Forestry, Faculty of
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Materials 15 (21): 7518 (2022)
Hu, Yuan
Xiong, Luyao
Li, Yanbo
Semple, K. E. (Kate E.)
Nasir, Vahid
Pineda, Hugo
He, Mei
Dai, Chunping, 1963-
Controlling the variability in mat structure and properties in bamboo scrimber (BS) is key to producing the product for structural applications, and wide strip scrimber (WBS) is an effective approach. In this study, the effects of scrimmed bamboo bundle morphology and product density on the properties of WBS were investigated. WBS panels were manufactured and tested using wide (200 to 250 mm) bamboo strips with different fiberization intensity. Maximum strength properties (flexural, compressive, and shear strength), and lowest thickness swelling and water absorption were achieved with three or four passes due to the higher resin absorption by strips. For balanced product cost and performance, we recommend 1–2 fiberization passes and a panel density of 0.9–1.0 g/cm³. Panel mechanical properties were compared with other common bamboo composites. Bamboo scrimber products were highly variable in properties due to differing manufacturing processes, element treatments, and suboptimal mat structure. Products including laminated bamboo lumber and flattened bamboo made from nonfiberized elements show markedly different relationships between strength and elastic properties mostly due to inadequate bonding between the laminae, which causes premature bond-line failure. This study helped improve the understanding of the structure–property relationship of engineered bamboo products while providing insights into process optimization.
Citation: Hu, Y.; Xiong, L.; Li, Y.;Semple, K.; Nasir, V.; Pineda, H.;He, M.; Dai, C. Manufacturing andCharacterization of Wide-BundleBamboo Scrimber: A Comparisonwith Other Engineered BambooComposites. Materials 2022, 15, 7518.https://doi.org/10.3390/ma15217518Academic Editors: Joji Okazaki,Satoshi Komasa, Tohru Sekino andYoshiro TaharaReceived: 3 October 2022Accepted: 24 October 2022Published: 26 October 2022Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).materialsArticleManufacturing and Characterization of Wide-BundleBamboo Scrimber: A Comparison with Other EngineeredBamboo CompositesYuan Hu 1,2, Luyao Xiong 3, Yanbo Li 1, Kate Semple 1, Vahid Nasir 1 , Hugo Pineda 1, Mei He 1,2,*and Chunping Dai 1,*1 Department of Wood Science, University of British Columbia, Vancouver, BC V6T 1Z4, Canada2 Research Institute of Forest Products, Jiangxi Academy of Forestry, Nanchang 330013, China3 Jiangxi Provincial Forest Resources Conservation Center, Nanchang 330038, China* Correspondence: mhe07@mail.ubc.ca (M.H.); chunping.dai@ubc.ca (C.D.)Abstract: Controlling the variability in mat structure and properties in bamboo scrimber (BS) is key toproducing the product for structural applications, and wide strip scrimber (WBS) is an effective approach.In this study, the effects of scrimmed bamboo bundle morphology and product density on the propertiesof WBS were investigated. WBS panels were manufactured and tested using wide (200 to 250 mm)bamboo strips with different fiberization intensity. Maximum strength properties (flexural, compressive,and shear strength), and lowest thickness swelling and water absorption were achieved with three orfour passes due to the higher resin absorption by strips. For balanced product cost and performance, werecommend 1–2 fiberization passes and a panel density of 0.9–1.0 g/cm3. Panel mechanical propertieswere compared with other common bamboo composites. Bamboo scrimber products were highlyvariable in properties due to differing manufacturing processes, element treatments, and suboptimal matstructure. Products including laminated bamboo lumber and flattened bamboo made from nonfiberizedelements show markedly different relationships between strength and elastic properties mostly due toinadequate bonding between the laminae, which causes premature bond-line failure. This study helpedimprove the understanding of the structure–property relationship of engineered bamboo products whileproviding insights into process optimization.Keywords: bamboo composites; wide-bundle bamboo scrimber; engineered bamboo products;fiberization; mechanical properties; dimensional stability1. IntroductionBamboo is one of the strongest, fastest-growing plants in the world. Bamboo forestsare widely distributed across the subtropical regions of Asia, Africa, and Latin America [1].According to the International Network of Bamboo and Rattan (INBAR), the total area ofglobal bamboo forests was more than 35 million hectares in 2020 [2]. Bamboo has a shortergrowth/harvest cycle than trees and can be harvested within 5 years. Bamboo products areextremely versatile and can be manufactured to high strength and stiffness for potentialuse in sustainable construction development [3].In North America and Northern Europe, timber has been traditionally and widelyused for construction [4,5]. As lumber prices increase, developing engineered bamboomaterials may help supplement timber building materials. Engineered bamboo uses woodadhesives and hot pressing to convert round, hollow, and variable bamboo culms into di-mension ‘lumber’ and panels for construction giving a wide range of properties suitable fordifferent applications. Engineered bamboo is promising as a rapidly renewable, bio-basedconstruction material that can also offer enhanced and uniform mechanical performance.Research and development of engineered bamboo are essential for regions in countriessuch as China and India with abundant bamboo but low timber resources. EngineeredMaterials 2022, 15, 7518. https://doi.org/10.3390/ma15217518 https://www.mdpi.com/journal/materialsMaterials 2022, 15, 7518 2 of 16bamboo for building construction needs to be lower in density than some existing productssuch as scrimber (for fastening); more standardized, uniform, and predictable in densityand properties; and lower in cost. Understanding the factors contributing to the high vari-ability in bamboo composite properties and manufacturing products with better-controlledand standardized properties and performance is crucial for their acceptance as structuralmaterials in the building and construction industry.Engineered bamboo products can be generally classified into two groups: (1) productswith laminated structures such as laminated bamboo lumber (LBL); and (2) strand andfiber-based products with more random their structure, such as bamboo oriented strandboard (OSB) and scrimber. Most structural wood composites used in building framing arelaminated, including plywood, laminated veneer lumber (LVL), cross-laminated timber,and glued-laminated timber. These are multilayer products with continuous glue lines,but failure to adhere to standardized manufacturing and quality control protocols such asANSI PRG 320 (developed for softwood glulam and CLT) can lead to premature bond-linefailure and reduced modulus of rupture (MOR) [6]. Strand and fiber-based products have ahigher degree of randomness in their mat structures and discontinuous glue lines. Thispaper deals with a fiberized bamboo scrimber (BS) product called wide bundle scrimberor WBS, designed to fit the laminated structure category to overcome the limitations anddeficiencies caused by random mat structure.BS adapts the crushing and flattening technology developed in Australia to convertsmall diameter pine thinnings to bundles of splinters that are coated in resin and hot-compressed back into solid composite lumber [7,8]. BS is made from crushed or ‘fiberized’bamboo bundles soaked in water-soluble phenol-formaldehyde (PF) resin and compressedto billets or panels with the required density and thickness [9]. BS has a high utilization rateof raw bamboo, high density, and good physical and mechanical properties [10,11]. It alsohas a high PF resin solids content (15 to 17%) compared with most other wood and bamboocomposites and is very heavily compacted at >10 MPa, giving it a tight grain texturesuitable for high-wear flooring and decking applications as well as garden constructionand certain civil engineering applications [12–14]. Approximately 60 bamboo scrimbermanufacturing enterprises presently exist in China, with an annual production rate of600,000 m3 [15]. The earliest recorded research on bamboo scrimber materials dates back tothe 1980s [16]The fabrication process consists of cold molding and hot