Effects of microwave softening treatment on dynamic mechanical and chemical properties of bamboo

Bamboo is a high-quality biomass material, but its thin walls and hollowness, limit subsequent processing. Softening can �atten and bend bamboo without causing cracks, and thus enables e�cient value-added use. In this study, the effects of microwave softening parameters (microwave power, processing time, and initial moisture content) on the glass transition temperature, compression ratio, microstructure, chemical composition, and surface wettability of Phyllostachys. edulis and Dendrocalamus. sinicus were studied. Microwave softening parameters (microwave power, treatment time, initial moisture content) improved the �exibility and processability of both bamboo species. Dynamic mechanical analysis showed the storage modulus of D. sinicus slices was reduced from 7846 to 4498 MPa, which was 1.06 times higher than that of P. edulis. The glass transition temperature was lowered from 221.25 to 123.67°C, which was1.07 times higher than that of P. edulis, indicating bamboo stiffness was reduced and elasticity was enhanced. Moreover,P. edulis has higher thermoplastic quality than D. sinicus. Water molecules have a wetting and swelling effect on the cell wall. With a rise in water content, the free hydroxyl group increased, and the compression ratio of P. edulis with a moisture content of 90% increased from 15.65–45.54%, and that of D. sinicus increased from 11.31–41.67%. Hence, choosing the most adaptable bamboo species and moisture content, and increasing the softening temperature and softening time can improve the �attening quality of bamboo and effectively reduce the number of hydroxyl and carbonyl groups, and thus offers a theoretical basis for the industrial processing of bamboo timbers.


Introduction
The emergence of plastic products has brought great convenience to people's life, but because plastic products are undegradable, plastic pollution caused serious harm to marine ecosystems and terrestrial ecosystems and a serious threat to humans.Therefore, nding alternatives to plastic products has become particularly urgent.Bamboo has fast growing ability, a short cultivation cycle, high strength and toughness, and thus can replace plastics to meet people's need and the requirements of environmental protection.There are more than 1,600 species of bamboo plants known globally, which cover more than 35 million hectares, and are widely distributed in Asia, Africa, and the Americas (Bian et al., 2020).However, bamboo is thin-walled, hollow, and structurally heterogeneous, and tissue structure, chemical composition, appearance and morphology largely vary among bamboo species.Therefore, the processing method of bamboo is facused on the use of bamboo slices and bundles, with a large amount of planning, and sawing, cumbersome procedures, and a general utilization rate of less than 50% (Kumar and Mandal, 2022).The emergence of attened bamboo panels transforms the bamboo processing unit into bamboo panels, and can effectively improve the utilization rate of bamboo materials.Softening is an important pretreatment process for bamboo spreading and bending, and is widely used in construction, decoration, furniture, paper, packaging, food, textiles, and other elds (Nguyen et al., 2018).
Bamboo softening can effectively improve the plasticity of bamboo by increasing the water content and temperature.Bamboo can be softened using chemical agents, such as ammonia, urea, NaHCO3, NaOH, and KOH (Cheng et al., 2006;Pawlak and Pawlak, 1997).Unfortunately, these chemicals are not environmentally friendly and even affect the appearance and mechanical properties of bamboo.
Microwave treatment has become increasingly popular in bamboo and wood processing and is a practical solution to bamboo and wood bending (Peres et al., 2016).Microwave softening provides high energy to bamboo to generate internal heat, which produces uniform heating.Microwave softening is reportedly superior over conventional methods in terms of processing e ciency, including shorter processing time and lower energy consumption (Gao et al., 2021b).In the 1990s, microwave softening of wood was rst studied comprehensively, which showed microwave softening can make the wood be quickly and uniformly heated, without causing a gradient in moisture content, and effectively improved product quality (T, 1983).Later, scholars focused on the key factors affecting microwave softening.Studies on the changes of moisture content and temperature during microwave softening of Fagus, showed that increasing moisture content was the key to microwave softening, and the microwave softening rate was only a few minutes (GaSparik and Barcik, 2014; Gašparík and Gaff, 2013).Microwave power and softening time also affect the softening effect.When the moisture content is 60%,the best microwave softening process for Pinus massoniana is at microwave power of 400W and heating time is of 2.4 minutes (Daian, 2010).At this stage, more studies focus on microwave softening of wood rather than bamboo.According to the method ued in wood, scholars carried out microwave softening of Phyllostachys edulis with different moisture contents and found that microwave power, processing time, and moisture content all affected the softening effect (Rui-min et al., 2014).However, studies on the parameters of the bamboo softening process are incomplete and focused on the macroscopic mechanics, and lack of studies on the microscopic as well as chemical areas.There is also a relative lack of research on different bamboo species.
Bamboo is a polymeric material consisting of lignin, cellulose, and hemicellulose (Liu et al., 2019).Hightemperature exposure affects the viscoelasticity and plasticity of these chemical components.The viscoelasticity and polymericity can be referred to as the glass transition.Hence, the softening is directly related to the glass transition temperature (T g ) of bamboo (Börcsök and Pásztory, 2021).The glass transition temperature of bamboo in the fully dry state is mainly determined by cellulose, hemicellulose, and lignin together, but varies mainly with moisture content because water is a very good plasticizer (Gao et al., 2018).The modulus of elasticity of bamboo after microwave softening decreases with the increase of the initial moisture content of bamboo, and the bamboo slices are softened better when the initial moisture content is higher (Yuan et al., 2021;Zhichao et al., 2020).Meanwhile, the glass transition temperature is also affected by temperature.After the softening treatment of P. edulis at different temperatures and different times lengths, the higher the heating temperature and the longer time, decreased the longitudinal bending strength and exural modulus of elasticity.The glass transition temperature varies among different bamboo species (Kadivar et al., 2022).At present, the effects of microwave process parameters on the glass transition temperature are unknown, and the glass transition temperatures of different bamboo species have not been su ciently studied.
Given the low softening e ciency and lack of industrially applicable process parameters for bamboo, as well as the advantages of microwave softening(e.g., high e ciency and uniform heating), this study was aimed to achieve high e ciency and uniform heating.Speci cally, microwave softening methods were used to explore the effects of microwave power, time and initial moisture content on the glass transition temperatures, compression ratios, microstructures, chemical compositions and surface wettability of P. edulis and D. sinicus.The optimized process parameters of bamboo slices were analyzed and compared with the adaptability of different bamboo species to provide a theoretical basis for the industrial processing of softened components.
2 Materials and methods

