Although the basic composition of paper is plant fiber, in order to improve the physical and mechanical properties of paper and make it more suitable, ordinary paper is not made of fiber alone. Generally, some auxiliary materials are added in the papermaking process according to the needs, such as adding certain minerals to the pulp . This process is called "filling". The purpose of filling is to increase the opacity of the paper, increase the smoothness, improve the whiteness and elasticity of the paper, reduce the hygroscopicity and keep the size specification stable, improve the printing adaptability and so on. Of course, it can also save the amount of pulp and reduce the cost at the same time.
Commonly used fillers in the paper industry include clay (kaolin), talc, etc., and sometimes calcium carbonate (chalk), calcium sulfate (gypsum), barium sulfate, zinc white, titanium white, etc. The dosage is generally between 20-27%.
After paper is filled, its opacity , which is a very annoying problem in the printing industry.
Although the addition of filling is beneficial, it also has the disadvantages of reducing the strength of the paper, not being able to withstand folding, and becoming brittle. This is because the filling hinders the interaction of hydroxyl groups between paper fibers, thereby reducing the hydrogen bond.
In addition to fillers in paper fibers, there are so-called sizing materials. Because there are many capillaries between the paper fibers, they can absorb water, and the fibers themselves are hydrophilic, so there is a phenomenon of "water bleeding" when writing. Sizing is to fill the capillary between fibers to improve the water resistance of paper. In industry, paper is often divided into high sizing (such as banknote paper), heavy sizing (writing paper), light sizing (printing paper) and no sizing (absorbent paper, toilet paper) according to different uses of paper. kind.
There are two methods of sizing: internal sizing and surface sizing. Internal sizing is to add some sizing material to the pulp during the papermaking preparation process to make it settle on the fibers . When the paper sheet goes through the drying process, the surface sizing particles will melt into a thin film. , In this way , the water resistance of the paper is improved.
Surface sizing is to use sizing rollers to coat .
There are roughly four types of sizing agents commonly used in papermaking: (1) Vegetable sizing materials, rosin is the most , and alumite (aluminum sulfate) is generally added as a precipitating agent. Of course, the rosin and aluminum sulfate liquid added to the pulp can also be collectively referred to as the rubber compound. In addition, glues such as starch and other plant-based yellow hollyhock roots belong to this category; (2) animal glues, including bone glue, hide glue, casein, stearic acid, etc.; (3) mineral glues, which refer to The most common is water glass (Na2SiO5), paraffin, etc.; (4) Synthetic rubber materials, there are many varieties of this type, and the price is relatively expensive, such as melamine, urea-formaldehyde, formaldehyde resin, hydroxymethyl cellulose, polyvinyl alcohol etc.
How does rosin, as an internal sizing agent, precipitate onto the fiber? This can be explained with knowledge of colloid chemistry. Rosin gum is a negatively charged particle, and cellulose is negatively charged in water. Therefore, if only rosin gum is added to the pulp, it will "negative and negative repel each other" and cannot be combined. The result must be washed by water. Lose. After adding aluminum sulfate, the negative charge of the fiber surface can be changed. With the help , the surface of the fiber becomes positively charged. Because the positive and negative charges attract each other, the rosin particles are adsorbed by the fiber and precipitated.
In order to make the paper colorful, some colored papers also contain colored bodies (such as pigments , dyes, etc.).
