rus
Issue #7-8/2019
T.P.Sherbakova, I.N.Vaseneva
Modification of epoxy composite material by biogenic silica
Modification of epoxy composite material by biogenic silica
Samples of biogenic silica (SiO2) were obtained with a target component content of 76.4– 99.9% by means of calcination in oxidative ashing and alkaline extraction. Influence of the prepared silica on the heat resistance and mechanical strength of a composite material was studied based on the ED-20 epoxy oligomer cured by iso-methyltetrahydrophthalic (iso- MTHPA) anhydride.
Теги: epoxy composite material epoxy resin filler silica кремнезем наполнитель эпоксидная смола эпоксидный композиционный материал
INTRODUCTION
Amorphous silica has a high chemical activity and it makes the basis for its widespread use as a filler and modifier [1]. Analysis of the global market of synthetic (precipitated and pyrogenous) amorphous silica shows a number of technical descriptions and trademarks with specific physical and chemical parameters, such as "Aerosil", "silica white", etc. [2]. Nevertheless, it is necessary to spend a lot of electrical energy and apply security measures to ensure explosion safety in order to obtain pyrogenous silica (with the content of base substance, SiO2, 99.5% and higher). Usually, the precipitated silica (SiO2 content varies in the range of 60.0–90.0%) and can be obtained from "liquid glass" (Na2SiO3) because of technical simplicity and safety [3]. The production metod determines the properties of the end product, such as dimensions and shapes of the particles, presence or absence of pores and surface properties [2].
The widespread usage of silica as a filler for polymer materials makes it a perspective object of modern scientific research [4–7]. Perspectives of biogenic silica and methods of its extraction from the plant biomass are considered in [8–17]. Russian specialists have developed the production methods of high-purity (up to 99.99%) amorphous silicon dioxide from rice hull with a yield of up to 22.0 weight percentage [2, 8, 10].
The aim of the study was to extract silica from plants of silicophiles (rice hull, bottlebrush) and its study as a filler for the epoxy anhydride polymer.
METHODS OF RESEARCH
The research objects were: silica extracted from rice hull (RH) and bottlebruch (BB) by oxidative ashing and precipitation from alkaline solutions. The component composition of plant samples was studied according to the methods described in the monograph [18].
The lignin content was determined by the sulfuric method modified by Komarov, the cytase content was determined by the Kürschner method (extraction of the plant sample with a nitrogen-alcohol mixture). The total content of silica was determined by melting with alkalies [19].
The studied objects (with variants of the preliminary preparation, such as shredding, extraction with an acid solution, extraction with a sodium hydroxide solution) were subject to ashing in a muffle furnace in a stream of air. The samples of 0.2–0.3 g were heated from 25 to 600–1000°С at the rate of 8°С/min in a dynamic air atmosphere at flow rate about 5 l/min.
Samples were extracted with water and/or 0.1 N hydrochloric acid solution (НСl) at 90°С during one hour followed by filtration and drying [11–12]. Samples were extracted by 2.5 N with sodium hydroxide solution (NaOH) at 90°С during one hour followed by preparation of the alkali-soluble components extract of the plant matrix (silica, lignin, pectic substances, hexosan) and solid residue (cellulose).
Micrographs of the samples were obtained using a Tescan Vega III SBU scanning electron microscope, Tescan, Czech Republic, 2010. Qualitative elemental composition was determined using an AZTECENERGY / X-ACT TESCANVEGA 3 SBU X-ray energy dispersive microanalyzer.
The characteristics of the porous structure (specific surface area (Sуд), total pore volume (VS), pore diameter (dпор)) were determined using a NOVAStationA instrument. The specific surface area according to the BET method was calculated by the isotherm of thermal nitrogen sorption.
X-ray diffraction analysis was performed using a SHIMADZU XRD-6000 X-ray diffractometer, (SHIMADZU, Japan, 2007).
The composition based on the ED-20 epoxy oligomer cured with iso-methyltetrahydrophthalic anhydride (iso-MTHFA) was chosen as a modifiable object.
The compositions were prepared as follows: in the epoxy binder of the composition – ED-20 epoxy resin (100 wt.%), Iso-MTHFA (80 wt.%), accelerator 2,4,6, -tris(dimethylaminomethyl)phenol (1.5 wt.%), fillers were introduced (test samples of biogenic silica) in the amount of 0.5–10 wt.% of the main composition.
The components were mixed at a temperature of 70–90 °C, homogenization of the system was achieved by dispersing the filler particles in a low-viscosity hardener using an IL-10-0.1 ultrasonic generator with a frequency of 22 kHz and a power of 1000 W, followed by mixing with resin and other components. The compositions were cured in a stepwise mode at a temperature of 120 °C for 1 hour and at a temperature of 160 °C for 3 hours.
