Evaluation of Uniformity of Bamboo Bundle Veneer and Bamboo Bundle Laminated Veneer Lumber (BLVL)

Article Evaluation of Uniformity of Bamboo Bundle Veneer and Bamboo Bundle Laminated Veneer Lumber (BLVL)

Haiying Zhou 1 , Xin Wei 1, Lee M. Smith 2, Ge Wang 1 and Fuming Chen 1,* 1 Key Laboratory of Bamboo and Rattan Science and Technology of the State Forestry Administration, Department of Bio-Materials, International Centre for Bamboo and Rattan, Beijing 100102, China; [email protected] (H.Z.); [email protected] (X.W.); [email protected] (G.W.) 2 Mechanical and Energy Engineering, University of North Texas, Denton, TX 76203-1277, USA; [email protected] * Correspondence: [email protected]; Tel.: +86-010-847-8743

Received: 3 September 2019; Accepted: 17 October 2019; Published: 19 October 2019

Abstract: The lack of an effective and practical quality control method for industrialized bamboo bundle veneers is the key restriction in the application of bamboo bundle composite materials in the field of construction. In this work, the density uniformity and mechanical properties of bamboo bundle veneers were systematically evaluated by the combination of light transmittance and mechanical stiffness. It was found that the number of broomings, dippings, and high-temperature heat treatments had different effects on the bamboo bundle veneers. On this basis, the uniformity of the density and mechanical properties of the bamboo scrimber (BS) that underwent hybrid paving, and the bamboo bundle laminated veneer lumber (BLVL), were analyzed. The results showed that the performance stability of bamboo bundle composites could be greatly improved by bamboo bundle veneer laminated paving. A large-scale quality evaluation system for bamboo bundle veneers was established in this work, and it provides conditions for the manufacture of bamboo bundle composites with stable and controllable performance.

Keywords: quality evaluation; woven; bamboo bundle veneer; light transmittance; mechanical stiﬀness; uniformity

1. Introduction Bamboo scrimber (BS) is a bamboo-based composite material that is made from bamboo bundle or fiberized bamboo veneers that have been layered-up in parallel, glued, and hot-pressed or cold-pressed. BS possesses good durability, good mechanical strength, and a high density (1.15 g/cm3 or more) [1–3]. It is widely used in outdoor flooring, landscaping, and structural applications [4–6]. At present, bamboo scrimber is manufactured either by cold pressing or hot pressing, from bamboo bundle units or fiberized bamboo veneers that are dried, impregnated with adhesive, and manually paved according to the weighing method. The weighing method can easily cause uneven paving, which results in an uneven density, uneven stress, and other defects within the boards [1,7,8]. Currently, bamboo scrimber manufacturing requires high-pressure, high-density, and high-energy rough processing in order to ensure a good dimensional stability, glueing interface, and surface quality. In order to reduce energy consumption and improve the uniformity of bamboo scrimber manufacturing, Wang et al [7–11]. carried out a study on the processing of bamboo bundles from which bamboo bundle laminated veneer lumber (BLVL) was developed based on bamboo scrimber. The density of BLVL can be reduced to 1.05 g/cm3, and its mechanical strength was comparable to the density of 1.25 g/cm3 of bamboo scrimber made with hybrid pavement, which was a breakthrough in the quality control of bamboo

Forests 2019, 10, 921; doi:10.3390/f10100921 www.mdpi.com/journal/forests Forests 2019, 10, 921 2 of 14 scrimber [7–11]. The quality of the bamboo bundle units/bamboo bundle veneers had an important influence on the physical and mechanical properties of the bamboo scrimber. The uniformity of bamboo veneers is what determines the uniformity of both the density and mechanical properties of the BLVL. Effective quality evaluation of bamboo bundle veneers was a prerequisite for the manufacture of high-performance outdoor bamboo scrimber where the performance and structural design could be controlled. The bamboo bundle veneer was generally manufactured by fine-brooming, and the wide-sized bamboo bundles were then sewn together [7,10–13]. Various studies on the types of fiber mesh connectors of the bamboo bundle veneers were performed, and they found that the bamboo bundle veneers with a cotton fiber mesh had the highest efficiency. The spacing of the fiber mesh had a direct influence on the quality of the bamboo bundle veneers, and it possessed better tensile properties at a spacing of 40 cm. However, there were problems due to the sewing method used to prepare the bamboo bundle veneers. As bamboo bundles have a high hardness, the needles used to sew the fiber mesh to the bamboo were easily broken, which affected work efficiency. The low degree of automation and efficiency are the bottlenecks restricting the industrialized production of bamboo bundle veneers, which is why the process of bamboo bundle veneer is still in the research stage of development. It is a well-recognized fact that the morphology and degree of brooming on the bamboo bundle affects the physical and mechanical properties of the bamboo bundle composites. The holistic quality evaluation of bamboo bundle veneers was of great significance for guiding the production of quality bamboo bundle composites that possess uniform properties. When subjected to different brooming times, the degree of separation for bamboo vascular bundles and parenchyma cells was different. As the bamboo bundles are glued, they have different amounts of water-soluble phenolic resin applied, which leads to variations in the physical and mechanical properties of the bamboo bundle units/bamboo bundle veneers [6,11,14,15]. In order to reduce mold and increase the dimensional stability of bamboo scrimber, bamboo bundles are often subjected to high-temperature treatment, which can destroy the chemical structure of bamboo and affect its lignin and hemicellulose [16–19]. The physical and mechanical properties of the bamboo bundle units/bamboo bundle veneers changed due to the accuracy of the processing [20–23], as well as the surface density of the bamboo bundle veneers. Studies on the width, number of broomings, shape of fiber, fineness of fiber, transmittance, angle of the crack, and cross-sectional area of the bamboo bundles have led to some guiding conclusions on bamboo bundle veneers [1,2,6,24–27]. However, these studies were quality evaluations of bamboo bundle units and the results were affected by various factors such as software selection, processing accuracy, and material differences. At present, there is no thorough research on the quality evaluation of bamboo bundle veneers. The lack of standardized bamboo bundle veneer processing technology and high-efficiency quality evaluation methods of bamboo bundle veneers are key restrictions preventing the improvement in quality, reduction in density, and improvement in production efficiency of bamboo scrimber, which could lead to the automation and continuous improvement of the bamboo scrimber industry. In this study, narrow bamboo bundles with a width of about 2–3 cm were woven into a bamboo bundle veneer. This method of preparing the bamboo bundle veneer greatly improved the production efficiency of the bamboo bundle composites and can be facilitated in industrial applications. A real-time online evaluation system was used to evaluate the bamboo bundle veneers with light transmittance and mechanical stiffness as the main evaluation indexes, which can not only evaluate the uniformity of the bamboo veneers but also evaluate the mechanical properties of the bamboo veneers. This quality assessment standard for bamboo bundle veneers provides the basis for the manufacture of high-performance and special-purpose bamboo bundle composites in the future. A comparison of density uniformity and mechanical property uniformity between BLVL prepared by the lamination of bamboo bundle veneers and traditional bamboo scrimber with the hybrid paving was carried out. This study focused on the quality evaluation of bamboo bundle veneers in order to quantitatively characterize their quality and, thus, stabilize the performance of their final product of bamboo bundle veneer laminated composites. This stabilization of bamboo bundle veneers will allow the manufacture Forests 2019, 10, x FOR PEER REVIEW 3 of 14 the manufacture of high-performance bamboo bundle veneer laminated composites with controllable performanceForests 2019, 10 and, 921 structural design. 3 of 14

