Combined effects of ZnO particle deposition and heat treatment on dimensional stability and mechanical properties of poplar wood
SciENTiFic RepoRts |
Combined effects of ZnO particle deposition and heat treatment on dimensional stability and mechanical properties of poplar wood
OPEN This study proposed a one-step wood modification method by combining the deposition of ZnO particles on wood surface and heat treatment. The effects of ZnO particles and heat treatment on mechanical properties and dimensional stability of poplar wood were examined. Samples were sorted into 4 groups, i.e., control, heat-treated, impregnation/heat-treated, and hydrothermal-treated samples. The mechanical properties and dimensional stability of impregnation/heat-treated and hydrothermal-treated wood samples were measured in comparison with those of the control and heat-treated wood samples. Compared with the control ones, the reduction of the flexural strength of the heat-treated, impregnation/heat-treated and hydrothermal-treated samples were about 11.57%, 8.53% and 15.90%, respectively. The modulus of elasticity of the heat-treated and hydrothermaltreated samples decreased by 13.78% and 23.78%, respectively, while the impregnation/heat-treated samples increased by about 8.57% due to the ZnO particles growth. The dimensional stabilities of the heat-treated, impregnated/heat-treated and hydrothermal-treated samples were improved in comparison with that of the control sample. All samples were characterized by the scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X - ray diffraction (XRD).
Heat treatment is considered as an eco-friendly modification process, which improves some wood properties,
referring to the enhanced hygroscopic properties and dimensional stability1, decreased wettability2, and increased
durability3. However, the drawbacks of heat treatment are that the modulus of rupture (MOR) and modulus of
elasticity (MOE) are reduced4, 5. During the heat treatment, the chemical composition and structure of the wood6
were changed, resulting in a mass loss7. With the increases in temperature and time of the heat treatment, the
compression strength8 and the density of the wood decreased9. Therefore, it is necessary to compensate for the
decrease in wood mechanical properties during heat treatment.
A series of physical and chemical methods have been proposed to alleviate the negative effects of heat
treatment, such as integrating heat treatment with densification10, borate salt11 and silver nanopaticles treatments12,
but these methods need complex process and difficile-manipulation. Another approach was to deposit the
nanomaterials on wood surfaces using the hydrothermal method13, for instance, sedimentation of ZnO nanorods,
TiO2/Cu2O composite coating on the wooden substrate14, 15. The hydrothermal methods can be described as the
use of high temperature, high pressure aqueous solution to make the normally soluble or insoluble materials
dissolved and re-crystal16. However, the hydrothermal reaction is carried out at a high temperature and high
pressure consuming great amounts of energy, and an expensive sealed autoclave is a prerequisite for the hydrothermal
reaction17. Meantime, the preparative methods for forming particles on wood surfaces are generally complex and
include at least two or three steps18, which limits its application in the large sized wood modification.
The objective of this study was to develop a one-step wood modification method by combining the
deposition of ZnO particles on wood surfaces and heat treatment to alleviate the mechanical property loss caused by
heat treatment. The microcosmic characterization and formation mechanism of ZnO particles of the
impregnation/heat treatment were discussed in comparison with the hydrothermal treatment. The examination results of
mechanical properties and dimensional stability showed that the impregnation/heat treatment generating zinc
oxide on the wood surface compensated the loss of mechanical properties that caused by the heat treatment, and
the dimensional stability was improved in comparison with the heat-treated poplar.
Results and Discussion
Figure?1 shows the flexural strength and modulus of elasticity of the control, heat-treated, impregnation/
heat-treated and hydrothermal-treated samples. Compared with the control samples, the reduction of the
flexural strength of the heat-treated, impregnation/heat-treated and hydrothermal-treated were 11.94%, 7.94%, and
19.19%, respectively, indicating that the impregnation/heat-treated method reduced the loss of flexural strength.
Compared with the control samples, MOE of the heat-treated and hydrothermal-treated samples decreased by
14.29% and 23.81%, respectively, while the MOE of the impregnated/heat-treated samples increased by 7.48%.
The fact of the reduction of the flexural strength due to the heat-treatment was consistent to that of previous
studies19?21, which could be reasoned by the degradation of hemicelluloses in the high temperature process22. The
impregnation/heat treatment generating zinc oxide on the wood surfaces could compensate the loss of
mechanical properties that caused by the heat treatment. It was probably because of the zinc hydroxides deposited in
wood catheters and cell lumens during the process of impregnation/heat treatment as shown in Fig.?2(c). Since
the Zinc hydroxide could be easily dehydrated to produce Zinc oxide particles at 170 ?C, a large amount of Zinc
oxide particles grew in wood catheters, which increased the surface density of the wood. At the same time, the
rigidity of the wood increased in high temperatures, and the combined effects made a compensation for the loss
of mechanical properties during the heat treatment. As for the hydrothermal method, it was possible due to the
high relative humidity23 of the treatment environment to accelerate the degradation of hemicelluloses, resulting
in more significant reduction of flexural strength and MOE than that of the heat-treated wood.
