Research Progress of Reinforced Modification of Fast-Growing Wood

Abstract

The public’s requirements for a high-quality residential environment and the general improvement in ecological safety awareness have made renewable resource wood and products more favored by furniture, construction and other industries. However, on the one hand, the supply of natural forests is extremely limited, and on the other hand, the materials of artificial forests have defects such as low surface strength and poor dimensional stability due to a loose fiber structure, which restricts their promotion as an alternative to high-quality wood. In this paper, based on the mechanism of wood reinforcement, several reinforcement techniques, such as impregnation, compaction and surface modification, which are widely used in industry, are briefly introduced. On this basis, the possibility of impregnation as a pretreatment method, surface modification and densification to enhance wood was prospected. It is expected to provide reference for improving the added value of plantation wood and alleviating the contradiction between wood supply and demand.

1. Introduction

As an environmentally friendly renewable material, natural wood products not only give users excellent comfort in color, texture and touch, but also have good processability and a high strength/density ratio. These characteristics make natural wood a preferred material for furniture and construction industries. With the increasing requirements of the public for the residential environment and the development of related industries, the demand of wood materials is gradually increasing. However, the distribution of global forest resources is uneven, and a considerable part of natural forests play an irreplaceable role in atmospheric regulation, soil and water conservation, etc.; many countries (for example, Russia, Canada, Cameroon, etc.) implement close restraint or even a total ban on commercial logging policies of natural forests, coupled with the restrictions and prohibition attitude of wood-exporting countries on the export of some logs, and the contradiction between supply and demand of natural wood is becoming increasingly acute. At the same time, the wide distribution of plantation forests provides sufficient output, and the planted forest area in China alone has reached 7.9 million hm² as of 2018. In such a situation, the use of fast-growing forest wood has become an alternative that is more in line with the needs of all parties. A fast-growing forest, also known as a plantation forest, refers to the formation of seedlings, direct or air seeding, including the formation of the forest after cutting and nurturing, mainly Chinese fir, poplar, eucalyptus and so on. An artificial forest is a kind of vegetation type with obvious human traces; its management purpose is clear, and tree species selection, space allocation and afforestation technical measures are arranged according to people’s requirements [1]. Unlike natural wood, which has to go through a long growth cycle, the growth cycle and rotation period of a fast-growing forest are generally only one to several years. However, there are generally material defects such as low strength, loose fiber structure, poor dimensional stability and poor corrosion resistance, which restrict the wide use of fast-growing forest wood [1,2], not being compatible with expectations as an alternative to natural forest wood.

In order to improve the material defects of fast-growing forest wood and relieve the pressure of wood supply, more and more attention has been paid to the enhanced modification of fast-growing forest wood in recent years [1]. At the same time, based on the commonality of wood materials, researchers consider improving more traditional natural forest modification methods to apply to the wood enhancement of fast-growing forests.

2. Wood Reinforcement Mechanism

Wood is a kind of capillary porous finite dilatant colloid composed of tubular cells with uneven size and low directivity. Tubular cells have cell walls and cell cavities, and the ribbed pores serve as the connecting structure between cell cavities, forming a large capillary system of wood with an average pore diameter of 0.1 μm~4 mm. The cell wall is composed of microcrystals and microfibers, and the pores in the cell wall constitute a microcapillary system with an average pore diameter of less than 0.1 μm. The existence of these two kinds of pores not only provides channels and places for impregnation and filling of modifiers [3], but also serves as the theoretical source of compacted wood [4].

Wood surface modification refers to the use of chemical substances, physical coating or chemical coating and other surface processes to prepare a coating or film on the surface of wood to improve the overall performance of wood or obtain new properties, such as hydrophobic, UV resistance, chemical corrosion resistance, flame retardant, dimensional stability, etc., properties [5].

