Evaluation of different strengthening methods in the mechanical and functional properties of soy protein-based bioplastics

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Journal of Cleaner Production on Jiménez‐Rosado, M., Bouroudian, E., Perez‐Puyana, V., Guerrero, A., & Romero, A. (2020). Evaluation of different strengthening methods in the mechanical and functional properties of soy protein-based bioplastics. Journal of Cleaner Production, 262, 121517, available at: https://doi.org/10.1016/j.jclepro.2020.121517 1 Evaluation of different strengthening methods in the mechanical and functional properties of soy protein-based bioplastics M. Jiménez-Rosado1*, E. Bouroudian1, V. Perez-Puyana2, A. Guerrero1, A. Romero2 1Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, 41011, Sevilla, Spain. 2Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, 41012, Sevilla, Spain. *MERCEDES JIMÉNEZ-ROSADO Departamento de Ingeniería Química Universidad de Sevilla, 41011, Sevilla (Spain) e-mail: [email protected] Phone: +34 954557179 Fax: +34 954556441 2 Abstract 1 The bioplastics manufactured are still not competitive compared to conventional 2 plastics, due to their higher price and poor mechanical properties. For this purpose, 3 different physical strengthening methods have been studied to evaluate the 4 reinforcement of soy protein-based bioplastics: mold temperature increase (from 5 70 °C to 130 ºC), as the application of a dehydrothermal (4 and 24 h at 50 ºC) or 6 ultrasound (for 5 and 45 min at 20 kHz) treatment. In this sense, the crosslinking 7 degree, the mechanical properties, water uptake measurements and scanning 8 electron micrographs of the different bioplastics were compared. The results 9 conclude that suitable processing conditions and post-treatments could favor some 10 characteristics of bioplastics (such as maximum stress (~5 times) or strain at break 11 (~3 times)), although worsening others (such as water uptake capacity (~50%)). 12 Specifically, thermal treatments improved the mechanical properties of the 13 structures obtained, whereas the ultrasound treatment leads to the formation of a 14 structure with smaller pores. 15 16 keywords: Soy bioplastic; pea bioplastic; mold temperature; dehydrothermal 17 treatment; ultrasounds. 18 3 1. Introduction 19 Nowadays, plastics have an ever-increasing production and consumption due to 20 the large number of domestic and industrial applications, such as packaging or 21 hygienic products (Ahmed et al., 2018). However, this trend involves great 22 contamination due to the low degradability of plastics according to the studies 23 driven by Ahmed et al. (2018) or Derraik (2002). Thus, the weight of plastics in the 24 ocean will be greater than that of fish in 2050 (Ellen MacArthur Foundation, 2016). 25 Furthermore, most products we eat contain traces of plastics (Carrington, 2019). 26 The substitution of these petroleum-based plastics with bioplastics (bio-based 27 plastics) is seen as a potential alternative (Russo et al., 2019), since it will minimize 28 the dependency of plastic on petrol fuels and the pressure on landfills from solid 29 wastes (Álvarez-Chávez et al., 2012). 30 Bioplastics can be either bio-sourced (from biomass) or biodegradable, including 31 fossil resources, but also there are bioplastics that are both bio-sourced and 32 biodegradable (European Bioplastics, 2019). The latter, which have both 33 characteristics, are the most interesting and the most recently studied by using 34 different raw materials as soy protein (Jiménez-Rosado et al., 2019), cornhusk 35 fibers (Saenghirunwattana et al., 2014) or porcine plasma protein (Álvarez-Castillo 36 et al., 2019), among other alternatives (Flieger et al., 2003), by injection molding or 37 extrusion processing (Verbeek and van den Berg, 2010). In this context, bioplastics 38 can be made of polymers derived from biomass, such as proteins, which, in 39 combination with a plasticizer, can be easily processed (Tummala et al., 2006). In 40 addition, these proteins could come from byproducts or waste of agri-food industry, 41 generating added value to these products and closing its life cycle 42 (Mohammadhosseini and Tahir, 2018a), therefore encouraging the circular 43 4 economy. However, their mechanical properties are much lower (i.e. they are a 44 young’s modulus of 7 MPa in contract with 25 MPa of PE or 35 MPa of PS) 45 (Zhuang et al., 2014), and their price is often higher than that of conventional 46 plastics (around 10 times more expensive), thus their use is still limited (Confente 47 et al., 2019). These mechanical properties can be improved by using chemical, 48 enzymatic or physical treatments. Thus, formaldehyde, glutaraldehyde or glyoxal 