijing, China. The initial moisture content

Received: Revised: Accepted: Published: 9721

February 13, 2014 May 9, 2014 May 14, 2014 May 14, 2014 dx.doi.org/10.1021/ie5006349 | Ind. Eng. Chem. Res. 2014, 53, 9721−9727

Industrial & Engineering Chemistry Research

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Scanning Electron Microscope (SEM) study. The morphological features of untreated and treated wood samples were observed using the scanning electron microscope. The oven-dried wood samples were cut into tangential slices by using a sharp knife. After being gold coated, the observation was carried out under the S-3400N (Hitachi, Tokyo, Japan) SEM. Contact Angle Analysis. The water repellency of wood surfaces before and after treatment was analyzed through water contact angle measurements using a FTA 200 dynamics (First Ten Ångstroms, Portsmouth, VA, U.S.A.) contact angle analyzer. Sessile droplets of distilled water (70 μL) were deposited on the surface of wood samples. The contact angles between each droplet and the sample surface were measured (on both, the left and right side, and the mean contact angles were calculated) every 15 s. Measurements were repeated on 10 samples. Hygroscopicity. Rectangular specimens were prepared with dimensions of 20 (l) × 20 (t) × 20 (r) mm3. The specimens were weighed after drying in an oven at 105 °C until the weight change was less than 0.02 g after 2 h. Then the specimens were immersed in distilled water. After immersion, the excess water on the surface was removed by a soft cloth, and the weights and volumes of the specimens were taken during 72 h; the water uptake was given as weight gain/volume percentage. Measurements were repeated on 10 samples. Weight Percent Gain (WPG). Weight percent gain after pulse−pressure impregnation was calculated according to the formula below; measurements were repeated on 10 samples:

(MCs) of the wood ranged from 70 to 80% before impregnation. Synthesis of Phenolic Methylolurea. A reaction vessel was charged with solid urea, 37% formaldehyde, and 25% ammonia. The reaction mixture was stirred and kept at 30 °C for 2 h. At the same time, phenol and 37% formaldehyde were charged into another reaction vessel, in which only two-thirds of the total amount of formaldehyde was charged at this time. A calculated amount of 40% sodium hydroxide was added slowly into the vessel for 8−10 min with constant mechanical stirring, and reaction temperature was maintained at 45 °C for 1 h. After that, the temperature was gradually lowered to 30 °C within 30 min and maintained at 30 °C for 30 min more. At last, two reactive prepolymers were intermixed, and the second part of 37% formaldehyde was then added into the reactor. The temperature was kept at 30 °C for another 1 h. The final product was obtained with pH value of 9 ± 0.5, the molar ratio of formaldehyde to urea and phenol was 2.8/1.0/1.0, and the viscosity was 18 mPa·s. All chemicals were of analytical reagent grade and purchased from the Xilong Co., China. Pulse-Pressure Impregnation and Kiln Drying. The poplar logs for impregnation were chosen around 200 mm in diameter and then sawn into 1000 mm in length. Figure 1

WPG(%) = (m2 − m1)/m1 × 100%

(1)

where m1 is the oven-dry weight of untreated wood specimens and m2 is the oven-dry weight of wood specimens after impregnation. Antiswelling Efficiency (ASE). Rectangular specimens were prepared with dimensions of 20 (l) × 20 (t) × 20 (r) mm3. ASE was measured by the water-soaked method reported by Rowell and Eills.24 Triplicate specimens were run, and the values were averaged. Volumetric swelling coefficients were calculated as

Figure 1. Schematic diagram of pulse−pressure impregnation procedure.

