Summary of Chemistry doctoral thesis: Manufacture, investigate the properties and morphology of composite material based on glass fiber E and nanosilica-reinforced epoxy resin

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Manufacture composite based on epoxy reinforced by glass fiber with organ – modified nanosilica using organotitanate curing agent that have good mechanical strength, thermal stability, flame restraint ability; improve the toughness of epoxy resin by incorporating reinforcing agent such as organ – modified nanosilica and glass fiber with appropriate manufacturing condition and ratio.

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Nội dung Text: Summary of Chemistry doctoral thesis: Manufacture, investigate the properties and morphology of composite material based on glass fiber E and nanosilica-reinforced epoxy resin

MINISTRY OF VIETNAM ACADEMY EDUCATION AND OF SCIENCE AND TRAINING TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ---------------------------- Ho Ngoc Minh PROJECT NAME: MANUFACTURE, INVESTIGATE THE PROPERTIES AND MORPHOLOGY OF COMPOSITE MATERIAL BASED ON GLASS FIBER E AND NANOSILICA-REINFORCED EPOXYRESIN Major: Theoretical Chemistry and Physical Chemistry Code: 9 44 01 19 SUMMARY OF CHEMISTRY DOCTORAL THESIS Ha Noi - 2019
The thesis was completed in : Graduate University Science and Technology/ Vietnam Academy of Science and Technology. Supervisors: 1) Assist. Prof. PhD. Tran Thi Thanh Van 2) Prof. PhD. Thai Hoang Reviewer 1: …………………………. Reviewer 2: …………………………. Reviewer 3: …………………………. The thesis will be defended at the doctoral thesis committee at the Academy level, meeting at the Graduate University of Science and Technology - Vietnam Academy of Science and Technology on ....’, date … month … year The thesis can be found at: - Library of Graduate University of Science and Technology - Vietnam National Library
LIST OF PUBLICATIONS 1. Ngoc Minh Ho, Thi Thanh Van Tran, Thuy Chinh Nguyen, Hoang Thai "Characteristics and morphology of nanosilica modified with isopropyl tri (dioctyl phosphate) titanate coupling agent", Journal of Nanoscience and Nanotechnology Vol18, No 5, 2018, pp. 3624- 3630(7). (ISI) 2. Ngoc Minh Ho, Thi Thanh Van Tran, Thuy Chinh Nguyen, Hoang Thai, Effect of surface-modified nanosilica on the characteristics, poroperties and morphology of silica/epoxy nanocomposites, The 6th Asian Symposium on Advanced Materials: Chemistry, Physics & Biomedicine of Functional and Novel Materials (ASAM6), pp 343- 348, 2017. 3. Ngoc Minh Ho, Thi Thanh Van Tran, Thuy Chinh Nguyen, Hoang Thai, Epoxy/titanate modified nanosilica composites: morphology, mechanical properties and fracture toughness. Tạp chí Khoa học Công nghệ, 56 (2A),133-140, 2018. 4. Hồ Ngọc Minh, Trần Thị Thanh Vân, Nguyễn Thúy Chinh, Thái Hoàng, Chế tạo, nghiên cứu đặc trưng, tính chất của nhựa epoxy đóng rắn bằng hợp chất cơ titan và một số hợp chất amin, Tạp chí Hóa học, 56(3), 401-406 (2018). 5. Ngoc Minh Ho, Thi Thanh Van Tran, Thuy Chinh Nguyen, Hoang Thai, Epoxy-silica nanocomposite: Creep resitance and toughening mechanisms, Emerging Polymer Technologies Summit (EPTS) and Emerging Material Technologies Summit 2018 (EPTS/EMTS'18) / 2018. 6. Ngoc Minh Ho, Thi Thanh Van Tran, Thuy Chinh Nguyen, Hoang Thai, Ternary nanocomposites based on epoxy, modified silica, and tetrabutyl titanate: Morphology, characteristics, and kinetics of the curing process, Inc. J. Appl. Polym. Sci. 2019, 136, 47412. (ISI)
