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Melt flow and dripping of polymeric materials can be both beneficial and detrimental during fire. In the vertical UL 94 test, a well adjusted dripping behaviour of flame retarded polypropylene (PP-FR) resulted in a non-flaming dripping V-0 classification. Melt flow and dripping reduce flame spread and even result in extinction, as mass and heat are removed from the actual pyrolysis zone. For the polymer samples and their drops collected in UL 94, the decomposition and viscosity was investigated. Particle finite element method (PFEM) was successfully used to simulate the material behaviour in the UL 94 test. The competition between gasification, combustion and melt dripping is evaluated.
The fire behaviour of carbon fibre (CF) reinforced polymers differs in comparison to polymers. Fibres behave often inert with respect to pyrolysis, they change the melt flow and dripping behaviour, the heat absorption and transfer, the amount and properties of the fire residue and so on. Flame and fire retardancy concepts are needed not only suitable for the different fire protection goals typical for each application, but also tailored for composites. This field is illuminated by examples taken from different projects carried out in the group of the author in the recent years. The examples target on different applications through achieving reduction in reaction to fire controlling the fire risks (flammability, heat release) in the beginning and development of a fire and investigating the fire stability, when a severe flame is directly applied (key property in fully developed fires). Approaches to halogen-free flame retardancy in CF reinforced thermosets are presented as well as building up a bench and an intermediate scale testing of composites in fire applying mechanical load (up to 1 MN compression) and direct flame exposure (180 kW/m2) simultaneously. Indeed, e.g. we have investigated the fire stability of stringer reinforced shell components taken out from the fuselage of an aircraft.
The understanding of fire behaviour, fire resistance, and fire retardant modes of action in composites is a promising basis for target-oriented development. The role of flame inhibition, charring, and protective layer formation is discussed. Successful concepts are presented for fire retardancy tailored for different application as well as general guidelines for future development. Different phosphorus flame retardants are proposed to achieve halogen-free flame retardancy with respect to ignition and developing fires. Different protective approaches are sketched for addressing the fire stability of composites that is the most important fire risk for the fire resistance in structural applications.
Since thermoplastics’ melt dripping is connected directly with their ability to make a fire scenario more or less dangerous, it has been investigated in greater detail. However, it has still remained a question mark in the field of the flammability of plastics. Once it was recognized that each polymer has its own dripping characteristics under equal, standardized conditions, efforts were made to better understand the specifics of this phenomenon. Several different polymers were tested under fire in order to study the relations between molecular structure and the dimensions of the drops. In particular, the relationship between the polymer decomposition mechanisms and their drops was investigated. New quantification methods helped to study drop yields from materials upon exposure to fire or heat. Polymers and flame-retarded polymers showed variations in dripping, indicating that this behaviour is a combination of melting and the decomposition of the polymer and additives. From this, the main material characteristics that influence dripping under fire were highlighted: temperature, viscosity and decomposition.
As Polyamide 6 (PA6) is a thermoplastic polymer used in a wide range of products, including electronic devices, interest in enhancing the fire retardancy of this type of polymer is high. Melamine cyanurate (MCA) has been reported to be the most important nitrogen-containing flame retardant used in polyamides, and is especially attractive because it is halogen-free. In addition, glass fibres (GF) are commonly used in PA6 to improve its mechanical properties, mainly for engineering applications. In order to contribute to the scientific discussion on dripping behaviour, this work reports on an experimental investigation of a set of PA6 materials compounded with different amounts of MCA and GF content. In UL 94 conditions, dripping quantifications were carried out during First Ignition (FIG) and Second Ignition (SIG). These flammability tests were recorded with an infrared camera. The thermal decomposition and rheological properties of all of the materials and collected drops were evaluated.
Dripping behaviour under fire is controlled by the polymer, the flame retardant and the reinforcement filler. Pure PA6 achieved a V-2 classification in UL 94 test, PA6/MCA V-0, PA6/GF HB, and the flame-retarded composites are also classified as V-2. In spite of these classifications, dripping behaviour observed during the flammability test was distinct for each type of samples, as described in the paper. As for decomposition, the PA6 drops collected had a starting decomposition temperature lower than pure PA6 (Figure 1.a), indicating partly decomposed PA6. PA6/MCA shows a clear, early decomposition step for MCA in thermogravimetry, which vanishes for the collected PA6/MCA drops, proving the complete decomposition of MCA during the UL 94 test. In PA6, glass fibres acted as inert filler, prolonging the time to dripping, and leading to a constant flaming flow (Figure 1.b) in the samples with high concentrations. When GF and MCA were combined in PA6/GF/MCA, the modes of action of both the GF and the MCA interfered with each other (Figure 1.c). GF reduces the effectiveness of MCA and tends to force its main action into the condensed phase. There the dripping behaviour is well restricted through the formation of thermally stable residues. Rheological properties and scanning electron microscopy images are consistent with these results.
FIGURE 1. Thermal decomposition of samples and drops of: a) PA6, b) PA6/30GF, and c) PA6/20GF/20MCA