Browsing by Author "Jivkov V."

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    Flexural and Specific Properties of Acrylic Solid Surface (PMMA/ATH) Composites: Effects of Thermoforming-Relevant Heating and Cooling
    (2025-11-01) Jivkov V.; Petrova B.; Yavorov N.; Makyov Y.
    Acrylic solid surface composites made of poly (methyl methacrylate) (PMMA) and aluminum trihydrate, Al(OH)3 (ATH) are widely used in furniture and interior applications. However, independent brand comparative data, especially on density-normalized (“specific”) properties, remain limited. This study quantifies the flexural response of 11 commercial sheets (6, 8, and 12 mm, including one translucent) under ISO 178 three-point bending and evaluates the effects of heating and cooling relevant to thermoforming. The density is concentrated in the range 1680–1748 kg/m3 (weighted mean of 1712 kg/m3). The flexural strength ranged between 51 and 79 MPa, divided into three groups—high (76–79 MPa), medium (63–67 MPa), and low (51–56 MPa) levels, while the modulus ranged between 7700 and 9400 MPa with a narrow dispersion. The strength showed no significant correlation with density, while the modulus increased with density, indicating that stiffness is composition-dominated, while strength is influenced by factors related to microstructural defects/particle boundaries. Heating at 160 °C and subsequent cooling have a significant influence on flexural strength and strain. Flexural strength increased by an average of approximately 7%, and flexural strain increased by approximately 12%, while the modulus remained virtually unchanged (within ±0.5%); additionally, shock cooling did not bring any benefits. The density-normalized parameters (σ/ρ, E/ρ) reflected these trends, allowing for a more accurate comparison when limited by mass or deformation. Overall, the results are broadly consistent with manufacturers’ declarations and demonstrate that thermoforming-relevant heating at 160 °C, followed by cooling, can be used not only to improve formability but also to modestly increase flexural strength and strain without compromising stiffness.
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    Possibilities for Efficient Furniture Construction Made of Thin and Ultra-Thin Materials by Using Mitre Joints
    (2023-11-01) Petrova B.; Jivkov V.; Yavorov N.
    One of the biggest challenges for designers and manufacturers of furniture is to reduce the thickness of conventional furniture materials such as particleboard (PB), medium-density fibreboard (MDF) and plywood. Designing furniture based on thin (less than 16 mm) and ultra-thin materials (less than 10 mm) is desirable for aesthetic reasons and because of the substantial material savings. However, the use of thin and ultra-thin materials reduces the strength of the furniture, especially the strength and deformation resistance of the joints. This study aimed to establish the possibilities for efficient furniture construction made of thin and ultra-thin materials using mitre joints. For this purpose, 14 types of L-type joints were tested: 12 glued and 2 detachable. The joints were made of eight wood-based panels and one non-wood panel. The bending moments and the stiffness coefficient under compression were determined. The obtained results show that the mitre joints made of laminated material with high-pressure laminate (HPL), 8 mm thick, MDF achieved the highest bending moment, and the highest stiffness coefficient was achieved by joints made of 10 mm thick compact HPL. Compact HPL joints were significantly affected by the type of adhesive used. Detachable joints had a relatively high bending strength but very low stiffness.
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    Structural application of eco-friendly composites from recycled wood fibres bonded with magnesium lignosulfonate
    (2020-11-01) Antov P.; Jivkov V.; Savov V.; Simeonova R.; Yavorov N.
    The pulp and paper industry generates substantial amounts of solid waste and wastewater, which contain waste fibres. The potential of using these recycled wood fibres for producing eco-friendly composites that were bonded with a formaldehyde-free adhesive (magnesium lignosulfonate) and their use in structural applications was evaluated in this study. Fibreboards were produced in the laboratory with a density of 720 kg·m−3 and 15% magnesium lignosulfonate gluing content, based on the dry fibres. The mechanical properties (bending strength, modulus of elasticity and internal bond strength), physical properties (thickness swelling and water absorption) and formaldehyde content were determined and compared with the European Standards requirements for wood-based panels. In general, the laboratory-produced panels demonstrated acceptable mechanical properties, such as bending strength (18.5 N·mm−2) and modulus of elasticity (2225 N·mm−2), which were higher than the minimum requirements for type P2 particleboards and equal to the requirements for MDF panels. The moisture properties, i.e., thickness swelling (24 h) and water absorption (24 h) significantly deteriorated. The free formaldehyde content of the laboratory-produced composites (1.1 mg/100 g) reached the super E0 grade (≤1.5 mg/100 g), which allowed for their classification as eco-friendly, low-emission wood-based composites. The L-type corner joints, made from the developed composites, demonstrated significantly lower bending capacity (from 2.5 to 6.5 times) compared to the same joints made from MDF panels. Nevertheless, the new eco-friendly composites can be efficiently utilised as a structural material in non-load-bearing applications.