preserve the advantages of fluoride release and clini- cal adhesion of the conventional GIs yet provide the ease of light curing and good esthetics of resin-based materials. The use of nanotechnology in RMGI has resulted in enhanced esthetics of these materials.
MULTIPURPOSE RESIN COMPOSITES
The coupling agent, an organosilane (often referred to as “silane”), is applied to the inorganic particles by the manufacturer to surface treat the fillers before being mixed with the unreacted
monomer mixture. Silanes are called coupling agents, because they form a bond between the inor- ganic and organic phases of the composite. One end of the molecule contains functional groups (such as methoxy), which hydrolyze and react with the inorganic filler, whereas the other end has a meth- acrylate double bond that copolymerizes with the monomers.
The role of the initiator-accelerator system is to polymerize and cross-link the system into a hard- ened mass. The polymerization reaction can be triggered by light activation, self-curing (chemical activation), and dual curing (chemical and light curing).
Resin Matrix
METHACRYLATE MONOMERS
The vast majority of monomers used for the resin matrix are dimethacrylate compounds. Two mono- mers that have been commonly used are 2,2-bis[4(2- hydroxy-3-methacryloxypropyloxy)-phenyl]
propane [bisphenol A-glycidyl methacrylate (Bis- GMA)] and urethane dimethacrylate (UDMA).
Both contain reactive carbon double bonds at each end that can undergo addition polymerization initi- ated by free-radical initiators. The use of aromatic groups affords a good match of refractive index with the radiopaque glasses and thus provides better overall optical properties of the composites.
A few products use both Bis-GMA and UDMA monomers.
A
B
FIG. 9.1 Two-dimensional diagrams of composites with (A) fine and (B) microfine particles. (From Powers JM, Wataha JC. Dental Materials: Foundations and Applications. 11th ed.
St. Louis: Elsevier, 2017.)
O
O O O O
O
OH
Structure of Bis-GMA
OH
Structure of UDMA O
O O
O O
O
O O HN
HN
The viscosity of the monomers, especially Bis- GMA, is rather high and diluents must be added, so a clinical consistency can be reached when the resin mixture is compounded with the filler. Low- molecular-weight compounds with difunctional car- bon double bonds, for example, triethylene glycol dimethacrylate (TEGDMA), or bisphenol A ethox- ylate dimethacrylate (Bis-EMA6), are added by the manufacturer to reduce and control the viscosity of the compounded composite.
O O
O
Structure of TEGDMA
O O
O
O
O O
O O
Structure of Bis-EMA6
O O O
O O
LOW-SHRINK METHACRYLATE MONOMERS A variety of other methacrylate monomers have been used in the newer commercial products intro- duced since 2008 for controlling the volumetric shrinkage and polymerization stress of compos- ites. The general approach relies on increasing the
distance between the methacrylate groups resulting in lower cross-link density or increasing the stiff- ness of the monomers. Some examples include the use of dimer acids, incorporation of cycloaliphatic units, and photocleavable units to relieve stress after polymerization.
O
O O
O
O
O O
O
Structure of a Monomer with Cycloaliphatic Units O
O O
O
O n
O O
O
NH N
H
NH N
H n
O
O O
O
O O
O
Structure of Monomer with Photocleavable Units
O O
O
O
O O
O
The bonding agents widely used with these com- posites are also prepared from similar organic mono- mers so that they are compatible with the composites.
LOW-SHRINK SILORANE MONOMER
A new monomer system called “silorane” has been developed to reduce shrinkage and internal stress build-up resulting from polymerization. The name silorane was coined from its chemical building blocks siloxane and oxirane (also known as epoxy).
The siloxane functionality provides hydrophobicity
to the composite. The oxirane functionalities undergo ring-opening cross-linking via cationic polymeriza- tion. Special initiator systems are required for the polymerization of the siloranes (described in the sec- tion Initiators and Accelerators). Care has to be taken in choosing the filler system. If the filler surface has any residual basicity (as with some glasses and sol- gel-derived systems), the composite may become unstable. Furthermore, specific adhesive system has to be used for bonding these materials during clinical placement.
O O
O O
O
O
O O
O O O O Si Si Si
Si Si
Si Si
Siloxane
Silorane Structure of Silorane
Oxirane
Fillers and Classification of Composites
Fillers make up a major portion by volume or weight of the composite. The function of the filler is to rein- force the resin matrix, provide the appropriate degree of translucency, and control the volume shrinkage of the composite during polymerization. Fillers have been traditionally obtained by grinding minerals such as quartz, glasses, or sol-gel-derived ceramics. Most glasses contain heavy-metal oxides such as barium or zinc so that they provide radiopacity for visualization when exposed to x-rays. It is advantageous to have a distribution of filler diameters so that smaller par- ticles fit in the spaces between larger particles and pro- vide more efficient packing. Recently, nanofillers have been introduced into composites. These are described in the section Nanofillers and Nanocomposites.
