ABSTRACT THE INFLUENCE OF CHEMICAL AND MECHANICAL FLOCCULATION ON PAPER FORMATION AS ASSESSED BY THE GRAMMAGE PROBABILITY DISTRIBUTION By Jing, Yan This investigation tested the influence of the mechanical and chemical variables of papermaking to the resulting paper formation based on statistical and transmission radiography methods. Results were analyzed by using the formation number and grammage probability distribution. Increasing the degree of refining caused fiber fibrillation and shortened fiber length without changing coarseness. This made the distribution of fibers in the suspension and resulting handsheets more uniform as described by the crowding factor. The excessive concentration of C-PAM retention aids and the addition of A-PAM formation aids can all benefit the uniformity of the paper as determined by the formation number as attributed to the electrostatic repulsion. Conversely, at low ph or when the C-PAM dosage is below 0.5% paper formation was worse due to van de Waals forces and charge neutralization. The asymmetry of the grammage distribution was explained as the sum of different normal distributions with different mean values.
THE INFLUENCE OF CHEMICAL AND MECHANICAL FLOCCULATION ON PAPER FORMATION AS ASSESSED BY THE GRAMMAGE PROBABILITY DISTRIBUTION A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Paper and Chemical Engineering by Jing, Yan Miami University Oxford, Ohio 2009 Advisor Dr. D. S. Keller Reader Dr. S. Lalvani Reader Dr. D. Coffin
Table of Contents 1. Introduction... 1 2. Background... 3 2.1 Flocculation in Papermaking... 4 2.2 Fiber Flocculation Characterization... 5 2.3 Mechanical Causes of Fiber Flocculation... 7 2.3.1 The Effect of Fiber Length... 7 2.3.2 The Effect of Fiber Coarseness... 8 2.3.3 the Effect of Refining... 9 2.4 Chemical Reasons for Fiber Flocculation... 10 2.4.1 The Effect of ph... 10 2.4.2 The Effect of Retention Aids... 11 2.4.3Fines Aggregation Mechanism... 12 2.4.4 Formation Aids... 15 2.5 Paper Formation... 16 2.6 Paper Formation Characterization... 17 2.6.1 Statistical Methods to Analyze Paper Formation... 17 2.6.2 Spectral Analysis for Paper Formation... 20 3. Problem Statement... 21 4. Experimental Procedures... 22 4.1 Fiber Preparation... 23 4.1.1 Preparation Approaches... 23 4.1.2 Fiber Length Analysis... 24 4.3 Dilute Stock Preparation... 26 4.3.1 Settling Time... 26 4.3.2 ph... 26 4.3.3 Adding Chemical Additives... 27 4.4 Handsheets Testing and Analysis... 32 4.4.1 Transmission Radiography... 33 4.4.2 Using Statistical Method to Quantify the β-radiography Images... 34 5. Results and Discussion... 38 ii
5.1 The Effect of Settling Time and Degree of Refining... 38 5. 1.1 The Effect of Setting Time and Degree of Refining on Formation Number... 41 5.1.2 The Effect of Settling Time and Degree of Refining on Skewness of the Grammage Distribution... 43 5.2 The Effect of Fiber Length on Formation and Skewness... 47 5.3 The Effect of ph... 50 5.3.1 The Influence of ph to Formation Number... 50 5.3.2 The Influence of ph to Skewness... 51 5.4 The Influence of Chemical Additives... 51 5.4.1 The Identification of Maximum flocculation for Retention Aids... 51 5.4.2 The Influence of Retention Aids to the Resulting Paper Formation... 53 5.4.3 The Effects of Retention Aids on the Skewness... 55 5.4.4 The Effect of Formation Aids (A-PAM) to Formation Number... 55 5.4.5 The Effect of A-PAM on Skewness... 56 6. Conclusions... 59 6.1 Mechanical Aspect... 59 6.2 Chemical Aspect... 60 7. Suggestions for Continued Research... 61 Reference Literature... 62 Appendix 1.68 iii
LIST OF TABLES Table 4-1 Fiber Length Test for Original and Cut Fiber 24 Table 4-2 Meaning of Each Column in One calibration File 35 Table 5-1 Fiber Length Change after Different Degree of Refining 40 Table 5-2 Parameters for Grammage Probability Distribution of Handsheets (2.5Krev 2min) 45 Table 5-3 Parameters for Grammage Probability Distribution of Handsheets (2.5Krev 2min) 49 Table 5-4 Parameters for Grammage Probability Distribution of Handsheets (0.17% A-PAM) 57 iv
LIST OF FIGURES Page Figure 1-1 the Relationship of Paper Making Process and the Product Properties 2 Figure 2-1 Top: Random Fiber Distribution Left: Fiber Flocculation; Right: Fiber Dispersion 3 Figure 2-2 the Same Fiber before and after Beating in a PFI-mill 9 Figure 2-3 Patch Model 13 Figure 2-4 Bridge Model 14 Figure 2-5 the Relationship of Polymer Degree and Flocculation Degree 15 High Charge Density Cationic Polymer (left figure) Low Charge Density Cationic Polymer (right figure) Figure 2-6 Newsprint Sample Comparison 18 Figure 2-7 the Grammage Frequency Distribution of Newsprint Samples 19 Figure 4-1 Schematic Illustration of the Experiment Plan 22 Figure 4-2 Fibers Cut into 4mm 4mm Squares in order to Change Fiber Length 23 Figure 4-3 Chemical Structure of CIBA ALCOFIX 159 27 Figure 4-4 Process Flow for Test the CPAM s Influence to Fiber Flocculation and Resulting Formation 28 Figure 4-5 Diagram of a Particle Charge Detector, (PCD) 30 Figure 4-6 Schematic Illustration of the Storage Phosphor Process 33 Figure 5-1 Radiographic Image of Two Handsheets 38 Figure 5-2 Histogram Comparison for Handsheets with or without Settling Time 39 v
Figure 5-3 Formation Number as a Function of Beating Degree and Setting time (Original length) 41 Figure 5-4 Formation Number as a Function of Beating Degree and Settling time (cut fiber) 42 Figure 5-5 the Effect of Degree of Refining to the Crowding Factor 42 Figure 5-6 Skewness as a Function of Beating Degree and Settling Time (Original Length) 44 Figure 5-7 Skewness as a Function of Beating Degree and Settling Time (Cut Fiber Length) 44 Figure 5-8 the Grammage Probability Distribution of Handsheets without Settling Time 45 Figure 5-9 the Grammage Probability Distribution of Handsheets with Settling Time 45 Figure 5-10 the Effect of Fiber Length of the Crowing Factor 48 Figure 5-11 Grammage Probability Distribution of Cut fiber Handsheet with 2mins Settling Time 49 Figure 5-12 Grammage Probability Distribution of Original Handsheets with 2mins Settling Time 49 Figure 5-13 Formation Number as a Function of ph 50 Figure 5-14 Skewness as a Function of Different ph 52 Figure 5-15 Effect of C-PAM Dosages onto the Sample Charge Concentration 53 Figure 5-16 the Influence of C-PAM to the Formation Number 54 Figure 5-17 the Influence of C-PAM to the Histogram 54 Figure 5-18 the Influence of C-PAM to Skewness 55 Figure 5-19 the Influence of A-PAM to Paper Formation 56 Figure 5-20 the Influence of A-PAM Dosage to the Skewness 57 vi
Figure 5-21 Grammage Probability Distribution of Handsheet with 0.05% A-PAM 58 Figure 5-22 Grammage Probability Distribution of Handsheet with 0.17% A-PAM 58 vii
To my parents, Huijun Jing and Min Liang, with love viii
ACKNOWLEDGEMENTS I would like to give my sincere gratitude to Dr. Keller, my advisor, for his support, patient guidance and encouragement throughout all my two years at Miami. I would also like to thank my committee members Dr. Lalvani and Dr. Coffin, for their valuable editorial advice and support. Furthermore, I d like to thank Mr. Hart, for his help in using the experimental equipment properly and the fiber length test. I would like to extent the thanks to Payal Sood and all the faculty members and graduate students at Paper and Chemical Engineering department, for their support and help. ix
1. Introduction Like all types of material, paper exhibits structural hierarchy, and this characteristic is very important for paper. The term structural hierarchy describes the existence of structure at a variety of scales [1]. From higher scale to lower scale, paper can be divided into layers of paper, flocs of fibers, individual fibers, the fiber cell wall, micro fibrils, cellulose aggregate and polymeric chains. Each single structural level of paper can directly influence the properties of the whole sheet. For example, the behavior of fiber flocs can influence the opacity, the grammage uniformity and mechanical response. These properties can also affect the quality of paper. In our daily life, people are mainly concerned with the end product s performance. However, by choosing a suitable structural level and finding the specific factors that can improve the end product s properties, it may be possible to control the properties of paper to make it more suitable to meet the user s needs. The flocs caused by fiber flocculation are the suitable structural level to analyze the paper formation. The scale of fiber flocs is between about 1 mm to 50mm. This is the scale region that is important for visual uniformity and for printing uniformity defects such as print or gloss mottle. This scale is characteristic of the formation of paper. Paper has been defined as a felted sheet of fibers formed on a fine screen from a water suspension, [2]. Each step of papermaking can influence the properties of the resulting paper and each step can be controlled by a set of process parameters. For example, the use of softwood or hardwood in the pulp furnish, the degree to which the pulp is refined and even the consistency of the pulp suspension delivered to the forming wire can influence many of the properties of the paper, such as grammage uniformity, opacity, elastic modules, tensile strength and so on. This kind of relationship can be illustrated by Fig. 1-1. The diagram demonstrates how each step of paper making influences the intermediate properties separately and the end product s properties. For example, pulp has its own properties like fiber origin, lignin content, fiber length and wood fiber type. All of these intermediate properties can influence the resulting paper formation. Aggregates of fibers or flocs have their own properties 1
