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About VCNC

VCNC is the developer of Tada, a premium ride-hailing app. On October 8, 2021, Toss acquired a majority stake in VCNC. Terms of the transaction were not disclosed.

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12F, Room 110 507 Teheran-ro, Samseong 1(il)-dong, Gangnam-gu


South Korea

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Fabrication of untreated and silane-treated carboxylated cellulose nanocrystals and their reinforcement in natural rubber biocomposites

Feb 13, 2023

Abstract In this study, cellulose nanocrystal (CNC) was extracted from Napier grass stems and subsequently functionalized to carboxylated cellulose nanocrystal (XCNC) by using an environmentally friendly method, namely, the KMnO4/oxalic acid redox reaction. The XCNC was subsequently modified with triethoxyvinylsilane (TEVS), called VCNC, by using ultrasound irradiation. The characterization of the prepared XCNC and VCNC was performed. The needle-like shape of XCNC was observed with an average diameter and length of 11.5 and 156 nm, respectively. XCNC had a carboxyl content of about 1.21 mmol g−1. The silane treatment showed no significant effects on the diameter and length of XCNC. When incorporated into natural rubber (NR), both XCNC and VCNC showed very high reinforcement, as evidenced by the substantial increases in modulus and hardness of the biocomposites, even at very low filler loadings. However, due to the high polarity of XCNC, tensile strength was not significantly improved with increasing XCNC loading up to 2 phr, above which it decreased rapidly due to the filler agglomeration. For VCNC, the silane treatment reduced hydrophilicity and improved compatibility with NR. The highly reactive vinyl group on the VCNC’s surface also takes part in sulfur vulcanization, leading to the strong covalent linkages between rubber and VCNC. Consequently, VCNC showed better reinforcement than XCNC, as evidenced by the markedly higher tensile strength and modulus, when compared at an equal filler loading. This study demonstrates the achievement in the preparation of a highly reinforcing bio-filler (VCNC) for NR from Napier grass using an environmentally friendly method and followed by a quick and simple sonochemical method. Introduction Napier grass (Pennisetum purpureum) is one of the most important fodder crops for livestock due to its low water and nutrient requirements for rapid growth. This crop is considered a highly cellulosic material because it is composed of approximately 46% cellulose and 34% hemicellulose 1 , 2 , 3 . Various techniques, i.e., chemical and/or mechanical treatments, have been used to extract cellulose by separating and removing lignin and hemicellulose 4 , 5 , 6 . Alkali treatment with concentrated sodium hydroxide (NaOH) followed by bleaching with sodium hypochlorite (NaClO2) is one of the most popular methods for obtaining high purity cellulose. The purified cellulose can then be turned into nano-structured cellulose via various chemical reactions, e.g., sulfate acid hydrolysis 7 , 8 , 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO)-mediated oxidation 9 , and ammonium persulfate (APS) oxidation 10 . These methods have been widely used and suggested to be effective for the preparation of high purity cellulose with a high crystallinity of over 70%. However, the sulfate acid hydrolysis requires a massive amount of concentrated sulfuric acid, which has a negative impact on the environment. TEMPO-mediated oxidation is complicated and needs to be performed at a high pH value of 10–11 with several toxic reagents that can pollute the environment. APS oxidation also wastes a large amount of APS 11 . Due to the great concern about the environment, a new environmentally friendly method, namely, the potassium permanganate (KMnO4)/oxalic redox reaction, has been recently introduced 12 , 13 . Generally, KMnO4 in dilute sulfuric acid is used as a green oxidant because MnO4− and Mn3+ can oxidize the amorphous component of cellulose. However, Mn3+ can be easily reduced to Mn2+ and, thus, the use of KMnO4 alone needs a relatively long reaction time. The addition of oxalic acid will turn Mn3+ to [Mn(C2O42−)]+, which is a stronger oxidant, leading to a shorter reaction time and the formation of carboxylated cellulose nanocrystal (XCNC). Cellulose nanocrystal (CNC) is one of the sustainable alternative fillers for natural rubber (NR). Due to its extremely small particle size and high stiffness, the NR biocomposites containing CNC generally possess a higher modulus and hardness. It has been previously uncovered that the addition of 2.5 wt% CNC, isolated from soy hulls, into NR considerably enhanced the tensile modulus of the composites (approximately 