Modification of Polylactic Acid/ Halloysite Bionanocomposites Using Electron Beam Radiation: Physical, Barrier and Thermal Properties Abdulkader M. Alakrach1,a, Nik Noriman Zulkepli1,b*, Awad A. Al-Rashdi2,
Sam Sung Ting3, Rosniza Hamzah1, Omar S. Dahham1
1Center of Excellence Geopolymer and Green Technology (CEGeoGTech), Faculty of Engineering Technology (FETech), Universiti Malaysia Perlis (UniMAP), Level 1 Block S2, UniCITI Alam
Campus, Sungai Chucuh, Padang Besar, 02100, Perlis, Malaysia.
2Chemistry Department, Umm Al-Qura University, Al-qunfudah University College, Al-qunfudah Center for Scientific Research (QCSR), Saudi Arabia
3School of Bioprocess Engineering, Universiti Malaysia Perlis (UniMAP), Kompleks Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia
a[email protected], b*[email protected]
Keywords: Polylactic Acid, Halloysite Nanotubes, Bionanocomposites, Electron Beam Radiation, Physical properties, Thermal Properties.
Abstract. This study aimed to develop novel Polylactic acid/ Halloysite (PLA/ HNTs) films which showed better properties when they were used for food packaging. They also displayed better mechanical, barrier, morphological and structural properties when the researchers analysed the impact of the electron beam irradiation on the nanomaterials. They prepared PLA-based nanocomposites containing 5 % w/w of HNTs using the solution casting process. These nanocomposites were further exposed to different ebeam doses (i.e., 0, 20, 40 and 60 kGy). The researchers assessed the effect of the electron beam irradiation on the various properties of the PLA. All the composites showed a homogenous dispersion and distribution of the HNTs in this PLA matrix. Results indicated that the nanocomposites showed better barrier properties in comparison to the neat PLA. Furthermore, the ebeam irradiation could increase the glass-transition temperature and lead to the development of more crosslinks, which increased the degradation temperature and hydrophilicity of the nanocomposites.
In this study, the researchers showed that the PLA/HNTs films were effective materials that could be used for the electron beam processing of the pre-packed foods. The best effect was noted for the 20 kGy dosage which was used in the study.
Introduction
In the past few years, there has been an increased interest in the use of different biodegradable materials for applications like medicine, packaging, agriculture and industrial purposes. Out of these applications, the biodegradable polymeric films have garnered a lot of interest for packaging. The biopolymer blends could be used as an alternative to synthetic polymeric materials. Some of the potential and common biopolymers that can be used include chitosan, starch, alginate, gelatine, etc.
Many researchers believe that the use of the biodegradable polymeric materials could decrease the widespread application of the synthetic and non-biodegradable polymeric materials, thereby decreasing the environmental pollution. The non-degradable polymers, made from petrochemical- based materials and used in various packaging applications, can lead to a waste disposal issue. Hence, the application of biodegradable polymer materials for packaging could help in resolving the issue related to the accumulation of the non-degradable wastes [1].
Many studies used PolyLactic Acid (PLA) as the controlled release carrier material for food packaging since it displayed better biodegradability, biocompatibility and mechanical strength [2].
PLA was seen to be hydrolytically unstable. Though it was not water-soluble, it gets degraded due to the hydrolytic attack on its ester bonds, which leads to the formation of glycolic and lactic acids [3].
The hydrolytic degradation rate of PLA was generally controlled by making changes in its physical properties, like molecular weight, glass transition temperature (Tg) and the degree of crystallinity [4].
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (#541126106-22/07/20,13:28:34)
Some of the issues that were noted in the use of this polymer included its poor processing ability, poor availability, high costs, a lower toughness and lower moisture stability [5]. A reinforcement of the material with nanofillers could decrease the costs, which helped in improving the stiffness and enhanced the thermal stability of the material.
