Pilkington float glass process

Question 1

What is the Pilkington float glass process?

Answer 1

~ continuous ribbons of glass are formed using a molten tin bath

~ quiescent molten tin bath yields plateof remarkable flatness and defect free surfaces

~ float tank is 30 x 1600 ft, holds 1000 tons, 1000 ton output/day

Question 2

When was it formed?

Answer 2

~ Alistar Pilkington developed it in 1959

~ in 1974, PPG developed a slightly different competing process

Question 3

What are the steps of the process?

Answer 3

1. ~1065C glass is gravity fed onto molten tin bath, flow regulated by a fused silica tweel gate
2. Al2O3-SiO2 refratory lined tank, Sn fills the cracks between blocks and solidifies
3. glass wants to contract together, which competes against gravity based spreading to establish an equilibrium thickness of 6-7mm
4. glass is picked up by rollers at 600C and brought into an annealing lefr (550C)
5. on exiting and cooling, glass is cut (scribed and snapped)

Question 4

Why is Sn chosen to fill the cracks between blocks in Al2O3-SiO2 tanks?

Answer 4

~ Tmelt = 231.9C
~ low vapor pressure (10^-6 ATM at 1050C)
~ higher density (6.5g/cm^3)
~ Sn metal causes glass to float
~ high nobility of Sn makes it unreactive with glass
~ glass is non-wetting to Sn, so doesn’t stick when picked off by rollers

Question 5

How do you get thicker glass?

Answer 5

~ graphite fenders can restrict ribbon spread

Question 6

How do you get thinner glass?

Answer 6

~ water-cooled toothed wheels grip the ribbon near its edges

Question 7

What is the possible thickness range of the glass?

Answer 7

~ 1-25mm

Question 8

What pressure is the tank maintained at?

Answer 8

~ in a positive pressure of 90% N2 10% H2 to protect the tank Sn from oxiation

~ up to 30,000 ft^3/hr, batch changes atmosphere many times per hour

Question 9

What is the refractory fiber curtain?

Answer 9

~ at the bath exit

~ positioned to not touch the glass

Question 10

What is the result of oxygen contamination?

Answer 10

~ oxygen contamination to 10ppm can form SnO

Question 11

What is the effect of the presence of SnO?

Answer 11

~ SnO has 100x higher vapor pressure than Sn, it condenses on coller parts of the bath roof, falls to form a “tin speck” defect

Question 12

What happens to oxygen solubility at higher temp?

Answer 12

~ oxygen solubility is higher at higher temp

can precipitate at the cold end to form a haze on the glass underside surface called “bloom”

Question 13

How do we reduce Na2SO4 fining agent?

Answer 13

~ SnS has a high vapor pressure, depositing on roof, yielding a “tin drip” effect