Application Status of Halogen-free Flame Retardant Polyolefin Power Cable Materials

Application Status of Halogen-free Flame Retardant Polyolefin Power Cable Materials

With the rapid development of the domestic economy, electric power, construction and communications and other industries have an increasingly high demand for wire and cable, polyolefin cable production was unprecedented exponential growth trend, it is expected that by 2024, the scale of China's demand for wire and cable is expected to exceed 1.9 trillion yuan, the compound annual growth rate are about 4%, the halogen-free flame-retardant wire and cable materials market demand is about 200kt, it is expected that over the next 3~5 years It is expected that in the next 3~5 years, the demand for this kind of cable will increase at a rate of about 10%, and by 2025, the demand for halogen-free flame-retardant wire and cable materials will reach about 350kt. Polyolefin materials commonly used in cables have relatively high flammability, and are prone to decomposition and combustion at high temperatures, and produce a large number of molten droplets during combustion, igniting other combustibles and expanding the scope of the fire, which can bring about serious casualties and huge economic losses. Therefore, polyolefin cable flame retardant research has been a hot spot of concern.

Polyolefin flame-retardant cable materials are flame-retardant modified by flame-retardant additives for cable insulation materials and sheath materials. Polyolefin base materials for flame retardant cables should meet the following requirements:

(1) have good electrical properties, mechanical properties and processing properties;

(2) is conducive to increasing the filling amount of flame retardant additives.

Additives for flame retardant cable should meet the following requirements:

(1) itself has flame retardant properties;

(2) Reduce the amount of smoke and toxicity of polyolefin cable materials;

(3) maintain the electrical properties, mechanical properties and processing properties of flame retardant cable materials.

This article begins with the selection of halogen-free flame retardant cable polyolefin matrix materials to analyze the pyrolysis of polyolefins and flame retardant mechanism, and discusses the types of flame retardant additives and flame retardant mechanism, and finally discusses the current status of research on flame retardant polyolefin cable material technology, advantages and disadvantages and specific applications.

1.    Flame retardant cable material base material selection

As a carrier of power transmission, the insulation performance and mechanical strength of the cable is crucial, and the cable matrix material directly affects the electrical and mechanical properties of the cable material. Now widely used polyolefin cable matrix materials for polyethylene and cross-linked polyethylene, in addition to polypropylene as the base of the power cable material is also the focus of research.

PE

PE has no polar groups in its molecular structure, and is a non-polar polymer material, while halogen-free flame retardants generally have strong polarity and poor compatibility with PE. Therefore, improving the polarity of PE cable base material or utilizing coupling agent to deal with flame retardant additives, so as to improve the filling amount and compatibility of non-halogenated flame retardants, is a key step to improve the flame retardant performance of PE. The commonly used method to improve the polarity of PE base material is to blend and modify PE with more polar polymers, including: EVA, EEA and EPDM. On the one hand, the addition of these copolymers introduces polar groups, which in turn improves the affinity between the polymer matrix and the inorganic flame retardant, and can increase the filler amount of the flame retardant and improve the flame retardant properties of the composite material. On the other hand, these copolymers have good flexural, toughness, resistance to environmental stress cracking and adhesive properties, especially mechanical properties can be improved.

PP

PP is widely used in a variety of industries for wires and electrical equipment cables due to its excellent electrical properties, chemical reagent resistance, and oil resistance. The usual PP materials have poor toughness and impact resistance, and researchers have modified them through copolymerization, blending, grafting, nano addition, etc., and the mechanical and electrical properties have been greatly improved. Compared with XLPE cables, the modified thermoplastic PP is simpler to process and has the characteristics of recycling, which improves the cable production rate and significantly reduces production costs. In addition, the temperature resistance of PP cable is higher than XLPE, can significantly improve the cable capacity, in the field of power cable is highly anticipated.

