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High performance green composites of poly(lactic acid) and waste cellulose fibers prepared by high-shear thermo-kinetic mixing Oguzhan Oguz, Kaan Bilge, Eren Simsek, Mehmet Kerem Citak, Abdulmounem Alchekh Wis, Guralp Ozkoc, and Yusuf Z. Menceloglu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02037 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Industrial & Engineering Chemistry Research

High performance green composites of poly(lactic acid) and waste cellulose fibers prepared by high-shear thermo-kinetic mixing

Oguzhan Oguz1,2, Kaan Bilge1,2, Eren Simsek1,2,†, Mehmet Kerem Citak1,2, Abdulmounem Alchekh Wis3, Guralp Ozkoc3 and Yusuf Z. Menceloglu1,2,* 1

Faculty of Engineering and Natural Sciences, Materials Science and Nano Engineering, Sabanci University, 34956, Orhanli, Tuzla, Istanbul, Turkey

2

Sabanci University Integrated Manufacturing Technologies Research and Application Center & Composite Technologies Center of Excellence, Teknopark Istanbul, 34906 Pendik, Istanbul, Turkey 3

Department of Chemical Engineering, Kocaeli University, 41380, Kocaeli, Turkey †

Curretly at Quantag Nanotechnologies, Urla, Izmir, Turkey

* Corresponding author: [email protected] ( Y. Z. Menceloglu) 1

ACS Paragon Plus Environment


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Abstract Green composites of Poly(lactic acid) (PLA) and waste cellulose fibers (WCF) were produced by using a facile technique comprising high-shear mixing within relatively short processing times that facilitates the ease of processing of such materials and ensures the homogeneous dispersion of such fibers in thermoplastics due to shear rates as high as 5200 rpm. Key parameters, such as optimal concentrations, homogeneous dispersion, direct and indirect mechanical contributions of the fibers, interfacial interactions and crystallinity of PLA matrix, were examined for the sustainable production of PLA/WCF green composites with enhanced stiffness, strength, toughness and impact resistance. Briefly, around one-fold, 50% and 20% increase in the elastic modulus, tensile strength and impact strength of PLA, respectively, were achieved by the addition of 30 wt.% WCF. In addition, 87% increase in the impact strength of PLA was also achieved by the incorporation of 5 wt.% WCF.

Keywords: High-shear mixing, green/eco composites, PLA, waste cellulose fibers

1. Introduction

Critical environmental and economical issues have been stimulating research in the mass production of sustainable materials for plastics market that favours low costs and high production rates for decades. Along with new industrial regulations and growing technological needs, biocompatible and biodegradable polymers have attracted particular interest as promising alternatives to petrochemical based plastics.1-7 In this regard, poly(lactic acid) (PLA) is one of the most attractive (bio)polymers due to its high stiffness and strength combined with biodegradability, recyclability and ease of processing that are vital requirements in the rational design and mass production of sustainable materials.3-6 PLA is generally prepared by ring opening polymerization of lactic acid monomers obtained from the fermentation of renewable agricultural raw materials.4-6 It is completely biodegradable. The biodegradation of PLA occurs via hydrolysis to lactic acid that is metabolised by micro-organisms to water and carbon monoxide within a few weeks.6 In contrast to petroleum based plastics, the use of PLA does not contribute a lot to air pollution and greenhouse effect.4 However, inherent brittleness of PLA, which is evidenced by poor impact resistance and fairly low elongation at break, is one of its main limitations in substituting commodity plastics. In addition, PLA is unable to crystallize easily, and thus, has very low degree of crystallinity as compared to conventional semi-crystalline polymers which considerably restricts its employment in durable industrial applications such as automotive and electronic parts.4 Moreover, moisture sensitivity, fast physical ageing, high cost can be listed as other drawbacks of PLA that limit its use in a wide range of disposable and semi-durable applications. Consequently, various design strategies that comprise the physical and/or chemical modification of PLA must 2


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be developed to achieve desired functional properties demanded by various industries. Recently, PLA has been used as matrix in composites composed of different types of fillers employed as reinforcing agents, impact modifiers and nucleation sites to develop high performance materials with versatile functional properties.4 Various design approaches and production techniques, such as solution casting, lay-up/press molding, pultrusion, and extrusion, have been applied to incorporate the fillers into the PLA matrix.4, 5, 8-15 Particularly attractive is the incorporation of natural fibers since they are also renewable, recyclable and biodegradable like PLA. Among the natural fibers, cellulosic fibers are of essential interest for mass production of sustainable materials since they are the most abundant biomaterial with superior physical properties.1-4, 6, 7, 16-35 Over the past years, several approaches have been proposed to combine cellulose fibers with PLA including twin-screw extrusion,6, 16, 18, 21, 26, 35-38 solution casting,31, 33, 39-41 film stacking19, 32, 42 and papermaking-like processing.27 Among these techniques, the twin-screw extrusion is the most common one to produce the cellulose fiber-PLA composites. In most of the studies comprising extrusion process, although the elastic modulus is generally improved in the composites obtained by this technique, unfortunately, the tensile strength is not notably increased by the addition of cellulose fibers.6, 16, 21, 26 For instance, Oksman et. al.6 prepared flax fiber reinforced PLA based composites using extrusion and compression molding. The authors found that the elastic modulus of PLA was increased from 3.4 GPa to 8.3 and 7.3 GPa by the addition of 30 and 40% fiber, respectively. They also reported that the tensile strength of the composites with 30 wt.% fiber (53 MPa) was slightly higher than that of pure PLA (50 MPa). However, the tensile strength of the pure PLA decreased to 44 MPa by the addition of 40 wt.% fiber.6 Later on, different chemical treatments, compatibilizers and coupling agents have been used to improve the mechanical properties of cellulose fiber reinforced PLA based composites by removing non-cellulosic species in cellulose fibers and employing functional groups to enable better polymer-filler interaction and higher interfacial strength in the composites.43-46 For instance, Garcia et. al.43 used maleated-PLA in kenaf reinforced PLA based composites which were prepared by extrusion and injection molding. It was found that the addition of 30 wt.% fiber resulted in a decrease (approx. 10%) in the tensile strength of the pure PLA, although a notable increase (almost 71%) in the elastic modulus was recorded for the same composites. To address this issue, most recently, different forms of cellulose, such as microcrystalline cellulose (MCC), cellulose nanocrystals (CNC) and nanowhiskers (CNW), have attracted great interest to enhance the mechanical properties of PLA while maintaining other properties.39-41, 47 However, the preparation of MCC, CNC and CNW fillers significantly affects the mechanical properties of these composites. To provide better dispersion and interfacial strength in the final materials, they are required to be chemically treated by different methods before adding into the composite materials. Although promising 3



