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An Energy-Saving and Fast Synthesis Technology of Polyvinyl Butyral in a Microreactor System Baiyang Zhou, Xiyan Lin, Kai Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03906 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Industrial & Engineering Chemistry Research

An Energy-Saving and Fast Synthesis Technology of Polyvinyl Butyral in a Microreactor System Bai Yang Zhou†, Xi Yan Lin†, §, Kai Wang†, Guang Sheng Luo* , † †

The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

§

China Science and Technology Exchange Center, Ministry of Science and Technology of China, Beijing 100045, China

ABSTRACT

In this work, we established a cascade cooling and two-step feeding technology for energysaving and fast synthesis of polyvinyl butyral (PVB) with low molecular weight in a microreactor system. Because of the strong mixing capacity of the microreactor and the relatively higher initial reaction temperature of 60 °C compared to existing crafts, the condensation reaction between the raw materials of polyvinyl alcohol (PVA) and n-butanal was highly intensified and the reaction rate was faster along with less energy consumption for cooling. By reducing the aging temperature to 40 ~ 55 °C which is far below the glass transition temperature of PVB, we could also save energy consumption, and improve the morphology of

*

Correspondence concerning this article should be addressed to G. S. Luo at

[email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

PVB. Benefiting from the new technology, the acetalization degree (AD) of PVB reached more than 78 %. By applying the two-step n-butanal feeding method in the new technology, the utilization ratio of n-butanal was dramatically increased to more than 99.7 %, which was the best result among current public reports. Moreover, the PVB product with low molecular weight has been successfully prepared with the new technology. This work demonstrates a promising continuous microreaction process for greener and energy-saving synthesis of PVB.

Keywords: Polyvinyl butyral (PVB), Microreactor system, Cascade cooling, Energy-saving

INTRODUCTION Polyvinyl butyral (PVB) is a kind of synthetic resin which is widely used in laminated glass, solar cell packaging materials, high-power LED heat sink materials and so on, for its strong adhesion, good transparency, excellent mechanical strength, low glass-transition temperature, and potential resistance to light and heat. 1-4 Figure 1 shows the PVB molecular structure, and Figure 2 shows the reaction mechanism of the synthesis of PVB. As shown in Figure 1, the molecular chain of PVB contains three functional groups, namely, alcoholic hydroxyl group, butyraldehyde group and vinyl acetate group. Wherein the alcoholic hydroxyl group and the vinyl acetate group come from the raw material polyvinyl alcohol. The catalyst hydrogen ions first attack the carbonyl carbon to produce pronated n-butanal. Then the positive-charged carbonyl carbon is to attack the hydroxyl groups on PVA molecular chain and lose a water to form hemi-acetal. Upon the hemi-acetal reaction finishes, positive charge will transfer to the oxygen ion and form a new active center,


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another attack happens to the adjacent hydroxyl to produce a six-membered ring acetal structure, meanwhile the hydroxyl releases a hydrogen ion (Figure 2). 5

Figure 1. PVB molecular structure

Figure 2. Reaction mechanism of the synthesis of PVB To meet the application requirements, the acetalization degree (AD) of PVB is typically required to achieve over 78 %.

6-8

Although widely adopted in many studies, the method of

precipitation for PVB production9-12 not only meet the application requirements with great difficulties, but also increase the difficulty of scaling up and the cost of production. In a typical precipitation process, it is necessary to dissolve PVA at about 95 °C and then cool it to about 50 °C to disperse n-butanal in the PVA solution. In order to prevent the locally ununiform reaction at the same time as mixing, the temperature must be further reduced to about 15 °C before



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adding the catalyst (HCl) to slow down the reaction rate. However, after mixing and reaction for a period of time, the system needs to be heated up to about 60 °C for aging. Four major problems are hidden in this synthesis technology. Firstly, the acetalization degree (AD) of PVB is hard to meet the application requirements. As the reaction temperature is low and the reaction speed is slow, the reaction system begins to precipitate at very low AD. The reaction system changes from original liquid-liquid phase to the liquid-liquid-solid phase, and the mass transfer is greatly limited. Secondly, the product is easy to agglomerate, mainly because of poor mixing effect. Thirdly, the energy consumption is considerable. The whole process has experienced the process of heating, cooling and heating up, as shown in Figure 3. The production process is tedious and the reaction time is long, usually more than eight hours. Last, the reproducibility of the technology is poor due to the batch operation in the aging vessels, especially in a large scale stirring reactor. Therefore, to solve these problems and synthesize high-quality PVB, the key is to speed up the reaction rate. A good idea is to strengthen the mixing effect, reduce energy consumption, and change the batch operation to a continuous operation. Microreactors have shown outstanding advantages on reducing the dispersed scale from millimeter to micrometer, mixing rapidly, transmitting heat efficiently, improving the apparent velocity, shortening reaction time, controlling the process safely, and so on.