Kinetics of curing the epoxy polymer matrix and the glass transition temperature was studied with the aid of the differential scanning calorimetry (DSC) method and a Shimadzu DSC-60 instrument.
Thermal effects during polycondensation of ED-20 with iso-MTHFA in the presence of biogenic silica were determined based on DSC data obtained in the temperature range 25–250 °C at a heating rate of 5 °C/min. To calculate the activation energy, the following regime was used: temperature range – 25–250 °C, heating rate – 3, 5, 10 °C/min. Thermograms recorded the temperature of the beginning of the Тнач process, the temperature of the maximum heat release Тпик, and the heat release power Q.
The activation energy of the interaction reaction between ED-20, iso-MTHFA and biogenic silica was calculated by the method of multiple heating rates, which makes it possible to determine the activation energy regardless of the reaction sequence at a number of heating rates [20, 21].
The glass transition temperature of the obtained polymers was determined by DSC curves (from 25 to 300 °C, heating rate – 10 °C/min).
The mechanical properties of the obtained composite materials (ultimate tensile and bending stresses) were studied by standard methods [22, 23] using an IR 5057-60 testing machine.
RESULTS AND DISCUSSION
Sources of biogenic silica, rice hull (RH) and bottlebrush (BB), are characterized by a specific component composition (see Table 1) and silica content (see Table 2).
The main biomass content is represented by cellulose, polysaccharides and lignin, with which silicon forms complexes [24]. Moreover, the content of the main components in plant samples varies widely. Silica was extracted from the plant mass by ashing and sedimentation. The technological characteristics of the obtained silica are presented in Table 3.
The degree of silica purity depends on the preliminary preparation of plant biomass. Silica with the content of the main substance (SiO2) – 86.0% and ChL1 – 83.0% was obtained by ashing the initial plant mass RSh1.
Fig.1 shows the results of scanning electron microscopy (SEM) and energy dispersive analysis (EDA) of silica samples (RSh, ChL1).
Studies of the morphological structure of the obtained silica showed that silica particles form amorphous globules (PSh1) or cups joined in agglomerates (CL1). The degree of sample purity was analyzed by EDA (see Fig.1). Presence of the accompanying mineral components in the ash was shown, which also varies widely depending on the type of plant mass.
Extraction of biomass with solutions of mineral acids makes it possible to remove all soluble mineral components from the lignocellulosic matrix, except silicon, and obtain amorphous silica with a purity of up to 99.9% in the course of ashing [8, 10] (see Fig. 2).
Considering the physicochemical characteristics of silica, alkaline extraction methods are widely used when it is extracted from mineral or plant materials [8, 10, 25]. When silica was leached from the lignocellulosic matrix with subsequent mineralization, high purity SiO2 was obtained – 76.4% (PSh3), and preliminary acid leaching increased the silica purity to 98.6%. Fig.3 shows the results of SEM and EDA of silica (RSh4).
In addition to silica, a second target product was obtained – rice cellulose, which is characterized by low silicon content – 1.4–2.3%.
The resulting silica are X-ray amorphous (see Fig.4). X-ray diffraction patterns are characterized by a wide diffuse maximum in the region of the main extremum (peak of α-cristobalite) 2q=20–26, which indicates the amorphous state of the samples.
The resulting silica are characterized by a specific surface area of 240–260 m2/g, a total pore volume of 44 cm3/g, and the average pore radius of 8.29 A.
Silica samples were tested as filler modifiers of epoxy polymers.
The nature of the modifier produces a significant effect on the structure and properties of epoxy compositions. Using differential scanning calorimetry (DSC), the process of curing an epoxy mixture with modifying silica was studied. Table 4 shows thermal effects and activation energy parameters of the polycondensation of the epoxy oligomer reaction (Еа) with iso-MTHFA in the presence of modifiers (RSh1, XL1, RSh2, XL2, RSh3, RSh4).
Introduction of a modifier into the epoxy composition increases the curing start temperature by 20.0–37.8% (to 114.0–130.8 °C), which may be due to formation of bonds between the functional groups of silica and the components of the epoxy anhydride mixture, since the hydrophilic groups of the filler surface are polar and can form hydrogen communication with the same groups of components of the mixture.
The final curing temperature decreases slightly for all modified compositions, with the exception of XL2 (at the same level), which indicates an acceleration of the curing process due to a change in the stoichiometry of the polycondensation reaction of the epoxy oligomer with anhydride in the presence of additional OH-groups not taken into account.
