The pore size distribution of naturally porous cigarette papers and its relation to air permeability and diffusion capacity – Part 2
Several publications have shown in the past, on theoretical grounds and by actual measurements, that the perforation of cigarette papers causes a substantial increase in air permeability, while the diffusion capacity increases comparably less. These findings have been simplified and popularised in the probably questionable statement that large pores are responsible for air permeability and small pores for diffusion capacity. It is the aim of this study to investigate the substance of such statements by correlating the pore size distribution of naturally porous cigarette papers with their air permeability and diffusion capacity, respectively. To this end eight cigarette papers were selected that differed in permeability, diffusion capacity, fibre furnish, filler content and burn additive content. The pore size distribution of these papers was measured by mercury porosimetry before and after the papers had been exposed to 230 °C for 30 minutes. The pore size distribution was multiplied with a Gaussian weighting function and integrated to obtain a weighted pore volume. The two parameters of the weighting function, mean value and standard deviation, were chosen to maximise the correlation of the weighted pore volume with air permeability and diffusion capacity, respectively. The results show a good correlation with correlation coefficients greater than 0.9 for the air permeability as well as for the diffusion capacity. The optimal mean values of the weighting functions were at a pore radius of 2.5 µm for air permeability and 1.0 µm for diffusion capacity. These results indicate that in fact large pores are better correlated with changes in air permeability, while small pores are more strongly correlated with changes in diffusion capacity. They also demonstrate the tight relation between pore size distribution, air permeability and diffusion capacity, which makes the pore size distribution a tool to further optimise cigarette papers, for example, with respect to carbon monoxide yields.