The potential use of recycled PET bottle in nanocomposites manufacturing with modified ZnO nanoparticles capped with citric acid: preparation, thermal, and morphological characterization

Abstract

We devised a fast and facile potential practical application by incorporating the guest ZnO nanoparticles (NPs) into poly(ethylene terephthalate) (PET) as a host polymer that recycled by dissolution/reprecipitation method. Due to the high surface energy and tendency for agglomeration, surface of ZnO NPs was treated with biocompatible citric acid (CA) molecule. Different NCs of organo-modified ZnO NPs (1, 3, 5 wt%) and PET were constructed by means of an ultrasonic process. The resulting hybrid NCs were characterized by Fourier transform infrared spectroscopy, UV-Vis spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. From TEM images, it can be found that the surface-treated ZnO/CA NPs with diametric size of about 21 nm, uniformly dispersed in the recycled PET matrix and led to distinct properties. FE-SEM photographs of sonicated recycled PET in ethanol and DMAc were approved solvent effect.

---

1 Introduction

Poly(ethylene terephthalate) (PET) is globally recognized as a synthetic recyclable fiber and resin material, also a member of the polyester family of polymers. This valuable polymer has premier properties such as good flexibility, high transparency in blown containers, resistance to shrinkage, heat stability, notable stiffness and strength that can be due to presence of a large aromatic ring in its repeating units.^1,2 PET is popular for produce fibers, films, and packaging materials with high barrier properties.^3,4 A main contribution of PET that produced each year is used to make disposable substances of packaging or other short-lived products such as plastic bottles that are discarded within a year of manufacture. In view of the increasing environmental awareness, recycling remains the most viable option for the treatment of waste PET. Chemical recycling can leads to the formation of the materials which can be applied in the other industrial areas such as filling material for lightweight concrete.^5,6 PET fibers obtained from waste bottles are in different shapes such as circular fibers and long strips that can be added to the concrete in substitution of steel bars.⁷ Fiber-reinforced concrete is a composite material arises from the incorporation of reinforcing fibers like polystyrene and PET to the brittle matrix of ordinary concrete. PET fiber due to sufficient alkali resistance compared to the other fibers, is most suitable candidate for this purpose.⁸ Reinforcing them into concrete, lead to significant increases in structural characteristics such as compressive and flexural strengths and ductility.^9–12 Also, recycled PET fibre-reinforced cement composites are largely unaffected by salt, CaCl₂, and sodium sulfate environments.⁶ Currently, this reinforced material have been applied in various areas including road pavements, sidewalks, bridges, lining.¹³

Zinc oxide is a multifunctional inorganic material which has attractive properties such as low cost, chemical stability, ultraviolet and infrared absorption, effective antibacterial properties also biocompatibility.^14–16 Combination of unique features and high surface-to-volume ratio of ZnO NPs (less than 100 nm) can lead to more distinct antimicrobial activities but exhibit minimal effects on human cells.¹⁵ Furthermore, colorless and transparent ZnO NPs owing wide direct band gap, high exciton binding energy (60 meV) are of special interest, that can be used as an ideal UV blocker, and usually added into cosmetics for ultraviolet protection.^14,16

