9 Things Need to Be Considered When Choosing Flame Retardant

Nowadays, people attach great emphasis on green material due to the increasing requirement for environmentally friendly material. For example, in the building material industry, the application of flame retardant fabric, FR aluminum composite panel, and flame retardant cable are getting wider and wider. Because I am involved in the flame retardant industry, and we got many customers to consult about how to choose a flame retardant for plastics compound. So in this passage, I would like to introduce to you some factors that need to be considered when you choose a flame retardant for the formula of flame retardant plastics.

1. Compatibility

Firstly, you need to choose a flame retardant that has good compatibility with resin so that it can make its fire resistance performance into the best use. For example, in flame-retardant polypropylene(PP) system, adding 15 phr DBDPO shall have the same flame retardant effect as adding 5 phr octa-PBDE, because octa-PBDE has better compatibility with PP. But please be noted that DBDPO and octa-PBDE belong to Rohs restricted list now.

2. Impact to the property of the resin

For inorganic flame retardants such as magnesium hydroxide and aluminum hydroxide, it requires for high loading amount to meet the UL-94 V0 level. At the same time, the mechanical property of resin will be affected by a large amount of inorganic flame retardant. For example, in EVA/LDPE formula, when the loading amount of magnesium hydroxide (MDH) is higher than 80 phr, MDH is not evenly dispersed in the EVA/LDPE matrix, which tends to form an agglomeration phenomenon, resulting in apparent defects in the material. Under the action of external forces, the MDH becomes the stress concentration point, thus resulting in a sharp decrease in the tensile property of the material. 

So when choosing flame retardant, you need to balance the flame retardant performance and mechanical property of the product and find the best plastics formula.

3. Low cost

Choosing flame retardant that could provide good flame retardant but with a lower price.

4. Durability in resin

The flame retardant you choose should be durable and compatible with resin. Otherwise, if the flame retardant migrates too fast, it will separate with the resin before it takes effect.

5. Temperature matching

The decomposition temperature of the flame retardant shall match the temperature of the resin. On the one hand, the decomposition temperature of the flame retardant should mate the processing temperature of the resin. Generally speaking, the decomposition temperature shall 20℃ higher than the processing temperature. On the other hand, the decomposition temperature of flame retardant shall match the thermal degradation temperature of the resin, which is 60℃ lower generally.

6. Property of flame retardant

Same flame retardant shall affect differently if its attributes are various.

(1)Ultrafine particle size 

Using superfine flame retardant could not only improves flame retardant performance but also improves mechanical performance. For example, by adding 80 phr ATH into LDPE and the flame retardant performance will be affected along with the particle size of ATH. See the below table.

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(2)Microencapsulation technology

Using organic material, polymer, oligomer, and inorganic material to coat the flame retardant particle could improve the compatibility of flame retardant with resin.

(3) Surface modification

In the inorganic flame retardant formula, doing surface modification to flame retardant, and adding dispersing agent, plasticizer, etc., could improve the compatibility and reduce its impact on the resin property.

7. Main and auxiliary flame retardants

By adding different flame retardant into one formula, could complement each other’s advantages. According to the FR performance, the flame retardant is classified as major and auxiliary flame retardants.

(1) Main flame retardants including flame retardant resin, organic halogen, organic phosphorus, and metal hydroxide, etc. The adding quantity is different. Organic flame retardant is 20 phr, the flame retardant resin is 30~40 phr when mental hydroxide could add more than 100phr.

(2）Auxiliary flame retardants including synergist flame retardant and smoke suppression. For synergist flame retardant, it includes antimony trioxide, Zinc Borate, inorganic phosphorus, graphite, organosilicon, etc. Smoke suppression including Molybdenum compound, an iron compound, zinc oxide, etc. The adding quantity of auxiliary flame retardant is small. For example, the loading amount of inorganic series is 5 phr, and 10 phr for organic series.

8. Synergistic effect

In some formulas, adding different flame retardants will have much better FR performance than adding merely one flame retardant because of the synergistic effect. For example:

(1) halogen/antimony compound: Antimony oxide added alone does not have a flame retardant effect. Only when added with halogen flame retardant will have a flame retardant effect. The compound proportion of halogen: antimony is 3:1, the synergistic effect is good.

(2) ATH/MDH Compound: When magnesium hydroxide and aluminum hydroxide were added in cooperation, in addition to their respective flame-retardant effects such as heat absorption during decomposition, generation of water vapor to dilute combustible gas, residue deposition, and oxygen isolation, aluminum hydroxide can also promote the carbonation of magnesium hydroxide, accelerate the carbonation of the plastic surface, the composite ratio of these two is between 3:1-2:1.

(3) Phosphorous series flame retardant compound

The addition of nitrogenous flame retardants can promote the carbonization of phosphorus flame retardants. For the synergistic effect see the below equation.

3.5%P=2%P+2.5%N=1.4%P+4%N=0.9%P+5%N

Phosphorous series flame retardant has a synergistic effect with ATH, MDH, Zinc borate hydrate, etc. Because during the combustion of carbohydrates, the crystalline water vaporizes and absorbs heat to cool down, which provides a favorable condition for phosphate-containing flame retardants to produce polyphosphoric acid.

9. Counteraction 

(1) Halogenated flame retardants shall not be used with silicon flame retardants. If both were added together, the oxygen index decreases by 6~7.

(2) Red phosphorus can not be used with silicone, and the oxygen index decreases by more than 20% when added together.

(3) Brominated flame retardant can not be used with zinc stearate, which will decrease the flame retardant performance.

(4) Calcium carbonate and magnesium carbonate filler shall not be added to the formula with brominated flame retardant. Because these two will counteract with brominated flame retardant.

That’s all for today’s feed, hope this passage can help you when you design your plastic formula.

Thank you for your time and if you have any questions please feel free to contact Author Elena: kmt_elena@163.com.