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Characterization of polypropylene composites using yerba mate fibers as reinforcing filler
Composites Part B 174 (2019) 106935
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Characterization of polypropylene composites using yerba mate fibers as reinforcing filler Andre� Luis Catto a, *, Marcos Aur�elio Dahlem Júnior a, Betina Hansen a, Edson Luiz Francisquetti b, Cleide Borsoi a a b
Centro de Ci^encias Exatas e Tecnol� ogicas, Universidade do Vale do Taquari -UNIVATES, Lajeado, RS, Brazil Instituto Federal de Educaç~ ao, Ci^encia e Tecnologia do Rio Grande do Sul - IFRS, Farroupilha, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Yerba mate Composite Neat and post-consumer PP Mechanical properties
This study presents the preparation of polypropylene (PP) composites filled with yerba mate (YM) stick particles. Neat and post-consumer PP are used as the polymer matrices with 20, 30 and 40 wt% (w/w) YM fiber. Me chanical properties, including tensile, flexural and impact resistance, are determined, in addition to chemical and morphological analyses. The main findings show that the addition of filler from YM fiber increased the tensile and flexural strength of the neat and post-consumer PP composites up to a 30 wt% fiber content. The neat PP composites show better results than those with recycled PP, with PP20YM and PP30YM composites showing the best results. There is a gradual reduction in impact resistance with increasing YM content. Scanning electron microscopy shows a better adhesion between the matrix and filler in composites with 20 and 30 wt% YM, corroborating the tensile and flexural strength results.
1. Introduction The amount of solid waste that has been generated by humanity in recent years has attracted attention to the problem associated with its disposal, challenging academia and companies to seek effective solutions to the issue, allied with social awareness. Disposal in landfills and incin eration are the cheapest methods for the destination of post-consumer polymer products, but this destination creates a serious problem for the environment [1]. In the context of concerns regarding sustainability, there are two problems: waste generation and depletion of natural resources. As the planet cannot renew what is consumed in modern times, it must then reconsider the standards of production, with the aim of sustainable con sumption. This can be achieved through less polluting energy sources, with emphasis on the life cycle of products, reducing waste production and recycling as much as possible [2]. Post-consumption polymer materials represent one of the largest categories of waste, and their reuse in the production of natural fiber-reinforced polymer composites is of great economic and environmental importance [3]. Polyolefins are the most commonly found plastics in urban solid waste, among which, polyethylene (PE) and polypropylene (PP) are particularly prevalent [4]. Due to the excellent combination of thermal
and mechanical properties, PP is widely exploited in a variety of appli cations, providing a rapid expansion in the use of this material. The physical characteristics of the polymer can be varied to achieve a wide range of thermal and mechanical properties, and its ease of processing allows economical use in most commercial manufacturing techniques [5]. PP has been one of the most common polymers used as a matrix in composites of natural fibers in the last decade, according to the literature [6–12], and is able to compete with higher cost plastics on the market. It can be used in various applications because of its low processing tem perature, mechanical properties, crystallinity, relatively high melting point, crystalline phase that maintains resistance mechanics at high temperatures, availability, cost, low density and high stiffness [13–16]. The development of composites involving the use of lignocellulosic materials as reinforcements in recycled polymer matrices has grown in response to environmental conservation, becoming more common in the polymer industry, because lignocellulose can be renewable, inexpensive, biodegradable and non-toxic [14,15]. The use of natural fibers continues to expand due to the higher prices for petroleum products, the reduction of global warming and acceptance among consumers due to the use of renewable and non-polluting raw materials and waste [12]. Among the lignocellulosic materials most used for this purpose are sawdust, a timber
* Corresponding author. E-mail addresses: [email protected] (A.L. Catto), [email protected] (M.A. Dahlem Júnior), [email protected] (B. Hansen), edson. [email protected] (E.L. Francisquetti), [email protected] (C. Borsoi). https://doi.org/10.1016/j.compositesb.2019.106935 Received 8 March 2019; Received in revised form 18 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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industry waste, vegetable fibers and lignocellulosic agricultural and agroindustrial waste [14,15]. Their availability, renewability, low den sity and price, as well as satisfactory mechanical properties make them attractive ecological alternatives to glass, carbon and synthetic fibers used for the manufacturing of composites. Their disadvantages, however, are low compatibility with hydrophobic polymer matrices, much lower tensile strength than glass fibers, thermal sensitivity at the temperature of compounding processes, moisture absorption and flammability [17,18]. Therefore, interest in natural fiber-reinforced polymer composite materials is rapidly growing both in terms of their industrial applications and fundamental research. Natural fiber-reinforced thermoplastic composites are gaining popularity in applications for automobile in dustries, packaging, assembly boards, paneling, fencing and furniture [19]. Brazil, which represents one of the richest biodiversities on the planet, stands out in the production of lignocellulosic materials. The term vegetable fibers is used to refer to fibers that are obtained from �, cotton, sisal, flax, jute, pineapple, plants, such as eucalyptus, curaua bamboo, banana and various agricultural by-products, such as sugar cane bagasse, corn straw, grape stalks, yerba mate sticks, tobacco and wood industry residues, and other sources [20,21]. Yerba mate (Ilex paraguariensis, Saint Hil.) is a perennial shrub-tree (Aquifoliaceae) native to the subtropical region of South America, where it is grown and harvested to obtain its leaves and twigs for the commercial processing of yerba mate products. Yerba mate is consumed mainly in Paraguay, Argentina, Uruguay and southern Brazil. The infusion of yerba mate is the most popular tea-like beverage of southern Latin American ~o” countries, where its leaves are used in the elaboration of the “chimarra (hot water extract of green dried leaves; mate in Spanish speaking coun tries), “terer^ e” (cold water extract of green dried leaves) and mate tea (hot water extract of toasted leaves), which are very appreciated drinks that are part of the culture of these countries [22–25]. Argentina presents the highest production among the countries that produce yerba mate, with an estimated value of 809000 tons in 2018 [26], while Paraguay produced ~91600 tons in 2014 [27]. According to the Brazilian Institute of Geog raphy and Statistics (IBGE), Brazil produced ~346000 tons of yerba mate in 2016, of which 80% is destined for the domestic market [28]. The yerba mate stick residues correspond to ~2% of mass production and currently their main use is as a boiler energy source and/or as organic fertilizer [29]. Considering the high levels of consumption of yerba mate in these Latin American countries, any attempt to increase the use of these agroindustrial wastes will add value to this material, with a more sustainable destination. Therefore, the incorporation of yerba mate sticks into higher value prod ucts, such as polymer composites, becomes a strategic route to explore options to develop the industrial sector and increase the economic value of yerba mate [30]. In this context, the use of agricultural waste becomes a source of ecological and sustainable materials, and can be used in a wide range of industrial applications, replacing polymers