All Ayoub, Christophe Bliard
Laboratoire de Pharmacognosie, Faculte de Pharmacie, University of Reims, Reims, France
Cationisation of Glycerol Plasticised Wheat Starch under Microhydric Molten Conditions
Cationisation of glycerol plasticised wheat starch was performed in a molten mixture under the
action of thermo-mechanical energy using two reagents: 3-chloro-2-hydroxypropyltrimethylammonium chloride and 2-epoxypropyltrimethylammonium chloride. The reaction
was catalysed with sodium hydroxide under microhydric conditions. The results showed that under
these conditions the reaction proceeds rapidly and reaches completion within a few minutes. The
reaction efficiencies were higher for lower degrees of substitution (DS). The epoxide showed a
higher reactivity than the chloro derivative. Both destructurisation of starch granules and increased
sodium hydroxide concentration enhanced the reaction, but an increase in plasticiser concentration
had a negative effect. Measurements of intrinsic viscosity showed a decrease of the average
molecular weight of the product, which was attributed to the thermo-mechanical melting process.
An increase in water affinity with DS was seen in the water sorption properties of the chemically
modified starch.
Keywords: Cationic starch; Plasticised starch; Microhydric; Glycerol
1 Introduction
Chemically modified starches are widely used in the food and non-food industries. In the paper and
textiles industries cationic starches are used on a large scale [1, 2]. Commercial cationic starches are
synthesised in diluted suspension or with concentrated starch gels. These heterogeneous phase
reactions are carried out by direct conversion of either the semi-crystalline granules in aqueous
suspension or as dry processes [3-5]. Use of cationic starches in papermaking has been described by
several authors [6-8]. Numerous ways of synthesising or modifying polymers, including starches,
using reactive extrusion can also be found in the literature. Carrand Cunningham [9] successfully
employed a twin-screw extrusion technique to produce glycol glucosides by acetalisation of
starches depolymerised by extrusion. Meuser et al. [10] have used the extrusion technique to
prepare cationic and anionic derivatives of starch. Carr[11] describes the production of cationic
starch using amino, ammonium, sulfonium and phosphonium groups, one of the advantages being
that the cationic starches synthesised by the extrusion technique are soluble in cold water. The most
common reagent used to effect Cationisation of starch is 3-chloro-2hydroxypropyltrimethylammonium chloride (I). The feasibility of the reaction of water-plasticised
native or pre-extruded starch with this reagent has been studied
Correspondence: Christophe Bliard, Laboratoire de Pharmacognosie, Faculte de Pharmacie. UMR
6013 CNRS. Bat 18 Eu-ropol'Agro, Moulin de la Mousse. URCA, BP 1039, Reims 51097 cedex 2,
France. Phone: +33 3 26 91 34 95, Fax: +33 3 26 91 35 96, e-mail: christophe.bliard @ univreims.fr.
by Delia Valle et al. [12], among others. Delia Valle et al. used a twin-screw extruder to mix the
components and trigger the chemical reaction using sodium hydroxide as a co-reagent at a total
water content of 30 to 85%. The reaction's progression was then monitored for several days.
Starch can also be plasticised with organic plasticisers such as glycol, glycerol, sorbitol or urea in
order to obtain thermoplastic behaviour. Under the action of thermo-mechanical energy, the starch
granule containing a plasticis-ing agent will melt at low temperature and a very low water content.
Previous work in our laboratory dealt with plasticising starch at low water content (microhydric
conditions) [13]. As water decomposes the Cationisation reagents under alkaline conditions (Fig. 1),
we were led to study the Cationisation reaction under these microhydric molten conditions.
