Chemical Modification of Natural Rubber in the Latex Stage by

(15, 16) have also reported that cardanol acetate and epoxidated cardanol ... The initiator potassium persulfate (K2S2O8), sodium thiosulfate (Na2S2O3...
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Chemical Modification of Natural Rubber in the Latex Stage by Grafting Cardanol, a Waste from the Cashew Industry and a Renewable Resource Sunita Mohapatra and Golok B. Nando* Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India 721302 S Supporting Information *

ABSTRACT: Cardanol, an agricultural byproduct of the cashew industry, is a cheap and abundantly available renewable resource. The multifunctional activity of cardanol in the rubber and polymer industries has been well-established in recent years. The present study highlights the chemical grafting of cardanol onto natural rubber in the latex stage. The cardanol grafted natural rubber is characterized by FTIR and NMR spectroscopies. The grafting parameters have been optimized for maximum yield in terms of percent grafting and grafting efficiency by the Taguchi method. Four control factors, i.e., initiator concentration, cardanol concentration, reaction temperature, and reaction time, are chosen at three different levels. The optimum parameter combination is found to be the initiator concentration 2 phr, cardanol concentration 10 phr, reaction temperature 65 °C, and reaction time 6 h. The analysis of variance method is used to evaluate the percentage contributions of the different control factors on the percent grafting and grafting efficiency. nontoxic, low-leaching, halogen-free fire retardant polymer. It has been reported that cardanol can also be used along with lignin-based compounds for the synthesis of polyurethanes that exhibit good thermal and mechanical properties.2 Calo et al.3 have reported a novel benzoxazine prepolymer derived from cardanol which is employed in the synthesis of phenolic resins that exhibits good thermal properties and flame retardant characteristics with improved mechanical properties and greater molecular design flexibility. Bhunia et al.4,5 have reported the synthesis of the difunctional monomer 4-[(4-hydroxy-2-pentadecenylphenyl)diazenyl]phenol (HPPDP) derived from cardanol, based on which they synthesized a novel polyurethane and a copolyester.6 Bhunia et al.7 have also reported the synthesis of a monomer known as glycidyl 3-pentadecenyl phenyl ether (GPPE) from cardanol, based on which a novel polyether was synthesized. Since cardanol is an important natural renewable resource containing phenolic groups and possesses fairly antioxidant properties, derivatives of cardanol have shown equal promise as antioxidants in the stabilization of gasoline.8 Rios Façanha et al.9 have studied the antioxidant properties of phosphorated cardanol on mineral oils NH10 and NH20. Menon et al.10 have reported that natural rubber (NR) modified with phosphorylated cardanol is superior to that obtained by diethyl hexyl phthalate plasticizer in terms of higher tensile properties, better flame retardancy, and resistance to thermooxidative decomposition. Also, phosphorylated cardanol has been proven to be an effective plasticizer for ethylene−propylene−diene rubber,11 polychloroprene and polybutadiene rubber,12 natural rubber/

1. INTRODUCTION The use of aromatic oils as plasticizers and process aids has been prevalent in the rubber industry for more than a century. However, their use has been restricted in recent years because of carcinogenic effects due to the presence of polycyclic aromatic hydrocarbons in the aromatic oils. Therefore, there is a need to substitute these hazardous aromatic oils. Moreover, polymers from renewable resources are becoming more popular and acceptable due to environmental concerns. Cardanol obtained by double vacuum distillation of cashew nut shell liquid (CNSL), an agricultural renewable resource and a byproduct of the cashew industry, is not only cheap but also abundantly available and biodegradable. Cardanol, chemically known as m-pentadecenylphenol, has a phenolic moiety and a long aliphatic side chain containing 15 carbon atoms which may be a saturated hydrocarbon, a monoene, a diene, or a triene (nonconjugated) as shown in Figure 1. These double bonds are the sites for its chemical reactivity, apart from the phenolic −OH group which accounts for its bifunctional character. Cardanol, and its derivatives, has a wide range of applications in the form of brake linings, surface coatings, paints, and varnishes because of its bifunctional moiety and high chemical reactivity. Recently it has been used in polymer and rubber industries as a multifunctional additive. Ravichandran et al.1 have reported the synthesis of polycardanol, which is a

