Insights into the Silanization Processes of Silica with and without Acid

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Insights into the Silanization Processes of Silica with and without Acid-base Additives via TG-FTIR and Kinetic Simulation Jiwen Liu, Cong Li, Chong Sun, and Shugao Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04866 • Publication Date (Web): 16 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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Industrial & Engineering Chemistry Research

Insights into the Silanization Processes of Silica with and without Acid-base Additives via TG-FTIR and Kinetic Simulation Jiwen Liu,† Cong Li,‡ Chong Sun,† Shugao Zhao†, * †

Key Laboratory of Rubber-plastics, Ministry of Education/Shandong Provincial Key Laboratory

of Rubber-plastics, Qingdao University of Science ＆ Technology, 53 Zhengzhou road, Qingdao 266042, China ‡

Bio-Materials and Technology Lab, Kansas State University, 1980 Kimball Ave, BIVAP

Innovation Center, Manhattan, Kansas 66506, USA

ACS Paragon Plus Environment

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ABSTRACT: The silanization of silica with and without acid-base additives were investigated via TG-FTIR (Thermogravimetry coupled with Fourier transform infrared) technology and reaction kinetic simulation. The results showed that two parallel reactions were included in the silanization process of silica: one is the hydrolysis of ethoxy group of TESPT followed by the condensation with the silanol of silica (the activity energy E＝57kJ/mol); and the other is the direct condensation between the silanol of silica and the ethoxy group of TESPT (E ＝ 90.4kJ/mol). The hydrolysis of ethoxy group is much easier in comparison with the direct condensation with the silanol. The silanization was effectively promoted by means of adding stearic acid (HST) or diphenyl guanidine (D), and the reaction rate increased with the increase of HST or D in a reasonable range. HST promoted the condensation between ethoxy group and silanol (E ＝ 69.5kJ/mol), without affecting the hydrolysis of ethoxy group. However, D promoted both of the two reactions obviously (E＝59.5kJ/mol and 70.8kJ/mol, respectively). When HST and D coexisted in the system, the condensation between ethoxy group and silanol was promoted more obviously by the acid-base complex (E＝53.5kJ/mol). The two-step parallel reaction model was used to simulate the silanization processes and good fitting results were achieved.


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1. INTRODUCTION It is well known that silane coupling agents are widely used in many industrial fields as a class of important functional additives to improve the binding capacity of the interface between the materials1-3. Especially in rubber industry, green tires with excellent wet skid resistance, low rolling resistance and wear resistance can be achieved with the aid of silane coupling agents 4-7. Generally, this perfect performance is attributed to the good dispersion of silica particles and the filler-polymer interaction8-10. In other words, it is dependent on the effect of silanization between the silica and silane. Consequently, the study of silanization reaction process became a hot topic in the last decades 11-19. For the traditional TESPT-silica system widely used in the tire industry, the most representative silanization reaction mechanism is the two-step reaction model proposed by Hunsche according to the results of the solid-state NMR11, 13: (1) the TESPT molecular is grafted on the surface of silica; (2) the adjacent molecules further condense into shell layer. The first step is called the effective silanization, and the second step is called the non-effective silanization. In each step, it consists of two parallel competing reactions, i.e., the direct condensation between ethoxy group and silanol, and the ethoxy group is first hydrolyzed, followed by condensation with the silanol. This model is accepted and referenced by numerous researchers in the rubber and elastomer field. However, this model did not investigate the contribution of the two parallel reactions to the whole silanization process.


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It is difficult to find reports about the two parallel reactions in the silica silanization process. Recently, Franck Vilmin and his co-workers characterized the silanization process of silica and TESPT system, and described the change of several products during the sialnization reaction process by in situ infrared spectroscopy combined with chemometric analysis17. However, they think that the hydrolyzed TESPT tends to condense with the adjacent TESPT molecules rather than graft onto the silica surface. In this case, the silanization caused by the hydrolysis reaction, should mainly be the non-effective silanization. Therefore, it is still necessary to investigate the two parallel reactions further. In the mixing process, in addition to the main components, like rubber, silica and coupling agent, the effect of some additives on the silanization reaction can not be ignored. Especially for the common acid-base additives, they may even play a crucial role in the process of silanization. There are extensive research reports about the promotion of acid and basic catalysts on the hydrolysis reaction and the condensation reaction of alkoxysilanes in recent years20-27. The acid and base catalysts are mostly protonic acid and alkali, acetic acid, ethylamine, triethylamine and so on, and the research system was mostly carried out in solution. In the reports on the silanization of silica, some researchers expect that the silane can quickly and efficiently coat on the silica surface in solvent-free conditions, the organic amines and ionic liquids are also used to promote the silanization of silica28-31. The HST and D are the most commonly used vulcanization additives in the rubber industry. However, very few systematic studies are conducted to clarify the effect of these acid-base curing additives on the silanization of silica. If we can reveal the impact on the silanization of silica and their mechanisms, a better silanization effect may be achieved according to the rational use of these additives in the rubber mixing process.