Figure?2(a) shows the surface morphology of the control samples, illustrating that the wood surface was
smooth with some clearly visible pits. Figure?2(b) presents the surface morphology of the heat-treated samples,
which also have some visible pits, illustrating that no significant change occurred compared with that of the
control ones. Figure?2(c) shows the surface morphology of ZnO particles after the impregnated/heat treatment.
In the upper left corner in Fig.?2(c), it can be found that the flower-like ZnO crystal was composed of small-sized
needle and sheet-like ZnO. This may be due to heat treatment conditions. The heat and mass transfer was more
intense than that samples treated by the hydrothermal method, resulting in the faster Zn(OH)2 dehydration
reaction, sharper formation of ZnO and more uneven appearance. The formation of ZnO increased the wood surface
roughness, resulting in the improvement of dimensional stability of wood. Figure?2(d) shows that the surface
morphology of the wood was covered by ZnO particles after the hydrothermal treatment. The ZnO-treated
surface was uniform and the particles were compact and spherical, suggesting that ZnO particles can be formed on
the wood surface by the hydrothermal method under certain reaction conditions. This was mainly due to the rich
hydroxyl on wood surface and ZnO on the hydrophilic wood substrate. The ZnO particles can effectively
nucleation in cellulose fiber and the nuclear began to grow.Therefore, when the wood was immersed in the reaction
solution, the Zn2+ in the precursor solution was connected to the surface of the wood by the hydrogen bonding
energy under the action of hydrothermal energy. As the reaction time prolonged, Zn2+ was aggregated into ZnO
particles. As the continuous action of hydrothermal energy continued to grow, the self - assembly of surface
activity continued on the wood surface.
Figure?3 shows the FTIR spectra of the control, heat-treated, impregnation/heat-treated and
hydrothermaltreated samples. The peaks near 3400cm?1 was assigned to stretching vibrations of hydroxyl groups on the wood
surface, in which, the band at 3340 cm?1 (a curve) and 3343 cm?1 (b curve) corresponding to the stretching
vibrations of hydroxyl groups in the wood shifted to larger wavenumbers of 3421 cm?1 (c curve) and 3383 cm?1
(d curve), indicating a strong interaction between the hydroxyl groups of the wood surface and ZnO particles
through hydrogen bonds24. For impregnation/heat-treated samples, the two strong absorption peaks at 2920 cm?1
and 2850 cm?1were ascribed to the asymmetrical stretching vibrations of ?CH3 and ?CH225. New bands at 1604
cm?1 (c curve) and 1597 cm?1 (d curve) appeared in the spectrum of the ZnO particles deposited wood,
corresponding to the asymmetric and symmetric stretching of zinc carboxylate at the surface of the ZnO particles14.
In impregnation/heat-treated samples, the characteristic absorption peak of ZnO appeared around 459 cm?1,
indicating that the wood surface had the ZnO formation after the impregnation/heat treatment26.
Figure? 4 shows the XRD patterns of the control, heat-treated, hydrothermal-treated, impregnation/
heat-treated and ZnO samples. In Fig.?4, the characteristic diffraction peak of cellulose (2? = 15, and 22?)
appeared in the control, heat-treated, impregnation/heat-treated and hydrothermal-treated samples, indicating
that the crystal form of cellulose was not changed27. The characteristic diffraction peaks of cellulose and ZnO
diffraction peaks were observed at the same time in the impregnation/heat-treated samples. It was also found
that the characteristic diffraction peak intensity of the cellulose modified by the impregnated-heat treatment
was weaker than that of the control and heat-treated samples. The location of the diffraction peaks of ZnO in the
XRD patterns of the impregnation/heat-treated sample was consistent with the diffraction peak position of pure
ZnO. It can be seen that the ZnO-formed on wood surface was hexagonal wurtzite structure (JCPDS card No.
36-1451)28. The relatively strong and sharp XRD diffraction peaks showed that the ZnO crystals formed in the
wood were well crystallized. Therefore, it can be deduced that the particles formed on the wood surface by the
impregnation/heat treatment were pure ZnO crystals.
ZnO particles were successfully deposited on wood surfaces by one-step impregnation/heat treatment method.
) The impregnation/heat treatment generating zinc oxide on the wood surface compensated the loss of
mechanical properties that caused by heat treatment. Compared with the control ones, MOE of the
heat-treated and hydrothermal-treated samples decreased by 14.29% and 23.81%, respectively, while the
impregnation/heat-treated samples increased by about 7.48% due to the ZnO growth.
) The dimensional stabilities of the heat-treated, impregnated/heat-treated and hydrothermal-treated
wood were improved in comparison with that of the control samples. The dimensional stabilities of the
impregnated/heat-treated and hydrothermal-treated samples were also improved compared with that of
the heat-treated wood, indicating that the deposition of ZnO particles on wood surface enhanced wood
resistance to moisture.
) After the impregnation/heat treatment, the wood surface of the zinc oxide showed flower-like crystals.