shows the pulse−pressure impregnation as a schematic diagram. First, the fresh poplar log was stabilized on the pulse−pressure machine. Then, the modifier was impregnated from one side of the log by pulse−pressure (0.7−0.8 MPa) which was provided by a pneumatic diaphragm pump. The tree growth orientation was first chosen as the impregnation direction, so that the tree sap and diluted modifier could flow out of the other side of the log (typically 5−10 min after the impregnation process began). After that, the impregnation direction turned around, and the same procedure was performed for another 20 min. Step by step, the tree sap in the xylem was continually replaced by the original modifier. After soaking, the impregnated wood logs were sawn into boards about 50 mm thick (t) × 120 mm wide (r) × 1000 mm length (l) for kiln drying. The highest drying temperature was 135 °C, and the pressure on the timber was 0.2 MPa. Ten specimens were also cut from the boards to determine the formaldehyde emission according to JIS A1460-2003 standard (Building boards determination of formaldehyde emissiondesiccator method). Optical Microscopy Study. Cross and tangential slices with a thickness of 10 μm on xylem were cut by a model 860 sliding microtome (American Optical Co., Buffalo, NY, U.S.A.) and then stained with 1% safranin in alcohol solution for 2 h. In the next stage, the slices were dehydrated with gradient ethanol, from 50%, 70%, 85%, 95%, 100% into xylene, and then cemented with neutral gum. After that, they were observed under a BH-2 (Olympus, Osaka, Japan) optical microscope.

S(%) = (V2 − V1)/V1 × 100%

(2)

where V2 is the volume of the water-saturated wood specimens and V1 is the volume of the oven-dried wood specimens. The values were obtained by measuring the dimensions of the ovendried and water-saturated wood specimens with a slide caliper. The ASE was calculated from the wet and oven-dried volumes of treated and untreated wood specimens as ASE(%) = (Sc − St)/Sc × 100%

(3)

where Sc is the volumetric swelling coefficient of untreated wood specimens, and St is the volumetric swelling coefficient of the treated wood specimens. Fourier-Transform Infrared Spectroscopy (FT-IR). The KBr technique was applied for the milled specimens (120 mesh size) using Tensor 27 (Bruker, Karlsruhe, Germany) spectrometer (64 scans were accumulated with a resolution of 4 cm−1). X-ray Photoelectron Spectroscopy (XPS). The surface examination of untreated and treated wood was carried out by an X-ray photoelectron (Kratos, Manchester, England) spectrometer. The anode of XPS was Al, and the step was 1000.0 meV. 9722

dx.doi.org/10.1021/ie5006349 | Ind. Eng. Chem. Res. 2014, 53, 9721−9727


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CP/MAS 13C NMR Experiments. Solid State CP/MAS 13C NMR spectra were obtained by the DMX-400 NMR (Bruker, Karlsruhe, Germany) spectrometer. The spectra were recorded with MAS spinning at 7 kHz, with a contact time of 1 ms, a recycling delay of 3 s and a 90° pulse (6.0 μs). Wood powder prepared by a Wiley mill (Thomas Scientific, Swedesboro, NJ, U.S.A.) was used for NMR analysis. X-ray Diffraction (XRD). The crystalline structure of the untreated and treated wood samples was obtained by XRD6000 (Shimadzu, Kyoto, Japan; CuKα radiation with graphite monochromator, 30 kV, and 40 mA). The patterns were obtained between 5° to 40° 2θ with 0.05° steps and scan speed of 2°·min−1. The degree of crystallinity was calculated as the ratio of the intensity differences in the peak positions.

