Coupling effect of starch coated fibers for recycled polymer/wood composites
Graphical abstract
Introduction
The societies’ development leads to an increased demand for raw materials to products and services aiming at the well-being of the consumers [[1], [2], [3]]. However, these processes lead to the depletion of non-renewable raw materials and generation of a large number of residues [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 [6]. 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 [7]. 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 [8], 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], [12], [13], [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 [16]. To surpass these issues, the production of composites emerges as a possibility, increasing the properties of the polymers and reducing the total cost of the product by the addition of low-cost reinforcements [17].
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 [20], sisal [21], cotton [22], sugar bagasse and others [23]. 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 hydrophilic behavior due to the chemical composition of its components, reducing the interaction in the interface with the hydrophobic matrix leading to lower composites properties [18]. This interface can be modified by coupling agents to promote surficial interaction by modification on the fibers or in the polymers [[24], [25], [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 [6].
To create new alternatives, researchers have been working on the development of new coupling agents base in natural sources [27]. 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 [28]. 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 [29].
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 [30]. It has been extensively used commercially as a gum in food industry presenting a broad range of applications [31]. 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], [33], [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. [35] 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.
Section snippets
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. [35] with a solution of corn
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 formation, 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
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
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, 447180/2014-2, 163593/2015-9) and FAPESP (grant number 2018/11277-7).
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