With the use of Paulownia wood as a substrate and a vacuum impregnation method, CaCO3/wood composites were prepared. The XRD results showed that a variety of types of calcium carbonates coexisted in the composite and the elevated temperature was favorable for the aragonite type. The SEM results showed the modified wood retained the original wood structure and that CaCO3 was uniformly distributed in the wood cell cavity and clung to the wood cells. The mechanical testing results showed that the mechanical properties of all the composites were obviously improved; the maximum value of strength of compression (SC) and modulus of elasticity (MOE) of the CaCO3/wood composites were 32.23 MPa and 0.92 GPa, respectively, values 44.2% and 53.3% higher than those of the original wood. The maximum value of proportional limit (PL) of the composites was 29.4 MPa (38.5% increase from original wood). A cone calorimeter was used to investigate the flame retardation properties of CaCO3 on the composites. The value of the heat release rate and total heat release of composites were lower than those of the original wood; the CaCO3 wood composites showed good flame retarding effects.
The Preparation of CaCO3/Wood Composites Using a Chemical Precipitation Method and its Flame-Retardant and Mechanically Beneficial Properties
Lili Huang,a Xiaolin Yao,b,d,* Yongtong Huang,d and Qingsong Wang c,*
With the use of Paulownia wood as a substrate and a vacuum impregnation method, CaCO3/wood composites were prepared. The XRD results showed that a variety of types of calcium carbonates coexisted in the composite and the elevated temperature was favorable for the aragonite type. The SEM results showed the modified wood retained the original wood structure and that CaCO3was uniformly distributed in the wood cell cavity and clung to the wood cells. The mechanical testing results showed that the mechanical properties of all the composites were obviously improved; the maximum value of strength of compression (SC) and modulus of elasticity (MOE) of the CaCO3/wood composites were 32.23 MPa and 0.92 GPa, respectively, values 44.2% and 53.3% higher than those of the original wood. The maximum value of proportional limit (PL) of the composites was 29.4 MPa (38.5% increase from original wood). A cone calorimeter was used to investigate the flame retardation properties of CaCO3 on the composites. The value of the heat release rate and total heat release of composites were lower than those of the original wood; the CaCO3wood composites showed good flame retarding effects.
Keywords: Wood composites; Calcium carbonate; Chemical precipitation method; Mechanical performance; Flame retardant
Contact information: a: School of Engineering, Anhui Agricultural University, Hefei, 230036, P.R. China; b: School of Science, Anhui Agricultural University, Hefei, 230036, P.R. China; c: State Key Laboratory for Fire Science, University of Science and Technology of China, Hefei, 230026, P.R. China; d: Anhui Key Laboratory for Science and Technology of Wood, Anhui Agricultural University, Hefei, 230036, P.R. China; *Corresponding authors:email@example.com; firstname.lastname@example.org
Woods from natural forests usually have good properties in terms of their strength and durability. However, those trees grow slowly, and their harvesting disrupts the sustainability of this high-quality wood supply. However, the gap between wood supply and demand is increasing rapidly. Fast-growing wood is a good alternative to meet the large demand for wood resources, but its inferior quality (lower density, poor mechanical properties, etc.) limits its applications (Kojima et al. 2009). Specific modifications are needed to tailor the inherent physical properties of low quality, fast growing wood (Gaff et al. 2015). Chemical modification is an effective approach for improving the inherent properties of polymers (Worgull et al. 2013). There are abundant mesopores and macropores inside the wood. Also there are active groups, such as OH groups, that associate with inorganic particles and thereby offer opportunities for modification (Merk et al. 2015; Fu et al. 2017). Eco-friendly inorganic materials can enhance the mechanical properties without compromising the beneficial properties (Devi and Maji 2013).