Nanotechnology can significantly improve the properties of the plastic materials (either bioplastics or petroleum-based plastics), like the costs, polymer performance and their environmental compatibility. Furthermore, it can also improve the barrier properties, enable a low energy input for production, decrease the storage and transportation-related issues, decrease the waste volume and also reduce the carbon dioxide emission [6]. For instance, a reinforcement of the pristine PLA material with the nanoparticles could significantly improve the mechanical and barrier properties of the material [7]. Thus, the properties of the biopolymer can be improved by nano reinforcement, which helps in producing eco-sustainable materials. These could be effectively used in various food packaging applications.
Halloysite NanoTubes (HNTs) were seen to be 1:1 type (silica tetrahedral and gibbsitic octahedral), naturally-occurring clay particles with a tubular shape. Though the HNTs commonly showed a nanoscale tubular structure, they could display other shapes, like a spheroidal or plate-like structure, and other morphologies based on the geological and crystallisation conditions. The HNTs showed a chemical formula of Al2Si2O5(OH)4.nH2O, wherein n denoted a value of 0 or 2 for the
dehydrated and hydrated states, respectively. The hydrated HNTs showed an interlayer space of 10 Å, while the dehydrated state showed an interlayer space of 7 Å. The HNTs consist of hydroxyl
groups in their inner surface, whereas their outer surfaces display the Si groups [8]. HNTs are a novel candidate for the reinforcing filler material that could be used for the PLA matrix nanocomposites and are used for various food packaging applications. It is very difficult to produce the polymer nanocomposites which show a higher performance but require a low production cost, due to the poor interfacial compatibility seen between the highly polar nanofiller material and non-polar PLA.
The mechanical properties and compatibility of the nanocomposites are significantly improved by the surface etherification, esterification, and graft copolymer or by pre-treating the filler with the coupling agent. All these pre-treatment procedures are complicated, environmentally-harmful and do not display very obvious effects. Hence, these are not considered perfect strategies for modifying the surface properties of the filler and preparing high-performance nanocomposites [9]. Hence, a more effective technique is needed, which displays a better post-processing performance for developing the polymer nanocomposites.
Many researchers have suggested the use of the Electron Beam (EB) technique for modifying the surface properties of the polymer materials like films, fibres, composites and plastics [10]. This technique helps in removing the surface impurities and alters the surface chemical characteristics under proper irradiation conditions. Electron beam processing was seen to be a clean, dry and cold technique which showed many advantages like a high throughput rate, energy saving, uniform irradiation and environment-friendly.
In this study, the researchers have highlighted the effect of different doses of the EB irradiation on the PLA/HNTs bionanocomposite films. They aimed to determine the effect of 3 different doses of the EB irradiations (i.e., 20, 40 and 60 kGy) on the thermal and physicochemical characteristics of the PLA/ HNTs bionanocomposite films. Here, the researchers used a constant 5 % w/w concentration of the Ultra HalloPure (UHP) type of HNTs.
Experimental Materials
A grade of 4032D PLA was obtained (supplier NatureWorks LLC, USA) with an Mn of 133,000 and dispersity (i.e., Mw/Mn) of 1.94 (where Mw and Mn represented the weight and an average number of the molar mass of PLA). The PLA showed the following major characteristics: relative viscosity = 3.94; D isomer = 1.4% and the residual monomer = 0.14%. The HNT nanotubes were acquired from Aldrich and they displayed the following properties: nanotube length ranged between 1-3 mm; surface area was 64 m2g-1; and the pore volume ranged between 1.26 and 1.34 mL g-1. The
HNT nanotubes existed in a hydrated state and water was present between the interlayer spaces (i.e., molecular formula of the nanotubes was Al2Si2O5(OH)4.nH2O where n = 2). The HNT nanofiller used in the study was further characterised using different analytical techniques like SEM, TEM, TGA, XPS and XRD.
