Computation of starch cationization performances by twin-screw extrusion.

INTRODUCTION

Modified starches are widely used for food and non-food products, for example in the domains of paper (bedding and sizing), adhesives (stamps, book-binding, labels, plywood), textile (clothing), cosmetic (make-up), pharmaceutical products (dispersing agents), flocculation (preventing loss of oil from drilling fluids), and a lot of other applications [1].

Extrusion was first introduced in the starch industry because, contrary to the process of drum drying, it allows one to obtain starch soluble at low temperature. More recently, a lot of work has been dedicated to reactive extrusion in order to chemically modify starch, using for example oxidation [2, 3], hydrolysis [4], carboxymethylation [5], phtalation [6], acetylation [7], phosphorylation [8], and cationization [5, 9-11]. Generally speaking, the success of the reactive extrusion process is due to the fact that polymers can be directly modified in the molten state, during a single stage of transformation. Twin-screw extruders provide great flexibility and are often used to perform these chemical modifications. The modular construction of screw-barrel blocks is particularly favorable for the design of screw profiles, which are specific to the studied reactions and include, if necessary, separated zones for melting, homogenization, reaction, and gas release [12-14].

In the present study, we will focus on wheat starch cationization using a twin-screw extruder. In paper-making industry, cationic starches can increase strength, filler and fines retention, and drainage rate of the pulp. They can also lower biological oxygen demand of the white water when used as wet-end additives. Sizing agents based on cationic starches offer unique advantages due to their ionic attraction to cellulose fibers. Moreover, because of the tenacity of the ionic bonding, these modified starches are not removed during repulping of broke. As a result, the biological and chemical oxygen demands of mill effluents are lower than with other starches [1, 15].

Cationic starches are produced from starch with reagents containing amino, ammonium, sulfonium, or phosphonium groups, which are able to carry a positive charge [15]. Starch cationization consists of substituting the hydroxyl groups of the glycosyl units by one of these functional groups (see Fig. 1). The degree of substitution (DS) indicates the average number of sites per anhydroglucose unit on which there are substituent groups. Thus, if one hydroxyl on each of the anhydroglucose units has been cationized, DS is equal to 1. If all three hydroxyls are cationized, DS is maximum and equal to 3. Cationic starches used in industry usually have low DS, in the range 0.02-0.05. These values are generally sufficient for requested end-use properties.

In a first part of this work [11], we presented an experimental study of starch cationization process, carried out on a laboratory scale twin-screw extruder. We characterized not only the influence of different processing parameters such as barrel temperature, screw speed, and feed rate, but also the influence of the type of reagents, their concentrations and the type of starch plasticizer on the degree of substitution (DS) and on the reaction efficiency (RE). Depending on these parameters, DS between 0.005 and 0.6 and RE between 25 and 90% were obtained. However, even it was shown that starch cationization is possible by using a reactive extrusion process, optimization remains difficult by pure trial and error procedure. Consequently, we decided to build a theoretical model of this process, which is able to serve as a predictive tool for optimizing screw profile, processing conditions and for solving difficult scale-up problems.

[FIGURE 1 OMITTED]

We have already developed theoretical models for reactive extrusion in twin-screw extruders [16-23]. These models have been successfully applied to different types of chemical reactions, such as the transesterification of ethylene-vinyl acetate copolymer [18], the peroxide initiated controlled degradation of polypropylene [19-21], the [epsilon]-caprolactone polymerization [22], and the carboxyl terminated polyamide 12 chain extension using a dioxazoline coupling agent [23]. We will use in the following, for the case of starch cationization, a similar approach, close to the one developed for copolymer transesterification, because there is no coupling between reaction extent and rheological properties.