21 folds greater than that of the neat NR) 5 . However, the application of CNC in rubber reinforcement is still limited because the hydrophilicity of CNC, due to the abundancy of hydroxy groups, leads to not only the poor interaction between CNC and nonpolar NR, but also the high tendency of filler agglomeration, especially at high filler loadings. To improve the reinforcement of CNC, various surface modifications have been studied 14 , 15 . Silanization is one of the most promising surface modifications that can be achieved in aqueous media to prepare silane-treated nanocellulose. Various types of silane coupling agents have been employed 16 , 17 , 18 , 19 . Silane coupling agent is a bifunctional chemical that can react with both hydrophilic filler and hydrophobic polymer 20 . The surface modification of CNC with silane coupling agents can significantly improve the interaction between rubber and filler as well as the degree of filler dispersion in the rubber matrix 21 . Triethoxyvinylsilane (TEVS) is a bifunctional silane coupling agent that can be hydrolyzed to form silanol groups that can bond with hydroxy groups on the surface of hydrophilic fillers via a condensation reaction. Meanwhile, the vinyl group of TEVS can bond with NR during the sulfur vulcanization reaction 22 . According to the strict legislation on environmental, health, and safety, green technologies for nano-filler preparation and surface modification are more desirable. In this study, XCNC was prepared from Napier grass stems through three chemical treatments, i.e., alkali treatment, bleaching, and KMnO4/oxalic acid redox reaction. The surface treatment of XCNC with TEVS was then carried out, to reduce the hydrophilicity of XCNC, by using a green synthesis technique, called ultrasound irradiation. This irradiation induces acoustic cavitation that causes the formation, growth, and collapse of gas bubbles. At the end of the collapse, localized hot spots are generated which increase temperature and pressure, resulting in a fast reaction rate (low reaction time) and improved reaction yield at relatively low input energy 23 , 24 , 25 . Both XCNC and TEVS-treated XCNC (VCNC) were subsequently characterized by various techniques before being added to NR. The properties of the filled NR biocomposites were then investigated and discussed. Materials and methods Napier grass stems were supplied by the Bureau of Animal Nutrition Development, Khon Kaen, Thailand. Prevulcanized natural rubber (PNR) latex with approximately 60% dry rubber content (DRC) was obtained from VK industry, Nakhon Ratchasima, Thailand. Sodium hydroxide (NaOH) and potassium permanganate (KMnO4) were received from KemAus Co., Ltd., Australia. Hydrogen peroxide (H2O2), sodium chlorite (NaClO2), sulfuric acid (H2SO4), glacial acetic acid (CH3COOH), and hydrochloric acid (HCl) were received from Qrec Co., Ltd., New Zealand. Oxalic acid dihydrate (C2H2O4·2H2O), 97% triethoxyvinylsilane (TEVS), and AR-grade ethanol were obtained from Merck Co., Ltd., Germany. Preparation of XCNC First, raw Napier grass stems were cut and placed in an oven at 60 °C for 24 h. The dried Napier grass stems were later milled by a knife mill, sieved through an 80-mesh screen, and subjected to the alkali treatment. The Napier grass sample (10 g) was soaked in 200 mL of 4 wt% NaOH solution under vigorous stirring for 24 h. The mixture was then filtered by using a Buchner funnel and repeatedly washed with hot distilled water until the pH of the filtrate was 7. The alkali-treated sample was then bleached using 200 mL of the bleaching solution, a mixed solution (1:1 v/v) of 1.7 wt% NaClO2 and acetate buffer, at 80 °C for 2 h, and filtered using a Buchner funnel. After three cycles of bleaching, the sample was rinsed with hot distilled water and filtered to obtain white cellulose powder. The preparation of XCNC was conducted through a KMnO4/oxalic acid redox reaction based on the literature with some modification 11 . The white cellulose powder was suspended in 300 mL of 1 M H2SO4 solution and mechanically stirred for 30 min in an ice bath before adding 10 g of KMnO4. Next, 5 g of oxalic acid was dissolved in 50 mL of 1 M H2SO4 solution, and then slowly added to the cellulose suspension, causing the color change from dark purple to dark brown. The suspension was stirred vigorously and refluxed at 50 °C for 8 h. The oxidation reaction was terminated when hydrogen peroxide (5 mL) was added dropwise into the dark brown suspension, turning it into a white suspension. The suspension was filtered and rinsed with deionized water to neutralize it. Finally, the filter cake was homogenized in deionized water to obtain an XCNC suspension with 2 wt% solid content using a homogenizer for 30 min. Preparation of VCNC The XCNC suspension (50 g) was sonicated for 10 min before the modification. 