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Combination of TG and FTIR is one of the most developed analysis technologies. It has been widely used in qualitative and quantitative analyses, pyrolysis reaction mechanism and reaction kinetics studies 32-38. This work first reports the investigation of the silanization of silica via a combination of on-line TG and FTIR technique. The reaction process and mechanism of the two kinds of silanization reaction modes were distinguished through the ethanol gas trace analysis and the reaction kinetic simulation. Most reported silanization mechanisms of silica and reaction kinetics by now are carried out in solution system18, 39-41. This is different from the practical rubber mixing process. Consequently, the solid phase mixtures were used as the research subject in this work. HST and D, the commonly used additives in silica filled rubber system, were selected and the effects of them on the silanization process as well as their promotion mechanism were deeply investigated in this paper. 2. EXPERIMENTAL SECTION Raw Material. High dispersive silica, Zeosil 1165MP, with a specific surface area of 165 m2/g, was produced by Solvay (Rhodia) Co. Ltd.. The coupling agent TESPT was produced by Evonik Degussa Co. Ltd. The stearic acid, diphenyl guanidine and the solvents such as cyclohexane (AR) and tetrahydrofuran (LC) were all supplied by Sigma-Aldrich (Shanghai) Trading Co., Ltd. Sample Preparation. Hydrated samples: TESPT (8 wt %) and variantious weight ratios of additives were added into the silica matrix as shown in Table 1. All the components were fully mixed and ground in an agate mortar, and then used for the TG-FTIR measurement immediately.


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Table 1. The components of the hydrated samples (mg) N.O

1

2

3

4

5

6

7

8

9

10

11

Silica

50

50

50

50

50

50

50

50

50

50

50

TESPT

4

4

4

4

4

4

4

4

4

4

4

TTT HST

-

1

2

4

-

-

-

-

1

2

4

D

-

-

-

-

0.5

1

2

4

2

2

2

*The components of the dehydrated samples are the same as No. 1, 2, and 7 Dehydrated samples: TESPT solution was prepared in a concentration of 10 mg / ml with cyclohexane as solvent, and the content of other additives (if any) were also listed in Table 1. 12.5mg silica was weighed accurately and added into the alumina crucible for TG testing, and then the TG measurement was carried out from room temperature to 400℃ with a heating rate of 20℃/min in N2 condition to get rid of the absorbed water on the silica surface. Sample was then taken out when the TG furnace was cooled down to about 30℃, and the TESPT solution (0.1ml) was dropped into the crucible as quickly as possible. TG-FTIR measurement started when the solvent was completely volatilized in the TG furnace. Measurement and Data Analysis. TG-FTIR measurement: TG209F1 was supplied by NETZSCH instrument company, Germany, and VERTEX70 FTIR was supplied by BRUKER instrument company, Germany. TG measurement was conducted as follows: Nitrogen was used as the purge gas with a flow rate of 20 ml / min, and the heating rates of 5, 10, 15, 20 k/min were


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selected respectively in a temperature range from 30℃ to 400℃. The FTIR measurement was conducted as follows: the MCT detector with high scanning speed (20kHz) and sensitivity (Signal to noise ratio is better than 10000:1) was used and the single spectrum scanned 8 times with a resolution of 4cm-1. TG-FTIR spectra for each sample were obtained for the reaction kinetic analysis. GPC measurement: Gel permeation chromatography was supplied by TOSH Company, Japan. The 20mg silica/TESPT compounds were heated to 150℃ and 170℃ respectively, and then 2 ml tetrahydrofuran was used to extract the reaction product for GPC measurement. The mobile phase was tetrahydrofuran and the injection volume was 20μ l. FTIR measurement: FTIR spectra of HST, D and their blend were measured respectively using a KBr tablet by the VERTEX70 FTIR spectrometer. DTGS detector was used and the single spectrum scanned 32 times with a resolution of 4cm-1. DSC measurement: DSC204F1 was supplied by NETZSCH Company, Germany. Differential scanning calorimetry measurement was conducted via a heating rate of 10k/min from 30℃ to 160℃ under a nitrogen gas protection with a flow rate of 20 ml / min. 3. RESULTS AND DISCUSSION The Hydrated Silica/TESPT System. On-line TG-FTIR spectrum of the hydrated silica/TESPT sample is shown in Fig. 1(a), and two groups of peak absorptions can be recognized with the increase of time, indicating that two main products are released in this process.


