The effect of the densification process on dowel tensile performance with different types of glues

In particular, the fast-growing softwood species create highly hollow textures, and therefore, their density is low (Parham et al. 1984). High demand for quality material in the wood industry makes it important to expand the usage area of forest assets by improving their properties (densification modification) due to rising prices and increasing environmental awareness (Pelit 2014).

In scientific studies related to furniture, data on mechanical, for example, joint performance, is used. The mechanical properties of furniture products depend on the materials from which the elements are produced, and the joining techniques applied to assemble these elements. For jointing furniture elements, several types of fitting systems are used. One of these is using wood dowels. Dowel joints have been used with glue for many years. This technique is widely preferred in serial production factories and small workshop furniture enterprises due to its low cost and high-performance features.

A dowel is a wooden material that is used to connect two furniture elements. It can be applied with or without glue and can be prepared in different diameters (Kasal et al. 2020).

Dowel joining is the most preferred, easy, and economical method of joining in furniture production. For this purpose, multiple dowel drilling machines are used in serial production (Imirzi et al. 2014).

Research has been carried out to determine the effects of joining methods in determining the resistance at the joints of furniture elements. The dowel tensile performance values of wooden length joints prepared using dowels produced from ash (Fraxinus excelsior Lipsky), chestnut (Castanea sativa Mill) and sessile oak (Quercus petraea Lieble) wood were investigated. According to the test results, the highest dowel tensile strength value (N/mm²) was obtained in the test samples as 7.70, prepared using sessile oak using polyvinyl acetate (PVAc-D4) glue, and the lowest dowel tensile strength value (N/mm²) was obtained in a chestnut dowel using polyurethane (PU-D4) glue (Karaman 2019).

Some physical, mechanical, and technological properties of wood materials, thermo-mechanically compressed and heat-treated have been investigated. Scots pine (Pinus sylvestris L.) and Eastern beech (Fagus orientalis L.) woods were densified at 110 C and 150 C by compressing 20% to 40% radially. It has been found that resistance properties have increased from 4% to 81% (Pelit 2014).

Compressive strains of 12%, 16%, 24%, and 32% were applied to white spruce woods at temperatures of (20°C, 100°C, 150°C, and 200°C). At 150°C and 200°C, modulus of rupture (MOR) and modulus of elasticity (MOE) values showed minor differences between 150°C and 200°C (Tabarsa and Chui 1997).

After bleaching the colors of Scotch pine (Pinus sylvestris L.), eastern beech (Fagus orientalis L.), ash (Fraxinus excelsior L.) and sessile oak (Quercus petraea spp.) wood in which PVAc glue was used, the adhesion resistance was determined. As a result of the bonding resistance tests, it has been determined that there is a decrease in the adhesion performance of all tree species whose color is lighter compared to natural materials (Atar and Özçiftçi 1999). Wood samples prepared from wild cherry (Cerasus avium (L.) Monench) were heat treated by the Thermo Wood method at 190°C and 212°C for 1 and 2 hours. It has been determined that the average surface roughness decreases with heat treatment (Aytin 2013).

Different wood species are statically densified in a quietus press up to 140 MPa pressure. The performance potential index at bending varied between 0.65 and 1.03, the highest for oak (Fagaceae) and the lowest for alder Alnus (Blomberg 2006).

Beech (Fagus silvatica), Norway spruce (Picea abies) and coast pine (Pinus pinaster) trees are densified in the radial direction at 150°C temperature with 13 MPa pressure. After densification, the density of beech (0.67 g/cm³), Norway spruce (0.42 g/cm³), and beach pine (0.50 g/cm³) increased to 1.27 g/m³, 1.30 g/cm³, and 1.32 g/cm³, respectively. As a result of mechanical tests, the modulus of elasticity, Brinell hardness, and shear resistance are found to be much higher than the control samples (Navi and Girardet 2000).

Wooden materials prepared in different thicknesses have been subjected to the densification process with the thermo-mechanical method at 175°C press temperature and 13% and 22% compression level. It has been reported that hardness values increased by 23% and 31%, respectively, compared to the control samples (Adlam 2005).

In the study investigating the effect of high press temperatures on beech wood, it has been reported that with increasing pressing temperatures and pressing time, there has been a significant improvement in the dimensional stability of the beech wood samples. The most suitable press temperature value is 200°C and the most suitable pressing time is 6 minutes (Kudela and Resetka 2011).

Experimental samples obtained from Pinus Caribaea var. Hondurensis wood were subjected to the thermomechanical condensation process with 25% and 50% compression ratios for 50 minutes. As a result of the research, it was determined that a moderate improvement occurred in surface roughness and an improvement was observed in mechanical properties (Santos et al. 2012).

Samples prepared from Scotch pine (Pinus sylvestris L.) wood were subjected to surface densification at 150°C and in three different levels. A 90% increase was observed in Brinell hardness values with a 3.8% mass loss in the samples (Rautkari et al. 2009).

Polyvinyl acetate (PVA) glue was used as an adhesive in the study investigating the adhesion strength of yellow pine (Pinus silvestris L.), beech (Fagus orientalis L.), and oak (Quercus rubra) wood.

In terms of wood species, the highest adhesion resistance was found in beech, then in oak and pine, respectively (Altnok 1998).

