Physical properties and bonding quality of laminated veneer lumber produced with veneers peeled from small-diameter rubberwood logs

The peeling of small-diameter rubberwood logs from the current short-rotation practices undoubtedly will produce lower grade veneers compared to the veneers from conventional planting rotation. Hence, this raises the question of the properties of the produced laminated veneer lumber (LVL) from veneers peeled from small-diameter rubberwood logs using the spindleless lathe technology. Different thicknesses of rubberwood veneers was peeled from rubberwood logs with diameter less than 20 cm using a spindleless lathe. Three-layer LVLs were prepared using phenol formaldehyde (PF) adhesive and hot pressed at different temperatures. During the peeling of veneer, lathe checks as deep as 30–60% of the veneer thickness are formed. Owing to deeper lathe check on 3 mm rubberwood veneer, higher pressing temperature significantly increased the gluebond shear strength of the PF-bonded LVL. In addition, lathe check frequency was also shown to influence the bond strength. The presence of higher lathe check frequency on 2 mm veneer increased the wettability, thus facilitating optimum penetration of adhesive for stronger bonding. These findings stress the importance of measuring and considering the lathe check depth and frequency during the lamination process to get a better understanding of bonding quality in veneer-based products.

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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Kind  Comments to the Author(s) The paper can be interesting for the readers and brings new information on the use of rubber wood in wood technology. The analyzed process parameters are important in future development of the technology. However, some drawbacks occur. These are: 1. The phrase "rubber log" is misleading and not precise. I suggest use "rubber wood log" throughout the text (title, abstract and main text). 2. Two styles of reference citations occur in the text. Both numbers in square parentheses and name and year (p. 3 lines 5, 24; p. 6. line 56, 58; p. 7 lines 32, 35; 44. 3. Table 1 headings: "frequency" -are there any units? and "contact angle": I suggest use "contact angle after 10 seconds" or "contact angle in 10th second".. When these edits are done, I suggest accept.

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Royal Society Open Science, has been reviewed. The comments from reviewer(s) are included at the bottom of this letter.
In view of the criticisms of the reviewer(s), I must decline the manuscript for publication in Royal Society Open Science at this time. However, a new manuscript may be submitted which takes into consideration these comments.
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06-Nov-2019
Dear Miss KHOO, I am pleased to inform you that your manuscript entitled "Physical Properties and Bonding Quality of LVL Produced with Veneers Peeled from Small Diameter Rubberwood Log" is now accepted for publication in Royal Society Open Science.
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Authors' contributions
This paper has multiple authors and our individual contributions were as below KHOO, PUI SAN-data collection, analysis and interpretation of data, drafting the article ii.
CHIN, KIT LING-editing and revising the article critically for important intellectual content iii.
H'NG, PAIK SAN-agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved iv.
BAKAR, EDI SUHAIMI-editing and revising the article critically for important intellectual content v.
LEE, CHUAN LI-analysis and interpretation of data vi.
GO, WEN ZE-final approval of the version to be published vii.

Summary
The peeling of small diameter rubber logs from the current short-rotation practices, undoubtedly will produced lower-grade veneers compared to the veneers from the conventional planting rotation. Hence, this raises the question of the properties of the produced laminated veneer lumber (LVL) from veneers peeled from small diameter rubber logs using spindleless lathe technology. Different thickness of rubberwood veneers were peeled from rubber logs with diameter less than 20 cm using spindleless lathe. Three layer LVLs were prepared using phenol formaldehyde (PF) adhesive and hot pressed at different temperature. During the peeling of veneer, lathe checks as deep as 30 to 60% of the veneer thickness are formed. This study showed that deep lathe check of 3 mm rubberwood veneer significantly reduced the gluebond shear strength of PF bonded LVL. In addition, lathe checks frequency were also shown to influence bond strengths. The presence of higher lathe check frequency on 2 mm veneer increased the wettability, thus, facilitated optimum penetration of adhesive for stronger bonding. These findings stress the importance of measuring and considering the lathe check depth and frequency during lamination process to get a better understanding of bonding quality in veneer-based products.

