Rubber combustion and flame retardant research

1, rubber combustion and flame retardant

Combustion is a natural phenomenon in the objective world. Lightning strikes can cause forest fires. Combustion is also an artifact of human life or production. To make the burning happen and proceed, there are three conditions that are indispensable.

A certain temperature Any substance can only start to ignite when the ambient temperature reaches the burning point. The combustion characteristics of different characteristics vary, and the degree of difficulty in achieving the combustion is also not one, so it is difficult to burn and flammable.

Oxygen It is a combustion aid and it is an indispensable factor to ensure that combustion continues.

Combustible materials are the bodies on which combustion can proceed. They are generally hydrocarbons, biological materials (such as grass and wood), and synthetic polymer materials (such as rubber, plastics, and fibers).

1.1 Combustion of rubber Combustion of rubber has commonality with the combustion of other materials, but there are also special features. Rubber as a polymer material, the combustion process is more complex, and its combustion temperature is also higher than the general material. Even if the fire ignites, the temperature should reach 3160 or more. After rubber fires, its combustion process can usually be divided into three stages.

(1) Thermal decomposition reaches the point of combustion (different types of combustion for different types of rubber), such as the NR of 6200C to 6700C). It begins to soften and melt, and decomposes into low molecular weights. No open fire can be seen at this stage and can be considered as a prelude to burning.

(2) Combustion The thermal decomposition products react violently with oxygen in the atmosphere and a flame appears, which marks the official start of combustion. With the release of light and heat, new low-molecular combustibles (such as CO) incombustibles (such as CO2) and smoke are generated.

(3) Continuation of combustion This stage can be continued until all flammables have burned out. Most rubber species go through these three stages, but the halogen-containing rubber may only go to the second stage, because the hydrogen halide generated in the combustion plays a deterrent role.

1.2 The level of rubber combustion is usually divided by the degree of difficulty of combustion, which can be divided according to the oxygen index (see Table 1).

Table 1 Oxygen Index Flammability Rating of Rubber

Flame rating oxygen index range example

Not flame retardant <20 flammable rubber (such as natural rubber), no flame retardant

General flame-retardant >20<30 Combustible rubber, Add flame retardant

High flame retardant (flame retardant) ≥30 Halogenated rubber, halogenated rubber with added flame retardant

The commonly used types of rubber, such as the Oxygen Index (OI), are listed in Table 2 for the order of burning (ease of difficulty).

Table 2 Oxygen Index of Commonly Used Rubbers

EPDM BR, IR NR SBR, NBR CSM, CHR CR

17 18 to 19 20 21 to 22 27 to 30 38 to 41

Note: BR-cis butadiene rubber CHR epichlorohydrin rubber CSM - chlorosulfonated polyethylene IR-isoprene = ene rubber NR = natural rubber SBR - styrene butadiene rubber NBR - nitrile rubber EPDM - EPDM rubber

It can be seen from Table 2 that the oxygen index of the halogen-containing rubber is higher than that of the halogen-free rubber; the oxygen index of the side groups containing no halogen rubber is higher than that of the halogen-free rubber. The basis of rubber combustion is a hydrocarbon-based structure (all kinds of rubber are combustible materials), and only the degree of difficulty of combustion is different.

The only exception to the halogen-free rubber that has flame resistance is silicone rubber. The main chain structure is composed of silicon and oxygen atoms. It has a certain value in terms of flame retardancy, but its physical properties are poor and its application area is narrow.

Self-burning is not the only way for rubber to obtain flame-retardant effect. Non-flame-retardant rubber can also be used for general flame-retardant if flame retardant is added. Of course, the addition of a retardant to the flame retardant rubber can further increase the flame retardant rating, as if it were icing on the cake.

1.3 Selection of Flame-Retardant Rubber Halogen-containing rubber is generally the first choice, and its flame retardancy is beyond reproach, but the combustion is produced by hydrogen halide gas, which is corrosive and toxic. Its products are only suitable for applications in open space; and Space-limited places (such as transport or underground facilities) are not safe. The higher the halogen content of the raw rubber, the higher the oxygen index, but the unsafeness increases.

