The objective of this project was to develop an innovative and cost-effective coating removal technique for removing thick elastomeric coatings from Department of Defense (DoD) Weapon Systems. The developed technology aims to eliminate the need for chemical- or media-based treatments, thus reducing environmental problems. This was achieved through an innovative approach involving the use of tailored infrared and ultrasonication treatments of the coated substrates. The effect of different operating parameters such as time of irradiation, surface temperature, sonication power/intensity were investigated to determine the optimal conditions for coating removal without damaging the underlying substrate or causing environmental problems.
The approach was based on the concept of introducing bond failure at the surface coat-substrate interface by selective absorption of infrared (IR) and/or impinging of the topcoat with the use of ultrasonication technique. The degraded interfacial adhesion strength then allowed the specialty coating to be completely removed with the application of a pull at a certain angle to the substrate-coating interface. The development of tailored IR-Sonication coating removal technology was achieved through:
- Characterization of the surface coat and the underlying substrate coat,
- Measuring Bond/Adhesive Strength for treated and untreated substrates,
- Fabrication and Testing of IR-Sonication probe, and
- Characterization of surface coat and underlying substrates after treatment.
Optimum ultrasonication frequency/amplitude and infrared spectral range, where the surface coat showed the maximum absorption and the underlying substrate the minimum absorption, were determined. Based on these results, tailored IR and sonication treatments were applied on several different specialty coating samples provided by Air Force Research Lab and Naval Air Systems Command (NAVAIR). All the tasks were completed, and milestones were achieved as mentioned in the project plan. One of the key deliverables, i.e., introducing interfacial bond failure between the topcoat and the underlying substrate, was accomplished with a significantly better result than that was originally proposed in the proposal (25-50% bond strength reduction, Milestone 2.1 and Final Deliverable 2). On an average, a 65-75% peel-strength reduction was achieved by the use of tailored IR treatment alone and IR-Ultrasonic combination. Both, the shortwave infrared (SW-IR) and medium wave infrared (MW-IR) treatments demonstrated peel-strength reductions in the range of 75-50% with varied exposure time. The desired peel-strength reduction was achieved significantly quicker with SW-IR than that achieved with MW-IR exposure.
Two approaches were used for ultrasonication experiments; 1) directly applying ultrasonic energy to the top layer of the sample, 2) applying sonication while maintaining a very thin layer of water between the ultrasonic horn and the top layer. First approach was most successful in removing thick elastomeric sealants from the NAVAIR samples. The second approach was adopted for the IR-sonication hybrid treatment. The latter procedure showed a higher peel strength reduction than that achieved with the application of IR alone.
Cost analysis was carried out using the average energy usage of each technology. To remove one square feet of coating using IR treatments, the energy required was in the range of 100-200 kJ/ft2. This is accounted to $0.001-0.004 /ft2 for an electricity tariff of $0.065/kWh. Ultrasonic approach gives energy consumption of 200-300kJ/ft2 and would account to $0.002-0.006/ft2. This approach is an economically viable option considering the basic operation cost of the technology. Further studies are needed to investigate the complicated shapes and the initial machine fabrication costs.
In order to determine any adverse effect on the underlying substrates, thermogravimetric analysis (TGA), FTIR and scanning electron microscope imaging of the treated and as-received samples were investigated. The results established that IR and ultrasonication exposure had no detrimental effect on the physical and chemical makeup of the underlying substrates. These results not only provide a solid proof-of-concept of the proposed technology but also provide optimized parameters to successfully implement this technology on an industrial scale.
The developed coating removal technology is an environmental-friendly method as it does not produce any toxic gas or suspended solid waste in the air. Operation cost for this technology is very low compared to the conventional coating removal technologies. Coating removal is achieved at a much lower temperature than some of the energy-intensive technologies such as laser ablation. The project team has successfully addressed SERDP’s goal by developing a large-scale non-chemical, non-media removal process for thick, elastomeric coatings and treatments on DoD Weapon Systems.