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Five commonly used modified flame retardants for polylactic acid

Five commonly used modified flame retardants for polylactic acid


       Phosphorus flame retardant


Phosphorus-based flame retardants are environmentally friendly flame retardants that are widely used and have good practicability. Phosphorus flame retardants can be divided into organic phosphorus flame retardants and inorganic phosphorus flame retardants according to different compositions and structures. Organic phosphorus flame retardants mainly include phosphate ester, 9,10-dihydro-9-oxa- Organophosphorus compounds such as 10-phosphaphenanthrene-10-oxide (DOPO). Inorganic phosphorus-based flame retardants mainly include red phosphorus, ammonium polyphosphate, phosphate, etc. Phosphorus-based flame retardants can also be divided into two types: additive type and reactive type according to different ways of use. Additive phosphorus-based flame retardants directly mix flame retardants with polymer materials, and then process them into composite flame-retardant functional materials, while reactive phosphorus-based flame retardants bond flame retardant groups through chemical reactions. Combined with polymer materials to achieve the purpose of flame retardant.


Studies have shown that the flame retardant mechanism of phosphorus-based flame retardants is that the flame retardants will pyrolyze at high temperatures to produce acidic substances, which are conducive to the degradation of polylactic acid (PLA) polymers to form a carbon layer and prevent heat from entering the polymer. The matrix is transferred, and the phosphorus-containing free radicals generated during the combustion of phosphorus-based flame retardants can also quench the free radicals generated during the pyrolysis of PLA, and can form a polymer network with high thermal stability with the polymer matrix. burning effect. Phosphorus flame retardants have been widely used in the flame retardant modification of PLA and other polymer materials.


The currently reported phosphorus-based flame retardants for PLA are mainly derivatives of DOPO and other compounds, such as DOPO and epoxidized polybutadiene, DOPO and cinnamamide, DOPO and formaldehyde, etc. Another phosphorus-based flame retardant is a compound flame retardant based on ammonium polyphosphate (APP). APP is compounded, or nickel phytate (PA-Ni) and meglumine (N) are combined with APP. All of these phosphorus-based flame retardants can enable PLA to obtain excellent flame retardant properties.


       Nitrogen flame retardant


Nitrogen-containing flame retardant is a halogen-free flame retardant, which is in line with the current development concept of green flame retardant. Nitrogen-based flame retardants have the advantages of low toxicity and low smoke. At present, melamine, dicyandiamide, guanidine salts and their derivatives are more common.


When the decomposition temperature is reached, the incombustible gases such as CO2, NH3, N2 generated by the nitrogen-based flame retardant can dilute the flammable gas generated by the thermal decomposition of the PLA compound and the O2 around the polymer. At the same time, due to the decomposition of the flame retardant, it will absorb The heat effectively reduces the surface temperature of PLA, thereby achieving the purpose of flame retardant. Due to the low flame retardant efficiency of nitrogen-based flame retardants alone in actual flame retardant modification, nitrogen-based flame retardants are often used together with other types of flame retardants to achieve good flame retardant effects by synergistic effect.


       Silicon flame retardant


Silicon-based flame retardants are halogen-free and environmentally friendly flame retardant materials containing silicon, which can not only improve the flame retardant properties of the materials, but also improve the mechanical properties, processing properties and heat resistance properties of the materials. Based on the many advantages of silicon-based flame retardants, it has become a hot research topic. Studies have shown that silicon-based flame retardants can delay or even interrupt the thermal decomposition of materials in the condensed phase, so as to achieve a good flame retardant effect.


Silicon-based flame retardants have low smoke production and high safety, but the flame retardant effect is not good when used alone, which limits the application of silicon-based flame retardants in flame-retardant PLA to a certain extent. Silicon-based flame retardants can be compounded with other flame retardants for synergistic flame retardant, so the development of high-efficiency synergistic flame retardant systems for silicon-based flame retardants has a very broad development prospect.


       Inorganic nano flame retardant


Inorganic nano flame retardants have unique advantages compared with ordinary inorganic particles due to their special size characteristics, which provide a new idea for the preparation of flame retardant materials. However, when inorganic nanoparticles are used alone as flame retardants, there is a disadvantage of large amount of addition. In practical applications, inorganic particles are usually compounded with other types of flame retardants to improve the flame retardant effect.


Nano-magnesium-aluminum hydrotalcite can be used for flame retardancy of PLA. Generally, intercalating agent is used to intercalate it to prepare sulfamic acid intercalated hydrotalcite, phosphotungstic acid intercalated modified magnesium-aluminum hydrotalcite, etc., and then combined with intumescent flame retardant. It is used for flame retardant modification of PLA to improve the flame retardant properties of PLA. In addition, the organic intercalated nano-montmorillonite has obvious smoke suppression effect, which can effectively improve the flame retardant properties of PLA/wheat straw fiber composites.


In recent years, the new nano flame retardant carbon nanotubes (CNTs) has also been applied in the field of PLA flame retardant. For example, 10-hydroxy-9,10-dihydro-9-oxa-10-phosphinophenanthrene 10 oxide (DOPO-OH) can be covalently embedded on the surface of multi-walled carbon nanotubes (MWCNTs) to prepare DOPO-functionalized The MWCNTs (MWCNT-DOPO-OH) were introduced into the PLA/aluminum phosphate (AHP) system by melt blending, thereby improving the flame retardancy of PLA/AHP.


       Intumescent flame retardant


Intumescent flame retardant (IFR) is a kind of non-toxic and efficient green flame retardant. IFR is mainly composed of phosphorus, nitrogen and carbon, and consists of three parts: acid source, gas source and carbon source. At present, in the intumescent flame retardant system used for PLA flame retardant modification, the acid source is mainly APP; the gas source mainly includes melamine (MA), triazine derivatives, urea, etc.; the carbon source is polyhydroxy polymer starch (ST ), lignin (LIG), cellulose, etc.


IFR can increase the char formation of PLA and effectively reduce the phenomenon of burning droplets. The flame retardant mechanism of IFR flame retardant PLA is that the carbon and foam on the surface of PLA will expand when it is burned, resulting in a multi-foamed carbon layer, and the multi-foamed carbon layer on the surface hinders the combustion of the underlying material. Due to the good heat insulation and smoke suppression effect of IFR, it has received more and more attention in recent years.


Biomass flame retardant


In recent years, with the increasing awareness of environmental protection, people have gradually shifted their research direction to the use of biomass flame retardants to improve the flame retardant properties of PLA. Common biomass flame retardants include starch, cellulose, cyclodextrin, etc. Biomass flame retardants contain a large number of active hydroxyl groups. After adding PLA, they cross-link into charcoal during combustion to form a porous charcoal layer, so as to achieve the purpose of flame retardant. However, the use of biomass materials alone also has problems such as large amount of addition and poor compatibility, so the modification of biomass materials is of great significance.


Biomass flame retardants are generally used in combination with other flame retardants: for example, phosphorus pentoxide, methanesulfonic acid, and melamine can be used to modify the biomass flame retardant chitosan, and then the modified chitosan and melamine can be used. Polyphosphate (MPP) is compounded to form IFR; phosphorus-based flame retardant aluminum diethylphosphinate (AlPi) and starch are compounded to form a synergistic flame retardant. PLA with good flame retardant effect; IFR is obtained by compounding phytic acid modified chitosan (PAMC) and MPP, and then organic montmorillonite is used as a synergist, which can effectively improve the flame retardant performance of PLA composites.



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