ES Materials & Manufacturing, 2020, 8, 46-53
Published online: 31 May 2020
Received 27 Apr 2020, Accepted 31 May 2020
Rajib Das,1,2 Sravanthi Vupputuri,2 Qian Hu,2 Yun Chen,2 Henry Colorado,3 Zhanhu Guo2 and Zhe Wang,4,*
1 Process Engineer III, OXEA Corporation, Bay City, Texas 77414 USA
2 Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37997, USA
3 CCComposites Laboratory, Universidad de Antioquia UdeA, Calle 70 N°. 52-21, Medellín, Colombia.
4 Chemistry Department, Xavier University of Louisiana, New Orleans 70125, USA
Email: [email protected] (Z. Wang)
Polymer composites play a significant role in developing flame retardants to prevent fire accidents. The current work aims at investigating the flame retardancy of vinyl ester resins (VER) reinforced with nanotitania (nano-TiO2) nanofillers. The surface functionality of nano-TiO2 was modified by adding Si and N to improve its flame retardancy. The chemical structure and thermal stability of nanocomposites were studied using Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Peaks of Si and N2 in the modified TiO2-VER nanocomposite and weight loss of the modified composite confirmed the addition of Si and N2. The tensile strength results showed that the modified TiO2-VER nanocomposites didn’t make any significant impact on the tensile strength in comparison with pure VER. The flammability and thermal stability behaviors of these nanocomposites were evaluated using microscale combustion calorimetry (MCC). At high percent loadings of nanofiller, the normalized heat release capacity (HRC) of modified TiO2-VER nanocomposites was decreased by 27.7%, whereas the HRC of unmodified TiO2-VER nanocomposites was only reduced by 9.8%. The normalized total heat release of modified TiO2-VER was found to be 21.4%, whereas the unmodified TiO2-VER was 12.4%.
Table of Content
This work reported the significant reinforcement of flame retardancy of vinyl ester resins (VER) by adding the nanotitania (nano-TiO2) nanofiller.
Owing to multipurpose use of different anti-flammable materials in industry and household, the studies around flame retardants have become one of the most promising research fields in science and engineering.[1,2] Halogens, phosphorus, nitrogen, and inorganic compounds are the most commonly used flame retardants. However, due to the long-term ecological problems associated with the halogen-based materials, these materials are no longer used.[4,5] After the 20th century, carbon-based materials[6,7] and nanoclays have generated considerable interests in fabricating nanocomposites with significantly improved mechanical, thermal, and flame-retardant performance. Researchers have studied the thermal and mechanical properties and the particle size effect on the flame-retardant performance. For example, Zhang et al. have shown that the fire retardant properties of Mg(OH)2 filled rubber nanocomposites was better than microcomposites. Chen et al. investigated the effect of silica loadings on thermal and mechanical properties of the silica/epoxy resin nanocomposites. Mechanical testing demonstrates a 30% enhancement in toughness at lower loadings. However, as the loading was increased, the samples continued to increase in modulus but decreased in strength and fracture toughness.
In addition, the epoxy resin nanocomposites reinforced with silica nanoparticles have gained more and more attention due to high stability of silica at high temperatures and in strong acid/base environments and strong adhesion between silica and epoxy matrix.[11,12] As the designing of fire retardants to minimize fire hazards becomes an urgent issue, polymers have received increasing interests due to their use in a wide varity of applications, where specific mechanical, thermal, and electrical properties are required. However, to make a material anti-flammable, it is necessary to either mix the material with a base material (additive flame retardants) or chemically bonded to it (reactive flame retardants). Vinyl ester resins (VERs) are thermosetting resins consisting of a multi-methacrylate oligomer derived from epoxy (typically bisphenol-A based) with styrene as reactive diluents. The development of VERs has attracted more interest due to their unique physicochemical properties, which combine the mechanical and thermal properties of epoxy resins with the rapid cure of unsaturated polyester resins. VER has superior mechanical properties together with high moisture and chemical resistance. In polymer research, it has been used everywhere to fabricate high-quality nanocomposites filled with organic or inorganic nanoparticles.
