What does paper filler do to paper?

Paper is composed of fibers, but for some papers, it is difficult to achieve the purpose of paper using only fibers. For this reason, for cultural paper, printing paper, decorative base paper, white cardboard, cigarette paper, etc., Part of the filler must be added to the pulp to meet the requirements of the paper.

The main role of fillers
1) Improve the whiteness of the paper, because the whiteness of the filler is higher than that of the pulp;
2) Increase the opacity of the paper;
3) Improve the smoothness of the paper;
4) Improve the softness of paper;
5) Improve the printing performance of sheets;
6) Reduce the degree of deformation of the paper, that is, improve the flatness and dimensional stability of the paper;
7) Reduce resin barriers;
8) Meet the special requirements of some paper;

Types of fillers and their properties

1. Talc

Talc is a layered structure silicate mineral, and the functional properties of talc are mainly derived from its structural characteristics: lipophilic, hydrophobic and lamellar structure. The chemical stability, low hardness, lubricity, lamellar structure, lipophilicity, hydrophobicity and other structural characteristics of talc determine that it is a rare functional papermaking mineral material. In my country, more than 50% of the output of talc is used in the paper industry. Mainly used for papermaking filler, a small amount for papermaking resin (stickiness) control agent and paint pigment. Talc smoothes the paper, weights and enhances the adsorption of printing inks and pigments.

Different types of talc have different properties and are suitable for different paper products. Low-grade talc and its compound processing products will be mainly used for ordinary papermaking fillers; mid-grade talc is mainly used for high-quality functional papermaking fillers and primer coatings; high-quality talc is mainly used for viscosity that better reflects its functional characteristics Control agent products and coating grade products. And after ultra-fine processing, talc has good adsorption and coverage. At present, the new talc products that can be used in the paper industry mainly include cationic modified talc, solid paraffin and AKD modified talc, black talc, magnesium aluminum hydrotalcite, etc.

2. Porcelain clay (kaolin)

Kaolin is mainly used in papermaking fillers and surface-coated pigments in the papermaking industry, and its characteristics have a great impact on the operability of papermaking and the quality of the finished paper. According to different processing methods and product characteristics, kaolin for papermaking coatings is divided into hydro-separated kaolin and calcined kaolin. Among them, calcined kaolin is a functional material for paper making with high whiteness, porous and certain adsorption. ). The former is mostly used for papermaking fillers, and the latter is mostly used for papermaking coating materials. Its functionality mainly comes from the dehydration and deorganization process of kaolinite (soil) roasting. The porosity of the roasted structure can improve the bulk, opacity, and ink absorption of paper.

Kaolin is one of the most consumed white non-metallic mineral materials in the paper industry. More than 75% of the refined kaolin in the world is used in papermaking.

3. Wollastonite

Unlike traditional fillers, wollastonite is not simply filled into paper, but intertwined with plant fibers by virtue of its high aspect ratio to form a “plant fiber-mineral fiber” network structure, which partially Substitute vegetable staple fiber. Wollastonite is used in papermaking, and has the advantages of improving paper opacity, reducing production cost, improving evenness, and improving printability.

Fibrous wollastonite has many special properties that cannot be replaced by traditional fillers, and is suitable for replacing traditional fillers in cultural paper and some special papers and high-grade papers. The chemical modification technology to reduce product abrasion, improve fiber softness, and how to fully dissociate fiber bundles on the premise of maintaining fiber length is an important research field for wollastonite processing.

4. Calcium carbonate

Calcium carbonate has many advantages such as low price, can improve the opacity of paper, and increase the ink-absorbing performance of paper. It is one of the important traditional fillers in the paper industry.

Calcium carbonate or modified calcium carbonate fillers can endow paper with special functions under certain conditions, such as magnetic properties, photocatalytic properties, flame retardancy, antibacterial properties, electrical conductivity, and high bulkiness.

The advantages of calcium carbonate as a paper filler are:
1) Can make the paper have higher whiteness;

2) can make the paper have higher opacity;

3) Good coverage;

4) High bulk;

5) Good printing adaptability and ink absorption;

6) Excellent air permeability;

7) Good combustion-supporting properties;

5. Titanium dioxide

The trade name is titanium dioxide, which is processed and refined from ilmenite (FeO.TiO2). The refractive index of light is as high as 2.55~2.70, the whiteness is high, and the covering performance of paper ranks first among all fillers, but its price is expensive. .

Titanium dioxide has rutile and anatase types, and their chemical properties are different. The former has poor water solubility, so the paper industry (especially tissue paper) uses the latter as a filler.

6. Iron oxide pigment
Iron oxide pigments used as fillers for some special papers are a kind of inorganic pigments, which are not widely used at present. They are part of iron-based pigments. Generally, only red iron oxide, iron oxide yellow and iron oxide black are used. Four kinds of iron brown.

The common feature of iron oxide pigments is that they are insoluble in water and have no affinity with pulp fibers, but rely on the action of alum to be fixed on the fibers to obtain coloration and at the same time play a filling role. Iron oxide pigments have strong lightfastness and good hiding properties.

