Increasing PVAc emulsion polymerization productivity-an industrial application

Transcription

1 Indian Journal of Chemical Technology Vol. 17, January 2010, pp Increasing PVAc emulsion polymerization productivity-an industrial application Ashwini Sood Department of Chemical Engineering, Harcourt Butler Technological Institute, Kanpur , India Received 21 April 2009; revised 5 November 2009 A major objective in the operation of emulsion polymerization processes is that of faster and safer operation with consistent polymer product quality. The reaction is very fast and highly exothermic. This puts a constraint on the speed of the reaction, due to the limited heat removal capacity of the cooling jacket. The high viscosities of the latexes produced in many industrial processes aggravate the heat-removal problems. A systematic, experimental study in a 50 L pilot reactor is conducted where the monomer feed time is changed from 5.5 h to 2.5 h in various runs. The final samples are characterized for various properties which include appearance of emulsion, %solids, particle size, viscosity, ph, free monomer, film appearance and the application property which in this case is the bending length. The final viscosities are in the range of poise. The reactor temperature during these runs could be safely controlled between C. With decreasing monomer feed time, the final product shows improved application property and lower viscosity. The samples are withdrawn during the course of polymerization for two runs and analyzed for various properties. These provide fundamental understanding about the polymerization process. In conclusion, 30% reduction in batch time is possible. Keywords: Emulsion polymerization, Kinetics, Nucleation, Viscosity, Calculations An extensive literature exists on the subject of the emulsion polymerization of vinyl acetate. This is hardly surprising in view of the industrial importance of the polymer. Because of its good adhesion to a number of substrates, and to some extent because of its cold flow, a large quantity is produced for use in emulsion paints, adhesives and various textile finishing 1. Poly (vinyl acetate) is also used in floor tile, chewing gum bases, paper coatings. In the textile field the polymer goes into bodying and stiffening agents, binders for pigments, fabric sizes, bonding agents for non-woven textiles, and as a material which improves the abrasion resistance of the substrate. Its durability, transparency, flexibility, and stability to weathering and sunlight are attributes that contribute to its industrial acceptance 2. In industry, exact repeatability of emulsion polymerization processes within narrow limits is desirable, which means that the final solids content should be constant within ± 1%, the particle size, emulsion viscosity and polymer average molecular mass should vary little from batch to batch; and any residual monomer should be maintained within minimum possible narrow limits. In order to achieve these conditions, the formulation of emulsion polymerization should not be subject to variations such as minor changes of raw materials or of operative conditions. There are mainly two types of VAc emulsion polymers produced; a fine particle size ( microns) grade by using anionic and/or nonionic emulsifiers and without a protective colloid and a large particle size (0.5-3 microns) emulsion which is often only stabilized by protective colloids such as polyvinyl alcohol (88% hydrolyzed) and hydroxyethylcellulose. The process of preparation of polyvinyl acetate using poly (vinyl alcohol) as the principle protective colloid has been well described by Geddes 3. Poly (vinyl alcohol) plays an important role as a protective colloid in the emulsion polymerization of vinyl acetate. In general, the stabilizer of polymer emulsions strongly affects not only the colloidal properties but also the film properties. An emulsion stabilized with poly (vinyl alcohol) has many advantages over surfactants, including Newtonian fluidity, superior primary wet tackiness, high strength and creep resistant film properties. These phenomena stem from the existence of poly (vinyl alcohol) in emulsion, and therefore the chemical structure of poly (vinyl alcohol) has a strong influence on the emulsion properties 4. Poly(vinyl alcohol) is often used in vinyl acetate emulsion polymerization as a protective colloid, but its role is complex and controversial since it partakes in grafting reactions with the monomer, influencing process mechanisms, and affecting the