In this paper the results of a study on the Cationisation reaction of plasticised starch are presented,
the reaction being carried out in a microhydric melted phase using the reactive extrusion technique
with glycerol as a plasticiser. With reagent (I), two steps are involved (Fig. 1). In the presence of a
base, the chlorine atom undergoes an intramolecular nucleophilic substitution by the neighbouring
hydroxyl group to form the epoxide; then the epoxide (II) formed in-situ reacts with the hydroxyl
groups of starch under alkaline conditions. Therefore we decided to compare the reaction efficiency
for the two reagents 3-chloro-2-hydroxypropyltrimethylammonium chloride (I) and its oxirane
derivative 2-epoxypropyltrimethylammonium chloride (II). The reactions were carried out in a
bench top twin-screw microcompounder. This technique was
Fig 1. Cationisation reaction of starch and degradation reactions of (I) and (II).
chosen for its simplicity as compared to other methods operating on much larger scales. Several
parameters such as reagent concentration and type, molar ratio sodium hydroxide/reagent,
plasticiser concentration, starch granule destructurisation, temperature and reaction time were
examined. The process involved several steps. First a dry compound was prepared by adsorption of
the plasticiser in starch, then the catalyst and the reagent were added with the total water content
kept below 15%. This compound was transformed in the reactor under controlled thermomechanical conditions. The results presented in this paper deal with modified starches prepared
according to this process.
2 Materials and Methods 2.1 Reagents and Equipment
Native wheat starch (12.5% moisture) was provided by Chamtor (Bazancourt, France). The reagents
3-chloro-2-hydroxypropyltrimethylammonium chloride QUAB188® (I) and 2epoxypropyltrimethylammonium chloride QUAB 151® (II) were provided as 70% aqueous
solutions by the Degussa Corporation (Antwerp, Belgium). Sodium hydroxide, glycerol and other
chemicals were purchased from VWR International, (Fontenay\ Bois, France). The dialysis tubes
were purchased from Membranes Filtration Products Inc. (Seguin, TX, USA). The elementary
analyses were performed on a CHN 2400 Analyser (Perkin Elmer, Norwalk, CO, USA). The proton
NMR spectra were recorded at 313K on a 500 MHz AC-500 spectrometer from Bruker
(Wissembourg, France). Intrinsic viscosities were obtained using an AVS 400 semi-automatic
viscosimeter (Schott, Mainz, Germany). The reactions
were performed in a "Minilab Rheomex CTW5" (RHEO S.A., Champlan, France).
2.2 Reactor
The reactor is a device designed for compounding and analysing the rheological behaviour of
polymers on a 5 g scale. It consists of a sealed body containing two co-rotating conical screws. The
system is fed once by compacting the mix loaded in a compartment with a pneumatic piston at the
beginning of the cycle. The system's temperature is regulated by electric resistors and airflow. Via
an integrated back flow channel the filled-in mix can be reintroduced in the system, upstream in a
loop. A pneumatically controlled bypass flushes the sample to the air through a die at the end of the
cycle after a chosen reaction time. The measurement of the motor torque and pressure from the
sensors in the loop channel allow the monitoring of the sample's rheological behaviour.
2.3 Preparation of the samples and reaction
The plasticiser is adsorbed to the starch by heating a mixture of glycerol and starch at 175 °C for 45
min prior to adding the reagents in aqueous solution. In a typical experiment sodium hydroxide was
dissolved in half of the total amount of water, while the reagent was dissolved in the other half. The
reagent and the sodium hydroxide solutions were added to the starch/glycerol binary blend and the
resulting powder was introduced into the reactor. The beginning of the reaction was calculated from
the piston lowering time. After a given reaction time the modified mix was extruded out of the die
as a supple thread that rapidly became hard and brittle at room temperature. These samples were
immediately washed by dialysis in 10 KD
MWCO tubes against a large quantity of deionised water for 48 h before being lyophilised and
powdered. The fused starch (F) used in the comparative study with native starch was obtained by a
single extrusion under standard conditions (120 °C, 15% glycerol, 15% water, 120 rpm, 60 bar, 5
min).
2.4 Analysis of degree of substitution
Cationic starches are characterised by their degree of substitution (DS) corresponding to the average
number of hydroxypropyltrimethylammonium (HPTMA) cationic groups linked through ether
bonds to each anhydroglu-cose (AG) monomer. The DS of cationic starch can be determined from
the nitrogen content with a Kjeldahl type procedure. Depending on the DS, this method requires a 5
to 10 g sample, thus consuming the whole reaction mass. Other methods for determining the DS,
such as elementary analysis or proton NMR, require only 10 - 20 mg samples to obtain results of
the same accuracy. The samples were dried under vacuum (13,3 Pa, 0.1 mm Hg) at 60 °C for 16 h
over P2O5 prior to analysis.
