GB/T 2406.4-2025Plastics - Determination of burning behaviour by oxygen index - Part 4: High gas velocity test (English PDF)

5.1 Chimney

The chimney should consist of a heat-resistant glass tube, which is vertically fixed to a base that can introduce an oxygen/nitrogen mixture (see Figure 1).

The chimney should ideally be 450mm-500mm high and 75mm-100mm in inner diameter. The top outlet should be open and unrestricted.

Note. It has been found that turbulence is generated inside the chimney at high gas flow rates in the outlet cup specified in 5.1 of ISO 4589-2:2017.

If the specified gas flow rate is within the allowable error range, chimneys with a height greater than 500 mm or less than 450 mm can also be used.

The lower end of the chimney or the base supporting the chimney should include a device for uniformly distributing the gas mixture entering the chimney, as shown in Figure 1.

As shown. Installing a porous screen under the sample holder helps prevent combustion debris from falling and blocking the gas entry and distribution path. One approach is to...

The chimney is designed as a two-part structure that can be separated to make it easier to place samples and clean them.

5.2 Sample holder

Used for vertical support specimens at the center of chimneys.

The sample holder can be hung on the upper edge of the chimney (see Figure 2) or supported by a vertical rod of the chimney base (see Figure 3). The vertical...

Partially made of stainless steel fork type, the specimen should be able to be fixed in the stainless steel fork. The specimen support frame should be 20mm, 100mm and [other dimensions] in the vertical direction.

A reference mark is provided at 140 mm, as shown in Figures 2 and 3.The surface of the sample holder should be smooth to minimize the impact of gases inside the chimney.

Turbulence.

5.3 Gas Supply

The oxygen/nitrogen mixed gas flow velocity in the chimney should be at least 1000 mm/s, and the deviation of the gas flow velocity should not exceed ±50 mm/s.

The oxygen/nitrogen mixture flow rate in this document is higher than the flow rate specified in ISO 4589-2, and the gas supply capacity should be higher than that specified in ISO 4589-2.

5.4 Gas Control System

It should be able to set and adjust the volume fraction of oxygen in the mixed gas entering the chimney. At 23°C±2°C and a gas flow rate inside the chimney of [missing information].

At a gas flow rate of 800 mm/s ± 50 mm/s, the resolution of gas regulation is 0.1%, and the accuracy is ± 0.2%. The gas flow rate should be between 600 mm/s and...

1000mm/s is controllable.

The flow rate of the mixed gas should also be controllable at 100 mm/s or below (see 8.3).

The temperature of the gas mixture entering the chimney should be ensured to be 23°C±2°C. If a gas mixture temperature probe is present, its location and profile should be designed to maximize...

To minimize gas turbulence inside the chimney.

5.5 Oxygen Analyzer

It should be suitable for measuring the volume fraction of oxygen in the mixed gas entering the chimney, with a resolution of 0.1% and an accuracy of ±0.1%.

Note 1.The paramagnetic oxygen analyzer has been found to meet the accuracy requirements.

Note 2.If the oxygen volume fraction around the sample location differs from that controlled by the gas supply line, it is necessary to troubleshoot the problem.

5.6 Flame Igniter

It should include a tube that can be inserted into a chimney, with an end diameter of 2 mm ± 1 mm, for igniting the sample.

The flame fuel should be propane, unmixed air, with a purity of at least 98%. The fuel supply should be adjustable to match the set oxygen level.

The integral numbers match, and when the tube is vertically downward inside the chimney, the total flame height is 16mm ± 4mm.

A flame height gauge should be used.

5.7 Timing Device

The measurement range is at least 5 minutes, and the accuracy is ±0.5 seconds.

5.8 Smoke Exhaust System

It should have sufficient ventilation capacity to remove smoke or soot from the chimney without affecting the gas flow or temperature inside the chimney.

5.9 Gas Flow Rate Measurement Device

That is, the anemometer should be able to measure the vertical gas velocity at the sample location inside the chimney, with a resolution of 0.01 m/s.

Note. A hot-wire anemometer suitable for this purpose has been discovered. Studies have found that this device can also typically measure the gas temperature specified in 5.10.

5.10 Gas Temperature Device

The accuracy of measuring the gas temperature inside the chimney should be ±0.5°C.

7.1 Sample Pattern

This method uses only the "V"-shaped specimens given in Table 2 of ISO 4589-2:2017.The "V"-shaped specimens are flexible films or sheets, and are [length missing].

The diameter is 135mm~140mm, and the width is (52±0.5)mm. The thickness of the sample should not exceed 2mm.

7.2 Sampling

At least 15 samples should be prepared for each gas flow rate test. Sampling should be performed according to the material specification; otherwise, it should be performed according to ISO 2589-1 or ISO 2589-2.

The applicable standards shall be followed for processing.

