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DENTAL COMPOSITESDENTAL COMPOSITESChemistry and DesignChemistry and Design

Stephen C. BayneStephen C. BayneDepartment of Operative DentistryDepartment of Operative DentistrySchool of DentistrySchool of DentistryUniversity of North CarolinaUniversity of North CarolinaChapel Hill, NC 27599Chapel Hill, NC 27599--74507450

© 2004, Bayne and Thompson, UNC School of Dentistry.

Dental composites are an excellent example of composite theory in action. The objectives are to review composite theory, examine the specific components of composites (matrix phase, filler phases, interfacial coupling agents, bonding systems), and consider the importance of the interfaces (internal and external).

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20,0

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REVIEWRule-of-Mixtures

Composite theory is based the rule-of-mixtures (simple version or modified rule). The simple approach of the rule-of-mixtures (ROM) is that the “whole is equal to the sum of the parts” or “the properties of the composites are equal to the volume fraction contributions of the components.”Most composites can be envisioned as based on a solid continuous phase (matrix) and solid dispersed phase (filler). In almost all cases, the solid dispersed phase is one with the better properties.Therefore, as you increase the content of filler, the properties improve. The ROM can be written as a mathematical equation (X = X1V1 + X2V2) or represented as a 2-dimensional graph that shows the property of interest versus the volume of phases. As an example, if the compressive strengths of the matrix and filler phases are 20 MPa and 400 MPa respectively, and you make a composite that contains 50 volume percent (v/o) of each one, then the overall composite is predicted to have acompressive strength of X = X1V1 + X2V2 = (20 MPa)(50 v/o) + (400 MPa)(50 v/o) = (10 MPa) + (200 MPa) = 210 MPa.

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REVIEWModified Rule-of-Mixtures

Most composites depend not only on the contributions of the components (composition factors) but also on the “quality of the interfaces” involved with energy transfer between the phases (bonding factors), the arrangement of the phases with respect to each other (arrangement factors such as particle sizes and distributions), and defects (defect factors such as pores and cracks). All of these can be incorporated into the ROM to create a modified-rule-of-mixtures (MROM). This can be represented as X=(X1)(V1)(I1) + (X2)(V2)(I2) or as a graph in which the actual relationship between properties and composition is less that the straight line in the ROM case. Porosity has an exponential effect and is included as a special term in the mathematical equation. The quality of the interfaces (both internal and external) can vary from physical, mechanical interdigitation, mechanical interlocking, pseudo-chemical, to chemical.

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REVIEWPolymer Classification Systems

Matrices in actual dental composites are almost always acrylic polymers (created from acrylic monomers). It is quite common to classify these polymers (and therefore estimate their properties) on the basis of 9 standard sub-classifications as shown above. Linear polymers are relatively weak compared to crosslinked ones (which are typical for dental composites). The setting reaction of these materials involves the free radical polymerization (activation, initiation, propagation, termination) of difunctional acrylic monomers (e.g., BIS-GMA or BIS-GMA-like monomers).

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95-98% Conversion 55-65% Conversion

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REVIEWPolymerization and Properties of Key Monomers

As a reminder, consider the extremes for acrylic monomer systems in dentistry (linear PMMA versus crosslinked BIS-GMA). The strengths for BIS-GMA polymer are just about twice as much as for PMMA. Even so, BIS-GMA could produce a stronger composite if the actual degree of conversion was advanced much closer to 100%. With PMMA the monomer units are much smaller and mono-functional so they can continue to diffuse to the growing polymer chain ends and become incorporated. With BIS-GMA, as soon as one end is tied into one chain, the other end of the molecule is severely restricted. It can no longer diffuse and must simply be lucky enough that a growing polymer chain encounters it to become consumed. The unreacted chain ends tend to plasticize the polymer.

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DISPERSED PHASEinterface

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Coupling Agent

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CONTINUOUS PHASE

GENERALIZED VIEWComposite Restoration Structure

As a starting point for consideration of the chemistry of dental composites, imagine a simple view of the microstructure for the margin of a composite restoration. To the left in the figure above is enamel. The surface above reflects both unfinished and finished portions. Below this small portion is more composite that interfaces with dentin. The major portion of the composite consists of ceramic particles (non-crystalline SiO2) coated with silane coupling agents (to chemically bond to the particles and to the matrix) which is dispersed in a mixture of difunctional acrylic monomers (mostly BIS-GMA-like monomers). After polymerization the solid may contain defects that are within the matrix phase, at the surfaces of particles, and/or at the external interface with tooth structure.