Often in the actual experiment and production process, a single wood modification method cannot meet the needs, or the use of a modification method will cause inherent defects of the product and the use of contrary requirements, which has spawned the idea of combining the existing more mature enhancement methods to learn from each other. Moreover, with the further normalizing of industry standards and the gradual improvement in users’ requirements for product quality, the adaptability of the single modification method to task-based enhancement has been gradually weakened. More and more experiments on wood processing using several wood enhancement modification methods have been reported.

3. Wood Impregnation Modification

Cellulose, hemicelluloses and lignin are the main components of wood, which provide abundant hydroxyl (-OH) and other active groups for wood. Depending on whether the modifier reacts chemically with these active groups, the impregnation modification methods can be divided into physical methods and chemical methods [6].

Physical impregnation is when the modifier enters the pores through penetration, and is physically filled to increase density and reduce water diffusion, thereby enhancing the dimensional stability of wood. Commonly used inorganic nanoparticles are SiO₂, ZnO, TiO₂, nanoclay and so on [7]. After the modifier enters the pores, it reacts with the active groups of wood including esterification, etherification and acetalization to form a new stable structure to strengthen wood under the conditions of heating and catalysis. Regardless of physical or chemical osmotic modification, the permeability of wood is crucial to obtain enhanced wood with ideal properties [8]. Therefore, scholars have studied the factors affecting the permeability of wood. At present, it is believed that the difference of tree species, the distribution of extracts and water content are the key factors [9]. Methods such as steam treatment [10], boiling treatment [11], microwave pretreatment [12,13], compressed air micro-blasting treatment [14] and biological decomposition treatment [15] were mainly used to improve wood permeability (Figure 1 [16]).

According to the different agents used, it can be divided into organic modification, inorganic modification and organic/inorganic composite modification. Most of the studies on organic modifiers focus on urea–formaldehyde resin (UF) [17,18], phenolic resin (PF) [19,20,21], melamine formaldehyde resin (MF) [22] and isocyanate resin. Inorganic salts, inorganic oxide nanoparticles and natural mineral soil are used as inorganic modifiers [23]. Both organic modification and inorganic modification show great potential for the enhancement of fast-growing wood, but there are some shortcomings such as a high cost of organic modification, toxic substances, poor binding stability of inorganic modification and easy moisture absorption. Therefore, in recent years, researchers have paid more attention to organic/inorganic composite modification [24], for example, the combination of silica sol and acrylate [25], boride and melamine urea formaldehyde resin [26], in order to reduce the impact of the above deficiencies and make the modified wood not only have good mechanical properties and dimensional stability, but also have good performance in water resistance and structural stability.

4. Densification

The loose porous structure of the wood not only provides a channel and a place for the impregnation of the modifier, but also provides a space for the densification of the wood. Compression densification modification refers to plasticizing wood under hot and humid conditions using the principle of wood plasticity, and then compression treatment improves its strength, density, hardness and other mechanical properties [27,28].

For most fast-growing forest wood, the softening treatment result is a prerequisite for the performance of compressed wood [29,30]. According to the mechanism, it can be divided into physical softening (hydrothermal softening treatment and high-frequency microwave heating method, etc.) and chemical softening (chemical agents, such as ammonia, urea, lye treatment). In the early stage, it was generally integrated compression, that is, after the overall softening of the wood, the wood was compressed and densified at a certain compression rate [4]. Due to the large overall compression volume loss and the requirements of some industries on the elastic–plastic core layer of wood, a method that only softens the surface layer of wood and then densifies the surface layer is needed, namely, surface compression [31,32]. Due to the natural non-uniform structure of wood, in order to avoid product failure caused by load concentration during surface compression, the control of water content, compression layer thickness and pressure parameters during surface compression is higher than that of overall compression. Liu et al. [33] carried out a finite element analysis on wood surface densification theory using the cylindrical compression method, and analyzed the optimal surface compression conditions produced by Oguman wood produced in Gabon. When the volume loss was only 12%, the density of the dense layer increased by 82.6% and the hardness increased by 91.7%. Che [34] discussed the optimal conditions for the surface densification of poplar and Pinus radiata using sucrose and dimethylethylene urea (DMDHEU) as effective impregnating components, either alone or in combination.