7.3 Sample Preparation

Specimen preparation shall conform to any applicable procedures of the relevant material specifications (see Note 1) or other standard methods (see Note 2), which are used for molding.

Samples are prepared by molding or cutting.

Note 1.Some material specifications may require the selection and determination of the "state of the specimen" used (e.g., for styrene-based polymers or copolymers, it must be specified as "specific").

(State or basic state).

Note 2.In the absence of relevant standards, one or more procedures from ISO 293, ISO 294, ISO 295, ISO 2818 and ISO 3167 may be used.

These standards provide detailed guidance on how to mold or cut specimens for preparation. The choice of which standard to use depends on the specific application and testing requirements.

The sample surface should be clean and free of defects that could affect combustion behavior (such as flash or burrs from processing).

When cutting samples from finished products, the non-uniformity of the material should be taken into account (see Note 3).

Note 3.Due to the inhomogeneity of the material, the ease of ignition or combustion behavior of the sample may be significantly affected. (For example, anisotropic thermoplastic films, etc.) when heated, samples cut in different directions may shrink to varying degrees.

7.4 Specimen Marking

The test specimens should be marked at distances of 20 mm and 100 mm from the ignition end.

7.5 State Adjustment

The sample should be conditioned for 24 hours at a temperature of (23±2)°C and a relative humidity of (50±5)%. If the sample mass change is no greater than 0.1%, If the test is successful, the test may be conducted; otherwise, adjust the test for at least 88 hours (maximum duration) according to ISO 291:2008, section 8.1.

For foamed plastics that may contain volatile flammable substances, it is advisable to place the sample in a suitable, well-ventilated oven before conditioning.

In a chamber, pre-treat at 60°C for 168 hours to remove volatile substances. For larger pieces of this material, a longer pre-treatment time may be required.

The equipment used to cut such specimens should be suitable for the risks involved.

8.1 Equipment Inspection and Validation

8.1.1 Maintain the ambient temperature of the testing instrument at (23±2)°C. If necessary, store the sample at a temperature of (23±2)°C and relative humidity of [missing information].

In an environment of (50±5)%, samples were taken out one by one during the test.

8.1.2 If necessary, equipment components should be recalibrated according to Chapter 6 and Appendix A. Before testing, clean the chimney to ensure good visibility.

Clean the air intake or air filter and temperature sensor (if applicable) if necessary.

8.1.3 Ensure the chimney is installed vertically (see Figure 1).

8.2 Setting the oxygen volume fraction and gas flow rate

8.2.1 Select the initial volume fraction of oxygen, if possible, based on past experience. You can also try igniting the sample and recording the combustion behavior, such as...

If the sample burns rapidly, choose an initial oxygen volume fraction of approximately 18%; if the sample burns slowly or unstable, choose an initial oxygen volume fraction of approximately 21%.

Integral score; if the sample does not continue to burn in air, then select at least 25% of the initial value based on the difficulty of ignition or the duration of continuous combustion in air.

Oxygen volume fraction.

8.2.2 Select the required gas flow rate within the range of 600 mm/s to 1000 mm/s.

Note. It has been found that testing at gas flow rates of 600 mm/s, 800 mm/s, and 1000 mm/s is appropriate. See Appendix D.

8.2.3 Close the bypass valve (10 in Figure 4). Set the mixing ratio of nitrogen and oxygen and the mixing flow rate (3-8 in Figure 4) to achieve the desired mixture.

An oxygen/nitrogen mixture of (23±2)°C integral flows within the chimney at a selected flow rate.

8.2.4 The oxygen volume fraction was measured using an oxygen analyzer (5.5), which collected a mixed gas sample at the gas sampling port (15 in Figure 4).

Record the oxygen volume fraction.

8.2.5 When using the gas flow rate measuring device specified in 5.9, the sample should be placed at a position 20 mm from the center of the chimney and at the top horizontal plane of the sample.

Set the gas flow rate to be measured. The gas flow rate should be within ±50 mm/s of the set value. The gas flow rate should be able to pass through the flow control valve (Figure 4).

16) Adjustments are made.

8.2.6 Once the predetermined oxygen volume fraction and gas flow rate are reached, keep the oxygen and nitrogen flow controllers (7 and 8 in Figure 4) stationary.

When necessary, use gas control valves (4 and 5 in Figure 4) to shut off the oxygen and nitrogen flow.

8.3 Ignition procedure for the test specimen

8.3.1 Install the specimen vertically at the center of the chimney, ensuring that the top of the specimen is at least 30 mm from the top of the chimney and the bottom of the specimen is at least 30 mm from the base of the chimney.

The top of the gas distribution device of the seat should be at least 100mm.

8.3.2 Open the bypass valve (10 in Figure 4) and set the gas flow rate to 100 mm/s or less (5.4).

8.3.3 Open the gas supply valves (4 and 5 in Figure 4) to introduce oxygen and nitrogen into the chimney. Before igniting the sample, purge the chimney with gas for at least 20 seconds, and then ignite...