INTRODUCTION 1. Necessity of the thesis Polymer composite based on glass fiber E-reinforced epoxy resin were commonly used in transportation, electronics, mechanical engineering, machine, building, and chemicals … However, the disadvantages and limitations affecting the application of this composite material are their brittleness and poor impact resistance. Therefore, improving toughness / toughening for epoxy resins is very important. Some nanoscale additives have been applied to manufacture epoxy-based composites as products for industries. Among nano additives, surface modification of nanosilica with organic coupling agent is one of the most commonly used for polymers, rubbers, and plastics because it is easy to find, easy to use, relatively cheap Titanium-based hardened epoxy resin can work long time at high temperature. Research on composite materials based on epoxy / glass fiber combined with surface modification of nanosilica by titanate coupling and titanium curing agent is very new, promising to create material system with high mechanical, physical, thermal properties. And good electrical. Therefore, the PhD student chooses the thesis topic "Manufacture, investigate the properties and morphology of composite material based on glass fiber E and nanosilica-reinforced epoxy resin”. 2. Purpose of the thesis Manufacture composite based on epoxy reinforced by glass fiber with organ – modified nanosilica using organotitanate curing agent that have good mechanical strength, thermal stability, flame restraint ability. Improve the toughness of epoxy resin by incorporating reinforcing agent such as organ – modified nanosilica and glass fiber with appropriate manufacturing condition and ratio. 3. The main research content of the thesis Study on grafting nanosilica surface with KR-12 titanate coupling agent to enhance their dispersion ability in epoxy resin. 1
Study on curing reaction of epoxy resin YD-128 by tetrabutyl titanate and properties of post-hardening products. Manufacture, investigate the properties and morphology of nanocomposite material based on epoxy, nanosilica and tetrabutyl titanate. Manufacture, investigate the properties and morphology of composite material based on glass fiber E and nanosilica-reinforced epoxy resin. 4. New contributions of the thesis Successfully grafted K200 nanosilica particle surface with KR- 12 titanate coupling agent. Nanosilica nanoparticles after modified have good dispersion ability in epoxy resin. We studied the curing reaction of YD-128 epoxy resin with tetrabutyl titannate and clarified the advantages of this curing agent compared to conventional amine compounds Explained the positive effect of KR-12/nanosilica to mechanical properties, dynamic mechanical properties, toughness, toughness, toughening mechanism of composite material based on epoxy/m- nanosilica/ tetrabutyltitanate and glass fiber. CHAPTER 1. OVERVIEW OF EPOXY RESIN, NANOCOMPOZIT AND COMPOZIT MATERIALS BASED ON EPOXY/ NANOSILICA / GLASS FIBER This chapter present the following: 1. Epoxy resin: Classification, physical and chemical properties. Curing agents, curing mechanisms and applications of epoxy in various fields. 2. Nanosilica: introduction about composition, properties, structure, applications of nanosilica in industry and surface modification methods to increase their dispersion ability in plastic matrix. 3. Introduction about polymer composites, epoxy resin, reinforcements and some parameters affected on the durability of materials. 2
4. Domestic and foreign research situation and application of composite materials based on epoxy / nanosilica / fiberglass. CHAPTER 2. EXPERIMENTAL 2.2. Methods 2.2.1. Determine coupling efficiency of KR-12 on nanosilica K200 Was determined by thermogravimetry: H = (mbt.750 – mbd.750)/ mo where: mbt.750 is the mass of SiO2 after modifying at 750 oC. mbd.750 is the mass of unmodified SiO2 at 750 oC. mo is the mass of initial SiO2. 2.2.2. Determine particle size and Zeta potential Particle size distribution and zeta potential of nanosilica before and after modifying was determined by Zetasizer Nano ZS (Malvern-UK) using laser scattering method. 2.2.3. Determine gel content Gel content of the samples after curing was determined by Soxhlet extraction and calculated using the following formulation: GC = 100. (m1/m0) where: m0 is the mass of initial sample (g); m1 is the mass of the sample after extracting (g); GC: gel content (%). 2.2.4. Viscometry The viscosity was determined on the viscometer Brookfield Model RVT- Series 93412 (American), at 25 oC following the standard DIN 53018. 2.2.5. Transmitted Electronic Microscopy (TEM) TEM image was recorded on JEM1010 of JEOL (Japan). The sample was cut into ultrathin layers having the size of 50÷60 nm by specialized knife Leica Ultracut S microtome, then take TEM image at acceleration voltage of 80 kV. 2.2.6. Field Emission Scanning Electronic Microscopy Was done on high resolution Model HITACHI S-4800, Japan, acceleration voltage of 5 kV. 3
