Assessment of Mechanical and Morphological Properties of New Poly Lactic Acid ( PLA ) / Wood Fibers / Nanographene Composite

The present study analyzed the effect of nanographene (NG) application at three rates (0.75, 1.5, and 3 %) on the production of PLA85/F15 and PLA70/F30 composites. The incorporation of NG into the PLA/wood fi ber composites signifi cantly improved the tensile and bending properties. The results indicated that composites made from PLA68.5/F30/NG1.5 had the highest tensile and bending properties. Furthermore, it was revealed that the addition of fi bers to PLA decreased the composite impact strength in comparison with pure PLA, whereas the addition of NG improved its impact strength, but not as much as pure PLA. The highest impact strength was observed in the composites containing 15 % fi bers and 1.5 % NG (PLA83.5/F15/NG1.5 composites). The composites made from 30 % wood fi ber and 1.5 % (PLA68.5/F30/NG1.5 composites) NG had the best interfacial surface between the wood fi ber and the matrix of composites.


INTROUDUCTION 1. UVOD
As environmental issues are growingly addressed, the use of materials, which are helpful for sustainable development, increases worldwide.One of the most signifi cant improvements in the recent years has been the use of biodegradable polymers instead of crude oil-based polymers such as polypropylene and polyethylene.Recently, one of the most important biodegradable polymers has been poly lactic acid (PLA).
PLA has some advantages, including relatively high mechanical strength and processing feasibility on most equipment, but it needs to be strengthened for practical uses owing to its inherent brittleness (Petinakis et al., 2009).The use of natural fi bers can be a proper solution for strengthening its mechanical properties (Puglia et al., 2004;Mohanty et al., 2000) and reducing its production costs (Chaharmahali et al., 2014).Lignocellulosic fi bers are a suitable substitution for traditional synthetic fi bers in the production of polymer composite, because they feature such advantages as low density, renewability, biodegradability, wide availability, and low cost (Kim et al., 2009).
In spite of the advantages of PLA for producing PLA/wood fi ber composite, some problems have been reported during the processing, including low fi bers/ matrix interphase adhesion for PLA/natural fi bers composite (Bledzki et al., 2010).The use of nanoparticles in trace quantities is a solution to improve these properties, so that it has been demonstrated to be highly effective.Good dispersion of lignocellulosic fi bers is guaranteed in polymers by the use of nanoparticles because of their high surface area, low density and high Young`s modulus.Among a wide range of nanoparticles, the use of NG has been considerably increased because of their unique mechanical and physical properties.It has been reported that the mechanical and electrical properties of NG-based composites are much better than nanoclay-and other nanocarbon-based composites.It has been shown that polylactic acid (PLA)/ wood fi ber /nanographene has better mechani-cal, thermal, gas penetration and electrical properties than pure polymer due to the extraordinary properties of graphene in comparison with polymer (Balandin et al., 2008;Ansari and Giannelis, 2009).Moreover, it has been reported that the improvement of mechanical and electrical properties of polymer/graphene composites has been much higher than those of nanoclay and other carbon fi ller-based polymer composites (Kim et al., 2009).Despite relatively good graphene distribution in polyethylene matrix, a 20 % improvement is observed in tensile modulus and a 13 % improvement is observed in tensile strength (Lin et al., 2011).
The objective was to add NG to PLA/wood fi ber composite and to improve mechanical and morphological properties.

MATERIJALI I METODE
The present study used the natural fi bers obtained from MDF factory as fi ller for PLA matrix.Then, NG was added to the material at the rates of 0.75, 1.5, and 3 %.

Materijali
The PLA, which was used as the matrix, was procured from Chinese Shanghai Freeman Chemical Co., Ltd.It had the density of 1.25 g•cm -3 , the melting point of 150-170 °C, and the melting fl ow index (MFI) of 15-30 g•10min -1 .The fi bers used in this study were a mixture of 70 % broad-leaves from northern Iran gardens and 30 % spruce fi bers procured from Khazar (Caspian) MDF plant.Table 1 shows some details about fi bers used in this study.

