Thermal Insulation Properties of Deligniﬁed Balsa and Paulownia Wood “Foams” with Polylactic Acid Coverings

: The energy-conserving performance of a building, normally realized by a variety of insulation materials, needs to be carefully considered, from the design to construction operations. Traditional mineral or chemically synthesized insulation materials are facing severer environment or health challenges. Hence, this work aims at developing an alternative thermal insulation material from wood. Two species, balsa and Paulownia, were chosen for their naturally low densities. Lignin and hemicellulose components were removed to create a “foamed” structure with more air induced. Polylactic acid (PLA) was applied to cover the deligniﬁed wood samples to further improve the hydrophobicity. The microstructure, physical properties, thermal conductivity and insulation properties of the treated wood samples were assessed. After lignin and hemicellulose removal, the original porous micro skeleton of balsa and Paulownia wood was retained, along with an increased porosity to 96.6% and 94%, respectively. Meanwhile, the thermal conductivity was successfully decreased by 22% to 0.053 W/(m · K) for balsa wood and by 27% to 0.067 W/(m · K) for Paulownia wood. PLA-covering treatment further enhanced the water resistance of the deligniﬁed wood samples without an evident change in the thermal conductivity. The above ﬁndings demonstrated the feasibility of applying deligniﬁed wood as a potential insulation material in modern construction operations, which may help set up a new pathway for a low-carbon and energy-saving construction industry.


Introduction
Energy conservation for a variety of buildings has aroused increasing concerns worldwide, in terms of setting up a clean, safe and salubrious planet.In 2020, the energy consumption of building operations in China accounted for 45.5% of the national total quantity consumed.Along with the building operations, over 5 billion tons of carbon dioxide (CO 2 ) were produced, accounting for 50.9% of the national total quantity [1].Similar situations can also be seen in other regions in the world [2,3].Evidently, innovative building materials and processes are important points for conserving a green earth.
For a closed building, the envelope system, composed of walls, roof, doors and windows, dominates the energy flow with the outer environment.More and more new buildings choose prefabricated wall systems for their multiple benefits, where foamed insulation materials such as polyurethane, polystyrene, phenolic resin, wood fiber, hemp, rock wool or glass wool are applied [4][5][6].Those petrochemical or mineral-based materials are inconducive to sustainable development and may be harmful to residents' health [7,8].For example, styrene burning will produce a lot of smoke and toxic gases, polyurethane in the foaming process will produce toxic and harmful substances, glass wool will stimulate the eyes and respiratory tract.Therefore, finding more "green" and safer materials for thermal insulation is of great interest to researchers [9][10][11].
Some materials have the positive effect of reducing CO 2 in the atmosphere and are called negative carbon materials.As a special "negative carbon" material, wood has excellent characteristics such as a high specific strength, specific rigidity and an impact load-absorbing performance [12].Moreover, wood possesses a natural pore microstructure, which ensures its potential value in heat insulation applications [13,14].However, untreated solid wood still has higher thermal conductivity values when compared with the abovelisted traditional insulation materials, which are closely related to wood species.
Usually, the thermal conductivity of wood relies on its density or porosity level at an equal moisture content.Therefore, the further development of wood to improve its thermal insulation effect has been widely discussed.Fan et al. used a "bottom-up" method to synthesize aerogels from nano-cellulose in wood [15].The cellulose aerogels have excellent thermal insulation properties.However, this kind of "bottom-up" method usually produces isotropic aerogels with poor mechanical properties, and the preparation of nano-cellulose requires considerable chemical raw materials and energy, which is unconducive to the development of large-scale applications [16][17][18].
If the lignin and hemicellulose components are selectively removed from wood directly, a "foamed" wood can be acquired, where the cellulose skeleton is retained and the bulk density is decreased.The "foamed" wood prepared following this "top-down" strategy has a anisotropic structure, self-supporting strength, a good thermal insulation performance, and low processing costs.Li et al. prefabricated a wood-based lightweight thermal insulation material (termed as "nanowood") by removing the lignin and hemicellulose from American basswood [19].The "nanowood" showed anisotropic thermal conductivities, i.e., an extremely low value of 0.03 W/(m•K) in the transverse direction and 0.06 W/(m•K) along the cellulose alignment direction.A tested compression strength of 13 MPa confirmed the self-supporting ability of the treated wood, once installed in an insulation wall or roof.Similarly, Han et al. obtained super-hydrophobic and heat-insulating rattan aerogels, by deeply removing lignin and hemicellulose components, as well as further chemical modification [20].
When following the "top-down" pathway in setting up an insulation wood, it is crucial to control the density at a proper level.Usually, the thermal conductivity of materials will decrease with the decrease in density.However, when the wood density decreases to a certain value without a change in the temperature or relative humidity, although the solid phase thermal conductivity of the materials will decrease, the gas phase thermal conductivity, radiation thermal conductivity and convection thermal conductivity through the materials may increase [21].
In addition, after the treatment to remove hemicellulose and lignin, it becomes easier for wood to absorb water, due to the exposure of surface hydroxyl and the increase in porosity, which may weaken the thermo-hygroscopic stability and long-period durability of in-service insulation materials in an actual environment.Although some researchers use chemical [22,23] or physical [24,25] methods to enhance wood hydrophobicity, technical limitations still exist, from the perspective of environmental protection.At present, most studies on wood hydrophobic modification use expensive and environmentally unfriendly modification agents or petroleum-based polymer materials.Therefore, it is necessary to seek a simpler, more efficient and environmentally friendly modification method.Polylactic acid (PLA), as a biobased and biodegradable polymer material, has attracted extensive attention in recent years [26,27].The introduction of polylactic acid to cover the treated wood "foam" supplies another "green" solution to pay back the thermo-hygroscopic stability.
For insulation applications, low density wood, especially some fast-grown planted species, may be more attractive, considering their rich sources, low strength, and more economic "top-down" treatment.The purpose of this work is to improve the feasibility of wood as a construction thermal insulation material by modifying it.Therefore, this work chose two local wood species, i.e., balsa wood and paulownia, to fabricate insulation "foams".Their components were selectively removed to different degrees to explore suitable treatment methods.Polylactic acid was applied to coat the treated wood to improve its hydrophobicity.The microstructure, physical properties, thermal conductivity and thermal insulation properties were specifically characterized.