Received: Revised: Accepted: Published:

Figure 1. Structure of cardanol. © 2013 American Chemical Society

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EPDM tercopolymer blends,13 and LLDPE/EVA copolymer blends.14 Greco et al.15,16 have also reported that cardanol acetate and epoxidated cardanol acetate, the esterified derivatives of cardanol, are efficient plasticizers for PVC. Natural rubber, a well-known renewable resource obtained from the tree popularly known as Hevea brasiliensis in the form of a milky white fluid, is a versatile material used mostly in the manufacture of tires. Because of its high unsaturation (each repeat unit contains one double bond in its structure), it is less resistant to oxidation, ozone, weathering, various chemicals, and solvents in comparison to other synthetic rubbers. Also, processability for a good surface finish and dimensional stability are poor. Hence, chemical modification of NR is essential to overcome some of its drawbacks. Graft copolymerization is one such technique used to modify natural rubber. Menon et al.17 have established that cardanol and its derivatives act as good plasticizers in NR. They have also established the multifunctional activity of cardanol in NR. It has been also reported by Menon et al.18 that cardanol and its derivatives incorporated into the rubber act as plasticizers, process aids, cure promoters, antioxidants and tackifiers. However, incorporation of cardanol and its phosphorylated derivatives is a tedious and timeconsuming process. Besides, it causes cure retardation due to absorption of activators by this additive and thus needs additional doses of ZnO to compensate for the loss.18 In order to overcome this problem, Vikram et al.19 have reported grafting of cardanol onto the NR backbone by a solution technique. The present study focuses on the grafting of cardanol onto NR in the latex stage, which in turn makes this process commercially more viable than the earlier methods cited in the literature.17−22 Moreover, the present study emphasizes the optimization of the grafting reaction conditions by an economic and viable experimental strategy based on Taguchi’s parameter design.23

aqueous solution of sodium dodecyl sulfate. The cardanol emulsion thus prepared was added to the natural rubber latex mixture, and then it was stirred for at least 1 h with nitrogen purged for 15−20 min. Then the initiator potassium persulfate was added. After 15 min of mixing, 10 wt % aqueous solution of sodium thiosulfate was added (the ratio of K2S2O8/ Na2S2O3 was 1:0.6). The reaction was carried out at the desired temperature, for the desired time, with constant stirring at 300 rpm. After the reaction was over, the cardanol grafted natural rubber latex was precipitated using dilute acetic acid, washed with distilled water at least six times, and dried under vacuum for a minimum period of 12 h. Then the dried coagulum was Soxhlet extracted with methanol to remove any unbound cardanol present in the rubber coagulum. Grafting parameters such as percentage grafting and grafting efficiency were calculated gravimetrically using eqs 1 and 2. percent grafting (PG %) =

weight of cardanol grafted ·100 weight of NR taken (1)

grafting efficiency (GE %) weight of cardanol grafted = ·100 weight of cardanol taken

(2)

2.4. High Performance Liquid Chromatography (HPLC). HPLC analysis of cardanol before and after purification was carried out with the help of an HPLC instrument (Agilient 1100) equipped with a variable wavelength detector. A Zorbax Eclipse XDB-C18 (4.6 × 150 mm i.d., 5 μm particle size) column was used. The mobile phase was acetonitrile/water/acetic acid (80:20:1) at a flow rate of 1.8 mL/min, and absorbance was monitored at 280 nm. 2.5. IR Spectroscopy. IR spectroscopy of natural rubber (NR) and cardanol grafted natural rubber (CGNR) was studied using a Fourier transform infrared (FTIR) spectrophotometer (Model Spectrum RX-I, PerkinElmer Life and Analytical Sciences, USA) in the range 700−4000 cm−1. The samples were dissolved in chloroform, and then a film was cast on the KBr disk. 2.6. NMR Spectroscopy. 1H NMR spectra of natural rubber (NR) and cardanol grafted natural rubber (CGNR) were recorded on a Bruker 400 MHz NMR Spectrometer using CDCl3 as solvent and tetramethylsilane as an internal standard. 2.7. Taguchi Method. The Taguchi method is a systematic approach to design and analyze experiments for improving the quality characteristics.23 It is highly effective in studying the effects of multiple factors on the performance characteristics. It also determines which factor has more influence and which has less. The Taguchi method drastically reduces the number of experiments that are required to model the response function compared with the full factorial design of experiments. Hence, it is a technique for designing and performing experiments to investigate processes where the output depends on many factors (variables or inputs) without resorting to all possible combinations of values which are tedious and uneconomical in the process. Figure 2 represents the major steps of implementing the Taguchi method.23 In the present work, the Taguchi method has been employed to study the effect of four control factors, viz., initiator concentration (A), cardanol concentration (B), reaction time (C), and reaction temperature (D), with each set at three different levels of 1, 2, and 3 as shown in Table 1. With four

