Tek Tip - Defining Density vs. Specific Gravity
¹Density (D) -The mass (or weight) per unit volume of a material at a given temperature. Typical units are:
- grams per cubic centimeter (g/cc or g/cm3)
- kilograms per cubic meter (kg/m3)
- pounds per cubic foot (lb/cu ft or lb/ft3)
- pounds per cubic inch (lb/cu in or lb/in3)
Specific Gravity (sp gr) - AKA relative density. The ratio of the density of a material at a given temperature to the density of an equal volume of water at the same temperature. Units = none.
Test reference: ASTM D 792, D 1505
Density and specific gravity are both ways of describing the weight (mass) of a certain quantity of material. They are useful in determining yield and comparing different materials.
SO WHAT'S THE DIFFERENCE?
NOT MUCH. The main difference is that density has units and specific gravity does not. Why not? Specific gravity is determined by dividing the density of a material by the density of an equal volume of water using the same units. The units therefore cancel each other out. This means you don't have to worry about conversions when comparing materials that have densities using different units.
The specific gravity value of any given material is going to be the same in the US, Germany or China! (Also, materials with a specific gravity of less than 1 will float on water)
The down side is that without units you can't do anything but compare different materials. It's hard to determine, for instance, the weight of a 100 sheet stack of 24" x 48", 0.010" polycarbonate. This is where density comes in handy. The density of polycarbonate is 0.0433 lb/cu in. In the above example the stack weighs about 50 lbs.
100 x .010" x 24" x 48" = 1152 cu in x 0.0433 lb/cu in = 49.88 lbs.
Fortunately, there is a loophole in the "no units" rule for sp gr. It just so happens that the density of water in grams per cubic centimeter (g/cc) is very close to 1 (0.9976). This means that the specific gravity of a material is virtually the same as its density in g/cc.
Material density g/cc ÷ 1 g/cc = material density = specific gravity
Therefore, knowing the specific gravity and with the help of a conversion table on the shared drive one can determine the weight* of any given quantity of material in almost any units. Example:
What is the approximate weight (in pounds) of 3000 - 25" x 38" sheets of .010" press polished, clear RPVC?
Specific gravity = 1.35 (from data sheet)
Density = 1.35 g/cc (based on loophole)
Density = 0.0488 lb/cu in (from conversion table)
Volume of material = 28500 cu in (3000 x .010"x 25"x 38")
Weight = 1391 lb (28500 cu in x 0.0488 lb/cu in)
* The weight will vary slightly due to small variations in film thickness and sp gr.
Here is a list of densities of some common plastics:
polyetherimide (PEI) 1.27 1.27 0.0459
Type of Plastic sp gr¹ Density¹ g/cc Density¹ lb/cu in ABS 1.04 1.04 0.0376 acrylic (polymethylmethacrylate - PMMA) 1.19 1.19 0.043 cellulose triacetate (CTA) 1.30 1.30 0.047 cellulose acetate butyrate (CAB) 1.21 1.21 0.0437 polyamide (Nylon 6) 1.13 1.13 0.0408 polyamide (Nylon 12) 1.02 1.02 0.0368 polycarbonate (PC) LEXAN™ 1.20 1.20 0.0433 polyethylene naphthalate (PEN) 1.36 1.36 0.0491 PET polyester Melinex, Mylar 1.40 1.40 0.0506 low density polyethylene (LDPE) 0.91 0.91 0.0329 high density polyethylene (HDPE) 0.95 0.95 0.0343 polyimide (PI) Kapton 1.42 1.42 0.0513 polypropylene cast (PP) 0.89 0.89 0.0322 polypropylene biaxially oriented (BOPP) 0.905 0.903 0.0326 polystyrene (PS) 1.05 1.05 0.0379 rigid vinyl (RPVC) 1.35 1.35 0.0488 flexible cast vinyl 1.22 1.22 0.0441 polyvinyl fluoride (PVF) Tedlar 1.44 1.44 0.0520
¹These values are approximations for comparison purposes. they will vary based on grade of resin and additives such as pigments and fillers. for greater accuracy, use the value from the data sheet or specification on actual product.