**1. Introduction**

Polyurethane (PU) can appear in many forms, such as foam, adhesive, paint, and elastomer. Because of its favorable mechanical properties and resistance to chemicals and wear, PU is widely used in the automobile, textile, sports equipment, shoe sole, and paint industries, among others [1–3]. PU is the most common commercial polymer material in daily life. However, it has some weaknesses, such as low thermal stability and high flammability. For example, when PU is heated to 80–90 ◦C, its mechanical properties rapidly deteriorate. When the PU matrix is heated to over 200 ◦C, severe pyrolysis occurs and it becomes flammable, increasing the risk of a fire hazard. Therefore, improving the flame retardance of PU is a critical task in the development of polymer materials [1,4,5].

Compared with halogenated flame retardants, non-halogenated flame retardants have attracted grea<sup>t</sup> attention because during their combustion a lower amount of hazardous gases or smoke is produced, which is safer for the people in the fire and will not destroy the ozone layer in the atmosphere. These are called eco-friendly flame retardants. Flame retardants can play a role in the gas phase or condensed phase. Halogenated flame retardants are typically in the gas phase, whereas flame retardants containing phosphorus and nitrogen can be in either phase [4,6,7]. Flame retardants containing siloxane have a main chain of Si–O. The energy of the Si–O bond is high and the Si–O chain is resilient when

**Citation:** Shen, M.-Y.; Kuan, C.-F.; Kuan, H.-C.; Ke, C.-Y.; Chiang, C.-L. Flame Retardance and Char Analysisof an Eco-Friendly Polyurethane Hyperbranched Hybrid Using the Sol–Gel Method. *Sustainability* **2021**, *13*, 486. https://doi.org/10.3390/ su13020486

Received: 18 December 2020 Accepted: 30 December 2020 Published: 6 January 2021

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forming SiO2; as a result, these flame retardants exhibited outstanding thermal stability under high temperatures [8]. Rao et al. [9] reported an organic–inorganic phosphorus– nitrogen–silicon flame retardant that was synthesized by the Kabachnik–Fields reaction and the sol–gel method, and then it was used as a reactive flame retardant to prepare flameretardant and smoke-suppressant epoxy resins (EPs). The results certified the charring effect of the phosphaphenanthrene group and the enhancing effect of the silicon group. Subsequently, the flame inhibition effect and lesser combustible gases release enhanced the flame-retardant properties of the EP.

In the last few decades, the hyperbranched hybrid has gained more attention due to its special structure. Many hyperbranched materials have been synthesized and used in many fields, including flame retardants. Yang et al. [10] reported a novel hyperbranched phosphorus/nitrogen-containing flame retardant (HPNFR) that was facilely synthesized via the transesterification reaction of dimethyl methylphosphonate and tris (2-hydroxyethyl) isocyanurate. The sample with 4 wt% HPNFR can achieve a V-0 rating in the UL-94 test and possesses a limiting oxygen index (LOI) value as high as 34.5%. HPNFR would not significantly damage the transparence of EP thermosets; consequently, it reserved its application value in some special fields. Hu et al. [11] showed a phosphorus/nitrogen-containing hyperbranched polymer (PN-HBP) that was synthesized via the esterification reaction of 2-carboxyethyl (phenyl) phosphinic acid (CEPPA) and tris (2-hydrooxyethyl) isocyanurate (THEIC). A higher LOI and a V-0 rating in the UL-94 vertical burning test were realized, which indicated an apparent synergistic effect. The peak heat release rates (PHRRs) of composites were reduced significantly compared with that of PU.

The PU used in this study has a wide range of applications. However, it has fatal disadvantages such as flammability and dripping. This study employed the sol–gel method to prepare a reactive flame retardant containing nitrogen, phosphorus, and silicon and classified the flame retardance of the organic–inorganic polyurethane hybrid.