Introduction
Rubberwood is a wellknown species and highly requested by wood industry particularly for biocomposite production [1]. However, there have been a short supply of rubber logs for veneer production due the inadequate diameter of the logs for veneer peeling. Instead of the conventional planting rotation of 25 to 30 years, nowadays, rubber plantations in Malaysia are managed under short planting cycle and the trees will be likely felled when they are around 15 years old due to the high demand of latex and rubberwood [2] which resulted with small diameter logs are harvested. The invention of spindleless lathe in recent years had encouraged the utilization of fast growing plantation species with small diameter logs in the production of structural composite lumber such as laminated veneer lumber (LVL) [3].
The mechanical properties of LVL generally depend upon a variety of factors particularly veneer quality (moisture content, density, lathe checks, surface roughness, thickness variation), adhesive quality (type of adhesive, mixture of resin, and viscosity) and bonding quality (wettability, glue spread rate, pressure, time and temperature) [4,5]. Numeroues factors contribute to the veneer quality. The most common factors are due to log's characteristics (species, log diameter, log quality, specific gravity, wood pores, grain structure, juvenile and mature wood) [4].  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60 Owing to younger age and shorter planting cycle, juvenile wood is proportionally higher than mature wood [6]. This higher proportion of juvenile wood can have significant effect on the log quality, as well as veneer quality [6,7]. The presence of huge proportion of juvenile wood in a log will caused excessive shrinking and swelling, problems of warping, fuzzy grain, and general instability in the manufacture and use of the wood. These problems may occur in wood after sawing, veneering, drying and machining [8]. Darmawan et al. (2015) revealed that veneer with severe lathe checks will be produced when fast growing species with higher proportion of juvenile woods are peeled. Peeling veneers from small diameter log produced deeper lathe checks, longer lathe checks and larger interval between lathe checks [2,9,10]. Veneer produced from small diameter rubber log using spindleless lathes undoubtedly have very different properties compared to veneers produced from large diameter logs using conventional spindled lathes.
To date, however, there has been no published report on the production of LVL with veneers peeled from small diameter rubber logs using spindleless lathe. Due to the unique properties of veneer peeled from small diameter rubber log, the optimum parameters required for the production of LVL have to be determined. Therefore, the purpose of this research was to evaluate the physical properties and gluebond shear strength of rubberwood LVL manufactured from different veneer thicknesses at different pressing temperature. The LVL properties are important in determining the utilizing potential of LVL produced with veneers peeled from small diameter rubber log using spindleless lathe technology.

Materials and Methods
Small diameter rubber logs (between 15 and 18 cm) were peeled according to the method by Khoo et al. (2018), to produce veneer thickness with 1, 2 and 3 mm and the veneer properties were illustrated in Table 1; Three layer LVL specimens were prepared from 1, 2 and 3 mm thickness rotary peeled rubberwood veneers with moisture content of 8±2% [11]. Phenol formaldehyde (PF) adhesive with 45% solid content were obtained from Aica Chemicals (M) Sdn. Bhd. Poperties of the PF adhesive were as follows: specific gravity of 1.232 at 30°C; pH of 12.90 at 30°C; viscosity of 69 Cps at 30°C and gel time of 21 minutes at 105°C. Commercial filler was used with the PF adhesive. Double glueline spread rate of 200 g/m² was applied on the veneer surface, the veneer sheets were pressed together parallel to each other and the loose side of the veneer was placed towards the center of the boards. The three plies LVLs were hot pressed at 120, 140 and 160°C for 5 minutes with 7 kgf/cm² specific pressing pressure. After hot pressing, the LVLs were conditioned at temperature of 20±3°C and relative humidity of 65±1% until they reached the equilibrium moisture content of 10±2%.

