Polypropylene/calcium carbonate nanocomposites

Abstract

Polypropylene (PP) and calcium carbonate nanocomposites were prepared by melt mixing in a Haake mixer. The average primary particle size of the CaCO₃ nanoparticles was measured to be about 44 nm. The dispersion of the CaCO₃ nanoparticles in PP was good for filler content below 9.2 vol%. Differential scanning calorimetry (DSC) results indicated that the CaCO₃ nanoparticles are a very effective nucleating agent for PP. Tensile tests showed that the modulus of the nanocomposites increased by approximately 85%, while the ultimate stress and strain, as well as yield stress and strain were not much affected by the presence of CaCO₃ nanoparticles. The results of the tensile test can be explained by the presence of the two-counter balancing forces—the reinforcing effect of the CaCO₃ nanoparticles and the decrease in spherulite size of the PP. Izod impact tests suggested that the incorporation of CaCO₃ nanoparticles in PP has significantly increased its impact strength by approximately 300%. J-integral tests showed a dramatic 500% increase in the notched fracture toughness. Micrographs of scanning electron microscopy revealed the absence of spherulitic structure for the PP matrix. In addition, DSC results indicated the presence of a small amount of β phase PP after the addition of the calcium carbonate nanoparticles. We believe that the large number of CaCO₃ nanoparticles can act as stress concentration sites, which can promote cavitation at the particle–polymer boundaries during loading. The cavitation can release the plastic constraints and trigger mass plastic deformation of the matrix, leading to much improved fracture toughness.

Introduction

The use of inorganic fillers has been a common practice in the plastics industry to improve the mechanical properties of thermoplastics, such as heat distortion temperature, hardness, toughness, stiffness and mould shrinkage. The effects of filler on the mechanical and other properties of the composites depend strongly on its shape, particle size, aggregate size, surface characteristics and degree of dispersion. In general, the mechanical properties of the composites filled with micron-sized filler particles are inferior to those filled with nanoparticles of the same filler [1], [2]. In addition, the physical properties, such as surface smoothness and barrier properties cannot be achieved by using conventional micron-sized particles. In the recent years, intensive research efforts have been devoted to the development of nanocomposites [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

It is known that the mechanical properties of the composites are, in general, strongly related to the aspect ratio of the filler particles. Based on this reasoning, layered silicates, such as montmorillonite, which has a fairly large aspect ratio, have been extensively studied in recent years [3], [4], [5], [6]. Nanocomposites prepared with montmorillonite show improved strength, modulus, heat distortion temperature and barrier properties. In spite of many attractive improvements in physical and mechanical properties of the polymer/(intercalated or exfoliated) clay nanocomposites, a significant drawback—low fracture toughness—has greatly limited their engineering applications. We could hardly find any convincing evidence in the open literature reporting enhanced fracture toughness for polymer/clay composites. In most cases, a dramatic decrease in toughness due to the addition of clay has been reported. This represents a major challenge to researchers in the field of polymer toughening.

Other nanoparticles, such as silica [7], [8] and calcium carbonate [9], [10], [11], [12], have been used to prepare nanocomposites. Among them, calcium carbonate has been one of the most commonly used inorganic fillers for thermoplastics, such as poly(vinyl chloride) and polypropylene (PP). Historically, it has been used to merely reduce the cost of the expensive resins. The particle size of most commercially available CaCO₃ varies from 1 to 50 μm. The results of numerous studies have indicated that the improvement in the mechanical properties of micron-sized-CaCO₃-filled composites is found to be minimum. One of the key factors is believed to be the poor filler–polymer interaction. Many efforts have been devoted to surface-modified CaCO₃ particles [14], [15] to increase the polymer–filler interactions. The effects of surface modification on mechanical properties have been positive. The use of nano-CaCO₃ particles may bring new insights in the study of polymer–filler interaction, because of the dramatic increase in the interfacial area between the filler and polymer. In addition, when surface smoothness and high gloss are required, micron-sized CaCO₃ cannot be used. Nano-CaCO₃ particles can be good filler that can provide surface smoothness and high gloss. In addition, the mechanical properties of nano-CaCO₃-filled composites, which may be very different from those of the micron-sized-CaCO₃-filled composites, are rarely studied.

PP is one of the commodity plastics that has the highest growth rate [13]. In an early work of Levita et al. [12], the fracture toughness of PP/CaCO₃ composite with and without surface treatment was evaluated. Both untreated and surface-treated CaCO₃ with a particle size of about 70 μm was used. The authors found that the fracture toughness, in terms of the mode-I stress intensity factor (K_IC), of the PP with surface-treated CaCO₃ increased slightly. Compared with pure PP, a 20% increase in K_IC was noticed at 10% filler content. Addition of more than 10% filler, however, decreased the K_IC of the nanocomposites drastically. In a recent work reported by Rong and co-workers [9], [10], very fine SiO₂ nanoparticles (∼7 nm) were compounded with PP. The tensile strength of the nanocomposite with 0.65 vol% SiO₂ filler was 18% higher than that of pure PP. A further increase in the filler content did not have much influence on the tensile strength of the nanocomposites. The authors also reported a substantial increase in toughness owing to the incorporation of SiO₂ nanoparticles. However, it is worth noticing that the toughness reported by the authors is actually the energy-to-break measured in a uniaxial tensile test. It is well known that high tensile toughness does not necessarily mean high fracture toughness. The latter is measured with sharply notched specimens under a strictly defined test condition. Generally speaking, notched fracture toughness of a given polymer will be lower or much lower compared with tensile toughness, simply because many energy-dissipating events occurring during a plane-stress testing (such as in uniaxial tension) cannot take place easily, when the specimen is subjected under plane-strain condition (e.g. in notched fracture toughness test). Unfortunately, many catastrophic material failures in engineering applications are caused by the low plane-strain fracture toughness of the materials. Hence, the notched fracture toughness is always regarded as a critical parameter in material selection.

Another important reason to study nanoparticle-filled composites is that the fracture mechanisms for nanocomposites may be quite different from that for the composites containing micron-sized inorganic particles. The toughening of the polymers by using inorganic particles has been explained by the crack front bowing mechanism [15], [16], [17]. Because the rigid particles will resist the propagation of the crack, the primary crack has to bend between the particles. However, in the case where the size of rigid particles is of the order of 50 nm or less, the applicability of the bowing mechanism is questionable, because such small size rigid particles may not be able to resist the propagation of the crack. Hence, a new mechanism may be needed to explain any toughening effect [18], [19], if indeed it is observed for the nanocomposites.

In this study, the mechanical and thermal properties of nano-CaCO₃-filled PP were investigated. The physical and chemical properties of the nano-CaCO₃ particles were fully characterised. Fracture toughness of the nanocomposites was tested by the J-integral method and impact strength was evaluated using notched specimens following ASTM standard. The correlation between the mechanical and thermal properties of the nanocomposites and the physical and chemical properties of the nano-CaCO₃ particles was established. The toughening mechanisms involved during the fracture of the nanocomposites were proposed.