Halogen-free rubber itself is not flame-retardant, but after adding a certain amount of flame retardant, it can still meet the flame-retardant requirements. Some resins contain halogens in their structure, and when blended with rubber, they may also increase the flame retardancy of rubber (such as PVC).

1.4 Single use and combined use of rubber The single use and combination use of the host material in flame retardant rubber is very common.

1.4.1 Single use Focused halogen rubbers have single use examples, especially CR. Halogen-free rubber alone, it is not flame-retardant, but can be made up after adding flame retardant. In recent years, the use of halogen-containing rubbers has been banned on certain occasions. For example, in the United States, the use of halides generated from flame retardant materials containing halogenated rubber as the main material is prohibited in single-use facilities, aircrafts, and naval vessels. Fuel.

1.4.2 Combining Use Rubber alone is often difficult to achieve between the flame-retardant and physical purposes, and the performance requirements of flame-retardant products are often many, and need to be achieved through the use of rubber, as shown in Table 3.

Table 3 Application of Flame-retardant Rubber Body Material

Use component performance or function

Flame Retardant Oil Resistance Physical Properties Processing Properties Electrical Insulation Antistatic Heat Resistance Corrosion Reduce Costs Smoke Free

CR+NR √ √ √ √

CR+PVC √ √ √ √ √ √ √

+NR+BR √ √ √

CR+CPE+NR √ √ √ √

NBR+PVC √ √ √

CSM+CPE+CHRVA √ √ √ √

CSM+EPM+EVA √ √ √ √

NR+SBR √ √ √ √ √

EPDM+PE √ √ √

Note: "√" have this kind of performance or function

2, flame retardant

Flame retardant is a kind of special auxiliary agent for rubber, applicable to all rubber products that require flame retardant. In addition to flame retardant, some flame retardants also have plasticizing and filling effects.

2.1 Mechanism of action of flame retardants Flame retardants can play one or more of the following functions.

Cooling and endothermic combustion Thermal decomposition and oxidation reactions both result in significant heat generation. The heat provides the conditions for continued combustion, while the role of flame retardants is the opposite. For example, some flame retardants generate water when they act, and water absorbs the surrounding heat in the process of vaporization after heating, such as aluminum hydroxide is a typical representative of it.

Two molecules of aluminum hydroxide can release 3 molecules of water with a mass equivalent to 36.4% of aluminum hydroxide.

Separating the oxygen source Some flame retardants will decompose the non-flammable gases N2, CO2, etc. in the combustion. These gases surround the combustion, blocking the oxygen source and restraining the spread of the fire. Another example is the phosphoric acid ester flame retardant in the fire to generate phosphoric acid or metaphosphoric acid, the rubber surface covered with a hard transparent protective layer. The antimony trioxide and HCI or HB decomposed by the halogen-containing flame retardant generate SbCI3 or SbBr3, and thus are deposited on the surface of the rubber so as to form a flame retardant barrier.

Suppression of rubber flammability Some decomposition products of flame retardants can cause the rubber to lose flammability, such as halogen radicals released by paraffin wax.

2.2 Classification of rubber flame retardant It is customary to divide it into inorganic and organic categories.

2.2.1 Inorganic flame retardants These flame retardants generally have a cooling and barrier effect and are non-flammable. Due to its large amount, the concentration of the flammable material can be diluted. Such flame retardants are further divided into hydroxides, inorganic salts and metal oxides, all of which are non-combustible powders.

(1) Hydrated hydroxides Because they contain bound water, when the temperature rises to the critical point, the water separates and functions as heat absorption, cooling, and smoke elimination. The main varieties are aluminum hydroxide and magnesium hydroxide, and the moisture content is 36.4% and 30.9%, respectively. The advantages of aluminum hydroxide are not only the high moisture content, but also the low price. Magnesium hydroxide has the advantages of small specific gravity and good heat resistance.

Although aluminum hydroxide has broad prospects for development, its two major shortcomings have yet to be overcome.

