Hardwood timber is becoming increasingly important in Europe for its use as structural material, both as solid wood and engineered structural products. In Italy, the great abundance of beech forests has recently led to a research project to investigate the use of this species in construction. A first step is the characterization of the raw material for the production of structural glued products. This requires developing the strength grading of beech boards, both by visual and machine methods. To the aim, four provenances were collected across the distribution of beech in Italy. The main strength reducing characteristics were measured visually, and the dynamic modulus of elasticity was determined before performing the destructive bending tests of sampled boards. Correlation between properties were similar to what is usually observed for softwoods with the exception of density, which did not correlate with any other property. Several visual rules and machine settings were developed and compared, showing the effectiveness of both methods for the strength grading of beech. The grading yields for the high strength classes were very similar for machine and visual grading, while the machine permitted to reduce the number of rejected elements when two grades were selected at the same time.
Poplar wood is commonly used for many purposes due to its easy machinability, low density, uniform light colour, and relatively low cost. Here, vacuum thermal treatment is proposed for upgrading veneers in the manufacturing of plywood panels with resulting reduced hygroscopicity, improved durability, and dimensional stability. Thirty-eight batch processes with different treatment conditions (temperature ranging from 150 to 240 °C, time from 0.5 to 22.5 h and pressure from 100 to 1000 mbar) were performed to characterize the influence of process parameters on the product properties. Samples were characterized considering their appearance (colour) and their physical (mass loss and equilibrium moisture content), mechanical (bending strength), and chemical (investigated with near infrared spectroscopy (NIR)) properties. The darkening of poplar veneers and the reduction of mechanical strength were observed with increased treatment time and intensity. Mass loss closely correlated with colour change, resulting from chemical changes in wood components. Principal component analysis (PCA) and partial least squares (PLS) were used for evaluation of near infrared spectral data. Both were correlated with several technical properties, and thus NIR allowed the simultaneous prediction of several of these properties. Both colour change and NIR could be used to optimize the thermal treatment of poplar veneers at the industrial scale and for realtime statistical process control.
The interest in the use of beech wood in construction is growing steadily. Considering its high mechanical performance, it has a large potential for the production of glued timber structural products. However, the issue of structural bonding remains the one to be solved for an effective production and a safe use. Three adhesives (one-component polyurethane -PUR, PUR + primer and melamine-urea-formaldehyde -MUF) and two press systems (hydraulic and vacuum press) were investigated in the production of pure beech and combined beech-spruce-beech cross laminated timber (CLT). The evaluation of the bonding quality was performed with both standardized and optimized (the specimen layers were oriented with the wood grain forming an angle of 45° in respect to the load application) delamination and shear test methods. None of the adhesive tested met the requirements for delamination tests provided by the current standardization for softwood. As for the adhesive, PUR was that with the poorest performance in the production of the CLT panels entirely made of beech; the addition of a primer improved the bonding, permitting to achieve results comparable to those observed for MUF. On the contrary, in the production of beech-spruce panels, the three adhesive gave similar outcomes. The press system was not a relevant factor in terms of bonding quality. As for the testing methods, a size effect was noticed in the delamination test: the larger the specimen and the greater the delaminations observed. The shear tests on dry specimens were little sensitive, even if a 45° grain orientation seemed to reduce the rolling shear during shear test and to better highlight the effect of the bonding parameters. Combining delamination pretreatment and shear made the test more sensitive and subjective.
a Sawnwood of teak (Tectona grandis L. f.) from a Costa Rica plantation was thermally treated at different process conditions using thermo-vacuum technology. The main objectives of the study were to find the optimal combination of the process parameters, i.e. temperature (T), and duration (t), in order to minimise the colorimetric difference between sapwood and heartwood, and to evaluate the influence of the treatment on the modification pattern of physical properties of the material. The resulting mass loss (ML), hygroscopicity (H), dimensional stability (ASE), and lightness (L) were measured and compared. As expected, the temperature (T) is the main parameter influencing the extent of modification. The measured ML values turned out to be moderate even at high T values if compared with other hardwoods. The temperature range between 180 °C and 190 °C minimizes the colorimetric difference between treated sapwood and not treated heartwood.
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