Today's industry offers different alternatives. Let's know some novelties in its formulation.
by Eng. Orietta León*
With the growing trend of stricter fire safety regulations, the demands for fire risk reduction by highly combustible materials such as wood, plastics, textiles, among others, have gained importance in recent years. An adequate flame retardant treatment must be able to slow the ignition of such substrates and/or decrease the spread of the flame, thus reducing the risks of fire, loss of life and destruction of property [1].
Great efforts have long been invested in developing materials with flame retardant properties. Three types of approaches have been accepted and commonly used in various areas such as electrical, electronics, construction and transportation. The first approach involves the mechanical incorporation of flame retardant additives into the polymer matrix, which is a low-cost, fast-mixing technique. However, the flame retardant load required is usually too high, which significantly influences the elastic modulus and strength of the materials.
The second way to reduce the flammability of the matrix is to chemically bind the functional groups of the flame retardants. Through this approach the flame retardant element becomes an integral part of the polymer chain and typically results in increased efficiency and durability of the flame retardant effect. Such incorporation could change the morphology and physical properties of the polymer also presents some difficulties in the industrial manufacture of certain materials such as fibers and flexible foams.
The third approach basically involves surface modification and is widely used in different commercial sectors [2].
In many cases, the flame retardant coating represents the only barrier between the fuel and a possible source of fire, therefore, it must effectively resist fire, delaying the ignition of the substrate, reducing the transfer of mass and heat between the gaseous medium and the condensed phase, and preventing the spread of the flame. Based on the delay mechanism, flame-proof coatings are classified as intumescent or non-intumescent. The intumescent coating can be described as a mixture capable of swelling and forming a three-dimensional carbon layer on top of the substrate when exposed to fire. Traditional intumescent systems consist of a carbon source that acts as a carbon former (e.g., pentaerythritol), an acid source that behaves as a dehydrating catalyst (such as polyphosphate), and an expansion agent that helps form the porous barrier such as melanin and guanidine.
This characteristically porous carbonaceous residue acts as a barrier to heat, air as well as pyrolysis products and protects the substrate from fire expansion. Unlike intumescent systems, non-intumescent coatings can release active species that act in the gas phase to inhibit the flame, catalyze the decomposition of the material surface to form glassy non-bulky layers, or act as an insulating mirror for protection against heat source radiation [3].
Non-intumescent flame retardant coatings use halogen-containing compounds, e.g. chlorine or bromine, or phosphorus, or inorganic metal compounds as the main flame retardant component in the formulation [4].
In the case of intumescent flame retardant coatings, an optimal selection of compatible intumescent ingredients in terms of physical and chemical properties is necessary to obtain high fire protection efficiency. Non-intumescent flame retardant coatings typically contain fewer ingredients, making them more compatible with matrices, leading to better mechanical properties and fire performance [5].
In recent years new non-intumescent flame retardant coatings based on organic and inorganic compounds such as phosphorus, nickel, silica and metals have been developed, although some formulations still involve the presence of halogen. Flame retardant coating formulations may also contain combinations of the aforementioned elements to achieve synergistic interaction [6].
One of the most widely used flame retardant coating systems are halogen-based ones, however, their application has been restricted due to the environmental problems they cause. Halogenated retardant coatings, mainly, work in the gas phase, forming halogen radicals that act to remove free oxygen and hydroxyl radicals formed in the fire, so the combustion process is interrupted. Although it is rare to see new developments in flame retardant coatings that include halogens, recent advances include halogenated epoxy resins, which have been employed as flame retardant coatings for optical fibers [7].
Inorganic fillers can be added to flame retardant coatings as a coaddative to work in synergy with the primary flame retardant coating and help increase its effectiveness, for example magnesium or aluminum hydroxide is usually added in the formulation of the coating system along with the retardant to the main flame. On the market, there are formulations of flame retardant coatings containing a cross-linked thermoset polymer (based on polyurethane) and magnesium hydroxide in a concentration of approximately 30%, this coating provides the substrate with a self-extinguishing character. Recently, nanometric inorganic charges of layered silicates, derived silsesquioxanes, titanium dioxide, silicon dioxide and carbon nanotubes, among others, have been incorporated as inorganic charges [8].
