Study on Stability and Stability Mechanism of Styrene-Acrylic Emulsion Prepared Using Nanocellulose Modiﬁed with Long-Chain Fatty Acids

Article Study on Stability and Stability Mechanism of Styrene-Acrylic Emulsion Prepared Using Nanocellulose Modiﬁed with Long-Chain Fatty Acids

Heng Zhang 1,2,* , Hongyan Yang 1, Junliang Lu 1, Jinyan Lang 1 and Hongkun Gao 1

1 College of Marine Science and Biological Engineering, Qingdao University of Science & Technology, Qingdao 266042, Shandong, China 2 Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China * Correspondence: [email protected]

Received: 18 May 2019; Accepted: 28 June 2019; Published: 3 July 2019

Abstract: In this study, nanocrystalline cellulose (NCC) was grafted with lauric acid, palmitic acid, and stearic acid and used as stabilizer to prepare styrene butyl acrylate emulsion. The properties of the emulsion were determined, and the mechanism of modified NCC (MNCC) stabilized emulsion was analyzed. Results showed that long-chain fatty acids were grafted to NCC through esterification initiated at a low temperature. When the dosage of L-MNCC, P-MNCC, and S-MNCC was 0.05%, the styrene-acrylic emulsion had 92.5%, 94.2%, and 96.3% conversion rates, respectively, and exhibited good dilution, pH, Ca2+, and centrifugal stability. The particle size of styrene-acrylic emulsion was approximately 460 nm, and the absolute value of the Zeta potential increased with the MNCC concentration. According to the images of optical microscopy and the transmission electron microscope, the MNCC was adsorbed onto the surface of styrene-acrylic emulsion droplets. The synergistic effect from the electrostatic repulsion of MNCC, the hydrophile lipophilicity of MNCC, and the spatial hindrance of the MNCC adsorption layer provided good stability for the styrene-acrylic emulsion. Therefore, MNCC could replace traditional surfactants in stabilizing emulsion.

Keywords: modiﬁed nanocrystalline cellulose; long-chain fatty acids; hydrophilic lipophilicity; styrene-acrylic emulsion; stability mechanism

1. Introduction Emulsion with a large oil/water interface is a thermodynamically unstable system [1]. Adding surfactants could eﬀectively reduce the surface tension and promote the long-term stability of the emulsion [2,3]. Stabilized emulsions could also be achieved using highly dispersible solid powders, such as carbon black, silica, clay, and calcium carbonate [4]. In the past 10 years, the increasingly widespread application of emulsion in food and biomedicine has required stable particles with better biocompatibility, biodegradability, and sustainability [5]. Therefore, research on emulsion stability focuses on transforming traditional surfactants to biogenic derivatives, such as cellulose, hemicellulose, lignin, chitin, starch, protein, and their derivatives [6,7]. Cellulose derived from wood or cotton has attracted considerable attention in recent years due to its biodegradable, biocompatible, and renewable character [8,9]. Chmielarz [9] prepared brush copolymers of cellulose at low catalyst concentration by the ATRP method. It is expected that those polymer brushes may ﬁnd application as pH-and thermo-sensitive drug delivery systems. Nanocrystalline cellulose (NCC) has excellent properties, such as high purity, DP, crystallinity, and hydrophilicity [10]. Winuprasith [11] et al. reported that the microcrystalline cellulose (MFC) extracted from mangosteen peel stabilized the emulsion without the

Polymers 2019, 11, 1131; doi:10.3390/polym11071131 www.mdpi.com/journal/polymers Polymers 2019, 11, 1131 2 of 16 help of surfactants, and this effect increased with the decrease in fiber size and the increase in MFC concentration. Laitinen [12] et al. also found that NCC exhibited excellent stability in emulsion. Hydrophobically modified NCC could stabilize emulsions because its hydrophilic lipophilic property allows good adsorption on oil-water surfaces [13]. Stark [14] et al. chemically modified NCC using rapeseed oil fatty acid methyl ester through transesterification to obtain a modified NCC that had certain hydrophobicity and could be combined with polylactic acid in nano-materials with good plasticizing effect. Cunha [15] et al. surface modified NCC with chemically modified lauryl chloride to endow the NCC with a hydrophobic property and decreased hydrophilicity. They also proved that the modified NCC could stabilize the oil in water emulsion without surfactants. Many researchers also provided the mechanism for hydrophobically modified NCC stabilized emulsion. On the one hand, Hydrophobic groups of hydrophobic modified nanofibers extend into the interior of microemulsions, and hydrophobic groups associate with each other to induce thickening. Additionally, the elastic 3D network structure formation, which effectively inhibits emulsion flocculation [16–18]. On the other hand, the hydrophilic part of the nanocellulose chain greatly increased its solubility in water and enhances its dispersion in aqueous solution [16]. Therefore, the hydrophobic-modified NCC could be used as a stabilizer and emulsifier and had incomparable advantages over small molecular surfactants. The application of modified NCC instead of traditional surfactants in emulsion stabilization could expand the application scope of biomass resources, reduce the dependence on petrochemical products, and contribute to the protection of the environment [9,19]. In this study, NCC was prepared through sulfuric acid hydrolysis, and long-chain fatty acids were used to initiate NCC graft modification with FeSO4/H2O2 as an initiator. Styrene-acrylic emulsion was prepared using modified NCC (MNCC) instead of the traditional surfactant. The stability and stability mechanism of styrene-acrylic emulsion were analyzed by testing the solid content, conversion rate, physicochemical stability, particle size, and zeta potential as well as conducting SEM characterization.

2. Materials and Methods

2.1. Materials

Ammonium ceric sulfate and FeSO4 were purchased from China Shanghai Pierre Chemical Reagent Co., Ltd, Shanghai, China. H2O2 (30%) and ammonium persulfate were obtained from Tianjin Dingshengxin Chemical Co., Ltd, Tianjin, China. Concentrated sulfuric acid (98%) was purchased from Yantai Sanhe Chemical Reagent Co., Ltd, Yantai, China. Tianjin BASF Chemical Co., Ltd. (Tianjin, China) provided the lauric acid, palmitic acid, and stearic acid. Butyl acrylate and calcium chloride were obtained from Tianjin Bodi Chemical Co., Ltd, Tianjin, China. Chemical Reagent Co., Ltd. (Shanghai, China) provided the anhydrous ethanol, styrene, sodium hydroxide, and ammonia. All these chemical reagents were of an analytical grade. Medicinal grade microcrystalline cellulose was obtained from Chengdu Kelon Chemical Reagent Factory, Tianjin, China. The dialysis bag MD3500 was acquired from Viskase Company, USA (Lombard, IL, USA).

2.2. Experimental Method

2.2.1. Preparation of NCC

MFC was added to a one-neck ﬂask (250 mL) and heated at 45 ◦C in a water bath. The prepared H2SO4 (60%) solution was slowly added, and the solution was mixed well by a medium-sized propeller agitator (material: PTFE). A milky white NCC suspension was obtained after 2.5 h. The heating was stopped, and cooling to room temperature followed in half an hour. The reaction was terminated by adding 500 mL deionized water. The obtained NCC suspension was then centrifuged for 5 min at a speed of 10,000 r/min, and the acid solution in the upper layer was removed, which was repeated 2 to 3 times. Centrifugation was continued until an NCC colloid appeared in the upper layer, Polymers 2019, 11, 1131 3 of 16 which was deposited into the treated dialysis bag and subjected to dialysis to achieve neutral pH. Lastly, a blue-light NCC colloidal solution was obtained, which did not stratify for three months.

