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About paper aids

  The application of chemical additives in the wet end of papermaking has been increasing substantially in recent years, which is mainly affected by the following factors:
  1. The speed of the paper machine is increased.
  2. Increase in the use of fillers in alkaline papermaking systems.
  3. A large amount of waste paper is reused.
  4. The amount of filler used in newsprint is increasing.
  In order to make users more fully understand the performance and application of our company's paper auxiliaries series products, our technical department has compiled this material with reference to relevant technical materials, hoping to provide some reference for the technical application of customers, due to our papermaking. The professional knowledge is limited, and the inadequacies are inevitable. It is hoped that in the production practice, combined with their own successful experience, these chemical additives will play a more effective role in the papermaking process.
  First, retention aid
  The paper sheet forming process is the most important application field of wet end chemistry. Retention aids and filter aids are two of the most important process auxiliaries. Their use has been increasing significantly in recent years, and the filter aid can be retained. Used alone or in combination. The current development trend is toward the development of cationic polyelectrolytes and particulate retention systems. It must be borne in mind that the white water concentration should be divided into fine fibers, fillers, colloidal substances and dissolved substances. Ideally, by adding different types of retention aids, it should be possible to separately control the concentration of the above substances. However, for those paper stocks with high levels of dissolved and colloidal substances, there is still a lack of very effective retention aids.
  The flocculant with high molecular weight and low charge density (1 million to 5 million, 1% to 10%) is used in the paper industry, often used for retention of fine components under high shear and turbulent conditions; low molecular weight Products with high charge density (1000~100000, 40%~80%) (cationic type) are used for charge neutralization and play a role under low shear.
  There are particles of different sizes and shapes in the stock, and the flocculation kinetics depend on their collision frequency, collision efficiency, floc strength and external forces acting on the flocs. The collision frequency of the particles in the turbulent paper wet end is greater than several thousand times per second, and the adsorption of the polymer can be completed in 1 second. It has been calculated that the collision frequency of paper particles at a shear rate of 1000 s-1 shows that the collision frequency of different particles is significantly different, as shown below:
  Collision frequency between fiber and filler particles 109s-1
  Collision frequency of fine fibers to each other 106s-1
  Collision frequency between fiber and fiber 5000s-1
  Collision frequency between fillers 1s-1
  Even if the concentration of the polymer in the paper stock is only 10 mg/L, the collision frequency between the fiber and the polymer can be as high as 1010 s-1, on the paper machine, even if the contact time is short (a few milliseconds to several seconds), the particles The possibility of collisions between them is very high, and the flocculation process can be completed very quickly.
  In the turbulent state, the kinetics and practice of adsorption, remodeling and disconnection of polymers show that the residence time of CPAM on the fiber surface should be as short as possible, such as less than 10s, which makes the addition point of CPAM. The request. For a single polymer flow aid system, CPAM should be added after the centrifugal sieve; in the particulate system, the polymer should be added before the fan pump and the centrifugal sieve, and the particles should be added after the centrifugal sieve.
  The aggregation mechanism of each fine component in the paper stock:
In general, the aggregation mechanism of fine components has three basic forms of charge neutralization, patching effect, and bridging flocculation, but these three mechanisms are not independent, and their relative roles in different systems are different.
  Charge neutralization: fine fibers and fillers are negatively charged in the stock suspension and mutually repelled. The electrolyte is added to neutralize the positive charge, and the mutual repulsive force between the particles is reduced. When particles collide, Causes agglomeration between particles. Polyelectrolytes for charge neutralization, usually low molecular weight, high charge density electrolytes, such as polyaluminum chloride (PAC), polyethyleneimine (PEI), polydiallyl hydrazine methyl ammonium chloride (PDADMAC) ), polyamines and polyamide polyamines such as epichlorohydrin,
  Patch effect: The key to this mechanism is the formation of cationic patches. The patch should have a high charge density and a thickness greater than the double layer of the fiber surface. Patches can be formed by charge density, low and medium molecular weight cationic polyelectrolytes such as polyethyleneimine, polyamine. When selecting a polymer, it must be noted that the polymer should have sufficient charge density and molecular weight so that it does not diffuse into the pores of the fiber very quickly. Generally, the suitable molecular weight ranges from 100,000 to 1,000,000. If there is a branch, it will also prevent it. Remodeling and diffusion of polymers.
