Why are the two major hurdles in low-smoke halogen-free cable materials even more difficult to overcome?
In halogen-free systems, there are many flame retardants that can be selected, such as ammonium polyphosphate(APP), aluminum hypophosphite, and red phosphorus. However, those familiar with low-smoke halogen-free cable materials know that these flame retardants are rarely used in cable materials. The main reasons are as follows:
(1) Although phosphorus-based flame retardants have higher flame retardant efficiency than inorganic flame retardants, they produce a large amount of smoke and are prone to precipitation;
(2) Higher cost: Inorganic flame retardants are cheaper and can be added in large quantities, thus significantly reducing the cost of modified materials. Therefore, the most commonly used flame retardants in halogen-free cable materials are aluminum hydroxide (ATH) and magnesium hydroxide (MDH). Their flame retardant mechanism is "cooling + dilution": they decompose under heat, releasing water vapor and diluting combustible gases and oxygen.
The mechanism itself is sound and has minimal impact on smoke density, but it suffers from two inherent flaws:
1. Extremely poor charring ability: Unlike PVC, the polyolefin substrates (such as EVA, PE, POE) used in low-smoke halogen-free cable materials have only carbon-hydrogen bonds in their molecular structure, resulting in almost no self-forming char layer during combustion. The magnesium oxide (MgO) left after the decomposition of magnesium hydroxide is a loose, fragile powder that cannot form a dense heat and oxygen barrier.
2. Amplified "wick effect": In bundled combustion tests, multiple cables are arranged vertically. When one is ignited, the flame spreads upwards along the cable bundle. Without a strong char layer skeleton, the molten polymer drips, flows, or even detaches, acting like "candle wax" and fueling flame propagation. Simply put: ATH/MDH can suppress the flame of a single cable (depending on the amount added), but under the "fire magnifying glass" of bundled combustion, its lack of a "char-forming skeleton" is glaringly obvious. At the same time, because polymers cannot burn completely, a large amount of incompletely burned carbon particles (smoke) are produced. Low smoke and halogen-free are essentially twins, but halogen-free does not naturally equal low smoke.