Wednesday, November 25, 2009

Specific gravity and what every purchaser should know. On most Polycarbonate (and other polymer) technical data sheets there is a number called the specific gravity.   The specific gravity of Polycarbonate is 1.2 and is one of the most important numbers on the data sheet for purchasers.

Specific gravity is the ratio of the density of the material to the density of water.  As the density of water is 1000 kg/m3, this means that the density of Polycarbonate is 1200 kg/m3.

To convert the density from kg/m3 to lbs/ft3 you need to multiply by 0.0624.  This calculation gives a density of Polycarbonate of 74.88 lbs/ft3.  In this way we can easily calculate the density of Polycarbonate in lb/ft3 from the specific gravity figure on the data sheet.

The density is an important value for purchasers of Polycarbonate sheet.  It can help answer questions such as "If a supplier says that the Polycarbonate resin prices have gone up 10 cents per pound, what effect would that have on the cost of 1/8" sheet?"

To answer this question we need to be able to convert from \$/lb to \$/sqft.  We can do this conversion with the help of a simple equation:

Cost (\$/sqft) = Cost (\$/lb) x Density (lb/ft3) x Sheet thickness (inches) / 12

So to answer our question on the effect of an increase of \$0.10/lb for resin on the price of 1/8" sheet, we can plug the numbers into our equation:

Cost (\$/sqft) = \$0.10/lb x 74.88 lb/ft3 x 0.118 inches /12

Cost (\$/sqft) = \$0.074 /sqft

So a 10 cent per pound increase in Polycarbonate resin price will increase the cost of 1/8" sheet by 7.4 cents per sqft.

[Note: Nominal 1/8" Polycarbonate has a thickness of 0.118"]