Wood is a macromolecular polymeric material consisting of flammable cellulose, hemicellulose, and lignin, which makes it a safety hazard (Rowell and Dietenberger 2012; Lowden and Hull 2013). The associated health and environmental concerns limit its wide utilization for residential and non-residential buildings (Fu et al.2010; Ren et al. 2015). Therefore, flame-retardant and smoke-suppressant wood products have attracted great public attention (Marney and Russell 2008; Stark et al. 2010; Sharma et al. 2013). The development of wood modification methods that increase the flame retardance and smoke suppression performance of wood without compromising its physical properties is urgently needed (Deka and Maji 2011). In addition to their flame retarding effects, inorganic flame retardants inhibit the generation of smoke and hydrogen halide and have thus been widely used for the preparation of non-toxic and non-corrosive composite materials (Liodakis et al.2010; Liodakis et al. 2013). Additionally, rigid inorganic materials are promising reinforcing materials for polymers (Beyer 2002); the addition of inorganic materials without changing the nature of the original wood can improve the flame retardance, smoke suppression, and mechanical properties of composites (Qu et al. 2005; Jiang et al.2015). CaCO3 is a non-toxic, environmentally friendly, and economical compound that has good compatibility with polymer substrates and endows the substrate with many excellent properties; it has been widely used in plastics and rubber packing (Gutman et al.1995; Gao et al. 2009).
Paulownia trees, which are widely distributed in most provinces of China, could be used as short rotation woody energy crops due to its high heat value of almost 18 MJ/kg (Hidayat et al. 2017). The modification of Paulownia wood heat treatment was found to increase the weight loss and volume shrinkage, but decrease the modulus of rupture and modulus of elasticity (Chen et al. 2017).
In this study, Paulownia wood was used as the substrate to prepare CaCO3/wood composites. A precursor solution was infused into the wood through vacuum impregnation, with the goal of achieving chemical precipitation, which generates CaCO3 inside the wood in situ to develop a new wood composite. The structure and morphology of the composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties of the composites were tested using a universal mechanical testing machine. Cone calorimetry was used to investigate the flame retardation properties of CaCO3 on the composites.
All chemicals were of analytical grade and used as received without further purification. All experiments were conducted under air atmosphere. CaCl2 was purchased from the Guangdong Xilong chemical plant (Guangdong, China). Na2CO3 was purchased from Tianjin Bodi chemical plant (Tianjin, China). Paulownia wood was purchased from Longxi wood plant in Anqing, Anhui, China. Wood specimens were peeled and sawed until 50 mm thick, dried in a ventilated place for 1 month, then machined to the size of 20 mm (radial) by 20 mm (T) by 30 mm (R) for mechanical tests and 100 mm (L) by 100 mm (T) by 10 mm (R) for Cone tests.
Preparation of CaCO3/Wood Composites
Wood specimens were immersed in CaCl2 solutions (concentration: 0.4, 0.8, 1.2, 1.6, and 2.0 M) for 2 h. The concentration of the precursor solution was used to control the CaCO3 content in the wood. The pressure of the solutions were kept at 0.0 to 0.1 atm using a circulating water multi-purpose vacuum pump (SHB-B95, Zhengzhou Changcheng), and kept at atmospheric pressure for 6 h.
The wood specimens were removed and dried at 70 °C for 12 h in a vacuum drying oven (DZF-6020, Shanghai Jinghong). Repeating the above steps, under vacuum pressure, Na2CO3 solutions (the ratio of CaCl2 and Na2CO3 concentration was 1:1) were impregnated into the wood specimens at 30, 60, and 90 °C for 3 h and then kept at 70 °C for 12 h under atmospheric pressure and, finally, dried in a vacuum oven for 48 h. The mode of action of CaCO3 is described in Eq. 1.
Characterization of the Wood Composites
The modified wood specimens were cooled with liquid nitrogen and ground into powder in an agate mortar, then sieved through 180 mesh filters. X-ray powder diffraction (XD-3, Puxi, Beijing, China) was performed with a scan rate of 1 °/min ranging from 10° to 80° (Cu target, tube voltage: 36 kV, tube current: 20 mA, rated power: 1.5 kW, λ = 0.15406 nm).
Particle size and morphology
Wood slice samples of 20 μm were cut through the transverse, square, and tangential sections of the wood composites, using a wheeled slicing machine. The morphology of CaCO3 crystals in the composites were observed using a scanning electron microscope (Sirion 200, EFI, Holland) under an accelerating voltage of 10 kV and a working distance of 15 mm.