Properties of nanographene 2.1.1. Svojstva nanografena
The NG powder (grade AO-4) was procured from the Graphene Supermarket Co (USA).The average thickness and lateral size of NG was about 12 nm and 4.5 μm, respectively.The purity was 99.2 wt % and the specifi c surface area was less than 15 m 2 /g.According to Table 2, the materials composed of different rates of cellulosic fi bers and NG and PLA were made by an inner mixer.Before the mixing process, the material was oven-dried for 24 hours at 75 °C to remove the moisture.The mixture process was carried out by a mixer at three thermal rates of 170, 175, and 180 °C at 30 rounds per minutes (rpm) for 11 minutes (Internal mixer, Brabender, 2002 element, the W50 model).Compression molding was used for fabricating the standard samples and doing various mechanical experiments.As pre-compression, the samples were heated to 180 °C for 5-6 minutes.Then, they were compressed to 30 MPa for 4-5 minutes.Afterwards, the pressure was slowly removed in 5-6 minutes and the temperature was decreased to ambient temperature (mini test press, Japan, 2002, theWCH model, pressure of 25-35 MPa).Bending and tensile strengths were tested at loading rate of 5 mm•min -1 by INSTRON machine (model 4489) in accordance with D790 and D638 procedures of the ASTM standard, respectively.Notched impact strength was tested by INSTRON machine (model 5102, ZWICK Co.) in accordance with D256 procedure of the ASTM standard.

Scanning Electron Microscopy 2.2.3. Skeniranje elektronskim mikroskopom
Refraction sections of tensile samples were imaged by Scanning Electron Microscopy (SEM) (VEGA, ITESCAN Co., the Czech Republic).The microscope uses the voltage of 20 W. Before shooting, the samples were covered (encrusted) with a 15 nm gold layer for fi ve minutes by Covering Machine (Emitech k450x Quorum Technologies, UK).

Statistical data analysis 2.2.4. Statistička obrada podataka
Data collected from mechanical tests were analyzed by SPSS Software Package on the basis of a one-way analysis of variance.The mean values were compared by Duncan's multiple range test at the 95 % confi dence level.

