Hydrophobic modification of bagasse cellulose fibers with cationic latex: Adsorption kinetics and mechanism

Chemical Engineering Journal 302 (2016) 33–43

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Hydrophobic modification of bagasse cellulose fibers with cationic latex: Adsorption kinetics and mechanism Yuanfeng Pan a,⇑, Futao Wang a, Tengyou Wei a, Chaolan Zhang c, Huining Xiao b,⇑ a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China b Department of Chemical Engineering, University of New Brunswick, Fredericton, NB E3B 5A3, Canada c College of Materials and Chemical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China

h i g h l i g h t s
Latex with core–shell structure was

synthesized to render bagasse fibers hydrophobic.
Two-stage equilibrium of adsorption was observed and fitted with kinetic models.
The extra-added cationic surfactant enhanced the adsorption efficiency of latex.
Various driving forces for adsorption were discussed to reveal adsorption mechanism.

a r t i c l e

i n f o

Article history: Received 25 December 2015 Received in revised form 1 May 2016 Accepted 6 May 2016 Available online 7 May 2016 Keywords: Bagasse cellulose fibers Core–shell latex Hydrophobic modification Kinetic model Adsorption mechanism

g r a p h i c a l a b s t r a c t OOH OH

1

OH

Cellulose

Hydrogen bonding of H-N Hydrogen bonding of H-CI π−π

OOH

Cellulose

Adsorption H2O

OH

O-

Surfactant

O-

OH

Small latex particles

O-

OH OH

2

n tio sorp x A d Late

O-

Medium or lagre latex particles N

N+ CICellulose

Enlarge

O O-

O

H N+ CI-

O

N

H H

N

a b s t r a c t The modification of the bagasse cellulose fiber by the adsorption of cationic poly latex with core–shell structure on fiber surfaces was conducted in attempt to enhance the interfacial compatibility between hydrophilic fibers and the hydrophobic substrates for various applications. A two-step semi-batch emulsion polymerization was employed for constructing core–shell latex. TEM and dynamic light scattering (DLS) analysis indicated that the core–shell latex possessed a broad particles size distribution, which led to the adsorption of the latex on fiber proceeding via two-stage equilibrium mainly due to the effect of latex particle size. The pseudo-first-order kinetic model was appropriate for the description of the kinetics of the first stage equilibrium at high latex concentration and the entire adsorption process at low concentration. In contrast, the pseudo-second-order kinetic model fitted the second equilibrium at high latex concentration well. The activation energy calculated suggested that the number of activated molecules was increased as the increase of adsorbent concentration, which might change the dominant adsorption mechanism. This also addressed the reasons for the abnormal adsorption behavior with the extra-added emulsifier. In addition, the surface property of the modified cellulose fibers was also investigated via contact angle measurements. The results proved that the cellulose fiber modified with the latex became hydrophobic, allowing the as-modified cellulose to be potential reinforcements for biocomposite. Finally, the adsorption mechanism was proposed according to the adsorption process mainly governed by electrostatic interactions, hydrogen bond, p–p stack and chain entanglement. Ó 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Pan), [email protected] (H. Xiao). http://dx.doi.org/10.1016/j.cej.2016.05.022 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

Electrostatic

OH OH

34

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

1. Introduction It is well known that a series of unprecedented challenges from environment, energy and materials have been encountered with the rapid development of modern industry. This has raised the high demand on the development of alternative technologies for preparing traditional materials [1–4]. Over the past decades, polymer composites are being prepared in combination with various reinforcing materials in order to improve the mechanical strength. With the exhaustion of petroleum resources, polymer industry is under the pressure to utilize more cost-effective or green-based raw materials. One of the effective approaches to address this problem is to use low-cost filler including fibers, for polymer [5]. Recently extensive studies have been performed on exploring the ways to use cellulose-based fibers in place of synthetic fibers as reinforcements for synthetic polymer such as polypropylene (PP) composites [6]. These cellulose-based materials are abundant, inexpensive and easily obtained from renewable natural resources. They also possess other advantages, such as relatively low density, high specific stiffness and strength that allow the production of low-density composites with higher filler content [7,8]. Most importantly, cellulose fibers are biodegradable in comparison to manmade fibers such as glass and carbon fibers. Therefore, plant fibers obtain from wood or non-wood have recently attracted much attention from both industry and academia. The cellulosebased composites are generating a strong impact on various industrial and consumer sectors. This trend has been prompted due to various environmental, economic and performance issues [9]. However, the fiber surface accompanies with a great number of hydroxyl groups, leading to the fiber with a strong polarity and high water sorption [10]. Most of the thermoplastic polymeric matrices used for composites such as PP and polyethylene (PE) are nonpolar [2]. Therefore, the interfacial compatibility is poor between fiber reinforcement and matrix phase. Unmodified fiber added into the composites makes a significant reduction on impact strength due to the poor compatibility between hydrophilic fibers and the hydrophobic substrate [11,12]. It is important to ensure that the bonding or adhesion between fibers and polymeric matrix is sufficiently high to enhance the interface compatibility, which is important for improving the mechanical properties of composites, such as the tensile and impact strengths, by transferring the stresses more effectively from the polymer matrix to fibers. An easy and effective approach to address this problem is to find the means of reducing the surface energy of the nature fibers or render the fiber hydrophobic. However, a number of approaches via grafting or surface modification of cellulose have been attempted in the past, but several unsolved issues or challenges still remain, including the high efficiency of grafting or coupling [9]. Vehvilainen et al. [13] modified enzyme-treated cellulose in alkaline aqueous tertbutanol using allyl glycidyl ether as a modifying reagent to obtain 3-allyloxy-2-hydroxypropyl substituted enzyme-treated cellulose. The modification was performed in homogeneous system and could attain cellulose with high share of substituents. However, it was still restricted in application due to the strong alkaline system and complicated operation. Homogenous acylation and carbanilation reactions of wood-based lignocellulosic materials had also been investigated in ionic liquids, resulting in highly substituted lignocellulosic esters [14]. It is a promising approach for surface modification of cellulose if the ion liquids could be applied beyond the laboratory bench. Treating fiber with latex is another approach for surface modification, particular those prepared by the emulsion polymerization using cationic surfactants duo to the fact that the cellulose fibers are negative-charged. The positive-charged latex could readily adsorb on negative cellulose surface by electrostatic interaction