2. Materials and Methods of high-performance bamboo bundle veneer laminated composites with controllable performance and 2.1.structural Materials design. 2. Materials and Methods 2.1.1. Bamboo Bundle Veneers 2.1.Moso Materials bamboo (Phyllostachys pubescens [28,29]) that is 3–4 years old was harvested from Yong'an, Fujian Province in China. Bamboo strips 2–3 cm wide were obtained by splitting the bamboo tubes 2.1.1. Bamboo Bundle Veneers longitudinally by rolling and brooming them into bamboo bundles with a rolling machine. By controllingMoso thebamboo number (Phyllostachys of broomings pubescens (one [and28,29 two]) that times), is 3–4 bamboo years old bundle was harvested units with from different Yong’an, morphologicalFujian Province features in China. were obtained. Bamboo stripsThen, 2–3the cmbroomed wide werebamboo obtained bundles by were splitting prepared the bamboo into bambootubes longitudinallybundle veneers by by rolling using and a broomingweaving themmachine. into To bamboo prepare bundles bamboo with bundle a rolling veneers, machine. commercialBy controlling cotton the threads number were of broomingspurchased (onefrom andYong twoan times),City, Fujian bamboo Province, bundle China units and with used diﬀerent to connectmorphological the bamboo features bundles.[30] were obtained. Then, the broomed bamboo bundles were prepared into bambooCotton bundle threads veneers were byselected using ato weaving function machine. as a connecting To prepare line bamboo that would bundle hold veneers, the commercialbamboo bundlescotton together. threads wereFive lines purchased of thread from were Yongan arranged City, along Fujian the Province, 1.9 m long China bamboo and used bundle to connect veneers. the In bambooorder to bundlesavoid the [30 bamboo]. bundle veneers loosening, the distance between the cotton threads and the edgeCotton of the threads bamboo were bundle selected veneers to function was 15 as cm, a connecting and the threads line that were would set hold40 cm the apart bamboo from bundles each othertogether. across Five the length. lines of The thread bamboo were arrangedbundles were along transversely the 1.9 m long woven bamboo into bundlebamboo veneers. bundle Inveneers order to (Figureavoid 1). the bamboo bundle veneers loosening, the distance between the cotton threads and the edge of the bamboo bundle veneers was 15 cm, and the threads were set 40 cm apart from each other across the length. The bamboo bundles were transversely woven into bamboo bundle veneers (Figure1).

Figure 1. Schematic diagram of the weaving process of the bamboo bundle veneer. Figure 1. Schematic diagram of the weaving process of the bamboo bundle veneer. According to industrial production methods, two diﬀerent pressures of heat treatment were used onAccording the bamboo to bundles.industrial A producti horizontalon methods, pressure vesseltwo different (Zhenghe pressures Xiongfeng of heat Boiler treatment Manufacturing were usedCo., on Ltd., the bamboo in Fujian, bundles. China) A was horizontal used to pressure apply a high-pressurevessel (Zhenghe heat Xiongfeng treatment Boiler of 0.32–0.34 Manufacturing MPa at a Co., Ltd., in Fujian, China) was used to apply a high-pressure heat treatment of 0.32–0.34 MPa at a temperature of 130–150 ◦C for a time of 150 min. The low-pressure heat treatment used a pressure temperature of 130–150 ℃ for a time of 150 min. The low-pressure heat treatment used a pressure of of 0.10–0.12 MPa at a temperature of 110–120 ◦C for a period of time of 120 min. After the thermal 0.10–0.12treatment, MPa the at moisturea temperature content of (MC) 110–120 was ℃ reconditioned for a periodto of 10% time in of a drying120 min. room After with the an thermal ambient treatment, the moisture content (MC) was reconditioned to 10% in a drying room with an ambient temperature of 50–60 ◦C. temperatureBamboo of 50–60 bundle ℃. veneers were impregnated with a phenolic resin (PF) that had a solid content ofBamboo 18%, which bundle was veneers diluted were by a impregnated commercial water-soluble with a phenolic phenol resin (PF) formaldehyde that had a resinsolid content with a solidof 18%,content which of was 48.54% diluted supplied by a bycommercial Taier Corporation, water-solu Guangdong,ble phenol China.formaldehyde The bamboo resin bundlewith a veneerssolid contentwere thenof 48.54% dried supplied to a MCbetween by Taier 8% Corporation, and 12% in Guangdong, an ambient environment. China. The bamboo bundle veneers were then dried to a MC between 8% and 12% in an ambient environment. 2.1.2. Bamboo Scrimber (BS) and Bamboo Bundle Laminated Veneer Lumber (BLVL) 2.1.2. Bamboo Scrimber (BS) and Bamboo Bundle Laminated Veneer Lumber (BLVL) A CARVER M-3895 hot press (Carver Inc, Wabash, IN, USA) was used to press the BS and BLVL at aA temperatureCARVER M-3895 of 155 hot◦C, press for a (Carver press time Inc, ofWabash, 30 min IN, (1 minUSA) for was press used closing, to press 14 the min BS of and pressure BLVL at at thea temperature target thickness, of 155 and ℃, for 15 mina press for presstime of opening). 30 min (1 BS min and for BLVL press were closing, prepared 14 min by of glue-drying pressure at the thebamboo target thickness, bundle unit and hybrid 15 min paving for press and opening). whole-sheet BS bambooand BLVL bundle were laminatedprepared by veneers, glue-drying respectively. the

Forests 2019, 10, x FOR PEER REVIEW 4 of 14 Forests 2019, 10, 921 4 of 14 bamboo bundle unit hybrid paving and whole-sheet bamboo bundle laminated veneers, respectively. TheThe dimensionsdimensions andand targettarget densitydensity ofof thethe boardsboards werewere300 300mm mm (length) (length) × 150150 mmmm (width)(width) × 1414 mmmm 3 × × (thickness)(thickness) andand 1.01.0 gg/cm/cm3,, respectively.respectively.

