Since most polymers do not meet on-site fire protection standards, currently flame retardant plastics are being developed that can be used throughout the transportation industry in the electrical / electronics industry, construction industry, consumer products, and automotive, rail vehicles and aircraft manufacturing. Methods can be used to test flame retardant polymers: UL94 V flammability test (vertical test); taper calorimetry; micro combustion calorimetry (MCC); thermogravimetric and thermogravimetric analysis.
Flammability test based on UL 94 V
UL94 standards from the U.S. Safety Testing Laboratory (UL) are often used as benchmarks or as a test standard to guide the development of flame-retardant plastics for chemical and composite manufacturers UL initially used UL94 V Flame-retardant tests approve plastics that enter the US electrical / electronic field, but as the process of globalization progresses, this test has become an internationally accepted grade that demonstrates the flammability of polymers in all applications.
Figure 1 Test set (left) and test standard (right) (Source: LKT) used to determine the UL94 V-0, V-1,
This test requires a test strip (125 mm x 12.5 mm x thickness) and a 20 mm long, 50-watt methane flame.In a vertical combustion test (V test), the flame ignites the test sample twice for 10 seconds each time the flame is ignited Afterwards, with the aid of cotton, the burn-up time and the dripping of the melt are evaluated.Figure 1 shows the standard for pretreatment of the sample, the test procedure and the flammability rating of plastic materials.
Depending on its thickness, the material was rated V-0, V-1 or V-2:
◆ UL94 V-0: self-extinguishing within 10 seconds, no melt dripping, no more than 30 seconds remaining flame.
◆ UL94 V-1: 30 seconds self-extinguishing, no melt dripping, no more than 60 seconds remaining flame.
◆ UL94 V-2: self-extinguishing within 30 seconds, a melt dripping.
UL 5 V, 5 VA, and 5 V B represent tighter plastic fire ratings and a 125 mm long, 500-watt methane flame ignites vertically-oriented test strips (125 mm x 12.5 mm x thickness) Plastics above Class -2 are subject to additional ratings based on the UL 5V standard for larger wall thickness materials. The criteria for achieving this level are:
◆ 5V: The flame ignites five times, each lasting 5 seconds and then pause for 5 seconds. After igniting five times, there is no residual or residual flame within 60 seconds; no melt drips, including cotton ignition.
◆ 5VA, 5VB: Ignite the flame below the horizontal plate in addition to the requirement of 5V. 5VA: No burn-through point (hole) allowed on the board; 5VB: Allowable burn-through point (hole) allowed after flameout.
Unlike other fire tests, the unparalleled advantage of the UL94-V test is that the plastic grades are based on wall thickness.The results of flammability tests on flame-retardant and non-flame-retardant PC + ABS specimens of different wall thicknesses As shown in Table 1.
Table 1 Wall thickness based UL94 V fire rating test: Test results for flame retardant and non-flame retardant PC + ABS (Source: LKT)
The downside is that the UL94-V test device, which operates and evaluates only empirical and scientific evidence, has unique advantages in approvals and therefore is suitable for use in a wide range of applications.
The biggest problem with UL fire ratings is that it primarily tests molded parts (test specimens) and flammability levels are based on plastic raw materials.Therefore, the test results are very much dependent on the processing conditions (up to two fire levels) , Mold design (deviation up to a fire rating), relative position of gate and cavity, subjective evaluation between laboratories within and around the world, and test evaluation without statistical analysis (once a sample fails, tests Considered as unqualified).
Therefore, since the test specimen is not always produced under the same conditions, the test may be used to mold the part and the result may be biased. A reasonable way to avoid this is to include the process conditions in the fire rating test.
Cone calorimeter fire test
The procedure for conducting a fire test with a cone calorimeter is described in detail in the standard rules.
The pyramidal heating coil uniformly radiates the surface of a sample having a size of 100 mm × 100 mm × d (d preferred thickness = 3 mm) under a condition of a variable heat radiation of 0 to 100 kW / m 2 and burns in the thickness direction (FIG. The amount released is determined by the oxygen consumption method based on the principle that the amount of heat released per kg of oxygen consumed is 13.1 MJ.
Figure 2 Cone calorimeter test setup (Source: LKT)
During the test, the amount of heat released per unit area and the corresponding burn-in time were reported.The test results obtained by the cone calorimeter had the following parameter characteristics: ignition time (TI), heat release rate (HRR) Maximum heat release rate (PHR); Total heat release (THR); Total volume of CO and CO2; Smoke concentration.