like dimension and strength. Once the flocs are formed to the fibers web and are dried, they create a complex structure that gives paper its final strength and elastic response. Pulp, Fiber Suspension Properties Flocs of Fibers Properties Handsheets End Properties Figure 1-1 the Relationship of Paper Making Process and the Product Properties Additionally, paper formation is defined as the variation of local grammage. It can be determined by measuring the mass distribution in the plane of the sheet [3]. The scale of formation includes the scale of the floc structure and ranges from 1mm to 100mm. Consequently, by examining the intermediate structure by looking at the behavior of fibers (fiber flocculation or suspension) before the drainage of water in the forming zone, one can analyze how it influences the resulting paper formation. In papermaking, before the water is drained, fibers can tangle together and flocs of fibers can be formed. Fiber flocculation can directly influence the paper formation, making it less uniform in mass distribution. A lot of factors can influence the flocculation of fibers, such as the fiber length, different refining degrees [4] and chemicals additives such as wet-end retention aids [5] that promote adhesion between fibers. Reasons for each of these will be discussed below. In this investigation, the fiber suspension is chosen as the intermediate structure to analyze the resulting paper formation, because the fiber suspension can directly influence the form and the dimension of fiber flocs. 2
2. Background By researching the behavior of fibers before the sheet is formed, the origin of paper formation can be revealed. Flocculation and dispersion are two competing effect in the papermaking process as illustrated in the following Fig. 2-1 As shown in the Fig. 2-1 (top), the random fiber distribution is generated by the stochastic distribution of fibers in the plane of paper. One can see how regions of low and high grammage formed by this natural process. There is a certain level of flocculation within random fiber distribution, but they are not necessarily generated by a tendency of fiber aggregation through physical or chemical forces. Figure 2-1 [6] Top: Random Fiber distribution Left: Fiber Flocculation; Right: Fiber Dispersion 3
Generally speaking, flocculation occurs in the dilute stock as it passes through the approach system to the headbox, through the forming zone to the point where fibers are locked into the wet web. However, dispersion can also occur during the drainage of the dilute stock, and compete with flocculation. Dispersion is generated by drainage element under the forming fabric which can induce turbulent flow that breaks apart any flocs that have formed. Furthermore, the shearinducing devices above the forming wire can also induce dispersion such as dandy roll or top former [2]. Gap on twin wire formers have forming elements on both sides that can also redistribute fibers in plane. During the drainage of water, the web structure is formed layer by layer. According to Norman et al., [7], when the sheet is formed onto the wire, fibers in suspension are inclined to migrate to the low grammage areas. This is referred to as the self-healing process. Because higher grammage sheet contains more fiber layers, the self-healing process repeatedly happen during the sheet forming process. Consequently, the high grammage sheet is more uniform (with low formation number). The reason for the self-healing process is that low grammage areas have high volumetric flow of water through the porous web, and the suspended fibers have a tendency to accumulate at those areas [8]. In summary, flocs can break apart by mixing in strong shear flow so that fiber dispersion occurs. Preferential drainage at the forming wire can be due to the self healing where the low pulp density contributes to the uniform dispersion of fibers [9]. Additionally, by adding formation aids which cause an increase in negative charge of fibers, repulsion due to same charge type can also cause fiber dispersion. 2.1 Flocculation in Papermaking The pulp suspension, where fiber flocculation can occur, is the intermediate state between the raw material and the end product, and it can control both process runability and product quality [10]. According to Chatterjee [11], even one cubic meter of 1% mass concentration pulp suspension has more than 3 10 16 fibers of assorted size and shapes. One can conclude that the fiber suspension is very complex. Normally, there are three types of flocculation as follows, these are: fiber to fiber flocculation, fiber to fine flocculation and fine to fine flocculation [12]. Fines are 4
small particles which can pass through 75 μm screen, and fillers are included as fines. The flocculation types above have different mechanisms. According to Chatterjee[11], flocculation of fibers is mainly due to a mechanical process. Because of the relatively large size of fibers, inter fiber frictional resistance has been identified as the dominant force in fiber network strength in recent years [13]. The inter fiber frictional resistance is the result of is the normal force at fiber contact points due to elastic bending of fibers in the network. Fiber to fine flocculation and fine to fine flocculation are mainly due to chemical interactions because of the small mass of fines. The particle size of fillers and fines found in the pulp furnish are considered in the colloidal size region. A colloidal is defined as any particle that has some linear dimension between 10-9 m (10 Å) and 10-6 m (1μm) [14]. Colloids are present as dispersed system, and characterized by slow sedimentation and diffusion [15]. Adding retention aids into a fiber suspension can cause the fines retains to the surface of fibers and other fines. Fine to fine and fiber to fiber flocculation are not desirable to papermakers because of the negative effect on optical properties and paper uniformity respectively. The research described in this thesis focused on fiber to fine flocculation and investigating a method to predict its origin as measured by paper formation. Retention aids are used to cause fiber to fine flocculation, so that fines are retained in the web and do not pass through the wire into the white water loop. Fiber to fine flocculation can be classified into patch model, bridge model and microparticle model. The detailed explanation of each model mentioned above will be discussed in chapter 2.4.3. 2.2 Fiber Flocculation Characterization Because fines, fillers, and the participating polymers are all involved in fiber flocculation process, it is quite complex and has been the focus of many investigations [16-22]. Beghello et al., investigated some factors that influence fiber flocculation and used floc size to analyze the extent of flocculation [16]. Cadotte, et al. [17] conducted research in measuring fiber flocculation by the focused reflected beam technique (FBRM). The FBRA measures an average cord size, which, for well dispersed fibers, closely corresponds to the fiber diameter. When fibers flocculate or fillers deposit on fibers, the chord length increases. Thus the FBRM is well-suited to follow fiber flocculation and filler deposition. It was found that initial fiber flocculation rates for cationic polyacrylamide (cpam), polyethylimine (PEI), polyaluminum chloride (PAC) and polyethylene oxide 5