Chemical Treatment of Wood 2.2.1. Delignification
The natural wood (NW, Figure 1) was first oven-dried at 60 • C until its mass no longer changed with time, and we then immersed the wood in a 2 wt% sodium chlorite solution with a pH value of 4.6 at 80 • C. The pH of the solution was adjusted with acetic acid, and the solution was changed every 6 h.The treatment time was 8 h for balsa wood and 12 h for paulownia wood.The treated wood was cleaned with deionized water until the residual solution was completely removed.
its hydrophobicity.The microstructure, physical properties, thermal conductivity and thermal insulation properties were specifically characterized.

Delignification
The natural wood (NW, Figure 1) was first oven-dried at 60 °C until its mass no longer changed with time, and we then immersed the wood in a 2 wt% sodium chlorite solution with a pH value of 4.6 at 80 °C.The pH of the solution was adjusted with acetic acid, and the solution was changed every 6 h.The treatment time was 8 h for balsa wood and 12 h for paulownia wood.The treated wood was cleaned with deionized water until the residual solution was completely removed.

Further Hemicellulose Removal
The delignified samples were further immersed in 8 wt% sodium hydroxide solution at 80 °C to remove hemicellulose.The treatment time was 6 h for balsa wood and 7 h for paulownia wood.Then the treated wood was cleaned with deionized water and was frozen at −20 °C for 24 h.The samples were subsequently transferred to a vacuum freeze

Further Hemicellulose Removal
The delignified samples were further immersed in 8 wt% sodium hydroxide solution at 80 • C to remove hemicellulose.The treatment time was 6 h for balsa wood and 7 h for paulownia wood.Then the treated wood was cleaned with deionized water and was frozen at −20 • C for 24 h.The samples were subsequently transferred to a vacuum freeze dryer at −40 • C for more than 24 h, where the water in the samples (DHW, Figure 1) was completely removed.

Characterization 2.3.1. Scanning Electron Microscopy (SEM)
The samples were first sputtered by gold using a sputter coater (Oxford Ultim Max 65, Oxford Instruments, Oxford, UK), and they were then observed by SEM instruments (Quanta 200 FEI, Beaverton, OR, USA) at an accelerated voltage of 10 kV.

Fourier Transform Infrared Spectroscopy (FTIR) Microscopy
Chemical bonds were obtained from a Fourier transform infrared (FTIR) spectrometer (Nicolet IS10, Thermo Fisher Scientific, Waltham, MA, USA), at a resolution of 4 cm −1 in the wavenumber range of 4000 cm −1 .

