Topical Collection "Wood Composites"

A topical collection in Polymers (ISSN 2073-4360). This collection belongs to the section "Biomacromolecules, Biobased and Biodegradable Polymers".

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Editors

Laboratory of Wood Science, Chemistry and Technology, Department of Forestry and Natural Environment, School of Geotechnical Sciences, International Hellenic University, Thermi, Greece
Interests: wood; wood composites; lignocellulosic materials; chemical and thermal modification technologies; nanotechnology and nanomaterilas; adhesives
Special Issues, Collections and Topics in MDPI journals
Department of Mechanical Wood Technology, Faculty of Forest Industry, University of Forestry, 1797 Sofia, Bulgaria
Interests: wood and wood-based composites; eco-friendly wood-based composites; lignocellulosic composites; wood technology; wood sciences; biobased adhesives; advanced formaldehyde-based wood adhesives; formaldehyde emission; formaldehyde scavengers; recyclable materials; sustainability
Special Issues, Collections and Topics in MDPI journals

Topical Collection Information

Dear Colleagues,

The fibrous nature of wood has made it one of the most appropriate and versatile raw materials for various uses. However, two properties restrict its much wider use, namely dimensional changes when subjected to fluctuating humidity, and susceptibility to biodegradation by micro-organisms. Wood can be modified chemically or thermally so that selected properties are enhanced in a more or less permanent fashion. Another option to improve these properties is to exploit the solutions that nanotechnology can offer. The small nanoparticles of nanotechnology compounds can deeply penetrate into the wood, effectively altering its surface chemistry and resulting in a high protection against moisture and decay. In addition, the use of lignocellulosic materials for the production of advanced wood composites is an innovative avenue for research. This collection seeks high-quality works and topics focusing on (among others) the latest approaches to the protection of wood and wood composites with chemical or thermal modification technologies, the application of nanomaterials to wood science, the application of carbon fiber fabrics, and the use of lignocellulosic materials for the production of advanced wood composites.

Dr. Antonios Papadopoulos
Dr. Ľuboš Krišťák
Prof. Dr. Roman Réh
Collection Editors

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Keywords

  • wood
  • wood composites
  • lignocellulosic composites
  • chemical or thermal modification
  • nanotechnology and nanomaterials

Published Papers (7 papers)

2023

Jump to: 2022, 2021

14 pages, 3585 KiB  
Article
Effects of NCO/OH Ratios on Bio-Based Polyurethane Film Properties Made from Acacia mangium Liquefied Wood
Polymers 2023, 15(5), 1154; https://doi.org/10.3390/polym15051154 - 24 Feb 2023
Cited by 2 | Viewed by 1311
Abstract
The compatibility between isocyanate and polyol plays an important role in determining a polyurethane product’s performance. This study aims to evaluate the effect of varying the ratios between polymeric methylene diphenyl diisocyanate (pMDI) and Acacia mangium liquefied wood polyol on the polyurethane film [...] Read more.
The compatibility between isocyanate and polyol plays an important role in determining a polyurethane product’s performance. This study aims to evaluate the effect of varying the ratios between polymeric methylene diphenyl diisocyanate (pMDI) and Acacia mangium liquefied wood polyol on the polyurethane film properties. A. mangium wood sawdust was liquefied in polyethylene glycol/glycerol co-solvent with H2SO4 as a catalyst at 150 °C for 150 min. The A. mangium liquefied wood was mixed with pMDI with difference NCO/OH ratios to produce film through the casting method. The effects of the NCO/OH ratios on the molecular structure of the PU film were examined. The formation of urethane, which was located at 1730 cm−1, was confirmed via FTIR spectroscopy. The TGA and DMA results indicated that high NCO/OH ratios increased the degradation temperature and glass transition from 275 °C to 286 °C and 50 °C to 84 °C, respectively. The prolonged heat appeared to boost the crosslinking density of the A. mangium polyurethane films, which finally resulted in a low sol fraction. From the 2D-COS analysis, the hydrogen-bonded carbonyl (1710 cm−1) had the most significant intensity changes with the increasing NCO/OH ratios. The occurrence of the peak after 1730 cm−1 revealed that there was substantial formation of urethane hydrogen bonding between the hard (PMDI) and soft (polyol) segments as the NCO/OH ratios increased, which gave higher rigidity to the film. Full article
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2022