On the other hand, titanium dioxide (TiO2) is one of the most attractive inorganic material with high capacity, high safety, and low cost. Nanocrystalline titanium dioxide (nano-TiO2) or nanotitania has been presented as an environmentally friendly additive that improved the decomposition temperatures and flame-retardant properties. Nano-TiO2 is very attractive in practical applications because of iits advantages such as permanent stability under UV exposure and chemical stability at high temperatures. In the current work, the VER nanocomposites reinforced with Si and N surface-modified TiO2 nanoparticles were prepared. Multi-amino silane was used as a coupling agent to functionalize the surface of nano-TiO2. Two types of nanocomposites were prepared; (i) amino- modified TiO2- VER nanocomposite, and (ii) unmodified TiO2-pure VER nanocomposite. Compared to traditional flame retardant materials, newly prepared material contains unique structures to improve both the thermal and mechanical properties. The modified nanoparticle and the as-received nanoparticle-based nanocomposite were named as AMT and UMT, respectively. UMT and pure-VER were used as comparator groups. The chemical structure and thermal stability of nanocomposites were studied using Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The mechanical properties, including dynamic mechanical analysis (DMA) and tensile tests of these nanocomposites, were studied. The ﬂame- retardant behaviors of these nanocomposites were investigated using microscale combustion calorimetry (MCC).
The Derakane™ momentum 411–350 epoxy vinyl ester resin (VER) (manufactured by the Ashland Inc.), which is a mixture of 55 wt% epoxy vinyl ester with an average molecular weight of 970 g/mole and 45 wt% styrene monomers, was used as a polymeric matrix. Epoxy vinyl ester monomers with two reactive acryl end groups enable the cross-linking for network formation and styrene monomers with only one vinyl group serve as a solvent and provide the linear chain extension. The liquid resin has a density of 1.045 g/cm3 and a viscosity of 350 centipoises (cps) at room temperature. The curing agent, methyl ethyl ketone peroxide (MEKP), was purchased from Miller-Stephenson Chemical Company. Isopropanol (99.5+%) was purchased from Fisher Scientific.
TiO2 nanoparticles with an average diameter of 30–40 nm (99% purity, 89 vol% anatase + 11 vol% rutile) were obtained from Nanostructured & Amorphous Materials Inc., USA. Four different silanes were used as coupling agents for nanoparticle surface functionalization. 3-[2-(2- aminoethylamino)ethylamino]propyl-tri methoxysilane, (3- methacryloxy) propyltrimethoxy-silane (98%) and (3Chloropropyl) tri methoxysilane (98+%) were purchased from Fisher Scientific, and (3-Mercaptopropyl) tri methoxysilane was purchased from Sigma Aldrich. All chemicals except nanoparticles were used as received without any further treatment.
Isopropanol (80 mL), silane (7 g), and deionized water (0.7 g) were added to a three-necked flask equipped with a condenser and mechanically stirred for 20 min at room temperature. Later, 10 g of nanoparticles diluted in 120 mL of isopropanol were added to the flask under stirring. The mixed suspension was then heated to 80 °C and refluxed under mechanical stirring for 3 h. After the reaction, the mixture was cooled to room temperature, and the modified particles were collected by vacuum filtration, washed with isopropanol to remove any unreacted reactants, dried under vacuum at 50 °C overnight, and then grinded for further application.
The modified and unmodified TiO2 nanoparticles of 1, 3, 5, 7, and 10 wt% were added to VER and stored in the ice-water bath under ultrasonication for forced wetting at room temperature overnight without any disturbance. The curing agent MEKP was added to the VER or above prepared TiO2 nanoparticle-VER nanosuspension with a resin to curing agent weight ratio of 100:1. The mixture was stirred at room temperature for 5 min, and then poured into silicone rubber molds and cured isothermally at 40 °C for 4 h. After that, the temperature was raised to 85 °C for 30 min to accomplish the post-curing process. Finally, the samples were cooled down naturally to room temperature before being trimmed and grinded for further characterizations.
