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EurAsian Journal of BioSciences Eurasia J Biosci 13, 323-332 (2019)

Eco-friendly flame retardant additives for epoxy resin nanocomposites

Nguyen Tuan Anh 1*, Nguyen Quang Tung 1, Nguyen Xuan Canh 1, Nguyen Van Hoan 1 1 Faculty of Chemical Technology, Hanoi University of Industry (HaUI), No. 298 Cau Dien, North District Tu Liem, Hanoi, VIETNAM *Corresponding author: anhtn@haui.edu.vn

Abstract Flame retardants (FRs) are a diverse group of chemicals used as additives in a wide range of products to inhibit, suppress, or delay ignition and to prevent the spread of fire. We have developed a flame-retardant epoxy resin whose flame retardancy is greatly enhanced by adding fly ash (a by-product from thermal power plants). Applying the fly ash eliminates the need to use current retardants such as organic halogen compounds and thus promotes the perceived environmental safety of flame retardant epoxy resin. Some commercial additives used today are hazardous to humans and the environment, and their extensive application is limited by their negative effects on polymer mechanical properties. Accordingly, identifying materials that are environmentally friendly and harmless to humans has become urgent need. Some alternative additives that are gaining research interest are clay and fly ash, which are natural and recyclable resources that can enhance the FR properties of other polymers. However, room for improvement is always present as the related technology is continually being developed. This review focuses on studies aiming to enhance the flame retardancy of epoxy resin using eco-friendly additives (Fly ash, Multi-Walled Carbon Nanotube, nanoclay). Fly ash have been demonstrated to act as an effective/synergistic co-additive in some FR applications and could thereby contribute to reducing the loading of FRs in products and improving their performance. Flame retardant additives of Fly ash were embedded in epoxy resin-Fly ash to improve the resin’s resistance to oxidation. Keywords: epoxy Epikote 240, flame retardant, fly ash, limiting oxygen index Anh NT, Tung NQ, Canh NX, Hoan NV (2019) Eco-friendly flame retardant additives for epoxy resin nanocomposites. Eurasia J Biosci 13: 323-332. © 2019 Anh et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License.

INTRODUCTION Fly ash, composed of mainly silica and alumina, is a

by-product obtained when ground coal is fed into burners to generate power and shows dark color because of unburned carbon content. Some fly ash is used in the construction industry, mainly in cement and concrete manufacturing, while the remainder is disposed of in landfill sites. Using fly ash in a beneficial way would be preferable to dumping it, so many researchers have been looking for useful applications (Mohd Ali et al. 2018, Soyama et al. 2007). The global market consumption revealed that trends now are shifting towards eco-friendly FR due to the regulation imposed by authority, environment, safety and health factors. Fly ash is mainly composed of silica along with alumina and has a promising potential as fire retardant. Halogenated FRs (HFRs) are widely used because of their low impact on other material properties and the low loading levels necessary to meet the required flame retardancy (Aschberger et al. 2017, Singla and Chawla 2010). Health and environmental hazards associated with some halogenated FRs have driven research for identifying safer alternatives. A variety of halogen-free

FRs are available on the market, including organic (phosphorus and nitrogen based chemicals) and inorganic (metals) materials (Anh et al. 2018, Phuc et al. 2014). Fly ash (FA) have been demonstrated to act as an effective/synergistic co-additive in some FR applications and could thereby contribute to reducing the loading of FRs in products and improving their performance. Hence, in this study, ultrafine fly ash has been used. Fly ash has been surface treated with NaOH; HCl and stearic acid to improve dispersion in HDPE matrix. Epoxy functionalized epoxy resin has been used to enhance matrix-filler adhesion (Dhotre et al. 2015, Mirzamasoumzadeh and Mollasadeghi 2013, Zengin et al. 2015).

MATERIALS AND METHODS Materials - Epoxy Epikote 240 (EP) resin of Dow Chemicals

(USA) with epoxy group content of 24.6%, Mw: 5100-

Received: November 2018 Accepted: April 2019

Printed: May 2019

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5400 mmol/kg; density: 1,12,viscosity at 25 0C: 007-1.1Pa.s. Diethylenetriamine (DETA) of Dow Chemincals (USA), density 0.95, boiling point 207 oC, Mw: 103. HCl, NaOH of Dow Chemicals (USA).

- The fly ash primarily consisted of inorganic materials such as silica, alumina, and calcium oxide. Fly ash taken from the ash waste of Pha Lai Thermal Power Station was collected n Song Da Joint Stock Company 12- Cao Cuong, Vietnam.

