Abstract
9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) graft γ-amino propyltriethoxy silane (APS), γ-glycidoxy propyltrimethoxy silane (GPS), and methacryloyl propyltrimethoxysilane (MPS) were synthesized and introduced on the surface of wood fiber. DOPO-g-APS (GPS and MPS) treated wood fibers (DAWF, DGWF, and DMWF) were used to prepare composite phenolic foams (DAWFCPF, DGWFCPF, and DMWFCPF). The chemical structures of DOPO-g-APS (GPS and MPS) were confirmed by Fourier transform infrared (FT-IR) spectra and nuclear magnetic resonance. The structures of DAWF, DGWF, and DMWF were confirmed by FT-IR. The diffraction peak positions were basically unchanged, but the crystallinity was slightly increased. Compared with WF, thermal stabilities were increased. The residue (800 °C) was slightly reduced. Compared with phenolic foam, the properties of DAWFCPF and DGWFCPF were better than others. Besides compression strength, the mechanical properties were increased dramatically, limited oxygen index was slightly enhanced, the cellular pore distribution was more regular, and the size of bubble cell was smaller and more uniform. By comprehensive analysis, the properties of DAWFCPF were best in all samples. The interfacial compatibility was significantly improved between DAWF and phenolic resin. These results confirmed that DOPO-g-APS is a suitable material for treating wood fibers and preparing composite phenolic foams.
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Effect of Novel DOPO-g-Coupling Agent Treated Wood Fibers on Properties of Composite Phenolic Foams
Yufeng Ma,* Xiang Geng, and Xi Zhang
9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) graft γ-amino propyltriethoxy silane (APS), γ-glycidoxy propyltrimethoxy silane (GPS), and methacryloyl propyltrimethoxysilane (MPS) were synthesized and introduced on the surface of wood fiber. DOPO-g-APS (GPS and MPS) treated wood fibers (DAWF, DGWF, and DMWF) were used to prepare composite phenolic foams (DAWFCPF, DGWFCPF, and DMWFCPF). The chemical structures of DOPO-g-APS (GPS and MPS) were confirmed by Fourier transform infrared (FT-IR) spectra and nuclear magnetic resonance. The structures of DAWF, DGWF, and DMWF were confirmed by FT-IR. The diffraction peak positions were basically unchanged, but the crystallinity was slightly increased. Compared with WF, thermal stabilities were increased. The residue (800 °C) was slightly reduced. Compared with phenolic foam, the properties of DAWFCPF and DGWFCPF were better than others. Besides compression strength, the mechanical properties were increased dramatically, limited oxygen index was slightly enhanced, the cellular pore distribution was more regular, and the size of bubble cell was smaller and more uniform. By comprehensive analysis, the properties of DAWFCPF were best in all samples. The interfacial compatibility was significantly improved between DAWF and phenolic resin. These results confirmed that DOPO-g-APS is a suitable material for treating wood fibers and preparing composite phenolic foams.
Keywords: DOPO; Wood fiber; Coupling agent; Phenolic foam; Characterization
Contact information: Nanjing Forestry University, College of Materials Science and Engineering, Nanjing 210037; *Corresponding author: mayufeng@njfu.edu.cn
INTRODUCTION
As an excellent retardant material, phenolic foams (PFs) have been widely applied in fields of thermal insulation such as buildings, oil and gas pipelines, and central air conditioning systems (Lei et al. 2010; Yang et al. 2012). However, the greatest defect of phenolic foam is its high brittleness, which greatly limits its large-scale application (Rangari et al. 2007; Ma et al. 2017a). As a kind of environmentally friendly natural material, wood fibers are characterized by low density and relatively high tensile and flexural moduli (Bledzki and Gassan 1999; Canche-Escamilla et al. 1999; Mitra et al. 1998; Rana et al. 1998; Xie et al. 2010), and treating wood fibers with a coupling agent not only can improve the interfacial compatibility between wood fibers and phenolic resin, it can also composite PFs to improve the mechanical properties and toughness of composite PF. The limited oxygen index (LOI) of wood fibers composite PFs are reduced compared to PFs (Ma et al. 2017a,b).
As a halogen-free phosphorous flame retardant, 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) has received notable attention from researchers worldwide due to its advantages of smokeless, non-toxic, non-migration, durable flame retardant properties, and the multiple structural diversification by functionalization (Perret et al. 2011; Zhang et al. 2011; Sun and Yao 2011; Dumitrascu 2012; Wang and Cai 2017). The molecular structure of DOPO contains diphenyl and phenanthrene ring structures. The lateral phosphorous group is a circular O=P-O bond; therefore the thermal stability, chemical stability, and flame resistance of DOPO are higher than the normal organic phosphate ester. DOPO contains six-membered phosphorus heterocyclic structure, and phosphorus atoms have active lone pair electrons, which are prone to nucleophilic addition reaction. The active hydrogen of the P-H bond can be reacted with a variety of electron-deficient derivatives, leading to a wide range of compounds with a phosphaphenanthrene skeleton (Yuxiang and Jianjun 2006; Carja et al. 2014).
