Rockwell verses Barcol hardness tests
Rockwell and Barcol hardness are measured on different types of test equipment. Both measure the depth of indentation caused by an "indentor" when applied to the material under a specific load for a specified period of time and at a specified temperature. In the Rockwell test, several different "indentor" geometeries and loads can be used. Each combination is designated by a scale. Common scales for plastics include "R", "L", "M", "E" and "K".
The Rockwell Hardness on the "R" scale for CYROLON® ZX polycarbonate sheet is R-118. Its Barcol Hardness is approximately 75-80.
Acrylics are usually measured on the "M" scale. Standard ACRYLITE® extruded (FF) acrylic sheet and ACRYLITE® cast (GP) acrylic sheet measure between 93-94 on the "M" scale. ACRYLITE® LED sign grade acrylic sheet and ACRYLITE® Resist 65 acrylic sheet measure approximately 65-75 on the "M" scale. On the "M" scale, polycarbonate sheet has a Rockwell hardness of approximately 55-63.
The various hardness tests and scales cannot be accurately correlated to each other. Therefore, when cross referencing scales, values are not reliable. Each Rockwell scale involves the use of a different diameter indenting device and a different applied weight. Softer plastics will tend to produce more favorable results in comparison to harder ones as the diameter of the indentor increases or as the applied weight is reduced. Therefore, two plastics that measure nearly the same on one scale, could measure quite differently on another.