Methods Sample preparation Then this fly ash was kept in a drier for 100˚C, 24

hours to remove the moisture present in it. After 24 hr. the fly ash was taken out from the drier. Then the amount of fly ash, epoxy resin (Epoxy Epikote 240) and hardener (DETA) was calculated for five different composites (0, 5, 10 and 20 vol% fly ash). Curing was done at room temperature for approx. 24 hrs. After curing the mould was opene slab taken out of the mould and cleaned. cured under room temperature for 1 and 7 days.

Characterizations The morphology of the samples was carried out by

scanning electron microscope (SEM, Evacseq error codes, S-4800, Japan). An ultramicrotome (Leica microsystem) was used to prepare cut ultra-thin sections (80 nm) of samples before recovered on a copper grid. The flame retardant properties was tested by the limiting oxygen index (LOI) (Yasuda Seiki Seisakusho Ltd, Japan) according to JIS K720-1976, with sheet dimensions of 120× 6.5 × 3.2 mm. The UL-94 rating was tested according to the UL-94 (ASTMD635-12) with sheet dimensions of 125 ±5 mm long by 13.0 ±0.5 mm wide, and provided in the minimum thickness and 3.0 (-0.0 +0.2) mm thick. The combustion rate was measured by COMBUSTION RESISTANCE COD 6145000 according to ASTM D757-77 standard. Specimen’s dimensions 3.17 x 12.7 x 121 mm3. Mechanical properties were measured on an INSTRON-5582 100

KN (USA) according to ISO 527-1993 at an extension speed of 5 mm/min. All data were the average of five independent measurements; the relative errors committed on each data were reported as well. IR spectrum was measured at the Department of Chemistry - University of Natural Sciences - Hanoi National University.

RESULTS AND DISCUSSION Investigate Characteristics of Fly Ash Fly ash taken from the ash waste of Pha Lai Thermal

Power Station was collected n Song Da Joint Stock Company 12- Cao Cuong, Vietnam. Before investigated, fly ash was washed by acetone to remove impurities. Scanning Electronic Microscope (SEM) method was used to determine fly ash particles’ morphological structure. The results are shown in Fig. 1.

As seen in Fig. 1, fly ash particles are have spherical smooth structure. There is an abundance and difference in size.

The IR spectroscopy of fly ash with vibration bands 400- 4000 cm-1 shows that there is a peak at 3649 cm-1

and 1065 cm-1corresponding to -OH (free)and Si-O, respectively, proving the presence of -OH in fly ash’s surface. Thus, making it easier for the modification process to attach silane groups to fly ash’s surface to enhance compatible ability between inorganic material and polymer substrate. Besides, there are also peaks at 792 cm-1, 562 cm-1 and 480 cm-1, 452 cm-1 corresponding to the presence of quartz, O-Fe and O-Al, respectively.

Fig. 1. SEM image of fly/Epoxy Epikote 240 composites

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Treat Fly Ash’s Surface Treat fly ash’s surface with NaOH Analyze IR spectroscopy of fly ash modified with

NaOH NaOH is strong base including active hydroxide

groups. NaOH with concentration of 3M was used to modify fly ash. In the modification process, fly ash particles were stirred in NaOH 3M solution at 80oC within 5 hours. Efficiency of the process is qualitatively

evaluate through IR spectroscopy of modified fly ash (Fig. 3).

Fig. 3 shows that there are new peaks at 3451 cm-1 and 1653 cm-1 in IR spectra of modified fly ash in comparison to one of initial fly ash. In IR spectra, there is the appearance of a peak at 3451 cm-1 and 1653 cm-

1 corresponding to the stretching vibrations of -OH groups of NaOH and H2O, respectively. In other hand, there is no change in the fingerprint region, indicating

Fig. 2. IR spectroscopy of the original fly ash sample

Fig. 3. IR spectroscopy of the original fly-NaOH ash sample

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that metallic oxides exist on modified fly ash. Therefore, it can be said that the structure of fly ash is quite stable.

Effects of alkaline treatment on contribution of fly ash’s size and surface area

The surface of initial fly ash is smooth, making it difficult for adhesion between fly ash and polymer substrate. So, it’s necessary to treat fly ash surface to increase roughness and surface area or form active groups on the surface.

Fly ash modified with NaOH was filtered, washed and dried. The contribution of fly ash particles is observed by SEM method in low resolution.

In Fig. 4, SEM image of fly ash modified with NaOH shows that there is higher contribution, more even size of modified fly ash particles in comparison to initial ones. In other words, NaOH makes the size of fly ash particles smaller.

The morphological structure of treated fly ash showed by Fig. Fly-NaOH is rough in comparison to initial ones. There are many cracks and small particles which is aluminum silicate or aluminum sulfate formed from abrasion process on the surface. It was caused by reactions between NaOH and oxides of fly ash. Although the size of fly ash particles of experiments is not similar,

there is no difficulty in enhancing properties of fly ash by this method. The surface becomes rough and the increase of surface area may make the interaction between fly ash and epoxy resin better.