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results have been recorded in these composite materials,30 the necessity of control over the additional reactions, such as acid hydrolysis, chemical treatments, and processing steps to produce nanocrystalline cellulose presents some other challenges and increases the total production cost during the whole process. The high prices of commercially available nano-cellulose products also generate an additional cost in the production of commercial green/eco composites. When they are used as reinforcing agents in the composites without any pre-treatment, it is quite difficult to achieve a homogeneous dispersion and a better interfacial strength, and thus, enhanced mechanical properties in the final materials. For instance, Matthew et. al.26 prepared MCC, which was used as received without any chemical treatment, reinforced PLA based composites using twinscrew extrusion and injection molding. It was found that the tensile strength of pure PLA significantly decreased from 49.6 MPa to 36.2 MPa with the addition of 25 wt.% MCC, which resulted in a slight increase in the elastic modulus of the pure PLA from 3.6 GPa to 5.0 GPa.26 Consequently, it still remains a challenge to find an economical and efficient way to produce cellulose reinforced PLA based composites with enhanced mechanical properties. To address this issue, we have developed an economical design/processing approach that relies on the fundamental principles of sustainable production. Our approach suggests the reuse of waste cellulose fibers as an effective reinforcing agent in PLA, in order to reduce the total production costs arising from the cost of cellulose fibers and processing. Considering the market share of primary cellulose fibers in relevant industries, such as textiles, it is worth noting that the reuse of waste cellulose fibers as a reinforcing agent in thermoplastics, such as PLA, provides an effective way of addressing the world-wide disposal problem arisen from the vast use of these materials in a wide range of applications. To highlight how much waste can be recycled via this method, a quantitative estimation can be done based on cotton fibers since it is the largest segment of textile production. Roughly, cotton accounts for 90% of all natural fibers exploited in the textile industry. As of 2015/16, annual production of cotton is about 101.4 million bales (1 bale is equal to 480 pounds) which is estimated to rise 4% in 2016/17 to 105.5 million bales all over the world.48 Even though higher percentages are available in the literature, generally 10% of the cotton is classified as processing waste which also depends on the quality of manufacturing facilities. Hence, total amount of the waste cotton fibers approximately corresponds to 10 million bales, which can be recycled as reinforcing filler in thermoplastics. Apart from this, the reuse of waste cotton fibers as a costeffective reinforcing agent in thermoplastics, particularly at high filler concentrations, also corresponds to the reduced consumption of thermoplastics during the mass production of such composite materials. For instance, the reuse of 30 wt.% waste cotton fibers in a PLA based product basically corresponds to 30 wt.% reduction in PLA consumption during its production. Furthermore, our approach also suggests a facile technique comprising high-shear mixing that facilitates the ease of processing of such composites due to shear rates as high as 5200 rpm and relatively short processing times (less than 4


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1 minute per 100 gram). More importantly, significantly high shear rates also ensure a fairly homogeneous filler (fiber) dispersion and better polymer-fiber interaction, and thus, higher interfacial strength in the final materials. On this basis, the main aim of this study is the production of PLA/WCF green/eco composites with enhanced mechanical properties by using waste cellulose fibers as a cost-effective reinforcing agent. By the point of view, waste-cellulose fiber reinforced PLA based composites were prepared by high-shear thermo-kinetic mixer. These masterbatches were granulated and used in the preparation of standard test specimens by injection molding. From a processing perspective, PLA/WCF composites were chosen as a model system in order to demonstrate an economical and effective way of producing costefficient green composite materials. The processability of the green/eco composites at 1-30 wt.% fiber contents, the reuse of natural wastes as notable reinforcing agent and also as a way of reducing PLA consumption during the composite production, which provides an additional cost-saving during processing, and the advantages of the high-shear mixing technique were reported. Furthermore, the effect of waste-cellulose fiber content on the structure-property behavior of PLA/WCF green/eco composites was systematically investigated by several characterization techniques. To provide a comprehensive overview on the structure-property behavior of the materials, the results obtained from the thermal, mechanical, morphological and rheological studies were discussed in the following sections. 2. Experimental 2.1. Materials A commercial grade of PLA (Ingeo 4043D) with a weight average molecular weight of 150 kDa was purchased from NatureWorks LLC (Minnetonka, MN). Waste cellulose fibers (WCF) with an average diameter of 7-20 μm were kindly supplied by a local textile company in Istanbul, Turkey. The SEM micrograph of WCF was given in Figure 1. Along with this, the fibers were also characterized by FT-IR, XRD and DSC analyses as provided in Supporting Information (see Figure S1-S3, Figure S5 and Figure S8). 2.2. Fabrication of composite materials All masterbatches were prepared by a Gelimat G1 laboratory scale high-shear thermo-kinetic mixer (Draiswerke, USA). In this technique, the particles are generally accelerated by the blades on a high-speed shaft. This acceleration basically imparts a fairly large amount of kinetic energy to the particles. This kinetic energy is mainly converted to the thermal energy when the particles collide with the chamber wall. The mixing of compounds most likely takes place between the tip of the blade and the wall mainly because of the centrifugal forces generated by the rotating blade. The shear rate calculations were previously reported in detail by Gopakumar and Page. 49 In accordance with these calculations by the authors, the dimensions of the Gelimat mixing chamber/shaft, and the rotational speed, imply that significantly high shear rates (about 5



