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Phosphorus-based flame retardancy mechanisms - Old hat or a starting point for future development?
(2010)
Different kinds of additive and reactive flame retardants containing phosphorus are increasingly successful as halogen-free alternatives for various polymeric materials and applications. Phosphorus can act in the condensed phase by enhancing charring, yielding intumescence, or through inorganic glass formation; and in the gas phase through flame inhibition. Occurrence and efficiency depend, not only on the flame retardant itself, but also on its interaction with pyrolysing polymeric material and additives. Flame retardancy is sensitive to modification of the flame retardant, the use of synergists/adjuvants, and changes to the polymeric material. A detailed understanding facilitates the launch of tailored and targeted development.
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.
Intumescent coatings are used for decades to increase the fire resistance of steel or wood constructions. Intermediate and full scale tests are used to assess their protection performance. For the product development and screening, cheaper and faster bench-scale tests are demanded that provide information about thermal protection, foaming dynamics and mechanical resistance. In the recent years, we have developed several bench-scale fire resistance tests and used them in different research and developing projects. The influence of distinct binders and fillers, respectively, was studied in intumescent coatings using the Standard Time Temperature modified muffle furnace (STT Mufu+). This bench-scale test evaluates the fire resistance (by means of temperature measurements) and the foaming behaviour (by means of a high-temperature endoscope) during a standard time-temperature exposure. The fire residues were suitable for advance residue analysing techniques like nondestructive μ-computed tomography (μ-CT). Also, scanning electron microscopy was used to investigate the microscopic structure of the surface and inside of the residues. The mechanical resistance of the residues was tested by an impact resistance experiment.
The binder influence on the insulation of the coating was small for the investigated systems. Nevertheless, it was interestingly noted, that coatings with high expansion did not provide the best protection. The great influence of the binder material on the inner structure of the foamed residues was revealed by the μ-CT images. Clear differing morphologies were observed. These led to distinct mechanical resistance properties of the tested coatings. Also the change of a low amount of fillers, such as fibres and clay was investigated with similar effects. What is more, a transition of the residue from black, carbonaceous foam with closed cells into an inorganic, residual open cell sponge occurs at high temperatures during the test. This transition is due to the loss of carbon; the change in microstructure is analysed by scanning electron microscopy.
The bench-scale tool presented outreaches screening; the investigation based on the STT Mufu+ delivers a deeper understanding of the phenomena controlling the performance of intumescent coatings.
The exploration of condensed phase mechanisms such as charring and intumescence has been pushed forward in the last decades, since it is believed that focusing on these concepts will bring materials closer to an efficient and ecologically friendly fire retardancy. They promise to concentrate efficient fire retardancy at the key position between pyrolysis zone and gas phase. Examples of residue/char forming and intumescent materials are used to illustrate the influence of mass and heat barrier effects on the fire behavior of materials and general results are presented. A comprehensive understanding of the mechanisms and structure-property relationships in fire retardancy is presented.
The cone calorimeter has become one of the most important and widely used instruments for the research and development of fire retarded polymeric materials. The paper addresses three important ways in which the principal setup influences the results factors which sometimes do not receive due consideration when drawing conclusions. The paper discusses in detail the impact on cone calorimeter results of the choice of external heat flux, the influence on the peak of heat release rate of sample thickness and thermal feedback from the back of the sample, and the influence on irradiance of the horizontal and vertical distances from the cone heater.
FR additives were incorporated during tanning process. The mode of action of an intumescent system and each component individually, was methodically investigated in a semi-finished leather. Vertical flammability test results showed that the investigated semi-finished leather satisfies the regulations of the Federal Aviation Administration; in addition, afterglow time is strong minimized and forced fire behavior pointed out an important reduction on smoke production.
The passive fire protection of steel structures and other load-bearing components will continue to gain importance in future years. In the present contribution, novel intumescent aluminosilicate (geopolymer-bound) composites are proposed as fire-protective coatings on steel. Steel plates coated with these materials were exposed to the standard temperature-time curve as defined in ISO 834 – 1:1999. The coatings partially foamed during curing and expanded further during thermal exposure, demonstrating their intumescent characteristic.Thermogravimetryandoscillatory rheometry determined that the intumescent behavior is attributed to a transition to a viscous state (loss factor > 1) in the temperature range of major water release, differing from conventional geopolymers. XRD and SEM images showed that the coatings had characteristics of ceramic or glass-ceramic foams after fire resistance testing, suggesting superior performance under challenging conditions. The thickness of the coatings influenced their foaming and intumescent behavior and thus the time for the coated steel plates to reach 500 °C. A number of additives were also studied with the best performance obtained from samples containing sodium tetraborate.Acoating of just 6mmwas able to delay the time it takes for a steel substrate to reach 500 °C to more than 30 minutes.