制浆造纸专业单词3

Chapter 9 Non-fibrous Additives to Papermaking Stock
A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. A general classification of wet-end chemical and mineral additives is given in Table 9-1. Such additives as alum, sizing agent, mineral fillers, starches and dye are commonly used. Chemicals for control purposes such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides, and corrosion inhibitors are added as required. The order of addition must be taken into account to prevent interaction at the wrong time and enhance retention in the paper sheet.
Not all papermaking chemicals are added to the vet stock. Sizing solution is often applied to the tried sheet at a later stage in the process (e.g., at the size press): and pigment coatings are used for the better quality publication grades. Increased papermill chemical and mineral consumption is anticipated mainly for coatings. The highest tonnage additive is clay, over half of which is used as part of surface coating formulations.
It is of interest to put an economic perspective on the chemical and mineral contribution to papermaking. Perhaps, on average, 10% of the cost of making paper can be attributed to chemicals. Figuring the value of annual North American paper and board shipments at $80 billion, the industry probably uses some $8 billion wirth of additives per year.
9.1 RETENTION ON THE PAPER MACHINE
Two parameters are used to measure the retention of fibers and additives during paper forming: These formulas apply to the overall furnish or to any single component of the furnish. The cost of utilization for various additives is mainly related to their overall retention, because the portion not retained with the sheet is lost with the white water overflow from the system. Even though modern papermaking systems have a high degree of “closure” to reduce the volume of effluent, losses of certain constituents can still be substantial.
Paper quality and paper machine operation are more affected by single-pass retention. A low level of single-pass retention indicates a high recycle rate of furnish materials with the recirculating white water, it gives rise to non-uniform distribution in the cross-section of the sheet and may contribute to two-sidedness (i.e., different surface propertise on the tow sides) in fourdrinier-made paper. The accunulation of fines and additives in the headbox loop retards drainage, and the fines fraction absorbs a disproportionate amount of certain additives by virtue of its high specific surface. Also, pitch and slime have a greater propensity for buildups and agglomerations, and are generally more difficult to control. The major factors that affect the retention of such additives as rosin sizes, starches, resins and fillers during the sheet forming process anr listed in Table 9-2. Retention of nonfibeous additives occurs through the mechanisms of filtration, chemicalbonding, colloidal phenomena, and adsorption. Filtration (i.e., mechanical interception) is important for retaining large particles, but smaller panicles must be retained by other means (see Figure 9-1). For example, it is estimated that only about 2% of titanium dioxide pigment particles (average size 0.2 microns) are retained by the mechanism of filtration.
A number of retention aid chemicals are available to the ppaermaker (see also next section). Since these chemicals act mainly through flocculation and entanglement, some care should be exercised in their utilization. Some effects on stock drainage and anticipated. Primarily, the dispersive action of the headbox system must be adequate to avoid overflocculated stock that would be detrimental to
sheet formation.
9.2 WET END CHEMISTRY
Wet end chemistry deals with all the interctions between furnish materials and the chemical/physical process occurring at the wet end of the paper machine. While the wet end of the paper machine. While the subject is complex, it is possible to gain a basic understanding of the major concepts without delving too deeply into the tachnical aspects. The major interactions at the molecular and colloidal level are surface charge, flocculation, coagulation, hydrolysis, microbiologucal cativity. These interactions are fundamental to the papermaking process. For example, to achieve effective retention, drainage, sheet formation, and sheet properties, it is necessary that the filler particles, fiber fines,size and starch be flocculated and/or adsorbed onto the large fiber with minimal flocculation between the large fibers themselves. These is a wide range of ohenomena (as listed in Table 9-3) which can influence these fundamental interactions.
There are three major groups involved in wet-end chemistry: solid, colloids and solubles. Most attention is focused on the solid and their retention. In order to maximize retention, it is important to cause the fines and fillers to approach each other and form bonds or aggregates which are stable to the shear forces encountered in the paper machine headbox and approach system. In modern papermaking, this is usually accomplished by using synthetic polymers.
Certain colloidal materials derived from cellulose and hemicelluloses are released from pulps or watepapers or added deliberately. The pulping process breaks down the cellulosic structures into smaller molecules which are potentially soluble. These smaller molecules have a negative impact on process control and runnability, and their natural retention is effectively zero. They concentrate within the system, consume chemicals and are generally a nuisance.
