Morphology and properties of polybutylene succinic and cassava starch blend: Effect of esterification catalysts on graft copolymer formation

Cite this paper: Vietnam J. Chem., 2021, 59(1), 90-97 Article DOI: 10.1002/vjch.202000125 90 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Morphology and properties of polybutylene succinic and cassava starch blend: Effect of esterification catalysts on graft copolymer formation Vu Minh Duc , Nguyen Chau Giang* 1School of Chemical Engineering, Hanoi University of Science and Technology 1, Dai Co Viet, Hai Ba Trung, Hanoi 10000, Viet Nam Submitted July 26, 2020; Accepted October 21, 2020 Abstract The purpose of this research is to produce of esterified thermoplastic starch (ETPS) for reactive extrusion melt- blending with polybutylene succinate (PBS) in film blowing application. Esterification TPS named M-TPS and T-TPS were prepared from cassava starch, glycerol (plasticizer), and maleic anhydride (MA), tartaric acid (TA) as reactive compatibilizers in a twin extruder, respectively. FTIR results showed that esterification reactions were successfully formed between both MA and TA with cassava starch. The esterified TPSs was then melt blended with PBS in the presence of four kinds of esterification catalysts. The effect of compatibilizer contents on the blend properties was studied in terms of tensile properties, melt flow index (MFI), and electron scanning microscopy (SEM). The results show that TA induced better compatibilization effect than MA did, as evidenced by the PBS/T-TPS blend possessing higher strength than these of PBS/M-TPS, although SEM images showed structural morphology of both blends is similar. The effect of the esterification catalyst type and contents on properties of the PBS/T-TPS blend was also studied. The results from the Molau test confirmed the formation of graft-copolymer (PBSgT-TPS) at the interface of two phases of the blend when compatibilizers were added in the presence of an organotin compound catalyst. Keywords. Cassava starch, esterified TPS, polybutylene succinic (PBS), blends, compatibilization. 1. INTRODUCTION Vietnam is the fourth most polluted plastic waste to the sea in the world, after China, Indonesia and the Philippines.[1] According to the IUCN report in 2018, Vietnamese produces around 1.2 kg of waste per day, and 16 % of them are plastic. At present, these solid wastes are mainly buried (95 %) at more than 500 landfill sites and thousands of small landfills across the country but they are mostly open landfills. Many landfills are located right next to dikes, near-surface water. In many places waste was thrown directly into rivers and canals. All these results in gathering of plastic wastes in the natural environment and contaminates a wide range of natural terrestrial, freshwater, and marine habitats. Besides, non-degradable plastics have also caused an overload of land-fillings, bad seeing, methane emission etc. more and more seriously.[2] Due to increasing concerns on sustainable development and the impact of materials on the environment, biodegradable plastics have attracted intense interest in recent years in Vietnam. Among them, starch is one of the most popular and available biopolymers which can be biodegraded to carbon dioxide and water in a relatively short time compared with most synthetic polymers. Thermoplastic starch (TPS) obtained from the heat and shear process combining with an addition of a suitable amount of plasticizers is probably the most preferable application of starch- based material in packaging.[3] Starch is a highly hydrophilic polymer, so thermoplastic starch is very sensitive to environmental humidity, leading to a deterioration in physical properties when the material is dehumidified. Therefore, to overcome this disadvantage, TPS is often blended with polymers that are hydrophobic and have high mechanical strength.[3] Polybutylene Succinate (PBS) is an interesting biopolymer that shows balanced mechanical properties, high flexibility, heat resistance, and especially excellent biodegradability under industrial condition, according to EN 13432.[4,5] A wide processing window makes the resin suitable for extrusion, injection moulding, thermoforming, fibre spinning and film blowing. However, the high cost of PBS still limits its extensive application.[4,6] Blending TPS with PBS seems to be a perfect solution because it can reduce the end-product costs since cassava starch is at a low price and available in Vietnam. Vietnam Journal of Chemistry Nguyen Chau Giang et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 91 To create a blending system with high performance from two very different polarity polymers such as TPS (very hydrophilic) and PBS, a hydrophobic plastic, it is crucial to control the interfacial surface tension between the two constituents to create fine dispersion phase and have good adhesion with matrix.