2.2.7. Energy Dispersive X-rays Was determined on Model HORIBA 7593H (England). 2.2.8. Infrared Spectroscopy FT-IR spectrum was recorded by TENSOR II (Brucker) with wave number from 4000 cm-1 to 400 cm-1 at atmospheric temperature. 2.2.9. Thermal Analysis * Thermogravimetry analysis (TGA): Use NETSZSCH STA 409 PC/PG (Germany), in nitrogen and atmosphere, heating rate of 10 o C/min. * Differential Scanning Colorimetry (DSC): Was done on Netsch DSC 204F1, in nitrogen, temperature range 30–300 oC with the heating rate of 5, 10, 15, and 20 oC/min. 2.2.10. Dynamic Mechanical Analysis Was done on DMA-8000 (Perkin Elmer, America) by single bending method, with heating rate of 4 oC/min, temperature range 30-200 oC, vibration frequency 1 Hz. 2.2.11. Determine toughness and destroying energy Fracture toughness of the sample was determined following the standard ASTM D 5045-99 on LLoyd 500 N (England), the stress applied rate of 10 mm/min at room temperature. 2.2.12. Determine bending strength Was determined following ISO 178:2010 on Instron 5582-100 kN (England), bending rate of 5 mm/min. 2.2.13. Determine tensile strength Was determined on Zwick (Germany) following ISO 527-1:2012 with the dragging rate of 5 mm min. 2.2.14. Determine impact resistance Was determined following ASTM D6110 on Ray Ran (America). Each sample was measured six times and take the average. 2.2.15. Determine Interlaminar Fracture Toughness Was determined following ASTM D 5528-01 [85], on Lloyd 500 N (England) with the interlaminar pull off rate of 2 mm/min. 2.2.16. Preparation of the samples 4
2.2.16.1. Modify nanosilica Weigh nanosilica in the beaker, adding toluen and stir thoroughly at the speed 21.000 round/min for 5 minutes, then sonicate the mixture for 10 minutes. Adding slowly KR-12 with different contents (5; 10; 15; 30; 45 % compared to nanosilica) into the system, repeat the process of stirring and sonicating 3 times. Then, the mixture was separated from the solvent by centrifuging with the speed of 7000 round/min, obtaining the gel then using toluen to wash KR-12 that does not react, the process was repeated 3 times then dry to remove toluen at 90 oC for 24 hours. 2.2.16.2. Prepare nanocomposite based on epoxy and m-nanosilica Mix thoroughly epoxy YD-128 and m-nanosilica with different contents by mechanical stirrer, adding curing agent TBuT with the studying ratio, then pour into the mold that has been cleaned and anti-stick. Curing process was done at different temperatures and times then machined to determine mechanical properties (tensile strength, bending strength, impact resistance). 2.2.16.3. Prepare epoxy with different curing agents Weigh epoxy resin and curing agents into beakers, with the composition given in Table 2.1, stir the mixture for 5 minutes then vacated to remove bubble. The mixture was poured into the mold (clean, antistick) curing and determining mechanical stability. Table 2.1. Composition of epoxy resin and curing agents Resin – curing Epoxy Curing Curing condition agent YD128, g agent, g EP-TBuT 100 5-20 3 hours 150 oC 8 hours (25 oC); 10 EP-PEPA 100 20 hours (70 oC) 8 hours (25 oC); 10 EP-TETA 100 10 hours (70 oC) 8 hours (25oC); 10 EP-mPDA 100 10 hours (70 oC) 5
