Review Articles

The production of fiberboard, particleboard, and related hot-pressed biomass products can convert small, relatively low-valued pieces of wood into valuable products. There is strong interest in being able to manufacture such products without the addition of formaldehyde, which is a health hazard during both production and use.  This article reviews literature describing various challenges that need to be faced in order to achieve satisfactory bonding properties in hot-pressed bio-based board products without the addition of formaldehyde.  Bonding mechanisms are examined in the form of a hypothesis, in which the strength development is represented by a chain with four links.  Failure of a board is expected to occur at the weakest of these mechanistic links, which include mechanical contact, molecular-scale wetting and contact, various chemical-based linkages, and structural integrity.  The most promising technologies for environmentally friendly production of hot-pressed board with use of lignocellulosic materials tend to be those that favor success in the development of at least three of the mechanistic links in the hypothetical chain.

Wood is an abundant and renewable natural resource. Its use is promoted as a way to reduce the carbon footprint in building construction. Wood structures are degraded by their environment due to weathering. This review is a meta-analysis of the main factors of degradation that belong to this phenomenon. The impact of irradiation, the role of water, oxygen, temperature, and colonization by fungi are explained. To protect against these factors, the use of coatings is the most common solution. Since currently the trend is to maintain the grain and the natural color of the wood, the use of transparent coatings is favored. This review presents the main technologies used in clear wood coatings. The durability of this protection against weathering is approached. The whole of knowledge gathered has made it possible to begin a discussion on the multifactorial aspect of wood weathering. Schemes were created to synthesize the synergistic and antagonistic effects between the degradation factors.

The machining mechanisms in milling for medium-density fiberboard (MDF) and wood-plastic composites (WPC) are reviewed in this article. The study focuses on milling tool wear, chip formation mechanisms, processing stability, and machined surface roughness. The influence law of cutting parameters (cutting speed, feed rate, and cutting thickness), tool materials and geometry (rake angle, relief angle, and size parameters), temperature, and other factors on tool wear and machined surface roughness were considered. Concrete measures to improve tool life and machined surface quality are summarized as well as an online monitoring system of tool wear and machined surface roughness. Future research of tool wear and surface quality in milling wood-based materials is proposed to provide important references for wood-based materials researchers.

With the improvement of living standards, the human demand for antibacterial materials has increased. Cellulose, as the most abundant polymer in the world, is natural, biodegradable, and renewable, which makes it a promising raw material for the production of antibacterial materials. Silver nanoparticles (AgNPs)-cellulose antibacterial composites exhibit good biocompatibility and antimicrobial properties. These materials are easily degraded chemically and are environmentally friendly. Therefore, the AgNPs-cellulose antibacterial composites exhibit broad utilization prospects in environmental protection, medicine, chemical catalysis, and other fields. Several methods are used to manufacture such materials. This paper reviews three common techniques: the physical method, the in situ chemical reduction method, and the covalent bonding method. The differences and relationships are identified, and the advantages and disadvantages are compared among these three methods. Lastly, the present situation and the development potential of the AgNPs-cellulose antibacterial composites are discussed in this review.

The relevant literature is reviewed concerning eucalyptus wood collapse, with a focus on lumber drying technology. Potential future research is summarized regarding where potential future work may focus. Eucalyptus is often limited as a solid wood products material due to microstructural collapse and interior cracking that may occur during drying. To prevent the drying collapse, studies have focused on the mechanism of collapse, the morphological characteristics of collapse, the control of collapse, amongst other criteria. Because the surface tension of water results in wood cell collapse, the shape of collapsed cells should be recovered after the liquid tension disappears. Therefore, pretreating green timber (such as pre-heating, pre-steaming, microwave treatment, pre-freezing, or boiling) prior to drying results in the modification of wood cell tissue and inhibits the conditions for collapse. Thus, there is improved wood permeability, drying rate, shortened drying time, as well as reduced collapse during the drying process. In addition, applying process control in regards to a suitable drying schedule (especially the drying temperature), relative humidity, drying time, intermittent drying process, combined drying technology, etc., tends to reduce the amount of collapse and improve drying quality. Reconditioning, such as steaming during the drying process, can aid collapse recovery. Generally, reconditioning or other treatment can help recover 50% of the collapse.