curing, or, for panels, of alengthy cool-in and cool-out pressing procedure. Products range in density from about 0.85to 1.25 g/cm3, with high variability in mechanical properties [17–19]. Many studies haveinvestigated in depth the mechanical properties [20–23], flame retardant properties [24],dying properties [25], anticorrosion and antimildew properties [26], and supplementaryelement heat treatments for bamboo scrimber [27]. Various processing equipment such asflattening and fiberization devices, cold-compress machines, special curing kilns, multilayerhot-presses with water cooling systems, and hot-press molds with movable stops havebeen developed to suit BS fabrication [28].Bamboo scrimber can be made with high mechanical properties but at the expenseof heavy processing—mechanical culm flatting, cracking requiring much resin usage to‘repair’, heavy compaction, and lengthy press times to consolidate. The high densificationleads to excessive thickness well. Conventional BS is made from narrow flattened bamboostrips which have had the inner and outer skins, or ‘cortex’, removed to improve impedi-ments to resin bonding [29], but this also reduces the culm utilization rate. Narrow strips,randomized strip placement in mats, and lack of standardization of the manufacturingprocess result in an overly densified product with a wide range of properties which limitsits application as a structural building material. Narrow strips do not align as well in themat and create more random gaps and overlaps within each layer, or ‘laminate’, requiringexcessive compaction to provide adequate contact pressure between elements.Wide bundle scrimber (WBS) and stitched bundle scrimber or bamboo laminatedveneer lumber (BLVL) are more recent and effective approaches to convert BS to moreMaterials 2022, 15, 7518 3 of 16of a plywood or LVL-type laminated mat structure and therefore reduce the compactionrequired for adequate element contact. The objective of this study was to examine theeffects of fiberization extent (number of passes through toothed rollers) on resin absorptionby strips as well as associated panel density on mechanical properties including flexuralstrength, shear strength, and thickness swell. Modifications to the manufacturing processto enhance dimensional stability and mechanical properties within a suitable densityrange are discussed. The study also aims to contrast the properties of WBS with those oftraditional BS and other engineered bamboo products to gain broader understanding ofthe processing-structure-property relationships of bamboo-based composites.2. Materials and Methods2.1. MaterialsFresh 4–5-year-old Moso bamboo (Phyllostachys pubescens) culms were obtained fromYingtan city, Jiangxi province, China. The bamboo trees were 20 m in height with a culmdiameter of 80 to 100 mm and wall thickness of 7 to 11 mm. The moisture content of theraw bamboo was 35% to 45%, and density was between 0.5 g/cm3 and 0.6 g/cm3. Theharvested bamboo was delimbed and cut to 2-meter-long culms which were then splitlongitudinally into two to four sections for flattening and fiberization.2.2. Experimental DesignTwo factors were tested in the study: (Experiment a) extent of fiberization—four levels;and (Experiment b) board density—five levels, as shown in Table 1. Three replicate panelswere made for each level. The different levels of board density were achieved by adjustingthe weight of resin-soaked fiberized bamboo layers used in the mat and compressing themto the same thickness. The extent of fiberization for the board density variation was 3passes through the rollers which were considered to be the optimum fiberization extent tobalance resin usage with panel properties.Table 1. Experiment design for fiberization frequency and density sampling.Expt. Experimental Factors LevelaBoard density (g/cm3) 1.00Fiberization frequency (times) 1 2 3 4bFiberization frequency (times) 3Board density (g/cm3) 0.90 1.00 1.10 1.20 1.302.3. FiberizationThe culm halves were flattened and decorticated into fiberized bundles (Figure 1) mea-suring between 200 and 250 mm in width while in the fresh green state using a multi-purposefiberization machine [30] and then cut into 460 mm segments. The fiberized strips were driedin an oven at 85–90 ◦C for 3 h to a moisture content of 6–7% and kept at room temperaturefor 1 week before testing and panel fabrication. During the fiberization process, a series ofparallel cracks were formed in the culm wall which separates the wall tissue into a series oflarger or smaller interconnected fiber bundles. The rollers have discontinuously dislocatedconvex teeth to generate cutting and peeling forces in the longitudinal direction of the bamboowall. Linear and point cracks were formed longitudinally along the bamboo culm, whileextrusion forces were applied to the radial section resulting in the loosening and stretching ofthe broomed strip in the transverse (across the grain) direction [31]. Several pieces of fiberizedbamboo veneer were randomly selected, and the diameter of the thickest bundle of fibers ineach piece was measured using vernier calipers. Information on bundle sizes and utilizationrate of different groups is given in Tables 2 and 3). Utilization rate was estimated from themass of the culm sections before and after passing through the rollers. The green removal rate(also given in Table 3) was estimated from a randomly cut 5 cm × 5 cm specimen and imagedMaterials 2022, 15, 7518 4 of 16under a depth-of-field 3D super-deep-scene microscope. The lasso tool in Photoshop softwarewas used to identify and measure the remaining green skin area seen in Figure 1.Materials 2022, 15, x FOR PEER REVIEW 4 of 17 mass of the culm sections before and after passing through the rollers. The green removal rate (also given in Table 3) was estimated from a randomly cut 5 cm × 5 cm specimen and imaged under a depth-of-fiel s per-de p-scene microscope. The lasso tool in Photos p software was used to identify and measure the re aining green skin area seen in Figure 1. Figure 1. Culm flattening and fiberization process for wide bamboo scrimber (WBS). Image resolution = 300 dpi. Table 2. Morphological characteristics of WBS, NBS, and traditional Moso bamboo bundles. Bundle Type Number of Strips per Culm Width (mm) Thickness (mm) Diameter (mm) Utilization Rate (%) Photo of Bundles Wide-bundle Bamboo Scrimber 3–4 200–250 (234) 10–12 (10.95) 1.35–1.72 (1.54) 92 Narrow-bundle Bamboo Scrimber 4–8 30–60 (45) 10–12 (10.32) 1.15–1.52 (1.34) 80 Traditional Bamboo Scrimber bundles 6–10 20–30 (23) 8–10 (8.32) 2–3 (2.32) 50–55 The numbers in parentheses are average values. Image resolution = 300 dpi. The data in the figure are summarized from experimental data from the Wood-based Panel and Adhesive Laboratory of the Chinese Academy of Forestry and the laboratory of the Institute of Forest Industry, Jiangxi Acad-emy of Forestry. Figure 1. Culm flattening and fiberization p ocess for wide bamboo scrimb (WBS). Imageresolution = 300 dpi.Table 2. Morphological characteristics of WBS, NBS, and traditional Moso bamboo bundles.Bundle TypeNumber ofStrips perCulmWidth(mm)Thickness(mm)Diameter(mm)UtilizationRate(%)Photo ofBundlesWide-bundleBambooScrimber3–4 200–250(234)10–12(10.95)1.35–1.72(1.54) 92Materials 2022, 15, x FOR PEER REVIEW 4 of 17 mass of the culm sections before and after passing through the rollers. The green removal rate (also given in Table 3) was estimated from a randomly cut 5 cm × 5 cm specimen and imaged under a depth-of-field 3D super-deep-scene microscope. The lasso tool in Photoshop software was used to identify and measure the remaining green skin area seen in Figure 1. Figure 1. Culm flattening and fiberization process for wide bamboo scrim er (WBS). I age resolution = 300 dpi. Table 2. Morphological characteristics of WBS, NBS, and traditional Moso bamboo bundles. Bundle Type Number of Strips per Culm Width ( m) Thickness ( m) Diameter (mm) Utilization Rate (%) Photo of Bundles Wide-bundle Bamboo Scrimber 3 200–250 (234) 10–12 (10.95) 1.35–1.72 (1.54) 92 Narrow-bundle Bamboo Scrimber 4–8 30–60 (45) 10–12 (10.32) 1.15–1.52 (1.34) 80 Traditional Bamboo Scrimber bundles 6–10 20–30 (23) 8–10 (8.32) 2–3 (2.32) 50–55 The numbers in parentheses are average values. Image resolution = 300 dpi. The data in the figure are summarized from experimental data from the Wood-based Panel and Adhesive Laboratory of the Chinese Academy of Forestry