Viscoelastic Thermal Compression Research Articles

Influence of Densification Treatment on The Morphology and Density Profile of Paraserianthes falcataria Laminas

Fast-growing wood species have a limited range of applications due to their low density. Despite that, densification technology can be applied to improve its density. In this study, laminas from Paraserianthes falcataria, a low-density wood species, were subjected to viscoelastic- thermal compression (VTC), a densification treatment. The objective of this study was to evaluate the impact of different pre-steaming durations in the densification treatment on the cell lumen deformation and density profile of Paraserianthes falcataria laminas. The laminas were pre-steamed with different durations ranging from 0–30 min, compressed with heat, and cooled down. For cell lumen area measurement, laminas pre-steamed for 30 min had the lowest cell lumen area, where the decrease in the lumens contributed to an enhancement in density. The finding from the density profile measurement indicated that densification treatment had enhanced the density of the laminas by up to 50%, whereby laminas pre-steamed for 10 min achieved the highest density.

Correlation between hardness and density profile of glue-laminated timber manufactured from viscoelastic-thermal compression modified Paraserianthes falcataria laminas

A fast-growing tree species, Paraserianthes falcataria, possessed low value in density. Therefore, it was subjected to a densification treatment, i.e., viscoelastic-thermal compression (VTC), to enhance its density and mechanical performance. The objective of this study was to investigate the correlation between the density profile and hardness of VTC-modified glue-laminated timber. The laminas underwent VTC, whereby five different pre-steaming durations were applied, ranging from 0-30 minutes prior to forming into glue-laminated timber panels. The panels were cut into pre-determined test pieces and subjected to density determination using an X-ray densitometer and hardness test in the tangential direction using the ball indentation method. The correlation analysis found that increasing density slightly enhanced the hardness of the tangential surface of the glue-laminated timber test pieces, whereby the Pearson’s correlation values, r were .074 (0 minutes, non-densified), .154 (0 minutes, densified), -.027 (10 minutes, densified), .088 (20 minutes, densified), and .225 (30 minutes, densified). Nevertheless, the associations were found to be statistically insignificant because the p-values were greater than 0.05, thus suggesting densification treatment did not enhance hardness.

Densification of Bamboo: State of the Art.

Densification processes are used to improve the mechanical and physical properties of lignocellulose materials by either collapsing the cell cavities or by filling up the pores, consequently reducing the void volume fraction. This paper focuses on an extensive review of bamboo densification process, which is achieved by compressing the material in the direction perpendicular to the fibers using mainly two different techniques: an open system, thermo-mechanical (TM), or a closed system, viscoelastic-thermal-compression (VTC). The main aim of bamboo densification is to decrease its heterogeneity, as well as to improve its mechanical and physical performance. In addition, densification may occur during the manufacturing of bamboo products in which hot-pressing processes are used to mold bamboo panels. There are over 1600 publications about bamboo, concentrated in the recent decade, mainly about engineered materials. Although several papers regarding bamboo and wood densification are available, very few studies have comprehensively investigated the densification process solely through compression of natural bamboo culms. According to the literature, applying a combination of compression of 6–12 MPa at temperatures between 120–170 °C for 8–20 min can produce materials with higher strength in comparison to the mechanical properties of natural bamboo. The majority of research on bamboo densification indicates that the modified material results in improved properties in terms of density, hardness, bending strength, stiffness, and durability. This paper provides a review that consolidates knowledge on the concept of bamboo culm densification, discusses the roles of parameters that control the process, ascertains the best practice, and finally determines gaps in this field of knowledge.

