Thermal and thermo-mechanical properties of polypropylene composites using yerba mate residues as reinforcing filler
Graphical abstract
Introduction
Fiber reinforced polymer (FRP) composites, composed of vegetable fibers or lignocellulose residues, have been a promising alternative to conventional materials due to various factors such as lighter weight, lower cost, ease of handling and good thermal and acoustic insulation properties (Ardanuy et al., 2012; Dittenber and Gangarao, 2012).
Besides the advantages with regards to their properties, another important benefit of natural FRPs is the environmental concern. There are two main problems that must be considered nowadays: the generation of waste and the depletion of natural resources. Concerning the generation of waste, polymer materials represent an increasing fraction of solid waste (Lei et al., 2007) and among them, polyolefins such as polyethylene (PE) and polypropylene (PP) are the most commonly found plastics in urban waste (Zadeh et al., 2017).
PP is widely used in a variety of applications due to the excellent combination of thermal and mechanical properties and its ease of processing (Jones, 1994). Furthermore, this polymer has a low processing temperature, low density, high stiffness and low cost which makes it a good alternative to more expensive plastics found on the market (Kim et al., 2007; Kumar et al., 2017; Naghmouchi et al., 2015; Yeh et al., 2015). The automobile industry is one that has been increasing the application of PP, mainly in the form of composites, which leads to an increase of this plastic waste coming from end-of-life vehicles (Wang et al., 2016).
In addition to waste generation, and the fact that these polymers have low degradability and reduce the space of urban landfills, the depletion of natural resources is another important factor. Petrochemical resources are not renewable and in the near future they will not be able to meet production demand at an affordable price (Zhang et al., 2017). Therefore, sustainable development has become a major issue in recent years, motivating research for sustainable alternatives such as the reuse of polymeric wastes and production of green composites. According to Singh et al. (2017), green composites can be fully, or partially biodegradable, the latter being generally composed of natural fibers, used in a randomly oriented short-fiber form, reinforcing a thermoplastic matrix, that can be a virgin or post-consumer polymer. These green composites can be a good alternative use of growing PP residues, in combination with natural fibers.
The use of lignocellulose natural fibers, as reinforcement of composites, has grown greatly, because it meets the demands of sustainability, as stated before, since they are inexpensive, renewable and biodegradable (Ninomiya et al., 2017). Besides that, lignocellulose fibers have low density, competitive mechanical properties and low cost, which make them an attractive ecological alternative to glass synthetic fibers (Hietala and Oksman, 2018). The disadvantages of natural fibers remain in the low compatibility with hydrophobic polymer matrices, more limited processing temperatures and higher heterogeneity (Pappu et al., 2019; Zhang et al., 2017; Zhang and Li, 2016).
There are many possible sources of natural fibers such as wood, bagasse, cereal straw, vegetables and agricultural residues (Xie et al., 2010). The use of agricultural residues is of special interest since it combines the possibility of more worthy destination for these materials, with the fact that it is not necessary to burn or decompose these wastes, so providing a low-cost alternative to wood fiber composites (Ardanuy et al., 2012). These lignocellulose materials are mainly composed of cellulose, hemicellulose, lignin and pectin, with a small quantity of extractives and they are often ground into smaller particles to facilitate the feeding into the processing equipment (Hietala and Oksman, 2018; Xie et al., 2010).
Brazil has one of the richest biodiversities on the planet and therefore has a large variety of plants from which lignocellulose fiber materials can be obtained: eucalyptus, curauá, cotton, sisal, jute, pineapple, banana and various agricultural by-products such as tobacco, corn straw, grape stalks, yerba mate sticks, and other sources (Mo et al., 2009; Singh et al., 2017).
Yerba mate (Ilex paraguariensis St. Hil.) is a commercially important tree in South America. It is consumed as an infusion, brewed from the hot water (85 °C – 95 °C) extract of green dried leaves, known for its diuretic, anti-inflammatory and stimulant properties (Arrieta et al., 2018; Deladino et al., 2008). According to the Brazilian Institute of Geography and Statistics (IBGE), Brazil produced about 346 thousand tons of yerba mate in 2016, of which 80% is destined for the domestic market (IBGE, 2014). Its high consumption leads to the generation of a large quantity of residues, especially yerba mate sticks, that correspond to approximately 2% of mass production (Gonçalves et al., 2007). According to Dahlem Júnior et al. (2019), yerba mate sticks are composed of 34.85% of α-cellulose, 24.77% of hemicellulose, 2578% of lignin, 10.11% of extractives and 4.49% of ashes. It is necessary to search for alternative uses for these wastes, in order to provide a more worthy destination. Their incorporation into polymers, producing composites, becomes an option to develop the industrial sector and increase the economic value of yerba mate (Sanjay et al., 2018).
In this context, the novelty of this work lies in the combination of two different residues, post-consumer PP and yerba mate fibers, in order to produce a green composite that meets the current demands of sustainability. As far as is known, there is no similar research in the literature that involves the use of yerba mate residues in a recycled thermoplastic matrix. Composites, based on virgin and post-consumer PP filled with yerba mate fibers, were prepared by extrusion, followed by injection molding, and the effect of the fibers on the thermal and thermo-mechanical properties of the resultant composites were investigated.
Section snippets
Materials
Virgin polypropylene (PP), H-401 type, was obtained from Braskem, with a melt flow index of 7.5 g/10 min and density of 0.905 g/cm3. Post-consumer PP (rPP) was obtained from the packaging of buckets, bowls and dumpster products, collected at University of Vale do Taquari (Univates).
Ground sticks of yerba mate (Ilex paraguariensis, St Hil) residue were obtained from Elacy company, located in Venâncio Aires city (Rio Grande do Sul, Brazil). These sticks are considered a by-product from the
Thermogravimetric analysis (TGA)
Thermogravimetric analysis yields information about how the thermal stability of the material changes according to the incorporation of natural fibers. The results of TGA and derivative of the thermogravimetric analysis (DTG), for YM residues and composites with different contents of YM in virgin and post-consumer PP, are shown in Fig. 1. Data for thermal degradation such as Tonset, % residue at 650 °C and maximum degradation temperature (Tmax) are shown in Table 2.
For YM the first mass-loss
Conclusions
PP and rPP composites with YM residues were successfully produced, however the addition of YM residues to the polymers has reduced their thermal stability, but the production and use of the composites is still possible, since the degradation starts at temperatures higher than 250 °C for both matrices. It was also observed that the degradation of rPP composites occurs at higher temperatures than PP composites. The rPP40YM sample presented similar Tonset (262 °C) to the PP20YM, indicating higher
Acknowledgments
The authors would like to thank Univates for the financial support and Elacy company manufacturer for donating the residues of yerba mate.
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