Treat fly ash surface by HCl acid Analyze IR spectroscopy of fly ash modified by HCl

acid HCl is a strong acid used to corrode metallic oxides

included in fly ash. The modification process used HCl solution 2M. Fly ash particles were stirred in HCl 2M, at 95oC, within 6 hours to modify. By observing peaks appearing on IR spectroscopy of modified fly ash in Fig. 5, efficiency of this method was qualitatively evaluated. There is little difference in IR spectroscopy of modified fly ash in comparison to initial one. This prove a bad compatibility between acid HCl and fly ash. So, using acid HCl to change the structure of fly ash is ineffective.

Effects of acidic treatment on contribution of fly ash’s size and surface are

Observing SEM image in Fig. 5 to more clearly understand the change of fly ash particles after modified with HCl. Images in other resolutions of modified fly ash particles show that there are crystals attached to fly ash

Fig. 4. SEM image of Fly ash modified with NaOH

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particles, making fly ash surface area rough, thus increasing contact area between fly ash particles and polymer substrate (Fig. 6).

Besides, the size of modified fly ash particles is more even than initial ones, making the dispersion of fly ash particles into resin better (Fig. 6). However, there is only crystals but no crack on fly ash surface. Therefore, it can

Fig. 5. IR spectroscopy of the original fly-HCl ash sample

Fig. 6. SEM image of fly ash modified HCl

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be concluded that acid HCl does not strong impact with fly ash and this method is not optimal one.

Study on Preparation Fly Ash/Epoxy Composites

Effects of the amount of fly ash modified by inorganic compounds on mechanical properties, flame retardant ability and structure of the materials

Effects on mechanical properties Effects of the amount of fly ash modified by inorganic

compounds (NaOH, HCl) on tensile strength of the materials are shown in Fig. 7. As a result, materials’ tensile strength follows the order: materials including fly ash modified by NaOH > materials including fly ash modified by HCl > materials including fly ash without modification. Besides, increasing the amount of fly ash makes tensile strength decrease but it is insignificant. This shows that the compatibility with resin of modified fly ash is better than initial fly ash, leading to the increase of tensile strength. Concretely, tensile strength of the materials including 5, 10, 20 wt. % of fly ash modified with NaOH is 82.43; 79.20; 75.75 MPa, respectively. Tensile strength 7 of the materials including 5, 10, 20 wt.

% of fly ash modified with HCl is 79.98; 78.39; 75.37 MPa, respectively.

Effects of the amount of fly ash modified by inorganic compounds (NaOH, HCl) on tensile modulus of the materials are shown in Fig. 8. As a result, materials’ tensile modulus follows the order: materials including fly ash modified with NaOH > materials including fly ash without modification > materials including fly ash modified with HCl. Besides, increasing the amount of fly

Fig. 6 (continued). SEM image of fly ash modified HCl

Fig. 6 (continued). SEM image of fly ash modified HCl

Fig. 7. Comparison of Flexural strength

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ash makes tensile modulus decrease but it is insignificant. This shows that the compatibility with resin of modified fly ash is better than initial fly ash, leading to the stability of tensile modulus. Concretely, tensile modulus of the materials including 5, 10, 20 wt. % of fly ash modified with NaOH is 52.27; 49.98; 47.75 MPa, respectively. Tensile modulus of the materials including 5, 10, 20 wt. % of fly ash modified with HCl is 45.09; 43.79; 41.08 MPa, respectively.

Fig. 9 shows effects of the amount of fly ash modified with inorganic compounds (NaOH, HCl) on compressive strength of the materials. As a result, materials’ compressive strength follows the order: materials including fly ash modified by NaOH > materials including fly ash modified by HCl > materials including fly ash without modification. Moreover, impressive strength of the materials significantly increases with the increase of the amount of fly ash. This is caused by the increase in compatibility, adhesion and filling ability of modified fly ash with resin, thus leading to the improvement of compressive strength. Concretely, compressive strength of the materials including 5, 10, 20 wt. % of fly ash modified with NaOH is 176.01; 189.90; 197.07 MPa, respectively. Compressive strength of the materials including 5, 10, 20 wt. % of fly ash modified with HCl is 145.33; 180.07; 196.78 MPa, respectively.

Fig. 10 shows effects of the amount of fly ash modified with inorganic compounds (NaOH, HCl) on

compact strength of the materials. As a result, materials’ compact strength follows the order: materials including fly ash modified by HCl > materials including fly ash modified by NaOH > materials including fly ash without modification. Moreover, compact strength of the materials insignificantly decreases with the increase of the amount of fly ash. After modification, compatibility ability of fly ash with resin is better, leading to the improvement of compact strength. Concretely, compact strength of the materials including 5, 10, 20 wt. % of fly ash modified with NaOH is 6.89; 6.01; 5.70 MPa, respectively. Compressive strength of the materials including 5, 10, 20 wt. % of fly ash modified with HCl is 6.54; 6.55; 5.98 (kJ/m2) respectively.

