Characterization and enzymatic degradation study of poly(ε-caprolactone)-based biocomposites from almond agricultural by-products
Graphical abstract
Introduction
Over the last few years, the petroleum crisis has made biocomposites significantly important. The worldwide capacity of bio-based polymers is expected to increase from 2.33 million metric tons, Mt, (2013) to 3.45 Mt in 2020 [1]. Among biodegradable polymers, poly(ε-caprolactone), PCL, has emerged as an important candidate for polymer-based biocomposites in various applications due to its biodegradable character and ease of processing using conventional plastics machinery [2], [3]. However, the relative high price (ranging 4.50–6.00 € per kg) of PCL and its poor mechanical properties have limited its large-scale production as a substitute of traditional polymers [3], [4].
The hydrolytic degradation and biodegradation by microorganisms in the environment of PCL have been extensively investigated, being them very slow due to the polymer hydrophobicity and crystallinity. Highly crystalline PCL was reported to totally degrade in 4 days, observing a decrease in crystallinity during degradation [5], [6]. Three types of lipase were found to significantly accelerate the enzymatic degradation of PCL: Rhizopus delemer, Rhizopus arrhizus, and Pseudomonas. The result of this process is increasingly smaller molecules, which enter into cellular metabolic processes, generating energy and turning into water, carbon dioxide, biomass and other basic products of biotic decomposition. It is extremely important to know the exact rates of degradation and mineralization, not only in terms of use since certain properties of plastic materials must be guaranteed; but also in terms of the environmental impact of the decomposition products (e.g. fragments) [7].
Agricultural by-products are normally incinerated or dumped causing environmental problems such as air pollution, soil erosion and decreasing soil biological activity [8]. The incorporation of agricultural residues into polymer matrices is currently a trending topic in research due to the relatively high strength, stiffness and low density of natural fibres present in these residues. Different studies can be found regarding the valorisation of agricultural residues, such as orange tree pruning fibres, bamboo, jute, flax, wood, among others; as reinforcement in composite materials due to their desirable properties: availability, recyclability, low-cost, environmental friendliness, no toxicity, lower pollution emissions and energy consumption for their production and disposal, biodegradability and mechanical performance [9], [10], [11], [12]. Nut by-products, such as macadamia [13] and walnut shells [14] have been also used as reinforcing agents improving physical and mechanical properties of several polymer matrices. Regarding PCL, different composites were obtained by the incorporation of various natural fibres (wheat gluten, rice, wood) with a remarkable improvement in mechanical properties [15]. In this context, almonds are a very important crop throughout the world's temperate regions, being the worldwide almond production in 2012 about 1.9 Mt [16]. The research on almond reinforced composites from the literature is not very extensive. Pirayesh et al. studied the suitability of using different walnut/almond shell ratios (0, 10, 20, 30, 100 wt%) in wood-based composites manufacturing [8]. Almond shell particles were used as reinforcement in polypropylene by using particle contents up to 30 wt%; obtaining a clear improvement in mechanical and rheological properties [17].
Almond skin (AS) is industrially removed from the nut by hot water blanching, and it constitutes 4–8% of the total shelled almond weight. Almond-processing industries are interested in the valorisation of AS by-products, which at present are mainly used in cattle feed and in gasification plants to produce energy [18]. This residue is considered to have one of the highest fibre contents of all edible nuts (around 12%), among other interesting compounds such as flavonoids and phenolic acids with high antioxidant activity [18]. In a preliminary study, the evaluation of morphological, mechanical, thermal, barrier properties and degradation in composting environment of PCL-based biocomposite films, obtained at laboratory scale, containing AS residue at contents up to 30 wt% were carried out [19]. Results showed that PCL-based composite films reinforced with AS residue at 10 wt% loading showed a high disintegration rate with a clear improvement in mechanical properties, corresponding to a gain in elastic modulus of 17%, increasing the added-value potential of agricultural wastes and reducing the packaging cost. The aim of this study was the development and characterization of biocomposites based on PCL and AS by-products obtained by extrusion and injection moulding in order to revalorize this agricultural residue and to obtain reinforced bio-materials with added value. AS filler and the obtained PCL-based biocomposites at different AS contents (0, 10, 20, 30 wt%) were fully characterized by the use of different techniques to evaluate their main properties. Moreover, the influence of AS by-product addition on the biocomposites enzymatic degradation was evaluated in the presence of Pseudomonas lipase, which is capable of cleaving the ester bonds of PCL [3].
Section snippets
Materials
Poly(ε-caprolactone) PCL-CAPA 6800 (Mn = 80,000, density = 1.1 g cm−3) was supplied in pellets by Perstorp Holding AB (Sweden).
AS industrial by-product was obtained by “Almendras Llopis” (Alicante, Spain) and it was pulverized to a fine powder with a high speed rotor mill (Ultra Centrifugal Mill ZM 200, RETSCH, Haan, Germany) equipped with a 1 mm sieve size. The AS fraction obtained was then dried in a laboratory oven at 40 °C for 24 h. Morphological, structural, thermal and water absorption
Morphological study of AS by FESEM
AS powder, ASP (Fig. 1b), was obtained from AS industrial by-product (Fig. 1a) after milling and sieving. The morphological characterization of ASP particles was studied by FESEM. Fig. 1c shows the FESEM micrograph obtained for ASP surface at 300×. A heterogeneous fibrous microstructure was observed with different morphologies and dimensions, due to the organization of different compounds present into the matrix. According to the literature, AS contains 21.90 ± 1.12% cellulose, 12.19 ± 0.60%
Conclusions
Biodegradable composites based on poly(ε-caprolactone) (PCL) and almond skin (AS) by-product were successfully produced by an extrusion followed by injection-moulding method. The addition of AS particles to PCL resulted in biocomposites with improved mechanical properties compared to neat PCL, showing a reinforcement effect. The best general performance was obtained for PCL10 showing an improved value for Young's modulus compared to neat PCL, but maintaining tensile strength and elongation at
Acknowledgements
Authors would like to thank “Almendras Llopis S.A” for kindly providing the almond skin by-products as well as to Spanish Ministry of Economy and Competitiveness for financial support (MAT-2015-59242-C2-2-R). A. Valdés acknowledges Conselleria de Educación (Spain) for ACIF/2010/172 Predoctoral Research Training Grant.
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