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AVERAGE COMPOSITIONAuto-Cured (Self-Cured) Composite

Here is a relatively simple example of an actual dental composite formulation that is designed to be self-curing (chemically curing). Note the overall complexity (and sophistication) of this formulation. We will examine the details of each component shortly.

The continuous phase contains primarily high MW monomer (e.g., BIS-GMA) that has been diluted with some low MW monomer so that the mixture is usable and will flow. In addition, the matrix phase includes initiator (to provide free radicals), an accelerator )to chemically break down the initiiator into free radicals), a retarder (to cancel out unwanted free radicals that arise before the reaction is intended to start), and UV stabilizers (to keep the materials from undergoing discoloration on exposure to UV rays or other oxidizing events). Note that the weight percent of resin is about 31 percent. By volume this would be about 50 percent. Originally, the BIS-GMA manufactured for use by dental companies was produced by Freeman Chemical Corporation in Wisconsin under the code of Nupol 46-4005. Most of the BIS-GMA used today is produced by Esschem in Philadelphia. Composites contain difunctional monomers other than BIS-GMA which are often purchased but sometimes produced by dental manufacturers themselves as well.

The dispersed phase is almost entirely silica filler (non-crystalline) with minor amounts of colored ceramic or other colorants.

Particles of ceramic are chemically coupled to the matrix phase by right of being coated with silane coupling agents. The silane that is used is A-174 and has been the primary one for almost all dental composites for 40 years. It has one end that reacts with hydroxyls on the surface of glass particles. The other end contains a double bond that is capable of co-polymerizing with BIS-GMA.

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CHEMISTRY OF COMPOSITESMatrix Phases

Now lets examine the chemistry of each of the components just discussed – high MW monomers, low MW monomers, polymerization control additives, filler particles, and silane coupling agents.

BIS-GMA was the first high MW monomer to be incorporated into commercial dental composites. Its advantage is that with relatively high molecular weight, the polymerization shrinkage associated with the reaction of its double bonds (one at each end of the molecule) has relatively little effect on the overall mass. A simple molecule such as MMA with its double bond being a major portion of the molecule, undergoes 21 v/o shrinkage on polymerization. BIS-GMA undergoes about 6-7 v/oshrinkage. Since the matrix represents about 50%, the overall composite shrinkage would be about 3.5%.

BIS-GMA is an acronym for the reaction product between one molecule of Bis-Phenol-A and two molecules of Glycidyl Methacrylate. The center of the molecule contains two aromatic rings which add stiffness to the structure. This is the same basis of many commercial epoxy compositions. The unfortunate property of BIS-GMA is that it is very viscous and requires the addition of a diluent monomer to make the composition flowable. It also contains a couple of unwanted side-groups that are hydroxyls that make the material absorb small amounts of water. That water tends to plasticize the composition and lower the elastic modulus. Finally, this monomer cannot be purified. It contains a variety of unwanted reactants. Most monomers are purified by crystallization – and BIS-GMA does not crystallize. Therefore, it is about 93% monomer and 7% original reactants that can be biologically irritating if leached out.

Because of these problems, a variety of BIS-GMA like molecules have been synthesized. These include ethoxylated BIS-GMA to remove the hydroxyls and lower the water absorption. Other versions are described on the next slide.

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CHEMISTRY OF COMPOSITESMatrix Phases (cont.)

Terephthalate dimethacrylate is version that is more aliphatic in the central portion to reduce the monomer viscosity. However, it is not quite as stiff either. A variety of other analogues have been evaluated.

Probably the most successful alternative is known as urethane dimethacrylate (UDMA). Is has a linear structure in the middle based on urethane linkages. It can be purified, may make color stability easier, but still has many of the other properties of BIS-GMA-like systems. For all practical purposes, BIS-GMA and UDMA systems are about equal. In the US over many years, BIS-GMA-like monomers have been very popular. European composites are more often based on UDMA matrices.

To make any of these systems thin enough to be easily placed and manipulated, a diluent or thinning monomer must be added. This is a low MW monomer. While one could use just about anything that is soluble in the system, it is best if a monomer is chosen that has many of the same features as BIS-GMA. The most popular diluent monomer is triethylene glycol dimethacrylate (TEGDMA or TEGDM). A typical dental composite contains about 70% high MW monomer and about 30% low MW monomer (TEGDM). The lower MW monomer undergoes more polymerization shrinkage by nature of the fact that its double bonds represent a larger portion of the molecule. Therefore, if you shift the mixture to 50:50 then you get one with even more shrinkage, and that is less desirable.

