Effects of Filler Size on the Mechanical Properties of Polymer-filled Dental Composites: A Review of Recent Developments

Resin composites are widely used in esthetic restorative dentistry. Since their introduction in the mid-1960s, these composites made steady gains in popularity. Their increased use is attributed to their excellent biocompatibility, absence of taste, odour, tissue irritation and toxicity, insolubility in body fluids, easy operation, excellent aesthetic properties, stable colures, optical properties, easy pigmentation, low cost and repairability. The composite resins in current use still suffer from several shortcomings such as poor mechanical properties. In order to improve these properties, microparticles have been used as fillers for a long time. However, the inadequate mechanical properties of resin composites remain problematic. Recently, researchers have utilised nanoparticles as dental composite fillers. This article reviews the relevant literature on the mechanical properties of polymer dental composites filled with microand nano-scale particles. The effects of particle size on fracture toughness, flexural strength, and hardness were examined with emphasis on other important factors for improvement. The second section focused on the toughening mechanisms of particulate-polymer composites.


INTRODUCTION
Polymer-based composites have been widely used in dentistry since their introduction in the late 1950s.Recently, nano-composites were introduced as dental polymers. 1,24][5] Excellent biocompatibility, superior aesthetic qualities as fillings, moderate cost compared with ceramics, and strong bonding ability to the tooth structure, make resin composite the preferred material in dental restorative applications. 1,6,7Most dental composites consist of an organic matrix (polymer phase), inorganic fillers (dispersed phase) and interphase (coupling agent). 80][11] The filler is added to enhance polymer properties and usually consists of different compositions, sizes and size distributions of glass or ceramic particles, nanotubes, whiskers, fibres and nanoclusters. 12,13e coupling agent such as silane is designed to strongly bond the matrix to the filler, thus improving composite performance.The most commonly used silane in dental restorative composites is 3-methacryloxypropyltrimethoxysilane (γ-MPS).Filler content, type, shape, size and morphology are important factors enhancing the desirable mechanical properties of dental composites. 14Multiple fillers have been employed in dental composites to improve strength, toughness and durability.However, some problems continue to persist, such as inadequate mechanical properties, water uptake, polymerisation shrinkage, and poor wear resistance of large occlusal restorations during use. 1,15,16Dental composites fail because of surface and/or bulk cracks, degradation of the matrix and fillers, water uptake, and insufficient mechanical properties. 14The degradation of the bond between the fillers and resin after long-term water absorption is the main reason for the failure of dental composites. 17ding filler nanoparticles to the resin matrix of dental composites improves aesthetic, optical and mechanical properties, such as tensile strength and resistance to fracture, as well as reduces polymerisation shrinkage. 18Moreover, nanoparticles enhance wear resistance and gloss retention and also improve the fatigue properties of dental composites. 7Reduced interparticle spacing may increase obstacles for dislocation motions and decrease strain localisation. 19However, nano-composite properties are significantly affected by various factors, including the degree of conversion of the polymer matrix and interphase, which requires a high level of silanisation because of the high surface area of nanoparticles. 19,20This review focuses on several mechanical properties of dental composites, namely fracture toughness, flexural strength and hardness.However, the effect of particle/matrix interface adhesion and particle loading on the mechanical properties of polymer dental composites are not covered here.Fracture toughness strongly depends on toughening mechanisms, such as crack deflection, crack pinning, matrix-filler interactions and crack bridging, which increase crack propagation resistance.
Fillers with smaller particle sizes can improve flexural strength because of increased particle surface area, which results in a high surface energy at the fillermatrix interface.Hardness is readily improved by adding either micro-or nanoparticles because rigid inorganic particles generally have considerably higher stiffness than polymer matrices.The novelty of the present paper is that, no review has been done on the effects of different scales of fillers (i.e., nano-and microfillers) on the mechanical properties (fracture toughness, flexural strength and hardness) of polymeric dental composites.It elaborates on the effects of toughening mechanisms in improving the mechanical properties of particulatepolymer composites at these two scales.This review article aims to present the results of recent efforts to improve the mechanical properties of polymer-filled dental composites, compare and discuss in depth the reinforcing effects of nanoand micro-particles, as well as provide some basic understanding of the toughening mechanisms of these composites.To our knowledge, no review papers on these topics have been published yet.