2. EXPERIMENTAL SECTION 2.1. Materials. Natural rubber latex (60.02% dry rubber content) was supplied in kind by Rubber Board, Kottayam, India. Cardanol was procured from M/S Satya Cashew Chemicals Limited, Chennai, India. The initiator potassium persulfate (K2S2O8), sodium thiosulfate (Na2S2O3), and sodium dodecyl sulfate, the anionic surfactant, were obtained from EMerck. Other solvents and reagents from E-Merck were used directly without further purification. 2.2. Purification of Cardanol. Cardanol (50 g) was dissolved in methanol (160 mL), and ammonium hydroxide (25%, 100 mL) was added and stirred for 15 min. This solution was then extracted with hexane (4 × 100 mL). The organic layer was washed with 5% HCl (100 mL) followed by distilled water (100 mL). Activated charcoal (5 g) was added to the organic layer, stirred for 10 min, and filtered through Celite (15 g). The filtrate was dried over anhydrous sodium sulfate and concentrated to get pure cardanol.24 The purity of cardanol was confirmed by high performance liquid chromatography (HPLC). 2.3. Grafting of Cardanol onto Natural Rubber Latex. In a three-necked flask natural rubber latex (5 g, dry rubber content (DRC) 60.02%) was taken. Then 10 mL of 10 wt % potassium hydroxide solution and sodium dodecyl sulfate (1 phr) as an emulsifier was added followed by the stabilizer isopropyl alcohol, and the mixture was stirred. The cardanol was made into an emulsion by mixing mechanically with 10% 5952

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3. RESULTS AND DISCUSSION 3.1. High Performance Liquid Chromatography (HPLC). HPLC profiles of cardanol before purification (Figure 3a) and after purification (Figure 3b) reveal that there is an

Figure 2. Scheme of the major steps of implementing the Taguchi method.

Table 1. Control Factors and Levels levels code

control factors

1

2

3

A B C D

initiator concn (phr) cardanol concn (phr) reaction temp (°C) reaction time (h)

1 5 35 6

2 10 50 8

3 15 65 10

Figure 3. HPLC profiles of cardanol (a) before purification and (b) after purification.

absence of peaks in the cardanol profile after purification corresponding to cardol, methyl cardol, and other trace impurities after purification.24 This confirms the purity of the isolated cardanol. Moreover, the three peaks in the cardanol chromatogram after purification indicate that cardanol is composed of mainly these three components: 3-[8(Z),11(Z),14-pentadecatrienyl]phenol, 30.8%; 3-[8(Z),11(Z)pentadecadienyl]phenol, 20.3%; and 3-[8(Z)-pentadecenyl]phenol, 43.7%. 3.2. IR Spectroscopy. The FTIR spectrum of natural rubber (Figure 4a) shows some important absorption bands as

factors each with three levels, the full factorial design requires 34 = 81 runs or experiments. In the present study, with the help of the Taguchi method, the L9 orthogonal array was designed which involves only nine runs/experiments as shown in Table 2. The response variables chosen were percent grafting and grafting efficiency. Table 2. The L9 Orthogonal Array Layout run no.

initiator concn (phr)

cardanol concn (phr)

reaction temp (°C)

reaction time (h)

