Elementary units “bamboo bundle” and “bamboo sliver” were processed and cross-linked as “bamboo-bundle veneer (BBV)” and “bamboo-sliver veneer (BSV)” for preparation of laminated composites. The concept of “high-content-adhesive surface treatment” was raised to improve boards’ performance, rather than increasing adhesive absorption of every layer’s porous unit. That is, some BBVs experienced an extra “dipping & drying” to absorb more resin (named “HBBV”). The effect of the amount of knitting threads was also discussed for influencing BBV’s quality. Results indicated that light transmittance of BBVs decreased as the amount of threads added from 3 to 8, while mechanical stiffness increased. Adding two skin layers of HBBVs symmetrically was helpful to enhance 24-hour underwater and 28-hour “boil-dry-boil” dimensional stability for boards with BSVs as core, while more than two pairs of HBBVs were needed to improve 28-hour “boil-dry-boil” dimensional stability of boards with BBVs as core. Two symmetrical surface layers of BBVs/HBBVs provided BSV-boards/BBV-boards with greater bending resistance, while such “surface treatment” would not raise shearing strength of BSV-boards upon 28-hour “boil-dry-boil” treatment. Besides, the data obtained from drop-hammer impact test indicated that more than two pair of surface BBVs or HBBVs were required for significant improvement in anti-impact property.
As one of the most important alternative raw materials to wood, fast-growing bamboo has consistent orientation of vascular and parenchyma tissues, while no piths and transverse rays, which contributes to high axial tensile strength, superior toughness, relatively high specific stiffness, as well as convenient processing [
Among these, bamboo bundle sheet has received increasing attention as elementary unit for the last decade, due to its advantage of improving utilization rate of bamboo resource and maintaining longitudinal mechanical property of bamboo fibers. Inspired by the structure of laminated veneer lumber (LVL) [
Many researches focused on the effect of raw material BBV’s general characteristics (bamboo species, vascular bundle tissue ratio, effect of removing extent of bamboo green, etc.), technological parameter (adhesive quantity, structure design, hot-pressing temperature, pressure, time, board density, etc.), and external environment (coupled hydro-thermo, high-heat, mold, termite, combustion, etc.) on the performance of LBL [
Due to the mechanical preparation process, BBV and other kinds of elementary unit possessed much absorption sites towards adhesive, fire retardant, preventive and other functional additives, which led to better permeability or adhesiveness of these additives and further affected bamboo-based composites’ performance positively [
In this study, the effect of the amount of knitting threads on light transmittance and mechanical stiffness of BBV, was studied using previously proposed quality evaluation system [
A Three- or four-year-old bamboo (
Bamboo tubes (breast height diameter ≥ 100 mm) were first split into several pieces with bamboo nodes being removed, then processed into a size of 2100 mm (length) × 20 mm (width) × 2.8 mm (thickness) by a cutting machine along the radial direction. These slivers were cross-linked into BSVs by above-mentioned cotton threads (
Notes: S, B, H mean one layer of BSV, BBV and HBBV respectively. The value in parenthesis represents serial number of different laminated design in sequence.
Phenol formaldehyde (PF) resin was obtained from Beijing Dynea Chemical Industry Co., Ltd., Beijing, China. PF resin (pH 11~12) was diluted with water to a solid content of 30% as the adhesive, in which BBVs and BSVs were immersed for 4 min and then belt-dried to a MC of 8%~12%. To control gradient difference in AL, some BBVs were subject to an extra “glue dipping-belt drying” procedure to absorb higher content of resin, named “HBBVs”. The adhesive loading of BBV was calculated in
where
The “surface treatment” was applied herein that one or two layers of HBBVs were placed on the surface and back symmetrically during parallel-to-grain assembly. In this way, several groups of LBLs were manufactured (
The uniformity of the brooming morphology of BBV was evaluated with optical transmittance detection, while the mechanical properties was characterized by means of mechanical crimping method (
Herein,
Thickness swelling and water absorption rate after 24-hour underwater immersion, were obtained referring to Chinese national standard GB/T 17657-2013 [
Three-point bending and shearing tests were conducted in accordance with Chinese national standard GB/T 17657-2013 [
Abrasion tests were conducted in accordance with Chinese National Standard GB/T 17657-2013 [
Different assembly patterns of LBLs, with a size of 100 mm × 100 mm ×
Microstructures of bamboo units were captured by scanning electron microscope (SEM; Quanta 2000, FEI Company, Hillsboro, OR, USA) at an accelerating voltage of 5.0 kV, and the distribution of resin adhesive inside BSV, BBV and HBBV was analyzed for different adhesive loading.