MATERIALS AND METHODS

Cationization Reaction

Two reagents were used: 2,3-epoxypropyltrimethylammonium chloride (Quab[c] 151) and 3-chloro 2-hydroxypropyltri-methylammonium chloride (Quab[c] 188). Both reagents were supplied by Degussa (Belgium). The reaction of cationization involves two stages with the Quab[c] 188, first for activating the reagent in alkaline medium (sodium hydroxide) under an active epoxy form, and, second, to operate the substitution on starch backbone. With the Quab[c] 151, under the epoxy form, the first stage is eliminated (see Fig. 1). In the experiments, solid sodium hydroxide (NaOH) was purchased under anhydrous pellets form Sosa Caustica Aragoneses (Spain).

Native wheat starch, with 13 wt% initial moisture content, was provided by Chamtor (Bazancourt, France). This starch contains 74% amylopectin and 26% amylose, with residual protein and lipid contents less than 0.2% and 0.7%, respectively. It is plasticized with 40% water (on dry basis).

Before analysis, the extruded cationic starch was milled, then washed with methanol for removing residual reagents. It was then filtered and dried in an oven at 100[degrees]C for 2 h. Then, the nitrogen content (%N, value between 0 and 100) of each sample of cationic starch was determined by Kjeldahl method (Gerhardt, Kjedatherm). DS is then calculated by:

DS = [[M.sub.s] x %N]/[100[M.sub.N] - [M.sub.R] x %N] (1)

where [M.sub.s], [M.sub.N] and [M.sub.R] are the molar masses of starch anhydroglucose monomer (162 g [mol.sup.-1]), nitrogen (14 g [mol.sup.-1]) and reagent once fixed on glycosyl unit (152.5 g [mol.sub.-1]), respectively. The theoretical degree of substitution, [DS.sub.th], is the theoretical value that would be obtained if the reaction efficiency was 100%. It corresponds to the molar ratio between reagent and anhydroglucose monomer. In experiments, it will define the target and allow one to adjust the values of the starch and reagent flow rates, according to:

[DS.sub.th] = [[[[rho].sub.r][Q.sub.r]]/[Q.sub.s]][[M.sub.S]/[M.sub.fr]][[I.sub.pr]/[I.sub.ps]] (2)

where [Q.sub.r] (L [h.sup.-1]) and [Q.sub.s] (kg [h.sup.-1]) are the flow rates of reagent and starch as fed in the extruder. [M.sub.fr] is the molar mass of free reagent (188 and 151 g [mol.sup.-1] for Quab[c] 188 and Quab[c] 151, respectively), [[rho].sub.r] the reagent density (1166 and 1129 kg [m.sup.-3] for Quab[c] 188 and 151), [I.sub.pr] and [I.sub.ps] the purity indices of reagent (69 and 72% for Quab[c] 188 and 151) and starch (87%). The reaction efficiency RE is logically defined by:

RE = DS/[DS.sub.th]. (3)

Reactive Extrusion Experiments

We used a laboratory scale co-rotating twin-screw extruder (Clextral BC 21, Firminy, France). Its main characteristics are as follows: centerline distance 21 mm, screw diameter 25 mm, length/diameter ratio 36. Two screw profiles were used. They are depicted in Fig. 2 and Table 1. They are composed of two-flighted conveying screw elements of various pitches, and different mixing zones; the first profile is simpler, with only a block of 8 kneading discs, negatively staggered (-45[degrees]) to insure the melting of the starch. The second one has 2 blocks of kneading discs, negatively staggered (-45[degrees]). The barrel is made of 9 sections each 100 mm, regulated at a fixed temperature (40[degrees]C for Barrel Section 1 and 80[degrees]C for the others). The die used (except for viscosity measurements) has a diameter of 10 mm and a length of 46 mm.

[FIGURE 2 OMITTED]

The starch is fed into Barrel Section 1 using a volumetric K-TRON feeder. The liquid reagents (Quab, water, eventually sodium hydroxide) are mixed together and injected in Barrel 2 (Profiles 1 and 2) or 5 (only Profile 2) using a volumic pump.

Screw speed is controlled with a variable speed-motor. After steady-state conditions are achieved, extrudates are collected directly at the die exit, stored (24 h at 25[degrees]C in sealed bags), and ground for subsequent analyses.