2.5 Recording of NMR spectra
The samples were dissolved in D2O (10 mg in 500 uL D2O +100 uL of a 10 mM NaOD solution).
The DS value was obtained by comparing the relative integrations of the quaternary
trimethylammonium protons (nine protons at 3.4 ppm) with those of the glucose anomeric protons
(one proton, from 4.8 to 5.9 ppm). This DS value can also be obtained from the integration of the
4.55 ppm peak corresponding to the hydrogen atom near the HPTMA's hy-droxyl group of the
grafted residues.
The comparative analysis of these three methods was performed in duplicate on several commercial
and syn-thesised samples providing results with less than 10% relative variation. The high and low
DS samples used in the intrinsic viscosity measurement and the water sorption analysis were
prepared using standard conditions and the following molar ratio: NaOH/ (I) = 1 and (I) / AG = 0.5
for DS 0.13 and (I) / AG = 0.05 for DS 0.02.
2.6 Reaction efficiency
The reaction efficiency (RE) of each experiment, calculated as the percentage of reacted (I) or (II),
was obtained from the ratio measured DS : theoretical DS, the theoretical DS corresponding to the
molar ratio reagent: AG.
2.7 Measurement of intrinsic viscosity
The intrinsic viscosities were obtained at 25 °C with a 100 mm length and 1 mm diameter capillary.
The starch solutions were prepared by dilution in a 1 M aqueous
KOH solution for 24 h followed by magnetic stirring for another 24 h. The obtained solutions were
filtered at 0.2 urn. The concentration of the solutions were in the 1 to 5 g/L range. Measurements
were performed starting with the lower concentrations.
2.8 Measurement of water sorption properties
The powdered starch samples, dried at constant mass over phosphorus pentoxide (P2O5), were
placed in a container kept at a 98% relative humidity (RH) at 21 °C by a saturated aqueous CuSO4
solution. The relative humidity level was controlled with a hygrometer. The mass increase of the
samples was recorded over 200 h.
3 Results and Discussion
The two reagents (I) and (II) were reacted with glycerol plasticised starch in the presence of sodium
hydroxide under various conditions. The influence of parameters such as molar ratio NaOH :
reagent and reagent : AG, temperature, plasticiser content, starch conditioning, reagents addition
order and reaction time on the reactivity were studied.
3.1 Influence of sodium hydroxide concentration
The influence of the sodium hydroxide concentration on RE was measured for the two reagents by
varying the ratio NaOH : reagent from 0.1 to 3 equivalents with a theoretical DS set at 0.05.
The evolution of the resulting RE is shown in Fig. 2 for both reagents. The results show that RE
depends on the molar ratio NaOH : reagent and the nature of the reagent. With reagent (I), different
regions can be distinguished. There is an initial increase for molar ratio 0 to 0.3, followed by a
plateau between 0.3 and 0.7 and a second increase between 0.7 and 2, followed by a second plateau
between 2 and 3. If the molar ratio is smaller than 1, part
of the base is consumed in the in-situ oxirane formation (Fig. 1), the few remaining hydroxyl ions
catalyse the reaction, leading to a low RE. The reason for the occurrence of the first plateau is
unclear. However, when the molar ratio is higher than 1, after the consumption of one equivalent of
base in the intramolecular nucleophilic substitution of (I), there are still enough hydroxyl ions available to catalyse the cationisation.
With reagent (II), higher RE value were obtained. The RE rapidly reaches a maximum level at
molar ratio >1.
3.2 Influence of the reagent concentration
The influence of the reagent concentration has been studied with a ratio reagent: AG in the range of
0.015 to 0.5 for (I) (Fig. 3A) and from 0.015 to 0.1 for (II) (Fig. 3B). The NaOH : reagent ratio was
set at 1 for (I) and 0.3 for (II) in order to match RE as seen in the previous section. For both
reagents, with an increase of the reagent concentration, the final DS increases whereas the RE
decreases. The highest RE obtained with (I) was 70% for a molar ratio reagent: AG of 0.015. The
RE rapidly decreases with increasing reagent concentration down to 20% for a molar ratio of 0.5. In
the case of (II) the RE remains higher than 70% up to a molar ratio of 0.03, and decreases to 45%
with a molar ratio of 0.1.
ig. 3. Influence of the reagent concentration A) with reagent (I) and B) with reagent (II) (NaOH : (I)
=1, NaOH: (II) =0.3; standard conditions, RE are indicated on the curve).