7


5

Absorbance Units 0.000 0.002 0.004 0.006

H2O

3

4 1053

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3 2 1 0

CH3CH2OH

50

3500

3000 2500 2000 1500 Wavenumber cm-1

C:\Documents and Settings\liujiwen\桌面\CH3CH2OH-1.0 C:\Documents and Settings\liujiwen\桌面\H2O-2.0

100

150

200

250

300

350

o

Temperature/ C

10.450 | NETZSCH TG 209 F1 | E:\ngbwin\data5\MINE\ 课题\120801\Sil-Si-20K-1.dt3 01/08/2012

Sil+Si

Sil+Si

1000

10.450 | NETZSCH TG 209 F1 | E:\ngbwin\data5\MINE\ 课题\120801\Sil-Si-20K-1.dt3 01/08/2012

Page 1/1

(a)

(b)

(c)

Figure 1. TG-FTIR spectrum of hydrated Silica/TESPT system (a), the extracted FTIR spectrum (b) and ethanol trace (c) Figure 1 (b) is the extracted spectra from these two groups of peaks, showing the infrared spectra of water and ethanol gas, respectively. Water was released first, followed by ethanol gas. The water signal was derived from the absorbed water on the silica surface, and the ethanol signal was derived from the product of silanization. The ethanol trace is extracted from the characteristic absorbance of ethanol gas at 1053cm-1, which is not overlapped with the peak of water (Figure 1 (c)). This trace reflects the releasing process of ethanol with the increase of temperature, i.e., the silanization reaction process. For the purpose of understanding the contribution of hydrolysis and direct condensation to the ethanol releasing process, dehydrated sample was investigated at the same condition in comparison with the hydrated counterpart. Figure 2 is the TG curves and ethanol traces of hydrated and dehydrated samples (TG-FTIR spectra of all the studied systems in this paper see the supporting information Figure S1-Figure S9).


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Hydrated Silica/TESPT Dehydrated Silica/TESPT

102

5

100

4

98

3

96

2

94

3

1

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TG/%

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92 0 90 50

100

150 200 250 o Temperature/ C

300

350

Figure 2. The silanization reaction of the hydrated and dehydrated silica/TESPT systems (solid lines: TG curve; hollow symbols: ethanol trace) There are two steps for the weight loss of the hydrated sample: the first-step weight loss is water volatilization, and the second-step weight loss is mainly derived from the ethanol removal at ca. 250℃. For the dehydrated sample, there is no weight loss due to the water volatilization, and there is only one weight loss step at ca. 200℃, which is not corresponded to the ethanol trace. In fact, this weight loss is given rise to the volatilization of excessive TESPT molecular rather than the ethanol removal. It indicates that the silanization reaction degree decreases obviously for the dehydrated sample, according to the volatilization of TESPT. It is easier to see from the comparison of ethanol trace that the reaction degree is much higher and the beginning of reaction is much earlier for the hydrated sample. The peak temperature of ethanol release is at 250℃ for these two samples, which confirms that the peak is derived from the condensation reaction between silanol and TESPT. However, the reaction is very slow below 150℃, as evidenced by the ethanol trace of the dehydrated sample. Therefore, it is much easier to achieve hydrolysis than direct condensation for the binary blending system, and the grafting reaction after hydrolysis should be important to the effective silanization. The role of water on the effective


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silanization of silica was also confirmed by some previous research results19, 42-45. Even so, as shown in Figure 2, it takes at least 16 minutes for the system to achieve complete silanization at a heating rate of 20°C /min, that is, the temperature should rise to at least 350°C.

500 Raw TESPT o Extracts after heated to150 C o Extracts after heated to170 C

400 300

A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 100 0 0

500

1000

1500

2000

Molecular weight

Figure 3. The molecular weight distribution curves of TESPT and the products extracted from the heated samples In order to further confirm that TESPT tends to self-condense or graft onto the silica after hydrolysis, the hydrated sample is heated to 150℃ and 170℃ respectively, and then is extracted by THF to detect the molecular weight. The molecular weight differential distribution curves of the extracted products and the raw TESPT are showed in figure 3. As the TESPT curve shows, there is a peak at 550g/mol, corresponding to the TESPT monomer. Moreover, there is a weak peak at ca. 1100g/mol, indicating that there is a small amount of dimer molecules. There are also TESPT monomer peaks at 550g/mol for the extracted samples being heated to 150 and 170 ℃ respectively and the relative content of monomer molecule decreases with the increase of reaction temperature. It was noted that there was no obvious TESPT dimer or multimer peaks on the curves of the extracted products. According to the experimental result of Franck Vilmin and


