Introductory Chapter: Engineering Wood Review

1. Introduction

Forests exist in four major climatic zones (boreal, temperate, subtropical, and tropical) (Figure 1). According to the Global Forest Resources Assessment (FRA) report published by the Food and Agriculture Organization of the United Nations (FAO) in 2020, the total area of forests globally amounted to 4.06 billion hectares, representing 31% of the total land area [1]. Meanwhile, the total global area of planted forests is estimated to be 294 million hectares, accounting for 7% of the world's forest area. Asia has the largest area of planted forests with 135.23 million hectares, accounting for 46% of the total global planted forest area, followed by Europe, North and Central America, South America, Africa, and Oceania.

Figure 1.

Global forest distribution by climatic domain [1].

Planted forests are usually defined as forests consisting primarily of planted and/or intentionally seeded trees. Planted forests can provide benefits for traditional timber and fiber production, economic development, and employment in rural areas [2], while they can serve as a key means of combating climate change, restoring degraded land, and maintaining sustainable ecosystems in the short to medium term [3, 4]. In a broader geographic and economic context, well-managed planted forests contribute to sustainable development toward a forest-based circular bioeconomy and healthy ecosystems [5].

Wood is one of the traditional materials used in construction applications, and there is a wide range of engineered wood products (EWPs) available for construction, from sawn lumber to structural lumber. Light-frame systems are the most common type of wood-frame construction, using EWPs such as dimension lumber, placed at regular intervals and fastened together to form floor, wall, stair and roof members. Due to cost advantages, timeliness, and convenience, light-frame wood construction is commonly used in single-family homes, multi-unit dwellings, commercial buildings, and light industrial buildings. Construction costs and lead times are lower than traditional methods because more and more structural components are prefabricated in factories and often shipped to the jobsite along with plumbing fixtures, electrical systems, paints, flooring accessories, and other materials.

The development of wood-frame construction has been uneven globally, with major concentrations in North America, Australia, Japan, and some countries in South-East Asia. In developing countries, although wood is still considered a typical building material, there are a number of constraints that hinder the development of EWPs, such as consumer perception bias, which is usually associated with deforestation; high costs, which are higher when EWPs or more modern building systems are used; a lack of professional builders, who are accustomed to masonry and concrete buildings; and a lack of special regulations and standards. Nevertheless, some changes can be observed, especially in some developing countries with high forest cover. In Brazil, for example, the number of companies producing EWPs continues to grow, and the strong links between civil engineering and forestry have resulted in more wood-frame construction.

2. Types of EWPs

EWPs are a man-made composite material made from hardwoods and softwoods. There is a wide variety of EWPs with different manufacturing processes and applications. Examples of EWPs include particleboard, plywood, fiberboard, oriented strand board (OSB), laminated veneer lumber (LVL), glued laminated timber (GLT), and cross-laminated timber (CLT) (Figure 2).

Figure 2.

Types of EWPs.

2.1 Particleboard

Particleboard is a man-made board made of wood or other lignocellulosic materials made of scraps, applied adhesive, and then glued under the action of heat and pressure; the physical photograph of particleboard is shown as Figure 3. As the population grows, so does the market demand for particleboard, and over-exploitation of forest resources has led to a shortage of wood supply in most developing countries. The scarcity of timber resources has limited the development of the particleboard industry; therefore, in addition to timber by-products, some plant raw materials (e.g., bagasse, bamboo, bark, rice husk, etc.) are gradually being used. Bekalo and Reinhardt [6] investigated the process and properties of particleboard prepared from coffee husks. Coffee husk is a coffee processing residue and currently, coffee husk is rarely utilized in Germany and is usually incinerated or landfilled or poured into river water. Akinyemi et al. [7] prepared composite particle boards from waste materials such as corn cobs and wood chips. The effect of waste dosage on the physical and mechanical properties of particleboards was investigated by fixing the volume of adhesive. The results showed that the higher the composition of corn cobs, the faster the boards were saturated by water. The best physical properties were obtained at 50% corn cob composition and the worst at 100%. Nikvash et al. [8] investigated the properties of three crop processing residues, bagasse, rape straw, and industrial hemp straw, for the preparation of 3-layer structured particleboard. Bagasse was purchased from Iran and rape straw and industrial hemp straw were purchased from Germany. Bagasse, rape straw, and industrial hemp straw were processed into 6 mm long shavings and dried to a moisture content of 3–4%. The surface layer shavings were wood shavings made in Germany, and the core layer shavings consisted of bagasse, rape straw, and industrial hemp straw mixed with wood shavings according to a certain proportion, respectively.

Figure 3.

Physical photograph of particleboard.

2.2 Plywood

Plywood is generally made of rotary cut veneer or planed thin wood to adjacent layers of veneer fiber direction perpendicular to the group of blanks by the adhesive gluing into a multi-layer wood-based composite material, with a small coefficient of deformation, excellent mechanical properties, etc. are widely used in construction, packaging, furniture, flooring, car, and shipbuilding industries, the physical photograph of plywood is shown as Figure 4. Over the years, plywood has been one of the leading products in China's wood-based panel industry. With the technological progress and industrial restructuring, the development of plywood industry has entered the key stage of transformation and upgrading, and value-added function has become one of the important ways to increase the added value of plywood, expand its application areas, and enhance the competitiveness of plywood products.