3.3 Influence of temperature
In order to study the influence of temperature, the reaction was carried out at 20, 100, 120 and 140
°C (Fig. 4), the molar ratio reagent: AG being 0.05, and NaOH : AG
Fig. 4. Influence of the temperature on the DS (reagent: AG = 0.05; NaOH : AG = 1 for (I) and 0.3
for (II), temperature modulated standard conditions).
for (I) and (II) being 1 and 0.3, respectively, with a reaction time of 5 min. At room temperature the
reaction proceeds directly on the semi-crystalline granules and not in a melt and a DS of 0.002 was
obtained with both reagents. Between 100-120 °C the DS increases and remains constant at a
temperature >120 °C. At 160 °C, extensive degradations were observed with intensive browning of
the product.
3.4 Influence of the plasticizer
The influence of the plasticiser level on the RE was studied by modifying the percentage of glycerol
in the compound from 14% to 28%. Results shown in Fig. 5A show a decrease of RE with larger
glycerol concentration for both reagents. This observed decrease is higher than decrease calculated
from the dilution factor. It is interesting to note that the effect of the plasticiser amount is stronger
with reagent (I). Increasing glycerol concentration from 14% to 28% decreases RE from 40% to
12% with (I) and from 96% to 36% with (II). By lowering the glass transition temperature of starch,
the plasticiser facilitates the chain's movements, leaving hydroxyl functions more accessible to
undergo reaction. An increase in the plasticiser concentration may hinder these functions.
3.5 Influence of a starch premelting step on the reactivity
The effect of the granular organisation of starch on the reaction was studied by performing the
chemical modification on starch that had been previously destructurised by fusion under standard
conditions. The comparative results obtained for native (N) and fused (F) starches using a NaOH ;
reagent ratio = 1 and a reagent: AG ratio = 0.05 are presented in Fig. 5B. By using fused starch, an
increase of 22% was noted with (I), indicating that destructurisation enhanced the reaction. The
results obtained with (II) showed no noticeable increase, indicating that the reaction was probably
close to completion.
Fig. 5. A) Influence of the glycerol percentage on RE and B) comparison of RE using native (N) or
fused (F) starch on both reagents (NaOH : reagent = 1; reagent : AG = 0.05, standard conditions).
3.6 Influence of the addition mode
The way in which the reagents are added influences the RE. Starch was reacted with both (I) and
(II) via two different addition modes (Fig. 6), with the ratio NaOH : reagent varying from 0.5 to 2:
- Mode "mixed": The reagent and sodium hydroxide were mixed in aqueous solution then left for 5
min at room temperature before addition to the starch/glyc-erol system.
- Mode "not mixed" : Sodium hydroxide in solution was added to the starch/glycerol system and
the resulting blend was left for 5 min at room temperature; then an aqueous solution of the reagent
was added to this suspension.
The results indicate that with (I) the RE obtained via the "mixed" mode are higher than those
obtained by the "not mixed" one for molar ratios 0.5 and 1. The RE increase with the base content
in both modes, but the increase is steeper with the "not mixed" mode so that at molar ratio 2, the RE
becomes higher than with the "mixed" mode. In the case of (II) the "mixed" mode gives lower RE.
Moreover, increasing
Fig. 6. Influence of the addition mode on the RE at various NaOH contents for both reagents
(reagent : AG =0.05, standard conditions).
the NaOH content will increase the RE in the "not-mixed" mode and decrease it in the "mixed"
mode.
These results could be explained as follows. With reagent
(I) the "mixed" mode favours the epoxide formation using more than one equivalent of base. Higher
base concentration will enhance the reaction but in the "mixed mode" at molar ratio 2, the
degradation of the reagent will impair the result. Similarly, with (II), the "not mixed" mode will
give better results since the hydroxyl groups activated by the base will readily react on the epoxide.
In contrast, in the "mixed" mode, the reagent will start to decompose before reacting with the
polymer; secondary reactions tend to dominate when the base content increases and consequently
the RE diminishes.
