SÁNCHEZ CASAS,  Ester  rd

23 July 2012

 

     

MASTER THESIS

Chemical Engineer Escola Tècnica Superior d’Enginyeria Industrial de Barcelona (ETSEIB) Universitat Politècnica de Catalunya (UPC)

by Mlle. Ester SÁNCHEZ CASAS

MONITORING CONDUCTIVITY OF EMULSION POLYMERIZATION    

DIRECTED BY:

Mme. Nida S. OTHMAN M. Tim F. McKENNA

    CPE Lyon Laboratoire d’Automatique et de Génie des Procédés (LAGEP) Laboratoire de Chimie et Procédés de Polymérisation (LCPP) 43, Boulevard du 11 Novembre 1918 69616 Villeurbanne, cedex, France 1   

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I.

Abstract

In a polymerization reaction it is essential to control the physical properties of the particles which have been obtained. These physical properties: molecular weight distribution, particle size, polymer composition and morphology are fundamental parameters which determine the properties of the polymer.

In this project we attempt to develop on-line measurements controlling these parameters throughout the polymerization: we are interested in studying the conductivity of the reaction medium during the emulsion polymerization of styrene by radical (ascorbic acid, H2O2) and with the presence of surfactant (SDS). For this study we will rely on such measures as coupled calorimetry, dry mass and zetasizer (to determinate the particle size), which will allow us to parameterize the conductivity measurement. Initially, we will study the conductivity of a solution of surfactant without monomer to determine the CMC (critical micelle concentration) of SDS at different temperatures (Series 0). In the second part of the study we will investigate the reaction of styrene polymerization by adjusting various parameters: - The concentration of surfactant (Series 1) - The introduction flow rate of the initiator (Series 2) - The initial concentration of monomer (Series 3)

To reach a conclusion from these experiences: the conductivity can control precisely the presence of micelles in the medium. These micelles are responsible for the control of physical parameters of the resulting polymer. The conductivity should be calibrated according to the medium, the monomer and the surfactant. We will do additional tests to delve into the essence of this study, and understand the behavior of the conductivity. We will also use a video probe to observe how could the particle size change size due to different variations in the parameters.

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II.

Summary

  I.  Abstract 

3 

II.  Summary 

4 

III.  Nomenclature 

5 

IV.  Introduction 

6 

V.  Theoretical part 

7 

1)  Reaction et Kinetics 

7 

2)  Monitoring the calorimetry polymerization 

8 

3)  Monitoring the polymerization by dry mass 

9 

4)  Emulsion polymerization 

9 

5)  Formation of micelles and Conductivity 

10 

6)  Formation of micelles and particle size 

11 

7)  SDS on the surface of droplets 

11 

VI.  Experimental part 

13 

1)  Equipment 

13 

2)  Experimental process 

15 

2.1 Series 0 

15 

2.2 Series 1,2 and 3 

16 

2.2 Video measurements 

21 

VII.  Results and Discussion 

22 

1)  Effect of temperature on the CMC (Series 0) 

22 

2)  Effect of surfactant concentration (Series 1) 

24 

3)  Effect of initiator’s flow rate introduction (Series 2) 

30 

4)  Effect of monomer concentration (Series 3) 

35 

5)  Experiment using APS as initiator 

40 

6)  Experiments using KPS (Series A) 

43 

7)  Experiments using VA-086 (Series B) 

46 

8)  Video measurements 

47 

9)  Different studies of conductivity 

55 

10) 

58 

Miniemulsion (M01) 

VIII. Conclusion 

59 

IX.   Bibliographic references 

60 

X.   Vocabulary 

61 

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III.

Nomenclature

 

Symbol [ ]

Name Concentration

Units mol.m-3

F

Flow rate

g/h

rpm

Agitation

Tour/min

G.P.

Feed rate of the initiator

%

M

Mass

kg

Cp

Heat capacity

J.kg-1.°C-1

U

Heat transfer coefficient

J.mol-1.°C-1

A

Exchange surface

m²

Q

Amount of heat

J

σ

Conductivity

S.m-1

λ

Conductance

S.m².mol-1

MW

Molar mass

g.mol-1

Np

Number of particles

Dp

Particle diameter

nm

Enthalpy

J

T

Temperature

°C

v

Speed

mol.s-1

k

Rate constant

m3.s-1

H

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IV.

Introduction

This study is focused on monitoring conductivity of a polymerization. Nowadays, polymers are very important in the chemical industry, thus it is important to control the polymerization. One way to control these reactions by monitoring conductivity coupled with calorimetry. Actually, during an emulsion polymerization takes place the micelle formation which can be controlled by conductivity. The essential particularity of these micelles is the particle. Firstly, we will consider the CMC (critical micelle concentration) of our surfactant and we will try to establish a relationship between micelle formation and the particle diameter obtained and the amount of polymer formed. The parameterization of the conductivity sensor will be using calorimetric measurements. Calorimetry allows us to obtain Qr (the heat of reaction) and allows us to analyze the rate of polymerization, the conversion throughout the reaction and the size of the particles. We will do different measurements in order to study the polymerization, from samples taken at different times during the reaction. Additionally, we will do some studies in order to understand the meaning of the anomalies found during the previous experiments.  