X-ray Diffraction (XRD)
The crystal structure was examined by an X-ray diffractometer (XRD, Bruker D8 Advance, Bruker, Karlsruhe, Germany), equipped with Cu K+radiation (+0.15406 nm), scanned in the range of 10-50 • at a scan rate of 10 • /min.Crystallinity is calculated as: where I 002 represents the highest intensity and I am represents the lowest intensity between the primary and secondary peaks.

Mass Loss Rate and Porosity
The mass loss percentage (MLP) was calculated using the gravimetric analysis method, to judge the removal effect of various components in the wood.The calculation formula is as follows: where M 0 is the weight of the sample before treatment and M 1 is the weight of the sample after the freeze-drying treatment.
The porosity of wood is estimated according to the following equation: where ρ 1 and ρ 2 are the density of the wood samples and solid wood, respectively.ρ 2 is 1570 kg/m 3 [28].

Water Contact Angle and Water Absorption Mass Change Rate
The contact angle of the sample was tested with a JY-82C (Chengde Dingsheng Testing Machine Testing Equipment Co., Chengde, China) contact angle goniometer under indoor conditions.The sample was placed on a movable platform and ~2 µL of distilled water was dropped onto the surface to characterize its hydrophobic properties.The change rate of water absorption mass of the sample is estimated according to the following equation: where W is the change rate of water absorption mass, m 2 is the mass of the sample after water absorption and m 1 is the mass of the sample before water absorption.The change rate of water absorption quality of the sample was tested at 2 h, 4 h, 6 h, 9 h, 12 h and 24 h.

Thermal Performance
The thermal conductivity of the sample was measured using the Hot Disk (Hot Disk AB, Göteborg, Sweden) instrument and the sample was measured at an ambient temperature of 25 • C. The infrared thermal imaging images of the sample were taken by HM-TPH11 (HIKMICRO, Hangzhou, China) thermograph.The sample was placed on a plane heat source at 50 • C, and the temperature change was measured every minute.The total thermal conductivity (λ t ) can be described by four different contributions: where λ s is the conduction across the cell walls and struts of the solid material, λ g is the conduction along the gas phase, λ c represents the convection within the cells and λ r is the thermal radiation term.

SEM Analysis
Wood is mainly composed of cellulose, lignin and hemicellulose, accounting for 40%-50%, 20%-35% and 20%-30% of the total mass of wood, respectively, although this varies depending on the wood species.Lignin is one of the main factors causing wood to be light opaque, because of its chromophore group, with a 1.61 refractive index [29].In the process of wood delignification in an acidic sodium chlorite solution, the aromatic ring in lignin is oxidized by the free radical of ClO 2 , which leads to the continuous reduction in lignin content [30,31].And because holocellulose is colorless [32], the removal rate of lignin can be preliminarily evaluated by the change in wood color.As shown in Figure 2b,e, the wood is white under LED light transmission after being treated with acid sodium chlorite, so the removal of lignin can be preliminarily judged.
OR PEER REVIEW 6 of 13 solution was volatilized, the PLA formed a mechanical interlock structure with the wood, which greatly improved the adhesion of the PLA, as shown in the yellow area of Figure 2.

Fourier Transform Infrared Spectroscopy and X-ray Diffraction Analysis
The infrared spectra of wood samples are shown in Figure 3a,b.The NW characteristic peaks at 1502 cm −1 and 1462 cm −1 were derived from the aromatic skeleton vibration of lignin.After NaClO2 treatment, the characteristic peaks of balsa wood and paulownia In order to investigate the microscopic level changes in the cell wall, caused by the removal of the components in wood, SEM tests were performed on each sample.As shown in Figure 2a-f, by observing the transverse and longitudinal sections of the wood, it can be intuitively seen the NW cell wall was intact and tightly bound across the wood's crosssection.In DW, the inner cavity of each cell was clear and well defined, but due to the removal of lignin, cracks began to appear in the cell wall, which shows that delignification created more pore structure in the wood.After the further removal of hemicellulose, more damage to the wood cell wall was obvious.The wood cells began to separate from each other, and some of the cell walls were distorted.The original structure of the wood was damaged to a certain extent after the drastic removal of components.The fracture structure in wood may have been due to the removal of lignin and hemicellulose, as the structure of the wood cell wall became too loose, and the supporting function of the cell wall was reduced, so it was difficult to fully maintain the original structure.However, it is still clear that the skeleton of the treated wood was perfectly retained.
The microstructure of PDW is shown in Figure 2g,h.After DW is coated with PLA solution, the PLA adhered to the surface of wood to form a protective layer.The long chain of PLA penetrated into the surface cells of the wood with the solution, and after the solution was volatilized, the PLA formed a mechanical interlock structure with the wood, which greatly improved the adhesion of the PLA, as shown in the yellow area of Figure 2.

