Small scale screening test and optimization
Before presenting the results, it is important to keep in mind that the choice has been made to tests samples issued from industrials processes and not produced in the laboratories of Nord Composites or Sicomin. So, the material compounds are therefore the result of a compromise between the mechanical and fire performance and the processing limitations to sheets of great dimensions made by Polyecim.
The results obtained are compared with a polyester/glass reference material commonly used in the railway industry. The sample was also produced by Polyecim, using the same tools in order to have same production conditions.
For each material (epoxy and polyester), different production methods have been used:
- For epoxy: RTM, Resin Infusion and vacuum compaction
- For polyester: RTM and vacuum compaction
For the evaluation of fire performance, the tests with cone calorimeter and with tubular furnace (according to ISO 5660-1&2 standard) have been realized as planned.
The first one is used to measure the MAHRE (Maximum Heat Release) which is one of the parameters used to meet some of the requirements of the standard EN 45545-2. The second one is used to make the analysis of gases emitted and to estimate the conventional toxicity index (ITC PNL).
Globally, whatever the samples, the values obtained for the samples with flax fiber are higher than the polyester/glass reference (See table 1).
During the test with the cone calorimeter, the behaviour of the polyester-based materials and epoxy-based materials are quite different. The first ones presents two or three peaks (see figure 1) with a relatively closed intensity, while for the second ones two peaks of different intensity. The first peak is caused by surface ignition, and then a thick intumescent char is formed (roughly between 60 and 150 sec). Finally, the char is consumed in the final peak, which is much more energetic than the first peak (see figure 2).
The presence of nitrogen within the polymer chain of epoxy induces the production and release of nitrogen-based compounds (NO, N2O, HCN and NH3). These species are mostly absent in the degradation gases of polyester-based composites. Epoxy-based composites also generates more carbon monoxide (roughly three times more) than polyester-based composites. This can be explained by the intumescent charring behaviour of the epoxy-based composites, as char oxidation generates more CO. This explains why the toxicity of epoxy samples is higher than that of polyester.
For the mechanical strength two tests have been made: tensile test according to ISO 527-4 standard or flexural strength according to EN 14125 standard. This last one has been used because in the NF F 01-281 standard (Railway rolling stock — Parts in fiber-reinforced thermosetting composites — Specifications, test methods, manufacturing, qualification and assessment of conformity), for mechanical strength there is only specification on flexural.
The results obtained are not necessarily up to what was expected due to the fabric and the number of layers used (see tables 2 and 3).
Cross sections of the various samples show that unfortunately the resin impregnation of the fabric is not optimal for some of the samples, therefore degrading mechanical properties.
However, when the impregnation is rather good as for INFU1, and with the appropriate reinforcement, it is possible to obtain at least equivalent results.
In conclusion, the fire performances for some materials are not very far from those of the polyester/glass reference material even if they are ower. This may possibly reduce the range of applications but is not necessarily a reason to reject them as the specifications for different applications require different levels of severity.
Regarding the mechanical aspect, the choice to produce large parts sometimes leads to difficulties in implementation (imperfect impregnation) distorting the results. With better impregnation, the resistance of the materials should be at least equivalent. With the feedback gained from these different productions, this should be resolved for the next phase.