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his co-workers17, the hydroxyl arising from the TESPT hydrolysis product tends to disappear when the temperature is higher than 170℃. Therefore, it is grafted onto the surface of silica rather than self- condensation after hydrolysis for the TESPT. In the whole silanization process, no matter the graft reaction after hydrolysis or the direct condensation, the released product is ethanol. Therefore, we can reveal the two parallel reaction processes and their mechanism through the kinetic simulation of ethanol release trace. The basic kinetic equation can be used to express the reaction rate for each single step reaction, as shown in Equation (1) ，where α represents the conversion rate of reaction, and β represents the heating rate. The purpose of kinetic simulation is to obtain the kinetic parameters of each step reaction, such as the activation energy E, pre-exponential factor A and the conversion rate function f (α) which is related to the reaction order n46-51.

d   A   RTE    e f ( ) dT   

（1）

The different calculating methods of kinetic parameters can be obtained based on the basic equation. The classical kinetic equations are the differential and integral expressions, which are proposed respectively by Friedman (expression 2) and Ozawa-Flynn-Wall (expression 3). The activation energy E at different conversion rate can be calculated by means of multiple heating rates method.

 d    ln     ln  f ()  E / RT  dT  

（2）

ln   ln  AE / RG      5.3305  1.0516 E / RT


（3）

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Because of the linear relationship between the logarithm of heating rate and the reciprocal of temperature in expression 3, the activation energy (E) at different conversion rate can be obtained by means of the slope of the line, and the pre-exponential factor (A) is estimated according to the assumed first order reaction. On this basis, the multivariate nonlinear regression analysis is used to do kinetic simulation, and the accurate reaction model and kinetic parameters can be obtained30, 31, 35, 52-54. In this paper, the kinetic simulation was performed by NETZSCH thermo kinetics 3, the software includes 77 kinds of reaction model from one step to six steps reaction. In each step, there are 21 kinds of reaction type to be chosen.

log Heating rate/(K/min) 0.98 1.4

E/(kJ/mol) 110

0.02

log(A/s^-1)

Absorbance/(10^-4*mg) 4.0

100

6.5

1.2

3.0

1 2

A

20.0 K/min 15.0 K/min 10.0 K/min 5.0 K/min

B B

90 5.5

1.0

80 4.5

70

0.8

60

3.5

0.6 1.8

2.0

2.2 1000 K/T

(a)

2.4

2.6

0.1

0.3 0.5 0.7 Fractional Reaction

2.0

1.0

0

0.9

(b)

100

150

200 250 Temperature/℃

300

350

(c)

Figure 4. The silanization reaction kinetic simulation results of the hydrated silica/TESPT system, (a) relationship of lnβ ~1/T at each equal conversion rate, (b) the E and A values at different conversion rate, (c) the experimental curves (point) and fitting curves (line) of the ethanol trace The kinetic simulation results of ethanol trace for the binary phase blend system are shown in figure 4 (detailed simulation process see the supporting information Page S11-Page S19). There is a nice linear relationship between the logarithm of heating rate and the reciprocal of temperature at each equal conversion rate point for the four heating rates (figure 4 (a)). The E


12

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values at different conversion rates and the A values, which are estimated according to the simple first order reaction, are shown in figure 4 (b). The activation energy increases with the increase of fractional reaction, which can be divided into two stages according to the variation of slope: the first stage of the activation energy is ca. 60KJ/mol, and the second stage of the activation energy is ca. 80KJ/mol. These values are used as the initial parameters for the following model fitting. The four different heating rates curves are fitted by means of two-step parallel reaction model and simple order reaction type, and the results are shown in figure 4 (c). The fitting curves are almost the same as the experimental curves with a correlation coefficient of 0.997. The fitting results of kinetic parameters of two-step parallel reaction are shown in table S1 (see the support information). The quantity of reaction active site is reflected by pre-exponential factor A, which is positive proposal to the reaction rate. The value of E indicates the degree of difficulty of the reaction, and from this point of view, the hydrolysis of TESPT is much easier than the direct condensation between TESPT and silanol of the silica. However, the A value of hydrolysis is much lower, which indicates the active site is much less and the reaction rate is not fast enough although it is easy to react. The hydrolysis reaction order is too high according to the fitting result, which is probably related to the complexity of TESPT hydrolysis itself. Because three ethoxy groups are connected with each Si atom in TESPT molecular, there is a multi-step hydrolysis process. The hydrolysis process may be the superposition of multi-step hydrolysis reaction. The direct condensation reaction shows a quasi-second order reaction, however, it requires high temperature to realize the fast reaction speed because of the high reaction activation energy.


13


In the silanization process, the formation of a silane monolayer with complete coverage on the surface of silica is our desired goal. The effective coating of silane not only ensures the good dispersion of silica in the rubber matrix, but also plays an important role in improving the enhancement effect of silica. The results of silica and TESPT binary blending system show that no obvious self-condensation tendency of silane molecules was found and the silane mainly was grafted on the surface of silica. However, during the linear heating test, the temperature required to achieve complete silanization is very high and the time taken is relatively long. Therefore, it is necessary to promote the silanization of silica. The Hydrated Silica/TESPT/HST System. In the practical mixing process, the silanization reaction process is affected by other additives, especially for the acid-base additives, such as HST and D in the silica filled system. It is imperative to investigate the effect of these additives on the silanization reaction.

