Phenolic foam (PF), one of the best thermal insulation materials, offers excellent flame retardant properties, as well as low smoke and low toxicity, and is widely used in aviation, construction, industrial pipelines, and transportation [1
]. Nevertheless, large-scale promotion and application are greatly restricted because of its fragility [3
]. In order to reduce the fragility of PF, the toughening modification of PF is imperative. During the preparation process of PF, petroleum based products (such as glass fibers, aramid fibers) are introduced in PF to improve the toughness of PF [6
] on the one hand, and on the other hand, long and flexible molecular chains (such as polyurethane prepolymer [9
], epoxy [3
], cardanol [10
], etc.) are introduced into the molecular structure of PF to reduce fragility. However, there are few reports about toughening modification of phenolic foam using renewable cellulose.
Cellulose is one of the most abundant, renewable, and environmentally friendly natural macromolecular resources, which offers low price and density, high specific strength, degradability, and non-toxicity. Cellulose has become one of the most concerned polymer reinforcing materials [11
]. However, due to its supramolecular structure, cellulose cannot dissolve in water or most organic solvents, which greatly restricts the modification of cellulose [15
]. As a cellulose derivative, cellulose ethers have been proved to be particularly useful as intermediates [15
]. Ethyl cellulose (EC) is a kind of cellulose ether that has been widely used as biomedical or intelligent materials due to its nontoxicity, biocompatibility, and high mechanical strength [16
]. Since EC is not a flame retardant material, the use of flame retardant to modify EC is necessary with the aim to improve the mechanical properties of the composites without reducing its flame retardancy,
9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is an excellent flame retardant, and mainly exerts the fire retardant quenching effect by releasing free PO radicals and terminating the chain reaction of combustion in gas phase [22
]. The DOPO contains very active phosphor hydrogen bonds, which are prone to result in the reaction of nucleophilic addition. Therefore DOPO has attracted extensive attention in the field of flame retardant modified polymers [23
]. Itaconic acid (ITA) is an important renewable unsaturated dicarboxylic acid and is produced via fermentation with starch. ITA consists of one unsaturated bond and two carboxy functionalities. The conjugacy relation between an unsaturated bond and one carboxy endows ITA with a strong reaction capacity. Therefore, ITA has been used to synthesize polymers by addition, esterification, or polymerization reactions, and has widely utilized in the production of synthetic fibers, resins, adhesives, etc. [29
Herein, this work aims to introduce a phosphorus compound into the structure of ITA, followed by the modification of EC. Sequentially, the composite PF is prepared from using the modified EC. DOPO and ITA are chosen for their versatilities in organic synthesis [30
]. It was hypothesized that it could not only improve the mechanical properties of composite PF, but also without reducing the flame retardancy. The structure of DOPO-ITA was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (1
H NMR) spectroscopy. The properties of DOPO-ITA modified EC were measured including molecular structure, microstructure, crystallinity, and thermal stability. The curing kinetics of DIMEC composite phenolic resin was studied by the differential scanning calorimetry (DSC) at different heating rates. The mechanical and fragile properties, flame resistance, and microstructure of composite PFs were investigated as well.
2. Materials and Methods
Phenol (P > 99%), formaldehyde (37 wt %), calcium oxide (CaO), and sodium hydroxide (NaOH) were obtained from Nanjing Chemical Reagent, Ltd. (Nanjing, China). 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was obtained from Shenzhen jinlong chemical technology Co., Ltd. (Shenzhen, China). Itaconic acid (ITA), Deuterium dimethylsulfoxide (d6-DMSO) and ethyl cellulose (EC) were purchased from Aladdin (Shanghai, China). Polysorbate-80, petroleum ether, and Paraformaldehyde (≥95%) were obtained from Sinopharm group Chemical Reagent Co. Ltd. (Shanghai, China). Mixed acid curing agent were obtained from Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry (Nanjing, China).
