Introduction and History

Flake Glass

Flake glass is material made from glass.  It is used as a reinforcing agent for many plastic products.  The resulting composite material, properly known as flake-glass reinforced plastic (FGRP) or Flake-Glass reinforced epoxy (FGRE) is called Glassflake in popular usage.

Glassmakers throughout history have experimented with glass flakes, but manufacture of glass flakes was only made possible with the advent of finer machine tooling.  What is commonly known today as “flake glass” was invented in 1938 by Russell Games Slayter of Owens-Corning as a laboratory oddity.  Glassflake remained on the laboratory shelf until 1956-57 when a product development team from OCF of Ohio experimented with light diffusing glazing panels and structural coatings.  James N. Hume became a part of the team in 1958 with responsibility for coatings.

Glass flake is formed when silica based or other formulation glass, it is extruded into an ultra thin tube that is then crushed.  Glass is unlike other polymers in that, even as a flake, it has little crystalline structure.  The properties of glass in its softened stage are very much like its properties when extruded into flake.  One definition of glass is “an inorganic substance in a condition which is continuous with, and analogous to the liquid state of that substance, but which as a result of a reversible change in viscosity, during cooling, has attained so high a degree of viscosity as to be for all practical purposes rigid (Lowenstein, 4).

The technique of heating and extruding glass into fine flakes has been known to exist since 1938; however, the concept of using these flakes for reinforcement applications is more recent.  The first commercial production of glass flakes was in 1956.  In 1938, Owens-Illinois Glass Company and Corning Glass Works joined to form Owens-Corning Fiberglas Corporation.  GLASSFLAKE INTERNATIONAL began producing Glassflake in 2003.

Chemistry

The basis of glass flake is silica, SiO₂.  In its pure form it exists as a polymer (SiO²) n.  It has no true melting point but softens up to 2000°C, where it starts to degrade.  At 1713°C, most of the molecules can move about freely.  If the glass is then cooled quickly, they will be unable to form an ordered structure (Gupta, 544).  In the polymer it forms SiO₄ 4- groups which are arranged in a tetrahedron with the silicon atom at the center and four oxygen atoms at the corners.  These atoms form a network bonded at the corners by sharing the oxygen atoms.

The vitreous and crystalline states of silica (glass and quartz) have similar energy levels on a molecular basis, also implying that the glassy form is extremely stable.  In order to induce crystallization, it must be heated to temperatures above 1200°C for long periods of time (Lowenstein, 6).

Although pure silica is a perfectly viable glass flake, it must be worked with at very high temperatures, which is a drawback unless its specific properties are needed.  It is usual to introduce impurities in the form of other materials into the glass to lower its working temperature.  These other materials also impart various other properties to the glass which may be beneficial in different applications.  The first type of glass used was soda-lime glass or A glass.  It was not very resistant to alkali.  A new type, E-glass, was formed that is alkali-free (<2%) and a aluminum-borosilicate glass (Volf, 338).  This was the first glass produced for continuous filament formation.  E-glass still makes up most of the fiberglass production in the world.  Its particular components may differ slightly in percentage, but most fall within a specific range.  The letter E is used because it was originally for electrical applications.  S-glass is a high-strength formulation whose tensile strength is the most important property.  C-glass was developed to resist attack from chemicals, mostly acids which destroy E-glass.

Properties:

Glass flakes are useful because of their high surface area to weight, better known as “aspect ratio”.  However, the increased surface area makes them much more susceptible to chemical attack.

Glass strengths are usually tested and reported of “virgin” fibers which have just been manufactured.  The freshest, thinnest fibers are the strongest and this is thought due to the fact that it is easier for them to bend.  The more the surface is scratched, the less the tenacity is (Volf, 351).  Because glass has an amorphous structure, its properties are the same along the fiber and across the fiber (Gupta, 546).  Humidity is an important factor in the tensile strength.  Moisture is easily absorbed, and can worsen microscopic cracks and surface defects and lessen tenacity.

In contrast to carbon fibers, glass can undergo more elongation before it breaks (Gupta, 546).

The viscosity of the molten glass is very important.  During drawing the viscosity is relatively low.  If it is too high, the fiber will break during drawing.  If it is too low the glass will form droplets rather than drawing.

Manufacturing Process:

Flake glass production begins with the raw materials in solid form, called marbles.  The materials are mixed together in a furnace.  Then the molten glass goes into a bushing to be formed into a tube.  The bushing is the most important part of the machinery.  The bushing is almost always made of platinum alloyed with rhodium for durability.  Platinum is used because the glass melt has a natural affinity for wetting it.  When bushings were first used they were 100% platinum and the glass wetted it so easily it ran under after exiting the nozzle and accumulated on the underside.  Also, due to its cost and its tendency to wear, it was alloyed with rhodium.  In the direct melt process, the bushing serves as a collector for the molten glass.  It is heated slightly to keep the glass at the correct temperature.  In the marble melt process, the bushing acts more like a furnace as it melts more of the materials (Lowenstein, 91).