Data were analyzed using a one-way analysis of variance (ANOVA) with the Duncan test using SPSS 18 software (SPSS, Inc., Chicago, IL, USA). Probability values of less than 5% were considered to be significant (
The testing results of
As
Performanceindex | Amount of knitting threads | Model item | Sum ofsquares | Degree ofFreedom | Squareerror | Significant | |||
---|---|---|---|---|---|---|---|---|---|
3 | 5 | 9 | |||||||
7.60a |
6.31ab |
3.46b |
Inter-group | 71.85 | 2 | 35.92 | 5.82 | 0.009 |
|
Intra-group | 148.11 | 24 | 6.17 | ||||||
Sum | 219.96 | 26 | |||||||
891.51a |
908.83b |
927.94b |
Inter-group | 2.48E4 | 2 | 1.24E4 | 9.72 | 0.001 |
|
Intra-group | 2.68E4 | 21 | 1.27E4 | ||||||
Sum | 5.15E4 | 23 |
Notes: Parentheses in the table is the coefficient of variation (CV); Different lower-case letters indicate a significant difference (
Patterns | 24-hour underwater immersion | 28-hour “boil-dry-boil” cyclic treatment | ||||||
---|---|---|---|---|---|---|---|---|
Thickness swelling rate (%) | Water absorption rate (%) | 4h’ thickness swelling rate (%) | 4h’ water absorption rate (%) | 24h’ thickness swelling rate (%) | 24h’ water absorption rate (%) | 28h’ thickness swelling rate (%) | 28h’ water absorption rate (%) | |
1 | 4.88 ± 0.29 A | 23.75 ± 2.53 A | 25.54 ± 1.79 A | 33.19 ± 3.53 A | 21.50 ± 1.88 A | 7.75 ± 1.41 A | 31.89 ± 3.01 A | 40.38 ± 5.38 A |
2 | 3.80 ± 0.20 B | 10.79 ± 0.82 B | 24.68 ± 2.20 A | 29.98 ± 3.26 A | 19.34 ± 1.28 A | 7.12 ± 0.69 A | 28.87 ± 2.71 A | 33.59 ± 3.80 AB |
3 | 2.00 ± 0.09 C | 6.25 ± 0.52 C | 18.81 ± 1.02 B | 20.42 ± 2.35 B | 15.25 ± 0.64 B | 4.85 ± 0.18 B | 21.96 ± 1.32 B | 22.41 ± 2.38 B |
4 | 2.10 ± 0.12 C | 6.85 ± 0.48 C | 21.47 ± 1.48 AB | 29.90 ± 3.86 A | 19.30 ± 0.87 A | 6.44 ± 0.42 A | 28.14 ± 2.64 A | 32.91 ± 3.67 AB |
5 | 1.97 ± 0.12 C | 5.61 ± 0.43 CD | 17.44 ± 0.86 B | 18.19 ± 2.03 B | 14.71 ± 0.53 B | 4.35 ± 0.25 B | 20.95 ± 1.27 B | 20.23 ± 2.66 B |
6 | 1.44 ± 0.03 D | 5.13 ± 0.41 D | 12.08 ± 0.64 C | 10.74 ± 0.82 C | 10.50 ± 0.25 C | 3.61 ± 0.26 BC | 16.62 ± 1.03 C | 13.25 ± 1.07 C |
7 | 1.43 ± 0.07 D | 4.98 ± 0.38 D | 11.92 ± 0.46 C | 9.96 ± 0.86 C | 9.44 ± 0.25 C | 3.58 ± 0.20 BC | 16.63 ± 0.90 C | 11.54 ± 1.08 CD |
8 | 1.21 ± 0.08 E | 3.98 ± 0.30 E | 11.58 ± 0.36 C | 9.32 ± 1.01 D | 9.11 ± 0.44 C | 3.34 ± 0.30 C | 15.60 ± 0.83 C | 10.60 ± 1.05 CD |
9 | 0.64 ± 0.05 F | 2.96 ± 0.27 F | 8.31 ± 0.26 D | 6.90 ± 0.56 D | 5.86 ± 0.13 D | 1.68 ± 0.13 D | 11.60 ± 0.71 D | 9.33 ± 0.81 D |
Notes: Different capital letters indicate a significant difference (
Thickness swelling and water absorption rate after 24-hour underwater immersion are displayed in
From an overall perspective, the boards with BBVs as base boards (Patterns 6~9) had smaller dimensional change or less water absorption than those with BSVs as base (Patterns 1~5). There were bamboo green and bamboo yellow in BSVs, rather than BBVs. The presence of different ingredients in bamboo units resulted in different degree of drying shrinkage and hydroscopic swelling upon temperature-humidity change [
Three-point bending properties of LBLs are displayed in
Through the comparison between HHBBBHH (Pattern 8) and HHHHHHH (Pattern 9), it can be concluded that excessive addition of HBBVs led to certain improvement in MOR, but non-significant raise in MOE. The differential part (BBB or HHH) appeared near or in neutral layer, which did not reflect significant difference in MOE, as the job of resisting bend-deformation was undertaken primarily by the same surface layers of bamboo units (HH). As the loading continued, crack initiation and propagation occurred that the inner material did experience load bearing. Thermo-cured HBBVs possessed higher value of strength than BBVs due to gradient adhesive loading, that HHHHHHH (Pattern 9) performed higher MOR than HHBBBHH (Pattern 8).