and the laboratory of the Institute of Forest Industry, Jiangxi Acad-emy of Forestry. Narrow-bundleBambooScrimber4–8 30–60(45)10–12(10.32)1.15–1.52(1.34) 80Materials 2022, 15, x FOR PEER REVIEW 4 of 17 mass of the culm sections before and after passing through the rollers. The green removal rate (also given in Tabl 3) was stim te from a randomly cut 5 cm × 5 cm specimen and imagedunder a depth-of-field 3D uper-deep-scene microscope. The lasso tool in Photoshop softwarewas use to ident fy and measure the remaining gre n skin area see in Figure 1. Figure 1. Culm flattening and fiberization process for wide bamboo scrimber (WBS). Image resolution = 300 dpi. Table 2. Morphological characteristics of WBS, NBS, and traditional Moso bamboo bundles. Bundle Type Number of Strips per Culm Width (mm) Thickness (mm) Diameter (mm) Utilization Rate (%) Photo of Bundles Wide-bundle Bamboo Scrimber 3–4 200–250 (234) 10–12 ( .95) 1.35–1.72 (1.54) 92 Narrow-b l Bamb o Scrimb r 4 30–60 (45) 10–12 ( .3 ) 1.1 –1.52 (1.34) 80 Traditional Bamboo Scrimber undles 6–10 20–30 (23) 8–10 ( .32) 2–3 ( .32) 50–55 The numbers in parentheses are average values. Image resolution = 300 dpi. The data in the figure are summarized from experimental data from the Wood-based Panel an Adhesive Laboratory of the Chinese Academy of Forestry and the laboratory of the Institute of Forest Industry, Jiangxi Acad-emy of Forestry. TraditionalBambooScrimberbundles6–10 20–30(23)8–10(8.32)2–3(2.32) 50–55Materials 2022, 15, x FOR PEER REVIEW 4 of 17 mass of the culm sections before and after passing through the rollers. The green removal rate (also given in Tabl 3) was stim te from a ra domly cut 5 cm × 5 cm specimen and imagedunder a depth-of-field 3D super-deep-scene microscope. The lasso tool in Photoshop softw r was use to ident fy and measur the remaining gre n skin area see in Figure 1. Figure 1. Culm flattening and fiberization process for wide bamboo scrimber (WBS). Image resolution = 300 dpi. Table 2. Morphological characteristics of WBS, NBS, and traditional Moso bamboo bundles. Bundle Type Number of Strips per Culm Width (mm) Thickness (mm) Diameter (mm) Utilization Rate (%) Photo of Bundles Wide-bundle Bamboo Scrimber 3–4 200–250 (234) 10–12 ( .95) 1.35–1.72 (1.54) 92 Narrow-bundle Bamb o Scrimber 4–8 30–60 (45) 10–12 ( .3 ) 1.15–1.52 (1.34) 80 Traditional Bamboo Scrimber undles 6– 20–30 (23) 8–10 ( .32) 2–3 ( .32) 50–55 The numb rs in p rentheses are average v lues. Image resolution = 300 dpi. T data in the figure are summarized from experimental data from the Wood-based Panel an Adhesive Laboratory of the Chinese Academy of Forestry and the laboratory of the Institute of Forest Industry, Jiangxi Acad-emy of Forestry. The numbers in parentheses are average values. Image resolution = 300 dpi. The data in the figure are summarizedfrom experimental data from the Wood-based Panel and Adhesive Laboratory of the Chinese Academy of Forestryand the labor tory of the Institute of Forest Industry, Jiangxi Academy of Forestry.Materials 2022, 15, 7518 5 of 16Table 3. Morphological characteristics of WBS bundles with different fiberization frequency.FiberizationFrequency(Times)Outer GreenRemoval Rate(%)Diameter ofBambooBundles(mm)Resin Loading(%)Specific SurfaceArea(1/mm)Cross-Sectional Photo of BambooFiber Bundles (5 mm Scale Bar)1 34.23 (4.75) 4.21 (0.32) 26.45 (4.32) 1.741 2 56.76 (8.37) 3.13 (0.21) 47.21 (5.13) 2.411 3 79.11 (10.88) 2.54 (0.13) 58.17 (4.55) 3.321 4 94.23 (13.23) 1.89 (0.15) 72.44 (6.73) 3.541 Numbers in parentheses are coefficients of variation. Data summarized from experiments at the Wood-basedPanel and Adhesive Laboratory of the Chinese Academy of Forestry and the laboratory of the Institute of ForestIndustry, Jiangxi Academy of Forestry.2.4. Calculation of Specific Surface Area (SSA)The concept of specific surface area was proposed here to quantify the extent to whichthe bamboo strips are fiberized. Pictures of the cross-section of the bundles were taken foreach level of fiberization, and their average thicknesses were measured. The pictures werethen cropped to enclose only the cross-section and then binarized to obtain black-and-whiteimages using the image processing and analysis library available in MATLAB® R2019b.The number of white pixels was counted and divided by the total pixel area to obtain theoccupied portion. The white portion was also divided into non-touching regions, and theperimeter of each region was calculated. The value of SSA was calculated as the sum ofthe perimeters of all regions divided by the total occupied area. Since the units of the SSAresults are 1/pixel, the thicknesses of each photographed bundle were measured and usedas a scale to convert the results from pixels to mm. The average resolution of the pictures is80 pixel/mm.2.5. Panel FabricationThe dried bundles were dipped in liquid phenolic resin for 3–4 min and drained for4 min to remove excess resin for reuse. The phenolic resin was supplied by Beijing TaierChemical Co., Ltd. (Beijing, China), with a solids content of 46.56%, viscosity of 42 CPs,and pH of 10–11. The resin dosage (of liquid resin in Table 3) was estimated by measuringthe increase in mass. The resin-coated strips were then oven-dried to an MC of 10–12%.The resinated, dried strips were hand-laid into a mold with side pressure baffles with theinner face of the strips facing the core and the outer faces facing the surfaces for uniformsymmetry in the vertical direction [32]. To adjust density, between 6 and 8 strips were usedper panel, which resembled unidirectional LVL in configuration. The 460 mm × 200 mmmats were hot-pressed to a 20 mm target at 145 ◦C under 5 MPa compaction pressure for 30min (1.5 min/mm hot press time) using a cold-in–cold-out (water-cooled) schedule. Notethe compaction pressure is less than half that of traditional BS due to the uniform laminatesand there being no gaps or overlaps between strips within the mat. The implications of thisare discussed further in Section 3.4. Total panel pressing time was over an hour, accountingfor precompaction and postcooling to 60 ◦C before removal. The laboratory press wassupplied by Shanghai Wood-based Panel Machinery Factory Co., Ltd. (Shanghai, China).The panel was further cooled to ambient temperature then and conditioned before cuttingfor density and mechanical properties specimens.Materials 2022, 15, 7518 6 of 162.6. Physical and Mechanical Properties TestingThe physical and mechanical properties of the WBS panels were tested according to Chinesenational standards GB/T 30364 and GB/T 20241. For dimensional stability, the thicknessswelling rate (TS), width swelling rate (WS), and water absorption rate (WA) after immersion inboiling water for 4 h was followed by oven drying at 63 ◦C for 20 h then reimmersion in boilingwater for 4 h. Specimen size was 20 mm × 50 mm × 50 mm, with 12 specimens measuredand averaged per group. Flexural strength, parallel-to-grain compressive strength, and shearstrength were measured using a Jinan Meters microcomputer-controlled wood-based paneluniversal testing machine (MWD-w10). Test specimen size was 20 mm × 20 mm × 450 mm(MOR), 20 mm × 40 mm × 120 mm (SS), and 60 mm × 20 mm × 20 mm (CS). The averageof 6 specimens was taken for each group. The internal morphology of each type of panel wasobserved in 10 mm× 10 mm blocks using a scanning electron microscope (SEM, S-3400, Hitachi,Tokyo, Japan) after sputter-coating with gold.3. Results3.1. Structural Characteristics of WBSThe difference between WBS strips and earlier iterations of BS scrimmed strips isshown in Table 2. WBS has much wider fiberized strips of up to 250 mm, whereas traditionaland narrow bamboo scrimber strips are between 30 and 60 mm wide. Narrow BS ischaracterized by variability in planar panel density caused by poorly controlled mat lay-upwhich creates overlaps and gaps requiring very high compaction pressures to achieveadequate contact and bonding of elements [19,33–35]. WBS allows for more controlledmat lay-up—an intermediary between haphazardly-laid traditional BS and well-alignedbamboo bundle LVL (see Figure 2). The method also permits lower compaction pressures(~5 MPa) used here, leading to more uniform density and improved panel properties andperformance. From BS factory production data, the production efficiency of WBS is about5–6 times higher than that of traditional BS. The utilization rate of WBS was estimated tobe 92% since