Processing high-performance woody materials by means of vacuum-assisted resin infusion technology

Journal of Cleaner Production 241 (2019) 118340

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Processing high-performance woody materials by means of vacuumassisted resin infusion technology Changlei Xia a, b, c, 1, *, Yingji Wu a, 1, Ying Qiu d, 1, Liping Cai a, b, Lee M. Smith b, Maobing Tu c, Weihuan Zhao b, Dongwei Shao b, e, Changtong Mei a, Xu Nie d, Sheldon Q. Shi b, ** a

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, Jiangsu, 210037, China Department of Mechanical and Energy Engineering, University of North Texas, Denton, TX, 76203, USA Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221, USA d Department of Mechanical Engineering, Southern Methodist University, Dallas, TX, 75275, USA e College of Mechanical Engineering, Jiamusi University, Jiamusi, Heilongjiang, 154007, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2019 Received in revised form 28 August 2019 Accepted 7 September 2019 Available online 9 September 2019

High-performance materials derived from natural wood have been vigorously developed, however, their commercialized products are very limited owing to the lack of industrial production efficiency and increase of environmental concern. This study is aimed at developing a clean process for large-scale production of high-performance woody materials without involving any chemical methods. The industrialized vacuum-assisted resin infusion (VARI) process was initially applied to solid wood for manufacturing woody materials with excellent flexural performances. VARI process can provide better control over harmful volatiles generated by resins than that of the traditional open mold techniques, making them compliant with “clean production”. The porosity was reduced from 66.0% to 33.8% after infusing epoxy into wood through the VARI process, and further reduced to 22.6% after being compressed. The flexural modulus and strength were significantly increased by 142.2% and 142.8%, respectively. The flexural strength (304.9 MPa) reached the range of alloy, between Alloy 2024-T3 (345 MPa) and Steel alloy 1040 (260 MPa). Due to its relatively low density, the specific flexural strength was only slightly lower than diamond. The dramatic reduction of porosity was deemed to be the major contribution to those improvements. The fabrication of high-performance woody materials does not use chemical treatment; therefore, it could be considered as a clean production. Additionally, the contribution to clean production of this study was further confirmed by life-cycle assessment (LCA). The comparisons of environmental impacts of the woody material, steel alloy and aluminum alloy were carried out using LCA, revealing that the global warming, acidification, human health (HH) cancer, HH noncancer, HH criteria air pollutants, eutrophication, ecotoxicity, smog, natural resource depletion, habitat alteration, and ozone depletion, were reduced by 1.72e667.06 times when the woody material was used to replace steel alloy, while were reduced by 7.34e636.21 times when the woody material was utilized to replace aluminum alloy, respectively. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor. M.T. Moreira Keywords: Green/sustainable engineering Wood Porosity Resin infusion Flexural property Environmental impacts

1. Introduction

* Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, Jiangsu, 210037, China. ** Corresponding author. E-mail addresses: [email protected] (C. Xia), [email protected] (S.Q. Shi). 1 These authors contributed equally to the manuscript. https://doi.org/10.1016/j.jclepro.2019.118340 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

In order to enhance the performance of woody materials for extending their applications, modification of wood has become an important industrial practice to produce high performance woodbased products (Priadi and Hiziroglu, 2013). Densification of wood for enhancing mechanical properties has been widely investigated (Bekhta et al., 2018). The property enhancement of densification is mainly contributed by the porosity reduction.

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C. Xia et al. / Journal of Cleaner Production 241 (2019) 118340

Nomenclature list

sf sfM stM εf εfM

ra rg 2q

ATR BEES CIXRD Da Dm DMA E0

Fracture stress (MPa) Fracture strength (maximum fracture stress) (MPa) Tensile strength (MPa) Strain at fracture test (
) Fracture strain at break (
) Apparent density (g cm
3) Gas density determined by gas pycnometer (g cm
3) The diffraction angles of the various cones in XRD tests ( ) Attenuated total reflection (
) Building for environmental and economic sustainability (
) Crystallinity index determined from XRD pattern (
) Average pore diameter from mercury intrusion tests (mm) Median pore diameter from mercury intrusion tests (mm) Dynamic mechanical analysis (
) DMA storage modulus (MPa)

Currently, Song et al. applied a high pressure to the kraft treated solid wood for achieving ultra-high mechanical properties, being published in “Nature” (Song et al., 2018). After being compressed by 80% of thickness, the tensile strength could be 548.8 MPa, which is close to the value of SS304 (510e620 MPa) and the flexural strength can be approximately 300 MPa, considered as a promising alternative to structural metals due to its low-cost, high-performance, and lightweight. However, the abundant of delignifying agents (NaOH and Na2SO3) used in the pretreatment and long processing time (more than 7 h) may increase the environmental pollution and limit its applications (Song et al., 2018). Traditional wood modifications mainly utilize chemical modifications or thermal treatments to improve the wood properties. Thermal treatment was reported to prevent swelling and shrinking of the wood (Stamm, 1956), however, the mechanical properties are usually decreased and the adhesion properties of wood surface are altered (Mizukoshi et al., 2018). Chemical modifications have been raised the interests on the wood modification (Dong et al., 2017), which can usually improve antibacterial, water resistance, UVresistance and thermal-stable properties with high brittleness and strength losses of wood (Xiao et al., 2010). However, there was the conflict of views for the high brittleness and strength losses, claiming there have been reports on wood modification by chemical treatment, which not only led to decreased shrinking and swelling and also enhanced mechanical properties (Deka et al., € 2012; Hazarika and Maji, 2013, 2014). Ozmen and Çetin (2002) reported that chemical treatment improved bond quality between hydrophilic wood and hydrophobic monomer, but also led to a detrimental effect on the tensile strength. Kyziol (2016) reported that the surface-treatment of pinewood with polymerized polymethylmethacrylate (PMMA) could significantly increase the strength. When 0.35 kg or 0.56 kg PMMA was added to 1 kg pine wood, the tensile strength along fibers was increased from 95 MPa to 102 or 118 MPa, respectively (Kyziol, 2016). By adding wood wastes to gypsum, the thermal and acoustic properties of ceiling ~ o-Rojas et al., 2017). Usman et al. plates were improved (Pedren (2018) utilized wood sawdust to improve self-compacting cementitious systems. Although the strength was reduced by 11%e34%, the shrinkage strain was greatly reduced because the sawdust could greatly absorb water. The tensile strength of engineered

