1. DENTAL COMPOSITES Chemistry and Design Stephen C. Bayne
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).
Stephen C. Bayne
Professor and Chair
Dept of Cariology, Rest Sciences, & Endo
School of Dentistry
University of Michigan
Ann Arbor, MI
 2006, Bayne and Thompson.

2. REVIEW Rule-of-Mixtures 120,000 20,000
Composite theory is based the rule-of-mixtures (simple version or modified rule). [CLICK] 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. [CLICK] As an example, if the compressive strengths of the matrix and filler phases are 20,000 psi and 120,000psi, respectively, and you make a composite that contains 50 volume percent (v/o) of each one, then the overall composite is predicted to have a compressive strength of X = X1V1 + X2V2 = (20,000 psi)(50 v/o) + (120,000 psi)(50 v/o) = (10,000 psi) + (60,000 psi) = 70,000 psi.

3. Modified Rule-of-Mixtures
REVIEW
Modified Rule-of-Mixtures
Most composite behavior relies not only on the contributions of the components (composition factors) but also on the [CLICK] “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). The graph generated for a two-phase system is no longer the straight line predicted by the ROM but is a curve that shows a negative departure. [CLICK] This can be represented as X=(X1)(V1)(I1) + (X2)(V2)(I2). [CLICK] 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.

4. Polymer Classification Systems
REVIEW
Polymer 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). [CLICK] The setting reaction of these materials involves the free radical polymerization [CLICK] (activation, initiation, propagation, termination) of difunctional acrylic monomers (e.g., BIS-GMA or BIS-GMA-like monomers).

5. Polymerization and Properties of Key Monomers
REVIEW
Polymerization 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 an even stronger composite if the actual degree-of-conversion was advanced much closer to 100%. [CLICK] For MMA forming PMMA, the monomer units are small and mono-functional -- so they can continue to diffuse to where ever the growing polymer chain ends are and become incorporated. [CLICK] 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.
95-98% Conversion
55-65% Conversion 2X

6. Composite Restoration Structure
GENERALIZED VIEW
Composite Restoration Structure
interface
DISPERSED PHASE
Coupling Agent
CONTINUOUS PHASE
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 (as shown above). 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.
[CLICK] 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).
[CLICK] 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.
Now, let’s look at the specific chemistry of each of the major internal portions of a composite.

7. Auto-Cured (Self-Cured) Composite
AVERAGE COMPOSITION
Auto-Cured (Self-Cured) Composite
A relatively simple example of an actual dental composite formulation that is self-curing (chemically curing) is shown above. Even so, note the overall complexity (and sophistication) of this formulation. We will examine the details of each component shortly.
[CLICK] The continuous phase contains primarily high MW monomer (e.g., BIS-GMA) that has been diluted with some low MW monomer [CLICK] so that the mixture is usable and will flow. In addition, the matrix phase includes [CLICK] 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 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.
[CLICK] The dispersed phase is almost entirely silica filler (non-crystalline) with minor amounts of colored ceramic or other colorants.
[CLICK] 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.

8. CHEMISTRY OF COMPOSITES
Matrix Phases
BIS-GMA was the first high MW monomer to be incorporated into commercial dental composites. [CLICK] Its advantage is that since it is relatively high in molecular weight, [CLICK] 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 per unit volume. 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/o shrinkage. 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. [CLICK] Because it was originally discovered by Dr. Ray Bowen (a dentist and chemist) it is called Bowen’s resin. 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. [CLICK] 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.

9. CHEMISTRY OF COMPOSITES
Matrix 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). [CLICK] 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). [CLICK] It is lower viscosity because the middle of the molecule is linear. [CLICK] 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 [CLICK] 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.

10. CHEMISTRY OF COMPOSITES
Matrix 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 included together within a single component.
The most popular initiator for self-cured systems is benzoyl peroxide (BPO). [CLICK] It is accelerated with N,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 [CLICK] in combination with dimethaminoethyl methacrylate (DMAM) accelerator. [CLICK] 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. [CLICK] Butylated hydroxytoluene is included as well as a retarder to prevent premature polymerization.

11. CHEMISTRY OF COMPOSITES
Filler Phases
Quickly, let’s review the major points about ceramics, classification, and compositions. [CLICK] 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). While a range of different crystalline silicate types are available [CLICK], for dental composites, the fillers are primarily non-crystalline (glassy) silicates.
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. [CLICK] We will focus on irregular particles for the next part of this discussion.