2 SOOD: INCREASING PVAc EMULSION POLYMERIZATION PRODUCTIVITY 35 colloidal properties of the latex. Furthermore, in industrial operations, the wide scatter of macromolecular properties of the commercial types of poly (vinyl alcohol) causes process irreproducibility. Carra et al. 5 used different types of poly (vinyl alcohol) to perform a series of polymerizations, and compared their kinetics. A selective solubilization procedure was applied to separate the three fractions of poly(vinyl alcohol) in the final latex: free in the water phase, physically adsorbed onto the polymer particles and chemically grafted. These results were compared with those obtained from pure adsorption measurements of polyvinyl alcohol onto emulsifierfree polyvinyl acetate dispersions. The rheological behaviour of the different latexes was also compared, and the results were used to formulate a hypothesis on the interaction mechanisms acting in these systems. Gilmore et al. 6,7 developed and validated a mathematical model that accommodates the emulsion polymerization of vinyl acetate stabilized with poly (vinyl alcohol) in an isothermal semi-batch reactor. In addition to the particle nucleation and growth mechanisms governing ionically stabilized polymerizations of relatively water-soluble monomers, the model accounted for grafting onto the poly (vinyl alcohol) backbone during nucleation, and polymeric stabilization. The model predicted the experimental conversion and particle size data with reasonable accuracy. Model predictions of measurable variables exhibited sensitivity to variables affecting either primary un-grafted particle nucleation or flocculation kinetics, but were relatively insensitive to variables affecting poly (vinyl alcohol) grafting reactions and the resulting primary grafted particle concentration. Semi-batch simulations indicated that independent increases in the vinyl acetate, poly (vinyl alcohol), and initiator levels all increase the primary grafted particle population. The styrene minisuspension polymerization at 70ºC using AIBN as initiator and poly (vinyl alcohol) and mixture of poly (vinyl alcohol) sodium dodecyl sulphate as stabilizers was studied by Ramirez et al. 8 and Ramirez and Herrera-Ordonez 9, focusing on the kinetic behaviour of the process after the sodium dodecyl sulphate was added. It was confirmed that the addition of sodium dodecyl sulphate to the system initially stabilized with poly (vinyl alcohol) highly enhances the colloidal stability of the polymer particles because of the association formed between sodium dodecyl sulphate and poly (vinyl alcohol) molecules that provides electro-steric stabilization. It was observed that when sodium dodecyl sulphate was added, the rate of polymerization, the average molecular mass and final latex viscosity increased. The earlier the addition of sodium dodecyl sulphate, the more marked these increments. Earhart et al. 10 conducted infrared studies on the grafting reactions of poly (vinyl alcohol). They studied the emulsion copolymerization of vinyl acetate and butyl acrylate with poly (vinyl alcohol) of different degree of hydrolysis as the sole emulsifier. It was found that grafting reactions of the poly (vinyl alcohol) and the vinyl acetate in the aqueous phase affected the rates of individual monomer consumption and the overall polymerization kinetics. The particle nucleation mechanism in this system was also illustrated. Magallanes Gonzalez et al. 11 performed GPC analysis of the serum obtained after latex centrifugation and found that high and low molecular mass oligomers are formed during emulsion polymerization of vinyl acetate using poly (vinyl alcohol) as surfactant, and they remain in the aqueous phase. The quantitative separation and characterization of the grafted waterinsoluble poly (vinyl alcohol) was also carried out. It was concluded that grafting reactions do not occur on all the poly (vinyl alcohol) chains. Only a small fraction of the poly (vinyl alcohol) chains (about 21.8 wt%) participates in the grafting process. Egert et al. 12 extended the work of Magallanes Gonzalez et al. which was conducted on low solids content latexes to high solid content latexes. Budhlall et al. 13 investigated the role of grafting during the emulsion polymerization of vinyl acetate with partially hydrolyzed poly (vinyl alcohol) as an emulsifier and potassium persulphate as an initiator. It was concluded that particle nucleation was continuous and was accompanied by extensive aggregation during the particle growth stages. The final number of particles was independent of the degree of blockiness of the poly (vinyl alcohol). The amount of grafted poly (vinyl acetate) were about the same in the final latexes (37-39%). In these low solids recipes, grafting appeared to be primarily a solution event, occurring predominantly in the aqueous phase and not at the particle/water interface, as was previously speculated. The study showed that the hydrogen abstraction from poly (vinyl alcohol) with the sulphate radical is the fastest reaction, resulting in the grafting onto poly