Contact Angles of Viscoelastic-Thermal Compression (VTC) Modified Paraserianthes falcataria (L.) Laminas

Wood is recognized as hygroscopic material, which tends to absorb moisture from surrounding, thus affecting both physical and mechanical properties of the material itself. The aim of this study was to evaluate the effect of viscoelastic-thermal compression (VTC) on the contact angles of modified laminas of Paraserianthes falcataria (L.), in correlation with density and wettability of the wood. This low-density wood species was subjected to densification treatment in order to improve its density as well as mechanical properties. VTC is a densification treatment which involved pre-steaming for softening purpose and compression via hot pressing. There were four different pre-steaming durations alongside one control (NS/D: no pre-steaming (control); S1/D: 10 minutes; S2/D: 20 minutes; S3/D: 30 minutes). The laminas underwent contact angle test (sessile drop method) by referring to ASTM D7334-08: Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement. In addition, basic morphological feature of the laminas was determined by using Scanning Electron Microscope (SEM). The contact angle of S1/D (10 minutes) laminas indicated the lowest degree of contact angle, which means it had better wettability; while S3/D (30 minutes) laminas recorded the highest degree of contact angle, therefore having poor wettability.

Density of Densified Paraserianthes falcataria Wood Pre-treated with Alkali

Wood densification is done to improve its mechanical and physical properties, by compressing the porous structure under steam, heat and pressure called Viscoelastic-Thermal Compression (VTC). This study had been done to evaluate the effect of alkaline pre-treatment using different concentrations of Sodium hydroxide (NaOH) on Paraserianthes falcataria (local name: Batai) and its density. Alkaline pre-treatment was done before the densification process, using three different concentrations of NaOH (3%, 6% and 9%) with 0% NaOH as the control. There were 30 replicates used for the study. The lamina was prepared by trimming the lamina sawn timber into specific size. Then, laminas cooking process in different NaOH concentrations for 30 minutes, followed by hot water extraction with acetic acid for 20 minutes and 24 hours of oven dried under temperature 103 ± 5°C. The laminas were then pressed using hot press machine under 105°C temperature and 6 MPa pressure for 30 minutes before cooling phase were conducted for 10 minutes. The density was evaluated continuously for 7 days and the results showed that densified P. falcataria without alkaline pre-treatment (0% NaOH) has the highest density compared to treated densified P. falcataria with different alkaline pre-treatments (3%, 6% and 9%); meanwhile the highest concentration of NaOH (9%) has the lowest density value compared to 3% and 6%, as 3% has the higher value than 6% NaOH.

Effect of viscoelastic thermal compression (VTC) treatment on density and moisture content of laminas from Paraserianthes falcataria

ABSTRACT Paraserianthes falcataria, a low-density wood species underwent viscoelastic thermal compression (VTC) treatment, which was the combination of pre-steaming and compression by hot-pressing, in order to increase its density and mechanical performance. The aim of this study was to evaluate the impact of VTC treatment on the density and moisture content of Paraserianthes falcataria laminas. Several process conditions which consisted of pre-steaming duration (0–30 minutes) and hot-pressing pressure (0–8 MPa) were applied on Paraserianthes falcataria laminas. Density and moisture content of Paraserianthes falcataria laminas were measured for 7 days. This study found out that process condition of 30 minutes pre-steamed combined with 8 MPa pressing pressure obtained the highest percentage of densification degree (83.71%) based on time period of before the treatment until the next 7 days after VTC process; and the lowest percentage of density improvement was recorded on non-pre-steamed combined with 4 MPa pressing pressure (54.23%). In terms of moisture content, process condition of non-pre-steamed combined with non-hot-pressed recorded the highest equilibrium moisture content (9.5%); while non-pre-steamed combined with 4 MPa pressing pressure had the lowest equilibrium moisture content (6.3%).