E'' Ef FT-IR HH I002 Imin L LCA m Micro-CT Na2SO3 NaOH Pg PHg Pm SEM tan(d) Va VARI Vg XRD

DMA loss modulus (MPa) Fracture modulus (GPa) Fourier-transform infrared spectroscopy (
) Human health (
) XRD intensity at (002) (
) Minimum XRD intensity between (101) and (002) (
) Gauge length for fracture tests (cm) Life-cycle assessment (
) Sample mass (g) X-ray micro computed tomography (
) Sodium sulfite (
) Sodium hydroxide (
) Porosity determined by gas pycnometer (
) Porosity from mercury intrusion tests (
) Porosity from Micro-CT analysis (
) Scanning electron microscope (
) Mechanical loss factor from DMA test (
) Apparent volume (cm3) Vacuum-assisted resin infusion (
) Gas volume determined by gas pycnometer (cm3) X-ray powder diffraction (
)

cementitious composite was increased using polypropylene fiber (Khoso et al., 2018). The service life of wood components of buildings was increased by accurately determining and controlling their moisture content (Crean, 2017). Mechanical properties of some wood products were improved through the using of the modified phenol formaldehyde resin and metal adsorption of modified resin derived from lignin (Arasaretnam and Kirudchayini, 2019). The vacuum-assisted resin infusion (VARI) is an excellent technology for fabricating fiber (e.g., glass fiber, carbon fiber, natural fiber, etc.) based composites (Halimi et al., 2013; Menta et al., 2013; Wu et al., 2018b), which was applied to the glass-fiber/carbon fiber skin layers to fabricate wood-core sandwich structures to enhance the strength of the products (Pirvu et al., 2004; Qi et al., 2017). As a clean production technology, VARI process provided better control of harmful volatiles generated by resins compared to the traditional open mold techniques. However, no studies regarding the use of VARI technology on solid wood have been reported. As an ideal technology for reducing the porosity, the promising application in solid wood was anticipated. Being distinct from the work from Song et al. (2018), the VARI process will not generate chemical waste and much shorter processing time, without compromising flexural performance of the fabricated woody materials. The clean production of the biomass-based composites could be exhibited through life-cycle assessment (LCA) that has been accepted in the evaluation of environmental impacts of different materials globally. Recently, many studies using LCA to compare the environmental performance of biomass-based materials, e.g., kenaf fiber and glassfiber reinforced cement panels (Zhou et al., 2018), natural fiberreinforced composite and automotive glass-fiber sheet compound (Wu et al., 2018a), four different construction materials, i.e., bamboo, brick, concrete hollow block, and engineered bamboo in buildings (Zea Escamilla et al., 2018). Here, the solid wood samples were impregnated by epoxy resin through the VARI process and then compressed, expecting to reduce porosity of woody materials. The properties of the fabricated woody materials were examined. The analyses of ATR FT-IR, XRD, Micro-CT, SEM, gas pycnometer, and mercury intrusion porosimetry were used to characterize the samples and explore the mechanism of property enhancement. The effects of porosities of

C. Xia et al. / Journal of Cleaner Production 241 (2019) 118340

the woody materials on their properties were investigated. In addition, the environmental impacts were evaluated by LCA.

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compression are compared in Table 1. The compositions of the Wood/Epoxy and Compressed Wood/Epoxy samples were calculated by the weight difference before and after processing.

2. Experimental section This section describes the preparation and characterization of woody materials, and the examinations of micromorphology, porosity, mechanical property and water absorption of the resulted specimens. 2.1. Specimen preparation Poplar (Populus tremula) was utilized in this modification study because it is widely available throughout Eastern USA. Poplar boards were obtained from a furniture mill in Dallas, TX, USA, with a dimension of 6.35  88.9  609.6 mm3 (thickness  width  length). The Poplar boards were cut into 165 mm length as samples to be used for further treatments or control ones. Epoxy (1159 epoxy and 1160 harder) resin was purchased from Composite Envisions LLC., USA. Four groups of samples were prepared and examined: sole epoxy (named Epoxy), sole wood as a control group (named Wood), wood impregnated with epoxy using VARI (named Wood/Epoxy), and wood impregnated with epoxy using VARI and then compressed (named Compressed Wood/ Epoxy). The flowchart for preparing Wood/Epoxy and Compressed Wood/Epoxy is shown in Fig. 1A. Firstly, the wood sample with a dimension of 88.9  165 mm2 was wrapped by a peel ply and a vacuum bag. Two spiral tubing was placed on both ends of the board for dispersing the resin through vacuum force. The vacuum bag was then sealed by Airtech AT200Y tape. The detailed assembly was described in Fig. 1B. The fresh mixed epoxy resin was infused in the longitudinal direction by the vacuum pump (1.3e1.6 kPa). The curing temperature was 100 and 150  C (duration for 1 h each). After cooling down, the epoxy infused wood was prepared, namely, Wood/Epoxy. The fabrication of the Compressed Wood/Epoxy samples were similar to the Wood/Epoxy samples. After the resin infusion process, the samples were compressed at approximately 13.2 MPa and simultaneously cured with heating. The changes of thickness and compositions of the wood after resin infusion and

2.2. Chemical structure characterization The attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR, PerkinElmer Spectrum II Spectrometer) and X-ray diffraction (XRD, Philips X'Pert X-ray diffractometer) were used in this study. The wavenumbers used in FT-IR tests were 500e4000 cm
1 (interval 0.1 cm
1). The Cu Ka radiation wavelength was 1.5405980 Å. The X-ray was operated with a tension at 45 kV and a current at 40 mA. The scanning range was 10 e50 (2q). In terms of the method from Segal et al. (1959), the crystallinity index (CIXRD) was determined by Eq. (1):

CIXRD ¼

ðI002
Imin Þ  100% I002

(1)

where “I002” is the intensity at (002), and “Imin” is the minimum intensity between (101) and (002).