49 (Marquié, 2001) as well as even citric acid (Awadhiya et al., 2016) have been used 50 such as chemical crosslinker and transglutaminase (Giosafatto et al., 2018) have 51 been used like enzymatic ones. In addition, fillers (i.e. metalized plastic fibers) can 52 be added, which without interacting with proteins, improve the final properties of 53 the bioplastic (Mohammadhosseini et al., 2018). However, the use of some of 54 these materials can cause toxicity or the transmission of harmful properties to the 55 product they are being used for (Gerrard, 2002). In this context, the use of physical 56 treatments is the most recommended to could prevent these disadvantages and 57 improve the mechanical properties of bioplastics without causing toxicity problems 58 that may affect its subsequent use. Among the different physical treatments (i.e. 59 ultrasound, microwaves or thermal treatments), thermal treatments are the most 60 used, both during processing (Mo et al., 1999) and post-processing (Álvarez- 61 Castillo et al., 2018). In addition, there are also works where ultrasound 62 (Jayasooriya et al., 2004) or microwaves (Ghasri et al., 2019) are used to increase 63 the degree of crosslinking of bioplastics. However, these improvements normally 64 affect, apart from the mechanical properties, other properties such as water uptake 65 capacity, which could decrease between 1000-2500%, (Álvarez-Castillo et al., 66 2018) or even the morphology of the final structure obtained and the degradation 67 5 rate. Therefore, an optimization is required to reach the final desired properties 68 (Perez et al., 2016). 69 Among the possible applications, protein-based bioplastics can develop a 70 sustainable product that may act as superabsorbent materials (SABs) (Cuadri et 71 al., 2017). SABs are materials with the ability to absorb and retain more than 10 72 times their own weight of water, keeping their integrity without dissolving (Cuadri et 73 al., 2016). The forces which allow water absorption by polymers are mainly the 74 osmotic pressure and the formation of hydrogen bonds (Barbe and Storz, 2005). 75 There are several applications for these materials, such as construction 76 (Mohammadhosseini and Tahir, 2018b), waste management, agriculture, 77 electronics and horticulture (Nnadi and Brave, 2011). Among them, personal 78 hygiene products constitute most of the market of these materials (Magnay et al., 79 2010). 80 Specifically, soy protein has a high content of glutamic and aspartic acids, more 81 than other proteins such as rice or pea, which favor the formation of these 82 hydrogen bonds and, therefore, making soy protein-based bioplastics a potential 83 option as SABs (Fernández-Espada et al., 2016a). In addition, soy protein normally 84 used in this field is a co-product with soy oil and one of the cheapest proteins in 85 nature, which is another advantage to consider it the best alternative to plastics 86 (Fernández-Espada et al., 2016b). However, the properties (especially the 87 mechanical properties) shown by soy protein-based bioplastics (Gironi and 88 Piemonte, 2011) or other compostable alternatives (Song et al., 2009) 89 manufactured to date are not good enough to compete with conventional plastics; 90 therefore, further research is necessary to find a methodology that can generate an 91 adequate relationship between the properties of these bioplastics (Karana, 2012). 92 6 In this context, the main objective of this work was the use of different physical 93 strengthening in order to enhance the mechanical properties of soy protein-based 94 bioplastics without worsening their functional properties (i.e. water uptake 95 capacity). To this end, the manufacturing conditions were evaluated and different 96 post-treatments (dehydrothermal or ultrasounds treatments) were conducted to 97 reinforce the structure of the bioplastics. In order to compare the results, 98 mechanical and microstructural measurements were carried out, as well as water 99 uptake capacity tests. 100 101 2. Materials and methods 102 2.1 Materials 103 Bioplastics are usually composed of three different elements: matrix, plasticizer 104 and additives (optional). In this work, the matrix was soy protein isolate (SPI, 91 105 wt.% protein), which was supplied by Protein Technologies International (SUPRO 106 500E, Belgium). This raw material is a byproduct of a soybean oil production, so its 107 use in this work gives it an added value. Its chemical composition was evaluated 108 obtaining a minimum of 90 wt.% of protein, maximum of 6 wt.% of humidity, 1 wt.% 109 lipids, maximum of 0.1 wt.% carbohydrates and 5 wt.