[7] In this respect, many studies were carried out to functionalize starch or polyester by grafting highly active groups such as maleic anhydride (MA) and carboxylic acid on to the backbone.[8-13] These grafted functions could react with the hydroxyl groups of starch macromolecules to form covalent bonds, thus they provided better control of the size of phase and strong interfacial adhesion. Yin et al.[14] fabricated TPS/PBS composites with MA grafted PBS as a compatibilizer to improve the interfacial miscibility, resulting to increase the strength and elongation at break for the composites. Suttiruengwong et al.[15] produced PBA/TPS by one- step extrusion process. The compatibility of PBS/starch blends was improved by both MA/peroxide and MDI as the reactive compatibilizers, demonstrated by increasing mechanical properties, good adhesions of PBS/starch interface, and the small evenly dispersed starch particles. The results from the Molau test and FTIR spectra confirmed the formation of graft- copolymer at the interface of PBS and starch when reactive compatibilizers were added. Zhang et al. designed and developed a bio-based elastomer from mixtures of starch, glycerol and tartaric acid (TPS- TA) using reactive extrusion.[16] Then TPS-TA was extruded with PBS to fabricate the bio-composites and the impact strength of PBS/TPS-TA was superior to that of PBS. They revealed that TA reduced the molecular weight of starch and shear viscosity of TPS were beneficial for TPS-TA uniformly dispersing in PBS and TA also served as coupling effect to improve the compatibility of TPS and PBS matrix. Fahrngruber et al.[17] were produced thin TPS/PBS films with two compatibilizer systems prepared from grafting reaction between PBS with native starch and destructured TPS in addition of N,N′- dicyclohexylcarbodiimide. The addition of the TPS- based compatibilizer resulted in improved incorporation of TPS within the polyester resulted in higher tensile strength and tear resistance compared to native starch counterpart. Explanations for this observation could be that pre-plasticized starch provides a larger reaction surface and enables better homogenization during the course of compounding. Although the formation of graft copolymer between poly(butylene adipate co-terephthalate) (PBAT), a member of the compostable polyester family, and MTPS created by the transesterification reaction during blending extrusion process was reported by some researchers.[9-11,13] There are rare papers that can be found concerning the study of formation graft copolymer between PBS and esterified TPS in detail, except Suttiruengwong’s studies.[15] Also, since PBS molecules possess OH end groups and esterified TPS chains contain unreacted COOH groups, there is a high potential of forming grafted copolymer from PBS and esterified TPS caused by ester linkage formation between these two functional groups. The objective of works is to present a detailed study on the preparation and study of esterified TPS/PBS blend by reactive extrusions. Furthermore, this work aims to evaluate the influence of four kinds of esterification catalyst to graft copolymer formation generating from esterification reaction between esterified TPS and PBS, which was not mentioned in the previous studies. The resulting melt-blends were used in blown film applications. 2. MATERIALS AND METHODS 2.1. Materials The polybutylene succinate (PBS) Bionolle 1001D in the form of pallets was purchased from Showa Denko. The native cassava starch (moisture content of 11 % by weight) in the form of powder was supplied by Duy Hung Co., (Vietnam). Glycerol (99.5 %) as a plasticizer for starch was purchased from Aladdin Reagent (Shanghai, China). Maleic anhydride and tartaric acid as compatibilizer agents for starch were obtained from Sigma-Aldrich with purities of 99 %. The catalyst for esterification reactions included sodium hydroxide, sodium bicarbonate and organotin compound reagent grade 97 % were purchased from Aladdin Reagent (Shanghai, China). 