Jump to: 2023, 2021

28 pages, 11147 KiB  
Article
Effects of Adding Methods of Fluorane Microcapsules and Shellac Resin Microcapsules on the Preparation and Properties of Bifunctional Waterborne Coatings for Basswood
Polymers 2022, 14(18), 3919; https://doi.org/10.3390/polym14183919 - 19 Sep 2022
Cited by 8 | Viewed by 1189
Abstract
In this paper, urea-formaldehyde resin microcapsules with shellac resin as core material were prepared by in-situ polymerization. Morphologies of shellac resin microcapsules were characterized by optical microscope (OM) and scanning electron microscope (SEM). Both microcapsules were spherical in shape. The encapsulation property of [...] Read more.
In this paper, urea-formaldehyde resin microcapsules with shellac resin as core material were prepared by in-situ polymerization. Morphologies of shellac resin microcapsules were characterized by optical microscope (OM) and scanning electron microscope (SEM). Both microcapsules were spherical in shape. The encapsulation property of shellac resin was proved by Fourier transform infrared (FTIR). Shellac resin microcapsules and fluorane microcapsules were added to waterborne primer or topcoat at the same time to prepare waterborne coatings with thermochromic and self-healing dual functions. The effects of microcapsules on optical properties, mechanical properties, self-healing properties, anti-aging performance, and thermoreversible discolouration mechanism of coating films were studied. These results showed that the topcoat with 10.0% fluorane microcapsules and 5.0% shellac resin microcapsules had a better comprehensive performance. At this time, the colour of coating transformed yellow into colourless at 32 °C, and it had a good colour recovery. Shellac resin microcapsules endowed the coating with self-healing performance, and the self-healing rate was 35.9%. The research results provide a reference for the progression of multifunctional wood coatings. Full article
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10 pages, 1749 KiB  
Article
Morphological, Chemical, and Physical–Mechanical Properties of a Clumping Bamboo (Thyrsostachys oliveri) for Construction Applications
Polymers 2022, 14(17), 3681; https://doi.org/10.3390/polym14173681 - 05 Sep 2022
Cited by 1 | Viewed by 1450
Abstract
In view of the long-term utilization history as a building and furniture making material in southeast Asian countries, Thyrsostachys oliveri is considered to have great utilization potential. However, little is known about the quantitative morphological characteristics and comprehensive material properties of its culm. [...] Read more.
In view of the long-term utilization history as a building and furniture making material in southeast Asian countries, Thyrsostachys oliveri is considered to have great utilization potential. However, little is known about the quantitative morphological characteristics and comprehensive material properties of its culm. In this study, we systematically investigated the morphological characteristics, the chemical components, and the physical–mechanical properties of the three-year-old culm of T. oliveri. The morphological analysis result showed that the internode length, the diameter of internodes and the wall thickness changed with the culm height. The volume of the culm wall of a single internode increased before the 10th internode, and then it decreased to a significant level at the 20th internode. The basic chemical compositions (cellulose, hemicellulose, lignin and silicon content) of the culm wall were 346.19 mg/g, 95.32 mg/g, 33.17%, and 3.39 mg/g, respectively. These component contents were relatively stable in the bottom and middle part of the culm, but changed significantly in the upper part of the culm. The moisture content and the base density of the culm wall were 73.01% and 0.64 g/cm3, respectively. The culm wall shrinkage rate in the radial, tangential direction as well as the volumetric shrinkage reached the minimum value in the middle part of the culm. The average compressive strength, modulus of rupture and modulus of elasticity of the culm wall were 67.03 MPa, 143.74 MPa, and 7.99 GPa, respectively. These results provide valuable reference data for more rational use of this bamboo resources. Full article
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13 pages, 1271 KiB  
Article