12. CHEMISTRY OF COMPOSITES
Filler Phases (cont.)
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 m in diameter. Early composites were based on relatively large filler particles (macro-fillers) that had particle sizes in the range of m. Smaller fillers (midi-fillers) immediately became more popular to improve composite finishing characteristics (1-10 m). Next dentistry manufactured composites with only very fine fillers, microfillers ( m). 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.

13. CHEMISTRY OF COMPOSITES Interfacial Coupling Agents
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) [CLICK] and three methoxy groups on the other end (right). [CLICK] 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.
[CLICK] Now let’s represent silane schematically, and then look at what is happening during polymerization reactions of composite via the animation in the next slide. S1

14. SILANES
Managing the problems: (1) bonding, (2) multilayers, and (3) dimerization. S1
SiO2 in
Ceramic B1 B2 T1 T2 S1
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. [CLICK] Ideally, they should form a monomolecular layer onto a surface, [CLICK] completely react with the surface, and completely react with the matrix phase. [CLICK] 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 also is very low. [CLICK] 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. [CLICK] Finally, excess silane can react with itself to form a dimer. It then behaves as a difunctional molecule with just double bonds -- and that effectively dilutes the local polymer and reduces the overall composite strength.

15. DESIGNING COMPOSITES Class Exercise CONTINUE COMPOSITE TYPE: Posterior
NAME = COM-POST
Components in Kit:
>
COMPOSITE TYPE: Esthetic
NAME = COMP-EST
ACTUAL COMPOSITION:
Matrix:
> High MW Monomer =
> Low MW Diluent =
Curing System:
> Initiator =
> Accelerator =
> Inhibitor =
Filler:
> Particle Type 1 and Distro =
> Total Filler Level =
> Coupling Agent: =
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. TRY TO ANSWER THE QUESTIONS BELOW BEFORE [CLICK] PRESSING THE BUTTON ABOVE TO CONTINUE. LOOK AT YOUR HANDOUT IF NECESSARY.
For the “posterior composite” shown on the LEFT, 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)? PAUSE 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” on the RIGHT, 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?
CONTINUE

16. DESIGNING COMPOSITES Class Exercise COMPOSITE TYPE: Posterior
NAME = COM-POST
Components in Kit:
>
COMPOSITE TYPE: Esthetic
NAME = COMP-EST
ACTUAL COMPOSITION:
Matrix:
> High MW Monomer =
> Low MW Diluent =
Curing System:
> Initiator =
> Accelerator =
> Inhibitor =
Filler:
> Particle Type 1 and Distro =
> Total Filler Level =
> Coupling Agent: =
Directions; Shade Guide
Compules of Composites
(Bonding system)
(Finishing system)
BIS-GMA
TEGDMA
UDMA
TEGDMA CQ
NNDMPT
BHT
70% Mini-Filler
30% Micro-Filler
60 wt%, 40vol%
Silane
For the “posterior composite” shown on the LEFT, [CLICK] it would be wise to use BIS-GMA like monomers (or UDMA) and include a diluent of TEDGMA in about a 60-40% ratio. [CLICK] To make it visible light curing, include camphoroquinone, dimethylaminomethacrylate (DMAM) and some butylated hydroxytoluene (BHT) for inhibition. [CLICK] To maximize the packing fraction of the filler, include two sizes, in a ratio of 70% medium-sized filler and 30% very fine filler. We will talk more about these in the next module. The total filler level will probably approach 75% by weight overall (which is 50% by volume). A small amount of coupling agent will have already been reacted to coat the surfaces of the filler particles.
For the “anterior composite” on the RIGHT, [CLICK] try selecting UDMA – just to be different from the first composite. You will still need TEGDMA in about the same amount. For visible light curing, the material will contain the same types and amounts of initiator, accelerator, and retarder/inhibitor. However, in this case, the average particle size would be smaller to produce better finishing and apparent esthetics. So it would be preferable to use about 70% fine filler and about 70% very fine filler. The filler content would probably be less.
[CLICK] The kit should contain directions, could contain a bonding system but usually does not any more. For infection control, it is preferable to dispense the material from disposable unidose compules. Generally, 3 to 5 shades are included in the kit. Since tooth structure tends to become grayer and darker when it is dried in preparation for bonding procedures, you must match shades before you start the procedures. The kit could include finishing and polishing materials, but generally these are sold separately now. CQ
NNDMPT
BHT
70% Midi-Filler
30% Micro-Filler
75 wt%, 50vol%
Silane

17. Thank
You
Now you understand the design features of building a composite. In the next module we will examine actual commercial compositions and their properties. [CLICK] THANK YOU.