% of ash. In addition, its pH is 110 between 6.9 and 7.4, being negatively charged (isoelectric point 4.5). Thus, this 111 source of soy protein have a higher protein content than others such as used by 112 Tummala et al. (2006), 91 vs. 52 wt.%. 113 In addition, glycerol (Gly) was used as plasticizer, which was purchased from 114 Panreac Química Ltd. (Spain). The use of other plasticizers has been proved 115 (Tummala et al., 2006), but glycerol is the one that generate bioplastics with the 116 best mechanical properties. 117 7 2.2 Preparation of soy protein-based bioplastics 118 The soy protein-based bioplastics were prepared in a two-step process, which was 119 used and optimized in previous studies related to the production of soy-based 120 bioplastics combined with albumen (Fernández-Espada et al., 2016b), 121 lignocellulose (Gamero et al., 2019) or even functionalized soy protein (Cuadri et 122 al., 2017). Firstly, SPI and Gly in a 1:1 mass ratio was homogenized in a mixing 123 step. For this, both raw materials were introduced in a two-blade counter-rotating 124 ThermoHaake mixer Polylab QC (Germany) for 10 min at 50 rpm and adiabatic 125 conditions. In this step, the temperature and torque were controlled to prevent the 126 plasticization of the blends in the chamber. In this case, the temperature was below 127 37 ºC and the maximum torque was 5 Nm, so the blends were always under their 128 plasticization parameters (Fernández-Espada et al., 2016b). 129 The second step consisted in an injection molding of the dough-like blend obtained 130 in the mixing. For this, a ThermoHaake MiniJet Piston Injection Molding System II 131 (Germany) was used to obtain rectangular bioplastics (60x10x1 mm3) and dumb- 132 bell-type bioplastics (UNE-EN ISO 527-2, 2012). The parameters used, according 133 to previous studies, were cylinder and mold temperature of 40 and 70 ºC, 134 respectively, injected for 20 s at 500 bar and using a postinjection pressure of 200 135 bar for 300 s. These parameters were selected to highlight the water uptake 136 capacity of soy protein-based bioplastics, but they have poor mechanical 137 properties (Fernández-Espada et al., 2016a). In this work we aimed to enhance 138 these mechanical properties without significantly affecting the water uptake 139 capacity. 140 2.2.1 Strengthening modifications 141 8 To enhance the mechanical properties, three different experiments were carried 142 out. In the first case, the mold temperature was increased. Previous studies state 143 that protein-based bioplastics have a better crosslinking with a higher mold 144 temperature (Thakur, 2017). This greater crosslinking could improve the 145 mechanical properties of these bioplastics. In this context, an increment of 60 ºC in 146 the mold temperature was applied during the injection molding step, so the final 147 mold temperature used was 130 ºC instead of 70 ºC. It worth mentioning that 148 higher temperatures could cause protein disintegration without achieving the 149 desired effect. 150 The second experiment consisted in the addition of a post-treatment. Thus, the 151 bioplastics were subjected to a thermal step at 50 ºC in a conventional oven 152 (Memmert, Germany). The application of this step, called dehydrothermal 153 treatment (DHT), for a long time (12-14 h) can favor the formation of covalent 154 bonds in it (Álvarez-Castillo et al., 2018). In order to study the effect of this step 155 with the time of treatment in the bioplastic properties, different times (4 and 24 156 hours) were evaluated. 157 Finally, the last experiment was an ultrasound post-treatment (US). US treatment 158 produces localized cavitation, which facilitates the disintegration of the particles 159 (Karki, 2009), allowing the structure to rearrange and, consequently, improving the 160 mechanical properties of the bioplastics. To carry out this experiment, the 161 bioplastics, after the injection molding step, were inserted into vacuum bags. Then, 162 these bags with the bioplastics were immersed in an ultrasound bath (JP Selecta) 163 at 50 kHz with 100 W (power) and 0,4 A (intensity), using water as medium. In this 164 case, the treatment time was also evaluated at 5 and 45 min. 165 15 mold temperature increase (130 ºC), dehydrothermal treatment (DHT) at 4 and 24 295 h and ultrasound treatment (US) at 5 and 45 min. Different letters show significant 296 differences (p ≤ 0.05). 