Solution Casting Procedure and Sample Preparation. The researchers used the solution casting technique for preparing the PLA/ HNT bio nanocomposite films [11], with some modifications. The PLA matrix and nanofillers were dried for 24 h at 50 °C. Initially, a PLA solution (5% w/w) was prepared in chloroform (3.5 g PLA in 70 mL CH3Cl), which was constantly stirred on a magnetic stirrer for 4 h (500 rpm). The researchers prepared a PLA/ HNT film blend by dissolving HNTs (5%
w/w) in the PLA solution and stirring it at 800 rpm for 16 h. The researchers placed this blend in the ultrasonic bath for 30 min for homogeneously dispersing the HNTs and preventing a bubble formation. This solution was poured in a glass petri dish (diameter = 20 cm), which was covered with an aluminium sheet for decreasing the rate of evaporation of the solvent and preventing the formation of cracks on the film’s surface. These blends were dried at 25 °C overnight. Then, they were placed in a vacuum desiccator and dried for 10 h for extracting the residual solvent from these films. Finally, the films were peeled from the surface of the petri plates by adding a small quantity of cold distilled water.
Electron Beam Irradiation. These moulded films were irradiated with the help of a 3 MeV electron beam accelerator (NHV-EPS- 3000), using different doses, i.e., 0, 20, 40 and 60 kGy. The values for the acceleration energy, beam current and the dose absorbed by every pass were seen to be 2 MeV, 5 mA and 50 kGy, respectively. This ebeam irradiation process was carried out in the air.
Fig. 1. Electron beam accelerator (model NHV-EPS- 3000).
Characterization Method
Contact Angle. The researchers determined the water contact angle (θ) (Rame´-Hart model 250 - Standard Goniometer/Tensiometer, USA) for the PLA/HNTs specimens. All angles were determined at the 2 drop sides, and an average value was considered. The droplet volume of the distilled water
was seen to be 4 ± 0.5 µL. 5 different water contact values were acquired at different regions in the films for increasing the reliability of the results.
Fig. 2. Contact angle (Rame´-Hart model 250-Standard Goniometer/ Tensiometer equipment) device.
Water Vapour Permeability (WVP). In this study, the researchers determined the Water Vapour Permeability (WVP) value based on the ASTM E96-95 standards and a few modifications. For this purpose, they placed silica gel (20 g) in cylindrical cups. Circular films were used for sealing the open ends of these cylinders. In this test, the researchers measured the weights of the cups before placing them in the relative humidity chamber, which showed a 75 ± 2% relative humidity and 25 °C temperature. For obtaining this value of humidity, they made use of a saturated NaCl solution at 25 °C. The cups were measured periodically until a constant weight was noted. The increasing weight of the cups was noted and the WVP value was determined using the following formula:
WVP = (m × d) / (A × t × P) (1) Wherein; m (g) denoted the increase in the weight of the test cup; d (m) represented the film’s thickness; A (m2) represented the area of the exposed film; t (s) represented the time duration required for permeation; while P (Pa) indicated the partial pressure noted across all films (for the water vapour). These results were expressed as g/s.m.Pa.
Thermogravimetric Analysis (TGA). The Thermo Gravimetric Analysis was carried out (TGA) (TGA Q500, TA Instruments) of all the samples. These samples were heated at the rate of 10 °C/min from 25 to 650 °C, under a steady nitrogen gas flow (Sanyang et al., 2015).
Differential Scanning Calorimetry (DSC) Analysis. In this study, the researchers also conducted the Differential Scanning Calorimetric analysis (DSC, METTLER TOLEDO). These samples were heated at the rate of 10 °C/min from 25 to 200 °C, under a steady nitrogen gas flow. Thereafter, the samples were maintained for 3 mins at 200 °C for eliminating their thermal history, before being cooled at the ramping rate of 10 °C/ min to the final temperature of 25 °C. Then, these samples were reheated to 200 °C. The researchers evaluated the cold crystallisation Temperature (Tcc), glass Transition temperature (Tg) and the melting Temperature (Tm) of the samples.
Results and Discussion
Contact Angle (CA). The fundamental wetting feature of CA or the water "Contact Angle"
indicates the extent of hydrophobicity pertaining to a polymeric film or surface hydrophilicity. These are generally applied to assess the film resistance against water [12].
