Paper bulk is defined as the volume occupied by a given mass of paper (the inverse of density). The bulk of paper and paperboard is important because it contributes to the paper’s thickness, also known as caliper. Increased paper thickness leads to higher rigidity or bending stiffness, an important measure of strength. Higher bulk also allows the papermaker to calender the paper under greater pressure, which is done by passing it through rollers. Higher calendering pressure creates a smoother sheet that, in turn, produces a sharper printed image. In copy paper, a bulky, rigid sheet is desirable for improved operability of the copier with fewer paper jams. In paperboard, higher bulk allows for increased calender pressures and improved surface smoothness and print quality, while maintaining the thickness specification. Lastly, high bulk enables the papermaker to reduce the cost of making the paper. Since cut-size copy paper is produced to specified dimensions of length, width, and thickness, increasing paper bulk, by addition of filler or some other component allows a commensurate portion of the actual fiber to be removed to maintain sheet caliper. This causes the basis weight (weight per unit area) to decrease with a proportionate decrease in cost.
Fillers such as precipitated calcium carbonate (PCC), ground calcium carbonate (GCC), clay, and others can affect the bulk of paper and paperboard in various ways. The best bulking pigments are variants of clustered scalenohedral PCC. This is due primarily to two phenomena:
- Like all Specialty Minerals Inc.’s (SMI’s) PCC particles, which are produced via chemical syntheses, the distribution of particle sizes is very narrow—that is, the vast majority of PCC particles are very close in size. With such same-size particles, there is little opportunity for smaller particles to wedge themselves between larger ones, thus maximizing the packing volume taken up by the particles. This leads to a bulkier overall mass (and a reduction in the quantity of fiber used).
- Scalenohedral PCC particles are best for increasing bulk because they are not a solid mass, but instead are individual crystals emanating from a central core. Therefore, a large part of the volume taken up by any particle is actually empty space.
A third factor that contributes to bulky paper is not unique to PCC but is relevant for all fillers. Larger particles tend to increase bulk more than smaller ones simply because the fibers in the paper are kept farther apart by the large particles. Nonetheless, only PCC can deliver all three important attributes--large particle size, clustered structure and entrained void space. This explains why PCC is the single best filler for bulking the sheet, and also why PCC offers more options than any other pigment for balancing sheet bulk against other important characteristics, like porosity and smoothness.
Sheet porosity is a measure of how easily or quickly a volume of air can pass through the matrix of cellulose fibers and mineral particles that make up a sheet of paper. Generally, porosity is inversely related to bulk. That is, the bulkier the sheet, the more open. Conversely, a closed sheet is one that resists the passage of air. Porosity is a parameter that must be carefully controlled. If a sheet is too closed it may not be receptive to inks used in the printing process. Too open, and the sheet may jam in the air handling systems used by modern photocopiers, or may be too receptive to inks resulting in poor print quality.
One means of controlling sheet porosity is to blend larger pigments with smaller ones. By properly selecting the larger pigment, good sheet caliper and stiffness can be obtained also. The smaller filler particles, which fill in the void spaces created by the larger ones, help reduce the air flow through the sheet, that is, the porosity.
Smoothness is a characteristic of paper that affects its look, feel and especially printability. In practice, the quality of paper that is actually measured is roughness, which of course is the opposite of smoothness. Values for smoothness therefore are expressed as the inverse of measured roughness. Various techniques involving the flow of air over the paper surface are employed to measure roughness. One of the most common measurements used in the United States is Sheffield roughness, expressed as Sheffield units (SU). This method is based on air leakage that occurs when a piece of paper is used as the seal in a device that provides a constant flow of air at constant pressure. Another air leak measurement, called Bendtsen roughness is commonly used in Europe rather than the Sheffield roughness measurement. In these methods, when a paper or paperboard is rough, it allows more air to flow across its surface and out of the instrument. In such cases the Sheffield or Bendtsen roughness values, expressed as ml of air, will be relatively high. Sheffield roughness is expressed as the volume of air flowing over the sheet in a given time at constant pressure. Another smoothness measurement technique commonly used is the Parker Print Surf measurement. This technique is frequently used for printing grades of paper and paperboard, and is designed to simulate conditions on a printing press. The roughness value given by the Parker Print Surf measurement measures the “topography’ of the paper surface, and is expressed as a distance measured in micrometers.
The roughness (or smoothness) of paper and paperboard is important for producing high quality printed images. Smoothness also affects the textured appearance of the printed product. Higher roughness will often produce a very textured appearance, which detracts from the print quality. Smoothness or roughness is affected by a number of factors. First is the uniformity of the paper as it is being formed, called the formation. However, once the paper is formed, smoothness can be subsequently improved. For instance, the product can be coated, but this is costly and is usually done only for premium paper grades. Another way is to flatten the paper surface between two high pressure rolls in a process called calendering. But calendaring reduces sheet bulk. Thus, the papermakers must balance calendering against sheet caliper (bulk). Low caliper can result in low sheet stiffness and poor runnability on a printing press.
As discussed here, PCC tends to yield the bulkiest papers. Papermakers who use PCC can calender their paper at greater pressures to obtain maximum smoothness and maximum bulk, while still maintaining acceptable porosity. PCC is unique in this respect.