The principle of plastic smoke burning polymer to produce a lot of smoke, and some polymer combustion smoke is toxic. When the polymer is added with flame retardants, especially halogen and antimony flame retardants, combustion will produce more smoke and toxic gases.

Smoke is a comprehensive reaction of black smoke and white smoke produced during the combustion of materials.

Black smoke refers to the solid particles and agglomerates suspended in the combustion gas product. It is generally believed that there are three ways to reduce the concentration of black smoke: First, learn from the cover effect, transfer effect, inhibition of free radicals, accelerated charcoal and other flame retardant principle to change its combustion mode; Second, a large number of filled inorganic materials, by reducing the content of combustibles Reduce the amount of smoke, but the amount of over the General Assembly will cause serious damage to the performance of the product; third is the use of synergies, the compound.

White smoke is mainly due to the combustion of materials such as water vapor aggregates suspended in the air of small particles. Water vapor, although harmless to the human body, but the decline in light transmittance, smoke density increased. Aluminum hydroxide, magnesium hydroxide due to the absorption of heat and play a role in the combustion, but the resulting water vapor is the main reason for the production of white smoke. It is therefore essential to strike a balance between flame retardant and smoke suppression.

The maximum specific optical density (Dm) is an indicator of the size of the amount of plastic smoke, also known as the maximum smoke density. The greater the density of plastic than the greater the density, indicating that the greater the smoke, the more black smoke when burning, the greater the environmental pollution.

The maximum specific optical density of commonly used plastics is shown in Table 1.
 
As can be seen from Table 1, with double bonds, benzene ring and chlorine-like plastic smoke is relatively large. It is customary to use resin such as POM, PA6, PA66, PMMA, LDPE, HDPE, PP, PTFE, PVDC, etc., which are the most common when the combustion is greater than the optical density Dm below 300. On the other hand, the resin with the maximum density Dm greater than 300 when burning is called smoke plastic, such as PET, PC, PS, PPO, ABS and PVC in the table. In fact, we have to carry out the smoke suppression is only for the smoke plastic, in particular, PVC resin is a large smoke suppression.

Plastic smoke suppression method

In the contemporary flame retardant technology, "flame retardant" and "smoke suppression" compared to some high polymer, the "smoke suppression" than "flame retardant" is more important, so the development of smoke suppression flame retardant is very important.

What kind of material to have the conditions of smoke suppression agent? When the material burns, the flame is diffuse and the convection of the air takes the generated carbonaceous material into the air, which is the root cause of the increase in smoke.

If the carbonaceous material that has been formed can be fixed on the surface of the combustion material rather than floating in the air, the smoke density of the material will be greatly reduced. The key to achieving this idea is to synthesize or seek a compound that can be melted between 700 and 1000 ° C and has properties similar to that of the binder, and the resulting particulate carbonate is adhered to the surface of the combustibles.

In the design of smoke-free flame retardant formula, as far as possible to choose low smoke flame retardant, halogen / antimony flame retardant system and coated red phosphorus are increased smoke and toxic gas diffusion. Therefore, in the use of bromine flame retardant system, it is best to add smoke suppressants at the same time. Commonly used various types of flame retardants own smoke as shown in Table 2.
 
Plastic commonly used smoke suppressants

Can be used for a lot of smoke suppression material, summed up the smoke suppressants can be divided into inorganic and organic two categories, and inorganic smoke suppressants are the most commonly used. The following describes the commercial varieties of smoke suppressants.

Inorganic smoke suppression additives

(1) molybdenum

This is commonly used in the suppression of auxiliaries, the specific varieties of ammonium molybdate AOM, molybdenum trioxide, calcium phosphomolybdate, etc., the smoke suppression principle in the combustion process with PVC and other resin to form residual carbon, covering the polymer Surface, to achieve the effect of flame retardant smoke suppression. Add molybdenum-based smoke suppressants in the general amount of 2% to 3%, can reduce the 30% to 80% of the amount of smoke. For example, calcium molybdophosphate synthesized from ammonium molybdate, phosphoric acid and calcium hydroxide, adding PVC to reduce the maximum smoke density of combustion combustion to 297, and the maximum smoke density of incombustible combustion is reduced to 351.

So far, it has been found that the best smoke suppressant is molybdenum compounds, so the development and application of molybdenum compounds has become a hot spot in the field of flame retardants. The United States developed a series of ammonium molybdate no smoke inhibitor, the ability to withstand more than 200 ℃ processing temperature. At present, molybdenum compounds as flame retardants in China is still in its infancy.

(2) metal hydroxides

The main varieties of aluminum hydroxide, magnesium hydroxide, the specific smoke suppression principle:

① in the process of heating the formation of alumina and magnesium oxide has a large surface area, can absorb dust;
② in the solid phase to promote the formation of charcoal:
③ a large number of heat absorption will greatly reduce the temperature of the polymer, slow down the thermal decomposition rate;
④ exothermic water into water vapor, you can dilute the combustible gas, dilute the smoke, play a flame retardant, smoke double role.
⑤ can react with the halogenated hydrocarbons liberated by the halogen-containing compounds (to capture the hydrogen halide), thereby reducing the amount of toxic gas hydrogen halide in the smoke.

A single metal hydroxide has a very good effect of smoke suppression, but the two directly with the use or with molybdenum, metal oxide, metal composite effect is better.

For example, Al (OH) 3 and Mg (OH) 2 are used as the ideal flame retardant and smoke suppressor for soft PVC materials. Metal oxide and hydroxide are used in combination. Metal and metal hydroxide can further promote the PVC in the combustion process of solid phase into carbon, that is, to strengthen the smoke suppression effect. Such as Fe and Ni complex can improve the oxygen index of the system, and Ni and Cu smoke the best effect. Red phosphorus and zinc borate can also produce synergistic effect with Al (OH) 3 and Mg (OH) 2, and further enhance the effect of flame retardant and smoke suppression. The use of zinc borate, antimony oxide and Al (OH) 3 flame retardant Effect can also be prepared low smoke low halogen PVC cable material.

(3) metal oxide

The main varieties of antimony, zinc, tin, magnesium, aluminum, silicon, iron, copper some of the metal oxides, such as antimony trioxide, tin oxide, zinc oxide, copper oxide, the current more prone to complex use.

Sb2O3 smoke suppression mechanism for PVC combustion and PVC together with the formation of SbOCl, SbCl3, played a capture free radicals, isolated from the air, thus contributing to the role of PVC carbonization.