Use of a Twin-Screw Extruder as a Chemical Reactor for Starch Cationization Guy Della Valle, Paul Colonna, and Jean Tayeb, Nantes (France) zyx The objective of this work was to study the obtaining of cationic starch by using a pilot scale twin-screw extruder Clextral BC45 as a chemical reactor. The reagent was 3-chloro 2-hydroxypropyltrimethyl ammonium chloride (CHPTMA) and catalyst was sodium hydroxide (NaOH). In a first set of experiments extruded starch gave higher chemical efficiencies than native starch zyxwvutsrq (+15%) both being fed at the same location of the barrel. Influence of extrusion conditions on reaction kinetics was studied starting from extruded starch in order to optimize the reaction without interfering with starch melting phe- nomenon. In a second set of experiments, a special screw design was implemented in order to obtain a homogeneous starch molten phase in a first stage, and to achieve starch modification in a second stage. Changes of reacting conditions were performed through the modifica- tion of reagent quantities and molar ratios related to starch and NaOH, location of reagent injection, temperature of reaction and Specific Me- chanical Energy input. Samples were evaluated through their degree of substitution (final DS < 0.09); their homogeneity and rheological be- havior were studied by ion exchange chromatography and Brabender viscoamylograph,resp.. Reaction efficiency was computed to optimize the process, leading to values up to 82%. Extruded samples were then compared to commercial cat ionic starches. Verwendung eines Zweiwellen-Extruders als chemischen Reak- tor for die Kationisierung von Starke. Das Ziel dieser Arbeit war, unter Verwendung eines Pilot-Zweiwellen-Extruders Clextral BC45 zy als chemischem Reaktor kationische Starke zu erhalten. Das Reagenz was z 3-Chlor-2-hydroxypropyl-ammoniumchlorid (CHPTMA). und als Ka- talysator Diente Natriumhydroxid (NaOH). In einer ersten Versuchsrei- he extrudierte zeigte hBhere chemische Wirksamkeit als native Starke (+15%), wobei beide am gleichen Ort des Rohres zugefiihrt worden waren. Der EinfluB der Extrusionsbedingungenauf die Reaktionskine- tik wurde untersucht, beginnend mit der extrudierten StLke, um die Reaktion ohne Storung durch Stake-Schmelzerschinungen zu optimie- ren. In einer zweiten Versuchsreihe wurde eine besondere Konstruktion eingefuhrt, um eine homogene Phase geschmolzener Starke in einer ersten Stufe zu erhalten und in einer zweiten Stufe die Starkemo- difizierung durchzufuhren. Veranderungen der Reaktionsbedingungen wurden durch Variation der Reaganzmengen und der molaren Ver- halmisse, bezogen auf Starke und NaOH, der Stelle der Reagenzin- jektion, der Reaktionstemperatur und der Spezifischen Mechanischen Energiezufuhrung vorgenommen. Die Proben wurden nach ihrem Sub- stitutionsgrad (DS) (End-DS < zyx 0,09) bewertet; ihre Homogenitat und ihr rheologisches Verhalten wurde mittels der Ionenaustausch-Chro- matographie und dem Brabender-Amyloviskographen untersucht. Der Wirkungsgrad der Reaktion wurde zur Optimierung des Verfahrens be- rechnet, was zu Werten von bis zu 82% fuhrte. Die extrudierten Proben wurden dann mit handelsublichen kationischen Starken verglichen. 1 Introduction Chemically modified starches have been developed to overcome shortcomings of native starches (water repellency, uncontrolled viscosity, cohesive texture, sensitivity to shear and low pHs). Furthermore, the chemical modifications have extended the use- fulness of starch for many industrial applications. An increasing number of chemical modifications have been introduced in the last ten years [ 11, but the processes under which they have been achieved, wether in dry or wet phase, always refer to heteroge- neous phases, with starch as swollen semi-crystalline granules. Concurrently, there has been a growing interest in chemical and polymer engineering for the extrusion process as a new possi- bility to perform chemical reacting and compounding for syn- thetic polymers in homogeneous molten phase. An overview of the possible reactions has recently been made by zyxwvutsrq Larnblu zyxwvuts [2]: extruders designed for specific applications can overcome the drawback of short residence times. Moreover, compared with the classical batch processes, extrusion offer the advantages of continuous processing of highly concentrated polymers. In the biopolymers field, most studies on extrusion-cooking have been devoted to physicochemical transformations of starch [3]. In contrast, there has been quite few studies focused on the use of extruders for chemical reacting. Among them, Carr and Cun- ningham [4] used succcessfully a corotating intermeshing twin screw extruder to produce glycol glucosides from starch, by an acetal reaction on starch depolymerization products partly result- ing from extrusion. Meuser et al. [5] performed starch derivati- zation to anionic and cationic starches on a twin screw extruder: they emphasized the limits of system analytical approach for such studies, since Specific Mechanical Energy, residence time and product temperature were insufficient to provide a full de- scription of the phenomena. In both studies, chemical modifi- cations and starch melting were achieved simultaneously which might lead to the difficulty linked to the interpretation of the phenomena ocuring. The aim of this work was to examine the possible use of a twin screw extruder as a chemical reactor for starch chemical mod- ification, as far as possible, on an homogeneous molten phase, resulting from a melting in a first stage of the extruder. Starch cationization was chosen as a model reaction since it is of large interest for the paper industry ([6][7]). The art teaches that starch is usually slurried in water at a dry weight (d.b.) from about 10 to 42%. Reaction times are generally 10 to 20 h, at a pH of 11- 12; the temperature must be low enough to prevent gelatinization during the process. Improvement in this conventional procedure has been proposed by Procfor [8]: an aqueous starch disper- sion is continuously and simultaneously gelatinized and reacted for cationic substituent groups introduction. Necessary adapta- tions were brought to the extrusion system for give feasability to the process. Experimental samples were compared to commer- cial ones in order to assess the functionality of extruded samples. Adding to the potential advantages previously mentioned, the ex- trusion process would also lead to cold soluble cationic starches. The reaction of cationization involves two stages, first for ac- tivating the reagent in alkaline medium under an active epoxy form, and, second, to operate the substitution on starch backbone (Fig. I). For each anhydroglucose monomer, three locations are theoretically available, so the maximum throretical degree of substitution is 3. The reaction has been extensively studied by Can. and Bagby [9], on a batch process with an excess of water, and their results will be further used for comparison purpose. 300 starchhtarke 43 Nr. 8, S. 300-307 zyxwvutsr 0 VCH Verlagsgesellschaft mbH. D-6940 Weinheim, 1991 0038-9056/91/0808-0300$03.50+.25/0