Experimental 1. Modified set up - An engineering study of supercritical CO 2 assisted continu

An engineering study of supercritical CO 2 assisted continuous polymer micron-size particles production using an SMX mixer

6.2. Experimental 1. Modified set up

A schematic drawing of the set up used in a continuous particle production is shown in Fig. 6.1. Apart from a few modifications, the set up is similar to the one used in case of the Kenics type mixer. The modifications mainly involve a gear pump (MAAG, USA) and an SMX mixer (Sulzertech, Switzerland) to overcome practical limitations of the old set up. In the old set up, it has not been possible to run the extruder at high polymer feed rates due to a high pressure build up, which causes a high backflow of material. Therefore, the gear pump was implemented between the extruder and the static mixer. The pump can work up to 35 MPa with a maximum pressure difference of 25 MPa for a material having viscosity as low as 5 Pa-s. This construction allows keeping a low pressure on the extruder side even at high polymer feed rates and a high pressure at the downstream of the pump. A CO₂injection tube is located after the pump just before the first element of the mixer.

Motor

Fig. 6.1. A schematic drawing of the modified continuous particle production set up The static mixer used in this study is shown in Fig. 6.2. It consists of an array of 18 similar stationary elements placed behind each other inside a pipe having a diameter of 22.5 mm. The diameter of the elements is 22 mm. In the mixer the flow inside the mixing elements is split up in multiple streams where every element is rotated by 90° relative to its previous element. Consequently, the mixing occurs through continuous redirecting, splitting, and stretching of the fluids as they pass through available openings. This type of mixer is known for its excellent mixing and high dispersion effect with a narrow residence time distribution. In the SMX mixer, the striation thickness is reduced due to frequent splitting and reorientation, which in turn reduces the diffusion distance for CO2

considerably. The striation thickness is defined as an average distance between interfaces of two components in the mixture. Because of a smaller striation thickness, better mixing can be expected in an SMX mixer than in a Kenics type mixer. Pressure and temperature sensors are mounted at the beginning as well as at the end of the pipe.

Fig. 6.2. The SMX static Mixer used in the continuous particle production process In the old setup, a temperature has been controlled using a hot gun where hot air has been circulated through a tube surrounding the tube of the Kenics type mixer. Although it is possible to achieve a desired temperature, the time required to achieve a constant value is long and increased at high flow rates of polymer and CO₂. Therefore, electrical heating elements are provided in the new set up, which allows a better control over a temperature within a short time.

Different cores and nozzles used in this study have already been reported in Chapter 5.

6.2.2. Experimental method

Except a few gear pump adjustments, an experimental procedure similar to the exploratory work was adopted in this study. First, a polymer was fed at a particular flow rate to the extruder. A low pressure was possible at the end of the extruder even at high flow rates using the gear pump. The pump was always kept at the lowest possible temperature because a basic working principle of the gear pump is that the higher the viscosity of polymer melt the higher is the pressure difference possible over the pump. The rest of the set up after the pump was kept at a desired processing temperature using temperature controlled heating elements. Then, CO₂ was injected into the mixer at a desired gas to polymer mass ratio. Stable pressures and CO2 flow rate were the indications of a steady condition. After the nozzle the particles were collected in a drum.

6.2.3. Particle analysis

A wet laser diffraction (WLD) apparatus, Malvern Mastersizer®, was used for the particle size measurement. Better results are obtained when the particles are spherical. During measurements, a continuous recirculation of a liquid containing a suspension of particles is provided in order to scan the particles for a large number of times . A few drops of a surfactant (a commercial soap) were added to demi-water (solvent) to prevent agglomeration of particles during the measurements. However, the possibility of agglomeration can not be avoided due to a hydrophobic nature of the polymer. The average diameter of the particles (dp,0.5) was determined from the cumulative volume fraction. A scanning electron microscope (SEM) was used to observe the morphology and shape of the particles.

6.3. Results and discussion

The results obtained using different diameter nozzles in the absence and in the presence of a core are listed in Table 6.1 and 6.2, respectively. The CO2

dissolved in the polymer is not sufficient to produce the particles due to its very high viscosity. An excess of CO₂ is required in order to break up the polymer solution into particles. As discussed in Chapter 5, a flow pattern inside a nozzle ultimately determines the product quality. Unlike with the Kenics type mixer, a relatively low quantity of a byproduct (foam) along with the particles has been obtained in the absence of a core using the SMX mixer at nearly same conditions. The expansion product has always been screened through a 900 μm size mesh. With the SMX mixer, this byproduct is not present if the nozzle is equipped with a core depending on processing conditions. Therefore, it can be conjectured that the mixing efficiency of the SMX mixer is better tha/n that of the Kenics type mixer.