Iron oxide pigments play a filler role in high-grade decorative base paper (or tipping base paper), and can also partially replace expensive titanium dioxide. At the same time, it plays a role in dyeing, and can replace organic dyes with high price and serious pollution.

7. Bentonite

Bentonite is a hydrous aluminosilicate mineral containing small amounts of alkali and alkaline earth metals. Its multiple properties such as large specific surface area, high dispersion and thixotropy make it suitable for various applications in the paper industry to show considerable advantages. For example, when used as a papermaking filler, it has high whiteness and purity, and using a large amount of addition will not reduce the strength and tearing of the paper; when used as a papermaking coating, it can improve the water retention and suspension of the coating. and rheological properties. After separation, purification and modification, various properties of bentonite are greatly improved, and it can be made into deep-processed products such as sodium-based bentonite, granular bentonite, and organic bentonite. The modified bentonite has better adsorption and dispersion properties, and has a higher retention rate than talc, and has less influence on the paper strength.

8. Diatomaceous earth

The domestic paper industry mainly uses diatomite to produce filter paper and cardboard for liquid beverages, sugar refining, and winemaking by using the process characteristics of diatomite, such as large porosity, strong adsorption, small bulk density, heat insulation, sound absorption, chemical stability, and non-toxicity. It is also used in construction. Decorative cardboard, battery separator, super absorbent tissue paper, flame retardant paper, pet pad paper, wastewater purifier, coagulation aid, etc.; after organic modification, it can also be used as micro retention and drainage aid, etc. There are many impurities in diatomite ore. In order to improve its functional performance, diatomite is usually modified.

Reasonable selection of fillers

1) Talc
Because talc powder has the characteristics of fine texture, high whiteness, good coverage and low wear, and the smoothness and gloss of talc powder are high, and the price is cheap, talc powder is used as filler in general cultural paper and printing paper; Talc powder has good chemical stability, so disperse rosin is used as acid sizing for sizing, and talcum powder is used.

The calcium carbonate content in talc is generally required to be less than 2.5%, and at most 1.5%, because higher calcium carbonate content is unfavorable for sizing and dyeing. For decorative base paper, high calcium carbonate content is not conducive to subsequent The processing procedure has an adverse effect; as the white pigment of the coating, the powder has an adverse effect on the whiteness of the coating.

According to research, for every 10% increase in its dosage, the whiteness value decreases by 1.0%. The reason is that the talc powder is slightly hydrophobic, so the dispersibility in the coating system is poor, and the fine talc particles are easy to trap air, thus forming small bubbles , When the phenomenon of aggregation occurs, the coating will be uneven, so try not to use (less use) talcum powder as a pigment in the production of coated paper.

2) Calcium carbonate
In the application of new medium (alkaline) sizing agents such as AKD, calcium carbonate, a filler that is difficult to use in acid papermaking, has its place. The refractive index of calcium carbonate is as high as 1.65, which can significantly improve paper whiteness and opacity. .

In the pH range of 7.5~8.5, calcium carbonate has good stability, so the paper with medium (alkaline) sizing uses calcium carbonate as filler; for tissue paper, such as cigarette paper, printing tipping base paper, etc. Due to the requirement of high opacity, calcium carbonate is used as filler. For some papers with high printing performance requirements, calcium carbonate can also be used as filler.

Because calcium carbonate has less ultraviolet absorption, it is beneficial to improve the aging of paper. Therefore, for some papers that need to be stored for a long time (collections), calcium carbonate is used as filler; for paper with acid sizing, calcium carbonate cannot be used as filler; calcium carbonate can be used in large quantities. It is used as a color pigment for coated paper. Because of its cheap price, good fluidity of coating, and better printing performance than PCC, ground calcium carbonate is often used for white coating;

3) Titanium dioxide
Due to its high refractive index, high whiteness (97%~99%), and hiding power ranks first among pigments (fillers), it is used in the production of high-grade tissue paper or high-grade coated paper, but due to its high price , for which the dosage is less;

4) Porcelain clay
Due to the good chemical stability of china clay and its sheet structure, it can improve the smoothness and gloss of paper. Therefore, it can be used as a filler for various papers. It is only suitable for acid sized paper. However, due to its relatively high viscosity , so the addition amount is less, for example, it can be used together with talc; it can be used as a white pigment for coating, the higher the clay content in the flake, the better the coating performance (coating fluidity), and the coated paper (cardboard) ) its gloss and smoothness are better.

5) Iron oxide pigments
It is suitable for the production of tissue paper, such as high-grade decorative base paper and tipping base paper, which is characterized by its high coverage.

Papermaking is a complex technological process. Different types of paper have different quality requirements. In order to meet certain requirements of paper, it is necessary to add fillers; there are many types of fillers, each with different properties, different types of paper and process conditions. Different fillers or combinations of fillers must be selected to obtain the best results.