Effect of the injection moulding processing conditions on the development of pea protein-based bioplastics

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is a accepted manuscript of a paper published in: Journal of Applied Polymer Science (2016) vol.133 n.20 DOI: https://doi.org/10.1002/app.43306 Copyright: 1999-2025 John Wiley & Sons El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. “This is the peer reviewed version of the following article: Pérez-Puyana, Victor M; Felix Ángel, Manuel; Romero García, Alberto; Guerrero Conejo, Antonio: Effect of the injection moulding processing conditions on the development of pea protein-based bioplastics: from Journal of Applied Polymer Science, vol.133 n.20 which has been published in final form at https://doi.org/10.1002/app.43306. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited." 1 Effect of the injection moulding processing conditions on the 1 development of pea protein-based bioplastics 2 V. Pérez, M. Felix*,A. Romero,A. Guerrero 3 Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Química, 41012 4 Sevilla, Spain 5 Abstract 6 Bioplastic materials from renewable polymers, like proteins, constitute a highly 7 interesting field for important industrial applications such as packaging, agriculture, 8 etc., in which thermo-mechanical techniques are increasingly being used. Pea protein-9 based bioplastics can be made through a mixing process followed by an injection 10 moulding. The objective of this study was to investigate the influence of different 11 injection parameters (moulding time and injection pressure) on the properties exhibited 12 by the final bioplastics obtained. A dynamic mechanical analysis and tensile strength 13 measurements were performed, along with water absorption capacity and transparency 14 tests. The results indicated that the major differences between bioplastics obtained at 15 different moulding times are in transparency and in the Young's Moduli, exhibiting 16 lower values as moulding time increases. On the other hand, modifying the injection 17 pressure leaded to more consistent bioplastics which differed mainly in the elastic 18 component (E’ profiles) and in the strain at break. Furthermore, the water uptake was 19 more than 100% in almost all the different bioplastics processed due to its hydrophilic 20 character, so they could be considered as potential sources for absorbent material. 21 22 Keywords: Pea Protein; Bioplastic; Dynamic Mechanical Analysis; Tensile strength 23 test; Water Absorption. 24 _______________________ 25 *M. FÉLIX 26 2 Departamento de Ingeniería Química, 27 Universidad de Sevilla, Facultad de Química, 28 41012 Sevilla (Spain) 29 E-mail: [email protected] 30 Phone: +34 954557179; fax: +34 954556447. 31 1. Introduction 32 Proteins, polysaccharides and lipids are suitable raw materials for the production of 33 bioplastics.1,2,3 In particular, starch is widely used as a packaging material, usually 34 mixed with biodegradable polyesters4. Regarding proteins to manufacture bioplastics, 35 research studies have investigated not only plant proteins such as zein, wheat gluten 36 and soybean,5,6,7 but also, in some cases, animal proteins, such as milk proteins, 37 collagen, gelatine, etc.8,9.Plant proteins have a great potential for applying for many 38 manufacturing processes.4 Nowadays, soy protein dominates the market for bio-based 39 plastic materials because of its low price, quality and versatile applications. However, 40 pea protein has increasingly become an adequate raw material due to its price and 41 excellent properties.10,11,12 Protein concentrates have been widely used as raw 42 materials, but those bioplastics obtained simply by the action of pressure and 43 temperature. However, the combination of intermolecular disulphide bonding, hydrogen 44 bonding, hydrophobic interactions and electrostatic forces between proteins chains 45 typically leads to a fragile and brittle protein structures.13For that reason, protein 46 concentrates are generally mixed with a plasticizer. 