6.3.1. Principle component analysis

It has been found that the production of particles or agglomerated fibers using different diameter nozzles has not been possible under the same processing conditions. The principle component analysis (PCA) method, a data clustering tool, has been used to test the effect of various processing parameters on the morphology of the product. Two principal components as a function of standardized variables are given in equation 1)-2) and 3)-4) for different nozzles in the absence and presence of a core, respectively. The results for different diameter nozzles in the absence and presence of a core are shown in Fig. 6.3.

The detailed discussion of the PCA method has been given in Chapter 5.

Table 6.1. The operating conditions and the results obtained using different nozzles in

Exp d_n (mm)

(kg/hr)

GTP (m_CO2/m_P)

T (K)

P (MPa)

dp,0.5

(μm)

Product

39 0.57 3.33 1 386 10.30 - foam

40 0.57 3.33 2 389 7.05 - foam

41 0.57 7.23 3 382 12.35 - foam with particles 42 0.57 5.07 3 385 9.55 - foam with particles 43 0.57 3.33 3 388 7.35 - foam with particles 44 0.57 6.73 3 391 11.45 - foam with particles 45 0.57 8.46 3 394 14.35 118.78 particles 46 0.57 8.46 5 394 14.65 108.94 particles 47 0.57 4.59 10 392 14.70 93.54 particles 48 0.57 8.46 5 398 14.60 137.76 particles 49 0.57 8.46 3 404 13.45 153.10 particles 50 0.57 8.01 7 412 21.40 199.78 particles 51 0.81 5.63 10 374 11.80 - fibers with particles 52 0.81 7.86 7 391 11.90 73.62 particles 53 0.81 9.04 5 395 10.50 115.44 particles

Table 6.2. The operating conditions and the results obtained using different nozzles in

Exp dn

(mm)

m_P (kg/hr)

GTP (m_CO2/m_P)

Core -

T (K)

P (MPa)

d_p,0.5 (μm)

Product

91 0.81 9.35 2 B 390 9.80 - foam

92 0.81 9.35 3 B 392 11.70 116.92 particles (fibrous)

93 0.81 9.35 1 B 394 7.75 - foam

94 0.81 9.35 3 A 394 14.20 - aggl. fibers with particles 95 0.81 7.86 7 C 394 12.80 84.12 particles (fibrous)

96 0.81 9.35 2 A 395 12.00 - foam

97 0.81 9.35 3 B 403 11.40 127.50 particles

98 0.81 7.86 7 B 403 18.40 110.21 particles

99 0.81 5.48 10 B 401 18.80 114.80 particles

100 0.81 7.86 7 C 406 13.50 75.20 particles

101 0.81 9.35 1 B 406 6.80 - foam

102 0.81 9.35 2 B 407 9.10 - foam

103 0.81 7.86 7 A 412 25.65 79.89 particles (fibrous)

104 0.81 9.04 5 B 413 16.40 117.80 particles

105 0.81 5.48 10 C 412 13.60 64.27 particles 106 0.81 5.48 10 B 414 18.30 80.04 particles

107 0.81 9.35 3 A 416 13.20 123.83 particles

108 0.81 5.48 10 A 414 25.10 83.03 particles

109 0.81 9.35 3 B 419 11.00 156.67 particles

Z1=-0.033*Ts+0.739*GTPs+0.026*Ps +0.673*dns 1) Z2=-0.434*Ts-0.401*GTPs -0.673*Ps+0.445*d_ns 2) Z1=-0.152*Cs-0.389*Ts-0.434*GTPs-0.648*Ps+0.458*dns 3)

Z2=-0.589*Cs+0.457*Ts+0.385*GTPs-0.015*Ps+0.544*d_ns 4)

-4

Fig. 6.3. Plots of the results obtained using the PCA method for the two principal components, Z1 and Z2 a) core is absent b) core is present

It is clear from Fig. 6.3 that foam are mostly produced for the Z1<0 and Z2>0 in the absence of a core. In the presence of a core foam is present for the positive values of Z1 (>1). Though a clear transition can not be observed between particles and agglomerated fibers, distinct regions in which only foam is produced can be clearly seen. Accountability of multivariable dependency of the product quality on several processing parameters is the major advantage of the PCA method.

