Polyurethane is the abbreviation of polyurethane, which is a heat-insulating material widely used in the construction industry and industrial equipment, and an important material in industries such as automobiles, shipbuilding, and packaging. However, due to the special structure of polyurethane foam itself, it is extremely easy to burn, and its oxygen index is only about 17%. When burning, it releases a large amount of smoke and poisonous gas, which is prone to dripping. Its flammability problem has always been an important subject of polyurethane industry research. Various countries in the world have formulated various flame retardant standards and regulations for testing polyurethane, and gradually improved the test methods, and produced various testing instruments and equipment. Since the oxygen index is one of the signs to determine the flame retardant performance, the oxygen index tester is commonly used Testing equipment, Standard Group (Hong Kong) Co., Ltd. is a manufacturer, welcome customers in need to inquire in time. In addition, the oxygen index test experiment will be affected by factors such as environmental conditions, operators, and sample preparation differences. In order to more accurately determine the oxygen index value of the material, the technical research and development engineers of Standard Group have an analysis of the influence of various factors on the oxygen index during the test. The impact was discussed and analyzed.
Reference standard:
Oxygen index is in accordance with GB/T2406-1993, and the standard environment for sample state adjustment and testing is in accordance with GB/T2198-1988.
Experimental principle:
Limiting oxygen index (LOI) is abbreviated as oxygen index. It is one of the signs for judging flame retardancy. It refers to the low oxygen concentration required for the sample to maintain a balanced combustion in the oxygen and nitrogen mixed gas flow under specified conditions. The percentage of the occupied volume is expressed.
The principle of the test is to vertically hold a sample of a certain size in a transparent combustion cylinder with a sample holder, in which there is a flow of oxygen and nitrogen mixed in a certain proportion and flowing upward. Ignite the upper end of the sample, observe the subsequent burning phenomenon, record the continuous burning time or the burned distance, and compare it with the specified value. If the specified value is exceeded, the oxygen concentration will be reduced, and if the specified value is insufficient, the oxygen concentration will be increased. Repeat the operation from the upper and lower sides. Gradually approaching the specified value, until the concentration difference between the two is less than 0.5%.
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Analysis of influencing factors:
1. The influence of state adjustment time
According to the requirements of the national standard GB/T2918-1988, the state of the test materials is adjusted to minimize the impact of environmental accidental factors on the test results. It can be seen from the test data that as the sample conditioning time increases, the average value of the oxygen index obtained in the test continues to decrease. When the conditioning time meets a certain time limit, the oxygen index remains stable. The test results of the samples under different adjustment times are different. Therefore, the samples should be strictly adjusted according to the standard requirements, and then tested, in order to obtain an accurate oxygen index value under the standard state adjustment time.
2. The influence of sample ignition method
According to GB/T2406-93, two ignition methods are selected for the oxygen index test of polyurethane foam samples, and the ignition time is 15s.
Top ignition method: Make the low visible part of the flame contact the top of the sample and cover the entire top surface. Do not make the flame touch the edges and side surfaces of the sample.
Diffusion ignition method: Fully lower and move the igniter so that the visible part of the flame is simultaneously applied to the top surface and the vertical side surface of the sample about 6mm long.
It was found that when the same material was tested under the same conditions, different ignition methods had little or no effect on the oxygen index measurement data, which was not enough to change the evaluation of the sample oxygen index results. Regardless of the ignition method, when to remove the ignition source and start timing is to visually judge the ignition situation of the sample with the naked eye, there is a certain error, and may cause an impact on the inspection data. Only by increasing the number of tests can we try our best Reduce the error. In actual operation, the top ignition method is more labor-saving when igniting the sample.
3. The influence of sample length
The length of the sample of the tested product in the national standard is 70-150mm. The purpose is to keep the length of the glass cylinder of the test instrument consistent, and make the oxygen concentration in the small environment mixed gas in the glass cylinder uniform and constant. The length of the sample has a great influence on the inspection data. Within the range required by the national standard, the oxygen index value measured as the length of the sample increases. When the length of the sample is short, there is still a considerable distance from the top of the burning area of the tested sample to the top of the cylinder, so the oxygen required for its flame combustion is completely supported by the instrument itself and is not affected by the airflow outside the cylinder. However, as the length of the sample increases, the distance between the combustion area of the tested sample and the top of the cylinder becomes shorter, and the flow of oxygen required for flame combustion is gradually affected by the airflow outside the cylinder. In order to be able to support flame combustion, the instrument itself must provide more oxygen to satisfy the flame combustion. As a result, the measured oxygen concentration value gradually increases. Therefore, the length of the tested sample should be specified as a specific value instead of a range value to reduce the degree of influence of the indoor ambient air flow on the combustion conditions in the cylinder of the instrument.
4. The influence of flame retardants
Flame retardant is an expensive additive in polyurethane foam. Its role is very important. The amount of flame retardant is not the better, and the influence of the amount of flame retardant on the physical and mechanical properties of the foam should also be considered. In view of the consideration of cost, safety, meeting flame retardant technical indicators and foam physical and mechanical properties, it is necessary to optimize the use of appropriate flame retardants to maximize their effects.
Influence of flame retardant addition: data shows that for TDCPP flame retardant, based on polyether polyol, 10Phr of TDCPP can make the oxygen index of PUF at 23%, and the oxygen index of PUF increases with the addition of flame retardant. However, after the amount of TDCPP is greater than 15Phr, if the amount of TDCPP is increased, the oxygen index of the sample can be increased very limited.
According to the flame-retardant mechanism, phosphorus can interact with the polymer or ambient oxygen during the combustion process to generate a series of derivatives such as phosphoric acid, metaphosphoric acid and polymetaphosphoric acid. These glassy liquid derivatives have high thermal stability and can be used in An isolation film is formed on the surface of the polymer. Its formation can not only block the heat from the flame zone to decompose the polymer, but also prevent the decomposed combustible gas from entering the flame zone. Moreover, polymer decomposition and combustible gas combustion tend to promote or weaken each other. Therefore, the formation of this liquid film covering layer will inevitably promote a substantial increase in the flame retardant effect. The flame-retardant effect of melamine is mainly that it can catalyze the formation of a solid coating of carbon, and the flame-retardant effect of aluminum hydroxide is that it can form a solid protective layer of aluminum oxide on the one hand, forming a non-combustible barrier between the fire source and the foam. So it has a flame retardant effect. On the other hand, it contains a lot of crystal water, which is very stable during the foam production process, but it will quickly decompose when the foam burns and absorb heat and cool down.
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