The Effect of Particle Size on Fracture Toughness
Fracture toughness is a fundamental property of material to predict the strength of material when a crack is present. 21It is expressed with the critical stress intensity K IC . 22The crack driving force and critical value (fracture toughness) are equated, to obtain the relationship between applied load, crack size and structure geometry which provide the necessary information on structural design. 23The units of K IC are units of stress (force/length 2 ) × units of length 1/2 , or force × length −3/2 and are often reported as MNm −3/2 or MPa m 1/2 . 24The measurement of this fracture mechanics was applied to a number of problems associated with dental materials.It analysed the behaviour of materials containing cracks or flaws.These flaws and cracks may grow naturally or nucleated after a time in service, and sudden fractures can occur at stresses below the yield stress.Such fractures exist in brittle materials that are unable to plastically deform and redistribute stresses.The fracture mechanics analyses are performed during these types of failures. 5In the neat resin, there is a high stress concentration in front of the notch.Whereas composites with welldistributed nanoparticle have more uniform stress distribution, thus enhancing the toughness. 25corporating fillers in the polymer matrix increases fracture toughness, elastic modulus and tensile strength. 5,26Particle size has a distinct effect on the mechanical properties of particulate-polymer composites. 27,28Fu et al. reported that particle size significantly affected the fracture toughness of particulate-filled polymer composites. 27According to Tanimoto et al., increasing filler particle size increased the fracture toughness of resin-modified glass ionomers. 29In another study, adding up to 50 wt% glass particles with average sizes of 105-210 μm to acrylic bone cement significantly increased fracture toughness. 33Dental composites filled with nanoparticles showed enhanced fracture toughness. 34Nanoparticle fillers can be dispersed uniformly in polymer matrix, which increases fracture toughness compared with micro-filled polymer composites. 35,36Theoretical results obtained by Chan et al. indicated an increase in fracture toughness of dental composites because of silanisation and nanoparticle loadings. 37Ahmed and Ebrahim concluded that adding nano-sized ZrO 2 particles significantly increased the fracture toughness of PMMA denture base. 38The fracture toughness of PMMA resin for provisional restorations, increased remarkably with addition of 0.25 wt% of SiO 2 nanoparticles with average size of 12 nm. 39tanabe et al. demonstrated that hybrid and nanoparticle composites had significantly higher fracture toughness compared with micro-filled composites, particularly at high-volume fractions. 40Hosseinalipour et al. investigated the mechanical properties of bisphenol A-glycidyl methacrylate/triethylene glycol dimethacrylate (Bis-GMA/TEGDMA) dental composites reinforced with SiO 2 nanoparticles with sizes of 20-50 nm.Their results showed significantly increased mechanical properties compared with a conventional composite control containing SiO 2 particles with sizes of 10-40 μm.The fracture toughness of GMA/TEGDMA dental composite remarkably increased compared with that of the control when the weight fraction of the filler increased to 40 wt%, indicating the significance of the filler weight fraction in determining the mechanical properties of composites. 7owever, Elsaka et al. reported that glass ionomer filled with 3 wt% and 5 wt% TiO 2 nanoparticles with average sizes of 21 nm had improved fracture toughness compared with the unmodified glass ionomer. 41These findings are supported by Protopapa et al., who observed a significant increase in the fracture toughness of PMMA dental composite filled with a low-volume fraction of nanodiamond particles. 424][45][46] Silanisation and nanoparticles improve the fracture toughness of dental polymer nano-composites through a combination of enhanced interface toughness through silanisation, crack deflection and crack bridging. 35The fracture toughness of dental composites can be increased by improving the interfacial bond between the nanoparticles and matrix through a larger surface area-to-volume ratio and high particle strength. 37Du et al. illustrated that Al 2 O 3 nanoparticles (8 nm) with fixed filler content (1 wt%) were well dispersed in polyester resin and promoted crack front trapping that increased fracture toughness. 47The increase of volume-specific debonding energy also increases crack resistance with reduced particle size.Particles near the crack plane under high stresses are too small to be debonded from the matrix, indicating the importance of particle size distribution. 48o summarise, filler particle size significantly affects the fracture toughness of polymer dental composites.This property of dental composites can be improved by incorporating nanoparticle fillers at low concentrations, in addition to several factors.