[15,16]. However, the electrostatic sorption is physisorption, and the desorption often occurs [17]. This phenomenon can be attenuated via the synthesis of amphiphilic polymer and long branched chain polymer. The long branched chain can twine the strands of latex molecules around each other to form cable-like structure. Moreover, the amphiphilic polymer (the hydrophilic less than hydrophobicity) could combine with cellulose fiber easily [18,19]. However, the research on revealing adsorption behavior of cationic latex particles on cellulose fibers surface is still inadequate [20–22], partly because the latex particle was multi distribution. Therefore, the latex adsorption is more sophisticated than single substance due to the small size latex particles having higher specific surface area than large size particles [15]. In this work, we aimed at tackling the problem above using a segmentation method which can clearly comprehend the adsorption mechanism. The cationic polymer latexes with the core–shell structure and broad particle size distribution were firstly prepared; and then used in rendering bagasse cellulose fiber hydrophobic for various applications (e.g., to improve the compatibility between cellulose fiber reinforcement and polymeric matrix as biocomposites). Positive-charged latex (PSt-co-AH/BA-co-DMAEMA-co-BMA) was synthesized using a cationic surfactant and a cationic initiator. The monomers were selected based on the consideration of appropriate hydrophobicity and film formation after adsorption. The key objective was to investigate the adsorption behavior of cationic latex particles on cellulose fibers. To this end, the different influencing factors on latex adsorption were investigated, including latex concentration, pH, ionic strength and temperature. Finally, the adsorption kinetic models were fitted and the mechanism of the latex on cellulose fibers was proposed. 2. Materials and methods 2.1. Materials The bagasse cellulose fiber was obtained from Guangxi state farms sugar industrial Corp., China. Styrene (St), butyl acrylate (BA), and butyl methacrylate (BMA) were purchased from the Aladdin Reagent Co., Ltd. (Shanghai, China), and washed with alkali solution and distilled under reduced pressure prior to use. 2-(Dimethylamino)ethyl methacrylate (DMAEMA), from Aladdin Reagent Co., Ltd. (Shanghai, China) and distilled under reduced pressure prior to use. Allyl hexanoate (AH) and 2,20 -Azobis(2-met hylpropionamidine) dihydrochloride were from Aladdin Reagent Co. The emulsifier, hexadecyl trimethyl ammonium chloride (CTAC) was obtained from Sinopharm Chemical Reagent Co., Ltd. The crosslinker ethylene glycol dimethacrylate (EGDMA) was from Sigma–Aldrich. AR grade Nitric acid (HNO3), Potassium hydroxide (KOH) and Potassium chloride (KCl) were from Guangdong Guanghua Sci-Tech Co., Ltd. china and used as received without further purification. Deionized water was used in all the experiments. 2.2. Synthesis of cationic PSt-co-AH/BA-co-DMAEMA-co-BMA latex with core–shell structure A two-step semi-batch emulsion polymerization was employed for constructing core–shell latex. A typical emulsion copolymerization process is described as follows. Firstly, 75 mL of distilled water, 13.89 g of St/AH mixed monomers at mass ration of 5, 0.070 g of initiator (0.5% of the monomer weight), 0.139 g of EGDMA (1% of the monomer weight) were used in core latex preparations. The amount of surfactant was fixed 8% to the mass of the core monomers. The system was equipped with an inlet of nitrogen gas, a reflux condenser, thermometer and a mechanical stirrer. When the mixture was heated to 70 °C,

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

0.070 g of initiator dissolved in 5 mL of H2O was introduced into the reactor. The reaction was maintained at 70 °C for 2 h under nitrogen atmosphere with mechanical stirring at 200 rpm in order to achieve a high conversion of the core copolymer. Secondly, 17.75 g of BA/BMA/DMAEMA mixed monomers at mass ration of 10:10:3, 30 mL of distilled water, and 0.089 g of initiator (0.5% of the monomer weight). 8% of surfactant (wt on the mass of the shell monomers was added to pre-emulsify the mixture before dropped into the core latex. The initiator was dissolved in 10 mL of H2O, and dropped into the core copolymer latex with the shell pre-emulsion over 1 h at 70 °C. The emulsion polymerization was carried out at 85 °C for 3 h after the shell monomers and initiator were dropped completely. 2.3. Adsorption studies Adsorption kinetic experiment was performed at 303 K as follows: firstly, 5.0 g of bagasse cellulose fibers and 500 mL of distilled water were placed in a conical flask and the pH of system was adjusted to 6.0. The flask was sealed and shaken at 180 rpm in a water bath maintained at 303 K. Then, the latex was added to the flask and 1 mL of mixed solution was withdrawn at periods of time and diluted to measure the concentration using an UV–Vis spectrophotometer (UV, TU-1900, Beijing Purkinje General Instrument Co., Ltd., China) at a wavelength of 205 nm. The adsorbed amounts of the latex (qt, mg/g) and the efficiency of adsorption at different intervals were calculated by the following equation:

