of polyols on the FR activity of polymers and they observed that high-molecular
weight-containing poly(methyl methacrylate) (PMMA)/silica nanocomposite resins
showed high FR activity; furthermore, the melt viscosity increased with an increase
in the molecular weight of PMMA. Poly(1,3-phenylene phenyl phosphonate)
(PPP)-containing polyethylene terephthalate (PET) and polybutylene terephthalate
(PBT) polymers of different molecular weights were also investigated; however,
there was not much difference in their FR activities. The limiting oxygen index
(LOI) values of low and high molecular weight PPPs were different; it may also be
assumed that there were significant changes in the physical properties of
high-molecular weight PPP [106].
5.3 Phosphorus-Based Polyols for Flame-Retardant
Polyurethanes
Polyols are one of the main components for PU synthesis and the incorporation of
phosphorus functionalities into polyols via covalent bonding [65, 107] offers several advantages, such as permanent attachment, homogeneous distribution, low
smoke production, and FR efficiency at small quantities [36, 108–110]. FR PUs
were prepared using phosphorylated polyols and the effectiveness of phosphorylated polyols as reactive additives for coatings was investigated. It was observed
that at low phosphorus contents of *0.54 wt%, polyol PUs showed high LOI
values (*30%) and the ignition time was delayed [111]. Phosphorus-based polyols
were synthesized by a reaction between castor oil and tris(m-hydroxyl phenyl)
phosphate (THPP) [112–115] using an ester exchange method; this polyol was
further used to prepare FR-PU coatings with various diisocyanates, including
aromatic and aliphatic diisocyanates [116]. The prepared coatings were used as
coatings on mild steel surfaces and their FR properties, such as LOI values and
UL-94 test performance were analyzed. Aromatic diisocyanate-containing PUs
exhibited high LOI values and V-0 rating in the UL-94 test. However, it was not
clear why aromatic PUs exhibited better FR activity compared to aliphatic PUs.
Further, castor oil-based PU blends were prepared by introducing different percentages of THPP; the LOI values increased with THPP content and they could also
achieve a V-0 rating in the UL-94 test because of the presence of aromatic rings.
Polyester polyol PUs are well-known, but to prepare FR PUs, organo-phosphorus
functionalities are introduced into the polyol structure. Polyester polyols were
prepared from bisphenol-A and POCl 3 at suitable reaction conditions [117] and
used to synthesize polyester polyol PUs with different diisocyanates, as shown in
Scheme 4.11, and the thermo-mechanical and FR properties of these coating
materials were studied [118]. It was observed that TDI-based polyester PUs showed
high LOI values and achieved a V-0 rating in the UL-94 test due to their aromatic
nature [119]. Zhang et al. [120] prepared a castor oil-based FR polyol (COFPL) via
the epoxidation of castor oil and ring opening reaction with triethyl phosphate.
They prepared a series of PUs with varying percentages of COFPL and found that
54
5 Flame-Retardant Polyurethanes
weight-containing poly(methyl methacrylate) (PMMA)/silica nanocomposite resins
showed high FR activity; furthermore, the melt viscosity increased with an increase
in the molecular weight of PMMA. Poly(1,3-phenylene phenyl phosphonate)
(PPP)-containing polyethylene terephthalate (PET) and polybutylene terephthalate
(PBT) polymers of different molecular weights were also investigated; however,
there was not much difference in their FR activities. The limiting oxygen index
(LOI) values of low and high molecular weight PPPs were different; it may also be
assumed that there were significant changes in the physical properties of
high-molecular weight PPP [106].
5.3 Phosphorus-Based Polyols for Flame-Retardant
Polyurethanes
Polyols are one of the main components for PU synthesis and the incorporation of
phosphorus functionalities into polyols via covalent bonding [65, 107] offers several advantages, such as permanent attachment, homogeneous distribution, low
smoke production, and FR efficiency at small quantities [36, 108–110]. FR PUs
were prepared using phosphorylated polyols and the effectiveness of phosphorylated polyols as reactive additives for coatings was investigated. It was observed
that at low phosphorus contents of *0.54 wt%, polyol PUs showed high LOI
values (*30%) and the ignition time was delayed [111]. Phosphorus-based polyols
were synthesized by a reaction between castor oil and tris(m-hydroxyl phenyl)
phosphate (THPP) [112–115] using an ester exchange method; this polyol was
further used to prepare FR-PU coatings with various diisocyanates, including
aromatic and aliphatic diisocyanates [116]. The prepared coatings were used as
coatings on mild steel surfaces and their FR properties, such as LOI values and
UL-94 test performance were analyzed. Aromatic diisocyanate-containing PUs
exhibited high LOI values and V-0 rating in the UL-94 test. However, it was not
clear why aromatic PUs exhibited better FR activity compared to aliphatic PUs.
Further, castor oil-based PU blends were prepared by introducing different percentages of THPP; the LOI values increased with THPP content and they could also
achieve a V-0 rating in the UL-94 test because of the presence of aromatic rings.
Polyester polyol PUs are well-known, but to prepare FR PUs, organo-phosphorus
functionalities are introduced into the polyol structure. Polyester polyols were
prepared from bisphenol-A and POCl 3 at suitable reaction conditions [117] and
used to synthesize polyester polyol PUs with different diisocyanates, as shown in
Scheme 4.11, and the thermo-mechanical and FR properties of these coating
materials were studied [118]. It was observed that TDI-based polyester PUs showed
high LOI values and achieved a V-0 rating in the UL-94 test due to their aromatic
nature [119]. Zhang et al. [120] prepared a castor oil-based FR polyol (COFPL) via
the epoxidation of castor oil and ring opening reaction with triethyl phosphate.
They prepared a series of PUs with varying percentages of COFPL and found that
54
5 Flame-Retardant Polyurethanes