Synthesis and comprehensive study on industrially relevant flame retardant waterborne polyurethanes based on phosphorus chemistry
Graphical abstract
Introduction
Since the synthesis of the first synthetic polymer, they have been so widely used that global production is expected to exceed 500 million tonnes by 2050 mainly due to their low cost, durability, safeness, and processability [1]. This development brings also an inherent risk of fire as polymeric materials are mostly based on hydrocarbons which display a large fire load and high flammability [[2], [3], [4]]. Halogenated flame retardants have dominated the field because of their low price and high efficiency. However, in recent decades, works have been undertaken to develop halogen-free phosphorus-based flame retardant materials, as halogenated compounds produce large amounts of smoke and highly toxic gases in case of fire, damaging to human health and the environment [5]. The challenge is, therefore, to develop industrially relevant formulations with sustainable flame retardant character. One of the polymer families that has been most applied in this area is polyurethanes (PUs) due to their versatility and a wide range of potential applications such as foams, coatings, etc. [[6], [7], [8]].
As sustainable research is desired, water-based systems are gradually dominating the coating market as a consequence of their lower toxicity compared to solvent-based products. In line with this trend, considerable efforts have been devoted to the development of waterborne polyurethanes (WPUs) in both industries and academia [[9], [10], [11], [12]]. One of the main applications of WPUs is as a coating for leather, wood, and fabric which indeed are all combustible materials. So, the demand for flame retardant modification of WPUs has increased significantly to avoid enormous fire-related casualties and property losses [[13], [14], [15]]. As previously mentioned, among various flame retardants, halogen-free phosphorus-containing compounds have been believed to be the most promising, primarily because of high flame retardant efficiency and low production of toxic gases and smoke [[16], [17], [18]]. Therefore, great efforts have been expended on the development of this area in recent researches of flame retardant WPUs [19,20]. Over the past few years, some studies have been mainly focused on the synthesis of reactive-type flame retardant based on organophosphorus or phosphorus-nitrogen compounds for the use in WPUs [21,22]. However, few comprehensive works have been carried out to investigate the effect of flame retardant not only on thermal and flame retardant behaviors but also on other significant characteristics such as dispersion, physical, and mechanical properties of WPUs [15,20]. Wu et al. [23] prepared bis(4-aminophenoxy)phenyl phosphine oxide (BPPO) and synthesized flame retardant WPUs with a post-chain extension of BPPO. While the WPU with a chain extension ratio of 100% (containing 100 wt% of BPPO) had a good flame retardant property with the LOI value of 30.1%, an increase in particle size up to 127 nm with a little precipitate after being centrifuged was observed. Moreover, the synthesis of flame retardant material is usually complicated and hence raises the cost of final WPU products. Therefore, the use of commercially available reactive-type flame retardants might be considered as an economically efficient alternative [24,25].
To compete with solvent-borne PUs, some properties of WPUs such as mechanical strength still need to be improved especially for the use as high-performance coatings [26]. Self-crosslinking of WPUs through hydrolysis and condensation reactions of alkoxysilane groups attached to the WPU chains to form a stable siloxane network has been recently reported as a suitable method to enhance mechanical properties [[27], [28], [29]]. Experimental findings revealed that an increase in siloxane crosslinking density improved not only mechanical properties but also thermal stability and fire resistance [15,30]. We envisioned an improvement in the flame retardant behavior of WPUs with this type of crosslinking.
The aim of this study was the design of innovative, cost-effective, and sustainable WPUs for the use as flame retardant coatings. For this reason, Exolit® OP560 (OP560) was chosen as a commercially available reactive-type flame retardant. OP560 is oligomeric phosphonate diol commonly used in PU foam manufacturing [[31], [32], [33]]. However, introducing OP560 into the WPU structure has not been reported in the literature so far. Due to ethylene oxide-based structure of OP560, it was expected that by addition of this diol into WPU structure, water resistance property of coatings deteriorated. To overcome this defect, castor oil was considered as an appropriate candidate for enhancing the hydrophobicity of the WPU system owing to long nonpolar fatty acid chains. Castor oil has gained much attention in PU production due to its abundance and evenly distributed hydroxyl groups that can directly react with isocyanate without further modification [[34], [35], [36], [37]]. This will increase the hydrophobicity of the PU as well as the amount of renewable feedstock in the formulation in line with the current concerns about fossil fuels as finite resources [[38], [39], [40]].
To achieve these purposes, a series of self-curable WPUs were synthesized by means of acetone process using the same weight percent of alkoxysilane crosslinker. Moreover, comprehensive studies were conducted to investigate the effect of flame retardant diol and renewable polyol inclusion on the dispersion properties, mechanical performance, thermal behavior, and flame retardancy of resulting WPUs.
Section snippets
Materials
Isophorone diisocyanate (IPDI), poly(tetramethylene ether glycol) (PTMEG) (Mn = 650 g mol−1), castor oil (CO), 2-bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BDO), triethylamine (TEA), dibutyltin dilaurate (DBTDL), (3-aminopropyl)triethoxysilane (APTES), and acetone were purchased from Sigma Aldrich. Exolit® OP560 (OP560) was obtained from Clariant AG (OH-number ≈ 450 mg g−1, functionality approx. 2, viscosity = 500 mPa s and phosphorus content of approx. 11.5% (w/w)). Prior to
Results and discussion
In this work, firstly, a series of self-curable WPU dispersions were synthesized using APTES (10 wt% based on total polymer mass) as a crosslinker with different amounts of OP560 and castor oil. Then, the effects of flame retardant diol and renewable polyol on the structure and functional properties of WPUs were investigated.
Conclusions
In summary, a series of industrially relevant bio-based flame retardant self-curable WPU dispersions were prepared by means of acetone process using 10 wt% APTES as a crosslinker. The effects of flame retardant diol (OP560) and castor oil (CO) insertion on the properties of WPUs were investigated. The formation of siloxane network structure was confirmed by 29Si-NMR study after self-crosslinking of dispersions at room temperature. All dispersions were well stabilized at room temperature for
Acknowledgements
The authors would like to thank Iran National Science Foundation (INSF) and Diputación Foral de Gipuzkoa (OF218) for financial support. The authors gratefully acknowledge Dr. Ehsan Mehravar for helping with contact angle measurement, Dr. Nora Aranburu for mechanical analysis, and Mr. Alvaro Iregui for SEM study.
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