Table 1 The changes of thickness and compositions of the wood after resin infusion and compression. Sample

Wood Wood/Epoxy Compressed Wood/Epoxy

Thickness

Composition

(mm)

(wt%)

6.28 (0.01) 6.26 (0.01) 3.78 (0.16)

a

b

Wood

Epoxy

100 50.4 (2.5) 65.5 (1.7)

e 49.6 (2.5) 34.5 (1.7)

Notes. a Mean (standard deviation). b The composition in weight was calculated by the weight difference before and after the composite fabrication.

Fig. 1. (A) Flowchart of modified wood preparation. (B) Display of original wood, resin-filled wood with the assembly for VARI process, and modified wood.

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C. Xia et al. / Journal of Cleaner Production 241 (2019) 118340

2.3. Micromorphology observation and porosity analysis The SkyScan 1172 X-ray micro computed tomography (microCT) was employed for the investigation of inner structures and compounds of the samples, which were scanned at a tube with an acceleration voltage of 40 kV and a current of 250 mA. During the scan, the sample was rotated with a speed of 0.08 per step until the complete rotation (360 ). Those scans could result in 4500 projection radiographs (pixel size ¼ ~2 mm), which were reconstructed by the NRecon software. In terms of Data Viewer and CTvox, the 3dimensional I mages of the samples were generated. The scanning electron microscope (SEM, Quanta 200) was used for analyzing the surface micromorphology of the samples. The acceleration voltage of 20 kV was applied to the tests. Prior to the scanning, the samples were gold sputtering coated for 20 s. The dwelling time of 10 ms and magnifications of 100, 200 and 500 were used to collect the images. The Quantachrome Ultrapyc 1200e automatic density analyzer (±0.03% accuracy) was used to determine the porosity of the samples based on the bulk and gas densities. The single station automatic gas pycnometer was employed for the volume measurements of the samples using He gas as filler. The gas volume (Vg) of each sample was measured for calculating the gas density (rg) as follows:

rg ¼

m  100% Vg

(2)

where “m” is sample mass. Apparent density (ra) is calculated by m and apparent volume (Va) of the samples as follows:

ra ¼

m  100% Va

(3)

The gas porosity (Pg) was then calculated in according to the rg and ra as follows (Krus et al., 1997):

  r Pg ¼ 1
a  100%

rg

(4)

In addition, the contents of open and closed cells of the samples were afforded from the gas pycnometer analysis. The mercury intrusion porosimetry was used to determine the porosities and pore distributions of the samples. The Micromeritics Autopore IV 9500 was employed for the mercury intrusion tests. The 3-mL penetrometer (0.387IV, and 0.412SV) was used for the tests. The data at the pressures from 0.5 psi to 33000 psi were collected. 2.4. Mechanical property analysis Tensile tests (eight replicates, sample size of 12.5  165 mm2, gauge length of 50 mm, crosshead speed of 1.3 mm min
1) were carried out by following the procedures according to the ASTM D638 standard. The Shimadzu AGS-X universal machine (within ±0.5% indicated test force, ± 0.1% crosshead speed accuracy, ± 0.1% positional accuracy) was employed in these tests. Specific strength was calculated by Eq. (5):

Specific stM ¼

stM ra

(5)

where “st” is tensile strength; ra determined by Eq. (3). Fracture (3-point bending) tests (eight replicates, sample size of 12.5  165 mm2, gauge length (L) of 100 mm, bending rate of 5 mm min
1) were performed by means of the ASTM D790

standard. Using the resulted stain-stress curve, the fracture modulus (Ef), strength (sfM), and strain at break (εfM) were calculated by Eqs. (6)e(8), respectively.

Ef ¼

L3 m 4bd3

(6)  



sfM ¼ max sf ¼ max

3PL 2bd2



    6Dd εfM ¼ max εf ¼ max L2

(7)

(8)

where “m” is the slope of the curve; “b” is the width of sample; “d” is the thickness of sample; “P” is the load at a given point on the curve; and “D” is the maximum deflection. Three-point dynamic mechanical analysis (DMA) (Q800, TA Instruments) was performed for the Epoxy, Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples (size of 1  4  30 mm3 each, gauge length of 25.4 mm, temperature ranging from 35  C to 200  C, temperature ramping rate of 5  C min
1, frequency of 1 Hz, temperature accuracy of ±0.5  C). The curves of the E0 , E'', and tan(d) of the samples via temperatures were recorded. The force, strain, and tan(d) resolutions of Q800 DMA were 0.0001 N, 1 nm, and 0.00001, respectively, and the modulus precision was ±1%. 2.5. Water absorption test The ASTM D1037 standard was followed for water absorption tests of the Epoxy, Wood, Wood/Epoxy, and Compressed Wood/ Epoxy samples (six replicates, sized 20  20 mm2 each, oven-dried at 103 ± 2  C before tests). The data were collected at 0 (oven dried), 2, 4, 8, 16, 24, 48, 72, and 120 h. The mass of sample was weighted in a Fisher Scientific analytical balance with 220 g capacity and 0.1 mg readability. 2.6. A comparison of environmental impacts using life-cycle assessment Since the developed high-performance compressed wood material had the comparable flexural strength with the alloys, namely, between Aluminum alloy 2024-T3 and Steel alloy 1040, there was a need to compare the environmental impact of the developed compressed wood with these two alloys using LCA. The environmental impacts of three types of materials, namely, compressed wood material developed in this study, steel alloy and aluminum alloy, were compared by taking into account all stages of material life from raw material extraction to the end of the material service life using the SimaPro (Version 8.5.2) software. During the LCA process, to fairly compare the system performance, a function unit is required to be defined as a reference based on the stipulation of ISO standards (ISO, 2006). In this study, LCA was used for evaluating three materials including compressed wood material, steel alloy and aluminum alloy. Because they are usually utilized as panels for construction, 1 m3 was defined as the function unit. As a commonly used methodology in the LCA applications, the evaluation technology of Building for Environmental and Economic Sustainability (Lipiatt, 2007) has been accepted in the worldwide. The BEES index can be expressed by the biogenic CO2 uptake to characterize the global warming potential in the SimaPro software. Three densities of three types of materials (compressed wood material, steel alloy and aluminum alloy) were 1120, 7850 and 2770 kg m
3, respectively, as shown in Table 2. It was assumed that