PET as a host in combination with suitable guest metal oxides gives hybrids that are valuable for the development of advanced applications such as antimicrobial textile products and UV protection of PET components.^15,17 The specific surface area, volume and numerous –OH functional groups on the surfaces of NPs, usually leads to highly reactive and colloidal instability compared to their bulk counterparts. Strong tendency of NPs to agglomerate can effect on their physiochemical properties, reactivity, transport, biological interactions and many other transformations.¹⁸ The general class of NC organic/inorganic materials can provide innovative high-performance materials that are applied in many industrial fields because of their remarkable properties such as elasticity, strength, toughness, modulus and thermal stability.¹⁹ In NCs, strong interactions between the polymer and inorganic component for example chemical bonding, van der Waals forces, hydrogen bonding or electrostatic lead to improve property of the material and new usage due to their individual parents but also on their morphology and interfacial characteristics.^20,21 Comprised with conventional mixing, ultrasonic is faster, safe and time saving method with higher yields.²² Consequently, ultrasonication and modification are two simple methods those are able to increase the concentration of NP suspensions without forming aggregates. Modification by organic compounds can change surface structure which reduces the surface force of hydroxyl groups.^23,24 This aim achieved by different interactions such as hydrogen bonds between the –OH groups on the surface of NPs and the functional groups of coupling agents; also, changing NPs surface to lipophilic surface lead to NPs dispersity due to steric hindrance between NPs.²⁵ Some modifiers have received significant attention due to their effect on dispersion and NCs features such as stearic acid. Capping this organic molecule on ZnO surface in PET matrix compared to the NC filled with unmodified ZnO, exhibited a better dispersion of the NPs, improved thermal stability and crystallinity.²⁴ Among the modifiers, green materials such as citric acid (CA) can also refer for the functionalization of metal oxides. CA (2-hydroxy-1,2,3-propanetricarboxylic acid) is an essential tricarboxylic acid with antioxidant properties. It is a chemical-product which is produced by fermentation but basically, most of the CA that's used as a food additive is extracted. This safe organic acid has been used as acidifier, flavoring, for microbial fermentations, and chelating agent due to its different functional groups, and as reducing agent used in photolytic and photocatalytic systems.^26,27 This modifier has been used for ZnO NPs and the capped ZnO/CA NPs were employed to study PVC NCs futures.²⁸

In the present study, first PET was recycled by dissolution/precipitation method that offers the ideal manner for PET recycling. Then, nontoxic, biocompatible, and biodegradable CA was grafted on the safe ZnO NPs surface. CA has been used to give a more stable and uniform dispersion of protected ZnO NPs. Different percentages of surface-treated NPs (1, 3, 5 wt%) were employed as guests in the PET-based NCs. The prepared compounds were investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and UV-Vis spectroscopy. In addition, thermal stability and surface morphology of the synthesized materials were investigated by different techniques.

2 Experimental

2.1 Materials

CA (C₆H₈O₇, M_w: 192.13 g mol⁻¹), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc) and ethanol were purchased from Merck Chemical Co. and were used without further purification. Nano-sized ZnO powder with average particle size of <25 nm and specific surface area >80 m² g⁻¹ was obtained from Neutrino Co. (Iran). NPs were dried at 400 °C for 4 h before surface modification. The PET used in all experiments was procured from PET beverage bottles by recycling.

2.2 Surface modification of ZnO NPs with CA

For the surface modification of ZnO NPs, first 0.10 g of NPs was suspended in 10 mL of deionized water (DI) and the mixture was sonicated for 15 min. Then 0.01 g of CA was dissolved into another 10 mL of DI and sonicated 15 min. Then, both of them were blended and the suspension was sonicated for 1 h. The coated ZnO-CA NPs were dried at room temperature. Scheme 1 has illustrated different interactions between the COOH groups of the CA and –OH on the surface of ZnO NPs.

Scheme 1 Surface modification process of bare ZnO NPs with CA.

2.3 PET recycling

Before charging the PET in the reactor, the bottles were cut down into 1 mm × 3 mm to 6 mm × 4 mm flakes and washed with hot distilled water. DMSO has been used as a solvent in the recycling process. The dish containing DMSO and PET flakes was shaking vigorously under heating to solve PET flakes into DMSO. The vapor products can be collected in a cold to liquid products. Then cold distilled water was added directly and the obtained white powder was washed with hot distilled water several times and transferred to a glass dish. The obtained combination was allowed to dry at 100 °C overnight.

2.4 Preparation of PET@ZnO/CA NC

In order to obtain PET@ZnO/CA NCs, first 0.10 g of the PET (powder) was added into 12 mL DMAc and the mixture was ultrasonicated for 15 min to be partially dissolved. Then, different concentrations of ZnO-CA filler (1, 3, and 5 wt%) subsequently were added into PET solution and the mixture ultrasonicated for 1 h at 50 °C. The resulting homogenous mixture was centrifuged for 10 min at 6000 rpm and poured into glass Petri dishes. Finally, it was further dried by placing the container at 100 °C to obtain PET@ZnO/CA NC as white solid powder. Scheme 2 shows manufacturing way of PET@ZnO/CA NC.

