Cotton has been used for textile fibre for a long time. It is cool and comfortable to wear. The air spaces within the fibres allow the fibre to absorb liquids. Cotton can be chemically treated to make it fire-resistant by soaking it into chemicals mixed with water. A fibre is a strand composed of polymer chains twisted around each other.
All fabrics will burn with some being more combustible than other, as shown in Figure 2, combustion requires energy, fuel source and oxygen (), but their burning rates can be reduced with flame-retardants applied through chemical-treatment. Flame retardants are usually added to consumer products for furnituZhZhre, insulation, textiles and electronics to meet the flammability standards. The flammability is how easily something will burn-or-ignite, causing fire-or-combustion (Wikipedia, 2014). There are complete, incomplete combustion and charring occurs during incomplete combustion, which refers to burning in a lack of sufficient air. Not all carbon atoms form carbon dioxide, some or all may turn into carbon monoxide or forms pure carbon particles (soot) or deposits (char).
Flame retardants are used for preventing fires from starting or for delaying fire, as well as limiting the spread of fire and minimise the fire damage. Solid-materials do-not burn directly; they must be decomposed by heat (pyrolysis) first to release flammable gases. When the flammable gas burns with oxygen in the air, visible flames will appear. However, if solid-materials do-not break-down into gases, they will only be slow smouldering and usually extinguish themselves. Especially if materials ‘char’, then form a stable-carbonaceous barrier which prevents-access of the flame to the-underlying material (EFRA, 2014). When materials have been ignited, the heat generated breaks down from long-chain solid molecules to smaller molecules which transpire as gases.
Ammonium sulphate [(NH4)2SO4] is an inorganic salt with various commercial uses (Wikipedia, 2014), and ammonium dihydrogen phosphate (NH4H2PO4) forms when a phosphoric acid solution is put into ammonia till the solution is significantly acidic. These are used as flame retardants in our experiment in investigate which chemical is a better flame retardant at their maximum and half concentration.
The most effective flame retardant was judged by the average differences of mass before and after burning of ammonium sulphate (AS) and ammonium dihydrogen phosphate (ADP) at their maximum (M) and half maximum (H) concentrations. The control has the highest differences of mass of 1.48g, because it fully burned after it is put on fire for three seconds. 5.62mol/L (M) AS and 3.48mol/L (M) ADH have average differences of mass of 0.2g and 0.073g, which are less than the 2.81mol/L (H)AS and 1.74mol/L (H)ADH. This show the chemicals were more effective at their maximum concentration.
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Due to an anomaly that occurred in the average area burnt data, the average area burnt result in Table 3.1.3 and Graph 3.2.2 for ADH solution cannot provide a high accuracy data, so this result will be disregarded. The percentage differences between the mass before burning and average mass were calculated for comparing and justified 3.48mol/L ADH was the best flame retardant in this experiment. Water had the highest percentage difference which proves that water has little or no flame retardant ability. Obviously, MADH has the least percentage difference which supports that ADH is a good flame retardant.
A higher molarity resulted in a better flame retardant. Both chemicals had better results in resisting the burning process at maximum concentrations. There was lots of charring on fibres soaked in MADH after burning (black substances showed it was only burnt on the surface), and only a little amount of charring occurred on MAS’s fibre. The layer of carbon (black substances) on fibre’s/polymer’s surface is charring which proves the chemical is a good flame retardant. The more char fibre has, the more effective the chemical of that fibre coated with. When the fibres soaked in the other three solutions were burned, charring was only present on the edges. Charring occurs during incomplete combustion, which proves the lack of sufficient air during the burning, and therefore supports the result that 3.48mol/L ADH was the best flame retardant. [(NH4)2SO4] and ADH (NH4H2PO4) both have NH4 and hydrogen, however, the main difference is sulphate and phosphate. Although AS has one more nitrogen and two more hydrogen atoms than ADH, from the results, element phosphate can be predicted as having a better flame retardant ability than element sulphate.
Evaluation & Improvements
The results have a high level of consistency, as the range of values between trials was small, with a few anomalies. Table 7.3.1 shows the mass before and after burning of Trial 2 of the control (0.24g), which had a significant large disparity with the other trials’ data (0.04g & 0.03g). Although it could be removed from calculation of average, due to its small effect on the overall results, it was kept. Table 7.3.1 also shows the amount of burnt area of Control Trial 2 (88 squares) was lower than the other two trials (both fully burned), and is therefore a significant anomaly. This occurred as the fibres weren’t always steadily put at the same spot in the flame. This could be improved by placing the fibre in the flame more carefully, and performing a few more trials to ensure the overall accuracy.
Part of our group experiment was done twice, due to a systematic error. At first, the fibres coated with MADH and HADH were not timed, so the time that the fibres were soaked varied. The beakers might be put in the wrong concentration of ADH as it was marked. Therefore, those fibres might contain less or more solution, which explains the HADH solution’s results that appeared to be a better flame retardant than the MADH solution. A second attempt on the ADH solutions was successful, and showed much better results, matching the expected results. These results were used for analysing with the water and AS solutions’ data.
At half maximum concentration, the molarity is lower than the maximum concentration, but in Table 3.1.3 result of 1.74 mol/L HADH had a smaller area burnt than the MADH. Table 7.3.2 also shows the unexpected results of area burnt for ADH. This may be a systematic error as human judgement was required to count the number of burnt squares. Burning time can be recorded to the future experiment, to determine the best flame retardant. The mass retained of chemicals at their maximum concentration should be twice of the half concentration mass retained. Table 3.1.5 shows fibres’ masses at maximum concentration is 1.236 (AS) and 1.289 (ADH) times of their half maximum concentration; Graph 2.3.4 shows the growing trend of the mass retained at their half and maximum concentration. The exponential trend due to when chemicals at their maximum concentration the mass retained won’t grow any higher. In this experiment AS and ADH weren’t actually done at the same concentration, so in future experiment the ultimate test of the best flame retardant is to do the chemicals at the same concentrations. Alternatively, aluminium potassium sulphate-12-water and disodium tetraborate-10-water can be added into the experiment, and investigate the best flame retardant.
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