2.2.2.2. Methods

2.2.1.2.2.1. Quality Evaluation MethodMethod ofof BambooBamboo BundleBundle VeneersVeneers TheThe lightlight transmittancetransmittance andand stistiffnessﬀness werewere usedused asas thethe evaluationevaluation indexesindexes toto quantitativelyquantitatively characterizecharacterize thethe qualityquality ofof bamboobamboo bundlebundle veneers.veneers.

LightLight TransmittanceTransmittance DetectionDetection TheThe uniformityuniformity ofof thethe broomingbrooming morphologymorphology usedused toto makemake thethe bamboobamboo bundlebundle veneerveneer waswas observedobserved byby thethe opticaloptical transmittancetransmittance detectiondetection method,method, whichwhich usesuses aa lightlight generatorgenerator toto projectproject lightlight ontoonto oneone sideside of of the the bamboo bamboo bundle bundle veneer veneer and and an an optical optical analyzer analyzer to collectto collect the the light light transmittance transmittance on theon otherthe other side. side. The uniformityThe uniformity of the of brooming the brooming morphology morphology was estimated was estimated by calculating by calculating the optical the lossoptical of the loss light of the passing light passing through through the bamboo the bamboo bundle veneerbundle (Figureveneer2 (Figure). 2). 𝑆 𝐿=S × 100% (1) L = 𝑆 100% (1) S0 × wherewhere LL isis thethe lightlight transmittance,transmittance, inin %;%; SS isis thethe powerpower ofof thethe actualactual opticaloptical signalsignal receivedreceived byby thethe photodiode (PD), in V; S0 is the power of the initial optical signal received by the photodiode (PD), photodiode (PD), in V; S0 is the power of the initial optical signal received by the photodiode (PD), in V. in V.Eight pairs of photodetectors were set-up according to the lateral width of the bamboo bundle veneer,Eight which pairs would of photodetectors pass between were the photodetectorset-up according pairs. to the The lateral photodetector width of the pairs bamboo consisted bundle of twoveneer, components, which would a light-emitting pass between diode the photodetector (LED) light emitter pairs. onThe the photodetector bottom below pairs the consisted bamboo bundle of two veneerscomponents, and a a photodiode light-emitting that functionsdiode (LED) as anlight optical emitter analyzer on the on bottom the top. below The LED the flashesbamboo at abundle fixed frequencyveneers and that a photodiode only the photodiode that functions (PD) as recognizes an optical as analyzer the optical on the signal. top. The When LED the flashes gap size at a of fixed the frequency that only the photodiode (PD) recognizes as the optical signal. When the gap size of the bamboo bundle veneer changes, the intensity and duration of the optical signal received by PD will bamboo bundle veneer changes, the intensity and duration of the optical signal received by PD will differ. Based on the characteristics of the optical signal received by the PD, the uniformity of the density differ. Based on the characteristics of the optical signal received by the PD, the uniformity of the of the whole bamboo bundle can be estimated. density of the whole bamboo bundle can be estimated.

(a) (b)

Figure 2. Test schematic diagram of the light transmittance. (a) Frame structure of evaluation equipment; Figure 2. Test schematic diagram of the light transmittance. (a) Frame structure of evaluation (b) test schematic of light transmittance. equipment; (b) test schematic of light transmittance. The spot size emitted by the light emitter was 5.0 cm, which was twice the width of the bamboo The spot size emitted by the light emitter was 5.0 cm, which was twice the width of the bamboo bundle units. This method not only monitored the transmittance of the bamboo bundle itself, but also bundle units. This method not only monitored the transmittance of the bamboo bundle itself, but also detected gaps between the bamboo bundle units. According to the width of the materials, the step detected gaps between the bamboo bundle units. According to the width of the materials, the step length of the transmittance detection was 5.0 cm. The maximum transmittance of each photodetector length of the transmittance detection was 5.0 cm. The maximum transmittance of each photodetector channel was recorded in each step, allowing the gap size and mechanical property uniformity of the channel was recorded in each step, allowing the gap size and mechanical property uniformity of the bamboo bundle veneers to be monitored in real time. In order to avoid the inﬂuence of bamboo chips bamboo bundle veneers to be monitored in real time. In order to avoid the influence of bamboo chips and other impurities on the light transmittance, the control system automatically reset to zero before and other impurities on the light transmittance, the control system automatically reset to zero before the light transmittance detection of each bamboo bundle veneer. The actual light transmittance of the the light transmittance detection of each bamboo bundle veneer. The actual light transmittance of the

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bamboo bundle veneer was calculatedbased on the light transmittance at the moment to ensure the bambooaccuracy bundle of the test veneer results. was calculatedbased on the light transmittance at the moment to ensure the accuracy of the test results. Mechanical Stiffness Detection Mechanical Stiﬀness Detection The mechanical crimping method was used to characterize the mechanical properties of the bambooThe mechanicalbundle veneers. crimping A certain method pressure was used according to characterize to a requirement the mechanical was applied properties to the of theset bamboopositions bundle of the veneers. bamboo Abundle certain veneer, pressure and according the disp tolacement a requirement at the wasapplication applied site to the was set measured. positions ofThe the ratio bamboo of force bundle to displacement veneer, and the was displacement used to determine at the application the mechanical site was stiffness measured. of the The bamboo ratio of forcebundle to displacementveneers. was used to determine the mechanical stiﬀness of the bamboo bundle veneers.

FF MSMS== (2)(2) DD wherewhereMS MSis is the the mechanical mechanical sti stiffnessﬀness (N (N/mm);/mm); F Fis is the the force force (N); (N); and andD Dis is the the displacement displacement (mm). (mm). AA pushpush barbar withwith aa built-inbuilt-in pressurepressure sensorsensor waswas mountedmounted onon thethe bamboobamboo bundlebundle veneerveneer conveyorconveyor andand usedused to detect the pressure pressure value. value. The The accuracy accuracy of of the the mechanical mechanical sensor sensor was was determined determined by the by theaccuracy accuracy of the of stepper the stepper motors motors that record that record the displacement. the displacement. According According to the stepper to the stepper motor reducer motor reducersetting settingand lead and parameters, lead parameters, the displacement the displacement of every of everystep was step 33.33/2000 was 33.33/ 2000mm/step, mm/step, that thatis, the is, theaccuracy accuracy of the of the pressure pressure sensor sensor was was 0.0167 0.0167 mm. mm. The The pressure pressure sensor sensor calculates calculates the displacement ofof thethe bamboobamboo bundle bundle veneer veneer due due to to the the force force applied applied (Figure (Figure3). According3). According to the to preliminarythe preliminary test, iftest, the if setthe pressure set pressure was toowas high, too high, the bamboo the bamboo bundle bundle veneer ve wouldneer would break, andbreak, if the and pressure if the pressure was too low,was thetoo errorlow, wouldthe error be would too large. be Therefore,too large. Therefore, an optimized an optimized pressure of pressure 30 N was of selected 30 N was totest selected the mechanical to test the stimechanicalﬀness of the stiffness bamboo of bundlethe bamboo veneers. bundle veneers.