2.2.2. Preparation of MNCC Fifty mL ethanol was used to dissolve lauric acid 0.50 g (palmitic acid 0.64 g, stearic acid 0.71 g). Additionally, an appropriate amount of NCC suspension were mixed with fatty acid ethanol solution by stirring with a speed of 180 r/min. The stirring temperature was increased to 50 ◦C in half an hour. Additionally, 26 µLH2O2 (30%), 0.0070 g FeSO4, and 1–2 mL 0.5 mol/LH2SO4 solution were added successively. The grafting reaction was initiated, and the reaction lasted for 8 h with stirring under 50 ◦C. The mixed suspension containing grafted NCC was then obtained. The suspension was filtered by a Vacuum filter bottle and washed repeatedly with water and ethanol. Then the upper filter cake was collected with tweezers and placed into the Rapid Glass Dryer (containing discolored silica gel) to wait for the ethanol solvent to evaporate naturally. After 2 hours, the MNCC was obtained (lauric acid modified nanocrystalline cellulose, L-MNCC, palmitic acid modified nanocrystalline cellulose, P-MNCC, and stearic acid modified nanocrystalline cellulose, S-MNCC). The NCC was modified with different fatty acids as follows: with lauric acid under the reaction conditions of 0.8% FeSO4/H2O2 (molar ratio 1:1) as an initiator and reacted at 50 ◦C for 8 h, with palmitic acid under the reaction conditions of 1.2% FeSO4/H2O2 (molar ratio 1:1) used as an initiator and reacted at 55 ◦C for 20 h, and with stearic acid under the reaction conditions of 1.2% FeSO4/H2O2 (molar ratio 1:1) as an initiator and reacted at 60 ◦C for 24 h.

2.2.3. Preparation of MNCC Suspension Then, 0.1 g MNCC powder was added into 100 mL deionized water and disposed into 1 g/L MNCC suspension, which was dispersed by ultrasound for 30 min and stored in bottles for reserve.

2.2.4. Preparation of Styrene-Acrylic Emulsion As shown in Table1, a certain amount of MNCC suspension was dispersed by an ultrasonic disperser for 30 min and added to the ﬂasks, and the temperature was increased to 75 ◦C. A small amount of ammonium persulfate initiator was added, and the temperature was increased to 86 ◦C. The monomer mixture was made of styrene and butyl acrylate was dripped with a constant pressure funnel for 4 h, and ammonium persulfate initiator was simultaneously incorporated. A small amount of initiator was added during heat preservation for 1 h and then cooled to room temperature. Lastly, the styrene-acrylic polymer emulsion was obtained through ﬁltration.

Table 1. Proportion of emulsion polymerization reactants.

Monomers and Reactants Ratio of the Reaction (wt%) Mass of Reactants (g) styrene 20 10 butyl acrylate 20 10 ammonium persulfate 1 0.5 MNCC 0–0.15 0–0.075 hydroquinone 0.001 0.0005 water 58.95 29.48 ≈

2.3. Reaction Principle The reaction process of esterification and grafting modification of nanocellulose with fatty acids is shown in Figure1. Hydroxyl radicals can be produced by H 2O2 and FeSO4 in the reaction system. Hydroxyl radicals can capture hydrogen atoms on fatty acids and NCC to form water, fatty acid radicals, and NCC radicals, respectively [20–22]. Fatty acid radicals and NCC radicals can be esterified to obtain MNCC [22]. Polymers 2019, 11, x FOR PEER REVIEW 4 of 17

Polymersradicals,2019, 11 ,and 1131 NCC radicals, respectively [20–22]. Fatty acid radicals and NCC radicals can be4 of 16 esterified to obtain MNCC [22].

H+ Fe2+ 3+ + + H2O2 Fe + H2O + OH·

O O + OH· + H2O OH O·

HO ·OCH2 O O O + OH· O O OH + H2O HO OH HO OH

O ·OCH2 C O + O + O O + H H O O· OH + 2 HO OH O O O HO OH FigureFigure 1. 1.Reaction Reaction principleprinciple of of synthetic synthetic MNCC. MNCC.

2.4. Methods2.4. Methods The NCCThe NCC colloidal colloidal suspension suspension was was dried dried andand po powdered,wdered, and and small small amounts amounts of NCC of NCC and MNCC and MNCC solidsolid powders powders were were obtained. obtained. The The infrared infrared spectrumspectrum was was then then determined determined through through the KBr the KBrpressing pressing method with a resolution of 4 cm1 −1 in the region of 4000 cm−1 1to 500 cm−1 by1 using Fourier infrared method with a resolution of 4 cm− in the region of 4000 cm− to 500 cm− by using Fourier infrared spectrometer (VECTOR22). The powders were then analyzed by X-ray diffraction (XRD) tester (DX- spectrometer (VECTOR22). The powders were then analyzed by X-ray diffraction (XRD) tester 2700) at room temperature with a scanning rate of 0.02°/min and a diffraction angle of 2θ within a θ (DX-2700)10° to at60° room range. temperature with a scanning rate of 0.02◦/min and a diffraction angle of 2 within a 10◦ to 60◦Smallrange. amounts of NCC suspension, MNCC suspension, and styrene-acrylic emulsion were Smallseparately amounts dilutedof in NCCdistilled suspension, water at a ratio MNCC of 1:10 suspension, until a transparent and styrene-acrylic solution was obtained. emulsion The were separatelyparticle diluted size distribution in distilled and waterpotential at of a ratiolatex particles of 1:10 untilwere measured a transparent by the solution Malvern wasZetaSizer obtained. The particleNano-ZS90 size laser distribution particle size and analyzer potential (Shanghai, of latex China). particles were measured by the Malvern ZetaSizer Nano-ZS90Exactly laser 1.5000 particle g sizeof the analyzer prepared (Shanghai, emulsion China). was placed in a weighted drying bottle and, Exactlysubsequently, 1.5000 inside g of the an preparedoven and dried emulsion at 100 was °C until placed a constant in a weighted weight drying was achieved. bottle and, Solid subsequently, content was then calculated, followed by the conversion rate, according to the theoretical solid content. inside an oven and dried at 100 ◦C until a constant weight was achieved. Solid content was then The emulsion was diluted with different multiple times, stirred evenly, and sealed for 48 h. In a calculated, followed by the conversion rate, according to the theoretical solid content. beaker, 20 mL of emulsion and different pH acid-alkali (H2SO4, NaOH, and ammonia) solutions were Theadded, emulsion and the mixture was diluted was allowed with to di standfferent for multiple 48 h. A 0.5% times, Ca2+ solution stirred evenly,was mixed and with sealed emulsion for 48 h. In a beaker,in a 4:1 20ratio mL and of sealed emulsion for 48 and h. Then, different 20 mL pH of acid-alkali emulsion was (H2 centrifugedSO4, NaOH, at andspeeds ammonia) of 1000, 2000, solutions 2+ were3000, added, 4000, and and the 5000 mixture r/min for was 10 allowedmin. Lastly, to standthe occurrence for 48 h. of A delamination 0.5% Ca andsolution flocculation was mixed in the with emulsionemulsion in a 4:1was ratio observed. andsealed for 48 h. Then, 20 mL of emulsion was centrifuged at speeds of 1000, 2000, 3000,The 4000, emulsion and 5000 was rcentrifuged/min for 10 at min. 4000× Lastly, g r/min the for occurrence 10 min and of delaminationthen placed in andthe separation flocculation in the emulsionfunnel. The was lower observed. layer of emulsion was released, and the volume of the upper layer (oil) was Themeasured. emulsion The wasproportion centrifuged of stable at 4000emulsiong r /wasmin calculated. for 10 min and then placed in the separation funnel. × The lowerThe layer microscopic of emulsion morphology was released, of emulsion and the micr volumeospheres of thewas upper observed layer using (oil) the was optical measured. microscope (PM6000, Nanjing Jiangnan Yongxin Optical Co., Ltd., Nanjing, China) and the The proportion of stable emulsion was calculated. transmission electron microscope (JSM-2100Plus, JEOL, Tokyo, Japan). The microscopic morphology of emulsion microspheres was observed using the optical microscope (PM6000,3. Results Nanjing and Discussion Jiangnan Yongxin Optical Co., Ltd., Nanjing, China) and the transmission electron microscope (JSM-2100Plus, JEOL, Tokyo, Japan). 3.1. Preparation and Analysis of MNCC 3. Results and Discussion 3.1.1. Infrared Characterization of MNCC 3.1. Preparation and Analysis of MNCC