Scheme 2 Fabrication steps of PET@ZnO/CA NCs.

2.5 Characterization

The reaction was occurred on a TOPSONICS ultrasonic liquid processor, UP-400 series with a wave of frequency 20 ± 1 kHz and power 100 W (Tehran, I. R. Iran). FT-IR spectra for samples were recorded on Jasco-680 (Japan) spectrophotometer with KBr pellets. The vibration transition frequencies are reported in the range of 4000–400 cm⁻¹. Thermogravimetric (TG) analysis was performed on a STA503 TA instrument by heating rate of 20 °C min⁻¹ from room temperature to 800 °C at argon atmosphere. The XRD patterns were collected by using a Philips X'Pert MPD X-ray diffractometer. The diffractograms were measured for 2θ, in the range of 10–80°, using a voltage of 40 kV and Cu Kα incident beam (k = 1.51418 Å). The dispersion morphology of the hybrids were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4160), and transmission electron microscopy (TEM, Philips CM 120, 100 kV). UV/Vis measurements of NCs were carried out on a UV/Vis/NIR spectrophotometer, JASCO, V-570 in the spectral wavelength range between 200 and 800 nm at room temperature.

3 Results and discussion

Surface modification of NPs was used as an effective way to reduce their surface tension and increase the compatibility between them and PET matrix. For this aim, CA was selected as a biosafe, biocompatible and low cost modifier. PET was recycled in easy procedure (dissolution/reprecipitation recycling process) that during this technique the polymer can be separated and recycled using a solvent/non-solvent system.²⁹ Recycled PET was used as a polymeric matrix for the NCs preparation. The attained consequences showed that the matrix properties are influenced by the presence of ZnO/CA. As can be seen from the picture dispersity of prepared NCs in DMAc decreased with more inorganic NPs contents (Fig. 1).

Fig. 1 Dispersion of PET@ZnO/CA NCs in DMAc.

3.1 FT-IR analysis

The FT-IR spectra of neat ZnO confirms the presence of O–H on the surface of the NPs by broad band at 3500 cm⁻¹ and 472 cm⁻¹ points to the Zn–O bond stretching (Fig. 2a).²⁸ The vibration bands for the CA (Fig. 2b) appears as a very broad band from 2900 to 3400 cm⁻¹ due to hydroxyl groups and inter-molecular H-bonding. The peak about 1752 cm⁻¹ is assignable to the C=O vibration in neat CA from the COOH group which can be conjugated or in resonance. The peak in 1112–1316 cm⁻¹ is due to C–O stretching. Other smaller vibration bands observed are also the bands of the organic parts.³⁰ Modified ZnO spectrum shows peaks related to neat ZnO and CA (Fig. 2c).

Fig. 2 FT-IR spectra of (a) pure ZnO, (b) CA, (c) modified ZnO.

The degradation of PET, with respect to the changes in the structure, is investigated using the differences of the characteristic bands of the recycled PET (Fig. 3a) and original PET bottles (Fig. 3e). For recording FT-IR spectrum, the PET bottle was cut into small pieces and direct FT-IR spectrum recorded on the solid sample. In the present study the most significant peaks are in different positions from the original PET bottle for the recycled PET. For recycled PET there are peak at 3550 cm⁻¹ for hydroxyl end groups and 3430 cm⁻¹ related to C=O overton. The peaks at 3063 cm⁻¹ and 2965 cm⁻¹ indicate C–H bond stretching of methylene groups. The spectrum of recycled PET confirms the presence of a carbonyl group in conjugation with aromatic ring. Strong and broad peak at 1730–1722 cm⁻¹ is related to carbonyl group bond stretching. The appeared peaks at 1090 and 727 cm⁻¹ can be due to the methylene group and aromatic bands in-plane/out-plane bending.^31–33 Many of the medium and weaker bands have been attributed to chain conformation and are sensitive to the crystallinity nature of the sample.^30,31 PET bottle spectrum displays unlike bands that can be because of some additives that lost in the recycling procedure. The FT-IR spectra of the NCs with 1, 3, 5 wt% of ZnO/CA (Fig. 3b–d) compared with pure PET exhibited different absorption peaks about 3500 cm⁻¹ that can be related to hydroxyl group on the surface of NPs and modifiers structure. The peaks at 1730 cm⁻¹ in the NCs spectra related to carbonyl group in conjugation with aromatic ring tends to be weaker which has been assigned to interaction with modified ZnO NPs and CA modifier. The intensity of the band at 422 cm⁻¹ has been increased which has been assigned to ZnO NPs contents.