3 36 INDIAN J. CHEM. TECHNOL., JANUARY 2010 (vinyl alcohol), while the initiation reaction resulting mainly in homo-polymer is slower. The propagation of the poly (vinyl acetate) radical in the aqueous phase is a much slower reaction. The grafted molecules coagulated with each other to become a particle. From an industrial perspective, one of the major objectives in the operation of emulsion polymerization processes is that of faster and safer operation with consistent polymer product quality. The polymerization reaction proceeds as a classical double bond addition reaction initiated via a freeradical mechanism. The reaction is very fast and highly exothermic. This puts a constraint on the speed of the reaction, due to the limited heat removal capacity of the cooling jacket. With industrial reactors becoming as big as 20 m 3 (10 metric ton), this limitation is of increasing importance in comparison to lab scale reactors of a few 100 ml in volume. The high viscosities of the latexes produced in many industrial processes aggravate the heat-removal problems. Semi-batch reactors are often used to carry out strongly exothermic emulsion polymerization reactions. The aim of the present experimental study, in an industrial 50 litre semi-batch pilot plant reactor involving emulsion homo-polymerization of vinyl acetate in the presence of poly (vinyl alcohol) as the protective colloid, and potassium persulphate as the initiator, is to demonstrate productivity improvement in the process by increasing the monomer feed rate (or reducing the monomer feed time). It is also shown that these strategy can be used to control the particle size and hence, the viscosity of the latexes. For new as well as existing processes, first principles understanding is essential for optimizing and improving product quality and enhancing the company s cost position. Variations with time of the % solids content, particle size, viscosity, ph, free monomer content and the reactor temperature are reported. Process knowledge of the variation of the polymerization rate, total number of particles and average number of radicals per particle during the course of polymerization of this system is also developed in this work. Process description Vinyl acetate is highly flammable, and its vapor is explosive in most concentrations and hence safety factors must be carefully considered. Vinyl acetate has one of the highest heats of polymerization of the common commercial monomers at 21 kcal/mol and it has been emphasized 3 that many recipes require alert process operators, and appearance of the surface of the emulsion (with regard to monomer pooling, foams, or graininess due to entrained pockets of liquid or vaporized vinyl acetate beneath the surface) can indicate the need for prompt action. Many incidents involving the runaway polymerization of vinyl acetate monomer are known which have been reviewed by Gustin 15. In processes where the polymerization initiator was dissolved in the monomer, the initiator premix polymerized violently in the premix vessel. In polymerization processes where vinyl acetate monomer conversion ratio was not 100%, storages of recycled monomers containing no polymerization inhibitor and possibly some traces of polymerization initiator exploded due to vinyl acetate monomer violent bulk polymerization. Incidents happened either in batch or semi-batch polymerization processes in connection with wrong initiator introduction. In the laboratory, the polymerization is carried out in a 250 ml to 2 l, three necked glass reactor immersed in a constant temperature water bath at typically 50 to 75 C and equipped with a reflux condenser, a two bladed Teflon stirrer and a dropping funnel. Deionised water, protective colloids and emulsifiers and buffer are initially charged and maintained under a constant agitation of RPM for 2 h to ensure solubilization of the poly (vinyl alcohol). Then approximately 25% of the initiator dissolved in deionised water is added, followed by addition of 5% of the total