1.INTRODUCTION
Additives are materials used to improve the fifinished paper itself or aid in the process of papermaking. Functional additives such as dyes, internal sizing agents, adhesives to increase wet or dry strength, and fifillers are used to improve or impart certain qualities to the paper product and must be retained on the sheet to be effective. Control additives such as biocides, drainage aids, retention aids, pitch control agents, and defoamers are added to improve the papermaking process but do not directly affect the product and are not necessarily retained on the product. Of course, many additives have several effects at the same time; for example, alum is required for rosin sizing under acid conditions but also serves as a drainage and retention aid. Both types of additives are added to the stock before papermaking.
Metering and Pumping of Additives
Liquid additives are expensive chemicals that must be used at exact levels; thus they are usually metered into the system from positive displacement metering pumps. Like most positive displacement pumps, be sure to use appropriate fifilters (such as a 10 mesh or fifiner screen) in front of the pump inlet to keep the pump from fouling by clogging the check valves in an open (and useless) position. Use a pump inlet line (at least 12 mm 1/2 in.) for additives with viscosity above 100 cps. Use a flflow measurement device of some sort to insure flflow of each additive. As it is often more effective to pump above ambient pressure, a pressure gauge can be used with a pressurized output to at least give a qualitative flflow rate, which is better than no indication of flflow. Surprisingly, many vendors of additives supervise the addition of their additives even to the extent that they set
the flflow rates at mills. While one might rationalize that they know their product best, they do not know how their product will affect other (unknown) wet end additives.
- FUNCTIONAL ADDITIVES
Fillers
Fillers are pigments that are added to stock for opacity and brightness improvements of printing papers. The ideal properties of pigments for printing papers are high brightness, high index of refraction (to help scatter light and increase opacity), small and uniform particle size for smooth paper, low water solubility, inertness, low cost, low abrasion, low specifific gravity, and high retention levels. About 50% of the fifiller is retained in the sheet. Fillers are often ground or precipitated calcium carbonate (PCC) (with paper machines operating at pH of 7 or higher), titanium dioxide, or clay. Filler is used at 10%-30% to replace expensive fifiber; at the higher levels of addition, the paper becomes limp as fifillers interfere with fifiberefifiber bonding and do not impart strength themselves. Fillers are not used in linerboard or other papers where strength is the principal desired property.
Table 4.1 summarizes the properties of paper fifillers and coating pigments. Clay (or kaolin) is an inexpensive fifiller, mined from natural deposits, used in magazine and book paper. It is not as bright (80%e92%) as calcium carbonate or titanium dioxide, has an index of refraction of 1.55 and a specifific gravity around 2.58, and is abrasive. Typically, 40%e90% of clay particles are less than 2 mm. More clay is used than any other fifiller in paper traditionally, accounting for 90% of all fifillers and coating pigments. Clay consists of hydrated SiO2 and AI2O3. Calcium carbonate (chalk or limestone) is becoming an extremely important fifiller to the industry. The mineral form is called calcite and occurs in limestones, chalks, marbles, and other forms. As it reacts with HCl to give CaCl2 and CO2, it must be used in alkaline papermaking systems at pH 7.0 or higher. The brightness is about 92%e95%, with an index of refraction of 1.65 and a specifific gravity of 2.7e2.85. Median particle sizes for the various forms of calcium carbonate are as low as 0.5 mm to as high as 3 mm or larger depending on how fifine it is ground. Finer particle sizes give higher gloss and may improve brightness. Calcium carbonate is inexpensive (about 25%e50% of the cost of bleached pulp) and has low abrasion. It is made by grinding one of the naturally occurring mineral forms or by precipitation from solution to form CaCO3, PCC. PCC has a high scattering coeffificient (compared with ground calcium carbonate) that increases the opacity of paper. Calcium carbonate compares well at $200e$500/ton to bleached wood pulp at $600e$800/ton and titanium dioxide which is as high as $2000/ton. PCC is not a new product but is new to the North American pulp and paper industry.
Fig. 4.1 shows some examples of PCCs with different properties. PCC is produced by mixing CaO with water to form Ca(OH)2 in solution. Addition of CO2, for example, from flflue gases of the lime kiln, causes formation of the insoluble CaCO3. PCC offers advantages over chalk because of its fifiner particle size and uniform size distribution, higher chemical purity, and control over the zeta potential (ionic charge) of the surface. The shape of the particle can be controlled so as to give plate shapes that are ideal for papermaking. It is said that the pH of PCC tends to be somewhat higher than ground calcium carbonates.