the inner and outer cortex layers are not removed. The method leads to a 40%increase in the utilization rate compared to traditional BS. x FOR PEER REVIEW 6 of 7 The physical and mechanical properties of the WBS panels were tested according to Chi-nese national standards GB/T 30364 and GB/T 20241. For dimensional stability, the thick ess swelling rate (TS), width swelling rate (WS), and water absorption r te (WA) after immersion in boiling water for 4 was followed by oven drying at 63 °C for 20 h then rei i i ili ter for 4 h. Specimen size was 20 mm × 50 mm × 50 mm, with 12 specime s meas-ured and averaged per group. Flexural strength, parallel-to-grain compress ve strength, an shear st ength were measured using a Jinan Met rs microcomputer-controlled wood-based panel univ sal t sting machine (MWD-w10). Test specimen size was 20 mm × 20 mm × 450 mm (MOR), 20 m × 40 mm × 120 mm (SS), and 60 mm × 20 mm × 20 mm (CS). The average of 6 specimens was taken for each group. The internal morphology of each type of panel was observed in 10 mm × 10 mm blocks usi g a scanning electr n microscope (SEM, S-3400, Hitachi, Tokyo, Japan) after sputter- oati with gold. 3. Results 3.1. Structural Characteristics of WBS The difference between WBS strips and earlier iterations of BS scrimmed strips is shown in Table 2. WBS has much wider fiberized strips of up to 250 mm, whereas traditional and narrow bamboo scrimber strips are between 30 and 60 mm wide. Narrow BS is charac-terized by variability in planar panel density caused by poorly controlled mat lay-up which creates overlaps and gaps requiring very high compaction pressures to achieve adequate contact and bonding of elements [19,33–35]. WBS allows for more controlled mat lay-up—an intermediary between haphazardly-laid traditional BS and well-aligned bamboo bundle LVL (see Figure 2). The method also permits lower compaction pressures (~5 MPa) used here, lead-ing to more uniform density and improved panel properties and performance. From BS fac-tory production data, the production efficiency of WBS is about 5–6 times higher than that of traditional BS. The utilization rate of WBS was estimated to be 92% since the inner and outer cortex layers are not removed. The method leads to a 40% increase in the utilization rate compared to traditional BS. Figure 2. Cross-sections of mat lay-up and pressed panels for narrow BS, WBS, and bamboo bundle LVL. The percentage weight of liquid resin absorbed by the WBS strips with different broom-ing passes is shown in Table 3. Liquid resin absorption after one pass was about 26.5% but increased to 72.4% with four passes due to the larger extent of cracks, higher specific surface area [33] (see estimates in Table 3), and greater damage and removal of inner and outer cortex. While the resin absorption increased 2.7 times, the value of SSA was more than doubled after four passes, with a majority of cracks occurring during the early rounds of brooming. More fiberization passes decrease the fiber bundle diameter and increase the SSA. Greater SSA also leads to higher-bonded surfaces and resin-infused mechanical interlocking networks. The av-erage diameter of the fiber bundles after four passes through the fiberization rollers was 1.89 mm, giving an estimated contact surface area of about twice that of strips with one pass. i . Cross-sectio s of at lay- an press l f r rr , , l .The percentage weight of liquid resin absorbed by the WBS strips with differentbrooming passes is shown in Table 3. Liquid resin absorption after one pass was about26.5% but increased to 72.4% with four passes due to the larger extent of cracks, higherspecific surface area [33] (see estimates in Table 3), and greater damage and removal ofinner and outer cortex. While the resin absorption increased 2.7 times, the value of SSAwas more than doubled after four passes, with a majority of cracks occurring during theearly rounds of brooming. More fiberization passes decrease the fiber bundle diameterand increase the SSA. Greater SSA also leads to higher-bonded surfaces and resin-infusedmechanical interlocking networks. The average diameter of the fiber bundles after fourMaterials 2022, 15, 7518 7 of 16passes through the fiberization rollers was 1.89 mm, giving an estimated contact surfacearea of about twice that of strips with one pass.The fiberization extent must be balanced with the correct amount of resin uptake to‘repair’ the cracks and provide adequate properties without becoming uneconomic in termsof resin consumption. Excessive fiberization requires greater energy consumption andcost and reduced utilization rate to balance the need to mechanically flatten and createsupple, conformable strips with excessive fiberization. The mechanical properties’ resultsalso indicate no further gains from more than three passes, suggesting that only up to twoor at most three passes are needed.3.2. Effect of Fiberization on the Scrimber PropertiesStrength properties for each fiberization level are shown in Figure 3, and thickness andwidth swelling and water absorption rate of WBS after 4 h and 28 h are shown in Figure 4.Materials 2022, 15, x FOR PEER REVIEW 7 of 17 The fiberization extent must be balanced with the correct amount of resin uptake to ‘repair’ the cracks and provide adequate properties without becoming uneconomic in terms of resin consumption. Excessive fiberization requires greater energy consumption and cost and reduced utilization rate to balance the need to mechanically flatten and create supple, conformable strips with excessive fiberization. The mechanical properties’ results also indi-cate no further gains from more than three passes, suggesting that only up to two or at most three passes are needed. 3.2. Effect of Fiberization on the Scri ber Properties Strength ro erties for each fiberization level are sho n in Fig re 3, an thickness an i t s elli a ater absorption rate of WBS after 4 h and 28 h are shown in Figure 4. (a) (b) (c) Figure 3. Mechanical properties of WBS with different fiberization frequency: (a) flexural modulus of rupture (MOR), modulus of elasticity (MOE), (b) parallel shearing strength (SS||), perpendicular shear strength (SS⊥), and (c) compressive strength (CS). (a) (b) Figure 4. Dimensional stability at 4 h and 28 h of WBS with different fiberization frequency: (a) thick-ness swelling rate (TS), width swelling rate (WS), and (b) water absorption rate (WA). Flexural properties (modulus of rupture, MOR, and modulus of elasticity, MOE) in-creased between one and three passes, with no further gain at four passes. Compression strength (CS) and shear strength (SS) showed a steady increase with the number of passes up to three, as this is governed by the interface strength between two laminated faces. More passes mean more fissures in the adherend surfaces and more resin in the panel, creating stronger and better resin-interlocking between adherends and stronger bond in-terfaces. However, more than three passes likely compromises the structural integrity of the bamboo tissue to the extent that it starts to degrade composite mechanical properties and become uneconomic in terms of resin usage required to ‘repair’ accompanied by loss of strength properties. Fiberization with few passes retains larger and solid bamboo tissue Figure 3. Mechanical properties of WBS with different fiberization frequency: (a) flexural modulus ofrupture (MOR), modulus of elasticity (MOE), (b) parallel shearing strength (SS||), perpendicularshear strength (SS⊥), and (c) compressive strength (CS).Materials 2022, 15, x FOR PEER REVIEW 7 of 17 The fiberization extent must be balanced with the correct amount f resin uptake to ‘repair’ the cracks and provide adequate properti s without b coming uneconomic in terms of resin consumption. Exce sive f berization requir s greate nergy consumption and cost and reduced utilization rate to balance the need to mec anically flatten d create supple, conformable strips with excessive fiberization. The mechanical properties’ results also indi-cate no further gains from more than three passes, suggesting that only up to two or at most three passes are n ded. 3.2. Effect of Fiberization on the Scrimber Prop rties Strength properties for each fiberization level are shown in Figure 3, a d thickness and width swelling an water absorption rate of WBS after 4 h and 28 h are s ow in Figur 4. (a) (b) (c) Figure 3. Mechanical properties of WBS with different