Sand paper was used to smooth the wood surfaces. The test specimens were put on the center of the spherical activities support of the universal mechanical testing machine (BTF-266, Bitefu, Changchun, China), pressed uniformly, and damaged in 2 min to test for the strength of compression (SC), modulus of elasticity (MOE), and proportional limit (PL) of the composites.
Cone Calorimeter Test
The combustion performance parameters of the composites were measured with a cone calorimeter (FTT, UK). The experiments of cone calorimeter were based on ISO 5660 standard. The thermal radiation power was 50 kW·m-2 and the corresponding temperature was 780 °C. The heat release rate (HRR), peak heat release rate (pk-HRR) and total heat release (THR) were measured to investigate the effects of CaCO3 on the combustion behavior of the wood composites.
RESULTS AND DISCUSSION
Weight Gain Rate of CaCO3/Wood Composites
Figure 1 is the diagram of the weight gain rate of the CaCO3 wood composites under different CaCl2 concentrations and different preparation temperatures. The weight gain rate of the composites increased with increasing precursor concentrations at different temperatures, but the pattern of increase was nonlinear. With the precursor concentration increasing, more solution infiltrated the wood to yield more CaCO3. When the CaCl2 concentration was 0.4 M at 30, 60, and 90 °C, the corresponding average value of weight gain rates were 4.36%, 15.5%, and 17.7%, respectively. When the CaCl2concentration increased, the weight gain rates of composites increased remarkably, but in the order of 60, 90, and 30 °C. The impregnation effect of the precursor solutions into the wood was the best when the precursor concentration was between 0.8 M and 1.2 M. When the temperature was 90 °C, more CaCO3 was attached to the surface of the wood. This blocked the wood pore accessibility, which hampered Na2CO3 movement into the wood interior and reduced the generation of calcium carbonate, resulting in a lower calcium carbonate content in wood composites.
Fig. 1. Diagram of the weight gain rate of the CaCO3 wood composites under different CaCl2 concentrations and preparation temperature (30, 60, and 90 °C)
Figure 2 shows XRD images of CaCO3/wood composite materials prepared with different CaCl2 concentrations and different temperatures. Figure 2(a) shows composites prepared with solutions of CaCl2 with concentrations: 0.4, 1.2, and 2.0 M at 30 °C. Figure 2(b) shows composites prepared at different temperatures (30, 60, and 90 °C) and a CaCl2 concentration of 2.0 M. When 2θ was 16.5° and 22.5°, there were characteristic diffraction peaks of the wood cellulose.
Fig. 2. Diagrams showing XRD results of CaCO3/ wood composites: (a) at different CaCl2 concentrations (0.4, 1.2, and 2.0 M at 30 °C) and (b) at different temperatures (2.0 M at 30, 60, and 90°C)
There were obvious characteristic diffraction peaks of calcium carbonate (2θ being 29.40°, 47.79°, and 48.50° for calcite and 26.21 and 37.27 for aragonite) (Wu et al. 2018; Yaseen et al. 2018). Thus, there were a variety of crystal types, such as calcite, aragonite, and vaterite, which coexisted in the composite.
Fig. 3. Scanning electron microscope images of transverse sections of CaCO3/wood composite at different preparation temperature. (a, b) original material, (c, d) composite at 30 °C, (e, f) composite at 60 °C, and (g, h) composite at 90 °C
With increases of the CaCO3 content in composites, the characteristic peaks of the cellulose were weakened gradually due to the high precursor concentration reducing the crystallinity of cellulose. As shown in Fig. 2a, with the increase of CaCl2 concentration, crystal types of calcium carbonate did not change obviously; they remained primarily calcite. Additionally, calcite content was high at low CaCl2concentrations. As the concentration of CaCl2 was increased, the characteristics of the diffraction peaks intensity of calcium carbonate increased, especially for aragonite types. As shown in Fig. 2b, increasing the preparation temperature caused the crystal forms of calcium carbonate to change. At 30 °C, the calcium carbonate in composites was mainly composed of the calcite crystal shape. When the temperature rose, aragonite calcium carbonate content gradually increased; higher temperatures were beneficial to the formation of aragonite type of calcium carbonate. Calcite is the thermodynamically stable phase of CaCO3 among of the polymorphs.
