Electrochemical Surface Treatment of Carbon Fiber

Basics_Electrolysis Anodizing_priciple Electroplating_process Extraction of sodium Electrolysis of aqueous solutions Purification of copper

Basics_Electrolysis Anode 2H2O → 4H+ + O2 + 4e- 4OH-  2H2O + O2 + 4e- The positive electrode. It is positively charged because electrons are drawn away from it. 2H2O → 4H+ + O2 + 4e- 4OH-  2H2O + O2 + 4e- Cathode The negative electrode. It is negatively charged because an excess of electrons move towards it.

Basics_Electrolyte(USP4,839,006) Electrolyte 5 % aqueous solution pH Electric conductivity of solution (m·mho/cm) Phosphoric acid 1.6 27.5 Nitric acid 0.9 184.5 Sulfuric acid 0.8 216.2 Potassium tertiary phosphate 12.4 47.0 Potassium secondary phosphate 8.8 32.5 Potassium primary phosphate 4.3 28.1 Sodium tertiary phosphate 12.3 25.3 Sodium secondary phosphate 9.1 17.7 Sodium primary phosphate 17.8 Ammonium tertiary phosphate 30.3 Ammonium secondary phosphate 8.0 42.4 Ammonium primary phosphate 4.1 30.7 Potassium nitrate 5.5 49.7 Sodium nitrate 5.4 50.5 Ammonium nitrate 4.8 48.1 Potassium sulfate 6.4 52.7 Sodium sulfate 5.9 Ammonium sulfate 5.3 62.5 Sodium hydroxide 12.8 142.0 Potassium hydroxide 13.2 144.2 Ammonium carbonate 8.5 32.7 Ammonium hydrogencarbonate 7.5 34.3 Barium hydroxide (3%) 12.7

Basics_Anodising of Aluminium Equations of the anode reactions Al → Al3+ + 3e- (1) 2Al3+ + 3O2- → Al2O3 (2) 2Al3+ + 3OH- → Al2O3 + 3H + (3) For which the overall process is: 2Al + 3H2O → Al2O3 + 6H+ + 6e- (4)

Basics_Anodising of Aluminium Sulphuric acid begins to decompose, the hydrogen ions moving to the cathode where they are reduced to hydrogen gas. Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminium ions(Al3+) to be generated in the anode and in turn move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to form aluminium oxide. The sulphate ions also play some part as the oxide coating contains 12 - 15% sulphate ions. It is suggested that the sulphate ions facilitate the movement of hydrogen ions reducing the cell voltages required.

Basics_Electroplating Similar Process

Basics_Electroplating Process Development

Examples

Basics_Calculations The quantity of electricity flowing through an electrolysis cell is measured in coulombs (C). If one ampere (A) is passed for one second, the quantity of electricity is said to be 1 coulomb. In the purification of copper you saw that copper was deposited at the cathode. The electrode equation is: This equation tells us that 1 mole of copper (II) ions combines with 2 moles of electrons to produce 1 mole of copper metal atoms (63.5 g). A mole of electrons is called a faraday. From accurate electrolysis experiments it has been found that: 1 faraday = 96 500 coulombs Therefore, the quantity of electricity required to deposit 1 mole of copper atoms (63.5 g) is: 2 × 96 500 = 193 000 coulombs (2 faradays)

Electrolytic Oxidation of Carbon(Graphite) Chemical Effect : Chemical modification of carbon fiber surface

Physical Effect : Peel off defected skin Removal of surface flaw Wave shaped surface Improvement of tensile strength

Current flow(Current density) The treatment current for a specific fiber type can be specified in coulombs/meter(C/m). The total coulomb(C) flowing in 1 h would be 3600 X I (C/h). m is the total length of all fiber processed in 1 h with n tows running through the bath at a speed of s (m/h). C/m is specific to a particular d’tex tow and C/m should be adjusted to compensate for any change in the surface area of the tow. HM fiber needs about 10 ~ 20 times the treatment level used for a HT(high strength) fiber.

Parameters HT or HM Current density : greater than 1 A/m2 Voltage : greater than electrolyte decomposition V Speed : depend on carbonization Electrolyte selection: ammonium carbonate compound Distance between CF and Cathode : < 50 mm Anode contact design Electrolyte solution flow rate

HT or HM 1,000℃ 1,500℃ 2,500℃ High strength Carbon fiber (Easy oxidation) 1,500℃ High modulus Carbon fiber (Hard oxidation) 2,500℃

Current density !! Constant current density !! : greater than 1 A/m2 The fineness ↓  C.D. ↑ Running speed ↑  C.D. ↑ HM fiber > HT fiber

Voltage Greater than electrolyte decomposition V Depends on the type of electrolyte on the distance between Fiber and Cathode

Electrolyte Ammonium bicarbonate NH4HCO3  NH3 ↑ + CO2 ↑ + H2O Easy removal by heating Neutral pH(not acidic and alkaline) But Bad smell Relatively low conductivity

Distance between CF and Cathode

Anode contact design Contact or Noncontact Uniform current transfer

Electrolyte solution flow rate Maintain constant electrolyte concentration Prevent chemical polarization Remove air bubble on electrode surface (O2 and H2  flammable and explosive)

Example Electrolyte NH4HCO3 Conc. 10, 20, 30, 40 % Voltage 2V Current density 0.01 Time 60sec

Example