Tech Focus: Fire Retardation in Polyester Resin: Part 2
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R.Raghavan of Satyen Polymers educates us on Fire Retardation (FR) on Polyester Resins. This is a continuation of the section on Mechanism of flame Retardation.This is the last of a 2-part series.
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Endothermic Decomposition
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Decomposition occurs once the high binding energies between the individual atoms in the polymer are overcome. In general, decomposition occurs via free radical chain reactions initiated by oxygen or oxidizing impurities which are trapped in all plastics during manufacture. These free radicals are responsible for the flame spread in the combustion process.
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When Ignition takes place!
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The flammable gases that are formed by the decomposition process are mixed with atmospheric oxygen and ignited by an external flame, or alternatively by self-ignition if the temperature is sufficiently high compared to the oxygen to flammable gases ratio. When ignition occurs there is a thermal feedback due to the exothermic nature of the combustion process.
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Flame spread
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Thermal feedback from the combustion process provides more energy to the decomposition process during which new flammable gases are produced, thus feeding the combustion process. During flame spread the temperature of the polymer is typically 5000C-6000C while the temperature of the flame where the reaction with oxygen takes place is approximately 12000C.
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How Smoke develops?
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Smoke is a result of incomplete combustion and is a dispersion of solid/liquid particles in a carrier gas consisting of combustion gases and air. The liquid particles are tar-like droplets or mist composed of liquid product from pyrolysis; their partially oxidized derivatives, and water. The solid particles contain soot, ash, sublimed pyrolysis product and oxides of inorganic compounds.
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Combustion product
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The composition of gases during combustion of solid plastics is mainly dependent on the combustion temperature as well as the availability of oxygen.
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The supply of oxygen and the temperature will vary constantly during the fire As a result the composition of the combustion gases will also vary. It is too complex to measure the composition of the gases through out the fire’s duration.
During combustion of a halogenated fire retardant polyester laminate, formation of gaseous Hydrogen Chloride, oxy chloride and/or Hydrogen Bromide, oxy Bromide will result in addition to the above mentioned combustion products.
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When the fire occurs, what happens once a fire starts and how product design can render a product more resistant to ignition.
In addition to behaving as a fire retardant, it is very effective as a smoke suppressant in a wide range of polymers, most especially in polyesters, acrylics, ethylene vinyl acetate, epoxies, PVC, rubber.
An intumescent layer is a physical hindrance between the solid phase and the gaseous phase. This layer is formed by the combustion process and consists of inert gases and/or a solid crust that cools the solid phase by reducing heat transfer as well as shielding it from oxygen, breaking one side of the fire triangle. An intumescent layer may be formed by the introduction of phosphorous additives into the product.
Combination of these modes of action by use of different base resins and additives often results in a greater benefit (synergy) than if they were used individually. A common synergist with halogenated products is Antimony Trioxide (ATO).
Consequently it is possible to design composites with different degrees of smoke and flame spread performance to meet the various standards required by the end user.
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Laminate Construction and Resin Usage
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When designing and producing a laminate using a fire retardant resin, one should select a resin based on the required fire test criteria and end application.
Once this is done the following points which influence the performance of the end product should be taken into consideration – Content of glass, air voids, geometry and surface smoothness/thickness of laminate
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Post curing
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It’s important to maximize the glass content in finished laminates requiring fire retardancy not only because it acts as reinforcement,
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but also due to its inertness to fire.
In general, one should aim to reduce air voids and ensure a smooth surface since these factors prevent the propagation of flames. By limiting the laminate thickness one ensures that the total amount of combustible material is low. On the other hand, a thin laminate heats up to the temperature at which decomposition occurs much sooner than a thick laminate.
Due to the low thermal conductivity of GRP laminates it takes a longer time to reach the decomposition temperature in a laminate of 4-8 mm, thus delaying both time to ignition and the propagation of fire.
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The process of post curing reduces the amount of residual styrene in a laminate.
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This concludes the 2 part series on FR Resins. We have a useful table on Fire tests and Classification sent in by Mr. R. Raghavan Please email us if you wish to have a copy.
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Financial Year-end and
New year wishes
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We are approaching the end of the current financial year, and we hope you will close on a high note! We wish you a successful Financial Year 2012-2013.
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Contact us
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