Development of eco-friendly biodegradable superabsorbent materials obtained by injection moulding

1 Development of eco-friendly biodegradable superabsorbent materials obtained by injection moulding V. Perez-Puyana1, M. Felix2, A. Romero3, A. Guerrero2 1 Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Química, 41012 Sevilla, Spain 2 Departamento de Ingeniería Química, Universidad de Sevilla, Escuela Politécnica Superior, 41011 Sevilla, Spain 3 Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Física, 41012 Sevilla, Spain _______________________ *M. FELIX Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Química 41012 Sevilla (Spain) E-mail: [email protected] Phone: +34 954557179; fax: +34 95455644 2 Abstract The development of eco-friendly materials requires a deep knowledge of physical, chemical and functional properties of biopolymers and fillers. Soy protein (SPI), which is one of the cheapest protein in global markets, was used in combination with montmorillonite (MMT‑Na+) to obtain fairly attractive green biocomposite superabsorbent materials, leading to biodegradable superabsorbent materials. Natural MMT-Na+ was used since it is widely available as micron-size tactoids and gives rise to highly functional nanoparticles. However, the dispersion of such particles within a polymer structure is complex, showing a high impact on techno-functional properties. An easy-scalable processing technique was used with a mixing stage to obtain dough-like materials which were subjected to an injection-molding process that led to biodegradable biocomposite materials. The structure of dough-like materials was characterized by scanning electron microscopy and X-Ray diffraction, whereas mechanical properties of green biocomposites were evaluated by dynamic mechanical analysis (DMA) and tensile tests. Moreover, the techno-functionality of the new materials obtained was determined by means of water uptake capacity (WUC). Results suggested that the addition of nanoclay led to an increase of the elastic modus although the overall improvement of mechanical properties is not clear. Interestingly, water uptake capacity was greatly enhanced. In this sense, these green biocomposites could be considered as an excellent candidate for the development of novel bio‑based superabsorbent materials. Keywords: Biocomposites; Mechanical properties; Rheology; Superabsorbent; Tailormade polymers; 3 1. Introduction Plastic materials have many different applications according to their extraordinary properties (Plastics-Europe, 2008). Furthermore, the combination of different materials with different specifications allows to modulate their properties, modifying the features of the final product (Sehaqui et al., 2011). However, most plastics present two main disadvantages: their non-renewable source (petroleum) and non-recyclable character, which cause huge wastes. Because of that, the use of bio-based materials instead of those based on synthetic polymers is seen as a promising way to reduce pollution (Lee et al., 2014). New challenges in this field are devoted to the research of new technologies which would allow the development of renewable materials for the existing requirements. In addition, the fabrication of these new materials by using proteins or polysaccharides (renewable resources) is seen as an interesting alternative (Wang and Sain, 2007). The development of new protein-based materials, for the production of bioplastics, is focusing researchers in this field to investigate promising applications other than those related to the food, medical or agriculture industries. As an example, biopolymers are being used for the replacement of PVC pipes or the coating of cell-phones (Mano et al., 2003). Conventional plastics control the market, with the production of 240,000 million tons (Plastics-Europe, 2008), although the production of bioplastics in Europe is continuously growing, reaching 509,000 tons in 2013 (Martínez et al., 2003). Bioplastics are based in two main elements: a polymer matrix and a plasticizer, although some additives could be added to endow the bioplastics with specific properties. Protein isolates from soy bean (SPI) constitute the biggest by-product from the soybean oil industry, being one of the cheapest