A tough UV-curable clearcoat protects the lens in the facepiece of self-contained breathing apparatus (SCBA) for fire fighters. This equipment is worn by the fire fighters to protect them from backdraft and flashover fire conditions as well as toxic gases, smoke, and extreme heat.

The outstanding feature of the coating is that it upgrades the lens to meet severely tightened National Fire Protection Assn. (NFPA) safety requirements. Under the new requirements, the lens must resist abrasion and withstand a 10-sec direct contact with open flame and temperatures between 1500 and 21020F. After the test, a person must be able to look through the lens and read a standard eye chart at a distance of 20 ft. The SCBA lens is manufactured by Fosta-Tek of Lexan polycarbonate and measures 7 by 5 in.

Threat of shutdown
Before the NFPA tightened its SCBA standard, the facepiece lenses had been made for 10 years under the old NFPA specifications, using a thermally cured siloxane hardcoat to coat the lens. However, exposure to flame and very high temperatures caused this hardcoat, and others subsequently tested, to crinkle and impair visibility. The new standard threatened to shut down lens production.

To replace the siloxane hardcoat, chemists at Fosta-Teks longtime coating supplier, GE Silicones (Waterford, NY), suggested a promising new radiation-cure hardcoat with exceptional resistance to abrasion, scratching, and chemicals. At the time, however, the product was under development and considered to be at least 18 months away from commercial introduction.

Crash program
With less than a year before the new standard was to take effect, Fosta-Tek and GE Silicones established a team program to expedite development of the coating and move it into production. Initial testing showed that the coating performed beyond expectations in withstanding heat and flame testing, and in resisting crinkling and loss of visibility.

Modifications were needed to improve both the flow characteristics and pot life of the hardcoat. Experimenting with viscosity-by diluting the coating with isopropyl alcohol- as well as the withdrawal rate, line speed, and power output of the UV-curing lamps, yielded the coating chemistry and film thickness required.

The UV-curing hardcoat was tested initially on GE's equipment. Then Fosta-Tek set up a small prototype system that could only cure one side of the lens at a time. Successful performance testing on the prototype system led to the installation of the conveyorized UV line presently in operation, which cures both sides simultaneously and can handle parts up to 12 in. square in large volumes.

The proprietary UV hardcoat provides primerless adhesion to the Lexan polycarbonate substrate, and cures in seconds under UV radiation to a clear, high-gloss finish. The cured coating thickness on the lens is 4 to 5 mils. The UV hardcoat also has excellent resistance to chemicals and will not frost up and become opaque when exposed to Freon, acetone, sodium hydroxide, and other corrosive chemicals.

With its new hardcoat and UV curing line, Fosta-Tek is now supplying a third of all SCBA lenses manufactured under the new NFPA standards.