 

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V.

Theoretical part 1) Reaction et Kinetics

Reaction scheme:

HO

OH

OH

CH2

+

OH

HO

CH2

OH

HO

CH2

CH2

CH2

HO

* Decomposition of the initiator (H2O2 / ASCA) of the polymerization: kd 

ka 

H2O2

2 HO°

vd = 2 HO-M °

= 2 f kd [H2O2]

va= ka [M][HO°]

We suppose that ka is higher than kd and we have define the efficacy of initiator f

* Propagation: kp  HO-M ° + M

* Termination:

HO –M-M°

vp = ‐ 

-

By disproportionation

-

Par coupling :

 = kp × [M] × [M°] 

vt = ‐ 

 = 2 kt × [M °]² 

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 We define the rate of polymerization:

 

  Rp = kp × [M]p n ×

=-

=

• x is the conversion  • M0 is the initial mass of monomer  • MW represents the molecular weight of monomer  • n is the average number of radical particles  • Np is the number of particles per liter  • NA is Avogadro's number  • [M] p the concentration of monomer in the particles 

2) Monitoring the calorimetry polymerization Our reaction proceeds in a batch reactor (hall mock), the reaction temperature is controlled by a jacket. We can write the energy balance as follows:  

micpi ×

=



QR is the heat generated by the polymerization reaction



UA(TR-Tj) is the heat exchanged through the jacket



Qloss Qloss is the heat lost during the reaction through the device

Ficpi(Ti-TR) + QR – UA(TR-Tj) – Qloss

We neglect the other terms of energy due to agitation, and other reactor components.  We can define the progress calorimetry as follows:

X

  calorimetry

(t)=

, where Qmax is the

maximum heat maximum heat generated by the reaction. It is calculated using the following equation:   Q max =

.

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3) Monitoring the polymerization by dry mass We can define progress by measuring the dry mass from the equation above:   X masse= Mlatex represents the mass of the solution Msèche represents the mass remaining after drying Mmonomère represents the monomer mass

4) Emulsion polymerization An emulsion polymerization consists in an aqueous solution of water in a double-shell reactor. This water is heated to a fixed temperature and is stirred at 300 rpm. We add to this water, a sufficient quantity of a surfactant in order to form micelles. The surfactant is SDS in this study (see Figure 1). Once the mixture has reached the set temperature the monomer (styrene) and the initiator (H2O2) were added into the aqueous phase by the distilled water if necessary. The surfactant stabilizes the monomer, and then there are formed micelles and monomer droplets in the reaction medium because the monomer is less soluble in water than in the monomer (these droplets are very big and will become reservoirs for monomer for the polymerization). The initiator is present in the aqueous phase. Then we should allow the mixture to be homogenized for several minutes. Then we start by running the reaction rate of ascorbic acid content in a syringe (the speed is also a parameter that we will study). Ascorbic acid will allow the formation of radicals in the aqueous phase by reacting with the monomer forming oligoradicals. The radicals or oligoradicals have statistically more probabilities to enter into the micelles than in the monomer droplets and do not stay in solution. As their size increase, they have less affinity with water.

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  Figure 1: Nucleation mechanisms during an emulsion polymerization.

  The particle growth is then effected by transfer through the monomer from the aqueous phase drops. The monomer in the aqueous phase is gradually consumed to increase the size of the micelles in oligoradicaux precipitates or in solution. This implies the dissolution of monomer droplets which are thus progressively consumed by displacement of equilibrium. Growth of primary particles can also occur by coagulation of particles. Is obtained at the end of the reaction a polymer latex, i.e., a stable emulsion of polymer particles whose size can range from 0.05 to 5 microns typically.

5) Formation of micelles and Conductivity A surfactant is composed of a hydrophilic part which is soluble in water and a hydrophobic portion (insoluble in water) soluble in polar solvents:

OSO3‐   Na+   Figure 2: Schematic of surfactant: SDS



 When the surfactant is introduced into the reactor (containing water) to a concentration below the CMC: Assuming that the SDS is fully ionized, the solution contains a surfactant mixture 10 

 

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  composed of a hydrophilic surfactant. The ionic part is in the form of sodium ions and iondodecyl C12H25SO4 .Cette ionic part is at the origin of the conductivity of the reaction mixture. The conductivity of the solution then follows a linear law: σ = λNa+ x [Na+] + λSDS- x [SDS-] Units : σ (S.m‐1), λ (S. m2.mol‐1), [] (mol.m‐3)  (Q: we made several assumptions: There is no HO-ions in our environment because of H2O2 and ascorbic acid has a negligible conductivity in the reaction medium)



If the surfactant concentration in the reactor is greater than the CMC, micelles are formed in the reaction. The reason for their formation is because a surfactant molecule can reduce the solvation energy by assembling the hydrocarbon chains in the form of a droplet. This droplet is excited about the hydrophilic parts of surfactants, and is then soluble in water. The conductivity decreases since then the concentration decreases surfactant free.