(PEO) with cofactors were similar, but the fiber floc strength was the highest for PEO and the weakest for PEI and PAC. Furthermore, many investigations on the flocculation are based on image analysis [18, 19]. Yan et al. [18] designed a flow loop system and used imaging equipment to investigate a bleached softwood kraft pulp suspension. They demonstrated that the flow loop can be used to simulate the real forming conditions of flocculation, and wet end retention aids effects on fiber flocculation. Floc rheology was also studied. The effect of wet end chemical additives on fiber flocculation was also studied by comparing the image information of the fiber flocculation with or without chemical additives. Wågberg and Eriksson [19, 20] designed new equipment for detection of polymer induced flocculation of cellulosic fibers which is also based on image analysis. By analyzing the grey-scale values of the collected images, when fiber passing through a long transparent tube with the dimension 390 170 3 mm. It was possible to evaluate both a degree of flocculation of the fibers and an average diameter of the formed flocs. Furthermore, based on the gray level response, the following equation was used to quantity the fiber flocculation by comparing the relative difference between a flocculated and unflocculated state [21, 22]. Flocculation Index = V 2 2 2 V 1 V 1 Where, V 1= σ(e 1 )/e 1 V 2= σ(e 2 )/e 2 (Equation 2-1) σ(e 1 )= standard deviation of grey-value of images without polymer addition σ e 2 = standard deviation of grey-value of images with polymer addition e 1 =Average grey-value of transmitted light in the absence of polymer When V 1 is bigger than V 2, the negative sign will be added to indicate fiber dispersion, refer to figure 2-1 right; otherwise, the positive value indicates the extent of fiber flocculation, refer to figure 2-1 left. 6
2.3 Mechanical Causes of Fiber Flocculation The mechanical reasons mainly influence the fiber to fiber flocculation in the dilute stock. The fiber length, coarseness and degree of refining can all influence the extent of flocculation due to different mechanisms and are discussed with detail in the following parts. 2.3.1 The Effect of Fiber Length Fiber length is influenced by fiber species, i.e. hardwood or softwood, and any mechanical treatment that may act to reduce fiber length. Fiber species are generally divided into the softwood category that has longer fiber length (around 3mm to 5mm) and the hardwoods category that has shorter fiber length (1.5 to 2.5mm). Compared to hardwood fibers, softwood fibers more easily entangle with one another causing the resulting web structure to be less uniform. A less uniform (more flocced) structure with poor paper formation will result in the deterioration of tear strength and tensile strength [23]. At this time hardwood fibers with short length are commonly added to make more uniform structure. The reasons that fiber length has an effects on formation can be explained by the relationship known as the crowding factor [24-26] or the crowing number [27]. The crowding factor, Nc has been defined by Kerekes [26] as the number of fibers in a spherical volume of diameter equal to the length of a fiber. It is used to characterize the extent of papermaking flocculation in a water suspension. Crowing number can be calculated using the equation: Nc=(2/3)Cv(L/d) 2 [24] (Equation 2-2) Where Cv is the volume fraction of fibers, L is fiber length, d is fiber diameter. A larger crowding factor indicates that fibers are more likely to flocculate, and the same crowding factor indicates that fibers have the same tendency to form flocs. It has been found that when Nc ranges from 1 to 60, the frequency of collisions increase with Nc. Furthermore, when Nc 60, the contact among fibers in the suspension becomes nearly continuous and thus flocs appear persistently [24]. From the equation 2-2 we can see that the fiber is squared, so even a minor change 7
of the fiber length will bring immense change in the crowding factor, thus flocculation. This explains the increased flocculation observed in softwood pulp suspensions. Some studies have been performed by different methods to investigate how fiber length will influence the fiber flocculation [4, 28]. Yan et al. [4] studied the fiber length effect on fiber suspension flocculation and sheet formation by mixing softwood pulp and hard wood pulp to get different fiber length distribution. They draw the conclusion that the fiber length increase when increasing the ratio of softwood pulp in the mixture, and fiber floc size in the suspension also increases. But in their research, Yan et al. neglected the different diameter of hardwood fiber and softwood fiber. Thus they did not consider the influence brought on by the change in fiber coarseness. Luciano et al. [28] used refining to get different fiber length distribution and indicated that as the fiber length increased at the same fiber diameter, the floc size also increased. Fiber length played the dominate role to control the floc size. All the research discussed to this point was conducted in the environment without the presence of retention aids. Adding polymers can increase inter fiber adhesion and thus the shear resistance of the floc. This promotes agglomeration [29]. Hartley et al. [29] studied the flocculation of fibers by applying cationic polyacrylamides (c-pam) into pulp made of different length fibers. In that research, they used different mesh screens to isolate different fiber lengths. The product of the 60 mesh screen (50 mm) was recognized as short fiber; on the other hand, that from 12 mesh screen (1.68 mm) was called long fiber. Studies with fibers of different lengths showed that the degree of flocculation increases with fiber length, with the most firm flocs, that being stronger and denser, were formed with mixtures of short and long fibers. This is because short fibers did not flocculate by themselves but were captured by flocs formed with longer fibers and make the flocs more stable. 2.3.2 The Effect of Fiber Coarseness Fiber coarseness can be defined as weight per unit length and is normally expressed in units of mg/m or g/m. Coarseness depends on fiber diameter, cell wall thickness, cell wall density and fiber cross section [30]. The coarseness value has a great influence on the paper structure. A high coarseness value indicates a thick fiber wall, giving stiff fibers. Ones they are hard to collapse, thus the paper has higher tensile strength. Based on the research of Ramezani, et al. [30], higher 8
coarseness values represent a poor formation in the formed paper. Furthermore, Kerekes et al. [31], pointed out those coarser fibers also tends to be longer, and therefore it is not always easy to separate the resulting formation caused by coarseness or fiber length. 2.3.3 The Effect of Refining Mechanical refining brings structural change to fibers, and the flocculation behavior of refined fibers is changed. The most obvious change brought by refining is internal fibrillation [32]. Beating can expose more of the fiber s specific surface area. The increase of surface area of the fiber makes the fiber more likely to have increased bonded area. The process of fibrillation is shown as the Fig. 2-2. As seen from Fig. 2-2, the refining or beating process causes the fibers to be shorted, soften, fibrillated, and allows the fibers collapse to a ribbon like form. Refining destroys the primary and S1 layer of the cell wall, and makes the fiber more flexible and more easily to entangle with each other. The PFI mill can give fiber different refining degree in the laboratory. Furthermore, it was found that increased refining led to improved formation which was attributed to the change in fiber characteristics, particularly because of fiber shortening, and slower drainage [33]. This may also results from fine fiber fraction flowing to void spaces, and the so called self healing affect described in section 2 Background. Based on the research of Yan et al., [4] with the refining degree increasing, the number of fibers with shorter lengths increases. On the Figure 2-2 the same fiber before and after beating in a PFI-mill 9
other hand, the number of fibers with longer length decreases. So the refining contributes to shorten the fiber length can be verified. Furthermore, since refining affects several fiber properties, such as shortening the fibers, curling or decurling of fibers, increasing the external fibrillation and all of the properties may influence the fiber flocculation together. The corresponding effects on fiber suspension flocculation may be too complex to interpret for the papermaker [4]. According to Store, et al. [34], formation worsens as a result of external fibrillation when fiber length is constant. Pulp refining improves formation only when the influence of fiber shortening outweighs the affects of fibrillation and fiber straightening. 2.4 Chemical Reasons for Fiber Flocculation Paper can be defined as a random network of arbitrary rectangular fibers, and the mass distribution within the paper can be derived analytically [35]. Normally, after drainage of water, the fibers can form a sheet randomly. On the other hand, by adding wet-end chemical additives, the random distribution of fibers can be disturbed. Adding retention aids can accelerate fiber flocculation. Conversely by adding formation aids, the dispersion of fiber can be enhanced. These were discussed in section 2 Background part with Figure 2-1, and the examples of the resulting fibrous networks were illustrated. The details of how the retention aids influence the flocculation is discussed below. In the flocculation process, where fibers, salts and different polymers are involved, the role of salts, acids and other chemical additives have to be taken into consideration. They can strongly influence the chemical environment of fiber suspension. In Beghello s research [36], he tried to clarify which salts can influence the floc size. They studied the contribution of chemical environment to the fiber flocculation. The conclusion was drawn that floc size is increasing slightly with the increasing of electrolyte concentration such as CaCl 2, FeCl 3, NaCl. This is due to the absorption of cations to the carboxylic groups of the fibers. 2.4.1 The Effect of ph Changing ph by adding alkaline or acid when fibers are in suspension can also influence the fiber flocculation. According to Wågberg et al. [37], decreasing ph causes a decrease in the extent of flocculation. The reasons for this can be explained from the following two aspects. Firstly, 10
