Composites Part B 172 (2019) 1–8
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Coupling effect of starch coated fibers for recycled polymer/ wood composites Daniel Belchior Rocha, Derval dos Santos Rosa * Environmentally Friendly Polymer Laboratory - Center or Engineering, Modeling and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo Andr�e, 09210-180, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Recycling Interface/interphase Mechanical properties
In this work is proposed the use of starch gum (SG) as a coupling agent for polymer matrix composites (virgin and recycled) reinforce with wood flour. The coupling agent is prepared using an aqueous starch solution containing 3 and 5 %wt. forming a coating over the reinforcement, and compared with maleic anhydride grafted (MAPP), a usual coupling agent. The starch gum coating characterization indicates that 5%wt. presents more cohesive coating on the reinforcement. The mechanical properties of the composites also indicate that 5 %wt. produces higher properties than 3 %wt. The starch coupling agent presents a lower performance compared to MAPP for the virgin polymer; however, for the recycled polymer, it increases the elastic modulus in 37%, overcoming MAPP. Computational tomography indicates that the starch gum promotes compatible interaction at the interface with a low formation of empty spaces in the recycled polymer, indicating that it can be an exciting alternative to produce low-cost composites with eco-friendly solutions.
1. Introduction The societies’ development leads to an increased demand for raw materials to products and services aiming at the well-being of the con sumers [1–3]. However, these processes lead to the depletion of non-renewable raw materials and generation of a large number of resi dues [4,5]. As a possibility to fulfill the consumption of the products, polymers receive intense attention due to its versatility, obtaining high productivity and lower costs . These characteristics lead the world production of polymers to harshly increase reaching a total of 336 million tons of different polymeric compositions in 2017 and have predictions of continuing in this trend . This expressive consumption leads to environmental problems as the destination of the residual polymers after the use. Because of the strong chemical stability, these materials tend to non-degrade when disposed off in soil , persisting in the environment leading, to the effect of weather and leaching, to contamination of groundwater, rivers, and oceans [9,10]. The use of recycled polymers appears as a possibility, allowing substitution in the production chain for polymers after the product disposal and reinforces the potential act over the main issues [11–14]. Recycled polymers have good acceptance; in Europe, the recycling rates of polymer used as raw material reach values around 25% while other countries still have the
potential to increase this value [7,15]. However, the process to obtain the recycled raw material applies temperature and high shearing to melt the polymer and to shape it into pellets. This process produces structural modification in the polymer molecular chains, breaking and oxidizing the molecular chain or creating cross-linking between molecules, and consequently modifies and reduces the polymer properties . To surpass these issues, the production of composites emerges as a possibility, increasing the prop erties of the polymers and reducing the total cost of the product by the addition of low-cost reinforcements . Due to the enormous appeal in the sustainability, natural fibers emerged as one of the most promising materials that can be used as reinforcement in composites, because of the high availability, low cost, and low density, ideal for products with a short lifetime [18,19]. These fibers are applied mainly to improve mechanical properties as having been reported in the literature; the most used are jute , sisal , cotton , sugar bagasse and others . Despite the benefits of using natural fibers, it presents a significant drawback due to the incompatible interface with polymeric matrices. The fiber exhibits a substantial hy drophilic behavior due to the chemical composition of its components, reducing the interaction in the interface with the hydrophobic matrix leading to lower composites properties . This interface can be
* Corresponding author. E-mail address: [email protected]
(D.S. Rosa). https://doi.org/10.1016/j.compositesb.2019.05.052 Received 11 December 2018; Received in revised form 26 March 2019; Accepted 5 May 2019 Available online 7 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Table 1 List of composites samples produced, indicating the use or not, and which coupling agent was applied. Sample
Virgin Polymer VP (%)
Recycled Polymer RP (%)
Wood Flour (%)
Starch gum (pph)a
VP VP-WF VP-SG3% VP-SG5% VP-MAPP RP RP-WF RP-SG3% RP-SG5% RP-MAPP
100 80 80 80 80 – – – – –
– – – – – 100 80 80 80 80
– 20 20 20 20 – 20 20 20 20
– – 5 8 – – – 5 8 –
– – – – 5 – – – – 5
The values were based in starch gum coating deposition from section 3.1.