C. Xia et al. / Journal of Cleaner Production 241 (2019) 118340

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Table 2 Input information of LCA for the three materials. Sample

Compressed Wood/Epoxy Steel alloy Aluminum alloy

Epoxy

Wood

Total mass

Distance/weight

(kg m
3)

(kg m
3)

(kg m
3)

(tkm)

386.4 e e

733.6 e e

1120 7850 2770

560 3925 1385

the distance between the material manufacturers and the construction service sites was 500 km and the 30-ton truck was used for transportation. Since the weights of the compressed wood material, Steel alloy and Aluminum alloy were 1120 kg, 7850 kg and 2770 kg, calculating the distance/weight as 560 tkm, 3925 tkm and 1385 tkm, respectively, as shown in Table 2. 3. Results and dissection This section illustrates the characteristics and the effects of resin infusion on the porosity, porosity reduction on mechanical performance and porosity on the water resistance of the woody materials. 3.1. Characterization of the woody materials Fig. 3. XRD patterns of the Epoxy, Wood, and fabricated woody materials.

The chemical structures were analyzed by FT-IR (Fig. 2) and XRD (Fig. 3). In the FT-IR spectra (Fig. 2), the wide peaks around 3335 cm
1 belonged to the OeH stretching of hydroxyl groups of epoxy and lignocellulose (Ding et al., 2018; Karumuri et al., 2015). The FT-IR transmittance at approximately 3052 cm
1 was contributed to the CeH symmetric stretching of epoxide group (Rocks et al., 2004), however, which was hard to be seen in the Wood/ Epoxy and Compressed Wood/Epoxy samples. CeH in epoxide group was not observed on the wood/epoxy samples because the

Fig. 2. ATR FT-IR spectra of the Epoxy, Wood, and fabricated woody materials.

un-cured epoxy group could be consumed in the Wood/Epoxy and Compressed Wood/Epoxy interfaces due to the reactions between wood and epoxy (Karumuri et al., 2015). The absorbing at 2966 cm
1 belonged to the CeH stretching of the methyl groups in epoxy, which was also found in the FI-IR transmittance of the Wood/Epoxy and Compressed Wood/Epoxy samples. The IR transmittances at 2917 cm
1 and 2850 cm
1 were contributed by the stretching of CeH in methylene groups of epoxy and lignocellulose (Pandey, 1999). The peak at 1734 cm
1 belonged to the C]O stretching in the aliphatic ketone/aldehyde groups of epoxy and lignin (Pandey and Theagarajan, 1997). The peaks at 1606 cm
1 and 1507 cm
1 were the skeletal vibration of phenyl groups in epoxy and lignin (Karumuri et al., 2015). The peaks around 1376 cm
1 could be the CeH symmetric deformation in cellulose and/or the CeH deformation in epoxide group of epoxy resin (Pandey, 1999). The peak at 1296 cm
1 was contributed by the skeletal CeC vibration of epoxy (Karumuri et al., 2015), which is shown in the FT-IR spectrum of Epoxy but not in the Wood/Epoxy and Compressed Wood/Epoxy samples. This phenomenon was consistent with the FT-IR peak at 3052 cm
1. It was further indicated that epoxy groups were not completely reacted in cured epoxy, however, which were fully reacted in the composites. The reason could be the reaction between epoxy groups from resin and eOH groups from wood, which would consume the extra epoxy groups (Karumuri et al., 2015). This unexpected reaction might cross-link wood and epoxy matrix, benefiting the mechanical properties of composites. The FTIR peak at 1236 cm
1 represented to the CeO stretching in the phenol/phenoxy groups in lignin and epoxy (Karumuri et al., 2015). The FT-IR peaks at 1030 cm
1 were contributed by the CeO symmetric stretching in epoxy and lignocellulose. Fig. 3 shows the XRD patterns of Epoxy, Wood, and fabricated woody samples. A broad XRD peak was detected at approximately 17.6 for the Epoxy sample, indicating a highly amorphous structure of the cured epoxy resin. Two obvious peaks are shown in curves of the woody materials, i.e., 2q ¼ 22.2 and 15.2 , representing the crystal planes of (002) and (101)/(101), respectively. These XRD patterns were mainly contributed by the crystalline cellulose from the wood, which were close with the high-crystallinity cellulose

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nanocrystals (CNCs), e.g. 22.6 , 14.2 , and 16.4 for (002), (101), and (101) crystal planes, respectively (Chen et al., 2015). The broad humps at 2q ¼ 18.5 were referred to the amorphous nature of cellulose. Using Eq (1) (Segal et al., 1959), the CIXRD values of the Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples were calculated to be 70.6%, 53.3%, and 60.1%, respectively, indicating that the CIXRD values were reduced after the amorphous epoxy resin infused into wood. 3.2. Effects of resin infusion on the porosity of the fabricated woody materials The microstructures of the samples were investigated using the micro-CT analysis. The 3D images of the samples, and the pore size distribution are shown in Fig. 4. It was shown that the original poplar wood owned 39.2 vol% wood phase and 61.8 vol% pores, being called as the micro-CT porosity (Pm), which was close to the Pg (66.3%) of the wood determined by the gas pycnometer (Table 3). The reported porosities of different species of wood were various, which could be 23.3%e71.4% (Plotze and Niemz, 2011; Zauer et al., 2013). The resin infusing and compressing processes brought down the Pm to 32.6 vol%, and 16.6 vol%, respectively, which was matched with the trend of Pg (Table 3). The slight difference of the porosities between the micro-CT (Fig. 4) and gas pycnometer (Table 3) analyses were explained to be the resolution variance of two types of instrument (Adedeji and Ngadi, 2011), i.e., micro-CT owning micron-scaled resolution compared with helium pycnometer measuring pores as small as 0.22 nm. As shown in Fig. 4, the pores of wood were divided into small pores (<50 mm) and large pores (50 mm). After the epoxy infusion and compression, the large pores were filled by epoxy, which could be observed in the images of Pore distribution, Epoxy phase and Pore phase in Fig. 4. It was indicated that the resin infusion preferred to the large pores. The average small pore sizes of the samples were calculated to be