2.2. Preparation of thermoplastic starch (TPS) and esterified thermoplastic starch (ETPS) Thermoplastic starch was prepared in a twin-screw co-rotating Leistriz extruder with a screw diameter of 27 mm and L/D ratio of 36. Glycerol of constant loading of 15 wt. % of starch as a plasticizer was mixed with starch using a kitchen blender for 10 min. This mixture was then blended with two kinds of compatibilizer maleic anhydride and tartaric acid for further 10 min before being introduced into the extruder via screw feeder. Vietnam Journal of Chemistry Morphology and properties of polybutylene succinic © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 92 In the case of modified TPS vacuum was applied at the vent port to remove any unreacted MA and TA. The temperature profile was 100/120/135/135/135/135/135/135/120/110 °C from the feed throat to the die. The screw speed was kept constant at 125 rpm. The resulting TPS and ETPS were cooled by air, pelletized and then dried in an oven overnight at 70 °C. 2.3. Preparation of PBS/TPS blends PBS and ETPSs with a fixed weight ratio of 60/40 and four types of esterification catalyst were mixed by kitchen blender in 10 min. before melt-blended in a Leistriz twin extruder (D = 27 mm, L/D = 36) at temperature profile of 110/120/135/150/150/150/150/150/130/120 oC from the feed throat to the die. The extrudates were cooled using a water tank and pelletized. 2.4. Blowing films Films made of pure PBS and PBS/ETPS blends were blown by using a Labtech single screw blown film extrusion unit at a melt-temperature of 165 oC under a screw speed of about 22 rpm. The blend pellets were dried at 80 °C overnight before blowing. 2.5. Fourier transform infrared (FT-IR) spectroscopy The FTIR spectra were performed with IRAffinity- 1S, Shimadzu to conduct in the wavenumber range of 500-4000 cm-1 with a 4 cm-1 resolution. The maleate and tartarated thermoplastic starch were subjected to soxhlet extraction using boiling acetone for two days to remove any unreacted esterification agent before characterizing by FTIR spectroscopy. 2.6. Mechanical properties of the blends The tensile test of the PBS/ETPS films was carried out according to ASTM D638-14 test method at a strain rate of 200 mm/min using a Lloyd instrument with a load cell of 5KN under room temperature. Ten measurements were performed for the average value of tensile strength and elongation of the films. The tensile properties of films were only determined in the machine direction. 2.7. Melt flow index of PBS/ETPS extrudates Melt flow index values of the various samples of PBS/ETPS blend were measured by using a Melt Flow Indexer (at 190 °C, load 2.16kg) according to the ASTM standard D 1238-04 using a Tinius Olsen Extrusion Plastometer. The molten material flowed through an orifice of 2.0 mm diameter for 1 minute, and the values were reported in g/10 min as standard. The MFI of each sample was determined by the average of the three measurements. 2.8. Molau test Molau test was used to analyze the emulsifying effect of a graft copolymer in the PBS/ETPS blends. 1 g of PBS/TPS blend with and without esterification catalyst was thoroughly shaken with 20 ml dichloromethane and then left to rest at room temperature for 24 h. The PBS was extracted from PBS/ETPS blends by dichloromethane solvent. A digital camera was used to observe the emulsifying effect of the polymer blends solution. 2.9. Scanning electron microscopy (SEM) Morphology PBS/ETPS blends in the form of blowing films were characterized by a scanning electron microscope, Jeol 6360 LV, operated at an acceleration voltage of 15 kV. Only morphology of the uncompatibilized blend PBS/TPS was examined in the extrudate surface. The surface of the films and extrudate was coated with a thin platinum layer for SEM. 3. RESULTS AND DISCUSSION 3.1. TPS and esterified TPS analysis Because the polarity difference of PBS and TPS leads PBS/TPS blends having phase separation and poor mechanical properties, this work aims to eliminate this difference by in-situ chemical modification of starch in thermoplastic starch preparation. To investigate changes of the starch in every functional group after subjection to modification with anhydride maleic and tartaric acid in the reactive extruder, FT-IR was used to analyze native starch and esterified starch in thermoplastic form. The results were shown in figure 1. Figure 1 shows the FTIR spectra of pure TPS and the extracted maleate starch (M-TPS) and tartarated starch (T-TPS). For pure TPS, the infrared absorption peak positions in the spectrogram are as follows: 3300 cm-1 belongs to O─H stretching and vibration of the hydrogen bond association; 2920 cm-1 belongs to C-H asymmetrical stretching and vibration; 1625 cm-1 belongs to H-O bending vibration; 1150 cm-1 belongs to C─O─C Vietnam Journal of Chemistry Nguyen Chau Giang et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 93 asymmetrical stretching and vibration, and 1050 cm-1 belongs to d-glucopyranose. Figure 1: Infrared spectrum of thermoplastic starch and esterified starch For both M-TPS and T-TPS infrared spectrums, not only all of the above characteristic absorption peaks can be observed, but also exhibited two sharp peaks at 1720 cm-1 corresponding to the vibration of the carbonyl group (C=O) and 1270 cm-1 associated with C–O stretch of the ester group. Also, the peak at position 1640 cm-1 corresponding to the bending vibration of the –OH linkage in the starch molecule has been disappeared on the M-TPS and T-TPS spectrum.