HST-0 HST-2% HST-4% HST-8%

5

-3

4

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3 2 1 0 50

100

150

200

250

300

350

o

Temperature/ C

Figure 5. The effect of HST content on the silanization of hydrated Silica/TESPT/HST System


14

The effect of HST on silanization reaction of the hydrated silica/TESPT system is shown in figure 5. There is an obvious ethanol release peak at ca. 150℃. The peak intensity increases with the increase of HST. However, the position of the peak at ca. 250℃ does not change obviously, while the peak intensity decreases with the increase of HST, which corresponds to the ethanol release peak arising from the direct condensation between TESPT and silanol. In order to clarify the mechanism of the peak at 150℃, the dehydrated sample is also investigated for this system. The comparison of the TG results of the hydrated and dehydrated samples (figure 6) indicates that there is no water weight loss for the dehydrated sample. Therefore, the release of ethanol is only related to the direct condensation between TESPT and silica. It also can be recognized that the peak at 150℃ still exists from the ethanol trace, even no water existing. The two ethanol release peaks can be distinguished more obviously without the contribution of TESPT hydrolysis. It indicates that this peak is not related to the TESPT hydrolysis, but to the direct HST promoted condensation between ethoxy group and silanol. Under the same test conditions as the silica/TESPT system, it takes about 14 minutes for the hydrated silica/TESPT/HST system to achieve complete silanization .

102

Hydrated Silica/TESPT/HST Dehydrated Silica/TESPT/HST

100

5 4

98

3

96

2

94

3

1

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TG/%

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92 0 90 50

100

150 200 250 o Temperature/ C

300

350


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6. The silanization reaction of hydrated and dehydrated silica/TESPT/HST systems (solid lines: TG curve; hollow symbols: ethanol trace) The kinetic simulation of the reaction process for this system was carried out, in which the content of HST was normal, i.e., 2% of the silica content, and the heating rate was 10, 15, 20k/min respectively. It also shows a good linear relationship between the logarithm of heating rate and the reciprocal of temperature at each equal conversion rate point for these three heating rates (figure 7 (a)). The E and A values, which are calculated according to the slope of each line, are shown in figure 7 (b). The activation energy can be divided into two stages. The first stage of the activation energy is ca. 70KJ/mol, and the second stage of the activation energy is ca. 85KJ/mol.


E/(kJ/mol)

log(A/s^-1) 7.6

0.02

Absorbance/(10^-4*mg)

110 7.2 100

1.25

1 2

A

20.0 K/min 15.0 K/min 10.0 K/min

3.0

B B

6.8 90 1.15

6.4

2.0

80 6.0 1.05

70

1.0

5.6 60 0.95

5.2 1.8

2.0

2.2 1000 K/T

(a)

2.4

2.6

0.1


0.9

(b)

0 50

100

150 200 250 Temperature/℃

300

350

(c)

Figure 7. The silanization reaction kinetic simulation results of the hydrated silica/TESPT/HST system, (a) relationship of lnβ ~1/T at each equal conversion rate, (b) the E and A values at different conversion rate, (c) the experiment curves (point) and fitting curves (line) of the ethanol trace In this system, in addition to the two direct condensation reactions promoted or non-promoted by HST, the TESPT hydrolysis should not be ignored. However, it is overlapped with the direct


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condensation promoted by HST. There is no more parallel reaction model to be chosen, and it is considered that the rate of parallel reaction 1 is mainly controlled by the direct condensation promoted by HST. Therefore, two-step parallel reaction model is still used for this system, and the fitting correlation coefficient is 0.997. The fitting parameters are listed in table S2. The two parallel reactions both show a normal norder reaction. Although the HST mainly promotes the condensation between the silanol and ethoxy group in the parallel reaction 1, it is certain that it inescapably contains the contribution of TESPT hydrolysis reaction by itself. Therefore, the corresponding reaction order is also very high. The activation energy is 69.5kJ/mol，lower than that of the parallel reaction 2 not promoted by HST. The kinetic parameters of the parallel reaction 2 are almost the same as the silica and TESPT binary system. Based on the results, it is efficient to enhance the degree of silanization reaction in the normal mixing conditions by adding HST in the system. The Hydrated Silica/TESPT/D System. It has been proved by several research results that the rate of silanization reaction can be enhanced obviously by adding D in the mixing process. In order to understand the promotion mechanism, the silanization reaction process of silica/TESPT/D three-phase blend system was investigated. The effect of D content on silanization reaction is shown in figure 8. There are two peaks in the silanization reaction process, same as the HST system. However, compared to the previous two systems, these two peaks appear at lower temperatures. Furthermore, the position of the first peak moves to a lower temperature, while the second peak keeps the same position with the increase of D content. The reaction rate increases initially with the increase of D content, and then decreases when the D content is too much.