3.1.1. Infrared Characterization of MNCC

As shown in Figure2, the stretching vibration peak of the –CH 2– long-chain alkyl appeared at 1 2918 and 2850 cm− in the infrared spectrum of MNCC, which indicates that long-chain fatty acids had been grafted on the NCC. Moreover, the vibration peak of saturated ester bond appeared at 1 1 1740 cm− , and the characteristic peak of –COO– appeared at 1582 cm− . This ﬁnding indicates that the Polymers 2019, 11, x FOR PEER REVIEW 5 of 17

As shown in Figure 2, the stretching vibration peak of the –CH2– long-chain alkyl appeared at Polymers29182019 and, 11 ,2850 1131 cm−1 in the infrared spectrum of MNCC, which indicates that long-chain fatty acids5 of 16 had been grafted on the NCC. Moreover, the vibration peak of saturated ester bond appeared at 1,740 cm−1, and the characteristic peak of –COO– appeared at 1.582 cm−1. This finding indicates that the hydroxyl of NCC reacted with the acid group of long-chain fatty acids to produce ester groups, and the hydroxyl of NCC reacted with the acid group of long-chain fatty acids to produce ester groups, and nanocellulosethe nanocellulose and long-chain and long-chain fatty acidsfatty acids successfully successfully underwent underwent esteriﬁcation esterification copolymerization copolymerization [22]. Hence,[22]. long-chain Hence, long-chain fatty acids fatty couldacids could be grafted be grafted to to NCC NCC under under certain reaction reaction conditions conditions by using by using FeSOFeSO4/H2O4/H2 2systemO2 system as as an an initiator initiator to to obtain obtain stable MNCC MNCC products. products.

S-MNCC

P-MNCC

L-MNCC Transmitance

NCC

FeSO /H O 4 2 2 1740 4000 3500 3000 2500 2000 1500 1000 Wavenumbers (cm-1)

Figure 2. IR spectra of MNCC. Figure 2. IR spectra of MNCC. 3.1.2. XRD Analysis of MNCC 3.1.2. XRD Analysis of MNCC Figure3 provides a comparison of the XRD images of NCC with those of L-MNCC, P-MMCC, Figure 3 provides a comparison of the XRD images of NCC with those of L-MNCC, P-MMCC, and S-MNCC, which was prepared by using FeSO4/H2O2 system as an initiator. The L-MNCC, and S-MNCC, which was prepared by using FeSO4/H2O2 system as an initiator. The L-MNCC, P- P-MNCC, and S-MNCC exhibited diﬀraction peaks at I002 (2θ = 22.58◦), Iam (2θ = 16.38◦), and I004 MNCC, and S-MNCC exhibited diffraction peaks at I002 (2θ = 22.58°), Iam (2θ = 16.38°), and I004 (2θ = (2θ = 34.54◦), respectively, which indicated that the FeSO4/H2O2 system successfully initiated the 34.54°), respectively, which indicated that the FeSO4/H2O2 system successfully initiated the NCC NCC grafting reaction. Furthermore, the basic crystal structure of the NCC was also maintained [23]. grafting reaction. Furthermore, the basic crystal structure of the NCC was also maintained [23]. The The XRDXRD image of of the the L-MNCC L-MNCC was was substantially substantially similar similar to that to of that NCC, of NCC,and the and crystal the structure crystal structure did did notnot remarkablyremarkably change. change. The The XRD XRD images images of P-MNCC of P-MNCC and S-MNCC and S-MNCC showed showeddiffraction di peaksﬀraction at other peaks at otherpositions positions and and changed changed the thecrystal crystal structure. structure. This phenomenon This phenomenon occurred occurred because becausethe palmitic the acid palmitic acid andand stearic acid acid have have a longer a longer carbon carbon chain chain structur structuree than lauric than acid. lauric The acid. steric The hindrance steric hindrance was large, was large,Polymersand and the the 2019 crystal crystal, 11, x structureFOR structure PEER REVIEWof NCC of NCC was slightly was slightly changed changed after the after grafting the graftingat NCC [20,23]. at NCC [20,236 of]. 17

NCC

S-MNCC Intensity count Intensity

P-MNCC

L-MNCC

10 20 30 40 50 Scattering angle (degrees of 2θ)

Figure 3. XRD patterns of MNCC. Figure 3. XRD patterns of MNCC.

3.1.3. Particle Size Test of MNCC Figure 4 showed that the average particle size of MNCC, which was modified by fatty acids, was slightly increased than NCC. The average particle size, the particle size distribution coefficient, and the specific surface area were 168 nm, 0.255, and 19,260 m2/kg, respectively, for NCC; 255 nm, 0.297, and 17,500 m2/kg, respectively, for L-MNCC; 244 nm, 0.308, and 17,400 m2/kg, respectively, for P- MNCC; and 247 nm, 0.319, and 17,430 m2/kg for S-MNCC. The difference of particle size, the particle size distribution coefficient, and the specific surface area of L-MNCC, P-MNCC, and S-MNCC were very small. The particle size of MNCC were slightly different from that of NCC, likely because the molecular weight of MNCC increased slightly after the hydrophobic group was grafted, so the particle size of MNCC increased slightly. Although the particle size of MNCC was slightly increased, the particle size was still in a nanometer range. Moreover, the distribution coefficient of the particle size of MNCC was larger than that of NCC, which indicates that the particle size distribution of MNCC was wide, and a certain degree of flocculation occurred during the reaction of fatty-acid- grafted NCC. The particles of MNCC were enlarged, and the specific surface area of MNCC was reduced relative to that of NCC. After being grafted with long-chain fatty acids, NCC reactivity was reduced to some extent. In summary, the average particle size of grafted NCC was 244 to 255 nm, which was not a large increase relative to that of NCC. The particle size remained at the nanometer scale and had a large specific surface area.

Polymers 2019, 11, 1131 6 of 16

3.1.3. Particle Size Test of MNCC Figure4 showed that the average particle size of MNCC, which was modified by fatty acids, was slightly increased than NCC. The average particle size, the particle size distribution coefficient, and the specific surface area were 168 nm, 0.255, and 19,260 m2/kg, respectively, for NCC; 255 nm, 0.297, and 17,500 m2/kg, respectively, for L-MNCC; 244 nm, 0.308, and 17,400 m2/kg, respectively, for P-MNCC; and 247 nm, 0.319, and 17,430 m2/kg for S-MNCC. The difference of particle size, the particle size distribution coefficient, and the specific surface area of L-MNCC, P-MNCC, and S-MNCC were very small. The particle size of MNCC were slightly different from that of NCC, likely because the molecular weight of MNCC increased slightly after the hydrophobic group was grafted, so the particle size of MNCC increased slightly. Although the particle size of MNCC was slightly increased, the particle size was still in a nanometer range. Moreover, the distribution coefficient of the particle size of MNCC was larger than that of NCC, which indicates that the particle size distribution of MNCC was wide, and a certain degree of flocculation occurred during the reaction of fatty-acid-grafted NCC. The particles of MNCC were enlarged, and the specific surface area of MNCC was reduced relative to that of NCC. After being grafted with long-chain fatty acids, NCC reactivity was reduced to some extent. In summary, the average particle size of grafted NCC was 244 to 255 nm, which was not a large increase relative to that of NCC. PolymersThe particle 2019, 11 size, x FOR remained PEER REVIEW at thenanometer scale and had a large specific surface area. 7 of 17