proteins in global markets. Interestingly, their hydrophilic amino acids lead to superabsorbent properties (Tian et al., 2010). SPI systems have been used for the development of bio-materials, showing suitable properties such as 4 high elastic modulus and high water uptake capacity (Liu et al., 2005; Song et al., 2011). Moreover, these properties have been improved through the addition of lamellar nanofillers (Alexandre and Dubois, 2000; Angellier-Coussy et al., 2013; de L. Freitas et al., 2017; Guillard et al., 2013). Related to this, natural montmorillonite (MMT-Na+) is commonly used (Peelman et al., 2013). The structure and arrangement of these particles cause the formation of plates, with maintain a 1 nm gap between them, forming layers (Bagheri Marandi et al., 2011). One of the key factors is the efficient arrangement of these nanoclays in biopolymer matrices, where improving the mechanical properties of biocomposites have been related to the exfoliation of nanoclays (Yang et al., 1999). In this sense, it has been demonstrated that the arrangement of these particles within the polymer structure is complex. Several factors, such as the protein/plasticizer ratio, the amount of filler used or some processing variables, such as the moulding time or temperature, can modify the properties of the final bioplastics (Felix et al., 2018). Previous studies have been focused on analysing the influence of different additives on the properties of biocomposites (Laadila et al., 2017; Selvaraju et al., 2017). However, in the literature there are no data about the evaluation of the effect of nanoclays on the properties of superabsorbent materials. Thus, the main novelty and objective of this work was the development of a new family of green superabsorbent materials based on soy protein isolate (SPI) and montmorillonite (MMT-Na+), which were processed by injection moulding. The structure of the blends (after the mixing stage) was characterized by scanning electron microscopy and X-ray diffraction. The biocomposites were characterized by dynamic mechanical thermal analysis, tensile tests and water uptake capacity (WUC). In this study, MMT-Na+ concentrations were modified in order to obtain biocomposites with suitable mechanical and water absorption properties. 5 2. Materials and methods 2.1. Materials Soy Protein Isolate (SPI) Supro500E (91 wt.% of protein content) was purchased from Protein Technologies International (Leper, Belgium). Glycerol (GL), supplied by Sigma- Aldrich (St. Louis, Missouri, USA), was used as the plasticizer. Cloisite®-Na+ was selected as the nanoparticle to be introduced and named as MMT-Na+ (Southern Clay Products, Inc., USA). 2.2. Preparation of dough-like materials The biocomposite materials were processed by a thermo-mechanical two-stage process: the blends were obtained by the mechanical mixing of SPI, the plasticizer (ratio 50/50) and MMT-Na+, and subsequently the dough-like materials were injected (Fernandez- Espada, 2016; Fernández-Espada et al., 2016a). Hence, the first stage consisted of a mixing at 25 ºC, 50 rpm and 60 min in a rotating batch mixer (HaakePolylab QC; ThermoScientific, Germany). The samples were produced by adding different nanoclay (montmorillonite) concentrations to the dough-like materials with a protein/glycerol ratio of 50/50: 0 wt.% (0%), 3 wt.% (3%), 6 wt.% (6%) and 9 wt.% (9%). 2.3. Characterization of dough-like materials 2.3.1. Dynamic Mechanical Thermal Analysis (DMTA) A RSA3 rheometer (TA Instruments, USA), equipped with 8 mm parallel plates (for compression measurements) was used to determine the viscoelastic properties of the samples. E’, E” and tan δ were obtained after applying compression tests combined with a thermal cycle. Prior to temperature ramp tests, strain sweep tests were carried out to determine the linear viscoelastic region (LVR). Subsequently, temperature ramp tests were performed from 20 to 150ºC (3ºC/min), where temperature was controlled using a 6 Chiller PGC-100 (Polycold, Seattle, USA). All these measurements were carried out at constant strain amplitude (0.2 %) within the LVR. 2.3.2. X-ray powder diffraction (XRD) XRD studies, with a scanning range between 2 and 40º, and a step size of 0.05º, were performed in the dough-like materials obtained after the mixing step. The equipment used was a D8 Discover (BRUKE, USA) (40 kV, 30 mA) equipped with Cu-Kα radiation (λ=0.1516 nm). 