The activation energy of the polycondensation reaction of the epoxy anhydride polymer with all the studied modifiers is reduced by 30.3–33.7%. Thus, we can say that there is a mechanical and chemical interaction of the modifiers with the epoxy anhydride matrix.
In accordance with the DSC data, the optimal technological (temperature-temporal) mode for producing an epoxy anhydride composite with a filler flow rate of 0.5 was compiled; the filler consumption was 0.5; 1; 5; 10 wt.%: at a temperature of 120 °C – 1 hr, and at 160 °C – 3 hr.
When studying the influence of biogenic silica on the physical and mechanical properties of the epoxy anhydride composite material, a dependence of the relative change in Young’s modulus (see Fig.5, Fig. 6) was obtained, which has a similar character regardless of the origin of plant silica.
As can be seen from the presented dependences, the elastic modulus of the epoxy material for all applied modifiers in the range of low concentrations (0.5–1.5 wt.%) increases by an average of 10%. When the modifier concentration is increased to 5% and further to 10%, it leads to a sequential decrease in the relative modulus of elasticity to the value of the control sample, or decreases slightly. This may be due to defects in the spatial network of the three-dimensional matrix formed during the chemisorption of reaction components on the surface of the filler. The distinctive behavior of the sample with the RSh1 modifier can be explained by the finer dispersed state of the modifier and, as a consequence, by a more developed surface.
The glass transition temperature characterizes the structural changes of the polymer and its thermal and physical properties (see Fig.7).
Fig.7 shows that the glass transition temperature of the XL silica modified epoxy anhydride polymer increases by 6.0–9.0%, and that of RSh silica – by 19.0–22.7%. The upper limit of the heat resistance of all epoxy anhydride composite materials increases and, consequently, the working temperatures of the plant-modified silica-modified epoxy anhydride materials increase.
CONCLUSIONS
A method for the integrated processing of silica-containing plant mass and production of cellulose and high purity amorphous silica is proposed.
The prospects of using biogenic silica of various plant origin as a modifying additive of epoxy-polymer composite materials are shown. Using differential scanning calorimetry, it was found that the introduction of biogenic silica at the stage of polymerization of ED-20 oligomer with iso-methyltetrahydrophthalic anhydride reduces the activation energy of the process by up to 30% due to the mechanical and chemical interaction of the modifier with the epoxy anhydride matrix. It was shown that the relative Young’s modulus increases by 10% when keeping the modifiers in the range of low concentrations (0.5–1.5 wt.%), and the heat resistance of the composite increases to 25.0%, which is very important for epoxy compositions. ■
The work was carried out using the equipment of the Chemistry CCE of the Federal State Budgetary Institution of Science of the Federal Research Center "Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences".
Amorphous silica has a high chemical activity and it makes the basis for its widespread use as a filler and modifier [1]. Analysis of the global market of synthetic (precipitated and pyrogenous) amorphous silica shows a number of technical descriptions and trademarks with specific physical and chemical parameters, such as "Aerosil", "silica white", etc. [2]. Nevertheless, it is necessary to spend a lot of electrical energy and apply security measures to ensure explosion safety in order to obtain pyrogenous silica (with the content of base substance, SiO2, 99.5% and higher). Usually, the precipitated silica (SiO2 content varies in the range of 60.0–90.0%) and can be obtained from "liquid glass" (Na2SiO3) because of technical simplicity and safety [3]. The production metod determines the properties of the end product, such as dimensions and shapes of the particles, presence or absence of pores and surface properties [2].
The widespread usage of silica as a filler for polymer materials makes it a perspective object of modern scientific research [4–7]. Perspectives of biogenic silica and methods of its extraction from the plant biomass are considered in [8–17]. Russian specialists have developed the production methods of high-purity (up to 99.99%) amorphous silicon dioxide from rice hull with a yield of up to 22.0 weight percentage [2, 8, 10].
The aim of the study was to extract silica from plants of silicophiles (rice hull, bottlebrush) and its study as a filler for the epoxy anhydride polymer.
METHODS OF RESEARCH
The research objects were: silica extracted from rice hull (RH) and bottlebruch (BB) by oxidative ashing and precipitation from alkaline solutions. The component composition of plant samples was studied according to the methods described in the monograph [18].
The lignin content was determined by the sulfuric method modified by Komarov, the cytase content was determined by the Kürschner method (extraction of the plant sample with a nitrogen-alcohol mixture). The total content of silica was determined by melting with alkalies [19].
The studied objects (with variants of the preliminary preparation, such as shredding, extraction with an acid solution, extraction with a sodium hydroxide solution) were subject to ashing in a muffle furnace in a stream of air. The samples of 0.2–0.3 g were heated from 25 to 600–1000°С at the rate of 8°С/min in a dynamic air atmosphere at flow rate about 5 l/min.