tangentially, intervessel or intervascular bordered pits will form, which particularly are alternate pits (Figure 2c). Where the ends of the vessel elements come in contact with one another, perforation plate will form (Figure 2a). Where vessel elements come in contact with ray cells, simple or bordered pits called vessel-ray pits will form (Figure 2d). Meanwhile, these pits will be in much larger size or shape than the intervessel pits. Also, procumbent ray cells here are homocellular rays which have only one type of ray cell. Fiber pits are generally inconspicuous and may be simple or bordered (Figure 2b). The pit membrane is the thin semiporous remnant of the primary wall; it is a carbohydrate membrane.1 Figure 2h shows the sketches of the pulse−pressure impregantion. When green wood is impregnated with phenolic methylolurea by pulse−pressure, the vessels, which are presented in much larger scale, become infiltrated first. The impregnation will be evenly distributed on the horizontal of the xylem since there is no clear distinction of vessel distribution between earlywood and latewood (diffuse porous wood). Based on the ample alternate pits between vessel elements tangentially and the perforation plate formed at ends of the vessel elements, the modifier can flow fast through the vessel lumens from one to another. However, there still could be a bottom of a particular vessel (Figure 2c). Then, it would be the rays on horizontal that ensure the impregnation to be persistent. The modifier could go across the vessel−ray pits and permeate along the ray cell until reaching the adjacent vessels. By the pit connections within the vessel−ray−vessel, the wide-bore capillary column system in xylem will be established. This whole process is interpreted as the modifier could impregnate through the green wood xylem powered by pulse−pressure in a few minutes. Moreover, except for the dominant flow in vessels, by the ray-fiber pits (Figure 2b), there is minor impregnation in fibers synchronously. According to the SEM study, Figure 2f and g give a view of the anatomical characteristics of untreated and treated wood. The precipitation which occupies the vessel−ray pits supports the hypothesis of pulse−pressure impregnation in the way we proposed. Also, the clearly visible precipitate on the vessel lumen surface formed a layer rather than a bead. It is self-evident that the layer is important because covering more of the surface is the prerequisite for a deeper penetration in the cell wall as well as a stronger bond. In summary, this kind of impregnation of green wood could be feasible with three main mechanisms. In the first case, the foundation is the physical environment of wood (wide-bore capillary column system) in the fast-growing poplar xylem which allows modifier to flow through and fill the vessel lumens. Then in the second case, the modifier should be a lowmolecular weight oligomer with low viscosity. In the third case, the modifier should intimately encounter the wood cell surface, for diffusion and subsequent polymerization to nonleachable material within the cell wall.27 Hygroscopicity. The treated wood surface showed higher contact angles which indicated hydrophobicity was acquired after impregnation modification (Figure 3a). The decreasing rate of contact angles was rapid in the untreated samples and showed that water was absorbed into the samples accompanied by the collapse of the surface structures, whereas the treated wood surface showed strong structures with slight collapse of the surface skin. The absorption test of liquid water attests to the hydrophobicity of treated wood surfaces. The water uptake data are presented in Figure 3b as a function of time. As can be seen, the treated sample surface prohibited liquid water

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RESULTS AND DISCUSSION The aim of the study is to determine if poplar green log could be impregnated with phenolic methylolurea aqueous solution by pulse−pressure and establish in situ polymerization in the structure of wood interfibrillar. A key question is how the water-soluble modifier interacts with the wood components. Although cell wall penetration is a purely diffusion-controlled process, penetrating throughout the wood cell lumens should be the first step. So it will be interesting to determine whether the water insoluble polymer will precipitate throughout the cell wall structure or only in the lumen. Poplar Wood Morphology and Induction of Pulse− Pressure Impregnation. A robust understanding of the interrelationship between the wood morphology and impregnation dynamics can be useful to predict the efficiency of the method impregnation.25,26 It is necessary to define and delimit the wood anatomical structure because there is a significant difference in the quality and quantity between different wood cells. For this reason, the structure of wood will be explained in this section, and the impregnation rationale on xylem will also be discussed. As typical hardwood, the poplar axial or vertical system is composed of various fibrous elements, which are dominated by vessel elements and fibers; the radial or horizontal system is the rays, which are composed of ray parenchyma cells. Poplar is diffuse porous wood; its vessels do not significantly change in size or distribution from earlywood to latewood. In addition, the vessel may occur alone (solitary arrangement) or in pairs or radial multiples of up to three or more vessels in a row (Figure 2e). Where the vessel elements come in contact with each other

Figure 2. Poplar wood morphology. (a) Perforation plate of vessel. (b) Ray-fiber pits. (c) Alternate pits of vessel. (d) Vessel-ray pits. (e) Transverse section of wood. (f) Radial section of untreated wood. (g) Radial section of treated wood. (h) Simplified diagram of pulse− pressure impregnation way in wood cells. 9723



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Figure 3. (a) Contact angle versus time performed with water. (b) Water uptake versus time.

