Flame Retardant Plastics Are Often Said To Have High CTI, High Burning Wire, What Exactly Is It?

Flame Retardant Plastics Are Often Said To Have High CTI, High Burning Wire, What Exactly Is It?

In order to ensure the safety of people's lives and property, countries around the world have introduced regulations on the electronic and electrical equipment used in plastic insulation materials, flame retardant properties, heat resistance and electrical properties, such as harsh provisions.

Electronic and electrical products for plastic insulation materials for flame retardant heat resistance and electrical performance, usually have two requirements - high CTI, high burning wire.

For friends who do electrical and electronic products, CTI, burning wire is a very common word. But for the layman it is more obscure and difficult to understand. So what exactly does high CTI, high scorch wire mean?

1. What is CTI

CTI is called Comparative Tracking lndex (CTI), which is the maximum voltage in V that the surface of insulating material can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without forming leakage traces.

In other words, CTI is the highest voltage value at which the surface of an insulating plastic can withstand 50 drops of a conductive liquid under energized conditions without creating a carbonization short circuit. Generally speaking, the higher the CTI value of an insulating plastic, the better its leakage resistance.

2. Why CTI test?

Plastic insulating materials have a special electrical damage phenomenon: when the surface of the material is wet, there are debris and the electric field is large enough, the surface will form a carbonized conductive path, and ultimately form a short circuit, which will most likely lead to a fire.

The mechanism is: when the surface of the material is relatively wet, there are debris and the electric field is large enough, the surface of the debris may be energized, energized heat generated by the evaporation of water, and then the formation of a dry band (does not contain water conductive tape); due to the evaporation of water can take away a large amount of heat, the dry band does not contain water, a large number of heat aggregation; enough heat to make the insulation of the plastic surface carbonization, the formation of carbonization of the conductive path, and ultimately the formation of short circuits. The short circuit is eventually formed.

CTI simulates this process and measures the minimum voltage at which an insulating plastic can produce a leakage trace, thus determining whether the material is suitable for use in a particular environment.

3. What does CTI testing look like?

As shown in the figure: the upper figure shows the electrode device, and the lower figure shows the experimental equipment. The size of the specimen should be larger than 15mm x 15mm, and the thickness should be not less than 3mm; the distance between the two electrodes should be 4.0 mm 士 0.1mm, and the angle of clamping is 60°; the height of the droplet mouth and the sample should be 30-40mm.

After the experimental setup is installed, it is time for testing:

Given a voltage (generally from 300V to start), the droplet size control in 20-23mm³ is good, droplets to 30s Shi 5s interval drop on the sample surface; until the formation of electrical tracing and damage or until the drop 50 drops of electrolyte until.

Repeat the test by increasing or decreasing the voltage (by 25V or multiples of 25V) until two adjacent voltages are obtained (smaller voltage with no leakage after 50 drops, larger voltage with leakage), and determine the smaller voltage as the CTI value of the material.

4. Why improve CTI?

With the rapid development of the Internet of Things (IoT), the user side has put forward more and more stringent requirements on the safety and reliability of the electrical system, and the requirements on the insulating properties of the materials such as thermal aging, flame retardancy, dielectric strength, CTI, and electromagnetic compatibility (EMC) have increased dramatically.

The most typical example is electric vehicles. In the process of vehicle electrification, CTI greater than 400V is the most common requirement. In order to increase the range of electric vehicles, it is necessary to use a higher DC voltage, which requires materials with a CTI of more than 600V, or even 700V or 800V, in order to meet the requirements of DIN 60664-1 for the electrical clearance and creepage distance.

Therefore, material modification engineers must carefully evaluate these requirements at the formulation stage in order to increase the safety and reliability of the electrical system for each application.

5. What are the factors that affect CTI?

1) Base material

The higher the carbon content of the resin, the worse the CTI. PE, PP, PA6(66) all have a CTI of about 600V, which is related to their low carbon content. On the contrary, PPS containing benzene rings only has a CTI of 125V, therefore, the PPS/PA6/GF system is better than PPS/GF. Because the so-called relative leakage starting trace index (CTI) is the trace formed by carbonization, so all the factors favoring carbonization will reduce the CTI.

2) Organic small molecule additives

Among organic additives, bromine-containing flame retardants have a great influence on CTI. This is because bromine-containing flame retardants are unstable to heat and are easily carbonized. In addition, the organic small molecules (especially organic waxes) that are easily precipitated from the flame retardant are also easily carbonized on the surface of plastic products.

3) Fillers

Fillers, especially those flaky fillers, can cover the surface of the plastic, so that the resin exists in a discontinuous form, interrupting the charring pathway, which is not conducive to the formation of conductive channels (charring trails).

4) Other factors

- Surface quality: Plastic surface flatness, finish and cleanliness and other factors will also affect the leakage starting trace performance. Surface defects, scratches or contaminants may result in a greater susceptibility to leakage starting traces.