decreasing ph by adding acid brings protonates to the carboxyl groups of the fibers. Consequently, the dissociated charge density of the fibers is lower than before. This results in relatively less anionic sites on the fibers surface that can only absorb less cationic retention aids. Secondly, when the ph decreases, the resulting higher ionic strength makes polymers compress to a contracted structure and this lowers the bridge forming ability. Sometimes, the two explanations above can occur at the same time. Furthermore, according to Beghello [38], when ph changes, no obvious change of floc size can be observed. 2.4.2 The Effect of Retention Aids Wet-end retention aids are used in papermaking to capture fine particles onto the fibers. Therefore retention aids allow fines and fillers to be captured in a forming web and thus prevents the fine solid particles from passing into the white water loop [39]. Retention aids are normally cationic or non ionic polymers with high molecular weight. Some retention aids can be used alone, and others must accompany other compounds referred to as cofactor [17]. If the fines are not retained properly, they will negatively influence the runability, cleanliness, and chemical efficiency of a paper machine and pass through the wire into white water. The flocculation brought by retention aids is due to charge neutralization of fibers and fines. Normally, the highly charged wet-end chemical additives serve as the following functions. Firstly, because fibers always have a negative charge, retention aids help in the neutralization of excess negative colloidal charges in the suspension. Secondly, retention aids can coagulate colloidal fibers so that they became large enough to be filtered by the fibrous mat. Thirdly, the retention aids can help site preparation of solid surfaces in the suspension especially the fiber surfaces, making them more suitable for interaction with secondary retention aids such as dual polymer or microparticle systems [40]. According to Wågberg et al. [41], the charge densities of cationic polyacrylamides (C-PAM) can influence the flocculation of cellulosic fibers. The lowest charge density of C-PAM and highest fiber concentration can make the flocculation extent more severe. However, the flocculation speed is irrespective of charge density. Solberg et al. [42] also did some research on the CPAM induced fiber flocculation, and drew the conclusion that when 50% of the surfaces of the fibers and fines are covered by C-PAM, the flocculation extent achieved a maximum. Furthermore, 11
some other researchers focused on the dual polymer retention system. Lars et al. [43], performed some research on using anionic silica particles together with CPAM. They pointed out that when the dosage of micoparticle increases, a flocculation maximum was achieved. Consequently, adding anionic silica particles into C-PAM increases the flocculation. Some new structures of polymers improve the flocculation result. According to Broillette [44], the same paper formation can be achieved by using less structured C-PAM compared with linear C-PAM, especially at the high turbulence levels. Structured polymers include branched and cross linked cationic polyacrylamides (C-PAM). Dual polymer systems are also commonly used in the papermaking process, and sometimes the effect is better than when only one kind of polymer is used. From the research of Cadotte et al. [45], when using polyethylene oxide (PEO) with the cofactor the fiber and fine flocs are much larger than when only polyethylenimine (PEI), polyaluminum chloride (PAC) or PEO are used separately. The sequence of adding chemical additives sometimes can also influence the fibers behavior while in suspension. Based on the research of Ryoso et al. [46] adding retention aids before fillers improve the resulting paper formation. Furthermore, they indicated that the time delay of the adding fillers after retention aids plays an important role determines the efficiency of the retention aids. A long delay deteriorates the retention aids efficiency. However, adding retention aids will negatively influence the paper formation, because it can influence the flocculation of fiber to each other. So there is great interest in finding the balance between good paper formation and a desirable retention result. This can be solved by finding retention aids with high selectivity to only retain the fines and fillers, and not cause the fiber flocculation. 2.4.3 Fines Aggregation Mechanism Fines aggregation mechanism is categorized into charge neutralization, patching and bridging flocculation mechanism mainly based on the charge and molecular weight of retention aids. The detail for the mechanisms is discussed in the following parts. 2.4.3.1 Charge Neutralization Charge neutralization is the simplest mechanism. When the low molecular weight polymers or electrolyte salts added into fiber suspension can compress the fibers electrical double layer, the repulsion among fibers can be diminished. Then, van der Waals forces can attract fibers causing 12
coagulation [47]. If the charge of the low molecular weight polymers is strong enough to attract the adjacent particles, the mechanism will changed to patching as described in part 2.4.3.2. 2.4.3.2 Patching The patching mechanism is based on charge neutralization, and the polymers under this mechanism usually have high charge density and relatively low molecular weight. The process can be illustrated by Fig 2-3. The cationic polyelectrolyte can attach to the surface of the fiber and neutralize the anionic charge of the fibers or fines. The neutralized areas usually remain strong cationic charge brought by retention aids. These areas like patches on the surface of the fines and fibers and they can attract the anionic sites of the adjacent particles only when patches on the surface of the particles are large enough to compete the electrostatic double layer s thickness. Under this mechanism, aggregation is caused by electrostatic attraction [47, 48]. Furthermore, when half of the particle surface is covered by patches, the maximum flocculation occurs for low molecular weight retention aids. The coverage decreases as the molecular weight of the retention aids increases. The flocs formed by patch model are stronger than that of the charge neutralization model [48]. Figure 2-3 Patch Model 13
2.4.3.3 Bridging Flocculation Under this mechanism, a polymer performs like a bridge to connect one polymer to another one or several adjacent particles. Normally, polymers with high molecular weight can perform this mechanism. Besides the characteristics of retention aids, bridging flocculation extent also depends on contact time, the property of solution and the characteristics of the particles. This type of mechanism can be illustrated by Fig. 2-4 below. In this figure, the mechanism of bridging can be divided into three parts: Firstly, the beginning of the adsorption occurs when retention aids are attracted on the surface of one particle. Secondly, the rest of the polymer is extended into the surrounding like a tail. Thirdly, a second particle is absorbed by the same polymer, thus flocculation occurs. The second particle can also perform the same mechanism to connect another one or several particles together. The charge density of retention aids which perform bridge model influence the maximum flocculation dosage greatly. Furthermore, the initial charge of the whole system can also influence maximum flocculation greatly [48]. 2.4.3.4 Comparison of Patch and Bridge Mechanism The relationship of cationic retention aids added into pulp stock and flocculation degree can be explained by the following Fig 2-5. Figure 2-4 Bridge Model 14