qt ¼

VðC 0  C t Þ W

Adsorption ð%Þ ¼

ð1Þ C0  Ce  100% C0

ð2Þ

where C 0 is the initial solution concentration (mg/L), and C t is the solution concentration at time t (mg/L), V is the volume of solution (L), Ce is the latex equilibrium concentration and W is the weight of bagasse cellulose fibers (g). The effect of medium pH on latex adsorption onto cellulose was investigated in the pH range 5.0–9.0 at 30 °C. It is worth noting that the Zeta potential and particle size of latex and Zeta potential of cellulose fibers are also influencing by pH. The concentration of potassium chloride was changed between 0.0 and 0.5 M to investigate the influence of ionic strength on latex adsorption. The effect of the temperature was studied at three different levels (i.e. 30, 40, 50 °C) at pH 6, CKCI = 0.1 mM. To determine the effect of extra addition emulsifier on the adsorption efficiency and capacity on the fibers, the concentration of the emulsifier extra-added was varied between 0 and 300 mg/L in the adsorption medium at pH = 6, CKCI = 0.1 mM and 30 °C. 2.4. Characterization of latex and modified bagasse fiber The Zeta potential (f), mean particle size and size distribution of the latex particles and the Zeta potential of fibers at various pH values were determined using NanoBrook Omni Particle Sizer and Zeta Potential Analyzer (BrookHaven, American) in 0.1 mM KCl aqueous solution and at 25 °C. The concentration of fibers in the latex/fiber suspension was fixed at 1.0 g/L. The supernatant containing fine fiber particles was collected for Zeta-potential measurements [23]. Three repeats were conduct to get an average value for each sample. The morphology of latex particle was observed using a Transmission Electron Microscope (TEM, Tecnai G2 F30 S-Twin, FEI, American) operated at 300KV. The samples were diluted and stained with 2% phosphotungstic acid, mounted on 400-mesh carbon coated copper grids and then dried prior to observation. TEM images were recorded on a Gatan 4k  4k digital

35

camera. The chemical structures of cellulose and modified fibers were characterized with Fourier transform infrared (FTIR) analysis. FTIR spectra were obtained on a FTIR spectrophotometer (Nicolet FTIR IS10, American) using KBr discs to prepare the samples. The surface morphology of the cellulose fiber treated with the latex, coated with gold, were observed using Scanning Electron Microscope (SEM, S-3400N, Hitachi, Japan). The hydrophobic property of modified cellulose fibers were characterized by the contactangle measurement (JC2000P, Shanghai power each digital technology equipment Co. Ltd., China). The hydrophobic properties of the latex treated fiber were investigated by water contact angle measurements, which were conducted with the Sessile Drop test method using a tensiometer and contact angle meter (JC2000P, Shanghai power each digital technology equipment Co. Ltd., China). A drop of distilled water (3 lL) was deposited on the surface of the test specimen. The static contact angles were recorded after 1 min. Each sample was measured on 5 different points and the average values and standard deviation were obtained. The surface properties of cellulose fibers and modified cellulose, specific surface area in particular, were estimated from N2 adsorption/desorption isotherms at 77 K and P/P0 from 0 to 0.3 using Brunauer–Emmett–Teller (BET) (Gemini VII 2390, Micromeritics, America). Prior to the measurement, the sample was heated at 383 K for 8 h under vacuum of 133.3 Pa to remove adsorbed water and gases. 3. Results and discussion 3.1. Observation on the morphology of latex particles TEM was used to visualize the morphology of latex. The image is presented in Fig. 1a, which is a representative image of the core–shell latex particles addressed above. As can be seen, the core–shell latex structure was clearly revealed by TEM observation. The particles distribution revealed by TEM appears to be inhomogeneity. The hard core of the latex, the copolymer of St and AH, has high Tg (glass transition temperature) can enhance the mechanical properties. The relatively soft shell, shown as the dark edges of the latex in TEM image, consists of the copolymer of DMAEMA, BA and BMA which has low Tg for better formation of thin elastomer layer to enhance the compatibility. Meanwhile, the cationic surfactant on peripheral surfaces along with cationic shell polymer chains renders the latex cationic, thus ensuring the strong adsorption or high retention of the latex on fiber. Fig. 1b shows the particle size distribution of latex particle. From Fig. 1b, it can be seen that the peak of particle size distribution is broad. Moreover, the particle dispersion index (PDI) of the latex shown in Table 1 is 0.304, which is thought as ‘‘polydispersity” when the PDI is more than 0.3. It can be found that the mean diameter of latex larger than that from TEM due to the hydrated layer formed on latex. Fig. 1b also shows the particle size distribution is changed from bimodal to unimodal after adsorption, which might be attributed to polydispersity of the particle size of latex. The number of small particle size (particle size <100 nm) was decreased evidently after adsorption. It can be explained that the small particles have less obstruction than large ones during the adsorption process, which facilitates the adsorption of small particles, leaving only relatively large particles unadsorbed. As a result, the particle size distribution is alternated from bimodal to unimodal after adsorption. 3.2. Batch adsorption of latex on cellulose fibers 3.2.1. Effect of initial pH on latex adsorption on fibers The pH affects the surface charge of the latexes or adsorbents as well as the aggregation of latex particles. The particle mean size

36

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

a

b

1.0

Intensity

0.8

G(d)-before adsorption C(d)-before adsorption G(d)-after adsorption C(d)-after adsorption

0.6

0.4

0.2

0.0 10

100

1000

Size (nm) Fig. 1. TEM image corresponding to core–shell particles (a) and particle sizes distribution of latex (b), G (d) the signal intensity of emulsion particle, C (d) the cumulate content percentage of emulsion particle.

Table 1 The basic properties of latex. Sample

Solid content (wt%)

Average diameter ZD (nm)

PDI

f-potential (mv)

Latex

20.9

84.95

0.304

32.8

and Zeta potentials of latex and fiber were determined at different pH values to investigate colloid stability. As shown in Fig. 2, the mean size of latex particles was changed for pH values ranging from 2 to 11, but remained stable for pH ranging from 2 to 6 and 10 to 12; whereas the sharp increase was observed around neutral pH, corresponding to the low Zeta-potentials of latex. Obviously, this is attributed to the particle aggregation induced by the low electrostatic repulsion. As can be seen from Fig. 2, the particles exhibit positive surface charges in the pH range from 2 to 8. After the isoelectric point (at pH around 7.8), the charge reversal occurred due to the protonation of CTAC decreased or OH association with cationic surfactant at a high pH (>9). The cellulose exhibited negative surface charges in the investigated pH range (4–11) and the Zeta-potential of cellulose was decreased as pH increased. It can be explained based on the ion adsorption and ion exchange sites on the surface of cellulose fiber. The ionization of hydrogen ions on surface makes fiber negativecharged upon the adsorption of water on cellulose. The ion