6.3.2. Vital roles of processing parameters in the particle production

As it has not been possible to perform the experiments under the same conditions for all the nozzles, a simple data-fitting model (equation 5)) has been used to present the particle size data in terms of different processing parameters.

The model predicts the average particle diameter (d_p,0.5) as a function of temperature (T), pressure(P), nozzle diameter (d_n), and gas to polymer mass ratio (GTP).

d_p,0.5 = 93+ exp(a+b*T+d*GTP+e*P+m*d_n) 5)

Where, a, b, d, e, and m are the fitting parameters. The value of a, b, d, e, and m are 28.712, 0.0934, -0.0803, -0.0666, and -0.007, respectively. A non-linear least-square regression procedure (MatLab 7) has been used in order to predict the average particle diameter. A procedure involves a minimization of the sum of the deviations between predicted and experimental data. The model fits to the experimental data with an average relative deviation of ~ 9 %, Fig. 6.4.

50 100 150 200 250 300

d_p,0.5 exp., μm dp,0.5 pred., μm

Fig. 6.4. The fitting of the experimental data obtained in the absence of the cores (for 0.4 and 0.57 mm nozzle) to the proposed model

Three dimensional curves, the average particle diameteras a function of two other variables, can be obtained using this model. This model has not been applied to the data obtained using cores. The amount of data is not sufficient as the particles are produced only at high temperatures. Some of the results predicted using the model for 0.41 mm and 0.57 mm diameter nozzles in an absence of a core are shown in Fig. 6.5.

Effect of nozzle diameter and core-slot width

From Fig. 6.5a-b and Table 6.2 it is clear that the particle size decreases with increasing nozzle diameter in the presence as well as in the absence of a core.

With decreasing nozzle diameter, one would expect that the break up of a polymer melt should be possible at low pressures as the shear viscosity is decreased and hence, the diameter of the molten polymer film inside the nozzle is reduced. Nevertheless, opposite results have been obtained due to an increase in extensional (elongational) viscosity that resists the break up of the polymer melt. High pressures are required for smaller diameter nozzles under same isothermal conditions. Moreover, a large foaming in particles due to more amount of CO2 dissolved at high pressures may also be responsible for bigger particles.

50 100 150 200 250 300 350 400 450 500

0.08 0.085 0.09 0.095 0.1 0.105 0.11

CO2 wt. fratcion

dp,0.5, μm

3 GTP, 17.5 MPa 5 GTP, 17.5 MPa 10 GTP, 17.5 MPa

50 150 250 350

0.108 0.118 0.128 0.138 0.148

CO₂wt. fraction

dp,0.5, μm

3 GTP, 25 MPa 5 GTP, 25 MPa 10 GTP, 25 MPa

Fig. 6.5a. The particle size predicted at different processing conditions for a 0.4 mm diameter nozzle in an absence of a core

80 100 120 140 160 180 200

0.08 0.085 0.09 0.095 0.1 0.105 0.11

CO₂wt fratcion

dp,0.5, μm

3 GTP, 17.5 MPa 5 GTP, 17.5 MPa 10 GTP, 17.5 MPa

80 100 120 140 160

0.107 0.1145 0.122 0.1295 0.137 0.1445

CO₂ wt fraction

dp,0.5, μm

3 GTP, 25 MPa 5 GTP, 25 MPa 10 GTP, 25 MPa

Fig. 6.5b. The particle size predicted at different processing conditions for a 0.57 mm diameter nozzle in an absence of a core

Relatively higher temperatures are required in the presence of cores compared to that in the absence of cores to produce particles. An additional elongation caused by the slots on the cores is mainly responsible for such results. The particles produced using a 0.4 mm nozzle in the absence and the presence of a core are shown in Fig. 6.6. Particles together with fibers are present in the presence of a core even at a high temperature. However, the major advantage of using a core is that the quantity of byproduct is substantially reduced and is relatively absent at high pressures. This is due to extra mixing caused inside the core-slot before the nozzle entrance.

a) d_n=0.4 mm, GTP=5, T=381 K, and P=24.95 MPa (experiment 9)

b) d_n=0.4 mm, GTP=3, T=404 K, and P=23.50 MPa (experiment 61) Fig. 6.6. The SEM pictures of the products obtained in a) absence and b) presence of the core, C

Smaller particles are obtained in the presence of a core at elevated temperatures close to 413 K at the CO₂pressures lower than the pressures used in the absence.