2.2.16.4. Manufacture composite epoxy/m-nanosilica/TBuT/glass fiber m-nanosilica was dispersed in epoxy resin YD128 with the ratio 0÷7 % by weight, then add 15 fraction per weight (pkl) of curing agent TBuT. Glass fiber was dried 100 oC for 3 hours to remove moisture. Epoxy resin or epoxy-nanosilica were prepared as in 2.3.2. Glass fiber was cut into rectangle sheet having the size (150 x 200) mm then put layer by layer in the mold and pour the resin with the different ratios of glass fiber/resin. Distribute the resin to permeate into the fiber by roller and brushes. The samples of composite was then cured at 120 for 3 hours in vacuum dryer. CHAPTER 3. RESULTS AND DISCUSSION 2.1. Determination of coupling efficiency of KR-12 onto nanosilica nanosilica. The reaction of KR-12 with the surface of nanosilica is described in Figure 3.1. Carried out in Toluen Fig. 1. Functionalization of silica nanoparticles with tiatanate agent The result reveals that easy methodology for functionalization of SiO2 nanoparticles with titanate agent KR-12 in toluene solvent. The surface reaction was found to be rapid, less energetic demanded thus 6
less depends on reaction temperature and completes in a short reaction period. The loading amount of titanate was found to be strongly depending in relative concentration of titanate agent. Grafting efficiency was determined via thermal analysis, the appropriate content of KR-12 to modify nanosilica is 15 % in weight After the period of 45 minutes , the efficiency of 13,16%. 3.1.1. Size Distribution Size distribution of nanosilica and modified nanosilica was expressed in Figure 3.2, in which nanosilica modified by 0–15 wt.% of KR-12 correspoding to U-SiO2, SiO2-KR.12 (5), SiO2-KR.12 (10) and SiO2-KR.12 (15), respectively. Before being modified, the size distribution of silica (U-SiO2) was not homogeneous with large particles (the average particle size was found at 656.7 nm (72.7%) and5078 nm (27.3%)) due to the aggregation of nanosilicaparticles during storage. When using titanate coupling agent to modify nanosilica, the size distribution after stirring and sonicating indicated the much smaller size than in the case of unmodified nanosilica. The particle size of nanosilica has a tendency of reduction symmetrical arcording to the amount of titanate coupling agent KR-12 grafted onto nanosilica surface. For nanosilica modified by 5 wt.% of KR-12 (SiO2-KR.12 (5)), the average particle size was 408.8 nm(99.7%) and 4962 nm (0.3%), nanosilica-KR-12 (10), the particle size was decreased to 149.5 nm, and for nanosilica modified by 15 wt.% of KR-12 (SiO2-KR.12 (15)), there was only 1 peak corresponding to size distribution by intensity peaks at 84.58 nm. This demonstrated that the use of titanate coupling agent KR-12 plays important role in increasing the dispersiveness of nanosilica by reacting with hydroxyl groups on the surface to form a polymer layer preventing aggregation of nanosilica. Surface modification followed by stirring and sonicating helps to decrease the size of the particles to the nano scale. 7
3.1.2. Morphology of unmodified nanosilica and modified nanosilica Morphology of nanosilica and nanosilica modified by titanate coupling agent KR-12 were pererformed in Figures 3.2 and 3.3. Figure 3.2 indicated nanosilica particles of solid sphere with the heterogenous size ranging from 20 nm to 30 nm. However, the formation of hydrogen bond between molecules of nanosilica particles has a tendency to aggregate into clusters with the larger size of 600–1000 nm as determined by laser scattering.2 It is the aggregation during storage that limits the application of nanosilica. After being modified by titanate coupling agent KR-12, incorporated with stirring and sonicating, silica nanoparticles had much more smaller size than 100 nm (Fig. 3.3). The agglomeration of nanosilica modified by KR-12 decreases remarkably due to the physical interactions between the nanoparticles is instead of chemical interactions between nanosilica and KR-12. This can be explained due to the surface of nanosilica had been covered by a layer of organic titanate that increased the hydrophobicity as well as decreased the surface energy of nanosilica. Here may be the KR-12 layer thickness on the surface of modified silica is too thin, thus, this can not see morphology of KR-12 on these TEM images. Figure 3.2. TEM image of unmodified nanosilica 8