REZULTATI I RASPRAVA
According to Figure 1, the addition of 15 % fi bers to PLA increased the tensile modulus of PLA/natural fi bers composites.The addition of more natural fi bers (30 %) brought about higher stiffness of composites.As NG was applied, tensile modulus of composites kept its ascending trend.When NG level was increased from 0.75 % to 1.5 %, the highest tensile modulus was obtained among all composites, whilst further increases in NG up to 3 % resulted in the loss of tensile modulus as compared to the composites containing 1.5 % NG.The highest tensile modulus or stiffness was related to PLA composites with 30 % natural fi bers and 1.5 % NG, which was 30 % higher than that of 67 %.PLA70/F30 composites resulted in tensile modulus higher by about 32 % than the PLA83.5/F15/NG1.5 composites.It can be claimed that NG had higher stiffness than matrix polymer, which was the reason for the higher stiffness of PLA/natural fi bers composite (Puglia et al., 2004).Statistical surveys indicate signifi cant changes in tensile modulus with the variations of fi ber/ NG ratio.Duncan classifi cation shows that the addition of fi bers and NG to pure PLA entailed signifi cant differences among various groups.Duncan test classifi ed pure PLA in a separate project, while the use of fi bers at 15 % and three various levels of NG did not present much difference and all projects were almost placed in one class.However, the application of 30 % fi bers and various rates of NG resulted in greater differences among classes.
The analysis of the tensile strength of fabricated composites and their comparison to each other and to pure PLA revealed that, when 15 % wood fi bers were added to PLA composites, the tensile strength of composites was diminished.Even when NG was added to composite structures at the rates of 0.75, 1.5, and 3 %, their tensile strength was lower than that of pure PLA.The reduction in the tensile strength of polymer with the addition of natural fi bers has been confi rmed in several studies (Nourbakhsh and Ashori, 2009).The addition of 30 % wood fi ber to PLA decreased the tensile strength as did the addition of 15 % wood fi ber.The addition of 0.75 % NG to composites increased the tensile strength as compared to the pure PLA and graphene-free composites.A higher level of tensile strength was observed in PLA68.5/F30/NG1.5 composites.The tensile strength of these composites was 26 % higher than that of PLA composites containing 15 % wood fi ber and 1.5 % NG (PLA83.5/F15/NG1.5 composites) and 46 % higher than that of PLA composites containing 30 % wood fi ber (PLA68.5/F30/NG1.5 composites).According to statistical analysis, the changes in the tensile strength were signifi cant.Sun et al. (2005) reported that the rate of fi bers and interfacial adhesion affected the tensile strength.It can be said that, in addition to the application of NG to composite structures, the rate of wood fi ber application can be a determinant of tensile strength.The enhancement of tensile properties among composites containing NG can be related to the higher apparent coeffi cient and high interfacial areas (Balandin et al., 2008).
The overuse of NG (i.e., 3 %) resulted in the accumulation of this particle in composite structures and decreased the composite properties as compared to composites containing 1. Figure 3 shows the variation of bending modulus of the compositions used in the fabrication of PLA composites.The same as seen in tensile modulus in the study on the bending modulus of the fabricated composites and pure PLA, it was revealed that the bending modulus was increased considerably as wood fi ber was applied to PLA.By increasing wood fi ber from 15% to 30% in composite compositions, the bending modulus was remarkably improved.Nevertheless, the addition of NG increased the signifi cance of PLA composites (GhajeBeigloo et al., 2017; Chaharmahali et al., 2014).The composites containing 30 % wood fi ber and 1.5 % NG indicated the highest bending modulus.Adding 1.5 % NG to PLA68.5/F30 composite into wood structure, fi ber showed a 99 % increase as compared to pure PLA and a 46 % increase as compared to PLA83.5/F15/NG1.5 composites.Duncan classifi cation showed no signifi cant difference in stiffness between pure PLA and composites containing PLA percentages and 15 % fi bers.These two structures were placed in one class, while the addition of NG and the application of 30 % fi bers rendered the difference between classes significant, so that they were placed in separate classes.
Figure 4 indicates that the addition of 15 or 30 % wood fi ber did not improve bending strength of PLA composites when compared with pure PLA.On the other hand, none of the studied rates of NG could increase bending strength to that of pure PLA.The application of NG at the rate of 30 % of fi bers applied in PLA composites resulted in a good improvement of PLA composites.One of the most important factors in composite fabrication with NG is the distribution of nanoparticles across the matrix.Improper distribution of nanoparticles can decrease the NG composite properties at high rates of NG application in NG composites (Sheshmani and Amini, 2013).Moreover, the higher bending properties in composites containing 1.5 % NG as compared to other compositions can be attributed to higher stiffness, higher apparent coeffi cient, and higher interfacial connections.
Amani and Seshmani (2013) reported that the application of high rates of NG did not improve the bending properties due to the accumulation of nanoparticles.Statistical surveys showed signifi cant changes in bending properties (modulus and strength) with the variations of the level of fi bers and NG.According to the Duncan classifi cation, the use of 1.5 % NG and 15 % fi bers could enhance the bending strength to as high as that of pure PLA and these two classes were nearly placed in one class and higher rates of fi bers and nanoparticles application increased the bending strength as compared to pure PLA.
Figure 5 depicts the amounts and trend of impact strength of fabricated composites.The strength of material against breakage and the crack initiation and gap in weak spots refl ects the impact strength of composites.These weak spots are mainly located at the interface of fi bers and polymer.The application of wood fi ber decreased PLA composite impact strength as compared to pure PLA.The enhancement of composite impact strength, achieved by increasing wood fi ber, can be related to the accumulation of wood fi ber in composite structures (Nourbakhshand Ashori, 2009).Nevertheless, the application of NG could not increase the impact strength to a level as high as that of pure PLA.However, it should be noted that the application of NG increased the impact strength to a greater extent in PLA composites than in PLA-free composites.Among fabricated composites, PLA83.5/F15/NG1.5 composites had the highest impact strength.When NG is increased by more than 1.5 %, the stress is concentrated and, consequently, some cracks are created in the composites.In a study on polymer/fi bers/NG composites, Sheshmani et al. (2013) reported lower impact strength of composites at the presence of NG.Duncan classifi cation shows signifi cant differences between groups and classifi es each one in a separate class.