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104 s-1) can be generated between the tip of the blade and the walls in this system.49 For instance, these authors calculated the shear rates of 16400 and 13900 s-1 at the tip of the blade and at the wall, respectively, at a blade speed of 6000 rpm.49 In our study, the mixer was operated with a shaft speed of 5200 rpm until the compounds temperature reached to 190 ˚C. Using the same approach reported by Gopakumar and Page, 49 the shear rates at the tip of the blade and at the wall, respectively, were calculated as 14200 and 12060 s-1 at a blade speed of 5200 rpm. Details related to thermokinetic mixing can also be found elsewhere.49-51 All batches were prepared with an identical load of 100 g. The compounds were then granulated with a typical lab-scale granulator under ambient conditions, and subsequently injection molded using an Xplore Instruments 12 mL injection molding machine. The injection molding temperature was 190 ˚C. The injection pressure was 10 bars and the mold temperature was 30 ˚C. The cycle time was totally 10 seconds. The compositional details of the materials were listed in Table 1 along with the applied processing conditions. The code of PLA/WCF-X was used in the sample notation where the X represents the content of waste cellulose fibers in the composites as given in Table 1. 2.3. Characterization Techniques Melt flow index (MFI) measurements of the materials were determined according to ISO 1133 standart at 190 ˚C under 2.16 kg load. At least five specimens were tested for each composition. Differential Scanning Calorimetry (DSC) analyses were performed by a TA Q2000 instrument calibrated with indium standard and equipped with Tzero functionality. All measurements were performed between 20 and 250 °C at a heating rate and a cooling rate of 10 °C/min under the nitrogen atmosphere. We applied a typical procedure for the sample preparation. For each composition, a small piece of injection-molded sample, weighting around 10 mg, was encapsulated in a standard DSC pan prior to measurement. Glass transition temperatures were determined from the inflection points. Degree of crystallinity values were calculated by using Equation 1; 𝑋𝑐 =

∆𝐻𝑚−∆𝐻𝑐𝑐 ° 𝑤𝑓 ∆𝐻𝑚

×100%

(1)

° where 𝑋𝑐 , ∆𝐻𝑚 , ∆𝐻𝑐𝑐 , 𝑤𝑓 and ∆𝐻𝑚 represent the degree of crystallinity, the melting enthalpy, the cold-crystallization

enthalpy, the weight fraction of PLA in the composite materials and the latent heat of fusion value of 100% crystalline PLA that is denoted as 93 J/g in the literature, respectively.52, 53 A SUPRA 35VP, LEO field-emission Scanning Electron Microscope (FE-SEM) was used to monitor fiber and composite morphologies. For the imaging of tensile and impact fracture surfaces, fractured samples were used after stress-strain and impact tests, respectively. All the samples subjected to tensile and impact tests were coated with an ultra-thin layer of Au/Pd alloy and carbon prior to analyses, respectively. The tensile tests were performed in accordance with ASTM D3039 standard at a cross-head displacement rate of 2 mm/min 6


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on a Zwick Z100 model universal tester. The impact tests were conducted according to ISO 179 unnotched Charpy standard for injection molded samples using a CEAST impact tester. At least five specimens were tested for both analyses. All the tests were carried out under ambient conditions. Moreover, the samples were also characterized by infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The experimental procedures and corresponding results were provided in Supporting Information (see S1 and S2). 3. Results and Discussion 3.1. Characterization of waste cellulose fibers Figure 1 shows the SEM micrograph of waste cellulose fibers used in the production of PLA/WCF green composites by high-shear thermo-kinetic mixer. The fiber diameter was determined as 7-20 μm. The lengths of fibers ranged from several hundred microns to millimeters. 3.2. Melt Flow Index The MFI of neat PLA was found to be 6.2 g/10 min. Figure 2 displays the change in the MFI as a function of WCF content by weight. As shown in Figure 2, MFI, i.e. the flow rate decreases by increasing WCF content as expected. The MFI values of the green/eco composites were measured as 5.9, 5.5, 4.9, 4.3 and 3.9 g/10 min for PLA/WCF-1, PLA/WCF-5, PLA/WCF-10, PLA/WCF-20 and PLA/WCF-30, respectively. As a clear-cut evidence, it should be noted that these results mainly suggest that the green composites produced in this work can be easily melt-processed by various manufacturing techniques. 3.3. Thermal Properties DSC analyses were performed to investigate the thermal properties of the samples. Particularly, glass transition (Tg), coldcrystallization (Tcc) and melting (Tm) temperatures were evaluated by using the data obtained from the first heating scans of the specimens. They were also used to determine the degree of crystallinity values of the molded samples with regard to the waste cellulose fiber content since the mechanical properties can only be influenced by the thermal histories of the molded materials rely on the applied processing conditions. The first heating curves of the molded samples were presented in Figure 3a. The corresponding results were quantitatively listed in Table 2. As reported in Table 2, although the Tg values of the green composites are slightly higher than that of neat PLA, the Tg of PLA was not significantly affected by the addition of waste cellulose fiber up to 30% by weight. Slight differences were also observed in the peak temperatures of Tcc and Tm. The degree of crystallinity of molded sample of neat PLA was calculated as 10.3% using the equation 1. As shown in Figure 3b, it was significantly increased by the addition of waste cellulose fibers up to 5 wt.%, which could be due to the increase in the number of available nucleation sites giving rise to the formation of higher crystallinity.46 The changes in the degree of 7