The Effect of Particle Size on Flexural Strength
The addition of ceramic fillers to dental composites improves flexural strength. 49,50he particle size of fillers significantly affects the mechanical properties of particulate-polymer composites. 28,51Incorporating 50 wt% Al 2 O 3 (<10µm) in dental composites increased flexural strength by more than 100%. 52Tanimoto et al. reported that adding 70 wt% SiO 2 (3.3 µm) increased the flexural strength of dental composites, whereas adding larger microparticles reduced flexural strength (4.3, 7.9 and 15.5 µm). 29Two similar studies also reported that adding HA (18.1 µm) to PMMA denture base reduced flexural strength. 53,54According to Oral et al., the flexural strength of groups reinforced with large microparticles (>315 µm) decreased. 55ioactive glass (315 to 1000 µm) and biostable glass (915 to 1000 µm)

Decrease
As shown in Table 2, composites reinforced with high-volume fractions of small microparticles have high flexural strength values.The improvement in flexural strength can be attributed to the increased surface area of filler particles because of reduced particle size, which results in high surface energy at the filler-matrix interface.Meanwhile, the reduction in flexural strength can be attributed to the following factors: 1. Increased stress concentration at the interface between the filler and polymer matrix as a result of increased particle size. 29 Poor interfacial interaction between the matrix and filler.Mechanical interlocking is the only bonding mechanism holding the filler in the matrix because of the cooling shrinkage of the matrix.54 3. Agglomerations of the filler act as stress concentration points and lead to inefficient stress distribution; therefore, more stress is concentrated on adjacent particles, causing cracks in the material.53,54 4. Agglomerations restrict molecular motion in the polymer under load-bearing applications, causing deformation.54 5. Presence of particle-matrix interfacial defects. According to Foroutan et al., dental nanocomposites reinforced with three loadings (10, 20 and 30 wt%) of Al 2 O 3 (25-40 nm) had significantly increased flexural strength.52 Two similar studies evaluated the effects of adding TiO 2 nanoparticles (<20 nm and 21 nm) to dental composites at different low loadings (<20 nm: 0.5 and 1 wt%; 21 nm: 3, 5 and 7 wt%).The mixture consisted of HA (5 wt% and 10 wt%) and each percentage was added to 1% of Al 2 O 3 (i.e., 0, 3, 6 and 8 wt%).The hybrid nano-composites with 5 wt% and 10 wt% of HA and 6 wt% of Al 2 O 3 had the maximum flexural strength.60 Two similar studies evaluated the effects of incorporating silver (Ag) particles (38 nm) into PMMA denture base.61,62 Ag filler was incorporated at very low loadings (0.5 wt%) and (0.05 wt% and 0.2 wt%).The results showed an insignificant increase in flexural strength.Table 3 shows the positive effects of nanoparticles on the flexural strength of dental composites.  66 Tesults of these four studies showed that neat resins have higher flexural strength than nano-composites.Table 4 shows the negative effects of nanoparticles on the flexural strength of dental composites.As shown in Tables 3 and 4, composites filled with nanoparticles at low/high loadings exhibit high flexural strength, whereas some composites at low loadings have decreased flexural strength.The increased flexural strength of dental composites reinforced with nano-sized fillers can be attributed to five reasons: [69][70][71][72] 2. Formation of a strong bond between inorganic fillers and organic matrix. 27,35,72his bond is formed by covering the fillers with a functional silane coupling agent, such as MPS, to chemically link fillers with the matrix. 35,73,74The chemical linkage contains a siloxane bond between the filler and silane, as well as a covalent bond between the reactive groups of the matrix and organofunctional group of silane. 75 Decreased particle size at the same volume fraction results in covalent linkage, strong physical interaction, and increased contact area, which enhance the interfacial adhesion between matrix and nanofiller.52,72,76,77 Furthermore, smaller sizes cause more particles to share the applied stress in a specific region.77 These factors result in effective stress transfer from the soft resin to the hard nanofiller.52,72,76,77 4. Increased rigidity and decreased ductility of nano-composites result from the addition of highly rigid nanoparticles, such as nano-Al 2 O 3 and nano-TiO 2 , as well as the capability of these nanoparticles to withstand higher stresses.72,78,79 5. Uniformly dispersed nanoparticles prevent crack propagation and significantly improve flexural strength.27,58,72,80 Flexural strength may decrease because of the following reasons: 1. Nano-sized oxides affect the internal structure of polymerised by acting as impurities. 60,62,63. Dispersed nanoparticles within the acrylic resin decrease the degree of conversion and increase the amount of residual unreacted monomer that acts as plasticiser. 63,65 Agglomerated nanofillers in the matrix may enhance crack propagation.Under applied load, slippage may exist within the agglomerate.81,82 The surface area for the interaction between nanofillers and matrix decreases and fractures in the agglomerate sites are initiated.66,83 4. Decreased cross-section of the load-bearing matrix.60,61,84,85 5. Changes in the modulus of elasticity of the matrix and crack propagation mode of the sample because of increased filler content.[84][85][86] 7. Incomplete wetting of the filler by the matrix. However, polymer composition (chemical formulation) also significantly affects flexural strength.62 Dental composites showed increased flexural strength when Bis-GMA or TEGDMA was replaced with urethane dimethacrylate (UDMA) and decreased flexural strength when Bis-GMA was replaced with TEGDMA. 87 To summae, low contents of nanoparticles and high contents of small microparticle fillers can increase the flexural strength of dental composites.Particle size should be selected based on the properties affected by the filler volume fraction, such as required viscosity and curing shrinkage.Polymer composition is another factor with a marked effect on flexural strength.