obtained from nonrenewable sources [31]. Plastic waste, in general, takes too long to degrade and, when burned, it may produce different degrees of toxicity [32]. Polyolefins used for product packaging are often discarded after a single use, resulting in a large amount of polymeric residues in munic ipal solid waste, presenting as a promising possibility for recycled products [33,34]. Recycling is directly related to the environment, aiming to reduce direct and indirect costs and the amount of pollution generated during production [35]. With this, the perspective of a new source of income makes people to transform part of the waste into a new product of value [36]. Therefore, there is a general tendency to use these lignocellulosic and polymeric wastes, considering the immense potential value of the materials processed and the implications and pollution arising from the nonuse of waste [32,37,38]. The objective of this study is to evaluate the potential production of composites with neat and post-consumer PP filled with lignocellulosicbased particles obtained from yerba mate residues. Thus, there is the possibility to explore its use in thermoplastic composite materials for civil construction, automobiles and other industries. The properties of
the produced composites are evaluated for their mechanical properties of tensile and flexural strength, the impact energy absorption, in addi tion to a chemical and morphological analysis. 2. Materials and methods 2.1. Materials The polymeric materials used were neat PP, H-401 type, a homo polymer obtained from Braskem, with a melt flow index of 7.5 g/10 min and a density of 0.905 g/cm3, and post-consumer polypropylene (rPP) from packaging of buckets, bowls and dumpsters products collected at University of Vale do Taquari (UNIVATES). Yerba mate (Ilex paraguariensis, St Hil) residue was obtained from ^ncio Aires city (Rio Grande do Sul, Brazil), pro Elacy, located in Vena vided in the form of ground sticks. The yerba mate sticks are considered a by-product from the production process in industry, where the leaves of the plant are predominantly used. 2.2. Methods 2.2.1. Composite preparation The yerba mate sticks were oven dried at 60 � C for 24 h and then ground in a mill from Tecnal brand, TE 631 model, at a speed of 27000 rpm and classified using a 60 mesh sieve. PP plastic waste was pre-washed with water and dried at room temperature for 24 h. After that, they were ground to size reduction in the form of flakes in a knife mill Retsch SM 200. For the preparation of the composites, a pre-blend of neat or postconsumer PP polymer matrix with the yerba mate (YM), previously dried in an oven at 60 � C for 12 h, was prepared in a single-screw extruder, with the aim of decreasing the volume of the fibers and thus improving the uniformity of YM fibers in the composite. In the prepa ration of thermoplastic composites, 20, 30 and 40 wt% (w/w) YM fiber were used for each type of polymer matrix (neat and post-consumer PP). The materials were first processed in a single-screw extruder, ES 35FR model, from the SEIBT manufacturer. The temperatures in the different heating zones were 160, 170, 180, 185 and 190 � C, with a screw speed of 40 rpm. After this processing, the composites were ground in a knife mill from Primot�ecnica (1001 model) and oven dried for 12 h at 70 � C. Thus, the material was reprocessed, now in a twin-screw extruder, COR 20-32LAB, co-rotational with L/D ¼ 32 and D ¼ 20 mm, from MH Equipments, with a feed speed in the extruder of 90 rpm. The extruder has eleven heating zones, where the following temperature profile was used in the processing: 115, 150, 180, 185, 185, 190, 190, 195, 200, 200 and 220 � C, respectively, with a screw speed of 200 rpm. The specimens were obtained by injection molding with a LHS 150–80 from Himaco Hydraulic Machines Ltd. Three different temper atures were used in the heating zones, namely, 150, 165 and 180 � C. The rotation speed of the screw was 100 rpm and the mold temperature was ~20 � C. The composites were dried for 24 h at 70 � C in an oven before the injection molding process. The specimens were made for tensile, flexural and impact tests. Table 1 presents the identification and composition of the produced composites. Table 1 Identification and composition of produced PP composites with YM fibers.
2
Samples
Matrix Content (wt.%)
Fiber Content (wt.%)
PP PP20YM PP30YM PP40YM rPP rPP20 YM rPP30 YM rPP40 YM
100 80 70 60 100 80 70 60
_ 20 30 40 _ 20 30 40
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2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectroscopy analysis was per formed using a PerkinElmer Frontier spectrophotometer with an atten uated total reflectance (ATR) accessory. Each spectrum was obtained using 32 scans ranging from 4000 to 400 cm 1 at a resolution of 4 cm 1. 2.3.2. Mechanical tests The tensile and flexure tests were performed in a universal EMIC DL 2000 test machine using seven different specimens of each composite sample. The speed used for the tensile test was 5 mm/min, performed according to ASTM D-638-10 [39]. For the flexure test, the speed was 2.0 mm/min and performed in accordance with ASTM D790-03 [40]. The Izod impact was performed on CEAST equipment, model Impactor II, in accordance with ASTM D256-10 [41]. The specimens were pre pared without notch and pendulum of 2.75 J for the composites and of 11 J to PP samples were used, at room temperature. 2.3.3. Morphological properties Morphological characterization for the YM fiber and PP composites with YM fibers was carried out with a CARLS ZEISS LS-10 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. All samples were previously covered with a thin layer of gold. SEM images were obtained of the fracture surface from specimens after the tensile test. 3. Results and discussion The FTIR-ATR spectra of the produced composites with YM fibers are showed in Fig. 1. The FTIR spectra of the PP (Fig. 1(a)) and rPP (Fig. 1 (b)) matrices show characteristic bands of PP [42], with stretching of CH and CH2 bonds between 2960 and 2920 cm 1, and asymmetric and symmetrical deformations of CH and CH3 at 1461 and 1377 cm 1, respectively. The YM fibers show absorption bands related to stretching and bending vibrations of characteristic chemical groups of lignocellu losic compounds: cellulose, hemicellulose and lignin. However, as can be seen in Fig. 1, the chemical compositions of samples differ and the intensities of some bands are different. These bands for the YM fiber are in accordance with the ones found in the literature [22,43–49] and can be seen in 3690–3000 cm 1 (O–H stretching vibration of the hydroxyl groups in cellulose molecules), 2900 cm 1 (CH3 stretching vibration of alkyl groups in aliphatic bonds of cellulose, lignin and hemicellulose), 1740 cm 1 (acetyl and ester groups in hemicelluloses and in aromatic components of lignin), 1608 and 1518 cm 1 (C–C in plane symmetrical stretching vibration of aro matic ring present in lignin), 1440 cm 1 (CH2 symmetric bending pre sent in cellulose), 1248 cm 1 (C–O stretching vibration of hemicellulose component or arylalkyl ether compounds present in lignin) and 1170–1030 and 890 cm 1 (C–O stretching and C–H deformation vibra tions of the pyranose ring skeletal of cellulose) [22,46–49]. Table 2 presents a summary of the main absorption bands attributed to the lignocellulosic fibers and PP samples. It is observed by the spectra that there were no significant changes in the samples of the neat and recycled PP composites. In Fig. 1(a), with neat PP and its composites, there are alterations observed in the region of 3600–3000 cm 1, related to the –OH groups, appearing with larger and subtler peaks in the PP20YM and PP30YM composites, which may be an indication of the presence of moisture in these samples. In Fig. 1 (b), with the post-consumer PP, the rPP20 YM composite also presented similar behavior. According to Zulkifli et al. [50], the decrease of the peaks in the region at 3600–3000 cm 1 in the composites can also be related to the good interaction between the fiber and the polymer ma trix. In the region of 2900 cm 1, characteristic of the PP matrix, there is a more pronounced peak in Fig. 1(b), which may be related to the types of recycled PP used.