47 With regard to plasticizers, they are used in order to reduce intermolecular forces 48 among polymer chains,14,15 reducing the cohesion within the matrix and facilitating the 49 mobility of protein chains.16 The use of hydrophilic plasticizers with low molecular 50 weight improves the flexibility of the final bioplastics obtained, but they cannot support 51 a lower mechanical stress. The most effective plasticizer, for biopolymers, is water 52 because reduces the glass transition temperature facilitating the processing. Without 53 3 water addition, the degradation temperature would be easily reached before bioplastics 54 would be finally processed.17 Besides water, glycerol is a plasticizer widely used in 55 thermomechanical processing of proteins.18,19 Its effect is related to the facility of 56 glycerol to be inserted inside the three-dimensionaltridimensional structure of 57 biopolymers.20 58 Considering the processing method, classical thermoplastic polymer processing 59 techniques (extrusion, compression moulding, etc.) have been used to obtain different 60 protein-based bioplastic materials.21,22,23 Among these thermomechanical techniques, 61 injection moulding is one of the most important and suitable processes for systems that 62 may exhibit a mixed character such as proteins,24,25,26,27 but it needs a previous mixing 63 process in order to obtain a suitable protein-plasticizer blend. It is important to select 64 the optimum injection parameters (injection pressure and moulding time),28 but also the 65 temperature in the pre-injection cylinder, high enough to reduce the viscosity of the 66 blend (facilitating the subsequent injection) and leading to heating changes the three-67 dimensional structure of proteins (protein unfolding and denaturation) by disrupting 68 hydrogen bonds and non-polar hydrophobic groups.29 Regarding protein films, resulting 69 electrostatic interactions, hydrogen bonding, van der Waals forces (non-covalent 70 forces) and covalent disulfide bridges can improve the matrix stability.30 However, it is 71 important to avoid so high temperature in the cylinder to avoid protein crosslinking by 72 covalent intermolecular disulfide bonds or even protein degradation.26 Furthermore, 73 exposure to alkaline conditions, particularly when coupled to thermal processing, 74 induces formation of non disulphide covalent crosslinks, such as dehydroalanine, 75 lysinoalanine and lanthionine.31,32 On the other hand, it is important to control the 76 conditions in the packing stage and in the previous injection process, selecting the 77 appropriate conditions to ensure an optimum injection speed related with the lowering 78 speed of the piston, allowing the blend to be inserted into the mould. Depending on the 79 conditions selected, the bioplastics fabricated would exhibit adequate properties to 80 consider them for specific applications. In this way, not only preparation conditions are 81 4 important, but only other components in the formulation such as plasticizers, pH, 82 chemicals, enzymes, nanocomposites, lipids and as well as cross-linking by 83 irradiation.33 84 The main objective of this work was to explore the potential development of biobased 85 plastic materials from pea protein processed by injection moulding and to study the 86 influence of injection conditions in the packing stage (moulding time and injection 87 pressure) on their mechanical properties. Furthermore, a mechanical characterization 88 (water absorption and transparency measurements) was useful to evaluate the effects 89 of the modification of the injection parameters on the final bioplastics properties. A 90 small-scale-plunger-type injection moulding machine was used in this study to obtain 91 pea protein-based specimens from pea protein/glycerol blends, previously mixed by 92 means of a mixing rheometer that allows the torque and temperature to be recorded 93 during mixing process. 94 95 2. Material and methods 96 2.1. Materials 97 Pea flour was provided by Roquette (Lestrem, France). Its protein content, obtained in 98 quadruplicate as % N x 6.25 using a LECO CHNS-932 nitrogen micro analyser (Leco 99 Corporation, St. Joseph, MI, USA) was so close to 90% (89.5±0.7%) that it can be 100 considered as a protein isolate (PPI).33 Besides, microanalysis results revealed a 101 sulphur content of 0.45±0.02%, related to the presence of methionine and cysteine and 102 its importance on generating crosslinking. The ash and lipids content of the protein 103 isolate were 3.5±0.2% and 1.4±0.6%, respectively. Besides, the pea protein isolate 104 presents a moisture content close to 5% (5.1±0.1%). Glycerol (GL) with residual water 105 content ≤0.3% was purchased from Panreac Química, S.A. (Spain). 106 2.2. Characterization of blends 107 5 2.2.1. Rheological measurements. Dough-like blends were characterized by small 108 amplitude oscillatory shear (SAOS) measurements, using a controlled-strain rheometer 109 (ARES, TA Instruments, USA). A plate-plate geometry (dia: 40 mm) with a rough 110 surface has been used, selecting a gap between plates of 2 mm. Low viscosity Dow 111 Corning 200 fluid was used as sealant to avoid sample drying. Strain sweep SAOS 112 tests were also performed in order to establish the linear viscoelasticity range. 