Dynamic rheological measurements were performed both isothermally and non-isothermally by using an environmental test chamber aluminum parallel-plate geometry (25 mm in diameter). For the isothermal test, shear storage and loss moduli were obtained by operating the measurement isothermally at 20, 40, 60, and 80 ºC with a comprehensive time range between 0 and 700 s at a low strain (1%) in a rheometer (AR 2000ex, TA Instrumental Company). For the non-isothermal test, the shear storage and loss moduli were measured at different temperature ramp rates (0.5, 1, 2, 5 ºC/s).
Dynamic mechanical analyses (DMA) measurements were carried out in the rectangular torsion mode using a AR 2000ex (TA Instrumental Company) with a strain of 0.05%, a constant frequency of 1 Hz and a heating rate of 2 ºC/min in the temperature range of 30–200 ºC. The sample dimensions were 12×3×40 mm3. Tensile tests were carried out following ASTM (Standard D 412-98a, 2002) in a unidirectional tensile testing machine (ADMET tensile strength testing system). The parameters (displacement and load) were controlled by a digital controller (MTEST Quattro) with MTEST Quattro Materials Testing Software. The samples were prepared as described for the nanocomposite fabrication in silicone rubber molds, which were designed according to the standard ASTM requirement. A crosshead speed of 1.0 mm/min was used, and the strain (mm/mm) was calculated by dividing the crosshead displacement by the original gauge length.
Microscale combustion calorimetry (MCC) was conducted on 3 ± 1 mg samples using a Govmark Microscale Combustion Calorimeter (Model: MCC-2) operated at a heating rate of 1 °C/s until 750 °C in the pyrolysis zone. The samples were tested according to ASTM guidelines (ASTM D7309−07). The combustion zone was set at 900 °C. The oxygen and nitrogen flow rates were set at 30 and 70 mL/min, respectively. The heat release rate (HRR) in W/g of the sample was calculated from the oxygen depletion measurements. The heat release capacity (HRC) in J/gK was obtained by dividing the sum of peak HRR by the heating rate in K/s. The total heat release (THR) in kJ/g was obtained by integrating the HRR curve. The char yield was obtained by weighing the sample before and after the test.
The chemical structures of the modified and unmodified TiO2-VER nanocomposites were analyzed by Fourier transform infrared spectroscopy (FT-IR, a Bruker Inc. Vector 22 coupled with an ATR accessory) in the range of 500−4000 cm-1 with a resolution of 4 cm-1. The thermal stability was studied in thermogravimetric analysis (TGA, TA Instruments, Q-500) with a heating rate of 10 °C min-1 under an air and nitrogen ﬂow rate of 60 mL min-1 from 25 to 800 °C, respectively.
In current work, FTIR was conducted to study the interaction between the organic group and TiO2 nanoparticles. From the FTIR spectrum in Fig. 1, we see that the amine group addition to TiO2 nanoparticles showed a peak. From the curve of pure amino silane and unmodified TiO2 nanoparticles, it is observed that –CH=CH-(ethylene group) and -NH-(amine) group peaks are present in modified TiO2 nanoparticles.
Fig. 1 FTIR spectra of amino silane, modified (AMT) and unmodified (UMT) TiO2 nanoparticles.
TGA analysis was performed to confirm the presence of an amino group in the modified TiO2 nanoparticles. Fig. 2 shows that the weight loss in the amino-modified TiO2 nanoparticles was 2.75 %. In contrast, the weight loss of 1.25 % in the unmodified TiO2 nanoparticles was almost half compared to the modified filler. The weight loss in the modified nanoparticles took place in two stages. The first stage was from 100 to 300 °C, and the second stage was from 300 to 500 °C. In the first stage, the heat loss was due to the water loss from the amino-modified TiO2 nanoparticles, and the second stage was due to the organic group decomposition. The reason for the more significant amount of weight loss in amino-modified TiO2 nanoparticles was due to the loss of water and decomposition of the organic group upon heating.