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PLA crystallinity are almost 27% and 70% with respect to the addition of 1 and 5 wt.% WCF, respectively. Apart from this, further addition of WCF resulted in a gradual decrease in the degree of crystallinity. However, all the degree of crystallinity values recorded in the composites are higher than that of the pure PLA. These results mainly suggest that the incorporation of WCF into PLA matrix contribute to its crystallization particularly at low concentrations up to 5 wt.% under the applied processing conditions. Although it is not the main focus of the study, these results lead us to investigate the role of WCF fibers on the PLA crystallization without any contribution arisen from the thermal histories of the samples. For this purpose, the samples were heated up to 250 ˚C at a heating rate of 10 ˚C/min. They were kept at this temperature for 3 minutes and then cooled to 20 ˚C at cooling rate of 10 ˚C/min. After 3 minutes at 20 ˚C, again they were heated up to 250 ˚C at the same heating rate. For particular interests, corresponding thermograms and data obtained from the second heating scans were provided in Supporting Information (see Section S3, Figure S9 and Table S1). 3.4. Tensile properties Representative stres-strain curves of the PLA and PLA/WCF composites recorded at ambient temperature (T < Tg) were given in Figure 4. Accordingly, the elastic modulus (E), ultimate tensile strength (σmax) and strain at break (ε) values for the samples were listed in Table 3. As the common matrix for all composites, the tensile response of neat PLA was elastoplastic with distinctive elastic deformation, yield point, necking followed by a strain-softening regime. Along with the pure PLA samples, the composites with 1, 5 and 10 wt.% WCF contents (PLA/WCF-1, PLA/WCF-5 and PLA/WCF-10) also showed a similar necking and strain-softening behavior. On the contrary, this behavior was not observed for the samples with 20 and 30 wt.% WCF contents. It has been proposed that the nucleation and motion of screw dislocations in the crystalline domains govern the yielding in the semi-crystalline polymers which also depends on the stress transmitters concentration.54, 55 The stress transmitters (ST) have been defined as a group of all the essential elements, such as tie molecules, entanglements and crystal-amorphous interphase, which influence the mechanical coupling between the crystal and amorphous phases.54 As direct methods have not been reported yet to measure the ST concentration, Humbert et. al. 54 proposed that the “neck width” can be used as an indicator to associate the intensity of plastic deformation with the ST concentration. Basically, the higher ST concentration corresponds to the broader neck. On the other hand, it has been well documented that the yielding is associated with the crystallinity in terms of yield stress. As the crystallinity comprises both the dislocation model and the stress transmitter density by means of the crystal thickness and the long period, respectively.54 Considering overall stress-strain properties of composites as a result of mechanical contributions of matrix and fiber phases, this indicator can be used to identify the governing parameters in such materials. As it can be clearly seen in Figure 4, the neck width is increased by the addition of 8


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1 and 5 wt.% WCF into PLA matrix. This mainly indicates that the ST concentration in PLA/WCF-5 is higher than that of PLA/WCF-1, which is also higher than that of pure PLA. This is in accordance with the change in crystallinity as shown in Figure 3b. Therefore, it should be noted that mechanical contributions of the stress transmitters increased as the crystallinity, the neck width, and thus, the ST concentration increased by the addition of 1 and 5 wt.% WCF into PLA matrix. Accordingly, the intensity of plastic deformation and the yield stress in PLA/WCF-5 are higher than those of PLA/WCF-1, which is also higher as compared to those of pure PLA. The increase in the intensity of plastic deformation and in the yield stress with regard to the addition of 1 and 5 wt.% WCF mainly suggests that the stress transmitters play a key role on the overall stress-strain properties of the pure PLA and the composites with 1 and 5 wt.% WCF. Along with the role of stress transmitters, the mechanical contribution of the fibers must be also noted for PLA/WCF-1 and PLA/WCF-5. The role of fibers in these two composites can be understood from two different mechanical aspects, which can be referred as direct and indirect contributions of the fibers to the overall mechanical properties of the composites. The direct contribution of the fibers is quite straightforward which is basically arisen from the intrinsic mechanical properties of the fibers. In contrast, the indirect contribution of the fibers is a bit complex. However, it can be highlighted as follow: The fibers at low concentrations, such as 1 and 5 wt.%, contribute to the matrix crystallinity as shown in Figure 3b. As the crystallinity increases, the ST concentration increases too. This can be followed by the increase in the neck width as it is clearly seen in Figure 4. As the ST concentration increases, the ability to transmit the applied stress through the lamella stacks increases too. This leads to the distribution of stresses on the edge of crystallites more effectively. For this reason, the intensity of plastic deformation and the yield stress increase simultaneously. However, this phenomenon is no longer valid after the addition of 10 wt.% WCF into PLA matrix since the yield stress continues to increase while the intensity of plastic deformation begins to decrease and totally disappears with the addition of 20 and 30 wt.% WCF as can be clearly seen in Figure 4. In this respect, it can be concluded that the ST concentration, and thus, the crystallinity, i.e. the matrix properties, become more effective on the overall mechanical characteristics of the composites with 1 and 5 wt.% WCF which are mostly effected by the indirect contribution of the fibers. In contrast, the direct mechanical contribution of the fibers becomes more prominent on the overall stress-strain properties of the highly filled composites like PLA/WCF-20 and PLA/WCF-30. At this stage, although it is not clear for the composite with 10 wt.% WCF, governing role of the direct mechanical contribution of the fibers can be easily revealed by monitoring the fracture behavior of the material. We will discuss this point later. Figure 5 displays the effect of WCF content on the elastic modulus, tensile strength and strain at break values of the materials. As shown in Figure 5a, the elastic modulus of PLA linearly increased by increasing WCF content which is in 9



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accordance with the literature.46 The maximum E value (6.2 GPa) was obtained in the composites with the highest WCF content (30% by weight) which is one-fold higher than that of the pure PLA samples (3.1 GPa). Along with this, the incorporation of WCF also resulted in a remarkable enhancement in the ultimate tensile strength of PLA matrix. The maximum tensile strength (94.6 MPa) was also obtained in PLA/WCF-30 composites which is approximately 55% higher than that of the pure PLA specimens (60.8 MPa) as listed in Table 3 Moreover, to the best of our knowledge, the tensile strength of PLA/WCF-30 composites is also higher as compared to those of both treated and untreated (natural) fiber reinforced PLA based composites with similar fiber contents reported in a fairly large number of studies available in the literature.3,

