The silicon rubber is not flame retardant, it is necessary to modify its flame retardation. The normal physical method is adding fire retardant and flame-retardant filler in the silicon rubber material to be fireproof.
Selection of the flame retardant system
The requirements of choosing the flame retardant: high-efficiency (small dosage of fire retardant for unit property); Non-toxic /low toxicity; good thermostability (decomposition temperature: 250〜400℃); do not influence the flamed retardant material too much; As little of toxic gas and dense smoke as possible when burning. Thus, it is necessary to take the smoke suppression and fire retardant into consideration while choosing the flame retardant.
Flame retardant divides to organic and inorganic flame retardant. The organic flame retardant: Organic halide series (organic bromine, organochlorine), organic phosphorus, nitrogen flame retardants series. Inorganic flame retardants including aluminum-magnesium series, antimony series, boron series and so on.
The Organic halide flame retardant, with low price, good efficient of fireproofing, wide adaptation range, easy to mix with polymer evenly. The most common series is organic bromine , which with the most efficient of fire retardant, however, the shortage is lots of smoke when burning and cracking. Inorganic aluminum-magnesium series, with good performance of decreasing the smoke, especially inhibit the hydrogen halide, thus we consider to combine the aluminum-magnesium with organic halide into the fire retardant.
From picture 1, It shows that magnesium hydrate is perfect to the inhibition of HC1. Meanwhile, the decomposition temperature of magnesium hydrate is the highest (340℃), thus, magnesium hydrate is very suitable for fire retardant in high temperature environment.
The flame retardants mechanism of antimony and boron in inorganic flame retardants are almost same. Antimony oxide is the most commonly used flame retardant, It reacts with halide to form antimony halide at high temperature, which is a volatile substance with higher boiling point, so it can stay in the burning area to isolate oxygen for a long time. Antimony halide can also promote dehydrohalogenation and surface carbonization of polymer- flame retardant in the liquid-solid phase. Thus, it will be good flame retardant effect to combine antimony oxide with halogen flame retardant.
Pic 1， Different kinds of flame retardant inhibits HC1
Because of the own limitations of carious types of lame retardant, and in order to achieve the good flame retardant effect, using the magnesium hydroxide as the main flame retardant, which with high temperature resistance and smoke suppression, meanwhile, combining the organic brominated and antimony series flame retardants, and to get the most efficient flame retardant.
The confirmation of silicon rubber flame retardant formulation and evaluation.
After confirming the the flame retardant system through many times of experiments, we get the final silicon rubber flame retardant formulation. The purpose is to get the best effect of flame retardant, one the other side, the mechanics performance of silicon rubber coated fabric will not be effected largely. What’s more, introducing the dispersing agent — metallic soap, which can improve the dispersion uniformity of flame retardant in the silicone rubber.
The dosage of main flame retardant magnesium hydroxide and performance evaluation
The dosage of Magnesium hydroxide to the oxygen index of silicon rubber
The evaluation of flame retardant can be taken from Oxygen index. According to formulary requirement, the oxygen should be greater than 26, the materials will be with fire retardant property. After changing the dosage of Magnesium hydroxide, and observing the Oxygen index, we get the curve of picture 2.
After increasing the dosage of the Magnesium hydroxide in the silicone rubber system, the Oxygen Index is increased in the whole system. Thus, for the flame retardant effect, the more dosage of Magnesium hydroxide, the better effect will be.
The dosage of Magnesium hydroxide to the retention rate of silicone rubber coated fabric strength.
Pic 2 The dosage of Magnesium hydroxide to the oxygen index of silicon rubber
Using the Magnesium hydroxide as the main flame retardant, and combining with a kind of halogen flame retardant. Changing the dosage of Magnesium hydroxide, the retention rate of tensile strength of fireproof silicon rubber coated fabric will be turned out as picture 3.
The mass fraction of Magnesium hydroxide
The blank sample: silicone rubber coated fiberglass fabric without flame retardant
Pic 3 Tensile strength retention of flame retardant silicone coated fiberglass fabric
From the above picture, the tensile strength of silicon rubber coated fiberglass fabric is decreasing under the high temperature and high humidity environment, and the strength is decreasing much obviously when there is halogen; Using the Magnesium hydroxide, the dosage of Magnesium hydroxide is increasing, the retention rate of strength of silicon rubber coated fiberglass fabric is also increased. When the dosage of Magnesium hydroxide is over 20, the quantity of Magnesium hydroxide is enough to absorb halogen hydride, the excessive Magnesium hydroxide can only decrease the air permeability of silicon rubber, however, the effect is not obvious as the halogen hydride during the absorbing, thus, the strength retention rate increased with slow speed.
The effect of Magnesium hydroxide to the strength of the silicon coating
In order to evaluate the magnesium hydroxide to the strength of the silicon rubber coating, fabric folding tester is used to test the flame retardant silicon rubber coated fabric.
According to the experimental results, which is concluded that the dosage of magnesium hydroxide and coating toughness curve, and shown in picture 4. As to the picture, when the dosage of Magnesium hydroxide is less than 15, and before breaking of the base material, the coating is still not fractured, and this is similar to property of silicon rubber which not adding any flame retardant , so excluding the variability of the data to affect the number of fracture, the toughness of silicon rubber does not change. When dosage of magnesium hydroxide is more than 15, as a measure of coating toughness index, the number of fracture with unilateral decline, so the toughness of the magnesium hydroxide of silicone rubber, which with a negative effect, and it has a little influence under the certain range, in the toughness of the silicone rubber coating has no obvious change. However, dosage of Magnesium hydroxide beyond a certain range, however, the toughness of coating declined directly, so the toughness of silicone rubber is associated negatively with the dosage of the magnesium hydroxide.
The mass fraction of Magnesium hydroxide
The Blank sample: silicone rubber coated fiberglass fabric without flame retardant
Pic 4, the curve of Magnesium hydroxide dosage and coating toughness
According to the comprehensive evaluation of magnesium hydroxide dosage to oxygen index, strength retention under high temperature and high humidity environment, and the influence of the coating toughness, eventually, the dosage of magnesium hydroxide is 10%～25%.
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