  The cationic polymer is adsorbed on the surface of the negatively charged fiber, where a positively charged region is formed, and the locally positively charged particles are electrostatically attracted to the negatively charged portion of the surrounding particles, thereby causing condensation, and the degree of attraction depends on the polymer. Charge density and coverage on the surface of the particles
Bridge flocculation: The electrolyte used for bridging flocculation is high molecular weight, low medium charge density. Polymer molecular weight is a key factor in bridging flocculation, and charge density, charge density on the surface of the particles, and ionic strength of the system are also important.
  (1) Application conditions of retention aids
  The effect of the retention aid is influenced by many factors and must be selected according to the type of pulp used and the other chemical additives added. The main factors affecting the retention effect can be summarized as follows:
  1, the type of polymer and the relative molecular mass Because the retention process is very complicated, there are various mechanisms, different ions and different molecular mass of additives, the role is also different. The role of the different focus, and affected by the other polymer, the ion type is not a key factor for retention, but because it is basically an acidic papermaking process, cationic polymers are generally more advantageous. One reason is that it is not sensitive to changes in pH. The high molecular weight retention aid should be added to the slurry in an aqueous solution of 0.1% to 1% due to its high viscosity.
  2. Addition of retention aids Because the polymer retention aid has three functions of retention, drainage and flocculation, and the first two effects decrease with time, the latter increases with time. In order to obtain the maximum retention and drainage effect, the flocculation phenomenon should be minimized. Under the premise of uniform mixing with the paper stock, the retention agent should be added as close as possible to the head box of the paper machine, so that the wet paper sheet forming area can be formed. Flocs, without causing the paper to flocculate, affecting the uniformity of the paper.
  3. Filler particle size and shape The particle size of the filler has an important effect on the retention effect. When no retention aid is added, the filler particles can only be retained on the forming sheet by filtration. The retention and particle size have an approximately linear relationship, and the retention is reduced as the degree of fiber dispersion decreases. improve. Natural fillers such as calcium carbonate and the like have surface charge properties similar to those of paper fibers, i.e., their zeta potential is negative, and if they do not interact with the cationic polymer, they exhibit a negative charge on the surface like the fibers in the pulp. When the retention aid is added, the fibers and filler can be retained by coagulation, coagulation or agglomeration. The filler retention is independent of particle size and is more dependent on the loss of fiber-microparticle floes, while the retention of fines is reduced.
  If an oppositely charged polymer retention aid is added, the charge on the surface of the filler is neutralized, so that the interaction between the filler and the fiber is enhanced, and the retention is improved.
  Mechanical pulp and low-slurry chemical pulp, the point of action between the filler and the fiber is very limited, and the filtration retention mechanism plays a major role, so that the particle size has a significant effect on the retention rate, and the larger particles can be more Keep it. In general, when the filler retention rate is less than 40%, the filtration mechanism plays a major role, and if the filler retention rate is above 40%, the filler has physical adsorption with the fiber bundle and the fiber, and at this time, the particle Size has little effect on retention.
The shape of the filler particles also has an effect on retention, and the surface is relatively rough and easy to retain.
  (2) Synthetic polymer retention aid
  Polyacrylamide and its modifications are currently the most commonly used retention aids.
  The retention effect of anionic polyacrylamide is not as good as that of cationic polyacrylamide, but its use is convenient. Therefore, it is still widely used in papermaking wet processing. It is also one of the earliest retention aids. The relative molecular mass of polymer has a retention effect. It plays a vital role.
  1. Monocationic polymer system: As a retention aid, it is mainly a cationic polymer. Such a retention aid can be directly electrostatically adsorbed with fibers and fillers, and can also be bonded to fibers by bridging. Commonly used low-charge-density, high-molecular-weight cationic polyacrylamides, the use of cationic polymers for different filler retention rates is different, in general, when the dosage is 0.15% to 0.2%, the retention rate is increased by more than 10%, white water The sedimentation rate also improved to a considerable extent, while the effect on talc powder was not obvious, and the clarification effect of white water was not satisfactory.
  2. Cationic (or amphoteric) plus anionic polymer retention system: In the study of improving retention efficiency, it was found that the combination of two different charge polyelectrolytes can produce high-strength hard-stiffness flocs, and firstly add low Medium-molecular-weight, high-charge-density cation (amphoteric) polyelectrolyte, which produces a cationic patch, and then an anionic polyelectrolyte, which has the best retention effect. The retention process of this retention agent is called a dual retention system. In general, the average molecular weight of the cationic polyelectrolyte is 2×104~5×105, the average molecular weight of the anionic polyelectrolyte is 5×106~10×106, and the amount of cation added in the system is 0.07%, and the amount of anion added. It is 0.12%.