6, 45, 56-58

Nevertheless, similar results have also obtained in a limited number of studies comprising different

types of fibers and processing techniques.18, 32 For instance, Bledzki et. al.18 prepared man-made cellulose fiber reinforced PLA based composites using single-screw extrusion and injection molding along with a special pre-coating technique. The authors reported the highest tensile strength as 92 MPa, which have been achieved by the addition of 30 wt.% man-made cellulose fibers into PLA matrix.18 The elastic modulus in these composites have been recorded as 5.8 GPa,18 which is fairly comparable to that of PLA/WCF-30 specimens as listed in Table 3. The referred studies mainly indicate that the incorporation of natural fibers usually results in a notable increase in the elastic modulus of PLA. However, the ultimate tensile strength of the composites significantly depends on the fiber type and processing method. In our case, we believe that the remarkable enhancements in both elastic modulus and tensile strength of PLA are mainly originated from the strong polymer-fiber interactions favoured by the fairly homogeneous dispersion of WCF in the PLA matrix as a direct result of high-shear mixing technique. This basically ensures the effective stress transfer from matrix to fibers, which leads to the significant improvement in overall strength of the composites. This claim can be evidenced by following the change in the tensile strength of PLA as a function of WCF content. As also shown in Figure 5a, the tensile strength of pure PLA gradually increased with increasing WCF amount. Unlike the elastic modulus, the tensile strength is not a linear function of increasing WCF content, which mainly implies a threshold with the further addition of WCF into PLA matrix. Similar trend in the tensile strength of PLA as a function of increasing fiber content was also observed in the literature.46 The gradual increase in the tensile strength also indicates that the total (direct and indirect) contribution of WCF to the overall composite strength was somewhat decreased by increasing WCF content. For instance, the addition of 10 wt.% WCF into PLA matrix resulted in 18 MPa increase in the tensile strength of pure PLA. However, only 6 MPa increase was recorded in the tensile strength when the amount of WCF was increased from 20 to 30 wt.% in the composites. This trend can be explained by the inevitable increase in the probability of fiber-fiber interactions with regard to increasing fiber content in the composites. This claim is supported by the SEM studies on the fracture 10


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surfaces of tensile specimens as shown in Figure 6. The results obtained from the SEM studies will be discussed in detail later. However, the fiber dominated fracture behavior of the composites with 20 and 30 wt.% WCF contents (Figure 6.e and 6.f) can be considered as a clear-cut evidence for this argument. On the other hand, Figure 5b displays the change in the strain at break (failure strain) of PLA as a function of WCF content. As it can be seen in Figure 5b, the failure strain of PLA increased with the incorporation of 1 and 5 wt.% WCF and then subsequently decreased by the further addition of 10, 20 and 30 wt.% WCF. The failure strain in PLA/WCF-1 and PLA/WCF-5 are 22 and 34% higher than that of pure PLA, respectively. In contrast, the ones in PLA/WCF-20 and PLA/WCF-30 are 28 and 37% lower as compared to that of pure PLA, which has a similar value with PLA/WCF-10. In most of the studies based on natural fiber reinforced PLA based composites, the fiber content have been varied between 10 and 40% by weight. In these reports, it is noted that the failure strain decreases with increasing fiber content, which is also valid for the composites comprising similar amounts of waste cellulose fibers reported in this study. In contrast, to best of our knowledge, there is no clear physical explanation for the abrupt change in the failure strain as a function of low fiber concentrations such as 1 and 5 wt.% as it can be seen in Figure 5b. However, this behavior can be explained by following the key mechanisms behind the strain-softening regime as in line with the phenomenon based on the dislocation model and the stress transmitter concentration discussed above. Recently, Jabbari-Farouji et. al. demonstrated that the strain-softening regime is mainly governed by deformation of crystallites via reorientation of chain-folded lamellae toward the tensile direction and fragmentation of some of the larger crystalline domains.55 From this perspective, we basically consider that as the crystallinity increases, the number of crystallites, and thus, the strain required deforming/reorienting the crystallites increases too. As the crystallinity increases with the addition of 1 and 5 wt% WCF (Figure 3b), the strain-softening regime becomes broader and broader in PLA/WCF-1 and PLA/WCF-5 as compared to that of pure PLA (Figure 4). Hence, these two composites are eventually broken at higher strain values in comparison to pure PLA. This is also somewhat valid for the composite with 10 wt.% WCF, however, it is not easy to identify in a clear manner since its behavior has begun to be dominated by the fibers. However, the governing role of the fibers with regard to increasing fiber concentration can be clearly revealed by the loss of a strain-softening regime in the composites with 20 and 30 wt.% WCF. Although the crystallinity values of these two samples are slightly higher than that of pure PLA (Figure 3b), they do not display a strainsoftening behavior (Figure 4) as they are mainly dominated by the fibers. This can also be understood by monitoring the fracture surfaces of these composites (PLA/WCF-20 and PLA/WCF-30) after tensile tests as shown in Figure 6e and 6f, respectively. The SEM studies on the tensile fracture surfaces provide clear evidences for the tensile properties of the materials. Ductile 11



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to brittle transition in mechanical behavior was also observed in the fracture analysis done on tensile specimens. For neat PLA the failure surface (Figure 6a) included deformation marks corresponding to both brittle and ductile failure events. The discrete hills and holes in the fracture pattern represent the brittle failure mode whereas the wavy formation of streamlines mainly corresponds to the ductile failure mode as shown in Figure 6a. Figure 6b representatively shows the SEM micrograph recorded on the fracture surfaces of PLA/WCF-1. With only 1 wt.% WCF addition, the failure pattern of PLA is highly altered and typical plastic deformation marks near WCF reinforced regions was observed as given in Figure 6b. This mainly suggested that the presence of WCF forced the specimens to go through shear especially when fiber/matrix debonding had occurred during tensile testing. When WCF content was increased to 5 wt.%, the amount of debonding holes and plastic deformation marks increased significantly as it can be seen in Figure 6c. Furthermore, smaller plastic deformation marks were observed to concentrate on the debonding regions in accordance with the stress-strain curves which suggested that the specimens encountered a longer ductile deformation. The plastic deformation marks and debonding holes were highlighted in detail on the SEM micrograph given in Figure S10a in Supporting Information (see Figure S10a). Along with this, the additional mechanisms like fiber pull-out and fiber fracture were also observed as shown in Figure S10a in Supporting Information (see Figure S10a). On the other hand, Figure 6d displays the SEM micrograph recorded on the fracture surfaces of PLA/WCF-10 composites. With the presence of 10 wt.% WCF, density and sizes of the plastic deformation marks as well as the debonding hole regions decreased. Instead, a more brittle failure pattern, where relatively large parts of the specimens were instantaneously broken and separated from the failure surface, was observed. This behaviour can be identified from the roughness of the failure surface. Sharing the same magnification with previous cases, the fracture region in Figure 6d corresponds to a large hill/hole formed with the separation of neighbouring parts during the ultimate fracture. As suggested in stress-strain curves, the ultimate failure stress increases with increased WCF amount which means that more load was being carried by WCF. With that in mind, after the failure of fibers (either by debonding or individual fiber fracture) PLA matrix experienced a stress higher than it can carry. Hence a sudden brittle crack formation occurred leaving hills/holes on the fracture surface. However, the composite with 10 wt.% WCF still displays some local plastic deformation marks, such as tearing of matrix, particularly in the matrix phase adhered to the fibers as shown in Figure S10b in Supporting Information (see Figure S10b). On the other hand, fracture surfaces of PLA/WCF-20 and PLA/WCF-30 specimens were dominated by aforementioned brittle fracture indicators and local fiber debonding holes as well as given in Figure 6e and 6f, respectively. The plastic deformation marks were almost totally lost. The specimens could not go through a significant ductile deformation but failed after the yield point instead. These observations are also in accordance with the results obtained from the tensile tests of these two specimens as shown in Figure 4. 12