modified by coupling agents to promote surficial interaction by modi fication on the fibers or in the polymers [24–26]. Commercial coupling agents are large-scale produced; however, they have a cost of production and involve processes that can generate environmental impacts. The most used coupling agents are the silanes and the anhydrides . To create new alternatives, researchers have been working on the development of new coupling agents base in natural sources . Chun et al. produced composites of PP reinforced with residues from cocoa husk using fatty acids modified by ethylene diamine as coupling agents, producing composites with improved stiffness and maximum stress resistance . Pang et al. developed an eco-friendly coupling agent, based on the use of coconut oil after saponification and the reaction with epichlorohydrin lead to a chemical bond with the kenaf natural fiber, with a reinforced blend of linear low-density polyethylene (LLDPE) and poly(vinyl alcohol) (PVOH). The composites with the new coupling agent presented improved rheometric and mechanical properties, with lower water absorption and higher thermal stability . Among the natural materials that are suitable to be used as the coupling agents, starch appears as a viable choice. Starch is an abundant natural material; consequently, it presents a very low cost . It has been extensively used commercially as a gum in food industry present ing a broad range of applications . As a polymer, the thermoplastic starch has been highlighted to produce commercial biodegradable films and composites or even used as a blend with other polymeric materials [32–34]. It presents the similar chemical composition, amylose, and amylopectin, with cellulose from the natural fibers, and due to the characteristic of the macromolecular structure, and the amylose chains, it can entangle with polymer chains in the matrix producing physical interaction with the polymer matrix [35,36]. In this work, is proposed the application of starch as a coupling agent for wood polymer composites based in a recycled matrix. The method is based in the previous work reported by Macedo et al.  and uses a
starch solution to coat the fiber surface with a layer of starch gum that promotes interaction in the interface between wood flour and the polymeric matrix. The tensile properties and the obtained structures and phases interactions were evaluated. The use of starch gum can produce a new possibility to compatible natural fiber composites as an eco-friendly coupling agent from a natural source with a low cost of production and non-toxicity after disposal. 2. Experimental procedure 2.1. Starch gum coating The wood flour (WF) used as reinforcement was obtained from wood pallets from industrial packages transportation after milling in a wood waste grinder to obtain wood chips and followed by the knife milling in a model T90 (Tecnal, Brazil). The WF, with sizes lower than 850 μm, as described in the supporting information file, was oven dried (80 � C for 12 h) to remove absorbed water. The starch gum was produced as described on the method reported by Macedo et al.  with a solution of corn starch (Penetrose 80, with 27 % wt. of amylose, Ingredion) containing two concentrations of starch 3 %wt. and 5 %wt. in distilled water. The WF flour was added in a proportion of 1:10 (wt.) to the so lution. The system was treated in a water bath at 70 � C for 24 h. After that, the samples were dried in an oven for 48 h at 60 � C for water removal. Infrared Spectroscopy evaluated the coated fibers by Fourier Transform (FTIR) (IS 10 Nicolet, Thermofisher) in ATR mode with 64 scans from 400 to 4000 cm 1, and Scanning Electron Microscopy (JSM-6010LA, JEOL) after gold sputtering (ACE200, Leica). 2.2. Composite processing There were used two polymer matrices, a virgin polymer (PP HP503,
Fig. 1. FTIR Spectra of the samples WF, 3 %wt., 5 %wt., and pure starch separated by two regions: a) Full spectra and b) Fingerprint region. 2
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Fig. 2. Photomicrographs of the starch gum coated samples in the magnification of 100� (up) and 500� (down) for samples a) Wood flour, b) 3 %wt. and c) 5 %wt.
Braskem) and a recycled polymer from a local company (RP, Lucro Certo). As a comparison for the coupling effect there was used a usual coupling agent, maleic anhydride grafted polypropylene, MAPP, (Poly bond 3000, Addivanty) with a mass of 5 %wt. The composites where twice processed in single-screw extruder with L/D 24 and two heating zones, 175 and 185 � C. To improve the dispersion of the reinforcement, it was added a mesh mixture element of 20 mesh size. Firstly, the polymer and the reinforcement were pre-mixed manually considering the relation 80:20 of matrix and fiber, the starch gum coated re inforcements were processed with the matrices while MAPP was added to WF/matrix mixture and then processed. After each extrusion, the samples were air cooled and chopped in pellets. The samples are iden tified as present in Table 1. To evaluate the properties, the samples were compression molded using a hot-press (SL11, SOLAB Equipamentos) using the temperature of 180 � C and 7 ton for 5 min, producing samples with a thickness of 1.5 mm, approximately. The tensile properties were measured using a universal testing machine (3369, Instron) based in the ASTM D638-14 standard. The samples structures were evaluated using scanning elec tron microscopy (JSM-6010LA, JEOL) performed after cryo-fracture, with liquid N2 immersion, and sequential gold sputtering (ACE200, Leica), and x-ray microcomputed tomography was performed in a Micro-CT (SkyScan 1272, Bruker) operating 20 kV/175 μA. The software CTvox (Bruker) was used to construct the cross-sectional images to form the 3D images.
