Analysis of glue line and correlations between anatomical characteristics of Eucalyptus grandis × Eucalyptus urophylla glued-laminated timber

The main goal of this study was to analyze glue line on eucalyptus wood. In order to do that, thickness of main and secondary glue lines were measured as well their interaction with apparent density of elements glued with resorcinol-formaldehyde (RF) and castor polyurethane (CP) adhesives. Anatomical wood characterization of Eucalyptus grandis × Eucalyptus urophylla was performed by correlating glue line thickness. According to normative instruction, specimens were produced for delamination tests. The experiment was conducted in a completely random 2 × 2 design factorial scheme (two classes of apparent density and two adhesives). Pearson correlation (t < 0.01) was performed among variables. It was found that there was adhesive penetration into wood pots and rays. Glue line thickness was higher in woods with density higher than 0.58 g cm−3 glued with RF adhesive. There was low correlation among density, vessel diameter, main and secondary glue lines (t < 0.01).


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To perform the tests, 4 specimens were removed from each timber board, 105 totaling 160 specimens, which were sectioned in dimensions of 3 × 2 × 5 cm 3 in the 106 radial, tangential and longitudinal directions, respectively.

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Adhesive and glued elements 109 There were tested two commercial adhesives, a thermoset, the Cascophen RS-

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To produce glulam, timber boards were divided according to their apparent 118 density into two groups: G1) apparent density less than 0.58 g cm -3 ; and G2) apparent 119 density equal or higher than 0.58 g cm -3 . After dividing the groups, timber boards were 120 sectioned and transformed into 56 lamellae with dimensions of 21 × 21 × 2.5 cm 3 .

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Each group had 28 lamellae, resulting in 14 Glulam elements. 7 Glulam blocks 122 were used for each adhesive. A spatula was used to apply 300 g m -2 to each pair of 123 timber board in a single glue line. They were joined and pressed in manual press 124 (capacity 15 tons) for 48 hours at a pressure of 1 MPa and temperature of 20 °C [13].
129 the radial, tangential and longitudinal directions, respectively, made by microtome.
131 In addition, it was performed microscopic identification using a 10x magnification lens 132 [14].

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Visualization of the wood-adhesive interface 135 The study of wood-adhesive interface was carried out based on a methodology 136 known [7]. The timber blades came from glued joints of a 0.5 × 0.5 × 0.5 cm 3 dimension 137 central hub, on a microtome. Each cube had samples collected in the glue line.

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A total of 28 cubes were prepared, 14 cubes for each group of apparent density Delamination test on blocks of glued laminated timber 146 Delamination test was executed arranging the specimens inside an autoclave.
147 After that, glue lines were exposed to stresses that came from vacuum and pressure 168 Samples were placed outside in natural environmental conditions to dry out for 169 36 hours at 28 ± 2 ° C of temperature, aiming to reduce their weight, no more than 5 to 170 6% of the initial weight range. For each specimen, it is recommended that the total glue 171 line delamination should not exceed 10% of total length at the specimen top, to glulam 172 approval for exterior use [15].

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178 also performed a descriptive quantitative statistical analysis.

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Pearson correlation coefficient (t < 0.01) was used to find out the relationship 180 between the following variables: apparent density, adhesive type, main glue line, 181 secondary glue line, vessel diameter, ray height, width of rays, number of cells.

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Physical and anatomical wood characteristics 187 The hybrid Eucalyptus grandis × Eucalyptus urophylla showed an apparent 188 average density (ρ) of 0.59 g cm -3 and an average moisture content of 9.34% (Table 1) 189 These characteristics have influence on adhesive penetration [10], pressure need, 190 pressing time and adhesive cure (how long it needs to be ready for use), making its 191 determination an indispensable factor to produce glulam elements [32]. 200 glued with RF adhesive, due to its reddish color ( Figure 3C and 3D).

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When it comes to CP adhesive, the contrast between average apparent density 234 levels was statistically zero. Therefore, glulam elements in Group 1 and 2 with CP 235 adhesive showed no significant difference for both glue line thicknesses.

Pearson's correlation on studied variables
238 Main glue line thickness showed strong and significant correlation (t < 0.01) 239 with adhesive (0.739) ( Table 3). Correlation is a statistical analysis that measures the 240 association between variables. Therefore, it was possible to say that the behavior 241 presented by glue line thickness was expected due to the adhesive.
242   283 Thereby, glued elements possibly had ideal conditions in the surface to create a strong 284 connection [7] and the amount of used adhesive was enough.

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There was adhesive penetration into rays and vessels elements, which was 286 possible due to perforations on their walls that allowed the inter-cells communication. Adhesive penetration into ray cells may have happened through transfer cells, 290 since they are made of parenchymatic cells and they work as conducting elements in the 291 radial direction. Parenchyma cells have a modified wall to allow transportation, which is 292 generally short-distance [7]. The specie anatomical condition, high or low density, 293 reflects on adhesive penetration [25,26].

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When adhesive has corrected viscosity and finds in substrate ideal conditions of 295 surface and microscopic structure, the product created from this gluing must have equal 296 or superior characteristics than the sum of individual characteristics from the materials 297 that it was made from.

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Highest thickness averages to main and secondary glue lines were found in 310 Group 2, 42.50% and 35.73% higher than group 1 averages for RF (Table 2). These 311 numbers are higher than those reported in previous studies for double glue lines 150 g the thickness data of the glue line (Table 1).

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A wood increase causes a reduction of the porosity, consequently, the wood will 343 have a greater contact area and increase in the mechanical resistance to the worth which 344 it is submitted.

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The height of rays had a moderate correlation with cells number, also, rays width 346 and number of cells, respectively 0.503 and 0.608, both under significant (t <0.01).
347 Since the rays are formed by parenchyma cells and have an indeterminate length, it can 348 be said that, increasing the height and width of rays will increase their capacity to create 349 new storage cells [31], which can explain the correlation between these variables. 360 -There was a significant interaction between glue line thickness and apparent density. better to this interaction; characteristic qualifies the wood as favorable to gluing, since larger rays will contribute 365 in the adhesive permeability.