Commonly used chemical additives for papermaking
I. Surfactants used in waste paper recycling
1.1 Deinking mechanism
First, the fibers are separated and swelled by physical and chemical methods, and then the ink is saponified and emulsified by chemicals, dispersed in the slurry, and finally the ink is removed by washing or flotation.
1.1.1 Basic composition and chemical properties of ink
The main components of ink are pigments in the dispersed phase and binders in the continuous phase. The latter mainly plays a role in bonding and film formation. Pigment particles are particles aggregated by thousands of molecules, generally composed of inorganic substances such as porcelain clay and carbon black. Organic pigments are also used at present. Binders are generally viscous liquid substances prepared from drying vegetable oils and various resins. Ink chemicals are driers, viscosity regulators, color enhancers, etc. added to improve the performance of inks and adjust the printing adaptability of inks. Generally, they are used in small amounts. The ink particles to be removed by waste paper deinking are tiny particles formed by the bonding of pigments (especially carbon black) by the binder. The resin in all inks has the greatest impact on the deinking of waste paper, that is, the binder has the greatest impact on the deinking of waste paper.
Binder type Preparation method Features
Oily binder Heat the dry vegetable oil to a certain temperature and keep it warm to polymerize it into dimers and trimers. Such as linseed oil, tung oil, castor oil, etc. Cross-linking occurs through the action of oxygen in the air and solidifies into a film. It has good adhesion and certain water resistance, the film has good gloss, slow fixation speed, and long drying time.
1.1.2 Classification of deinking agents
According to the different deinking, it can be divided into flotation deinking agent and washing deinking agent.
The ink formula of newsprint letterpress printing is different from that of offset printing. The ink of letterpress printing is dispersed and penetrated through mineral oil, and the content of resin binder is low, which is relatively easy to deink. Offset printing is to heat the resin paint and carbon black together, and through bonding and film-forming, a hard film of ink is formed on the surface of the fiber. Letterpress printing can use alkali and industrial soap to saponify and emulsify the resin and mineral oil for deinking. For offset printing, the ink hard film combined with the fiber should be destroyed first, and then the resin and ink should be dispersed by saponification and emulsification.
1.2 Selection of deinking agent raw materials
Deinking agents are mainly composed of surfactants, caustic soda, sodium silicate, hydrogen peroxide, etc. Surfactants are mainly nonionic surfactants and anionic surfactants. Cationic surfactants are usually weakly acidic or neutral in aqueous solution, and are often used as bactericides, softeners, antistatic agents, dyeing aids, etc., but the washing performance is poor and the price is also very expensive, so cationic surfactants are generally not used for deinking. Anionic surfactants have good oil removal performance and are also relatively cheap; nonionic surfactants have good emulsification and dispersion effects at lower concentrations. Therefore, deinking agents generally use anionic-nonionic surfactant compounds.
2. Defoaming agent
2.1 Mechanism of action
The defoaming agent enters the bimolecular oriented film of the foam, destroys the mechanical balance of the oriented film and achieves the purpose of defoaming or foam suppression.
①, Chemical reaction method The defoamer reacts with the foaming agent. For example, when the foaming agent is soap, acid can be added to turn it into stearic acid, or metal ions such as calcium and magnesium can be added to form water-insoluble stearates, which causes the foam to burst and disappear.
②, Reduce film strength method The defoamer is a non-polar solvent, such as kerosene, diesel, gasoline, etc. These organic hydrocarbons can spread quickly on the surface of the liquid, take away part of the foaming agent, make the liquid film thinner and reduce the strength, causing rupture. This defoamer must be emulsified before it can be used in papermaking, otherwise kerosene and other substances will pollute the fiber.
Most defoamers use small molecule alcohols, such as ethanol and octanol, which can enter the foam bimolecular oriented film, reduce the film strength, and bring part of the foaming molecules into the water through the diffusion of these polar molecules, causing the foam to burst.
③. Cause local tension difference Surfactants and solid hydrophobic particles that can significantly reduce surface tension, such as fluorinated surfactants, silicone oil, polyether, high carbon alcohol, colloidal silica, distearyl ethylenediamine (EBS), can enter the foam bilayer, resulting in a local reduction in the surface tension in the membrane, while the rest of the membrane still maintains a large surface tension. This tension difference causes the stronger tension to pull the weaker part, resulting in the rupture of the foam.
2.2 Occasions for the use of defoamers
During the alkaline pulping process, a large amount of foam will be generated. At this time, the defoamers added are mainly polyethers and fatty acid esters.