1 2 3 4 5 6 7 8 9

1 1 1 2 2 2 3 3 3

5 10 15 5 10 15 5 10 15

35 50 65 50 65 35 65 35 50

6 8 10 10 6 8 8 10 6

The Taguchi method employs a generic signal-to-noise (S/ N) ratio which measures the effect of noise factors on performance characteristics. A larger S/N ratio represents better quality characteristics and less variation. The S/N ratio characteristics may be divided into three categories: smaller is better, larger is better, and nominal is the best characteristic. In the present study, since both the response variables percent grafting and grafting efficiency are intended to be maximized, the larger-the-better target characteristic for the S/N ratio is chosen which has been calculated as follows:23 S/N = − 10 log

⎛ n ⎞ 1⎜ 1⎟ ∑ n ⎜⎝ i = 1 yi 2 ⎟⎠

Figure 4. IR spectra of (a) NR and (b) CGNR.

follows: 2960−2854 cm−1 (aliphatic C−H stretching), 1634 cm−1 (aliphatic CC stretching), 1448 and 1375 cm−1 (C−H bending vibration), 1260 cm−1 (C−C stretching), and 801 cm−1 (=C−H bending vibration). However, the FTIR spectrum of cardanol grafted natural rubber (Figure 4b) shows an additional peak at 3446 cm−1 which has been attributed to the −OH stretching vibration of the phenolic moieties present in

(3)

where yi is the observed data and n is the number of observations. 5953

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the cardanol. This infers that the double bonds present in the side chain of cardanol have taken part in the grafting reaction, leaving behind the intact phenolic moiety. 3.3. NMR Spectroscopy. The 1H NMR spectrum of NR (Figure 5a) shows a singlet resonance signal at 5.12 ppm

Figure 6. Mechanism of cardanol grafting onto NR backbone.

Table 3. Response Variables and Results for the S/N Ratios

Figure 5. 1H NMR spectrum of (a) NR, (b) cardanol, and (c) CGNR.

run no.

PG (%)

GE (%)

S/N ratio for PG

S/N ratio for GE

1 2 3 4 5 6 7 8 9

4.72 5.06 6.62 4.34 7.47 4.99 3.49 4.77 6.35

81.94 52.38 54.62 84.92 78.93 36.47 67.79 43.96 42.85

13.48 14.08 16.42 12.75 17.47 13.96 10.86 13.57 16.06

38.27 34.38 34.75 38.58 37.94 31.24 36.62 32.86 32.64

signal-to-noise (S/N) ratios for each series of experiments are also calculated using eq 3, and the results are presented in Table 3. The response of each factor to its individual level has been calculated by averaging the S/N ratios of all experiments at each level for each factor. In order to evaluate the influence of each factor on the yield, the S/N ratio for each factor should be computed. The S/N ratio for a single factor can be calculated by averaging the values of S/N ratios at different levels. For example, the mean S/N ratio for cardanol concentration at level 1 can be calculated by averaging the S/ N ratios for experiments 1, 4, and 7. The mean S/N ratio for every factor at different levels is calculated similarly. Figures 7 and 8 represent the effects of the four control factors on the percent grafting and grafting efficiency, respectively. Monitoring for the higher S/N ratios for the different levels of the

corresponding to the unsaturated methyne proton. The signal at 2.04 ppm may be due to the methylene protons, and the singlet resonance signal at 1.67 ppm may be due to the methyl protons. The spectrum of CGNR (Figure 5c) shows signals at 1.83, 2.17, and 5.12 ppm corresponding to the NR backbone. In addition, it shows a signal at 6.65−6.99 ppm corresponding to the aromatic protons due to the presence of phenolic moiety in cardanol that is absent in the 1H NMR spectrum of NR. This signal for aromatic protons is also seen in Figure 5b, which corresponds to the 1H NMR spectrum of cardanol. Thus it is confirmed that cardanol has been grafted onto NR. Moreover, a shift in peak positions toward higher δ values is observed in the CGNR spectrum which may be attributed to the space deshielding effect of the polar hydroxyl group present in the phenolic moiety of cardanol. Hence, it can be predicted that the unsaturation present in the cardanol must have taken part in the grafting reaction, leaving behind the intact phenolic moiety. Among the allylic protons present in the NR backbone, the most labile one is the −CH2− group in the fifth position in comparison with the −CH3 group because of the existence of the maximum number of resonance structures for the radical which forms upon loss of a hydrogen atom. Hence, the preferred grafting site at the backbone NR is the carbon which is attached to the most labile hydrogen atom (fifth position). The probable mechanism of the grafting of cardanol onto NR and the structure of CGNR are given in Figure 6. 3.4. Optimization of the Reaction Conditions Using the Taguchi Method. The response variables such as percent grafting (PG) and grafting efficiency (GE) are calculated by using eqs 1 and 2, and the results are shown in Table 3. The