3. Results and discussion 3.1. Characterization of the starch gum coated fibers The formation of the starch coating over the wood flour is the base to ensure the coupling effect on the composites. To evaluate the gum for mation, it was performed the FTIR spectra evaluating the presence of starch components at the fiber surface. Fig. 1a shows the spectra for the different coating processes. The spectra of WF and starch present similar peaks since they have cellulose and amylose, however, cellulose presents β(1–4) glucan bonds while amylose presents α(1–4) glucan bonds; also the presence of lignin and hemicellulose or the cross-linked bonds at amylopectin allows observations of differences between these structures . With fiber coated the peaks related to the lignocellulosic struc tures, 2880 cm 1 from C–H stretching and 898 cm 1 related to C–H out-of-plane deformations of cellulose and hemicellulose are reduced as the starch concentration increases in the coating process [37,38]. In Fig. 1b, the peaks at 930, 863 and 760 cm 1 are related to the skeletal vibrations of starch; these peaks are more intense for the sample with a higher concentration of starch during the coating process, indicating the formation of a more significant starch presence at the surface . This effect can also be observed by thermal decomposition, as described in the support information file. The electron photomicrographs of the coated fibers show the surface of the natural WF and the samples 3 and 5 %wt. In Fig. 2a is possible to observe WF, with its surface presenting continuous segments of ligno cellulose and several longitudinal fractures, with dimensions varying
Fig. 3. Schematic of starch gum coating formation over wood flour surfaces. 3
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fiber surface. The process of coating formation is related to the retro gradation of the starch granule in the presence of water, as presented in Fig. 3. As reported by Ratnayake and Jackson , the starch granule in solution will swallow by absorbing water and heat increasing the mobility of the molecules leading to the disruption of the granule. The molecules of amylose and Amylopectin tend to segregate. Due to the cross-linked structures, the amylopectin will tend to agglomerate while amylose, with more mobility, will involve this amylopectin core and interact with amylose structures from other disrupted grains, forming a network in the fiber surface. Since the sample of 5%wt. has more available granules, the network will be more cohesive during the gela tinization process . The amylose molecules will interact with fiber surface, being absorbed in the fiber surface, filling the fractures and pores, forming mechanical anchoring for the coupling effect . 3.2. Characterization of coupling agents for recycled polymer To evaluate the interface effect, obtained for samples of starch gum coated with different concentrations, a tensile strength test was per formed to evaluate the mechanical properties of the obtained compos ites. In Fig. 4a the values of Elastic modulus are presented, while in Fig. 4b the values of tensile strength are to be seen. The values of elastic modulus present different behaviors for each matrix and each coupling agent. The composite with the virgin polymer (VP-WF) presents a reduction of 35% when only WF is added to the matrix. When the coupling agents are applied, the use of MAPP leads to increase of 20% of elastic modulus, while the starch gum coupling agents present a reduction of 12% for coated WF of 3% wt. and an increase of 9% for coated WF of 5 %wt. These results indicate that the virgin polymer presents better interaction with MAPP coupling agent then the starch gum, corroborating with other reports of this combination, as presented in several studies [44–47]. For the recycled polymer, the coupling effect was inverted. The addition of WF in the matrix reduced the elastic modulus in 29%, like VP-WF samples. When applied the coupling agents, the starch gum results showed an increase of 7% for RP-SG3% sample and an increase of 37% for the samples of RP-SG5%, while the samples of RP-MAPP presented an increase of 11%. These results indi cate that the recycled polymer presents an improved interaction with starch gum when it is present in a higher amount at the surface since the samples with 5 %wt. presented exciting results, even for the virgin polymer. As cited by Zaman et al.  the presence of oxidized struc tures, as the carbonyl groups, which are inserted in recycled polymer molecules in the mechanical recycling process , is favorable to the compatibilization with starch molecules due to the presence of the hy droxyl groups present. For the tensile strength, present in Fig. 4b, the results present an
Fig. 4. Tensile properties of composites using the different coupling agents a) Elastic modulus and b) Tensile strength.
