Filler is the most important material of paper mill. It is usually used in paper mill to reduce cost. Depending upon the performance characteristics of the particular filler as well as the percentage added these materials can enhance the physical, optical, price-performance & esthetic properties of the finished paper. Fillers is used in the wet end operation of paper making.
A good paper filler should be exhibit good distribution within the sheet, no dusting, maximize strength, maximize bulk etc.
One of the survey shows that the filler used in North America PCC 70%, GCC 13%, Kaolin 15%, TiO2 2%, Silica/Silicates 0.3%, talc 0.1% and Aluminium Trihydrate 0.1%. In Europe used of PCC increased day by day.
Advantages of paper filler
Fillers that are dominantly applied in the paper mill are mostly minerals. There are quite a lot of reasons why fillers are used in papermaking. The main reasons are their low cost compared to fiber; since the cost of paper filler is one third or one fourth of fiber. The application of fillers also increased brightness, opacity, productivity, improve printability (smoothness, ink absorption, show through), gloss of paper, sheet formation (fill in void), dimensional stability, paper’s appearance etc. In addition they increased water drainage and drying rate of the formed sheets.
Problems of using paper filler
There are some negative effects on paper with the use of filler such as reduced paper strength, increasing the demand for sizing agents, retention, abrasion, dusting, sheet two-sidedness and reduced bulk. Moreover Calcium Carbonate filler is unsuitable in acid papermaking and wood-containing paper. A final product of higher density is also a drawback.
The choice of filler material and the loading level are determined according to kind of pulp furnish, paper grade, process and paper machine configuration.
Important characteristics of paper filler
Particle size, particle shape, specific surface area, light absorption properties, particle charge, refractive index and abrasion are more important characteristics of filler materials.
Particle size: It is plays an important role on the optical properties of any paper filler. If particle size of filler is fine, then it scattered good light. Particle size also affect on drain-ability and resultant reduction in steam consumption.
Particle shape: Particle shape is a significant factor for optical properties. It is also affects abrasion values. Minor the abrasion value longer the wire-life.
Specific surface area: The specific surface area of paper filler depends on the particle size, shape & degree of agglomeration. It is helps in light scattering & also influences the strength & printing characteristics of the paper.
Effect on paper strength: filler material has negative effects on paper strength. If more filler uses, then it is interfering with fiber to filler bonding within the sheet. Hence the bond formation is weak, so reduced the paper strength. Moreover weak bond will be increasing the dusting tendency.
Abrasion: The abrasive characteristics of paper fillers increase wear to wire on the paper machine and to the printing plates in the printing press. The crystalline nature, hardness, impurities and large particle of the filler tend to be more abrasive.
Refraction Index: Refraction index is one of the important characteristics of paper filler. Light scattering is influenced by this property. If refracted index of a filler is higher, then it will be scattered more light. As a result opacity improves.
Retention: Higher fine material content in the circulating water system reduced retention. Hence some retention aids have to be used, as a result increased paper production cost.