# Paint Removal with Pulsed Laser: Theory Simulation and Mechanism Analysis
## 3 Results and Discussion
### Discussion  
Paint removal using laser technology is a complex process. Laser beams are used to work on the paint layer with photons as the energy carrier. The photons transfer energy to the surface of the paint layer, causing a series of physical and chemical changes. The paint layer absorbs most of the  
Figure 13: Scanning electron micrograph of the cleaned layer: (**a–d**) products protruding from the cleaned surface; (**e**) elemental composition of the ball-particle; (**f**) size distribution of ball-particles.  
laser energy, activating and converting the molecules or atomic groups in the paint to thermal energy. Instantaneous expansion and vaporization, among other processes, occur on the surface of the paint. In this experiment, the photon energy of the fiber laser with wavelength of 1064 nm was found to be 1.16 eV. The laser energy absorbed by the paint surface is transmitted and accumulated in the paint. The area closest to the laser beam's focus shows the fastest response and a rapid rise in temperature. Figure 14 shows the temperature change curves at different depths in the x-axis direction at t = 0.000851 s. The temperature of the paint layer was found to decrease rapidly with increase in depth, as shown in Figure 14. Compared with the temperature of the paint surface, the temperature below surface 5 \(\upmu\)m was lower than 600 \({}^{\circ}\)C.  
When the temperature of the paint rises to a certain level, the chemical bonds thermally crack [11; 12; 13]. The cleavage of chemical bonds, such as C-C, C-H, C-O, and C-N, generate free radicals, which then became intermediates for subsequent chemical reactions. The reaction between activated free radicals and free radicals with oxygen in the air formed new organic compounds, such as (-CH\({}_{2}\)CH(C(O)OCH\({}_{3}\))-)\({}_{n}\), (-CH\({}_{2}\)CH(C(O)OH)-)\({}_{n}\), (-CH\({}_{2}\)CHC(CH\({}_{3}\))CH\({}_{2}\)-)\({}_{n}\), (-CH\({}_{2}\)C(CH\({}_{3}\))(C(O)OCH\({}_{2}\)CH\({}_{3}\))-)\({}_{n}\), (-CH\({}_{2}\)C(CH\({}_{3}\))(C(O)OC(CH\({}_{3}\))\({}_{3}\))\({}_{n}\), and (-CH\({}_{2}\)CH\({}_{2}\)NH\({}_{n}\)); oxides such as C\({}_{96.8}\)O\({}_{3.2}\), and C\({}_{93.1}\)O\({}_{6.9}\); and other substances containing N, such as [Mg(C\({}_{5}\)H\({}_{4}\)NCOO)\({}_{2}\)] and C\({}_{48}\)H\({}_{26}\)N\({}_{8}\) on the laser cleaning surface. The little molecular gas substance generated during the cleaning process became volatile [14; 15; 16; 17]. A large number of C atoms from the gaseous substances formed uniform nanoparticles when they encountered cold air, as shown in Figure 13c,d. The paint removal mechanism is primarily a chemical bond breaking combustion method.  
As laser energy is continuously absorbed, as shown in Figure 3, the breaking speed of chemical bonds in the polymer is accelerated. At the same time, gaseous products, such as CO\({}_{2}\), CO, and H\({}_{2}\), above the surface of the paint layer, gradually increase. The large amount of steam that is generated strongly absorb the laser energy, and a laser plasma is formed by physical processes such as nuclear collisions and target electron excitation in high-energy gases. This process is accompanied by outward expansion, which produces shock waves that directly act on and generate impact damage on the paint layer [13], as shown in Figure 12a,b. The large amount of bond cleavage causes a gradual increase in electron density and energy in the laser plasma. The high-input energy thus results in a rise in temperature of the electron in the plasma, which is high enough to exceed the Vander Waals force and the chemical bond energy in the polymer. This further promotes the cracking of the inside of the polymer and the breaking of the chemical bond, resulting in destruction of the polymer structure [18; 19].