Chitosan is widely used across various industries, including medicine, textiles, dyeing, papermaking, food, wastewater treatment, metal recycling, environmental protection, cosmetics, and daily chemicals. As a result, there has been growing interest in its production and application.
The production process of chitosan starts with shrimp shells, which go through several stages: pretreatment, digestion, acid leaching, decolorization, drying, deacetylation, and final drying to obtain the final product.
First, raw materials must be properly pre-treated. Shrimp shells are cleaned thoroughly to remove meat, dirt, and other impurities. After washing with water, they are dried in the sun and stored if not processed immediately.
Next comes the digestion stage. The main goal is to eliminate proteins and fats from the shells. This is done by placing the pre-treated shells into a reactor and adding a 18–19% sodium hydroxide solution. The mixture is then boiled for about 1.5 hours while being stirred.
Following digestion, acid leaching is performed. The purpose is to dissolve and soften the inorganic salts present in the shells. The digested material is soaked in a 4–6% hydrochloric acid solution until it becomes soft. This usually takes 1 to 2 days. If no bubbles form during the process, it indicates that more acid is needed, and the soaking should continue.
Decolorization follows. One method involves exposing the acid-leached material to sunlight after rinsing it with water until it is neutral. The shells are spread out and turned regularly until they turn white. Alternatively, oxidation using agents like potassium permanganate or sodium bisulfite can also be applied.
Deacetylation is the most critical step in the process. It directly affects the quality of the final chitosan. During this stage, the chitin reacts with a strong base, leading to both deacetylation and possible degradation of the molecular chain, which reduces the molecular weight. Key indicators of chitosan quality include the degree of deacetylation (which reflects the free amine group content) and viscosity (which reflects molecular weight). Several factors influence this step:
- **Alkali concentration**: Typically between 40% and 60%. Lower than 30% is ineffective. While increasing concentration boosts deacetylation, beyond 50%, the reaction rate slows, and the polymer chain may break, reducing viscosity.
- **Temperature**: Higher temperatures speed up the reaction. For example, at 150°C, deacetylation reaches 80% in 15 minutes, but higher temps also cause faster viscosity loss.
- **Time**: Longer time increases deacetylation but also decreases molecular weight and viscosity.
Optimal results are achieved by balancing these parameters—using around 50% alkali, moderate temperature (around 120°C), and controlled reaction times.
Finally, the product must meet certain quality standards. It should appear as a colorless or yellowish translucent solid. Physical and chemical properties include ash content below 1%, moisture less than 12%, viscosity above 600 centipoise, a deacetylation degree over 80%, and complete solubility in 5% acetic acid within 30 minutes.
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