Materials
Four-years-old P.edulis and D.sinicus were harvested.P. edulis with an outside diameter of 80-120 mm came from Hangzhou, Zhejiang Province, and D. sinicus with an outside diameter of 200-250mm came from Cangyuan, Yunnan Province.The samples were taken from the fth section of the bamboo, and the part of the ground containing the bamboo nodes was the rst section.Bamboo samples were removed from bamboo green and yellow and cut into 60×10×5mm 3 (length×width×thickness) slices.

Microwave softening treatment process.
The effects of microwave power, microwave treatment time, and initial moisture content on the softening effect of P.edulis and D.sinicus were investigated.

Microwave power
Single-factor experiments with P. edulis and D. sinicus were conducted at four microwave powers, 300 W (medium-low re), 500 W (medium re), 700 W (medium-high re), and 1000 W (high re) with 0W as a blank experiment.The moisture content of bamboo chips was 30%, and the microwave treatment time was 30 seconds.Three parallel tests were carried out.

Microwave time
Single-factor experiments with P. edulis and D. sinicus were conducted at four microwave times lengths, 20, 30, 40 and 50 s, while a blank comparison experiment was done.The moisture content of bamboo chips was 30%, and the microwave power was 500W.Three parallel tests were carried out.

Initial moisture content
The moisture contents of the specimens was controlled using the weight method.Fresh bamboo slices were weighed, recorded and placed in an oven at 40°C.The moisture contents were calculated by weighing at a half-hour interval until the target moisture content was reached.Then the slices were taken out sealed, and stored in cling lms.The target moisture contents were 7% (air-drying), 18% (standard), 30% ( ber saturation point), 60% (fresh), and 90% (saturated state).
The experiments were carried out using a microwave device (Koyamaki-30MX67) with a frequency of 2450MHz and an adjustable power range of 300W-1000W for microwave treatment.To prevent excessive evaporation of water during the softening, the bamboo slices were placed in microwave oven-speci c freshness bags and put into the microwave oven together.(Fig. 1) 2.3Characterisation

Dynamic mechanical analysis
The specimens were sanded to standard specimens in size of 60 x 10 x 3 mm 3 with at surface and uniform thickness.Energy storage modulus, dissipation modulus, and damping factor of the bamboo sheets were examined using a dynamic mechanical analyzer.The tests were conducted using double cantilever bending mode at amplitude of 15 µm, frequency of 3 Hz, from ambient to 260°C, and heating rate of 5°C /minute.The peak temperature of the E" curve was considered as the glass transition temperature (T g ) according to the ISO standard recommendations(ASTM, 2007).