Fig. 1. FTIR-ATR spectra for the YM fiber, neat PP (a) and post-consumer PP (b) composites. Table 2 FTIR absorption bands assigned to YM and PP present in the composites. Wavenumber (cm 1) Lignocellulosic fiber 3400 2800–3000 1728 1650 1606 1505–1511 1462 1375 1268
1163 1059 897 PP 2952, 2918 and 2838 2720 1456 1376 1165 974, 841 and 808
3
Peak Assignment
Reference
O–H stretch (hydrogen-bonded) C–H stretch in methyl and methylene groups C¼O stretching of acetyl or carboxylic acid Absorbed O–H and conjugated C–O C¼C stretching of the aromatic ring (lignin) C¼C stretching of the aromatic ring (lignin) CH2 deformation in lignin and carbohydrates C–H deformation in cellulose and hemicellulose Guaiacyl ring breathing, C–O stretch in lignin and for C–O linkage in guaiacyl aromatic metoxyl group C–O–C asymmetric vibration (in cellulose) C–O stretch in cellulose and hemicellulose Glucose ring stretch, C–H deformation in cellulose
[46–49] [46–49] [46–49] [46] [46–48] [46,47] [46,47] [46]
C–H stretching
[42,46]
CH bending and CH3 stretching CH3 asymmetric deformation CH3 symmetric deformation Bending vibration of tertiary carbon C–H deformation out-of-plane
[42,46] [42,46] [42,46] [42,46] [42,46]
[46–49] [46–48] [46] [46,47]
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Fig. 2. Tensile strength (a) and Young’s modulus (b) for the PP polymer matrix and composites with YM fibers.
Mechanical properties are extremely important in the composites, because they strongly influence the applications of these materials [51]. Fig. 2 shows the tensile strength and Young’s modulus of the composites with YM fibers. In relation to the tensile test (Fig. 2(a)) it was observed that the postconsumer PP matrix had a higher tensile strength than neat PP. This result may be related to the variability of types of PP found in the postuse residues of PP used. Among the specimens of neat PP with YM, the PP30YM composite showed the highest tensile strength at break, with a value of 22.5 MPa. The PP20YM composite also presented a similar result, with 21.5 MPa; however, increasing the amount of YM fiber to 40 wt% resulted in a reduction in the strength of the material, where the PP40YM composite presented a value of 19.2 MPa. According to Aseer et al. [52], the decrease in tensile strength at higher loading may be due to the imperfection in filler dispersion at higher loading. The mechanical properties of polymer composites reinforced with short vegetable fibers depend on many factors that determine the surface area able to transfer the tensile strength of the matrix to the fibers [53,54]. These factors include geometric parameters (length, diameter and aspect ratio), orientation and distribution of fibers in the polymer matrix, and these parameters are dependent on processing conditions used for the com posite manufacture [55]. In addition, the adhesion between fiber and
matrix is responsible for the efficient transfer of the tensile strength of the matrix to the reinforcement [53]. By analyzing the modulus of elasticity (Fig. 2(b)) of the samples, it was also verified that the PP30YM composite presented a higher value of Young’s modulus, with 816 MPa, indicating a higher stiffness of this composite. The improvement of the mechanical properties of composites depends not only on the composition, such as the percentages used and the individual components produced, but is also affected by the strength and effectiveness of the interfacial adhesion between the cellulose fibers and the matrix. This adhesion can be improved with prior treatment and also can be attributed to the localized tensions that are generated due to the low fiber concentration in the polymer matrix, which causes a mixture not so homogeneously dispersed, causing a smaller module [56]. With the increase of the fiber content, the tension is better distributed and the material becomes more resistant and with the addition of 20 and 30 wt% YM fibers, an increase in the tensile strength and in the modulus is observed when compared to neat PP [57]. For the rPP composites, rPP20 YM presented the highest tensile strength at rupture, with 18.6 MPa, followed by rPP30 YM, with a similar value of 17.4 MPa. Furthermore, in these composites it was observed a decrease in the resistance with the increase in the content of fibers used, as reported by Yan et al. [58]. Comparing the PP and rPP composites, it was found that those with neat PP presented higher results 4
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of tensile strength than those with recycled PP. In the case of composites with 40 wt% YM fibers, the decrease of the tensile strength can be due to the agglomeration of the fibers. According to Lee and Cho [59], this agglomeration indicates the low interfacial adhesion between the load and the matrix, evidencing that the insertion of an amount equal to or greater than 40 wt% fiber weakens the composite. Among the rPP composites, the rPP20 YM, rPP30 YM and rPP40 YM samples presented very similar elastic moduli, with values between 600 and 650 MPa, but higher than the post-consumer PP polymer matrix, indicating a higher stiffness of these samples with the increase in fiber content. Regarding the results of flexural strength and flexural modulus, showed in Fig. 3, it can be verified that the composites with neat PP presented higher values than the composites with post-consumer PP. Among the samples with neat PP, PP30YM presented the better resis tance to flexural strength, with a value of 39.4 MPa, as showed in Fig. 3 (a). However, by analyzing the standard deviations, it is observed that the PP20YM composite also presented a very similar result, with a value of 38.5 MPa, followed by the composite PP40YM, with a value of 37.4 MPa. Among the rPP composites, rPP20 YM presented an increase of ~8% in flexural strength in relation to the rPP matrix, but with the increase of the fiber content, there is a gradual decrease in this property in the samples.