2.4. Preparation of biocomposites Once the dough-like materials were prepared, they were injected into a mould by means of MiniJet Piston Injection Molding System (ThermoHaake, Germany), obtaining biocomposite probes. Suitable processing conditions were selected on the basis of rheological properties found in previous studies. Thus, the mould temperature was 70 ºC and the injection pressure 500 bar over 20 s (200 bar as post-injection pressure was selected over 200 s) (Fernández-Espada et al., 2016a, 2016b). Two different specimens were produced: one was a 60×10×1 mm rectangular shaped and the other had the shape of a dumb-bell. 2.5. Characterization of biocomposites 2.5.1. Rheological Measurements A RSA3 rheometer (TA Instruments, USA) fitted with a dual cantilever geometry (for flexion experiments) was used to carry out two different tests: (i) frequency sweep test (in a frequency range between 0.02 and 20 Hz) at room temperature and (ii) temperature ramp (in a temperature range between 20 and 140 ºC at 3 °C/min and 1Hz). All measurements were carried out within the linear viscoelastic range, studying the viscoelastic properties of the composites. 7 The critical stress was calculated in previous stress sweep tests. A stress was selected in order to perform the frequency sweep and temperature ramps into the LVR (0.2 %). 2.5.2. Tensile strength measurements Stress-strain curves were obtained according to ISO 527 at room temperature (ISO, 2012). Three different parameters were obtained from these curves: (i) The Young’s Modulus, (ii) Strain at Break and (iii) maximum tensile strength. Five replicates for each system were obtained. The device used was an Insight 10 kN (MTS, USA). 2.5.3. Scanning Electron Microscopy (SEM) After water immersion over 24 h at 25±1ºC, the biocomposites were lyophilized and then observed under SEM with JSM 6460 LV (JEOL, Japan) with a secondary electron detector (using 20 kV as voltage). 2.5.4. Water uptake capacity (WUC) The water uptake capacity of the composites was tested at 25±1ºC according to the standard ASTM D570 method for determining water absorption in plastics (ASTM, 2001). To carry out these tests, rectangular specimens (60×10×1 mm) were used. Before the WUC, a conditioning step was also performed to the different samples obtained. This step consisted in a drying at 50±2ºC in an oven for 5-6h (obtaining the dry weight). Then, the samples were immersed in water for 24 h and weighed again (wet weight). Finally, they were again subjected to drying and weighed (final dry weight). The water uptake capacity was determined by the following equation (Eq. 1): % 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 𝑊𝑊𝑎𝑎𝑎𝑎𝑎𝑎𝑊𝑊𝑎𝑎𝑊𝑊𝑎𝑎𝑎𝑎𝑎𝑎 = 𝑊𝑊𝑊𝑊𝑊𝑊 𝑊𝑊𝑊𝑊𝑎𝑎𝑊𝑊ℎ𝑊𝑊−𝐹𝐹𝑎𝑎𝑎𝑎𝑊𝑊𝐹𝐹 𝐷𝐷𝑊𝑊𝐷𝐷 𝑊𝑊𝑊𝑊𝑎𝑎𝑊𝑊ℎ𝑊𝑊 𝐹𝐹𝑎𝑎𝑎𝑎𝑊𝑊𝐹𝐹 𝐷𝐷𝑊𝑊𝐷𝐷 𝑊𝑊𝑊𝑊𝑎𝑎𝑊𝑊ℎ𝑊𝑊 (1) 2.6. Statistical analysis Statistical analyses were carried out using t tests and one-way analysis of variance (p < 0.05). For that reason, all the experiments were performed in triplicate. Standard deviations were calculated for the selected parameters. 8 3. Results and discussion 3.1. Preparation and characterization of dough-like materials 3.1.1 Preparation of dough-like materials The samples with a 50/50 SPI/GL ratio were mixed with different nanoclay (MMT-Na+) contents to form SPI/GL/MMT-Na+ blends. The MMT-Na+ concentrations used were, 3%, 6% and 9%. Both torque and temperature were recorded along mixing time (Figure 1). Two different behaviours were observed: the samples without nanoclay and the lowest nanoclay concentration (3%) showed torque and temperature profiles which were almost constant during the whole mixing process (although the 3% system showed a soft increase from 50 min). On the other hand, it is worth pointing out that the most concentrated systems (6% and 9%), after an initially constant period, displayed a remarkable increase in both temperature and torque until they reached a maximum value, from which both parameters remained constant again. This relevant thermal event may be associated to some crosslinking effects which might be induced by the combination of shear forces and the