This study was devised to investigate the effects of chemical modification of wood fiber with DOPO-g-coupling agents on the properties of wood fibers and their composite phenolic foams. The aim was to improve the comprehensive performance of composite foam without reducing its flame retardancy. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic spectra (1H NMR) of DOPO-g-coupling agents were performed, and FT-IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) of modified wood fiber were determined. The mechanical properties, limited oxygen index (LOI), and microstructure of wood fiber composite phenolic foams were measured.
EXPERIMENTAL
Materials
9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Shenzhen Jinlong Chemical Technology Co., Ltd. (Nanjing, China). Phenolic resin (PR) was obtained from the Institute of Chemical Industry of Forestry Products (CAF; Nanjing, China). Polysorbate-80, petroleum ether, formic acid, and dicumyl peroxide (DCP) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. (Shanhai, China); γ-amino propyltriethoxy silane (APS) was obtained from the United States Carbon Company (Danbury, USA). γ-Glycidoxy propyltrimethoxy silane (GPS) and methacryloyl propyltrimethoxysilane (MPS) were purchased from Dow Corning (Midland, USA). Eucalyptus fiber was obtained from Guangxi Fenglin Wood Industry Group Co., Ltd. (Nanning, China).
Methods
Preparation of DOPO-g-coupling agent
DOPO (0.26 mol) was introduced into a 250 mL four-necked round bottom flask fitted with a stirrer, aerated with nitrogen for 5 min, and heated to 140 °C to 150 °C. After the DOPO melted, a coupling agent (0.1 mol APS, GPS, MPS) was introduced into the reactor, aerated with nitrogen for 5 min, and reacted for 5 h in an anaerobic environment. The temperature was then brought to 120 °C, toluene was added (60 mL), and vacuum suction filtration was performed. The temperature was brought down to 75 °C, tetrahydrofuran was added (50 mL), and vacuum suction filtration was performed. Subsequently, the reaction product (DOPO-g-APS, DOPO-g-GPS, DOPO-g-MPS) was dried to a constant weight in a vacuum drying oven at 50 °C. The yields of DOPO-grafted silane (APS, GPS, and MPS) were 87.1%, 87.5% and 87.7%, respectively.
DOPO-g-coupling agent pretreatment of wood fiber
The composition of coupling agent water solution incorporated 2 wt.% DOPO-g-APS or DOPO-g-GPS or DOPO-g-MPS and 2 wt.% DCP. The pH was adjusted to 3.5 with formic acid. The wood fibers were immersed in the solution for 4 h. Finally, treated wood fibers were filtrated and dried at 60 °C for 24 h.
Fig. 1. Scheme of DOPO-g-GPTS modified wood fiber composite PFs
Table 1. Nomenclature of Wood Fibers Pretreatment
Preparation of wood fiber composite PFs
Surfactants (Polysorbate-80, 5%/PR), acid curing agents (22%/PR), blowing agents (petroleum ether, 5%/PR), wood fibers (6%/PR), and treated wood fibers (6%/PR) were added into phenolic resin and mixed well, then poured into the mold. The PFs were obtained after foaming for 40 min at 70 °C.
Table 2. Nomenclature of Wood Fibers Composite Phenolic Foams
FT-IR measurements
Fourier transform infrared spectra were taken on a Nicolet IS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).
1H NMR Measurements
The 1H NMR spectra were recorded at 400 MHz on a Bruker DRX 500 NMR spectrometer (Karlsruhe, Germany) with tetramethylsilane (TMS) as an internal reference.
X-ray diffraction measurements
All wood fibers were used for X-ray diffraction (Shimadzu 6000 X, Kyoto, Japan) measurements. The measurement parameters were as follows: Cu target, pipe pressure, 40 kV; flow, 40 mA; scanning speed, 4/min; the sampling step width was 0.02; and shaft dynamic continuous scanning was one time, 2θ = 5 to 50.
Scanning electron microscopy of wood fibers and foams
All wood fibers and foams were examined using a Hitachi S3400-N SEM microscope (Kyoto, Japan). Prior to the analysis, the samples were coated with gold to avoid sample charging under the electron beam.
Thermogravimetric analysis
Thermogravimetric analysis was conducted on a NETZCSH TG 209 F3 TGA system (Selb, Germany), ramping from 35 to 800 °C with a rate of 10 °C/min.