Fig. 6.7 shows the influence of the core on the particle size for a 0.4 mm nozzle.

In Fig. 6.7, the values without a core are predicted from equation 5) and the values with a core are experimentally obtained for similar processing conditions.

Similar to the nozzle diameter effect, the smaller the core-slot width the higher is the elongation. Such effect is absent at high temperatures most probably due to low extensional viscosity and low shear viscosity that allow a better distribution of excess CO₂ in the molten polymer. A large improvement over the particle size distribution can be seen in Fig. 6.8 when compared to that obtained using the traditional method.

0 100 200 300 400 500 600

dp,0.5, μm

no core core

Fig. 6.7. Influence of the core on the particle size for various conditions using a 0.4 mm nozzle. (P refers to the size predicted using the model, equation 5))

0 1 2 3 4 5 6 7

0 200 400 600 800 1000

d_p,0.5, μm

qln, %

grinding core B core A core C

Fig. 6.8. The particle size distributions obtained in the presence of different cores for a 0.81 mm nozzle (core A: exp 108, core B: exp 106, and core C: exp 105) and obtained using a grinding method

Effect of gas to polymer mass ratio

With increasing gas to polymer mass ratio, the expansion product transforms from foam (microcellular) to particles or agglomerated fibers. Also, the particle size decreases with increasing ratio in the absence as well as in the presence of cores. As the ratio is increased above the solubility, CO₂ bubbles are present and their size increases with increasing ratio. Consequently the thickness of the CO₂ saturated polymer film decreases. A sudden depressurization of the excess CO2

leads to disturbances on the surface of the film and helps in better atomization of the melt. A positive effect of the ratio is present also on the particle size distribution in the absence of a core, Fig. 6.9. The effect is insignificant in the presence of cores probably due to a better distribution of CO2 bubbles.

65P exp 65 66P exp 66 67P exp 67 68P exp 68 64P exp 64

Fig. 6.9. The effect of gas to polymer mass ratio (GTP) on the particle size distribution obtained a) in the absence of core b) in the presence of core B

Effect of temperature and pressure

Fig. 6.5 clearly shows that the temperature and pressure contribute significantly in the particle production process. The particle size is decreased with decreasing temperature and increasing pressure, which can be related to high CO₂ solubility. However, the effect of pressure at low temperatures, means high CO2

weight fraction, is insignificant, which is due to high shear and extensional viscosity of the material. The lower the shear or extensional viscosity of a polymer melt the easier is the expansion of the polymer melt.

It has been shown in Chapter 5 that a significant reduction in the shear viscosity can be achieved due to dissolved CO₂. Indeed, this assists in the production of particles. In the absence of CO2, it has been observed that the particle production

has not been possible despite very high pressure and temperature conditions (~30 MPa and ~413 K) both in the absence as well as in the presence of a core.

In the particle production, the break up of polymer melt is caused not only by the expansion of dissolved CO2 but also by the expansion of excess of CO2. The density of CO₂ increases with increasing pressure and decreases with increasing temperature. A sudden depressurization of dense CO₂ to atmospheric condition causes an abrupt enhancement in its specific volume. This leads not only to a rapid nucleation of CO2 bubbles inside the polymer melt due to supersaturation but also to intense disturbances on the surface of the polymer film. At low temperatures, a resistance from viscous forces to the disturbances and the expansion of dissolved CO2 is high despite a significant shear viscosity reduction caused by the dissolved CO₂. Therefore, the effect of pressure comes into the picture when a temperature is increased above a certain value. For isobaric conditions, an increment in the particle size with respect to increasing temperature in the absence of a core can be related to low CO2 solubility and low CO₂ density.

The particle size distributions obtained at different temperature and pressure conditions for different nozzles are shown in Fig. 6.10. In the absence of core, a narrower particle size distribution is obtained at the low temperature. It suggests that expansion of the gas saturated polymer solution at different temperatures is determined by the density and solubility of CO2.

In the presence of core, a completely different picture can be seen in Fig. 6.10.

At the high temperature, a relatively narrow particle size distribution is obtained.