For the flexural modulus (Fig. 3(b)), the rPP composites showed lower results than those with neat PP, as expected, where the PP20YM composite presented the largest value of all specimens. The flexural modulus was ~30–40% higher in the composites with neat PP compared to composites with recycled PP. The influence of the proportion of lignocellulosic material on the flexural strength in thermoplastic com posites has already been verified by some authors [60,61]. In these studies, the authors verified that the flexural strength increases with increasing natural fiber content in the composite. In general, there must be good adhesion between the fiber and matrix phases for this to occur, but even in a low adhesion system, this behavior is verified. According to Magalh~ aes et al. [62], the presence of fillers with an irregular shape does not support the stress transferred to the polymeric matrix. In this case, the incorporated material acts more as a filler than as a reinforcement. Espert et al. [63] reported that the length and shape of the fibers in fluence the mechanical properties of the composites. Fig. 4 shows the impact resistance results of the analyzed samples. Fig. 4(a) presents the impact absorption energy of the polymer matrix (PP and rPP), while Fig. 4(b) shows the impact absorption energy of natural fiber composites. It is observed that the PP polymeric matrix has an impact absorption energy much higher than the composites, where an 11 J hammer was used to break the test specimens, whereas for the
Fig. 3. Flexural strength (a) and flexural modulus (b) for the PP polymer matrix and composites with YM fibers.
5
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Fig. 4. Results of impact resistance of PP matrix (a) and PP and rPP composites (b).
composites, a 2.75 J hammer was used. The neat PP showed impact resistance well above the post-consumer PP, with a value ~40% higher. Lignocellulosic fiber is a kind of stiff organic filler, so the addition of fibers could decrease the impact strength of composites [64,65]. This indicates a higher tenacity of this PP in relation to PP-r. Among the composites, those with neat PP presented higher resistance to impact than those with post-consumer PP, a result expected by the higher stiffness of the neat PP. However, with the increase of the YM fiber content, there is a gradual reduction of the values of impact resistance for all composites. In general, addition of the filler introduces regions of
poor interfacial adhesion and stress concentration in the composite because the particles reduce the mobility of the polymer chains, thereby reducing the energy absorption [66] of the composite. The decrease of the tenacity during the degradation process occurs due to the presence of voids both on the surface and inside the specimens, facilitating the propagation of cracks and reducing the mechanical properties [67]. The SEM analysis of the YM fibers is shown in the micrographs of Fig. 5. It is possible to observe the structure of the YM fiber and the size of the particles used in the preparation of the composites. The particles had a rougher surface layer, probably due to the presence of amorphous
Fig. 5. SEM micrographs of YM fibers used: (a) magnification of 100 X and (b) magnification of 500X. 6
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seen that pieces of vegetal fibers of the YM adhered to the PP polymer matrix, in addition to uniform structures, indicating that there may have been good adhesion between the matrix and the vegetal fiber [50,69]. By analyzing the SEM micrographs (Fig. 6), with higher fiber con tent, an increase in fiber-fiber contact can be observed, which could decrease the interaction between the fiber and matrix. Higher concen trations of fiber can result in agglomerations, thereby decreasing the effective stress transfer between the natural fibers and the matrix [50]. Fig. 7 shows the micrographs of the rPP sample and its composites with YM fibers. Fig. 7(a) shows the rPP polymer matrix and Fig. 7(b–d)
components of the fiber [68], and showed the fibrillar structure of the yerba mate. It can also be seen the size of the fibers, which have a long length and has a microstructure with diameters above 100 μm. The morphological analysis of the PP polymer matrix and composites with YM is shown in the micrographs of Fig. 6, with a magnification of 300 � . It is possible to observe that the composites present structures that are rougher and with the presence of fibrils coming from the polymer matrix of the fracture region of the specimens. There are also small voids in some regions of the samples, which may indicate that there was a pull out of fibers in these regions. In Fig. 6(b–d), it can be
Fig. 6. SEM micrographs of PP (a), PP20YM (b), PP30YM (c) and PP40YM (d) samples.
Fig. 7. SEM micrographs of rPP (a), rPP20 YM (b), rPP30 YM (c) and rPP40 YM (d) samples. 7
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present the structures of the composites. It is observed that the morphological structure of the rPP polymer matrix (Fig. 7(a)) is different from the neat PP matrix (Fig. 6(a)). rPP presents small pores along the matrix, unlike the neat PP matrix, where there is greater homogeneity in the structure. In relation to the com posites, it is also possible to observe the presence of pieces of YM fibers adhered to the rPP matrix, but the structure of these composites is more heterogeneous, showing a larger amount of voids and a more irregular distribution. The heterogeneity of rPP used in these composites, from post-use packages, may have a greater influence on the adhesion of the fibers to the matrix, indicating a more uneven structure and making the interaction between composite phases more difficult, thus reducing the mechanical properties of these composites and corroborating the results of tensile and flexural strength of post-consumer PP.
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4. Conclusions Particles of YM fiber were successfully used as filler in neat and postconsumer PP to prepare composites. Chemical, mechanical and morphological evaluations were performed. The main results showed that the addition of filler from YM fiber increased tensile and flexural strength of neat and post-consumer PP composites up to 30 wt% fiber content, with a decrease in the properties of composites with 40 wt% of fiber. The PP20YM composite presented tensile and flexural strength 13 and 22% higher than the rPP20 YM composite, respectively, whereas PP30YM showed 20 and 22% greater than the rPP30 YM composite in these same mechanical properties. The neat PP composites showed better results than those with recycled PP, where PP20YM and PP30YM composites presented the best results. There was a gradual reduction in impact resistance with the increase of the YM content and it was observed that between the two PP matrices, the neat PP showed impact resistance well above than post-consumer PP, with a value ~40% higher. Among all the composites evaluated, the PP30YM and rPP30 YM composites presented the smallest impact resistance difference, pre senting more uniform results. Infrared analysis did not show significant changes, indicating that the difference in the matrices of pure and recycled PP did not influence the chemical characterization of the evaluated composites. The analysis of the fracture surface of the speci mens by SEM showed a better adhesion between the matrix and filler in the composites with 20 and 30 wt% of YM fiber, corroborating the mechanical results of tensile and flexural strength. The results showed that YM fiber has great potential to be used in polymeric composites, showing to be efficient for used as a filler. Thus, it is possible to replace 30–40% by mass of the matrix for fibers, depending on the desired properties, replacing a material of difficult decomposition and coming from nonrenewable source, like the PP, by a process residue and coming from a renewable source, such as YM residues. Acknowledgments The authors would like to thank Universidade do Vale do Taquari (Univates) for financial support, to Instituto Federal de Educaç~ ao, Ci^ encia e Tecnologia do Rio Grande do Sul for their support in charac terization analyzes and Elacy for donating the residues of yerba mate. References [1] Shin C. Filtration application from recycled expanded polystyrene. J Colloid Interface Sci 2006;302:267–71. [2] Srinivasan P, Sarmah AK, Smernik R, Das O, Farid M, Gao W. A feasibility study of agricultural and sewage biomass as biochar, bioenergy and biocomposite feedstock: production, characterization and potential applications. Sci Total Environ 2015;512:495–505. [3] Colom X, Carrasco F, Pages P, Canavate J. Effects of different treatments on the interface of HDPE/lignocellulosic fiber composites. Compos Sci Technol 2003;63: 161–9.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Characterization of polypropylene composites using yerba mate fibers as reinforcing filler Andre� Luis Catto a, *, Marcos Aur�elio Dahlem Júnior a, Betina Hansen a, Edson Luiz Francisquetti b, Cleide Borsoi a a b
Centro de Ci^encias Exatas e Tecnol� ogicas, Universidade do Vale do Taquari -UNIVATES, Lajeado, RS, Brazil Instituto Federal de Educaç~ ao, Ci^encia e Tecnologia do Rio Grande do Sul - IFRS, Farroupilha, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Yerba mate Composite Neat and post-consumer PP Mechanical properties
This study presents the preparation of polypropylene (PP) composites filled with yerba mate (YM) stick particles. Neat and post-consumer PP are used as the polymer matrices with 20, 30 and 40 wt% (w/w) YM fiber. Me chanical properties, including tensile, flexural and impact resistance, are determined, in addition to chemical and morphological analyses. The main findings show that the addition of filler from YM fiber increased the tensile and flexural strength of the neat and post-consumer PP composites up to a 30 wt% fiber content. The neat PP composites show better results than those with recycled PP, with PP20YM and PP30YM composites showing the best results. There is a gradual reduction in impact resistance with increasing YM content. Scanning electron microscopy shows a better adhesion between the matrix and filler in composites with 20 and 30 wt% YM, corroborating the tensile and flexural strength results.