Figure 3 displays the changes in contact angle as a function of electron beam irradiation.
PLA/HNTs film samples exhibited as low a value of CA, as θ = 63.5°. Hydrophilicity of HNTs stands behind this low value of contact angle. It is clear from the CA data that using nanoclay led to enhance ng the surface hydrophobicity of PLA film to a value of 14.7°. According to the results of Chapter 4, the hydroxyl domains of nanoclay can constitute more hydrogen bonds with PLA chains.
Figure 3 shows the effect of different EB irradiation doses on the value of contact angle pertaining to nanocomposite films. Surface hydrophobicity relevant to film samples that had been irradiated using 20 kGy seems to be close to that of non-irradiated nanocomposite film samples (θ=54.9°). Data
pertaining to CA marked that the irradiation of 40 kGy increased the CA value to become (θ=58.4°).
Out of all film samples that were subjected to irradiation processing, the EB 60 kGy film sample scored the highest CA increase reaching a value of θ=63.5°. Even though, the EB irradiation of the nanocomposites resulted to increase the hydrophobicity of the irradiated samples, by producing new terminal hydroxyl groups, which confirmed by FT-IR results, but there is another factor is the increasing of the roughness of the samples surfaces. The SEM illustrated that the roughness of the irradiated samples increased as a function of irradiation doses.
This result goes in line with other results in literature. De Silva et al. (2014) that reported that increasing a material's surface roughness leads to reducing its interfacial tension that is there between water and polymer, aka; surface energy. This makes it hydrophobic, which is a similar phenomenon just like for super-hydrophobic materials [11]. Cwikel et al. (2010) explained the increase of the CA by the increasing of surface roughness in other way, which they stated that the more surface roughness is able to trap more air the more its chances of increasing the hydrophobic properties. Accordingly, it may be deduced that the increase of surface roughness caters for the matrix's hydrophobic properties to be enhanced [13].
0 kGy 20 kGy
40 kGy 60 kGy
Fig. 3. Contact angle (θ) of non-irradiated and irradiated PLA/HNTs samples by EB radiation with different doses.
Water Vapor Permeability (WVP). Water Vapor Permeability (WVP) is a very essential property prone for assessment in this class of materials since water has a very significant role to play in matters of microbial growth and degradation too. In food coating field a very crucial factor is to preserve natural and fresh food products as long as can be with the added need of avoiding any type of microbial attack and/or oxidation [14].
One of the familiar facts in this field of science is that incorporating a nano-filler within a polymeric matrix can reduce the WVP. The high efficiency of that nano-filler also reduces the WVP of PLA/HNTs bionanocomposite film and this is explained in terms of " tortuous path" (Pinnavaia &
Beall, 2000). However, to improve the WVP of PLA/HNTs bionanocomposites film, The EB irradiation treatment was applied in this research. Figure 4. shows the variation in WVP of PLA/HNTs films with different irradiation doses. There is a notable and clearly evident decrease in WVP for non- irradiated PLA/HNTs (1.43×10-10 g/m.s.Pa) to (1.15×10-10 g/m.s.Pa) for PLA/HNTs film irradiated by EB 20 kGy which means that the WVP has been reduced by ~ 20%. The new bonds are formed between PLA chains or might be between PLA chains and HNTs, providing a denser structure in the film matrix. Consequently, H2O molecules hardly pass through the film matrix.
After the irradiation dose 20 kGy, Figure 4 shows that there is an increase in the WVP of EB 40 film relative to other films that had been irradiated too. There is a possibility of occurrence of considerable breakdown of the chain-chain intermolecular interaction in the chitosan. This could be a result of irradiation at 40 kGy. Thus, there is no rigid structure for the film to block the passage of the moisture transport. These results were confirmed by tensile properties and thermographic analyses which proofed the confirmation of crosslinking in PLA/HNTs films till the EB irradiation dose 20 kGy. Research resources for the influence of EB irradiation on barrier property pertaining to biodegradable films are very scarce. Shahbazi et al (2017) demonstrated that the EB irradiation decreased the WVP of chitosan/cloisite (20) film at the level of EB20 and EB30 kGy. In 2005 study conducted by Lee et al, a Gamma irradiation dose of 50 kGy was found to have enhanced barrier property of gluten film. The amount of enhancement was around 29% [15].