Evaluation 3.1.1 Moisture Content and Specific Gravity
Moisture content of the produced LVL was determined using conventional drying method according to ASTM D 4442-03. The specimens were oven-dried at 103±2°C until constant weight in order to determine the ovendry weight. Moisture content of the specimens was calculated as follow: Specific gravity of test samples with dimension of 50 mm by 50 mm were determined according to ASTM D 2395-02. Oven-dry density and specific gravity of the specimens were calculated using formula as follow:

Water Absorption and Volumetric Swelling
Test specimens with dimension of 50 mm by 50 mm were weighed and the radial (thickness), tangential and longitudinal directions were measured before submerged in 25 mm of distilled water maintained at temperature of 20±1°C. After a 2 hours submersion, the water was being removed and the specimens were suspend to drain for 10±2 minutes in order to remove excess surface water. The specimens were weighed and the radial, tangential and longitudinal of the specimens was measured immediately. After that, the specimens were submerged for an additional 22 hours and followed by the weighing and measuring procedure mentioned above. After submersion, the specimens were put in oven at 103±2°C to calculate the moisture content based on oven-dry weight. Based on ASTM D 1037-12, the percentage of water absorption, radial, tangential, longitudinal and volumetric swelling were determined using formula as follow:

Gluebond Shear Strength
For gluebond shear strength, the cutting pattern for testing specimens as shown in Fig. 1.
Testing specimens were tested according to the ASTM D 906 using INSTRON Universal Testing Machine. Load was applied continuously throughout the test at a uniform rate of motion of the movable crosshead of the testing machine of 4 mm/min. The shear strength of each specimen was calculated from the following formula: Where, = failing force of the specimen, in Newtons; = length of the shear area, in mm; = width of the shear area, in mm.
The glue penetration of the rubberwood veneer was examined using an Olympus SZX12 stereo microscope to evaluate the interaction between the adhesive and rubberwood veneers. Specimens were taken at the cross section of each LVL panel. The LVL specimens of size 10 mm wide x 10 mm long were cut for assessment. Specimen was examined under microscope at 75× magnification.

Results and Discussion
As shown in Table 2, the interaction between veneer thickness and pressing temperature was analyzed using one-way analysis of variance (ANOVA). The interaction of veneer thickness and pressing temperature has highly significant effect on the physical properties and gluebond shear strength of 3-plies rubberwood LVLs. The significant mean values were compared using Tukey's test and the results were tabulated in Table 2.

Moisture Content and Specific Gravity
After conditioning for two weeks, the equilibrium moisture content of LVLs produced from different thickness of rubberwood veneers were 13±2%. Based on the verdicts there was an increase in the density of rubberwood LVLs compared to solid rubberwood. From previous research, air dry density of solid rubberwood generally ranged from 650 to 706 kg/m3, stated by [2]. In this study, it shows the air dry density  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60 of rubberwood LVLs ranged from 736 to 857 kg/m3 was higher than the density of solid rubberwood. The increase in density of 3-ply rubberwood laminated veneer lumber probably due to addition of adhesive. The density of phenol fromaldehyde resin used in this study was 1.2 g/cm3, which is much higher than that of the substrate. Hence, the total mass of the panels was greatly affected by the density of the adhesive [12].
One way analysis of variance ( Table 2) clearly shows that the specific gravity of rubberwood LVLs are highly significantly influenced by the interaction between veneer thickness and pressing temperature. Highest specific gravity can be found in LVL produced with 1 mm veneer thickness at 120°C whereas the lowest specific gravity rubberwood LVLs were produced using 3 mm veneer thickness at 140°C. With increasing pressing temperature,reduction in specific gravity was more drastically in rubberwood LVL produced with 1 mm veneer thickness compared to rubberwood LVL produced with 2 and 3 mm veneer thickness. Reduction in specific gravity is highly related to the degradation and changes of both chemical and physical properties due to the exposure of wood to higher temperature [6]. Between 100 to 200°C, dehydration and decarboxylation processes happen and wood generate water vapour, carbon dioxide and volatile organic compounds (VOC); thus the carbohydrate polymers in wood depolymerize and degrade slowly as the temperature increase [13].
Regardless of pressing temperature, LVL produced with 3 mm veneer thickness has significantly lower specific gravity compared to 1 and 2 mm veneer thickness. This variation is mainly attributed to the presence of more pores and void volume in thick veneer, resulted in lower specific gravity. High specific gravity in LVL produced with thin veneer generally due to the presence of less pores and void volume [14].