Fourier Transform Infrared Spectroscopy and X-ray Diffraction Analysis
The infrared spectra of wood samples are shown in Figure 3a,b.The NW characteristic peaks at 1502 cm −1 and 1462 cm −1 were derived from the aromatic skeleton vibration of lignin.After NaClO 2 treatment, the characteristic peaks of balsa wood and paulownia disappeared, which proved the successful removal of lignin [33].The characteristic peaks at 1737 cm −1 and 1234 cm −1 in NW were derived from hemicellulose, and the hemicellulose in DW was also successfully removed after further NaOH treatment, which corresponded to the disappearance of the characteristic peak here in the infrared spectrum [34].
At the same time, in order to further evaluate the impact of chemical treatment on the wood, XRD tests were conducted on the wood, and the results are shown in Figure 3c,d.Although the lignin and hemicellulose in the wood were removed by chemical treatment, it can be intuitively seen from the figure that the positions of I 002 and I am of DW and DHW did not change relative to NW, thus demonstrating that the chemical treatment of the wood did not affect the cellulose crystallization zone [35].However, compared with NW, the crystallinity of balsa wood and paulownia wood increased to different degrees after chemical treatment.It can be inferred that after chemical treatment, the lignin and hemicellulose in the wood were removed, leaving only the highly oriented cellulose.With the removal of lignin and hemicelluloses, the hydroxyl groups between the molecular chains in the amorphous region of cellulose were combined with each other through hydrogen bonding, so that the cellulose in the amorphous region was more closely arranged, so the crystallinity of the wood after chemical treatment was relatively improved [36].

Mass Loss Rate and Porosity Change Analysis
As can be seen in Figure 4a, after lignin removal, the mass loss rate of balsa wood was 24%, while that of paulownia wood was 40%.According to the study of Qin et al., this is because paulownia contains more extractive components, which are extracted together in the delignification process [37].After further removal of hemicellulose, the total mass loss rate of balsa wood was 55%, and the total mass loss rate of paulownia was 58.5%, which was roughly similar to the sum of the content of lignin and hemicellulose in wood.Therefore, the complete removal of lignin and hemicellulose can also be verified from the side.Since the wood sample was freeze-dried to remove the water, the wood retained its original size and structure after the water is removed by sublimation, however, because the composition of wood was reduced and the volume was not changed, the density was also reduced.Correspondingly, the change in wood porosity was calculated according to formula (3) and presented in Figure 4b.Whether it was balsa wood or paulownia wood, the porosity of DW was higher than that of NW, and the porosity of DHW was higher than that of DW.In combination with Figure 4a,b, it can be seen that the higher the component removal rate, the greater the porosity obtained, under the premise that the volume of wood remains unchanged during chemical treatment.

Mass Loss Rate and Porosity Change Analysis
As can be seen in Figure 4a, after lignin removal, the mass loss rate of balsa wood was 24%, while that of paulownia wood was 40%.According to the study of Qin et al., this is because paulownia contains more extractive components, which are extracted together in the delignification process [37].After further removal of hemicellulose, the total mass loss rate of balsa wood was 55%, and the total mass loss rate of paulownia was 58.5%, which was roughly similar to the sum of the content of lignin and hemicellulose in wood.Therefore, the complete removal of lignin and hemicellulose can also be verified from the side.Since the wood sample was freeze-dried to remove the water, the wood retained its original size and structure after the water is removed by sublimation, however, because the composition of wood was reduced and the volume was not changed, the density was also reduced.Correspondingly, the change in wood porosity was calculated according to formula (3) and presented in Figure 4b.Whether it was balsa wood or paulownia wood, the porosity of DW was higher than that of NW, and the porosity of DHW was higher than that of DW.In combination with Figure 4a,b, it can be seen that the higher the component removal rate, the greater the porosity obtained, under the premise that the volume of wood remains unchanged during chemical treatment.