Control of wet-end chemistry is vital to ensure that a uniform paper priduce is manufactured. If the system is allowed to get out of balance (say, by over-use of cationic polymers), the fibers thenselves will become flocculated and sheet formation will suffer. Also, functional additives (e.g., size, wet-Strength agents) are often added at the wet end; if the chemistry is not under control, the functionality may not be adequately imparted and the product will be off-quelity. Unfortunately, wet-end chemistry is made more complex because of soluble materials. Whehter they are organic or inoragic, added deliberately or inadvertantly, they change the action of the polymers, In particular, wastepaper furnishes content a relatively high concentration of solubes and a greater variety of chemical species which have an adverse impact on the controllablity of the wet end.
Electrokinetics
The term zeta potential，applies to the electrical charges existing in finr dispersions. A solid particle (e.g., fiber, starch, mineral) suspended in a papermaking stock is surrounded by a dense layer of ions having a specific electrical charge. This layer is surroubded by another layer, more diffuse than the first, that has an electrical charge of its own. The bulk of the suspended liquid also has its own electrical charge (see Figure 9-2). The difference in electrical charge betweeen the dense layer of ions surrounding the partical and the bulk of the suspended liquid is the zeta potential, usually measureed in millivolts.
The best retention of the fine panicles and colloids in the papermaking system normally occurs when the zeta potential is near zero. Pulp fibers, filler and size panicles usually carry a negative charge, but the zeta potential can be controlled by absorbing positive ions from solution. Ployvalent cations such as aluminum (A3+) and ferric (Fe3+) are most effective.
Papermakers lum, Al2(SO4)3, is still a commonly used agent for wet end chemistry because it effectively neutralizes the negatively-charged fiber and pigment particales to zero zeta potential. At the proper p . it also hydrolyzes to form an ionic polymer:
Al2(SO4)3 +6H2O 3H2SO4 + 2Al(OH)3
This aluminum polymer has a significant flocculating effect by bridging from particle to panicle and thereby forming large ionically-attracted flocs. However, the retentioneffect is sensitive to shearing forces or strong agitation. With higher paper machine speed, alum has become less effective. Fortunately,synthetic polymers have been developed with good shear resistance. These polyelectrolytes are available either as cationic or anionic retention aids. The retention mechanism is a combination of ionic charges and long molecular chains linking fibers and particles together. Synthetic polymers have less p dependence than alum and are used in very dilute form. Instruments are now available to measure both zeta potential and single-pass retention. Therefore, the economics of utilizing polyelectrolytic polymers to optimize zeta potential and retention can be monitored continuously and evaluated under commercial conditions.
Some researchers have found that simple adjustment of the papermaking system close to zero zeta potential will lead to optimum results (e.g., Figure 9-3). Others have found the optimum zeta potential to be approximately -9 mv. However, in commercial practice these findings have not always been confirmed. It appears that zeta potential is an indirrct measure of a number of interacting factors, each of which could be dominant under certain conditions. As such, it cannot be relied upon to provide unequivocal information for operation of the papermaking system. Perhaps the best role for an online zeta potential measurement is to chzracterize a well-operated system, and then flag upsets by showing deviations from the norm.
9.3 APPLICATIONS OF NON-FIBROUS ADDITIVES
Sizing
The purpose of sizing is to enable paper products to resist penetration by fluids. Sizing can be achieved either by using wet-end additives or by applying a suitable coating to the surface of the dried paper. Sometimes a combination of treatments is required. The action of a wet-end sizing agent also imparts other desirable properties to the paper; however, the sheet remains porous. For products that require a vapor barrier, a surface coating must be used.
A fundamental factor influencing the rate of liquid penetration is the contact angle formed between the impinging liquid and the sheet surface, as illustrated in Figure 9-4. The action of a wet-end sizing agent is to provide the fiber surface with a hydrophobic, “low-energy”coating that discourages aqueous liquids from moving extensively. The traditional wet-end sizing agent is a modified rosin, most often in a saponified form to make it water soluble. Rosin size is usually shipped to the paper mill as a high-solids thick paste and is diluted through an “emulsifier”for metering into the stock. Natural rosin is the amber-colored resin obtained from southern pines. Formerly, it was tapped from growing trees or extracted from stumps. Now, more commonly, it is processed from tall oil (see also Section 10.6). Rosin is an amphipathic material, meaning that it has both hydrophillic and hydrophobic parts. To provide good sizing, it is essential that the hydrophobic parts are oriented outward, as shown in Figure 15-5. In practice, the rosin is precipitated onto the fibers by the action of alum as an oriented monolayer of aluminum resinate molecules.