Maintain a constant flow rate during the fire.

8.3.4 After purging, ignite the sample using a flame igniter (5.6), sweeping the flame along the top surface of the sample, ensuring combustion only on the top surface. Ignition process it should not last longer than 30 seconds.

8.3.5 After igniting the sample, close the bypass valve (10 in Figure 4) to make the gas flow rate in the chimney the selected flow rate.

8.3.6 The sample is considered ignited if the visible burning portion reaches the upper reference mark.

8.3.7 Observe the combustion behavior of the sample and record whether it exceeds or does not exceed the combustion limit under a predetermined oxygen volume fraction and gas flow rate.

Limits (see 8.4).

8.4 Evaluate the combustion behavior of the test specimens

When the burning continues and exceeds the lower reference mark, observe the corresponding burning phenomenon, extinguish the flame, and record it as an "×" response.

When the combustion does not exceed the lower reference mark, observe the corresponding combustion phenomenon and record it as a "0" response.

Note. The lower reference mark is located 40 mm above the bottom edge of the specimen.

8.5 Select a continuous oxygen volume fraction

The steps described in 8.6 and 8.7 are based on reference [17], which specifically addresses the special case of NT-NL=5 (see 8.7.2 and 8.7.3), and are modified accordingly.

Arbitrary step size can be used when changing the oxygen volume fraction.

During the experiment, the oxygen volume fraction of the next sample was selected as follows.

a) If the combustion behavior of the previous sample gave an "×" response, then reduce the oxygen volume fraction;

b) If the combustion behavior of the previous sample gave a "zero" response, then increase the oxygen volume fraction.

Choose the magnitude of the change in oxygen volume fraction based on 8.6 or 8.7.

8.6 Determination of the initial volume fraction of oxygen

Repeat steps 8.3–8.5, changing the oxygen volume fraction using any convenient step size, until the difference in oxygen volume fraction is found to be less than or equal to 1.

1.0%, one gives a "0" response, the other a "×" response. Record the "0" response from this pair of oxygen volume fractions.

The volume fraction of oxygen is taken as the initial oxygen volume fraction.

Note 1.When the difference in oxygen volume fraction is less than or equal to 1.0%, two opposite results do not need to come from samples measured consecutively.

Note 2.The volume fraction of a "0" response need not be lower than the volume fraction of a "×" response.

Note 3.For a format that facilitates recording the required information, see Appendix B.

8.7 Changes in oxygen volume fraction

8.7.1 Using the initial volume fraction of oxygen (see 8.6), test one sample by repeating steps 8.3–8.4.Record the volume fraction of oxygen used.

The number (co) and the response of "×" or "〇" are the results of the first series of NT and NL.

8.7.2 Change the volume fraction of oxygen according to 8.5, with a step size (d) of 0.2% (see 8.7.2.2). Test it according to 8.3~8.5.

He tested the samples, recorded the co values and corresponding responses, until the responses differed from those obtained in 8.7.1.

8.7.2.1 The NL series results consist of the results in 8.7.1 and the responses in 8.7.2 (see the example in B.3).

8.7.2.2 When the requirement of 8.7.4 is satisfied by a d value other than 0.2%, then that value can be used as the initial value of d.

8.7.3 Following steps 8.3-8.5, test four more samples, maintaining d=0.2%; and record the oxygen volume fraction and response for each. Then test the last sample...

The volume fraction of oxygen used in the sample is specified as cf.

These four results, together with the last result in 8.7.2 (i.e., the result with a different response than in 8.7.1), constitute the remainder of the NT series.

Partially, as shown in formula (1).

9.1 Calculation of HOI

9.2 Determination of k value

The value and sign of k depend on the sample response tested according to 8.7 and are determined by Table 2 as follows.

a) If the sample response tested according to 8.7.1 is "0", then the first opposite response (see 8.7.2) is "×", and when tested according to 8.7.3, in in column 1 of Table 4, find the row corresponding to the last four response symbols. Locate the "〇" in the NL series (according to 8.7.1 and 8.7.2).

The number of responses is the number of "0"s in row a) of this table. The k value and sign are given in columns 2, 3, 4 or 5.

b) If the sample response according to 8.7.1 is "×", then the first opposite response is "〇". When testing according to 8.7.3, see item 6 in Table 2.

In the column, find the rows corresponding to the last four response symbols. Find the number of "×" responses in the NL series (according to 8.7.1 and 8.7.2).

The number of "×" signs in row b) of Table 2, the value of k, and its sign are given in columns 2, 3, 4, or 5, but the sign of k is reversed.

Negative values of k become positive values, and vice versa.

Note. Appendix B provides examples of determining k and calculating HOI.

9.3 Standard deviation of oxygen volume fraction