17


8

D-0 D-1% D-2% D-4% D-8%

7 3

6

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4 3 2 1 0 50

100

150

200

250

300

350

o

Temperature/ C

Figure 8. The effect of D content on the silanization reaction of hydrated Silica/TESPT/D System Hydrated and dehydrated samples are compared in order to understand the attribution of the two peaks after adding D in the system. As shown in figure 9, there is no water weight loss on the TG curve for the dehydrated sample, therefore, the silanization reaction process is only related to the direct condensation reaction. Only one peak can be found at ca. 200℃ on the ethanol trace curve for the dehydrated sample, which corresponds to the second peak of the ethanol trace for the hydrated sample. This indicates that the first peak of the hydrated sample is derived from TESPT hydrolysis, and the second peak is derived from the condensation between silanol and ethoxy group. Compared to the silica/TESPT binary system, the D promotes not only the TESPT hydrolysis reaction, but also the direct condensation between silanol and ethoxy group, in which the peak temperature decreases from 250℃ to 200℃. Compared with the previous two systems, the hydrated silica/TESPT/D system can achieve complete silanization in less time of about 12 minutes.


18

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TG/%

102 100

Hydrated Silica/TESPT/D 8 Dehydrated Silica/TESPT/D

98

6

Absorbance*10

96 4 94 2

92

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

0

88 50

100

150

200

250

300

350

o

Temperature/ C

Figure 9. The silanization reaction of hydrated and dehydrated silica/TESPT/D systems (solid lines: TG curve; hollow symbols: ethanol trace) The silanization process consists of two parallel reactions, i.e., the hydrolysis and direct condensation, and both of them are catalyzed by D for the three-phase blend system. Figure 10 shows the kinetic simulation results of the silanization reaction at four different heating rates.

log Heating rate/(K/min) 1.4

E/(kJ/mol)

0.98

0.02

1.2

1.0

0.8

log(A/s^-1)

74

6.4

72

6.2

70

6.0

68

5.8

66

5.6

64

5.4

62

5.2

1.8

2.0

2.2

2.4 1000 K/T

(a)

2.6

2.8

A

1 2

50

100

B B

20.0 K/min 15.0 K/min 10.0 K/min 5.0 K/min

6

60

0.6

Absorbance/(10^-4*mg)

5.0 0.1


0.9

(b)

4

2

0 150 200 Temperature/℃

250

300

(c)

Figure 10. The silanization reaction kinetic simulation results of the hydrated silica/TESPT/D system, (a) relationship of lnβ ~1/T at each equal conversion rate, (b) the E and A values at different conversion rate, (c) the experiment curves (point) and fitting curves (line) of the ethanol trace


19


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Page 20 of 35

The activation energies are calculated by the slopes of lines shown in figure 10(a), decreasing with the increase of conversion rate in the initial reaction stage (figure 10(b)).The activation energies of the two parallel reactions are ca. 60-70kJ/mol and ca. 70kJ/mol, respectively. The fitting results are consistent with the experimental results very well with a correlation coefficient up to 0.999. The fitting results listed in table S3 accords with the autocatalytic reaction characteristic for the D promoted TESPT hydrolysis. This is probably due to the high melting point of D itself. At low temperature, few D molecules act as the catalyst, but with the increase of temperature, more D molecules achieve the catalytic effect. Because of multiple-step hydrolysis, the reaction order is also high. Compared to the TESPT hydrolysis itself, the activation energy of hydrolysis catalyzed by D does not decrease, however, the reaction rate increases. Therefore, it is speculated that the D is effective to reduce the activation energy of the reaction rate-controlled step. Similar to the HST, D also decreases the activation energy of the condensation between silanol and ethoxy group, but the effect of D on the hydrolysis reaction rate is more obvious. The Hydrated Silica/TESPT/HST/D System. The effect of individual agent on silanization reaction has been studied above. Considering the coexistence of all additives in recipes, the effect of HST/ D on silanization reaction needs to be further studied. The silanization reaction process is investigated with the increase of HST content, while the D content keeps constant in this case. As shown in Figure 11, the silanization reaction process also exhibits a bimodal under the normal HST / D ratio of 1: 2. The origin of the first peak is due to the promoted hydrolysis reaction, which is the same as the case when D is used alone. However, the origin of the second condensation peak between silanol and TESPT is not the same as the case when D or HST is used alone. Because of the strong peak, it does not look like a simple superposition of peaks


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derived simply from D or HST alone. Moreover, the condensation peak between silanol and TESPT moves to lower temperature with the increase of HST, and is overlapped with the hydrolysis peak at last. It seems that the change of condensation peak is related to the transformation degree of some more efficient catalyst. As can be seen from Figure 11, the system is fully silanized within 11 minutes. The silanization has been completed before the temperature rise to 250°C.