Figure 3.3. TEM image of modified nanosilica 3.2. Influence of Silica Nanoparticles on Changes in the Physical State and Viscosity of the Epoxy/m-Nanosilica Systems The content of silica nanoparticles had a strong effect on their dispersion in the epoxy matrix, characteristics, and properties of the cured epoxy–nanosilica–TBuT nanocomposites. Table I presents the weights of the components in the epoxy–nanosilica systems and the changes in the physical state and viscosity (at 25 oC) of the epoxy with and without silica nanoparticles. Table 3.1. Viscosities of the Epoxy/nanosilica systems Physical Viscosity Samples Epoxy, g Nanosilica, g state 25 oC, cP Epoxy 100 0 Liquid 53,509 Unmodified nanoslica (u-nanosilica) EP-SiO2 99 1 Gel - Modified nanoslica (m-nanosilica) EP-N1 99 1 Liquid 69,256 EP-N2 98 2 Liquid 145,873 EP-N3 97 3 Liquid 256,923 EP-N4 96 4 Liquid 546,345 EP-N5 95 5 Liquid 803,823 EP-N6 94 6 Gel --- EP-N7 93 7 Gel --- 9
As shown in Table I, epoxy resin containing 1 wt % unmodified silica nanoparticles was formed in the gel state; this led to difficulty in the combination of epoxy, unmodified silica nanoparticles, and hardener to form cured nanocomposites. This phenomenon was due to the fact that the silica nanoparticles had a very high specific surface area (> 200 m2/g) and contained a large number of hydroxyl groups on the surface, which interacted strongly with the hydroxyl and epoxy groups in the epoxy resin. In the case of silica nanoparticles modified by the KR-12 titanate coupling agent (m- nanosilica), the dispersion ability of mnanosilica into the epoxy matrix influenced the viscosity of the epoxy–nanosilica systems. At contents of 1–5 wt % m-nanosilica, the epoxy resin was still in the liquid state. The viscosity of the epoxy–m-nanosilica systems increased rapidly with increasing mnanosilica content and reached a maximum value of 803.823 cP at 5 wt % m-nanosilica. This could be explained by the organic layer grafted onto the surface of the silica nanoparticles, which led to the reduction of interactions between the hydroxyl groups (Si─OH) on the surface of m-nanosilica and hydroxyl and glycidyl groups in the epoxy; 3.3. Study factors affecting on the curing process of YD-128 epoxy resins by TBuT The effect of temperature, time, and curing agent on curing process is assessed through variations in the glass transition temperature and mechanical strength of the sample. The results were shown in Figure 3.4, which has determined the appropriate curing conditions for YD-128 epoxy resins by TBuT curing agent as follows: Curing temperature: 150 oC; time: 180 minutes; Curing content: 15 phr. After solidification, the glass transition temperature of 123.6 oC; flexural strength 88.7 MPa; impact resistance of 19.71 J/m2. 10
Flexural strength, MPa Tg, oC Flexural strength, MPa Temperature, oC Time, min Flexural strength, MPa Tg, oC TBuT content, % TBuT content, % Figure 3.4. Influence of temperature (a), time (b), content of curing agent (c) on mechanical strength and glass transition temperature of epoxy-TBuT system 11
3.4. The effect of nanosilica on the kinetics and properties of the epoxy resin system cured by TBuT 3.4.1. Effect of m-nanosilica on the curing temperature of epoxy- TBuT system The gel content (GC) of the cured epoxy–5 wt % m-silica–TBuT (EP–N5) nanocomposite was used to evaluate the effect of m- nanosilica on the curing of epoxy chains by TBuT in the temperature range 80–180 oC. It was obvious that the GC of the neat epoxy increased rapidly with increasing reaction temperature from 80 to 150 C. This could have been caused by the energy supplied to the curing reaction of the neat epoxy, which was smaller than that at lower temperatures; thus, it was not sufficient for the curing reaction to take place completely. This led to a lower network density and a lower GC of the cured neat epoxy. The GC reached a highest value of 98.9% (near