from 2 to 18 μm and longitudinal values of 100 μm, produced by the milling process . For both covered samples, it is possible to observe the formation of a layer over WF surface with a different appearance than the original WF, and this layer is similar to plasticized starch structures as presented by Karger-Kocsis . In Fig. 2b the samples with 3 %wt. are shown; it is possible to observe that the starch gum coated most of the surface, this coating maintaining the original char acteristics of the surface, the lignocellulosic structure, and surficial fractures. For sample 5 %wt., in Fig. 2c, the layer formed is more cohesive and uniform over the surface, removing the characteristics of WF surface; it indicates that for 5 %wt. samples, the starch gum is more deposited in the fibers. This result can be confirmed by evaluating the mass deposition over the fiber surface. After drying the samples, the weight was measured, and it was observed that samples 3 and 5 %wt. gained values of 23.4 � 0.6% and 40.0 � 2% of the initial mass, respectively. This behavior was also observed by Macedo et al. in a previous study eval uating the coating over cotton fibers . It was observed that the initial mass of starch in the solution rules the deposition of the coating in the
Fig. 5. Photomicrographs of the composite samples from virgin and recycled matrices with the magnification of 100� (left) and 500� (right): a) VP-SG 5%, b) VPMAPP, c) RP-SG5% and d) RP-MAPP. 4
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Fig. 6. Schematic of coupling effect from A) Virgin matrix and WF produced by the MAPP. B) Recycled matrix and WF produced by the starch gum coating.
opposite behavior, with the reduction of these values for composites from both matrices. For the virgin matrix, both coupling agents showed reductions of 52 and 29% for samples VP-SG5% and VP-MAPP, respectively. Recycled matrix had the same characteristic, reduction in tensile strength of 42 and 29% for samples RP-SG5% and RP-MAPP. These reductions are also cited in the literature for the use of different
natural fiber for wood polymer composites [50–52]. As presented by Krause et al. the properties and structures of the wood flour of Pinus may reduce the tensile strength, in their work PP composites were produced using several natural fibers, and their results indicate that the Pinus reinforcement tends to present lower properties than other fibers . Since the tensile properties are the result of interface formation, the
Fig. 7. a) Fully 3D construction of composites samples (Right: VP-MAPP and Left: RP-SG5%). b) Representation of composites with matrix suppressed and a highlight of the reinforcement. 5
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Fig. 8. a) Cross-section representation of the samples obtained in computed tomography. Top: Virgin matrix composites. Bottom: Recycled matrix composites. b) 3d construction with a highlight (in red) of the empty spaces in recycled composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
SEM of cryo-fractured samples can be used to evaluate the interactions of the composite components. In Fig. 5 the photomicrographs of the cross-section of the samples containing MAPP and SG 5 %wt. are pre sented. The samples with the virgin matrix, represented in Fig. 5a (VPSG5%) and Fig. 5b (VP-MAPP) present behavior that corroborates the tensile results. For VP-MAPP the interface of the phases is compatible with good interaction and empty spaces between them, which will lead to improved mechanical properties, as observed by Spiridon . However, the use of starch gum on the WF presented limited compati bility with the matrix, with incoherent interface forming a separation between the phases . For the samples with the recycled matrix, the compatibility is visually alternated for the coupling agents as shown in Fig. 5c, RP-SG5%, and Fig. 5d, RP-MAPP. The sample RP-SG5% presents an interface with improved interaction between the starch gum coating and the matrix with only a few empty spaces in this region, while the sample RP-MAPP presents several empty spaces at the interface, showing low interaction of the fiber with the matrix. These interface behaviors relate to the characteristic of the coupling of each agent. As presented in Fig. 6a, the MAPP is applied in the matrix, due to the chain of polypropylene that has good miscibility with the neat matrix of PP. Then in contact with the fiber, the maleic anhydride will promote the chemical bonding with fiber surface and the available hy droxyl groups, creating the necessary interaction in the interface for the composite properties . However, for the recycled polymer, repre sented in Fig. 6b, the presence of oxidized groups and the contamina tions tends to reduce fiber for bonding with maleic anhydride and reduces interfacial adhesion, and consequently the properties. For the starch gum, the interaction with virgin polypropylene is not favored, due to the incompatibility of surficial characteristic. However, as re ported in the literature, plasticized starch presents interaction due to polymeric chains entanglement in processing . This characteristic leads to TPS (Thermoplastic Starch) to be used in PP to promote cost reduction and possible pro-oxidizing effect [56,57]. This low interaction is increased as the presence of oxidizing molecules in the recycled matrix promotes interaction with hydroxyl groups from the starch, and, as amylose can entangle and mixture with the matrix chains, this adhesion is improved, generating the properties increase. The composites interface and structures were also verified by computational tomography. Fig. 7a represents the 3D construction of
Table 2 Volume percentage values of samples empty spaces ob tained in composites samples from both matrix composites. Sample
Empty spaces (%v.)