Samples were extracted with water and/or 0.1 N hydrochloric acid solution (НСl) at 90°С during one hour followed by filtration and drying [11–12]. Samples were extracted by 2.5 N with sodium hydroxide solution (NaOH) at 90°С during one hour followed by preparation of the alkali-soluble components extract of the plant matrix (silica, lignin, pectic substances, hexosan) and solid residue (cellulose).
Micrographs of the samples were obtained using a Tescan Vega III SBU scanning electron microscope, Tescan, Czech Republic, 2010. Qualitative elemental composition was determined using an AZTECENERGY / X-ACT TESCANVEGA 3 SBU X-ray energy dispersive microanalyzer.
The characteristics of the porous structure (specific surface area (Sуд), total pore volume (VS), pore diameter (dпор)) were determined using a NOVAStationA instrument. The specific surface area according to the BET method was calculated by the isotherm of thermal nitrogen sorption.
X-ray diffraction analysis was performed using a SHIMADZU XRD-6000 X-ray diffractometer, (SHIMADZU, Japan, 2007).
The composition based on the ED-20 epoxy oligomer cured with iso-methyltetrahydrophthalic anhydride (iso-MTHFA) was chosen as a modifiable object.
The compositions were prepared as follows: in the epoxy binder of the composition – ED-20 epoxy resin (100 wt.%), Iso-MTHFA (80 wt.%), accelerator 2,4,6, -tris(dimethylaminomethyl)phenol (1.5 wt.%), fillers were introduced (test samples of biogenic silica) in the amount of 0.5–10 wt.% of the main composition.
The components were mixed at a temperature of 70–90 °C, homogenization of the system was achieved by dispersing the filler particles in a low-viscosity hardener using an IL-10-0.1 ultrasonic generator with a frequency of 22 kHz and a power of 1000 W, followed by mixing with resin and other components. The compositions were cured in a stepwise mode at a temperature of 120 °C for 1 hour and at a temperature of 160 °C for 3 hours.
Kinetics of curing the epoxy polymer matrix and the glass transition temperature was studied with the aid of the differential scanning calorimetry (DSC) method and a Shimadzu DSC-60 instrument.
Thermal effects during polycondensation of ED-20 with iso-MTHFA in the presence of biogenic silica were determined based on DSC data obtained in the temperature range 25–250 °C at a heating rate of 5 °C/min. To calculate the activation energy, the following regime was used: temperature range – 25–250 °C, heating rate – 3, 5, 10 °C/min. Thermograms recorded the temperature of the beginning of the Тнач process, the temperature of the maximum heat release Тпик, and the heat release power Q.
The activation energy of the interaction reaction between ED-20, iso-MTHFA and biogenic silica was calculated by the method of multiple heating rates, which makes it possible to determine the activation energy regardless of the reaction sequence at a number of heating rates [20, 21].
The glass transition temperature of the obtained polymers was determined by DSC curves (from 25 to 300 °C, heating rate – 10 °C/min).
The mechanical properties of the obtained composite materials (ultimate tensile and bending stresses) were studied by standard methods [22, 23] using an IR 5057-60 testing machine.
RESULTS AND DISCUSSION
Sources of biogenic silica, rice hull (RH) and bottlebrush (BB), are characterized by a specific component composition (see Table 1) and silica content (see Table 2).
The main biomass content is represented by cellulose, polysaccharides and lignin, with which silicon forms complexes [24]. Moreover, the content of the main components in plant samples varies widely. Silica was extracted from the plant mass by ashing and sedimentation. The technological characteristics of the obtained silica are presented in Table 3.
The degree of silica purity depends on the preliminary preparation of plant biomass. Silica with the content of the main substance (SiO2) – 86.0% and ChL1 – 83.0% was obtained by ashing the initial plant mass RSh1.
Fig.1 shows the results of scanning electron microscopy (SEM) and energy dispersive analysis (EDA) of silica samples (RSh, ChL1).
Studies of the morphological structure of the obtained silica showed that silica particles form amorphous globules (PSh1) or cups joined in agglomerates (CL1). The degree of sample purity was analyzed by EDA (see Fig.1). Presence of the accompanying mineral components in the ash was shown, which also varies widely depending on the type of plant mass.
Extraction of biomass with solutions of mineral acids makes it possible to remove all soluble mineral components from the lignocellulosic matrix, except silicon, and obtain amorphous silica with a purity of up to 99.9% in the course of ashing [8, 10] (see Fig. 2).