(i) Must be filled in large quantities to show the effect. When used alone, it should be no less than 60, more than 120 can be more ideal results (see Table 4).

Table 4 Relationship between the amount of oxyaluminum hydride in SRB and flame retardancy

The amount of AL(OH)3,Phr 0 60 120 180 240

Oxygen Index, % 18.5 24 27 30 36

As can be seen from Table 4, the flame-retardant effect of aluminum hydroxide increases as the amount increases. When used alone, the amount of 240 parts can make the original flame-retardant styrene butadiene rubber close to a high level of flame retardancy. However, such a high amount of filling will bring difficulties to the refining process.

(ii) A large amount of filling leads to a decrease in the properties of the rubber

In order to overcome these two major shortcomings, the countermeasures are to improve the fineness so that the average particle size is less than or equal to 2 μm, and to make the surface active, the suitable surface modifiers are silane coupling agents and fatty acids. From Table 5, it can be seen that the miniaturization of aluminum hydroxide particles is beneficial to the mechanical properties and flame retardancy of the rubber.

Table 5 Effect of particle refinement on flame retardancy of AL(OH)3

Performance AL(OH)3 Fineness Tensile Strength Mpa Tensile Elongation % Hardness Shore (A) Oxygen Index %

Ordinary 7.9 450 87 21.5

Superfine 9.1 490 87 29.0

(2) Metal oxides These compounds have a certain degree of flame retardancy, but they are relatively expensive. Among them, the storage stability of magnesium oxide is poor, and it is not desirable to use it in a large amount. One of the most practical and widely used varieties is Sb2O3. It has limited effectiveness when used alone, but it has limited synergistic effects with halogen-containing flame retardants. However, when contacted with halogen-containing flame retardant decomposed HCI or HBr, SbCL3 with a higher specific boiling point will be generated. In addition to its good coverage, this halogenated antimony also captures -OH radicals in the system and inhibits further decomposition. Table 6 lists the flame retardant and synergistic effects of the ternary system composed of antimony trioxide, oxidized paraffin, and zinc borate.

Table 6 Sb2O3 single and combined flame retardant effect

Flame retardant dosage, phr

Clay 30 30 30 30 30

Oxidized paraffin 30 20

Sb2O3 30 5

Zinc borate 30 5

Flame retardant effect is larger and larger 15s self-extinguishing

(3) Inorganic salt inorganic salt itself is not flammable, and a large number of combustible components in the dilutable rubber compound are added. Some inorganic salts also have a hydration structure. For example, zinc borate and talc powder all contain crystal water, which helps inhibit heat. break down. In particular, the ternary system consisting of zinc borate, chlorinated paraffin and antimony trioxide has outstanding flame-retardant effects. Although calcium carbonate does not contain crystal water, it has a large amount of filler and is an excellent candidate for capturing HCL.

2.2.2 Organic Flame Retardants Organic flame retardants are divided into two categories: halogen and phosphorus. When they have the same amount of added organic flame retardants, the flame retardant effect is super-blocking inorganic flame retardants.

(1) Halogen-containing flame retardants These flame retardants liberate halogenated oxygen after combustion, because hydrogen halide weighs more than air and sinks, acting as a barrier. The representative chlorine-containing flame retardant is chlorinated paraffin, which is a large variety of traditional varieties used. It is divided into two types, liquid and solid, and can be selected according to the process requirements. The representative of brominated flame retardants is decabromodiphenyl ether, which is highly effective because it contains 10 bromine atoms per molecule. However, it is expensive and is used for products requiring high flame retardance and small size, such as TV accessories. Other brominated flame retardants include hexabromocyclododecane, decabromodiphenyl ethane, and tetrabromobisphenol A grade octabromoether.

(2) Phosphorous-containing flame retardants Phosphoric acid esters are the main ingredients and have plasticizing functions. In general, with the increase of the amount of alkyl carbon in the structure, the flame-retardant effect becomes stronger, and the benzene ring in the structure is the best. For example, TPP and TCP are commonly used varieties. The flame retardant effect of TCP is better than that of chlorinated paraffin, but the plasticization effect is poor. See Table 7 for commonly used species.