113 Temperature ramp tests were carried out at 5 ºC/min from 20 to 100 ºC. In these 114 measurements, complex viscosity (η*) was monitored at a constant frequency of 6.28 115 rad/s. All the systems studied had the same thermo-rheological history before 116 performing any rheological test. 117 2.3. Preparation of bioplastics 118 Bioplastics with a 60PPI/40GL ratio (lower protein/plasticizer ratios would lead to an 119 excess of plasticizer that yields too low consistent blends to be properly processed and 120 an increase of this ratio would produce some shear-induced crosslinking effects 121 leading to excessively brittle specimens) were manufactured by a two-stage thermo-122 mechanical procedure. Firstly, the selected blend was mixed using a two-blade 123 counter-rotating batch mixer, HaakePolylab QC (ThermoHaake, Karlsruhe, Germany), 124 at 25ºC and 50 rpm for 60 min, monitoring the torque and temperature during mixing to 125 obtain a dough-like blend. Secondly, bioplastics were obtained by an injection moulding 126 process in a MiniJet Piston Injection Moulding System (ThermoHaake) using the 127 blends previously prepared. A schematic illustration of the injection moulding cell can 128 be observed in Figure 1: before injection (A) and after injection took place (B).The 129 selected conditions for the pre-injection cylinder were 50 °C (see 3.1) and a residence 130 time of 100 s. As mentioned above, the temperature should not be increased 131 excessively but in addition the residence time should not be too long in order to prevent 132 thermally induced protein cross-linking effect before the injection stage. On the other 133 hand, as for the mould processing conditions, the mould temperature was 130 ºC and 134 6 different moulding times were selected (100, 200 and 300 s) to investigate their effect 135 on the properties of the final bioplastics obtained. It was also important to avoid 136 exposition to high temperatures for a long time in order to avoid protein degradation. In 137 addition, different injection pressures (100, 300, 500 and 900 bar) were also evaluated. 138 A pressure value of 200 bar was selected for the packing stage, to ensure a suitable 139 flow of blend and moulding of specimens. These conditions should allow the 140 development of protein crosslinking to achieve the final network structure. Some 141 injection conditions as the injection pressure or the moulding time were modified in 142 order to study their influence on the properties of the final bioplastics obtained. Two 143 moulds were used to prepare two different specimens: (1) a 60×10×1 mm rectangular-144 shaped specimen for dynamic mechanical analysis (DMA) experiments, water 145 absorption and transparency measurements and (2) a dumb-bell-type specimen by ISO 146 527-1:2012 for tensile properties of plastics. Bioplastic were stored at room 147 temperature and 50% RH for at least five days in order to reach equilibrium. 148 2.4. Characterization of bioplastics 149 2.4.1. Dynamic Mechanical Analysis (DMA). DMA tests were carried out with a RSA3 150 (TA Instruments), on rectangular probes using dual cantilever bending. Strain sweep 151 tests were also performed in order to establish the linear viscoelasticity range.The 152 selected heating rate was 5 ºC·min−1 and the temperature range covered was from 153 -30ºC by the use of an air chiller connected to the forced convection oven (Polycold, 154 TA Instruments) to 130ºC. Linear viscoelastic modulus (E’) and tan δ (E’’/E’) were 155 monitored at constant strain (0.05%, within the linear viscoelastic region) and 156 frequency (6.28 rad/s). All the samples were coated with Dow Corning high vacuum 157 grease to avoid water loss and showed the same thermo-rheological history. 158 2.4.2. Tensile strength measurements. Tensile tests were performed by using the 159 Insight 10 kN Electromechanical Testing System (MTS, Eden Prairie, MN, USA), 160 according toby ISO 527-2:1993 for Tensile Properties of Plastics. Young’s Modulus, 161 7 strain at break and maximum tensile strength were evaluated using type IV probes and 162 an extensional rate of 10 mm·min−1 at room temperature. 