Figure 4. (a) Illustration of the reaction and the functional groups in phenolic methylolurea. (b) FTIR curves of untreated and treated wood.

for outdoor wood composites (0.5 mg/L, grade F3‑star) according to the JIS standard A5908-2003. FT-IR Analysis. According to the former studies,29 the process of phenolic methylolurea we synthesized can be illustrated in a diagram (Figure 4a). First, urea and phenol reacted with formaldehyde to form methylol compounds, respectively. Then, after adding the extra formaldehyde, five typical structure units in the oligomer may form. Our discussion of the reaction mechanism between the modifier and wood components will be based on these basic structures. The FT-IR spectrum of untreated poplar wood was typical for hardwood in general. The broad band at 3400 cm−1 (−OH stretching), 2908 cm−1 (−C−H stretching) and 1738 cm−1 (CO stretching) were prominent; and the fingerprint region was dominated by the bands around 1054 cm−1 due to various polysaccharide vibrations.30,31 It was apparent that the treated wood samples had typical infrared absorption characteristics of amide spectral bands I (1653 cm−1), amide spectral bands II (1588 cm−1), and amide spectral bands III (1242 cm−1).32,33 The spectrum of treated wood illustrated the stretching vibration of functional groups −OH and −NH, indicating that there was a great potential for forming intermolecular hydrogen bonds. These gave rise to a shift of the stretching vibration absorption peak from 3400 to 3320 cm−1. In addition, −NH should dominate in the amide since the absorption peak of −OH and −NH were combined into one wider absorption peak (there was little −NH2 existing because we could not observe two small peaks corresponding to each N−H stretching). This point was also supported by the absorption peak at 758 cm−1 which was the plane bending

penetration that resulting in retarded rate of water uptake in a 72-h duration period. The final water uptake of the treated wood decreased from 31.9% to 10.7%. In addition, the ASE of treated wood is as much as 73.2% (±0.57) at 75.6% (±3.32) WPG. It has been reported that the ASE for PF and MF treated woods have an ASE around 70%, while the ASE of UF resin impregnated wood is 50%.17,28 Phenolic methylolurea treatment showed a positive effect for the wood water-repellent property. The modifier synthesized in the treated wood could not only block the water absorption on the wood surface as shown in the contact angle test but could also protect the cell wall inside, resulting in water persistently being hard to transport into the inner structures. The improved water-repellent property of treated wood was partly due to reactions happening within the cell wall, masking some of the hydroxyl groups. At the same time the entire modifier was not located in the cell wall, there were still parts polymerized and that additionally formed a barrier on the lumen surface (Figure 2g). According to the high ASE value, obviously the bonds formed (either intramolecularly within the modifier network or intermolecularly between the modifier and the cell wall components) were stable to hydrolysis; otherwise, the polymerized modifier would leach out, and the cell wall bulking effect would be lost. Considering all the factors discussed above, this increase in dimensional stability of treated wood appears to be caused both by the cell wall bulking effect and cross-linking reactions between the modifier and cell wall components. The formaldehyde emission of treated wood was 0.479 mg/L (±0.012), which was lower than the required value 9724



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Table 1. Mass Concentration of Each Element for Neat and Grafted Sample Correlated to Deconvolution C1s Obtained by XPS experimental values

decomposition of C1s C1 (%)

C2 (%)

C3 (%) O−C−O

C4 (%)

C−C

C−O

sample

C (%)

O (%)

N (%)

C−H

C−OH

CO

O−CO

bending energy (eV) untreated treated

286.60 66.85 60.85

533.05 31.39 32.35

400.12 0.78 5.4

284.80 34.00 26.25

286.60 54.22 55.19

288.12 9.12 11.16

289.35 2.66 7.4

Figure 5. Decomposition of the C1s signal into its constituent contribution for neat and grafted untreated and treated wood as mentioned in Table 1. (a) Untreated wood. (b) Treated wood.