Morfologija
To better analyze the results and study the morphology of break level and interstitial area of polymer and wood fi ber, electron imagery was prepared by SEM system from break sections of module in bending test.
Figure 6 shows the distension of fi bers from the matrix and the porosity between the matrix and fi bers.It presents a weak connection area between the two phases, where there was no proper adhesion between the polymer and the matrix, fi nally proving weaker composite properties than the other compositions.
In spite of the presence of 1.5 % NG in the composition of PLA and 15 % wood fi ber, the fi bers fl ow out the matrix with lower rate and less pores.Moreover, the images show that NG settled on matrix more than fi bers, implying better union between NG and matrix than between NG and fi bers.Also, NG did not accumulate.
Figure 6.C shows the PLA82/F15/NG3 composites with fracture surface.As can be seen, NG did not accumulate.The accumulation of NG results in the loss of adhesion between matrix and NG and the loss of proper distribution of NG on fi bers and matrix.Finally, the connection does not emerge between the two phases leading to the fracture and distension of fi bers from the matrix.Figures 6.E1 and E2 show a connection of good interface surface between two sample phases of PLA68.5/F30/NG1.5 composites.Figure 6E clearly shows that the use of 30 % wood fi ber resulted in higher uptake of NG and that NG can settle on matrix and fi bers, causing the adhesion of interface between fi bers and matrix.Here, no accumulation of NG was observed.The good distribution of fi bers among the matrix and NG among the fi bers and matrix was the reason for this good interface connection between the two phases (it can clearly be seen in Figure 6E2).Finally, it results in more effective transfer of tension from the matrix to fi bers.It is the effect of NG on fi bers and matrix that caused the adhesion of interface surfaces.The highest mechanical properties were observed in these composites.

ZAKLJUČAK
The application of natural fi ber in the PLA matrix increased the stiffness of PLA/natural fi ber composites.The increase in fi bers from 15 % to 30 % in composite structures improved the stiffness of the composites.The use of 30 % fi bers and 1.5 % NG (PLA68.5/F30/NG1.5 composites) resulted in the highest stiffness among composites.
As fi ber (15 %) was applied to PLA composites, its tensile and bending strength was decreased as compared to pure PLA, whilst further enhancements of fi bers to 30 % improved the tensile and bending strength.PLA68.5/F30/NG1.5 composites exhibit the highest tensile and bending strength.
The results of impact strength tests revealed that the use of natural fi bers at two rates of 15 and 30 % decreased the impact strength of PLA/natural fi ber composites and even the use of NG could not improve the impact strength of PLA composites as compared to that of pure PLA.
The SEM images show that the application of 30 % fi bers resulted in lower outfl ow of fi bers from matrix and their better dispersion in matrix when compared with the application of 15 % fi bers.Moreover, the application of NG at the rate of 1.5 % presented better dispersion of fi bers in matrix than other rates of NG.
5 % NG.The proper distribution of NG in composite structures has been reported in several studies (Chang et al., 2005; Coleman et al., 2006).These results demonstrate the better interfacial behavior between fi bers and PLA.The enhancement of tensile properties of composites containing nanoparticles is in agreement with previous studies (Li et al., 2011; Hajian et al., 2012; Mohamadi et al., 2013).The signifi cant difference in different groups can be clearly seen in Duncan classifi cation.

Table 1
Details of fi ber used in this study