paper review

4. Laser Decontamination of Epoxy Painted Concrete Surfaces in Nuclear Plants 

• Substrate: Concrete samples, including pure quartzite, calcite, and a 1:1 quartzite/calcite mix, aged by carbonation to an equivalent of approximately 30 years were used. 

• Paint Application: The surfaces were coated with a decontamination epoxy paint containing polychlorinated biphenyls (PCB). The thickness of these paint layers varied between 100 µm and 700 µm. 

• Laser Equipment: A 10 kW diode laser operating in continuous wave (CW) mode with a wavelength of 980–1030 nm was utilized. The system used a homogenizer optic to create a rectangular flat-top laser spot size of 45 × 10 mm² at a 400 mm focal length. 

• Optimal Processing Condition: The system achieved high paint ablation rates of up to 6.4 m²/h. Complete paint removal without melting the underlying concrete surface required achieving surface temperatures of at least 800°C, and effectively reaching over 1000°C. 

• During paint removal: The rapid heating from the laser led to the simultaneous ablation of the paint layer and the thermal decomposition of the PCBs in a single process step. The combustion initiated through six distinct phases, with a stable flame forming after an average ignition delay of less than 0.3 seconds. A 0.5-second time delay (offset time) was implemented before moving the sample to ensure a stable flame right from the start. 

• Thermal Quenching: The aspirated flue gas was immediately mixed with ambient air in the exhaust channel to rapidly cool it to temperatures below 250°C. This thermal quenching step successfully prevented the formation of highly toxic polychlorinated dioxins (PCDD) and polychlorinated furans (PCDF). 

• Process Monitoring: An in situ online measurement system based on laser-induced fluorescence (LIF) is being developed to monitor the thermal decomposition of PCBs and control the ablation process parameters. 

3. Laser fluence, repetition rate and pulse duration effects on paint ablation

1. Core Mechanism: Heat Accumulation 

When a high-repetition-rate laser (e.g., 10 kHz) is applied to a medium with low thermal conductivity like paint, "heat accumulation" occurs. Successive pulses strike the material before it can cool down to its initial temperature, continuously raising the base temperature for subsequent pulses. . 

2. Key Experimental Results

• Drastic Decrease in Threshold Fluence: Due to the heat accumulation effect, the ablation threshold at 10 kHz drops to approximately 0.2 J/cm². This requires only about one-sixth of the energy compared to low-repetition-rate environments.

• Advantage of Longer Pulses (100 ns): At high fluence levels, 100 ns pulses yield higher ablation efficiency than 5 ns pulses. If the pulse is extremely short and intense, the ejected matter or generated plasma shields the surface, absorbing subsequent laser beams.

• Necessity of an Air Jet: At high repetition rates like 10 kHz, ejected micro-particles can block the incoming laser before dissipating. Applying an air jet to sweep these particles is essential to prevent a decrease in removal efficiency.

3. Optimal Parameters & Practical Implications

• Optimal Conditions: The highest paint ablation efficiency (approx. 0.3 mm³/J) was achieved at a 10 kHz repetition rate, a 100 ns pulse duration, and a 1.5 J/cm² fluence.

• Practical Implications: Implementing this low-energy, high-repetition-rate setting for concrete wall cleaning systems in nuclear facilities enables the safe and precise removal of surface paint and contaminants while strictly preventing thermal damage to the underlying concrete substrate.

2. Process development and monitoring in stripping of a highly transparent polymeric paint with ns-pulsed fiber laser

Substrate: Aluminum alloy (EN AW 5005A) was used. 

Pre-treatment: Prior to the painting procedure, the surface was degreased in an alkaline bath and pre-coated with a ~2 µm layer of Titanium (Ti) and Zirconium (Zr). 

Paint Application: A highly transparent, white polymeric paint consisting of polyester and polyamide was used. It was applied in two distinct layers: 

• Undercoat: Applied with a thickness of 5 µm.

• Topcoat: Applied with a thickness of 15 µm, resulting in a final total coating thickness of 20 µm.

Laser Equipment: A Q-switched pulsed fiber laser with a 1064 nm wavelength and a 250 ns pulse duration was utilized. Operating at a pulse energy of 0.16 mJ (yielding an ablation diameter of ~39.4 µm) and a 50 kHz repetition rate, it achieved a paint removal rate of 11.7 cm²/min. 

Optimal Processing Condition (Overlap): Complete paint removal was successfully achieved using a single pass with the laser pulse overlap set to approximately 50%. 

• During paint removal: The superficial interaction with the Ti pre-coating emitted wavelengths in the 589–626 nm band. 

• Substrate reach detection: When the paint is fully removed, new emission lines appear in the 497–504 nm band. This signals the ionization of Aluminum (Al) and Iron (Fe), indicating the laser has reached the substrate. 

• Depth tracking via total intensity: The total emission intensity was continuously summed to track the machining depth 

• Cut-off threshold: A total intensity threshold of 1.5 × 10⁶ AU was established. Exceeding this value indicated complete paint removal, which can be used to immediately halt the process and prevent excessive substrate damage. 

1. Removal of chlorinated rubber coatings from concrete surfaces using an RF excited CO_2 laser

Substrate:  Standard concrete slabs were used, which underwent a 4-month curing process and were dried to match typical moisture levels.

Paint Application: Chlorinated rubber (CR) paint was applied in three distinct layers.

Primer: Applied with a dry film thickness (DFT) of ~25 μm and dried for 24 hours.

Undercoat: Applied with a DFT of ~100 μm and dried for 24 hours.

Topcoat: Applied in 1-2 layers with a DFT of ~75 μm each, resulting in a final total coating thickness of ~250-275 μm.

Laser Equipment: A Coherent Diamond 84 CO₂ laser (wavelength 10.4–10.8 μm, power 25–150 W) was utilized. An unfocused ~7 mm diameter beam was scanned at speeds of 2.5–100 mm/s. 

Side Gas Jet (Optimal Condition): Air at ~3 bar was injected perpendicularly to the scanning direction at a 5° angle. This immediately removed the generated ash and provided a strong cooling effect, enabling complete paint removal down to the substrate without any concrete damage or residual ash.