1. Introduction The amount of solid waste that has been generated by humanity in recent years has attracted attention to the problem associated with its disposal, challenging academia and companies to seek effective solutions to the issue, allied with social awareness. Disposal in landfills and incin eration are the cheapest methods for the destination of post-consumer polymer products, but this destination creates a serious problem for the environment [1]. In the context of concerns regarding sustainability, there are two problems: waste generation and depletion of natural resources. As the planet cannot renew what is consumed in modern times, it must then reconsider the standards of production, with the aim of sustainable con sumption. This can be achieved through less polluting energy sources, with emphasis on the life cycle of products, reducing waste production and recycling as much as possible [2]. Post-consumption polymer materials represent one of the largest categories of waste, and their reuse in the production of natural fiber-reinforced polymer composites is of great economic and environmental importance [3]. Polyolefins are the most commonly found plastics in urban solid waste, among which, polyethylene (PE) and polypropylene (PP) are particularly prevalent [4]. Due to the excellent combination of thermal
and mechanical properties, PP is widely exploited in a variety of appli cations, providing a rapid expansion in the use of this material. The physical characteristics of the polymer can be varied to achieve a wide range of thermal and mechanical properties, and its ease of processing allows economical use in most commercial manufacturing techniques [5]. PP has been one of the most common polymers used as a matrix in composites of natural fibers in the last decade, according to the literature [6–12], and is able to compete with higher cost plastics on the market. It can be used in various applications because of its low processing tem perature, mechanical properties, crystallinity, relatively high melting point, crystalline phase that maintains resistance mechanics at high temperatures, availability, cost, low density and high stiffness [13–16]. The development of composites involving the use of lignocellulosic materials as reinforcements in recycled polymer matrices has grown in response to environmental conservation, becoming more common in the polymer industry, because lignocellulose can be renewable, inexpensive, biodegradable and non-toxic [14,15]. The use of natural fibers continues to expand due to the higher prices for petroleum products, the reduction of global warming and acceptance among consumers due to the use of renewable and non-polluting raw materials and waste [12]. Among the lignocellulosic materials most used for this purpose are sawdust, a timber
* Corresponding author. E-mail addresses: [email protected] (A.L. Catto), [email protected] (M.A. Dahlem Júnior), [email protected] (B. Hansen), edson. [email protected] (E.L. Francisquetti), [email protected] (C. Borsoi). https://doi.org/10.1016/j.compositesb.2019.106935 Received 8 March 2019; Received in revised form 18 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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industry waste, vegetable fibers and lignocellulosic agricultural and agroindustrial waste [14,15]. Their availability, renewability, low den sity and price, as well as satisfactory mechanical properties make them attractive ecological alternatives to glass, carbon and synthetic fibers used for the manufacturing of composites. Their disadvantages, however, are low compatibility with hydrophobic polymer matrices, much lower tensile strength than glass fibers, thermal sensitivity at the temperature of compounding processes, moisture absorption and flammability [17,18]. Therefore, interest in natural fiber-reinforced polymer composite materials is rapidly growing both in terms of their industrial applications and fundamental research. Natural fiber-reinforced thermoplastic composites are gaining popularity in applications for automobile in dustries, packaging, assembly boards, paneling, fencing and furniture [19]. Brazil, which represents one of the richest biodiversities on the planet, stands out in the production of lignocellulosic materials. The term vegetable fibers is used to refer to fibers that are obtained from �, cotton, sisal, flax, jute, pineapple, plants, such as eucalyptus, curaua bamboo, banana and various agricultural by-products, such as sugar cane bagasse, corn straw, grape stalks, yerba mate sticks, tobacco and wood industry residues, and other sources [20,21]. Yerba mate (Ilex paraguariensis, Saint Hil.) is a perennial shrub-tree (Aquifoliaceae) native to the subtropical region of South America, where it is grown and harvested to obtain its leaves and twigs for the commercial processing of yerba mate products. Yerba mate is consumed mainly in Paraguay, Argentina, Uruguay and southern Brazil. The infusion of yerba mate is the most popular tea-like beverage of southern Latin American ~o” countries, where its leaves are used in the elaboration of the “chimarra (hot water extract of green dried leaves; mate in Spanish speaking coun tries), “terer^ e” (cold water extract of green dried leaves) and mate tea (hot water extract of toasted leaves), which are very appreciated drinks that are part of the culture of these countries [22–25]. Argentina presents the highest production among the countries that produce yerba mate, with an estimated value of 809000 tons in 2018 [26], while Paraguay produced ~91600 tons in 2014 [27]. According to the Brazilian Institute of Geog raphy and Statistics (IBGE), Brazil produced ~346000 tons of yerba mate in 2016, of which 80% is destined for the domestic market [28]. The yerba mate stick residues correspond to ~2% of mass production and currently their main use is as a boiler energy source and/or as organic fertilizer [29]. Considering the high levels of consumption of yerba mate in these Latin American countries, any attempt to increase the use of these agroindustrial wastes will add value to this material, with a more sustainable destination. Therefore, the incorporation of yerba mate sticks into higher value prod ucts, such as polymer composites, becomes a strategic route to explore options to develop the industrial sector and increase the economic value of yerba mate [30]. In this context, the use of agricultural waste becomes a source of ecological and sustainable materials, and can be used in a wide range of industrial applications, replacing polymers obtained from nonrenewable sources [31]. Plastic waste, in general, takes too long to degrade and, when burned, it may produce different degrees of toxicity [32]. Polyolefins used for product packaging are often discarded after a single use, resulting in a large amount of polymeric residues in munic ipal solid waste, presenting as a promising possibility for recycled products [33,34]. Recycling is directly related to the environment, aiming to reduce direct and indirect costs and the amount of pollution generated during production [35]. With this, the perspective of a new source of income makes people to transform part of the waste into a new product of value [36]. Therefore, there is a general tendency to use these lignocellulosic and polymeric wastes, considering the immense potential value of the materials processed and the implications and pollution arising from the nonuse of waste [32,37,38]. The objective of this study is to evaluate the potential production of composites with neat and post-consumer PP filled with lignocellulosicbased particles obtained from yerba mate residues. Thus, there is the possibility to explore its use in thermoplastic composite materials for civil construction, automobiles and other industries. The properties of