(a) (b)

FigureFigure 3.3. TestTest schematicschematic diagramdiagram ofof thethe mechanicalmechanical stistiffness.ﬀness. ((aa)) FrameFrame structurestructure ofof evaluationevaluation equipment;equipment; ((bb)) test test schematic schematic of of mechanical mechanical sti stiffness.ﬀness.

FourFour mechanical stiffness stiffness detection detection channels channels were were set-up set-up according according to the to theveneer veneer length. length. The Thedeformation deformation detection detection points points were wereset in setthe inmiddl thee middle of the support of the support rollers, rollers,and the andspacing the spacingbetween betweenthe neighboring the neighboring support support roller was roller adjustable. was adjustable. The shape The shape and size and sizeof the of theplane plane indenter indenter can can be beadjusted adjusted to suit to suit differen differentt materials. materials. The Thediameter diameter of the of indent the indenterer on the on push the pushbar was bar twice was twicethe width the widthof a single of a single bamboo bamboo bundle bundle unit in unit order in order to detect to detect the mechanical the mechanical stiffness stiffness of the of bamboo the bamboo bundles bundles and andthe thegap gap between between them. them. The The mechanic mechanicalal stiffness stiffness and and light light transmittance transmittance detection detection were performedperformed simultaneouslysimultaneously on on a a conveyor. conveyor. The The bamboo bamboo bundle bundle veneers veneers were were transferred transferred by a by conveyor a conveyor along along their widththeir width where where the length the length of the of veneers the veneers first passed first passed through through the photodetectors the photodetectors to measure to measure the light the transmittancelight transmittance and were and then were moved then downmoved the down conveyor the whereconveyor they where were subjectedthey were to subjected mechanical to stimechanicalffness testing. stiffness This testing. method This of detectingmethod of the detect lighting transmittance the light transmittance and mechanical and mechanical stiffness helpsstiffness to develophelps to adevelop uniformity a uniformity test that ensures test that the ensures quality the of quality the bamboo of the bundle bamboo veneers bundle being veneers used being (Figure used4): (Figure 4):

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Figure 4. Test schematic diagram of the quality evaluation process of bamboo bundle veneer.

2.2.2. QualityFigure Stability 4. Test schematic Evaluation diagram of BLVL of the and quality BS evaluation process of bamboo bundle veneer.

2.2.2.The Quality performance Stability stabilityEvaluation of theof BLVL boards and was BS mainly embodied in the density uniformity and mechanical properties. In order to visually and clearly express the profile density distribution of each The performance stability of the boards was mainly embodied in the density uniformity and part of the whole board, 10 specimens measuring 50 mm 50 mm 14.0 mm were cut from the finished mechanical properties. In order to visually and clearly express× the× profile density distribution of each bamboo bundle laminated veneer lumber (500 mm 300 mm 14.0 mm) and had a profile density part of the whole board, 10 specimens measuring ×50 mm × 50× mm × 14.0 mm were cut from the test performed on them, using a DENSE-LAB X-ray profile density tester from Hameln, Germany. finished bamboo bundle laminated veneer lumber (500 mm × 300 mm × 14.0 mm) and had a profile According to ASTM standard D1037-06a (ASTM 2006), modulus of rupture (MOR) and modulus density test performed on them, using a DENSE-LAB X-ray profile density tester from Hameln, of elasticity (MOE) of BLVL and BS were respectively determined. Three boards for both BS and BLVL Germany. were pressed and 24 test specimens measuring 300 mm 20 mm 14 mm were cut from them. Twelve According to ASTM standard D1037-06a (ASTM 2006),× modulus× of rupture (MOR) and modulus test repetitions were performed for both the MOR and MOE testing. of elasticity (MOE) of BLVL and BS were respectively determined. Three boards for both BS and 3.BLVL Results were pressed and 24 test specimens measuring 300 mm × 20 mm × 14 mm were cut from them. Twelve test repetitions were performed for both the MOR and MOE testing. 3.1. Quality Evaluation and Analysis of Bamboo Bundle Veneers 3. Results The light transmittance and mechanical stiffness data of the bamboo bundle veneers detected by the3.1. quality Quality evaluation Evaluation equipmentand Analysis were of Bamboo processed Bundle using Veneers the Origin software to obtain 3D empirical models of their distribution (Figure5). The figures show the light transmittance and mechanical stiffnessThe in light the full-scale transmittance bamboo and bundle mechanical veneers, stiffness and they data reflect of the the bamboo density bundle uniformity veneers and mechanicaldetected by propertiesthe quality of evaluation the bamboo equipment bundle veneers. were processed using the Origin software to obtain 3D empirical modelsIn order of their to test distribution the accuracy (Figure of the 5). light The transmittance figures show and the mechanical light transmittance stiffness, aand comparison mechanical of thestiffness bamboo in bundlethe full-scale veneer bamboo with the bundle bamboo veneer mat wass, and performed they reflect in order the to density determine uniformity the relative and qualitymechanical of uniformity. properties It of was the found bamboo that bundle the light veneers. transmittance of the bamboo mat was less than that of the bambooIn order bundle to test veneer,the accuracy and the of the transition light transmittance was smoother and in mechanical the full-scale stiffness, range, a indicating comparison that of thethe densitybamboo uniformity bundle veneer of the with bamboo the bamboo mat was mat much was higher performed than that in order of the to bamboo determine bundle the relative veneer. However,quality of theuniformity. mechanical It was sti fffoundness ofthat the the bamboo light transmittance mat was much of the smaller bamboo than mat that was of less the than bamboo that bundleof the bamboo veneer, bundle but its uniformityveneer, and wasthe transition higher than was that smoother of the bambooin the full-scale bundle range, veneer. indicating The quality that evaluationthe density method uniformity can eofffectively the bamboo evaluate mat andwas classifymuch higher bamboo than bundle that of veneers. the bamboo bundle veneer. However, the mechanical stiffness of the bamboo mat was much smaller than that of the bamboo bundle veneer, but its uniformity was higher than that of the bamboo bundle veneer. The quality evaluation method can effectively evaluate and classify bamboo bundle veneers.

