From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 6

Shrimp and crab
shells

Acid treatment Alkali treatment Crude chitin

Chitosan Deacetylation Chitin Decolor

Figure 1.3
Preparation process of chitosan.

180°C for 30 min under nitrogen protection, transferring the mixture into ethanol solvent
to form gelatinous depositions, washing the depositions with water to neutral pH in order
to form rough chitosan, dissolving the rough product in 5% formic acid, neutralizing the
solution by using diluted NaOH to form deposits, filtering the mixture, washing the
deposits to neutral, and forming refined chitosan by repeating the said steps. The product
undergoes serious main chain degradation and hence the relative molecular weight is
small; also, the method is complicated. As a result, it was abandoned. The concentrated
alkali solution method is the most popular one, in which raw materials react with 40-50%
NaOH solution at 100-130°C for 0.5-6 h and chitosans with different DDs are generated.
The concentration of alkali solution, reaction temperature, reaction time, and shape of
solid chitin are closely related to DD.
The concentration of alkali solution, reaction temperature, and reaction time are the
main factors affecting chitosan performance (viscosity and DD). Orthogonal experiments
show that all three factors can influence the performance to varying degrees, and the
most important one is NaOH concentration [17]. Considering the main quality standards
of chitosan, which are viscosity and DD, the preparation method includes the following
steps: mixing milled chitin with 45-50% NaOH in a weight/volume ratio of 1:10, reacting
the mixture at about 90°C for 8-10 h, controlling the temperature carefully and stirring the
mixture continuously during the reaction, washing the product with water to neutral, and
drying the product to form white chitosan powder. To accelerate deacetylation, discontin-
uous water washing can be used.
In homogeneous phase, when DD is about 50%, chitosan will have good water solubil-
ity. However, when the reaction occurs in heterogeneous phase, the product is water
insoluble despite 50% DD [18]. Analysis of chemical structure proves that acetamino and
amino irregularly distribute in chains of the water-soluble chitosan with 50% DD, break-
ing molecular orderliness; that is why the product is water soluble. The water-soluble
chitosan has high solubility and is alkali soluble, so that the modification reaction can
be carried out in alkali conditions, expanding the range of chitosan research and
application [19]. Although deacetylation in homogeneous phase can generate water-sol-
uble chitosan, the reaction must occur in concentrated alkali solution and a large amount
of solvent is needed for desalting in the late stage; therefore, this method is not applica-
ble for industrial use.
The concentrated alkali solution method requires largely excess NaOH, which is a waste
product. Organic solvent can strongly permeate chitin and help alkali enter the chitin as a
diluting medium; thus organic solvent will reduce the amount of alkali while chitosan
with high DD can still be formed. A batch process can form chitosan with high DD and
high quality. When the reaction temperature is 60°C, acetone will produce higher DD and
relative molecular weight than ethanol. However, the product is yellowish and difficult to
wash when the solvent is acetone. When the reaction temperature is 80°C, water is worse
than ethanol as reaction medium because it causes low DD, unsatisfactory color, and makes