Cellulose Latex Latex mean size

1400 1200

40 1000 20 800 0

600 2

-20

Size (nm)

Zeta potential (mv)

60

4

6

pH 8

10

12 400 200

-40 0

Fig. 2. The Zeta potentials of cellulose and latex, and latex mean size as a function of pH (KCl concentration of 0.1 mM).

exchange of surface cellulose is accelerated because the amount of hydrogen ions is reduced as pH increase. As a result, the Zeta potential became more negative (>30 mV at pH 8). As can be seen that the maximum difference in terms of the Zeta-potentials of latex and cellulose was found at pH around 5; and the electrostatic force had slight variations at pH value of 6–7. Therefore, the maximum adsorption amount tends to occur at pH = 5 and the amount adsorption should be close to those at pH 6–7. Fig. 3(a) illustrates the effect of pH on the adsorption of latex on cellulose. As can be seen, the amount of adsorption was decreased with increased in pH, and the maximum adsorption at the pH = 5 and the amount of adsorption had a slight variations at pH value of 6–7. Substantial decrease occurred at pH 8–9, which is mainly due to the charge reversal of latex or the reduction of electrostatic association. The results are clearly consistent with the findings from Zeta-potential measurements. The adsorption amount remained could be driven by electrostatic force, van de Walls force and hydrogen bond.

3.2.2. Effect of ionic strength on adsorption The effect of ionic strength on Zeta potential and particle aggregation were also investigated (Fig. 4). For salinity from 0 to 1 mM, no significant change of Zeta potential and particle size was observed, which is probably due to the fact that the particles are highly charged (i.e. the Zeta potential is above 30 mV). Above 1 mM KCl, the increasing ionic strength induces a decrease in Zeta potential due to the reduction of double layer thickness. As a result, the latex emulsion become less stable and the aggregation of latex occurred. The effect of ionic strength on the amount of latex adsorption on cellulose was studied at 30 °C and pH = 6. As seen in Fig. 3(b), increasing ionic strength has no significant effect at the electrolyte concentration up to 10 mM, but the adsorption amount deceased from 97.1 mg/g to 77.3 mg/g with the increase of KCl concentration from 10 to 100 mM. This is consistent with the Zeta-potential results shown in Fig. 4. The lowering of Zeta-potential of latex reduced its adsorption amount on fiber correspondingly. The slight salinity effect (between 0.0001 and 10 mM KCl) could be explained by the high charge density of latex particles and the strong electrostatic interactions between particles and fibers. The decrease of adsorption amount is due to the strongly compressed thickness of electrical double layer when electrolytes concentration over 10 mM. For higher salt concentration (>10 mM), latex particles

37

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

a

100

b

100

pH=6 80

Qe (mg/g)

Qe (mg/g)

80

60

60

40

40

20

20

0

0 5

6

pH

7

8

0

9

1E-4

1E-3

0.01

0.1

CKCI (mol/L)

Fig. 3. Effects of (a) pH and (b) ionic strength on the adsorption of latex for cellulose fibers at 303 K. The initial latex concentration was 1000 mg/L and the fiber concentration was 10 g/L; contact time 2 h.

40

30 40 1600

20

20

15 0 0

10

30

100

150

200

300

5

-20 0 0

1E-4

1E-3

0.01

0.1

10

CCTAC (mg/L)

400

0

50

Qe (mg/g)

800

Zeta potential (mv)

1200

20

Size (nm)

Zeta Potential (mv)

25

0

0.5

CKCI (mol/L) Fig. 4. Latex particle mean size and Zeta potential as a function of KCl concentration (pH = 6).

became rather instable so that no further experiments were conducted under such conditions.

Fig. 5. Effects of extra-added emulsifier on the adsorption amount of latex on cellulose fibers and Zeta potential for cellulose fibers at 303 K; the initial latex concentration of 300 mg/L, pH = 6, and a contact time above 2 h.

without extral emuisifier

100

with extral emusifier

Adsorpation (%)

80

3.2.3. Effect of extra-added emulsifier on adsorption The effect of the extra addition of emulsifier on the Zeta potential of cellulose fibers and adsorption amount of latex was investigated (Fig. 5). The Zeta potential of fibers was increased with the amount of emulsifier increased due to the positive charges of emulsifier adsorbed on the cellulose. Generally, the Zeta potential of cellulose and adsorption amount of latex had a common change tendency at low emulsifier dosage. As can be seen from Fig. 6, the efficiency of adsorption increased, reached plateau stage and decreased with further increasing latex concentration. The efficiency of adsorption of the latex at low concentration was less than 60% without extra-added emulsifier. This phenomenon was abnormal and the abnormal phenomenon was eliminated with added extra cationic emulsifier before latex adsorption. The results, shown in Fig. 6, indicated that the efficiency of adsorption was increased to 90% at low latex concentration by added extra emulsifier and the efficiency of adsorption was increased and then decreased with latex concentration increasing. Therefore, adsorption amount of latex could be enhanced by the impregnation of fiber with the surfactant at a very low dosage. It may be interpreted that the hydrophilic side of CTAC was combined with fibers

60

40

20

0 0

100

300

600

1000

2000

4000

6000

7000

Latex concentration (C (mg/L)) Fig. 6. Effects of extra addition emulsifier concentration of 10 mg/L on the efficiency of adsorption at 303 K, pH = 6, and a contact time above 2 h.

and the alkyl chain can twined the strands of latex molecules around each other to form cable-like structure, and involved some hydrophobic interaction between latex and surfactant molecules impregnated in the cellulose [24–27].