14 HST/D (1:2) HST/D (1:1) HST/D (2:1)

12

3

10

Absorbance*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4 2 0 50

100

150

200

250

300

o

Temperature/ C

Figure 11. The effect of the ratio of HST and D on the silanization of hydrated Silica/TESPT/HST/D System This kind of efficient catalyst is probably the resulting product from the reaction of HST and D. DSC result shows that the melting peaks of HST and D disappear after being blended at room temperature (figure 12-a), indicating that the acid-base neutralization reaction probably occurred. The chemical reaction has been confirmed by FTIR spectrum. As figure12-b shows, the carboxyl group characteristic peaks of HST (2657cm-1, 1700cm-1 and 941cm-1) disappear in the blend. At the same time, the intensity of amino group in D (3300-3500cm-1 ) are weakened


21


remarkably, while the characteristic peaks of ammonium ion (2780cm-1 ) and carboxylate ion (1597cm-1,1403cm-1 ) appear in the HST and D blended system. So the reaction of producing carboxylic acid ammonium salt is fast once the HST and D are blended. It is the acid-base complex that effectively promotes the condensation reaction between silanol and ethoxy group.

1597

exo

20

HST D HST/D

40

Absorbance Units 0.5 1.0 1.5

9 8 7 6 5 4 3 2 1 0

0.0

DSC/(mW/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 80 100 120 140 160 o Temperature/ C

1403 2780

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3360

D

1700 2657

941

HST

3500

3000

D:\TEST\MINE\课题\HST+D\HST-D(1-1).1 D:\TEST\MINE\课题\HST\HST(30C).0 D:\TEST\MINE\课题\D\D.0

D

2500 2000 1500 Wavenumber cm-1

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DTGS

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16/03/2015

DTGS

12/02/2015

DTGS

15/02/2015

(a)

(b)

Page 1/1

Figure 12. DSC curves (a) and FTIR spectra (b) of HST, D and the HST/D blend Based on the analysis above, the silanization reaction process mainly consists of two parallel reactions in the system, i.e., the hydrolysis and direct condensation reactions, and both of them are catalyzed by the HST/D complex. The kinetic simulation results of silanization reaction at three different heating rates with HST/D ratio of 1:2 are shown in figure 13.


E/(kJ/mol)

log(A/s^-1)

0.02

1.2

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(a)

(b)

(c)

Figure 13. The silanization reaction kinetic simulation results of the hydrated silica/TESPT/HST/D system, (a) relationship of lnβ ~1/T at each equal conversion rate, (b) the E and A values at different conversion rate, (c) the experiment curves (point) and fitting curves (line) of the ethanol trace The activation energy calculated from figure 13(a) decreases with the increase of conversion rate and also shows two step reactions. The activation energies of the two step reactions are ca. 54-56kJ/mol and ca. 52-54kJ/mol, respectively, as shown in figure 13(b). The fitting results are almost the same as the experimental results by means of two-step parallel reaction model, and the correlation coefficient is also up to 0.999. It can be seen from the fitting results that they accord with the autocatalytic reaction characteristic for the two parallel reactions (table S4). The parallel reaction 1 is the ethoxy group hydrolysis promoted by HST/D complex. Its reaction order becomes much higher because of the complexity of hydrolysis reaction process. The parallel reaction 2 is the condensation between silanol and ethoxy group, promoted by the HST/D complex. Because the concentration of acidbase complex increases with the increase of temperature, it presents the autocatalytic reaction characteristic with the activation energy of 53.5kJ/mol. Compared to the above systems, the condensation activation energy decreases further, same as that of the hydrolysis catalyzed by HST/D complex. Therefore, HST/D complex promotes the two parallel reactions obviously, especially the direct condensation between silanol and TESPT that enhances the contribution to silanization greatly.


23


According to the above results, the following reaction mechanism can be proposed to explain the catalytic effect of HST and D on the silanization reaction. The alkoxysilanes hydrolysis mechanism promoted by basic was proposed by K.J. Kim,55 which can be used to explain the TESPT hydrolysis promoted by D. The HST, D and their complex promote the condensation mechanism between the silanol and ethoxy group, which can be explained by the three intermediate products shown in scheme 1. OH OH Si O Si O O Si O

O

Si OH O

Silica O

O Si

OH Si

O O

Si OH O

O

Si

O

D

O

O Si

O

O OH OH

Si O

OH

Si

O

OH O

O

Si

Si O

H

O

R

O

Si

NH

O H

O

OH

Si

O

OH

Si

O

O

O

O

NH

O

O Si OH

OH

Si

Silica

Si

Si

O

O H

NH

Sx

H

OH

Sx

O

OH

O

O R

Si

Silica

Si

H

Sx

OH

OH

HS T Si

Silica

O Si O O

T/D HS

O

Si

Sx

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

NH NH

O Si

NH

O Si O O

O Si O O

O

O

(I)