13-17, 23

These

advantages may directly solve those problems in the synthesis process of PVB as mentioned above. In the previous studies of our group,

5, 18-19

we improved the method of precipitation by

utilizing a microstructured chemical system containing a membrane dispersion microreactor


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and an aging vessel reactor to implement the condensation reaction between polyvinyl alcohol (PVA) and n-butanal, which is able to increase the initial reaction temperature from 15 °C to 35 °C. In this way, the energy consumption for cooling dropped and the reaction time was cut by half which also helped to save energy consumption. By developing a two-step n-butanal feeding method, the utilization ratio of n-butanal could be increased to 98.7%. However, the energy consumption for cooling the temperature from 95 °C to 35 °C is still large, especially in the industrial process. Meanwhile, the second heating stage in the stirred vessel at 60 °C of 1.5 ~ 2.5 h needs extra heating energy, and the high temperature closing to the glass transition temperature (Tg) of PVB (the Tg of commercial PVB is 66 ~ 84 °C) which will lead to PVB particles to agglomerate partly. Particularly, almost all previous studies have focused on the synthesis of PVB particles with molecular weights larger than 75000.1-4 The larger molecular weight can bring benefits such as increased strength, but not the larger molecular weight, the better application. With the relatively smaller molecular weights of PVB particles, the permeability of the polymers will increase, and the surfaces will be smoother when the PVB particles are pressed to the PVB films, especially suitable for applications in packaging materials of solar cells, heat dissipation materials of high power LED and so on. Herein, we utilize the raw materials PVA with average molecular weight about 20000 to synthesize PVB particles with molecular weights about 20000 ~ 40000, which have never been reported before. In this work, we increased the initial mixing temperature in the micromixer to 60 °C to further reduce the energy consumption for cooling, the high temperature can speed up the hemi-acetal reaction and reduce the viscosity of the raw material in the microreactor system. After that, the aging process will further increase the AD value at 40 ~ 55 °C controlled by a water bath. This



temperature was obviously below the Tg of PVB which was able to prevent agglomeration and improve the morphology of PVB. As a whole, the reaction system is in a cascade cooling mode, as shown in Fig. 3, and the process could save much energy for heating and cooling in different periods. Besides, we developed a two-step n-butanal feeding method, so that we can increase nbutanal utilization ratio up to more than 99.7 %. Finally, by developing the new technology in this work may reduce energy consumption, intensify the reaction process, improve the utilization of n-butanal, and simplify the production process.

Figure 3. Variation of temperature in the synthesis process of PVB EXPERIMENTAL SECTION Materials The raw material PVA was provided by Aladdin, Co., Ltd. Its average molecular weight (Mn) is 20000 and the degree of hydrolysis of vinyl acetate is 98 ~ 99 %. Unlike previous studies, they usually used the PVA with a molecular weight of about 75000 as a reactant. As Mn of PVA decreased, the Mn of PVB decreased also. In addition, the conditions required for the experiments changed also. Hydrochloric acid (37.5 % HCl, provided by Beijing Chemical


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Works) working as the catalyst was mixed with the PVA aqueous solution, and the PVA-HCl aqueous solution acted as the continuous phase in the microreactor. n-butanal (98%+, provided by J & K Chemical Technology) acted as the dispersed phase in the microreactor. Synthetic procedures and devices The microreactor system in our experiments is shown in Figure 4. The raw material PVA was first dissolved in deionized water (obtained using a Center 120FV-S water purification device), at 95 °C for 2h. The resulting PVA solution was cooled down and then mixed with hydrochloric acid in a 500 mL vessel according to a certain proportion. After that, the 500 mL PVA - HCl aqueous solution as the continuous phase and n-butanal as the dispersed phase were pumped simultaneously into the membrane dispersion microreactor using. The flow rate of the continuous phase (FA) was 100 mL/min, and the flow rate of the dispersed phase (FB) was set under different experimental conditions. In the microreactor, n-butanal passed through the membrane and dispersed into the continuous phase. The microreactor was designed by our group, and the details of the membrane dispersion microreactor were illustrated in Figure 5. 5 In the whole setup, every single unit was connected between each other by the Teflon tube, and the length of the Teflon tube connecting the membrane dispersion reactor and the aging tank is 1.29 m. After mixing completely, the mixed solution would enter the aging vessels for aging.



