Table 1.

Advantages and limitations of conventional flame retardants.

Category	Halogen	Non-Halogen
Type	Brominated Chlorinated	Phosphorus-based	Nitrogen-based	Metallic hydroxide
Reaction mechanism	Releases gaseous hydrogen halide into the air, isolating oxygen and interacting with active free radicals OH• and H•	Active phosphorus-containing radicals such as PO• and PO2• act in the gaseous phase, reacting with free-radical OH•	Mostly melamine based. Condensed-phase melamine transforms into cross-linked structures, producing solid-phase char layers	Acts as a catalyst to produce carbonized residues during combustion
Advantages	Low-cost, high flame-retardant efficiency and wide applicability	High efficiency, less influence on light stability, and hinder reignition	Low volatility, halogen-free, low toxicity, no corrosive gas	Non-toxic, good stability, no toxic gas produced at high temperature, low price, a wide range of sources
Disadvantages	Produces large quantities of smoke and toxic, corrosive gases when burned	High volatility, poor heat resistance, and unsatisfactory compatibility	Poor compatibility, easy-to-absorb moisture, and affects the physical performance of the substrate	Poor acid resistance and susceptibility to acid