Layered particle-based polymer composites for coatings: Part I. Evaluation of layered double hydroxides

https://doi.org/10.1016/j.porgcoat.2008.09.021Get rights and content

Abstract

Layered double hydroxides (LDH) suitable as fillers for the formulation of waterborne polyurethane (WPU) nanocomposites in coating applications are designed and characterized. Their elaboration follows a simple and reproducible process leading to samples without impurity. The attention is paid to the impact of the LDH nature (MxAl/CO32−, M = Mg and/or Zn, and x = 2, 3 and 4) on the structure characteristics, i.e. cell parameters and coherent domain dimensions. Focusing on two end-member phases M2Al/CO32−, M = Mg or Zn, the microstructural characterization performed from X-ray diffraction peak profile analyses permits to point out larger coherent domain sizes for Zn2Al species than for Mg2Al ones, and then to correlate with the “macroscopic” crystallinity of the samples. The evolution of LDH slurries over time is tentatively considered in a prediction interest. The stability of a chosen organic–inorganic hybrid, taken Mg2Al as inorganic host structure with anions of the 4-aminobenzene sulfonic acid (4-ABSA), is studied as function of its carbonate contamination in time. Finally, the dispersion of LDH fillers in WPU is scrutinized in terms of WPU/LDH structure revealed by indirect and direct observations, XRD and TEM, respectively.

Introduction

Since the first works of Toyota's R&D in the 1990s [1], [2], polymer nanocomposites based on intercalated and exfoliated layered particles have increasingly attracted research activities due to improved properties of the resulting polymer materials like stiffening, diffusion barrier and thermal stability [3], [4], [5], [6]. The vast majority of such nanocomposites is based on smectite clays bearing an anionic layer charge, like montmorillonite or hectorite. One of the main challenges for an effective dispersion of these particles in the polymer matrix still remains the design of the interphase between polymer and particle by usually rendering the particle surface less polar, and by increasing the interlayer spacing through the chemical modification of silanol groups, or via an ion exchange reaction using organic cations.

Inversely charged layered particles are easily accessible in the form of synthetic layered double hydroxides (LDH), and a special attention is now paid to LDH materials. Different authors have actually pointed out their versatility for fabricating nanocomposites [7], [8], [9], [10], [11], as the lamellar nature of LDH permits host–guest chemistry and intercalation reactions, which invoke considerable attention from material designers [12], [13], [14]. In comparison to smectite-type materials, widespread in nature but generally difficult to synthesize, LDH are naturally not abundant but rather easily to prepare. The two main differences between these species are (i) the polyhedra layer building and (ii) the layer charge density range.

Regarding the first point, smectite-type materials belong to the 2:1 phyllosilicate class of minerals for which the layer consists of central octahedral sheet sandwiched between two tetrahedral sheets. Cations such as Al3+, Fe3+ and Mg2+ are located in the first polyhedra, whereas Si4+ and Al3+ are present in the second one. For smectite of ideal composition MinterMOhMTdO10(OH)x.nH2O, the occupancy may be di- or tri-octahedral such as montmorillonite, Mx(Al2−xMgx)Oh(Si4)TdO10(OH)2·nH2O, and hectorite Mx(Mg3−xLix)Oh(Si4)TdO10(OH)2·nH2O, respectively. Due to isomorphic substitution of the site by cations of lower valence, a negative charge is present in the layers, compensated by the presence of cations in the interlayer space. Conversely, LDH materials, also referred to hydrotalcite, are described according to the ideal formula [M1−xIIMxIII(OH)2]intrax+ [Ax/mm·nH2O]inter, where MII and MIII are the metallic divalent and trivalent cations, A the anions, intra and inter denote the intra- and interlayer domains. The structure consists of brucite-like layers built from edge-sharing M(OH)6 octahedra. Partial MII to MIII substitution induces positive charges within the layers, which are counterbalanced by interlamellar anions.

Depending on the divalent to trivalent metal ratio present in the LDH layer, the ion exchange capacity is tunable in a wide range between 450 to 200 meq/100 g, while smectite-type materials usually are restricted to an exchange capacity close to 100 meq/100 g. In the latter case, it is associated to an average area per charge unit of 70 Å2, whereas it is ranging between 25 and 40 Å2 for LDH with MII/MIII compositions usually between 2 and 4. That pictures hydroxide layers tightly stacked via attractive forces with the interlayer anions filling the gallery. This situation is unfavorable for an exfoliation process to occur; however, new synthetic routes have been developed in different media [15], [16], [17], [18]. In polymer nanocomposites, intercalation and exfoliation require either external forces which separate the layers, e.g. shear stress, and/or thermodynamical driving forces like a gain in entropy or favorable interactions at the interface between platelets and polymer.