Method of Stabilizing Heavily Spalted Big Leaf Maple as a Decorative Coating Veneer Layer for Engineered Wood Flooring

Spalted wood, wood colored by fungi, has been popular in woodcraft for centuries. Most spalted wood, however, is found in an advanced state of decay and cannot be utilized. This project describes the use of viscoelastic thermal compression (VTC) to investigate the potential increase in spalted woods’ strength and stiffness, with the main objective of converting so-called “punky” wood into the top layer (veneer layer or “coating”) on commercial flooring. Spalted Acer macrophyllum logs were cut into veneers of size 7 mm × 7.8 cm × 25 cm and were then VTC-treated at 150 °C and 50 psi for 11.5 min. Statistical analysis on the mixed linear models showed significant increases for both the density and hardness of spalted wood (p < 0.0001). Density and Brinell hardness increased by 84% and 209%, respectively. FTIR analysis revealed that the wood polymers present in spalted wood were more susceptible to degradation imposed by the heat of the VTC treatment compared to sound wood. Additionally, the color analysis of the wood specimens showed statistically significant changes in color after the VTC treatment (p < 0.0001), which turned the wood surface darker and redder. The use of the VTC technology to transform spalted wood into wood flooring is viable. However, when exposed to moisture, the VTC-treated spalted wood showed a high percentage of set recovery (78%), which was significantly different from the set recovery of the sound wood (71%, p = 0.004). Successful use of VTC-treated spalted wood for flooring will require addressing of the swelling issue, and additional studies are needed to fully characterize the anatomy of VTC-treated spalted wood.

Potential for Using Essential Oils to Protect Viscoelastic Thermal Compression–Treated Hybrid Poplar

Abstract The potential for using pretreatment with cinnamon leaf oil or juniper foliage oil to improve the durability of viscoelastic thermal compression (VTC)–treated hybrid poplar was explored in...

Resistance of resin-impregnated VTC processed hybrid-poplar to fungal attack

Viscoelastic thermal compression (VTC) is an excellent method for improving the physical properties of low density woods, but it has little effect on resistance to microbial degradation. The potential for improving durability through pre-impregnation with either phenol formaldehyde resin (PF) or tung oil was investigated in laboratory tests. Tung oil had little effect on decay resistance; while PF resins markedly improved durability against both white and brown rot fungi. The results suggest that resin pre-treatment was compatible with the VTC process and produced markedly more durable materials.

The influence of thermal-hydro-mechanical processing on chemical characterization of Tsuga heterophylla

Viscoelastic thermal compression (VTC) is a type of thermal-hydro-mechanical (THM) processing that requires only a short processing time. THM processing causes some chemical transformations, the nature and extent of hydro-thermolysis depends on the special treatment conditions and the chemical nature of wood species. In the present study, the chemical transformations of the cell wall components and wood extractives during VTC treatment were investigated, and correlation between chemical characterizations and observed property changes was analyzed. For this purpose, the content of extractives and pH values were determined, and FTIR analysis was performed on extractable substances, extract-free wood, holocellulose, α-cellulose and lignin. Two temperatures and two steam exposure times were adopted to determine the influence of processing conditions on chemical characterization of Tsuga heterophylla. The results revealed that THM treatment caused a series of chemical reactions in extractives. Treatment temperature and conditioning time have significant influence on chemical changes of extractives. For all of the VTC treatments used in this study, no significant changes occurred in the lignin and α-cellulose components. The only significant chemical changes occurred in the hemicelluloses, which were primarily reduction of carbonyl and acetyl functional groups. This study also confirmed that the chemical transformation of wood correlates with property changes of VTC wood.

Bending performance of 3-layer beech (Fagus sylvativa L.) and Norway spruce (Picea abies (L.) Karst.) VTC composites bonded with phenol–formaldehyde adhesive and liquefied wood

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.