18 NCC 16 L-MNCC P-MNCC 14 S-MNCC

Intensity/% 6

0 10 100 1000 10000 Size/nm Figure 4. ParticleParticle size size distribution distribution of MNCC. 3.2. Emulsification and Stability Analysis of Styrene-Acrylic Emulsion 3.2. Emulsification and Stability Analysis of Styrene-Acrylic Emulsion 3.2.1. Effect of MNCC Dosage on the Solid Content and Conversion of Emulsion 3.2.1. Effect of MNCC Dosage on the Solid Content and Conversion of Emulsion In emulsion polymerization, styrene and butyl acrylate were the reaction monomers, and ammoniumIn emulsion persulfate polymerization, was the initiator. styrene Table and1 (frombutyl Sectionacrylate 2.2.4 were) showed the reaction the main monomers, compositions and ammoniumand indicated persulfate that the theoreticalwas the initiator. solid content Table 1 of (from styrene-acrylic Section 2.2.4) emulsion showed was the 40%. main compositions and indicatedCertain amounts that the theoretical of L-MNCC, solid P-MNCC, content andof styrene-acrylic S-MNCC were emulsion weighed was and 40%. added to deionized waterCertain and dispersed amounts byof ultrasoundL-MNCC, P-MNCC, for 30 min. and Styrene-acrylic S-MNCC were emulsion weighed polymerization and added to deionized was then waterconducted, and dispersed according by to theultrasound proportion for shown 30 min. in Styrene-acrylic Table1, and the solidemulsion content polymerization and conversion was rate then of conducted,the emulsion according obtained to were the calculated.proportion Theshown test in results Table are1, and shown the insolid the content following and figure. conversion rate of the emulsion obtained were calculated. The test results are shown in the following figure. Figure5 showed that, when MNCC dosage was less than 0.05%, the emulsification of MNCC was Figure 5 showed that, when MNCC dosage was less than 0.05%, the emulsification of MNCC weaker. With the increase of MNCC dosage, the emulsifying effect was enhanced, a large number of was weaker. With the increase of MNCC dosage, the emulsifying effect was enhanced, a large micelles were generated, and the nuclear growth process accelerated, which made the emulsion solid number of micelles were generated, and the nuclear growth process accelerated, which made the content and conversion rate increase. When the dosage of MNCC was 0.05%, the solid contents of emulsion solid content and conversion rate increase. When the dosage of MNCC was 0.05%, the solid contents of styrene-acrylic emulsion were 37%, 37.68%, and 37.68%, and the conversion rates were 92.5%, 94.2%, and 96.25%, respectively. When the dosage of MNCC was higher than 0.05%, the solid content and conversion rate of styrene-acrylic emulsion showed a decreasing trend with the increasing MNCC dosage. It might because the viscosity of styrene-acrylic emulsion increased significantly as a result of the thickening effect of MNCC, which resulted in the contact between monomers being less than when the little dosage of MNCC was used. Higher solids and conversion rates were observed for emulsions containing P-MNCC and S-MNCC compared with L-MNCC, because the carbon chains of the hydrophobic groups of P-MNCC and S-MNCC were longer than those of L-MNCC. They had better hydrophobic ability, which could better form micelles and help with emulsion polymerization [24]. This finding indicated that the length of the hydrophobic carbon chain had a certain effect on the conversion rate of the emulsion. The longer the hydrophobic carbon chain was, the better the emulsification, and the higher the conversion rate of styrene-acrylic emulsion that would be obtained.

Polymers 2019, 11, 1131 7 of 16 styrene-acrylic emulsion were 37%, 37.68%, and 37.68%, and the conversion rates were 92.5%, 94.2%, and 96.25%, respectively. When the dosage of MNCC was higher than 0.05%, the solid content and conversion rate of styrene-acrylic emulsion showed a decreasing trend with the increasing MNCC dosage. It might because the viscosity of styrene-acrylic emulsion increased significantly as a result of the thickening effect of MNCC, which resulted in the contact between monomers being less than when the little dosage of MNCC was used. Higher solids and conversion rates were observed for emulsions containing P-MNCC and S-MNCC compared with L-MNCC, because the carbon chains of the hydrophobic groups of P-MNCC and S-MNCC were longer than those of L-MNCC. They had better hydrophobic ability, which could better form micelles and help with emulsion polymerization [24]. This finding indicated that the length of the hydrophobic carbon chain had a certain effect on the conversion rate of the emulsion. The longer the hydrophobic carbon chain was, the better the emulsification,Polymers 2019, 11, x and FOR the PEER higher REVIEW the conversion rate of styrene-acrylic emulsion that would be obtained.8 of 17

100 39 L-MNCC-Solid content P-MNCC-Solid content 38 S-MNCC-Solid content 95 L-MNCC-Conversion rate 37 P-MNCC-Conversion rate S-MNCC-Conversion rate 36 90 35

34 85

33 Solid content/% Conversion rate/% 32 80

30 75 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Concentration of MNCC(%) Figure 5. EffectEffect of MNCC dosage on the solid content and conversion ofof styrene-acrylicstyrene-acrylic emulsion.emulsion. 3.2.2. Effect of MNCC Dosage on Emulsion Particle Size 3.2.2. Effect of MNCC Dosage on Emulsion Particle Size A certain amount of styrene-acrylic emulsion was diluted for 10 times. Then, the particle size was A certain amount of styrene-acrylic emulsion was diluted for 10 times. Then, the particle size tested. The test results are shown in the following figure. was tested. The test results are shown in the following figure. Figure6 showed that, when the dosage of MNCC was less than 0.05%, the particle size of the Figure 6 showed that, when the dosage of MNCC was less than 0.05%, the particle size of the emulsion gradually decreased with the increase of the dosage of MNCC. When the dosage of MNCC emulsion gradually decreased with the increase of the dosage of MNCC. When the dosage of MNCC was 0.05%, the emulsion particle size was stable at approximately 460 nm. With the increase of the was 0.05%, the emulsion particle size was stable at approximately 460 nm. With the increase of the dosage of MNCC, the emulsion particle size did not change. When the concentration of MNCC was dosage of MNCC, the emulsion particle size did not change. When the concentration of MNCC was small, the dispersion and emulsifying ability of MNCC was weak, and the stable emulsion could not small, the dispersion and emulsifying ability of MNCC was weak, and the stable emulsion could not be formed. As a result, droplets tend to aggregate, and the size of the emulsion formed was larger. be formed. As a result, droplets tend to aggregate, and the size of the emulsion formed was larger. When the concentration of MNCC increased, its emulsifying and dispersing ability increased. Hence, When the concentration of MNCC increased, its emulsifying and dispersing ability increased. Hence, the aggregation of emulsion decreased, and the particle size of the emulsion decreased. When the the aggregation of emulsion decreased, and the particle size of the emulsion decreased. When the dosage of MNCC reached 0.05%, MNCC could play a good emulsifying function, which could form dosage of MNCC reached 0.05%, MNCC could play a good emulsifying function, which could form micelles better and form stable emulsion. When the dosage of MNCC was greater than 0.05%, the effect micelles better and form stable emulsion. When the dosage of MNCC was greater than 0.05%, the of MNCC concentration on emulsion micelles decreased. MNCC was only dispersed in the emulsion. effect of MNCC concentration on emulsion micelles decreased. MNCC was only dispersed in the Hence, the particle size of the emulsion no longer changed. The three modified products of L-MNCC, emulsion. Hence, the particle size of the emulsion no longer changed. The three modified products P-MNCC, and S-MNCC had a little effect on the particle size of emulsion. When the emulsion was of L-MNCC, P-MNCC, and S-MNCC had a little effect on the particle size of emulsion. When the stable, the particle size of the emulsion was approximately 460 nm. The difference among the three emulsion was stable, the particle size of the emulsion was approximately 460 nm. The difference modified products was reflected in the carbon chain length and hydrophobicity of hydrophobic groups. among the three modified products was reflected in the carbon chain length and hydrophobicity of The formation of emulsion droplets was encapsulated by MNCC droplets, and the structure of the hydrophobic groups. The formation of emulsion droplets was encapsulated by MNCC droplets, and the structure of the three modified products was not very different. Therefore, the particle size difference between the three modified products as surfactants was almost absent. Moreover, when the MNCC dosage was 0.05%, the particle size distribution coefficients particle size distribution coefficient of the emulsion were 0.297, 0.319, and 0.308, respectively. A small particle size distribution coefficient value indicates uniform particle size distribution. Therefore, a narrow distribution indicates the uniform particle size of emulsion. In summary, when the dosage of L-MNCC, P-MNCC, and S-MNCC was 0.05%, the particle size of emulsion was the smallest at 460 nm.