Patterns | Three-point bending properties | Vertical shearing strength (MPa) | |||
---|---|---|---|---|---|
MOE (GPa) | MOR (MPa) | Before cyclic treatment | After cyclic treatment | Decline rate (%) | |
1 | 10.10 ± 0.76 A | 121.78 ± 8.16 A | 18.17 ± 1.15 A | 5.91 ± 0.42 A | 67.47 |
2 | 11.21 ± 0.70 AB | 125.28 ± 5.90 A | 18.33 ± 1.41 A | 7.77 ± 0.74 B | 57.61 |
3 | 11.31 ± 0.25 B | 129.90 ± 7.84 A | 19.09 ± 0.70 AB | 9.28 ± 0.60 BC | 51.39 |
4 | 11.22 ± 0.82 AB | 129.60 ± 6.79 A | 18.38 ± 0.95 A | 8.31 ± 0.54 B | 54.79 |
5 | 11.88 ± 0.85 B | 134.01 ± 5.19 AB | 20.06 ± 0.87 AB | 10.11 ± 0.88 BC | 49.60 |
6 | 12.03 ± 0.61 B | 135.06 ± 4.51 AB | 21.62 ± 0.35 B | 12.43 ± 0.86 C | 42.51 |
7 | 12.59 ± 0.96 B | 141.43 ± 10.38 BC | 21.97 ± 0.36 B | 13.40 ± 0.68 CD | 39.01 |
8 | 14.85 ± 0.67 C | 151.85 ± 10.69 C | 24.99 ± 0.94 C | 13.92 ± 0.60 D | 44.30 |
9 | 15.72 ± 1.17 C | 174.98 ± 9.34 D | 25.70 ± 1.58 C | 14.96 ± 0.69 D | 41.79 |
Notes: Different capital letters indicate a significant difference (
Sample weight was measured after 100 and 200 r (revolutions), and mass loss within 1st and 2nd 100 r was calculated, as
Boards within 3rd Group were subjected to the most severe mass loss under the same wear environment, which could be caused by gradient adhesive loading. Initial morphology of bamboo unit BBV and HBBV were the same, however the latter possessed larger amount of cured PF resin, whose brittleness went against excellent abrasion. It can be concluded herein that symmetrical outer layer of BBV or HBBV did not help much in surface abrasion resistance of LBLs.
Notes: Different capital and lower-case letters indicate a significant difference (
Several performance parameters of LBLs during drop-hammer impact test, had been illustrated in
Notes: (a) Total energy is the sum of the absorption energy of impact failure and impact machine, energy at peak load is the absorption energy of specimen at the peak load; (b) Different capital lower-case letters indicate a significant difference (
Boards in this paper were manufactured with all the fibers parallel in the length direction, which had a lower strength in the transverse direction due to the lack of bamboo fibers (bamboo slivers or bamboo bundles) as reinforcement. Therefore, no enhanced fibers could be ruptured firstly in the width direction under the effect of longitudinal stress wave, and then the board had to bear uniaxial load only in length direction, which brought about stress distribution imbalance. Once micro-cracks developed, many micro-fissures extended and merged in laminated layers to form cracks along the fiber direction, leading to the fracture failure of the board (
Notes: The Arabic numbers represent different patterns of assembly for LBLs.
As
Notes: The lowercase letters represent different patterns of assembly for LBLs. (a) means “BSVs + 0 BBVs/HBBVs” (pattern 1); (b) means “BSVs + 1/2 BBVs/HBBVs” (Patterns 2~5); (c) means “BBVs + 0 HBBVs” (Pattern 6); (d) means “BBVs + 1/2 HBBVs” and “pure HBBVs” (Patterns 7~9).
Distribution of adhesive inside BSV, BBV, HBBV was captured by SEM, and gradient adhesive loading of different porous laminates (8.51% ± 0.19%, 12.23% ± 0.27% and 24.02% ± 0.60%) can be clearly reflected through
As the amount of knitting threads added, light transmittance ( Adding two outer layers of high-content-resin bamboo bundle veneers (HBBVs) symmetrically was helpful to enhance 24-hour underwater dimensional stability and 28-hour “boil-dry-boil” dimensional stability for boards with bamboo-sliver veneer (BSV) as core material. More than two layers of surface HBBVs were needed to improve 28-hour “boil-dry-boil” dimensional stability for those boards with BBVs as core material. For 1st and 2nd 100 revolutions’ wear test, the abrasive mass loss of 1st Group (LBLs with BSVs as surface) was less than that of 2nd Group (LBLs with BBVs as surface), followed 3rd Group (LBLs with HBBVs as skin layer). Two symmetrical surface layers of BBVs/HBBVs provided BSV-boards/BBV boards with greater non-deformability (MOE) for the surface enhancement. Shearing strength (SS) of BBV-boards undergoing 28-hour “boil-dry-boil” treatment would be raised upon the addition of two laminates of HBBVs, whereas that of BSV-boards would not be increased with the corresponding BBVs due to the poorer internal bonding themselves. Drop-hammer impact was kind of test on composites’ ability to absorb energy that only one or two pair of surface BBVs or HBBVs would be not enough to help enhance anti-impact property significantly.