## 3 Results and Discussion  
### Flammability Test  
Three typologies of Radel R specimen series were tested: as-received (R), double layered (R-2L) and Ceramer-coated (R-HY). The as-received material was also tested for several flame exposition times; in Table 1, the characteristics of the different sets of measurement are specified, including the double duration test (R-2T). The time/length measurement results are also reported: Such values represent averaged results by testing at least three samples for each class. The char length is measured after cleaning operations needed to remove the smoke deposition onto the samples surface. Besides the aforementioned parameters (char length and after-flame/glow times), further information is provided, such as the smoke length (measured before the sample cleaning), the damaged length (referred to the part of material which is visibly damaged but not charred) and the char width (the  
Figure 2: Picture of the UV/AtOx simulator test facility. length of the base line of the charred area, at the bottom edge of the sample). Moreover, the test chamber is thoroughly analyzed after each test, in order to check if the specimen sputtered some particles due to the flame exposure (material spread). Some pictures of the tested samples are reported in Figure 3. All the uncoated Radel samples showed no glow and did not spread visible amount of material during the test. The exposure to the flame gradually damaged the materials: A thin line distinguishing the contour of the charred area is clearly visible. The char length is generally not much extended, but there is an area where matter melting occurred. The obtained findings are in good agreement with results reported in the literature for this kind of thermoplastic material (see, in particular, Table 3 in Reference [12]. The damaged length describes such phenomena: In the melted area, the materials still show a visible transparency; this area is smooth and it is not detectable by handle touch. The charred area is more evident at the external sides of the slabs: It is irregular, and little bulges are detected, likely due to gas releasing during combustion (Figure 3a). The behavior of the double-layered samples (R-2L) is more effective than what observed for the reference R class: The part of the samples between the two layers seems to be not affected by the flame, and the bottom edge of the samples shows non-uniform burns, while only the external sides of the samples are melted and charred (Figure 3b). The behavior of the coated Radel samples (R-HY) revealed some differences in respect to the other tests. Smokes are not uniform, and the damaged area is not marked by a clear boundary between melted area and undamaged part (Figure 3c): It is composed of a first charred area with some bulges, a second charred area without bulges, and a white-yellow area just around, related to the coating degradation. The charred area without bulges can be removed by means of abrasive cloth, since the friction of simple paper tissues (used to remove smokes) is useless. Moreover, it was observed that the R-HY specimens spread visible particles of material around the final seconds of the flame test, while, at the end of each test, a little quantity of ash was discovered; the spread material could be likely identified with the hybrid coating. In the longer flame exposure time tests (R-2T), some traces of increasing damaging were founded. In particular, the higher char length and the presence of darker zones of shape similar to that of the smoke areas (that is, the inner trace of the smokes) indicates the worsening of the Radel R when exposed to flame for longer times (Figure 3d). Anyway, it has to be stressed that all the R-2T specimens have shown to be self-extinguishing and that no melted material dripped off the samples.