STIC • Self-stratifying intumescent coatings
The ‘self-stratifying’ coating approach means just one application of a complex formulation is needed to achieve multi-layer or gradient coating structures. This coating can be applied directly to plastic or steel, delivering both good substrate adhesion and surface barrier properties. This approach had never previously been considered for fire-retardant coatings for any substrate
The STIC project objective was therefore to develop model self-stratifying formulations with adhesive properties for a given substrate, surface barrier properties (protection from water, UV light, etc.), and fire-retardant properties to encourage, from an ecological standpoint, the development of specialist flame-retardant coating industries.
A fire-retardant coating can be applied to a material’s surface to protect it from fire. However, three different layers have to be applied one after the other to achieve a coating that is fire retardant, durable and adheres to the substrate.
It would therefore be highly desirable, from a profitability as much as an ecological standpoint, to reduce the number of layers to a minimum while maintaining equivalent or better overall performance than the current coating system.
As no academic paper or patent had been found on this specific subject, the door was wide open for a technological breakthrough.
Two very different substrates were chosen:
- a plastic substrate – polycarbonate (PC) – which is hard to fireproof (both in bulk and on the surface)
- a steel substrate, which, without protection, loses its mechanical properties at temperatures over 500°C
The first objective was to obtain a type I model self-stratifying coating (perfect stratification giving rise to two distinct layers) or type II (stratification characterised by a homogeneous concentration gradient), using two or three resins depending on the results achieved.
The second objective was to incorporate flame-retardant fillers into these model formulations, in order to fireproof polycarbonate (UL94-V0 class and an LOI of at least 30) and obtain a fire barrier effect on steel similar to that offered by a three-layer commercial coating, while maintaining the self-stratifying system, and therefore the adhesion to substrates and the surface barrier properties.
Self-stratifying coatings that met the project specifications were developed for PC. In addition, a self-stratifying coating containing a flame-retardant filler displayed very promising properties and is under investigation.
- A type I self-stratifying coating designed for PC and containing a silicone resin and an epoxy resin was developed. Such a coating had not been mentioned in academic publications until now. Another type I self-stratifying coating containing fluoropolymer and epoxy resins, which was less innovative but perfectly characterised, was also obtained.
- Contrary to what had previously been reported in academic publications, pigments/extenders do not necessarily remain in the phase in which they were initially dispersed. Instead, they migrate to the phase with which they have the most affinity. This is the case with the epoxy/silicone/iron oxide coating studied by this project; a paper is being written on the subject.
- A very interesting result for the next step of the study was obtained. The incorporation of a pigment (iron oxide) shows extremely promising fire-retardant properties in both the self-stratifying epoxy/silicone coverings and epoxy/fluoropolymer coverings developed. Iron oxide and another additive with flame-retardant properties have shown no negative effect on the self-stratification process, contrary to data in academic publications that report that most of the additives commonly used in paint have a negative effect on self-stratification.
Project type: ANR (French National Research Agency)
Start date: September 2014
Duration: 42 months