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CHEMISTRY OF COMPOSITESMatrix Phases (cont.)

To control the reaction, a variety of polymerization control additives are incorporated. We will talk later about different types of curing systems (Self- or Chemically-cured; UV-light cured, Visible-light cured). Depending on the curing method, the components may either be separate components that must be mixed before use, or be together within a single component.

The most popular initiator for self-cured systems is benzoyl peroxide (BPO). It is accelerated withN,N-dimethyl-p-toluidine (NNDMPT) or N,N-dihydroxyetheyl-p-toluidine (DHPT or NNDHPT). The acrynyms can be confusing and do not follow a consistent pattern of abbreviations. If the system is a UV-light cured one (not used any more in dentistry) then methyl ether of benzoin (MEB) is used as the initiator instead of BPO. Most current composites are visible-light cured (VLC) systems and employ camphoroquinone (CQ) as the initiator in combination with dimethaminoethylmethacrylate (DMAM) accelerator. The advantage of this particular accelerator is that it becomes polymerized into the matrix and is less likely to cause discoloration of the system over time.

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CHEMISTRY OF COMPOSITESFiller Phases

Quickly review the major points about ceramics, classification, and compositions. A simple approach to classifying ceramics (metal and non-metal components) is to consider their arrangement (crystalline or non-crystalline), non-metallic component (oxide or non-oxide), metallic component (silicate or non-silicate type), and degree of alloying (main structure or derivative structure). For dental composites, the fillers are primarily non-crystalline silicates (silicate glasses).

A wide range of filler particle geometries has been explored – including spherical beads rods, plates, irregular particles, and porous particles. In most past cases, fillers are produced by grinding large blocks of silicate glass into finer and finer particle sizes. Therefore, the particles end up roughly round in shape (approximately equiaxed) but not necessarily smooth. We will focus on irregular particles for the next part of this discussion.

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CHEMISTRY OF COMPOSITESFiller Phases (cont.)

1 µm 0.1 µm 0.01 µm

MINIFILLER

MICROFILLER

Particle sizes (and ranges) can be easily classified in terms of orders of magnitude (e.g., on the basis of 10-fold differences). Remember that a bacteria is typically about 1 µm in diameter. Early composites were based on relatively large filler particles (macro-fillers) that had particle sizes in the range of 10-100 µm. Smaller fillers (midi-fillers) immediately became more popular to improve composite finishing characteristics (1-10 µm). Next dentistry manufactured composites with only very fine fillers, microfillers (0.01-0.1 µm). However, the particles were so fine that their high ratio of surface-area-to-volume increased the friction between the particles and surrounding resin to the point of increasing the viscosity into unusable ranges. Therefore, the filler contents were quite limited in those compositions. Microfill composites had good finishing characteristics but were weaker due to the lower filler content. Next, manufacturers began to combine filler particle types (hybrids = mixture of two filler particle types) using midi-fillers and micro-fillers. Mini-fillers have been very hard to produce and only recently were part of commercial composites. Compositions now exist that would be called mini-fill hybrids. We will discuss these in detail in the next presentation (Composites: Manipulation and Properties).

Filler particles that are based on silica have the distinct disadvantage of being radiolucent. If you take a radiograph of a tooth containing such a composite restoration, it will be impossible to distinguish the difference between the composite and any dissolved tooth structure next to the restoration. Therefore, it is very important, whenever possible, to use radiopaque composites. This is done by either alloying radiopacifying materials (e.g., Li2O or Al2O3 with SiO2) or adding radiopaque ceramics (e.g., BaSO4). The latter method is not recommended because only SiO2 containing particles can be bonded by silane coupling agents.

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CHEMISTRY OF COMPOSITESInterfacial Coupling Agents

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Silane coupling agents are relatively small molecules that must be added to the surfaces of the filler particles in advance of the filler being mixed into the monomer matrix. Silane is mixed into water, acidified, washed onto the particle surfaces, heated to encourage reaction, and rinsed. As shown above, silane coupling agent has a double bond on one end (left) and three methoxy groups on the other end (right). The methoxy groups can condense (etherify) with pendant hydroxyls on the surface of the silicate filler particles. Methanol is produced as a by-product and eliminated. On average only about 1.5 of the 3.0 methoxy groups actually react with the surface. While the silanation step has always been suspected to be poorly controlled in composite production (i.e., there is a lot of black magic here), most composites show good evidence of chemical bonding at the interfaces. Under rigorous basic conditions (pH>8.0) it is possible to reverse this reaction and degrade the silane. That type of condition is rarely encountered intraorally.

