Synthesis and characterization of well-dispersed polyurethane/CaCO3 nanocomposites

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Abstract

Well-dispersed and long-term stable nano-CaCO3/polyol dispersions were prepared by a mechanochemical approach with the aid of poly (propylene glycol) phosphate ester (PPG-P). Polyurethane (PU)/CaCO3 nanocomposites were prepared by further in situ polymerization with 6 wt% nano-CaCO3. The microstructure and dispersion of nano-CaCO3 in the nanocomposites were investigated. It was found that well dispersion was obtained up to 6 wt% of the surface treated CaCO3 loading for PU/CaCO3 nanocomposite. The segmented structures of PU were not interfered by the presence of nano-CaCO3 in these nanocomposites as evidenced by Fourier transform infrared. Compared with the pure PU, a significant improvement in thermal stability was observed with the addition of 6 wt% of the surface treated CaCO3. The experimental results suggested that the properties of nanocomposites were correlated with the dispersion of nano-CaCO3 in PU and the interfacial interactions between nano-CaCO3 and polymer matrix.

Introduction

Polyurethane (PU) elastomer is one of the most interesting classes of synthetic elastomers that have unique properties. Hence, it has received wide attention for its synthesis, morphology, chemical and mechanical properties [1], [2], [3]. PU generally consists of a soft segment which is a high molecular weight polyester macrodiol and a hard segment which is composed of diisocyanate and low molecular weight diol or diamine [4]. Due to the difference in the chemical structure of the soft and hard segment, microphase separation takes easily arising from the thermodynamic incompatibility in PU. The domain morphology in such phase-separated structures achieved by phase-separation or phase mixing has a great influence on the PU properties, which has limited its wide engineering usages. To extend the application fields of PU, researches are compelled to search for alternative PU with higher performance.

Polymer-based nanocomposites exhibit remarkable improvements in mechanical, dielectric magnetic, thermal optical and acoustic properties compared to the pure organic polymers [5], [6], [7], [8], [9], [10]. Meanwhile, there have been some studies on the interfacial interactions between fillers and polymer matrix [11], [12], [13], [14]. Therefore, incorporation of nanoparticles into PU is a promising approach to improve its properties. However, nanoparticles less than 0.7 μm in size tend to aggregate and show very poor dispersion in composites, and agglomeration becomes worse as the particle size reduced [15].

As one of the most abundant materials on our planet, CaCO3 has been quite early used to produce polymer composites. However, in literature mentioned previously, there are a few studies regarded to the usage of nano-CaCO3 in PU [16], [17], [18], [19]. This is possibly due to the difficulty in achieving good dispersion of nano-CaCO3 in PU matrix, which leads to the deterioration of PU properties. Therefore, the biggest challenge in making PU/CaCO3 nanocomposites is the uniform dispersion of CaCO3 in PU matrix without agglomerates and entanglements. According to the literature, the modifying agents used in the polymer-based nanocomposites are always a series of low molecular weight compounds, including silane, stearic acid, CTAB, etc [20], [21], [22], [23], [24]. A few high molecular weight modifiers are also used recently, for example PAA, PAAS, PMMA, etc. All of them could improve the dispersion of nano-CaCO3 to some extent, but they could not solve the dispersion problem in essence.

This study will endeavor to solve the dispersion problem of nano-CaCO3 in PU. PPG is one of the most popular polyols in PU industry because of its low cost. In this study, the poly (propylene glycol) phosphate ester (PPG-P) with –PO(OH)2 as an anchoring group and PPG as a soluble chain has been synthesized by the reaction of PPG with polyphosphoric acid (PPA). The reaction scheme of this synthesis is shown in Scheme 1. Compared with other modifiers used in the polymer, PPG-P could be easily synthesized at low cost. Meanwhile, it could also improve the flame retardant property of the composites. It is believed that PPG-P was synthesized by PPG which was one of the urethane monomers, the surface treated CaCO3 with PPG-P would disperse well in polyol and nanocomposites because of their good compatibility. In this paper, the effects of PPG-P on the dispersion quality of nano-CaCO3 in polyol and nanocomposites were studied. Simultaneously, the micro-domain structures and thermal property of nanocomposites were also investigated.

Section snippets

Materials

Nano-CaCO3 was synthesized in the laboratory, with the diameter of about 50 nm. 2,2-Bis (hydroxymethyl) propionic acid (DMPA) was purchased from Alfa Aesar, America. Toluene diisocyanate (TDI), dibutyltin dilaurate (DBTL), trimethylolpropane (TMP), 2,5-dimethylfuran (DMF) and acetone were all analytical reagents. Poly (propylene glycol) (PPG), with molecular weight 1000 and polyphosphoric acid (PPA) were used as received.

Synthesis of PPG phosphate ester

PPG phosphate ester was synthesized by reacting 1 mol of PPA with 3 mol of

Characterization of PPG-P and its influence on nano-CaCO3

FT-IR spectrum of the product of experiment in Section 2.2 is shown in Fig. 1. The characteristic group vibrations of functional groups commonly existed in phosphate ester are found in Fig. 1. The –OH stretching vibrations are observed around 3400 cm−1. The symmetric and antisymmetric stretching modes of –CH2 groups are in the region 2800–3000 cm−1. The bands observed between 1400 and 1480 cm−1 are attributed to –CH2 scissoring vibrations. The weak band found at 1374 cm−1 is connected to the

Conclusions

A high molecular weight phosphate ester (PPG-P) has been obtained by the reaction of poly (propylene glycol) (PPG1000) with polyphosphoric acid (PPA). It had successfully attached on the surface of nano-CaCO3 and influenced the decomposition of CaCO3. With the assistance of PPG-P, the good dispersion of CaCO3 in polyol has been obtained by the mechanochemical approach. Furthermore, polyurethane (PU) nanocomposites with the blank CaCO3 and surface treated CaCO3 have been successfully prepared by

References (24)

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