Water Contact Angle and Water Absorption Mass Change Rate Analysis
The water absorption mass change rate of wood samples is shown in Figure 5a,b.It can be intuitively seen from the figures that, for balsa wood and paulownia wood, the water absorption mass change rate of DW was higher than that of NW, and DHW was higher than that of DW.From the perspective of time change, whether balsa wood or paulownia wood, the change rate of water absorption mass of NW was still increasing at 24 h, DW reached saturation at 9 h, and DHW reached saturation at 2 h.Evidently, the increase in wood porosity created more passages for water entering into wood [38].Additionally, more exposed hydrophilic hydroxyl groups bound more water molecules through hydrogen bonds [39].
After PLA-coating treatment, however, the water absorption of the extracted wood was greatly reduced.It is understandable that the pathways of water entering the sample were blocked due to the PLA film wrapped on the surface.Also, the hydrophilic -OH groups on the surface of wood were fully covered.This can be further confirmed by water contact angle analysis of virgin and extracted wood samples (i.e., NW→DW→DHW→PDW, Figure 6).PLA-coated wood samples showed extremely high values of 95-107° (up to 60 s), which accounted for the low surface energy of PLA [40].Therefore, the PLA coating strategy was strongly proven effective in enhancing the water repellence of chemically extracted wood as an alternative "foamed" insulation material.

Water Contact Angle and Water Absorption Mass Change Rate Analysis
The water absorption mass change rate of wood samples is shown in Figure 5a,b.It can be intuitively seen from the figures that, for balsa wood and paulownia wood, the water absorption mass change rate of DW was higher than that of NW, and DHW was higher than that of DW.From the perspective of time change, whether balsa wood or paulownia wood, the change rate of water absorption mass of NW was still increasing at 24 h, DW reached saturation at 9 h, and DHW reached saturation at 2 h.Evidently, the increase in wood porosity created more passages for water entering into wood [38].Additionally, more exposed hydrophilic hydroxyl groups bound more water molecules through hydrogen bonds [39].

Water Contact Angle and Water Absorption Mass Change Rate Analysis
The water absorption mass change rate of wood samples is shown in Figure 5a,b.It can be intuitively seen from the figures that, for balsa wood and paulownia wood, the water absorption mass change rate of DW was higher than that of NW, and DHW was higher than that of DW.From the perspective of time change, whether balsa wood or paulownia wood, the change rate of water absorption mass of NW was still increasing at 24 h, DW reached saturation at 9 h, and DHW reached saturation at 2 h.Evidently, the increase in wood porosity created more passages for water entering into wood [38].Additionally, more exposed hydrophilic hydroxyl groups bound more water molecules through hydrogen bonds [39].
After PLA-coating treatment, however, the water absorption of the extracted wood was greatly reduced.It is understandable that the pathways of water entering the sample were blocked due to the PLA film wrapped on the surface.Also, the hydrophilic -OH groups on the surface of wood were fully covered.This can be further confirmed by water contact angle analysis of virgin and extracted wood samples (i.e., NW→DW→DHW→PDW, Figure 6).PLA-coated wood samples showed extremely high values of 95-107° (up to 60 s), which accounted for the low surface energy of PLA [40].Therefore, the PLA coating strategy was strongly proven effective in enhancing the water repellence of chemically extracted wood as an alternative "foamed" insulation material.After PLA-coating treatment, however, the water absorption of the extracted wood was greatly reduced.It is understandable that the pathways of water entering the sample were blocked due to the PLA film wrapped on the surface.Also, the hydrophilic -OH groups on the surface of wood were fully covered.This can be further confirmed by water contact angle analysis of virgin and extracted wood samples (i.e., NW→DW→DHW→PDW, Figure 6).PLA-coated wood samples showed extremely high values of 95-107 • (up to 60 s), which accounted for the low surface energy of PLA [40].Therefore, the PLA coating strategy was strongly proven effective in enhancing the water repellence of chemically extracted wood as an alternative "foamed" insulation material.