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3.5. Impact properties One of the particular challenges in PLA based composites is to improve the impact properties of the final products in an effort to promote their use in a wide range of industrial applications.59 Impact strength of a material basically defines its ability to resist fracture under force applied at high speed.1 As widely reported, natural fiber reinforced composites can be used as an alternative to the ones reinforced by glass fibers due to their comparable properties.1 However, relatively low impact properties of natural fiber reinforced composites have listed as one of their main drawbacks.1 On the other hand, contrary results on the effect of fiber content on the impact properties of natural fiber reinforced composites have reported by different research groups in the literature. For instance, Bledzki et. al.18 have achieved a significant enhancement (3.6 fold) in the impact strength of PLA by the addition of 30 wt.% man-made cellulose fibers. They have also reported 2.4 fold increase in the impact strength of PLA by the incorporation of 30 wt% abaca fibers.18 On the contrary, Oksman et. al. have reported a decreased impact strength for the PLA/flax composites (60/40).6 In this study, we investigated the impact properties (unnotched Charpy) of the pure PLA and PLA/WCF composites at 23 ˚C as a function of WCF content. The corresponding results obtained from the impact tests of the materials were listed in Table 3. The effect of WCF content on the impact strength of PLA was given in Figure 7. As shown in Figure 7, the impact strength of PLA significantly increased with the incorporation of 1 and 5 wt.% WCF. The impact strengths of PLA/WCF-1 and PLA/WCF-5 are 44 and 87% higher than that of pure PLA, respectively. The composite with 5 wt.% WCF (PLA/WCF-5) displayed the highest unnotched Charpy impact strength at 23 ˚C. The further addition of WCF resulted in a decrease in the impact strength as compared to that of PLA/WCF-5. The impact strengths of the composites with 10, 20 and 30 wt.% WCF, which were quite similar to each other, were slightly (15%) higher than that of pure PLA samples at ambient conditions. It has been well documented that the impact properties are mainly driven by a number of deformation and failure mechanisms in the composites.60-62 For the composites with semi-crystalline polymers and rigid fillers, remarkable studies have been reported to explain the key mechanisms behind the enhanced toughness and impact properties of such materials. For instance, Bartczak et. al.60 and Zuiderduin et. al.62 have been reported the toughening of high density poly(ethylene) and polypropylene with rigid calcium carbonate particles which mimics the rubber toughening mechanism widely discussed in the literature.63 As the waste cellulose fibers are intrinsically stiffer than the PLA matrix, enhanced toughness and impact properties of the composites reported in this study can be explained by this phenomenon, which comprises three main steps listed as stress concentration, debonding and shear yielding. The waste cellulose fibers behave like stress concentrators due to their intrinsically different elastic properties in comparison to the matrix. This gathers the stress around the fibers, which leads to debonding at polymer-fiber interphase. Debonding of the fibers gives rise to the formation of voids that changes the 13



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stress state in the matrix phase enclosing the voids. This mechanism also releases the volumetric strain in and stimulates the shear yielding in the matrix phase as an effective way to dissipate huge amount of energy upon deformation and failure resulting in an increase in toughness and impact properties of the composites. On this basis, such materials are usually expected to display extensive plastic deformation in the matrix phase accompanying crack propagation. Detailed interpretation of this phenomenon can be found in elsewhere.61, 62 To provide clear-cut evidences for the presence of these mechanisms in the green composites of PLA and WCF, SEM micrographs recorded on the fracture surfaces of the samples subjected to the impact tests were given Figure 8. The impact direction and deformation marks were highlighted by thick and dashed yellow arrows on the micrographs, respectively. As representatively shown in Figure 8a, typical deformation marks related to the brittle fracture were observed on fracture surfaces of the pure PLA samples. In contrast, deformation marks on the fracture surfaces significantly changed solely by the addition of 1 wt.% WCF into PLA matrix as given in Figure 8b. The debonding holes marked on the SEM micrograph in Figure 8b indicate the localized plastic deformation of the matrix phase which is mainly responsible for the energy dissipation during the impact tests of PLA/WCF-1 samples. Along with this, the additional mechanisms64 like fiber pull-out46, 64, fiber fracture46, 64 and fiber buckling,64 which contribute to the energy dissipation improving toughness and impact properties were also observed on the fracture surfaces of PLA/WCF-1 composites as highlighted in Figure 8b. Among these mechanisms, detailed view of fiber buckling in PLA/WCF-1 was representatively shown in the high magnification SEM micrograph, which also comprises several debonding holes as provided in Figure S11 in Supporting Information (see Figure S11). On the other hand, Figure 8c represents the SEM micrograph recorded on the fracture surfaces of PLA/WCF-5 composites. Extensive plastic deformation of the matrix formed by shear yielding mechanism was highlighted on the SEM micrograph (Figure 8c) along with the related debonding holes in PLA/WCF-5, which have the highest impact strength among the materials reported in this study. Along with this, the composites with 5 wt.% WCF also show additional mechanisms like fiber pull-out and fiber fracture as given in Figure S12a in Supporting Information (see Figure S12a). The SEM micrograph in Figure S12a also demonstrates the significant deformation marks mainly corresponding to the extensive plastic deformation of the matrix phase via shear yielding mechanism. Accordingly, the crazing-tearing deformation marks65 representing the ductile tearing of the matrix around a debonding hole were also highlighted as additional mechanism contributing to the energy dissipation by extensive plastic deformation of the matrix as given in Figure S12b in Supporting Information (see Figure S12b). The SEM micrograph recorded on the fracture surfaces of PLA/WCF-10 composites was also given in Figure 8d. Starting with the addition of 10 wt.% WCF into PLA matrix, the fracture behavior has begun to be dominated by the fibers. Hence, the mechanisms like fiber pull-out, fiber fracture, fiber buckling and fiber slippage have become operative instead of plastic 14