There has been increasing concern regarding environmental problems arising from the widespread use of petroleum-based plastic materials for packaging. Many efforts have been made to develop sustainable and biodegradable packaging materials to replace plastic products. The current review summarizes recent research progress in developing cellulose packaging materials to replace plastics used for cushioning and barrier packaging functions based on pulp fibers, cellulose nanofibers, and regenerated cellulose films to benefit from their renewability, sustainability and biodegradability. The cushioning packaging materials include molded pulp products and bio-based foams. Advanced cellulose films and paper can be good barriers for oxygen and carbon dioxide gases, as well as for water vapor. Several cellulose fiber-based packaging products have been commercialized in areas that used to be occupied solely by plastic products.

Lyocell fiber has emerged as an important class of regenerated cellulose that is produced based on the N-methyl morpholine-N-oxide (NMMO) dissolution method, and it has unique properties compared to viscose fiber. The NMMO technology provides a simple, resource-conserving, and environmentally friendly method for producing regenerated cellulose fiber. In this paper, the manufacturing process, environmental impact, and product quality of lyocell fiber are reviewed and compared with those of the conventional viscose fiber.

Hygiene tissue paper properties are a function of fiber type, chemical additives, and machine technology. This review presents a comprehensive and systematic discussion about the effects of the type of fiber and machine technology on tissue properties. Advanced technologies, such as through-air drying, produce tissue with high bulk, softness, and absorbency. Conventional technologies, where wet pressing is used to partially dewater the paper web, produces tissue with higher density, lower absorbency, and softness. Different fiber types coming from various pulping and recycling processes are used for tissue manufacturing. Softwoods are mainly used as a source of reinforcement, while hardwoods provide softness and a velvet type surface feel. Non-wood biomass may have properties similar to hardwoods and/or softwoods, depending on the species. Mechanical pulps having stiffer fibers result in bulkier papers. Chemical pulps have flexible fibers resulting in better bonding ability and softness. Virgin fibers are more flexible and produce stronger and softer tissue. Recycled fibers are stiffer with lower bonding ability, yielding products that are weaker and less soft. Mild mechanical refining is used to improve limitations found in recycled fibers and to develop properties in virgin fibers. At the same time that refining increases strength, it also decreases bulk and water absorbency.

This paper reviews ways that biomass can be converted by thermal depolymerization to make synthetic gas, i.e. syngas. Biomass, being carbon neutral, is considered as a form of solar energy stored during the growing season by photosynthesis. An effective biomass is one with low moisture and ash content, high lignin content, high calorific value, and small particle size. Woody biomass with low ash content (<1%), nut shells with high lignin content (30 to 40%), and municipal solid waste with synthetic polymers are effective at creating value-added synthetic gases. An allothermal downdraft gasifier produces a low tar syngas (99.9% tar conversion) at 850 oC and provides a simple and low-cost process. Integrated gasification combined cycle (IGCC) improves thermodynamic efficiency. To avoid thermal loss, a hot gas filtration system uses trona sorption material for sulfur and halogen compounds. Secondary systems can use multiple cyclones followed by reactors employing calcined dolomite, olivine, and others for adsorption or reaction with residual sulfur, ammonia, metals, and halogens. Reforming of residual tar to syngas can take place within chambers with ceramic tubes doped with nano-nickel particles. Syngas can then be used in boilers, gas turbines for production of electricity or production of chemicals by Fischer-Tropsch conversion.

The production of wood-based polymer composites has gained increasing interest in recent years, especially regarding sustainability issues, aiming at the recovery, reuse, and up-cycling of by-products from natural resources exploitation, as well as plastics. Due to their reduced cost, low density, and availability, wood components (fibers, flour) are attractive fillers for thermoplastic polymer matrices used in multi-component systems. Performance of wood-based thermoplastic materials mainly depends on the type and strength of interactions at the polymer-wood interface. Different low polarity polymers (high/low density polyethylene, polypropylene, polyvinylchloride) can be successfully used as matrices in such formulations. Various methods may be applied in order to obtain specified performance attributes of wood-based composites. Addition of appropriate compatibilizing agents, chemical and/or physical modification of the filler in order to improve its compatibility towards the matrix, or a judicious combination of these approaches may be employed. This paper briefly reviews some recent literature data, as well as research results by the authors, aiming at a comparative assessment of the materials properties (structure, thermal, mechanical and water sorption behavior) in correlation with the nature and type of components, processing, recycling options, and environmental impact.