Erratum to: The wettability and bonding performance of densified VTC beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) bonded with phenol–formaldehyde adhesive and liquefied wood

The influence of viscoelastic thermal compression (VTC) on surface wettability and bonding performance of wood was evaluated. Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with the VTC process to different degrees of densification. Control and densified strips were bonded with phenol–formaldehyde (PF) adhesive and liquefied wood (LW). Shear strength of bonded assemblies was determined after 1 week of conditioning at 20 °C and relative humidity of 65 %. Wettability was determined on the basis of the contact angle of water, PF adhesive, and LW using the Wilhelmy method. Results showed that densification of beech and spruce wood did not significantly affect the shear strength of specimens bonded with PF adhesive. In beech assemblies bonded with LW shear strength decreased significantly with increased density, whereas in bonded spruce specimens decrease of shear strength was not significant. It was found that degree of densification and bonding process used in the study were not appropriately chosen for spruce wood specimens, since major deformations after the bonding process occurred. Wettability changed significantly after densification. Contact angle of water and LW increased after densification, whereas contact angle of PF showed inverse trend and decreased after VTC process. Furthermore, the degree of densification had a minor effect on the wettability.

Integrated drying and thermo-hydro-mechanical modification of western hemlock veneer

High moisture content western hemlock veneer was modified using viscoelastic thermal compression (VTC) technology with an integrated drying stage. VTC is a type of thermo-hydro-mechanical treatment. This project determined the influence of five processing parameters on properties of VTC lamina. VTC processing significantly increased the mechanical properties of veneer, which mainly depended on the degree of compression. However, moisture-induced thickness swell increased substantially depending on the degree of compression and venting time. Increasing process temperature increased mass loss. Integrating the drying step improved process efficiency and eliminated drying defects. Process optimization was different depending on the desire to increase mechanical properties or increase dimensional stability. Statistical models were developed to predict properties of VTC treated products and drying time. This project will reduce the cost of VTC technology and provide guidance to design commercial manufacturing processes.

Microstructure of viscoelastic thermal compressed (VTC) wood using computed microtomography

The paper describes for the first time the analysis of the structure of compressed wood using computed tomography. The anatomical structures of Douglas-fir and hybrid poplar before and after densification with the viscoelastic thermal compression (VTC) process were described by pore size distributions and mean pore sizes and compared. The compression of Douglas-fir mainly affected earlywood, while the compression of hybrid poplar mainly occurred in the vessels. In both wood species, the densification resulted in a significant decrease in the pore volumes. The porosity decreased to less than half of the original value for Douglas-fir earlywood and to approximately one-quarter for the vessels in hybrid poplar. The relevant mean pore sizes also decreased dramatically to about one-quarter compared to the original values. In contrast, latewood in Douglas-fir and libriform fibers in hybrid poplar are quite stable under compression. Douglas-fir latewood retained its original structure after compression and did not show any reduction in pore size. The results confirmed that the anatomical structure of VTC densified wood can be described by pore size distributions and mean pore sizes. However, in the case of broad or bimodal distributions, the mean pore sizes are of less significance.

Apparatus for viscoelastic thermal compression of wood

Viscoelastic Thermal Compression (VTC) of wood is a treatment which greatly improves material properties by means of densification with mechanical compression perpendicular to the grain. VTC is similar to other wood densification processes, but is designed for thin lamina and employs dynamic temperature and steam pressure conditions. A device was constructed that is capable of producing VTC wood. Details of the construction and operation of the VTC device are presented. The device consists of a pressurized chamber with variable volume. Wood density may be modified from the initial density of the virgin wood up to approximately 1400 kg/m3. Control of steam pressure and temperature insures that the wood cell walls are not fractured during the process. The device is designed for batch operation and may be adapted to a conventional laboratory hot-press. An example operating schedule is described, but other procedures are possible within the design parameters of the device.

Fungal decay of viscoelastic thermal compressed (VTC) wood

Low-density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified with the viscoelastic thermal compression (VTC) process to three different degrees of densification (63%, 98%, and 132%). Durability of the VTC treated specimens was assessed by exposure to two white rot fungi (Pleurotus ostreatus and Trametes versicolor) for four or eight weeks. After incubation, mass loss in grams and percentage mass loss were determined. The results showed that VTC densification did not change decay resistance to Pleurotus ostreatus and Trametes versicolor. The mass losses caused by Trametes versicolor were more extensive than those by Pleurotus ostreatus. As expected, longer exposure time resulted in higher mass loss. Furthermore, the degree of densification did not influence susceptibility to fungal degradation.