fiberization frequency: (a) flexural modulus of rupture (MOR), mod lus of elasticity (MOE), (b) parallel shearing st ength (SS||), pe pendicular shear strength (SS⊥), and (c) compressive strength (CS). (a) (b) Figure 4. Dimensional stabil ty at 4 h nd 28 h of WBS with different fiber zation frequency: (a) thick-ness swelling rate (TS), width sw lling rate (WS), and (b) water absorption r (WA). Flexural properti s (modulus of rupture, MOR, and modulus of elasticity, MOE) in-creased between on and thr e passes, with no further gain at four passes. Compression strength (CS) and shear strength (SS) showed a steady increase with the number of passes up to three, as this is governed by the interface strength between two laminated faces. More passes mean more fissures in th adher nd surfaces and more resin in the pan l, creating stronge nd be ter resin-interlocking betw en adherends and stronger bo in-terfaces. However, more than th ee passes likely compromis s the structural int grity of the bamboo tissu to the ex ent that it star s to degrade composite m chanical prop rties and become uneconomic in terms of resin usage required to ‘r pair’ accompanied by loss of strength properties. Fiberization with f w passes retains larger and solid bamboo tissueFigure 4. Dimensional stability at 4 h and 28 h of WBS with different fiberization frequency:(a) thickness swelling rate (TS), width swelling rate (WS), and (b) water absorption rate (WA).l r l r erties (modulus of rupture, MOR, and modulus of elasticity, MOE)i creased betw en on and r e pa ses, with no further gain at f . r it t t t t i it tt thre , as this is gover ed by the interface strength betwe n two lamin ted f c s. Morep ss s m an ore fissures in the a he n u faces and more resin in the pa el, creatingstro ger and b tter resin-interlocking betwe n a h rends a d str ger b d interfaces.However, mor an th e p sses likely compromises the str ctural integri y of the bambooissue o the extent that i star s o degrade composite mechanical properties and becomeu ec n c i terms of resin usag required to ‘ pa ’ accom nied by loss of strengthrop rties. Fiberization with few pa ses retains large and olid bamboo tissue in the lengthMaterials 2022, 15, 7518 8 of 16direction, with the resin coating ensuring minimal loss of mechanical properties comparedto unbroken tissue.The dimensional stability indicators thickness swell (TS), width swell (WS), and waterabsorption (WA) were significantly reduced with fiberization frequency from one to threewith a further small gain after four passes. The greater resin uptake and content in panelswith more intensively fiberized strips (i.e., greater resin-to-bamboo ratio) explain the trend,as PF resin is water-resistant compared with the hydrophilic bamboo tissue. From theresults, no more than two to three fiberization passes are needed for optimal mechanicaland dimensional properties.3.3. Effect of Density on the Scrimber PropertiesFlexural properties MOR and MOE both increase with density, as shown in Figure 5.The greatest increase was from 0.9 g/cm3 to 1.1 g/cm3. Ideally, for minimizing processingcosts and time, engineered bamboo should be not much greater in density than the bambooparent tissue, and results from this study suggest high-recovery, resin-bonded BS productscould be manufactured to 0.9–1.0 g/cm3 density. If mat layering is carefully controlled, asdiscussed in Figure 2, then the minimum required product densification can be reducedand still maintain adequate element contact and bonding [36].Materials 2022, 15, x FOR PEER REVIEW 8 of 17 in the length direction, with the resin coating ensuring minimal loss of mechanical prop-erties compared to unbroken tissue. The dimensional stability indicators thickness swell (TS), width swell (WS), and wa-ter absorption (WA) were significantly reduced with fiberization frequency from one to three with a further small gain after four passes. The greater resin uptake and content in panels with more intensively fiberized strips (i.e., gre er resin-to-bamboo ratio) xplain the trend, as PF resin is water-resistant compared with the hydrophilic bamboo tissue. From the results, no more than two to three fiberization passes are needed for optimal mechanical and dimensional properties. 3.3. Effect of Density n the Scrimber Properties Flexural properties MOR and MOE both increase with density, as shown in Figu e 5. The greatest increase was from 0.9 g/cm3 to 1.1 g/cm3. Ideally, for minimizing processing costs and time, engineered bamboo should be not much greater in density than the bamboo parent tissue, and results from this study suggest high-recovery, resin-bonded BS products could be manufactured to 0.9–1.0 g/cm3 density. If mat layering is carefully controlled, as dis-cu sed in Figure 2, then the minimum required product densificati n can be reduced and still maintain adequate element contact and bonding [36]. Figure 5. Effect of density on MOR and MOE of WBS. The shear and compression strength (SS and CS) with board density are shown in Fig-ure 6a,b. Unlike the flexural properties, the increase in SS and CS with panel density is linear. This is likely due to the fact that both shear and compressive strengths are more directly related to bonding strength, which in turn correlates to density through its impact on interele-ment contact development. On the other hand, bending MOE and MOR are more dependent on inter-element contact and bonding when the density is low. As the density increases, suf-ficient bonding develops, and the MOE and MOR will be governed by other factors such as fiber length and orientation. Figure 5. Effect of density on MOR and MOE of WBS.The shear and compression strength (SS and CS) with board density are shown inFigure 6a,b. Unlike the flexural properties, the increase in SS and CS with panel densityis linear. This i likely due to the fact tha both shear and compressiv strengths are moredirectly related to bonding str ngth, which in tur correl tes to nsity through its impacton interelement contact development. On the other hand, bending MOE and MOR aremore dependent on inter-element contact and bonding when the density is low. As thedensity increases, sufficient bonding develops, and the MOE and MOR will be governedby other factors such as fiber length and orientation.Materials 2022, 15, x FOR PEER REVIEW 9 of 17 (a) (b) Figure 6. Effect of density on (a) shear strength (SS) and (b) compressive (CS) of WBS. The thickness swell at 4 and 28 h with panel density are shown in Figure 7. TS increased with density up to 1.1 g/cm3, then decreased as the ratio of water-resistant resin was high enough to hamper absorption and tissue swelling. The lowest 28 h TS was at 0.9 g/cm3 (6.5%), since the extent of cell compaction and internal stress is the lowest. Figure 7. Effect of density on 4 h and 28 h TS of WBS. Lower board density and higher void volume fraction allow easier penetration by wa-ter during soaking treatment. At a panel density of 0.8–1.0 g/cm3, there are still some open pores conducive to more rapid penetration and absorption of water. However, despite the increased liquid uptake, there is also less compacted tissue per unit volume of the panel to absorb moisture and swell, resulting in lower overall irreversible TS in the low-density pan-els. At the other end of the density range, the very high tissue compaction restricts moisture ingress and swelling during the limited moisture exposure time. Given unlimited water exposure time, the highest density panels would be expected to have the highest TS. Board density increases with higher volumes of bamboo bundles pressed to the same thickness due to a higher compaction rate. During hot-pressing and material softening, the vessels mostly undergo buckling and collapse, as seen in Figure 8b,c, which occurs during the viscoelastic softening and tissue relaxation during hot-pressing. If compaction and defor-mation occur rapidly at the beginning of hot-pressing before the material has heated suffi-ciently to plasticize and absorb the stress, then localized microfractures can occur, weakening the bamboo parent tissue. Similarly, if compaction is too slow and the resin heats and precures, then bond integrity is compromised. The fundamentals of BS hot-pressing, which should be similar to the consolidation behavior of wood composites [37–40], are under investiga-tion and will be published in a future paper. Figure 6. Effect