Preparation of Cellulose Modified Wall Material Microcapsules and Its Effect on the Properties of Wood Paint Coating
Polymers 2022, 14(17), 3534; https://doi.org/10.3390/polym14173534 - 28 Aug 2022
Cited by 2 | Viewed by 980
Abstract
An orthogonal experiment with four factors and three levels was designed. Nine different microcapsules were prepared by changing four factors: the core–wall ratio, emulsifier concentration, reaction temperature, and rotation speed. Through an analysis of the microcapsule yield and morphology, it was determined that [...] Read more.
An orthogonal experiment with four factors and three levels was designed. Nine different microcapsules were prepared by changing four factors: the core–wall ratio, emulsifier concentration, reaction temperature, and rotation speed. Through an analysis of the microcapsule yield and morphology, it was determined that the microcapsule of sample 6 performed the best in the orthogonal test and that the core–wall ratio was the largest factor affecting the microcapsule morphology and yield. In order to further optimize the performance of the microcapsules, single factor independent tests were carried out using the core–wall ratio as a single variable. It was found that the microcapsules with the core–wall ratio of 0.75:1 had good micro morphology and yield. The properties of the coating were the best when the microcapsules were added into the primer and the topcoat at the same time with an additional amount of 10.0%. The mechanical properties of the coating containing cellulose microcapsules and the coating without cellulose microcapsules were tested. Cellulose can enhance the toughness of the microcapsules, inhibit the generation of microcracks, and enhance the performance of the coating to a certain extent. The elongation at break of the coating with cellulose microcapsules was 9.49% higher than that without cellulose and was 11.1% higher than that without cellulose microcapsules. Full article
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19 pages, 2774 KiB  
Article
Influence of Lignin Content and Pressing Time on Plywood Properties Bonded with Cold-Setting Adhesive Based on Poly (Vinyl Alcohol), Lignin, and Hexamine
Polymers 2022, 14(10), 2111; https://doi.org/10.3390/polym14102111 - 22 May 2022
Cited by 21 | Viewed by 2400
Abstract
The sustainability, performance, and cost of production in the plywood industry depend on wood adhesives and the hot-pressing process. In this study, a cold-setting plywood adhesive was developed based on polyvinyl alcohol (PVOH), high-purity lignin, and hexamine. The influence of lignin content (10%, [...] Read more.
The sustainability, performance, and cost of production in the plywood industry depend on wood adhesives and the hot-pressing process. In this study, a cold-setting plywood adhesive was developed based on polyvinyl alcohol (PVOH), high-purity lignin, and hexamine. The influence of lignin content (10%, 15%, and 20%) and cold-pressing time (3, 6, 12, and 24 h) on cohesion, adhesion, and formaldehyde emission of plywood were investigated through physical, chemical, thermal, and mechanical analyses. The increased lignin addition level lowered the solids content, which resulted in reduced average viscosity of the adhesive. As a result, the cohesion strength of the adhesive formulation with 10% lignin addition was greater than those of 15% and 20% lignin content. Markedly, the adhesive formulation containing a 15% lignin addition level exhibited superior thermo-mechanical properties than the blends with 10% and 20% lignin content. This study showed that 10% and 15% lignin content in the adhesive resulted in better cohesion strength than that with 20% lignin content. However, statistical analysis revealed that the addition of 20% lignin in the adhesive and using a cold-pressing time of 24 h could produce plywood that was comparable to the control polyurethane resins, i.e., dry tensile shear strength (TSS) value of 0.95 MPa, modulus of rupture (MOR) ranging from 35.8 MPa, modulus of elasticity (MOE) values varying from 3980 MPa, and close-to-zero formaldehyde emission (FE) of 0.1 mg/L, which meets the strictest emission standards. This study demonstrated the feasibility of fabricating eco-friendly plywood bonded with PVOH–lignin–hexamine-based adhesive using cold pressing as an alternative to conventional plywood. Full article
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2021