In order to improve the shortcomings of the traditional densification method of molding resilience and poor stability in a wet environment, some scholars have carried out a two-step process to prepare compressed wood, which first removes part of the lignin and hemicellulose from natural wood by using solution cooking, and then makes microfiber, highly oriented and highly densified wood after compression. Song et al. [35] obtained dense wood by removing lignin and other chemical substances in the boiling aqueous solution of 2.5 M NaOH and 0.4 M Na₂SO₃, and then pressing it for 1 d. The lignin removal rate can be controlled by adjusting the time of immersion in the boiling solution, and the strength, fracture work and density of wood obtained are the largest when the removal rate is 45%. The scratch depth shown in the scratch hardness test is significantly smaller than that of natural wood, and is independent of the removal rate. Li [36] completely removed lignin from the wood and then mechanically compressed it. After treatment, the block turned bright white due to its high reflectivity to the sunlight segment in the spectrum, and various physical properties were improved. The mechanical strength and toughness were 8.7 times and 10.1 times that of natural wood, respectively. In a test of energy consumption patterns for mid-level apartments in 16 North American cities, the average energy saving for new apartments was 20%, and up to 35% for older apartments. Since the distribution of the selected cities basically covers all the climatic zones in North America, the test results are quite representative of the North American continent and are expected to be promoted as an energy-efficient sustainable building material. Wang [37] completed the work of introducing silica and preparing poplar/epoxy veneer laminates, respectively, on the basis of the previous two groups of studies. In addition to the mechanical properties of in situ grown silica wood being better than natural wood, the ignition time is 16 s, the limit oxygen index (LOI) value is 36.7% and the wood has a self-extinguishing tendency. After 5 days of immersion in water, the thickness expansion rate of the laminated wood prepared through the three-layer process is 28.0%, and the humidity of 95% is less than 10%, which shows good dimensional stability.

5. Wood Surface Modification

Wood surface modification refers to the coating or film formation of chemical substances on the surface of wood through impregnation, coating, spraying and other methods. It is a relatively simple and feasible modification method to solve the problems of wood water absorption and expansion, inflammability, photothermal aging, biological erosion and other problems, and can also give the surface of wood some special properties [5].

Surface modification can also be conducted by soaking the solution to form a coating or film, which is different from dip modification in that only the surface is treated with little or no impact on the internal structure, and there is no special requirement for wood permeability. At the same time, several other commonly used surface modification methods have a wide range of use and are more convenient to operate, which makes surface modification more suitable for large-scale production and has greater potential. Surface modification according to the different reagents used, organic polymer coating, inorganic nano/polymer composite coating, inorganic nanoparticle coating and corresponding film can be formed. The research and application of organic polymer coating in wood coating started early, and gradually developed into solvent- and water-based versions according to the different dispersion media. Among them, the water-based coating forms a film through evaporation and drying of water, and the volatiles are more friendly to the environment, and are more favored in indoor application than solvent-based coating. Organic compounds such as polyacrylates [38,39] and polyurethanes [40,41] are more widely used.