Thermal Conductivity and Infrared Thermal Imaging Analysis
As shown in Figure 7a, the overall thermal conductivity (λt) of balsa wood and paulownia wood decreased significantly after the removal of the components.This was due to the removal of lignin and hemicellulose and other components, resulting in more pores in the wood cell wall; these closed-cell structures in the cell wall locked part of the gas and prevented the heat transfer of the gas [41].Thus, the heat transfer by gas (λg) was reduced.At the same time, because the cell wall became thinner and more curved, the heat transfer path through the cell wall became more tortuous, so the heat transfer by solid λs was reduced.Moreover, the increase in porosity resulted in more air in the sample, and since air is a poor conductor of heat, the thermal conductivity was reduced.
It is noticed that the thermal conductivity of DHW was slightly higher than that of DW.According to SEM images (Figure 2c,f), it can be seen that the cell wall shape structure in DHW was damaged, due to the drastic removal of wood components, and large cracks were generated in the wood, as shown in the blue area of Figure 2. λg increased as the aperture increased, which led to more heat conduction through the gas [42].And, because the density of DHW was lower than that of DW, DHW's resistance to infrared radiation became smaller, so λr decreased.Since the pore diameters in wood were all smaller than the millimeter level, λc could be ignored for wood.These factors ultimately resulted in DHW having a slightly higher thermal conductivity than DW [21].
PLA coating promoted the heat transfer behavior of delignified wood, resulting in a higher thermal conductivity (Figure 7b,c).Accordingly, a higher amount of PLA resulted in increased λt values, regardless of wood species.However, a proper loading level of PLA (e.g., equivalent to 70%, PDW-1, Figure 7a) did not significantly change the thermal conductivity of delignified wood (DW).

Thermal Conductivity and Infrared Thermal Imaging Analysis
As shown in Figure 7a, the overall thermal conductivity (λ t ) of balsa wood and paulownia wood decreased significantly after the removal of the components.This was due to the removal of lignin and hemicellulose and other components, resulting in more pores in the wood cell wall; these closed-cell structures in the cell wall locked part of the gas and prevented the heat transfer of the gas [41].Thus, the heat transfer by gas (λ g ) was reduced.At the same time, because the cell wall became thinner and more curved, the heat transfer path through the cell wall became more tortuous, so the heat transfer by solid λ s was reduced.Moreover, the increase in porosity resulted in more air in the sample, and since air is a poor conductor of heat, the thermal conductivity was reduced.More practically, the temperature-changing behavior of different wood samples, subjected to 20 min of heating by a plane heat source, was recorded (Figure 8).Whether balsa or paulownia wood, after component removal, the final temperatures at the sample surface were lower than those of NW, which showed that DW, DHW and PDW had a better thermal insulation effect than the control.More interestingly, 70% PLA-coated wood samples were demonstrated to have the lowest ending temperature after being heated, which was comparable to the DW condition.The observed results reconfirm the advantage of PLA coating in protecting delignified wood "foams".From a future perspective, this is crucial in the consideration of constructing a full-size insulation "foam" by laminating It is noticed that the thermal conductivity of DHW was slightly higher than that of DW.According to SEM images (Figure 2c,f), it can be seen that the cell wall shape structure in DHW was damaged, due to the drastic removal of wood components, and large cracks were generated in the wood, as shown in the blue area of Figure 2. λ g increased as the aperture increased, which led to more heat conduction through the gas [42].And, because the density of DHW was lower than that of DW, DHW's resistance to infrared radiation became smaller, so λ r decreased.Since the pore diameters in wood were all smaller than the millimeter level, λ c could be ignored for wood.These factors ultimately resulted in DHW having a slightly higher thermal conductivity than DW [21].
PLA coating promoted the heat transfer behavior of delignified wood, resulting in a higher thermal conductivity (Figure 7b,c).Accordingly, a higher amount of PLA resulted in increased λ t values, regardless of wood species.However, a proper loading level of PLA (e.g., equivalent to 70%, PDW-1, Figure 7a) did not significantly change the thermal conductivity of delignified wood (DW).
More practically, the temperature-changing behavior of different wood samples, subjected to 20 min of heating by a plane heat source, was recorded (Figure 8).Whether balsa or paulownia wood, after component removal, the final temperatures at the sample surface were lower than those of NW, which showed that DW, DHW and PDW had a better thermal insulation effect than the control.More interestingly, 70% PLA-coated wood samples were demonstrated to have the lowest ending temperature after being heated, which was comparable to the DW condition.The observed results reconfirm the advantage of PLA coating in protecting delignified wood "foams".From a future perspective, this is crucial in the consideration of constructing a full-size insulation "foam" by laminating thin delignified wood veneers, where PLA may act as not merely a water-repelling coating but also as a bonding agent.More practically, the temperature-changing behavior of different wood samples, subjected to 20 min of heating by a plane heat source, was recorded (Figure 8).Whether balsa or paulownia wood, after component removal, the final temperatures at the sample surface were lower than those of NW, which showed that DW, DHW and PDW had a better thermal insulation effect than the control.More interestingly, 70% PLA-coated wood samples were demonstrated to have the lowest ending temperature after being heated, which was comparable to the DW condition.The observed results reconfirm the advantage of PLA coating in protecting delignified wood "foams".From a future perspective, this is crucial in the consideration of constructing a full-size insulation "foam" by laminating thin delignified wood veneers, where PLA may act as not merely a water-repelling coating but also as a bonding agent.