Jump to: 2023, 2022

23 pages, 5473 KiB  
Article
The Effect of Varying the Amount of Short Hemp Fibers on Mechanical and Thermal Properties of Wood–Plastic Composites from Biobased Polyethylene Processed by Injection Molding
Polymers 2022, 14(1), 138; https://doi.org/10.3390/polym14010138 - 30 Dec 2021
Cited by 12 | Viewed by 1726
Abstract
Biobased HDPE (bioHDPE) was melt-compounded with different percentages (2.5 to 40.0 wt.%) of short hemp fibers (HF) as a natural reinforcement to obtain environmentally friendly wood plastic composites (WPC). These WPC were melt-compounded using a twin-screw extrusion and shaped into standard samples by [...] Read more.
Biobased HDPE (bioHDPE) was melt-compounded with different percentages (2.5 to 40.0 wt.%) of short hemp fibers (HF) as a natural reinforcement to obtain environmentally friendly wood plastic composites (WPC). These WPC were melt-compounded using a twin-screw extrusion and shaped into standard samples by injection molding. To improve the poor compatibility between the high non-polar BioHDPE matrix and the highly hydrophilic lignocellulosic fibers, a malleated copolymer, namely, polyethylene-graft-maleic anhydride (PE-g-MA), was used. The addition of short hemp fibers provided a remarkable increase in the stiffness that, in combination with PE-g-MA, led to good mechanical performance. In particular, 40 wt.% HF drastically increased the Young’s modulus and impact strength of BioHDPE, reaching values of 5275 MPa and 3.6 kJ/m2, respectively, which are very interesting values compared to neat bioHDPE of 826 MPa and 2.0 kJ/m2. These results were corroborated by dynamic mechanical thermal analysis (DMTA) results, which revealed a clear increasing tendency on stiffness with increasing the fiber loading over the whole temperature range. The crystal structure was not altered by the introduction of the natural fibers as could be seen in the XRD patterns in which mainly the heights of the main peaks changed, and only small peaks associated with the presence of the fiber appeared. Analysis of the thermal properties of the composites showed that no differences in melting temperature occurred and the non-isothermal crystallization process was satisfactorily described from the combined Avrami and Ozawa model. As for the thermal degradation, the introduction of HF resulted in the polymer degradation taking place at a higher temperature. As for the change in color of the injected samples, it was observed that the increase in fiber generated a clear modification in the final shades of the pieces, reaching colors very similar to dark woods for percentages higher than 20% HF. Finally, the incorporation of an increasing percentage of fibers also increased water absorption due to its lignocellulosic nature in a linear way, which drastically improved the polarity of the composite. Full article
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12 pages, 2350 KiB  
Article
Efficiency of High-Frequency Pressing of Spruce Laminated Timber Bonded with Casein Adhesives
Polymers 2021, 13(23), 4237; https://doi.org/10.3390/polym13234237 - 03 Dec 2021
Cited by 2 | Viewed by 1925
Abstract
This study identifies the importance of reducing press times by employing high-frequency pressing of spruce-laminated timber bound with sustainable casein adhesives. Spruce lamellas with dimensions of 12 × 10 × 75 cm were bonded into five-layered laminated timber and then separated into single-layer [...] Read more.
This study identifies the importance of reducing press times by employing high-frequency pressing of spruce-laminated timber bound with sustainable casein adhesives. Spruce lamellas with dimensions of 12 × 10 × 75 cm were bonded into five-layered laminated timber and then separated into single-layer solid wood panels. Three types of casein (acid casein from two sources and rennin) were used. To compare the effectiveness of the casein formulation, two control samples bonded with polyvinyl acetate (PVAc) adhesive were pressed at room temperature (20 °C) and also with high-frequency equipment. The tests included compression shear strength, modulus of rupture, modulus of elasticity and screw withdrawal resistance on the wood panel surface and in the glue line. The average values of casein-bonded samples compression strengths ranged from 1.16 N/mm2 and 2.28 N/mm2, for modulus of rupture (MOR) were measured 85 N/mm2 to 101 N/mm2 and for modulus of elasticity (MOE) 12,200 N/mm2 to 14,300 N/mm2. The screw withdrawal resistance (SWR) on the surface of the wood panels ranged from 91 N/mm to 117 N/mm and in the adhesive line from 91 N/mm to 118 N/mm. Control samples bonded with PVAc adhesive did not perform better for compression shear strength, MOR and MOE, but for SWR in the adhesive line with 114 N/mm. Casein-bonded spruce timber pressed with HF equipment represents a sustainable new product with reduced press times, hazardous emissions and improved workability. Full article
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