6) Formation of micelles and particle size Surfactants are compounds that reduce surface tension between two media. When a medium is saturated with surfactant molecules there will be formation of micelles. The CMC depends on the geometry and functionality of the surfactant. And a surfactant with a longer chain form micelles with a larger diameter and increase the CMC. When the micelle formation in a stirred medium there is a thermodynamic equilibrium constant this is set up between fragmentation and coalescence of the drops. This balance depends on various parameters including agitation but also the surfactant selected. We can then "control" the size of the micelles which allows controlling the particle diameter.

7) SDS on the surface of droplets In order to calculate the SDS amount into droplets we have done these calculations:

º

/ 4 3

4 11   

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40

where:

_

_





80

 

_

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VI.

Experimental part 1) Equipment

‐ ZETASIZER MALVERN: For measuring diameter particles.

  Figure 3: Zetasizer de malvern.

  ‐ Metler LJ16: To calculate the dry mass.

  Figure 4:Metler LJ16.

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  ‐ Conductivity probes:  Probe 1: Fisher bioblock scientific K10/PT1000/300mm :

Ampliation:

   

                    

 

Figure 5: Conductivity probe 1.

                  Probe 2: SKT21T-30  Probe 3: 856 Conductivity Module Metrohm :

  Figure 6: Conductivity probe 3.

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2) Experimental process 2.1 Series 0 1. Prepare the SDS + water and weigh. 2. Weight the water before putting into the reactor. 3. Put inside a syringe the solution of 1, and take out the bubbles inside of the surfactant; then don not forget to weigh it full of solution. 4. Open the condenser (water entry). 5. Add water. 6. Connect the conductimeter between the PC & reactor. 7. Change Tsp in the computer (Tsp = 20/70/60/50... °C). 8. Cover all the entries of the reactor by caps. 9. Turn on ‘Bain’ and ‘Agitation’ buttons.

  Figure 7: PC Supervision. Buttons.

  10. Open the programme ‘Conductimeter2’ in ‘stages’ folder and create an empty excel file in our own folder. 11. In the programme: Open the folder below and open the file already created. 12. When the Treactor is close to the estimated temperature:

-

Press Start in the programme.

-

Connect the syringe to the machine and turn on while timing by chronometer. 15 

 

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  (it goes slowly at the end, and the flow rate could change. Thus, stop the machine and the chronometer). 13. Stop the programme. 14. Weigh the empty syringe.

2.2 Series 1,2 and 3 1. Check the aspiration (the hood). 2. Start the program (Reacteur) on the “PC supervision”. 3. Check the connection with the balance. 4. Clean the reactor with water 3 times. 5. Weight the necessary amount of surfactant on the precise balance. 6. Add water using the balance for more weight. 7. Add the conductivity probe and check connections.

  Figure 8: Conductimeter.

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  8. Open nitrogen valve (1 bar), check the flow rate after few minutes.

  Figure 9: Nitrogen cylinder.

9. Insert into the reactor the surfactant + some water. 10. Open 2 valves of water (one for the condenser and another for cooling the bath), check the flow rates. Stirrer is at 300 rpm.

Figure 10: Valves of water (condenser and bath cooler).

  11. Press the green buttons of the bath and agitation. 12. Temperature set-point = 70°C; (Chauf_Inactif) with option auto.

13. Once temperature attains 70°C (30min)  Add H2O2 and styrene. Keep it inside for 15 min till you add the Ascorbic Acid. 17   

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  Figure 11: Hood to take the styrene from its bottle.

  14. At PC supervision: put on the program (Supervision IFix). Execute (). Modify set-point again to 70°C. Do not close this window! 15. At PC distant:

  Figure 12: PC Distant.

- Put on the program (Supervision pc distant). Execute (). - write 70 in the file c:\donneeslabview\commandeT - write 0 in the file c:\donneeslabview\commande_pompe - « Autorisation commande » in « double enveloppe » - « Autorisation commande » in « pompe manu » for semi-continuous exp. - « Effacer le fichier de mesure » 18   

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  16. Start MATLAB: - Directory C:\Documents and Settings\calorimetre\Bureau\stages\prog_matlab - Open folder / Control-T-et-debit 17. Execute Conductimetre2 in stages. Create an Excel file empty in the folder of conductivity and link it below. 18. Introduce Ascorbic Acid solution into the reactor by a syringe. 19. Put the azote (nitrogen) in the air of the reactor (or close it) 20. At PC distant: - in Labview program clic on « marche enregistrement ». - Execute Matlab program « control_T_et_debit” - Clic on (Début commande temperatire) 21. Write the hour on the « PC distant » 22. Take a sample every 10 min.

  Figure 13: Samples.

At the end: 23. PC distant : - Close the program conductimetre2. - clic on FIN in matlab figure. - write 20 in the file C:\DonneesLabview\commande_T. - click on « arrêter l’enregistrement » in labview. - copy the file « mesures » in c:\donneeslabview. 19   

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  24. Empty the reactor if T