Polymer/silica nanohybrids by means of tetraethoxysilane sol–gel condensation onto waterborne polyurethane particles
Introduction
Polyurethane materials show a unique combination of high performance properties, including excellent abrasion resistance, flexibility or hardness, which makes them suitable for many useful applications such as surface and textile coatings, adhesives, elastomers, foams and dispersions [1], [2], [3], [4]. In the past, polyurethanes were mainly synthesized in organic solvents [5]. However, due to current regulations, organic solvents must now be replaced by water based systems [6], [7], [8].
Unfortunately, it is difficult to synthesize polyurethanes in aqueous media due to the incompatibility between water and isocyanates [9]. As a consequence polyurethane dispersions are almost exclusively obtained using a two-step procedure where the first polymerization step is carried out in an organic solvent followed by its dispersion in aqueous media [10], [11]. One of the main drawbacks of these water-based systems is that in most of the cases carboxylic groups must be incorporated in order to stabilize polyurethane dispersions. The presence of these polar groups increases the susceptibility of the polymer toward hydrolysis, considerably reducing mechanical and thermal properties [11], [12]. One of the most efficient synthetic approaches to overcome these negative effects has been the incorporation of inorganic moieties such as silica nanoparticles [13], [14], [15] due to their synergetic effect on polymer properties [16], [17]. However, the interfacial interaction between the polymer matrix and dispersed silica particles is a crucial factor in order to improve the properties [18].
It is generally accepted that the preferred hybrid structure is one in which the organic and inorganic materials are linked by means of covalent or ionic bonds [19], [20]. The use of the sol–gel process to prepare highly intermingled inorganic–organic hybrid polymer networks using coupling agents, such as aminopropyl triethoxysilane (APTES), is of current scientific interest [21], [22], [23]. However, Sardon et al. have shown that using this strategy it was possible to incorporate only 2 wt% of silica without affecting the homogeneity of the films [20].
Several attempts have been made in order to incorporate greater amounts of silica into the polyurethane matrix without affecting the homogeneity of the films [24], [25], [26], [27], [28]. One possibility is the surface modification of the inorganic particles by chemical reaction with organic materials. This method has been found to be an effective strategy to improve homogeneity [29], [30]. Similarly, Jeon et al. [31] incorporated functionalized silanes into polyurethane particles to act as a coupling agent and subsequently reacted them with preformed silica particles by the sol–gel process. Although their properties were slightly enhanced, homogeneous silica distribution in the polyurethane matrix was not obtained. Yeh et al. [32] polymerized tetraethoxysilane (TEOS) by the sol–gel process in the presence of amine terminated polyurethane, which can also act as an internal base catalyst for the TEOS sol–gel process. Even though the condensation of TEOS was successfully obtained, the films were not totally homogeneous.
In order to enhance the silica distribution in the polyurethane matrix, the best solution is to polymerize silica “in-situ” in the presence of polyurethane particles [33]. Tissot et al. [34] showed that TEOS could be polymerized on the surface of polystyrene particles using 3-(trimethoxysilyl)-propyl methacrylate (MPS) as a functional comonomer. They showed that the silanol groups coming from the functional monomer allowed the creation of silica coating onto the surface of the polystyrene particles. Following a similar strategy, stable functionalized polyurethane dispersions containing silanol groups were generated in order to enhance TEOS grafting [20].
This work presents the first successful polymerization of TEOS onto waterborne polyurethane nanodispersions. The prepared polyurethane/silica colloids were studied by Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). Dried hybrid films were characterized using 29Si solid-state NMR, FTIR spectroscopy and TEM. Firstly, the best conditions for TEOS polymerization in the presence of polyurethane particles were established. Secondly, polyurethane silica systems containing different silica amounts were synthesized from functionalized and non-functionalized polyurethane particles. Finally, the coupling agent effect on the homogeneity of the films obtained by casting was evaluated.
Section snippets
Materials
Isophorone diisocyanate (IPDI), 2-bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BD), poly(1,4-butylene adipate) end capped diol (PBAD) (Mn ca. 1000 g mol−1), triethylamine (TEA), dibutyltin diacetate (DBTDA), (3-aminopropyl)triethoxysilane (APTES), tetraethoxysilane (TEOS), acetone (HPLC grade), ethanol (HPLC grade) and methanol (HPLC grade) were purchased from Aldrich Chemical Corporation. All materials were used as received.
Preparation of hybrid dispersions
Waterborne polyurethane dispersions without and with APTES
Optimization of TEOS sol–gel process in the presence of polyurethane aqueous nanodispersions
The effect of temperature and TEOS concentration on the sol–gel process was studied. Table 1 shows all the conditions employed for this study.
GC was employed to quantify the amount of ethanol released during the sol–gel process. The ethanol amount can be related to the condensation rate, allowing the comparison between different systems. In order to simplify the study, the best polymerization conditions for TEOS were established in the presence of non-functionalized polyurethane particles
Conclusions
In conclusion, a series of stable waterborne polyurethane/silica dispersions, containing 2, 4 and 8 wt% SiO2 were prepared. Tetraethoxysilane was successfully polymerized in the presence of functionalized and non-functionalized polyurethane particles, which acted as colloidal templates. The best polymerization conditions for obtaining stable polyurethane silica dispersions were established (75 °C and semi-batch polymerization conditions). TEOS polymerization was confirmed by FTIR, TGA and solid 29
Acknowledgments
The authors acknowledge the University of the Basque Country, UPV/EHU (UFI 11/56), the Ministerio de Ciencia e Innovación (MAT2010-16171) and the Basque Government (Ayudas a grupos de investigación del sistema universitario vasco ITT68-13) for the funding received to develop this work. H.S. gratefully acknowledges financial support through a postdoctoral grant (DKR) from the Basque Government.
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2020, International Journal of Adhesion and AdhesivesCitation Excerpt :In contrast to the solvent–borne systems, WPUs offer a significant reduction in terms of emissions of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), and are thus able to meet stringent environmental regulations [1–5]. Consequently, they have been widely applied as adhesives and coatings [6–12]. However, compared with conventional solvent–borne polyurethanes, WPUs have some drawbacks such as relatively low heat resistance, poor mechanical properties, and poor water resistance, which inhibits their extensive application.