Considering the physicochemical characteristics of silica, alkaline extraction methods are widely used when it is extracted from mineral or plant materials [8, 10, 25]. When silica was leached from the lignocellulosic matrix with subsequent mineralization, high purity SiO2 was obtained – 76.4% (PSh3), and preliminary acid leaching increased the silica purity to 98.6%. Fig.3 shows the results of SEM and EDA of silica (RSh4).
In addition to silica, a second target product was obtained – rice cellulose, which is characterized by low silicon content – 1.4–2.3%.
The resulting silica are X-ray amorphous (see Fig.4). X-ray diffraction patterns are characterized by a wide diffuse maximum in the region of the main extremum (peak of α-cristobalite) 2q=20–26, which indicates the amorphous state of the samples.
The resulting silica are characterized by a specific surface area of 240–260 m2/g, a total pore volume of 44 cm3/g, and the average pore radius of 8.29 A.
Silica samples were tested as filler modifiers of epoxy polymers.
The nature of the modifier produces a significant effect on the structure and properties of epoxy compositions. Using differential scanning calorimetry (DSC), the process of curing an epoxy mixture with modifying silica was studied. Table 4 shows thermal effects and activation energy parameters of the polycondensation of the epoxy oligomer reaction (Еа) with iso-MTHFA in the presence of modifiers (RSh1, XL1, RSh2, XL2, RSh3, RSh4).
Introduction of a modifier into the epoxy composition increases the curing start temperature by 20.0–37.8% (to 114.0–130.8 °C), which may be due to formation of bonds between the functional groups of silica and the components of the epoxy anhydride mixture, since the hydrophilic groups of the filler surface are polar and can form hydrogen communication with the same groups of components of the mixture.
The final curing temperature decreases slightly for all modified compositions, with the exception of XL2 (at the same level), which indicates an acceleration of the curing process due to a change in the stoichiometry of the polycondensation reaction of the epoxy oligomer with anhydride in the presence of additional OH-groups not taken into account.
The activation energy of the polycondensation reaction of the epoxy anhydride polymer with all the studied modifiers is reduced by 30.3–33.7%. Thus, we can say that there is a mechanical and chemical interaction of the modifiers with the epoxy anhydride matrix.
In accordance with the DSC data, the optimal technological (temperature-temporal) mode for producing an epoxy anhydride composite with a filler flow rate of 0.5 was compiled; the filler consumption was 0.5; 1; 5; 10 wt.%: at a temperature of 120 °C – 1 hr, and at 160 °C – 3 hr.
When studying the influence of biogenic silica on the physical and mechanical properties of the epoxy anhydride composite material, a dependence of the relative change in Young’s modulus (see Fig.5, Fig. 6) was obtained, which has a similar character regardless of the origin of plant silica.
As can be seen from the presented dependences, the elastic modulus of the epoxy material for all applied modifiers in the range of low concentrations (0.5–1.5 wt.%) increases by an average of 10%. When the modifier concentration is increased to 5% and further to 10%, it leads to a sequential decrease in the relative modulus of elasticity to the value of the control sample, or decreases slightly. This may be due to defects in the spatial network of the three-dimensional matrix formed during the chemisorption of reaction components on the surface of the filler. The distinctive behavior of the sample with the RSh1 modifier can be explained by the finer dispersed state of the modifier and, as a consequence, by a more developed surface.
The glass transition temperature characterizes the structural changes of the polymer and its thermal and physical properties (see Fig.7).
Fig.7 shows that the glass transition temperature of the XL silica modified epoxy anhydride polymer increases by 6.0–9.0%, and that of RSh silica – by 19.0–22.7%. The upper limit of the heat resistance of all epoxy anhydride composite materials increases and, consequently, the working temperatures of the plant-modified silica-modified epoxy anhydride materials increase.
CONCLUSIONS
A method for the integrated processing of silica-containing plant mass and production of cellulose and high purity amorphous silica is proposed.
The prospects of using biogenic silica of various plant origin as a modifying additive of epoxy-polymer composite materials are shown. Using differential scanning calorimetry, it was found that the introduction of biogenic silica at the stage of polymerization of ED-20 oligomer with iso-methyltetrahydrophthalic anhydride reduces the activation energy of the process by up to 30% due to the mechanical and chemical interaction of the modifier with the epoxy anhydride matrix. It was shown that the relative Young’s modulus increases by 10% when keeping the modifiers in the range of low concentrations (0.5–1.5 wt.%), and the heat resistance of the composite increases to 25.0%, which is very important for epoxy compositions. ■
The work was carried out using the equipment of the Chemistry CCE of the Federal State Budgetary Institution of Science of the Federal Research Center "Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences".
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