Polymers 2019, 11, 1131 8 of 16 three modified products was not very different. Therefore, the particle size difference between the three modified products as surfactants was almost absent. Moreover, when the MNCC dosage was 0.05%, the particle size distribution coefficients particle size distribution coefficient of the emulsion were 0.297, 0.319, and 0.308, respectively. A small particle size distribution coefficient value indicates uniform particle size distribution. Therefore, a narrow distribution indicates the uniform particle size of emulsion. In summary, when the dosage of L-MNCC, P-MNCC, and S-MNCC was 0.05%, the particlePolymers 2019 size, 11 of, x emulsion FOR PEER REVIEW was the smallest at 460 nm. 9 of 17

1100 L-MNCC 1000 P-MNCC S-MNCC 900

800

700

600 Particle size/nm

500

400

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 The dosage of MNCC(%) FigureFigure 6. E6.ﬀ Effectect of of MNCC MNCC dosagedosage on the the particle particle size size of ofemulsion. emulsion.

2+ 3.2.3.3.2.3. Effect Effect of pH, of pH, Ca Ca,2+ and, and Dilution Dilution on on thethe Stability of of Styrene-Acrylic Styrene-Acrylic Emulsion Emulsion The styrene-acrylicThe styrene-acrylic emulsion emulsion containing containing 0.05%0.05% L-MNCC was was obtained. obtained. In addition, In addition, pH dilution pH dilution and centrifugeand centrifuge stability stability tests tests were were conducted, conducted, according according to Table Table 2.2. Ca Ca2+ 2stability+ stability test testwaswas conducted conducted on theon styrene-acrylic the styrene-acrylic emulsion emulsion of of di ffdifferenterent amounts amounts of of L-MNCC.L-MNCC. The The test test results results are are shown shown in Table in Table 2. The centrifugal2. The centrifugal stability stability of styrene-acrylate of styrene-acrylate emulsion emulsion withwith different different dosages dosages of ofMNCC MNCC was wastested tested under 4000 r/min condition. The result is shown in Table 2. under 4000 r/min condition. The result is shown in Table2. Table2 showed that, whenTable 2. theConditions L-MNCC and results dosage of emulsion was less stability than test. 0.05%, di fferent degrees of delamination were observed following the addition of Ca2+ to the emulsion. When the L-MNCC Dosage of L- 0.01% 0.025% 0.05% 0.075% 0.1% dosage was higher than 0.05%,MNCC the emulsion was stable, and no oil phase was detached from the Ca2+ stability test Not emulsion system.(A) This finding indicatesLayered, that oil L-MNCC Layered, at oil a low concentrationNot layered, did not completelyNot layered, cover Stability layered, the droplet, and the emulsification andoccupies dispersion 1/3 occupies were weak. 1/4 Therefore, thestable formed emulsionstable was not stable 2+ stable. In the case of addingpH Ca (salt solution),3 oil-water 5 separation 7 was 9 (NaOH) easily generated, 11 (NaOH) which pH stability test Not results in demulsification. In Table2NotB, thelayered, pH stabilityNot layered, test was conductedLayered, Slight using a styrene-acrylicLayered, (B) Stability layered, Stable stable flocculation flocculated emulsion. In the acidic and neutral environment, the emulsionstable showed stability. However, when the NaOH solution was addedDilution to the emulsion, it was flocculated and stratified. This is possible because 1000 100 50 10 5 multiple - there wasDilution a peeling stability reaction between NaOH and L-MNCC [25]. Moreover, the addition of OH might Not test (C) Not layered, Not layered, Not layered, Not layered, have destroyed the stabilityStability of the emulsion internal charge,layered, which results in the instability of the stable stable stable stable emulsion to produce flocculation. Therefore, replacing NaOHstable with ammonia (ammonia could not Centrifugal 3000 cause NCC to initiate peeling reaction)5000retested r/min the 4000 alkalir/min stability and2000 found r/min that flocculation 1000 r/min and Centrifugal rate r/min delaminationoscillation did not occur. Therefore, it could be explained thatNot strong alkali solution could cause Layered, oil Not layered, Not layered, Not layered, stability (D) Stability layered, MNCC to occur peeling reaction andoccupies degrade, 1/4 destroystable the structure of MNCC,stable and makestable it no longer have emulsifying capacity, which results in micelle breakingstable and flocculating and delamination of emulsion.Table However, 2 showed the emulsion that, when was the stable L-MNCC under dosage weak was bases, less and than the 0.05%, addition different of aqueous degrees ammonia of to thedelamination emulsion did were not observed cause the following sedimentation the addition of emulsion. of Ca2+ to the emulsion. When the L-MNCC dosage was higher than 0.05%, the emulsion was stable, and no oil phase was detached from the emulsion system. This finding indicates that L-MNCC at a low concentration did not completely cover the droplet, and the emulsification and dispersion were weak. Therefore, the formed emulsion was not stable. In the case of adding Ca2+ (salt solution), oil-water separation was easily generated, which results in demulsification. In Table 2B, the pH stability test was conducted using a styrene- acrylic emulsion. In the acidic and neutral environment, the emulsion showed stability. However,

Polymers 2019, 11, 1131 9 of 16

Table 2. Conditions and results of emulsion stability test.

Dosage of L-MNCC 0.01% 0.025% 0.05% 0.075% 0.1% Ca2+ stability test (A) Layered, oil occupies Layered, oil occupies Not layered, Not layered, Not layered, Stability 1/3 1/4 stable stable stable pH 3 5 7 9 (NaOH) 11 (NaOH) pH stability test (B) Not layered, Not layered, Not layered, Layered, Slight Stability Layered, ﬂocculated Stable stable stable ﬂocculation Dilution multiple 1000 100 50 10 5 Dilution stability test (C) Not layered, Not layered, Not layered, Not layered, Not layered, Stability stable stable stable stable stable Centrifugal oscillation Centrifugal rate 5000 r/min 4000 r/min 3000 r/min 2000 r/min 1000 r/min stability (D) Layered, oil occupies Not layered, Not layered, Not layered, Not layered, Stability 1/4 stable stable stable stable Polymers 2019, 11, x FOR PEER REVIEW 10 of 17

when the NaOH solution was added to the emulsion, it was flocculated and stratified. This is possible because there was a peeling reaction between NaOH and L-MNCC [25]. Moreover, the addition of OH- might have destroyed the stability of the emulsion internal charge, which results in the instability of the emulsion to produce flocculation. Therefore, replacing NaOH with ammonia (ammonia could not cause NCC to initiate peeling reaction) retested the alkali stability and found that flocculation and delamination did not occur. Therefore, it could be explained that strong alkali solution could Polymerscause2019 MNCC, 11, 1131 to occur peeling reaction and degrade, destroy the structure of MNCC, and make it no10 of 16 longer have emulsifying capacity, which results in micelle breaking and flocculating and delamination of emulsion. However, the emulsion was stable under weak bases, and the addition of Table2C showed that the dilution of the solution did not destroy the stability of the emulsion. aqueous ammonia to the emulsion did not cause the sedimentation of emulsion. It showedTable that 2C the showed dilution that factor the dilution of the emulsion of the solution did not did a ffnotect destroy the stability the stability of the of emulsion. the emulsion. Table 2D showedIt showed that, under that the the dilution action factor of high-speed of the emulsion centrifugation, did not affect the oilthe phasestability of of the the emulsion emulsion. was Table slightly desorbed2D showed to form that, an under oily liquidthe action suspension of high-speed layer. centrifugation, The emulsion the exhibitedoil phase of a stablethe emulsion state whenwas the centrifugalslightly rotationdesorbed speedto form wasan oily decreased. liquid suspension Figure 7layer. showed The emulsion that, when exhibited the dosage a stable ofstate MNCC when was less thanthe centrifugal 0.05%, the rotation centrifugal speed was stability decreased. of the Figu emulsionre 7 showed continued that, when to the increase dosage of as MNCC the amount was of MNCCless increased.than 0.05%, Moreover, the centrifugal the centrifugalstability of stabilitythe emulsion of the continued emulsion to wasincrease best as when the amount the amount of of MNCCMNCC reached increased. 0.05%. Moreover, The hydrophobic the centrifugal properties stability of of the the emulsion three modified was best productswhen the amount were di offf erent. MNCC reached 0.05%. The hydrophobic properties of the three modified products were different. When the amount of MNCC was gradually increased, MNCC could cocoon the oil phase in the micelle. When the amount of MNCC was gradually increased, MNCC could cocoon the oil phase in the Hence,micelle. even Hence, if MNCC even was if MNCC subjected was tosubjected the centrifugal to the centrifugal force, most force, of most the oilof phasethe oil wasphase di wasfficult to detach.difficult When to thedetach. amount When of the MNCC amount exceeded of MNCC 0.05%, exceeded the 0.05%, emulsion the emulsion also exhibited also exhibited better centrifugalbetter stability.centrifugal However, stability. when However, the centrifugal when the speedcentrifuga reachedl speed 5000 reached r/min, 5000 instability r/min, instability occurred. occurred. The above resultsThe indicated above results that indicated the emulsion that th hade emulsion certain had stability. certain stability.