12.7 mm, 8.9 mm, and 8.7 mm, respectively, indicating that the resin infusion could partially fill the small pores of wood, which represented as the reduction of average small pore size. After being compressed, it was shown that the average small pore size almost remained unchanged (from 8.9 mm to 8.7 mm), pointing out that the small pores were hard to be compressed. Instead, the large pores were compressed considerably during the compression, which represented as the visible reduction of large pores in wood component (Wood phase in Fig. 4) in the Compressed Wood/Epoxy sample. The mercury intrusion porosimetry was employed for investigating the porosities and pore distributions of the samples (Fig. 5 and Table 4). The mercury intrusion porosities (PHg) were calculated in terms of the cumulative intrusion curves (Fig. 5A) of the samples, which were 60.5%, 28.4%, and 20.1% for Wood, Wood/ Epoxy, and Compressed Wood/Epoxy (Table 4), respectively. The difference of mercury intrusion porosities for Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples owned the same sort, compared to the Pm (Fig. 4) and Pg (Table 3) for these samples, and the closed values, i.e., Pm ¼ 61.8%, 32.6%, and 16.6% and Pg ¼ 66.0%, 33.8%, and 22.6%, respectively. The pore distribution curves from the mercury intrusion porosimetry (Fig. 5A) displayed more details than those from the micro-CT analyses (Fig. 4), especially when the pore diameters were smaller than 5 mm. The median (Dm) and average pore diameters (Da) of the samples are compared in Table 4, reflecting similar situation with that from the mico-CT analysis, i.e., the large pores were markedly decreased after resin infused into the wood, and the applying compression load to the Wood/Epoxy samples could slightly reduce the pore sizes. The mercury intrusion results furtherly confirmed the findings from the micro-CT and gas pycnometer analyses that resin infusion could effectively fill the large pores, resulting in significant reduction in porosity and the compression process was difficult to squash the small pores. The SEM pictures of the material surfaces perpendicular to the

Fig. 4. Micro-CT images of the Wood, Wood/Epoxy and Compressed Wood/Epoxy samples. X-ray computed tomography images include the entire phases and separated wood, epoxy and pore phases. The pore distributions are analyzed in terms of the images of Pore phases.

C. Xia et al. / Journal of Cleaner Production 241 (2019) 118340

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Table 3 The results from gas pycnometer analysis of the Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples. Sample

Wood Wood/Epoxy Compressed Wood/Epoxy

ra b

rg c

Pg

Open cell

Closed cell

(g cm
3)

(g cm
3)

(%)

(%)

(%)

1.39 (0.09) 1.19 (0.05) 1.44 (0.02)

66.0 (1.8) 33.8 (6.2) 22.6 (5.2)

66.3 (1.7) 34.1 (6.2) 23.2 (5.2)

33.7 (1.7) 65.9 (6.2) 76.8 (5.2)

0.47 (0.01) 0.79 (0.01) 1.12 (0.08)

a

d

Notes. a Mean (standard deviation). b ra ¼ apparent density, calculated from Eq. (3). c rg ¼ gas density, obtained by terms of Eq. (2). d Pg ¼ gas porosity, obtained based on the apparent and gas densities using Eq. (4).

Table 4 Mercury intrusion porosimetry results of Wood, Wood/Epoxy, and Compressed Wood/Epoxy. a

b

c

Sample

PHg (%)

(mm)

(mm)

Wood Wood/Epoxy Compressed Wood/Epoxy

60.5 28.4 20.1

21.2 0.0825 0.0554

1.51 0.0382 0.0248

Dm

Da

Notes. a PHg ¼ porosity from mercury intrusion tests. b Dm ¼ median pore diameter. c Da ¼ average pore diameter.

The small pores were still observed on the surfaces of both Wood/ Epoxy and Compressed Wood/Epoxy samples, indicating that the small pores in the wood were much more difficult to be compressed and squeezed than the large pores. A schematic diagram of the changes of pore structures after the resin infusion and compression is presented in Fig. 6B. The large pores would be filled by epoxy after the resin infusion, but not much in small pores. These resin-filled large pores would be largely compressed; however, the compression deformation of small pores could be very limited. The observation matched perfectly with the results from the micro-CT analysis. 3.3. Effects of porosity reduction on mechanical performance of the fabricated woody materials

Fig. 5. Log differential (A) and cumulative intrusion (B) of Wood, Wood/Epoxy, and Compressed Wood/Epoxy from mercury intrusion tests.

longitudinal direction of the wood samples were polished by sanding paper to obtain the blurry wood structure of the sample (Fig. 6A). However, the small pores (<50 mm) and large pores (50 mm) were still clearly observed. The large pores of wood structure of Wood/Epoxy were detected; however, no large pores were found on the SEM images of the Compressed Wood/Epoxy samples (Fig. 6A). It was clearly seen that the large pores of the wood structure were preferentially compressed during the compression, as presented in the yellow dotted-line box in Fig. 6A, which was also observed in the previous work (Song et al., 2018).