Figure 7. Effects of control factors on percent grafting. 5954

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due to all four factors are shown in Figure 9. It can be seen that the control factors such as cardanol concentration and reaction

Figure 8. Effects of control factors on grafting efficiency. Figure 9. Percentage contributions of the control factors to percent grafting and grafting efficiency.

control factors, it can be concluded that the factor combination of A2, B3, C3, and D1 gives the maximum percent grafting while for the maximum grafting efficiency the combinations are A2, B1, C3, and D1. 3.5. Analysis of Variance (ANOVA) Results. The analysis of variance (ANOVA) was performed to evaluate the influence and relative importance of the four control factors on the grafting reaction. Following the analysis, it becomes increasingly easier to identify the effectiveness of the control factors on the percent grafting and grafting efficiency. ANOVA has been established based on the sum of squares (SS), the degree of freedom (D), the variance (V), and the percentage of the contribution to the total variation (P) which can be calculated as follows.25,26 The total sum of squares SST can be calculated as m

SST =

time stand as the most significant factors for both the percent grafting and the grafting efficiency. Cardanol concentration is found to have the dominating effect on the yield with the high percentage contributions of 46.37% on percent grafting and 63.0% on grafting efficiency. Considering the response variable percent grafting, it is seen that cardanol concentration and reaction time have major effects while initiator concentration has the least effect with 7.26% contribution to the overall effect. Therefore, the order of the effect of control factors on percent grafting is cardanol concentration, reaction time, reaction temperature, and initiator concentration. While considering the grafting efficiency, it is observed that cardanol concentration has the highest influence with a contribution of up to 63.0% whereas the reaction temperature has the least influence with only 9.36% contribution to the overall effect. Both initiator concentration and reaction time have a nominal effect on the grafting efficiency. 3.6. Multiple Linear Regression Analysis. A correlation has been established between the input control factors and the yield such as the percent grafting and grafting efficiency by using multiple linear regression analysis. Linear regression is performed with the help of Minitab 15 software. The regression equation for percent grafting is obtained as follows:

m

∑ ni 2 − i

1 [∑ ni]2 m i=1

(4)

where m is the total number of the experiments and ni is the S/ N ratio at the ith test. t

SSp =

∑ j=1

(Snj)2 t

m

−

1 (∑ ni) m i=1

(5)

where SSp denotes the sum of squares for the tested factors, p represents one of the tested factors, j is the level number of this specific factor p, t is the repetition of each level of the factor p, and Snj is the sum of the S/N ratios involving this factor and level j. Vp (%) =

SSp Dp

percent grafting (%) = 4.26 − 0.298A + 0.18B + 0.034C − 0.234D

The regression equation for grafting efficiency is obtained as follows:

·100 (6)

grafting efficiency (%)

where Vp is the variance of the tested factors and Dp is the degree of freedom for each factor. SSp′ = SSp − DpVe Pp (%) =

SSp′ SST

(9)

·100

= 97.3 − 5.72A − 3.36B + 0.43C − 1.68D

(10)

where A = initiator concentration (phr), B = cardanol concentration (phr), C = reaction temperature (°C), and D = reaction time (h). The positive values of the coefficients suggest that the percent grafting and grafting efficiency increase with their associated variables, whereas the negative values of the coefficients suggest that the percent grafting and grafting efficiency decrease with an increase in the associated variables. Thus it can be inferred from eq 9 that percent grafting increases with increase in both cardanol concentration and reaction temperature as the coefficients are positive. This is also in agreement with the plot of the main effects (Figure 7) derived

(7)

(8)

where Pp is the percentage of the contribution of each individual factor to the total variation. The analysis of variance (ANOVA) for the response variables, viz., percent grafting and grafting efficiency, has been carried out. (The results are shown in the Supporting Information in Tables S1 and S2, respectively, for percent grafting and grafting efficiency.) The percentage contributions 5955