100 L-MNCC P-MNCC 95 S-MNCC

Proportion of stable emulsion/% stable of Proportion 70

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 The dosage of MNCC (%) FigureFigure 7. E7.ﬀ Effectect of of MNCC MNCC dosagedosage on the the centrifugal centrifugal stability stability of emulsion. of emulsion.

3.2.4.3.2.4. Zeta Zeta Electric Electric Potential Potential Test Test of of Styrene-Acrylic Styrene-Acrylic Emulsion Emulsion TheThe blank blank styrene-acrylic styrene-acrylic emulsion emulsion containingcontaining no no MNCC MNCC and and the the styrene-acrylic styrene-acrylic emulsion emulsion containingcontaining diﬀ erentdifferent contents contents of of L-MNCC, L-MNCC, P-MNCC,P-MNCC, and and S-MNCC S-MNCC were were diluted diluted for 1000 for 1000 times times into a into a transparenttransparent liquid. liquid. Then, Then, the the Zeta Zeta potential potential distribution distribution of styrene-acrylic styrene-acrylic emulsion emulsion was was tested tested with with a Malverna Malvern laser particlelaser particle size size potential potential analyzer, analyzer, and and the the test testresults results are shown shown below. below. Figure 8 showed that the zeta electric potential of the styrene-acrylic emulsion exhibited a Figure8 showed that the zeta electric potential of the styrene-acrylic emulsion exhibited a negative negative electric potential. As the amount of MNCC increased, the absolute value of the Zeta electric electric potential. As the amount of MNCC increased, the absolute value of the Zeta electric potential of potential of the styrene-acrylic emulsion increased, and the value of the zeta electric potential had a the styrene-acrylic emulsion increased, and the value of the zeta electric potential had a certain relation that, the greater the absolute value of the zeta electric potential had, the more stable the emulsion was with the stability of the emulsion [26]. According to the theory of colloidal stability, Van der Waals gravity made the droplets attract each other. When the droplets approached the surface of the electric double layer and overlapped, the electrostatic repulsion hindered further contact of the droplets [26]. The surface of the MNCC contains hydroxyl groups, according to the diffusion electric double layer theory. The MNCC was negatively charged in water. According to the styrene-acrylic emulsion test, when the amount of MNCC increased, the absolute value of the zeta electric potential of the styrene-acrylic emulsion increased. Therefore, the repulsion between the droplets increased, and the emulsion was more stable. When the amount of MNCC exceeded 0.05%, the absolute value of the zeta electric potential of the styrene-acrylic emulsion remained as increasing, but the increase was slow, because the excess MNCC could form micelles or dissociate in the emulsion, which can also increase the zeta electric potential of the styrene-acrylic Polymers 2019, 11, x FOR PEER REVIEW 11 of 17

certain relation that, the greater the absolute value of the zeta electric potential had, the more stable the emulsion was with the stability of the emulsion [26]. According to the theory of colloidal stability, Van der Waals gravity made the droplets attract each other. When the droplets approached the surface of the electric double layer and overlapped, the electrostatic repulsion hindered further contact of the droplets [26]. The surface of the MNCC contains hydroxyl groups, according to the diffusion electric double layer theory. The MNCC was negatively charged in water. According to the styrene-acrylic emulsion test, when the amount of MNCC increased, the absolute value of the zeta Polymers 2019electric, 11, 1131 potential of the styrene-acrylic emulsion increased. Therefore, the repulsion between the 11 of 16 droplets increased, and the emulsion was more stable. When the amount of MNCC exceeded 0.05%, the absolute value of the zeta electric potential of the styrene-acrylic emulsion remained as increasing, emulsionbut system. the increase By comparing was slow, thebecause three the curves excess MNCC of a, b, could and c,form with micelles the increase or dissociate of the in the carbon chain emulsion, which can also increase the zeta electric potential of the styrene-acrylic emulsion system. length of theBy comparing hydrophobic the three group curves of MNCC,of a, b, and the c, zetawith electricthe increase potential of the carb ofon the chain stabilized length of styrene–acrylic the emulsion alsohydrophobic had an group increasing of MNCC, tendency, the zeta because electric pote thential degree of the of stabilized substitution styrene–acrylic of L-MNCC, emulsion P-MNCC, and S-MNCC decreased.also had an increasing The hydroxyl tendency, content because of the the de surfacegree of ofsubstitution the modified of L-MNCC, nanocellulose P-MNCC, increased,and S- so the zeta electricMNCC potential decreased. increased. The hydroxyl This finding content of was the consistent surface of the with modified the results nanocellulose of the previousincreased, testsso on solid the zeta electric potential increased. This finding was consistent with the results of the previous tests content andon stability.solid content As and the stability. carbon As chain the carbon length chain of thelength MNCC of the MNCC hydrophobic hydrophobic group group increased, increased, the stability of the stabilizedthe stability styrene–acrylic of the stabilized emulsion styrene–acrylic increased emulsion sequentially. increased sequentially.

-26 a：L-MNCC -28 b：P-MNCC c：S-MNCC -30

-32

-34

-36 a -38 b -40

Zeta potential of emulsion/mV of potential Zeta c -42

-44 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 The dosage of MNCC (g/L) Figure 8.FigureEﬀect 8. Effect of MNCC of MNCC dosage dosage on on thethe zeta zeta potential potential of styrene-acrylic of styrene-acrylic emulsion. emulsion.

3.2.5. Optical3.2.5. MicroscopyOptical Microscopy Test Test of Styrene-Acrylic of Styrene-Acrylic Emulsion The styrene-acrylic emulsion containing 0.05% L-MNCC, P-MNCC, and S-MNCC was diluted The styrene-acrylic1000 times into a transparent emulsion liquid. containing Then, a 0.05% small diluted L-MNCC, emulsion P-MNCC, was obtained and S-MNCCand observed was diluted 1000 timesunder into an a transparentoptical microscope. liquid. The Then, test results a small were diluted as follows. emulsion was obtained and observed under an optical microscope.Figure 9 showed The that test the results oil was were dispersed as follows. into spherical droplets, and the NCC was coated on Figurethe9 surfaceshowed of thatthe droplets the oil when was dispersedthe styrene-acry intolic spherical emulsion was droplets, magnified and by the 1600 NCC times. was Hence, coated on the MNCC could effectively coat the oil droplets to form a stable emulsion. The MNCC had a small surface ofparticle the droplets size and when was stable the in styrene-acrylic the form of an emul emulsionsion. The wasoptical magnified microscope byshows 1600 that times. the stability Hence, MNCC could effectivelymechanism coat of the the MNCC oil droplets and the pickering to form emulsion a stable were emulsion. similar. In other The MNCCwords, the had nano-sized a small particle size and wassolid stableparticles in adsorbed the form onto of the an surface emulsion. of the Theemulsion optical monomer microscope to stabilize shows the emulsion that the stability mechanism[11,27,28]. of the However, MNCC andthe MNCC the pickering also had aemulsion part of hydrophobic were similar. groups, In which other made words, the MNCC the nano-sized easier to stabilize oily droplets and enhanced the stabilizing effect. According to the principle of solid particles adsorbed onto the surface of the emulsion monomer to stabilize the emulsion [11,27,28]. However, the MNCC also had a part of hydrophobic groups, which made the MNCC easier to stabilize Polymers 2019, 11, x FOR PEER REVIEW 12 of 17 oily droplets and enhanced the stabilizing effect. According to the principle of similar compatibility, the hydrophobicsimilar compatibility, group of the hydrophobic MNCC clung group to of thethe MNCC inside clung of the to the oil inside droplet, of the and oil droplet, the hydroxyl and group extendedthe to hydroxyl the water group phase. extended Hence, to the the water MNCC phase. He wasnce, coated the MNCC on thewas coated surface on ofthethe surface droplet of the to form an adsorptiondroplet layer. to Therefore,form an adsorption the droplets layer. Therefore, could be the stably droplets present could be in stably the aqueouspresent in the phase. aqueous phase.