Compression ratio
The microwave-softened samples were placed in a hot press and compressed along the radial direction for 30 s at 100°C and 3 MPa.Measurements were done before and after compression at an accuracy of 0.01 mm.The compression ratio (CS) was calculated by measuring the thickness of each bamboo chips in the hot press two times: before and immediately after the removal from the press (Fang et al., 2012).
where T 0 and T C are the thicknesses of bamboo strip before and after compression respectively.

Microstructure
The cross-sectional micromorphology of the two species of bamboo after microwave softening was observed by an SEM device (S-3400N, Japan) at 1.5 kV.Structurally intact samples (5×5×1mm 3) were prepared and dried at 40°C until reaching constant weight.They were xed on conductive carbon adhesives using tweezers and then placed in a vacuum coater to spray with gold lms for 30 min.

Fourier transform infrared spectroscopy (FTIR)
The chemical modi cations involved in the microwave softening of bamboo were monitored using FT-IR.bamboo samples were ground into powder and passed through a sieve with a mesh size of 40-80.The mass ratio of each sample to potassium bromide is 1:150.Spectra were recorded in absorbance units of 500-4000 cm -1 at a resolution of 4 cm -1 and 16 scans respectively.The samples were oven-dried at 103°C for 2 hours prior to measurement.

X-ray diffraction (XRD)
The crystal structure and crystallinity of two bamboo microwave-softend samples and control bamboo samples were analyzed using XRD.The sample powders of P. edulis or D. sinicus was ground and sieved, and about 0. where I 002 is the maximum intensity of the diffraction angle at the (002) lattice, and I am represents the intensity of amorphous background diffraction when 2θ is close to 18°.

Contact angle
A contact angle meter (DSA100, Japan) was used to measure the contact angle of two bamboo spcies with or without microwave softening.In the tests, distilled water was loaded into the contact angle meter after complete removal of air bubbles.The volume of the water droplets was 4µL.The contact angle was measured after the droplets touched the sample surface for 10 s.Five parallel measurements were done on each sample surface in different areas and the average value was taken as the result.
3 Results and discussion

Glass transition temperature analysis
Changes in E' and E'' under different microwave softening parameters and in different bamboo species with increasing temperature are shown in Fig. 3, E' represents the energy stored by bamboo during deformation due to elastic deformation, and a higher E' means, stronger rigidity.E" describes energy loss (transformation into heat) when a material undergoes deformation, and a lower E" the plant becomes more elastic P. edulis and D. sinicus showed the same pattern of changes in E' and E''.As shown in E' is higher than E'' in all cases, indicatin the bamboo samples are mainly elastic.Figure 3A (1)-F (1) shows the E' spectra of bamboo slices after microwave softening under different conditions.As the temperature rose, the E' of all the bamboo slices decreased to different degrees, which was negatively correlated with the degree of molecular motion.In the case of P.sedulis, E' at the initial temperature (30℃) varied in ascending order of power as 7763, 5826, 5647, 6372, and 6533 MPa, and in ascending order of time as 5910, 5647, 6624, 6540 MPa.The E' of the bamboo slices decreased slightly after microwave softening compared to the untreated sample.However, the E' after different softening processes was very close, indicating the softening treatment did not signi cantly affect the E'.Due to the plasticizing effect of water, the E' of 7%, 18%, 30%, 60%, 90%, and bamboo chips were 7695, 6542, 5647, 5344, and 4627 MPa.The 90% water content decreased by 40.41% compared to the absolute dry.Clearly, the water content can greatly reduce the stiffness.
Figure 3A (1)-F (1) show the E'' and spectra of bamboo slices after microwave softening under different conditions.Clearly the initial E'' of the bamboo slices after microwave softening was reduced compared with the untreated samples, and rst increased and then decreased.The effect of softening time was not obvious, while the effect of moisture content was most signi cant.The rst peak of the E'' pro le for microwave power of 500 W and time of 30 s was signi cantly higher than other process parameters, indicating the viscosity of bamboo akes increased after microwave softening under this condition, and T g decreased from 204.25° to 100.39° (Zhang et al., 2018).This is due to the combined effect of the increased free volume of molecules within the bamboo slices, cell wall softening, and hemicellulose degradation.The glass transition temperature of the bamboo akes decreased signi cantly with the increase of water content from 217.56°to 72.95°.This is related to the glass transition temperature being thermally softened by lignin and hemicellulose.