38

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

All in all, the efficiency of adsorption of latex reached plateau staged and decreased with latex concentration increased. It can be interpreted that the small size particles and cationic emulsifier have higher priority of sorption than large ones during the adsorption process; and the small particles (and/or emulsifier) rapidly provide a layer of activation. In other words, the cellulose surface could be activated by the sorption of emulsifier. The alkyl chains of emulsifier can be twined with the strands of latex molecules around each other to form cable-like structure, thus enhancing the adsorption of latex on fiber. However, the sorption sites and electrostatic force were dwindled, which led to the decreasing of adsorption efficiency.

3.2.4. Effect of temperature on bagasse cellulose fibers adsorption The results, shown in Fig. 7, indicated that the latex adsorption appeared to have two-stage equilibrium at the high latex concentration (i.e., 3000 mg/L). This might be mainly attributed to the polydispersity of the latex particle size. The small latex particles possess a high specific surface; therefore, tend to have the priority adsorption and reaches the dynamic equilibrium first; while the large latex particles do not arrive on the surface of cellulose fiber at the moment [15,28]. In other words, from a statistic point of view the first stage equilibrium of the adsorption was achieved by small latex particles; followed by the adsorption of the relatively large latex particles, leading to the second stage of equilibrium. As can be seen from Fig. 1(b), the particles with the size less than 100 nm almost disappeared after adsorption. This provides strong evidence that the small latex particles do have the priority in the adsorption on fiber surfaces; whereas the unadsorbent latex mainly consists of the particles with the average size larger than 350 nm. Such effect became the minimal at the low concentration (i.e., 600 mg/L) due to the less amount of latex particles involving in the competition of adsorption. Fig. 7 also shows that the temperature had no effects on adsorption amount at the low latex concentration (i.e., 600 mg/L) within the temperature range investigated. However, the temperature effect at the high latex concentration (3000 mg/L) is associated with the stage of equilibrium. The adsorption amount was decreased as the increase of temperatures during reaching the first equilibrium. In contrast, the adsorption amounts were closed at 303 K and 323 K for the second stage equilibrium. In fact, the first-stage equilibrium might be driven by electrostatic force; whereas the intermolecular forces could be dominants for the sec-

ond stage equilibrium. Moreover, the relatively high temperature could promote the colliding efficiency of small particles with cellulose fibers; but meanwhile might also induce the desorption of large latex particles. As a result, the relatively high amount of latex adsorbed was achieved at 313 K. 3.3. Characterization of modified fiber 3.3.1. FTIR spectroscopic analysis To further verify the adsorption of latex on bagasse fiber, FT-IR spectroscopic analysis was performed. The FT-IR spectra of the latex, bagasse fiber, and modified fiber are shown in Fig. 8. The new peak occurred at 1738 cm1 after the modification clearly demonstrated the presence of the latex on fiber surface as this peak is attributed to the stretching of C@O in the latex (contributed from acrylate type of monomers). The peak at 1158 cm-1 is C–O antisymmetric bridge stretching. The peak at 1030 cm1 is attributed to the C–O–C group of primary hydroxyl stretching. The strong adsorption peaks at 3430 cm1 are the stretching of H-bonded OH groups. For latex, the peaks at 3021 cm1 and 3061 cm-1 are the C–H stretching vibration of benzene ring. The absorption peaks at 757 and 703 cm1 were revealed that the benzene ring was monosubstituted. Overall, compared with unmodified cellulose fiber, the adsorption peak at 1738 cm1 appearing in the spectra of modified fiber indicated the success of the latex adsorption on cellulose fiber surface. 3.3.2. Adsorption isotherm of N2 and latex on fiber Fig. 9(a) and (b) present the adsorption isotherms of N2 and latex on fiber surface, respectively. Clearly, the N2 adsorption isotherm exhibited a type II isotherm characteristic of non-porosity material and multilayer adsorption. The amount of N2 adsorbed on untreated cellulose was more than that of modified fiber. The results are clearly consistent with specific surface area due to the formation of latex film on the fiber surface that lowers the specific surface area of the fiber. The saturated adsorption occurred at approximately 520 mg/g fiber, driven by the electrostatic association between cationic latex and negative-charged cellulose fiber. 3.3.3. Surface morphology of modified fiber The morphology of fibers, revealed by SEM observation (see Fig. 10), indicated that the polymer film from the latex seemed to cover the fiber surface. The modified fiber with non-heat treated

250 300

a

b 200

250

303 K-600 mg/L 313 K-600 mg/L 323 K-600 mg/L 303 K-3000 mg/L 313 K-3000 mg/L 323 K-3000 mg/L

150

303 K-600 mg/L 313 K-600 mg/L 323 K-600 mg/L 303 K-3000 mg/L 313 K-3000 mg/L 323 K-3000 mg/L

150

Qt (mg/g)

Qt (mg/g)

200

100

100 50 50

0

0 0

20

40

60

80

t (min)

100

120

140

160

0

2

4

t (min)

6

8

10

Fig. 7. The effect of temperature on the adsorption amount of latex on fibers, (a) the entire adsorption process (b) enlarged drawing of first stage equilibrium; latex concentrations: 600 and 3000 mg/L, respectively.

39

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

1069 757 703

1738 Bagasse fiber

Modified bagasse fiber

Latex

4000

3000

2000

1000

0

-1

Wavenumber (cm )

qt ¼ qe1 ð1  ek1 t Þ

Fig. 8. FTIR spectra of latex, bagasse cellulose fiber and modified cellulose fiber.

had inhomogeneous film, probably because the size of particles is rather small and Tg is relatively low. It should be addressed that the latex film could be formed during the sample preparation for SEM observation. From Fig. 10(c), it can be seen that the layer of latex covers the fiber surface and makes it smoother. The relatively low specific surface area of the modified cellulose fiber (see Table 2) also demonstrated the adsorption of latex on cellulose fiber. 3.3.4. The contact angle measurement of modified fibers To further demonstrate the adsorption of latex on cellulose fiber, the contact angle measurements were conducted. The results, shown in Fig. 11, indicated that the contact angles of water on the surface of fibers increased dramatically with the increasing of latex content; and reached 102° at 15 wt% (on dried fiber). The high contact angle suggested that a stable hydrophobic surface of the fibers was created in the presence of the latex with core–shell structure.