the produced composites are evaluated for their mechanical properties of tensile and flexural strength, the impact energy absorption, in addi tion to a chemical and morphological analysis. 2. Materials and methods 2.1. Materials The polymeric materials used were neat PP, H-401 type, a homo polymer obtained from Braskem, with a melt flow index of 7.5 g/10 min and a density of 0.905 g/cm3, and post-consumer polypropylene (rPP) from packaging of buckets, bowls and dumpsters products collected at University of Vale do Taquari (UNIVATES). Yerba mate (Ilex paraguariensis, St Hil) residue was obtained from ^ncio Aires city (Rio Grande do Sul, Brazil), pro Elacy, located in Vena vided in the form of ground sticks. The yerba mate sticks are considered a by-product from the production process in industry, where the leaves of the plant are predominantly used. 2.2. Methods 2.2.1. Composite preparation The yerba mate sticks were oven dried at 60 � C for 24 h and then ground in a mill from Tecnal brand, TE 631 model, at a speed of 27000 rpm and classified using a 60 mesh sieve. PP plastic waste was pre-washed with water and dried at room temperature for 24 h. After that, they were ground to size reduction in the form of flakes in a knife mill Retsch SM 200. For the preparation of the composites, a pre-blend of neat or postconsumer PP polymer matrix with the yerba mate (YM), previously dried in an oven at 60 � C for 12 h, was prepared in a single-screw extruder, with the aim of decreasing the volume of the fibers and thus improving the uniformity of YM fibers in the composite. In the prepa ration of thermoplastic composites, 20, 30 and 40 wt% (w/w) YM fiber were used for each type of polymer matrix (neat and post-consumer PP). The materials were first processed in a single-screw extruder, ES 35FR model, from the SEIBT manufacturer. The temperatures in the different heating zones were 160, 170, 180, 185 and 190 � C, with a screw speed of 40 rpm. After this processing, the composites were ground in a knife mill from Primot�ecnica (1001 model) and oven dried for 12 h at 70 � C. Thus, the material was reprocessed, now in a twin-screw extruder, COR 20-32LAB, co-rotational with L/D ¼ 32 and D ¼ 20 mm, from MH Equipments, with a feed speed in the extruder of 90 rpm. The extruder has eleven heating zones, where the following temperature profile was used in the processing: 115, 150, 180, 185, 185, 190, 190, 195, 200, 200 and 220 � C, respectively, with a screw speed of 200 rpm. The specimens were obtained by injection molding with a LHS 150–80 from Himaco Hydraulic Machines Ltd. Three different temper atures were used in the heating zones, namely, 150, 165 and 180 � C. The rotation speed of the screw was 100 rpm and the mold temperature was ~20 � C. The composites were dried for 24 h at 70 � C in an oven before the injection molding process. The specimens were made for tensile, flexural and impact tests. Table 1 presents the identification and composition of the produced composites. Table 1 Identification and composition of produced PP composites with YM fibers.
2
Samples
Matrix Content (wt.%)
Fiber Content (wt.%)
PP PP20YM PP30YM PP40YM rPP rPP20 YM rPP30 YM rPP40 YM
100 80 70 60 100 80 70 60
_ 20 30 40 _ 20 30 40
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2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectroscopy analysis was per formed using a PerkinElmer Frontier spectrophotometer with an atten uated total reflectance (ATR) accessory. Each spectrum was obtained using 32 scans ranging from 4000 to 400 cm 1 at a resolution of 4 cm 1. 2.3.2. Mechanical tests The tensile and flexure tests were performed in a universal EMIC DL 2000 test machine using seven different specimens of each composite sample. The speed used for the tensile test was 5 mm/min, performed according to ASTM D-638-10 [39]. For the flexure test, the speed was 2.0 mm/min and performed in accordance with ASTM D790-03 [40]. The Izod impact was performed on CEAST equipment, model Impactor II, in accordance with ASTM D256-10 [41]. The specimens were pre pared without notch and pendulum of 2.75 J for the composites and of 11 J to PP samples were used, at room temperature. 2.3.3. Morphological properties Morphological characterization for the YM fiber and PP composites with YM fibers was carried out with a CARLS ZEISS LS-10 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. All samples were previously covered with a thin layer of gold. SEM images were obtained of the fracture surface from specimens after the tensile test. 3. Results and discussion The FTIR-ATR spectra of the produced composites with YM fibers are showed in Fig. 1. The FTIR spectra of the PP (Fig. 1(a)) and rPP (Fig. 1 (b)) matrices show characteristic bands of PP [42], with stretching of CH and CH2 bonds between 2960 and 2920 cm 1, and asymmetric and symmetrical deformations of CH and CH3 at 1461 and 1377 cm 1, respectively. The YM fibers show absorption bands related to stretching and bending vibrations of characteristic chemical groups of lignocellu losic compounds: cellulose, hemicellulose and lignin. However, as can be seen in Fig. 1, the chemical compositions of samples differ and the intensities of some bands are different. These bands for the YM fiber are in accordance with the ones found in the literature [22,43–49] and can be seen in 3690–3000 cm 1 (O–H stretching vibration of the hydroxyl groups in cellulose molecules), 2900 cm 1 (CH3 stretching vibration of alkyl groups in aliphatic bonds of cellulose, lignin and hemicellulose), 1740 cm 1 (acetyl and ester groups in hemicelluloses and in aromatic components of lignin), 1608 and 1518 cm 1 (C–C in plane symmetrical stretching vibration of aro matic ring present in lignin), 1440 cm 1 (CH2 symmetric bending pre sent in cellulose), 1248 cm 1 (C–O stretching vibration of hemicellulose component or arylalkyl ether compounds present in lignin) and 1170–1030 and 890 cm 1 (C–O stretching and C–H deformation vibra tions of the pyranose ring skeletal of cellulose) [22,46–49]. Table 2 presents a summary of the main absorption bands attributed to the lignocellulosic fibers and PP samples. It is observed by the spectra that there were no significant changes in the samples of the neat and recycled PP composites. In Fig. 1(a), with neat PP and its composites, there are alterations observed in the region of 3600–3000 cm 1, related to the –OH groups, appearing with larger and subtler peaks in the PP20YM and PP30YM composites, which may be an indication of the presence of moisture in these samples. In Fig. 1 (b), with the post-consumer PP, the rPP20 YM composite also presented similar behavior. According to Zulkifli et al. [50], the decrease of the peaks in the region at 3600–3000 cm 1 in the composites can also be related to the good interaction between the fiber and the polymer ma trix. In the region of 2900 cm 1, characteristic of the PP matrix, there is a more pronounced peak in Fig. 1(b), which may be related to the types of recycled PP used.