The inorganic nano/polymer composite coating is formed by dispersing inorganic nanoparticles into a polymer matrix on the basis of an organic polymer coating. Due to the introduction of inorganic nanoparticles with better mechanical properties, the performance of physical properties such as wear resistance, heat resistance, scratch resistance, toughness and strength is generally better than that of simple polymer coatings [42]. At the same time, some inorganic nanoparticles can replace the copper, chromium and arsenic compounds or polychlorinated and phenolic compounds’ additives often contained in polymer coatings, avoiding the toxic effects of these organic components on organisms. The unique properties of some inorganic nanoparticles have also been used to provide new properties such as hydrophobic, anti-corrosion, anti-aging, and flame retardant properties to wood surfaces [43]. The inorganic nano/polymer composite coating not only reduces the harm to the human body and the environment while playing the same strengthening role as the pure polymer coating, but also can introduce some new characteristics more conveniently to expand the scope of use of wood and its products, which makes scholars pay more attention to the inorganic nano/polymer composite coating than the polymer coating. Liu et al. [44] used epoxy groups to anchor the amino functionalized SiO₂ particles and then modified the wood surface with octadecyl trichlorosilane. In the 120-day water immersion test, the maximum weight gain rate was 5.7%, which was 27 times lower than that of untreated wood (155.1%), and the weight gain rate reached a stable state after only 20 days. When Wu et al. [45]’s method was used to treat the wood surface, the coating surface had a rough hierarchical structure, showing that the contact angle of water, coffee, milk, cola, soy sauce and other common liquids was close to 150° or higher. Tu et al. [46] used the two-step method to build SiO₂/epoxy resin/fluorosilane composite film on the surface of wood, and the test showed that it not only had good hydrophobic (static contact angle of 153° and rolling angle of less than 4°) and oleophobic properties, but also maintained the micro-nanostructure after 10 cycles of the wear resistance test, showing excellent mechanical wear resistance. Zhou et al. [47] used ethylene glycol diglycide as a crosslinking agent to improve the stability of SiO₂ nanoparticles on the surface of polydopamine-modified wood, and the surface not only maintained a surface contact angle of more than 150° after 5 h of ultrasonic shock at 100 w and 42 kHz, but also maintained superhydrophobicity after 24 h of continuous radiation at 100 °C in the thermal radiation test. Using SiO₂ nanoparticles and silicone oil (hydroxyl silicone oil and hydrogen-containing silicone oil) as the main raw materials, Liu et al. [48] constructed a hydrophobic surface of heat-treated wood after vacuum and pressure impregnation, and showed that the water contact angle on the transverse and radial sections of the modified material with a mass fraction of 2.7% was close to or higher than 150°. There was a large amount of the Si element deposited on the surface of wood.

Inorganic nanoparticle coating means that inorganic nanoparticles are chemically bonded to the surface layer of wood or to a certain depth through the precursor solution (such as sol–gel method). Different from the inorganic nanoparticle/polymer composite coating, the inorganic nanoparticle is no longer used as the dispersed phase of the polymer matrix [49,50,51], and the commonly used inorganic nanoparticles include SiO₂, TiO₂, ZnO, Ag, etc. At present, the lack of low recombination and ease of falling off is still a challenge for the development of inorganic nanoparticle coatings.

However, in general, the pore size and hydrogen bond network of wood itself have higher requirements for the entry and fixation of reagents [8], and the heterogeneous structure of wood is also more likely to produce stress concentration and lead to premature failure [33]. Surface modification can minimize the adverse effects of these two points on the enhancement effect, which is a potential modification method.

6. Other Wood Strengthening Methods

There are also some methods to destroy a part of the wood, or no longer retain the original intuitive form of the wood; these methods can also give the wood some specific properties of the more excellent performance to expand its application prospects, but the former will make the wood lose a part of the mechanical properties, and the latter in some cases creates difficulty to meet the expectations as a natural wood alternative. Among them, the latter can improve the utilization rate of large-volume wood products processing residues to a certain extent, and can be used to improve the added value of fast-growing forest wood. It can be simply summarized as recombination enhancement and compound enhancement. Recombination reinforcement refers to the processing of fast-growing wood into different form units (wood strips, veneers, veneer strips, shavings, etc.), the application of composite material theory, recombination of wood units of different forms and the hot pressing of adhesives into products with high strength and good dimensional stability [52,53] (Figure 2). It includes reconstituted wood, orthogonal glulam wood (CLT), veneer laminated wood (LVL), integrated wood, parallel veneer laminated wood, directional particle boards, etc. Composite reinforcement is also required to process fast-growing wood into different form units, and then composites process it with other high-strength materials into new multiphase materials with various properties, applications and structures. Common reinforcement materials for composite reinforcement are fiber (glass fiber, carbon fiber, etc.), inorganic cementing materials (cement, gypsum, etc.), bamboo, metal, etc.