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(a) (b)

FigureFigure 5. 5. LightLight transmittance transmittance and and mechanical mechanical stiffness stiffness of bamboo of bamboo bundle bundle veneer veneer and and bamboo bamboo mat. mat. (a) 3D(a) model 3D model diagram diagram of light of light transmittance transmittance of ofbamboo bamboo bundle bundle veneer; veneer; (b ()b 3D) 3D model model diagram diagram of of light light transmittancetransmittance of of bamboo bamboo mat; mat; ( (cc)) 3D 3D model model diagram diagram of of mechanical mechanical stiffness stiffness of of bamboo bamboo bundle bundle veneer; veneer; ((dd)) 3D 3D model model diagram diagram of of mechanical mechanical stiffness stiffness of bamboo mat.

3.2.3.2. Influence Influence of of the the Degree Degree of of Brooming Brooming and and Glue Glue on on the the Quality Quality of of Bamboo Bamboo Bundle Bundle Veneers Veneers TheThe morphology morphology of of the the bamboo bamboo bundle bundle units units had had a a direct direct relationship relationship with with the the physical physical and and mechanicalmechanical properties properties of of the the bamboo-based bamboo-based boards boards they they were were used used to to make. make. The The different different degree degree of of broomingbrooming and and dipping dipping also also affected affected the the physical physical and and mechanical mechanical properties properties of of the the bamboo bamboo bundle bundle veneers.veneers. Figure6 6shows shows the the light light transmittance transmittanc of thee of bamboo the bamboo bundle veneersbundle withveneers di ff erentwith broomingdifferent broomingtimes before times and before after beingand after dipped being (the dipped bamboo (the bundle bamboo veneer bundle that veneer underwent that underwent brooming brooming once and oncetwice and is abbreviated twice is abbreviated as S and as D, S respectively, and D, respectively, while that while after that being after glued being is glued abbreviated is abbreviated as S-G and as S-GD-G, and respectively). D-G, respectively). The degree The of degree fiber bundle of fiber separation bundle separation of the bamboo of the bundle bamboo unit bundle could beunit changed could beby changed controlling by thecontrolling number ofthe broomings, number of whichbroomings had an, which effect had on the anlight effect transmittance on the light transmittance of the bamboo ofbundle the bamboo veneers. bundle As the veneers. proportions As the ofproportions parenchyma of parenchyma cells differ across cells differ the cross-section across the cross-section of bamboo, ofwhen bamboo, it is processed,when it is processed, it can cause it can the cause mechanical the mechanical stiffness stiffn of theess bamboo of the bamboo bundles bundles to vary. to Itvary. was Itobvious was obvious that the that light the transmittance light transmittance of the bambooof the bamboo bundle bundle veneers veneers after one after brooming one brooming was greater was greaterthan that than of thethat bamboo of the bundlebamboo veneers bundle that veneers underwent that underwent two broomings, two broomings, and the dipping and the reduced dipping the reducedlight transmittance the light transmittance of the bamboo of bundlethe bamboo veneers. bund Thele veneers. end face The of the end bamboo face of bundle the bamboo unit after bundle one unitbrooming after one was brooming still compressed was still but compressed had irregular but cracks,had irregular and the cracks, fiber bundles and the were fiber tightly bundles grouped were tightlybut not grouped separated. but After not twoseparated. broomings, After the two bamboo broomings, fiber bundlesthe bamboo had smallerfiber bundles diameters had and smaller were diametersintermittently and connectedwere intermittently to each other connected in a fine to and each uniform other in shape. a fine Therefore, and uniform the lightshape. transmittance Therefore, thewas light lowered transmittance and the uniformitywas lowered increased. and the unifor The gluedmity increased. bamboo bundle The glued veneers bamboo had bundle a smaller veneers light had a smaller light transmittance because the glue and dryness cause the bamboo bundles to contract

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Foreststransmittance 2019, 10, x FOR because PEER theREVIEW glue and dryness cause the bamboo bundles to contract laterally so8 of that 14 the gaps between the bamboo bundles were reduced. The light transmittance of the bamboo bundle laterally so that the gaps between the bamboo bundles were reduced. The light transmittance of the veneers that had been broomed twice was lower than that which had undergone one brooming. bamboo bundle veneers that had been broomed twice was lower than that which had undergone one The greater number of broomings also increased the uniformity of the bamboo bundles, which, in turn, brooming. The greater number of broomings also increased the uniformity of the bamboo bundles, helped to reduce the light transmittance. which, in turn, helped to reduce the light transmittance.

(a) (b)

Figure 6. ComparisonComparison of of light light transmittance transmittance of of bamboo bamboo bu bundlendle veneers veneers before before and and after after being being dipped. dipped. (a) 3D3D modelmodel diagram diagram of of light light transmittance transmittance of bambooof bamboo bundle bundle veneers veneers that th underwentat underwent brooming brooming once; (once;b) 3D (b model) 3D model diagram diagram of light of transmittancelight transmittance bamboo bamboo bundle bundle veneers veneers that underwent that underwent brooming brooming twice; (twice;c) 3D model(c) 3D diagrammodel diagram of light transmittanceof light transmittance of bamboo of bundlebamboo veneers bundle that veneers underwent that underwent brooming broomingonce after beingonce after dipped; being (d )dipped; 3D model (d) diagram 3D model of lightdiagram transmittance of light transmittance of bamboo bundle of bamboo veneers bundle that veneersunderwent that brooming underwent twice brooming after being twice dipped. after being dipped.

Comparing the mechanical stiffness stiffness of the bamboobamboo bundle veneers with different different numbers of broomings, itit was was found found that that the mechanicalthe mechanical stiffness stiffness curves curves of the bambooof the bundlebamboo veneers bundle fluctuated veneers fluctuatedup and down up and between down 800between and 1400 800 and N/mm 1400 (Figure N/mm7 ).(Figure However, 7). However, the mechanical the mechanical sti ffness stiffness of the ofbamboo the bamboo bundle bundle veneers veneers broomed broomed once once was was significantly significantly higher higher than than that that of of the the bamboobamboo bundle veneers broomed broomed twice. twice. As As the thetwo two broomings broomings processed, processed, it led itto leda greater to a greaterseparation separation of the bamboo of the fiberbamboo bundles, fiberbundles, causing causingmore breaki moreng breaking in the fibers in the and fibers stripping and stripping along alongthe parenchyma the parenchyma cells. cells.This Thisbreaking breaking in the in fibers the fibersand damage and damage to the tocells the reduced cells reduced the mechanical the mechanical stiffness sti offf nessthe bamboo of the bamboo bundle veneers.bundle veneers. The mechanical The mechanical stiffness of sti bambooffness of bundle bamboo veneers bundle increased veneers after increased impregnation after impregnation with resin. withThe flowing resin. The thermosetting flowing thermosetting phenolic resin phenolic (PF) was resin applied (PF) was to appliedthe veneers to the by veneers dipping by it dipping where it whereadhered it adheredto the surface to the and surface infiltrated and infiltrated into the into gaps the and gaps cell and gaps cell of gaps the bamboo of the bamboo bundles. bundles. After being After dipped,being dipped, the veneers the veneers were subjected were subjected to drying to drying at a te atmperature a temperature of 50–60 of 50–60 °C. The◦C. PF The was PF in was a solid in a solidstate onstate the on surface the surface and andinside inside of the of thebamboo bamboo bundle bundle ve veneers,neers, granting granting the the veneers veneers flexibility, flexibility, which increased the hardness andand mechanicalmechanical stistiffnessffness ofof thethe bamboobamboo bundlebundle veneers.veneers.