  3. Anionic, cationic (amphoteric) enhancer mixing formula (polyion complex PIC) retention system: a specific anion enhancer and a cationic enhancer are mixed in advance to react to form an ultrahigh molecular weight polymer ion complex (PIC) ), the PIC can be added to the slurry to adapt to the papermaking conditions, which can not only enhance the effect, but also exert a good retention and drainage effect. Since the ultra-high polymer PIC will have a strong bridging effect, it should be sufficiently dispersed during use, so that the fine components and fibers are strongly flocculated to destroy the sheet formation, so the addition position should be set in the pulp and white water. The inlet of the mixed slurry pump and the paper machine screen is suitable. It is mostly used for paperboard making.
  4. Particulate system: Generally, a cationic polymer CPAM is added to the slurry first, and then a special inorganic pigment particle (such as penmine soil or colloidal silica) having a large specific surface area and a high density and a negative charge is added. It can produce flocculation which is very beneficial to help retain and improve the sheet formation. It has been widely used in the production of paper and paperboard, especially in the production of high-speed paper machines and high-grade paper.
  5, three-stage Compozil system - the derivative system of the particle flocculation system: due to the increase of mechanical pulp in the paper stock ratio, waste paper deinking pulp (DIP), coating loss of pulp, the wettology of the paper machine is more complicated, The stock contains more hetero anions. On the basis of the particle flocculation system, a three-stage Compozil system was newly developed to add special cationic polymers to the characteristics of more anionic impurities in the paper to eliminate the influence and provide conditions for the particle system to function. A special low molecular weight, high cationic polymer is first applied to the slurry to form an anion trap system, reducing the adverse effects of the dissolution and dispersion of the magazine. Second, a high charge density cationic starch is added to the stock. The third stage is the addition of colloidal silica to the paper stock to produce good microflocculation and achieve the desired retention and sheet formation.
  6. Integra retention system: The system uses the coagulant, coagulant and micro-flocculant drugs alone or in combination, and achieves the effects of comprehensively satisfying the retention, filtration and good sheet formation of the papermaking system. The coagulating agent used in the system is PDADMAC with different molecular weights and different charge amounts. It is an anionic trapping agent. The suitable molecular weight of PDADMAC should be selected according to the characteristics of the anionic particles in the paper stock. The coagulant used is an anionic acrylic or cationic amine-based polymer introduced into the polyacrylamide structure. The form of the product is a W/O type emulsion, and the molecular weight is very high, ranging from 15 to 20 million. Up to 25%. The micropolymer used was a water-soluble medium molecular weight anionic polymer, sodium lignosulfonate, which was developed from the night of cooking wood. In the Integra system, the anionic impurities in the paper stock are first neutralized with PDADMAC to create conditions for the ultra-high molecular weight polymer to function. Then PAM is added, and the fine fibers and fillers in the paper form a large floc together with the fibers. However, when it is subjected to shearing force by the paper machine pressure screen, it splits into small flocs. In this state, sodium lignin sulfonate is added, and the floc is dispersed into a stable, flexible, uniform fine floc by the dispersing effect, thereby achieving good retention and sheet formation. The system has good filter aid.
  Zwitterionic polyacrylamides are also used effectively in retention. (1) The cationic polymer retention aid can be directly combined with negatively charged fibers, including fine fibers. (2) The anionic polymer retention aid is bonded to the fine fibers by the AL3+ in the slurry or to form a bridge with the filler. (3) The zwitterionic polymer retention aid has both of the above effects. (4) The nonionic polymer retention aid generates adsorption by fine bonds or van der Waals forces with fine fibers and fillers. As a result of the above actions, various cross-linking networks are formed, thereby providing a retention effect.
  Sometimes, chemical cross-linking and physical cross-linking occur simultaneously, and the retention effect is remarkably improved because the formed aggregate is not easily broken by mechanical force. The amphoteric polymer has a wide pH range for adaptation, and the anionic group can be combined with the fiber through AL3+; the cationic group is directly bonded to the fiber, so that it has a better retention effect.
  Second, the filter aid
  The filter aid is a chemical additive used to improve the dehydration of the paper sheet during the papermaking process, and its function is to improve the water filtration and dewatering speed of the wet paper from the papermaking net portion. The use of filter aids can increase the speed of paper machine production, improve sheet formation, and reduce steam consumption in the dryer section.
All additives commonly used as retention aids and charge neutralizers can be used as filter aids. Commonly used filter aids include charge neutralizers (alum, PAC), cationic polyelectrolytes, anionic microparticles (colloidal silicon and sodium based). Peng Runtu).