297 298 3.2.2 Bending tests 299 Figure 2 shows the results obtained from the bending tests. In Figure 2A, the 300 evolution of the elastic modulus (E’) with the frequency can be observed. All the 301 bioplastics studied showed a dependency of the flexural elastic modulus (E’) on the 302 frequency studied range. This means that the properties of the bioplastics vary 303 depending on the force applied and the application time, making it more difficult to 304 predict their behavior. This dependence is higher in bioplastic processed thermally, 305 since its rise is steeper than US processed bioplastics. Thus, it seems that higher 306 tensile parameters lead to less flexural stability. These results may be because the 307 bioplastic, although it supports higher stresses, deforms plastically when a certain 308 force is applied, changing its structure and flexural stability. Similar results were 309 obtained in a previous work (Jiménez-Rosado et al., 2018). On the other hand, 310 Figure 2B shows the elastic modulus and loss tangent at 1 Hz (E’1 and tan δ1, 311 respectively). As in the tensile tests, a higher mold temperature or the incorporation 312 of a thermal stage (DHT) caused an improvement in the flexural strength of the 313 bioplastics. However, the US treatment worsened the elastic modulus of the 314 bioplastics, which was higher when a longer treatment time was applied. These 315 results are consistent with those obtained in the tensile tests, showing that a 316 thermal treatment caused a strengthening of bioplastic bonds, whereas an 317 ultrasound treatment weakened them. As for tan δ1, it remained invariant below 1 318 in all the systems analyzed. This reflects the high solid character of the systems, 319 16 which was not altered by the change in the processing method. This solid 320 character was also observed in previous studies (Fernández-Espada et al., 2016a; 321 Perez-Puyana et al., 2018). 322 17 323 Figure 2: Bending tests of the different processed bioplastics. (A) Elastic modulus 324 (E’) in the entire frequency range and (B) elastic modulus and loss tangent at 1 Hz 325 (E’1 and tan δ1, respectively). Systems: Reference (Ref.), mold temperature 326 increase (130 ºC), dehydrothermal treatment (DHT) at 4 and 24 h and ultrasound 327 treatment (US) at 5 and 45 min. Different letters show significant differences (p ≤ 328 0.05). 329 If the mechanical properties of these bioplastics are compared with those of 330 conventional plastics, it can be observed that after thermal strengthening of the 331 bioplastics have better mechanical properties. In fact, it achieves properties similar 332 to that of polyethylene or polyvinylchloride (Young’s modulus 25 and 35 MPa, 333 respectively), but not so high as polystyrene (3-3.4 GPa). However, the US 334 treatment does not improve these properties. 335 3.3 Water uptake capacity & soluble matter loss 336 The measurements of water uptake capacity and soluble matter loss are shown in 337 Figure 3. Considering the soluble matter loss, there were no significant differences 338 between the systems, where, in all cases, all Gly is lost, due to its hydrophilic 339 character, and the soluble part of the protein (about 10 wt.%). These results were 340 similar to those obtained in previous works, where all Gly and part of the protein 341 were lost (Fernández-Espada et al., 2016b). 342 18 343 Figure 3: Water uptake capacity and soluble matter loss measurements for the 344 different processed bioplastics (systems): Reference (Ref.), mold temperature 345 increase (130 ºC), dehydrothermal treatment (DHT) at 4 and 24 h and ultrasound 346 treatment (US) at 5 and 45 min. Different letters show significant differences (p ≤ 347 0.05). 348 On the other hand, for the water uptake capacity, there were major differences 349 between the bioplastics studied. The greatest difference was observed with the 350 increase in mold temperature, losing the superabsorbent capacity (<1000%) of the 351 bioplastic. This is due to the greater structuring of the bioplastic while it is 352 conferred. It creates a more reinforced structure with more difficulty to swell, and 353 therefore, with less capacity to retain water inside (Fernandez-Espada et al., 354 2016a). 355 As for the application of DHT, it also caused a decrease of the bioplastic’s water 356 uptake capacity, which was more significant when longer DHT time was needed. 357 19 However, the functionality of the bioplastic was not lost (they were 358 superabsorbent). These results show that, although there is a strengthening of the 359 system, improving its mechanical properties, it was not sufficiently consolidated. 360 Consequently, the bioplastic could swell to retain a greater amount of water than in 361 the case of mold temperature change. 362 The US treatment also worsened the water uptake capacity of the bioplastics, 363 which remained at the limit of being considered superabsorbent. These results are 364 not consistent with those obtained in previous studies (Álvarez-Castillo et al., 365 2018), since these predicted that the mechanical properties of bioplastics were 366 closely linked to their water uptake capacity, which worsens when the better 367 mechanical properties are presented by the bioplastics. In this case, this prediction 368 is not fulfilled, which may be due to the protein denaturation that was previously 369 intuited. Thus, the protein loses the ability to create hydrogen bonds with water, 370 which worsens its retention capacity. 