A helpful method of classifying dental composites is by the particle size, shape, and the particle-size dis- tribution of the filler. This classification is presented in the following section.
MACROFILLS
The early composites were macrofills. These com- posites contained large spherical or irregular-shaped particles of average filler diameter of 20 to 30 μm. The resultant composites were rather opaque and had low resistance to wear.
HYBRID AND MICROHYBRID COMPOSITES In hybrid composites two types of fillers are blended together: fine particles of average particle size 2 to 4 μm and 5% to 15% of microfine particles, usually silica, of particle size 0.04 to 0.2 μm. In microhybrid compos- ites the fine particles of a lower average particle size (0.04 to 1 μm) are blended with microfine silica. The fine particles may be obtained by grinding glass (e.g., borosilicate glass, lithium or barium aluminum sili- cate glass, strontium, or zinc glass), quartz, or ceramic materials and have irregular shapes. The distribution of filler particles provides efficient packing so that high filler loading is possible while maintaining good handling of the composite for clinical placement.
Microhybrid composites may contain 60% to 70%
filler by volume, which, depending on the density of the filler, translates into 77% to 84% by weight in the composite. Most manufacturers report filler concen- tration in weight percent (wt%). A micrograph of a typical, fine glass filler is shown in Fig. 9.2A. Hybrids and microhybrids have good clinical wear resistance and mechanical properties and are suitable for stress- bearing applications. However, they lose their surface polish with time and become rough and dull.
NANOCOMPOSITES
Recently, the incorporation of nanotechnology in designing and manufacturing composites has greatly
improved their properties. Nanocomposites describe this class of composites. The nanofiller technology is described in the next section.
Nanofillers and Nanocomposites
The latest advancement in composite technol- ogy has been the utilization of nanotechnology in
A
C
1 mµ
1 mµ
1 mµ
B
FIG. 9.2 Scanning electron micrographs of types of filler. (A) Fine inorganic filler; (B) microfine silica filler; (C) microfine silica in organic polymer filler.
development of fillers. Nanotechnology is the pro- duction of functional materials and structures in the range of 1 to 100 nm—the nanoscale—by various physical and chemical methods. Nanotechnology requires devices and systems to create structures that have novel properties and functions because of their small sizes. Thus it implies the ability to con- trol and manipulate structures at the atomic and/
or molecular scale. Although true nanocomposites should have all filler particles in the nanometer size, the term nanotechnology has some hype associated with it and it is sometimes misused in describing a material. To date, oxide nanoparticles have been the most prevalent types of nanomaterials used in dental composites. At present, there are two dis- tinct types of resin composites available that contain nanoparticles:
1. Nanofills: These contain nanometer-sized particles (1 to 100 nm) throughout the resin matrix. Larger primary particles are not present.
2. Nanohybrids: These consist of large particles (0.4 to 5 μm) with added nanometer-sized particles. Thus they are hybrid materials, not true nanofilled composites.
NANOFILL COMPOSITES
All filler particles of true nanofilled composites are in the nanometer range. There are several purposes for incorporating nanofillers in dental composites.
First, the size of nanomeric particles is below that of visible light (400 to 800 nm), which provides the opportunity of creating highly translucent materials.
In addition, the surface area-to-volume ratio of the nanoparticles is quite large. The sizes of the smallest nanoparticles approach those of polymer molecules so they can form a molecular-scale interaction with the host resin matrix.
Two types of nanoparticles have been synthe- sized and utilized for preparing this class of com- posite. The first type consists of nanomeric particles that are essentially monodispersed nonaggregates and nonagglomerated particles of silica or zirco- nia. The surface of the nanoparticles is treated with silane coupling agents that allow them to be bonded to the resin matrix when the composite is cured after placement. Nanomers are synthesized from sols, creating particles of the same size. Because of this, if nanomeric particles alone are used to make highly filled composites, the rheological properties are rather poor. To overcome this disadvantage, one manufacturer has designed a second type of nano- filler, which is called nanocluster. The nanoclusters are made by lightly sintering nanomeric oxides to form clusters of a controlled particle size distribu- tion. Nanoclusters have been synthesized from sil- ica sols alone as well as from mixed oxides of silica
and zirconia. The primary particle size of the nano- mers used to prepare the clusters ranges from 5 to 75 nm.
It is important to remember that in a nanocluster, the nanoparticles still maintain their individual form, much as in a cluster of grapes. The clusters can be made to have a wide size distribution ranging from 100 nm to submicron level and have an average size of 0.6 μm. Fig. 9.3A shows a scanning electron micro- graph (SEM) image of a nanocluster of silica in the composite in a commercial composite after the resin matrix was removed by washing with acetone. In this material, the surface of the nanoclusters is treated with a silane coupling agent to provide compatibil- ity and chemical bonding with the resin system. Fig.