Figure 6. (a) CP/MAS 13C NMR spectra of untreated and treated wood. (b) XRD curves of untreated and treated wood.

vibration of amine functional groups −NH, instead of the strong wide peaks in 900−650 cm−1 for −NH2. The increased bands at 1161, 1113, and 1054 cm−1 (due to C−O−C asymmetric stretching, O−H association, and C−O stretching of cellulose and hemicellulose) could be a result of the reaction between −NHCH2OH in phenolic methylolurea and the O−H groups of wood.34 XPS Analysis. Table 1 compares the XPS survey spectra from poplar wood before and after treatment. Besides the expected C and O peaks at 286.60 and 533.05 eV, the spectrum from treated samples revealed a high peak around 400.12 eV which can be attributed to nitrogen (N 1s), this could demonstrate the presence of the modifier. The high-resolution C1s peaks for the untreated and treated wood samples are shown in Figure 5, the peaks showed the expected four components generally expressed as C1−C4. In class 1 (C1), carbon atoms are bonded only to carbon and/or hydrogen atoms (C−C/C−H, 284.80 eV). Class 2 (C2) refers to carbon

atoms bonded to a single oxygen, except for carboxyl oxygen (C−O, 286.60 eV). In class 3 (C3), carbon atoms are bonded to two noncarboxyl oxygens or to a single carboxyl oxygen (O− C−O or CO, 288.12 eV) while in class 4 (C4), they are bonded to a carboxyl and a noncarboxyl oxygen (OC−O, 289.35 eV). Chemical analysis of wood components shows that there is good stability up to a temperature of 100 °C with 48 h heating.35 Above this temperature, the holocellulose content decreases, which is associated with loss of hemicelluloses, since the cellulose content remains unchanged up to 150 °C (changes in DP can occur, however). The heating of wood in the presence of water or steam results in the accelerated formation of organic acids (primarily acetic acid) that catalyze the hydrolysis of hemicelluloses.36 As has been noted above, the production of cellular breakdown products derived from the more labile hemicelluloses attaching to the cell wall polymers will lead to a greater significance. It resulted in the reaction 9725



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difference between the maximum at 16° 2θ and the minimum at 19.1° 2θ was leveled, while the maximum diffraction peak (002) near 22.5° 2θ was enhanced, and the diffraction peak (040) near 35° 2θ was nearly not changed. The XRD spectrum was interpreted as the crystal structure of cellulose not being noticeably affected by the phenolic methylolurea treatment. Also, the crystallinity indices of treated wood (39.7%) and untreated wood (37.2%) were very similar. Generally, hemicelluloses are much more unstable in the course of the high temperature compared with other macromolecular components in wood; this will lead to a slight increase in the crystallinity of wood samples.36 Finally, the observation that the difference between the maximum at 16° 2θ and the minimum at 19.1° 2θ was leveled indicated that amorphous cellulose was changed, which helped confirm that in situ polymerization primarily happened in the wood amorphous region.