VP-WF VP-SG5% VP-MAPP RP-WF RP-SG5% RP-MAPP
6.9 � 0.5 2.5 � 1.0 1.3 � 0.7 2.7 � 1.7 1.4 � 0.2 5.0 � 0.7
the samples with the best mechanical properties for each matrix, VPMAPP, and RP-SG5%. The animated 3D constructions of the samples are also available at supporting information video. The 3D construction indicates that the composites present a proper distribution of the fibers in the composite, as present in Fig. 7b, showing that even with the use of a single screw extruder the processing was satisfactory, and this factor does not reduce the properties. The reinforcement is randomly distrib uted in the composite, and none signal of orientation is observed due to the compression molding process, which allows obtaining an isotropic distribution of properties in the material . The figures from recycled polymer matrix present several points of high electron density in the structure, for carbonyl groups, which generate noise in the region be tween the fiber and the matrix on the tomography, making difficult the isolation on the image construction. The observation of the cross-section images of the samples also al lows observing the interface obtained in situ for the composites. Fig. 8a presents the cross-section images for the obtained composites, while the construction in 3 dimensions allows the analysis of empty spaces formed in the sample volume. The results indicate similar results from those obtained for SEM photomicrographs. Table 2 presents the values, in percentage, of empty space volumes obtained for composite samples. The samples from both matrices produced with only WF lead to the formation of empty spaces in the composite due to the low compati bility. The sample VP-MAPP presented a good interaction with the virgin polymer with lower empty spaces, while the opposite was observed for the recycled polymer with RP-SG5% presenting a low fraction of these 6
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empty spaces. Fig. 8b presents, in red, the empty spaces found in volume evaluated by tomography for recycled polymer matrix. Besides the low compatibility, the empty spaces formation may be the result of water absorbed in the sample due to its high hydrophilicity, that is evaporated in the processing and leaving the empty spaces in the composite struc ture [59,60]. This indicates that even with improved interaction, the starch gum coupling agent still presents the formation of empty spaces, which leads to a limitation on properties. However, the result also in dicates that there is still a possibility to improve the properties and that the use of starch gum can be applied to produce composites from the recycled polymer with improved properties.
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4. Conclusion In the present work, it was proposed the use of starch gum as a novel coupling agent for wood flour as reinforcement and recycled polymer as a matrix, evaluating the process of coating of the reinforcement and the properties of the composite. The proposed coating of the reinforcement in aqueous solution, with different concentrations of starch, was confirmed by FTIR spectra. The SEM images indicate that both samples present the formation of the starch gum coating, occupying fractures and other morphologies of fiber surface after granule disruption and mole cules movement. The coating obtained for the sample with 5%wt. of starch was more cohesive, indicating an improved interaction of the starch structures between them and the reinforcement. The starch gum has a different effect for the matrices, the composites containing 20 %wt. of reinforcement and starch gum 3 %wt. presenting lower properties than 5 %wt. for all composites. Composites containing starch gum and the virgin matrix present a limited improvement on the elastic modulus, with lower values than traditional MAPP coupling agent. However, for the recycled matrix, the sample RP-SG5% presents higher values than MAPP, indicating an interaction with oxidized structures of the recycled matrix. The SEM images and the computed tomography showed that a reinforcement dispersion was obtained using a single-screw extruder and that the samples with higher values of tensile properties for each matrix also presented low formation of empty spaces and improved compatibility between the composite phases. The samples with starch gum, even with more compatible interaction still presented some empty space formations, indicating that the properties can be improved. The use of starch gum represents an exciting eco-friendly alternative coupling agent for recycled polymers composites, with advantages when compared to traditional coupling agents, presenting an easy adaptation on an industrial scale, leading to low cost of production and processing that will lead to obtaining of high-performance composites. Acknowledgments The authors are grateful to UFABC, Núcleo REVALORES, Brazilian Nanotechnology National Laboratory (LNNano) and UFABC Multiuser Central Facilities (CEM-UFABC) for the experimental support. This work was supported by CNPq (grant numbers 306401/2013-4, 447180/20142, 163593/2015-9) and FAPESP (grant number 2018/11277-7). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.05.052. References  Tilman D, Clark M, Williams DR, Kimmel K, Polasky S, Packer C. Future threats to biodiversity and pathways to their prevention. Nature 2017;546:73–81. https:// doi.org/10.1038/nature22900.  United Nations - Department of Economic and Social Affairs - Population Division. World population prospects: the 2017 revision 2017. https://population.un. org/wpp/DataQuery/.
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