The examination of SEM, FTIR and XRD confirmed that he zinc oxide particles could be grown on wood
surface by the one-step impregnation/heat treatment method, which caused the structural changes of the
The one-step impregnation/heat treatment method simplified the processing steps of the deposition of ZnO
particles on wood surface and improved the energy efficiency.
Materials and Methods
Materials. The Poplar (PopulusAdenopodaMaxim) wood was obtained from Mudanjiang, Heilongjiang
Province, China. The Zinc acetate, with a purity? 99%, was supplied by Tianjin Zhiyuan Chemical Reagent Co.,
Ltd. The Sodium hydroxide particles (analytical grade) were supplied by Tianjin Tianli Chemical Reagent Co,
Ltd. The Ammonia, with a concentration of 25%, was supplied by Tianjin Quartz Clock Factory Bazhou City
Methodology. The process of depositing zinc oxide on wood surface by impregnation/heat treatment is
presented in Fig.?7(a). A certain amount of 1 mol / L Zn (Ac)2 and NaOH solution were prepared firstly. The wood
samples were immersed in the solution of Zn (Ac)2 and remained at a vacuum of 0.01 MPa absolute pressure,
(75 Torr) for 8 hours and then dried in a drying oven at 60 ?C. The wood samples were taken out and immersed
in the NaOH solution for 8 hours in the vacuum pressure, and then dried in a drying oven at 60 ?C. After the
impregnation processes of Zn (Ac)2 and NaOH, the unreacted chemical substances were removed from the wood
sample surfaces using the deionized water. The ZnO-deposited wood samples were heat-treated at 170?C for 6 h.
The process of generating zinc oxide on poplar surfaces by hydrothermal treatment is presented in Fig.?7(b). The
major factors influencing the ZnO formation morphology on the surface of poplar were the reaction temperature,
reaction time and reaction solution Zn2+/OH? ratio. Based on the single factor test, the optimum conditions
ZnO morphology of poplar surface were as follows: the reaction temperature of 170 ?C, reaction time of 6 h, and
solution Zn2+/OH? ratio of 1: 1. Zn (Ac)2 ? 2 H2O (analytically grade) was dissolved in the deionized water. After
stirring at room temperature for 30 min, a certain amount of NH3 ? H2O was added and stirred for 10 min. The
resulted solution and the wood samples were transferred to a reactor equipped with a polytetrafluoroethylene
liner. The reaction was carried out at 170?C for 6 h and then the samples were removed out from the reactor
and cooled to room temperature. The wood samples were washed three times with deionized water to remove
unreacted chemicals and residues on the wood surfaces and then dried at 60 ?C for 24 h. The heat treatment were
performed at 170 ?C for 6 h using a temperature-controlled laboratory oven.
Mechanical property tests. From each treatment group, six samples sized 80 mm ? 13 mm ? 4 mm were
prepared for examining the flexural strength and MOE by means of a universal testing machine (Ruigeer,
RGT20A) in accordance with the procedure described in the ASTM D 7031 standard. Three-point bending set-up was
used with a span of 64 mm and a crosshead speed of 2.5 mmmin?1.
Dimensional stability tests. From each treatment group, forty samples sized 20 mm ? 20 mm ? 20 mm
were prepared for four different tests as follows. a) Room temperature-conditioning swelling: the oven-dried
samples were placed under the condition of a temperature of 20 ? 2 ?C and relative humidity of 65 ? 3%
until the dimensions were stable. b) Water-saturation swelling: the oven-dried samples were immersed in
distilled water at a temperature of 20 ? 2 ?C for 20 days until the stable dimensions were obtained. c) Room
temperature-conditioning shrinkage: the water-saturated samples were placed at the condition of the
temperature of 20 ? 2 ?C and relative humidity of 65 ? 3% until the dimensions were stable. d) Oven-dry shrinkage: the
Data availablility statement.
All data generated or analyzed during this study are included in this
pubwater-saturated samples were placed in an oven with a temperature of 103 ?C and dried until the constant weight
was obtained (bone dried). During the four tests, the dimensions were measured in three directions (longitude,
radius, and tangential as shown in Fig.?8) based on the Chinese standard31.
Characterization. From each treatment group, four samples were sputter-coated with gold layer, and the
morphology of wood sample surface was characterized by the scanning electron microscopy (SEM, FEI, and
Quanta200). The Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific, and Nicolet 6700)
measurements were used to explore the chemical changes. The wood samples were examined in the range of
4000?400 cm?1 with a resolution of 4 cm?1, scanning 32 times for each spectrum. The crystalline structure was
analyzed by the X-ray diffraction (XRD, Philips, and D/max2200) operating with Cu radiation and at the
acceleration voltage of 40 kV, the current of 30 mA, the scanning range (2?) from 5 to 70?, and the scan rate of 4?/min.
This research was supported by the National Natural Science Foundation of China (31670574).
M.X. conceived the project and revised the whole manuscript. L.C. revised the whole manuscript. W.C. and N.Z.
performed the experiments and wrote the paper. All authors reviewed the manuscript and agreed to submit the
Competing Interests: The authors declare that they have no competing interests.
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