Figure 2-5 the Relationship of Polymer Dosage and Flocculation Degree [40] High charge density cationic polymer (left figure) Low charge density cationic polymer (right figure) From the graph, we can infer the influence on the electronic double layers of fibers or fines as cationic polymer retention aids are added. On one hand, if the charges of fibers are neutralized by cationic polymers, the highest flocculation degree can be achieved. Fibers will tend to flocculate at this concentration. The representative of this mechanism is the patch model. On the other hand, when using low cationic charge retention aids, the highest flocculation degree appears when cationic retention aids are over dosed as compared to the dosage causing charge neutralization. The representative of this mechanism is the bridging model. Furthermore, the flocculation mechanism may vary under different circumstances. For instance, the different concentration of retention aids can determine the flocculation mechanism. From the research of Fuente et al. [49], it is reported that there are two polyethylenimines (PEI) doses that produce a maximum flocculation due to different flocculation mechanisms. Low PEI doses induce a fast flocculation through the bridge mechanism. However, high PEI doses induce a slow flocculation by charge neutralization. 2.4.4 Formation Aids Formation aids are normally water soluble linear polyelectrolyte of high molecular weight (anionic polyacrylamides) such as sugar gums. These can be used to promote dispersion of fibers [50]. 15
Unlike retention aids, formation aids are seldom used in the commercial paper making industry. According to Yan [51], the formation aids can be classified into the following groups: (1) additives which can increase the dispersion medium viscosity. (2) additives like mucilage and gums which can decrease the friction coefficient between the fibers, thus letting the final paper be more uniform. (3) polymers with relatively high molecular weight, affecting the rheological properties when the fibers are in suspension. The mechanism of the influence of A-PAM to fiber dispersions is to change the viscosity of the pulp before the drainage of water by altering the inter fiber friction. Normally, high liquid viscosity has higher shear stress. Shear stress is the stress which resists the deformation caused by external force. Based on the research of Zhao et al. [52], higher shear stress and high shear viscosity can improve the paper formation, thus make the flocs within a handsheet more uniform. The high shear viscosity can be achieved by simply adding formation aids. Furthermore, some formation aids with high molecular weight polymers can suppress the turbulence in suspension, known as the drag reduction effect [51]. 2.5 Paper Formation The extent of fiber flocculation or dispersion can directly influence the resulting paper formation. By analyzing paper formation, we can also infer the characteristics of fiber flocculation. It is obvious to tell what good or bad paper formation is by observing transmitted light through the paper using only the unaided eyes. However this is very subjective. A lot of research has been conducted to find the suitable method to quantify the paper formation. Of the various properties of paper, the most commercially applicable one is how paper formation influences print quality. The aim of printing is to present the intended image, which includes the reproduce of tone, color, print gloss, uniformity of color and so on. Good formation is important in most paper grades to obtain a uniform distribution of ink or coating, and also to prevent breaks on fast manufacturing, converting machines, and the printing press [53]. Paper formation affects many of the functional properties including strength, opacity and smoothness in a subtle way [54]. For instance, according to Hubbe s research [55], by using the laboratory results to infer typical machine-made papers results, the improved paper formation can potentially brings the tensile strength to achieve approximately 30% higher than before. 16
2.6 Paper Formation Characterization Formation is the distribution of fiber within the plane of the sheet, and may refer to the transmission of visible light (optical formation) or β-radiation (paper formation). The latter is preferred since it is less subject to scattering of incident rays. Since the wavelength of the visible light (400 to 700nm) is sometimes longer than the diameter of fines, the visible light can be scattered by them. This makes the test result more inaccurate. Furthermore, β rays can be attenuated and absorbed by thin materials like paper, and the extent of absorption is proportional to the mass of the materials being passed through. This relationship can be used to map paper formation accurately, based only on mass density [56]. The thicker part of paper transmits β rays to a less extent than the thinner part. The resulting radiographic image is a map with different gray levels. Based on the gray level at each location, we can easily convert the radiographic images into an array (or map) of the mass distribution of paper. Transmission radiography based on photostimulable storage phosphor screen can be adopted to analyze paper formation, and this is the major method used in this investigation. The detailed methods of how to use this technology will be discussed in Chapter 4. Photostimulable storage phosphor were developed to replace film for quantifying and recording radiographic images. Compared to the film, storage phosphor screens have the following advantages: linear response to 14 C source with five orders of magnitude and 10 to 250 times more sensitivity [57]. 2.6.1 Statistical Methods to Analyze Paper Formation Statistical analysis is a useful method to quantify paper formation objectively. Statistical values used to analyze paper formation normally involve the mean, standard deviation, skewness, and the coefficient of variation. Furthermore, histograms formed by a plot of the probability distribution of grammage contain all the important information listed above. Of main concern in this investigation, are the grammage, histograms and comparisons for different flocculated papers. Sometimes, statistical analysis can also help one discover and magnify the differences which are hardly detectable by visual methods. For example, in figure 2.6, it may be difficult for one person to notice the difference of the formation for the two newsprint samples. However, the histograms are a helpful tool to give all kinds of information in one graph, as seen in Fig. 2.7 below. 17
If the paper has a good formation, the histogram is narrow and tall which indicates a small standard deviation, thus small coefficient of variation. On the other hand, if the paper has a bad formation, the shape of the histogram is a wider bell shape, which suggests a larger standard deviation. Take the newsprint sample 1111-30-12 and 1111-27-4 for example. From the histogram we can clearly see the sample 1111-30-12 has a lower standard deviation thus better formation. Furthermore, the histogram represents a binormal distribution with minor skewness. The skewness measures the asymmetry of the distribution as compared to the normal distribution, when the median is not in the center of range of data values. A positive skewness means that more data spreads out to the right hand side, and the negative skewness means more data points spreads out to the left hand side. Theoretically, when a hand sheet is formed, the grammage distribution should be a normal distribution. However, from the experiments, skewness exists both in the paper from industry and those formed in the lab. The reasons for the existence of skewness will be investigated in chapter 4. Sample 1111-30-12 Sample 1111-27-4 Figure 2-6 Newsprint Sample Comparison 18
Frequency 4.5 4 3.5 1111-30-12 (56) 1111-27-4 (44.9) 3 2.5 2 1.5 1 0.5 0 70 80 90 100 110 Grammage (g/m2) Figure 2-7 the Grammage Frequency Distribution of Newsprint Samples Furthermore, calculating formation number is another simple method to quantify paper formation. The formation number, F, is usually used to denote the coefficient of variation of local grammage, that is, standard deviation divided by mean basis weight and it is often expressed as a percentage. In summary, the obvious advantage of using statistical methods is that it is simple to use and understand. It provides us a platform to compare any aspect of paper quality only by the comparison of some single numbers. For instance, the mean grammage value can be used to choose the paper with desired weight; coefficient of variation can help us quantify the uniformity of paper. Furthermore, statistical analysis is flexible, and can be designed to meet one s specific needs. For instance, in order to compare the skewness that exists in the paper, normal distribution histogram and the skewed histogram can be plotted 19
2.6.2 Spectral Analysis for Paper Formation Spectral analysis is another important tool to analyze the paper formation, and it is relatively complex compared to statistical analysis. However, using spectral analysis not only gives insight into the extent of flocculation and floc sizes, the pattern contained in the sample can also be quantified. According to Chatterjee [11], power spectra can be used to find the principal direction and period of the sample pattern by observing the location of prominent peaks of the power spectral result. While the geometrical scale is more easily perceived through the human senses, the wavelength spectrum is used to obtain the distribution of floc size and intensity in a graphical form [58]. Under the normalized wavelength spectrum result, the whole graph will shift to the right if the sample has a coarse flocculated structure, and the peaks of the graph represent the higher spectral density, thus indicates the existence of flocs at the certain places. 20