9.3B shows the micrograph image of a nanocluster composed of silica and zirconia. The differences in
ASupreme 6.0kV 16.1mm x20.0k SE[M] 2.00um
HV 15.00 kV mag
1 832 x HFW 81.4 µm Det
ETD WD
14.0 mm vac mode High vacuum
B Quanta 20020µm
FIG. 9.3 (A) Scanning electron micrograph image of a nanocluster of silica in a commercial composite 3M ESPE Filtek Supreme. (B) Image of a nanocluster of zirconia- silica in Filtek Supreme Ultra. (A, Courtesy Dr. Jorge Perdigao, University of Minnesota. B, From Rodrigues Jr SA, Scherrer SS, Ferracane JL, Della Bona A. Microstructural characterization and fracture behavior of a microhybrid and a nanofill composite.
Dent Mater. 2008;24(9):1281–1288.)
particle architecture of nanomers, nanoclusters, and conventional microhybrid fillers are readily appar- ent from transmission electron micrographs (TEMs) of composites prepared from these fillers. Fig. 9.4A shows the TEM image of a nanocomposite filled with 75-nm diameter nanoparticles only; Fig. 9.4B is that of a nanocomposite filled with a mixture of nanoclusters alone; and Fig. 9.4C is of a conventional microhybrid composite. To date, there is only one true nanofilled dental composite available. In this manufactured composite, a combination of nano- meric particles and nanoclusters is used in optimum combinations. Fig. 9.5A shows a schematic diagram of this nanocomposite containing a blend of nanoclu- ster and nanomeric fillers, whereas Fig. 9.5B shows a TEM of the nanocomposite showing the presence of the two types of nanofillers.
The uniqueness of the nanofilled composite is that it has the mechanical strength of a microhybrid but at the same time retains smoothness during service like a microfill. The initial gloss of many restoratives is quite good but in hybrid composites (microhybrids, nanohybrids) plucking of the larger fillers causes loss of gloss. By contrast, in the nanofilled composite, the nanoclusters shear at a rate similar to the surround- ing matrix during abrasion. This allows the restora- tion to maintain a smoother surface for long-term polish retention. Optical analysis of the polish reten- tion can be done using atomic force microscopy after extended toothbrush abrasion.
Nanofillers also offer advantages in optical prop- erties. In general, it is desirable to provide low visual opacity in unpigmented dental composites. This allows for the creation of a wide range of shades and
A B C
FIG. 9.4 (A) Transmission electron micrograph (TEM) image of composite with nanomeric particles (×60,000 magnifica- tion). (B) TEM image of composite with nanocluster particles (×300,000 magnification). (C) TEM image of composite with hybrid fillers (×300,000 magnification). (A, B, and C From Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. J Am Dent Assoc. 2003;134(10):1382–1390.)
B
Nanomers
Nanoclusters Nanocluster
Nanomer
A
FIG. 9.5 (A) Schematic diagram of a nanofilled compos- ite containing nanoclusters and nanomers. (B) Transmission electron micrograph image of a nanocomposite with nanocluster and nanomeric fillers in Filtek Supreme Plus.
(Courtesy 3M Company, St. Paul, MN.)
opacities so the clinician can design a highly esthetic restoration. In hybrid types of composites, the filler particles are 0.4 to 3.0 μm in size. When particles and resin are mismatched in the refractive index, which measures the ability of the material to transmit light, the particles will scatter light and produce opaque materials. Nanomeric fillers particles are far smaller than the wavelength of light, making them unmea- surable by refractive index. When light enters, long wavelength light passes directly through and mate- rials show high translucency. As shown by Fig. 9.6, the discs made with hybrid and microfill fillers are rather opaque. The nanofill composite sample made predominantly with the nanomeric filler is quite clear, as the background can be easily seen through the composite. In addition, when placed on a black background, the nanomeric and nanocluster parti- cles preferentially scatter blue light, giving the com- posite an opalescent effect. This gives a more lifelike appearance because natural enamel also exhibits the same effect.
The ability to create a nanocomposite with very low opacity provides the ability to formulate a vast range of shade and opacity options from the very translucent shades needed for the incisal edge and for the final layer in multilayered restorations to the more opaque shades desired in the enamel, body, and dentin shades. This allows the clinician the flexibility of choosing a single shade or a multi- shade layering technique depending on the esthetic needs. The wear resistance of this material after 3 and 5 years of clinical use was found to be similar to human enamel.
NANOHYBRID COMPOSITE
Several manufacturers have placed nano-sized particles in their microhybrids. These composites have been described as nanohybrids. Because the smoothness and wear of any composite is often determined by the size of its largest filler particles as with microhybrids, the surface of nanohybrids becomes gradually dull after a few years of clinical service.