completely) at 150 oC and increased insignificantly at curing temperatures above 150 oC. When 5 wt % m-nanosilica was added into the epoxy resin (EP–N5), the GC of the cured EP–N5 nanocomposite increased rapidly with increasing reactiontemperature from 80 to 120 oC and reached a value of 98.4%. Then, it was nearly constant at a reaction temperature of more than 150 oC. 3.4.2. Active energy and kinetic of curing epoxy and epoxy/m-silica by TBuT The active energy (E) of the curing process of epoxy / TBuT, unmodified nanosilica/epoxy/ TBuT and epoxy/m-silica/TBuT were determined according to Flynn-Wall-Ozawa (3.1) and Kissinger (3.2) equation, from differential scanning calorimetry data. The results are presented in Table 3.2 [ ] ) ) (3.1) 2 p   ( )  ( 2 )= ( ) (3.2) p ) p p  12
Table 3.2. The active energy (E) of the curing process of epoxy/TBuT, unmodified nanosilica/epoxy/ TBuT and epoxy/m- silica/TBuT Samples EFlynn-Wall-Ozawa EKissinger Eave Epoxy/TBuT 69,614 66,171 67,893 Epoxy/5% unmodified 63,3 59,75 61,53 nanosilica/TBuT Epoxy/5% m- nanosilica/TBuT 52,87 48,94 50,91 When nanocomposite system using m-nanosilica, the activation energy of the system is significantly reduced. This may be due to the catalytic effect of nanosilica for the epoxy curing reaction. With unmodified nanosilica, the activation energy of the curing reaction decreased by 4.59 (kJ/mol), on the other hand, m-nanosilica nanosilica showing a significant decrease of the activation energy to 15.02 (kJ/mol). The reason due to the unmodified nanosilica particles have the phenomenon of coherence, so only a part exists in nano size with catalytic effect. In case of m-nanosilica particles, they exist commonly in nano form with an average particle size of about 30 nm, so they have a larger catalytic effect, significantly reducing the activation energy of the curing reaction. 3.4.3. Morphology of nanocomposite materials TEM images show that, when not modified, nanosilica particles are distributed in the aggregate state, with micron size. m-nanosilicas are well dispersed in epoxy resins, the particles exist in the nanoscale with sizes in the range of 30 ÷ 60 nm. When the content of m- nanosilica is greater than 5%, (EP-N7 sample) shows the aggregation of some nanoparticles forming large clusters with the size of about 600 nm, corresponding to the state transition of the sample when not solidified from liquid to gel form. This phenomenon is due to the high concentration of m-nanosilica, the gap between the nanoparticles is narrowed, which increases the interaction between them, resulting in agglomeration and gelatinization. 13
Epoxy-unSiO2 Epoxy EP-N5 EP-N7 Figure 3.5. TEM image of epoxy/ m-nanoslica nanocomposite with different content of m-nanosilica 3.4.3. Effect of m-nanosilica content on tensile and flexural strength of epoxy / m-silica / TBuT nanocomposite materials: Mechanical strength was evaluate by impact strength and flexural strength, these factors can reflect the toughness of a material indirectly. The result was shown in fig 3.6. As can be seen in fig 1 (a) the impact strength of epoxy/silica nanocomposite was significantly increased with the addition of nanosilica particles. As an increase in the nanosilica content to 5.0 wt%, the impact strength reached a maximum value 36.95 kJ.m-2. Similarrly, in fig 1 (b) the flexural strength reached 116.6 MPa when the mass content of nanosilica was 5.0 wt%, which represented increase of 87.47% and 31.45% compared with that of pure epoxy resin. The improved mechanical strength could be attributed to nanosilica particles were dispersed well into epoxy resin and the composite exhibited good interfacial bonding, during the fracture process of nanocomposite, the extener force dissipated to interfacial debonding between the nanosilica and 14
