What are the methods for testing the flame retardant properties of textiles?

Assessing the flammability of flame-retardant treated fabrics is a complex task, influenced by numerous factors such as moisture absorption and weight. However, it can be primarily evaluated from two key aspects: ignition point (the ease with which the fabric ignites) and combustion performance (the rate at which a fabric sample burns in a specific direction under defined conditions).

A closer look at the flame-retardant testing methods used in various countries reveals inherent connections despite their differences. From the perspective of the environmental conditions of the test sample, the oxygen index method has unique advantages. From the perspective of the sample's placement, they can be broadly categorized into three types: vertical, 45°, and horizontal.

1. Limiting Oxygen Index Method

The limiting oxygen index method is currently a widely used method for testing the flammability of textiles. It refers to the minimum oxygen concentration required for a material to maintain combustion in a mixture of oxygen and nitrogen under specified experimental conditions, denoted by LOI, which is the volume percentage of oxygen in the gas mixture. GB/T5454-1997 specifies the oxygen index method for testing the flammability of textiles. The sample is clamped in a sample holder perpendicular to the combustion chamber. In an upward-flowing oxygen-nitrogen gas stream, the upper part of the sample is ignited, and its combustion characteristics are observed. The afterflame time or smoldering length is compared with the specified limit values. Through a series of tests on samples at different oxygen concentrations, the minimum oxygen concentration value, expressed as the percentage of oxygen required to sustain combustion, can be determined. 40%-60% of the tested samples should exceed the specified afterflame and smoldering times or smoldering lengths.

2. Combustion Test Method

The combustion test method is used to determine the extent of combustion (charred area, smoldering length), afterflame, and smoldering time of a sample. The specific operation is as follows: A sample of a certain size is placed in a specified combustion chamber, ignited with a specified flame source for 12 seconds, and after the flame source is removed, the afterflame and smoldering times are measured. After the smoldering stops, the smoldering length is measured.

Depending on the relative position of the sample and the flame, the test method is divided into three types: vertical, inclined, and horizontal. The vertical method is the most common and involves more intense testing. It is further subdivided into the vertical damage length method and the vertical flame spread performance test method, among others. GB/T5456-1997 specifies the method for determining the flame spread performance of vertically oriented samples. This involves igniting a vertically oriented textile sample with a specified igniter, flame, and ignition time, and measuring the time it takes for the flame to spread to the marked line. Other flame spread performance can also be observed and recorded. The vertical method is suitable for clothing, decoration, and tent fabrics; the inclined method is suitable for aircraft interior fabrics; and the horizontal method is suitable for carpets and other upholstery fabrics.

3. Surface Burning Test Method

The surface burning test method is used to determine the extent of combustion spread on the surface of thick textiles. Taking floor coverings as an example, initially, horizontal methods were commonly used abroad. For instance, the United States used the horizontal cigarette butt method and the urotropine method. The latter involves placing a urotropine tablet in the center of the sample, igniting it, and measuring the maximum distance from the extinguished flame to the center of the tablet. The United Kingdom used the hot metal nut method, placing a hot stainless steel nut on the sample surface and measuring the distance from the extinguished flame to the center of the nut and the ignition time. However, the horizontal method's combustion conditions were not intense enough, and many carpets could meet the standards without flame retardancy. Therefore, the thermal radiation method was adopted instead. The United States has specified critical radiant flux standards measured by the radiant plate method for residential and public facility carpets respectively; Japan uses the top-mounted flame method, which has requirements on the charring distance of the carpet.

The thermal radiation method involves placing a thermal radiation plate fueled by natural gas at a 30° angle to the horizontal sample in a 180°C chamber, generating a specified radiant flux. The sample is ignited with an igniter after a specified time, and the damaged length is measured after the flame extinguishes to calculate the critical radiant flux. Its characteristics include being conducted inside a chamber, the sample being subjected to specified radiant heat, and the ability to place actual lining materials, simulating real fire scenarios. The results more accurately reflect actual combustion performance; the higher the critical radiant flux value, the more difficult the material is to burn.

4. Smoke Generation Test

Based on long-term accumulated data on various fires, the smoke and toxicity of burning materials are analyzed. The hazard is often more severe than the flame and heat produced during combustion, and is a major cause of human death. Specialized instruments and equipment are available both domestically and internationally for this test, with the principle often employing the light transmission method. By measuring the smoke density and obtaining transmittance and time curves, various parameters can be obtained, including optical density, maximum smoke density, average smoke generation rate, and light transmittance, as well as the time required to reach 75% (specific optical density) from the maximum. This provides a more comprehensive evaluation of the smoke generation of flame-retardant textile materials. The construction industry and transportation departments frequently use this type of instrument and testing method to research and select flame-retardant materials.