Interfacial Phase and Coupling Agents
For a composite to have successful clinical perfor- mance, a good bond must form between the inor- ganic filler particles and the organic resin matrix during setting. This is achieved through the use of compounds called coupling agents, the most com- mon of which are organic silicon compounds called silane coupling agents. The surface of the filler is treated with a coupling agent during the manufac- ture of the composite. A typical silane coupling agent is 3-methacryloxypropyltrimethoxysilane, the chem- ical structure of which is shown below.
O O
O
Structure of 3-Methacryloxypropyltrimethoxysilane O
O Si
In the low-shrink silorane composite, an epoxy functionalized coupling agent, 3-glycidoxypropyltri- methoxysilane, is used to bond the filler to the oxi- rane matrix.
O O Si
O
Structure of 3-Glycidoxypropyltrimethoxysilane
O O
During the filler treatment process, the methoxy groups hydrolyze to generate hydroxyl groups through an acid-or base-catalyzed reaction. These hydroxyl groups then undergo condensation with the hydroxyl groups on the surface of the filler and become attached by covalent bonds (see the follow- ing schematic sketch). Condensation is also possible with the adjacent –OH groups of the hydrolyzed silanes or with water absorbed on the surface of the filler. This results in the formation of a very thin monolayer or multilayer polymeric film on the sur- face of the filler with unreacted double bonds. During the curing of the composite, the double bonds of the methacryloxy groups of the treated surface coreact with the monomer resins. The coupling agent plays a critical role in the composite. Its functions are as follows:
• It forms an interfacial bridge that strongly binds the filler to the resin matrix.
• It enhances the mechanical properties of the composite and minimizes the plucking of the fillers from the matrix during clinical wear.
Hybrid Microfill Nanocomposite
FIG. 9.6 Translucency of a hybrid composite, microfill composite, and a nanocomposite. (From Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced den- tal materials. J Am Dent Assoc. 2003;134(10):1382–1390.)
• The resulting interfacial phase provides a medium for stress distribution between adjacent particles and the polymer matrix.
• It provides a hydrophobic environment that minimizes water absorption of the composite.
R
OCH3
R
Matrix
Filler
R R R
O O O
OH
nCH3O-Si-OCH3 nHO-Si-OH Si-O-Si-O-Si
Initiators and Accelerators
The curing of composites is triggered by light or a chemical reaction, with the former being more com- mon. Light activation is accomplished with blue light at a peak wavelength of about 465 nm, which is absorbed usually by a photosensitizer, such as cam- phorquinone, added to the monomer mixture during the manufacturing process in amounts varying from 0.1% to 1.0%.
In methacrylate composites, free radicals are generated upon activation. The reaction is acceler- ated by the presence of an organic amine. Various amines have been used, both aromatic and aliphatic.
Examples of two such amines are shown below. The amine and the camphorquinone are stable in the pres- ence of the oligomer at room temperature, as long as the composite is not exposed to light. Although cam- phorquinone is the most common photosensitizer,
others are sometimes used to accommodate special esthetic considerations. Camphorquinone adds a slight yellow tint to the uncured composite paste.
Although the color bleaches during cure, sometimes clinicians find shade matching difficult with the color shift.
O O
O
O N
Camphorquinone A typical amine
Chemical activation is accomplished at room tem- perature by an organic amine (catalyst paste) reacting with an organic peroxide (universal paste) to produce free radicals, which in turn attack the carbon double bonds, causing polymerization. Once the two pastes are mixed, the polymerization reaction proceeds rapidly.
Some composites, such as core and provisional products, are dual-cured. These formulations contain initiators and accelerators that allow light activation followed by self-curing or self-curing alone.
In the silorane composite, the initiator system generates cations when irradiated with light. One of the components is the camphorquinone pho- tosensitizer enabling the composite to be cured by a dental curing unit. Other components of the initiation system are iodonium salts and electron donors, which generate the reactive cationic spe- cies that start the ring-opening polymerization process.
I O +
O
+ +
Electron-DonorA-
Reactive cationic species
Pigments and Other Components
Inorganic oxides are usually added in small amounts to provide shades that match the majority of tooth shades. The most common pigments are oxides of iron. Numerous shades are supplied, ranging from very light shades to yellow to gray. Various color scales are used to characterize the shades of the com- posites. A UV absorber may be added to minimize color changes caused by oxidation. Darker and more opaque shades of composites cannot be cured to the same depth as the lighter translucent shades.
Fluorescent agents are sometimes added to enhance the optical vitality of the composite and mimic the appearance of natural teeth. These are dyes or pigments that absorb light in the UV and vio- let region (usually 340 to 370 nm) of the electromag- netic spectrum, and reemit light in the blue region (typically 420 to 470 nm). These additives are often used to enhance the appearance of color causing a perceived “whitening” effect, making materials look less yellow by increasing the overall amount of blue light reflected.