163 2.4.3. Water absorption capacity. Water uptake capacity of bioplastics was measured 164 according to the standard method for determining water absorption in plastics 165 ASTMD570, 2001. Rectangular specimens of 60×10 ×1 mm were used. The 166 specimens were subjected to drying (conditioning) in an oven at 50±2 ºC for 5-6h to 167 determine dry weight, then introduced into distilled water and weighed after 24h 168 immersion. Finally, it was subjected to drying (reconditioning) again and weighed to 169 determine the soluble material loss. All the experiments were performed in triplicate at 170 room temperature. According to the methodology used, water absorption capacity and 171 soluble material loss were determined by the following equations: 172 % 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 𝑢𝑢𝑢𝑢𝑊𝑊𝑊𝑊𝑢𝑢𝑊𝑊 = 𝑊𝑊𝑊𝑊𝑊𝑊 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 − 𝐼𝐼𝐼𝐼𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼 𝐷𝐷𝑊𝑊𝐷𝐷𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 𝐼𝐼𝐼𝐼𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼 𝐷𝐷𝑊𝑊𝐷𝐷𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 ·100 (1) 173 % 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝐿𝐿𝑜𝑜 𝐿𝐿𝐿𝐿𝐼𝐼𝑢𝑢𝑠𝑠𝐼𝐼𝑊𝑊 𝑚𝑚𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼 = 𝐼𝐼𝐼𝐼𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼 𝐷𝐷𝑊𝑊𝐷𝐷𝐷𝐷𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 − 𝐹𝐹𝑊𝑊𝐼𝐼𝑊𝑊𝐼𝐼 𝐷𝐷𝑊𝑊𝐷𝐷𝐷𝐷𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 𝐼𝐼𝐼𝐼𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼 𝐷𝐷𝑊𝑊𝐷𝐷𝐷𝐷𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑊𝑊 ·100 (2) 174 2.4.4. Colour determination. A ColorimeterCM-700D (Konica, Japan) was used to 175 measure the colour of the bioplastics. According to EN ISO 11664-4, the CIE standards 176 are used to calculate colour differences. It is described by a three-dimensional 177 coordinate system (L*, a* and b*) that locates a colour in a colour space. The 178 parameter L* refers to the lightness of the colour (L* = 0 indicates black and L* = 100, 179 white). Parameters a* and b* can be either positive or negative: Parameter a* extends 180 from green (-a*) to red (+a*) and b* from blue (-b*) to yellow (+b*). 181 2.4.5 Transparency measurements. Transparency measurements were performed on a 182 Genesys-20 (Thermo Scientific, USA) spectrophotometer. In this device, the 183 transmittance (%) of rectangular specimens, 1mm thickness, at a selected wavelength 184 of 600 nm is measured. Air is used as blank (100% transmittance). In order to compare 185 the transparency of different bioplastics, a transmittance index (IT) was used: 186 𝐼𝐼𝑇𝑇=% 𝑇𝑇𝑊𝑊𝑊𝑊𝐼𝐼𝐿𝐿𝑚𝑚𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼𝑇𝑇𝑊𝑊 %𝑇𝑇𝑊𝑊𝑊𝑊𝐼𝐼𝐿𝐿𝑚𝑚𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼𝑇𝑇𝑊𝑊𝐿𝐿𝑜𝑜𝑊𝑊𝑊𝑊𝑜𝑜𝑊𝑊𝑊𝑊𝑊𝑊𝐼𝐼𝑇𝑇𝑊𝑊 𝑠𝑠𝑊𝑊𝐿𝐿𝑢𝑢𝐼𝐼𝑊𝑊𝐿𝐿𝑊𝑊𝑊𝑊𝑇𝑇 (3) 187 8 2.5. Statistical analysis 188 At least three replicates of each measurement were carried out. Statistical analyses 189 were performed with t tests and one-way analysis of variance (ANOVA, p < 0.05) by 190 means of the statistical package SPSS 18 (IBM, Chicago, IL, USA). Standard 191 deviations from some selected parameters were calculated. 192 193 3. Results and discussion 194 3.1. Preparation and characterization of blends 195 As indicated above, mixing is the first stage in the thermochemical processing of the 196 protein-based bioplastics studied. The 60PPI/40GL ratio was an adequate proportion 197 because doughs with a higher or lower content in protein would not be suitable for 198 processability because they were too consistent or the final bioplastics exhibit glycerol 199 exudation, respectively (results not shown). Besides choosing an appropriate 200 protein/plasticizer ratio, a suitable selection of the mixing conditions is very important, 201 however it is not always easy. An extensive mixing was required to obtain a 202 homogeneous dough-like blend, but long mixing periods must be avoided to limit shear 203 induced structuration effects. For that reason, both torque and temperature values 204 were monitored as a function of mixing time for the 60PPI/40GL system (Figure 2). The 205 profile shows a maximum torque value followed by a continuous decrease and a 206 tendency to reach an eventual constant value, whereas the temperature exhibited a 207 constant increase over the mixing time. 208 From the torque and temperature profiles, it may be deduced that a balance for the 209 mixing time, long enough for a suitable homogenization degree but short enough to 210 avoid premature cross-linking reactions of protein chains is needed. Therefore, three 211 different mixing times were studied: one related to the minimum torque (10 min), 212 another when the increase in torque was produced (21 min) and the third when the 213 15 Acknowledgements 378 This work is part of a research project sponsored by MINECO, “Ministerio de Economía 379 y Competitividad”, from the Spanish Government (Ref. MAT2011-29275-C02-02/01) 380 and by the Andalousian Government, (Spain) (project TEP-6134). The authors 381 gratefully acknowledge their financial support. The authors also acknowledge the 382 Microanalysis Service (CITIUS-Universidad de Sevilla) for providing full access and 383 assistance to the LECO-CHNS-932 (TA instruments). 