3.7 Influence of the reaction time
The influence of the reaction time was studied with both reagents (I) and (II) at 120 °C with the
molar ratios NaOH : reagent and reagent: AG set at 1 and 0.05, respectively. The results (Fig. 7)
indicate that the RE increases with time and the reaction is complete within 5 min with both
reagents. The highest RE were 40% and 94% with (I) and
(II), respectively.
Fig. 7. Influence of the reaction time on RE for (I) and (II) (NaOH : reagent =1; reagent: AG =0.05,
time modulated standard conditions).
Fig. 8. Intrinsic viscosity of native starch (N), fused starch (F), starch fused with 1% NaOH (FOH),
DS 0.02 and DS 0.13cationic starches.
water sorption capacity. The water sorption capacity of native starch (N), fused starch (F), and two
cationic starch samples with low and high DS values (DS 0.02, DS 0.13) at 98% RH was analysed
(Fig. 9). The sorption kinetics recorded over 200 h showed no significant difference in either the
kinetics or the equilibrium value for native or fused starch, unlike the two modified samples that
displayed an increasing water uptake with increasing DS. Although the equilibrium was not reached
within 200 h, the results clearly show a much larger water sorption affinity for the chemically
modified samples, which increased with higher DS.
3.8 Intrinsic viscosities
The intrinsic viscosity of the products was determined in order to observe the influence of the
melting process and the cationisation reaction on the molecular size. Fig. 8 displays the values of
intrinsic viscosities of native starch (N) compared with those of the following samples fused under
standard conditions: starch plastified with glycerol (F), starch plastified with glycerol and 1%
NaOH (FOH), and two cationic starch samples with low and high DS values (DS 0.02, DS0.13).
The comparative values of the intrinsic viscosities of fused and native starch indicate that
depolymerisation occurs during the fusion process as described by other authors [14]. This effect of
reduction of molecular size is enhanced by the presence of alkali. The values of intrinsic viscosities
obtained with the chemically modified samples are similar to those of fused starch indicating that
the chemical reaction does not affect depolymerisation.
3.9 Water sorption properties
One of the important features of cationic starches is their cold water solubility. This property can be
correlated with
Fig. 9. Water sorption of native starch (N), fused starch (F), and cationic starch samples (DS 0.02
and DS 0.13) at 98% RH.
water sorption capacity. The water sorption capacity of native starch (N), fused starch (F), and two
cationic starch samples with low and high DS values (DS 0.02, DS 0.13) at 98% RH was analysed
(Fig. 9). The sorption kinetics recorded over 200 h showed no significant difference in either the
kinetics or the equilibrium value for native or fused starch, unlike the two modified samples that
displayed an increasing water uptake with increasing DS. Although the equilibrium was not reached
within 200 h, the results clearly show a much larger water sorption affinity for the chemically
modified samples, which increased with higher DS.
4 Conclusion
In this study it was shown that starch cationisation can be performed on starch plasticised with
glycerol under mi-crohydric conditions in a molten medium. The chemical modification can reach
high degrees of conversion in a short period of time. The influence of various parameters such as
reagents concentrations, addition order, plasticis-er amount, temperature and starch granule
destructurisa-tion were examined. Reactivity depends on the reagent, the epoxide being much more
advantageous. The reaction efficiency was measured for both reagents. It was found that the
transformation of the hydrochloride (I) into the epoxide (II) in-situ is the rate determining step. The
excellent chemoselectivity of this reaction should be emphasised. Despite the presence of a
relatively large amount of the polyhydroxylated plasticiser glycerol, excellent REs were obtained,
particularly with reagent (II) (94%), which showed no interference of the plasticiser with the
reaction. Although the process of melting under thermo-mechanical energy clearly led to a
reduction of molecular size as shown by intrinsic viscosity, chemical modification did not appear to
cause any further depolymerisation. The chemical modification of starch strongly increased its
water sorption properties.
Acknowledgement
The authors wish to thank Europol'agro for financial support as part of the Amival project, G.
Stockton for careful revision of the manuscript and ARD for the Kjeldahl analysis.
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(Received: November 22, 2002) (Revised: January 24, 2003) (Accepted: February 4, 2003)