1. Introduction
Glass-fiber-reinforced polymer (GFRP) composites started to be used in shipbuilding in the 1970s. Their intrinsic properties such as corrosion prevention and good aging resistance in the marine environment make them ideal candidates to replace the traditionally used materials such as steels and aluminum alloys [
1]. Indeed, corrosion of metallic structures generates high maintenance costs with long period of unavailability of equipment. The weight reduction achievable with composites is a significant advantage for lightening structures (over 10 wt% lighter for similar size of boat between aluminum and GFRP composite-vinylester-based [
2]). However, the use of these materials in shipbuilding is accompanied by concern about their vulnerability to fire. The organic polymer matrix is responsible for the ignition and can release toxic gases during a fire [
3]. Until 1994, the International Maritime Organization (IMO) in the SOLAS convention (International Convention for the Safety of Life at Sea) excluded the possibility of using combustible materials (including GFRP) for ship construction. This convention specifies minimum standards for the construction, equipment and operation of passenger ships operating in international waters. It is applied to ships carrying passengers and merchandises over 500 gross tonnage navigating in international waters. Chapter II-2 of this convention, added on 1 July 2002, describes that unconventional materials used in ships must be “fire restricting”, meaning low release of heat and smoke and must not spread a fire to adjacent compartments. Test methods relating to fire reaction and fire resistance requirements are described in the document. It is specified that the fire safety design may deviate from the descriptive requirements provided that the design meets the fire safety objectives. Consequently, alternative solutions including GFRP materials for the construction of ship bulkheads must be proposed. Presently, a combustible wall can be found in ships as long as it is coated with a 100 mm of rock wool cladding considered as incombustible. This cladding constitutes a loss of space and a significant gain in weight. To limit the loss of space, it is necessary to develop intrinsically fire-resistant composite materials to meet regulatory requirements.
GFRP used in most marine applications consists of glass fabrics as reinforcement and generally polyester or vinylester resin as the matrix. The case of unsaturated polyester resin (UP) has been more particularly investigated. Cured UP resins rapidly thermally degrade at temperatures above 300 °C to give volatile products (initially styrene) that are easily ignited and burn with smoke [
3]. There are several flame-retardant (FR) solutions that can improve the flame retardancy of GFRP. A first way consists of modifying the resin by using comonomers with a flame-retardant action. Kandola et al. used modified novolac [
4] but also phenolic resoles [
5] cured with UP with styrene as a reactive diluent. The resulting resin exhibited better flame retardance than pure UP. Tibiletti et al. synthesized a phosphonated styrenic comonomer that can partially substitute the reactive diluent in UP. This comonomer led to the creation of an efficient char layer during combustion [
6]. A second way consists of incorporating FR additives into the polyester matrix. Interesting results have been obtained by combining classical FR with nanoparticles. Kandare et al. investigated the thermal behavior of flame-retarded UP containing ammonium polyphosphate (APP), zinc borate and organo-modified montmorillonite. An increase of the thermal stability as well as the char yield was observed [
7]. Tibiletti et al. studied an FR system composed of alumina trihydrate (ATH) and alumina nanoparticle. Synergistic effects were highlighted on the thermal stability and heat release rate that were attributed to size complementarity between micro- and nanoparticles [
8]. Most of the time, the improvement of composite fire properties comes with a reduction in the mechanical strength, which is detrimental for the lifespan of a ship [
3,
9]. A third possible way to improve the fire resistance of GFRP without changing the intrinsic properties of the material is the use of flame-retardant coatings [
3,
10]. The structural integrity of a composite wall can be maintained if the flame-retardant coating acts as a thermal barrier to protect the matrix from an increase in temperature due to a flame. Several studies have shown that ceramic [
11] or intumescent coatings [
5] are the most effective in forming a thermal barrier to limit the propagation of heat into the materials [
10,
12]. The result of the intumescence process is the swelling and growth of a char layer that insulates the material from the action of a heat source or a flame [
13].
