Mercury intrusion porosimetry is a useful technique for the characterization of coated and uncoated papers. Mercury porosimetry can provide structural information such as pore size distribution, pore volume, percent porosity, bulk density, and surface area on a wide variety of porous paper samples. The mercury intrusion technique is based upon the fact that a non-wetting fluid, such as mercury, will not penetrate the pores (or voids) of a porous material unless acted upon by a pressure large enough to cause intrusion. The equation governing this behavior and the working equation in mercury porosimetry is the Washburn equation:
Where d is the pore diameter, γ is the surface tension of the intrusion fluid, Θ is the contact angle formed by the intrusion fluid on the solid, and P is the applied pressure to the intrusion fluid. The surface tension of mercury is taken as a constant, and the contact angle formed by mercury on a wide array of solids has been shown to be approximately 140 degrees.
Mersury Porosimetry for Characterization of Specialty Papers
Mercury porosimetry can provide valuable information on specialty papers used in printing applications. Proper ties such as ink spreading, penetration, and adsorption (printability) correlate strongly with pore size, pore size distribution, and pore volume of the paper and the paper coating.
Since the aim of many paper producers and converters is to optimize these properties for a particular printing application, application of mercury intrusion porosimetry can assist in product development.
To illustrate, a series of coated and uncoated papers were analyzed using the Quantachrome Instruments PoreMaster® mercury intrusion porosimeter. The pore size distributions for two such papers are shown in Figure 1:
Figure 1. Pore size distributions for base paper and coated paper
The red curve in Figure 1 shows the pore size distribution of an uncoated, base paper sample. The distribution is bimodal, with a peak at 14.7 microns and a peak at 0.14 microns. The green curve shows the resulting pore size distribution for the same base paper after coating and calendering steps. The resulting distribution is similarly bimodal, but shifted to smaller pore sizes – one peak at 1.5 microns and the other at 0.11 microns. These curves clearly illustrate the reduction in pore size as a result of paper coating and calendering.
The total pore volume associated with pores in the size range from 950 microns down to 0.0036 microns of each paper was also determined during the experiments. The base paper had a total pore volume of 1.70 cc/g and the coated paper had a total pore volume of 1.64 cc/g.
Figure 2 below shows the weight normalized intrusion volume (cc/g) versus pore size for these papers.
Figure 2. Weight normalized intrusion volume (cc/g) versus pore size for base paper and coated paper
Again, the red curve shows the base, uncoated paper, and the green curve shows the coated, calendered paper. Figure 2 shows that the two papers have very similar total pore volumes – as evidenced by the approach of both curves to the same normalized volume on the upper right of Figure 2. The two samples differ, however, in how this pore volume is distributed in pores of given size. The majority of the total pore volume of the uncoated paper is present in pores larger than 1 micron, whereas the majority of the pore volume for the coated paper is present in pores smaller than 1 micron. This can be quantified by the median pore size (based on volume) which from mercury intrusion measurements was found to be 7.5 microns for the uncoated paper, and 0.12 microns for the coated paper. From these values, it is concluded that 50% of the total pore volume for the uncoated paper was found in pores larger than 7.5 microns (See Figure 3 below) – whereas only ca. 10% of the total porevolume was found in pores larger than 7.5 microns for the coated paper.
Figure 3. Percent intruded versus pore size
This data shows an interesting reduction in the pore size of a coated versus uncoated paper as displayed by the mercury intrusion pore size distribution (Figure 1), but a preservation of total pore volume (Figure 2) for these two papers.
The printability of papers depends in part upon the interaction of the ink with the porous structure of the paper and coating. Ink settling has been shown to correlate strongly with pore volume and pore size and indeed a similar relationship between the pore structure of paper used in pr int ing and its’ optical and printing properties has been established. Thus, mercury intrusion porosimetry is a useful tool for the characterization of coated papers used in printing applications.
This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.
For more information on this source, please visit Quantachrome Instruments.