Viscosity Measurements

For the modeling of the cationization process, it was important to check whether the reaction development could modify the shear viscosity of the wheat starch. In this case, a strong coupling between reaction extent and rheological properties should be considered, as in polymerization or degradation reactions. Therefore, viscosity measurements were carried out. It is now well known that starchy products are very sensitive to the thermomechanical history they support during extrusion. This effect can be accounted for by introducing a dependence on the specific mechanical energy (SME) in the viscosity law [24]. The best way to characterize the viscous behavior of a molten starch is to use a rheometric die, fixed at the end of the extruder. A few years ago, a specific device with a twin channel was specially developed for this type of application [25]. A new system, specifically designed for the small scale of the laboratory extruder, was built and installed on the Clextral BC 21. Different extrusion runs were performed, varying both screw speed and feed rate to obtain various levels of specific energy (from 190 to 460 kWh [t.sup.-1], covering the range of experimental conditions for starch cationization). For these values of SME, it has been checked by DSC that the starch was completely molten. In some cases, GPC measurements have also clearly confirmed a classical depolymerization of both amylose and amylopectin [26]. Consequently, extruded cationic starches will have a high level of solubility and a lower paste viscosity, which could be an advantage for some industrial applications. For example, low viscous cationic starches are useful paper binders that provide high strength, ink hold-out, and gloss at lower levels than other starches. However, for some properties such as fibre retention, low degraded products are preferable.

From experiments with the plasticized wheat starch, without any reagent, the expression of the viscous law was depicted according to a power law [27]:

[eta] = K exp [n([E/R]([1/T] - [1/[T.sub.0]]) + [beta](SME - [SME.sub.0]) + [alpha](MC - [MC.sub.0]))][[gamma].sup.n-1] (4)

where K is the consistency, n the power law index, E the activation energy, R the gas constant, T the absolute temperature, SME the specific energy, and MC the moisture content. [alpha] and [beta] are material parameters. Subscript "0" refers to reference conditions.

From our experiments, we obtained for the wheat starch plasticized with water: K = 1920 Pa [s.sup.n], m = 0.53, E/R = 5150 K, [T.sub.0] = 363 K, [SME.sub.0] = 325 kWh [t.sup.-1], [MC.sub.0] = 0.40, [alpha] = -10.91, and [beta] = 0.0028 t [kWh.sup.-1].

These experiments were repeated with the cationic molten starches, for DS between 0.013 and 0.020, and RE between 32 and 50%, indicating that the reaction was effectively developed. It can be seen in Fig. 3 that the experimental points are perfectly superimposed with those of the molten starch. Consequently, we can say that the cationization reaction does not influence the rheological behavior of the starch, which is not surprising if we consider the low modifications brought to the chain structure. Equation 4 can thus be used for the modeling, without coupling between rheology and reaction extent.

THEORETICAL MODELING

The different elements necessary for obtaining a theoretical model for reactive extrusion processes are a kinetic scheme for the considered reaction, a flow model for the description of the flow along the extruder and, if necessary, a rheokinetic model when the rheological behavior is affected by the reaction [16, 17]. In the present case, as explained before, only the two first elements are requested. They are rapidly introduced in the following.

[FIGURE 3 OMITTED]

Kinetic Scheme

The kinetic scheme of starch cationization has been characterized in a previous publication [26]. In a homogeneous medium, the irreversible starch cationization reaction of the hydroxyl groups of the anhydroglucose units by the cationizing reagent (Quab[c] 151 or Quab[c] 188) can be written as follows (if one considers a 2nd order reaction with k, rate constant):

[GRAPHIC OMITTED]

The disparition speed of reacting starch OH groups [v.sub.A] (unit: mol [L.sub.-1] [min.sub.-1]) equals the disparition speed of the reagent [v.sub.B] and the apparition speed of the cationic groups on starch [v.sub.C]:

[v.sub.A] = -[[d[[A].sub.t]]/dt] = -[[d[[B].sub.t]]/dt] = [d[[C].sub.t]]/dt = k[[A].sub.t][[B].sub.t]. (5)

