In this research, some practical aspects of densified wood glulam were investigated. For this aim, poplar wood specimens were densified by hygro-thermo-mechanical technique. Glulam specimens were produced by finger joint technique and polyurethane glue was used as resin. Physical and mechanical properties of glulams were investigated according to ASTM D 143-09, ASTM D 905-03 and ISO 8375 standards. Based on obtained results, using this process (steam and pressurize compaction) resulted in improvement of physical and mechanical properties of glulam specimens. Optical microscopic and scanning electron microscopic (SEM) pictures were proved structural changes on wood material during of applied technique. Generally, the increase of wood density and hygrothermal treatment has a positive role on the glulam properties especially its dimensional stability. This technique showed a significant effect on water absorption; the steaming temperature and holding time significantly decreased water absorption but densification process increased this parameter. Also, delamination of specimens was decreased by hygrothermal treatment while densification showed negative effect on delamination. Moreover , modulus of rupture, modulus of elasticity, shear strength and compression strength parallel to grain were decreased by this technique but it is depend to treatment temperature and holding time. Higher temperature and holding time cause to higher decrease in the glulam mechanical strength.
Considering environmental sustainability, using renewable materials like wood timber and other wood products for building construction have major priority than other materials. Glue laminated timber (glulam) is one of the most popular and most used type of engineered wood products in recent years [
However, glulam has some disadvantages and limitations of solid wood such as water absorption, swelling, etc. Therefore, the various methods of modification has been used in recent years such as chemical modification, thermal treatment, enzymatic modification, or a combination of them to improve these materials characteristics and their applications capabilities.
The modification generally targets dimensional stability, i.e., reduce moisture induced movement, and increase durability, i.e., resistance to degradation [
Heat treatments are the other eco-friendly modification methods usually which carried out at a temperature of 160–260°C [
Increasing the density of wood enhances most of its mechanical properties. Wood compression is an effective method to increase the density of wood [
Thermo-hygro-mechanical treatment is the other modification method in which used heat, steam, and pressure [
In the current research work, the applying necessity of hygro-thermo-mechanical treatment in glulam production is due to elimination of disadvantages and limitations of solid wood. Therefore, obtaining optimum conditions in this method can bring superior functional properties of the product. In other words, in hygro-thermo-mechanical technique, not only the proper condition of steaming and heating can provide suitable physical properties, but also densification process resolve some mechanical weaknesses which caused by hygrothermal treatment such as lose weight and strength reduction.
Wood blocks were prepared from poplar wood (
Hygrothermal temp. (°C) | 130 | 150 | 170 | Control | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Holding time (min.) | 20 (A) | 40 (B) | 20 (C) | 40 (D) | 20 (E) | 40 (F) | No treat | ||||||||
Compression set (%) | 40 | 60 | 40 | 60 | 40 | 60 | 40 | 60 | 40 | 60 | 40 | 60 | 40 | 60 | No treat |
Code | A1 | A2 | B1 | B2 | C1 | C2 | D1 | D2 | E1 | E2 | F1 | F2 | P1 | P2 | U |
The next step (after treatments and climatisation) was to make finger joint on the treated specimens. The specimens were then cut in desired longitudinal dimension to glulam production. 6 blade Groove cuter were used for preparing the finger joints. Polyurethane adhesive with consumption rate of 200 g/m2 were used to connect prepared blocks. The assembled glulams were placed in cold press with pressure 0.8 N/mm2 for 8 h [
The mechanical properties (bending strength, modulus of elasticity, shear strength of glueline and compression strength parallel to grain) of glulams which made from treated and untreated wood specimens were determined according to ASTM D 143-09, ASTM D 905-03 and ISO 8375. Physical properties such as density (5 × 5 × 5 cm3), water absorption, radial and tangential swelling, dimensional stability (Anti swelling efficiency) as well as delamination were also determined. The results were statistically analyzed based on a complete randomized design under factorial experiment and the mean values were grouped according to Duncan’s multiple range test.
The results showed that wood densification based on the compression sets (C.S.) of 40% and 60% significantly increased the glulam density in controls and treated specimens due to increment of wood material in equal volume (
The results showed that steaming temperature, holding time as well as compression set had significant effect on water absorption after 24, 48 and 72 h soaking in water.
As indicated in
The thickness swelling of glulams in radial and tangential direction was determined after 24 and 48 h of soaking in water. The results revealed that the radial swelling was reduced by increasing of the steaming temperature as well as the holding time after 24 and 48 h of soaking in water (
The interaction effect of compression set on radial swelling, as indicated in
According to the results, the tangential swelling of specimens after 24 and 48 h of soaking in water was reduced by increasing of hygro-thermo-mechanical treatment parameters as steaming temperature, holding time and compression set (
The dimensional stability of specimens was determined after 5 stages of soaking-drying (24 h soaking in water and then oven drying). The results showed significant effect of steaming temperature as well as the holding time on dimensional stability. As indicated in the
The delamination of glulam specimens was determined after two soaking-drying cycles (24 h soaking in water and then oven drying). The results revealed positive significant effect of hygro-thermo-mechanical treatment on delamination, so that, the delamination of treated specimens by this process were less than control specimens (
The result revealed all parameters of hygro-thermo-mechanical treatment had significant effect on bending modulus of rupture (MOR). According to the results shown in
The Modulus of elasticity (MOE) of glulam was increased due to temperature and holding time of hygrothermal treatment (
As indicated in
The results of shear strength of glueline also showed that all the failures occurred in the wood area (
Results revealed, among the hygro-thermo-mechanical treatment parameters, the steaming temperature was the only significant factor on compression strength parallel to the grain. As the results indicates in
Based on the findings of this study the application of hygro-thermo-mechanical process resulted improvement of physical and mechanical properties of glulam. In this process, the compression increases density and hygrothermal treatment has a positive role in this field. All of applied process parameters have significant effect on water absorption; so that the steaming temperature and holding time drastically reduce water absorption but densification process increases this feature. The radial swelling decreases by increasing of hygrothermal parameters (steaming temperature and holding time); while compression process increases it. Moreover, all parameters of hygro-thermo-mechanical process decrease tangential swelling. Dimensional stability of specimens improves due to the hygrothermal treatment of this process.
The delamination of specimens increase by densification process, while hygrothermal treatment parameters decrease the delamination. Also, this modification method has completely positive effect on MOR and MOE. In this technique, compression process increases MOR while hygrothermal treatment caused reduction of MOR; and this is vice versa about MOE. The shear strength of glueline generally decreases by hygro-thermo-mechanical process. This method completely improves the compression strength parallel to the grain. The compression process increases this mechanical property, while hygrothermal treatment caused its reduction.
Authors would like to express their special thanks to Research Institute of Forests and Rangelands, Agricultural Research, Education and Extension Organization (AREEO) and Tarbiat Modares University for their scientific support in this research.