Fig. 2 TGA graph of amino-modified (AMT) and unmodified (UMT) fillers.
Dynamic mechanical analyses (DMA) gives the information on the storage modulus (G¢), loss modulus (G²), and loss factor (tan d) of nanocomposites in the test temperature range. The G¢ and G² reflect the elastic property and viscosity behavior of nanocomposites, respectively. Fig. 3(a-c) and 4(a-c) show the dynamic mechanical behaviors of VER reinforced with different loadings of unmodified and modified TiO2, respectively. At 7% loading in glass plateau (below 75 °C), the G¢ values of unmodified and modified TiO2 -VER composites were almost the same but a little bit lower while the loading was 10%. In the glass transition range (6-120 °C), the G¢ curve of nanocomposites at less than 5 % loading shows a shoulder peak, which is attributed to the additional curing of VER because of the increased molecular mobility at high temperatures. As the loading increases, the shoulder peak decays gradually and vanishes when the loading is 10%. The TiO2 nanoparticles restrict the molecular mobility of VERs, thus decreasing the additional curing of nanocomposites.
Fig. 3(b) and 4(b) show the G² curves of VER nanocomposites reinforced by unmodified and modified TiO2 nanoparticles, respectively. It has been observed that all the G² curves of pure VER and unmodified TiO2-VER nanocomposites show two peaks (or shoulder peaks). The first peak is between the temperature range of 83 to 91 °C, which represents the start of the glass transition temperature of samples. The second peaks (or shoulder peak) above 100 °C were due to the friction between enlarged polymer chains caused by the additional curing of VERs.[22,23] This peak was shifted to the higher temperature as the loading of the TiO2 nanoparticles increased i.e., 105 °C for pure VER to 120 °C for VER at the samples with 10% loading of TiO2 nanoparticles. It has been noticed that the G² curves of modified TiO2-VER nanocomposites is similar to unmodified ones except that the two peaks merge into one at higher loadings, and move to a lower temperature. This phenomenon implies that modified TiO2 nanoparticles have increased the stiffness and constriction between the epoxy polymer chains that restrict the mobility at higher loadings.
Tand is the ratio of loss modulus to the storage modulus, and the peak of tan d is often used to determine the glass transition temperature, Tg. Tan d curves of unmodified and modified TiO2-VER nanocomposites are shown in Fig. 3(c) and 4(c), respectively. Pure VER has two tand peaks (i.e., 95 and 115 °C), which are due to the additional curing of VERs. As mentioned earlier, with an increase in temperature, the samples first enter the glass transition range, and with further increase in temperature, the additional curation becomes complete, and the samples enter into the second glass transition again. When the unmodified TiO2 nanoparticles are added to VERs, the further curing has been restricted. Therefore, the two peaks of tan d curves merged into one gradually and moved to the lower temperature at higher loadings. Tand curves of modified TiO2-VER nanocomposites show a similar tendency, except that the peak temperature at higher loadings moved to a much lower temperature (75 °C), which also confirms that modified nanofillers have a strong influence on the cured structured of VERs.
Fig. 3(a) Storage modulus (G¢) vs. temperature curves of unmodified TiO2-VER nanocomposites.
Fig. 3(b) Loss modulus (G″) vs. temperature curves of unmodified TiO2-VER nanocomposites.
Fig. 3(c) Tan δ vs. temperature curves of unmodified TiO2-VER nanocomposites.
Fig. 4(a) Storage modulus (G′) vs. temperature curves of modified TiO2-VER nanocomposites.
Fig. 4(b) Loss modulus (G″) vs. temperature curves of modified TiO2-VER nanocomposites.