Property Variation of Sodium Carboxymethyl Cellulose (CMC) under High Temperatures

The performance and chemical stability of Sodium Carboxymethyl Cellulose (CMC) are susceptible to temperature fluctuations. Exposure to high temperatures triggers a sequential transformation of CMC from physical changes to chemical degradation, which can be summarized as a progressive process: reversible viscosity reduction → irreversible viscosity loss → decarboxylation and chain scission → thermal decomposition and carbonization.

I. Physical Property Changes in Aqueous Solutions

The thickening capacity of aqueous CMC solutions is most sensitive to heat. At temperatures below 50 °C, heating reduces solution viscosity due to intensified molecular thermal motion, which weakens physical entanglement, hydrogen bonding and other intermolecular interactions among polymer chains. This process is generally reversible, and viscosity recovers upon cooling.

Nevertheless, prolonged heating above 80 °C leads to irreversible viscosity loss. High temperatures induce degradation and cleavage of macromolecular chains; shortened molecular chains can no longer form an effective thickening network, so viscosity fails to restore after cooling. Chain scission accelerates further in alkaline systems or in the presence of dissolved oxygen.

In addition, sustained high temperatures (150 °C and above) cause severe discoloration of the solution, gradually shifting from colorless or pale yellow to tawny and even brown, a visual indicator of polymer oxidation or carbonization.

II. Thermal Degradation of Solid-State CMC and Its Microstructure

Solid CMC exhibits relatively favorable thermal stability below 150 °C. Intermolecular crosslinking enables it to retain fundamental properties within this temperature range, so it can be applied as a heat-resistant thickener in processes such as petroleum well cementing and ceramic manufacturing.

When the temperature rises to 200–250 °C, degradation accelerates markedly. On one hand, decarboxylation occurs on carboxymethyl side chains, releasing volatile small molecules including carbon dioxide, which lowers the degree of substitution and alters chain structures. On the other hand, oxidative cleavage of the main chain takes place under heat and oxygen, leading to a sharp drop in molecular weight and rapid loss of intrinsic high-molecular-weight characteristics.

At 250–300 °C, CMC enters a violent thermal decomposition stage featuring extensive chain scission, depolymerization and carbonization accompanied by drastic mass loss shown on thermogravimetric curves. Carbonoxygen and carbonhydrogen bonds are completely broken; primary decomposition products include carbon dioxide, water vapor and trace lowmolecular-weight organics, leaving carbonized residues in the end. The original functional properties of the polymer are fully lost at this stage.

III. Factors Influencing High-Temperature Stability

The heat resistance of CMC is not fixed but governed by inherent product parameters:

Degree of Substitution (DS) It refers to the average number of hydroxyl groups substituted by carboxymethyl groups on cellulose anhydroglucose rings. A higher DS introduces more hydrophilic anionic groups and strengthens electrostatic repulsion and solvation between molecular chains, delivering higher solution viscosity as well as superior heat resistance and salt tolerance.

Molecular Weight High-molecular-weight CMC delivers high initial viscosity yet still undergoes chain scission under high temperatures. Moderate crosslinking or optimized polymerization techniques can improve thermal stability.

Ambient pH Value CMC achieves optimal stability under neutral to weakly alkaline conditions (pH 7–9). In high-temperature acidic media (pH＜4), sodium carboxylate converts into water-insoluble carboxylic acid form and precipitates out, accelerating performance failure.

IV. Practical Heat-Resistance Recommendations for Industrial Applications

Targeted measures should be adopted in industrial production to preserve CMC functionality:

Minimize heating duration and avoid prolonged high-temperature cooking when processing or operating above 80 °C.

For extreme working scenarios exceeding 150 °C such as deep-well drilling fluids and high-temperature printing pastes, select specially modified heat-resistant CMC grades with high substitution degree and high purity. Oxygen scavengers, antioxidants or thermal stabilizers may be incorporated as needed to suppress oxidative chain scission and decarboxylation, delaying irreversible viscosity decline.

Formulate systems to maintain a neutral-to-weakly alkaline pH, and avoid coexistence with strongly acidic materials under high-temperature conditions.

The deterioration of CMC under high temperatures is a progressive damage process dependent on temperature ranges. Insight into the mechanisms of physical viscosity attenuation, chemical degradation and thermal decomposition supports rational material selection and process optimization to maximize the application value of CMC.