The Effect of Particle Size on Hardness
9][90][91][92][93] The addition of 38.16 micro-sized Nb 2 O 5 particles to dental adhesive resin with loadings of 5, 10 and 20 wt% significantly increased hardness at 20 wt%, followed by 10 wt%. 94Vojdani et al. incorporated Al 2 O 3 with an average particle size of 3 µm into PMMA denture base; they noted that hardness significantly increased at 2.5 wt% and 5 wt%, followed by 0.5 wt% and 1 wt%. 85Variance results showed a decrease in the surface hardness of PMMA denture base reinforced with 10 wt% and 20 wt% ZrO 2 with particle sizes of 5-10 µm. 95Moreover, filler size affects the hardness of composites. 968][99] Liu et al. reported a significant increase in the hardness of dental composite reinforced with SiO 2 with a mean size of 30 nm and filler volume fraction of 1.5 wt%. 1 In another study, researchers added modified and unmodified TiO 2 nanoparticles (<20 nm) to dental composite at a filler volume fraction of 0.5% and 1%.Both groups showed increased hardness values. 58Moreover, Prentice et al. incorporated YbF 3 and BaSO 4 (25 nm and <10 nm) in glass ionomer cement at loadings of 1 wt% and 2 wt%.Both groups reported insignificant increases in hardness values. 100Incorporating ZrO 2 particles with average sizes of 5-15 nm in PMMA denture base at different loadings (1.5, 3, 5 and 7 wt%) significantly increased hardness at 7 wt%. 38Table 5 shows the effects of filler size on the hardness of dental composites.As shown in Table 5, hardness increased as the filler volume fraction increased.Microparticles improved hardness at relatively higher concentrations than nanoparticles.In addition to particle size, several factors significantly enhance the hardness of dental composites, such as: 1. Inherent properties of some filler particles, such as Al 2 O 3 and ZrO 2 .These particles exhibit strong ionic interatomic bonding to confer favourable properties, such as high hardness. 1,38,60,85Moreover, SiO 2 , Al 2 O 3 and TiO 2 nanoparticles show elastic, rather than plastic, deformation under indentation load. 104[107][108][109] 3. Strong interfacial interactions between the modified nanoparticles and polymer. 45,100,110 Uniform dispersion of nanoparticles provides enough distances between the particles, increasing composite reinforcement and hardness.45,96 5. Harder filler particles exhibit higher surface hardness in the composite.111,112 Therefore, filler particle size and filler content, in addition to various factors, can affect the hardness of dental composites.

Crack Deflection
4][115][116] This leads to increased fracture toughness because of nonplanar cracks. 117Crack deflection is a shielding mechanism that increases fracture resistance by reducing the stress intensity factor at the crack tip. 113Schematic drawing of crack deflection is shown in Figure 1.

Crack Pinning
This mechanism suggests that when crack propagation meets inorganic particles, crack propagation becomes pinned and bows out between the filler particles by generating secondary cracks.8][119] This mechanism has been detected in micro-and nano-composites. 120,121It occurs easily at a nanoscale level, particularly with more reduced interparticle distance resulting from relatively high nanofiller content. 119Medina et al. performed fractography on a nano-composite to illustrate that adding nanoparticles induced crack pinning. 115oreover, the river-like lines in the nano-composites possibly resulted from crack pinning and the blocking effects of nanoparticles. 119

Matrix-filler Interactions
The strength and toughness of the particulate-filled, polymer micro-and nanocomposites are strongly affected by bonding integrity at the filer/matrix interface and thus the stress transfer between the fillers and the matrix. 51,122,123Under perfect bonding conditions, a large quantity of energy is consumed at the filler-matrix interface. 123,124The existence of a thin and high-strength interphase layer results in effective stress transfer and causes crack deflection and propagation in the matrix.However, a thick and low-strength interphase layer causes crack propagation and crack blunting in the interphase material. 125Figure 3 shows a schematic drawing of filler-matrix interphase.