Poly(vinyl alcohol) (PVA) is a film-forming polymer used in several applications as packaging films, coatings, adhesives and biomaterials because of its good chemical resistance and moisture and oxygen barrier protection [
14]. In addition, PVA is a biodegradable and nontoxic polymer, easy to process with a relatively low production cost. As regards to the fire properties, PVA is highly flammable (LOI = 19%) [
15]. However, as a polyol polymer, it exhibits intrinsic charring properties that make it a good candidate as a charring agent in intumescent systems when combined with appropriate flame retardants. Several studies have shown the effectiveness of intumescent systems based on ammonium polyphosphate (APP) in the PVA matrix [
15,
16]. The authors investigated the morphology of residues of PVA/APP after a cone calorimeter or LOI (limited oxygen index) test. A swollen and compact structure was observed by SEM which appeared to provide a good barrier to the transfer of heat, mass and flammable gases during a fire. APP has been reported to mainly play a role in the condensed phase [
15,
17]. In the presence of a heat source, APP decomposes into polyphosphoric acid and promotes a dehydroxylation process in decomposing oxygen-containing polymers. Polyaromatic structures form and lead to a residual char which constitutes a protective layer at the surface of the decomposing material isolating the non-degraded polymer from the flame. However, an intumescent system with APP alone requires a high charge rate (typically greater than 30 wt%) to achieve good fire performance [
18]. In addition, the presence of a high level of flame retardant in a polymer matrix tends to result in a loss of mechanical properties. Nanoparticles have been incorporated into polymers because they are likely to improve mechanical performance as well as thermal stability and fire resistance for low loading rates [
17,
19,
20]. Moreover, it has been shown that nanoparticles can be used in intumescent FR systems in order to improve the mechanical strength of the expanded char layer [
21]. Sepiolite is a magnesium silicate clay material with the theoretical formula (Si
12Mg
8O
30)(OH
4)(OH
2)
4.8H
2O with H
2O representing zeolitic water present in an intra-crystalline cavity (tunnel). Sepiolite is characterized by blocks of octahedral sheets of magnesium oxides and hydroxides between two tetrahedral silica layers. In each tunnel, the octahedral sheets are bound with two H
2O molecules which correspond to coordinated water and are weakly bound with zeolitic water [
22]. Sepiolite exhibits a needle morphology which generates a high specific surface area of 200–300 m
2/g, lengths of 0.2–4 µm and a width of 10–30 nm and thickness of 5–10 nm [
23]. It is a hydrophilic nanoparticle—it can be added into PVA matrix by simply dispersing in water [
14]. However, only a few works have reported the use of sepiolite in flame-retardant systems in combination with APP [
24,
25,
26]. Vahabi et al. [
24] investigated the thermal behavior and flame retardancy of poly(methyl methacrylate) containing APP (Exolit AP422) and sepiolite (Pangel S9). The authors reported a decrease of 65% of the peak of heat release rate (pHRR) and an increase of 29 wt% of the residue with respect to the virgin matrix after cone calorimeter tests. The presence of sepiolite particles make the residue more compact and less porous which leads to an improved barrier effect of the residue during combustion. Pappalardo et al. [
25] reported in a propylene matrix a considerable decrease of the pHRR by 80% during cone calorimeter tests with the incorporation in an extruder of an APP (Exolit AP 766) and sepiolite (Sigma Aldrich). Carretier et al. [
26] investigated polyurethane/APP (Exolit AP423)/Sepiolite (Pangel S9) ternary systems prepared by casting in a mold at 155 °C and observed that the pHRR decreased by 88% and the residue increased by 17 wt%. The authors suggested the interest of choosing a ratio of 3 between APP and sepiolite allowing the presence of free sepiolite which could play an additional thermal barrier effect.
This work aims at assessing the effectiveness of sepiolite and APP as fire retardant agents in PVA for the manufacture of a surface coating containing nanoparticles to protect GFRP composites during a fire with the objective to improve both the reaction-to-fire and the thermal insulation. The influence of APP content in the PVA matrix was firstly studied for a rate of 10 to 40 wt%. Then APP/sepiolite mixtures were investigated at a fixed loading rate of 20 wt% in PVA. The substitution of a high rate of APP loading by sepiolite was carried out in order to investigate the impact on fire properties for an acceptable loading rate in terms of mechanical property. This study focuses on the mechanisms of fire retardancy induced by the presence of both additives in PVA as well as their influence on fire reaction and fire resistance of coatings designed to protect GFRP composites. Fire reaction and fire resistance were assessed at the macroscopic scale using cone calorimeter and surface temperature measurements. Moreover, investigations on cone calorimeter residues from coatings and coated composite laminates were carried out. XRD analyses, SEM observations and thermogravimetric analysis were achieved to account for the interactions occurring between the various components during thermal degradation.