The defoamers added during the wet end processing are generally emulsion-type, which are a combination of surfactants and hydrocarbons. Distearic acid amide, stearic acid polyoxyethylene ester and other defoaming effects are the best.
Silicone defoamers are usually used in coating treatments.
2.3 Types and main varieties of defoamers
2.3.1 Hydrocarbon defoamers
Hydrocarbons are generally not used alone, but are mixed with emulsifiers and waxes to form W/O emulsions as defoamers. They have strong diffusion ability and fast defoaming speed, but poor defoaming effect. They are mostly used in pulping processes and have good defoaming effect when combined with hydrophobic particles such as colloidal silica.
The main components of hydrocarbon defoamers are:
(1) Hydrophobic particles Surface-treated colloidal silica and distearyl ethylenediamine (EBS). EBS can directly replace silica, and the dosage is half of that of silica. The amount of hydrophobic particles added is about 1%.
(2) Oil-soluble surfactants Its function is to reduce the interfacial tension between the defoamer suspension and the treated liquid, so that the defoamer can spread on the liquid surface in the form of a thin film. Various fatty acid derivatives are commonly used as oil-soluble surfactants. Its dosage is about 1%~2%.
(3) Hydrocarbon solvents are generally paraffin hydrocarbons such as kerosene, gasoline, diesel, etc., accounting for about 40% of hydrocarbon defoamers. Adding a small amount of silicone oil to hydrocarbon defoamers has a significant synergistic effect.
2.3.2 Silicone defoamers
Generally, organic compounds such as hydrocarbons, ethers and phosphates have a large spreading coefficient, so they have a strong destructive effect (defoaming) but poor antifoaming ability. Silicone has a very small spreading coefficient. Pure silicone such as dimethylsiloxane has no defoaming effect, but after emulsification, the surface tension decreases rapidly, and a small amount can achieve a strong defoaming and foam suppression effect.
2.3.3 Polyether defoamers
Blocked polyethers are commonly used defoamers with extremely significant effects. These surfactants are often used in combination with silicone oil and mineral oil to reduce costs and achieve a comprehensive effect of antifoaming and defoaming.
III. Pulp control agents
The main types of pulp preservatives are as follows
3.1 Surfactants
Cationic and amphoteric surfactants have a more significant effect in this regard. Its mechanism of action is not yet clearly understood. It is generally believed that the cationic group of the surfactant adsorbs on the cell wall of the microorganism to destroy a certain enzyme in the cell wall, reacts with the protein and affects the normal metabolic process of the microorganism, eventually leading to the death of the microorganism.
Cationic surfactants, especially quaternary ammonium salts with benzyl groups in the molecular structure, have strong bactericidal properties, but in the case of pure other proteins or heavy metal ions, the bactericidal ability of some amphoteric surfactants exceeds that of cationic surfactants, especially in the case of compounding with anionic surfactants, which further shows the superiority of amphoteric surfactants.
3.2 Organic bactericides
Organic mercury, chlorine, chlorophenol, etc. have been basically eliminated due to their high toxicity and easy to pollute water sources. At present, organic sulfur, organic bromine and nitrogen-containing sulfur heterocyclic compounds are mainly used. Typical products include isothiazolones, methylene dithiocyanate and 2,2-dibromocyanopropionamide. The representative product is methylene dithiocyanate, referred to as MBT. MBT has a strong bactericidal effect and a wide sterilization spectrum. It has obvious bactericidal effects on bacteria, fungi and algae.
Benzisothiazol-3-one is less toxic to humans and animals, and the dosage is about 150PPM. It can be used for slurry and coating corrosion protection.
Organic bromine preservatives include 2,2-dibromo-cyanoacetamide (DBCA) and 3-bromo-3-nitro-pentane-2,4-diol.
Peracetic acid and hydroxybenzoic acid esters.
IV. Sizing agent
4.1 Rosin glue series
Rosin is a complex mixture, of which acidic substances account for about 90% (the main component is the isomers of resin acid, and there are a small amount of fatty acids), and neutral substances account for about 5%~10%. Resin acid is the main component of rosin, and there are many isomers. Its common molecular formula is C19H29COOH, and its chemical structure is divided into resin acid type and pimaric acid type.