Figure 4. The membrane dispersion microreactor system

Figure 5. The membrane dispersion microreactor. (a) Detailed sandwich-structure of the membrane dispersion microreactor. (b) SEM image of the membrane used in the microreactor. RESULTS AND DISCUSSION Influence of temperature and time on reaction performances In the new technology, temperature is the most important influencing parameter, so we first explored the influence of reaction and aging temperature on the AD of PVB. The results are


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shown in Figure 6, when the feeding temperature in microreactor (T1) was 60 °C, the concentration of PVA in solution (߱୔୚୅ ሻ was 8 %, the mass ratio of HCl to PVA (RHCl/PVA) was 0.1, the mass ratio of n-butanal to PVA (Rn-butanal/PVA) was 0.56, the aging temperature (T2) in aging vessels was varied at 40 °C, 45 °C, 50 °C, 55 °C respectively, and the aging time was 5 h. It can be found that as the aging temperature increases, the AD increases first and then decreases (Figure 6(a)). By measuring the residual molar concentration of n-butanal in the aging vessels during the whole reaction process (Figure 6(b)), and we could calculate the apparent AD corresponding to the reaction time (Figure 6(c)). By comparing the values with the actual AD, it can be found that the increase of aging temperature would make the condensation reaction more fully, but it would also accelerate the volatilization rate of n-butanal. If the temperature is too high, the influence of the latter would prevail, making the AD decline.



Figure 6. The influence of aging temperature on the AD. (a) Variation in AD values with the variation in different aging temperature. (b) Variation in the residual concentration of n-butanal (Cn-butanal,r) with the variation in different aging time. (c) Variation in apparent AD values (ADt) with the variation in different aging time. Influence of PVA concentration on reaction performances Theoretically, when Rn-butanal/PVA = 0.56 with one hundred percent of utilization ratio of butyraldehyde, the AD is about 78 %. But obviously, the actual AD is impossible to meet this value under this mass ratio in our experiments. There are some reasons for this result, such as the volatilization of n-butanal during the reaction, uneven mixing and so on. As the raw materials, the concentration of n-butanal and PVA would have a great influence on the AD. As shown in Figure 7(a), when Rn-butanal/PVA = 0.64, the AD had been significant increased. This is because with the increase in the amount of n-butanal, more n-butanal was dispersed around the alcohol hydroxyl group, so that the probability of the condensation reaction increased. But for industrial production, overdose of n-butanal is uneconomical. However, on the other hand, higher concentration of PVA could not only bring higher production efficiency, but also result in better water-saving, energy saving and environmental protection. Therefore, we tried to increase the concentration of PVA.


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Figure 7. The influence of PVA concentration on the AD. (a) Variation in AD values with the variation in different

Rn-butanal/PVA. (b) Variation in AD values with the variation in different

concentration of PVA. The initial PVA aqueous solution concentration of 8 % and 9 % were compared (see Figure 7(b)). The results show that the increase of PVA concentration is beneficial to the improve of AD. This is because the increase in the concentration of the PVA aqueous solution will make the viscosity of the solution become higher, making the force of the continuous phase to the dispersed phase become stronger. On the one hand, the volatilization of n-butanal decreased. On the other hand, it leads to smaller droplet size of n-butanal and larger mass transfer surface area.20-22 Besides, when the mass ratio of n-butanal to PVA is kept constant, the increase in the concentration of PVA will increase the concentration of n-butanal. The higher the concentration, the faster the reaction rate, which makes the prepared PVB to achieve higher AD prior to precipitation. By optimizing the reaction conditions in the microreaction system, we could obtain the following reaction conditions for high quality products: the feeding temperature in the



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microreactor (T1) is 60 °C, ωPVA = 8 %, Rn-butanal/PVA = 0.64, the aging temperature in aging vessels (T2) is 45 °C and 50 °C, or the feeding temperature in the microreactor (T1) is 60 °C, ωPVA = 9%, Rn-butanal/PVA = 0.64, the aging temperature is 40 °C, 45 °C, 50 °C and 55 °C. With all above mentioned conditions, the AD could exceed 78 %. Compared with other PVB particles in previous studies and a commercial product (Aladdin), the AD of PVB in this work improved obviously (see Table 1). Table 1 The Comparison of PVB’s AD in Different conditions