3. Problem Statement In this investigation, statistical methods are used to identify how the variables in the paper making process influence the resulting paper formation from the following six aspects: Chemical Aspects Different PH Environment Adding retention aids Adding Formation aids Mechanical Aspects Different fiber length as the same coarseness Different settling time before the drainage of water Refining degree Specifically the goal is to exam bleached softwood papers generated under different conditions using transmission radiography and statistical analysis. Consequently, the conditions of forming the paper in standard lab can be identified by analyzing the summarized analyzing results. 21
4. Experimental Procedures The experimental approach was to adjust different parameters when making laboratory handsheets in order to study how one particular parameter such as setting time, fiber length or the addition of flocculating retention aids influence the resulting paper formation. After collecting all of the data above, subsequent analysis was performed in an attempt to separate the effects of one or several process variables. Consequently, by simply performing paper formation tests, the parameter in the paper making process might be identified. The detailed procedures and explanations of each step are illustrated in the following parts. The experimental procedures used in this study were divided into four steps as shown in the schematic below, Fig 4-1. Step 1 Fiber Preparation Original Cut Step 2 PFI Milling Step 3 Dilute Stock Preparation Setting Time ph Chemical Additives Step 4 Handsheets Testing and analysis 4-1 Schematic Illustration of the Experiment Plan 22
During the first step, fibers were prepared from dry pulp mat by using either the original fiber or by cutting the pulp mat in order to shorten fiber length. Fibers then were processed in the second step by beating in the PFI mill, causing fibrillation. The third step involved fiber dispersion and treatment with different chemical additives or by adjusting the settling time after turbulence to affect the flocculation. Laboratory handsheets were then formed using standard methods. Finally, in step four, the formation of the handsheets were measured and analyzed. Each of the steps will be discussed in the detail in the sections that follow. 4.1 Fiber Preparation The material used in this study is dry, bleached, softwood kraft pulp. Fiber length was measured by two methods and found to be approximate 3.08 weight weighted average length. As discussed in Chapter 2.3.1, fiber length is a very important physical parameter which dominates the extent to which fiber flocculation occurs. Consequently, at this step, the fiber length was adjusted by cutting fibers, which has the affect of shortening fibers without bringing any change to the coarseness. 4.1.1 Preparation Approaches About 31.5 g lab pulp mat was weighted for the preparation of the thick stock at a consistency of 0.3%. That was used to make laboratory handsheets. For pulps that were cut to reduce fiber length, a cutter was used to cut the pulp lap into 4mm 4mm squares aimed at changing the fiber length into half of its original length. Figure 4-2 illustrates what the cut pulp lap looked like before dispersion in water. Figure 4-2 Fibers Cut into 4mm 4mm Squares in Order to Change Fiber Length. 23
4.1.2 Fiber Length Analysis The pulp samples were sent to Andritz Inc. Pilot Plant/R&D Laboratory, Springfield, OH for fiber length analysis. Details of the fiber length results are shown in the Table 4-1 Table 4-1 Fiber Length Test for Original and Cut Fiber SAMPLE O1 O2 O3 CLIENT IDENTIFICATION Sample 1 Sample 2 Sample 3 SWD SWD SWD original 4mm 4mm 3mm 3mm Date and Time 2-24-09 2-24-09 2-24-09 Length Weighted Avg Length (mm) 1.86 1.48 1.40 Average Width (µm) 26.39 26.11 25.36 Weight Avg Width (mm) 30.81 29.49 28.63 Arithmetic Avg Curl (mm) 17.95 17.45 17.23 Arithmetic Avg Length (mm) 0.774 0.719 86 Arithmetic Avg Width (mm) 20.88 21.91 21.55 Weight Weighted Avg Length (mm) 3.08 2.29 2.22 Surface Area (m 2 /kg) 900 1084 1034 Specific Volume (x10 3 m 3 /kg) 1.54 2.15 2.16 Different representations of fiber length are presented, and each one of them has its own calculation method. Arithmetic Average Length and Weight Weighted Average Length are explained as follows, Arithmetic Average length is rarely used to quantity the effect of fiber length to other experimental parameters, because large amounts of short fibers will greatly influence the average, while only representing a small part of the total weight [59]. The following equation can be used to calculate arithmetic average length of fibers: 24
Arithmetic Average Length, L 1 L 1 = n il i n i (Equation 4 1) Where: n is the number of fibers in each category i and l is the fiber length in each category [59]. Weight Weighted Average Length, takes the fiber weight into consideration, can be calculated by the following equation, Weight Weighted Average Length, L 2 L 2 = n 3 il i 2 (Equation 4 2) n i l i As seen from the table above, by cutting the fiber into 4mm 4mm squares, the fibers were 25.8% shorter than the original ones. By cutting the fiber into 3mm 3mm squares, the fibers were 27.9% shorter than the original ones. At the same time, only minor change of the fiber diameter can be seen from the table. Consequently, the expected cutting result was achieved and 4mm 4mm squares are used in the following experiments. 4.1.3 Dispersion In order to let fibers disperse better, and avoid dry fiber clumps remaining from the pulp dry lap, the pulp was soaked for at least 24 hours in order to fully wet and debond fibers. Pulp was then diluted to 0.3% percent to prepare the thick stock. Specific Procedures of Dispersion (1) 200 ML of water was added to soak the pulp mat. It was then placed into the disintegrator and fibers were dispersed sufficiently for about 3 minutes. (2) Water was then added to dilute the pulp to achieve around 0.3% consistency. 4.2 PFI milling A PFI mill was used to beat the fibers in order to simulate the commercial refining process. In the center of the PFI mill, the rotating internal beating roll gives fibers different refining degree by rotating a certain number of times. In the experiment 0, 2500, 5000 and 10,000 revolutions 25
were used to beat the pulp at a different extent. This provided realistic freeness values normally found in commercial pulps. Specific Procedures of PFI Milling (1) After disintegrating, the pulps were placed into a filtering flask to remove a certain amount of water until the final weight was 300 grams (2) The revolution count of the internal beating roll was set to give the fibers a desired degree of refining. (3) TAPPI Test Method T-227 om-94, Freeness of pulp (Canadian standard method) [60] was followed to test the freeness of the pulp with different degrees of refining. 4.3 Dilute Stock Preparation The Definition of settling time, how to adjust the ph and the proper method to add the chemical additives using Britt Jar are discussed with detail in the following parts. 4.3.1 Settling Time This step occurs in the process of making handsheets, when the fibers are in suspension in the handsheet mold. Under normal handsheet preparation, the dilute stock is aggregated before immediate drainage. By waiting a period of time after aggregation before draining, the fibers tend to flocculate due to entanglement and inter fiber forces. Some of the handsheets were made by the standard method and are indentified as settling time equals to zero. Handsheets that were allowed to settle were held for two minutes after turbulence before drainage are identified as settling time equal to two minutes. 4.3.2 ph In order to test how the ph changes influence the fiber flocculation, both alkaline and acid conditions were tested. Specific Procedures of adjusting ph (1) NaOH and HCL solution were prepared in order to adjust the ph of the pulp dilute stock in the handsheet mold. 26
