The intumescent flame retardants (IFRs) are one of widely used halogen-free flame retardants due to their advantages of little smoke and low toxicity. These are multicomponent systems which decompose on the effect of heat forming a blown char layer on the surface of a polymer. This char layer protects the polymer from oxygen and heat. Conventional IFRs consist of three ingredients:
- an acid source, usually ammonium polyphosphate (APP),
- carbon source, commonly pentaerythritol (PER),
- blowing agent, like melamine (MA).
PER can be substituted by cyclodextrin (CD).
CD itself has good charring ability. The amount of char formed upon thermal degradation depends on the size of the CD (beta > gamma > alpha) and on the presence of substituents (Trotta et al. 2000). There was an inverse correlation between the temperature of degradation and the amount of char: the tosyl beta-CD starts to decompose at 187 oC (the unmodified CD at 314 oC) leaving 45% char (vs 17%) at 400 oC.
When applied in IFR composite, CD promotes the formation of stable and compact carbonaceous char, preventing the melt dripping of the polymer. The phosphorous and nitrogen compounds, as FRs react with high energy radicals H and OH both in the gas phase and in the condensed phase. They are released from the CD-containing IFR formulations only at the temperature above the thermal decomposition of the CD (250-300 oC). Applying the same dose of FR complexed by CD improved properties, such as delayed ignition, increased limiting oxygen index (LOI, the minimum concentration of oxygen that supports combustion of a polymer), reduced heat release, better char formation, are observed (see Table 1). The improved performance allows using lower dose of flame retardant. The decreased dose and reduced volatility of FRs provide lower human health risk (Zhang et al. 2015). These advantagaeous properties are often accompanied by other relevant properties, such as improved mechanical resistance, washability resistance, self healing ability, thermal conductivity, etc.
The FR/CD complex is easy to incorporate into the production of polymer films. For instance, PLA composites with 20w% of APP/CD/melamine complex gave limiting oxygen index values (LOI = minimum concentration of oxygen that will support the combustion of a polymer) of 34.2, and passed UL 94 V0 rating (burning stops in 10 seconds in a standard test) (UL94 2013). The char residue of the PLA composite was 17.4 wt% at 700°C while only 7.4 wt% was achieved without CD (Feng et al. 2011).
Hydrophobically modified CD (esterified by ethylacetate) has improved compatibility with PP, reduced aqueous solubility and the acetyl-CD (AcCD)/polypropylene (PP) composite showed slightly better flame retardancy compared to PP (Zheng et al. 2016). Synergistic effect of expandable graphite (EG) on the flame retardant properties of PP/melamine phosphate (MP)/AcCD system was observed. When the content of EG was 10 wt.%, the flame retardant properties of PP system achieved 31.2 % in LOI tests and passed UL-94 V-0 rating in vertical burning tests. Adding 10 w% of BCD to the thermoplastic polyether ester elastomer (TPEE)/25 w% P-N FR systems the UL94 reached to V0 grade and the tensile strength of this blend remained above 10 MPa, which can be applied in practice (Zheng et al. 2016).
Double-network films were prepared on the surface of paper, textile or wood via the host–guest interaction between poly (acrylic acid)-adamantanamine and ammonium polyphosphate-cross-poly (ethylenimine)-β-cyclodextrin (PAA-AD/APP-co-PEI-β-CD) via layer-by-layer (LbL) assembly (Xuan et al. 2017). This coating provides self-extinguishing property.
Layered double hydroxide (LDH) is also a prominent flame-retardant nanoadditive for polymers. The dispersion state can be improved by modifying by biobased flame-retardant species. For instance, phytic acid (Ph) and (hydroxypropyl)sulfobutyl-β-cyclodextrin sodium (HPSB-BCD), and subsequently Fe3O4 nanoparticles are used for modification to obtain a Fe3O4 nanosphere@LDH hybrid. (Kalali et al. 2016) or HPSB-BCD, dodecylbenzenesulfonate (DBS) and taurine (T) are added (Kalali et al. 2015). With only 6 wt% functionalized LDH, the sCD-DBS-T-LDH/EP nanocomposite reached V0 rating in the UL-94 vertical burning test.
A recent review with 77 references on cyclodextrins and derivatives as green char promoters has been recently published (Luda & Zanetti, 2019).
Some examples are listed in the Table 1.
Table 1 Overview of the studies and patent applications on the IFR systems using CD as carbonaceous additive
epoxy resin (EP), ethylene vinyl acetate copolymer (EVA), polyamide 6 (PA6,6), PAA poly (acrylic acid), polyethylene (LDPE), PEI polyethyleneimine, polypropylene (PP), poly(lactic acid) (PLA), poly(methyl methacrylate) (PMMA), polyethylene tereftalate (PET), resorcinol bi(diphenyl) phosphate (RDP), thermoplastic polyether ester elastomer (TPEE)
aluminum diethylphosphinate (AlDP), ammonium polyphosphate (APP), dibasic ammonium phosphate (APb), N,N′-dibutyl-phosphate diamide (DBPDA), isopropylated triaryl phosphate ester (ITPE), diethyl phosphoramidate (PhEt), N, N’-diamyl-p-phenylphosphonicdiamide (P-MA), layered double hydroxide (LDH), melamine phosphate (MP), poly(propylene glycol) (PPG), resorcinol bis(diphenyl phosphate) (RDP), triethyl phosphate (TEP), triphenyl phosphite (TPP)
β-Cyclodextrin (BCD), acetyl β-Cyclodextrin (AcBCD), (hydroxypropyl)sulfobutyl-β-cyclodextrin sodium (HPSBCD), BCD cross-linked by polydiphenylmethane diisocyanate (BCD-PMDI),
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