Fig. 3 FT-IR data of (a) recycled PET, (b) PET@ZnO/CA NC 1 wt%, (c) PET@ZnO/CA NC 3 wt%, (d) PET@ZnO/CA NC 5 wt% and (e) PET bottle.

3.2 XRD patterns

XRD is a useful analytical technique used for the determination of the crystalline structure. XRD patterns of the neat and modified ZnO NPs, recycled PET, and PET@ZnO/CA NCs with 1, 3 and 5 wt% of ZnO/CA NPs content are presented in Fig. 4. As can be from XRD diffractograms (Fig. 4a), a series of characteristic peaks are observed at (100), 34.4 (002), 36.2 (101), 47.6 (102), 56.6 (110), 62.9 (103), 66.4 (200), 67.9 (112), 69.1 (201), 72.6 (004), and 77.0 (202) which diffraction peaks are in agreement with JCPDS data (No. 36-1451). These peaks correspond to the typical hexagonal wurtzite phase of ZnO NPs.³⁴ Diffractograms of the ZnO/CA NPs produces neither new peaks nor peak shifts. This fact suggests that the crystal nature of ZnO NPs has not modified due to the presence of modifier and ultrasonic irradiation (Fig. 4b).

Fig. 4 XRD patterns of (a) pure ZnO, (b) modified ZnO/CA, (c) recycled PET, (d) PET@ZnO/CA NC 1 wt%, (e) PET@ZnO/CA NC 3 wt%, (f) PET@ZnO/CA NC 5 wt%.

The XRD patterns of recycled PET (Fig. 4c) and PET@ZnO/CA NCs (Fig. 4d–f) indicate semi-crystalline triclinic structure for PET matrix. Semi-crystalline PET typically shows characteristic crystalline XRD peaks at 2θ = 16.1, 17.5, 21.5, 22.7, 24.0, 26.1, 27.8, and 32.5°, corresponding to the: 011, 010, 111, 110, 011, 100, 021, 002, 111 and 101 crystal planes, and a broad amorphous halo at 10–35°.³⁵ Intensity of the NCs peaks increases in compare with recycled PET as an evidence for the enhanced crystallinity with the increasing ZnO contents. The change in degree of crystallinity can be described with variables affecting such as molecular weight, crystallization temperature, co-monomer content and nucleating agents.³⁶ Through ultrasonic irradiation, energy creates from collapse sources a temperature of 5000 K and a pressure of 1000 bar that can be the reason for recrystallization.³⁷ Besides, the sensed change in crystallinity can be affected by the solvent due to polymer solubility. The interaction of the solvents with polymer is expected to recrystallization.³⁶ These results can be depended on the incorporated ZnO/CA NPs content that act as heterogeneous nucleation agent.¹⁶ By the uniform dispersion, the good interfacial adhesion between ZnO/CA NPs and polymeric matrix, also very large interfacial area of the NPs as the strong interconnection between the two phases reduces the mobility of PET chains.²⁴ These consequences can use as input for more sophisticated approaches to desire morphology and structures to improve application of PET polymer.³⁸