In batch experiments, kinetic study is very important to find out the contact time of the adsorbent with adsorbate and to under-

0.025

where qt is the amount of adsorbate adsorbed at time t (mg/g); qe1 is the adsorption capacity in equilibrium (mg/g); k1 is the rate constant of pseudo-first-order model (min1); and t is the time (min). The pseudo-first-order rate constants were used to calculated the initial sorption rate given by limit theorem.

h ¼ k1 qe1

ð4Þ

where h is the initial sorption rate. Equation of Elvoich kinetics model [34,35]:

qt ¼ b lnðabÞ þ b ln t

ð5Þ

where a is the initial sorption rate (mg(g min)1) and b is the desorption constant (mg/g). Equation of pseudo-second-order kinetics model [36]:

t 1 t ¼ þ qt k2 q2e2 qe2

ð6Þ

The pseudo-second-order rate constants were used to calculated the initial sorption rate given by limit theorem.

ð7Þ

where k2 is the rate constant of pseudo-second-order model g (mg min)1, qe2 is the adsorption capacity in equilibrium (mg/g) for pseudo-second-order adsorption. 600

a

b 500

Modified cellulose Untrated cellulose T=77 K

303 K 400

0.020

Qe (mg/g)

Amount adsorbed (mmol / g)

0.030

ð3Þ

h ¼ k2 q2e2

3.4. Adsorption kinetics

0.035

stand adsorption mechanism. The adsorption kinetics of latex on fibers was investigated at constant pH = 6 in 1 mM KCl solution. As shown in Fig. 7, the adsorption proceeded rapidly first; and most of the latex were retained within the first 2 h of contact. Three kinetic models were fit with experimental data, including pseudo-first-order, the Elovich equation and pseudo-secondorder. These models are most commonly used to describe the sorption of dyes as well as other pollutants on solid sorbents [29–34]. The application of single kinetic model to the sorption on solid sorbents may be questionable because of the heterogeneity of cellulose fiber surface, and diversity of adsorption phenomena (transport, surface interactions). Therefore, a two-stage of equilibrium on adsorption process occurs due to the poly-dispersed size of latex. Parameters of the kinetic models were estimated from the experimental date with the aid of the nonlinear curve-fitting and liner-fitting procedure. The adsorption kinetics was investigated at low, medium and high concentrations, respectively. Equation of pseudo-first-order kinetics model:

0.015

300

200

0.010 0.005

100

0.000 0

0.00

0.05

0.10

0.15

0.20

0.25

0

500

1000

1500

P / P0

(a) Fig. 9. Adsorption isotherm of N2 (a) and latex (b) on fiber surface.

2000

2500

Ce (mg/L)

(b)

3000

3500

4000

40

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

Fig. 10. SEM images of surface of (a) untreated fiber (b) modified fiber with non-heat treated (c) modified fiber with heat treated.

Table 2 The physical properties of cellulose fibers and modified cellulose. Parameters

Cellulose fibers

Modified cellulose fibers

Specific surface area (m2/g)

2.63 ± 0.15

1.76 ± 0.12

120

Contact angle (°)

100

80

60

40

20

0 0

10

20

30

40

50

Latex dosage (wt %) Fig. 11. Contact angle of cellulose fibers modified with latex.

The kinetic parameters qe1 ; qe2 ; a; b; k1 ; k2 ) and correlation coefficients R2 of latex were calculated; and the results are listed in Tables 3 and 4. The correlation coefficients for a low concentration of latex and the first stage equilibrium of high concentration on fibers were found in range of 0.94–1. Moreover, the theoretical uptakes qe(cal)

were in good agreement with the experimental data qe(exp) for the pseudo-first-order expression. Therefore, the experimental results do fit the pseudo-first-order kinetic model well at a low concentration of latex and the first stage equilibrium of high concentration. Tables 3 and 4 also illustrate that the adsorption rate constant (k1) increases as the concentration of latex and temperature increase. This could be due to the presence of a greater number of active molecules for adsorption of high concentration and the mass transfer promoted at high temperature. As seen in Table 3, the correlation coefficients for the Elvoich equation were small. This result has shown that the experimental data did not fit with the Elvoich equation. Apparently, the correlation coefficients of second equilibrium are closer to unity for pseudo-second-order kinetic model than pseudo-first-order kinetic model and Elvoich equation. Therefore, it is ascertained from a comparison of the predicted (best fitted) time dependencies with the experimental date that the pseudosecond-order kinetic equation describes the second stage of latex sorption more accurately, especially for long time periods. The results also demonstrated that the kinetic rate constants k2 were adversely affected by the initial concentration at the same temperature. This indicated that the systems with lower initial latex concentration reached equilibrium faster than those with higher initial concentration. Azizian et al. [36,37] stated that an additional criterion in the discrimination between the pseudo-first and pseudosecond order models was that the rate constant (k1) increased with an increase in initial concentration for the pseudo-first order model, while decreased if the reaction obeyed the pseudo-second order model [29,36]. The experimental data from this work agreed well with their criterion. The two stage equilibrium was induced by the greater diffusion of small particles compared to large one. It can be concluded that the pseudo-first-order well fit low concentration and the first equilibrium of high concentration, and the kinetics of low concentration only was equipped with first equilib-

41

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43 Table 3 Kinetics data calculated for adsorption of low initial concentration of latex on fibers. Parameters

*

Pseduo-first-order model

Pseudo-second-order model 2

Elvoivh equation 2

R2

T (°C)

[C0] (mg/L)

k1 (1/min)

qe(cal) (mg/g)

qe(exp) (mg/g)

R

h (mg/g min)

k2 (g/mg min)

qe(cal) (mg/g)

qe(exp) (mg/g)

R

30 30 40 50

300 600 600 600

5.02 1.68 1.85 2.07

26.2 58.8 58.8 57.3

26.2 59.0 59.0 57.8

0.997 0.941 0.995 0.999

131.1 98.8 109.1 119.8

1.18 0.181 0.235 0.325

25.9 58.3 59.0 59.0

26.2 57.8 59.0 59.0

1.00 1.00 0.999 1.00

–* – – –

The curves and the correlation coefficient data were not fitted with Elvoich equation in a perfect way.

