A composite is any material that is composed of hard, pebble-like filler particles similar to sand or pebbles, surrounded by a hard matrix of a second material which binds the filler particles together. The filler particles can be any coarseness varying from large rocks to microscopically fine powder made up of particles of virtually any shape, varying from spherical through fibers to flakes. The matrix material generally starts out as a paste, powder or liquid and begins to harden when it is activated, either by adding a catalyst (which may be mixed with the filler particles), or by adding water or another solvent to allow chemical reactions to take place.
Before it hardens, it can be pressed into a mold, or stuffed into a hole. The most commonly understood composite material is concrete, or “Portland cement”. It is composed of sand, sometimes mixed with pebbles, bound together by a matrix of lime, alumina and Iron. This material can be formed into bricks, poured into molds, or used to “cement” iron rods into the ground. Composites are an increasingly important part of everyday life, from wooden particle board to Corian® counter tops.
The image above shows the microscopic structure of a typical composite material. The filler particles are the darker, irregular granules. The matrix is the lighter material that surrounds them. This particular composite is not highly “filled”, which means that there is a low density of filler particles compared to the amount of matrix material. Compare that with the micrograph below. This shows another composite material with differently shaped filler particles which are much more closely packed together. This is a ” highly filled” composite. Because the characteristics and relative volumes of both the matrix materials and the various filler particles can be manipulated by the manufacturer of the composite, it is obvious that these materials show an almost infinite range of physical properties.
In dentistry, The material commonly called “composite” is made of an acrylic matrix called BIS-GMA mixed with a finely ground glass or quartz particle filler. The acrylic will harden with the addition of a catalyst, similar to the way fiber-glass hardens. In the case of light cured composites, the catalyst is already mixed into the paste, but does not become active until illuminated with a strong light. To ensure bonding between the filler and the matrix, the filler particles are coated with a silane-coupling agent that contain a methacrylic group able to co-polymerize with the matrix-forming dimethacrylate monomers and functional groups able to interact with the filler.
Dental amalgam is also a composite, although it is not customary to refer to it as such. It is made up of finely ground silver/tin metal powder mixed with mercury. The mercury dissolves the outside layers of the metal powder particles to form a matrix of silver-tin-mercury which hardens around the unreacted metal powder particles to form the finished amalgam composite. For much more on dental amalgam, please click here.
Dental cements are all composite materials made from different powders mixed with different liquids. The liquid partially dissolves the powder particles and forms a matrix which becomes hard enough to act as a “glue” and is used to cement Crowns and Posts. All non metallic composite filling materials are really just more highly filled versions of their respective cements.
Porcelain is not generally thought of as a composite material, but it is in fact composed of a glass matrix filled with crystalline particles. While ceramics are an extremely important part of dentistry, very few dental professionals really understand them. For this reason, I have written a Beginners course in dental ceramics to help fill this void.