(2) The measured amount of pulp in a bucket were adjusted to the ph of around 11 by adding the NaOH solution; or to a ph of pulp suspension to around 2 by adding the HCL solution. (3) The ph was allowed to mix and readjusted until it became stable. It was then poured into the handsheet mold to make the handsheets. 4.3.3 Adding Chemical Additives At this step retention aids and formation aids were added into the suspension to change the extent of flocculation. Retention aids are supposed to increase the degree of fiber flocculation; on the other hand, formation aids are supposed to make fibers in suspensions more dispersed. 4.3.3.1 Retention Aids Retention aids are commonly used in the paper industry not only to retain additives and fillers, but to maintain adequate efficiency of drainage on the paper machine. In this study, retention aids were used to create fiber to fine flocculation and fiber to fiber flocculation in order to affect paper formation. Material CIBA ALCOFIX 159, a low molecular weight cationic polyamine was added to aid in the drainage, retention, formation and runability in the papermaking process. The chemical structure of this chemical is shown in Fig 3.3 and it belongs to the polyamine (epichlorohydrin- dimethylamine) family. Principally, cationic runability aids can be used at the dosage rates of 0.01% to 5% based on dry weight of the furnish. The reason for selecting CIBA ALCOFIX 159 was because of its high cationic charge which can neutralize the negative charge contained on the fiber. Furthermore, CIBA ALCOFIX 159, polyamine performs as a patch model in the wet end, Figure 4-3[61] Chemical Structure of CIBA ALCOFIX 159 27
and it can fix on the surface of the fibers firmly in order to make it stable to turbulence. Furthermore, as mentioned in the Chapter 2.4.3.4, the dosage causing the maximum flocculation can be easily identified by simply finding the point when all of the charges contained in the fibers are neutralized. In order to model the real paper making process and to test how retention aids influence the fiber flocculation, the Brit jar geometry in order to add a continuous turbulence to the handsheet forming process. Also particle charge detector was measured to find the suitable dosage of the retention aid by detecting the isoelectric point of the zeta potential so that flocculation is at a maximum. When the Zeta potential approaches zero, the extent of electro static repulsion between the particle and the fibers in suspension reaches a minimum [47]. Consequently, the whole system became stable, and flocculation can achieve a maximum. The process used to measure surface charge is seen in Fig. 4-4. The Britt Dynamic Drainage Jar was introduced in the mid 1970 s by Britt [62], who provided an effective tool to simulate the paper machine by incorporating a certain level of turbulence in the laboratory scale test. It can be used to study the performance of retention aids under different turbulence and shear [62, 63]. In this study, the theory of the Britt jar was used. An impeller was added to the standard handsheet mold as a modified Britt Jar, in order to mix the pulp and retention aids for 5 minutes before the drainage of water. During the 5 minutes of mixing, the pulp and chemical are mixed thoroughly and the chemical has enough time to neutralize the negative charges of the fibers and to cause fiber flocculation. 1. Set up Modified Britt Jar 2. Prepare Retention Aids 3. Add retention aids into fibers solution mix it for 5 minutes 4. Take sample to test particle charge and make handsheets Figure 4-4 Process flow for test the CPAM s influence to fiber flocculation and resulting formation 28
Specific Procedures for Retention aids preparation (1) 2.4g of polyamine was weighed and 1000ml of deionized water was added and mixed to let the polyamine dissolved completely (2) 100ml of the polyamine solution from above was placed into a metric flask and diluted with water to 2L. The final concentration was 0.06g/L of retention aids. (3) The dilute retention aid solution was used to achieve 0.05%, 0.1%,5%,0.375%, 0.5% 0.75% (Based on the dry weight of fibers) additions in the dilute stock in the handsheet mold. 4.3.3.2 Particle Charge Testing In an aqueous suspension of pulp, the electrostatic charge at the solid and liquid interfaces play a very important role fiber flocculation. If a liquid is moved under pressure along the particle surface, counter-ions are also carried along and sheared off the particle, which can be converted in streaming potential. The Mütek PCD-03 (ph) Particle Charge Detector can be used to display a streaming potential either with positive charge or negative charge. The streaming potential is a relative value and may be affected by PH, viscosity, temperature and other variables. In order to quantify the exact charges at the surface, a titration is performed using an oppositely charged titrant until a streaming potential of 0mV is measured. The PCD technique can be widely used in water phase and is especially useful in the paper making industry for controlling coagulant dosage of chemicals and rheological properties of the liquid [64]. The description of the Particle Charge Detector device is shown in the following, Figure 4.6. The central element of Particle Charge Detector is a plastic measuring cell and a closely fitted displacement piston as shown in the illustration below. 29
Figure 4-5 Diagram of a Particle Charge Detector, (PCD) [65]. Where, 1. LED Cationic 8. Guide for measuring cell 2. Switch for displacement piston drive 9. Contact pins 3. LED anionic 10. Measuring chamber 4. Burette tip holder 11. Power LED 5. Displacement piston suspension 12. Front panel 6. Displacement piston 13. Potential mv display 7. Measuring cell After placing the liquid into the plastic measuring cell, the piston is inserted into the cell where there is a narrow annular gap. The piston oscillates up and down so that the fluid moves continuously. This causes an intensive liquid flow which free adsorbed counter ions from the surface of the cell and piston, thus separating them from the absorbed sample materials. Finally built in electrodes can detect the current induced by the flow of the counter ions which has been ampli- 30
fied. After the polyelectrolyte titration, we can also use streaming potential to identify the point of zero charge, then the charge of the sample can be calculated [65]. Specific Procedures of using Particle Charge detector The experimental procedures of using Particle Charge Detector in the research are listed as follows: (1) The displacement piston and plastic measuring cell were washed sufficiently then rinsed using deionize water twice then kept it dry. (2) 10ml of pulp sample was mixed with the chemical and placed into the measuring cell, then motor was turned on (3) Once the system was stable, the charge number was recorded and the proper titration liquid was selected. (4) The sample was titrated with a titrant of opposite charge until the charge achieves zero. If possible, the titrant was diluted to let the consumed titrant be between 2 to 20 ml (5) The value of particle charge is calculated using the volume of titration liquid added into the sample Specific Procedures for calculating the particle charge Titrant solution must carry the opposite charge of the sample, and the titrant consumption should be controlled between ml to 10 ml. Pes-Na (polyethene sodium sulfonate, with anionic charge) and Poly-Dadmac (Polydimethyl diallyl ammonium chloride, with cationic charge) were recommended as the standard reagents because of their charge densities that were practically phindependent [64]. (6) Calculation of cationic/ anionic titrant solution demand The following formula was used to calculate the charge concentration of the pulp by using the titrant known particle charge, volume and concentration C 2 = C 1 V 1 V2 (Equation 4 3) Where, C1 is the concentration of titrant in [N] or [eq/l]; C2 is the concentration of the pulp in 31
[eq/l]; V1 is the consumption of titrant in [l] and V2 is the pulp sample volume chosen to be titrated in [l] After getting the charge concentration of the pulp sample, the specific charge quantity of the fibers was calculated using the following formula, (7) Calculation of specific charge quantity q = V c m (Equation 4 4) Where, V is the consumed titrant volume [l]; C is the titrant concentration [eq/l]; m is the solids of the sample pulp or its active substance [g] and q is the specific charge quantity [eq/g] 4.3.3.3 Formation Aid Additions The formation aid used in this research was MAGNAFLOC 156, anionic flocculent. MAG- NAFLOC 156 is a fully anionic polyacrylamide flocculent with high molecular weight. For the fibers are surrounded by negative double layers, by adding MAGNAFLOC 156, excessive negative charge causes the fibers repelling one another, thus becomes a dispersed suspension. By adding formation aids, the paper formation can be improved. Specific Procedures for Formation aids preparation (1) 0.5g MAGNAFLOC 156 was placed into a 100mL dry bottle followed by 3ml ethanol, then 97mL of water (temperature below 40 degree centigrade) was poured rapidly. (2) The bottle was then placed on automatic stirrer for 2 hours in order to let the chemical mix with water thoroughly. The final concentration of the formation aid is 0.05g/L (3) The dilute formation aid solution was used to achieve 0.05%, 0.1% and 0.17% additions in the dilute stock in the handsheet mold 4.4 Handsheets Testing and Analysis In this investigation, transmission radiography was used to obtain the basis weight maps for each handsheet. Statistical analysis was the main method to quantify the experimental results. This will be discussed in detail below. 32