Expressing the concentration of reagent [[B].sub.t] in relation to that of the starch hydroxyl groups at time t, [[A].sub.t], and to initial concentrations, [[A].sub.0] and [[B].sub.0], incorporating this relation into Eq. 5, and integrating the resulting differential equation leads to [26]:

[[A].sub.t] = [[[[A].sub.0]/[[B].sub.0]]([[A].sub.0] - [[B].sub.0]) exp[k([[A].sub.0] - [[B].sub.0])t]]/[[[[A].sub.0]/[[B].sub.0]]exp[k([[A].sub.0] - [[B].sub.0])t] - 1]. (6)

Since the degree of substitution corresponds to the number of cationic groups linked per anhydroglucose unit (one anhydroglucose unit bears three OH groups), the theoretical degree of substitution is:

[DS.sub.th] = 3[[[B].sub.0]/[[A].sub.0]] (7)

Moreover, the relationship between degree of substitution and starch concentration is:

DS = 3[[[C].sub.t]/[[A].sub.0]] = 3[[[[A].sub.0] - [[A].sub.t]]/[[A].sub.0]] = [[DS.sub.th]/[[B].sub.0]]([[A].sub.0] - [[A].sub.t]). (8)

In fact, the entire reagent amount is not consumed for the cationization. Parasitic reactions, such as degradation of the epoxy form (Quab[c] 151) at high temperature, could occur, limiting the efficiency of the reaction. Moreover, the cationization ratio could reach a yield due to the hindered reagent diffusion toward reactive OH sites. This can be seen by considering the kinetic curves for the two reagents at different temperatures [26]. To take this effect into account, we introduce an additional parameter in the relation between efficiency RE (in %), the final degree of substitution, and the theoretical degree of substitution:

RE = [DS/[DS.sub.th]] [chi] (9)

where [chi] is the said additional parameter, varying between 0 and 100, deduced from the experimental data, and corresponding to the asymptotic value obtained for the final degree of substitution at very long times. This value depends on the temperature, the theoretical degree of substitution, and the type of the reagent [26]: for example, for Quab[c] 151 and a [DS.sub.th] of 0.1, [chi] varies from 40 at 70[degrees]C to 82 at 150[degrees]C. For Quab[c] 188 and the same [DS.sub.th], [chi] varies from 30 at 70[degrees]C to 70 at 150[degrees]C. Other values can be found in the work of Tara [28].

Twin-Screw Extrusion Model

The second element of a reactive extrusion model is a tool for computing the flow conditions along the twin-screw extruder. In such machine, the geometry and the local flow conditions are complex: the flow is unsteady, non-isothermal, and three-dimensional. Of course, even if 3D simulations are now possible, simplifications are necessary to obtain a user-friendly software. It has been done a few years ago by Vergnes et al. [29], to develop the Ludovic[c] model. This commercialized software (S & CC, Saint Etienne, France, http://www.scconsultants.com) is based on a one-dimensional (1D) approach. It allows one to calculate the main thermomechanical parameters of the process (pressure, temperature, residence time, shear rate, filled ratio ...), from the hopper to the die exit, including solid conveying, melting and melt conveying. It has been validated through comparisons with more developed numerical simulations and with experiments [21, 30]. As it is a 1D model, it provides only average local values of temperature and residence time. But it has been shown that it is largely sufficient for the prediction of the reaction extent along the screws [16-23].

[FIGURE 4 OMITTED]

In the present case, as the cationization reaction does not modify the rheological behavior of the extruded starch, a strong coupling between the reaction extent and the viscosity is not necessary. In a first step, the flow is computed, with a rheological law (Eq. 4) corresponding to the SME value of the considered case. Then, from the computed values of temperature and residence time, the degree of substitution or, equivalently, the efficiency of the reaction, are computed using Eqs. 6, 8, and 9. We assume that starch cationization starts once the reagents have been injected into the barrel, it means generally at the beginning of the melting zone.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