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(a) (b)

FigureFigure 7. 7.Comparison Comparison of of mechanical mechanical stiff stiffnessness of bamboo of bamboo bundle bundle veneers veneers with diwithfferent different brooming brooming times beforetimes andbefore after and being after dipped. being dipped. (a) 3D model (a) 3D diagram model diagram of mechanical of mechanic stiffnessal ofstiffness bamboo of bundlebamboo veneers bundle thatveneers underwent that underwent brooming brooming once; (b) once; 3D model (b) 3D diagram model ofdiagram mechanical of mechanical stiffness ofstiffness bamboo of bundlebamboo veneersbundle thatveneers underwent that unde broomingrwent brooming twice; (c) twice; 3D model (c) 3D diagram model ofdiagram mechanical of mechanical stiffness of stiffness bamboo of bundlebamboo veneers bundle that veneers underwent that underwent brooming once brooming after being once dipped;after being (d) 3Ddipped; model (d diagram) 3D model of mechanical diagram of stimechanicalffness of bamboo stiffness bundle of bamboo veneers bundle that underwentveneers that brooming underwent twice brooming after being twice dipped. after being dipped.

TheThe average average value value of theof the light light transmittance transmittance and mechanicaland mechanical stiffness stiffness was aff ectedwas affected by the number by the ofnumber broomings; of broomings; to understand to understand this effect, this interactions effect, interactions between between brooming, broom gluing,ing, andgluing, drying and ofdrying the bambooof the bamboo bundle veneersbundle wereveneers analyzed were (Figureanalyzed8). (Figure The light 8). transmittance The light transmittance and mechanical and stimechanicalffness of thestiffness bamboo of bundlethe bamboo veneers bundle that underwentveneers that one unde broomingrwent one were brooming 167.23% were and 8.32% 167.23% higher and than 8.32% those higher of thethan bamboo those bundleof the veneersbamboo broomed bundle veneers twice, respectively. broomed twice, After beingrespectively. dipped, After the mechanical being dipped, stiffness the ofmechanical the bamboo stiffness bundle of veneers the bamboo with onebundle brooming veneers increased with one bybrooming 13.77% andincreased the light by 13.77% transmittance and the decreasedlight transmittance by 50.95%. decreased The mechanical by 50.95%. stiffness Theof mechanical the bamboo stiffness bundle of veneers the bamboo that were bundle broomed veneers twice that increasedwere broomed by 10.99%, twice but increased the light by transmittance 10.99%, but increasedthe light transmittance by 72.33%. Due increased to being by subjected 72.33%. toDue two to broomingbeing subjected treatments, to two the separatedbrooming bamboo treatments, fibers the were separated very loose bamboo and uniform. fibers were Once dried,very loose the fiber and bundlesuniform. shrank, Once causingdried, the the fiber gaps bundles between shrank, adjacent ca fiberusing bundles the gaps to between increase, ad whichjacent led fiber to an bundles increase to inincrease, light transmittance. which led to Thean increase separation in light of the transmittance. bamboo bundle The fibers separation after one of the brooming bamboo was bundle very fibers low, soafter once one they brooming dried, the was shrinkage very low, in so the once fiber they occurred dried, to the the shrinkage entire bamboo in the bundle,fiber occurred which contributedto the entire tobamboo the reduction bundle, of which light transmittance.contributed to the reduction of light transmittance.

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S D S D (a) (b) (a) (b) FigureFigure 8. Average 8. Average value value of of light light transmittancetransmittance and and mech mechanicalanical stiffness stiff nessof bamboo of bamboo bundle bundleveneer. ( veneer.a) (a) TopographyTopography ofof bamboobamboo bundle bundle veneer veneer with with different different numbers numbers of broomings; of broomings; (b) average (b) average value of value Figure 8. Average value of light transmittance and mechanical stiffness of bamboo bundle veneer. (a) of lightlight transmittance transmittance and mechanical mechanical stiffness stiffness of ba ofmboo bamboo bundle bundle veneer veneerwith different with dibroomingfferent broomingtimes Topography of bamboo bundle veneer with different numbers of broomings; (b) average value of timesbefore before and and after after being being dipped. dipped. The bamboo The bamboo bundle bundle veneer veneerthat underwent that underwent brooming brooming once and twice once and light transmittance and mechanical stiffness of bamboo bundle veneer with different brooming times twiceis is abbreviated abbreviated as asS and S and D, respectively, D, respectively, while while that after that being after glued being is glued abbreviated is abbreviated as S-G and as D-G, S-G and before and after being dipped. The bamboo bundle veneer that underwent brooming once and twice respectively. D-G,is respectively. abbreviated as S and D, respectively, while that after being glued is abbreviated as S-G and D-G, respectively. 3.3. E3.3.ffect Effect of Heat of Heat Treatment Treatment on on the the Quality Quality of of Bamboo Bamboo BundleBundle Veneers Veneers During3.3. EffectDuring production, of Heat production, Treatment bamboo bamboo on the bundles Quality bundles alwaysof Bambooalways undergo Bundleunderg Veneerso heat heat treatment treatment inin orderorder toto reducereduce mold and and size changes of BS. This high-temperature heat treatment can destroy the chemical structure of size changesDuring of BS. production, This high-temperature bamboo bundles heat always treatment underg cano destroy heat treatment the chemical in order structure to reduce of the mold bamboo the bamboo by removing lignin and hemicellulose from the bamboo. The impact of heat treatment by removingand size changes lignin and of BS. hemicellulose This high-temperature from the bamboo. heat treatment The impact can destroy of heat the treatment chemical structure under different of under different pressures on the light transmittance and mechanical stiffness of the bamboo bundle pressuresthe bamboo on the lightby removing transmittance lignin an andd hemicellulose mechanical stiffness from the of bamboo. the bamboo The impact bundle of veneers heat treatment was studied veneers was studied and compared to the veneers that had not been heat-treated (Figure 9). It can be under different pressures on the light transmittance and mechanical stiffness of the bamboo bundle and comparedseen that the to light the veneerstransmittance that hadand mechanical not been heat-treated stiffness of the (Figure bamboo9). bundle It can veneers be seen before that the and light veneers was studied and compared to the veneers that had not been heat-treated (Figure 9). It can be transmittanceafter heat andtreatment mechanical were relatively stiffness uniform. of the bambooThe heat bundletreatment veneers at lower before pressure and had after little heat effect treatment on seen that the light transmittance and mechanical stiffness of the bamboo bundle veneers before and werethe relatively light transmittance uniform. The and heat stiffness treatment of the atbamboo lower ve pressureneers, but had the little heat effecttreatment on the at higher light transmittance pressure after heat treatment were relatively uniform. The heat treatment at lower pressure had little effect on and stiffnesshad an influence of the bamboo on the transmittance veneers, but and the mechanical heat treatment stiffness at higher of the bamboo pressure veneers. had an The influence influence on the the light transmittance and stiffness of the bamboo veneers, but the heat treatment at higher pressure on the mechanical stiffness could be due to the bamboo becoming plasticized after high-temperature transmittancehad an influence and mechanical on the transmittance stiffness of and the mechanical bamboo veneers. stiffness The of the influence bamboo on veneers. the mechanical The influence stiffness heat treatment, which results in the reorganization of the cellulose polymer components [30] and couldon be the due mechanical to the bamboo stiffness becoming could be plasticizeddue to the bamboo after high-temperature becoming plasticized heat after treatment, high-temperature which results in partial crystallization of amorphous cellulose [31] that would cause an increase in the elastic modulus the reorganizationheat treatment, ofwhich the celluloseresults in polymer the reorganization components of the [30 cellulose] and partial polymer crystallization components of [30] amorphous and (MOE) [32]. cellulosepartial [31 crystallization] that would causeof amorphous an increase cellulose in the [31] elastic that would modulus cause (MOE) an increase [32]. in the elastic modulus (MOE) [32].