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4. CONCLUSIONS The following conclusions may be drawn from the aforesaid work. Grafting of cardanol onto natural rubber in the latex stage has been carried out successfully using the redox initiator system potassium persulfate/sodium thiosulfate. The Taguchi method provides a simple, systematic, and efficient tool to evaluate the effect of four different control factors on the response variables percent grafting and grafting efficiency. The optimum combination of the parameters are found to be initiator concentration 2 phr, cardanol concentration 10 phr, reaction temperature 65 °C, and reaction time 6 h, considering both percent grafting and grafting efficiency. The percent grafting found is to be 7.47% and the grafting efficiency is 78.93% for the optimum parameter combination. The analysis of variance technique provides the percent contribution of different control factors on percent grafting and grafting efficiency. Cardanol concentration is observed to have the highest effect on the yield contributing to the extent of 46.37% on the percent grafting and 63.0% on the grafting efficiency. The experimental results of percent grafting and grafting efficiency are found to be in good agreement with the predicted values as derived from regression analysis.

from the S/N ratios. In the case of the grafting efficiency, it may be inferred from eq 10 that only with an increase in reaction temperature there is an increase in the grafting efficiency. This has been attributed to the fact that with an increase in the reaction temperature free radical generation is facilitated, leading to an increase in the grafting efficiency. 3.7. Confirmatory Test. The final step in the Taguchi method is to confirm the experimental results. Once all the control factors are optimized, the confirmatory tests are performed at the optimum level of each of the control factors for the grafting reaction as shown in Table 4. The tests are Table 4. Set of Control Factors for the Confirmation Tests test

initiator concn (phr)

cardanol concn (phr)

reaction temp (°C)

reaction time (h)

1 2

2 2

5 15

65 65

6 6

performed in duplicates, and the results are recorded in Table 5 for both the experimental results and the theoretical values calculated using the regression eqs 9 and 10 for percent grafting and grafting efficiency, respectively.

■

Table 5. Confirmation Tests and Their Comparison with Regression Model percent grafting (%)

ANOVA results are given in Tables S1 and S2 for percent grafting and grafting efficiency, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

grafting efficiency (%)

test

exptl

pred

% error

exptl

pred

% error

1 2

5.32 7.71

5.37 7.17

0.93 7.53

85.61 51.86

86.93 53.33

1.51 2.75

ASSOCIATED CONTENT

S Supporting Information *

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-3222-283194/ 282292. Fax: +91-3222-282292/255303.

Thus from Table 5 it can be inferred that the error in the calculations varies from 0.93 to 7.53% for percent grafting and from 1.51 to 2.75% for grafting efficiency. This concludes that the multiple regression equations derived (eqs 9 and 10) correlate the evaluation of percent grafting and grafting efficiency to a reasonable degree of approximation. Moreover, it is observed (from Table 5) that, for cardanol concentration of 5 phr, percent grafting is less but grafting efficiency is more. However, when the cardanol concentration is 15 phr, percent grafting is more and grafting efficiency is lower. Hence, considering both percent grafting and grafting efficiency, the cardanol concentration is taken as 10 phr with all other control factors remaining the same and the experiment is performed (which is with the same combination of parameters as run 5 in Table 3). In this case, the percent grafting is found to be 7.47% and the grafting efficiency is 78.93%, which shows that the optimum parameter combination shall be A2, B3, C3, and D1 considering both percent grafting and grafting efficiency together. It is seen that, with an increase in cardanol concentration from 10 to 15 phr, there is not much increase in percent grafting; on the other hand, the grafting efficiency decreases significantly. Percent grafting is related to the weight of cardanol grafted onto the natural rubber with respect to the initial weight of natural rubber, while grafting efficiency is related to the weight of cardanol grafted onto the natural rubber with respect to the initial weight of cardanol. Hence, it is observed that higher doses of cardanol concentration will not necessarily result in an increase in percent grafting while grafting efficiency falls, signifying that unreacted cardanol remains in the polymer system, unutilized.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the award of individual Senior Research Fellowship.

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REFERENCES

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dx.doi.org/10.1021/ie400195v | Ind. Eng. Chem. Res. 2013, 52, 5951−5957