3.3 Optical property analysis

UV-Vis spectroscopy for recycled PET and PET@ZnO/CA NCs (1, 3, and 5 wt%) was performed as presents in Fig. 5. Interaction with high energy radiation in the UV (200–400 nm) and visible (400–700 nm) range of the electromagnetic spectrum causes many organic molecules to undergo electronic transitions. This range is the main causes of the damage to human health, which may lead to erythema, certain skin cancers, keratitis, and cataracts. Colorless and safe ZnO NPs is considered as an ideal UV blocker and typically used for UV protection especially in cosmetics.³⁹ It is noticeable that the UV-protection property of the NCs was affected by the type of matrix polymer and the characteristics of the NPs used (size, distribution, dispersion, surface properties) together with the interfacial adhesion.^40,42 According to the shape of curves depicted in Fig. 5, absorption increases with more ZnO/CA NPs concentration due to absorption of ZnO and concentration of absorbing molecules. The Beer–Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing substance in the solution and the path length.⁴¹

Fig. 5 UV-visible absorption spectra of (a) recycled PET (b) PET@ZnO/CA NC 1 wt%, (c) PET@ZnO/CA NC 3 wt% and (d) PET@ZnO/CA NC 5 wt%.

ZnO as an antibacterial agent, expose to UV light damages the structure of bacterial cell membrane and caused bacteria die by produce radicals (O₂˙ and HO˙).^40,43 Photoactivity of the obtained PET@ZnO/CA NCs has been dramatically changed by combining PET polymer matrix and ZnO/CA. Band-gap absorption of ZnO related to the electron transitions from the valence band to the conduction band (O_2p → Zn_3d) herein shifted to the 255 nm.⁴⁴ The interfacial interaction between modified ZnO/CA NPs and PET matrix has been ameliorated by the increasing specific surface area of the filler and prevent agglomeration by using organic modifier.

3.4 Morphological characterization

TEM and FE-SEM are other useful tools for the analysis of the morphology and dispersion of the ZnO/CA NPs in the recycled PET matrix. Fig. 6 is the FE-SEM images of ZnO/CA at different magnifications and revealed that the shape of modified ZnO NPs is spherical and they are distributed uniformly. A number-frequency histogram is a typical way to present the particle size and its distribution. Histograms of the particle sizes accumulated from measurements of 100 particles and resulted that mean size of particles is about 44 nm.

Fig. 6 FE-SEM images of modified ZnO/CA.

FE-SEM images of the recycled PET samples sonicated in DMAc and ethanol were carried out at different magnifications which are shown in Fig. 7. Recycled PET images before using ultrasonication irradiation Fig. 7a–c and also ultrasonicated PET in ethanol Fig. 7d–f, have shown an irregular, broken, rock-like morphology.

Fig. 7 FE-SEM images of (a–c) recycled PET before ultrasonication, (d–f) ultrasonicated PET in ethanol, (g–i) ultrasonicated PET in DMAc.

It is clear that safe and easy ultrasonication process with using suitable solvent has changed morphology and reduced the particle size. Sonicated PET in DMAc Fig. 7g–i compared to the others, have relatively spherical and uniform morphology in smaller size.

The FE-SEM images of the prepared NCs were shown in Fig. 8. Both modified ZnO NPs and PET matrix are clearly observed in these images. ZnO/CA NPs has been observed as many little particles on the PET surface that are relatively homogeneous. The uniform dispersion of the NPs indicates that the modifiers grafted on the NPs surface can improve dispersion by adhesion between organic PET and inorganic metal oxide NPs.

Fig. 8 FE-SEM images of (a–c) NC 1 wt%, (d–f) NC 3 wt%, (g–i) NC 5 wt%.

Other direct evidence for the formation of true nano-scaled materials was provided by TEM photographs that shown in Fig. 9 and 10. It can be seen the mean size of about 85 nm from TEM observations of the modified NPs and related histogram (Fig. 9) which are made from relatively uniform particles.

Fig. 9 TEM images of modified ZnO/CA.

Fig. 10 TEM images of PET@ZnO/CA NC 8 wt% at different magnifications and related histogram.

From TEM images of PET@ZNO/CA NC 8 wt% at different magnifications (Fig. 10), it is clear that the ZnO/CA NPs are rather well dispersed in the PET matrix. From histogram resulted that the size of NPs in the polymer matrix has been reduced (∼21 nm) in homogenous dispersion which can be attributed to effective modifier.