(a) (b) (c)

Figure 9.FigureOptical 9. Optical microscopy microscopy of of styrene-acrylic styrene-acrylic emulsion emulsion stabilized stabilized by MNCC. by (a MNCC.) L-MNCC, (a ()b) L-MNCC, P- (b) MNCC, and (c) S-MNCC. P-MNCC, and (c) S-MNCC. 3.2.6. Transmission Electron Microscopy Analysis of the Emulsion A blank styrene-acrylic emulsion (the preparation method was shown at Section 2.2.4, but no MNCC was added) and a styrene-acrylic emulsion containing different contents of L-MNCC, P- MNCC, and S-MNCC were diluted into a translucent liquid. A small amount of diluted emulsion was obtained and dropped on the copper net to let it dry naturally. The morphology of the styrene– acrylic emulsion droplets was then observed using a transmission electron microscope. The test results were as follows. Figure 10 showed that several small droplets were brought together to form a large agglomerate after the droplets of the styrene-acrylic emulsion were diluted in the absence of MNCC (Figure 10a). The degree of aggregation of the droplets was reduced, but the diameter of the individual droplets was increased to some extent after the addition of MNCC (Figure 10b–10d). The MNCC coated around the droplets of styrene-acrylic emulsion could effectively stabilize and disperse the emulsion. Hence, the stability of the emulsion depended not only on the electrostatic repulsion of the MNCC but also on the hydrophilic and lipophilic structure of the MNCC. The hydrophobic group of the MNCC clung to the inside of the droplet, and the hydrophilic structure tended to the aqueous phase. Hence, it could be adsorbed on the surface of the droplet to stabilize the emulsion.

(a) (b)

Polymers 2019, 11, x FOR PEER REVIEW 12 of 17 similar compatibility, the hydrophobic group of the MNCC clung to the inside of the oil droplet, and the hydroxyl group extended to the water phase. Hence, the MNCC was coated on the surface of the droplet to form an adsorption layer. Therefore, the droplets could be stably present in the aqueous phase.

(a) (b) (c)

Figure 9. Optical microscopy of styrene-acrylic emulsion stabilized by MNCC. (a) L-MNCC, (b) P- PolymersMNCC,2019, 11 and, 1131 (c) S-MNCC. 12 of 16

3.2.6. Transmission Electron Microscopy Analysis of the Emulsion 3.2.6. Transmission Electron Microscopy Analysis of the Emulsion A blank styrene-acrylic emulsion (the preparation method was shown at Section 2.2.4, but no MNCCA blankwas added) styrene-acrylic and a styrene-acrylic emulsion (the emulsion preparation containing method wasdifferent shown co atntents Section of L-MNCC,2.2.4, but noP- MNCCMNCC, was and added) S-MNCC and were a styrene-acrylic diluted into emulsion a translucent containing liquid. di ﬀAerent small contents amount of of L-MNCC, diluted P-MNCC,emulsion andwas S-MNCCobtained wereand dropped diluted into on the a translucent copper net liquid. to let Ait dry small naturally. amount ofThe diluted morphology emulsion of was the obtainedstyrene– andacrylic dropped emulsion on the droplets copper was net tothen let itobserved dry naturally. using The a transmission morphology ofelectron the styrene–acrylic microscope. emulsionThe test dropletsresults were was as then follows. observed using a transmission electron microscope. The test results were as follows. Figure 10 showed that several small droplets were brought together to form a large large agglomerate after the droplets of the styrene-acrylic emulsion were diluted in the absence absence of of MNCC MNCC (Figure (Figure 10a).10a). The degree of aggregation of the droplets was reduced,reduced, but the diameter of the individual droplets was increased to some extent after after the the additi additionon of of MNCC MNCC (Figure (Figure 10b–10d).10b–d). TheThe MNCCMNCC coated around the droplets of styrene-ac styrene-acrylicrylic emulsion could effectively eﬀectively st stabilizeabilize and and disperse disperse the the emulsion. emulsion. Hence, the stability of the emulsion depended not only on the electrostatic repulsion of the MNCC but also on the hydrophilic and lipophilic structurestructure of the MNCC. The hydrophobic group of the MNCC clung to the inside of the droplet, droplet, and and the the hy hydrophilicdrophilic structure structure tended tended to to the the aqueous aqueous phase. phase. Hence, it could be adsorbed on the surface of the droplet toto stabilizestabilize thethe emulsion.emulsion.

Polymers 2019, 11, x FOR PEER REVIEW 13 of 17 (a) (b)

Figure 10. TransmissionTransmission electron electron microscope microscope im imageage of ofstyrene-acrylic styrene-acrylic emulsion: emulsion: (a) (Nonea) None MNCC, MNCC, (b) (L-MNCC,b) L-MNCC, (c) P-MNCC, (c) P-MNCC, and and (d) (S-MNCC.d) S-MNCC.

3.2.7. Mechanism Analysis of MNCC-Stabilized Emulsion 3.2.7. Mechanism Analysis of MNCC-Stabilized Emulsion The stability of the styrene-acrylic emulsion was mainly determined by The electrostatic exclusion The stability of the styrene-acrylic emulsion was mainly determined by The electrostatic of MNCC, the hydrophile lipophilic property of MNCC, and spatial hindrance of MNCC adsorption exclusion of MNCC, the hydrophile lipophilic property of MNCC, and spatial hindrance of MNCC layer through the measurement of the particle size of the styrene–acrylic emulsion, the zeta potential adsorption layer through the measurement of the particle size of the styrene–acrylic emulsion, the distribution, the observation, and analysis by optical and transmission electron microscopy. The test zeta potential distribution, the observation, and analysis by optical and transmission electron results were as follows. microscopy. The test results were as follows. Figure 11 showed that the particle size distribution of the styrene-acrylic emulsion droplets was Figure 11 showed that the particle size distribution of the styrene-acrylic emulsion droplets was not uniform, and the emulsion droplets of diﬀerent sizes and sizes were easily aggregated when not uniform, and the emulsion droplets of different sizes and sizes were easily aggregated when MNCC was not added (Figure 11a). The styrene-acrylic emulsion formed was unstable and was MNCC was not added (Figure 11a). The styrene-acrylic emulsion formed was unstable and was easily affected by external influences causing coalescence, instability, or flocculation at this time. Considering that MNCC had a certain hydrophilic and lipophilic ability, the hydrophilic group of the MNCC would cling to water, and the lipophilic group composed of the hydrophobic hydrocarbon long chain would extend to the inside of the droplet. Thereby, MNCC was adsorbed on the surface of the styrene–acrylic emulsion droplets (Figure 11b). However, the coating of the styrene-acrylic emulsion droplets could not be formed when the content of MNCC was small. Therefore, the stability was still not good, even though the stability of the styrene-acrylic emulsion at this time was improved to some extent. The MNCC could completely form the droplets of the styrene-acrylic emulsion when the concentration of MNCC reached a certain amount (Figure 11c). The contact between the droplets of the styrene-acrylic emulsion was greatly reduced, so the formed emulsion was further stabilized. Due to the hydroxyl structure on the surface of the MNCC, the surface of the MNCC exhibited negative electrons. Hence, the droplets and droplets were less likely to contact and aggregate due to electrostatic repulsion. Styrene-acrylic emulsions exhibited high stability. The excess MNCC existed in the aqueous phase in the form of micelles when the MNCC content continued to increase (Figure 11d).

