Glasses- Process Requirements for Making Commercial Glasses Flashcards
What is heated when melting soda-lime-silica glasses?
SiO2, soda ash (Na2CO3) and limestone (CaCO3) and other components
Problem of solid state reactions
They are slow
How is problem of solid state reactions overcome for melting glasses?
Na2CO3 melts at 852C. CaCO3 decomposes at 600-1000C to CaO. But Na2CO3 and CaCO3 together form a liquid carbonate phase at lower temperatures. SiO2 reacts with this liquid creating CO2 bubbles
What happens when SiO2 reacts with the liquid from soda ash and limestone?
The SiO2 of the liquid (and its viscosity) increases slowly with time and the dissolution rate falls. Now the melting temperature is 1550-1600C, takes a few hours for sand dissolution
Potential problems for melting soda-lime-silica glasses
Poor mixing of solid reactants spoils reaction sequence.
Poor grain size distribution slows initial liquid formations (ideally equal sized particles).
Gravity based segregation of liquid and solid components (light silica grains float and initial liquids formed can be dense and sink).
What is saltcake used for?
Is Na2SO4. In small amounts acts as surfactant to encourage dispersion and solution of last sand grains. If it’s concentration is too high it can separate out as a liquid floating on melt surface as it is immiscible
What does refining involve?
Removing bubbles from the melt
How do bubbles form and move?
Initially there is a large amount of CO2 gas produced in a low viscosity melt which forms large bubbles which coalesce and burst at the surface. Later the melt viscosity is much higher and a few small bubbles remain
Stokes law
Velocity=(1/12)(dg/η)x^2 Rise velocity of gas bubble d is difference between melt and gas density g is acceleration due to gravity η is viscosity of melt in Pa.s x is diameter of bubble
Why can’t small bubbles just be removed by rising?
They are too small and in too dense a liquid so the rise through the furnace would take many hours. Also viscosity is greater at the bottom and longer is needed for lower temperatures
How do refining agents work to remove bubbles?
Add chemicals (Na2SO4) to batch that decompose at later stages of melting (higher T) releasing a gas (O2). This oxygen diffuses into the few remaining bubbles causing their growth and aiding their rise to surface.
What happens to bubbles as melt is cooled before forming?
Reaction from refining agents proceeds in reverse. Oxygen in any remaining bubbles dissolves, bubbles shrink, surface tension causes internal gas pressure to rise, increases solubility of remaining gases.
How much soda in a commercial SLS glass is added as sulphate (refining agent)?
Roughly 4%
How to analyse bubbles in glass
Count them. Measure their size and distribution. Use mass spectrometers for bubble compositions
Problem of homogenisation
The liquids formed at different states of melting have a wide compositional range while high quality glass must be more chemically uniform for optical transparency