  The mechanism of action of the filter aid is:
  1. A positively-charged filter aid can reduce the surface charge of paper fibers and fillers (ie, electrical neutralization occurs), which reduces the polarity, and it is difficult for water molecules to be wetted and aligned on the surface of fibers and fillers.
  2. A filter aid (a filter aid that also acts as a retention aid) can promote the agglomeration of fibers and fillers, resulting in a decrease in the specific surface area of ​​the fibers or fillers, forming large aggregates and accelerating dehydration. .
  3, the filter aid is often also a polymer surfactant, or has the effect of reducing the surface tension, after adsorption or bonding on the surface of the fiber, filler, can reduce its surface tension, reduce the contact angle, making small molecules difficult to spread and wet It is easy to get rid of the papermaking net after being stressed.
  Low molecular weight, high charge density polyelectrolytes produce small, dense flocs that do not bind water strongly, thusGood water filtration effect; and high molecular weight, low charge density polyelectrolyte forms large flocs, these flocs have high water retention capacity, and the filtration aid is not obvious.
It has been found that cationic PAM and polyethyleneimine (PEI) increase the water repellency of the slurry, but do not change the equilibrium moisture content of the formed paper. The effect of the polymer is to increase the rate of equilibrium. Under static conditions, the first-order kinetics of the waste paper slurry was investigated by studying different additives: the filtration rate increased with the increase of the cationic charge of the polymer; decreased with the increase of the charge of the polymer anion; the presence of fine components The water filtration rate is reduced; the filtration rate can be maximized at a certain polymer dosage, and it is found that in order to increase the drainage water, the molecular weight of the polyelectrolyte is required to be as small as possible.
  Application of main polymer filter aid
  1. CPAM:
  CPAM is different for accelerating the water filtration effect of different slurries. The application results show that the effect of grass fiber with higher semi-fiber content is better; the pure pulp with higher α-fiber content is less effective.
  When the speed of the paper machine is low, the water filter aid with relatively low molecular weight should be selected, because the low or medium relative molecular mass and high charge density polyelectrolyte will produce small, tight floccules, which cannot bind water strongly. It has good water filtering effect, but can not withstand too much shearing force. When the relative molecular mass is too large,      floccules will form quickly and affect the uniformity of the paper.
When used as a filter aid, CPAM should be selected for medium molecular weight, medium point and density. This kind of retention and drainage aid can adsorb on the surface of various particles and fibers, which can reduce and neutralize the charge of the surface of the fiber filler. Destruction is filled with macromolecular structures oriented in the fibers and fillers, allowing water to be easily filtered out. In addition, the addition of the filter aid brings the zeta potential of the slurry closer to the isoelectric point, reducing the repulsive force between the particles and the particles and the fibers, thereby easily forming a bridge, and finally producing a good filter aid.
  CPAM and its mixture with modified bentonite are widely used to improve the water filtration performance of slurry, and are suitable for chemical pulp and mechanical pulp. The optimum amount of CPAM is strongly dependent on the charge density of the polymer and the pH of the slurry.
  2, APAM
  APAM is a product with high molecular weight and low charge density, but it must be combined with cationic polyelectrolyte to have a filter effect. If it is used alone, the water filtration is not ideal. APAM degrades the water filtration performance of the slurry. The reason is that APAM has the same anionic property as the fiber, and the repulsive force between the particles and the particles and the fiber is increased, resulting in deterioration of the drainage performance of the slurry.  Therefore, the APAM cannot be used alone. Used as a filter aid.
  3. PEI binary polymerization system
  PEI is generally a typical binary polymer system with APAM, and of course there are other combinations. However, for improving water repellency, two systems have been shown to be particularly effective, PEI-CPAM and PEI-CPAM-active pennite. It has been launched by BSF for newsprint. In addition, these two systems have achieved good drainage results for wrapping paper.
  4, the help of the Heideroco system
  The Hyderol Cole system is a new compound control system with a cationic polymer and a special inorganic pigment, which can simultaneously improve the water repellency and retention. This system uses the ionic polymer electrolyte as the first component. In order to maximize the performance, it is easy to select a polymer with a proper charge density and relative molecular mass, and it is only necessary to adjust the amount of the polymer to be controlled.
  Third, the enhancer
  There are two ways to enhance the paper. One is to add a reinforcing agent to the slurry, and the other is to add a surface enhancer during papermaking. The in-pulp enhancer is further divided into two types: a dry strengthening agent and a humidifying agent.