Crack Bridging
Crack bridging occurs because of the interparticle/intercluster crack growth when particles connect the crack faces at the crack wake. 43,113,116These uncracked bridges sustain part of the load, increasing fracture resistance. 114,126Essentially, the crack bridging mechanism minimises the stress concentration at the crack tip and therefore works as an extrinsic toughening source. 114Furthermore, when load is increased, a microcrack is created at some distance from the main crack, and an uncracked bridge exists between the microcrack and the main crack.The microcrack occurs very near to the tip of the main crack and grows in both directions, whereas the main crack stops propagating.Finally, both cracks will meet each other because of the extension of the microcrack.Toughening ceases at this point.However, for small particle-filled composites, crack bridging is not an expected powerful factor enhancing toughness. 43Crack deflection and bridging often work in harmony, given that crack deflection usually leads to crack bridging. 113,114Schematic drawing of crack bridging is shown in Figure 4.

CONCLUSION
This review article compares and highlights the effects of micro-and nano-scale particles on the mechanical properties, including fracture toughness, flexural strength and hardness, of particulate dental resin composites.Many types of nanofillers, such as SiO 2 , ZrO 2 , TiO 2 , Al 2 O 3 , BaSO 4, HA, Ag, YbF 3 and nanodiamond, have been used in dental composites.The results of the conducted review showed that the mechanical properties of dental nano-composites with lower filler loadings are superior to microfilled dental composites.The effects of nanoparticles strongly depend on many factors, such as the type and mechanical properties of inorganic nanofillers, uniform dispersal of nanofillers within the polymer matrix, volume fraction of the filler particles, and type of silane used.The key control parameters to enhance fracture toughness are toughening mechanisms (i.e., crack deflection, crack pinning/bowing, matrix-filler interactions and crack bridging).Good flexural strength requires effective stress transfer from resin to nanofiller, whereas increased filler volume fraction is needed for adequate hardness.
Therefore, future works on this subject demand a proper and systematic investigation on the effects of particle loading, morphology (shape) and particle/matrix interface adhesion that directly contributed to the enhancement of mechanical properties especially fracture toughness and hardness.The reason is obvious since composite strength depends on the load transfer between filler and matrix, and stiffness depends highly on particle loading.However, specific attention should be made on the particle shape of fillers since it has a remarkable effect on some mechanical properties of the dental composites.
31Asar et al. found that adding Al 2 O 3 , TiO 2 and ZrO 2 fillers with average sizes of 12.4, 9.6 and 8.6 µm, respectively, at different percentages (1% TiO 2 and 1% ZrO 2 , 2 wt% Al 2 O 3 , 2 wt% TiO 2 and 2 wt% ZrO 2 ) significantly increased the fracture toughness of poly(methyl methacrylate) (PMMA) denture base composites.31In the study, the fracture toughness of the test groups was significantly higher than that of control group (p < 0.05). Thst group containing 2 wt% ZrO 2 had the highest fracture toughness among all groups (p < 0.05) and increased fracture toughness by 30% compared with the control group.31

Table 1 :
The effect of filler size on fracture toughness of dental composites.

Table 2 :
The effect of microparticles on flexural strength of dental composites.
59Many researchers had attempted to enhance the mechanical properties of PMMA denture base.A recent study by Ahmed and Ebrahim evaluated the flexural strength of PMMA denture base reinforced with different low concentrations(1

Table 4 :
The negative effect of nanoparticles on flexural strength of dental composites.
601,102Safarabadia et al. evaluated the hardness of PMMA denture base reinforced with hybrid nanofillers consisting of HA and Al 2 O 3 (30/80 nm).Their results revealed that HA/Al 2 O 3 (10/8 wt%) significantly increased hardness.60Balos et al. ound that using SiO 2 with agglomerate size of 50 nm at very low loading (0.023 wt%) increased the hardness of PMMA denture base composites.

Table 5 :
The effect of filler size on hardness of dental composites.