of density on (a) shear strength (SS) and (b) compressive (CS) of WBS.Materials 2022, 15, 7518 9 of 16The thickness swell at 4 and 28 h with panel density are shown in Figure 7. TSincreased with density up to 1.1 g/cm3, then decreased as the ratio of water-resistant resinwas high enough to hamper absorption and tissue swelling. The lowest 28 h TS was at0.9 g/cm3 (6.5%), since the extent of cell compaction and internal stress is the lowest.Materials 2022, 15, x FOR PEER REVIEW 9 of 17 (a) (b) Figure 6. Effect of density on (a) shear strength (SS) and (b) compressive (CS) of WBS. The thickness swell at 4 and 28 h with panel density are shown in Figure 7. TS increased with density up to 1.1 g/cm3, then decreased as the ratio of water-resistant resin was high enough to hamper absorption and tissue swelling. The lowest 28 h TS was at 0.9 g/cm3 (6.5%), since the extent of cell compaction and internal stress is the lowest. Figure 7. Effect of density on 4 h and 28 h TS of WBS. Lower board density and higher void volume fraction allow easier penetration by wa-ter during soaking treatment. At a panel density of 0.8–1.0 g/cm3, there are still some open pores conducive to more rapid penetration and absorption of water. However, despite the increased liquid uptake, there is also less compacted tissue per unit volume of the panel to absorb moisture and swell, resulting in lower overall irreversible TS in the low-density pan-els. At the other end of the density range, the very high tissue compaction restricts moisture ingress and swelling during the limited moisture exposure time. Given unlimited water exposure time, the highest density panels would be expected to have the highest TS. Board density increases with higher volumes of bamboo bundles pressed to the same thickness due to a higher compaction rate. During hot-pressing and material softening, the vessels mostly undergo buckling and collapse, as seen in Figure 8b,c, which occurs during the viscoelastic softening and tissue relaxation during hot-pressing. If compaction and defor-mation occur rapidly at the beginning of hot-pressing before the material has heated suffi-ciently to plasticize and absorb the stress, then localized microfractures can occur, weakening the bamboo parent tissue. Similarly, if compaction is too slow and the resin heats and precures, then bond integrity is compromised. The fundamentals of BS hot-pressing, which should be similar to the consolidation behavior of wood composites [37–40], are under investiga-tion and will be published in a future paper. i re 7. ff t f sit 28 S of S.Lower board density and higher void volume fraction allow easier penetration bywater during soaking treatment. At a panel density of 0.8–1.0 g/cm3, there are still someopen pores conducive to more rapid penetration and absorption of water. However, despitethe increased liquid uptake, there is also less compacted tissue per unit volume of the panelto absorb moisture and swell, resulting in lower overall irreversible TS in the low-densitypanels. At the other end of the density range, the very high tissue compaction restrictsmoisture ingress and swelling during the limited moisture exposure time. Given unlimitedwater exposure time, the highest density panels would be expected to have the highest TS.Board density increases with higher volumes of bamboo bundles pressed to the samethickness due to a higher compaction rate. During hot-pressing and material softening,the vessels mostly undergo buckling and collapse, as seen in Figure 8b,c, which occursduring the viscoelastic softening and tissue relaxation during hot-pressing. If compactionand deformation occur rapidly at the beginning of hot-pressing before the material hasheated sufficiently to plasticize and absorb the stress, then localized microfractures canoccur, weakening the bamboo parent tissue. Similarly, if compaction is too slow and theresin heats and precures, then bond integrity is compromised. The fundamentals of BS hot-pressing, which should be similar to the consolidation behavior of wood composites [37–40],are under investigation and will be published in a future paper.Materials 2022, 15, x FOR PEER REVIEW 10 of 17 Figure 8. SEM images of raw bamboo and WBS: (a) raw bamboo density = 0.65 g/cm3, (b) WBS density = 0.9 g/cm3, (c) WBS density = 1.3 g/cm3. After pressing, some internal stress remains within the panel. With exposure to mois-ture, the compressed cells absorb water via diffusion and restore their original shape caus-ing swelling, delamination, and internal flaws. After continuous water boiling, elastic de-formation continues to increase as internal stress is gradually released [41]. Rupture of in-ternal bond lines may occur, caused by the heavy compaction, which exacerbates defor-mation and swelling, causing increased irreversible thickness swell rate. Such high-density and low-dimensional stability have in the past greatly limited the cost-effectiveness and commercial applications of BS. One reason for the high resin requirement and low cost-ef-fectiveness of BS is to help contain TS caused by high compaction. Panels in this study had relatively low TS due to the high ratio of water-resistant resin coating the tissue and reduced free void space restricting water entry. One strategy for reducing moisture uptake and swelling without needing more resin is to adjust the resin formulation. For example, Zhu et al. [42] discussed the ability of low-molecular phenolic resin to provide more uniform and effective penetration into bamboo tissue, and it was believed to be the primary contributor to the dimensional stability of BS that was made at different density levels. Higher dimensional stability may result from small resin molecules entering, polymerizing, and occupying smaller voids and more of the hydrophilic functional groups in the tissue. More frequent deposits of low molecular weight resin on bamboo cell walls and their internal pits can effectively decrease the swelling prop-erties of testing specimens when immersed in water. The deposit of polymers on the bam-boo cell wall is the fundamental contributor to the improvement of the dimensional sta-bility of the scrimber board. 3.4. Impact of Densification on the Microstructure of Scrimber Figure 8 compares the transverse section of raw bamboo (density = 0.65 g/cm3) and WBS at both ends of its density range of between 0.9 g/cm3 and 1.3 g/cm3 At the lower den-sification, localized deformations occurred in the parenchyma cells. As the densification in-creases, the deformation extends to vascular bundles, especially in the vessels as seen in the high-magnification image in Figure 8c. The reduction of large lumen space influences water absorption and TS. At excessive densification levels, cracks and debonding (Figure 8c) start to develop due to localized shear failures, particularly if the tissue has not sufficiently heated and softened during compaction and/or the resin has precured and become brittle. These failures do not necessarily weaken the global strength properties as observed in MOR and SS, but could negatively impact other structural properties such as fatigue and duration of load. Compaction reduces the between- and within-strip void fraction, creating more effec-tive contact and adhesion of the resin. The strength of the bamboo fiber in the length direc-tion and the bonding strength between fiber bundles largely determine the strength prop-erties of the scrimber. At the same panel thickness, higher density is achieved by adding more layers, resulting in a higher degree of element compaction ratio and densification dur-ing hot pressing. At too low a density, the weak bonding cannot provide sufficient stress Figure 8. SEM images of raw bamboo and WBS: (a) raw bamboo density = 0.65 g/cm3, (b) WBSdensity = 0.9 g/cm3, (c) WBS density = 1.3 g/cm3.After pressing, some internal stress remains within the panel. With exposure tomoisture, the compressed cells absorb water via diffusion and restore their original shapecausing swelling, delamination, and internal flaws. After continuous water boiling, elasticdeformation continues to increase as internal stress is gradually released [41]. RuptureMaterials 2022, 15, 7518 10 of 16of internal bond lines may occur, caused by the heavy compaction, which exacerbatesdeformation and swelling, causing increased irreversible thickness swell rate. Such high-density and low-dimensional stability have in the past