Etching behavior of carbon fiber in acid-base environment

In some specific use environments, carbon fiber may be exposed to chemicals such as acid and base, causing people to pay attention to its stability and performance changes in acid and base environments.

1. Surface morphology after etching When carbon fiber is etched in an acidic solution, its surface will change significantly. Scanning electron microscopy (SEM) observations show that the surface roughness increases, and some tiny pits and scratches appear on the otherwise smooth fiber surface. This is due to a chemical reaction between the acid and the surface of the carbon fiber, which gradually erodes the surface layer of the carbon fiber.

2. The mechanism of etching In acidic environments hydrogen ions (H +) play a key role. There are usually some active groups on the surface of carbon fiber, such as hydroxyl (-OH) and carboxylic (-COOH) groups. Hydrogen ions will undergo a displacement reaction with these active groups, such as for metal impurities on the surface of carbon fibers or some components that are easily reacted with acids, which will dissolve them, forming tiny defects on the surface. At the same time, the acid may also attack the graphite microcrystalline edge of the carbon fiber, making some carbon atoms free, resulting in the destruction of the surface structure and the increase of roughness.
In the common sulfuric acid (H₂SO₄) example, sulfuric acid ionizes hydrogen ions (H +) and sulfate ions (SO₄²⁻) in water. Hydrogen ions will be replaced with metal impurities on the surface of the carbon fiber (such as catalyst metal particles left over during the preparation of the carbon fiber), forming metal ions into the solution, while leaving a vacancy on the surface of the carbon fiber. In addition, hydrogen ions may also react with some weakly bonded carbon atoms on the surface of the carbon fiber, causing these carbon atoms to break away from the surface of the carbon fiber in the form of hydrocarbons, resulting in surface etching.
After the carbon fiber is etched by alkali solution, its surface will undergo oxidation reaction, the color may change to a certain extent, and the chemical composition of the surface will also change. From a microscopic point of view, the surface will form some oxygen-containing functional groups, such as carbonyl groups (-C=O) and ether bonds (-O-), which makes the surface hydrophilicity of carbon fiber enhanced.
The hydroxide ion (OH⁻) in alkali solution has strong oxidation property. In the etching process, hydroxide ions will react with carbon atoms on the surface of carbon fiber, oxidizing them into oxygen-containing functional groups, and may destroy part of the chemical bond of carbon fiber, making the structure of the fiber surface loose, and thus affecting the overall performance of carbon fiber. Moreover, alkali etching may preferentially act on some disordered areas or defect parts on the surface of carbon fiber, and gradually expand to the interior with the extension of etching time.
Taking sodium hydroxide as an example, hydroxide ions (OH⁻) in sodium hydroxide (NaOH) solution will undergo a REDOX reaction with carbon atoms on the surface of carbon fibers. Hydroxyl ions transfer electrons to carbon atoms, which increases the valence of carbon atoms and forms oxygen-containing functional groups, such as carbonyl groups. At the same time, due to the electron transfer generated during the reaction process, a certain change in charge distribution will be formed on the surface of the carbon fiber, which may also affect the interaction between the carbon fiber and the surrounding environment, such as the interface binding performance of the matrix resin in the composite material.

3, the effect of acid and alkali etching behavior will reduce the mechanical properties of carbon fiber to a certain extent. Surface etching will lead to a decline in the strength and modulus of carbon fiber, because the surface defects and structural damage will become stress concentration points, which is easy to cause the generation and expansion of cracks during the stress, thus reducing the carrying capacity of carbon fiber.
After acid and alkali etching, the chemical stability of carbon fiber will be changed. After acid etching, the adsorption and reactivity of some chemical substances may change due to the change of surface active groups. After alkali etching, due to the increase of surface oxygen-containing functional groups, its stability in oxidation environment may be affected, and further oxidation reactions are more likely to occur.
The etching of carbon fiber in acid-base environment is a complex process, which involves a variety of chemical reactions and physical effects between acid and base and carbon fiber surface. Acid etching results in pits and roughness increase on the surface of carbon fiber mainly through hydrogen ion replacement and attack on the surface active groups and the edge of graphite microcrystal. Alkali etching relies on the oxidation of hydroxide ions to form oxygen-containing functional groups on the surface of carbon fiber and loosen the surface structure. These etching behaviors will significantly affect the mechanical properties and chemical stability of carbon fiber. In practical applications, the acid and alkali environment that carbon fiber may be exposed to must be fully considered, and corresponding protective measures, such as surface coating, must be taken to ensure the performance reliability of carbon fiber during use.