Novel four components self-layering fire retardant systems on Polycarbonate, with excellent aging resistance
Epoxy/Silicone self-stratifying coatings
- The choice of solvent influence the stratification level, pigment dispersion, adhesion and the visual appearance of the coating: the higher the evaporation rate, the more sponteneous the stratification. Although, the rate of evaporation should not be too high because the system reaches the gel point before the completion of phase separation. The BuAc: xylene (at a 1: 1 ratio) solvent blend leads to very promising results: it gives rise to the best stratification pattern with a great adhesion (5B rating) to polycarbonate and a nice visual appearance.
- Layering can be influenced by crosslinkage induced by the curing agent: a too reactive hardener can be incompatible with the materials used in the system (the thermoplastic resin, pigments …). It can also react with the thermoplastic resin, thus preventing, in some case, the stratification process and also impacting the adhesion and the visual appearance of the resulting coating.
- Fillers show no negative effect on the stratification process, only the visual appearance and the adhesion of the coating can be affected, depending mainly on their dispersion and on the wettability of the additive.
With incorporation of solid fire retardant fillers:
- Iron oxide is the most versatile pigment: it is the less affected by the type of solvent or the incorporation phase compared to calcium carbonate, with which such modifications can completely prevent the occurrence of stratification or affect the visual appearance of the coating. Zinc phosphate is also a good candidate. It has been evidenced that pigments chosen in this work have a higher affinity with the silicone resin, and thus in most cases do not remain in their dispersing phase if they have been incorporated in the epoxy medium.
- The incorporation of a low amount (2.5 % PVC) of iron oxide and calcium carbonate led to a significant improvement of the fire properties with vertical fire tests compared to virgin PC: a 33 and 35 vol. % LOI value respectively with iron oxide and calcium carbonate, and a V0 ranking at the UL-94 test were achieved. The Time To Ignition is also significantly delayed during MLC tests, however, pHRR and THR are not enhanced compared to the PC coated with unfilled system.
- When fillers are introduced in a higher amount (5 and 10 % PVC), excellent fireproof properties are still obtained, however adhesion is slightly impacted: 4B rating is obtained compared to 5B with 2.5 % PVC.
- Iron oxide was reported to inhibit the thermal degradation of the silicone resin during combustion, probably through crosslinking reaction which accelerate the formation of char. This char formation, also increased in the presence of calcium carbonate, mainly prevents from the dripping and the propagation of the flame by forming a thermal barrier to protect the polycarbonate in a condensed phase mechanism.
- A great resistance over ageing under Temprature/Relative Humidity is obtained when both the unfilled and filled coatings are applied on PC. Whereas, under UV, the yellowing of the unfilled coating and of the polycarbonate is accelerated, the filled coating completely protects the underneath substrate from UV rays attack.
With incorporation of liquid fire retardant fillers
The composition, dissolved in solvents in the course of film forming process produced structures with sharply separated layers.
- Phosphorus based liquid fillers do not impact the stratification process when incorporated at 5 wt. %, however their migration to the upper layer of the coating (silicone phase) was not always complete: a concentration gradient of the additives though the film thickness is obtained, with a higher concentration toward the top of the coating.
- When fillers were incorporated at a lower amount (2.5 wt. %), incomplete phase separation was observed in some area of the film, although a high adhesion rating and nice visual appearance remained. Good fire performances wre obtained, with a charring more pronounced (condensed phase mechanism). Also, dripping is avoided and flame inhibition occurs (gas phase mechanism) compared to the unfilled system and to virgin polycarbonate.
Epoxy/Fluoropolymer self-stratifying coatings
- The solvents have proven to hugely affect the quality of the resulting film: both adhesion properties and pigment dispersion, thus visual appearance are affected.
- Incorporation of iron oxide in the epoxy phase has no negative effect on the layering behavior: the filler has a higher affinity with the fluoropolymer resin, and thus does not remain in the phase where it was initially dispersed (i.e. the epoxy resin).
- The evaporation rate of solvents influences the migration of fillers toward the fluoropolymer medium. When a blend of MIBK: xylene: 1-methoxy-2-propanol (50:30:20) is used, fillers have not enough time to migrate toward the topcoat layer and are thus trapped at the interface between the two resins. This poor pigment dispersion is detrimental to the film aspect and to the adhesion properties.
- The incorporation of a low amount (10 wt. %) of iron oxide particles leads to an outstanding improvement of the fire performances compared to virgin PC and unfilled coated PC samples.
- Iron oxide was reported to catalyze the thermal degradation of the fluoropolymer resin during combustion, probably through the formation of aromatic double bonds that may undergo crosslinking and accelerate the formation of char. This char formation mainly prevents from the dripping and the propagation of the flame by forming a thermal barrier to protect the polycarbonate in a condensed phase mechanism.
- A great resistance over ageing under Temperature/Relative Humidity is obtained when both the unfilled and filled coatings are applied on PC. Under UV, the yellowing of the unfilled coating and of the PC is accelerated but the filled coating completely protects the underneath substrate from UV rays attack.
Other self-layering systems designed
Epoxy/acrylic and silicone/fluorpolymer systems in various type of solvents have also been developed in the frame of this project. Very promissing self-layering compositions were obtained, hovewer some improvements have to be made in terms of adhesion properties onto the polycarbonate substrate.