3.5 Thermogravimetric analysis

The thermal stability of pure ZnO, modified ZnO with CA, recycled PET together with all prepared NCs was investigated by TGA techniques conducted at a heating rate 20 °C min⁻¹ under flow of argon from room temperature to 800 °C. As resulted from the curve (Fig. 11), weight loss of pure ZnO NPs is not considerable and observed weight loss is attributed to the elimination of absorbed water on the surface of the NPs.⁴⁵ From the weight loss steps in the TGA curve of modified NPs, first the absorbed moisture on the surface of NPs and CA molecules desorbs. Then the other two steps related to the attached CA on the ZnO NPs surface. Due to numerous functional groups in the CA structure, it can be double layered. One layer attached on the NPs surface and the second one is bound to the first layer via hydrogen bonds.^46,47 Besides, according to the TGA results, about 9% of CA was absorbed on the surface of the NPs.

Fig. 11 TGA curves of pure and modified ZnO NPs with CA.

The 10% and 5% weight loss temperatures (T₁₀ and T₅) and the residue at 800 °C (char yield) for recycled PET and PET@ZnO/CA NCs 1, 3 wt% are listed in Table 1. From TGA thermogram of recycled PET (Fig. 12a), the first small weight loss between 250 and 370 °C can be evident as a removal of the absorbed small molecules and moisture. Main step mass-loss of recycled PET undergoes about 370 and 460 °C correspond to oxidative elimination of the carbonaceous residue.^48,49 Results show that in the presence of incorporated ZnO/CA NPs, elimination of the absorbed moisture has been occurred with low slope that can be evident as a modifiers effect. Furthermore, ZnO NPs in the polymer may weaken the interactive force between polymer chains and accelerate the oxidative decomposition of surrounding carbon led to thermal decomposition at lower temperature in the NCs.⁵⁰

Table 1 Thermal properties of the pure PET and PET@ZnO/CA NCs

Samples set	T5 (°C) ^a	T10 (°C)^b	Char yield (%)^c	LOI (%)^d	ΔHcomb (kJ g^−1)
a Temperature at which 5% weight loss was recorded by TGA at heating rate of 20 °C min^−1 under an argon atmosphere.b Temperature at which 10% weight loss was recorded by TGA at heating rate of 20 °C min^−1 under an argon atmosphere.c Weight percentage of material left undecomposed after TGA analysis at a temperature of 800 °C under an argon atmosphere.d Limiting oxygen index (LOI) evaluating char yield at 800 °C.
Recycled PET	413	420	18.0	24.7	32.4
(PET@ZnO/CA)1 wt%	354	400	19.4	25.3	31.6
(PET@ZnO/CA)3 wt%	354	400	19.6	25.3	31.6

---

Fig. 12 TGA thermograms of (a) recycled PET and (b) NC1 wt% and (c) NC 3 wt%.

Char yield can be used as criteria for evaluating limiting oxygen index (LOI) of the polymers in accordance with Van Krevelen and Hoftyzer equation.⁵⁰

LOI = 17.5 + 0.4CR where CR = char yield

PET and the NCs LOI values calculated and on the basis of LOI values, prepared materials can be classified as self-extinguishing polymers. Also, according to Johnson, the ΔH value of compounds can determine from LOI:

LOI = 8000/ΔH_comb

ΔH_comb is the enthalpy (heat) of a compound combustion with oxygen via standard conditions in J g⁻¹.⁵¹

4 Conclusions

Due to environmental and economic considerations as well as energy conservation issues we describe a method that can be used to produce concrete-reinforcing PET fiber from used PET bottles. Modification of ZnO NPs with biocompatible CA performed in fast, clean and powerful ultrasonic irradiation protocol. NCs of PET@ZnO/CA with 1, 3 and 5 wt% of lipophilic ZnO/CA NPs were prepared; incorporation is achieved by ultrasonication. The obtained PET@ZnO/CA NCs were investigated using FT-IR spectroscopy, UV-Vis analysis, TGA and XRD. Results showed that ultrasonic irradiation is a beneficial method for particle size reduction in dispersions due to potential in the de-agglomeration. The morphology of the ZnO/CA NPs and PET@ZnO/CA NCs was examined using FE-SEM and TEM and the NPs appear to be embedded and uniformly dispersed in the PET. The obtained data by thermal analysis revealed that basis on LOI values prepared NCs can be categorized as self-extinguished materials. Crystallinity was improved with the addition of modified ZnO. According to UV-visible spectra, absorption increased with more content of modified ZnO moieties. Crystallinity of the NCs was improved as ZnO/CA content increased.