- Temperature and Humidity: Changes in ambient temperature and humidity may have an effect on the resistance of plastics to leakage marking. At high temperatures or high humidity, plastics may be more susceptible to leakage marking.

- Electrical stresses: The electrical stresses that plastics are subjected to during use, such as voltage and current, may also have an effect on their resistance to leakage marking. High voltages or currents may increase the risk of leakage marking.

- Conditions of use: The conditions of use of plastics, such as whether they are exposed to chemicals, UV radiation or mechanical abrasion, may also affect their resistance to leakage marking.

In conclusion, leakage marking is mainly caused by the unevenness of the electric field in electrical components, resulting in flashover discharge, which in turn causes the generation and buildup of free carbon, resulting in leakage marking of the insulating material. Reducing the generation and accumulation of free carbon caused by discharge, increasing the thermal decomposition temperature of various additives, and improving the surface gloss and flatness of the products are effective ways to obtain high CTI values for modified engineering plastics.

6. About Scorch Wire Test

Scorch wire test is to test the stability of electrical and electronic products in operation, and the scorch wire itself is actually a fixed specification of the resistance wire ring.

The test is performed by electrically heating the wire to a specified temperature, allowing the tip of the wire to contact the sample for a standardized period of time, and then observing and measuring its condition, the range of the test depending on the specific test procedure.

There are two main terms involved in the scorch wire test:

Flammability Index GWFI

Glow-Wire Flammability Index (GWFI), the maximum test temperature of a test specimen of a specified thickness in three consecutive tests, which shall satisfy one of the following conditions:

- The flame or burning of the test sample is extinguished within 30 seconds after the removal of the scorching wire, and the wrapping silk paper placed under the test sample does not ignite;

- The test sample does not ignite.

The way the GWFI is recorded:

For example, for a 3 mm thick test sample with a test temperature of 850°C , the GWFI is recorded as: GWFI: 850/3.0

If neither or both of the above conditions are met, it is necessary to select a lower test temperature and repeat the test with a new test sample.

Ignitability Temperature GWIT

Glow-Wire Ignition Temperature (GWIT) is a temperature that is 25K (30K between 900℃ and 960℃) higher than the maximum temperature of the top of the scorch wire that does not cause ignition of the test specimen of the specified thickness in three consecutive tests.

Simply put, the GWIT test is passed as long as no ignition occurs at the test temperature (3 consecutive samples) (25℃ is added to the temperature when the report is issued, and 30℃ is added to the temperature when the test is conducted at 900-960℃).

GWIT recording method:

For example, for a 3.0mm thick test sample, the highest test temperature that did not cause ignition is 825°C , then recorded as:

GWIT: 850/3.0 (Note: 850 ℃ = 825 ℃ + 25 ℃)

Note: The so-called ignition, IEC for the GWIT regulations, refers to the burning time of more than 5 seconds for ignition, that is, as long as the flame is not more than 5s are not ignited, this point please pay special attention to!

According to the European Union's International Electrotechnical Commission (IEC) requires the industry to implement the IEC60695 burning wire flame retardant test evaluation standards, the IEC organization in the IEC60335 safety standards for household and similar electrical appliances put forward by the long-term unattended appliances used in the plastic parts of the flame retardant properties of the material must meet the UL94V-0 level and 750 ℃ burning wire contact material within 30s does not ignite or combustion time of less than 5s, that is, the ignition temperature of the burning wire is not ignited, the flame time of 5s. That is, the burning wire ignition temperature (GWIT) is greater than 750 ℃. For specific components such as connectors, contact switches, motors and circuit breaker housings also require a GWIT temperature of 850°C and a Glow Wire Flammability Index (GWFI) of 950°C.

Conclusion

In the field of flame retardants, the Scorch Wire test has become an important criterion for measuring the stability and safety of materials at elevated temperatures. This test not only evaluates the thermal stresses that a material may encounter in practical applications, but also predicts its reliability in electronic and electrical equipment. Passing the scorch wire test at 850°C demonstrates that the material maintains its structural and functional integrity under extreme conditions, thus providing a strong guarantee of safe product use.

YINSU Flame Retardant's Red Phosphorus Flame Retardant 301Y and Piperazine Flame Retardant PPAP-15 have successfully passed the 850°C Scorch Wire Test, which highlights the company's professionalism and technological strength in the research and development of flame retardants. 301Y's and PPAP-15's high performance in the Scorch Wire Test not only meets the market's need for high-performance flame retardant materials, but also demonstrates the company's commitment to enhancing the safety and environmental friendliness of materials. material safety and environmental friendliness.

Flame retardants that pass these tests provide safer and more reliable material choices for the electrical and electronics, automotive, construction and aerospace industries. As the global demand for flame retardant materials continues to increase, YINSUs Flame Retardant's flame retardants will continue to play an important role in promoting the advancement of industry safety standards and safeguarding the lives and properties of users, and provide efficient and cost-effective flame retardant solutions for different customers.