Compression ratio analysis
The average compression of the two species by microwave power, treatment time, and moisture content is shown in Fig. 4. Results show that the microwave softening treatment parameters more greatly impact the softening effect of bamboo chips.The average compression ratio is up to 45.54% for P.sedulis, and is 23.68% for D. sinicus.The variation patterns of the two species are consistent, but the compression ratio of P. sedulis is larger than that of D. sinicus.This result is positively correlated mostly with basic density, and also with vascular bundle distribution density and thick-walled ber tissue ratio (Dixon and Gibson, 2014).As shown in In the case of P. edulis, the compression ratio rst rose and then decreased with the increasing microwave power/time.The lowest compression ratio of 26.41%/24.09% in the microwave softening test at 300W/20s was attributed to the low molecular motion and insu cient temperature inside the bamboo slices.When the microwave power/time increased to 500w/30s, the compression ratio maximized to 33.26%.This result is due to the increased internal temperature of the bamboo sheets, the disruption of hydrogen bonds between the complexes, the increase in free volume in the cell wall, and the change in the glass transition temperatures of lignin and hemicellulose (Yang and Park, 2019).With the continuous increase in power/time, the compression ratio decreased.This indicates the softening effect of the bamboo boards was reduced due to the carbonization condition that occurred as a result of the rapid evaporation of internal moisture.As the initial water content of the specimen increased, the compression ratio rose gradually after softening(Fig.1C).The reason is water, as a plasticizer, has a good wetting and swelling effect on the interior of the bamboo.Compared with the moisture content at the ber saturation point (30%), the compression ratio increase at 7% and 18% is small, from 10.52-19.4%/21.5%.The wetting and swelling of the non-crystalline zone are not obvious due to the low water content.When the water content reaches 60% and 90%, the compression ratio maximized to 45.54%.This is related to the fact that the free hydroxyl groups absorb more water and prolong the intermolecular distance (Song and Li, 2010).However, when the water content is too saturated, the water molecules inside the bamboo slices shrink sharply under the high-temperature microwave treatment, which can easily subject the specimens to deformation (Fig. 5(C)).Thus, when the microwave power is 500W, the processing time is the 30S, and the water content of the specimens reaches 30%, the bamboo slices are softened best by the microwave treatment.

Microstructural analysis
The cross-sectional micro-morphology of the two species of bamboo after microwave softening (microwave power, time, moisture content) is shown in Fig. 5.Although the cell morphology of different species varied considerably, leaf and tree bamboos had the same microscopic features, including deformation, cracks and collapse.From the original cross-sections of two species, the vascular bundles of P. sedulis are smaller and structurally open, and the parenchyma cells are mainly round.The vascular bundles of the D. sinicus are larger compared to P. sedulis, and the structures are broken-waisted with thin-walled cells, mainly long-ovate.
Under microwave treatment with low power and short time (300W/20s), the vascular bundles showed a complete and smooth morphology with no obvious changes, while the cell walls became puffed up and the boundaries were unclear under the pressure.As the temperature rose and the time was prolonged, the vascular bundles gradually blurred in shape and some began to crack.Moreover, the shape of the cells became distorted, These changes were due to the severe loss of internal water molecules in the heating and the difference in the degree of contraction between neighboring bamboo cells (Zhang et al., 2023).When the condition rose to 1000W/50s, the shape of vascular bundles became irregular and the cracks increased signi cantly and even collapsed.Furthermore the laminar structure of the cell wall almost disappeared or even destroyed.This change is mainly due to the rapid water vaporization in the bamboo under the action of microwave, which produced a large vapor impact, resulting in the rupture of the cell membrane.At the same time, the in ltrated components were partially volatilized under the action of heat, and a part of the surface shranks and migrated..According to the microstructural analysis, the high temperature environment, water evaporation and chemical transformations cause the cell wall to thin and the laminar structure to disappear.