Phosphorus-based flame retardants, on the other hand, are dominated by phosphate esters. During heat exposure, phosphate flame retardant coatings can interact with the coating matrix to improve surface protection. Within this class, particularly halogenated phosphate esters are widely used in flame retardant coating formulations. Phosphate flame retardant coatings are versatile, since they act in both the gas phase and the condensed phase. These flame retardant coatings generate less toxic gases and smoke during combustion [9].
However, nitrogen flame retardant coatings are being widely used because they are environmentally friendly and non-toxic substituents for halogenated flame retardant coatings. Many of the nitrogenous bases used in flame retardant coating formulations are derived from melanin and exhibit flame inhibition in condensed phase and gas phase. Melanin salts with phosphoric acid are widely used in intumescent flame retardant coatings while new developments include monomers containing melanin which are applied in non-intumescent coating systems [10].
As for the preparation of silicone-based flame retardant coatings, these involve the incorporation of silicone, silicates, organosilicates or silsesquioxanes as fillers and copolymers or as the main polymer matrix in the system. Hydroxyl-finished polydimethylsiloxane have been developed to modify silicone within an epoxy resin. These systems do not release corrosive smoke during combustion [11].
Currently, we also find multi-element flame retardant coating systems based on phosphorus, nitrogen, sulfur and chlorine for application in solvent-based alkyd paints and emulsion. These systems seek to improve the performance of the flame retardation level of the coating [12].
* Eng. Orietta León
Pintumaxx Paint Factory – Colombia
https://fabricatupintura.com
References
1. E.M. Pearce, R. Liepins. 1975. Flame retardants. Environmental Health Perspectives, 11, 59-69.
2. H. Qu, W. Wu, Y. Zheng, J. Xie, J. Xu. 2011. Synergistic effects of inorganic tin compounds and Sb2O3 on thermal properties and flame retardancy of flexible poly(vinyl chloride). Fire Safety Journal, 46, 462-467.
3. E.D. Weil. 2011. Fire protective and flame retardant coatings a state of the art review. Journal of Fire Sciences, 29, 259-296.
4. Z. Wang, E. Han, W. Ke. 2005. Influence of nano-LDHs on char formation and fire resistant properties of flame retardant coating. Progress in Organic Coatings, 53, 29-37.
5.M. Jimenez, S. Duquesne, S. Bourbigot. 2006. Multiscale experimental approach for developing high-performance intumescent coatings. Industrial & Engineering Chemistry Research, 45, 4500-4508.
6. I. Errifai, C. Jama, M. Le Bras, R. Delobel, L. Gengembre, A. Mazzah, et al. 2004. Elaboration of a fire retardant coating for polyamide-6 using cold plasma polymerization of a fluorinated acrylate. Surface and Coatings Technology, 180-181, 297-301.
7. A.R. Horrocks, G. Smart, S. Nazaré, B. Kandola, D. Price. 2010. Quantification of zinc hydroxystannate and tannate synergies in halogen containing flame retardant polymeric formulations. Journal of Fire Sciences, 28, 217-248.
8. T. Nosker, M. Mazar, J. Lynch, P. Nosker. 2007. Flame retardant coating. US Patent Application Publication, US 2007/0173583 A1.
9. X. Chen, C. Jiao. 2008. Thermal degradation characteristics of a novel flame retardant coating using TG-IR technique. Polymer Degradation and Stability, 93, 2222-2225.
10. H. Liang, A. Asif, W. Shi. 2005. Thermal degradation and flame retardancy of a novel methacrylated phenolic melamine used for UV curable flame retardant coatings. Polymer Degradation and Stability, 87, 495-501.
11.M. Iji, S. Serizawa. 1998. Silicone derivatives as new flame retardants for aromatic thermoplastics used in electronic devices. Polymers for Advanced Technologies, 9, 593-600.
12. H. Abd El-Wahab, M. Abd El-Fattah, M.Y. Gabr. 2010. Preparation and characterization of flame retardant solvent base and emulsion paints. Progress in Organic Coatings, 69, 272-277.
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