AD / %

In this work

In previous studies

Commercial products

81.4

79.9

75

※ The products come from Aladdin, and the product number is P105914. Moreover, the macro-structure of PVB demonstrates the morphology of PVB particles to be finely dispersed (Figure 8). Observing the morphology of PVB particles under different magnifications, we found that the sizes of the primary PVB particles could be reduced to 2 µm , which was beneficial from the rapid nucleation and precipitation of the high concentration system. The smaller sizes of PVB particles can further confirm that the highly efficient mixing and quick condensation reaction at the initial reaction stage are helpful to improve product quality.


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Figure 8. SEM images of the primary PVB particles obtained at different magnifications. Experiments carried out at ωPVA = 8%, FPVA-HCl = 100.0 mL ⋅ min −1 , Rn-butanal/PVA = 0.64, T1 = 60 °C, T2 = 45 °C, t = 5 h. Two-step feeding of n-butanal on reaction performances According to the theoretical calculation, when the mass ratio of n-butanal to PVA is 0.56, the apparent AD could reach 78 %. However, it was found that with the increase of the n-butanal added, the AD was increased, but the utilization ratio of n-butanal (Un-butanal) was not high enough, or even declined in some cases due to the limitation of other influencing factors (see Table 2). The loss of n-butanal has three main kinds of whereabouts. First, the high aging temperature (usually about 45 °C or 50 °C) led to much volatilization of n-butanal, which was the most important loss. Second, due to the excessive amount of n-butanal, a lot of n-butanal could stay unreacted in the aging vessels. Third, since the PVB particles is oleophilic, the adsorption of nbutanal contributes to part of the loss. In order to improve the utilization ratio of n-butanal as well as be more environmentally friendly, we need to further improve the utilization of n-



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butanal. The improvement plan is to add n-butanal in the reaction system by two steps. In the early stage of the reaction, only a certain part of n-butanal is added. Less n-butanal in the early stage not only prevents the agglomeration of the products due to the rapid reaction, but also reduces the volatilization of n-butanal. After maintaining reactions after a period of time, adding the residual n-butanal can further increase the AD. In this study, we proposed two different effective adding strategies. Table 2. AD and Utilization Ratio of n-butanal Under Different Conditions with the Cascade Cooling Technology Only ωPVA = 8 %

ωPVA = 8 %

ωPVA = 9 %

Rn-butanal/PVA = 0.56



Aging temp/ °C AD/ % Un-butanal / % AD/ % Un-butanal / % AD/ % Un-butanal / % 40

63.3

81.2

68.8

77.2

78.2

87.7

45

65.0

83.3

78.3

87.9

79.6

89.3

50

67.0

85.9

77.9

87.4

80.4

90.2

55

56.5

72.4

73.6

82.6

81.4

91.3

The first feeding method is adding a small part of n-butanal in the preparation of the initial PVA-HCl aqueous solution. Most n-butanal are still added through microreactor. As shown in Figure 9, when the mass ratio of the pre-added n-butanal to PVA (Rn-butanal/PVA,0) is 0.1, with the mass ratio of the total n-butanal to PVA (Rn-butanal/PVA = 0.56) and other operating conditions stay unchanged, this kind of feeding method succeeds increasing the utilization ratio of n-butanal and improving the AD results significantly.


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Figure 9. The influence of the feeding method on the AD and the utilization ratio of n-butanal (Un-butanal). (a) Variation in AD values by different feeding methods. (b) Variation in Un-butanal by different feeding methods. Unlikely the pre-addition way, the other feeding method is to retain part of n-butanal until one-hour aging after the main addition in the microreaction stage and then add it. As shown in Figure 10, when the mass ratio of the post-added n-butanal in aging vessels to PVA (Rnbutanal/PVA,2)

is 0.2, still keeping the mass ratio of the total n-butanal to PVA (Rn-butanal/PVA = 0.56)

and other operating conditions constant, similarly, this kind of feeding method can also improve the AD and the utilization ratio of n-butanal remarkably.



Figure 10. The influence of the second feeding method on the AD and the utilization ratio of nbutanal (Un-butanal). (a) Variation in AD values by different feeding methods. (b) Variation in Unbutanal

by different feeding methods.