monomer. The initiation occurs after 10 min. This stage is called the seeding stage. The remaining part of the monomer is then added drop-wise over a period of 5-6 h. The remaining 75% initiator is added in 3-12 parts after every 30 min to 2 h. This stage of monomer addition is called the feeding stage. After feeding, the batch is held for 1 h at the temperature of reaction and then cooled to C and held for 30 min. The same process is scaled up to pilot scale and plant scale reactors. The process that has been chosen can be described as follows. Deionised water, along with poly (vinyl alcohol) was charged in the reactor. The reactor was heated to C with the help of steam and held for 2 h. The monomer vinyl acetate was charged into the monomer tank from the monomer storage tank. At C, sodium dodecyl sulphate (emulsifier) and sodium bicarbonate (buffer) were added. The ph was checked ( ). If necessary, extra sodium

4 SOOD: INCREASING PVAc EMULSION POLYMERIZATION PRODUCTIVITY 37 bicarbonate was added to adjust the ph. At C, 25% of potassium persulphate (initiator) dissolved in deionised water was added and regular feeding of the monomer was started. Feeding rate of the monomer was 3 kg/h for 30 min and 5 kg/h for 5-6 h. Reactor temperature is maintained at C with the help of cold water and chilled water, fed into the external jacket. The reactor was also equipped with a reflux condenser which provided evaporative cooling. 1/12 th part of remaining initiator dissolved in deionised water was added after every 30 min of regular feeding. After monomer feeding was over, the last shot of initiator was added and the batch held for 1 h at C. The cooling of the reactor was then started and formaldehyde was added and the batch was held for 30 min. It took about 30 min to empty the reactor. The reactor was cleaned manually by scrapping off the coagulum deposited during the reaction, after every 5-6 batches. Due to proprietary reasons, the commercial recipe can not be disclosed but a typical recipe for this process is given in Table 1. The product that was chosen was a popular finishing agent that imparted body, handle and stiffness to cotton, synthetic, blends, buckroms and interlinings. The specifications for this product are given in Table 2. Thus, the total batch time comes to 9-10 h, out of which 50-60% was the monomer feed time. It was realized that since monomer is added very slowly or drop-wise in the laboratory set-up; the delayed addition of monomer translated into a very large feeding time in the pilot-plant set-up during scale-up, and therefore, reduction in batch time could be achieved by feeding the monomer at faster rates, while meeting the heat transfer and safety constraints. This strategy was tested in a 50 L pilot plant reactor, then newly constructed, in the plant of Jubilant Organosys Limited, situated in Gajraula, Uttar Pradesh, India. The above product was chosen as it had the highest viscosity among all the homo-polymer products that were being manufactured and thus, provided the maximum limitation to heat transfer. Since, the product specifications were not to be disturbed, the initial seeding stage and initial feeding of the monomer at slower rate for 30 min was not changed in any of the runs; these stages define the particle nucleation stage which determines most of the end-use properties of the final product. It was decided to change the feeding time of the monomer from 5 h to 4 h to 3 h to 2 h. Thus, in different runs, the total feed time of the monomer varied from 2.5 h to 5.5 h. Experimental Procedure All the ingredients listed in Table 1 were used as received, without any further purification. The polymerization procedure has been described above under process description. The pilot reactor reaction volume was 50 L. It was jacketed and had provisions for jacket heating with steam and cooling with cold water and chilled water. It was also provided with a reflux condenser which provided evaporative cooling from the latent heat of evaporation. The condensed monomer was cooled further before return. The monomer fed into the reactor from the storage tank at ambient temperature also provided cooling. The reactor was equipped with an agitator with provisions for two RPMs: 36 and 72. All the runs were made at 72 RPM to overcome the heat transfer limitations. The reactor was also equipped with a temperature gauge and the temperature was recorded manually at every 15 min during a run. Monomer was fed into the reactor from a storage tank and its flow rate was set using a valve