Additionally, the lateral dimensions of LDH platelets ranging from 100 nm to 1–2 μm, with a width of 1–3 nm for the organo-modified hybrid systems, give rise to aspect ratios in between 100 and 2000, comparable to smectite ones and interesting for the increase of barrier properties and for the mechanical reinforcement in nanocomposites.

Academic studies have actually shown that LDH filler compete well with smectite-type filler as exemplified with polymethylmethacrylate [19], [20], polyimide [21], [22], and epoxy [23], [24]. For polyurethane-based nanocomposites, the use of LDH is scarcely reported and mainly in connection with the permeability to gases and liquids [13] or in the field of corrosion-resistant primer coating composition [25], [26]. In fact, different aspects make LDH very promising fillers for various new applications in combination with polymer matrices: their crystallinity and cationic layer charge density can be varied over a broad range depending on the cation composition and the MII/MIII ratio. Besides, their lateral dimensions as well as their surface chemistry are tunable via the synthesis parameters and the anions chosen [27]. Different nanocomposite structures can be envisaged as depicted in Scheme 1.

In this study, the aim is to gain basic knowledge on LDH materials evaluated as fillers for nanocomposites suitable for coatings. The metallic cations selected for the layer composition are the divalent Mg2+ and/or Zn2+, and the trivalent Al3+. An in-depth structural characterization is firstly performed on inorganic LDH phases intercalated with carbonate anions, hereafter named MxAl/CO32−, M = Mg and/or Zn, x = 2, 3 and 4. Then, a focus is done on M2Al/CO32−, M = Mg or Zn, to get better insights on the influence of the LDH composition on coherent domain size, aspect ratio, and evolution of these characteristics with time. The latter criterion is not often evaluated, although it constitutes an important prerequisite for a successful transfer into industrial products and applications. A hybrid LDH organic–inorganic phase is afterwards characterized, the intercalated molecule being the anion of the 4-aminobenzene sulfonic acid (4-ABSA). Finally, the dispersions of Mg2Al/A, A = CO32− or 4-ABSA, into a waterborne polyurethane are compared.

Section snippets

Materials

MgCl2·6H2O (Acros, 99%), ZnCl2 (Acros, 98%), AlCl3·6H2O (Acros, 99%), Na2CO3·10H2O (Acros, 99%), NaOH (Aldrich, 97%), and 4-aminobenzenesulfonic acid (4-ABSA) (Aldrich, 99%) were used as-received. Deionized water was employed throughout all the experiments.

The aqueous polyurethane dispersion Daotan® VTW 1225 was provided by Solutia Inc. (St. Louis, MO), it includes hydroxy functionalities (hydroxy value of 47 mg KOH/g as indicated by the supplier), available for crosslinking with the methylated

Accurate structural characterization

The inorganic LDH phases MxAl/CO32− elaborated by varying M (Mg and/or Zn) and x (=MII/MIII = 2, 3 and 4) were characterized by X-ray diffraction. The resulted diffraction patterns (as exemplified in Fig. 3) were refined applying the Rietveld method to get accurate structural information on the phases, the corresponding data are reported in Table 1 and plotted in Fig. 4.

The cell parameter a, or interatomic distance between two neighbour cations in a layer plan (Fig. 4a), is related to the ionic

Conclusion

Inorganic and hybrid organic–inorganic layered double hydroxides (LDH) have been designed and thoroughly characterized to further evaluate their performance as fillers in nanocomposite formulations for coatings application [41]. The inorganic LDH phases MxAl/CO32− elaborated by varying M (Mg and/or Zn) and x (= MII/MIII = 2, 3 and 4) were studied by means of X-ray diffraction (Rietveld refinement and microstructural analysis). A detailed study was performed on Mg2Al/CO32− and Zn2Al/CO32−

Acknowledgements

The authors would like to thank Vanessa Prévot for the SEM observations. A.L.T. thank the BASF Coatings Company for funding through a post-doctoral fellowship.

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      The patterns are characteristic of organo-LDHs with basal reflections (0 0 l) at 2θ-low angles, due to the layered structure, and with the “saw-toothed” reflections (1 1 0 and 1 1 3) at higher angles of 2θ, indicating of the intra-layer structural ordering. The position of reflection (1 1 0) informs on the cell parameter a = 2d110, which may indicate any deviation for the non-stoichiometry with respect to the formation of the theoretical LDH [41]: the experimental results obtained are 3.06 Å for Zn/Al and 3.02 Å for Mg/Al, consistent with the fact that Zn2+ has an ionic radius higher (0.74 Å) than Mg2+ (0.65 Å) and that the cation ratio charged MII/MIII is 2 [37]. Zn2Al-HPPA and Mg2Al-HPPA present no significant differences in the interlayer distance (c′), i.e. the gallery size between two consecutive inorganic brucite type slabs, its common value of 17.4 Å, is in agreement with similar system containing p-hydroxycinnamic acid interleaved in Zn2Al-LDH by coprecipitation [42] and Zn2Al-HPPA prepared by ion-exchange [35].

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