We tried to change the proportion of two stages of n-butanal in the microreactor and the aging vessels. But interestingly, it was found that the different proportion of the two stages of n-butanal had little effect on the AD under identical experimental conditions (Figure 11(a)). Then, we changed the adding time of the second stage of n-butanal. As shown in Figure 11(b), the addition timing for n-butanal resulted in higher AD when it was set at 0.5 h of aging reaction. By measuring the concentration of n-butanal in the aging vessels over time (see Figure 12), it could be seen that the concentration of n-butanal declined most steeply in the first one hour. This showed that after entering the aging vessels, the first one hour was the rapidest period of the acteal reaction, which meant it was the best supplementary timing.


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Figure 11. The influence of different adding methods of n-butanal on the AD. (a) Variation in AD values with the variation in different proportion of the two stages of n-butanal. (b) Variation in AD values with the variation in adding time of the second stage of n-butanal.

Figure 12. In the new technology based on two-step n-butanal feeding method, the variation of n-butanal concentration (Cn-butanal,r) in aging vessels with time.



In this part of research, we improved the cascade cooling technology by changing feeding of n-butanal from one-step to two-step, we successfully increased the utilization ratio of n-butanal to high than 98% to make the process much greener. Moreover, the success of adding a small part of n-butanal in the preparation of the initial PVA-HCl aqueous solution gives us such enlightenment that we can recycle the remaining liquid phase in aging vessels and use it again as the raw materials to configure the new PVA-HCl aqueous solution so as to realize the cycle of the whole process. CONCLUSION In order to realize the goal of green chemical synthesis, this paper aimed at the synthesis of polyvinyl butyral (PVB), taking advantages of membrane dispersion microreactor system in strengthening the mixing and accelerating the reaction rate. With the new technology of cascading cooling, and two-step n-butanal feeding method, we systematically discussed the influences of aging temperature, the mass ratio of n-butanal to PVA, initial concentration of PVA aqueous solution and adding methods of n-butanal on the AD of PVB product with low molecular weight. Besides, we observed the macroscopic and microscopic morphology of the particles as another important property of the products. With fully optimization, we achieved appropriate synthesis conditions. Compared with traditional synthesis technologies, this new technology could not only raise the AD to over 78 %, but also effectively reduce the dosage of nbutanal, and increase the utilization ratio of vinyl acetate to more than 96 % even 99.7 %. In addition, the new technology also simplified the temperature control process, and shortened the reaction time from more than 8 h originally down to 4 h, which was proved to be more energysaving and environmentally friendly. The attempts to add the n-butanal by two steps are of great help to the realization of the process cycle. In future studies, we will continue to explore the


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continuous experiments based on this new technology and scale up the microrector system for industrial scale preparation. SUPPORTING INFORMATION Analytical methods used in this work. This material is available free of charge via the Internet at http://pubs.acs.org. NOTES AD

Acetalization degree, butyral group content (mass fraction), %

ADt

Apparent AD values

Cn-butanal,r

The residual concentration of n-butanal in aging vessels

FA

The flow rate of the continuous phase

FB

The flow rate of the dispersed phase

RHCl/PVA

Mass ratio of HCl to PVA

Rn-butanal/PVA

Mass ratio of n-butanal to PVA

Rn-butanal/PVA, 0

Mass ratio of the pre-added n-butanal to PVA in the two-step n-butanal

feeding method Rn-butanal/PVA, 2

Mass ratio of the post-added n-butanal in aging vessels to PVA in the two-

step n-butanal feeding method T1

The feed temperature in microreactor

T2

The aging temperature in aging vessels

Un-butanal

Utilization ratio of n-butanal

ωPVA

Mass concentration of PVA in the initial solution, %

ACKNOWLEDGEMENT We gratefully acknowledge financial support from National Natural Science Foundation of China



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(20) Wang, K.; Lu, Y. C.; Xu, J. H.; Luo, G. S. Droplet Generation in Micro-Sieve Dispersion Device. MICROFLUID NANOFLUID. 2011, 10(5), 1087-1095. (21) Wang, K.; Luo, G. S. Microflow Extraction: A Review of Recent Development. Chem. Eng. Sci. 2017, 169, 18-33. (22) Wang, K.; Li, L. T.; Xie, P.; Luo, G. S. Liquid-Liquid Microflow Reaction Engineering. React. Chem. Eng. 2017, 4, 1-17. (23) Noël, T.; Su, Y.; Hessel, V. Beyond Organometallic Flow Chemistry: The Principles Behind the Use of Continuous-Flow Reactors for Synthesis. Top. Organomet. Chem. 2015, 9(1), 2735.


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