and measured using a rotameter. The %solids (non-volatile content) was determined gravimetrically by heating 1-2 g of the emulsion in an oven at 105 C for 3 h. Viscosity of the emulsions was measured at room temperature (~ 30 C) using a Brookfield viscometer using spindles RVT 3-7. Particle size of the emulsions was determined by the use of disc centrifuge combined with light scattering. The ph of the emulsions was measured using a ph meter. Free monomer was measured through bromine titration. General appearance of the emulsions was observed visually. Films were casted using 100 micron film Table 1 Typical recipe for polymerization Ingredients Amount (% of total mass) Deionised water Vinyl acetate (monomer) 47.0 Poly (vinyl alcohol) (88% 5.0 hydrolyzed) (protective colloid) Potassium persulphate (initiator) Sodium bicarbonate (buffer) Formaldehyde (fungicide) Table 2 Specifications of the product Property Nature/Value Appearnce Milky white Solid (%) 50±1 ph 4-6 Viscosity (poise) Particle size (micron) 1-3 Film Hazy Free monomer <3% Application property Bending length

5 38 INDIAN J. CHEM. TECHNOL., JANUARY 2010 applicator on glass plates about cm and dried overnight at ambient temperature in a dust free atmosphere. Clarity of the film was observed visually. The bending length was measured as follows. 10% aqueous dispersions of the emulsions were made. Pieces of similar dimensions (6 1 inch) were cut from cotton cloth. These pieces were padded after dipping them in the 10% aqueous dispersion by using rollers. They were weighed. These pieces were dried in oven at 105 C for 5 min and again weighed. These pieces were then ironed and their bending length was measured using a bending meter. Results and Discussion In the beginning the final product obtained with the feed time of 5.5 h, manufactured in the 50 L pilot plant reactor and 5 ton plant reactor was tested. The comparison of their properties is given in Table 3. As can be seen, the various properties are comparable. The viscosity of the pilot plant sample is higher than the plant sample because in the pilot runs double the amount of poly (vinyl alcohol) (total mass percent basis) was used as compared to plant runs. Then, it was decided to change the total feed time of the monomer to 4.5 h in the pilot plant run. Table 4 Table 3 Comparison of the properties of the Pilot plant sample and Plant sample Property Nature/Value Pilot plant sample Nature/Value Plant sample Appearance Milky white Milky white Viscosity (poise) Solids (%) Film properties Hazy, water sensitive Hazy, water sensitive Particle size (micron) ph Free monomer (%) Bending length (cm) (10% aqueous dispersion) Table 4 Key properties of final samples obtained with different feed times Property Nature/ Value Nature/ Value Nature/ Value Nature/ Value Feed time (h) Appearance Milky white Milky white Milky white Milky white Particle size (micron) Viscosity (poise) Solids (%) ph Film appearance Hazy Hazy Hazy Hazy Bending length (cm) compares the properties of the product obtained with a feed time of 5.5 h and 4.5 h. Both the runs were made on the same day and in the same reactor, back to back. It can be seen that bending length obtained with feed time of 4.5 h was higher than that obtained with the feed time of 5.5 h. Also the viscosity was lower for the feed time of 4.5 h which could be attributed to larger particle size, and hence, larger interparticle distance. Then, it was decided to change the total monomer feed time to 3.5 h and 2.5 h. The comparison of the key properties of final samples obtained with different feed times are given in Table 4. It can be seen that as the feed time is reduced, there is an improvement in the application property i.e. bending length and there is decrease in the viscosity which is the result of increase in the particle size (and hence larger interparticle distance). It must be mentioned that latex exhibit thixotropic behaviour i.e. their viscosity decreases with time at constant shear rate. In Table 5, the viscosities after 1 min and 5 min of shear at 30 C, 20 RPM have been recorded with Brookfield viscometer with 7RVT spindle. It can be seen that viscosity decreases with time at constant RPM. All the viscosities reported in this work (except those in the third column of Table 5) were measured after 1 min of shear at 30 C and 20 RPM. In Fig. 1, the temperature of the reactor from the start of monomer feeding to 1 h after the monomer Table 5 Effect of the duration of shear on the viscosity of final samples of various runs Run time (h) Viscosity after 1 min (poise) Viscosity after 5 min (poise) Plant (5.5) Pilot (5.5) Pilot (4.5) Pilot (3.5) Pilot (2.5) Fig. 1 Variation of reactor temperature with time for runs with different monomer feed time. Feed time: Series 1, 2.5 h; Series 2, 3.5 h; Series 3, 4.5 h; Series 4, 5.5 h

