Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites
Introduction
To cope with the obvious limitations of polymers, for example, low stiffness and low strength, and to expand their applications in different sectors, inorganic particulate fillers, such as micro-/nano-SiO2, glass, Al2O3, Mg(OH)2 and CaCO3 particles, carbon nanotubes and layered silicates, are often added to process polymer composites, which normally combine the advantages of their constituent phases. Particulate fillers modify the physical and mechanical properties of polymers in many ways. This review is concerned with the latter properties, which are stiffness, strength and fracture toughness. The manufacturing processes and techniques for such particulate–polymer composites are, however, not covered here.
It has been shown that dramatic improvements in mechanical properties can be achieved by incorporation of a few weight percentages (wt%) of inorganic exfoliated clay minerals consisting of layered silicates in polymer matrices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Commonly used layered silicates have a thickness of ∼1 nm and lateral dimensions of ∼30 nm to several microns or larger. The large aspect ratios of layered silicates are thought to be mainly responsible for the enhanced mechanical properties of particulate–polymer nanocomposites. There have been many papers on layered silicate reinforced polymer composites including some reviews [1], [2], [13] and hence the case of layered nanoparticles will not be discussed here. In contrast, much attention has been paid to carbon nanotubes (CNTs) as reinforcing fillers for polymers. There are many research papers and several reviews on the mechanical properties of CNT reinforced polymer nanocomposites [13], [14], [15]. Improvements in mechanical properties have been observed by adding a few wt% of CNTs. In these studies, both layered silicates and CNTs have high aspect ratios.
Polymer composites containing particles with a small aspect ratio of 1 or thereabout have also been studied extensively because of their technological and scientific importance. Many studies have been conducted on the mechanical properties of these particulate-filled polymer composites. Stiffness or Young’s modulus can be readily improved by adding either micro- or nano-particles since rigid inorganic particles generally have a much higher stiffness than polymer matrices [16], [17], [18], [19], [20], [21], [22], [23], [24]. However, strength strongly depends on the stress transfer between the particles and the matrix. For well-bonded particles, the applied stress can be effectively transferred to the particles from the matrix [25]; this clearly improves the strength [16], [26], [27], [28], [29], [30]. However, for poorly bonded micro-particles, strength reductions occur by adding particles [17], [18], [19], [31], [32], [33], [34], [35], [36], [37]. The drawback of thermosetting resins is their poor resistance to crack growth [38], [39], [40], [41]. But inorganic particles have been found to be effective tougheners for thermosetting resins [22], [42], [43]. Though they do not increase the toughness as dramatically as rubber particle inclusions [44], [45], they increase the elastic modulus and hardness much better than rubber particles. In contrast, most studies on thermoplastics filled with rigid particulates reported a significant decrease of fracture toughness compared to the neat polymers [19], [35], [46], [47], [48]. There are, however, several studies that show toughness increase with introduction of rigid particles in polypropylene [49], [50] and polyethylene [50], [51], [52], [53], [54], [55], [56], [57]. Impressively enhanced impact toughness has been reported for polyethylene filled with calcium carbonate particles by Fu and Wang [53], [54], [55], [56] and Bartczak et al. [57]. Enhancement of impact properties of some pseudo-ductile polymers by the introduction of inorganic particles has also been achieved [57], [58].
The mechanical properties of particulate–polymer composites depend strongly on the particle size, particle–matrix interface adhesion and particle loading. Particle size has an obvious effect on these mechanical properties. For example, smaller calcium carbonate particles provide higher strength of filled polypropylene composites at a given particle loading [14]. Sumita et al. [58] underlined the interest of replacing microscale silica by its nanoscale counterpart, since nanoscale silica particles possess superior mechanical properties. They found that these nanoparticles give higher rigidity and superior yield strength to the polymers. Smaller particle size yields higher fracture toughness for calcium carbonate filled high density polyethylene (HDPE) [57]. Similarly, alumina trihydrate filled epoxy containing smaller particles show higher fracture toughness [21]. Particle–matrix interface adhesion and particle loading are two important factors that also affect mechanical properties. For example, the tensile strength of glass bead filled polystyrene composites depends on the particle–matrix adhesion and increases with it [17]. Thus, the use of coupling agents that increase the particle–matrix adhesion leads to higher strength [22], [38], [42], [59], [60], [61], [62]. When chemical treatment was applied to the silica particles in HDPE, the toughness of the filled polymer was significantly improved [46]. The strength of polyimide/silica composites increases with particle loading to 10 wt% [16] and decreases beyond that. However, their modulus increases monotonically with silica particle loading [16]. Moreover, the fracture toughness of glass bead filled epoxy composites increases initially with increasing filler loading till a plateau value is reached at a critical particle volume fraction [63].
From the preceding paragraphs, it is clear that the mechanical properties of particulate-filled polymer micro- and nano-composites are affected by particle size, particle content and particle/matrix interfacial adhesion. This review will focus on how these factors influence the mechanical properties of polymer micro- and nano-composites containing fillers with a small aspect ratio of approximately one. Meanwhile, theoretical models that have been proposed to predict elastic modulus, strength and fracture toughness of particulate–polymer composites are also critically examined.
Polymer composites are noted to show mechanical properties which depend on time, rate and temperature [64]. Viscoelastic moduli are mainly governed by the volume fraction of particles [65] and strain rate has important effects on matrix/particulate interface adhesion and other mechanical properties [66], [67], [68], [69]. Whilst this is a relevant topic, it is beyond the scope of this review and thus will not be discussed here. Interested readers may refer to the cited Refs. [64], [65], [66], [67], [68], [69] and others in the published literature.
Section snippets
Young’s modulus
Young’s modulus is the stiffness (the ratio between stress and strain) of a material at the elastic stage of a tensile test. It is markedly improved by adding micro- and nano-particles to a polymer matrix since hard particles have much higher stiffness values than the matrix.
Strength
The strength of a material is defined as the maximum stress that the material can sustain under uniaxial tensile loading. For micro- and nano-particulate composites this relies on the effectiveness of stress transfer between matrix and fillers. Factors like particle size, particle/matrix interfacial strength and particle loading that significantly affect the composite strength are discussed below.
Fracture toughness
Fracture mechanics concept to material design considers the effects of cracks and defects on strength. Strength concept works with critical stresses but in fracture mechanics the critical crack length is an extra parameter. There are two approaches to determine fracture toughness which is a material property: stress intensity approach and energy approach. The first approach yields a fracture toughness (Kc) which relates the crack size to fracture strength. The energy approach provides a
Concluding remarks
A critical review of the experimental results and theories of the mechanical properties including modulus, strength and fracture toughness of polymer based particulate micro- and nano-composites is presented. The effects of particle size, particle/matrix adhesion and particle loading on composite stiffness, strength and toughness of a range of particulate composites having both micro- and nano-fillers with small aspect ratios of unity and thereabout are examined in detail. It is shown that
Acknowledgements
Financial support from the National Natural Science Foundation of China (Grant Nos.: 50573090, 10672161, 10525210 and 10732050), the Overseas Outstanding Scholar Foundation of the Chinese Academy of Sciences (Grant Nos.: 2005-1-3 and 2005-2-1), the 973 Project (2003CB615603), the Australian Research Council and the Alexander von Humboldt Foundation is gratefully acknowledged.
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