(a) (b) (a) (b)

Figure 9. Light transmittance and mechanical stiﬀness of the bamboo bundle veneers that underwent heat treatment. (a) Average value of light transmittance and mechanical stiﬀness; (b) Fourier transform infrared spectroscopy (FT-IR) spectra of the bamboo bundle veneers before and after heat treatment. Forests 2019, 10, x FOR PEER REVIEW 11 of 14

Figure 9. Light transmittance and mechanical stiffness of the bamboo bundle veneers that underwent heat treatment. (a) Average value of light transmittance and mechanical stiffness; (b) Fourier Foreststransform2019, 10, 921 infrared spectroscopy (FT-IR) spectra of the bamboo bundle veneers before and after heat11 of 14 treatment.

3.4.3.4. The The Density Density and and Mechanical Mechanical Uniform Uniformityity of of Traditional Traditional BS and Novel BLVL TheThe quality of the bamboo bundle veneers was cl closelyosely related to the physical and mechanical properties ofof thethe bamboo bamboo bundle bundle fibers. fibers. To understandTo understand therelationship the relationship between between the fibers the and fibers veneers, and veneers,the density the and density mechanical and mechanic propertiesal ofproperties BLVL prepared of BLVL by laminatedprepared bambooby laminated bundle bamboo veneers bundle and BS veneerswith hybrid and pavingBS with were hybrid compared paving and were analyzed compared (Figure and 10 analyzed). The variable (Figure density 10). The profile variable curves density (VDP) profileof BS and curves BLVL (VDP) fluctuated of BS acrossand BLVL their fluctuated cross-section across with their the cross-section VDP of traditional with the BS (FigureVDP of 10 traditionala) having BSlarge (Figure dispersed 10a) having fluctuations. large dispersed The coe ffifluctuations.cient of variation The coefficient (COV) of of the variation average (COV) density, of the maximum average density,density onmaximum the edges density of the on board, the edges and minimum of the board, density and wereminimum larger density in BS than were in larger BLVL. in The BS COVthan inof BLVL. the maximum The COV density of the wasmaximum especially density larger was in especial BS thanly in larger BLVL, in with BS than the left in BLVL, and the with right the being left and2.79 the and right 7.15 timesbeing those 2.79 ofand BLVL, 7.15 respectivelytimes those (Tableof BLVL,1). The respectively profile density (Table curves 1). The of theprofile BLVL density were curvesrelatively of flatthe (Figure BLVL 10werec), and relatively the COVs flat were (Figure below 10c), 4% (Tableand the1), indicating COVs were that below the uniformity 4% (Table of the1), indicatingbamboo bundle that the veneers uniformity had greatly of the improved bamboothe bundle density veneers uniformity had greatly of the BLVL. improved the density uniformity of the BLVL.

(a) (b)

FigureFigure 10. ProfileProfile density density curves curves in in width of bamboo scrimber (BS) and bamboo bundle laminated veneer lumberlumber (BLVL).(BLVL).(a (a) Profile) Profile density density curves curves in width in width and bondand mechanismbond mechanism of BS; (bof) fiber BS; ( bondingb) fiber bondingmechanism mechanism of BS; (c) of profile BS; (c density) profile curves density in curves width in and width bond and mechanism bond mechanism of BLVL; of (d BLVL;) fiber bonding(d) fiber bondingmechanism mechanism of BLVL. of BLVL.

Table 1. Table 1. ProfileProﬁle density density values values and and mechanical mechanical properties of BS and BLVL. Maximum Maximum Average Maximum Maximum Minimum Average Density on Density on Minimum MOR MOE Types DensityDensity on the Density on the Density MOR MOE Types Density the Left the Right Density (MPa) (GPa) (g/cm3) Left Right (g/cm3) (g/cm3) (g/cm3) (g/cm3) (g/cm3) (MPa) (GPa) (g/cm3) (g/cm3) 1.02 1.04 1.01 0.66 104.57 12.57 BS 1.02 1.04 1.01 0.66 104.57 12.57 BS (12.36) (12.36) (10.16)(10.16) (19.53)(19.53) (9.90)(9.90) (26.68)(26.68) (39.71)(39.71) 0.98 1.00 1.03 0.81 121.31 15.83 BLVL 0.98 (3.89) 1.00 (3.64) (2.73)1.03 (2.71)0.81 (7.90)121.31 (10.79) 15.83 BLVL (3.89) (3.64) (2.73) (2.71) Note: The values in the table are the average of 10 samples, and the coeﬃcient of variation(7.90) in ( ), %. (10.79) Note: The values in the table are the average of 10 samples, and the coefficient of variation in ( ), %. Forests 2019, 10, 921 12 of 14 Forests 2019, 10, x FOR PEER REVIEW 12 of 14