PP heat of combustion is very high, low carbon rate, and limit the PP as a flame retardant cable material is the main drawback of its molecular chain is shorter, higher crystallinity, these lead to its compatibility with flame retardants is relatively poor, a small amount of flame retardant that will cause a significant decline in the processing and mechanical properties of PP. Especially when the cable needs to be bent, its bending resistance is also poor, in the transportation and laying process may cause mechanical damage. Therefore, under the premise of not affecting the mechanical properties of PP, improving its flame retardant properties has become a hotspot of PP flame retardant modification research.

PP flame retardant base material modification methods are varied, but used in the field of cable insulation blending and copolymerization modification is the most effective way. Blending modification is PP and other elastomers: PE, EVA, POE blending, copolymerization modification is PP molecular chain connected to ethylene or propylene molecular chain. These two methods of low cost, simple process, technical flexibility, both at home and abroad have good prospects for development.

2.    Polyolefin cable flame retardant types

Polyolefin combustion characteristics

PE and PP and other polyolefin constituent elements in the C, H content is extremely high, and therefore very easy to burn, the limit of the oxygen index is only 17%, and in the combustion process is prone to melting droplets and delayed fire phenomenon. Polyolefin combustion process exists in the softening, decomposition, combustion of three stages, in which the decomposition process will produce a large number of combustible substances, and the heat released during the combustion process to promote the decomposition of polyolefin. Therefore, the flame retardant mechanism of flame retardant polyolefin cable materials is mainly manifested in the use of flame retardants to slow down the thermal decomposition of the material, limiting the heat transfer to avoid the role of fire. According to the composition of flame retardants, halogen-free flame retardants can be divided into phosphorus, nitrogen, silicon, boron, inorganic metal hydroxide and expansion type.

Halogen-free flame retardants

At present, the commonly used halogen-free flame retardants for power cables include nitrogen-based, phosphorus-based, boron-based and silicon-based flame retardants.

I. Nitrogen-based flame retardants

Mainly melamine and its salt, its decomposition temperature is high, the combustion process mainly produces: NH3, N2, NO and water vapor and other non-toxic, non-corrosive products. Nitrogen-based flame retardants absorb a large amount of heat and release non-flammable gases through volatilization and thermal decomposition, which can greatly reduce the surface temperature of polymers and dilute the concentration of flammable gases and oxygen in the environment, and ultimately achieve good flame retardant effects. Nitrogen-based flame retardants and other flame retardants applied at the same time has a good synergistic effect, for example, nitrogen ⁃ phosphorus flame retardant which can promote the carbonization of phosphorus, the formation of expanded carbon layer, play a good role in heat insulation and flame retardant. Some nitrogen flame retardants such as melamine cyanurate are often used as lubricants and compatibilizers to improve the blending of other flame retardants in polyolefins.

II. Phosphorus flame retardants

The main use of phosphorus groups in the process of thermal decomposition can make the polymer surface dehydration carbonization, play a role in isolation flame retardant. Among them, ammonium polyphosphate (APP) is commonly used in power cable flame retardant system. The disadvantages of phosphorus-containing flame retardants are that phosphorus element has neurotoxicity, low stability, poor water resistance, poor compatibility with polymers, and relatively large impact on mechanical properties, so the application is limited.

III. Boron compounds

The flame retardant mechanism of boron compounds is to form a glassy isolation layer during the combustion process, to block the effect of oxygen and volatile combustible gases, to prevent further oxidation of the charcoal layer and to promote the formation of charcoal. Compared with phosphorus flame retardant, boron flame retardant has good thermal stability, low toxicity and small smoke, so it is more suitable for popularization and application. Zinc borate is commonly used as flame retardant co-efficient, but its effect alone is not good, and it is mainly used as flame retardant co-efficient.

IV. Silicon flame retardant

Silicon flame retardant is a new type of environmentally friendly flame retardant with high flame retardant efficiency, low toxicity, anti-melting droplets and smoke-free features. Commonly used in power cable flame retardant is inorganic silica-based flame retardant, including silicate minerals, such as talc, layered silicate and porous silicate, etc. They not only promote the formation of carbon in the combustion process, but also increase the role of smoke absorption.