Phenol–formaldehyde impregnation of densified wood for improved dimensional stability

To produce a highly stable wood-based product with increased mechanical properties, phenol formaldehyde (PF) resin impregnation was combined with the viscoelastic thermal compression (VTC) process. Dimensional stability and bending stiffness were evaluated. Two PF resins with weight average molecular weights of 172 and 780 were studied at three different concentrations, 5, 10 and 20%. After 24-h room temperature water soak and 2-h boil, both PF treatments at all concentration levels showed high levels of dimensional stability compared to non-impregnated VTC processed controls. The higher molecular weight PF provided greater stability with an average thickness swell value of 12% compared to 20 and 37% for the lower molecular weight PF resin treatment and control, respectively. High anti-swelling efficiency values were recorded for both low and high molecular weight resins, implying these modifications were effective at reducing the volumetric swelling which occurred in the unmodified control. PF treatments were also extremely effective at reducing irreversible swelling. The low and high molecular weight resin treatments had 1/5th and 1/7th the irreversible swelling than the unmodified VTC processed controls, respectively. All dimensional stability values improved as resin concentrations increased. Both resin types at all concentration levels reduced Young’s modulus.

The mechanical properties of densified VTC wood relevant for structural composites

The mechanical properties of densified wood relevant for structural composites were studied. Low density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified using the viscoelastic thermal compression (VTC) process to three different degrees of densification (63, 98, and 132%). The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the VTC wood were determined. The bonding performance of the control and VTC wood, using two phenol-formaldehyde (PF) adhesives, was studied. Four different 3-layer composites were also prepared from undensified and VTC wood, and tested in four-point bending. The results showed that the bending properties of the VTC wood (MOR and MOE) were significantly improved due to the increased density. The bonding performance of VTC wood with PF adhesives was comparable with or better than in the case of the control wood. Increased density of the face layers in the 3-layer VTC composites was advantageous for their mechanical performance.

The influence of viscoelastic thermal compression on the chemistry and surface energetics of wood

The influence of the viscoelastic thermal compression (VTC) of wood on its surface chemistry, wettability, and surface free energy was evaluated. The changes in surface chemistry were studied by means of Fourier transform infrared spectroscopy (FTIR). Wettability was determined on the basis of the contact angle of water, using the Wilhelmy plate method. Surface free energy was determined according to the Owens Wendt Rabel and Kaelble (OWRK) theory, using the contact angle data of diiodomethane and formamide. The studied material was wood of the low-density hybrid poplar ( Populus deltoides × Populus trichocarpa), which was densified to three different degrees of densification (63%, 98%, and 132%) using the VTC process. The results, based on the FTIR spectra, revealed that the VTC process caused minor chemical changes on the wood surface. However, wettability changed significantly. The VTC wood surface exhibits rather hydrophobic behaviour. Evaluation of the surface energetics using the OWRK approach showed that total surface free energy dropped significantly due to the VTC process.

Density profile and morphology of viscoelastic thermal compressed wood

The viscoelastic thermal compression (VTC) of low-density hybrid poplar (Populus deltoides × Populus trichocarpa) from fast growing trees was performed in order to produce specimens with three different degrees of densification (63, 98, and 132%). The morphology and density profile of the VTC specimens were studied. Three different methods for the preparation of specimens for microscopy were used in order to find a technique that makes it possible to examine the VTC wood microscopically in the completely deformed state. It was found that the abrasive surface preparation of oil-embedded blocks was the most promising technique. Microscopic observation revealed that the deformations in the VTC wood were mostly the result of the viscous buckling of cell walls without fracture. The volume of the void areas in the specimens decreased with the degree of densification. The results showed that the density profile of the VTC wood varied with the degree of densification as a consequence of different temperature and moisture gradients formed before and during wood compression. The density profile is also visible on the cross-section of the VTC specimens.