greatly limited the cost-effectivenessand commercial applications of BS. One reason for the high resin requirement and lowcost-effectiveness of BS is to help contain TS caused by high compaction. Panels in thisstudy had relatively low TS due to the high ratio of water-resistant resin coating the tissueand reduced free void space restricting water entry.One strategy for reducing moisture uptake and swelling without needing more resinis to adjust the resin formulation. For example, Zhu et al. [42] discussed the ability of low-molecular phenolic resin to provide more uniform and effective penetration into bambootissue, and it was believed to be the primary contributor to the dimensional stability of BSthat was made at different density levels. Higher dimensional stability may result fromsmall resin molecules entering, polymerizing, and occupying smaller voids and more of thehydrophilic functional groups in the tissue. More frequent deposits of low molecular weightresin on bamboo cell walls and their internal pits can effectively decrease the swellingproperties of testing specimens when immersed in water. The deposit of polymers on thebamboo cell wall is the fundamental contributor to the improvement of the dimensionalstability of the scrimber board.3.4. Impact of Densification on the Microstructure of ScrimberFigure 8 compares the transverse section of raw bamboo (density = 0.65 g/cm3) andWBS at both ends of its density range of between 0.9 g/cm3 and 1.3 g/cm3 At the lowerdensification, localized deformations occurred in the parenchyma cells. As the densificationincreases, the deformation extends to vascular bundles, especially in the vessels as seen inthe high-magnification image in Figure 8c. The reduction of large lumen space influenceswater absorption and TS. At excessive densification levels, cracks and debonding (Figure 8c)start to develop due to localized shear failures, particularly if the tissue has not sufficientlyheated and softened during compaction and/or the resin has precured and become brittle.These failures do not necessarily weaken the global strength properties as observed inMOR and SS, but could negatively impact other structural properties such as fatigue andduration of load.Compaction reduces the between- and within-strip void fraction, creating more ef-fective contact and adhesion of the resin. The strength of the bamboo fiber in the lengthdirection and the bonding strength between fiber bundles largely determine the strengthproperties of the scrimber. At the same panel thickness, higher density is achieved byadding more layers, resulting in a higher degree of element compaction ratio and densifica-tion during hot pressing. At too low a density, the weak bonding cannot provide sufficientstress transfer between elements and hence become the weakest link in the composite. Asdensity increases element surface contact and pressure, the bond quality increases to thepoint where it exceeds the strength of the bamboo tissue, and tissue strength becomesthe limiting factor. High compaction forces more resin into smaller pores and voids inthe tissue, creating better mechanical interlocking. High-density panels contain fewervoids, resulting in many more bond contact points for stress transfer between the elements.Properties must be balanced with increased panel weight and the time, energy, and resinrequired to press higher-density BS and the reduced dimensional stability that accompaniesvery high compaction of BS products. The results from this study with wide strips favorminimal passes through the rollers (1 to 2) to reduce tissue break-up and resin consumption,and densification to no more than 0.9 to 1 g/cm3 to achieve adequate strength propertiesfor structural purposes. Wide strips are recommended for BS to help reduce the number ofelements per mat, the problems associated with mat heterogeneity, and flaws caused bymisalignment and random overlaps and gaps between narrow strips, as illustrated anddiscussed earlier in Figure 2.Materials 2022, 15, 7518 11 of 163.5. Performance Comparison of WBS with Other Engineered Bamboo MaterialsTo further understand the structure–property relationships of bamboo composites, theMOR and MOE of different engineered bamboo materials reported from past studies inthe literature are compared using Ashby plots in Figure 9 and reported values in Table 4.Bamboo products may be classified into five product types: natural raw bamboo (RB),laminated bamboo lumber (LBL), flattened bamboo (FB) bamboo scrimber (BS and WBS),and bamboo bundle laminated veneer lumber (BLVL). Figure 9 shows mechanical propertiesare generally positively correlated with product density.Materials 2022, 15, x FOR PEER REVIEW 11 of 16 3.5. Performance Comparison of WBS with Other Engineered Bamboo Materials To further understand the structure–property relationships of bamboo composites, the MOR and MOE of different engineered bamboo materials reported from past studies in the literature are compared using Ashby plots in Figure 9 and reported values in Table 4. Bam-boo products may be classified into five product types: natural raw bamboo (RB), laminated bamboo lumber (LBL), flattened bamboo (FB) bamboo scrimber (BS and WBS), and bamboo bundle laminated veneer lumber (BLVL). Figure 9 shows mechanical properties are gen-erally positively correlated with product density. (a) (b) Figure 9. Density-flexural property relationships of bamboo products. (a) MOR, (b) MOE. Notes: BS—bamboo scrimber, WBS—wide bamboo scrimber, RB—raw bamboo, BLVL—bamboo bundle lami-nated veneer lumber, LBL—laminated bamboo lumber, and FB—flattened bamboo. All bamboo com-posites use Moso (Phyllostachys pubescens Mazel.) and RB includes Moso, Guadua and Dendrocala-mus asper. Figure 9. Density-flexural property relationships of bambo products. (a) MOR, (b) MOE. Notes:BS—bamboo scrimber, WBS—wide bamboo scrimber, RB—raw bamboo, BLVL—bamboo bundlelaminated veneer lumber, LBL—laminated bamboo lumber, and FB—flattened bamboo. All bam-boo composites use Moso (Phyllostachys pubescens Mazel.) and RB includes Moso, Guadua andDendrocalamus asper.Materials 2022, 15, 7518 12 of 16Table 4. The mechanical properties of different engineered bamboo products.Products Density(kg/cm3)Modulus of Rupture(MPa)Modulus of Elasticity(GPa) ReferenceWBS 1.02–1.3 253.23–398 26.35–32.3Yu et al. [30]Yu et al. [43]BS 0.85–1.3 119–398 13–32.3Huang et al. [31] Yu et al. [15]Sharma et al. [44] Shang et al. [27]Wei et al. [45] Zhang et al. [46]Yu et al. [17] Kumar et al. [47]RB 0.68–0.87 69.1–122.46 3.6–13.5Lorenzo et al. [48] Yu et al. [17]Ribeiro et al. [49] Sharma et al. [44]Chung et al. [50] Huang et al. [51]Dixon et al. [52] Yu et al. [30]BLVL 0.98–1 121.31–280 15.83–24.61Zhou et al. [53] He et al. [54]Chen et al. [55] Deng et al. [56]Deng et al. [57]LBL 0.62–0.78 67.7–88 7.4–17.3Andy et al. [58] Yu et al. [30]Rittironk et al. [59] Sharma et al. [60]FB 0.62–1.01 74–115.13 4.49–12.1Andy et al. [58] Huang et al. [31]Rittironk et al. [59] Sharma et al. [44]Wenji Yu et al. [30] Nugroho et al. [61]Raw bamboo (RB) is natural bamboo without any treatment or densification andtherefore has baseline density and strength properties for the bamboo type. RB has lowerstrength properties than most engineered bamboo products, especially BS and BLVL, due tothe densification and large resin addition to these products. Different species of raw bambooexhibit a large range in density but relatively low correlation with strength properties.Laminated bamboo lumber (LBL) has a low densification ratio, just enough to com-press and bond the flat smooth strips together, and is most similar to that of RB, as itrequires little densification for bonding [29]. LBL is made by stacking and gluing uniformlymilled bamboo elements, producing a uniform element shape, size, and orientation inthe product. The average strength of structural LBL is similar to flattened bamboo andhardwoods [62]. Several studies have demonstrated the unique properties of structuralLBL [62–66]. Structural members made of LBL can be created in any cross-section and shapeand are adaptable to different heights and spans of members. Of particular note is thatwhile the MOE of LBL increased with density, the MOR did not show a significant increasewith density because MOR was governed more by glue-line strength, which could failprematurely, rather than transferring stress to the tissue. Unlike BS, which is a compositemixture