  Increasing the strength of the paper is mainly to strengthen the bond between the layers of the fiber. The addition of various dry strength and moisturizing agents utilizes the chemical and physical bonding between them and the fibers to enhance the adsorption between the layers of the fibers and between the various particles. The bonding between fibers depends on the following energies: covalent bonds, ionic bonds, coordination bonds, hydrogen bonds, van der Waals forces, physical entanglements.
  In the paper industry, dry strength mainly refers to the tensile strength, crack length, tearing degree and bursting resistance of paper. Wet strength refers to the rewet strength of paper, which is the strength that dry paper retains after being wetted by water.
  Polyacrylamide type dry enhancement agents mainly include cationic (CPAM), anionic (APAM), and zwitterionic.
  CPAM has a lot of comonomers, and it has a large choice for its modification.
APAM is effective for all chemical pulps, but when the ratio contains a large amount of machine pulp and waste paper pulp, the reinforcing effect is not obvious.
The zwitterionic polyacrylamide molecule contains a cationic group and an anionic group, which contributes to the retention and filtration and enhancement of the cationic or anionic PAM alone.
  The use of polyacrylamide-based dry strength agents is influenced by many factors:
  1. Relative molecular mass: It is required to have a certain molecular structure length to provide good adsorption performance and a sufficient number of hydrogen bonding, and it is not possible to bridge the fibers to cause flocculation or even damage the uniformity of the sheet. The molecular weight is generally 10 to 1 million.
  2. Dosage: The general dosage is 0.5%, which can increase the bursting strength and tensile strength by about 20%, but has little effect on the tearing degree. At a concentration of 5%, it must be diluted sufficiently to mix well with the slurry.
  3. Two kinds of resin sharing: When APAM and CPAM are mixed, good reinforcement effect can be obtained. The specific method is to mix the anion and the cation enhancer before adding the material, and quickly add it to the slurry, but care should be taken not to place the two polymers for a long time after mixing, and the flocculation is quickly generated by the electrostatic adsorption. If it is not dispersed, it will affect the uniformity of the paper and cause a decrease in the effect.
  4, alum: can add a small amount of alum to improve the adsorption of APAM on the fiber, the amount of which depends on the rosin gum filler and other additives, generally 1.5% ~ 3.0%. When the amount of alum is increased, it is unfavorable for improving the dry strength, because the retention of alum floc becomes a new filler, which hinders the bonding of the fibers. However, Alum has no obvious effect on improving the enhancement of CPAM.
  5. PH value: The improvement of the maximum dry strength is achieved by adsorbing the resin in the fixed fiber, and the optimum PH value of the paper material is 4.2~5.0. CPAM must exhibit its cationicity under acidic conditions to form an electrostatic adsorption bond with the fiber.
  6, the order of feeding: first add rosin, then add alum, then add PAM. Because of the role of PAM, it requires a certain amount of contact time with the slurry. Because polymers are sensitive to high shear forces, PAMs are generally added as close as possible to the headbox.
  7. Degree of beating: With the increase of the degree of beating, the strength of paper with PAM is higher than that without PAM, indicating that the increase of beating degree is beneficial to increase the hydrogen bonding point between PAM and fiber.
  The best point of addition for PAM is with the paper machine, which depends to some extent on the purpose of use. The most common addition point is to add it to the final slurry. Studies have shown that it is more powerful to add a dry strength agent to the long fiber component. For the best results, good mixing is very important. The ideal method is to add the resin to the well-stirred paper machine or to the lowering tube of the headbox. Special care should be taken not to conflict with other auxiliaries, if it is not possible to add the resin after the final refiner, it should be added after the main refining (ie before minor refining).
  When the use of the drying enhancer is started, there is a possibility that the sheet strength does not increase or decrease, because the chargeability of the polymer increases the retention of the fine fibers and the filler, and the strength of the sheet increases. The increase effect of the resin has been exceeded. In this case, as long as the charge balance is not destroyed, further increasing the amount of the resin can overcome the above problem. Increasing the flocculation of the fibers will affect the formation of the sheet and will also result from similar effects as described above.
  Experience has shown that weak anionic PAM can achieve better enhancement than cationic PAM. However, the main disadvantage of using anionic PAM resin reinforcement is that under alkaline papermaking conditions (pH 8.0~8.5), the anionic PAM is fully charged, in order to obtain the ideal. The effect is to retain the resin with bauxite. Although it is possible to carry out papermaking under the condition of pH 6.5, it is not feasible to add a large amount of alumina to neutral or alkaline papermaking using calcium carbonate as a filler.
  Unlike anionic PAM, cationic PAM (quaternary ammonium salt type) can be completely ionized throughout the pH range of the papermaking and can be directly fixed to the fiber without the need for a special retention aid.