Table 4 Kinetics data calculated for adsorption of medium and high concentrations of latex on fiber. Parameters

Pseudo-first-order model The first stage of equilibrium

The second stage of equilibrium

T (°C)

[C0] (mg/L)

k1 (1/min)

qe(cal) (mg/g)

qe(exp) (mg/g)

R2

h (mg/g min)

k1 (1/min)

qe(cal) (mg/g)

qe(exp) (mg/g)

R2

30 40 50 30 40 50 30

1500 1500 1500 3000 3000 3000 5000

2.11 2.30 2.56 5.65 2.03 3.81 3.99

100.6 89.3 110.2 191.2 176.1 201.9 304.1

103.4 96.6 109.5 190.7 165.5 201.0 301.4

0.986 0.996 0.995 0.996 0.980 0.998 0.991

218.5 221.6 279.9 1077.6 335.1 766.6 1201.9

0.305 0.147 0.136 0.078 0.097 0.199 b

149.1 148.5 148.0 295.1 267.8 273.8 b

148.9 148.2 148.1 296.1 271.5 277.0 b

0.956 0.989 0.946 0.776 0.802 0.855 b

Parameters

Pseudo-second-order model The first stage of equilibrium

The second stage of equilibrium

T (°C)

[C0] (mg/L)

k2 (g/mg min)  103

qe(cal) mg/g

qe(exp) mg/g

R2

h

k2 (g/mg min)  103

qe(cal) mg/g

qe(exp) mg/g

R2

30 40 50 30 40 50 30

1500 1500 1500 3000 3000 3000 5000

9.82 51.3 62.3 9.40 14.0 482.7 14.1

133.3 114.8 91.66 216.9 190.8 194.9 320.5

103.4 109.5 96.6 190.6 165.5 201.0 301.4

0.951 1.00 0.991 0.993 0.991 0.984 0.998

50.4 139.6 1042.6 53.32 76.7 193.3 838.8

2.30 6.36 47.0 0.608 1.04 2.52 3.49

151.5 152.2 148.8 309.6 280.9 280.1 568.2

148.1 148.2 148.9 296.1 271.5 277.0 490.2

0.999 0.999 1.00 0.996 0.995 0.999 0.993

rium. Adsorption on solids is classified into physical adsorption and chemical adsorption, but the dividing line between the two is not sharp. Generally speaking, the pseudo-first-order and the pseudo-second-order models hint the physical adsorption and chemical adsorption dominate, respectively. It can be seen from Tables 3 and 4 that the initial sorption rate h in the first equilibrium increased with initial concentration at the same temperature. However, the value of h was increased as the temperature increased, which might attributed to the enhanced mass transfer at an elevated temperature. However, the initial sorption rate h of the first equilibrium was higher than that of the second equilibrium. It may be explained that the concentration of latex and adsorption site of fibers on the first stage were more than the second stage. Therefore, the first stages of drive of mass transfer and electrostatic force over the second stage. And, the results agreed well with the physical adsorption rate were quickly than chemical adsorption rate.

The range of 5–40 kJ/mol of activation energies corresponds to a physisorption mechanism and the range of 40–800 kJ/mol suggests a chemisorption mechanism. Fig. 12 shows the plots of ln k versus 1000/T based on the results in Table 3. As can be seen, the first stage of preexponential factor and the activation energy increased as the latex concentration at high latex concentrations. The pre-exponential factor and activation energy for the first stage were very close to those for the second stage at high latex concentrations. The result indicated that the first stage adsorption was physisorption; and the

4

600mg/L First equilibrium-1500mg/L First equilibrium-3000mg/L Second equilibrium-1500mg/L second equilibrium--3000mg/L

3

3.5. Activation parameters The pseudo-first-order and pseudo-second-order rate constants listed in Tables 3 and 4 have been used to estimate the activation energy of latex adsorption on cellulose fibers using Arrhenius equation:

ln k

2

1

0

Ea ln k ¼ ln k0  RT

ð8Þ

where Ea is activation energy (J/moL); k is the rate constant of sorption; k0 is Arrhenius factor, which is the temperature independent factor; R is the gas constant (J/mol K); and T is the solution temperature (K). The slope of plot of ln k versus 1/T is used to evaluate Ea.

-1 3.10

3.15

3.20

3.25

1000/T Fig. 12. Arrhenius plots for adsorption of latex on fibers.

3.30

42

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

Fig. 13. Adsorption mechanism of bagasse cellulose fibers.

3.6. Adsorption mechanism

Table 5 Activation parameters for adsorption of latex on cellulose fiber. [C0] mg/L

The first stage of equilibrium

The second stage of equilibrium

Pre-exponential factor k0 (1/min)

Ea kJ/mol

Pre-exponential factor k0 (g/mg min)

Ea kJ/mol

600 1500 3000

51.21* 45.29 3.46  107

8.628* 7.738 41.889

– 2.42  1018 5.03  106

– 122.436 55.115

* The adsorption process had a one stage of the equilibrium when the concentration of latex was 600 mg/L.

number of activated molecules was low. The pre-exponential factor and activation energy for the second stage was increased compared to those for the first stage equilibrium at 1500 mg/L of latex concentration. Clearly, as the number of activated molecules was increased, the adsorption behavior was transformed, i.e., the converting from physisorption to chemisorption. It can be interpreted that the cellulose surface was emulsifier layer and the layer can led to the activation phenomenon (see Figs. 5 and 6), which the second stage activation numbers increased. Secondly, the cellulose surface was covered by cationic latex particles; the particles combined benzene ring and lone pair electrons, which can be formed chemical bond, so the chemisorption as a leading adsorption process on the second stage. On the latex concentration of 3000 mg/L, the sorption mechanism was chemisorption according the activation energy and the pro-exponential factor indicated that the activation molecular numbers far more than the first stage of 1500 mg/L. The higher concentration of the latex had more emulsifier. So the cellulose surface was quickly adsorption emulsifier layer in short time at the high latex concentration, and the cellulose surface was activated by sorption emulsifier. Therefore, the activation energy and the pro-exponential factor of two stages equilibrium at 3000 mg/L were closed. It can interpret that the electrostatic adsorption was quickly and transiently. So the mainly sorption function was chemical sorption on the high concentration.