Polymers 2019, 11, 1131 13 of 16 easily affected by external influences causing coalescence, instability, or flocculation at this time. Considering that MNCC had a certain hydrophilic and lipophilic ability, the hydrophilic group of the MNCC would cling to water, and the lipophilic group composed of the hydrophobic hydrocarbon long chain would extend to the inside of the droplet. Thereby, MNCC was adsorbed on the surface of the styrene–acrylic emulsion droplets (Figure 11b). However, the coating of the styrene-acrylic emulsion droplets could not be formed when the content of MNCC was small. Therefore, the stability was still not good, even though the stability of the styrene-acrylic emulsion at this time was improved to some extent. The MNCC could completely form the droplets of the styrene-acrylic emulsion when the concentration of MNCC reached a certain amount (Figure 11c). The contact between the droplets of the styrene-acrylic emulsion was greatly reduced, so the formed emulsion was further stabilized. Due to the hydroxyl structure on the surface of the MNCC, the surface of the MNCC exhibited negative electrons. Hence, the droplets and droplets were less likely to contact and aggregate due to electrostatic repulsion. Styrene-acrylic emulsions exhibited high stability. The excess MNCC existed in the aqueous phasePolymers in 2019 the, 11 form, x FOR of PEER micelles REVIEW when the MNCC content continued to increase (Figure 11d). 14 of 17

MNCC

Styrene-acrylic emulsion droplet Styrene-acrylic emulsion droplets(a small amount of MNCC) a b

MNCC

Styrene-acrylic emulsion droplet (Excessive MNCC) Styrene-acrylic emulsion droplet (Moderate amount of MNCC) d c Figure 11. Mechanism of MNCC stabilizing styrene-acrylate emulsion.

Figure 10 showedshowed the transmissiontransmission electron microscopemicroscope image,image, wherewhere styrene-acrylicstyrene-acrylic emulsionemulsion droplets mostlymostly existedexisted inin thethe formform ofof twotwo dropletsdroplets stuckstuck together.together. ThisThis phenomenonphenomenon occurredoccurred because, when long-chain fatty acids were grafted to MNCC, they would be grafted on both sides of NCC. During the formation of the emulsion, the hydrophobic carbon long chain of the same MNCC macromolecule might be in the same oil phase droplet. It might also be adsorbed in two oil droplets, and the shapeshape ofof twotwo dropletsdroplets stuckstuck togethertogether waswas alsoalso formed,formed, asas shownshown inin FigureFigure 1212.. Due to the existence of double electric layers on the MNCC, the MNCC-coated styrene-acrylic emulsion droplets werewere formed,formed, and, considering the electrostatic ex exclusionclusion effect, eﬀect, coalescence did did not not occur occur easily. easily. Hence, the styrene-acrylic emulsion droplets were stable in the waterwater phase,phase, and thethe styrene-acrylicstyrene-acrylic emulsion exhibitedexhibited aa typetype ofof stability.stability.

O HO O O HO O

O OH O OH O O

O O HO O HO O Oil O Water Oil O OH O OH O Figure 12. Schematic diagram of the adsorption of MNCC on styrene-acrylic emulsion oil-water interface.

Polymers 2019, 11, x FOR PEER REVIEW 14 of 17

MNCC

Styrene-acrylic emulsion droplet Styrene-acrylic emulsion droplets(a small amount of MNCC) a b

MNCC

Styrene-acrylic emulsion droplet (Excessive MNCC) Styrene-acrylic emulsion droplet (Moderate amount of MNCC) d c Figure 11. Mechanism of MNCC stabilizing styrene-acrylate emulsion.

Figure 10 showed the transmission electron microscope image, where styrene-acrylic emulsion droplets mostly existed in the form of two droplets stuck together. This phenomenon occurred because, when long-chain fatty acids were grafted to MNCC, they would be grafted on both sides of NCC. During the formation of the emulsion, the hydrophobic carbon long chain of the same MNCC macromolecule might be in the same oil phase droplet. It might also be adsorbed in two oil droplets, and the shape of two droplets stuck together was also formed, as shown in Figure 12. Due to the existence of double electric layers on the MNCC, the MNCC-coated styrene-acrylic emulsion droplets were formed, and, considering the electrostatic exclusion effect, coalescence did not occur easily.

PolymersHence, 2019the ,styrene-acrylic11, 1131 emulsion droplets were stable in the water phase, and the styrene-acrylic14 of 16 emulsion exhibited a type of stability.

O HO O O HO O

O OH O OH O O

O O HO O HO O Oil O Water Oil O OH O OH O Figure 12.12. SchematicSchematic diagram diagram of theof the adsorption adsorption of MNCC of MN onCC styrene-acrylic on styrene-acrylic emulsion emulsion oil-water oil-water interface. interface. The stability of the emulsion was related to the following factors: the physical properties of the interfacial film, the electrostatic repulsion effect between the droplets, the space blocking effect of the polymer film, the viscosity of the continuous phase, the size and distribution of the droplet, and the volume ratio of the phase and the temperature. Among these factors, the physical properties, electrical properties, and space hindrance of the interfacial film were the most important. The surface MNCC showed negative electricity and had a certain zeta potential. The electrostatic repulsion between MNCC was conducive to the stability of the emulsion. As shown in Figure 11, MNCC was used as the interfacial layer between emulsion droplets and the aqueous phase, which forms a thick liquid affinity protective layer around the droplet. This protective layer constituted a space obstacle for the styrene-acrylic emulsion droplet to approach and contact. The hydrophilicity of the surface hydroxyl groups of the MNCC made the continuous phase liquid in the protective layer similar to that of the gel. The interface area formed a space barrier, which was conducive to the stability of the emulsion. Moreover, the MNCC had a certain size. The particle size of the MNCC was approximately 240 nm. The MNCC occupied a large area on the droplet surface, which was much larger than that of small molecular surfactants (0.27–0.45 nm2). Moreover, the particle size of styrene-acrylic emulsion stabilized by MNCC was larger than that of traditional surfactant stabilized styrene-acrylic emulsion. A big droplet relative to the small droplets, the interfacial area was smaller, the interfacial energy was lower, and the thermodynamic stability was greater. Therefore, the stability of the styrene-acrylic emulsion was caused by electrostatic repulsion between MNCC, the hydrophilic lipophilic ability of MNCC, and the space barrier of the MNCC adsorption layer. Thus, MNCC could be applied to the stability of emulsions.

4. Conclusions In this study, lauric acid, palmitic acid, and stearic acid were used to graft and modify NCC. The MNCC was used as a stabilizer to prepare styrene-butyl acrylate emulsion. The stability of emulsion was determined, and the mechanism of MNCC-stabilized emulsion was analyzed, according to the zeta potential and transmission electron microscope image. The following results were obtained. The average size of MNCC ranges from 244 to 255 nm. In the infrared images, ester bonds were formed, and XRD images showed slight changes compared with NCC. These findings indicate that long fatty acid chains were well grafted onto NCC through esterification initiated at low temperature. When the dosage of three MNCC was 0.05%, the stable styrene-acrylate emulsion was obtained with Polymers 2019, 11, 1131 15 of 16 conversion rates of 92.5%, 94.2%, and 96.3%, respectively. The emulsion had good physical and chemical stability. The electrostatic exclusion of MNCC, the hydrophile lipophilic property of MNCC, and the spatial hindrance of the MNCC adsorption layer worked together to provide good stability for the styrene-acrylic emulsion, according to the measurement of the particle size, Zeta potential, and optical and transmission electron microscopies. In addition, MNCC can be applied to the stability of emulsions and could replace the application of surfactants, which provides an important theoretical basis for the application of MNCC in the field of food and medicine. It is also conducive to the conservation of fossil resources and the reduction of environmental pollution.

Author Contributions: Data curation, J.L. (Junliang Lu); Investigation, H.Y.and J.L. (Jinyan Lang); Writing–original draft, H.G.; Writing–review & editing, H.Z. Funding: The work were supported by Shandong Provincial Natural Science Foundation of China (Grant No. ZR2017MC032), and the Foundation of Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University (Grant No. 2018BCE005). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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