Water Absorption and Volumetric Swelling
The interaction between veneer thickness and pressing temperature has highly significant effect on the percentage of water absorption, radial swelling, longitudinal swelling, tangential and volumetric swelling after two and 24 hours water immersion ( Table 2). Within the first two hours immersion, rubberwood LVL produced from 1 mm veneer thickness at 160°C has the highest percentage of water absorption and radial swelling due to the permeability of thin veneer which allow for efficient water transport compared to thicker veneer [15]. As water molecule forming hydrogen bonds with active hydroxyl groups in cell wall polymers, the cell wall expands to accommodate the water molecules [16]. This results in higher percentage of water absorption and radial swelling in thin veneer compared to thick veneer. At the same time, pressing LVL at higher temperature might accelerated the condensation process of glueline and more bubbles will be formed within the gluline. The void created by the bubbles might provide a void volume for water molecule to fill up; hence the percentage of water absorption and radial swelling increased with increasing pressing temperature [13,17].
After 24 hours immersion, rubberwood LVL produced from 3 mm veneer thickness at 140°C has the highest value in water absorption. This might be due to more porosity in 3 mm veneer thickness compared to 1 and 2 mm veneer thickness. This result was supported by the specific gravity result, which mentioned that the rubberwood LVL produced from 3 mm veneer thickness at 140°C has lowest specific gravity. Since the volume of the lumina increase as decreasing specific gravity, the void volume in cell lumina will be replaced by free water until the maximum moisture content is achieved [18]. This resulted in the highest percentage of water absorption after the sample was immersed for 24 hours. On the other hand, lowest percentage of water absorption after 24 hours immersion was significantly obtained by LVL produced from 1 and 2 mm veneer thickness at 120°C; which are 46.46% and 45.98%, respectively [19]. This result was in agreement with specific gravity result, which shown that as increasing specific gravity, the volume of the lumina decrease. This decreases the maximum moisture content because less void is available for free water [18].
LVL produced by 3 mm veneer thickness was found to be the most dimensional stable panels with the least radial swelling, tangential swelling and volumetric swelling after two and 24 hours. Nevertheless, LVL with 3 mm veneer thickness has significant increment in the percentage of water absorption from two hours immersion to 24 hours immersion. This increment due to the LVL had reached the fiber saturation point within first two hours immersion. As hydroxyl groups in cell wall polymers have been saturated with water molecules, the cell wall no longer expands to accommodate the water molecules [16]. Wood is dimensionally unstable within the first two hours immersion due to the moisture content below fiber saturation point (usually 30% moisture content). Once the moisture content is beyond the fiber saturation point, the excess free water will only fills lumens and makes wood heavier, but does not contribute to further expansion [20].

Gluebond Shear Strength
In terms of gluebond shear strength, the effect of veneer thickness and pressing temperature interaction was highly significant. Not only the information regarding to the gluebond shear strength is crucial, but percent wood failure is also important for the evaluation of bonding quality. The LVL failures after gluebond shear test were observed and evaluated visually. The result was shown in Fig. 2.
According to Table 2 Fig. 3(c), adhesive was unable to penetrate fully into the deep lathe checks on the surface of 3 mm veneer thickness in order to give stronger mechanical interlocking interaction and bonding quality [25,26]. Bonding quality was significantly affected by the pressing temperature as the adhesive penetration improved with the increasing of pressing temperature. At higher pressing temperature such as 160°C, the highest gluebond shear strength and percent wood failure were observed in LVL produced by 3 mm veneer thickness [1,17,27]. Darmawan et al. (2015) stated that the gluebond shear strength decrease significantly as the veneer lathe check frequency increases. In previous published research, it was found that 1 and 2 mm rubberwood veneer has more lathe checks formation but generally with shallower lathe checks compared to 3 mm veneer thickness [2]. Highest gluebond shear strength and percent wood failure were observed in LVL produced with 2 mm veneer thickness at 120°C which have a higher lathe check frequency compared to 3 mm veneer. However, the depthness of the lathe checks should be taken into consideration in this research. The presence of higher lathe check frequency on 2 mm veneer loose side increased wettability, thus, facilitated optimum penetration of adhesive for stronger bonding (Fig. 3(b)). For LVL produced with 1 mm veneer thickness, the adhesive penetration was limited on the veneer surface only. In Fig. 3(a), adhesive was distributed uniformly along the glueline between the surfaces of the veneers. The adhesive penetration in 1 mm veneer thickness was not as deep as in 2 mm veneer thickness.

