Effect of modified montmorillonite on biodegradable PHB nanocomposites

Abstract

Polymer nanocomposites, based on a bacterial biodegradable thermoplastic polyester, poly(hydroxybutyrate) (PHB), and two commercial montmorillonites (MT), Na-M (MT) and 30B-M (organically modified MT), were prepared by melt-mixing technique at 165 °C. Both clays minerals were characterized by morphology, crystallochemical parameters, and thermal stability. Lower specific surface area (determined by adsorption methods) values were found for 30B-M. The apparent particle size from light scattering measurements, scanning electron microscopy observations, and crystallite size (determined from XRD patterns) of 30B-M indicated a higher degree of particles exfoliation than of Na-M.

The nanocomposites PHBNa and PHB30B were characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffraction (XRD), transmission electron microscopy (TEM), mechanical properties, and burning behaviour. Intercalation/exfoliation observed by TEM and XRD was more pronounced for PHB30B than PHBNa, indicating the better compatibility of 30B-M with the PHB matrix. An increase in crystallization temperature and a decrease in spherullites size were observed for PHB30B. The intercalation/exfoliation observed by TEM and structure XRD increased the moduli of the nanocomposites. The burning behaviour of PHB30B was influenced by the aggregation of the clay mineral particles.

Introduction

Montmorillonite, hectorite and saponite are the most commonly used clay minerals for the preparation of polymer nanocomposites. Numerous studies have shown that the incorporation of small quantities of these clay minerals (5–10% in mass), with a certain degree of exfoliated structure, have a great influence on the properties of the final material, such as mechanical strength, stiffness, thermal stability, conductivity, and gas barrier properties (Ray & Okamoto, 2003, Ray & Bousmina, 2005, Tjong, 2006, Ruiz-Hitzky & Van Meerbeek, 2006, Xiong et al., 2007). The nanocomposite structure depends on the clay mineral–polymer compatibility and on processing conditions (Zenggang et al., 2002, Fornes et al., 2003, Homminga et al., 2005, Avella et al., 2006, Cervantes-Uc et al., 2007, Hablot et al., 2008). Mainly, two different structures are achievable: nanocomposites with intercalated or exfoliated clay mineral particles (Ray & Bousmina, 2005, Lagaly et al., 2006). Both structures coexist in nanocomposites, and it is believed that the properties are related to the high aspect ratio of exfoliated structure (Alexandre and Dubois, 2000). The term intercalation (Dennis et al., 2001, Lagaly et al., 2006) describes the case where a small amount of polymer moves into the interlayer space of the clay mineral particles. Exfoliation or delamination occurs when polymer further separates the clay mineral layers, e.g. by 80–100 Å or more.

The use of conventional plastics in the last years has been broadened to new areas. Due to their very low degradation rates, plastics can cause environmental problems when they are disposed as solid waste. An alternative to reduce this problem is the substitution of conventional plastics by biodegradable ones. Polyhydroxybutyrate (PHB) is an example of a biodegradable polymer, obtained from a renewable resource, showing a biodegradation rate of about 3 months after soil burial (Correa et al., 2008). The main applications of PHB are in biomedical, agricultural, and packaging products (Scott, 2002), being the last industry one of the most interested in the use of PHB in form of nanocomposites.

The aim of this study is to characterize two commercial clays minerals (one of them being organically modified) used to prepare PHB nanocomposites.