It has been widely accepted that the adsorption of cationic polymers on cellulose fibers is mainly governed by the electrostatic interaction. For the current system, the adsorption mechanism focusing on the driving forces is proposed according to the results addressed in previous sections and schematically illustrated in Fig. 13. Overall, the covalent and non-covalent interactions (including hydrogen bond, electrostatic, ion exchange, charge transfer interaction and chain entanglement) play crucial roles in latex adsorption. In the first step, the fibers surface became negativecharged due to the ion exchange of acid adsorption site (hydroxyl groups). In the second step, the latex adsorbed on the fiber surface is mainly driven by electrostatic force due to the ionization of the cationic surfactant layer on the latex outface. Moreover, the ion exchange can occur between the cationic surfactant and the hydroxyl groups of cellulose. Therefore, the surfactant was priority adsorption and the lone pair electrons of cellulose and latex can form hydrogen bonding. The electrostatic association was gradually weakened as the adsorption proceeds. However, the cellulose surface had more lone pair electrons and p track, which created more hydrogen bonding, p–p stacking and charge transfer interaction, and the activation of surfactant of the cellulose surface. More importantly, the latex particles with small particle size carry higher charge density than those with large size. Therefore, the small latex particles are priority adsorption; and the adsorption curves had two stages equilibrium, suggesting that the adsorption behavior was transformed, i.e., converting from physisorption to chemisorption. This is consistent with the findings shown in Table 5. 4. Conclusions In this study, cationic nanosized latex with core–shell structure, prepared via the emulsion copolymerization of St-co-AH/BA-co-

Y. Pan et al. / Chemical Engineering Journal 302 (2016) 33–43

DMAEMA-co-BMA using CTAC as surfactant was used for hydrophobic modification of cellulose fibers. The hydrophobicity of the modified cellulose fibers was enhanced significantly once the adsorption amount reached 196 mg/g. The saturated adsorption capacity of cellulose fibers was as high as 520 mg/g at room temperature due to the excellent combining effects of amphiphilic polymer with cellulose fiber. However, the abnormal adsorption phenomenon was observed, i.e., the adsorption efficiency was lower at a lower latex concentration, which was eliminated by adding a small amount of cationic emulsifier prior to latex adsorption. The two-stage equilibrium of adsorption process was successfully fitted with the pseudo-first-order and pseudo-second-order models at different latex concentrations. The activation energy determined in this work indicated that the physisorption mechanism was converted into chemisorption mechanism during the two-stage adsorption. The sorption mechanism was influenced by the latex concentration. However, it is essential that the adsorption forces were mainly governed by electrostatic interactions, hydrogen bond, p–p and chain entanglement on the sorption process. More importantly, the contact angle (CA) of modified cellulose fibers was remarkably enhanced. Therefore, the interfacial elastic thin film created from latex could improve the compatibility between hydrophilic cellulose fibers (as reinforcements) and hydrophobic polymer matrix as biocomposites for various applications. Acknowledgments The research was financially supported by National Natural Science Foundation of China (No. 21466005), Guangxi Natural Science Foundation (2014GXNSFAA118036); the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2014Z003). References [1] S. Ummartyotin, H. Manuspiya, An overview of feasibilities and challenge of conductive cellulose for rechargeable lithium based battery, Renewable Sustainable Energy Rev. 50 (2015) 204–213. [2] M.F. Rosa, B. Chiou, E.S. Medeiros, D.F. Wood, T.G. Williams, L.H.C. Mattoso, W. J. Orts, S.H. Imam, Effect of fiber treatments on tensile and thermal properties of starch/ethylene vinyl alcohol copolymers/coir biocomposites, Bioresour. Technol. 100 (2009) 5196–5202. [3] G. Canché-Escamilla, J. Rodriguez-Laviada, J.I. Cauich-Cupul, E. Mendizábal, J.E. Puig, P.J. Herrera-Franco, Flexural, impact and compressive properties of a rigid-thermoplastic matrix/cellulose fiber reinforced composites, Compos. Part A Appl. Sci. 33 (2002) 539–549. [4] F.P. La Mantia, M. Morreale, Green composites: a brief review, Compos. Part A Appl. Sci. 42 (2011) 579–588. [5] T.H. Mokhothu, M.J. John, Review on hygroscopic aging of cellulose fibres and their biocomposites, Carbohydr. Polym. 131 (2015) 337–354. [6] M. Abdelmouleh, S. Boufi, M. Belgacem, A. Dufreane, Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading, Compos. Sci. Technol. 67 (2007) 1627– 1639. [7] P. Wambua, J. Ivens, I. Verpoest, Natural fibres: can they replace glass in fibre reinforced plastics?, Compos Sci. Technol. 63 (2003) 1259–1264. [8] D. Liu, T. Zhong, P.R. Chang, K. Li, Q. Wu, Starch composites reinforced by bamboo cellulosic crystals, Bioresour. Technol. 101 (2010) 2529–2536. [9] Y. Pan, M.Z. Wang, H. Xiao, Biocomposites containing cellulose fibers treated with nanosized elastomeric latex for enhancing impact strength, Compos. Sci. Technol. 77 (2013) 81–86. [10] M. Mahbubul Bashar, M.A. Khan, An overview on surface modification of cotton fiber for apparel use, J. Polym. Environ. 21 (2013) 181–190. [11] Y. Pan, H. Xiao, Y. Zhao, Z. Wang, CTMP-based cellulose fibers modified with core–shell latex for reinforcing biocomposites, Carbohyd. Polym. 95 (2013) 428–433.

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