Fig. 4. Water vapor permeability of of non-irradiated and irradiated PLA/HNTs films by electron beam irradiation with different doses.
Thermogravimetric Analysis (TGA). Thermogravimetric Analysis (TGA) As given in Figure 5, thermogravimetric analysis was applied to investigate the thermal decomposition conduct of irradiated and non-irradiated PLA/HNTs bionanocomposites. A one-stage weight loss was the common theme among the thermal degradation behavior of all samples. Nevertheless, there is a significant difference between the initial decomposition temperatures pertaining to all samples under investigation. The initial decomposition temperature (T5 wt %) for the non-irradiated sample
(PLA/HNTs - 0 kGy) was around 310 °C while the same temperature for the non-irradiated samples having an absorbed dose of 20, 40 and 60 kGy was as high as 316 °C, decreasing down to 308 and to 239 °C, respectively. The rise in initial decomposition temperature could be corresponding to crosslink structure in nanocomposites that had been formed in times of electron beam irradiation. The decomposition is retarded by virtue of the network forming in PLA nanocomposites. This suggests that the thermal stability of PLA/HNTs bionanocomposite is improved by crosslinking of PLA by electron irradiation. The optimum absorbed dose could be a 20 kGy dose. Degradation at higher doses become more pronounced in contrast with crosslinking. As an example, it is evident from the results that a 60 kGy dose value is very high as for this very nanocomposite system. It shows the relatively low initial decomposition temperature made by the decomposition behaviour of the PLA chain at a high absorbed dose. The initial decomposition temperature after the radiation dose 20 kGy started decreasing. This may be ascribed to PLA degradation inflicted upon being treated with higher levels of energy EB. When treated with high energy electrons, PLA experiences less cross-linking and predominant chain-scission, hence the low reading of initial degradation temperature.
Moreover, the char residue for the non-irradiated PLA/HNTs nanocomposite taken at 400 ºC mounts to 6.02% while the same residues at the same temperature for the PLA/HNTs 20 kGy, PLA/HNTs 40 kGy and the PLA/HNTs 60 kGy irradiated nanocomposites are 8.17, 5.29 and 5.12 %, respectively. The differences are attributed to crosslinking structure during electron beam irradiation since such structure in polymers would lead to more char residues to remain during thermal decomposition. Another point worth noting here is that various absorbed doses influence char
residues and thermal stability at high temperature. The optimum absorbed dose could well be the 20 kGy dose. For dose values higher than that degradation gets more pronounced in contrast with
crosslinking. As an example, here; the dose of 60 kGy is very much high for this very nanocomposite system. It shows the char residues and relatively low initial decomposition temperature inflicted by the decomposition conduct of the PLA chain at a high absorbed dose.
Fig. 5. TGA thermograms of non-irradiated and irradiated PLA/HNTs bionanocomposites by different doses of electron beam.
Differential Scanning Calorimetry Analysis (DSC). Differential Scanning Calorimetry analysis (DSC) was performed to determine, in addition to the thermal properties of the PLA/HNTs bionanocomposite film, the influence of EB with a variety of irradiation doses on the glass transition (Tg), the crystallization (Tcc) and the melting (Tm) temperature of PLA/HNTs samples.
Table 1 shows DSC curves of PLA/HNTs and the different PLA/HNTs samples irradiated by radiation doses of 20, 40 and 60 kGy. Three peaks at temperatures corresponding to the glass transition temperature Tg (60.02 ºC), cold crystallization temperature Tcc (116.13 ºC) and melting
temperature Tm (149. 63 ºC) can be observed in DSC thermogram of non-irradiated PLA/HNTs film.