6 SOOD: INCREASING PVAc EMULSION POLYMERIZATION PRODUCTIVITY 39 Sample/Property Appearance Table 6 Product properties during the course of polymerization for run with monomer feed time of 3.5 h Time (h) Final Milky white Free flowing emulsion Milky white free flowing emulsion Milky white viscous emulsion Milky white viscous emulsion Milky white viscous emulsion Solids (%) Viscosity (poise) Particle size (micron) Free monomer (%) Polymerization rate (kg/h) Feed rate (kg/h) Table 7 Product properties during the course of polymerization for run with monomer feed time of 4.5 h Time (h) Final Sample/Property Appearance MWFFE* MWFFE MWVE** MWVE MWVE MWVE Solids (%) % Viscosity (poise) Particle size (μ) Polymerization Rate (kg/h) Feed rate (kg/h) *Milky white, free flowing emulsion **Milky white, viscous emulsion Fig. 2 Variation of %solids with time for the runs with monomer feed time of 3.5 h (rectangles) and 4.5 h (diamonds) feeding was stopped, for the runs with 5.5 h, 4.5 h, 3.5 h and 2.5 h feeding time, respectively, is given. As can be seen, the temperature could be maintained between C during monomer feeding. It was then decided to gain process understanding by withdrawing samples during the course of the run after every 0.5 to 1 h from the beginning of the monomer feeding. The samples were quenched by putting them in an ice-filled container and also by adding hydroquinone. The samples were withdrawn for two runs with monomer feed time of 3.5 h and 4.5 h and the various properties evaluated of these samples are given in Tables 6 and 7. These runs were conducted in 100 L pilot reactor. The sample size was 0.5 kg. The high viscosities of these runs were attributed to the poly (vinyl alcohol) lot that was used as it was reported that the same lot had given higher viscosities in the plant runs also. It should be noted that run with lower monomer feed time (3.5 h) gave lower viscosities than the one with the higher feed time (4.5 h). Since the particle size before 2.5 h could not be measured, they were obtained by fitting a linear regression line through the data for each of these runs with intercept equal to zero (the particle size at the start of monomer feed will be zero). The plots are given in Fig. 2. As can be seen, the particle grows at a faster rate at higher monomer feed rate as expected. The variations of % solids with time, for both the runs, are plotted in Fig. 3. In Fig. 4, the variation of viscosity during the course of reaction is plotted with respect to time, for both the runs. The variation of viscosity with % solid, for both the runs,

7 40 INDIAN J. CHEM. TECHNOL., JANUARY 2010 Fig. 5 Variation of viscosity with % solids for the runs with monomer feed time of 3.5 h (rectangles) and 4.5 h (diamonds) Fig. 3 Variation of particle size with time for the runs with monomer feed time of 3.5 h (rectangles) and 4.5 h (diamonds) Fig. 4 Variation of viscosity with time for the runs with monomer feed time of 3.5 h (rectangles) and 4.5 h (diamonds) is plotted in Fig. 5. A second order polynomial curve is fitted to the data points as shown in Fig. 5. The polymerization rate was calculated as follows. During every time interval, total mass of the recipe components in the reactor till that time (m tot ) was calculated. Also, the mass of water (m W ) and the monomer fed (m M ) was calculated. The mass of other components (m oc ) was calculated from m oc = m tot - (m W + m M ). The total solids in the reactor till that time (m solid ) were calculated from the measured value of %solid and m tot as m solid = %solid/100 m tot. The mass of polymer formed upto that time (m P ) was calculated from m P = m solid m oc. The mass of polymer formed (Δm P ) in a given time interval (Δt) was calculated by subtracting the mass of polymer formed after the end of this time interval (m P ) t+δt from that formed after the end of previous time interval (m P ) t. The rate of polymerization (R P ) in a given time interval was calculated from R P = Δm P /Δt. The rate of polymerization which provides information about the reaction kinetics of this process builds up during the course of polymerization to a maximum after an initial low value during the initial half hour when the particles are being formed and falls to zero