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Bonding aryl groups with carbon-nitrogen double bond is a critical structural factor for the self-crosslinking of Schiff bases at high temperature. In this study, a novel aromatic Schiff base “5-(benzylidene-amino)-isophthalic acid dimethyl ester (BA)” has been designed as a self-crosslinking monomer for poly(ethylene terephthalate) (PET) based copolyesters (BA n PETs). The cross-linking behaviors, flame retardancy and non-dripping performance of BA n PETs have been investigated by simultaneous thermal analysis (TG-DSC), dynamic rheology, limiting oxygen index (LOI), UL-94 vertical burning and cone calorimetry tests. It is found that BA units can cross-link during combustion resulting in high melting viscosity and enhance char forming for BA n PETs, which endows the copolyesters with excellent flame retardancy and non-dripping behavior. Incorporating of BA with a low content (only 7.7 mol%), the LOI value of copolyester increases from 22.0% to 31.0%, and at the same time it achieves V-0 rating in the UL-94 vertical burning test. The mode action of Schiff bases on flame retardation and anti-dripping of copolyesters also has been investigated, and the results show that the aromatic Schiff base groups can lead copolyesters to form nitrogen-containing cross-linkable networks and further transform into compact char layer during combustion. The char layer which acts as an effective barrier, cuts off oxygen, hinders the volatilization of combustible gas and isolates unburned polymer from fire resource. This work suggests that using Schiff base as a self-crosslinkable monomer is a new and effective way for fire-safe polymeric materials. [ABSTRACT FROM AUTHOR]

• A novel benzimidazole monomer was incorporated into the chain structure of PET. • Physical networks are formed in the copolyesters via H-bonding and π-π stacking. • Physical networks improve the melt strength and retard the melt-dripping greatly. • Copolyesters with no-dripping and low smoke release show excellent fire safety. • Copolyesters exhibit high T g , high tensile strength and good processability. Currently, it is still a huge challenge to prepare flame-retardant polymeric materials with excellent fire safety, high strength, good durability and processability. We here propose a H-bonding cross-linking strategy that can simultaneously endow polymers with excellent flame retardancy, non-dripping behavior and enhanced strength. In this work, a novel benzimidazole monomer, 2-(4-methoxycarbonyl-phenyl)-1H-benzimidazole-5-carboxylic acid methyl ester (PBM) has been designed as a H-bonding cross-linking monomer to prepare poly(ethylene terephthalate)-based copolyesters (PET- co -PBMs). Interestingly, along with the hydrogen bonding, the π-π stacking interactions from the benzimidazole groups also exist. Due to the formation of physical crosslinking network via hydrogen bonding and π-π stacking, the melt strength of copolyesters are greatly improved. Combining with char forming ability of benzimidazole, PET- co -PBMs display excellent fire safety. Moreover, PET- co -PBMs also exhibit higher glass transition temperature, higher mechanical strength and good processability. This work provides a promising strategy to design polymers with high strength, good processability, excellent flame retardance and anti-dripping performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • MS-K containing potassium sulfonate and benzimidozale groups was synthesized. • Incorporation of MS-K into the backbone of PET the migration of IRF is eliminated. • PET copolymer exhibits enhanced self-intumescent carbonization behavior. • Fire hazards of PET copolymer is significantly suppressed. Additive-type Intumescent flame retardants (IFRs) are widely used in many polymeric materials to reduce fire hazards. However, the migration behavior of IFRs in the long-term use process leads to poor flame retardance durability and undesirable health issue. To address these problems, a novel reactive monomer (MS-K) containing potassium sulfonate and benzimidozale groups was synthesized and chemically linked into the backbones of model polymer (Polyethylene terephthalate, PET). When the PET-copolymer burnt, the sulfonate group worked as acid and gas sources, and the benzimidozale group acted as carbon source. The trinity effect (acid, gas and carbon sources) of benzimidozale-potassium sulfonate structure gave rise to the self-intumescent carbonization behavior of PET matrix. As a result, the formed intumescent char-layer exerted a significant condensed-phase flame retardant effect that can hinder the oxygen exchange, isolate from the heat source and suppress the volatilization of combustible small molecules. Thus, the corresponding PET-copolymer exhibited extremely low fire hazards with UL-94 V-0 rating, a high limiting oxygen index of 32.5 vol%, greatly decreased peak heat release rate (decreased by 58%) and total smoke production (decreased by 39%). The strategy of reactive trinity-macromolecule not only exhibits excellent intumescent flame-retardant effect, but also completely eliminates the migration issue, which represents a promising way to replace the traditional additive-type IFRs. [ABSTRACT FROM AUTHOR]