Figure 4.

Physical photograph of plywood.

The flammability of ordinary plywood has limited its application in many fields. At present, flame retardant plywood production methods are mainly immersion method, veneer lamination composite method, and surface coating method. Among them, flame retardant plywood is most commonly prepared by veneer impregnation process (Figure 5), which is mainly to impregnate veneer or plywood with flame retardant components by pressurized (or atmospheric pressure) method. The current research mainly focuses on the development of new environmentally friendly flame retardant with high impregnation efficiency, good flame retardant effect, small impact on mechanical properties and not easy to precipitate. In addition, in order to meet the more demanding practical application environment, the development of flame retardant multifunctional (aldehyde reduction, low smoke, mold, moisture, antibacterial, etc.) plywood is of great significance [9]. Using ammonium dicyanide, phosphoric acid, magnesium sulfate, boric acid (BX), and other compound treatment of plywood, heat release is significantly reduced by 91.9%, and smoke release is reduced by 76.8%. It has a certain anticorrosive and anti-mold function, while the formaldehyde release of the plywood was reduced by 82.3%.

Figure 5.

Preparation of flame retardant plywood by impregnation method.

In addition, electromagnetic shielding plywood is a veneer and electromagnetic shielding materials with electromagnetic shielding material using stacking, mixing, and flexible pressurization and other methods of preparation with the electromagnetic shielding effect of wood-based composite materials. Copper powder, nickel powder, graphite powder, and other conductive powder and iron fiber, copper fiber and other metal conductive fibers added to the adhesive can be used to prepare electromagnetic shielding plywood (Figure 6). The metallic copper fibers within a certain size range can effectively improve the electromagnetic shielding effect by increasing the amount of fiber coating and glue coating, which has some practical value. Increasing the amount of conductive material coating can improve the electromagnetic shielding efficiency but is not conducive to the strength of the glue. Metal conductive fiber is more conducive to improving electromagnetic shielding performance than conductive powder under the same amount of conductive material.

Figure 6.

Preparation of electromagnetic shielding plywood.

Ordinary plywood is susceptible to insect and fungal attack and decay, and its service life and scene are limited. After anticorrosive and anti-insect treatment, plywood has certain anticorrosive, anti-insect, and anti-mold effects, which in turn extends the service life of plywood. The use of impregnation method of horsetail pine and poplar veneer preservative treatment with ammolysis alkylamine copper, borate and different additives compound as preservative, after phenolic or urea-formaldehyde glue gluing can be obtained after a good anticorrosive effect of plywood. It was found that the average drug loading capacity of sound brewing ammonia-soluble alkyl turned horsetail pine and poplar wood preservation plywood was the highest, reaching 7.80 and 9.10 kg/m^3, respectively, the average drug loading capacity of adhesive ammonia-soluble copper vanillylamine horsetail pine and poplar wood preservation plywood was 4.21 and 4.53 kg/m³, respectively, and that of UF adhesive BX poplar wood preservation plywood was 4.96 kg/m³, respectively. The average boron retention rate of the phenolic adhesive glyoxal/propanetriol and BX compounded horsetail pine plywood and the phenolic adhesive glyoxal/propanetriol and borax (BA) compounded poplar preservative plywood were 45.52% and 49.38%, respectively, and the preservative plywood produced under the most favorable conditions could reach the strong corrosion-resistant grade.

2.3 Fiberboard

Fiberboard is an artificial panel made from wood fibers or other vegetal fibers, cured by hot pressing under the bonding action of adhesives; the physical photograph of the fiberboard is shown in Figure 7. Fiberboard has excellent comprehensive performance and is an important part of packaging, indoor furniture, and decorative materials. However, it is easy to burn and produces smoke and toxic gases when burning, which may lead to fire, and the toxic smoke will cause secondary injury to the human body in the fire, which will bring serious harm to people's living environment as well as personal health and property safety. Therefore, it is necessary to choose safe and harmless green flame retardant to modify fiberboard.

Figure 7.

Physical photograph of fiberboard.

Inorganic flame retardants have the advantages of wide source, low price, environmental protection and safety, good flame retardant performance, and small toxic side effects when they play a flame retardant role, etc. They are the most widely used flame retardants at present and gradually become a hot spot of flame retardant research. Inorganic flame retardants mainly include boron flame retardants, phosphorus and nitrogen flame retardants, metal hydroxide flame retardants, and metal oxide flame retardants. Two borates such as BX and BA are the most commonly used flame retardants in the boron family of flame retardants, which have the advantages of low toxicity to humans and environmental friendliness [10]. Borates are widely used in fire protection because they reduce flame propagation [11]. In addition, the combined use of BX and BA has a synergistic flame-retardant effect [12]. However, inorganic flame retardants use the process of moisture absorption and loss and other shortcomings, and a single inorganic flame retardant is difficult to meet the application requirements, the use of the process is often used in a variety of composite, to obtain excellent performance of the flame retardant smoke suppressant.