In the actual production process, the use of a single modification method to enhance the wood to more demanding properties in a task-based context is often accompanied by a disproportionate increase in treatment conditions or a sharp increase in the cost of the reagent used. In the case of impregnation modification, it involves the introduction of a reagent with a high unit price and biological hazards. Densification is a strict interval control of water content, pressure parameters and other conditions. The cost of solving the stability problem of the surface modified reinforced layer is far more than the production expectation. The above situation has prompted researchers to explore the process of combining multiple modification methods to give full play to their respective advantages and weaken the limitations and cost margins of a single method. Impregnation modification can use a flexible solution form, and can especially be more convenient to remove adverse components for subsequent treatment and introduce functional groups or special properties of particles, and is more suitable than other modification methods as a pretreatment means to transport the precursor to the specified binding location. In the previous study, Wang [37] used the impregnation process to transport the silica precursor tetraethyl silicate (TEOS) and modifier KH550 into the wood, and then the nano-silica, which grew in situ in the pores of the wood under heating conditions, remained in the winding structure to form an inorganic skeleton when the wood was compressed. It becomes a structure providing high mechanical properties, water resistance and wear resistance. Shu [54] used ionic liquid (IL) Amim[Cl] 1-allyl-3-methylimidazole chloride salt and Bmim[Cl] 1-butyl-3-methylimidazole chloride salt as an impregnating solution to impregnate wood powder, 0.5 mm slices and single boards produced from poplar wood and then hot-press forming, successfully achieving glue-free bonding. No matter which ionic liquid impregnated wood components, the cellulose had a tendency to change to cellulose type II; FTIR spectra showed that no derivative reaction occurred, and 20% PEG6000 + 80% IL had the largest improvement in the shape and ductility of the veneer, and the least water absorption. Song [55] compared the densification of wood treated by boiling, alkaline sulfite solution impregnation (Figure 3) and white rot bacteria biological inoculation under the same hot pressing conditions. The results showed that the mass loss rate of solution impregnation pretreatment was between the other two kinds of pretreatment, and the highest density (1.25 kg/cm³), compression rate (81.58), compressive strength (105.6 MPa) and hardness (5614.3 N) and the lowest moisture absorption thickness increase rate (7.94%) were obtained after pretreatment for 5 h. Due to the material difference, poplar was pretreated for 3 h with the best performance except for the compression ratio. It can be seen that impregnation as a pretreatment method can rely on the free combination of the solvent solute to complete the swelling of cellulose, lignin removal and crystal transformation of cellulose under relatively mild reaction conditions. In the face of increasingly high performance requirements, impregnation pretreatment has broad prospects and needs to be further developed.

7. Conclusions

The current situation of relatively short natural forest resources and rich fast-growing forest resources will continue, while the development of wood-related industries requires sufficient wood supply; scientific and reasonable use of fast-growing forest wood is an inescapable solution to the balance of wood supply and demand, and research on fast-growing forest wood enhancement has not yet fully met the industrial demand. In this paper, wood impregnation modification, compression densification, wood surface modification and other wood enhancement methods are introduced based on the tasks undertaken by the fast-growing forest wood industry and the mechanism of wood enhancement. It can be seen that there are still some problems in these methods, such as the use of toxic reagents, complex and expensive equipment and research limited to the scale of the laboratory, which restrict the development of the fast-growing forest wood industry. According to the differences in tree species and wood use of fast-growing forests, the existing shortcomings of wood enhancement modification methods and the latest understanding of the wood structure [53], targeted improvement or innovative modification methods to enhance wood or functional modification of wood can provide a better technical basis for fast-growing forest wood to replace natural wood, so as to promote the sound development of wood-related industries in the process of growth, and better service for economic and social development.