FTIR analysis.
The FTIR spectra showed the chemical structural changes that occurred with different softening treatment parameters.As shown in Fig. 4, the characteristic infrared peaks of different bamboo species and their attributions are the same, except that the position of the characteristic peaks of each group move within a small range.Thus, these changes may be due to the slight difference in the extent of the in uence effects (e.g., inductive, conjugation, and hydrogen bond effects) on each group in different species.The characteristic peaks of the three main elements are consistent but slightly different in certain positions.
In the combined pro les of the two bamboos species, the absorption peak at 3450 cm − 1 is stronger in P. edulis than in D. sinicus, which may be because there are more O-H groups in P. edulis.With the increase of microwave power, time, and moisture content, the hydroxyl peak at 3450 cm − 1 was weaker than that of the untreated sample.This change was attributed to the reduction of free hydroxyl groups and the degradation of hemicellulose (Tianfang Zhang et al., 2021).The absorption peak at 1745 − 1 cm is the C = O stretching vibration of the acetyl group in hemicellulose, and is the same for both bamboos species, indicating the hemicellulose contents of the two species are the same.With the rise in temperature, time, and moisture content, the absorption peaks became weaker.The reason is due to the degradation of xylan.At the same time, the hot spot effect radiation of microwave leads to acetyl fracturing (Wang et al., 2022).The absorption peak near 1600cm − 1 is the lignin C = O stretching vibration absorption peak, which does not change signi cantly, indicating lignin has not changed signi cantly and the benzene ring skeleton in lignin is relatively stable.Similarly, the peaks at 1510 and 1430 cm − 1 are characteristic of lignin, and the changes are not obvious (Kotilainen et al., 2000).However, the absorption peaks of D. sinicus were sharper than those of P. edulis, indicating the lignin content of D. sinicus was higher.The bands at 1425 cm − 1 (CH2 bending in hemicellulose), 1328 cm − 1 (CH swinging vibration in cellulose) and 1162 cm − 1 (C-O stretching vibration in xylan) showed the same downtrend.The lower-intensity peak of C-C and C-O at 1038 cm − 1 is related to the degradation of the structural xylose unit in the hemicellulose backbone (Colom et al., 2003).The above results indicate the degradation of hemicellulose is higher after softening treatment.After microwave softening, hemicellulose is the least stable and degrades, while lignin is the most stable.

XRD analysis
Cellulose crystallinity re ects the physical and chemical properties to some extent and is an important basis for evaluating the mechanical properties and cellulose quality of biomass materials.Figure 7 shows the XRD curves of P. edulis and D. sinicus subjected to different microwave softening treatments.The difference in the crystallinity of cellulose between bamboo species was not signi cant, and the position of the cellulose diffraction peak in microwave softening treatment (power, time, moisture content) did not change much.This result indicates no transformation of the crystal type occurred during microwave softening.
The relative crystallinity rst increased and then decreased with the rise of power and time.In the presence of microwave heat, hydroxyl groups under the condensation effect caused the rearrangement of cellulose molecular chains and crystallization of amorphous regions, which increased the relative crystallinity (Yin et al., 2017).With further increase in microwave power and time up to 700W/40s, the crystallinity started to decrease.This result is related to the non-thermal effect of microwaves, which break the hydrogen bonds between cellulose molecules.At the same time, the acetyl conversion of acetic acid and propionic acid exacerbated the destruction of the cellulose structure.(Hanne Wikberg and Maunu, 2004).When the power/time maximized, the relative crystallinity increased again due to the charring of bamboo, recrystallization of degraded micro brils within the amorphous zone, and simultaneous pyrolytic crystallization of xylan and mannan in hemicellulose (Sun et al., 2022).At the same microwave power and time, the crystallinity of the treated samples decreased intensely as the initial water content increased.This was presumably owing to the water-induced swelling of hemicellulose in the amorphous region, which enlarged the speci c surface area and free hydroxyl content of hemicellulose.These free hydroxyl groups can bind to water molecules through hydrogen bonding and then oscillate in the presence of microwaves, causing more damage to the crystal structure(Zhichao et al., 2020).

