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Because of epidemic, polypropylene is known to almost everyone. As a material with light weight, non-toxicity, excellent physical properties, easy processing and chemical corrosion resistance, polypropylene (PP) is widely used in medicine. , communications, electronic appliances, home appliances and many other fields. However, limiting oxygen index of polypropylene is about 17%, and it is extremely flammable, which limits use of polypropylene in applications where high fire resistance is required. With increase in environmental protection requirements, traditional bromine-antimony-halogen flame retardant polypropylenes severely limit their application prospects due to large amount of smoke, formation of toxic and corrosive gases, and environmental pollution.

In recent years, intumescent flame retardant (IFR) has emerged in flame retardant field as a green and environmentally friendly flame retardant. It mainly consists of carbon, nitrogen and phosphorus. other types of flame retardants can be compounded. Plastics containing intumescent flame retardants form a layer of carbonaceous foam on surface when burned, which performs functions of thermal insulation, oxygen barrier, drip protection and smoke suppression, and is increasingly used in PCB. refractory material.

1. What do you know about combustion process of polypropylene?

Figure 1. Polypropylene combustion process

[1]Wei Tao, Zhang Daohai, Wu Bin, Qi Shuting, Qu Jin, Guo Jianbing. Progress in research on flame retardant polypropylene[J]. Plastic Industry, 2014, 42(09):15-18.

It can be seen from Figure 1 that polypropylene undergoes three processes of chain initiation, chain growth and chain branching during heating process, during which a large amount of free radical groups will be generated, further accelerating degradation of polypropylene. in process of thermal decomposition of polypropylene, suitable flame retardants can be selected to make it fire resistant.

Second, intumescent flame retardant

An intumescent fire retardant system generally consists of a carbon source (dehydrating agent), an acid source (dehydrating agent), and a gas source. The main components of IGF are ammonium polyphosphate (APF), pentaerythritol (PER) and melamine (MEL). The carbon source is basis for formation of carbon layer and is a substance that can be dehydrated by a dehydrating agent during combustion. , The acid source is mainly a substance that reacts with a carbon source to dehydrate it, and gas source is a substance that can release non-toxic and non-flammable gas during combustion. The main case is shown in Table 2-1:

Table 2-1 Intumescent Flame Retardant

Thirdly, effect of intumescent flame retardant on mechanical properties of polypropylene.

Use a high-speed mixer to evenly mix proportions of PP, flame retardant and other additives in formula shown in Table 3-1, and then use a twin screw extruder to extrude and pelletize. The extruder temperature is maintained at 180-200°C, and screw speed is 300-350 rpm. After drying resulting granules, standard test specimens were injection molded using an injection molding machine.

Table 3-1 Composition table

Table 3-2 Influence of flame retardant content on polypropylene properties

From Table 3-2, it can be seen that as amount of flame retardant increases, tensile strength and Izod impact strength of polypropylene gradually decrease, and fire retardant performance is achieved at a proportion of flame retardant of 21%. , Reach level V-0. This is because flame retardant is also used as a special filler, and polarity between it and polypropylene is different, and compatibility is poor, resulting in weak interfacial bonding between flame retardant and polypropylene matrix resin. defects such as voids and delamination more often appear at junction of two surfaces, which become points of stress concentration, which leads to a decrease in tensile strength and toughness; an increase in amount of flame retardants causes intumescent flame retardant to burn instantly when exposed to fire. A dense carbon layer forms, blocking contact of oxygen with sample and diffusion of combustible gas to surface of unburned sample, thereby playing a fire retardant role.

In addition, it can also be seen from TG curve that when an intumescent flame retardant is added, thermal decomposition temperature of polypropylene will increase accordingly, which is associated with a low decomposition temperature of flame retardant and fire retardant. Early decomposition of agent promotes formation of a carbon layer and promotes dehydration and carbonization of material with formation of a carbon layer, which plays role of heat and oxygen insulation, thereby improving fire resistance of PP. .

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