Acknowledgements

The research was financially supported by the Research Affairs Division of Isfahan University of Technology (IUT), National Elite Foundation (NEF), and Center of Excellency in Sensors and Green Chemistry Research (IUT).

References

1. M. Vakili and M. H. Fard, World Appl. Sci. J., 2010, 8, 839–846.
2. M. Li, Y. Huang, T. Yu, S. Chen and A. Ju, RSC Adv., 2014, 4, 46476–46480 and Mingqiao Geab.
3. M. D. Teli and R. D. Kale, Adv. Appl. Sci. Res., 2011, 2, 491–502.
4. C. Grady and T. Younos, International Water Technology Journal, 2012, 2, 185–194.
5. S. Akçaözoğlu, C. D. Atiş and K. Akçaözoğlu, Waste Manag., 2010, 30, 285–290.
6. J. P. Won, C. I. Jang, S. W. Lee, S. J. Lee and H. Y. Kim, Construct. Build. Mater., 2010, 24, 660–665.
7. D. Foti, Compos. Struct., 2013, 96, 396–404.
8. D. Foti, Mech. Adv. Mater. Struct., 2013, 20, 163–175.
9. F. Fraternali, V. Ciancia, R. Chechile, G. Rizzano, L. Feo and L. Incarnato, Compos. Struct., 2011, 93, 2368–2374.
10. Y. Mohammadi, S. P. Singh and S. K. Kaushik, Construct. Build. Mater., 2008, 22, 956–965.
11. S. B. Kim, N. H. Yi, H. Y. Kim, J. H. J. Kim and Y. C. Song, Cem. Concr. Compos., 2010, 32, 232–240.
12. H. Mazaheripour, S. Ghanbarpour, S. H. Mirmoradi and I. Hosseinpour, Construct. Build. Mater., 2011, 25, 351–358.
13. A. Khaloo, E. M. Raisi, P. Hosseini and H. Tahsiri, Construct. Build. Mater., 2014, 51, 179–186.
14. Y. C. Chang, RSC Adv., 2014, 4, 20273.
15. Y. Xie, Y. He, P. L. Irwin, T. Jin and X. Shi, Appl. Environ. Microbiol., 2011, 77, 2325–2331.
16. P. Maddahi, N. Shahtahmasebi, A. Kompany, M. Mashreghi, S. Safaee and F. Roozban, Mater. Sci.–Pol., 2014, 32, 130–135.
17. H. Agrawal, V. K. Saraswat and K. Awasthi, Adv. Electrochem. Sci. Eng., 2013, 1, 118–123.
18. W. Zhang, Adv. Exp. Med. Biol., 2014, 811, 19–43.
19. E. T. Thostenson, C. Li and T.-W. Chou, Compos. Sci. Technol., 2005, 65, 491–516.
20. S. Kamel, eXPRESS Polym. Lett., 2007, 1, 546–575.
21. S. Mallakpour and A. Barati, J. Polym. Res., 2012, 19, 1–8.
22. M. Dinari, S. Mallakpour and V. Behranvand, RSC Adv., 2013, 3, 23303–23309.
23. S. Mallakpour and M. Khani, Polym.–Plast. Technol. Eng., 2015, 54, 541–547.
24. W. Gao, B. Zhou, Y. Liu, X. Ma, Y. Liu, Z. Wang and Y. Zhu, Polym. Int., 2013, 62, 432–438.
25. S. Mallakpour and F. Marefatpour, Synth. React. Inorg., Met.–Org., Nano-Met. Chem., 2015, 45, 376–380.
26. V. Maharani, D. Reeta, A. Sundaramanickam, S. Vijayalakshmi and T. Balasubramanian, Int. J. Curr. Microbiol. Appl. Sci., 2014, 3, 700–705.
