Introduction:
The original purpose of adding filler to the fibre furnish was as a cost saving diluents with the quantity limited by the commensurate loss in strength properties. Now this use has become secondary to the selection & incorporation of designed fillers to achieve specific improvements in paper quality. Depending upon the performance characteristics of the particular filler as well as the % added these materials can enhance the physical, optical, price. Performance & esthetic properties of the finished paper.
The more common practice is to choose materials that provide the opportunity for both cost & quality improvement.
Functions of fillers:
Fillers can be useful in most types of paper they are especially beneficial in printing & writing papers. The addition of fillers can contribute the following properties to paper.
Better formation & sheet structure through improved fibre distribution & the fill in of void areas.
Improved texture & feel resulting from a smoother surface & more uniform composition.
Increased opacity from both the filler per se & its impact on fibre debonding.
Better prinlability for a number of reasons including a more uniform & smoother surface for receiving the impression. Less showthrough because of higher opacity increased & more rapid acceptance of ink as well as faster ink setting. Reduced ink penetration, feathering & strike through, improved dimensional stability.
Cost savings interrelated with the type & amount of filler used.
The performance of a filler is dependent upon essentially two factors, the properties of the material & the manner in which it is used.
Important characteristics of filler pigments:
Particle size & shape, specific surface area, light absorbing properties, particle charge, refractive index and abrasion are some of the more important characteristics of filler pigments.
Particle size: the optical properties of any pigment are strongly affected by the particle size distribution & the degree of agglomeration of the pigment. Results show that a narrow particle size distribution promotes good light scattering efficiency.
The mie theory predicts that the maximum scattering of light is obtained by spherical particles one half the wavelength of light or approximately 0.2 to 0.3 μm in diameter.
Spherical Particles→ Plastic pigment, titanium dioxide etc
Non spherical particles→ clay, talc, precipitated CaCO3 etc.
Particle shape : When particles deviale from a spherical shape their optimum equivalent spherical diameters may be outside the range predicted by the Mie theory. Also the packing orientation of the pigment will greatly influence its alignment within the fibre matrix of the sheet.
Specific surface area: the particle size, shape & degree of agglomeration all influence the specific surface area of a pigment. The pigment surface area aids in light scattering & also influences the strength & printing characteristics of the paper.
Effect on paper strength: filler pigment will tend to cause a reduction in the strength properties of the sheet. Min general the higher the specific surface area the weaker the paper will be at an equal degree of loading. The primary cause of this weakening effect is related to the pigments interfering with fibre to filler bonding within the sheet.
Abrasion: highly abrasive pigments will cause excess wear of both paper machine wires & printing plates; cutter & trimmer knives in the converting area of the mills are also susceptible to excess wear. The abrasiveness of a pigment is principally caused by two factors. The crystalline nature & hardness of the pigment is of importance to the abrasiveness of the pigment (strength of the atomic bonds, spatial arrangement, impurities etc) along with its physical properties (size, particle size distribution, shape surface area etc). Impurities such as quartz can cause severe abrasion problems & large particles tend to be more abrasive than smaller particles of the same crystalline form.
Types of filler
Kaolin clay: 39% Al2O3, 46% SiO2 & 13% H2O remaining impurities.
CaCO3: Despite its high brightness & other benefits for many years the traditional acid paper marking systems in most instances precluded the use of CaCO3 as a paper filler. Aside from cigarette paper where the incorporation of high % of precipitation CaCO3 as a filler serves to control the paper porosity & burning rate, the uses for CaCO3 in the paper were quite limited.
TiO2: TiO2 has the highest refractive index of any white pigmentary material. Its extremely high price.
Talc: In the US the major use for talc in paper is to control pitch. Becase of its unique properties. There is a limited but growing interest in high purity talc as a paper filler.
Unfortunately in commerce the term talc, perse , is commonly used to cover a wide range of mineral mixtures. Some of which contain very little pure talc. Most of these materials are not suited for use in paper & therefore are not refrred to in what follows