happening particularly between acetic acid derived from the hemicelluloses and the ample methylol from the phenolic methylolurea forming ester bonds, which could be demonstrated by C4 (OC−O) content increasing significantly from 2.66% to 7.40%. In addition, C2 (C−O) content increased due to forming C−O−C bonds; as well C3 (CO) content increased due to the urea constitutional unit [N−(CO)−N]. The formation of ester bonds masked some hydroxyls which made the whole system more stable and hydrophobic. CP/MAS 13C NMR Spectra and XRD Analysis. The chemical changes of wood specimens treated with phenolic methylolurea were examined by the solid state CP/MAS 13C NMR spectroscopy and XRD. Figure 6a shows the 13C NMR spectra of untreated control and treated wood. In the spectrum of untreated wood, all noticeable signals between 55 and 105 ppm are specific for the carbons of carbohydrates. The signals at 104.8 (C-1 of cellulose and xylan), 88.2 and 83.7 (C-4 of crystal cellulose and amorphous cellulose), 74.2 and 72.1 (C-2, C-3 and C-5 of cellulose; C-2, C-3 and C-4 of xylan), 64.6 and 62.9 (C-6 of crystal cellulose and amorphous cellulose), and 56.0 (methoxy group in lignin) are observed.37,38 As for the treated wood, the relative intensity of the absorption band at 62.9 ppm, which has been assigned to the absorption of C-6 position, disappeared after modification, indicated that the esterification and etherification of cellulose occurred mainly at the C-6 position in the amorphous region. In addition, the intensity of the overlapping signals at 74.2 and 72.1 ppm attributed to C-2, C-3, and C-5 of cellulose and C-2, C-3, and C-4 of xylan decreased after modification, and that at 104.6 ppm arising from C-1 of cellulose and xylan also decreased. The decreased intensity indicated that the reaction may also occur at other position of cellulose and xylan. The region between 105 to 160 ppm is specific for aromatic carbons of lignin. The peak at 171.4 ppm was resulted from carbons of −COOCH3, and −COOR in different kinds of lignin; while the ones at 152.8 and 134.0 ppm were resulted from the C4, C3, C5, C1 of phenolic ring carbons.39,40 Deacetylation of hemicelluloses in treated wood was detected as an intensity decrease at 171.4 ppm (carboxyl group signal) and 21.6 ppm (methyl group signal) even though the changes appeared to be minor.35 Apart from that, there were obvious changes in the region of 105−160 ppm due to the impregnation of phenolic methylolurea. The chemical shifts at 160.1 and 153.6 ppm in the spectrum of the treated wood were assigned to carbonyl in the urea unit and phenoxy carbon of the phenyl ring, respectively. The strong absorption peak at 129.7 ppm was the result from the substituted and unsubstituted aromatic carbons.40 Peaks at 116.2 and 47.1 ppm were thought to be due to the resonance dislocation caused by the different attached positions of methylene bridges to phenolic rings and urea units, respectively. All discussions above illustrated that, besides the reactions between the modifier and wood components, there was still a considerable amount of self-polymerized phenolic methylolurea structures maintained in wood after the drying process. The XRD analysis confirmed some results above (Figure 6b). The spectrum of untreated wood showed a maximum at 16° 2θ (cellulose crystal diffraction, 101) and it was extended to a minimum at 19.1° 2θ, a region, which was characteristic for the amorphous wood. The most significant diffraction peak (002) of the cellulose crystal was near 22.5° 2θ, while a small diffraction peak (040) occurred near 35° 2θ.41 However, the diffraction profile changed a little for the treated wood. The

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CONCLUSIONS We established a new method to directly impregnate green wood without preliminary drying. Our principle is that the impregnation is performed as the mobile low-molecular weight monomers impregnate throughout the wide-bore capillary column system in the fast growing poplar xylem and then the polymerization will be induced in the wood in situ. The swollen cell walls filled with nonleachable polymers enhanced the dimensional stability of wood. Furthermore, the results from spectral analysis confirmed the reactions happened in the amorphous carbohydrates. Wood and the modifier polymerized within the interfibrillar region of the cell wall due to the formation of covalent bonds between the methylol group, the wood hydroxyl, and acetic acid from hemicelluloses (esterification and etherification). Meanwhile, in treated wood spectrum, some of the connection status, such as the existence of the phenolic ring, the phenolic hydroxyl, methylene bridges, and methylol linkage, still dominated after polymerization. This indicated that reactions also occurred within the modifier itself which can be present as a thin layer observed in SEM of the cell walls. Moreover, the hydrogen bond may form between amide in the urea unit and phenolic hydroxyl in the phenol unit, which could make the whole system more stable and hydrophobic. Future work in this area could include evaluating the UV stability and biological deterioration of treated wood. Meanwhile, more tree species could be investigated. Theoretically, hardwoods which possess open vessels are the potential species to be treated by pulse−pressure impregnation. Some initial ideas could include developing different ways to open up the structure of the wood, such as using ultrasonic waves during the impregnation process to increase the liquid permeation. Other efforts could be collecting and concentrating the tree sap and diluted modifier for recycling. Last but not least, applying other novel or special modifiers may provide even higher value-added wood-based material in the industry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Forestry Public Industry Foundation 201204702-B2: the research of key 9726



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In Situ Polymerization of Phenolic Methylolurea in Cell Wall and

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