In addition, polyphosphoramide (APP) is a newly developed type of phosphorus-containing flame retardant and is particularly suitable for use in EPDM. When the added amount reaches 50 parts, the natural time is reduced to zero.

3, flame retardant rubber formula design

Generally, the flame-retardant rubber formulas follow the traditional principles and need to set up conventional systems such as protection, processing, reinforcement and filling. The design focuses on the selection of plastics, the selection of flame-retardant, and the possible processing problems that may occur.

3.1 Selection of glue type According to the product use requirements, it can be decided to use alone or in combination.

3.2 Flame Retardant Selection and Collocation

3.2.1 When flame retardants are used in combination, the balance between flame retardant, physical properties, and process costs should be considered. Under normal circumstances still more consideration and use, because it is difficult to take into account all aspects, and after the use of different flame retardants can also produce a synergistic effect, so it has a greater practical value. Flame retardants and can be summarized into three situations.

(1) Use of organic halogen-containing flame retardants in combination with inorganic flame retardants The use of the former's high efficiency and the latter's smoke-free, non-toxic complementary effects. Halogenated compounds are the most widely used chlorinated paraffin, and inorganic species can generally be selected from antimony trioxide, zinc borate, aluminum hydroxide or alumina. The use of decabromodiphenyl ether in combination with zinc borate can also achieve better results. According to information, the oxygen index can be as high as 42%.

(2) Combination of inorganic flame retardants and phosphate esters For example, a system consisting of 55 parts of phosphoric acid ester, 30 parts of aluminum hydroxide and 15 parts of antimony trioxide has a self-extinguishing time of less than 15 g.

(3) Inorganic flame retardants are used in combination with each other Although this practice is rare, there are also successful experiences. For example, when zinc borate is used together with aluminum hydroxide, a porous hard agglomerate is generated during combustion, which further prevents the thermal decomposition of the rubber and blocks the contact between the air and the flame.

3.2.2 Single use of flame retardants This is a new trend that has emerged in recent years. The flame retardants used are smokeless and halogen-free. The most used is AL(OH)3. According to reports, when AL(OH)3 is filled in 180 parts without halogenated rubber, the oxygen index can reach a high level of 30 samples.

Table 7 Phospholipid flame retardants

Name Description

Tributyl phosphate TBP colorless odorless liquid

Trioctyl phosphate TOP

Triphenyl phosphate TPP has low volatility and good flame retardant effect

Trimethyl phosphite TCP is excellent in flame retardancy, oil resistance and electrical properties

However, such a high filling rate leads to a decrease in the physical properties of the rubber material and a difficulty in processing. The countermeasures are particle refinement and surface modification. Phosphorus-containing flame retardants can also be used alone. The applicable varieties are TPP and APP (polyammonium phosphate). When the amount of APP reaches 60, the oxygen index can reach such an astonishing level of APP.

3.3 The impact of flame retardancy on rubber processing performance Flame retardant due to different ingredients, the impact on the rubber processing performance is also different.

(1) Some flame retardants tend to give sticky rolls to the compound. For example, liquid chlorinated paraffins have this drawback and should be controlled in their amount.

(2) Some varieties tend to be delayed, mainly inorganic salts such as clay.

(3) Both chlorinated paraffins and phosphate esters have a plasticizing effect and should be considered when designing the overall formulation to avoid abnormal plasticization.

4, rubber combustion test

The level of flame retardancy achieved needs to be assessed by specific tests. At present, there are two main combustion test methods commonly used in the rubber industry: oxygen index measurement and combustion test. The former provide the oxygen index (OL); while the latter are mainly used to determine the self-extinguishing properties of the rubber, ie (1) measured by the time that the test piece continues to burn after leaving the flame. The above three aspects of the data (oxygen index, self-extinguishing time and burning rate) are the currently recognized basis for evaluating the flame retardancy of rubber.