Specific Surface
Physical characterization of porous materials relies on gas adsorption isotherms to determine the available surface area per unit mass of a substrate. In the testing of coated paperboards, the Brunauer-Emmett-Teller method provides the theoretical framework for calculating this area by measuring how much inert gas, typically nitrogen, adheres to the sample surface at cryogenic temperatures. This analysis defines the boundary between surface adsorption and capillary condensation, which dictates how printing inks are absorbed by the coating layer.
Measurement Method
The process starts by degassing the paper sample under a vacuum at a moderate temperature to remove any pre-existing moisture or contaminants. Following this preparation, the system admits successive controlled doses of nitrogen gas into the sample chamber at liquid nitrogen temperature to build a relative pressure curve. Adsorbed molecules form a complete single layer before starting to build subsequent layers, allowing the calculation of the monolayer capacity of the material.
This measurement operates under the assumption that the first layer of molecules holds a specific adsorption energy while subsequent layers behave like a condensing liquid. Knowing this monolayer volume allows paper chemists to calculate the total surface area by multiplying the number of molecules by the cross-sectional area of a single nitrogen molecule.
Coating Assessment
Coated paper performance during high-speed printing depends on the micro-structure and total surface area of the mineral coating. Using the brunauer-emmett-teller model to verify this surface area helps predict ink solvent absorption times. Paperboard mills use this information to adjust the ratio of pigment to binder in their formulations.
If the surface area is too low, the ink dries too slowly, which causes smearing and set-off on the press.