4.4.1 Transmission Radiography Based on transmission radiography, β-radiographic imaging for all paper samples was obtained using Molecular Dynamics PhosphorImager System, serial number, 76236. A PhosphorImager System uses storage phosphor technology as a faster, more accurate replacement for x-ray film. Under this system the β-emitting 14 C source is poly 14 C methyl methacrylate film. The dimension of the storage phosphor screen is 102mm 102mm 1mm and the β source has a total activity of 6.17mCi and surface activity 11.8±0.1 mci/m 2. The mechanism of storage is that the BaFBr: Eu complex was be activated to an excited state when exposure to radiation source of sufficient energy such as 14 C [66]. The mechanism is illustrated in Figure 4-6. Storage Phosphor Screen was mounted onto thick layer of aluminum to be sure of the flatness of the surface [57]. Storage Phosphor Screens stored energy in the crystals (BaFBr: Eu 2+). When the conversion of Eu 2+ to Eu 3+ happens, electrons are escaped from the crystals conduction band and then kept in bromine vacancies to form temporary F centers. When scanned with about 633nm wavelength laser, the trapped electrons move back to the crystals conduction band. This process releases photons at 390 nm [67]. The light emitted in this process is detected and quantified accordingly. Figure 4-6 Schematic illustration of the storage phosphor process [67] 33
The following equation is used to calculate the grammage under Molecular Dynamics Phosphor Imager System. The following equation can be used to determine the thin materials with the grammage less then 120g/m 2 w = In R MD t In c I 0 µ (Equation 4 5) Where, w is the grammage [g/m 2 ]; R MD is the instrument response; T is the exposure time; I 0 is the surface activity of the radioactive source with no material, [mci/m 2 ]; µ is the mass absorption coefficient and c is a constant which can be calculated using c = R MD t I 0 Specific Procedures of β-radiographic Imaging for paper According to the Operations Manual edited by Kwon [68], the flowing steps were followed to get the β radiography image of samples, (1) The optimal exposure time was determined based on the arithmetic grammage of the handsheets. For handsheets with grammage 30g/m 2, the exposure time is 30 minutes; for handsheets with grammage 60g/m2, the exposure time is 45 minutes. (2) The scanner was turned on at least 30 minutes before use, and the storage phosphor screen was erased for about 10 minutes. (3) The handsheet was sandwiched between 14 C source and erased storage phosphor screen source for a certain time period and be sure the handsheet was flat and did not cover the calibration bars on the cassette (4) When the required exposure time was reached, the storage phosphor screen was quickly transferred to the scanner. (5) After scanning, the results were checked using Image Quant software and saved. 4.4.2 Using Statistical Method to Quantify the β-radiography Images A Matlab program and the grammage probability distributions are mainly used to quantify the resulting images of the handsheets, and the detailed methods are discussed in the following parts. 34
4.4.2.1 Matlab Program In order to convert the β-radiography Images into a basis weight map which can be used to perform statistical analysis, a Matlab Program based on Molecular Dynamics files is used in this study. The useful information obtained from the Matlab program includes mean basis weight, standard deviation, power spectra and the whole basis weight map. Specific Procedures of using the program based on Matlab (1) Each image was calibrated using the four bars generated by the Mylar Samples tested at the same time. A new calibration file was generated for each handsheet to avoid the influence of inaccuracy exposure time. An sample of a calibration file is as follows: 11.8, 0, 0, 0 30, 0, 5698.99, 19021.34 (Blank) 30, 8.16, 4308.18, 16538.68 (the fourth bar) 31, 21.35, 3035.53, 13945.52 (the third bar) 45, 27.21, 2560.32, 12748.9 (the second bar) 30, 32.97, 2096.11, 11535.24 (the first bar) Meaning of each number are described in the following Table 4-2 Table 4-2 Meaning of Each Column in One calibration File First Row Second row to m th row Second row to m th row Surface activity Exposure time Exposure tiem N/A N/A N/A 0 Instrument response in PIU Gravimetric grammage Instrument response in PIU Gray level in TIF file Gray level in TIF file (2) The basis weight of each calibration bar was measured, then each average phosphor image unite (PIU) and standard image file (TIF) number was recorded, and put in the proper place in the calibration file (3) Gravimetric grammage and exposure time was filled into the proper place of the program before running 35
(4) The central part of the handsheet was chosen as Region of Interest to analysis, then after the project file was saved, the program was run 4.4.2.2 Analyzing basis weight map After running the Matlab Program, a grammage map containing thousands of grammage data is saved under the result file for each handsheet. After copy and paste the basis weight map onto Matlab, we can see each number contained in the basis weight map. Thus copy and paste the number to excel can help us get the histogram of each handsheet and amplify then compare the differences easily. Specific Procedures of analyzing and compare histogram using Excel (1) The frequency number and grammage number along with the histogram generation by Better Histogram were recorded for analysis (2) The frequency number was normalized use the equation below F = F n F1 + F2 + Fn Where Fn is the frequency number in each category 100 (Equation 4 6) (3) Frequency was further normalized in order to compare with others using the following equation. F = F W a W i (Equation 4-7) Where, W i is the ideal grammage ( the grammage number desired to modify each histogram with desired mean in order to compare) and W a, is the average grammage number (4) Grammage number in each category was also normalized using the equation below G = G n Wi W a Where Gn is grammage number in each category; Wi is the ideal grammage (the mean grammage number desired to modify each histogram in order to compare) and W a, is the average grammage number (Equation 4-8) 36
(5) The histogram was drawn using the normalized grammage as the x- axis and normalized frequency as the y axis, to compare each handsheet Specific Procedures of analyzing Skewness using Matlab and Excel (1) Skewness is calculated using the Matlab command below Reshape (BASIS_WEIGHT, 1, A*B) (Where A is the number of data point in basis weight map each line Skewness (ans) B is the number of data point in basis weight map each column) (2) The histogram of each handsheet ( use grammage step 0.1) was drawn and standard deviation and mean were recorded (3) A normal distribution was generated using the same mean, and standard deviation of the sample handsheet (4) The normal distribution generated was adjusted in order to fit the actual distribution by adjusting the following three variables: the mean, the standard deviation and the area under the curve. By adjusting the three variables, the left side of the histogram was fitted as closely as possible (5) Another graph was generated using each frequency of the actual distribution by subtracting the normal distribution as y axis and the grammage accordingly as the y axis (6) The graph generated in step (5) was fitted using a normal distribution graph and the mean standard deviation and the area of this normal distribution was recorded. 37
5. Results and Discussion The results obtained by different experimental procedures discussed in Chapter 3 are analyzed with detail in this chapter. The way that changes of the process parameters influence the formation number and the grammage distribution are the main two aspects discussed in the following sections. 5.1 The Effect of Settling Time and Degree of Refining After making the handsheet samples using the standard method, each sample was tested by transmission radiography, and the grammage map for a selected region was obtained. A grammage map is a representation that uses different grey levels to indicate the mass distribution for each pixel within a region. In the images shown in Figure 5-1, the lighter color represents higher grammage distribution and the darker color represents lower local grammage. The figure shows two representative images of handsheets, one prepared using the standard procedure, and the other allowed to stand for two minutes after stirring in the handsheet mold. Figure 5-1 Radiographic image of two handsheets. The image on the left is for 2500rev BSWK pulp formed using the TAPPI standard method. The image on the right shows a handsheet allowed to settle for 2minutes after stirring. Both handsheets had grammage of 30g/m 2. 38