However, at the low temperature a positive effect of the pressure on the particle size distribution is present. One cannot compare the two results above as the diameter of the nozzle is different. A better expansion of uniformly dispersed excess CO2 due to low viscosity at the high temperature and a high super saturation of the dissolved CO₂ at the high pressure are responsible for the narrow particle size distribution in both cases.

6.3.3. Shape and morphology of PPB particles

The shape and morphology of particles is as important as the particle size in any powder application. The spherical shape is always preferred in various applications because of its good flow ability and of a good heat transfer.

However, the shape becomes less important as the particle size reduces to micron level. The effect of nozzle diameter on the shape of the particles has already been discussed earlier in this chapter. Particles with different shapes (irregular, nearly-spherical and fibrous) and different morphologies (dense or foam) can be created by playing with processing conditions. The particles with different morphologies and shapes obtained in this study under different processing conditions for different diameter nozzles are shown in Fig. 6.11a-e.

Fig. 6.10. The effect of temperature (a and b) and pressure (c) on the particle size distribution obtained using different diameter nozzles

dn=0.4 mm, GTP=5, T=401 K, and P=24.3 MPa (exp 21)

d_n=0.81 mm, GTP=10, T=374 K, and P=11.8 MPa (exp 51)

Fig. 6.11a. Scanning electron microscope pictures of different morphologies and shapes obtained using different diameter nozzles

d_n=0.57 mm, GTP=10, T=392 K, and P=14.7 MPa (exp 47)

d_n=0.57 mm, GTP=3, T=404 K, and P=13.45 MPa (exp 49)

dn=0.57 mm, GTP=3, T=371 K, and P=12.15 MPa (exp 33)

Fig. 6.11b. Scanning electron microscope pictures of different morphologies and shapes obtained using different diameter nozzles

d_n=0.81 mm, core=C, GTP=7, T=394 K, and P=12.80 MPa (exp 95)

dn=0.81 mm, core=B, GTP=7, T= 403 K, and P=18.4 MPa (exp 98)

d_n=0.81 mm, core=C, GTP=10, T=412 K, and P=13.6 MPa (exp 105) Fig. 6.11c. Scanning electron microscope pictures of different morphologies and shapes obtained using different diameter nozzles

d_n=0.81 mm, core=A, GTP=3, T=416 K, and P=13.20 MPa (exp 107)

d_n=0.81 mm, core=B, GTP=3, T=419 K, and P=11.00 MPa (exp 109)

dn=0.57 mm, core=B, GTP=10, T=405 K, and P=19.80 MPa (exp 81) Fig. 6.11d. Scanning electron microscope pictures of different morphologies and shapes obtained using different diameter nozzles

d_n=0.40 mm, core=C, GTP=4, T=404 K, and P=26.10 MPa (exp 63)

dn=0.40 mm, core=B, GTP=3, T=413 K, and P=18.40 MPa (exp 65)

d_n=0.40 mm, core=C, GTP=3, T=413 K, and P=23.4 MPa (exp 67)

Fig. 6.11e. Scanning electron microscope pictures of different morphologies and shapes obtained using different diameter nozzles

From the scanning electron microscope pictures shown in Fig. 6.11a-e and the processing conditions, morphology, and shape of the expanded product as a function of temperature and pressure can be represented schematically as shown in Fig. 6.12. At low pressures, because of low CO2 density, expansion of excess as well as dissolved CO₂ does not overcome the viscous forces if the temperature is low. Consequently, a break up of polymer melt is not possible and hence, it results into foam. At low temperatures and at high pressures irregular shaped foamed particles are formed in the absence of core. Foaming of particles is due to a large amount of CO₂ dissolved at high pressures. The irregular shape acquired by the particles is due to a rapid solidification caused by the evaporation of the dissolved CO2. The fibre shown along with the nearly spherical particle is the situation where a core is present. The additional elongation caused by the slot in the core is mainly responsible for the production of agglomerated fibers. A quantity of fibers reduces with an increase in the diameter of the nozzle, the core-slot width, and the temperature due to a decrease in the extensional viscosity. At high temperatures and low pressures, a reduction in the viscosity of a polymer caused by the temperature leads to an improved expansion. However, foam is formed along with particles due to a low pressure. At high temperatures and high pressures, nearly-spherical dense

In document Polymer melt micronisation using supercritical carbon dioxide as processing Nalawade, Sameer (Page 96-124)