64 mg of TEVS was dissolved in 50 mL of ethanol and then mixed with the pre-sonicated XCNC suspension. The pH of the mixture was adjusted with a drop of acetic acid and mechanically stirred for 10 min. The mixture was subsequently subjected to ultrasound irradiation for 30 min. A water bath (24 cm × 21 cm × 14 cm) was equipped with a 20 kHz and 200 W ultrasonic generator (AKHGZ, ACME ultrasonic tools, Thailand). This set-up allowed the sample to be held securely while the ultrasonic horn was turned on. The ultrasonic bath temperature was maintained at 30 °C by using a cooling system. The mixture was later centrifuged at 6000 rpm for 10 min. The obtained VCNC was washed with deionized water and ethanol at least three times to completely remove the un-reacted TEVS. The VCNC was finally re-dispersed in deionized water to obtain the VCNC suspension. Filler characterization During the XCNC preparation, the percent yields of the products after the alkali treatment, the bleaching process, and the KMnO4/oxalic acid redox reaction were calculated using Eq. ( 1 ): $$Yield\;(\% ) = \frac{{W_{1} }}{{W_{0} }} \times 100$$ (1) where W1 is the mass of the dried sample from each step and W0 refers to the mass of Napier grass stems. The microstructure of the samples obtained from each preparation step was also explored by scanning electron microscopy (SEM model 1450VP, Leo, UK) with a gold sputtering coating machine. The morphology of XCNC was observed by transmission electron microscopy (TEM; FEI Technai G2 20S Twin, Oregon, USA). The sample diluted with ethanol was dropped onto a 200-mesh carbon-coated copper grid and subsequently dried at ambient temperature before the examination. The average dimensions of XCNC were evaluated from 100 elements using ImageJ software. The density of XCNC was determined by a densitometer (MDS-300, Alfa Mirage, Japan). Determination of functional groups was performed by an attenuated total reflectance-Fourier transform infrared spectroscope (ATR-FTIR; Tensor 27, Bruker, Ettlingen, Germany). The carboxyl content of XCNC was measured by a conductometric titration method. Initially, 10 mg of XCNC was dispersed in 100 mL of 0.01 M hydrochloric acid (HCl). The suspension sample was mechanically stirred for 30 min prior to the titration with 0.01 M NaOH using a CDM210 conductivity meter equipped with a CDC866T electrode (Radiometer Analytical, France). The carboxyl content can be calculated using Eq. ( 2 ): $${\text{Carboxyl}}\;{\text{content}} = \frac{{C_{{{\text{NaOH}}}} \times \Delta V}}{W}$$ (2) where \(C_{{\text{NaOH}}}\) is the concentration of NaOH (mol L−1), \(\Delta V\) is the volume of titrant at the horizon section (mL), and \(W\) is the weight of XCNC (mg). Crystalline structure and crystallinity index (CrI) were studied by X-ray diffractometer (XRD; Malvern Panalytical Empyrean, Royston, UK), using Cu-Kα radiation (λ = 0.15406 nm). According to Segal et al. 26 , the CrI was calculated using Eq. ( 3 ): $$CrI(\% ) = \frac{{I_{200} - I_{{{\text{am}}}} }}{{I_{200} }} \times 100$$ (3) where I200 refers to the peak intensity of the (200) crystallographic plane and Iam represents the peak intensity of the amorphous domain at 2θ = 18°. Thermal decomposition was studied by a Mettler Toledo thermogravimetric analyzer (TGA), Schwerzenbach, Switzerland. The temperature was scanned from 50 to 700 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra DLD spectrometer (Manchester, UK). The survey spectra were used to calculate the atomic concentrations of oxygen (O), carbon (C), and silicon (Si). To determine the makeup of chemical bonds, high-resolution spectra of C 1s, O 1s, and Si 2p were analyzed. After the silane treatment, VCNC was characterized in a similar manner to XCNC before being added to NR. Preparation and testing of the NR biocomposites Table 1 represents the compounding formulation of the PNR latex. Various amounts of the pre-sonicated XCNC and VCNC suspensions were added to the PNR latex in order to prepare the biocomposite containing various filler loadings ranging from 0 to 4 parts per hundred of rubber (phr). Table 1 Formulation of the pre-vulcanized natural rubber (PNR) latex.

VCNC Frequently Asked Questions (FAQ)

  • When was VCNC founded?

    VCNC was founded in 2011.

  • Where is VCNC's headquarters?

    VCNC's headquarters is located at 12F, Room 110, Seoul.

  • What is VCNC's latest funding round?

    VCNC's latest funding round is Corporate Majority.

  • How much did VCNC raise?

    VCNC raised a total of $3M.

  • Who are the investors of VCNC?

    Investors of VCNC include Toss, Socar, 500 Global, Global Brain, DeNA and 6 more.

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