In this study, alder, spruce, and beech woods were used for homogeneous symmetric, inhomogeneous symmetric (combined) and inhomogeneous non-symmetric glued laminated timber (glulam) beams glued with resorcinol phenol formaldehyde (RPF) adhesive. The aim of this paper is to determine and compare the modulus of elasticity of glulam beams using three methods, i.e., analytical calculation, numerical model (FEM) and experimental testing. As an additional characteristic, the bending strength (MOR) of the beams was determined during experimental testing. Analytical calculation was used to calculate the modulus of elasticity (MOE) of glued laminated timber based on the knowledge of the modulus of elasticity of solid wood and to estimate the location of the neutral axis during bending. According to calculations, for symmetrical combinations, the deviation from the real neutral axis does not exceed 5%. In the case of the modulus of elasticity, the deviation is an average of 4.1% from that of the actual measured beams. The numerical model includes finite element modelling, where the deflection of the modelled beams can be calculated with a deviation of up to 10%. The last method was experimental testing of glued beams using four-point bending, in which, among homogeneous beams, beech glulam beams achieved the highest MOE and MOR, while alder glulam beams achieved the lowest. The combination of wood species resulted in an increase in both MOE and MOR compared to homogeneous spruce and alder beams.

This article investigates the effects of pressure vacuum impregnation using inorganic, organic, and natural flame retardants on enhancing the fire resistance and chemical composition of structural beech wood (Fagus sylvatica). The study examines fire resistance characteristics such as the limiting oxidation number and heat of combustion, which indicate the effectiveness of the flame retardants used. Chemical changes in the beech wood were characterized through various analyses, including changes in chemical composition, FTIR spectra, DSC thermograms, and SEM images. The relationships between combustion characteristics and chemical changes were assessed using multiple methods. The results demonstrate that using 5% potassium acetate achieved a lower heat of combustion compared to 15% sodium phosphate, and it was significantly lower than the heat of combustion observed with 5% arabinogalactan or the reference sample of beech wood. However, neither potassium acetate nor diammonium phosphate significantly affected the macromolecular structures of the wood when compared to the reference sample. Low concentrations of flame retardants reduce environmental release and environmental impact while increasing fire resistance, which could be used for structural solutions made of hardwoods.

The effect of a surface coating with an aqueous solution containing a synthetic diammonium hydrogen phosphate fire retardant and vacuum pressure impregnation with a synthetic diammonium hydrogen phosphate fire retardant, potassium acetate, and a natural polymeric retardant, arabinogalactan, to improve the fire resistance and selected properties of structural fir (Abies alba L.) wood was investigated in this article. The combustion characteristics were investigated, and the heat of combustion reflects the effect of the presence of fire retardants. Changes in the content of cellulose, hemicelluloses, holocellulose, lignin, and extractives characterize the chemical changes in wood caused by these factors. The relationship between the combustion characteristics and chemical changes in chemically modified wood as a consequence of the presence of flame retardants were assessed using Fourier transform infrared spectroscopy. The results showed that the effectiveness of the fire retardants against ignition and burning when applied by vacuum pressure impregnation was always higher than in the case of surface coating, even when using impregnation solutions of low concentrations. In the case of diammonium hydrogen phosphate, a low 5% concentration of retardant was sufficient to provide suitable flame retardancy. Further, degradation by depolymerization of cellulose occurred only at temperatures between 460 and 470 °C. Low concentrations of retardant limit the loss to the environment and consequent ecological impact.

This study delved into the combustion properties of combined glulam bonded using polyurethane (PUR) and resorcinol-phenol-formaldehyde (RPF) adhesives. The experiment involved three distinct wood species, namely, spruce, alder, and beech, which were combined in homogeneous, non-homogeneous symmetrical, and non-homogeneous asymmetrical arrangements. These species were selected to represent a spectrum, namely, softwood (spruce), low-density hardwood (alder), and high-density hardwood (beech). The varying combinations of wood species illustrate potential compositions within structural elements, aiming to optimize mechanical bending resistance. Various parameters were measured during combustion, namely, the heat release rate (HRR), peak heat release rate (pHRR), mass loss rate (MLR), average rate of heat emission (ARHE), peak average rate of heat emission (MARHE), time to ignition (TTI), and effective heat of combustion (EHC). The findings indicate that incorporating beech wood into the composite glulam resulted in an increase in heat release, significantly altering the burning characteristics, which was particularly evident at the second peak. Conversely, the use of spruce wood exhibited the lowest heat release rate. Alder wood, when subjected to heat flux at the glued joint, displayed the highest heat emission, aligning with the results for EHC and MARHE. This observation suggests that wood species prone to early thermal decomposition emit more heat within a shorter duration. The time to ignition (TTI) was consistent, occurring between the first and second minute across all tested wood species and combinations. Notably, when subjected to heat flux, the glulam samples bonded with PUR adhesive experienced complete delamination of the initial two glued joints, whereas those bonded with RPF adhesive exhibited only partial delamination.

This study investigates the valorization of post-consumer and post-industrial recycled cotton fibers from textile waste into porous fiber-based insulation composites using a low-temperature cold-pressing process and a water-borne hybrid binder based on polyvinyl acetate (PVAc) and modified cornstarch. Insulation boards were produced with target densities ranging from 300 to 340 kg·m⁻³, achieved by systematically adjusting the percentage weight fractions of recycled cotton fibers and binder components. The influence of board density on microstructure, inter-fiber bonding, and structure–property relationships was evaluated. The resulting boards exhibited thermal conductivity values between 0.0710 and 0.0739 W·m⁻¹·K⁻¹. Compressive strength measured at 10% relative deformation of the specimen thickness ranged from 46 to 162 kPa, while internal bond strength varied between 2 and 6 kPa. Water absorption decreased by approximately 18% with increasing density, indicating improved binder distribution and reduced open porosity. The PVAc–starch binder system enabled effective inter-fiber bonding without formaldehyde-based resins or energy-intensive curing, supporting a low-temperature and circular processing concept for textile waste valorization. Overall, the results demonstrate that recycled cotton fibers represent a viable feedstock for porous insulation composites combining balanced thermal, mechanical, and moisture-related performance with potentially reduced environmental impact.