2.2.1. Synthesis of DOPO-ITA
DOPO (0.13 mol), ITA (0.1 mol), and Xylene (50 mL) were added into a round bottom flask equipped with magnetic stirring. The reaction was performed for 5 h at 125~130 °C under an inert environment (N2). After the temperature decreased to 100 °C, the vacuum filtration was performed and tetrahydrofuran (50 mL) was added and obtained a crude DOPO-ITA. The wash of crude DOPO-ITA with tetrahydrofuran and vacuum filtration was repeated three times, and obtained the white and purified solid (DOPO-ITA). The final DOPO-ITA was obtained after the dry at 40 °C to a constant weight under vacuum.
2.2.2. Preparation of DOPO-ITA Modified EC (DIMEC)
Ethyl cellulose (0.1 mol) and DOPO-ITA (0.05 mol) were added into a round bottom flask equipped with magnetic stirring. Then dimethylformamide (50 mL) and potassium carbonate (0.01 mol) were added. The reaction was performed for 9 h at 120 °C, then the vacuum filtration was performed, and non-reactive materials were removed by extraction. Finally, DIMEC was obtained and dried to a constant weight at 50 °C in a vacuum oven. The yield of DIMEC was about 70.6%. The scheme of DIMEC was shown in Figure 1
2.2.3. Preparation of Composite PFs
The phenolic resin (PR) was synthesized according to the literature [35
]. During the processing of synthesis of PR, DIMEC (5 wt %/P, 10 wt %/P, 15 wt %/P and 20 wt %/P) was introduced in the system of reaction. After the end of the reaction, DIMEC composite PR (DCPR) was obtained. Surfactants (Polysorbate-80, 5%/DCPR), acid curing agents (20%/DCPR) and blowing agents (petroleum ether, 5%/DCPR) were added into the DCPRs and completely mixed, which was then poured into a mold. Phenolic foams were obtained after foaming for 40 min at 70 °C.
FT-IR spectra of DOPO-ITA and DIMEC were monitored by a Fourier transform infrared spectrometer (Nicolet IS10, Madison, WI, USA). 1H NMR spectra were performed on a DRX 500 NMR spectrometer (400 MHz) (Bruker, Karlsruhe, Germany) at room temperature using d6-DMSO as solvent, and tetramethylsilane (TMS) as an internal reference. XRD spectra of DIMEC were collected on a Shimadzu 6000× X-ray diffractometer (Kyoto, Japan). SEM were used to observe the micro-scale morphology of DIMEC and PFs by a Hitachi S3400-Nscanning electron microscope (Tokyo, Japan). Thermogravimetric analysis (TGA) curves were collected by a NETZCSH TG 209 F3 TGA system (Bavaria, Germany) under nitrogen atmosphere. Samples were heated from 35 to 600 °C (DIMEC) and 800 °C (CPFs) at a heating rate of 10 °C/min. DSC spectra were obtained on Diamond DSC (PerkinElmer, Waltham, MA, USA). DSC measurements were performed using freeze-dried samples. Heating rates were 5, 10, 15, and 20 °C/min. The scanning temperature ranged from 25 to 200 °C in flowing nitrogen atmosphere (0.02 L/min). Compression strength, bending strength, and tensile strength were measured according to the standard ISO 844:2014, ISO 1209-1:2012, and ISO 1926-2009, respectively. The test was repeated for 5 times. Limiting oxygen indexes (LOIs) of all samples were obtained at room temperature on a JF-3 LOI instrument (LOI Analysis Instrument Company, Jiangning County, China) according to ISO 4589-1-2017, the number of tests was five.
The structure of DOPO-ITA was confirmed by FT-IR and 1H NMR spectra. The esterification reaction between DOPO-ITA and EC was verified by FT-IR spectra and SEM, and DOPO-ITA was successfully introduced in the molecular structure of EC. Compared with EC, the crystallinity of DIMEC was dramatically decreased and the diffraction peak positions were basically unchanged. Additionally, thermal stability decreased, but the residual carbon (600 °C) increased significantly. With the dosage of DIMEC/P increased, the Ea values of DCPRs were increased gradually and the reaction orders were all non-integers. Compared with PF, the mechanical properties, flame retardancy, and the residual carbon (800 °C) of CPFs were increased. The cell size of CPFs was less, and the cell distribution was relatively regular. By comprehensive analysis, the suitable dosage of DIMEC/P was no more than 15%.