Polypropylene (PP) itself is a highly flammable hydrocarbon with a limiting oxygen index (LOI) of only 17.8%. It will continue to burn even after being removed from the fire source. The core principle of flame-retardant PP is to interrupt or delay its combustion cycle through physical and chemical means. Combustion requires the simultaneous existence of three elements: combustible material, heat and oxygen. The function of flame retardants is to destroy this "burning triangle".
In industry, flame retardancy is mainly achieved by adding flame retardants to PP. Different types of flame retardants function through the following mechanisms:
1. Gas-phase flame retardant mechanism
This is one of the most common mechanisms, especially applicable to traditional halogen-based flame retardants. When flame retardants are heated and decomposed, they can capture the free radicals (such as H· and HO·) that maintain the combustion chain reaction in the combustion reaction zone (flame), causing their concentrations to drop sharply and thus interrupting the combustion.
2. Condensed phase flame retardant mechanism
This is the most mainstream mechanism of halogen-free flame-retardant PP. Flame retardants promote the formation of a uniform and dense carbon layer on the surface of polymers. This layer of carbon has three major functions. The first step is to prevent external heat from entering the interior of the polymer. Secondly, it prevents the escape of flammable gases inside and the entry of external oxygen. Finally, it inhibits the further pyrolysis of the polymer and the generation of smoke.
When a fire occurs, the acid source promotes the dehydration, cross-linking and carbonization of the carbon source. Meanwhile, the large amount of gas produced by the decomposition of the gas source causes the softened carbon layer to expand, eventually forming a porous, dense and strong foam carbon layer, which protects the underlying PP like "armor".
3. Cooling/heat absorption mechanism
Flame retardants absorb a large amount of heat during the decomposition process, reducing the surface temperature of polymers and making it difficult for them to continuously pyrolyze and produce flammable gases. Typical representatives include aluminium hydroxide (ATH) and magnesium hydroxide (MH). When they decompose, they absorb a large amount of heat (endothermic reaction) and release water vapor. The water vapor can not only dilute flammable gases but also play a cooling role.
4. Dilution mechanism
Flame retardants decompose to produce a large amount of non-flammable gases (such as water vapor and CO₂, etc.), which can dilute the concentration of flammable gases and oxygen near the polymer surface, making combustion unsustainable. Both the gas sources of metal hydroxides and intumescent flame retardants have this function.
In conclusion, the working principle of flame-retardant PP in industry is a complex process involving the synergy of multiple mechanisms. Modern flame-retardant PP technology is developing towards halogen-free, low smoke, low toxicity and high efficiency. Among them, the condensed phase flame-retardant mechanism represented by intumescent flame retardants (IFR) is the core of current research and application. By carefully designing flame-retardant formulas, the best balance can be achieved among flame-retardant efficiency, material mechanical properties, processing performance and cost.
The core of the thermal resistance induction in high and low temperature test chambers also utilizes the physical property that the resistance value of platinum metal changes with temperature. The core logic of the control system is a closed-loop feedback control: measurement → comparison → regulation → stability
Firstly, the thermal resistance sensor senses the current temperature inside the chamber and converts it into a resistance value. The measurement circuit then converts the resistance value into a temperature signal and transmits it to the controller of the test chamber. The controller compares this measured temperature with the target temperature set by the user and calculates the deviation value. Subsequently, the controller outputs instructions to the actuator (such as the heater, compressor, liquid nitrogen valve, etc.) based on the magnitude and direction of the deviation. If the measured temperature is lower than the target temperature, start the heater to heat up; otherwise, start the refrigeration system to cool down. Through such continuous measurement, comparison and adjustment, the temperature inside the box is eventually stabilized at the target temperature set by the user and the required accuracy is maintained.
Due to the fact that high and low temperature test chambers need to simulate extreme and rapidly changing temperature environments (such as cycles from -70°C to +150°C), the requirements for thermal resistance sensors are much higher than those for ordinary industrial temperature measurement.
Meanwhile, there is usually more than one sensor inside the high and low temperature test chamber.
The main control sensor is usually installed in the working space of the test chamber, close to the air outlet or at a representative position. It is the core of temperature control. The controller decides on heating or cooling based on its readings to ensure that the temperature in the working area meets the requirements of the test program.
The monitoring sensors may be installed at other positions inside the box to verify with the main control sensors, thereby enhancing the reliability of the system.
Over-temperature protection is independent of the main control system. When the main control system fails and the temperature exceeds the safety upper limit (or lower limit), the monitoring sensor will trigger an independent over-temperature protection circuit, immediately cutting off the heating (or cooling) power supply to protect the test samples and equipment safety. This is a crucial safety function.
Lab thermal resistance sensor is a precision component that integrates high-precision measurement, robust packaging, and system safety monitoring. It serves as the foundation and "sensory organ" for the entire test chamber to achieve precise and reliable temperature field control.
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