The flexural property indicates the load bearing capability of a material without undergoing any permanent deformation, which is one of the most important indexes for a structural material (Callister and Rethwisch, 2008). The flexural strain-stress curves (flexural force-time curves in Fig. S1), modulus, strengths, and strains at break of the samples are presented in Fig. 7BeE (see detailed data in Table S1). Compared to the Wood sample (flexural modulus of 11.0 GPa and strength of 125.6 MPa), the flexural modulus and strength of the Compressed Wood/Epoxy samples (flexural modulus of 26.6 GPa and flexural strength of 304.9 MPa) were enormously increased by 142.2% and 142.8%, respectively. The flexural strain at break of the Compressed Wood/Epoxy sample was slightly decreased from 1.8% to 1.6%, mainly showing the brittleness of wood, and the flexural strain at break of epoxy (4.7%) was much higher than wood, which was consistent with the reported finding that wood fiber showed more rigid characteristic than epoxy (Hristov and Vasileva, 2003). It is observed that the Compressed Wood/Epoxy sample owned much higher flexural modulus (26.6 GPa) than the original wood (Poplar, 11.0 GPa) used in this research (Fig. 7C), as well as other reported wood species, e.g. Douglas fir (10.8e13.6 GPa) and red oak (11.0e14.1 GPa) (Callister and Rethwisch, 2008). The flexural modulus of the Compressed Wood/Epoxy sample was comparable

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Fig. 6. SEM observation and schematic illustration. (A) SEM images of the woody materials. Compressed large pore in the yellow dotted-line box. (B) A schematic diagram of the changes of pore structures after (i) vacuum-assisted resin infusion (VARI) and (ii) compression. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

with concrete (25.4e36.6 GPa) (Callister and Rethwisch, 2008), presenting high resistance to be deformed elastically, which might expand the applications of wood-based materials. Compared with the natural fiber-reinforced composites (flexural modulus of 6.9 GPa and flexural strength of 68.3 MPa) fabricated by VARI process (Xia et al., 2015), the flexural modulus (26.6 GPa) and strength

(304.9 MPa) of Compressed Wood/Epoxy exhibited much higher performance, although natural fibers own better modulus and strength than wood fibers (Mirbagheri et al., 2007). The reason might be the flexural tests were performed perpendicular to the longitudinal direction of isotropic wood, however, kenaf fiberreinforced composites were anisotropic structures. The flexural

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Fig. 7. Flexural performances of the Epoxy, Wood, Wood/Epoxy and Compressed Wood/Epoxy samples, and compressions of Compressed Wood/Epoxy with other materials. (A) Illustration of sample setup for flexural tests; the results of flexural tests (BeE); (F) A comparison of flexural strength between the Compressed Wood/Epoxy and other materials. The data were from William D. Callister, Jr.’s book (2008), pages 814e815. (G) A comparison of specific flexural strength (flexural strength normalized by density) between the Compressed Wood/Epoxy and other materials.

strength of the Compressed Wood/Epoxy was also comparable with the densified wood (flexural strength of 315.3 MPa) recently published in “Nature” (Song et al., 2018). In the report by Song et al. (2018), abundant of delignifying agents (NaOH and Na2SO3) and very long processing time (more than 7 h) were applied for producing the densified wood. While, comparatively, there were obvious advantages of no chemical residue and much shorter

processing time in the production of Compressed Wood/Epoxy in this work. Apart from the comparison with woody materials, a comprehensive comparison was performed between Compressed Wood/ Epoxy and other materials on their flexural strength (Fig. 7F), including alloys, ceramics, polymers, etc. (Callister and Rethwisch, 2008). As the comparison, the flexural strength of Compressed

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Wood/Epoxy was found to be much higher than those of polymers and comparable with some of the ceramics and alloys, which was between Alloy 2024-T3 (345 MPa) and Steel alloy 1040 (260 MPa) (Fig. 7F, see the detailed data in Table S2). However, it was necessary to point out the Compressed Wood/Epoxy sample owned much lower density (1.12 g cm
3) than those two alloys (densities of Alloy, 2024-T3 and Steel alloy 1040 are 2.77 and 7.85 g cm
3, respectively, Table S2). The normalized flexural strengths by densities (specific flexural strength) were performed and compared (Fig. 7G, see the detailed data in Table S2). It was very significant that the specific flexural strength of Compressed Wood/Epoxy (272 MPa cm3 g
1) was only lower than diamond (327 MPa cm3 g
1) but higher than all other materials (Fig. 7G). The comparison results indicated that the same weight of Compressed Wood/Epoxy could offer higher strength than all other materials listed in Fig. 7G except diamond, which may benefit its applications in the areas where lightweight is required, for instances, automobile, aircraft, etc. In addition, compared with the flexural strength of 153.94 MPa and flexural modulus of 19.77 GPa (Gao et al., 2016) and 7.48 GPa (Anshari et al., 2012) of the compressed wood obtained in the previous studies, it was demonstrated that the flexural modulus (26.6 GPa) and strength (304.9 MPa) of Compressed Wood/Epoxy in this study had substantial improvements. The tensile strain-stress curves (tensile force-time curves in Fig. S2), modulus, strengths, and strains at break of Epoxy, Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples are presented in Fig. 8. Similar to the flexural performances, the tensile strength was significantly enhanced from 44.3 MPa to 101.3 MPa after the epoxy infusion, and more enhancement was observed to 141.9 MPa after the compression (Table S1). The modulus and strength of the Compressed Wood/Epoxy samples were increased by 53.0% and 220.3%, respectively, compared with those of the Wood sample. It was calculated to have a 37.9% reduction of the elongation at break, owing to the epoxy fraction that had a much lower elongation at

break (3.7%) (Table S1). The tensile strength of the Compressed Wood/Epoxy sample was close to the pure Aluminum 1100-H16 (145 MPa) and Aluminum alloy 6063-T1 (150 MPa) (Kaufman, 2008), which can be used in aerospace, marine, cycling, automotive alloys, etc. (Karthikeyan and Kumar, 2011; Shevell, 1989). The results from DMA tests are shown in Fig. 9. In general, Epoxy owned a relatively lower storage modulus (E0 ) than the others, and its modulus approached to zero when temperature increased over around 85  C because of the softening of epoxy. Wood owned good energy storage capability during the temperature increased, representing as the E0 from approximately 6,000 MPa to 4,000 MPa. The E0 of the Wood/Epoxy and Compressed Wood/Epoxy samples had a similar drop at low temperatures, which was attributed to the softening of epoxy in the composites. When epoxy was softened, the E0 of Wood/Epoxy represented the wood energy storage capability, which was similar with the Wood's E'. The Compressed Wood/Epoxy sample showed higher E0 compared with Wood and Wood/Epoxy, which might be attributed to the thickness reduction from 6.28 mm/6.26 mme3.78 mm after the samples were compressed (Table 1). In the loss modulus (E'') (Fig. 9B) and mechanical loss factor (tan(d)) (Fig. 9C) curves, the peaks from wood component were determined around 50  C, which could be found in the Wood, Wood/Epoxy, and Compressed Wood/Epoxy samples. And the curve peaks of Epoxy were located at around 80  C, indicating the unambiguous glass transition temperature. The epoxy components in the Wood/Epoxy and Compressed Wood/Epoxy samples showed minor effect on E'', representing as unshaped peaks in E'' curves. That could be because of the minor contribution of epoxy to the modulus (Fig. 4) of the composites and the interaction between the epoxy and wood of the composites (Hazarika et al., 2014; Hristov and Vasileva, 2003). The mechanical-properties improvements of Compressed Wood/Epoxy could be attributed to the reduction of the porosities and the thickness of the woody material. The Pg from gas