V. Inorganic metal hydroxide flame retardants

At present, the commonly used inorganic metal hydroxides are aluminum hydroxide (ATH) and magnesium hydroxide (MDH), which have the characteristics of low smoke, non-toxic, green and environmental protection, and have received wide attention as new pollution-free flame retardants. ATH and MDH start to decompose and absorb a large amount of heat to reduce the local temperature of the combustion area when the temperature is higher than 200℃, and the decomposition of the water vapor generated by the decomposition of the combustible gases dilutes the concentration of oxygen, while the non-combustible gas generated by the decomposition of the combustible gases and oxygen. The water vapor generated by their decomposition dilutes the concentration of combustible gases and oxygen, and at the same time, the non-combustible oxides generated form an isolation film, which has the effect of flame retardant.

Inorganic metal hydroxides rely on their own decomposition to reduce the heat of combustion, while diluting oxygen, so its flame retardant efficiency is lower, the content is often more than 50%. At the same time, the polarity of inorganic metal hydroxide is large, and the compatibility of polyolefin cable base material is poor, resulting in its difficult to disperse during processing, easy to form mechanical stress points, making the mechanical properties of the cable material significantly reduced. At present, the research focus on ATH and MDH as flame retardants is still focused on improving the compatibility problem, and the commonly used methods include: particle thinning, surface modification and increasing the compatibilizer.

VI. Intumescent flame retardants

The composition of intumescent flame retardant is mainly based on phosphorus and nitrogen, which combines the advantages of the two kinds of flame retardants and is characterized by non-toxicity and less smoke. Phosphorus forms an isolation film after being heated, and nitrogen decomposes into water and gas after being heated, in which the gas decomposed by nitrogen facilitates the formation of foamy charcoal layer of phosphorus. The foamy charcoal layer can play the role of oxygen barrier and heat insulation, and at the same time, it can prevent molten droplets. Compared with other halogen-free flame retardant systems, in the field of flame retardant PE and PP, excellent flame retardant effect can be achieved when the content of IFR is 20%~30%.

The theoretical phosphorus content of APP is up to 31% or more, which is the most common acid source in IFR, and it also serves as a gas source. However, APP applied in PE and PP has many defects in performance, for example, thermal stability is not high enough, irritating odor and corrosive mold phenomenon during processing; poor compatibility with PE and PP, which can not meet the requirements of mechanical properties; poor water resistance, large moisture absorption, and so on. Therefore, surface modification of APP is one of the effective ways to solve the above problems.

The surface modification of APP can be divided into two kinds: physical coating method and surface chemical modification method. The key of physical coating is that the coating material needs to have good compatibility with APP to ensure the solidity of coating; the coating material also needs to have good thermal stability, interfacial compatibility and good water resistance. Compared with physical coating, chemical modification has stronger bonding force, but chemical modification will lead to higher cost of APP and raise the threshold of application.

Cable material flame retardant technology

According to the provisions in GB/T32129-2015 Halogen-free Low Smoke Flame Retardant Cable Material for Wires and Cables, modified PE and PP power cable materials should meet the performance of Table 1.

VII. Modified metal hydroxide flame retardants

Metal hydroxide is a widely used green flame retardant, and in view of its shortcomings such as poor mechanical properties and poor compatibility of modified flame retardant cable materials, researchers have tried to modify it in order to enhance compatibility and improve flame retardant efficiency. Taking MDH as an example, its surface is modified by using coupling agent with hydrophilic and lipophilic groups at the same time, and the coupling agent connects metal oxides and polyolefins together, which can improve the compatibility very well.

Conclusion

Polyolefin flame retardant cable materials have good insulating properties and are used in large quantities worldwide for power cable insulation, making an important contribution to power transmission. However, with the emphasis on environmental issues, the use of halogen-free flame retardant instead of traditional halogen flame retardant is an important trend in the development of cable insulation.