once the monomer feeding is over. A comparison between the rate of polymerization and feed rate can be also obtained from Tables 6 and 7. It can be concluded that initially the polymerization rate is slower than the monomer feed rate and the unreacted monomer in this stage leads to a higher polymerization rate than the monomer feed rate sometimes during the course of polymerization. In the remaining time, the polymerization rate is more or less equal to the monomer feed rate as is the characteristic of the monomer-starved semi-batch operations. In Table 8, the mass of solid formed at the end of a given time duration (m solid ), the particle size and the number of particles are given, for the run with monomer feed time of 3.5 h. The number of particles (N p ) were calculated using the formula: N p = m solid /(ρ P 3 π/6d p ). Here, ρ P is the density of the polymer (= 1.15 g/cm 3 ) and d p is the particle diameter. The particle sizes at 0.5 h and 1.5 h were obtained from the equation of the fitted regression line. In Fig. 6, the variation of number of particles with time is plotted. From initial trends, it can be concluded that a coagulative nucleation mechanism is operative in this process. As can be seen, there is continuous coagulation during the course of polymerization and even after monomer feeding is stopped. As the particles grow there is a tendency to reduce the surface area to maintain stability. The development of viscosity during the course of polymerization is a result of competition between increase in viscosity due to increase in dispersed phase volume fraction due to polymerization and decrease in viscosity due to

8 SOOD: INCREASING PVAc EMULSION POLYMERIZATION PRODUCTIVITY 41 decrease in total particle number as a result of coagulation. The extent of coagulation during a run with a particular monomer feed time will depend on the swelling of the polymer particles with the monomer. The swelling will make the particles softer and amenable to coagulation. The smaller the feed time, the larger the monomer feed rate and larger the monomer swelling. As a result, the stickier the particles 16 or the softer the particles and the greater the extent of coagulation. Thus, larger particles are obtained with smaller monomer feed times as given in Table 4. Also, the larger the particle size for nearly the same %solids, the smaller the number of particles and the greater the interspacing between them, and hence, the smaller the viscosity. This trend is also evident from Table 4. No of particles ( ) Fig. 6 Variation of number of particles with time for run with monomer feed time of 3.5 h Table 8 Variation with time of total solids, particle size and number of particles during the course of polymerization for the run with monomer feed time of 3.5 h Time (h) Total soilds Particle size No. of particles (micron) final Also, rate of polymerization, R P = k p [M] p n/n A N p where k p is the propagation rate constant, [M] p is the monomer concentration in the particle, n is the average number of radicals per particle, N A is the Avogadro s number and N p is the total number of particles in the reaction volume. Thus, one can obtain the product k p [M] p n = R P N A /N p by using the values of R P from Table 6, N p from Table 8 and N A = , for the run with the monomer feed time of 3.5 h. These are given in Table 9. To covert, R P in the units of gmol/s, the value of R P given in Table 6 is multiplied by a factor F=1000(g/kg)/[3600(s/h) MW M (g/gmol)] where MW M is the molecular mass of the monomer which for the case of vinyl acetate is 86. Thus, F= For vinyl acetate polymerization 17, k p = exp[-(19000±2900)/rt] 1/gmols. The average temperature for the run with the monomer feed time of 3.5 h can be taken as 78 C or 351 K. Substituting R = J/gmolK at this temperature and taking activation energy as J/gmol, one gets k p = l/gmols or cm 3 /gmols. Also, for vinyl acetate polymerization, maximum monomer volume fraction inside particle when aqueous phase is saturated with monomer, ф max = This can be multiplied by the density of monomer (ρ M = g/cm 3 ) and then divided by the molecular mass of the monomer (MW M = 86) to give the maximum monomer concentration in the particle, [M] p,max = 0.01 gmol/cm 3. Thus, dividing k p [M] p n by k p [M] p,max, one can get n min, the minimum number of average number of radicals per particle. Here, it has been assumed that the particles are having maximum saturated monomer concentration but under monomer-starved conditions, [M] p < [M] p,max and hence, through these calculations, we can get the lower bound of n (Table 9). It can be concluded that during most part of the polymerization (except the initial 0.5 h), Smith-Ewart case 3 kinetics is observed (n> 0.5). This can be attributed to large particle sizes ~ 1 micron. Table 9 Variation with time of minimum number of the average number of radicals per particle during the course of polymerization for the run with monomer feed time of 3.5 h Time R p R p N p k p [M] p n n min duration (h) (kg/h) (kg/h) final