After irradiation, only two endothermic peaks corresponding to Tg and Tm can be observed whereas the endothermic peak corresponding to Tcc was completely disappeared.
Tg is dependent on its chains mobility and is associated also with changes in the amorphous region
of PLA and is dependent upon the mobility of its chains, Tg is found to be 60.02 ºC, 62,50 ºC, 63.33 ºC and 64.19 ºC at 0, 20, 40 and 60 kGy irradiation doses respectively. It can be noticed that
the Tg of PLA/HNTs slightly increased with the increasing of the irradiation dose. This result could be ascribed to the crosslinking structure that had been introduced into samples of irradiated PLA/HNTs films. the higher the density of crosslinking the shorter the chain length between the formed crosslinks, also, the higher the glass transition temperature [16].
As for irradiated samples, the cold crystallization seemed to have disappeared upon increasing radiation dose rendering the samples as if non-crystalline. This could be a result of increasing temperature causing interference to exist between crystallization and crosslinking structure.
The decrease in Tm that is inversely proportional to irradiation dose is ascribed to chain scission and the forming of PLA chains having lower molecular weight in part. In another part it is ascribed to the number of crystallites in terms of reduction surfaces under the melting peaks and the narrowing crystalline size distribution. Finally, it is ascribed also to the declination in crystalline perfections. A greater mobility is granted to macromolecules due to irregularities forming next to irradiation. This enhanced mobility induces the advent of a disorder phase at lesser temperatures. This conforms to what is already there in the literature regarding other kinds of polymers such as polyhydroxybutyrate or polypropylene [17-18].
Table 1. Transition temperatures (Tg, Tcc and Tm) of non-irradiated and irradiated PLA/HNTs bionanocomposites by EB radiation with different doses.
Specimens Tg (℃) Tc(℃) Tm(℃) WVP ×10-10
(g/m.s.Pa)
PLA/HNTs 0 kGy 60.02 116.13 149.63 1.43
PLA/HNTs 20 kGy 62.50 - 145.31 1.15
PLA/HNTs 40 kGy 63.33 - 144.60 1.27
PLA/HNTs 60 kGy 64.19 - 144.13 1.54
Summary
This study aimed to design novel PLA-HNT films which showed better barrier, physical and thermal properties. These films were developed with the help of the Electron Beam technique for increasing the shelf life of the foods and for cavity treatment. Hence, they used only 4 doses, i.e., 0, 20, 40 and 60 kGy. All nanocomposite films were made from 5% w/w clay and were exposed to various electron beams. These samples were further characterised based on their TGA, DSC, water contact angle and WVPs, before and after they were subjected to an eBeam irradiation. Results indicated that the solution casting process could homogeneously disperse the filler molecules within the organic matrix. It was also noted that the PLA properties were affected by the clay addition and an ebeam irradiation treatment. After the electron beam exposure, all nanocomposite samples showed a higher Tg value and lower water permeability. It was seen that the clay present in these samples created a tortuous path, which enabled the migration of water through these films and increased the crosslink formation after the ebeam exposure. This further increased the crystallinity of the samples, after the clay addition and ebeam processing. Based on the results, it could be concluded that these PLA-based nanocomposite samples showed better barrier and thermal properties compared to only the PLA samples. These could be customised for various applications, as required in the food processing industries.
References
[1] A. M. Alakrach, N. Z. Noriman, O. S. Dahham, , R. Hamzah, , M. A. Alsaadi, Z. Shayfull, S.
S. Idrus, Chemical and Hydrophobic Properties of PLA/HNTs-ZrO2 Bionanocomposites. J. Phys.
1019 (2018) 012065.
[2] M. A. Elsawy, K. H. Kim, J. W. Park, A. Deep, Hydrolytic degradation of polylactic acid (PLA) and its composites. RENEW SUST ENERG REV., 79 (2017) 1346-1352.
[3] S. Lakshmi, D. S. Katti, C. T. Laurencin, Biodegradable polyphosphazenes for drug delivery applications. Drug Deliv. Rev.", 55 (2003) 467-482.