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deformation mechanisms such as shear yielding and ductile tearing in the matrix phase. However, several debonding holes indicating localized plastic deformation of the matrix phase was also observed on the fracture surfaces of PLA/WCF-10 as highlighted in Figure 8d. As the fiber pull-out, fiber fracture, fiber buckling and fiber slippage do not dissipate the energy as effective as the extensive plastic deformation of the matrix phase, a sudden decrease takes place in the impact strength of PLA/WCF-10 compared to that of PLA/WCF-5 (Figure 7). However, it is still slightly higher than that of pure PLA due to the presence of localized plastic deformation marks as shown in Figure 8d. The governing role of the fibers on the impact fracture behavior becomes more apparent with the addition of 20 and 30 wt.% WCF into PLA matrix. The SEM micrographs recorded on the impact fracture surfaces of PLA/WCF-20 and PLA/WCF-30 composites were shown in Figure 8e and 8f, respectively. As it can be seen from the micrographs, the fracture surfaces are mainly occupied by the fiber pullout, fiber fracture and so on. The fracture analyses done on the impact specimens are in parallel with the fracture analyses performed on the tensile specimens for all the materials reported in this study. More importantly, the trend observed in the impact strength (Figure 7) is in accordance with the trend found in the crystallinity as a function of WCF content (Figure 3b). In fact, both of the impact strength and the crystallinity changed in the following order with regard to the WCF content: PLA/WCF-5 > PLA/WCF-1 > PLA/WCF-10 > PLA/WCF-20 > PLA/WCF-30 > PLA. However, it should be also noted that the impact strengths of PLA/WCF-10, PLA/WCF-20 and PLA/WCF-30 are quite similar to each other. 4. Conclusion Green composites with enhanced tensile and impact properties were successfully produced by mixing PLA and waste cellulose fibers in a high-shear thermo-kinetic mixer. This approach mainly suggests an economical way for the sustainable production of high performance green composite materials, as it comprises the reuse of waste cellulose fibers and the implementation of a facile technique, which is available to scale up, ensuring the homogeneous filler dispersion and the ease of processing. In addition, this approach creates significantly high added-value for the mass production of green composites as the reuse of waste cellulose fibers also corresponds to the reduced consumption of thermoplastics during processing. The green composites of PLA and WCF display remarkably enhanced tensile and impact properties. For instance, the composites with 30 wt.% WCF show an average elastic modulus of 6.2 GPa, an ultimate tensile strength of 94.6 MPa and an impact strength of 18 kJ/m2, which are approximately 100, 50 and 20% higher than those of pure PLA, respectively. Besides, the composites with 5 wt.% WCF display average impact strength of 29.7 kJ/m2, which is around 87% higher than that of PLA. In addition, PLA/WCF-5 composites also show an elastic modulus of 3.6 GPa and an ultimate tensile strength of 73.9 MPa, which are also 16 and 23% higher than those of pure PLA, respectively. Furthermore, the key mechanisms behind the remarkable enhancements in tensile and impact properties of the composites 15



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were also demonstrated. The composites with WCF concentrations higher than 10 wt.% are mainly dominated by the fibers. For these composites, direct mechanical contribution of the homogeneously distributed fibers and strong polymer-fiber interfacial interactions are the key parameters to enhance the final properties, stiffness and strength in particular. On the other hand, the composites with WCF concentrations lower than 10 wt.% are mainly governed by the matrix properties, the degree of crystallinity in particular. However, indirect contribution of the homogeneously distributed fibers must be highlighted for these composites since they significantly influence the matrix crystallinity under the applied processing conditions as evidenced by thermal analyses. In addition, the increase in ST concentration also plays a key role on the final properties, particularly for the impact resistance, of the composites with relatively low WCF concentrations. The main conclusion of the study is that the facile technique comprising high-shear mixing gives us the ability to produce high performance green composite materials as well as addressing the world-wide waste disposal problem by reusing of waste cellulose fibers as a renewable and biodegradable alternative to non-renewable and non-biodegradable reinforcing agents like carbon, aramid and glass fibers, which is a great opportunity to ensure sustainability and reduce environmental and economical costs for many industrial applications. Acknowledgements We sincerely thank Dr. Bahattin Koc and Mr. Cemil Cicek for kindly providing the PLA and WCF samples. Associated Contents: Supporting Information: The Supporting Information is available free of charge on the ACS Publications website (http://pubs.acs.org/). Figure S1-S3. FT-IR spectra of the samples. Figure S4-S6. XRD spectra of the samples. Figure S7-8. DSC thermograms of pure PLA and WCF obtained from heat-cool-heat cycle. Figure S9. Second heating curves of the samples obtained from DSC analyses. Table S1. Thermal properties of the molded samples obtained from the second heating scans. Figure S10-12. SEM micrographs obtained from the fracture surfaces of the samples after tensile and impact tests. Author Information Corresponding author: E-mail: [email protected] Phone: +90 216 4839000-9501 ORCID: 0000-0003-0296-827X Notes: The authors declare no competing financial interest. References (1) Faruk, O.; Bledzki, A. K.; Fink, H.-P.; Sain, M. Biocomposites reinforced with natural fibers: 2000–2010. Prog. Polym. Sci. 2012, 37, 1552-1596. (2) Koronis, G.; Silva, A.; Fontul, M. Green composites: A review of adequate materials for automotive applications. 16