RESULTS AND DISCUSSION

Influence of Throughput

With Screw Profile 2 and the Quab[c] 151 at 400 rpm, for a targeted [DS.sub.th] of 0.04, the feed rate has been varied between 2.7 and 5.4 kg [h.sup.-1]. As expected, in these conditions, the computed residence time greatly decreases (from 80 to 48 s) when the temperature remains quite unchanged. Consequently, as observed in Fig. 4, the final DS and the efficiency decrease (from 0.0308 to 0.0231 and from 77.0 to 57.8%, respectively) when the feed rate increases. It can be seen in the figure that the main variations in DS occur in the two blocks of kneading discs, where the residence time is maximum. The symbols represent experimental values of DS measured at the die exit. These values are in good agreement with the computed data.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Influence of Screw Speed

Again with Screw Profile 2 and the Quab[c] 151, at 2.7 kg [h.sup.-1], for the same targeted [DS.sub.th] (0.04), the screw rotation speed has been varied between 150 and 400 rpm This case is particularly interesting because, as it can be seen in Fig. 5, an increase in screw speed induces both of the decrease in residence time and the increase in temperature by viscous dissipation. In other words, the residence time and the temperature which control the chemical reaction vary in opposite direction, making it impossible to foresee the final effect. As it can be seen in Fig. 6, the model indicates that, when the screw speed increases from 150 to 400 rpm, the computed DS increases from 0.0244 to 0.0308 and RE from 61.0 to 77.0%. The benefit due to the temperature increase (~13[degrees]C) largely compensates the decrease of residence time (~16 s). Once again, the experimental values are in good agreement with the computed values.

Influence of Barrel Temperature

In the same conditions (Screw Profile 2, Quab[c] 151, Q = 2.7 kg [h.sup.-1], N = 150 rpm, [SD.sub.th] = 0.04), the barrel temperature has been increased from 80 to 130[degrees]C. Obviously, this leads to a global increase in product temperature (die exit temperature at 136[degrees]C instead of 92[degrees]C), but without modification of residence time, as the latter is totally independent of the viscosity of the extruded product [31]. Consequently, a more advanced reaction is expected at 130[degrees]C, what is effectively observed in Fig. 7. At high temperature, a DS of 0.0331 is reached, instead of 0.0244 at 80[degrees]C, and RE also increases from 61.0 to 82.8% when the temperature is increased. The agreement with the experiments remains satisfactory.

Influence of Type and Concentration of Reagents

In this section, we have used the two reagents (Quab[c] 151 and Quab[c] 188) and three concentrations (corresponding to three values of [DS.sub.th]; 0.01, 0.04, and 0.1). Experimental data were obtained with Screw Profile 1, at 1.9 kg [h.sup.-1] and 400 rpm. This screw profile has only one mixing zone in Barrel 7, which explains the delay in the start of reaction, compared with preceding cases. Theoretical results are shown in Fig. 8. Obviously, the computed DS is higher when the [DS.sub.th] is higher, but the efficiency varies in the opposite way, that is, it is higher for a low [DS.sub.th]. Finally, if we compare the two reagents, we obtain with the model a higher DS and a higher efficiency with the Quab[c] 151, which is more reactive because of its primary epoxy form (94% instead of 79%, for [DS.sub.th] = 0.01, in the same conditions). As can be seen in Fig. 8, the theoretical predictions are well correlated with the experiments. It confirms the interest to use directly Quab[c] 151 rather than Quab[c] 188.

[FIGURE 9 OMITTED]

Influence of Position of Reagent Injection

On the preceding results, we see that the reaction develops when the starch is molten. We may imagine that the position of injection point of the reagents (Quab[c] and eventually sodium hydroxide) may play an important role on the cationization process. On Profile 2, we tested two different points, located either before or after the melting zone (first block of kneading discs). The computation was carried out for Quab[c] 151 at 400 rpm and 2.7 kg [h.sup.-1]. It can be seen in Fig. 9 that the computed DS values are higher when the reagent is injected before the melting zone. DS and ER are respectively 0.0308 and 77.0%, instead of 0.0239 and 59.8% when the reagents are injected after melting. These results are principally due to the increase in reaction time (~30 s) when the material is flowing through the first kneading block. A similar result was experimentally observed also by Della Valle et al. in 1991 [9], but the authors did not explain the phenomenon.