The results show that the amount of fly ash affect mechanical properties of the materials. Like fly ash without modification, there are effects on the materials in the case of modified fly ash such as increasing impressive strength, decreasing tensile modulus, compact strength and tensile strength. However, effects made by modified fly ash are more significant than initial fly ash because of good contribution and adhesion of modified fly ash particles with resin. In which, fly ash modified with NaOH is better than fly ash modified with HCl.

Investigate effects of the amount of fly ash on flame retardancy of the materials by UL 94 HB standard

Fig. 11 shows effects of the amount of fly ash modified with inorganic compounds (NaOH, HCl) on burning rate of the materials following UL 94 HB standard. As a result, burning rate of the materials follows the order: the materials including fly ash modified with HCl < the materials including fly ash modified with NaOH < the materials including fly ash without modifications. Besides, the higher the amount of fly ash is, the lower burning rate is or the better flame retardancy of the materials is. This proves that after modified, fly ash’s compatibility with resin is better, making flame retardancy of the materials better. Concretely, burning rate following UL 94 HB standard of the materials including 5, 10, 20 wt. % of NaOH is 21.15; 18.22; 13.45 mm/min, respectively. Burning rate

Fig. 8. Comparison of Tensile strength

Fig. 9. Comparison of compressive strength

Fig. 10. Comparison of Impact strength

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following UL 94 HB standard of the materials including 5, 10, 20 wt. % of HCl is 20.27; 15.37; 11.51 mm/min, respectively.

Fig. 12 shows effects of the amount of fly ash modified with inorganic compounds (NaOH, HCl) on LOI of the materials. As a result, LOI of the materials follows the order: the materials including fly ash modified with HCl > the materials including fly ash modified with NaOH > the materials including fly ash without modifications. Besides, the higher the amount of fly ash is, the higher LOI is or the better flame retardancy of the materials is. This proves that after modified, fly ash’s compatibility with resin is better, making flame retardancy of the materials better. Concretely, LOI of the materials including 5, 10, 20 wt. % of NaOH is 21; 21.5; 22.4 %, respectively. LOI of the materials including 5, 10, 20 wt. % of HCl is 21.5; 22.4; 23%, respectively.

These above images show that modified fly ash make flame retardancy of the materials better than initial fly ash. This is caused by the good contact and dispersion of modified fly ash. Among them, fly ash modified with HCl is better than with NaOH. However, the difference is insignificant. From above- said results, it can be said that the sample including 20 wt. % of fly

ash is the best with stable mechanical properties and good flame retardancy. Therefore, it will be selected for further experiments.

Effects of the amount of fly ash modified with inorganic compounds on structure of the materials

SEM images in Fig. 13 show the dispersion and adhesion of modified fly ash into resin. Fig. 13 shows fracture surface of the material including 20 wt. % of fly ash modified with NaOH and HCl. For fly ash modified with NaOH, it can be seen that fly ash particles have a broken shell to release smaller inside particles which adhere and contact with resin (Fig. A). Therefore, adhesion ability and contact surface area of fly ash modified with NaOH are better than initial one. Besides, there are high density and good contribution of spherical fly ash particles in resin (Fig. B). For fly ash modified with HCl, it can be seen that there are rough surfaces of fly ash particles (Fig. C) leading to the higher contact surface are of modified fly ash than initial ones. Besides,

Fig. 11. Flame retardation according to UL94 HB research method

Fig. 12. Investigate effects of the amount of fly ash on Limited oxygen index (LOI)

Fig. 13. Flame retardation according to UL94 HB research method

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there are also high density and good contribution of spherical fly ash particles in resin (Fig. D).

CONCLUSIONS - With the addition of fly-ash in epoxy resin –fly-ash

composite the compressive strength has been found to increase with increase in fly ash particles. Polymer matrix composite was successfully prepared by using fly ash.

- Interface between fly ash and epoxy matrix plays an important role in determining the mechanical strength of the composites. In SEM analysis it has been found that fly-ash particles has been uniformly segregated.

-The fly ash exhibited compatibility as a coupling agent, enhanced mechanical properties, has high heat resistance and reduced the amount of heat-release rate, smoke produce rate and total smoke production.

ACKNOWLEDGEMENT The authors wish to thank the Faculty of Chemical

Technology, Hanoi University of Industry for funding this work.

DATA AVAILABILITY Any questions relating to the data or the original data

requirements of the article can be contacted at email address: anhtn.haui@gmail.com.

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Fig. 13 (continued). Flame retardation according to UL94 HB research method

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