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58, 529-536. (58) Vila, C.; Campos, A.; Cristovao, C.; Cunha, A.; Santos, V.; Parajó, J. Sustainable biocomposites based on autohydrolysis of lignocellulosic substrates. Compos. Sci. Technol. 2008, 68, 944-952. (59) Nagarajan, V.; Zhang, K.; Misra, M.; Mohanty, A. K. Overcoming the fundamental challenges in improving the impact strength and crystallinity of PLA biocomposites: influence of nucleating agent and mold temperature. ACS Appl. Mater. Interfaces 2015, 7, 11203-11214. (60) Bartczak, Z.; Argon, A.; Cohen, R.; Weinberg, M. Toughness mechanism in semi-crystalline polymer blends: II. High-density polyethylene toughened with calcium carbonate filler particles. Polymer 1999, 40, 2347-2365. (61) Zuiderduin, W.; Huetink, J.; Gaymans, R. Rigid particle toughening of aliphatic polyketone. Polymer 2006, 47, 5880-5887. (62) Zuiderduin, W.; Westzaan, C.; Huetink, J.; Gaymans, R. Toughening of polypropylene with calcium carbonate particles. Polymer 2003, 44, 261-275. (63) Muratoglu, O.; Argon, A.; Cohen, R.; Weinberg, M. Toughening mechanism of rubber-modified polyamides. Polymer 1995, 36, 921-930. (64) Singleton, A.; Baillie, C.; Beaumont, P.; Peijs, T. On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite. Composites Part B. 2003, 34, 519-526. (65) Dasari, A.; Misra, R. The role of micrometric wollastonite particles on stress whitening behavior of polypropylene composites. Acta Mater. 2004, 52, 1683-1697.

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List of Figures Figure 1. SEM micrograph of waste cellulose fibers. Figure 2. The effect of waste cellulose fiber content on the melt flow index (MFI). Figure 3. (a) The first heating curves obtained from the DSC analyses of the materials and (b) the effect of waste cellulose fiber content on the degree of crystallinity calculated using the data obtained from the first heating scans. Figure 4. Stress-strain curves of pure PLA and PLA/WCF green composites. Figure 5. The effect of waste cellulose fiber content on the elastic modulus, tensile strength and strain at break. Figure 6. SEM micrographs obtained from the fracture surfaces of (a) PLA, (b) PLA/WCF-1, (c) PLA/WCF-5, (d) PLA/WCF-10, (e) PLA/WCF-20 and (f) PLA/WCF-30 after tensile tests. Figure 7. The effect of waste cellulose fiber content on the unnotched Charpy impact strength. Figure 8. SEM micrographs obtained from the fracture surfaces of (a) PLA, (b) PLA/WCF-1, (c) PLA/WCF-5, (d) PLA/WCF-10, (e) PLA/WCF-20 and (f) PLA/WCF-30 after Charpy impact tests.

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Figure 1. SEM micrograph of waste cellulose fibers.

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Figure 2. The effect of waste cellulose fiber content on the melt flow index (MFI).

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Figure 3. (a) The first heating curves obtained from the DSC analyses of the materials and (b) the effect of waste cellulose fiber content on the degree of crystallinity calculated using the data obtained from the first heating scans.

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Figure 4. Stress-strain curves of pure PLA and PLA/WCF green composites.

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Figure 5. The effect of waste cellulose fiber content on the elastic modulus, tensile strength and strain at break.

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Figure 6. SEM micrographs obtained from the fracture surfaces of (a) PLA, (b) PLA/WCF-1, (c) PLA/WCF-5, (d) PLA/WCF-10, (e) PLA/WCF-20 and (f) PLA/WCF-30 after tensile tests.

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Figure 7. The effect of waste cellulose fiber content on the unnotched Charpy impact strength.

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Figure 8. SEM micrographs obtained from the fracture surfaces of (a) PLA, (b) PLA/WCF-1, (c) PLA/WCF-5, (d) PLA/WCF-10, (e) PLA/WCF-20 and (f) PLA/WCF-30 after Charpy impact tests.

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List of Tables Table 1: Sample notations, compositional details and processing parameters. Table 2. Thermal properties of the molded samples obtained from the first heating scans. Table 3. Mechanical properties of pure PLA and PLA/WCF green composites.

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Table 1: Sample notations, compositional details and processing parameters. PLA Content

WCF Content

Speed

Time Temperature* (˚C)

Sample Notation (wt.%)

(wt.%)

(rpm)

(min)

PLA

100

0

5200

190

≤1

PLA/WCF-1

99

1

5200

190

≤1

PLA/WC-5

95

5

5200

190

≤1

PLA/WCF-10

90

10

5200

190

≤1

PLA/WCF-20

80

20

5200

190

≤1

PLA/WCF-30

70

30

5200

190

≤1

*This is defined as the final temperature that the compound reached during the high-shear mixing.

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Table 2. Thermal properties of the molded samples obtained from the first heating scans. Tg

Tcc

∆Hcc

Tm

∆Hm

χc

(˚C)

(˚C)

(J/g)

(˚C)

(J/g)

(%)

PLA

58.1±1

109.3

13.97

145.7

23.59

10.3

PLA/WCF-1

60.9±1

111.1

14.46

148.0

26.50

13.1

PLA/WCF-5

60.1±1

109.0

12.30

147.8

27.72

17.5

PLA/WCF-10

60.4±1

109.6

14.83

151.5

25.69

12.9

PLA/WCF-20

60.5±1

113.7

15.06

150.3

23.56

11.4

PLA/WCF-30

59.7±1

113.4

15.32

147.7

22.21

10.6

Sample

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Table 3. Mechanical properties of pure PLA and PLA/WCF green composites. E

ε

σmax

Sample

Charpy Impact Strength

(MPa)

(MPa)

(%)

PLA

3079±154

60.8±3.04

3.03±0.032

15.9±0.795

PLA/WCF-1

3283±164

67.3±3.37

3.69±0.046

22.8±1.140

PLA/WC-5

3618±181

73.9±3.70

4.06±0.059

29.7±1.490

PLA/WCF-10

4455±223

78.8±3.94

3.07±0.038

18.6±0.930

PLA/WCF-20

5165±258

88.9±4.45

2.17±0.038

18.5±0.930

PLA/WCF-30

6212±310

94.6±4.73

1.92±0.037

18.0±0.900

TOC Graphic / For Table of Contents use only

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(kJ/m2)

High-Performance Green Composites of Poly(lactic ... - ACS Publications

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