384 385 References 386 1. Tanrattanakul, V., Saithai, P. Mechanical properties of bioplastics and bioplastic-387 organoclaynanocomposites prepared from epoxidized soybean oil with different epoxide contents. J. 388 Appl.Polym. Sci.2009,114, 3057-3067. 389 2. Sakunkittiyut, Y., Kunanopparat, T., Menut, P., Siriwattanayotin, S. Effect of kraft lignin on protein 390 aggregation, functional, and rheological properties of fish protein-based material. J. Appl.Polym. Sci. 391 2013, 127, 1703-1710. 392 3. Jones, A., Mandal, A., Sharma, S. Protein-based bioplastics and their antibacterial potential. J. Appl. 393 Polym. Sci.2015,132, 41931. 394 4.Ke, T., Sun, X. 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Results from mechanical tests carried out for 60PPI/40GL biobased 477 specimens obtained at different moulding times (100, 200 and 300 s): (A) Storage 478 modulus (E’) and (B) loss tangent (tan δ) values from Dynamic Mechanical Thermal 479 Analysis (DMTA) temperature ramp measurements performed at constant frequency 480 (6.28 rad/s) and heating rate (3oC·min-1). Values for LDPE and Soy biobased 481 specimens (SPI) were also included. 482 Figure 5. Young’s modulus, maximum stress and strain at break from Tensile Strength 483 measurements carried out for 60PPI/40GL biobased specimens obtained at different 484 moulding times (100, 200 and 300s). Values for Soy biobased specimens (SPI) were 485 also included. Columns with different letters are significantly different (P ≤ 0.05). 486 Figure 6. (A) Evolution of water absorption capacity (%) after immersion for 24 h and 487 soluble material loss (%) and (B) Color standards and transparency measurements: 488 lightness (L*), yellow/blue value (b*) and transmittance carried out for 60PPI/40GL 489 biobased specimens obtained at different moulding times (100, 200 and 300s). 490 Columns with different letters are significantly different (P ≤ 0.05). 491 Figure 7. Results from mechanical tests carried out for 60PPI/40GL biobased 492 specimens obtained at different injection pressures (100, 300, 500 and 900 bar): (A) 493 Storage modulus (E’) and (B) loss tangent (tan δ) values from Dynamic Mechanical 494 Thermal Analysis (DMTA) temperature ramp measurements performed at constant 495 frequency (1 Hz) and heating rate (3oC·min-1). 496 Figure 8. Young’s modulus, maximum stress and strain at break from Tensile Strength 497 measurements carried out for 60PPI/40GL biobased specimens obtained at different 498 injection pressures (100, 300, 500 and 900 bar). Columns with different letters are 499 significantly different (P ≤ 0.05). 500 Figure 9. (A) Evolution of water absorption capacity (%) after immersion for 24 h and 501 soluble material loss (%) and (B) Color standards and transparency measurements: 502 lightness (L*), yellow/blue value (b*) and transmittance carried out for 60PPI/40GL 503 biobased specimens obtained at injection pressures (100, 300, 500 and 900 bar). 504 Columns with different letters are significantly different (P ≤ 0.05). 505 506 19 FIGURE 1 507 508 509 510 511 512 20 513 FIGURE 2 514 020 40 60 0 5 10 15 20 25 30 Torque (N·m) t (min) Torque (N·m) T(ºC) 20 40 60 80 T (ºC) 515 516 517 518 519 21 520 FIGURE 3 521 522 20 30 40 50 60 70 80 90 100 6.0x10 4 8.0x10 4 1.0x10 5 1.2x10 5 1.4x10 5 1.6x10 5 60PPI/40GL 70PPI/30GL T (ºC) η∗ (Pa·s) 523 524 525 526 22 527 FIGURE 4 528 -40 -20 020 40 60 80 100 120 140 10 6 10 7 10 8 10 9 A E' (Pa) T(ºC) LDPE E' (Pa) (100 s) E' (Pa) (200 s) E' (Pa) (300 s) E' (Pa) SPI (500 s) 529 -40 -20 020 40 60 80 100 120 140 0.0 0.1 0.2 0.3 0.4 0.5 T(ºC) LDPE tan δ (100 s) tan δ (200 s) tan δ (300 s) tan δ SPI (500 s) tan δ B 530 531 532 533 534 23 535 FIGURE 5 536 0 10 20 30 40 50 60 70 80 90 100 Strain at Break (%) SPI (500 s)300 s 200 s 100 s δ C b Moulding time Maximum Stress (MPa) Young's Modulus (MPa) Strain at Break (%) Young's Modulus (MPa) Maximum Stress (MPa) A a α B a β A a γ 0 2 4 6 8 10 537 538 539 540 541 24 542 FIGURE 6 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 100 s 200 s 300 s 0 40 80 120 Soluble Material Loss (%) a a a C B A Water Uptake (%) Moulding Time Water Uptake (%) Soluble Material Loss (%) A