Figure 3 shows the SEM images of the transverse section of original wood and CaCO3/wood composites prepared at 30, 60, and 90 °C. Figures 3a and 3b are SEM images of original wood, which show there were ample spaces in the wood fibers on the cell wall of the wood to hold particles of calcium carbonate. Figures 3c and 3d are SEM images of composites prepared at 30 °C. Calcium carbonate particles were distributed uniformly in the wooden fiber cell, were 1 to 2 μm in size, and were mainly rhombohedral shaped. CaCO3adhered to the cell wall tightly and were present in wood cell inner cavities and intercellular space. The flame retardant was distributed evenly through the entire cell wall, and numerous, minute granules were attached to the inner cell wall. The penetration of flame retardant into the cell walls could improve the dimensional stability of wood and flame retardancy.
When the temperature increased to 60 °C (Fig. 3e and 3f), granular shaped CaCO3 particles, 100 nm in size, were found dispersed on the surface of the lumen of the wood fibers and closely arranged together. Interestingly, at a preparation temperature of 90 °C (Fig. 3g and 3h), calcium carbonate particles with a column-like morphology, 100 nm in diameter and 200 to 500 nm in length, appeared on the surface of the cell wall and some of them were embedded into the interior of the cell wall. As is known, the column is a typical shape of aragonite.
Mechanical properties (SC, MOE, and PL) of CaCO3/wood composite materials prepared at different precursor solution concentrations (0.4, 0.8, 1.2, 1.6, and 2.0 M) and different temperature (30, 60, and 90 °C) are listed in Table 1. The SC, MOE, and PL of the original wood were 22.4 MPa, 0.60 GPa, and 21.2 MPa, respectively.
As shown in Table 1, the values of SC, MOE, and PL of all CaCO3composites were higher than those for the original wood. The SC and PL values of the composites prepared at 30 °C increased obviously with the increase of precursor concentrations. When CaCl2concentration was 2.0 M, the SC and PL values of the composites were 31.1 MPa and 27.7 MPa, respectively. When the preparation temperature was 60 °C and 90 °C, the values of SC and PL at low precursor concentrations (0.4, 0.8, 1.2 M) were higher than the values obtained at high precursor concentrations (1.6, 2.0 M). Furthermore, the values of MOE of the composites at 30 °C and 90 °C preparation temperatures were higher than the values at a 60 °C preparation temperature. The maximum values of SC and MOE of the CaCO3/wood composites were 32.2 MPa and 0.92 GPa, respectively, when the CaCl2 concentration was 0.8 M at 90 °C and 30 °C. Compared with the original wood, these were increases of 44.2% and 53.3%. The maximum value of PL of the composites was 29.41 MPa when precursor concentration was 1.2 M at 90 °C, an increase of 38.5% as compared with the original wood.
Table 1. Mechanical Performance of CaCO3 / Wood Composites
Cone Calorimetric Analysis
The combustion phenomena of the original wood and wood composites were obviously different. The combustion process of the unmodified wood was intense, with a large flame and a large amount of black smoke generated. A layer of carbon was formed when the composites began to burn and remained until the combustion ended. Only a small flame occurred on the surface of the composites, and it burned slowly; only white smoke appeared. The CaCO3 inside the wood did not take part in the reactions but changed the kinetics of the reactions that took place during the thermal degradation of wood. The formation of a char layer on the surface of the wood was observed during the combustion. Figure 4a is the picture of original wood after combustion and Fig. 4b is that of the CaCO3/wood composite. The original wood was easy to combust and it can be seen from Fig. 4a the combustion residue was grey. This showed that organic substance in original wood combusted completely and the gray ash left was the product of incombustible inorganic substances.
Figure 4b shows that the combustion residue of composites was black. When the modified wood burnt, the combustion of organic substance was suppressed by CaCO3, and the combustion was not complete with a layer of carbon left.