9 42 INDIAN J. CHEM. TECHNOL., JANUARY 2010 Conclusions Through a systematic, experimental study on a pilot reactor, it is shown that the monomer feed time could be safely decreased from 5.5 h to 2.5 h; thus, decreasing the total batch time by 30%. Reducing the monomer feed time gives an improvement in the application property which in this case was the bending length. Also, it leads to product with larger particle size and as a result with lower viscosity. The reactor temperature during these runs could be safely controlled between C. The effect of decreased monomer feed time on the particle size and viscosity was attributed to presence of coagulation during the course of polymerization. A fundamental understanding of the polymerization behaviour was provided by analyzing the samples withdrawn during the course of polymerization for various properties. The rate of polymerization, the number of particles and the minimum number of average radicals per particle during the course of polymerization were calculated using these measurements. The polymerization rate was compared with the monomer feed rate. It was found that initially the polymerization rate is slower than the monomer feed rate and the unreacted monomer in this stage leads to a higher polymerization rate than the monomer feed rate sometimes during the course of polymerization. In the remaining time, the polymerization rate is more or less equal to the monomer feed rate as is the characteristic of the monomer-starved semi-batch operations. The number of particles was found to initially increase and then decrease with time which indicates the presence of significant coagulation throughout the polymerization, even after the stoppage of monomer feeding. It was also found that during most of the polymerization, Smith-Ewart case 3 kinetics is followed due to large size of the particles ~ 1 micron. Nomenclature k p = Propagation rate constant m M = Mass of monomer fed m oc = Mass of other components m P = Mass of polymer formed m solid = Mass of total solids m W Mass of water MW M = Molecular weight of monomer [M] p = Monomer concentration in the particle n = Average number of radicals per particle N A = Avogadro s Number N P = Number of particles R = Universal gas constant R P = Rate of polymerization t = Time T = Temperature ρ M = Density of monomer ρ P = Density of polymer ф = Monomer volume fraction in the particle References 1 Brydson J A, Plastics Materials, 7 th edn (Butterworth Heinemann, Oxford), Sandler S R & Karo W, Polymer Synthesis, 2 nd edn (Academic Press, New York), Geddes K R, In Wood Adhesives: Chemistry and Technology, Vol. 2, edited by A Pizzi (Marcel Dekker Inc., New York), 1983, Nakamae M, Yuki K, Sato T & Maruyama H, Colloids Surfaces A: Physicochem Eng Aspects, 153 (1999) Carra S, Sliepcevich A, Canevarolo A & Carra S, Polymer, 46 (2005) Gilmore C M, Poehlein G W & Schork F J, J Appl Polym Sci, 48 (1993) Gilmore C M, Poehlein G W & Schork F J, J Appl Polym Sci, 48 (1993) Ramirez J C, Herrera-Ordonez J & Gonzalez VA, Polymer, 47 (2006) Ramirez J C & Herrera-Ordonez J, Euro Polym J, 43 (2007) Earhart N J, Dimonie V L, El-Aasser M S & Vanderhoff J W, In Infrared Studies on the Grafting Reactions of Poly(vinyl alcohol) edited by C D Craver & T Provder (American Chemical Society), 1990, Magallanes G S, Dimonie V L, Sudol E D, Yue H J, Klein A & El-Aasser M S, J Polym Sci Part A: Polym Chem, 34 (1996) Egert H, Dimonie V L, Sudol E D, Klein A & El-Aasser M S, J Appl Polym Sci, 82 (2001) Budhlall B M, Sudol E D, Dimonie V L & El-Aasser M S, J Polym Sci Part A: Polym Chem, 39 (2001) Suzuki A, Matsuda Y, Masuda T, Kikuchi K & Okaya T, Colloid Polym Sci, 285 (2006) Gustin J, Chemical Health and Safety, 12 (2005) Warson H, In Frontiers Between Theory and Industrial Practice in Vinyl Acetate Polymerizations, edited by M S El- Aasser & J W Vanderhoff (Applied Science Publishers, New York), 1981, Kamachi M & Amada Y, In Polymer Handbook, edited by J Brandrup, E H Immergut & E A Grulke (Wiley Interscience, New Jersey), 1999.