Fig. 8. Tensile test results of the Epoxy, Wood, Wood/Epoxy and Compressed Wood/Epoxy samples. (A) Illustration of sample setup for tensile tests; and the results of tensile tests (BeE).

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Fig. 9. DMA results of the Epoxy, Wood, Wood/Epoxy and Compressed Wood/Epoxy samples, including (A) storage modulus (E0 ), (B) loss modulus (E''), and (C) mechanical loss factor (Tan(d)).

pycnometer test, Pm from micro-CT test, and PHg from mercury intrusion porosimetry of the samples were determined to be 66.0%, 33.8% and 22.0% (Table 4), 61.8%, 32.6% and 16.6% (Fig. 4), and 60.5%, 28.4% and 20.1%, respectively. The previous reports about natural fiber-reinforced composites also showed that the reduction of porosity could improve the strength and modulus of the samples (Wu et al., 2018a; Xia et al., 2016). Proved by Ji et al. (2006), indicating that porosity plays an important role on microstructural feature in most natural materials so that significantly changing the properties of these materials such as modulus, strength, etc. Additionally, the thickness of the sample was reduced by 40% after the compression (Fig. 6B), resulting in the low-modulus epoxy being squeezed out from woody material (Figs. 7C, 8C, and 9A), thereby, increasing the mechanical properties of the woody materials. 3.4. Effects of porosity on the water resistance The water absorptions of the Epoxy and woody samples were performed (Fig. 10). Epoxy had the best water resistance compared

with the other samples, i.e., 0.6% for 2 h water absorption, 2.4% for 24 h, and 4.0% for 5 d. Among the samples used in this experiment, wood had the worst water resistance, i.e., 29.8% at 2 h water absorption, 65.6% at 24 h, and 99.3% at 5 d, respectively, because of the porous structure of natural fibers (Fig. 4 and 6) (Sun et al., 2010). After the resin infusing into wood, the water absorptions of the Wood/Epoxy samples were down to be 6.0% (2 h), 19.0% (24 h), and 26.5% (5 d), respectively. After being further compressed, those values of the Compressed Wood/Epoxy became 4.4%, 16.6%, and 26.8%, respectively. Compared to Wood, the water absorptions of the Compressed Wood/Epoxy were reduced by 85.2% (2 h), 74.6% (24 h), and 73.0% (5 d), respectively. The reason could be the epoxy filling of wood porous structure, which reduced water absorption (Fig. 6). As shown in Fig. 10, the water absorption of Wood/Epoxy sample was slightly higher before 48 h, compared with that of Compressed Wood/Epoxy sample, which was probably due to the higher porosity of the Wood/Epoxy sample that accelerated the water absorption at the beginning. After 48 h, the water absorptions of the Wood/Epoxy and Compressed Wood/Epoxy samples were similar, which was mainly contributed by the reduction of sample weight per unit area after the compression process. It was indicated that the water absorptions of the Compressed Wood/ Epoxy sample per unit area were lower than those of Wood/Epoxy at the absorption time from 2 h to 5 d. The reduction of water absorption had very significant benefits for the woody materials being used for various out-door applications (Ghasemi and Kord, 2009; Tamrakar and Lopez-Anido, 2011).

3.5. Comparison results of environmental impacts

Fig. 10. Water absorption curves of the Epoxy, Wood, Wood/Epoxy and Compressed Wood/Epoxy samples.

The BEES environmental impact LCA of three types of materials (the compressed wood material, Steel alloy and Aluminum alloy) were summarized and are presented in Table 5. The comparison results of Steel alloy Vs. compress wood and Aluminum alloy Vs. compressed wood are presented in the columns of “Reduce” in Table 5, indicating that, all the environmental impact BEES indices were significantly reduced when the compressed wood were applied for replacing Steel alloy or Aluminum alloy. As shown in Table 5, the Global warming, Acidification, Human health (HH) cancer, HH noncancer, HH criteria air pollutants, Eutrophication, Ecotoxicity, Smog, Natural resource depletion, Habitat alteration, and Ozone depletion were reduced by 1.72, 8.95, 667.06, 94.33, 6.35, 484.16, 17.94, 2.27, 1.90, 180.51, and 26.85 times when the

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Table 5 A comparison of environmental impact assessment among the three materials. Impact category

Compress Wood/Epoxy

Steel alloy

Reduce

Global warming (g. CO2 eq.) Acidification (Hþ moles eq.) HH cancer c (g. C6H6 eq.) HH noncancer (g. C7H7 eq.) HH criteria air pollutants (Micro DALYs) Eutrophication (g. N. eq.) Ecotoxicity (g. 2,4-D eq.) Smog (g. NOx eq.) Natural resource depletion (MJ surplus) Habitat alteration (T&E count) Ozone depletion (g. CFC-11 eq.)