27. J. M. Meichtry, N. Quici, G. Mailhot and M. I. Litter, Appl. Catal., B, 2011, 102, 555–562.
28. S. Mallakpour and M. Javadpour, Colloid Polym. Sci., 2015, 293, 2565–2573.
29. D. S. Achilias, A. Giannoulis and G. Z. Papageorgiou, Polym. Bull., 2009, 63, 449–465.
30. L. C. Bichara, H. E. Lanús, E. G. Ferrer, M. B. Gramajo and S. A. Brandán, Adv. Phys. Chem., 2011, 2011, 1–10.
31. A. Ptiček Siročić, A. Fijačko and Z. Hrnjak-Murgić, Chem. Biochem. Eng. Q., 2013, 27, 65–71.
32. S. Vijayakumar and P. Rajakumar, Int. Lett. Chem., Phys. Astron., 2012, 4, 58–65.
33. G. Mamoor, W. Shahid, A. Mushtaq, U. Amjad and U. Mehmood, Chem. Eng. Res. Bull., 2013, 16, 25–32.
34. A. Abdolmaleki, S. Mallakpour and S. Borandeh, Polym. Bull., 2012, 69, 15–28.
35. I. Strain, Q. Wu, A. M. Pourrahimi, M. S. Hedenqvist, R. T. Olsson and R. L. Andersson, J. Mater. Chem. A, 2015, 3, 1632–1640.
36. L. A. Baldenegro-Perez, D. Navarro-Rodriguez, F. J. Medellin-Rodriguez, B. Hsiao, C. A. Avila-Orta and I. Sics, Polymers, 2014, 6, 583–600.
37. S. Mallakpour and M. Madani, Polym.–Plast. Technol. Eng., 2014, 53, 423–428.
38. G. Oyeleke, A. Popoola and A. Adetuyi, J. Polym. Biopolym. Phys. Chem., 2015, 3, 6–11.
39. X. Jiang, S. Luo, K. Sun and X. Chen, eXPRESS Polym. Lett., 2007, 1, 245–251.
40. J. He, W. Shao, L. Zhang, C. Deng and C. Li, J. Appl. Polym. Sci., 2009, 114, 1303–1311.
41. P. Pisitsak and R. Magaraphan, Chiang Mai J. Sci., 2014, 41, 1317–1331.
42. M. Abdul Nabi, R. M. Yusop, E. Yousif, B. M. Abdullah, J. Salimon, N. Salih and S. I. Zubairi, Int. J. Polym. Sci., 2014, 2014, 1–6.
43. N. Padmavathy and R. Vijayaraghavan, J. Biomed. Nanotechnol., 2011, 7, 813–822.
44. A. K. Zak, R. Razali, W. Majid and M. Darroudi, Int. J. Nanomed., 2011, 6, 1399–1403.
45. S. Mallakpour and V. Behranvand, Colloid Polym. Sci., 2014, 292, 2275–2283.
46. M. E. De Sousa, M. B. Fernández van Raap, P. C. Rivas, P. Mendoza Zélis, P. Girardin, G. A. Pasquevich, J. L. Alessandrini, D. Muraca and F. H. Sánchez, J. Phys. Chem. C, 2013, 117, 5436–5445.
47. S. Mallakpour and F. Sirous, Polym.–Plast. Technol. Eng., 2015, 54, 532–540.
48. W. Cui, Q. Jiao, Y. Zhao, H. Li, H. Liu and M. Zhou, eXPRESS Polym. Lett., 2012, 6, 485–493.
49. S. H. Kaleel, B. K. Bahuleyan, J. Masihullah and M. Al-Harthi, J. Nanomater., 2011, 2011, 1–6.
50. S. Mallakpour, A. Abdolmaleki and M. Rostami, Polym.–Plast. Technol. Eng., 2014, 53, 1615–1624.
51. S. Mallakpour and A. Zadehnazari, J. Polym. Res., 2013, 20, 1–12.

---