[10,18,19] Additionally, in the FTIR spectrum of M-TPS there was a sharp absorption peak at 750 cm-1 region. This band can be attributed to C-H bending in butylene group which was formed in the starch by ring opening and esterification of maleic anhydride.[20] Besides, a weak intensity peak at the same position also can be observed in the FTIR spectrum of T-TPS, assigning to C-H bending in TA molecular.[21] Because samples had been extracted by acetone as mentioned above and the unreacted maleic anhydride and tartaric acid had been removed, it could be confirmed that appearing C=O and C–O came from esterified starch, attesting that MA and TA were covalently linked to the starch backbone. At the same time, the disappearance of the peak at 1640 cm-1 on M-TPS spectra also showed that the loss of the –OH group in starches was involved in the esterification reaction.[9] 3.2. Effect of reactive compatibilizers on the blend properties and morphology Due to a notable difference in polarity of unmodified TPS and PBS, the blend of these constituents was hardly produced and could not be blown into the film. Therefore, no data on mechanical properties of PBS/TPS blend were collected. The effect of reactive compatibilizer type, as well as its content on blend properties and its processability of the blown film, was studied by melt blending in the twin-screw extruder The melt blends were produced with fixed plasticizer content in the TPS mixture of 15 wt.% and fixed PBS/TPS ratio of 60/40 meanwhile maleic anhydride content varied from 1.2 to 2 wt.% with an interval of 0.4 % and tartaric acid content varied from 1.2 to 4wt.% with internal of 0.6 % compared to TPS mass. The melt flow index of the blend is a crucial factor in the optimization of the processing conditions, studying the rheology of these complex systems. Therefore, measurement of MFI of the blend versus reactive compatibilizer content was carried out, and results are shown in figure 2. The MFI is a determination of the flow-ability of thermoplastic materials. Blend of PBS with unmodified TPS hardly make a continuous flow of the melted plastic under the condition of the MFI test, therefore, MFI of PBS/TPS blend could not be measured and is not shown in figure 2. 0 4 8 12 16 20 4 5.1 7.2 10.6 4.8 6.9 6.3 8.8 18.4 ArẢ M F I ( g /1 0 m in ) PBS/TPS = 60/40 1.2 1.6 2.0 1.2 1.8 2.4 3.0 3.6 PBS Maleic Anhydride Tartaric Acid compatibilizer contents of PBS/ETPS blends, wt.% Figure 2: Effect of reactive compatibilizer on MFI of the blends From figure 2, it is found that all blends of PBS/modified TPS have higher melt flow index than that of pure PBS. In general, MFI of the blends with both types of modified thermoplastic starch increases with increasing content of MA and TA. For the blend of PBS/M-TPS the melt flow index of the blends increases gradually with rising of anhydride maleic content and reach the highest value of 10.6 (g/10 min) at 2 wt.% MA. This value of MFI is too high to be suitable for the blown film extrusion application. Therefore, MA content should be limited below 1.6 wt.%. For the blend of PBS/T- TPS, at low range of TA content from 1.2 to 3.0 wt.% the flow index increases slightly from 3.0 to Vietnam Journal of Chemistry Morphology and properties of polybutylene succinic © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 94 8.8 g/10 min. However further increasing TA content just up to 3.6 wt.% had resulted in a sharp rise of MFI up to 18 g/10 min. This rising can be explained by the presence of residual MA and TA may leading to partial PBS chains scissors at high temperature condition in the extruder and promoted the hydrolysis of the starch molecular which resulting in decreasing in the viscosity of the blend system.[9,10] The melt index is inversely proportional to the viscosity and is an accessible parameter in the plastic processing industry. 32 14.5 16.1 14.9 18.2 18.4 20.8 19.6 15.5 157 179 169 210 214 212 233 318 196 10 30 0 20 40 Stress Elongation 3.63.02.41.81.22.01.6 S tr e s s (M P a ) PBS Maleic Anhydride Tartaric AcidPBS 1.2 compatibilizer contents of PBS/ETPS blend, phr 0 50 100 150 200 250 300 350 E lo n g a ti o n (% ) Figure 3: Effect of reactive compatibilizer on tensile properties of the blends The tensile properties of films were studied from the blend of PBS/TPS containing 60 wt.% polyester, which had been prepared from TPS modified with different MA and TA contents, as mentioned above. These data were compared to the tensile properties of PBS and presented in figure 3. It is worth noting that the unmodified TPS/PBS melt-blend co