Fig. 1. FTIR-ATR spectra for the YM fiber, neat PP (a) and post-consumer PP (b) composites. Table 2 FTIR absorption bands assigned to YM and PP present in the composites. Wavenumber (cm 1) Lignocellulosic fiber 3400 2800–3000 1728 1650 1606 1505–1511 1462 1375 1268
1163 1059 897 PP 2952, 2918 and 2838 2720 1456 1376 1165 974, 841 and 808
3
Peak Assignment
Reference
O–H stretch (hydrogen-bonded) C–H stretch in methyl and methylene groups C¼O stretching of acetyl or carboxylic acid Absorbed O–H and conjugated C–O C¼C stretching of the aromatic ring (lignin) C¼C stretching of the aromatic ring (lignin) CH2 deformation in lignin and carbohydrates C–H deformation in cellulose and hemicellulose Guaiacyl ring breathing, C–O stretch in lignin and for C–O linkage in guaiacyl aromatic metoxyl group C–O–C asymmetric vibration (in cellulose) C–O stretch in cellulose and hemicellulose Glucose ring stretch, C–H deformation in cellulose
[46–49] [46–49] [46–49] [46] [46–48] [46,47] [46,47] [46]
C–H stretching
[42,46]
CH bending and CH3 stretching CH3 asymmetric deformation CH3 symmetric deformation Bending vibration of tertiary carbon C–H deformation out-of-plane
[42,46] [42,46] [42,46] [42,46] [42,46]
[46–49] [46–48] [46] [46,47]
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Fig. 2. Tensile strength (a) and Young’s modulus (b) for the PP polymer matrix and composites with YM fibers.
Mechanical properties are extremely important in the composites, because they strongly influence the applications of these materials [51]. Fig. 2 shows the tensile strength and Young’s modulus of the composites with YM fibers. In relation to the tensile test (Fig. 2(a)) it was observed that the postconsumer PP matrix had a higher tensile strength than neat PP. This result may be related to the variability of types of PP found in the postuse residues of PP used. Among the specimens of neat PP with YM, the PP30YM composite showed the highest tensile strength at break, with a value of 22.5 MPa. The PP20YM composite also presented a similar result, with 21.5 MPa; however, increasing the amount of YM fiber to 40 wt% resulted in a reduction in the strength of the material, where the PP40YM composite presented a value of 19.2 MPa. According to Aseer et al. [52], the decrease in tensile strength at higher loading may be due to the imperfection in filler dispersion at higher loading. The mechanical properties of polymer composites reinforced with short vegetable fibers depend on many factors that determine the surface area able to transfer the tensile strength of the matrix to the fibers [53,54]. These factors include geometric parameters (length, diameter and aspect ratio), orientation and distribution of fibers in the polymer matrix, and these parameters are dependent on processing conditions used for the com posite manufacture [55]. In addition, the adhesion between fiber and
matrix is responsible for the efficient transfer of the tensile strength of the matrix to the reinforcement [53]. By analyzing the modulus of elasticity (Fig. 2(b)) of the samples, it was also verified that the PP30YM composite presented a higher value of Young’s modulus, with 816 MPa, indicating a higher stiffness of this composite. The improvement of the mechanical properties of composites depends not only on the composition, such as the percentages used and the individual components produced, but is also affected by the strength and effectiveness of the interfacial adhesion between the cellulose fibers and the matrix. This adhesion can be improved with prior treatment and also can be attributed to the localized tensions that are generated due to the low fiber concentration in the polymer matrix, which causes a mixture not so homogeneously dispersed, causing a smaller module [56]. With the increase of the fiber content, the tension is better distributed and the material becomes more resistant and with the addition of 20 and 30 wt% YM fibers, an increase in the tensile strength and in the modulus is observed when compared to neat PP [57]. For the rPP composites, rPP20 YM presented the highest tensile strength at rupture, with 18.6 MPa, followed by rPP30 YM, with a similar value of 17.4 MPa. Furthermore, in these composites it was observed a decrease in the resistance with the increase in the content of fibers used, as reported by Yan et al. [58]. Comparing the PP and rPP composites, it was found that those with neat PP presented higher results 4
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of tensile strength than those with recycled PP. In the case of composites with 40 wt% YM fibers, the decrease of the tensile strength can be due to the agglomeration of the fibers. According to Lee and Cho [59], this agglomeration indicates the low interfacial adhesion between the load and the matrix, evidencing that the insertion of an amount equal to or greater than 40 wt% fiber weakens the composite. Among the rPP composites, the rPP20 YM, rPP30 YM and rPP40 YM samples presented very similar elastic moduli, with values between 600 and 650 MPa, but higher than the post-consumer PP polymer matrix, indicating a higher stiffness of these samples with the increase in fiber content. Regarding the results of flexural strength and flexural modulus, showed in Fig. 3, it can be verified that the composites with neat PP presented higher values than the composites with post-consumer PP. Among the samples with neat PP, PP30YM presented the better resis tance to flexural strength, with a value of 39.4 MPa, as showed in Fig. 3 (a). However, by analyzing the standard deviations, it is observed that the PP20YM composite also presented a very similar result, with a value of 38.5 MPa, followed by the composite PP40YM, with a value of 37.4 MPa. Among the rPP composites, rPP20 YM presented an increase of ~8% in flexural strength in relation to the rPP matrix, but with the increase of the fiber content, there is a gradual decrease in this property in the samples.