3.70  106 1.30  106 2.76  103 1.53  107 6.18  102 4.23  103 4.80  104 2.32  104 8.64  103 6.59  10
12 2.09  10
2

6.37  106 1.17  107 1.84  106 1.44  109 3.92  103 2.05  106 8.61  105 5.29  104 1.64  104 1.19  10
9 5.62  10
1

1.72 8.95 667.06 94.33 6.35 484.16 17.94 2.27 1.90 180.51 26.85

a

(times)

Aluminum alloy

Reduce

1.05  108 4.10  107 1.76  106 2.50  109 3.16  104 2.01  106 1.40  106 3.91  105 6.34  104 3.09  10
9 4.89

28.45 31.42 636.21 163.48 51.10 475.47 29.15 16.81 7.34 468.73 233.35

b

(times)

Notes. a Reduce ¼ Steel alloy/Compress Wood/Epoxy. b Reduce ¼ Aluminum alloy/Compress Wood/Epoxy. c HH ¼ human health.

compressed wood material was used to replace Steel alloy, respectively; while these indices were reduced by 28.45, 31.42, 636.21, 163.48, 51.10, 475.47, 29.15, 16.81, 7.34, 468.73, and 233.35 times as the compressed wood material was utilized to replace Aluminum alloy, respectively. 4. Conclusions The high-performance woody materials were successfully developed using a clean production method, namely, VARI process, and their characteristics/properties were investigated in this study. The release of the harmful volatiles generated by epoxy was reduced because the specimens were wrapped by the vacuum bag during the VARI process. The enhanced mechanical properties after the VARI process could be contributed to the porosity reduction of the woody materials. After the resin infusion and curing, the wood porosity was reduced from 66.0% to 33.8%, and further reduced to 22.6% by applying a compression during the epoxy curing process, which was counted a 65.8% reduction using the gas pycnometer analysis. Because of the reduction of porosity, the flexural and tensile modulus and strengths of wood were significantly enhanced, and the water absorption was reduced by more than 70%. The ultimate flexural strength (304.9 MPa) was increased to the values of alloy range, namely, between Alloy 2024-T3 (345 MPa) and Steel alloy 1040 (260 MPa). After being normalized by density, the specific flexural strength of the Compressed Wood/Epoxy sample (272 MPa cm3 g
1) exceeded the highest alloy (Tie6Ale4V) (205 MPa cm3 g
1), only being lower than diamond (327 MPa cm3 g
1). The micromorphology study from the micro-CT and SEM analyses indicated that the large pores of wood (50 mm) were easily filled by resin and compressed, however, the small pores (<50 mm) were difficult to be either filled by resin or squashed by compression. It led to a 22.6% porosity remaining in the sample, which might decrease the mechanical properties. Future study might be conducted to additionally reduce the porosity of woody materials. Using the proposed method in this study, no chemical pretreatment of the solid wood was involved before the processing, which has great benefits to environments. However, the limitation/challenge of the VARI system is the possible premature gelation of the epoxy resin if the size of product is large and long resin transfer time is required. The following three methods can address this challenge: a. selecting correct Part B of epoxy, which controls the gelation/ curing time; b. controlling the right temperature during the resin injection; c. selecting the optimum number of open injection gates and the correct flow rate that can be controlled by the vacuum pressure and temperature. The LCA results indicated that, all major indices of environmental impacts, including the global warming,

acidification, HH cancer, HH noncancer, HH criteria air pollutants, eutrophication, ecotoxicity, smog, natural resource depletion, habitat alteration, and ozone depletion, were reduced by 0.72, 7.95, 666.06, 93.33, 5.35, 483.16, 16.94, 1.27, 0.90, 179.51, and 25.85 times when the compressed wood material was used to replace Steel alloy, respectively. While these indices were reduced by 27.45, 30.42, 635.21, 162.48, 50.10, 474.47, 28.15, 15.81, 6.34, 467.73, and 232.35 times as the compressed wood material was utilized to replace the aluminum alloy, respectively. Acknowledgements National Natural Science Foundation of China (31901372), Natural Science Foundation of Jiangsu Province (SKB2019042796), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), National Science Foundation (NSF) CMMI 1247008. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.118340. References Adedeji, A.A., Ngadi, M., 2011. Porosity determination of deep-fat-fried coatings using pycnometer (Fried batter porosity determination by pycnometer). Int. J. Food Sci. Technol. 46 (6), 1266e1275. https://doi.org/10.1111/j.1365-2621.2011. 02631.x. Anshari, B., Guan, Z., Kitamori, A., Jung, K., Komatsu, K., 2012. Structural behaviour of glued laminated timber beams pre-stressed by compressed wood. Constr. Build. Mater. 29, 24e32. https://doi.org/10.1016/j.conbuildmat.2011.10.002. Arasaretnam, S., Kirudchayini, T., 2019. Studies on synthesis, characterization of modified phenol formaldehyde resin and metal adsorption of modified resin derived from lignin biomass. Emerg. Sci. J. 3 (2), 101e108. https://doi.org/10. 28991/esj-2019-01173. Bekhta, P., Sedlia cik, J., Jones, D., 2018. Effect of short-term thermomechanical densification of wood veneers on the properties of birch plywood. Eur. J. Wood Wood Prod. 76 (2), 549e562. https://doi.org/10.1007/s00107-017-1233-4. Callister Jr., W.D., Rethwisch, D.G., 2008. Fundamentals of Materials Science and Engineering: an Integrated Approach, 3ed. John Wiley & Sons. Chen, J., Lin, N., Huang, J., Dufresne, A., 2015. Highly alkynyl-functionalization of cellulose nanocrystals and advanced nanocomposites thereof via click chemistry. Polym. Chem. 6 (24), 4385e4395. https://doi.org/10.1039/C5PY00367A. Crean, A., 2017. Evaluation of various brands of moisture meters in gypsum and wood substrates at a range of moisture contents. Civ. Eng. J. 3 (9), 640e649. https://doi.org/10.21859/cej-03091. Deka, B.K., Mandal, M., Maji, T.K., 2012. Effect of nanoparticles on flammability, UV resistance, biodegradability, and chemical resistance of wood polymer nanocomposite. Ind. Eng. Chem. Res. 51 (37), 11881e11891. https://doi.org/10.1021/ ie3003123. Ding, Q., Xu, X., Yue, Y., Mei, C., Huang, C., Jiang, S., Wu, Q., Han, J., 2018. Nanocellulose-mediated electroconductive self-healing hydrogels with high

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