Eckelman (1979) has found a strong relationship between tensile performance and shearing resistance that is parallel to the fibers of the wood used in construction. As a result of the regression analysis, the dowel tensile performance of the wood material from the edge he proposed. (1.1) F=0.834 DL .89(0.95 S1−S2)ab {\rm{F}} = 0.834\,{\rm{DL}}\,.89\left( {0.95\,{\rm{S}}1 - {\rm{S}}2} \right)ab where:

F – dowel tensile performance (pound),

Samples from poplar (Populus tremula) and birch (Betula pubescens) woods have been subjected to the condensation process at a temperature of 200°C and pressures between 1.5 MPa and 7.3 MPa. Defects occurring in the condensation process are found in 75% of poplar wood and 25% of birch wood. Some physical and mechanical properties of Scots pine (Pinus sylvestris L.) are investigated after densification processes. It was found that the most suitable temperature level was 120°C while an increase in densification temperature decreased strength properties (Ülker et al. 2014).

However, it has been determined that these defects can be minimized by increasing the steaming time (Ahmed et al. 2013). The optimal condensation parameters for fir Abies wood densified by the thermohydromechanical (THM) method were determined to be a compression ratio of 60%, a temperature of 230°C, and a time of 20 minutes. At the end of the condensation process, hardness was found to be 30 MPa while adhesion strength was 8 MPa. (Li et al. 2012).

The properties of Eastern redcedar (Juniperus virginiana), southern pine (Pinus echinata), and yellow poplar (Liriodendron tulipifera) samples that had undergone heat treatment were assessed. For the testing, a total of 80 samples-20 for each temperature level-were used. The results suggest that, when compared to two other species of wood, eastern redcedar specimens exhibited the lowest levels of discolouration. Hardness values of the samples consistently demonstrated a negative impact of heat exposure (Ulker et al. 2018).

Viscoelastic thermal compression (VTC) was used to densify low-density hybrid poplar (Populus deltoides × Populus trichocarpa) to three distinct densification levels (63, 98, and 132%). The findings demonstrated that the increased density had a considerable positive impact on the bending characteristics of the VTC wood (MOR and MOE) (Kutnar et al. 2018).

Surface quality, adhesive bondline shear strength, hardness, and color changes were studied for materials compressed at temperatures ranging from 100°C to 180°C. According to these findings, compressed specimens’ surface roughness decreased with rising temperature (Ulker et al. 2017).

In this study, dowel tensile performance values on melamine-coated chipboard were investigated. As a result of the experiments, the test samples prepared using different glues, densification ratios, and wood species in different diameters showed different performance properties against tensile forces. The effects of wood species, densification, diameter, and glue type on dowel tensile performance were found to be significant.

The densification process had a positive effect on wood material tensile performance values. The highest tensile performance (kg/cm²) was determined in samples with a 75% densification ratio of 303.547. The lowest tensile performance (kg/cm²) was obtained with the test sample as 110.105, to which a 50% densification ratio was applied. According to the control samples, the 50% densification process increased the tensile performance by 4.79%. This increase was by 14.37% in the test samples with 75% densification.

Scots pine solid wood was densified at 40, 50, and 60% of the thickness. The hardness of densified samples with a ratio of 40, 50, and 60% was found to be 19.0, 29.3, and 23.9 N/mm², respectively, while control samples had a hardness of 15.5 N/mm² (Laine, 2016).

One of the aims of this study is to find out whether the type of glue is effective in dowel tensile performance. For this purpose, the two most used glues in joining operations in the wood industry were preferred, and the results were interpreted according to these glue types. It has been clearly observed that PVAC glue has superior adhesion properties to D-VTKA glue. According to the glue type, the highest tensile performance was obtained in PVAc glue (303.547 kg/cm²) while the lowest value was in D-VTKA glue (110.105 kg/cm²). PVAc is thought to penetrate deep into the wood material by adjusting the viscosity of the adhesive molecules with water, which is used as a thinner for PVAc glue.

The effects of Vacsol Azure (VA) on the bonding power of PVAc and D-VTKA adhesives on some impregnated wood materials were determined. The highest bonding performance was obtained in PVAc treated samples at 9.707 N/mm² (Keskin et al. 2009).

According to the dowel diameter, the highest tensile performance was obtained with a 10 mm dowel diameter (303.547 kg/cm²). The lowest tensile performance was obtained on an 8 mm dowel (176.582 kg/cm²).

Edge dowel holding performance at 10 mm dowels was found to be 11% higher than that at a diameter of 8 mm (Kasal, 2007). Wood material density values, elasticity properties, late wood ratios, resin presence, and glue type can all influence dowel tensile performance.

Eastern redcedar (Juniperus virginiana), southern pine (Pinus echinata), and yellow poplar (Liriodendron tulipifera) samples that had undergone heat treatment had their properties assessed. Hardness values of the samples consistently demonstrated a negative impact of heat exposure. The surface quality of the samples from all three species appears to have improved with rising temperatures (Ulker et al. 2018).

As a result, considering its widespread use, the properties of wood materials, dowel diameter, densification amount, glue type, and engineering design approach are important to make dowel joints in accordance with the purpose.