Mechanical properties measurements of foams
Compression strength was measured by the standard ISO 844 (2014), and the samples used for the test were 50 × 50 × 50 mm. Bending strength was measured using the standard ISO 1209-1 (2012); the samples used for the test were 120 × 25 × 20 mm. Tensile properties were measured by the standard ISO 1926 (2009). The number of parallel tests was five.
Limited oxygen index (LOI).
The LOI test was carried out with JF-3 oxygen index meter (LOI analysis instrument company, Nanjing, China) according to ISO 4589-1 (2017). The samples used for the test were 100 × 10 × 10 mm3. The test was repeated 5 times.
RESULTS AND DISCUSSION
The structure of DOPO-g-coupling agent
The FTIR spectra of DOPO-g-coupling agent are shown in Fig. 1. The FTIR analysis of DOPO was shown as follows: 3061 cm-1 (C–H in phenyl group), 2436 cm-1 (P-H), 1593 cm-1 and 1603 cm-1 (phenyl group), 1477 cm-1 (P-phenyl), 1236 cm-1 (P=O), 903 cm-1 (P-O-phenyl). In the structures of DOPO-g-coupling agent, a peak of P-H (around 2436 cm-1) disappeared. In addition, the absorptions around 1595 cm−1 and 1606 cm-1 corresponded to the phenyl group, the absorptions around 1238 cm−1 corresponded to P=O, and the absorptions around 906 cm−1 corresponded to P-O-phenyl. Figure 2 shows the 1H NMR spectra of the DOPO-g-coupling agent. In the structure of DOPO, around 6.85 ppm to 8.23 ppm corresponded to the phenyl protons. The phenyl protons also appeared in the DOPO-g-coupling agent structure. These absorptions and protons supported the idea that the reaction occurred between DOPO and the coupling agents (APS, GPS, and MPS), and that the DOPO-g-coupling agent could be successfully synthesized (Min et al. 2016; Ni et al. 2017; Shan et al. 2017; Tang et al. 2017).
Fig. 1. FTIR spectra of DOPO-g-coupling agents
Fig. 2. 1H NMR spectra of DOPO-g-coupling agents
The Properties of Wood Fibers
FTIR
Figure 3 shows the FTIR spectra of wood fibers. The characteristic absorption bands of DAWF, DGWF, and DMWF were enhanced at 2900 cm-1, 1425 cm−1, and 1050 cm-1, which were respectively ascribed to –CH2, P=O, and Si–O. These results indicate that reactions occurred between some of the hydroxyl groups on the surface of wood fiber and the hydroxyl groups of silanol (obtained after hydrolysis of silane), which thus generated an ether bridge, –Si–O–Si–, –Si–O–C– (around 1050 cm-1), and an aliphatic chain (CH2 in coupling agent) was introduced on the surface of fiber (Valadez-Gonzalez et al. 1999; Cui et al. 2008; Tang et al. 2017). These absorptions led to the appearance or the increment of the absorbance in the regions around 2900 cm-1, 1425 cm−1, and 1050 cm-1. These absorptions also suggested that DOPO-g-coupling agents were successfully introduced on the surface of the wood fiber.
Fig. 3. FTIR spectra of wood fibers
Fig. 4. XRD spectra of wood fibers
X-ray diffraction (XRD)
XRD spectra of wood fibers are shown in Fig. 4. There were two diffraction peaks: a sharp high peak (2θ = 22.6°) and a weaker diffraction peak (2θ = 15.6°) (Wang et al. 2007). The wood fibers’ peak positions were basically unchanged, which suggested that the crystal form of treated wood fibers were unchanged. But the treated wood fibers’ peak intensities were only slightly enhanced. The reason for this could be that during the process of pretreatment, wood fibers were immersed in the acidic solution, and so a part of the impurities, the small and soluble molecules on fiber surface were dissolved and removed by suction filtration (Valadez-González et al. 1995; Lu et al. 2005). Hence the cellulose could have a better arrangement, and the crystallinity of wood fiber was slightly increased.
SEM
As shown in Fig. 5, the surface of WF was not smooth, and it was covered with pectin, waxy substances, and impurities (Wang et al. 2007). The surface of DAWF, DGWF, and DMWF was etched and became smooth. The surface of DAWF was the smoothest. A thin layer of material covered the surface of DAWF, DGWF, and DMWF. This could be attributed that during the process of treatment, some of the small and soluble molecule impurities present on the surface of the wood fiber were dissolved and removed by suction filtration (Valadez-Gonzalez et al. 1999). In the meantime, DOPO-g-coupling agents were introduced on the surface of wood fibers, which was advantageous in enhancing the bonding stability between the fiber and polymer matrix (Lu et al. 2005).
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