In order to overcome the shortcomings of inorganic flame retardants, such as moisture absorption and loss, researchers have developed organic flame retardants on the basis of inorganic flame retardants. Organic flame retardants have good compatibility with the base material fiber and excellent anti-loss performance. Organic flame retardants mainly include organophosphorus and nitrogen, organophosphorus and boron, organophosphorus and nitrogen and boron. Due to the high production cost and unstable performance of organic flame retardants and other shortcomings affecting its application, the application of organic flame retardants in fiberboard research has rarely been reported, and more organic flame retardants and inorganic flame retardants composite, the preparation of better performance of the flame retardant.

2.4 Oriented strand board

OSB is a kind of wood structural board made from small diameter timber, mesquite timber, wood core, and other raw materials, after slicing, drying, gluing, oriented paving, hot press molding, and other processes, with high strength, high bending strength, good nail grip, less glue, less formaldehyde emission, anticorrosive, anti-moth-eaten, anti-deformation, heat insulation, sound insulation properties, etc. The physical photograph of OSB is shown in Figure 8. OSB can be used to replace structural plywood in applications. OSB can replace structural plywood in applications, but compared with structural plywood, the process of obtaining structural units through the planing process makes it less demanding on raw materials, so the raw material sources are more extensive. At the same time, in the OSB manufacturing process, the wood utilization rate is high, up to more than 80%, which can efficiently utilize the wood and achieve the purpose of “inferior wood, better use”. With its excellent overall performance, OSB is used as a building panel in Europe and the United States for flooring, wall panels and structural support materials, and is now also used in the wood packaging sector, mainly for packaging pallets and container floors. In China, OSB is mainly used for furniture and interior decoration, commonly used as door frames, shelves, and interior wall panels.

Figure 8.

Physical photograph of OSB.

The main research focuses on the theoretical and experimental studies of various processes in the OSB preparation process, including the influence of particle lay-up on the mechanical properties of the boards, the study of OSB lay-up structure and sectional density, the theory of bonding interface between particles and adhesive in OSB, the modeling study of OSB processing and expansion, and the influence of adhesive and its dosage on the overall performance of OSB [13, 14]. In addition, it also includes the modification of OSB by adding borate, the improvement and optimization of hot pressing process parameters, the optimization of directional paving process, the research and development of OSB adhesive for broadleaf timber, and the improvement of particle production process, etc.

Comprehensive analysis of the current research situation can be found; the current OSB research has achieved great results. Researchers on OSB manufacturing process of raw materials and basic process parameters for a more in-depth study; but at the same time should also be noted that, compared with other wood-based panel products, OSB research is still somewhat insufficient but also need to continue to strengthen the research on the theoretical and practical aspects of the two.

2.5 Cross-laminated timber

In recent years, CLT has gained popularity in Europe and is gradually gaining interest in the rest of the world due to its strength, appearance, versatility and sustainability. The material consists of sawn, glued, and layered wood panels where each layer is perpendicular to the previous one. The layers of wood are joined at a perpendicular angle, allowing the structural stiffness of the panel to be obtained in both directions, similar to plywood, but with thicker components. This gives the panel great tensile and compressive strength. A physical drawing of CLT is shown in Figure 9, and the dimensions of CLT for different applications are shown in Figure 10.

Figure 9.

Physical diagram of CLT.

Figure 10.

Dimension diagram of CLT for different purposes.

CLT is a sustainable material because it is composed of wood, a renewable resource (often from reforestation), and does not require the burning of fossil fuels during its production. It has been used for infrastructure and support on large construction sites, as a form of concrete bridge, and even as a foundation for tractors in unstable terrain during dam construction. Due to its interesting appearance and structural strength, its potential in smaller structures has been noted. Currently, there are even skyscrapers built using CLT parts.

In fact, CLT is not in competition with the existing timber building sector, with its focus on linear timber elements, but a direct competitor of mineral-based solid building materials. This position is expected to be further strengthened. This is due to the fact that local timber species can be sustainably utilized to the benefit of all regions of the world.

When designing CLT structures, it is necessary to consider not only specific knowledge about CLT and joint design but also the whole structure, utilizing integrated knowledge and interdisciplinary thinking.

2.6 Classification of adhesives for EWPs

Wood adhesives can be broadly categorized as petroleum-based or natural adhesives. Petroleum-based adhesives can be further categorized as thermosets, thermoplastics, and elastomers. Natural or bio-based adhesives can be derived from four main sources, namely lignin, proteins, starch, and tannin. Figure 11 shows the classification of wood adhesives and their subclassifications.

Figure 11.

The classification of wood adhesives.

Commonly used petroleum-based adhesives include UF, phenol-formaldehyde (PF), melamine formaldehyde (MF), resorcinol formaldehyde (RF), and isocyanate-based adhesives [15]. However, growing environmental concerns and the increasing depletion of petroleum-based resources have put pressure on the wood composites industry to develop environmentally friendly adhesives using renewable resources [16].