[4] H. K. Makadia, S. J. Siegel, Poly lactic-co-glycolic acid (PLGA) as biodegradable controlled drug delivery carrier. J. Polym., 3 (2011) 1377-1397.
[5] A. M. Alakrach, N. Z. Noriman, O. S. Dahham, R. Hamzah, M. A. Alsaadi, Z. Shayfull, S. S.
Idrus, The Effects of Tensile Properties of PLA/HNTs-ZrO2 Bionanocomposites. J. Phys. 1019 (2018) 012066).
[6] J. M. Lagaron, A. Lopez-Rubio, Nanotechnology for bioplastics: opportunities, challenges and strategies. Trends Food Sci Technol, 22 (2011) 611-617.
[7] E. Espino-Pérez, J. Bras, V. Ducruet, A. Guinault, A. Dufresne, S. Domenek, Influence of chemical surface modification of cellulose nanowhiskers on thermal, mechanical, and barrier properties of poly (lactide) based bionanocomposites. Eur. Polym. J., 49 (2013) 3144-3154.
[8] A. M. Alakrach, N. Z. Noriman, O. S. Dahham, A. A. Al-Rashdi, I. Johari, Z. M. Razlan, W.
Khairunizam, Physical properties of plasticized PLA/HNTs bionanocomposites: effects of plasticizer type and content. Mater. Sci. Eng. 557 (2019) 012067. IOP Publishing.
[9] A. Mautner, M. Hakalahti, V. Rissanen, T. Tammelin, Crucial Interfacial Features of Nanocellulose Materials. Nano. Cellu. Sust., (2018) 87-128.
[10] Y. H. Han, S. O. Han, D. Cho, H. I. Kim, Kenaf/polypropylene biocomposites: effects of electron beam irradiation and alkali treatment on kenaf natural fibers. Compos. Interfaces 14 (2007) 559-578.
[11] R. T. De Silva, P. Pasbakhsh, K. L. Goh, S. P. Chai, J. Chen, Synthesis and characterisation of poly (lactic acid)/halloysite bionanocomposite films. J. Compos. Mater., 48 (2014) 3705-3717.
[12] J. W. Rhim, Effect of clay contents on mechanical and water vapor barrier properties of agar- based nanocomposite films. Carbohydr. Polym., 86 (2011) 691-699.
[13] D.C wikel, Q. Zhao, C. Liu, X. Su, Marmur, Comparing contact angle measurements and surface tension assessments of solid surfaces. Langmuir, 26 (2010) 15289-15294.
[14] P. G. Seligra, C. M. Jaramillo, L. Famá, S. Goyanes, Biodegradable and non-retrogradable eco-films based on starch–glycerol with citric acid as crosslinking agent. Carbohydr. Polym., 138 (2016) 66-74.
[15] M. Shahbazi, G. Rajabzadeh, S. J. Ahmadi, Characterization of nanocomposite film based on chitosan intercalated in clay platelets by electron beam irradiation. Carbohydr. Polym., 157 (2017) 226-235.
[16] T. M. Quynh, H. Mitomo, L. Zhao, S. Asai, The radiation crosslinked films based on PLLA/PDLA stereocomplex after TAIC absorption in supercritical carbon dioxide. Carbohydr.
Polym., 72 (2008) 673-681.
[17] A. M. Alakrach, A. F. Osman, N. Z. Noriman, B. O. Betar, O. S. Dahham, Thermal properties of ethyl vinyl acetate (EVA)/montmorillonite (MMT) nanocomposites for biomedical applications.
MATEC Web Conf. 78 (2016) 01074).
[18] M. G. H. Zaidi, S. K. Joshi, M. Kumar, D. Sharma, A. Kumar, S. Alam, P. L. Sah, Modifications of mechanical, thermal, and electrical characteristics of epoxy through dispersion of multi-walled carbon nanotubes in supercritical carbon dioxide. CARBON LETT, 14 (2013) 218-227.