epoxy matrix, otherwise nanosilica particles promoted the generation of shear yielding. Interfacial debonding combine with shear yielding consumed a large amount of energy during deformation then the nanocomposite displayed higher strength. 140 Flexural strength (MPa) 40 120 Impact strength (MPa) 100 30 80 20 60 40 10 20 0 0 0 5 10 0 5 10 Nanosilica content (wt%) Nanosilica content (wt%) Figure 3.6. Impact strength (a) and flexural strength (b) of epoxy/silica nanocomposite 3.4.4 Fracture toughness and fracture energy of nanocompozit epoxy/m-nanosilica/TBuT Fracture toughness is a measure for the ability of a material to resist the growth of pre-existing cracks or flaws. Figure 3.7 and the fracture toughness (KIC), fracture energy (GIC), modulus of elasticity (E), and Poisson’s ratio (µ) of neat epoxy and epoxy/m-nanosilica composites loading different m-nanosilica content. 2,5 700 KIC (MPa.m1/2 ) 2 600 500 1,5 GIC (J/m2 ) 400 1 300 200 0,5 100 0 0 0 5 10 0 5 10 Nanosilica content (wt%) Nanosilica loading (wt%) Figure 3.7. Fracture toughness (KIC), fracture energy (GIC) of neat epoxy (a) and epoxy/m-nanosilica composites 15
In case of neat epoxy, the determined fracture toughness value was 1.06 MPa.m1/2, which correlates well with published literature for epoxy materials [2]. The addition of m-silica nanoparticles into the epoxy matrix causes an increase in fracture toughness (KIC) of the composites and a maximum value of 1.73 MPa.m1/2 at 5.0 wt.% m- nanosilica, which corresponds to a 91.51% increase in fracture toughness, compared with that of neat epoxy. At higher nanosilica content, the enhancement in KIC epoxy/m-nanosilica was diminished and at 7 wt. % m-nanosilica, the KIC of composite was reduced to 1.45 MPa.m1/2. This can be also explained by agglomeration of m- silica nanoparticles, the appearance of agglomerates in epoxy matrix reduced the effective volume fraction of m-silica nanoparticles and net surface area. Therefore, the KIC of epoxy/m-nanosilica composite was reduced. The relationship between elastic modulus (E) and fracture toughness (KIC) of the composites is reflected in the equation: GIC = [(1 - µ2)]/E, where µ is the Poison’s ratio, E value is obtained from the tensile test. The fracture energy (GIC) quantifies the energy required to propagate the crack in the material. Figure 4b indicated the GIC of neat epoxy was 243 J/m2, which typically shows relatively low values of the GIC for brittle polymers. The incorporation of m-silica nanoparticles into the epoxy caused a significant increase in the composite’s GIC up to 660 J/m2 at 5.0 wt.% m-nanosilica, corresponding to 171.6% increase in fracture energy. This improved critical energy release rate for the epoxy/m-nanosilica composites is comparable to that of tough polymers. These results expressed the potency of m-silica nanoparticles in toughening of the epoxy resin. 3.4.5. Effect of nanosilica on fire resistance and fire resistance mechanism of nanocomposite epoxy / m-nanosilica / TBuT The LOI of nanocomposite epoxy / m-nanosilica / TBuT materials depends on nanosilica content as shown in Figure 3.8. The results showed that the LOI value of the material increased 16
gradually with the increase of nanosilica content, the EP-N7 sample had the highest LOI value of 27.4, increasing by 1.21 times compared to the neat epoxy resin. In the presence of m-nanosilica, the material's ability to inhibit combustion has increased significantly. The cause of the increase in LOI is explained by the formation of a nanosilica layer on the combustion surface that prevents the penetration of oxygen into the material Figure 3.8. LOI of epoxy resin and nanocompozit epoxy/m- nanosilica/TBuT SEM image of the nanocomposite and epoxy resin surface in Figure 3.9 shows that there is a tight layer of nanosilica on the surface of the sample after decomposition, the distribution of nanosilica particles is quite even with a size of about 30-80 nm, this layer of material prevents the subsequent permeability of oxygen and heat to decompose the polymers, so that nanocomposite has a LOI value higher than the neat epoxy resin. The aggregation of particles creates a micron-sized structure. EP-N1 EP-N5 17