presence of MMT-Na+. Moreover, these systems also anticipated the increase in torque and temperature, which could be related to a lower lube capacity of glycerol in the presence of MMT-Na+ or cross linking due to shear-induced exothermic reactions developed during the mixing process (Felix et al., 2017a). In fact, the maximum was found at ca. 31 and 18 min for the 6% and 9% systems. This information allowed selecting optimal mixing times for each system. The selected time should allow a complete homogenization of the protein/plasticizer blends; however, long mixing time usually induces protein cross-linking, which should be avoided. Table 1 indicates the total mixing times selected for each system. In addition, the Specific Mechanical Energy (SME) related to the mixing stage carried out for each blend was also calculated, showing a 9 remarkable rise when nanoclay was added to the formulation. Obviously, this rising in SME was truncated by the selection of such short mixing time periods. 3.1.2. X-ray diffraction The results of the X-ray diffraction of the blends with different concentrations of nanoclay (0, 3, 6 and 9%) are plotted in Figure 2. The profiles obtained were similar in all cases, showing a good integration between the protein and the nanoclay present in the structure. Interestingly, the height of the first diffraction increased according to the nanoclay content. This first peak corresponds to the first reflection of the dhkl-spacing characteristic of MMT-Na+. Thus, as may be observed after the mixing process carried out, there were still coherent specular reflections, which indicates the existence of parallel layers. Interestingly, this peak appeared at lower 2θ angle than the one for the pure MMT-Na+, which suggests a variation in dhkl-spacing towards higher sizes (Sands, 2012). Polymer/MMT-Na+ interactions may yield three different scenarios (Chivrac et al., 2008): - If the interactions are very weak, tactoids remain unaltered and they are incorporated as a filler in the polymer structure. - When there are moderate interactions, the clay interlayer expands, and under these conditions, polymer chains penetrate into the gaps between the plates, leading to an intercalated structure. - If there are strong interactions, the original structure of the nanoclay is lost and nanoclay particles are exfoliated. This last assumption has been related to the greatest improvements in polymer performance (Chivrac et al., 2008). According to the X-ray measurements, the crystallinity of MMT-Na+ was not completely lost, and the SPI/MMT-Na+ blends could be set in the second case. 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X-Ray diffraction measurements for the different dough-like materials (without nanoclay and with 3, 6 and 9% nanoclay content), as well as for MMT-Na+. Fig. 3. Evolution of (A) viscoelatic moduli (E’ and E”) and (B) loss tangent (tan δ) obtained from temperature ramp tests performed at constant frequency (1 Hz) and heating rate (3ºC·min-1) for all dough-like materials without nanoclay and with 3, 6 and 9% nanoclay content. Fig. 4. (A) Dynamic frequency sweep test and (B) temperature ramps for biocomposites without MMT-Na+ and containing 3%, 6% and 9% MMT-Na+. Fig. 5. (A) Profile and (B) Parameters of tensile strength measurements: Maximum tensile strength, Strain at break and Young’s modulus for biocomposites without MMT-Na+ and containing 3%, 6% and 9% MMT-Na+. Fig. 6.SEM images for biocomposites without MMT-Na+ (A) and three different nanoclay concentration: 3% (B), 6% (C) and 9% (D). Fig. 7. Water uptake capacity after 24h for biocomposites without MMT-Na+ and containing 3%, 6% and 9% MMT-Na+. Table Captions Table 1. Mixing time and Specific Mechanical Energy (SME) for the dough-like materials processed without MMT-Na+ and containing 3%, 6% and 9% MMT-Na+. Table 2. Critical Strain at 20ºC and 80ºC for the different systems processed without MMT-Na+ and containing 3%, 6% and 9% MMT-Na+. 23 Table 1 Systems Mixing Time (min) SME (kJ·kg- 1 ) 0% MMT-Na+ 60 2061 3% MMT-Na+ 30 6515 6% MMT-Na+ 18 4966 9% MMT-Na+ 12 4115 24 Table 2 Systems Critical Strain (%) (20ºC) Critical Strain (%) (80ºC) 0% MMT-Na+ 0.37 0.72 3% MMT-Na+ 0.27 0.37 6% MMT-Na+ 0.05 0.19 9% MMT-Na+ 0.05 0.14 25 Figure 1