Rosin, along with wax emulsions, is sometimes categorized as bulk or nonreactive sizes. Their retention is dependent primarily on precipitate panicle size and electrostatic attraction to cellulose. Also, they depend on the drying process to promote flow and coverage of the fiber surface. But, they do not react chemically with the fiber.
Corresponding to the movement from traditional acidic papermaking toward a neutral or alkaline wen end. There is a trend toward greater use of synthetic sizing agents which react chemically with the cellulose hydroxyl groups to form stable ester linkages. These chemicals were introduced to the paper industry in the 1950’s and provideed the first opportunity to manufacture sized paper in an alum-free envioronment. Alkyl ketene limer was the first commercially available reactive size and is still the most widely used. Acrylic stearic anhydride size (derived from fatty acids) and alkenyl succinic anhyfride (derived from petroleum) are more reactive and have more selective applications.
Internal Strength
A number of natural and synthetic polymeric substances (refer to Table 9-4) may be admixed with the stock at the wet end to improve the physical properties of the dry paper sheet. Their action is to enforce fiber-to-fiber bonds and thereby improve the burst and tensile strength, provide greater resistance to erasure, reduce “fuzz”or limit on the paper surface, and reduce the rate of water penetration.
The traditional internal strength additives are natural and modified starches and gums. Starches are polymers of glucose and are derived from various plants, principally corn, tapioca, potato and wheat. Gums are polymers of mannose and galactose and are derived from locust bean and guar seeds. Both starches and gums are usually cooked at low concentration liquor to use to promote swelling and dispersion.
The trend today is toward increased use of such synthetic polymers as latexes and polyacrylamides either alone or in combinnation with starches and gums. By means of copolymerizing or cross-linking, these products have evolved over the past two decades to meet a wide range of specific requirements for greater paper strength with different degrees of stiffness and stretch.
Wet Strength Resins
Ordinary paper will retain a significant portion of its strength when immersed in most oils or solvents (see Table 9-5). But because of the special interaction between water and cellulose, the normal fiber-to-fiber bonds are destroyed in aqueous media. The action of wet-strength resins is to tie fibers and fines together with additional bonds that are not taken apart by water. Wet-strength paper is defined as such if it retains more than 15% of its tensile strength when wet. Some papers actually retain up 15%. Wet strength develops during aging: this effect is more pronounced with treated papers, but is also true for untreated papers.
The most common wet-strength agents are urea-formaldehyde, melamine-formaldehyde and polyamide resins; they are water-soluble and available in both anionic and cationic forms. These agents are applied at an intermediate degree of polymarization, so that the final “cure” is obtained in the fryers. Since wet-strength resins are water-soluble, they must be fixed onto the fibers. Retention can be poor under some conditions. Anionic resins are best added with alum, but only after rosin size has been precipitated. The best retention on the stock is achieved over a relatively long period of contact.
Fillers or Loadings
Finely-divided while mineral fillers are added to papermaking furnishes to improve the optical and physical properties of the sheet. The panicles serve to fill in the spaces and crevices between the fibers, thus producing a denser, softer, brighter, smoother and more opaque sheet. In some instances the paper can also be made cheaper because the filler are often less costly than fiber.
The proportion of filler in the sheet is limited by the resultant reduction in strength, bulk and sizing quality. The majority of filled papers contain between 5 and 15% of sheet weight, but some heavily loaded grades exceed 30% (refer to Figure 9-6). An important innovation of the 1980’s was the development of the multicomponent wet-end chemical system, described as a systems approach toward optimizing the balance between retention,dewatering and product strenghth. A number of proprietary two-component systems are currently being offered by chemical supplier which are purported to allow a higher level of loading without compromising machine productivity or product quality (see Figure 9-7)
The common papermaking fillers are clay (kaolin, bentonite), calcium carbonate tale (magnesium, silicate), and titanium dioxide. Clay is the most popular filler because it is cheap, plentiful, stable and provides generally good performance. Calcium carbonate is used only in neutral or alkaline systems because of its solubility at lower p levels. It is available at a higher brightness level than clay and is a better opacifier, it is especially useful for “permanent paper” because it neutralizes the acids which from during aging and cause deterioration. Titanium dioxide is the brightest and most effective opacifier; however, its relatively high cost limits its use to those application where high whiteness and opacity must be obtained at low filler level without loss of strength. Talc is notable as a “soft” filler, imparting a soft, silky feel to the paper product. Talc also has an affinity for pitch particaled and is effective in preventing pitch deposits in the papermaking system.