of fiber bundles encased in large amounts of resin that can withstand loading evenwhen its structure begins to fail, the stress transfer capability of LBL collapses, resultingin a masked density effect on bending strength. In contrast, the MOE of LBL increaseswith increasing density because the MOE reflects the material’s ability to resist elasticdeformation, i.e., the linear portion gradient of the stress–strain curve before the glue linebecomes affected.Similarly, laminated FB has comparable properties and lack-of-density effect on MORto RB. The product is produced by glue-laminating whole or half culms that have beensoftened under high heat and steam and progressively opened out flat through a series ofcustom-designed rollers [18,67]. Most engineered bamboo products have a low utilizationrate of bamboo biomass and/or high content of adhesive, which can obscure the textureand grain of the original bamboo surface. Advantages of FB technology include its highrecovery rate and maintenance of the original grain texture of bamboo without cracking,which makes it suited to appearance flooring and wall paneling applications. If the outercortex is retained, then the utilization rate of flattened bamboo increases to around 90% [29].Adhesive requirements are also greatly reduced compared with most other engineeredbamboo products [68]. FB sheets are generally not heat-treated or heavily densified beyondabout 1 g/cm3, giving them similar mechanical properties to RB, and can be glued intoMaterials 2022, 15, 7518 13 of 16LBL-type products. Note that the MOR of RB, LBL, and FB ranges from 69.1–122.46 MPaand MOE from 3.6–17.9 GPa. The mechanical properties of these products are low and mayonly be suitable for lower structural performance requirements.Bamboo scrimber (BS) is mostly denser and stronger than both the RB and LBL due toits higher densification ratio and required resin content. Note that BS and WBS have thehighest variation in mechanical properties, with MOR ranging from 119–398 MPa and MOEfrom 13–32.3 GPa. Their strength properties are comparable to other structural buildingmaterials. From Figure 9, it can be seen that BLVL is notably less variable in properties thanBS. Its mechanical properties are similar to BS but at a lower density. Other research showsBLVL is more uniform, allowing for better control and standardization properties [53]. BLVLis a relatively new product designed to improve the automation of the production processand the performance of the product. It also enhances material utilization to some extent.Its success lies in the use of stitched bamboo bundle mats to increase the laminate size anduniformity and hence achieve better bonding with less compaction requirement [36,69].The greater uniformity of these elements effectively reduces variation in properties.The width and extent of fiberization of BS strips also have a large impact on strengthproperties (as demonstrated here), as they affect both mat lay-up (see Figure 2) and resincontent. BS currently lacks uniform manufacturing and quality standards that adequatelyaccount for the variation in density and mechanical properties of these products [70]. Theuse of wide strips and stitched veneer go a long way toward controlling axial alignmentand the frequency of overlaps within layers, which requires greater compaction to achieveadequate surface contact and bonding than if there are no overlaps between layers, muchlike plywood or LVL [36].Furthermore, the bamboo bundles used in BS often undergo a modification treatmentsuch as carbonization prior to manufacture. Heat and steam treatments are used to improvethe corrosion and mildew resistance, adjust product color, and improve dimensionalstability. However, such treatments, particularly at higher temperatures, also contribute tovariability in the mechanical properties of BS. It can be seen from Figure 9 that MOR andMOE of BS materials that had undergone saturated steam treatment (SST) and hot dry airtreatment (HDAT) are in the range of 86–166.5 MPa and 4.43–29 GPa, respectively.4. ConclusionsThis study investigated the fiberization–density–property relationships of wide-bundlebamboo scrimber (WBS). The outer cortex of the culm was partially retained during fiber-ization, resulting in a utilization rate for WBS above 90% compared with 55% for normalscrimber. Fiberization creates cracks and exposes fiber surfaces in the bamboo strips. Assuch, the specific surface area (SSA) increased by 100%, and the resin absorption rate wentup 2.7 times after four brooming passes. Upon consolidation and curing, the induced crackswere more-or-less “repaired”, forming a resin-infused mechanical interlocking networkthat enhanced water absorption, dimensional stability, and mechanical properties of theresulting composites. To balance product cost and performance, strips with 1–2 broomingpasses and a panel density of 0.9–1.0 g/cm3 is recommended. This fiberization and densitylevel yielded properties of MOR = 163.96–210.73 MPa, MOE = 16.11–20.9 GPa, and 28 hTS = 6.54–6.97%. These properties are, on average, better than regular BS, representing a sig-nificant improvement in product quality at lower compaction if wide strips (200–250 mm inwidth) are used. This was believed to be largely due to improved mat layering with betterstrip alignment and fewer random edge overlaps within layers compared with normal BS.The properties could be further enhanced by using strip stitching and continuous lay-up,leading to laminated structures such as bamboo laminated veneer lumber (BLVL).The flexural properties (MOR and MOE) of WBS were compared with other engineeredbamboo products. Those that use crack-free elements such as flattened bamboo (FB) orlaminated bamboo lumber (LBL) have lower variations in density and flexural propertiesand are more similar to the bamboo parent tissue. While the MOE of LBL and FB increasedwith density, the MOR failed to show a significant increase with density, which is contraryMaterials 2022, 15, 7518 14 of 16to scrimber and counter-intuitive. This discrepancy can be attributed to the lack of sufficientbonding in the laminated products, causing premature failures in the glueline rather thaninter-element stress transfer. The high densification of ‘fiberized’ element products suchas bamboo scrimber or BLVL can improve mechanical properties due to high resin usageand mechanical interlocking. Further work will focus on the performance analysis oflow-density and low-fiberization bamboo scrimbers to obtain a more accurate balancebetween material performance, bundle morphology, and product density.Author Contributions: Conceptualization, C.D.; methodology, C.D. and Y.H.; software, Y.H. andM.H.; validation, Y.L. and M.H.; formal analysis, Y.H. and H.P.; investigation, M.H. and V.N.;resources, Y.H.; data curation, L.X.; writing—original draft preparation, Y.H., Y.L., K.S., and V.N.;writing—review and editing, K.S., V.N., L.X., and C.D.; supervision, C.D.; project administration,Y.H and C.D.; funding acquisition, C.D. and Y.H. All authors have read and agreed to the publishedversion of the manuscript.Funding: This work was financially supported by the National Natural Science Foundation ofChina (Grant No. 31960337), Key Program for S&T of Jiangxi Academy of Forestry and Key Pro-gram and S&T Cooperation Projects of Jiangxi province (Grant No. 20212BDH80003 and Grant No.20202BBFL63021). Financial support from the Natural Sciences and Engineering Research Council ofCanada (NSERC) through a Discovery Grant is also gratefully acknowledged.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.Data Availability Statement: No new data were created or analyzed in this study. Data sharing isnot applicable to this article.Conflicts of Interest: The authors declare that they have no known competing financial interest orpersonal relationships that could have appeared to influence the work reported in this paper.References1. 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Vancouver : University of British Columbia Library
Multidisciplinary Digital Publishing Institute
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bamboo composites
wide-bundle bamboo scrimber
engineered bamboo products
fiberization
mechanical properties
dimensional stability
Manufacturing and Characterization of Wide-Bundle Bamboo Scrimber: A Comparison with Other Engineered Bamboo Composites
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