Modification with tertiary amine catalysts improves vermiculite dispersion in polyurethane via in situ intercalative polymerization

Abstract

In situ intercalative polymerization is an environmentally-friendly process to produce polymer-clay nanocomposites with good clay dispersion. In this work, quaternary ammonium salts with tertiary amine groups were synthesized to modify clay as catalytically active modifiers for polyurethanes. Polyol dispersions with the catalyst-modified vermiculites were prepared by ultrasonication and examined by X-ray scattering and rheology. In the polyurethane elastomers synthesized from the above dispersions in a solvent-free process, X-ray scattering showed that the clay was highly intercalated/exfoliated without noticeable peaks for d < 9 nm, and TEM images revealed that the local dispersion of clay sheets was affected by the structure of the modifier. Addition of the modified clay to the polyurethane elastomers had only a small effect on the phase separation between hard and soft segments as deduced from DSC, FT-IR and DMA results. Thermo-mechanical and barrier properties of the composites were evaluated, and with one modifier, the nanocomposites showed a 390% increase in tensile modulus at 25 °C and a 40% reduction in CO₂ permeability at a loading of 5.3 wt% of catalyst-modified clay.

Introduction

Polymer-clay nanocomposites (PCNs) have attracted much attention ever since the pioneering work by Toyota that demonstrated the exfoliation of clay in polyamide-6 more than two decades ago [1], [2]. Since then, extensive studies have been carried out to investigate the reinforcing effects of clay in other polymer matrices, such as polyolefins, epoxies, polyesters, and polyurethanes (PU) [3]. PCNs exhibit improved mechanical properties, thermal stability, reduced flammability, and improved barrier properties [3], [4]. It is recognized that the compatibility between clay particles and either the monomers or the polymer matrix is crucial to achieve nanodispersion of clay. Therefore, to improve the compatibility, hydrophilic clays are modified by organomodifiers, commonly quaternary alkylammonium ions, that render the clay surface organophilic. More complex, multifunctional organomodifiers can also be designed to enhance the interfacial interactions between the organoclay and the polymer or specifically aid the interaction/exfoliation process. Reactive modifiers can form covalent bonds with polymer chains, creating a stronger interface; this approach has shown positive effects in a number of polymers, including polystyrene [5], polycarbonates [6], and polyurethanes [7].

To aid the clay dispersion and exfoliation, one can also modulate the relative reaction rate between intragallery and extragallery regions of the clay by intercalating a catalytically active modifier between clay sheets. Ziegler–Natta type catalysts can be fixed into the interlayer space by ion exchange [8], [9] or by anchoring them on the modifiers [9], [10], [11], enabling clay intercalation and exfoliation during in situ polymerization of olefins or acrylates. Alkylammonium modifiers have been shown to exhibit catalytic effects in epoxy systems [12], and Jana suggested that clay was exfoliated in epoxy by the elastic force of the growing polymer chain balanced by the viscous force of the extragallery matrix [12]. The polymerization reaction occurs faster in the intragallery space of catalytically modified clay. Due to the confined space, polymer chains exert an elastic force on the clay sheets, which acts as the driving force to break clay aggregates apart.

PU elastomers are among the most widely-used industrial polymers with great resistance to abrasion and solvents, and much effort has been expended to improve their mechanical, thermal, and barrier properties by incorporating organoclays [13], [14]. Extensive studies have been carried out to determine the effects of various modifiers [7], [15], [16], [17], [18], [19], [20] and processing parameters [21], [22], [23], [24] on clay dispersion in PU elastomers. However, most previous studies of elastomeric PU-clay nanocomposites required large amounts of solvents to achieve good clay dispersion/exfoliation, resulting in products with a high content of harmful volatile organics and preventing industrial implementation. This has driven us to pursue an alternate approach to achieve clay exfoliation without the use of solvents to swell and disperse nanoclays. The in situ intercalative polymerization process used in this study is an environmentally-friendly and promising route to produce PCNs [3]. Although this bulk polymerization approach has been employed to form nanoclay-filled PU elastomers, intercalated rather than exfoliated nanocomposites are usually obtained when clays are premixed with polyols [15], [22], [23], [25]. Pattanayak and Jana carried out comprehensive studies on clay dispersion in bulk polymerized thermoplastic polyurethane (TPU)-clay nanocomposites, and they found clay exfoliation was possible if reactive group-modified clays were added at the late stage of polymerization when the viscosity is high to produce high shear stress [21], [26], [27], [28]. However, for thermoset PU elastomers, such a late-mixing route is not practical because the functionality of polyols is higher than two and gelation may occur before the clay is dispersed well. Therefore, it is desirable to achieve clay exfoliation via a conventional in situ intercalative polymerization process for general PU elastomer syntheses. Given the fact that the abovementioned catalytically active modifiers have shown to promote clay exfoliation in various polymers, it is surprising that for PU elastomers, effects of catalytic modifiers have not yet been investigated. For rigid PU foams, one study describes organoclays modified with a grafted organotin catalyst, which provided better clay exfoliation [29].

Here, we explore the efficacy of interlayer tertiary amines as novel catalytic modifiers to enhance clay exfoliation in bulk polymerized thermoset PU elastomers. The type of clay studied is vermiculite (VMT), which has a structure similar to montmorillonite (MMT) but a higher charge density [30]. Generally used as packaging materials, VMTs are much less studied than MMTs as nanofillers, but could be another low cost alternative to MMTs for PCNs. PUs were synthesized starting from a polyol-OVMT dispersion, and this conventional process is well-suited for both thermoplastic and thermoset elastomers. All reagents for the modifier and the PU synthesis employed in this study were commercial products, making this solvent-free approach viable for industrial implementation. The structures of the modifiers and the OVMTs were characterized, and the dispersion of VMT in polyol monomer and PU nanocomposites was evaluated. Mechanical and diffusion barrier properties of the nanocomposites were also tested. A modifier was identified that raised the tensile modulus of nanocomposites at 25 °C by 390% and reduced CO₂ permeability by 40% at a loading of 5.3 wt% of catalyst-modified clay.