The mechanicalmechanical properties properties of BLVLof BLVL were were significantly significantly better better than those than of those BS. In of order BS. to In objectively order to compareobjectively the compare mechanical the mechanical properties ofproperties BS and BLVL of BS atand the BLVL same at density, the same the density, MOR and the MOE MOR were and dividedMOE were by thedivided density by the to obtain density the to specific obtain the MOR spec andific the MOR specific and MOE.the specific It was MOE. found Itthat wasthe found specific that MORthe specific and MOE MOR of and BLVL MOE were of 19.36%BLVL were and 29.64%19.36% higherand 29.64% than higher those of than BS, those respectively of BS, respectively (Figure 11). The(Figure COVs 11). of The MOR COVs and of MOE MOR for and BLVL MOE were for below BLVL 11%, were which below was 11%, significantly which was lower significantly than those lower for BSthan (COV those> 20%).for BS This(COV would > 20%). indicate This thatwould the indicate process that of using the process bamboo of bundles using tobamboo produce bundles bamboo to bundleproduce veneers bamboo with bundle a relatively veneers uniform with fibera relative arrangemently uniform can fiber improve arrangement the density can and improve mechanical the propertydensity and uniformity mechanical of the property bamboo uniformity bundle fiber of the composites. bamboo bundle The bamboo fiber composites. fibers were The arranged bamboo in thefibers same were direction, arranged eff inectively the same increasing direction, the effectively specific surface increasing area the and specific having surface an improved area and adhesive having interfacean improved (Figure adhesive 10b). However, interface in(Figure the hybrid 10b). pavement,However, thein the bamboo hybrid fibers pavement, were arranged the bamboo at a certain fibers angle,were arranged resulting at in a acertain decrease angle, in theresulting contact in areaa decrease between in the the contact fibers and area a between discontinuous the fibers bonding and a interfacediscontinuous (Figure bonding 10d). The interface thickness (Figure of the 10d). bamboo The bundles thickness was of uneven, the bamboo resulting bundles in a large was deviation uneven, inresulting the overall in a densitylarge deviation of the BS in so the that overall the mechanical density of properties the BS so ofthat the the BLVL mech wereanical higher properties than those of the of BS,BLVL and were the performancehigher than those stability of BS, of BLVLand the was performance also higher thanstability that of of BLVL BS. The was density also higher and production than that costof BS. of bambooThe density bundle and fiber production composites cost could of bamboo be reduced bundle by laminated fiber composites bamboo bundlecould be veneers, reduced and by it couldlaminated create bamboo more economic bundle veneers, value to and them. it could create more economic value to them.

Figure 11. Specific MOR and specific MOE of BS and BLVL. Figure 11. Specific MOR and specific MOE of BS and BLVL. 4. Discussion and Conclusions 4. DiscussionThe innovation and Conclusions of utilizing whole bamboo bundle veneers prepared via the weaving method providesThe aninnovation efficient of and utilizing convenient whole way bamboo for industry bundle to veneers develop prepared applications via the for weaving bamboo method bundle veneers.provides Thean efficient light transmittance and convenient and way mechanical for industry stiffness to develop were used applications as indicators for bamboo to evaluate bundle the uniformityveneers. The of light the physical transmittance and mechanical and mechanical properties stif offness the were bamboo used veneers, as indicators where theto evaluate density andthe mechanicaluniformity stiof fftheness physical of the entire and mechanical bamboo bundle proper veneersties of could the bamboo be observed veneers, in a nondestructivewhere the density manner. and Themechanical evaluation stiffness method of the of theentire veneer bamboo units bundle was an veneers effective could basis be that observed will promote in a nondestructive the industrial productionmanner. The of low-densityevaluation method and high-uniformity of the veneer bamboo units was bundle an effective veneers. basis The brooming, that will glueing,promote andthe heatindustrial treatment production of the bundles of low-density were varied and tohigh-u analyzeniformity their e ffbambooects on thebundle resulting veneers. veneers. The brooming, glueing,The numberand heat of treatment broomings of andthe dippingsbundles were had a varied significant to analyze effect on their the lighteffects transmittance on the resulting and mechanicalveneers. stiffness of the bamboo bundle veneers. The heat treatment had a greater effect on the mechanicalThe number stiffness of broomings and little e ffandect dippings on the light had transmittance. a significant effect Bamboo on the bundle light transmittance veneers with twoand broomingsmechanical hadstiffness a lower of lightthe bamboo transmittance bundle and veneers. better The uniformity. heat treatment had a greater effect on the mechanicalThe use stiffness of bamboo and bundlelittle effect veneers on the to ligh maket transmittance. BLVL can eff ectivelyBamboo improve bundle veneers the physical with andtwo mechanicalbroomings propertieshad a lower of light BLVL, transmittance while these improvements and better uniformity. to the processing of bamboo bundle veneers can helpThe improveuse of bamboo the effi ciencybundle of veneers BLVL production. to make BLVL The uniformitycan effectively of the improve density the and physical mechanical and propertiesmechanical of properties BLVL produced of BLVL, from while the bamboothese impr bundleovements veneers to the were processing superior toof thosebamboo of BSbundle with veneers can help improve the efficiency of BLVL production. The uniformity of the density and mechanical properties of BLVL produced from the bamboo bundle veneers were superior to those of

Forests 2019, 10, 921 13 of 14 hybrid paving. The investigation of the whole process and quality evaluation of the bamboo bundle veneers provided a reference for the preparation and performance prediction of high-performance, low-density, bamboo-bundle-structure engineered materials.

Author Contributions: Conceptualization, F.C. and G.W.; Methodology, F.C.; Software, H.Z. and X.W.; Validation, F.C., G.W. and H.Z.; Formal analysis and investigation, H.Z.; Resources, F.C.; Data curation, H.Z.; Writing—original draft preparation, H.Z.; Writing—review and editing, H.Z. and L.M.S.; Supervision, F.C.; Project administration and funding acquisition, G.W. Funding: This research was funded by the Serial Key Manufacturing Technology of Bamboo Bundle Laminated Composite Structural Materials for Prefabricated Buildings Program, 1632019003. Acknowledgments: The technical guidance from International Centre for Bamboo and Rattan is gratefully acknowledged. The authors would also like to thank Fujian Youzhu Technology Co., Ltd. for supplying the raw materials. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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