2025-06-23- readings1. Introduction
Hydroxypropyl methylcellulose (HPMC) is an important water-soluble cellulose ether. It is widely used in pharmaceutical, food, cosmetic and material industries due to its excellent biocompatibility, film-forming, thickening and solubility. Its thermal stability is crucial for processing and application. Clarifying the thermal decomposition temperature range helps optimize process conditions and avoid thermal degradation.
2. Molecular structure and thermal stability
HPMC is modified by replacing hydroxyl (-OH) with hydroxypropyl (-OCH₂CHOHCH₃) and methyl (-OCH₃) on the cellulose backbone (glucose units connected by β-1,4-glycosidic bonds). This structure gives it water solubility and other properties. As an organic polymer, its thermal stability is significantly affected by the molecular chain structure, molecular weight, substituent type and degree of substitution (DS/MS). The thermal decomposition process mainly involves molecular chain breakage and substituent thermal decomposition.
3. Thermal decomposition behavior and influencing factors
The thermal decomposition of HPMC presents stage characteristics:
Initial stage (about 150–200 °C): It is mainly the removal of physically adsorbed water and bound water, accompanied by the initial thermal decomposition of a small amount of substituents (such as hydroxypropyl dehydration), and the mass loss is relatively slow.
Main decomposition stage (about 200–300 °C): The stability of the molecular chain decreases, the cleavage of the glycosidic bond accelerates, and the hydroxypropyl and methyl groups undergo significant thermal decomposition reactions, releasing volatile products (such as small molecular alkanes, alkenes, aldehydes, ketones, water, etc.), and the mass loss rate accelerates.
Severe decomposition stage (>300 °C): The molecular chain is severely broken, the thermal decomposition reaction is violent, and a large amount of gaseous products (such as CO, CO₂, CH₄, C₂H₄, etc.) are released. The residue is mainly carbonaceous residue.
Key factors affecting thermal decomposition:
Molecular weight: HPMC with high molecular weight usually has higher thermal stability due to molecular chain entanglement, can disperse heat energy more effectively, and delay the breakage of the main chain.
Substituent type and degree of substitution: Hydroxypropyl and methyl have different thermal decomposition behaviors. The higher the degree of substitution, the more thermally decomposable groups on the unit chain segment, which usually leads to a slight decrease in the starting temperature of thermal decomposition or an intensification of the main decomposition stage.
Ambient atmosphere: In an oxidizing atmosphere (such as air), thermal decomposition is often accompanied by oxidation reactions, generating more oxygen-containing products (such as CO₂) and may reduce the apparent decomposition temperature. The thermal decomposition process under an inert atmosphere (such as N₂) is relatively "pure", and the products are mainly thermal cracking products.
4. Thermal decomposition temperature range
Combining literature and experimental data, the significant thermal decomposition temperature range of HPMC is usually 200 °C to 400 °C.
Initial water loss/slight decomposition can start at about 150 °C.
The main decomposition reaction occurs at 200–300 °C.
Violent decomposition and carbonization mainly occur above 300 °C, and tend to be completely decomposed when approaching 400 °C.
The specific decomposition temperature is regulated by the aforementioned molecular weight, degree of substitution, and environmental atmosphere.
Conclusion
The thermal stability of HPMC is closely related to its molecular structure parameters (molecular weight, degree of substitution) and external conditions (atmosphere). Its significant thermal decomposition mainly occurs in the range of 200–400 °C. Mastering this temperature range is crucial to guide the safe use of HPMC in high-temperature processing (such as hot melt extrusion, spray drying, sterilization) and high-temperature application environments, and can effectively avoid performance degradation caused by thermal degradation.