Chemical Dyes
The absorption of dye by pulp fibers depends on the chemical nature of the dye, the capillary pore structure of the fiber, and the nature and polarity of the fiber surface. The principal types of water-soluble dyes are known as acid, basic, or direct. Chemically, acid and direct dyes are similar, both being the sodium salts of colored acids. The difference is in other affinity (or substantivity) for cellulose fibers. While direct dyes are readily absorbed by cellulosic fibers, the acid dyes can only be retained by adding rosin size and alum. Acid dyes are more soluble in water than other classes of dyes and have the advantage over basic dyes that they do not mottle in mixed fiber furnishes. As a class, direct dyes are less soluble than acid dyes, tending to form colloidal systems, they are often duller than basic dyes and more expensive for producing a given shade.
Basic dyes are the salts of color bases, and generally appear as the chlorides, hydrochlorides, sulfates, or oxalates. They are the most important class used in coloring paper. They have the advantages of low cost, high tinctorial strength, and great brilliance. Sometimes basic dyes are used in small quantities to improve the brilliance of acid or direct dyes, in this way producing a small anount of insoluble color “lake”. Because there are severial chemical groupings of basic dyes, considerable variation is found in the physical and dyeing properties among individual members. However, as a class, basic dyes possess relatively poor fastness to light, acid, alkalis, and chlorine. All dyes are specific chemical compounds. The actual color produced in the paper from adding one or more of these compounds is affected by various processing conditions, such as the nature of the pulp used, the degree of refining, and the chemical balance. Therfore, color matching of products and control of color uniformity is a difficult task requiring experience and judgment. Often, consultation with a specialist in the dield is required to solve a specific problem.
Control of Pitch
A common problem in paper mills is the depositing of pitch panicles within the papermaking system. These panicles accumulate in the openings of the forming fabric, thereby producing holes in the finished product. They also collect within the structure of press felts, thus reducing the felt’s
permeability, and on roll surfaces causing pickouts and non-uniform peeling of the sheet. Once deposited, oitch can only be removed by scrubbing with solvents or special cleaning compounds. Pitch is composed of low-molecular-weight oleophilic materials (mainly fatty acids, rsin acids and esters) which are released from wood fibers during chemical and mechanical processing, and precipitated as calcium and magnesium salts. The concern in a papermaking system is with “depositable oitch”, rather than the total pitch content Pitch that is well dispersed usually causesno problems.
In acid systems, alum is used for cationic fixation of pitch on to the fibers. Nonionic wetting agents are commonly used in alkaline systems to disperse the pitch. Since oitch is precipitated by caicium or magnesium, any step to reduce the amount or impact if these ions will be helpful. Where possible, use of lard water should be avoided. In some cases, delating agents to inactivate the metallic ions re useful. To be effective, all chemicals used for pitch control must be added before agglomeration occurs.
9.4 ALKALINE PAPERMAKING
Traditionally, sized papers were manufactured only under acid conditions. The capability to produce papers ubder neutral or alkaline conditions has now existed for over 30 years, and about 75% of European fine paper mills have converted to this technology. North American papermakers initially were more reluctant to change, but interest has surged since 1980. The original impetus came mainly from customer demands for higher brightness and opacity. Another significant factor was that calcium carbonate fillers became readily available and more price-comprtitive. Approximately 30% of North American fine paper is now produced by the alkaline process.
As mills have changed over, a wide range of benefits have been documented. Now, the major driving force toward conversion is the greater force strength of the alkaline sheet which permits higher levels of clay and calcium carbonate filler. Typical ash content with alkaline sizing rage from 18% to 25% vs 7% to 12% with an acid system. Substitution od filler for fiber provides significant economic advantages. Since calcium carbonate serves to stabilize the papermaking system in the p range from 7.2 to 8.0, other means of p control are no longer necessary. Additionally, the alkaline system is less prone to corrision, so maintenance cost is lowed. Because of reduced chemical loading, alkaline systems are more easily closed than acid systems.
Along with synthetic reactive sizes, calcium carbonate fillers are at the heart od alkalin papermaking. Two forms of calcium carbonat having quite different morphologies are common used. Ground calcium carbonate is made from natural chalk deposits using sophisticated we grinding technology. Precipitated calcium carbonate is a manufactured product made by passing flue gas through a milk of lime solution. While either from is often used as the sole filler, blends can be used to optimize the opacity and production goals for a particular grade.
So-called alkaline papermaking systems actually operate in the p range from 7.0 to 8.0, just barely into the alkaline region. It is understandable that many industry people prefer to use the term “neutral papermaking”.。