Conclusion
Development of this LVL production will definitely increase the usability of these small diameter rubber logs, which are currently only used as a low grade woody material in particleboard or fiberboard production. The effect of veneer thickness and pressing temperature interaction was highly significant on the physical properties and gluebond shear strength of 3 plies rubberwood LVL. LVLs produced with thin veneer has the highest specific gravity but lowest dimensional stability. In terms of water absorption, LVL produced by 2 mm veneer thickness at 120°C has shown the lowest percentage after two and 24 hours immersion. LVLs produced with 2 mm veneer thickness at 120°C pressing temperature has the highest gluebond shear strength and percent wood failure. 2 mm veneer thickness was selected as the most suitable thickness for the LVL production. Pressing temperature at 120°C was sufficient in order to completely cured the glueline.

Acknowledgments
The authors are grateful for the financial support given by the Ministry of Higher Education Malaysia (MOHE) under the Higher Institution Centre of Excellence (HICoE) project at the Institute of Tropical Forestry and Forest Products.

Ethical Statement
I declare that the work submitted for the publication is original, has not been published elsewhere, accepted for publication elsewhere or under editorial review for publication elsewhere; and that all the authors mutually agree with its content and have approved the paper for release and submission. All the authors have declared no conflict of interest. The manuscript does not contain experiments using animals. In the same time, the manuscript does not contain human studies.

Royal Society Open Science Physical Properties and Bonding Quality of LVL Produced with Veneers Peeled from Small Diameter Rubberwood Log
Thank you for the thoughtful reviews of our manuscript. We take concerns seriously and have addressed them to the best our abilities. Changes have been made as suggested by the reviewers. Some of the more notable changes are listed as below;  (Table 2) has shown the effect of pressing temperature was highly significant on the specific gravity of LVL.  Based on Table 2, the effect of pressing temperature was highly significant on the specific gravity of LVL. With increasing pressing temperature, reduction in specific gravity was more prominent in rubberwood LVL produced with 1 mm veneer thickness.  This sentence had been added in the manuscript.
6) It is definitively not high for WA. Please consider revising the sentence (p. 6 lines 26 -29). (Reviewer #1)  Within the first two hours immersion, rubberwood LVL pressed with higher temperature has significantly higher percentage of radial swelling compared to others.  This sentence has been revised to address the reviewer's concerns.
 In general, the number of treatment replicates (sample size) depends on the type of materials and experiment you are working on. Mostly in experiments conducted under controlled conditions, your COV should be 20 % or less. Some also accept it even if it is around 30 % especially on research studies where issue of spatial heterogeneity is hard to control for. For all the results, we obtained COV less than 10 % for all testing properties.  The reviewer question about the need of having 30 replicates. We would like to explain further the differences of "treatment replicates" and "technical replicates". In the manuscript we had stated "each treatment was carried out in three replicates" (3 treatment replicates). A treatment replication is the repetition of an experimental condition so that the variability associated with the phenomenon can be estimated. ASTM, in standard D906, defines treatment replication as "the repetition of the set of all the treatment combinations to be compared in an experiment". While, technical replicate is "when you test the same sample multiple times -repeated measurements of the same sample (the same board) that represent independent measures of the random noise associated with protocols or equipment". We conducted 3 replications for each treatment (which means 3 boards for each treatment) and 5 -8 technical replicates for each testing properties on each board (total 15 -24 technical replicates for each testing properties). All the data of our experiments (results of treatment and technical replications) has been submitted together with the manuscript in the Dryad repository.