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SiOSiO22 inCeramic

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SILANESManaging the problems: (1) bonding, (2) multilayers, and (3) dimerization.

One of the practical problem with coupling agents such as silanes is that they do not act as neatly as the design shown on paper. Ideally, they should form a monomolecular layer onto a surface, completely react with the surface, and completely react with the matrix phase. However, silane does not completely cover the surface and only partially reacts. While it is capable of reacting at three points on the molecule, that is almost never realized. The likelihood of forming a mono-molecular film is very low. More times than not, the film is many molecular layers thick and so a silane shell is formed that is not as strong as the neighboring composite. The shell is predominately a silane polymer rather than a simple bridging layer. Finally, excess silane can react with itself to form a dimer. It will then behave as a difunctional molecule that effectively dilutes the local polymer and reduces the overall composite strength.

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DESIGNING COMPOSITESClass Exercise

ACTUAL COMPOSITION:

Matrix:> High MW Monomer = > Low MW Diluent =

Curing System:> Initiator = > Accelerator = > Inhibitor =

Filler:> Particle Type 1 and Distro = > Particle Type 1 and Distro = > Total Filler Level = > Coupling Agent: =

ACTUAL COMPOSITION:

Matrix:> High MW Monomer = > Low MW Diluent =

Curing System:> Initiator = > Accelerator = > Inhibitor =

Filler:> Particle Type 1 and Distro = > Particle Type 1 and Distro = > Total Filler Level = > Coupling Agent: =

COMPOSITE TYPE: Esthetic

NAME =

Components in Kit:> > > >

COMPOSITE TYPE: Posterior

NAME =

Components in Kit:> > > >

Now let’s put all of this design knowledge to work and design two different types of composites. We will produce a strong composite intended for use in posterior sites (called COM-POST ☺) and an anterior composites with fine finishing characteristics for excellent esthetics (called COMP-EST ☺). You will need to make the choices for these materials.

For the “posterior composite,” what type of high MW and low MW components do you want to use (pick discrete things) and how much of each should you combine (percentages). If it is to be VLC composite, what would you add for an initiator? What type of accelerator would you add? What type of inhibitor would you add? What type of filler (compositions and particle size) and how much would you use? What type of coupling agent would be needed?

For the “anterior composite,” what type of high MW and low MW components do you want to use (pick discrete things) and how much of each should you combine (percentages). If it is to be VLC composite, what would you add for an initiator? What type of accelerator would you add? What type of inhibitor would you add? What type of filler (compositions and particle size) and how much would you use? What type of coupling agent would be needed?

If you wanted to package and sell your composite, what would you put into the package (or kit)? How would you supply the composite (in jars, tubes, unidose compules)? Should you put a bonding system in the kit to be used with the composite? How many shades of composite should you provide? How will you match shades? Will you include finishing and polishing aids? Should there be an instruction sheet? Should there be an MSDS?

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ThankYou

Now let’s put all of this design knowledge to work and design two different types of composites. We will produce a strong composite intended for use in posterior sites (called COM-POST ☺) and an anterior composites with fine finishing characteristics for excellent esthetics (called COMP-EST ☺). You will need to make the choices for these materials.

For the “posterior composite,” what type of high MW and low MW components do you want to use (pick discrete things) and how much of each should you combine (percentages). If it is to be VLC composite, what would you add for an initiator? What type of accelerator would you add? What type of inhibitor would you add? What type of filler (compositions and particle size) and how much would you use? What type of coupling agent would be needed?

For the “anterior composite,” what type of high MW and low MW components do you want to use (pick discrete things) and how much of each should you combine (percentages). If it is to be VLC composite, what would you add for an initiator? What type of accelerator would you add? What type of inhibitor would you add? What type of filler (compositions and particle size) and how much would you use? What type of coupling agent would be needed?

If you wanted to package and sell your composite, what would you put into the package (or kit)? How would you supply the composite (in jars, tubes, unidose compules)? Should you put a bonding system in the kit to be used with the composite? How many shades of composite should you provide? How will you match shades? Will you include finishing and polishing aids? Should there be an instruction sheet? Should there be an MSDS?