( III )

( II ) O

OH

O

Si O Si O Si O

Silica

O

Si O O O Si Si O O

O Si OH

Si O

Sx

O

OH

O Sx

Si O

O

Scheme 1. Mechanism schematic of the direct condensation between silica and TESPT promoted by HST, D and their complex


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The HST is associated to the silanol of silica and the ethoxy group of TESPT through the hydrogen bond, acting as the acceptor and the donor of the electron at the same time.56 It decreases the silicon atom electric density of TESPT, meanwhile, the oxygen atom electric density of the silanol increases. By the formation of annular transition structure (Ⅰ), the HST enhances the nucleophilicity of the oxygen atom of the silanol to the silicon atom of TESPT, and also decreases the reaction activation energy. For the D system, the hydrogen bond is formed between amino group and silanol. The oxygen atom electric density of the silica silanol increases because of the electron donating of amino group.57 Therefore, the nucleophilicity of the oxygen atom is enhanced and is easy to form a five-coordination transition structure with the silicon atom of TESPT (Ⅱ). The HST/D complex acts as catalyst in the HST and D coexistence system. The promotion mechanism is similar with that of HST (Ⅲ). However, the electrophilic and nucleophilic effect of carboxylic acid ammonium salt is much stronger than that of HST, leading to much lower reaction activation energy and much higher nucleophilicity tendency between the oxygen atom of the silica silanol and the silicon atom of TESPT. 4. CONCLUSIONS Based on the on-line investigation of Silica/TESPT solid phase mixing reaction via TG-FTIR technique and kinetic simulation, the silanization of silica can be divided into two-step parallel reactions. The hydrolysis reaction of TESPT took place first, and then it condensated with the silanol of silica. On the other hand, the condensation reaction between TESPT and the silanol of silica took place directly. The hydrolysis reaction is easier to take place compared to the direct


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Page 26 of 35

condensation. The HST promoted the direct condensation between TESPT and silanol, possibly related to the reduced system energy by forming tri-molecule ring structure. Both the hydrolysis and direct condensation reactions are promoted by D, in which the nucleophilicity of oxygen atom in silanol or water is enhanced by D. Carboxylic acid ammonium salt derived from the combination of HST and D promoted the condensation between TESPT and silanol. Compared to HST, the corresponding mechanism is similar, however, the promotion effect is more obvious because of the stronger electrophilic and nucleophilic effects. Therefore, D/HST coexisting system can greatly enhance the contribution of direct condensation to the silanization reaction. Good results were achieved when the two-step parallel reaction model was utilized to simulate the reaction kinetics process for all the systems. This study provides a theoretical support for the more reasonable use of acid and base additives in rubber to further improve the silanization of silica. Moreover, the silanization reaction kinetic model established in this work has a good guidance for the mixing process of silica filled rubber and realizes an accurate prediction of the degree of silanization reaction in different mixing process. ASSOCIATED CONTENT Supporting Information TG-FTIR spectra of all the studied systems in this paper; detailed simulation process of the silanization kinetics; the silanization kinetic parameters of each system. AUTHOR INFORMATION Corresponding Author


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* Shugao Zhao: tel, +86-532-84022936; e-mail, [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Dr. T. Zhuang and Dr. G. S. Yu are sincerely thanked for the helpful discussion about GPC and DSC analysis. REFERENCE (1) Builes, S.; Lopez-Aranguren, P.; Fraile, J.; Vega, L. F.; Domingo, C. AlkylsilaneFunctionalized Microporous and Mesoporous Materials: Molecular Simulation and Experimental Analysis of Gas Adsorption. J. Phys. Chem. C 2012, 116, 10150–10161. (2) Garcia-Gonzalez, C. A.; Saurina, J.; Ayllon, J. A.; Domingo C. Preparation and Characterization of Surface Silanized TiO2 Nanoparticles under Compressed CO2: Reaction Kinetics. J. Phys. Chem. C 2009, 113, 13780-13786. (3) Li, H. L.; Perkas, N.; Li Q. L.; Gofer, Y.; Koltypin,Y.; Gedanken A. Improved Silanization Modification of a Silica Surface and Its Application to the Preparation of a Silica-Supported Polyoxometalate Catalyst. Langmuir 2003, 19, 10409-10413. (4) Wolff, S. Chemical Aspects of Rubber Reinforcement by Fillers. Rubb. Chem. Tech. 1996, 69, 325-346.


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Insights into the Silanization Processes of Silica with and without Acid

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