As in almost all things involving nature and its resources, papermaking is an art wherein a balance of properties is achieved in the final sheet. No one mineral pigment is capable of maximizing all the product qualities the papermaker desires, while minimizing those qualities that are unwanted. A classic example is the balance between sheet caliper, or bulk, and porosity. Increased sheet bulk, especially in uncoated wood-free copy papers, is highly desirable. High sheet bulk creates better mechanical stiffness in the sheet, resulting in better runnability on printing presses, copy machines and such. But higher bulk also creates a more porous sheet, and greater porosity tends to diminish printability, resulting in poorer handling on presses and in copy machines. Balance, thus, is key.
The synthetic process that yields precipitated calcium carbonate (PCC) is capable of producing a range of particle sizes. This is not unique to PCC. Ground calcium carbonate (GCC), for example, is also produced in a range of sizes by crushing, screening, and fractionating high-quality limestone. But though it is possible to control the size and size distribution of GCC, it is not possible to manipulate the shape (morphology). PCC, on the other hand, can be produced over a range of particle sizes and several different morphologies. This allows combinations of sizes and morphologies that are not possible with other fillers. It is a huge advantage to a paper mill to have on-site a Specialty Minerals Inc. (SMI) PCC plant capable of producing multiple products that are tailored to meet the needs of the host mill as single pigments or in blends.
Blends of pigments exhibiting different sizes and/or morphologies can sometimes impart a balance of properties that no single pigment can deliver. A simple example is a blend of ALBACAR® LO PCC and ALBACAR® HO PCC. The weight fraction of the first, which is a scalenohedral pigment that can be produced in controlled sizes around 2 um, will improve the caliper of the sheet. The latter is also scalenohedral, and is produced in controlled but smaller sizes around 1.2 um; it helps control porosity and also provides a slight improvement in brightness and opacity. This simple example shows how blended fillers have the potential to maintain optical and mechanical properties in the finished sheet that would be lost if a single pigment were used.
Coating formulations have always used blends of pigments to balance properties. As the brightness standards for paper increase, the choice of available pigments capable of achieving these standards is proportionally narrowed. In these cases too, SMI satellite PCC is capable of providing unique blends that balance brightness, gloss, opacity, printability, and cost.
Every paper mill and paper client is unique in its needs. The flexible design of SMI PCC satellites allows tailored blends of products that, plainly and simply, can meet more of these needs than any other pigment on the market.
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