For the flexural modulus (Fig. 3(b)), the rPP composites showed lower results than those with neat PP, as expected, where the PP20YM composite presented the largest value of all specimens. The flexural modulus was ~30–40% higher in the composites with neat PP compared to composites with recycled PP. The influence of the proportion of lignocellulosic material on the flexural strength in thermoplastic com posites has already been verified by some authors [60,61]. In these studies, the authors verified that the flexural strength increases with increasing natural fiber content in the composite. In general, there must be good adhesion between the fiber and matrix phases for this to occur, but even in a low adhesion system, this behavior is verified. According to Magalh~ aes et al. [62], the presence of fillers with an irregular shape does not support the stress transferred to the polymeric matrix. In this case, the incorporated material acts more as a filler than as a reinforcement. Espert et al. [63] reported that the length and shape of the fibers in fluence the mechanical properties of the composites. Fig. 4 shows the impact resistance results of the analyzed samples. Fig. 4(a) presents the impact absorption energy of the polymer matrix (PP and rPP), while Fig. 4(b) shows the impact absorption energy of natural fiber composites. It is observed that the PP polymeric matrix has an impact absorption energy much higher than the composites, where an 11 J hammer was used to break the test specimens, whereas for the
Fig. 3. Flexural strength (a) and flexural modulus (b) for the PP polymer matrix and composites with YM fibers.
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Fig. 4. Results of impact resistance of PP matrix (a) and PP and rPP composites (b).
composites, a 2.75 J hammer was used. The neat PP showed impact resistance well above the post-consumer PP, with a value ~40% higher. Lignocellulosic fiber is a kind of stiff organic filler, so the addition of fibers could decrease the impact strength of composites [64,65]. This indicates a higher tenacity of this PP in relation to PP-r. Among the composites, those with neat PP presented higher resistance to impact than those with post-consumer PP, a result expected by the higher stiffness of the neat PP. However, with the increase of the YM fiber content, there is a gradual reduction of the values of impact resistance for all composites. In general, addition of the filler introduces regions of
poor interfacial adhesion and stress concentration in the composite because the particles reduce the mobility of the polymer chains, thereby reducing the energy absorption [66] of the composite. The decrease of the tenacity during the degradation process occurs due to the presence of voids both on the surface and inside the specimens, facilitating the propagation of cracks and reducing the mechanical properties [67]. The SEM analysis of the YM fibers is shown in the micrographs of Fig. 5. It is possible to observe the structure of the YM fiber and the size of the particles used in the preparation of the composites. The particles had a rougher surface layer, probably due to the presence of amorphous
Fig. 5. SEM micrographs of YM fibers used: (a) magnification of 100 X and (b) magnification of 500X. 6
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seen that pieces of vegetal fibers of the YM adhered to the PP polymer matrix, in addition to uniform structures, indicating that there may have been good adhesion between the matrix and the vegetal fiber [50,69]. By analyzing the SEM micrographs (Fig. 6), with higher fiber con tent, an increase in fiber-fiber contact can be observed, which could decrease the interaction between the fiber and matrix. Higher concen trations of fiber can result in agglomerations, thereby decreasing the effective stress transfer between the natural fibers and the matrix [50]. Fig. 7 shows the micrographs of the rPP sample and its composites with YM fibers. Fig. 7(a) shows the rPP polymer matrix and Fig. 7(b–d)
components of the fiber [68], and showed the fibrillar structure of the yerba mate. It can also be seen the size of the fibers, which have a long length and has a microstructure with diameters above 100 μm. The morphological analysis of the PP polymer matrix and composites with YM is shown in the micrographs of Fig. 6, with a magnification of 300 � . It is possible to observe that the composites present structures that are rougher and with the presence of fibrils coming from the polymer matrix of the fracture region of the specimens. There are also small voids in some regions of the samples, which may indicate that there was a pull out of fibers in these regions. In Fig. 6(b–d), it can be
Fig. 6. SEM micrographs of PP (a), PP20YM (b), PP30YM (c) and PP40YM (d) samples.
Fig. 7. SEM micrographs of rPP (a), rPP20 YM (b), rPP30 YM (c) and rPP40 YM (d) samples. 7
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present the structures of the composites. It is observed that the morphological structure of the rPP polymer matrix (Fig. 7(a)) is different from the neat PP matrix (Fig. 6(a)). rPP presents small pores along the matrix, unlike the neat PP matrix, where there is greater homogeneity in the structure. In relation to the com posites, it is also possible to observe the presence of pieces of YM fibers adhered to the rPP matrix, but the structure of these composites is more heterogeneous, showing a larger amount of voids and a more irregular distribution. The heterogeneity of rPP used in these composites, from post-use packages, may have a greater influence on the adhesion of the fibers to the matrix, indicating a more uneven structure and making the interaction between composite phases more difficult, thus reducing the mechanical properties of these composites and corroborating the results of tensile and flexural strength of post-consumer PP.
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4. Conclusions Particles of YM fiber were successfully used as filler in neat and postconsumer PP to prepare composites. Chemical, mechanical and morphological evaluations were performed. The main results showed that the addition of filler from YM fiber increased tensile and flexural strength of neat and post-consumer PP composites up to 30 wt% fiber content, with a decrease in the properties of composites with 40 wt% of fiber. The PP20YM composite presented tensile and flexural strength 13 and 22% higher than the rPP20 YM composite, respectively, whereas PP30YM showed 20 and 22% greater than the rPP30 YM composite in these same mechanical properties. The neat PP composites showed better results than those with recycled PP, where PP20YM and PP30YM composites presented the best results. There was a gradual reduction in impact resistance with the increase of the YM content and it was observed that between the two PP matrices, the neat PP showed impact resistance well above than post-consumer PP, with a value ~40% higher. Among all the composites evaluated, the PP30YM and rPP30 YM composites presented the smallest impact resistance difference, pre senting more uniform results. Infrared analysis did not show significant changes, indicating that the difference in the matrices of pure and recycled PP did not influence the chemical characterization of the evaluated composites. The analysis of the fracture surface of the speci mens by SEM showed a better adhesion between the matrix and filler in the composites with 20 and 30 wt% of YM fiber, corroborating the mechanical results of tensile and flexural strength. The results showed that YM fiber has great potential to be used in polymeric composites, showing to be efficient for used as a filler. Thus, it is possible to replace 30–40% by mass of the matrix for fibers, depending on the desired properties, replacing a material of difficult decomposition and coming from nonrenewable source, like the PP, by a process residue and coming from a renewable source, such as YM residues. Acknowledgments The authors would like to thank Universidade do Vale do Taquari (Univates) for financial support, to Instituto Federal de Educaç~ ao, Ci^ encia e Tecnologia do Rio Grande do Sul for their support in charac terization analyzes and Elacy for donating the residues of yerba mate. References [1] Shin C. Filtration application from recycled expanded polystyrene. J Colloid Interface Sci 2006;302:267–71. [2] Srinivasan P, Sarmah AK, Smernik R, Das O, Farid M, Gao W. A feasibility study of agricultural and sewage biomass as biochar, bioenergy and biocomposite feedstock: production, characterization and potential applications. Sci Total Environ 2015;512:495–505. [3] Colom X, Carrasco F, Pages P, Canavate J. Effects of different treatments on the interface of HDPE/lignocellulosic fiber composites. Compos Sci Technol 2003;63: 161–9.
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