The eigenvalues of the flame-retardant and non-flame-retardant PC + ABS samples measured with a cone calorimeter are shown in Figure 3. It can be seen that the ignition time of flame retardant PC + ABS is about 75% longer, The maximum heat release is only about 50% of the non-flame retardant sample, which is why the flame retardant sample reaches the UL94 V-0 rating and the PC + ABS resin is not.
Figure 3 Cone calorimetry: Flame retardant and non-flame retardant PC + ABS curves are measured in a cone calorimeter at a heat flux of 50 kW / m2 (Source: LKT)
The wide range of properties that plastics require for fire reactions require more time, cost, and testing effort than the UL94 V test, which is limited by the method of sample production that is used to test thin-walled materials (d <1mm)时不够精准。
Micro-combustion calorimetry (MCC)
The advantage of the micro-calorimetry is that it is process-independent, and it is able to deduce the effects of processing by inspecting the pellets and part samples prior to processing. A small portion of plastic (2-3 mg) is treated with an inert gas (eg, Nitrogen) heated by a heating coil surrounding the chamber (Figure 4) .After the heating and nitrogen supply were interrupted, the external igniter ignited the released combustible gas and supplied oxygen.The amount of heat released was determined by the oxygen consumption method.During the test, The heat released per unit of material and the corresponding sample temperature are marked (Figure 5).
Figure 4 micro-combustion calorimeter test device icon (Source: LKT)
The eigenvalues of the tested flame retardant and non-flame-retardant PC + ABS blends are shown in Figure 6. It can be seen that the flame retardant mixture caused a temperature change corresponding to a maximum mass change of about 95 K (from about 445 ° C About 540 ° C. The heat release rate of the flame retardant mixture was reduced on average by about 130 W / g The scatter (peak difference) of the heat release rate of PC + ABS was 80 W / g, although the heat release rate was significantly reduced, The combustion mixture is 10 W / g higher The reason for the large scatter in the test results is the uneven distribution of additives in the pellets.
Figure 5 MCC with eigenvalue (Source: LKT)
Thermogravimetric and thermogravimetric analysis (TGA)
The thermogravimetric and thermogravimetric test setups and testing procedures were standardized in ISO 11385 and DIN 51006. Samples were prepared from 5-10 mg plastic and were observed at 0-50 K / min (typically 20 K / min) The sample mass is affected by temperature and time when heated to a maximum of 1000 ° C at the heating rate.For comparison and understanding, Figure 7 shows the differential signal dm / dt as a function of temperature (derived thermogravimetric method, in%, mass temperature Curve derived results) in the form of flame retardant and non-flame retardant PC + ABS mixture test results.
Figure 6 MCC measurement results: Non-flame retardant PC + ABS (left) and Flame retardant PC + ABS (right) (Source: LKT)
It can be seen that there are two distinct characteristic peaks for PC + ABS, one at 458 ° C for ABS and one at 538 ° C for PC. The peak temperature of the flame retardant mixture varies between 476 ° C and 547 ° C, Expressed as the largest change in mass and / or exotherm, which corresponds to a change of about 95K and is therefore within the range of the results of the micro-calorimetry.
Conclusion
These tests do not accurately assess the fire response of the above flame retardant and non-flame retardant PC + ABS blends but this does not matter, but rather, these results illustrate the limitations of the methods used either without a sound scientific basis (UL94 ), Or lack of a clear and consistent standard in practice to assess and compare the effectiveness of fire retardant plastic fire prevention measures. TGA and MCC gave similar results on decomposition temperature but TGA did not provide any information on plastic combustion behavior. Material development process is a practical aid, but can not explain the shape and structure of the combustion behavior.
Figure 7 Derivatization thermogravimetric method: Flame retardant PC + ABS (yellow) and non-flame retardant PC + ABS (green) (Source: LKT)
Therefore, in order to illustrate the relationship between flammability and material, shape and processing conditions, we need new ways to scientifically quantify the reaction to fire and to be able to execute quickly, and also to change the shape and structure, adjust the flame according to the part, and analyze the smoke. The test is ideal for samples of various wall thicknesses - samples produced by a Campus mold system or an injection molding machine equipped with a screw of 25-30 mm diameter, with variability in sample shape and manufacturing conditions Components and unchecked configurations can be predicted by neural networks.
The new Key Laboratory of the Bavarian Polymer Research Institute (BPI) at the University of Friedrichs in Nuremberg, Germany, is delving into the latest systems analysis methods to test pellets and two-dimensional specimens.