Rosin sizing mechanism: After free rosin acid ions adsorb AL ions, they are positively charged, or they are positively charged themselves. They are adsorbed on negatively charged fibers through electrostatic attraction and evenly distributed. After entering the drying section of the paper machine, due to its low sintering temperature, the free rosin acid particles with aluminum ions quickly soften and position to form a low-energy surface with the hydrophobic base facing outward. At the same time, the free rosin acid reacts with the aluminum ions adsorbed on its surface to form rosin acid aluminum, which makes the hydrophilic base firmly bonded to the fiber.
4.1.1 Saponified rosin glue
Saponified rosin glue is the first generation of rosin glue. It is made by reacting rosin with a measured amount of alkali. According to the amount of alkali or the degree of saponification of rosin, it can be divided into white rosin glue and brown rosin glue. The saponification degree of white rosin glue is about 75%, and the appearance is turbid and opaque white liquid; the saponification degree of brown rosin glue is close to 100%, and the appearance is a relatively transparent brown or yellow-brown paste. Both need to be diluted into white liquid and added to the pulp when used. Improved saponified rosin glue uses organic alkali as saponifier, has a completely transparent appearance, is easy to disperse in water, and has improved stability when placed.
4.1.2 Strengthened rosin glue
Use maleic acid (anhydride) or fumaric acid to react with rosin, modify the rosin, make strengthened rosin, and then saponify it to make rosin glue.
It is better to replace part of the saponified rosin glue when used.
4.1.3 Emulsion rosin glue
Emulsion rosin glue is the so-called high-dispersion rosin glue (anionic rosin glue), also known as the third-generation rosin glue, which is a white emulsion. It generally contains more than 90% free rosin acid, pH is below 7.0, and the solid content is about 50%. The latex particles are small and can be stored at room temperature for a long time. The sizing effect is excellent. The whiteness, weather resistance, alkali resistance and other aspects are better than rosin glue sizing agents, but alumina is also required for precipitation.
Emulsion rosin glue is also called high-dispersion glue. There are three preparation methods: solvent method, melting method, and reverse emulsification method.
(1) Solvent method: Dissolve rosin with an organic solvent, add a small amount of organic base and emulsifier, and then add water to form an unstable water-in-oil emulsion. Then, pass it through a high-pressure homogenizer or colloid mill to homogenize it, and remove all organic solvents under reduced pressure to obtain rosin latex.
(2) Melting method: Melt the rosin, and preheat the aqueous solution with surfactant to 80~90℃, mix the two and emulsify them through a high-pressure homogenizer, and quickly cool them to below 40℃ to obtain a stable rosin latex.
(3) Reversal method: Melt the rosin at high temperature (120~200℃), add oil-soluble surfactant and water-soluble surfactant, stir evenly, and then add a small amount of 80~90℃ water to form an oil-in-water emulsion. Then, under high-speed stirring, quickly add a large amount of hot water to change the emulsion from oil-in-water type to water-in-oil type, and quickly cool it to below 40℃ to obtain a stable rosin latex.
In order to improve stability, colloidal protective agents should be added to the highly dispersed rosin glue, and casein, polyvinyl alcohol and carboxymethyl cellulose are commonly used. During the process of chemical processing, the requirements for water quality are relatively high, the water hardness increases, and the latex is prone to demulsification and precipitation.
4.1.4 Cationic rosin glue
It is a highly dispersed rosin glue with a positive charge, which contains a large number of rosin acid molecules and has a medium charge density (Zeta potential is about 20mv). The main difference from traditional rosin glue is that cationic rosin glue is cationic. The carboxyl content in cationic rosin glue is reduced, which is due to the reaction of the carboxyl group in the rosin molecule with the cationic agent.
The advantages of cationic rosin glue also include: low latex viscosity and good stability; less aluminum sulfate can be added during sizing; fillers such as calcium carbonate can be added; it can stay on the surface of negatively charged fibers by itself; the pH value of sizing is 4.0~6.5, and it can be used in systems close to neutral, etc.
Cationic rosin glue can be divided into two types: cationic dispersion type and self-cationic type.
(1) Cationic dispersed rosin glue: Emulsify and disperse rosin through cationic surfactants.
(2) Self-cationic rosin glue: Introduce cationic groups into rosin molecules, use the reaction of carboxyl groups or copolymerize rosin with unsaturated cationic monomers to make it cationic, which actually forms cationic rosin surfactants to emulsify and disperse rosin.
The sizing effect of cationic rosin glue will decrease at a higher pH (6.5) because a large amount of rosin acid will become rosin acid soap at this time. Rosin soap has no sizing effect at high pH.