371 3.4 Scanning electron microscopy (SEM) 372 SEM imaging was performed in the different systems studied before and after the 373 water uptake measurements. The micrographs of the bioplastics before (A, B, C 374 and D) and after (A’, B’, C’ and D’) the water immersion can be seen in Figure 4. 375 Before water absorption, the different processes performed on the bioplastics had 376 an impact on the microstructure in each case. The reference system (A) was the 377 one with the greatest number of pores, which were also larger than in the rest of 378 the systems. This is due to the lower structuring of the bioplastic when softer 379 processing conditions were used. This lower structuring induced a greater water 380 uptake capacity, with the subsequent swelling and opening of the structure 381 20 reflected by the increase in the number and size of pores in the microstructure (A’). 382 Previous work also shows these results (Fernandez-Espada et al., 2016a). 383 384 Figure 4: Microstructure images of the different processed bioplastics obtained by 385 scanning electronic microscopy (reference without modification (A, A’), processed 386 with 130 ºC in mold temperature (B, B’), with 24 h dehydrothermal treatment (C, C’) 387 and with 45 min ultrasounds treatment (D, D’)) before (A, B, C and D) and after (A’, 388 B’, C’ and D’) water immersion. 389 On the other hand, any change in the processing method led to a less porous 390 structure (B, C and D), probably due to the reinforcement produced by the 391 crosslinking achieved in these bioplastics. The most solid and compact structure 392 was achieved with the highest mold temperature (B). The tightness obtained 393 implies a lower water absorption (as seen in Section 3.3) and, therefore, no 394 variation in the structure after the immersion (B’). A similar effect took place after 395 performing the DHT post-treatment (C), but with a slight modification of the 396 microstructure with the formation of some pores on it (C’). These results are similar 397 to these obtained by Álvarez-Castillo et al. (2018). As for the bioplastic submitted 398 to US (D), it can be seen that the ultrasound caused a modification of the 399 21 microstructure respect to the reference system, presenting a large number of small 400 pores that did not vary after the water immersion (D’). This was surely due to the 401 denaturation discussed above. 402 403 4. Conclusions 404 Soy protein-based bioplastics processed by injection molding have demonstrated 405 their potential to replace conventional plastics. However, suitable processing 406 conditions are extremely important to reach the required properties for these 407 bioplastics. 408 Thus, thermal treatments (both higher mold temperature and dehydrothermal 409 treatment) improve the mechanical properties of the bioplastics (specially the 410 Young’s modulus and strain at break). Nevertheless, higher mold temperature 411 leads to bioplastics more compact and without superabsorbent capacity, while 412 dehydroythermal treatment decreases slowly this capacity, allowing bioplastics to 413 still have superabsorbent capacity. Thus, the first ones could be used in 414 applications as packaging, and the second ones could be used in hygienic 415 applications. 416 On the other hand, ultrasound treatment leads to a change in the structure of 417 bioplastic with a decrease in the water uptake capacity and worse mechanical 418 properties. In this way, however a greater crosslinking normally results in an 419 improvement of the mechanical properties and a loss of the water uptake capacity, 420 this is only true if the microstructure of the materials is not altered too much. 421 Finally, this work opens the possibility of creating sustainable bioplastics 422 competitive with commercial ones, making the consumption of plastics cleaner 423 without creating pollution problems. However, further study is necessary where 424 22 combinations between the different strengthening used (i.e. mold temperature in 425 addition to post-treatment), as well as the use of other intermediate temperatures 426 and times, would be evaluated. 427 Acknowledgements 428 This work is part of a project funded by “Ministerio de Economía y Competitividad” 429 (Ref. RTI2018-097100-B-C21), the authors gratefully acknowledge their financial 430 support. 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