Influence of Screw Profile

As explained in the previous paragraphs, DS and efficiency are directly controlled by the evolutions of residence time and temperature along the screws. It is thus evident that the choice of the screw profile will play a major role in the cationization process. In order to confirm it, we will compare Screw Profiles 1 and 2 in the same processing conditions (Quab[c] 151, 400 rpm, 2.7 kg [h.sup.-1], [DS.sub.th] = 0.04, injection point before the melting zone, as indicated in Fig. 11).

[FIGURE 10 OMITTED]

Figure 10 shows the computed results concerning the flow conditions with the two screw profiles. Even if the melting is delayed on Profile 1, the temperature evolutions are quite similar. However, the residence time is 16 s higher on Profile 2, which has two blocks of kneading discs. This difference in residence time is the main reason for a higher performance of Profile 2, which allows obtaining a computed DS of 0.0308 with an efficiency of 77.0%, instead of 0.0226 and 56.5% for Profile 1 (see Fig. 11).

[FIGURE 11 OMITTED]

General Validation

If we want to resume the whole results obtained for different screw profiles, reagents, concentrations, feed rates, screw speeds, and barrel temperatures, we can plot the predicted values of efficiency as function of the experimental ones (see Fig. 12). We can see that, whatever the conditions, the agreement is satisfactory. The maximum error is about 14%. The theoretical model allows a correct prediction on a range of efficiencies between 30 and 90%.

[FIGURE 12 OMITTED]

CONCLUSIONS

In this paper, we have proposed a theoretical modeling of the cationization process of wheat starch using a twin-screw extruder. Based on a kinetic scheme and a flow simulation software (Ludovic[c]) and using a technique already developed in reactive extrusion modeling, the model allows one to calculate the degree of substitution and the efficiency of the reaction, as function of screw geometry, processing conditions, and material properties. We have systematically studied the influence on the reaction extent of the main processing parameters, such as type of reagent, concentration, feed rate, screw speeds, and barrel temperature. The computed results were, in all cases, compared with experimental measurements and they agreed with the latter. The proposed model has been proved to be satisfactory. It will be used in a next paper for the optimization of the cationization process and the solution of scale-up problems (extrapolation from a laboratory scale extruder (Clextral BC 21) to an industrial scale one (Clextral BC 45)).

REFERENCES

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16. F. Berzin, "Etude experimentale et modelisation d'une operation d'extrusion reactive," PhD Dissertation, Ecole des Mines de Paris (1998).

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23. Y. Chalamet, M. Taha, F. Berzin, and B. Vergnes, Polym. Eng. Sci., 42, 2317 (2002).

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Francoise Berzin, (1) Ahmed Tara, (1) Lan Tighzert, (1) Bruno Vergnes (2)

(1) ESIEC, CERME, Reims Cedex 2, France

(2) Ecole des Mines de Paris, CEMEF, Sophia Antipolis Cedex, France

Correspondence to: F. Berzin; e-mail: [e-mail]

Contract grant sponsor: Region Champagne-Ardenne, Conseil General de la Marne, France (through the research program Amival).

TABLE 1. Screw profiles (from hopper to die).

Profile 1
Screw pitch (mm)    33   25   16  -45/5 (a)   33  25  16
Screw length (mm)  250  200  200   50        100  50  50

Profile 2
Screw pitch (mm)    33   25   16  -45/5 (a)   33  25  16  -45/5 (a)
Screw length (mm)  250   50   50   50         50  50  50   50

Profile 1
Screw pitch (mm)
Screw length (mm)

Profile 2
Screw pitch (mm)   33   25   16
Screw length (mm)  50  125  125

(a) -45/5 denotes a block of kneading discs, with a staggering angle of
-45[degrees] (left-handed) and a disc thickness of 5 mm.