A course in Dental Composites page 5 (Al-Fl-Si Glass)-DoctorSpiller.com

A course in composites pages 1234567891011

Al-Fl-Si Glass

Alumino-Fluoro-Silicate glass (powder)

The glass powder that is used in the production of both Silicate cement and Glass Ionomer cement is made from a glass formulated with Sodium Fluoride andstabilized with minimal alumina.  It is technically known as Alumino-Fluoro-Silicate glass.  This glass is ground into a very fine powder.  While Al-Fl-Si glass is stabilized to make it insoluble in water, it is formulated to remain partially soluble in very highly acidic solutions.  (It is not soluble in saliva or in any food or liquid that can be consumed by mouth.)  By the use of different trace metals, zirconium, and other components, the glass can be fabricated to match the various colors and opacities of tooth structure.

One of the major characteristics of this type of glass is its ability to partially dissolve, and form a hard, waterproof matrix when mixed with either phosphoric acid or polyacrylic acid (See the table on page 3).  When the powder to liquid ratio is varied correctly, a stiff paste results  This paste can then be used to fill cavities, and the paste will set in time to form a very hard and insoluble solid..  A major characteristic of the cements made with Al-Fl-Si glass is that they are translucent, and their color characteristics can be controlled by varying the composition of the glass.  The hardness and durability of the resulting restoration is largely dependent on the nature of the chemistry of the matrix formed when the glass particles begin to dissolve in the acidic solution.  The appearance of the resulting restoration is dependent on the exact composition of the glass.

The two cements made with Al-Fl-Si glass particles are silicate cement, and glass ionomer cement.  Both will be discussed in more detail on the next page in this course.

silicate2
silica4Alumino-fluoro-silicate glass cements and restorations bond chemically with enamel, dentin and most metallic dental alloysRestorations and cements made with alumino-fluoro-silicate glass have a number of advantages and disadvantages:

    • This means that they can be applied directly to clean tooth structure without etching or bonding or even cutting retentive undercuts.
    • These materials will also chemically bond to metallic substructures such as gold and base metal crowns and bridges, so they can be used to anchor esthetic facings made of resin composite to these structures.
  • Alumino-fluoro-silicate glass cements will slowly release fluoride into the adjacent tooth structure.  This converts hydroxyapatite into fluoroapetite, thus strengthening the tooth structure and making it more resistant to decay.
  • The major disadvantages of restorations  and cements made from unmodified alumino-fluoro-silicate glass are:
    • The materials are very water soluble during the setting phases, and if they are allowed to get wet during placement, they can leach out allowing the final restoration to leak.
    • They are also not especially resistant to abrasion, and are not suitable as restorations on occlusal or stress bearing areas.

For a thorough understanding of glass and porcelain, students and dental professionals should consult  my five page course “Dental Ceramics for the beginner”

The chemistry of Al-Fl-Si glass

Silicate cement was probably the very first tooth colored filling material (if you discount whalebone). Glass Ionomer restoratives came later.  However, in order to understand silicate cement, and, indeed, in order to understand the characteristics of most modern composites, it is very important to understand the composition and chemistry of the glass powder that gives them their special characteristics.

Glass is composed of three major components.  The first, silica (silicon dioxide),  is a naturally occurring mineral known as quartz..  Silica is the chief component in ordinary sand.   Silica is the major glass former in most glass formulations (but it is not the only glass former).  The melting temperature of quartz is very high, but it was discovered early in human civilization that the addition of certain metallic oxides could serve to lower the melting point of the glass quite a bit.  These additional metallic oxides, when added to sand in order to lower the melting temperature are called “fluxes“.  Fluxes are the second major constituent in glass.

Fluxes are oxides of alkaline metals such as sodium, potassium, lithium, boron and lead.  They serve to dissolve the silica, a bit like water dissolves sugar.  This is important, since glass is composed of silicon dioxide which has a very high melting temperature. ( Pure quartz melts at 1713 degrees centigrade.  The addition of 25 % sodium oxide can lower the melting temperature to 793 degrees centigrade.)  The most common fluxes used in ceramics are sodium and potassium oxides, but there is a long list of fluxes, each one with its own set of characteristics and uses.

The third component of glass is a stabilizer.  Stabilizers make the glass strong and water resistant. Calcium carbonate, (limestone) is a stabilizer. Without a stabilizer, water and humidity attack and dissolve glass. Glass lacking a stabilizer is often called “waterglass” since it can dissolve in water.

  • When lead is used as the stabilizer, the resulting glass has superior clarity and durability, and will ring like a bell when tapped.  Historically, lead “crystal” has been used for years in the manufacture of fine tableware including drinking glasses and wine canisters (Reference Waterford crystal).  It is also fairly insoluble, even in acidic solutions, and has never been implicated in any cases of lead toxicity in humans.  However, the FDA (US food and drug administration) has recommended that lead stabilized glass not be used to store liquids as small amounts of lead have been known to leach out of the glass and into the liquid.  Lead is not used to flux or stabilize any dental glass manufactured in North America or Europe.
  • Boron oxide is, like silica, a glass former.  When added to silicon based glass at a minimum of 5% by weight, the glass becomes a borosilicate.  Glass fortified in this way is resistant to mechanical and thermal shock and is used to make baking pans (Pyrex), laboratory ware and sealed beam headlights.
  • Alumina (aluminum oxide) is found combined with silicon in naturally occurring glasses called feldspars.  It is used as a glass stabilizer in molecular form to toughen the glass, and when used in this fashion, it does not affect the transparency or color characteristics of the resulting glass.   It is also used in crystalline form (as opposed to the dissolved molecular form) dispersed throughout the  glass.  As crystalline inclusions, the alumina particles fuse together and act as a sort of framework or skeleton.  This “framework” stiffens the glass during firing and makes it less likely to slump.  The inclusion of crystalline structures transforms the glass into porcelain which is much tougher and less prone to fracture than the same glass without such a matrix.  Alumina is a major component in ordinary clay and is present in nearly all the ceramic products you buy such as the plates and cups in your dinnerware and your mother’s bone china.
  • The addition of trace metals can give color to the glass. Cobalt imparts a blue color, while gold imparts red and copper a green color.  (These metals are added as oxides, and they generally have fluxing qualities, but they are not alkaline metals, and are added in such small amounts that they are not considered fluxes for purposes of calculating glass formulas.)
  • The addition of zirconium and titanium oxides add opacity to the glass.  These oxides form a crystalline structure within the otherwise translucent glass, and this diffuses light as it penetrates, creating a milky or pure white appearance depending on the amount of zirconium or titanium oxides used.

Melting temperature vs. Solidus

When a glass melts, it becomes a liquid with the consistency of syrup on a cold day.  Glass does not have a specific melting temperature, and when it cools, it remains a “supercooled” liquid (think of a hard candy, like a lollipop), however contrary to mythology, it does not continue to flow at normal temperatures.  While any given glass does not have a definite melting temperature, it does have a definite solidustemperature.  The solidus is the lowest temperature at which a non crystalline material shows any characteristics of melting, including a tendency to flow.

For a thorough understanding of glass and porcelain, students and dental professionals should consult  my five page course “Dental Ceramics for the beginner”

Next page==>Alumino-fluoro-silicate glass cements

A course in composites pages 1234567891011