Due to the high surface energy, hydrophilic and oleophobic properties, and strong polarity of nano-calcium carbonate, it is easy to aggregate into agglomerates. Therefore, when the dispersion of nano-calcium carbonate is insufficient, flocculation will occur, thus affecting the application of nano-calcium carbonate in papermaking. If only the general mechanical dispersion method or organic dispersing agent is used to disperse it uniformly, sodium polyacrylate is used as the dispersing agent to disperse the nano-calcium carbonate, which has a better effect. This is because sodium polyacrylate is adsorbed to form an anion adsorption layer, which plays a dispersing effect through the repulsion of the electric double layer, and also forms a covering layer on the surface of the calcium carbonate particles, which blocks and disperses the agglomerated particles through its long polymer chain.
1 Suitable particle shape
The most basic particle shapes of calcium carbonate for papermaking are: aragonite shape, rhombic hexahedron, and scaleno triangle. Other shapes can be regarded as derived from these three shapes by controlling the production process conditions. At present, the typical forms of calcium carbonate commonly used in domestic papermaking are different when used as fillers, coatings and cigarettes.
The commonly used forms when used as fillers are: spindle shape, rice grain shape, column shape, chrysanthemum shape, polymer, amorphous, etc. Commonly used forms when used as coatings are: spindle shape, rice grain shape, rhombus cube, flake shape, short column shape, amorphous shape, needle shape, etc. Carbonate forms for cigarette paper fillers; spindle, chrysanthemum, needle.
Some production practices show that spherical PCC not only has high cost, but also has low filler retention and paper strength; chain PCC has poor uniform distribution in paper fibers and is only used in special occasions.
2 Crystal form
The crystal morphology of PCC is one of the most important factors affecting paper production operability, coating performance, paper quality and other aspects.
Various forms of PCC can be roughly divided into two existing forms: single transistor particles and aggregates. Previously, the influence on papermaking mainly came from the morphology and aspect ratio, and the latter mainly came from the pore structure characteristics in the body.
3 Suitable aspect ratio
The aspect ratio refers to the ratio of the length to the diameter of a single particle. Spindle, column and needle all belong to large aspect ratio crystal forms.
PCC with a large aspect ratio is used for paper fillers. When the calcium carbonate single crystal is combined with the paper fibers, it is easy to generate a bridge effect, so that the fibers are stretched and uniform pores are formed, which can improve the air permeability, thickness, opacity and stiffness of the finished paper. , reducing the two-sided difference, and obtaining a higher filler retention rate are all beneficial. However, if PCC with a large aspect ratio is used for paper coatings, it will show high viscosity, poor viscosity stability, and poor coating gloss. On the contrary, small aspect ratio PCCs such as rice grains and rhombus cubes are only suitable for paper coatings and show good coating gloss; flake PCCs have good gloss on paper, but due to the difficulty of production and high prices, currently Not commonly used.
4 suitable polymers
Multimeric PCC, also known as ligand PCC, is generally composed of monomers such as needle-shaped, ultra-fine spherical, cubic, and rice-shaped. Its main physical properties are: high porosity, high dispersion coefficient, high oil absorption value and low bulk density value formed between particles. At the same filler retention rate, it is beneficial to improve the opacity, bulk, stiffness, calendered paper smoothness, air permeability and ink absorption of the filled paper, showing a higher water retention value in the coating, for the filler system It is beneficial to adsorb viscous substances to purify white water, etc. The main negative effect is slightly lower strength and sizing of filled paper, followed by higher viscosity and lower surface strength when used in coatings. Therefore, the complex PCC is rarely used in papermaking coatings alone, and is often used in combination with other PCCs with small aspect ratios to improve the pore structure of the coating, and improve the bulk, air permeability, and ink absorption of calendered paper.
5. Viscosity effect of PCC with different morphologies
It is often hoped that PCC used in coatings has lower viscosity and viscosity stability, except for the cubic crystal form. The PCC of coatings in the paper industry is often short columnar, amorphous, chrysanthemum-like, etc. with small viscosity and viscosity coefficient.
6 Effects of different forms of PCC on the properties of filler paper
The effects of different forms of PCC on the properties of filler paper are relatively good, such as chrysanthemum shape and spindle shape. Except for tensile strength and elongation, the spherical shape has good performance, and the performance of the cube is generally poor. In the paper industry, the following aspects are mainly considered in the quality control of high-quality nano-calcium carbonate.
Particle shape
Any form of PCC requires regularity, integrity, consistency, and smooth edges without defects. There is no adhesion between the aggregated particles of the polymer; the number of chrysanthemum-shaped single crystal particles is similar, the opening degree is similar, and the end of the single crystal is sharp; the spindle-shaped and needle-shaped are sharp.
Major axis and minor axis dimension (length-to-diameter ratio or diameter-to-thickness ratio)
High-quality PCC such as spindle-shaped, rice-grain-shaped, column-shaped, and needle-shaped have certain limitations on their long-axis, short-axis dimensions, or aspect ratio.
Apparent porosity and porosity
The surface of PCC has small pores, and the microscopic pores of PCC with good quality are small and uniform, so that PCC with the same shape and particle size has a larger specific surface area, as well as high oil absorption value and astigmatism coefficient. The opening degree of the needle-like single crystals of chrysanthemum-like PCC has a direct effect on the apparent porosity and porosity of the particles. The macroporous water of the polymer formed by cooperating with other soiling particles should be uniform.
Morphological homogeneity
High-quality calcium carbonate should have regular order and uniformity of morphology, including consistency of shape, consistency of long axis and short axis, approximation of aspect ratio, consistency of particle size and pore size distribution, etc.