Contact angle analysis
Bamboo is a hydrophilic biomass material.Its surface wettability is determined by changes in its chemical composition, and is usually expressed in terms of the contact angle.A larger contact angle indicates a greater surface hydrophobicity(Bryne and Walinder, 2010).Figure 8 shows the variation rules in the effects of microwave power, treatment time, and moisture content on the surface wettability of two species of bamboo in three cases.The contact angle of the microwave softening treatment varied in a similar way for the two species, but the contact angle of D. sinicus was larger than that of P. edulis.This result is due to the relatively small or sparse pores on the surface of D. sinicus, with a denser structure and relatively weak water extension ability (Gao et al., 2021a).
In the case of D. sinicus, the contact angle increased and then decreased with the increment of power and time.The contact angle increased from 45.8° to about 65° at 300w/20s, indicating the microwave softening reduced the surface wettability of the bamboo slices, which was related to the decrease of the hydrophilic groups of the hemicellulose degradation (Gao et al., 2021a).The surface polymer molecular chains of bamboo were fractured by microwave softening at 500w/30s, the active groups were reduced (Ke et al., 2022), and the contact angle maximized to 73.8°.When the power/time was maximized, the internal tissue structure was destroyed, which led to a decrease in the surface wettability of bamboo.The contact angle gently decreased with the increasing moisture content.This is because the moisture content will affect the density of the bamboo sheets inside, and a higher moisture content lead to decrease in density and hydrophobicity.The moisture content decreases the more compact inside, and the hydrophobicity increases.

Conclusions
(1) The glass transition temperature of the bamboo akes was in uenced by the microwave process parameters, including microwave power, processing time, and especially the initial moisture content.The E'' of 90% moisture content bamboo slices of P. edulis and D. sinicus decreased from 6498/8545 to 4323/4981 MPa, which was 0.91 and 0.78 times the power and treatment time.E'' decreased from 134.1 to 108.9 /100.6 MPa.T g decreased from 221.25/204.67 to 123.93 /100.39°C.The stiffness of bamboo decreased, and its elasticity increased.
(2) The increase in the compression ratio from 10.52-45.54% is related to the increase in moisture content and the penetration of water into the crystalline zone of the bers to induce wetting, swelling, and cellulose dissolution.The vascular bundles and thin-walled cells of the specimens shrank and deformed to different degrees under different microwave softening parameters.Microwave softening led to an increase in the surface plasticity and hydrophobicity of bamboo slices.This result is also related to the number of free hydroxyl groups in the hemicellulose.
(3) The plasticizing effect produced by water is the most important factor affecting microwave softening.Water molecules at high temperatures can subject the molecular chains of cellulose, hemicellulose, and lignin to breakage and causes an increase in free hydroxyl groups.,The water content is more exible compared to the heat provided by the increase in power and time.
(4) The T g for microwave softening of P. edulis under different pretreatment conditions decreased by 103.86°, which was 1.14-times higher than that of D. sinicus, and the compression increased to 37.58%, 5g of the bamboo powder was taken and attened on the sample utes.The testing condition were Cu target Kα radiation (λ = 0.154 nm), a radiation tube voltage of 40 kV, a tube current of 40 mA, a scanning angle range of 10 − 40•, and a scanning speed of 5•/min.The relative crystallinity ( ) of bamboo was calculated according to the Segal method (Segal et al., 1957) 2

Figures Figure 1
Figures

Figure 4 Effects
Figure 4

Table 1 (
Wang et al., 2016),but lignin and hemicellulose contents are different between the two species and T g is different.

Table 2
As a result, the compression ratio of P. sedulis is higher than that of D. sinicus and is more easily softened.
(Dai et al., 2023), the basic density, density of vascular bundle distribution, area of individual vascular bundles, and tissue ratio of brous sheaths of D. sinicus are all larger than those of P.sedulis.

Table 2
Vascular bundle structural parameters and basic density of Phyllostachys edulis.and Dendrocalamus sinicus