Conclusions
To improve the feasibility of wood as a construction thermal insulation material, low-density wood was treated to obtain wood "foams", and it was hydrophobically modified with PLA.It turns out that only a delignification treatment without additional hemicellulose removal achieved a better thermal insulation effect, and this method is more economic.After delignification, the thermal conductivity of balsa wood decreased from 0.068 W/(m•K) to 0.052 W/(m•K), and that of paulownia wood decreased from 0.092 W/(m•K) to 0.066 W/(m•K).Furthermore, by utilizing PLA film wrapped on the surface of balsa wood and paulownia wood delignification samples, the hydrophobicity of the sample was improved, and the change rate of 24 h water absorption quality was greatly reduced.The thermal conductivity of balsa wood and paulownia wood delignified samples did not change when the amount of PLA applied was 70 wt% of DW, but excessive PLA application adversely affected the thermal conductivity.

Figure 1 .
Figure 1.Schematic procedures of wood treatment.

Figure 1 .
Figure 1.Schematic procedures of wood treatment.

Figure 3 .
Figure 3. FTIR and XRD images of the sample: (a) FTIR of balsa wood; (b) FTIR of Paulownia wood; (c) XRD of balsa wood; (d) XRD of Paulownia wood.

Figure 3 .
Figure 3. FTIR and XRD images of the sample: (a) FTIR of balsa wood; (b) FTIR of Paulownia wood; (c) XRD of balsa wood; (d) XRD of Paulownia wood.

Figure 4 .
Figure 4. Physical properties of different wood samples: (a) rate of mass loss of different wood samples; (b) the porosity of different wood samples.

Figure 5 .
Figure 5. Water absorption property of different wood samples: (a) change rate of water absorption quality of balsa wood sample; (b) change rate of water absorption quality of paulownia wood sam-

Figure 4 .
Figure 4. Physical properties of different wood samples: (a) rate of mass loss of different wood samples; (b) the porosity of different wood samples.

Figure 4 .
Figure 4. Physical properties of different wood samples: (a) rate of mass loss of different wood samples; (b) the porosity of different wood samples.

Figure 5 .
Figure 5. Water absorption property of different wood samples: (a) change rate of water absorption quality of balsa wood sample; (b) change rate of water absorption quality of paulownia wood sample.

Figure 5 .
Figure 5. Water absorption property of different wood samples: (a) change rate of water absorption quality of balsa wood sample; (b) change rate of water absorption quality of paulownia wood sample.

Figure 6 .
Figure 6.Water contact angle of various samples of balsa and paulownia wood.

Figure 6 .
Figure 6.Water contact angle of various samples of balsa and paulownia wood.

Figure 7 .
Figure 7. Thermal conductivity of different samples: (a) thermal conductivity of NW, DW, DHW, PDW-1; (b) thermal conductivity of balsa wood PDW changes with the addition of PLA; (c) thermal conductivity of paulownia wood PDW changes with the addition of PLA.

Figure 7 .
Figure 7. Thermal conductivity of different samples: (a) thermal conductivity of NW, DW, DHW, PDW-1; (b) thermal conductivity of balsa wood PDW changes with the addition of PLA; (c) thermal conductivity of paulownia wood PDW changes with the addition of PLA.

Figure 7 .
Figure 7. Thermal conductivity of different samples: (a) thermal conductivity of NW, DW, DHW, PDW-1; (b) thermal conductivity of balsa wood PDW changes with the addition of PLA; (c) thermal conductivity of paulownia wood PDW changes with the addition of PLA.