5. Flash Point, Autoignition Point, and Ignition Temperature Determination

The flash point refers to the lowest initial temperature of the surrounding air at which a material decomposes upon heating, releasing flammable gas, and can just be ignited by a small external flame. The auto-ignition point refers to the lowest initial temperature of the surrounding air at which a material, after being heated to a certain temperature, will spontaneously explode or burn without an external ignition source. The above measurements are used to assess the combustion performance of various fabrics under heat or flame, serving as a factor in evaluating fire hazard. Furthermore, the analysis and research on the toxicity of combustion gases from fabrics (which has received increasing attention in recent years) can be conducted using infrared spectroscopy, gas chromatography, and mass spectrometry, as reported internationally.

6. Cone Calorimeter

The cone calorimeter is a new type of combustion testing device that emerged in the early 1980s. It can simulate real combustion parameters and is mainly used to measure the heat release rate of materials during combustion. This rate is the most important parameter characterizing the fire hazard of materials, and related measuring instruments and methods have been continuously emerging in recent years. The cone calorimeter uses the oxygen consumption principle to measure the heat release rate and has replaced the traditional energy balance method, gaining widespread application. In addition, it can also measure parameters such as heat release rate per unit area, sample ignition time, mass loss rate, smoke density, effective heat of combustion, and content of harmful gases. These parameters are of great significance for evaluating the performance of flame retardants or systems, because in actual fires, victims will suffer from heat burns and smoke asphyxiation.

7. Thermal Analysis of Flame Retardant Finishing

When fabrics are heated or cooled according to a certain temperature program, a series of physical or chemical changes often occur. Thermal analysis technology studies or measures the functional relationship between the mass or energy of a substance and temperature (or time) during these changes. Thermal analysis techniques are extensive; commonly used methods in flame retardant testing include pyrolysis gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA can determine the thermal weight loss of fibers, allowing for relative comparison of the flame retardant effect of fabrics and providing a quantitative understanding. DSC can analyze changes in fiber decomposition temperature, indicating changes in the pyrolysis mode before and after flame retardancy. Chromatography-mass spectrometry can also be used in thermal analysis to study the thermal decomposition products of fibers.

8. Simple Testing Methods

For ease of operation, several simple testing methods are introduced. These methods require no complex equipment or conditions, are low-cost, and are suitable for preliminary observation of flame retardant effects or for reference or comparison when factories select process conditions. However, they cannot be used as standard test methods, nor can they be used as arbitration criteria.

9. Match Test Method

The match test method can evaluate the flame retardant effect of fabrics or compare their relative flame retardant performance. During the test, take a strip of fabric approximately 2.5cm × 30cm long. Place a lit match under the strip of fabric and let it burn until the match is completely burned (or a specified 5-12 seconds). Observe the burning situation or flame retardant effect. Sometimes, it can be specified that the sample is acceptable if it burns for no more than 5 seconds, and unacceptable if it burns beyond the center line or smolders for more than 15 seconds (other indicators can also be specified). The match size can be specified by the user or refer to standard methods. This method is similar to the vertical test method.

10. Lighter Test Method

The sample size can be determined according to the test requirements. A lighter is used as the heat source, and the burning time is generally 5 seconds. The placement of the heat source can be similar to the application conditions. After the flame extinguishes, observe the flame spread; if the spread is not severe, it is considered acceptable.

11. Ethanol Combustion Test Method

The heat source is 0.3 mL of anhydrous ethanol, placed in a small combustion cup (a bottle cap can also be used). The test can be conducted using the vertical method (5 cm × 30 cm), the horizontal method (20 cm × 25 cm), or the 45° inclined method (5 cm × 15 cm). The distance between the ethanol and the fabric is 2.5 cm. The test parameters can be determined according to requirements, such as char length, burning area, afterflame time, smoldering time, and the condition of the combustion residue.

12. Other Test Methods

Traditional flame retardant test methods cannot meet the needs; therefore, developing new test methods is an inevitable trend.

Japanese clothing burning tests utilize the umbrella method and the human model method, relying on thermal sensors to record the thermal conductivity of different parts of the clothing and the time to reach the maximum value, thus understanding the burn effects of combustion on the human body. The simulated bronze man test can evaluate the fire protection effect of underwear and robes. During the test, the bronze man wears protective clothing, sensors are connected to a computer, the gas nozzle is ignited, and the temperature and burn severity are monitored. A detailed report can be printed after the test.

Burning tests are complex. Apparent flame retardancy is related to the testing method. Domestic small-scale testing methods have limited data, only allowing comparison of the relative flame retardancy of fiber materials, and cannot evaluate behavior in real fires. Europe believes that the shortcomings of small-scale laboratory testing methods are difficult to overcome, and standard large-scale tests are necessary for special occasions. Some developed countries in flame retardant technology are building large-scale testing methods or special models, but these are costly. Therefore, it is necessary to gradually adopt larger-scale testing methods or combine multiple testing methods to comprehensively evaluate the flame retardant performance of materials.